New Horizons in the Synthesis, Properties, and Applications of MXene Quantum Dots

The progress of MXenes, a 2D layered structural material since its discovery in 2011 recently arouses a great deal of attention due to their plethora of applications in a diverse range of fields. Their excellent properties in terms of surface chemistry, optical activity, electrical conductivity, presence of abundant active catalytic sites, tunable band gap structure, ease of surface functionalization, and good biocompatibility make them an ideal candidate. The authors aim to guide the readers through various aspects such as their unique properties, synthesis techniques, and the recent advancements associated with MXene quantum dots (QDs). Herein, this review serves as a one‐stop point for prospective researchers to gain a better understanding of the application of MXene QDs in various fields including electrochemical detection, biomedical applications, and energy conversion and storage applications.


Introduction
Lately, the research fashion has taken a resounding shift from bulk materials to nano-sized, 1D, or 2D materials. Miniaturization has brought in much simplicity and flexibility along with exciting structural and electronic features. [1] The nano size of the particles, particularly the metallic nanoparticles have their maximum absorption in the UV-visible region. The absorption bands are sensitive to the chemical environment, and any changes in the environment immediately affect the spectra of the nanoparticles. The phenomenon allows nanoparticles to have varied optical and electrochemical properties. [2][3][4] Similarly, many other 2D inorganic materials such as graphene, graphitic carbon nitride, transition metal dichalcogenides, and black phosphorous are under passionate research to meet new application requirements. [5][6][7][8] Recently in 2011, Gogotsi and Bar-capacity especially in developing MXene QDs as fluorescent nanoprobe for diverse forms of application. [40,41] A timeline recording the development of MXene QDs for the fields of electrocatalysis, photocatalysis, and photoelectrochemical applications is seen in Figure 1.
The research interest of MXene QDs has been continuously growing owing to their diverse range of applications. Figure 2 represents the number of articles in journals by searching the keywords quantum dots, MXene using the logical operation "AND" in Web of Science.
Therefore, the authors of this article have aimed to highlight the properties and the latest promising research work on MXene QDs. The article also presents a critically analyzed compilation of synthesis techniques as well as insights into the diverse applications of different MXene QDs. This review gives an insight into the advantages and disadvantages of each synthesis technique so that future researchers can make a well-informed decision. An overall scan of the database shows only a limited number of reviews for MXene QDs and most of them focus only on one of the aspects. Hence, this article was written with the hope to provide all the imperative information and knowledge required to build novel MXene frameworks to satisfy the requirements of novel applications. The authors have also elaborated on the current limitations and future research prospects in this field.

Properties of MXene QDs
The interesting optical properties such as light absorption, photoluminescence, and electrochemiluminescence of MXene QDs have attracted much attention for application in multimodal detection, [42][43][44] optoelectronic devices, biomedicine, and catalysis. [1,4,45,46] MXene QDs generally show a size below 10 nm with the thickness ranging from single to very few layers. MXene QDs show efficient light absorption in the ultraviolet, visible, and near-infrared regions, owing to the tunable electronic band size. They can also convert the absorbed energy into other forms such as heat or chemical energy. [1] A strong dependence of the electronic structure on the size of the nanoparticles brings in tunability for the MXene QDs. [41] The energy levels of orbitals may be adjusted by controlling and modifying the synthesis strategy for obtaining nanocrystals of the desired diameter. [47] We understand that the optoelectronic activity of a QD is a function of its size, a strong deterministic effect caused by the spatial dimension's restriction. Hence, more energy is used up by the smaller QDs to get excited when compared to the larger-sized QDs. This results in a blue-to-red shift concerning the size of the QDs. [48,49] This is an advantage when compared to conventional dyes and fluorophores, where the absorption or emission spectra are observed at fixed wavelengths only. In the case of QDs, excitation is possible at any single wavelength below their characteristic wavelength just by varying the size of the nanoparticle. MXene QDs show strong excitationdependent fluorescence emission when excited at different wavelengths mostly in the range of 355-505 nm, mainly due to the size effects and surface defects. This property of MXene QDs is particularly exploited in many in vitro, in vivo imaging applications. [50] Another important aspect regarding MXene QDs is their colloidal stability which is of great significance for their diverse forms of applications. [51,52] Owing to their colloidal properties, a direct correlation of the absorption of MXene QDs with their concentration has been established using the Beer-Lambert law. Balanced attractive and repulsive forces between the colloidal MXene QDs are necessary to maintain their continuous phase stability in an aqueous medium, preventing any considerable aggregation or precipitation of the particles. It has been observed that the choice of synthesis route largely influences the colloidal stability of MXene QDs. For instance, MXene QDs obtained via HF-based etching produce several defects in the product, which cause contamination of the material with oxygenic or hydrophilic terminal groups, resulting in decreased colloidal stability. [53,54] MXene QDs prepared by the electrochemical etching method, along with fluorination, surpass degradation and show increased colloidal stability compared with that obtained by the HF etching. Synthesis of functionalized MXene QDs with hydrothermal treatment produces a wide active area and high colloidal stability in an aqueous solution. In addition, MXene QDs treated by ethanol refluxing reveal a lower degree of oxidation which results in enhanced colloidal stability and quantum yield compared to those synthesized by the microwave heating method. [55] However, smaller MXene QDs often show low long-time stability, which largely limits their applications.
MXene QDs are better catalysts and co-catalysts when compared to other QDs because of their diverse surface chemistry, availability of abundant adsorption sites, strong metallic conductivity, and high charge transfer ability. [56] The extremely small work function, hydrophilicity, high dispersibility and solubility in aqueous media, chemical inertness, conductorinsulator shift, reversibility, tunable band gap, surface functionalities, and biocompatibility, in comparison to their 2D equivalents, further broaden their electrocatalytic and photocatalytic applications. [57] Furthermore, modification of surface functional groups and doping with heteroatoms significantly improves the catalytic properties and analytical parameters. [58] Photocatalytic reactions at the surface of the MXene QDs involve redox reactions under light irradiation, which is one of the most efficient sustainable methods to address energy-related and environmental issues. Excellent wide spectral response to produce more photogenerated carriers, boosting the photogenerated carrier accumulation and carrier separation, decreasing the carrier resistance, and avoiding their recombination achieved by the MXene QDs make it a promising candidate for a large variety of photocatalytic applications including energy conversion, pollutant degradation, and biomedical applications. [19,59] A report shows the use of Ti 3 C 2 QDs as a co-catalyst to overcome the inferior spectral response range and carrier recombination of g-C 3 N 4 for enhancing its photocatalytic performance. Ti 3 C 2 QDs having high conductivity and plenteous catalytic active centers promote electron capture, increase the carrier lifetime, and accelerate the redox reactions at the surface of the catalyst. This method was applied for H 2 generation with a much higher reaction rate compared to that of g-C 3 N 4 , Pt/g-C 3 N 4 , and 2D MXene/g-C 3 N 4 under similar reaction conditions. [60] The tunable bandgap of the MXene QDs attributed to their size effects along with immense unsaturated sites helps to effectually absorb and activate the CO 2 moieties for highly selective photocatalytic reduction reactions. [61] Defect engineering strategies to MXene QDs-based photocatalysts have been found to improve their catalytic activity. For instance, Ti 3 C 2 QDs with rich oxygen vacant sites and Ti 3+ sites, bound to mesoporous C 3 N 4 hollow nanospheres resulted in higher adsorption and activation of N 2 species. The quantum confinement of the MXene QDs also amplified the photo-absorption of C 3 N 4 , steering superior carrier accumulation and excellent N 2 photo fixation ability. [62] In addition to that, the abundant carbon vacancies of the QDs favored its attachment to the 3D inverse opal g-C 3 N 4 ,

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generating an interfacial Schottky barrier. This elevated the work function of the catalyst, increasing the carrier separation and giving a much higher H 2 O 2 yield. [63] Another blessing for the MXene QDs is that they show excellent resistance to photobleaching on continuous excitations, unlike the other fluorescent tags. [20] For example, an article reports the fabrication of Nb 2 C MXene QDs for selective fluorescence detection. The studies report their excellent photobleaching capacity as well as enzyme-responsive biodegradability. [64] Here, the existence of various functional groups in different proportions is also responsible for the unrivaled diversity exhibited by them. The surface chemistry in turn depends on the etching (synthesis) conditions, delamination method, and the precursor material. [57,65] The impact is widely observed in the mechanical properties of the material. Mostly, MXene QDs are terminated with different surface functionalities such as OH, O, and F groups. The -F terminated MXene QDs are weak compared to those with OH or O groups because the F group gets replaced by either of these. In addition, different temperature treatments can lead to the conversion of OH to O groups, and further, if they get in contact with Mg, Al, and other metals, they tend to decompose into bare MXenes. [66,67] In the case of MXenes, more complex modeling is required to present the random adsorption and coexistence of numerous terminal groups, whereas in the case of MXene QDs, the modeling and DFT calculations could be done in a facile manner owing to their narrow and finite dimensions. [61] DFT calculations could be employed to study the atomic and electronic characteristics of MXene QDs. This method also provides insights into the density of states and quantum yields of MXene QDs based on their molecular orbital energies. It has been demonstrated that the HOMO density of the pure MXene QDs is primarily situated at the edges rather than the center of the material, while the LUMO occupies the center. This validates the edge particles being the donor and the center moieties being the acceptor. On doping the pristine MXene QDs with heteroatoms such as S and N, the energy gap between the donor and acceptor increases, and the HOMO density is positioned at the smaller edge portion while the LUMO density is at the smaller center portion. This suggests switching the edge particle's activity as donor or acceptor based on the type of functionality linked to the center moiety. It has been found that the HOMO-LUMO overlap in such heteroatomdoped MXene QDs is lower, resulting in a smaller ΔE value and higher emission quantum yield. The unsatisfied valency of these heteroatoms functions as the energy gradient, leading to higher charge transfer recombination, leading to enhanced luminescence. [20,68] DFT results further suggest that N-doping improves the radical capturing capabilities of the F and O functionalities in MXene QDs. [69] Another study on MXene QDs/Cu reveals the spontaneous flow of electrons from the QDs to the metal owing to the lower work function of the QDs compared to the metal. The electrons transferred are chiefly concentrated at their heterointerface, showing higher filled states crossing the Fermi level. This validates the higher conductivity of the MXene QDs/Cu interface compared to the individual constituents. [70] Such greatly conductive surfaces favor electron-trapping and facilitate rapid electron transport channels for diverse forms of applications. [71,72]

Synthesis Strategies and Characterizations
The challenge of fabricating MXene QDs lies in the dimensional reduction of layered transition metal carbide/nitrides into MXene QDs of a few nanometers. This involves alteration of the chemical as well as photo-physical properties such as increased surface area, quantum confinement, and edge effects. [73] Therefore, it is pertinent to learn about the different synthesis techniques and their specifications to take advantage of the strong influence that the synthetic parameters have on deciding the product properties. [1] The synthesis techniques can be categorized as top-down and bottom-up approaches. In the top-down approach (which is mainly preferred), the larger and bulk precursor molecules are broken down into smaller units by chemical, physical, or electrochemical methods to attain oxygen-containing MXene nanosheets. These then serve as sites for creating defects and chemically reactive sites. [1,5] Further, the layered structure can be easily destroyed into smaller counterparts by different cutting techniques, eventually leading to the formation of a quantum dot. [74] The studies reveal that different ways of breaking down the MXene sheets can also strongly influence the properties of the quantum dots. Examples include hydrothermal/solvothermal, ultrasonication, and electrochemical cutting as shown in Figure 3. [25,52,53,75,76] However, there can be instances where the procedure can be more complicated. Several parameters such as etchant type, etching time, temperature, and concentration determine the properties of the produced MXene QDs. [25,77] Several advantages such as low capital, lower temperature requirement, and the easy procedure have made this strategy more preferred. However, crystallographic damages, internal stress, multi-step, and time-consuming nature of the approach remain a challenge, preventing complete exploitation of the technique. [78][79][80] In case of the bottom-up approach, it is more efficient, cheap, and less laborious when compared to the top-down methods. Pyrolysis, chemical vapor deposition, and electrodeposition are some methods under the present category. The synthetic approach under the present discussion, because it starts at the molecular level and is built upward, assures lesser defects in the final product as well as a more homogeneous composition. It enables better control and improved structure but is an expensive, more energy-consuming and sophisticated technique. [81][82][83] Such challenges have limited the research on bottom-up strategies, which leaves an opportunity for the researchers to explore the techniques.

Ultrasonication for the Preparation of MXene QDs
Exfoliation techniques have emerged as an important synthesis step for the accelerated delamination of bulk material into smaller sheets or particles. [84] Chemical oxidation, thermal oxidation, and ultrasonic exfoliation are some of the types and these are mostly performed where achieving increased surface area/pore volume is the goal. [1,5] The sonication technique has become an energy-conserving alternative to conventional hydrothermal and solvothermal techniques. [19,85] Zhang et al. developed MXene-based fluorescence QDs by employing one-step ultrasonic exfoliation techniques. An excellent fluorescence www.advmatinterfaces.de quantum yield of 7.7% was observed for the hence-prepared QDs. The excitation-dependent and pH-independent emission characteristics of the QDs served as great advantages along with high selectivity and sensitivity toward Fe 3+ detection in seawater and serum. [80] Likewise, in an article, ultrasonication techniques were employed in an N 2 environment to produce MXene QDs and further centrifuged for collection. A non-aqueous ionic liquid and acetonitrile were mixed to prepare the electrolyte for this system. Before the preparation of MXene QDs, the authors developed MXene layers as precursors for the desired MXene QDs via electrochemical etching and synchronous fluorination strategy. A comparative study is reported here by the authors for the hence-prepared MXene QDs with that obtained synthesized via HF etching. The results concluded that the MXene QDs derived from the electrochemically etched and fluorinated MXene showed enhanced colloidal stability. [85] The routine etching methods involved the usage of toxic etchants such as hydrofluoric acid (HF) and zinc chloride along with other organic solvents for developing MXenes and their derivatives. Interestingly, the saturable absorption intensity for the henceprepared QDs was comparable to that of graphene; and thus, advantageous for a lower mode-locking threshold in lasers. [85] An article reports the synthesis of Nb 2 C QDs via simultaneous interlayer delamination and layer cutting with the help of a highly intense sonication treatment for 10 h to develop a uniform, ultrasmall nano-fluorophore for selective detection of metal ions and cell imaging. The characterization results showcased a well-defined structure containing abundant functional groups which caused the bright excitation-dependent fluorescence. The bright PL emission of the QDs was attributed to the quantum confinement effect of the ultrasmall lateral dimension as well as the surface defects resulting from the oxygencontaining species. From the pH studies, it is understood that the Nb 2 C QDs are deprotonated sufficiently in pH values above 5 which causes a high negative surface charge to develop. Thus, electrostatic repulsion was responsible for the stable nature of the QDs and fluorescence intensity in this pH range. [64]

Electrochemical Etching for the Preparation of MXene QDs
The random appearance of terminal groups on the MXene surfaces can be witnessed during the deployment of the HF acid etching technique. [86,87] These are environmentally unsafe; and therefore, the delamination and intercalation steps of the etching process need to be upgraded for it to become an environmentally benign process. The search for a fluorine-free synthesis of MXene, which also provides sufficient up-scale production with high yield has eventually set in the laboratories. [77,88] In addition to chemical exfoliation and ultrasonic exfoliation, nowadays, electrochemical exfoliation pathways are also preferred. [77] They have the advantage of being environment-friendly and non-hazardous. In addition, this method is a good alternative to prevent the irreversible defects induced by heavy oxidations and long-time exposure to ultrasonic radiations. The prior exfoliation techniques are also time-consuming and sometimes require sophisticated instrumentation. [89] As an alternative, in an article, the researchers propose the synthesis of chlorine and nitrogen co-doped MXene QDs via the electrochemical etching method shown in Figure 4a. Here, tetramethylammonium hydroxide and NH 4 Cl were added to make up the electrolyte environment which then interacted with the Ti and C layers of a large area. Inside the workstation, the applied electric potential caused the fragmentation of carbon layers at the defect edges through the interaction with electrolyte ions, while the Cl and Ti ions interacted to produce Ti-Cl termination. This advanced the stripping of bulk Ti 3 AlC 2 MAX phase (working electrode) to produce Cl, N Ti 3 AlC 2 MXene QDs. Further, the intercalation of N occurred simultaneously via new bond formation. Owing to the high conductivity of the bulk Ti 3 AlC 2 , the procedure required only low energy and what's more, allowed co-doping by selectively adding the appropriate electrolytes. The formation of the Cl, N-Ti 3 AlC 2 MXene QDs could be validated via the XPS analysis for C, N, O, and Cl (Figure 4b-f). Later, the authors proposed a model of the scavenging mechanism of Cl, N-Ti 3 AlC 2 MXene QDs for • OH. [36] The preparation of Ti 3 C 2 T X MXene QDs via the electrochemical exfoliation technique was proposed and they were highly fluorinated to attain stability. The TEM images were analyzed to study the structure and morphology of the final product and the absence of aluminium species in the XPS results confirmed the complete exfoliation of the Al layer from the precursor. The hence-prepared QDs were suggested to be used as saturable absorbers as the intensity noted was found to be 10 −3 GW cm −2 . Further, the fluorinated MXene QDs were introduced into the ytterbium-doped fiber laser and highly mode-locked pulses were achieved. [44]

Hydrothermal Technique for the Preparation of MXene QDs
Delamination of the MXene precursor by hydrothermal synthesis is one of the most popular procedures for building MXene QDs. [19,73,90,91] The process involves two major steps: crystal nucleation and its further growth. Under high temperatures and pressure, the 2D MXene sheets undergo cracking and assembling. The pH of the medium can be controlled to be acidic or basic, and then, they react with metal hydroxides which are at an accelerated pace, finally producing the required MXene QDs. Sometimes, the setup also allows a passage for argon gas to prevent the sample from undergoing oxidation. [19] Here, pure crystals can be obtained by the heterogeneous reactions which are carried out in the aqueous phase under high pressure and temperature conditions. The major advantage of the method is low energy requirements when compared with other techniques, size control, and high yield. The environmental conditions supplied in the system influence the dielectric constant and density of water, further affecting the morphology and reaction rate. Hence, the method allows controllability for obtaining the desired product. [92] The . a) Schematic diagram of the preparation of Cl, N-Ti 3 C 2 MQDs. b) XPS spectra of Cl, N-Ti 3 C 2 MQDs after reaction with KMnO 4 and highresolution XPS spectra of c) C1s, d) O1s, e) N1s, and f) Cl2p after the reaction. Reproduced with permission. [36] Copyright 2021, Elsevier.
www.advmatinterfaces.de hydrothermal method was reported for the cutting of bulk-layered MXene and further, the preparation of water-soluble Ti 3 C 2 MXene QDs (Figure 5a). [25] Here, the researchers have produced MXene QDs at different temperatures and obtained colloidal MXene QDs having differing morphology. The prepared MXene QDs showed a strong quantum confinement effect with a quantum yield of 10%. The surface composition, as well as functional groups of the prepared MXene QDs, were investigated via FTIR, XPS, and XRD (Figure 5b-d). The PL intensities remained mostly unchanged for changing pH values showcasing good surface passivation of the MXene QDs (Figure 5e-g). One of the parameters which had a deterministic effect on the size and composition of the MXene QDs synthesized via this method was the hydrothermal temperature. [26] A study by Xue et al. suggested that the increased temperature often leads to lower crystallinity in the product. This will alter the surface composition, and in turn, affect the fluorescence behavior. [25] Further study showed that the MXene QDs prepared at high temperature (150 °C) were more cytotoxic than the MXene QDs-100 and MXene QDs-120 but as per the luminescence records, the MXene QDs-150 was recommended to be applicable in Zn +2 detection. The PL intensity decreases significantly with the addition of Zn 2+ ions but no quenching or increased fluorescence is observed for other metals used. [25] Similarly, in a recent article, hydrothermal treatment was employed for the preparation of Ti 3 C 2 MXene QDs for developing PL sensors for detecting histidine in human samples. The prepared Ti 3 C 2 QDs displayed a bright blue fluorescence owing to their size effects and surface defects. The sensor could work in minimal detection time and gave a detection limit of 3.52 nm. It also showed high sensitivity and low interference in the presence of alkali, alkaline earth metals, and a few transition metals. [93] For alkaline phosphatase assay and embryonic stem cell identification, a Ti 3 C 2 MXene QD-based fluorescent probe was designed. Here, the precursor MXenes were prepared from Ti 3 AlC 2 powder by a two-step exfoliation method with HF etching and intercalation. Then, the MXenes were cut into MXene QDs via conventional hydrothermal www.advmatinterfaces.de synthesis followed by freeze-drying. The optical properties revealed that the main absorption for the hence-prepared QDs was in the ultraviolet region and an excitation-dependent fluorescence behavior was seen with a large red shift when there was a gradual increase in the excitation wavelength. It was suggested that the quantum confinement, small size, and surface defects introduced because of the hydrothermal treatment it underwent, together, are responsible for the fluorescence activity. [94] The production of Ti 3 C 2 MXene QDs to enhance the photocatalytic ability of g-C 3 N 4 was reported. The MXene sheets were prepared from layered Ti 3 C 2 MXene structure via sonication technique, and later, these were developed into MXene QDs under-protected atmospheric conditions by hydrothermal treatment. Further, the g-C 3 N 4 @Ti 3 C 2 QDs were prepared by the self-assembly process. TEM analysis reveals the phase compositions as well as the microscopic framework of the product. Here, intimate contact is observed between the QDs and the g-C 3 N 4 NSs which also results in an enhanced surface area. The photocurrent responses suggest an increased intensity in the case of g-C 3 N 4 @Ti 3 C 2 QDs and smaller carrier transfer resistance when compared to the pristine-g-C 3 N 4 . In addition, the co-catalyst Ti 3 C 2 QD allows the optimized composite to display a high solar light photocatalytic H 2 production as desired. [60] Likewise, a combination of etching and sonication-cutting techniques, followed by a hydrothermal method was employed to develop a new class of Ti 3 C 2 quantum dots, fabricated with polyethylenimine. Under the characterization results, the QDs displayed blue emission with a quantum yield of 7.13%. The results from UV-vis absorption studies suggested that a typical absorption peak for Ti 3 C 2 QDs was observed at 310 nm. Another strong peak at 310 nm along with an absorption band centering at 350 nm was also obtained which were speculated to be from the inter-band transitions according to the DFT calculations. The pH dependence of the developed QDs is attributed to the deprotonation of the surface defects, which causes the absorption increase/decrease for the non-radiation. Further, the size dependence of interband transitions, and also, the surface defect sites give rise to the excitation wavelength dependence of the quantum yield. [95]

Solvothermal Method for the Preparation of MXene QDs
Currently, instead of aqueous solvents, organic solvents such as ethanol, DMSO, and DMF are employed in the case of the solvothermal method and make it an enhanced technique. [5] The solvothermal technique is advantageous when compared to the hydrothermal method in the sense that it allows easy control of morphology, size, and dispersion of MXene QDs. [35,96] The research highlights the solvothermal technique for the preparation of Ti 3 C 2 T x MXene QDs with photoluminescence and electroluminescence properties. Three different types of MXene QDs were developed with different ratios of amine dosages. The synthesized QDs were made to undergo several centrifugation cycles followed by complete drying to obtain N-MXene QDs. The characterization records show that the hence-prepared MXene QDs displayed different sizes as a response to different solvothermal times, further impacting their intensity of luminescence. The results of the space lattice information display a high crystalline trait and confirm the tailoring of the N-MXene QDs (Figure 6a-e). [97] Similarly, an article discusses the cutting of MXene nanosheets via the simple solvothermal method into MXene QDs which then showed excellent photobleaching resistance. The prepared N-doped MXene QDs were employed as a fluorescent nanoprobe for dual detection of Cr (VI) and ascorbic acid. The characterization results from FTIR studies revealed that there are plenty of hydrophilic groups on the surface of the MXene QD which resulted in their excellent water solubility. [75] An interesting work by Siya Lu et al. demonstrates the preparation of Ti 3 C 2 MXene QDs which exhibit a bright two-photon white fluorescence. The researchers also conducted high-pressure studies and observed the color changes in the emitted light. Here, the Al MAX phase was etched to obtain the Ti 3 C 2 MXene sheets, which were further treated by a two-step solvothermal process for cutting the sheets and attaining functionalized MXene QDs (Figure 7a). [98] Transient absorption measurements were conducted to get an insight into the luminescence mechanism of the prepared QDs. Figure 7b-j highlights the photophysical characteristics of the prepared MXene QDs. The results pointed toward the synergistic effect of Ti 3 C 2 MXene QDs core and surface state to be responsible for the observed white fluorescence. The final product displayed no serious damage to the host layers and strong emissions that were recorded and all the different characterizations confirmed the safety of the preparation method. [98] This method allows strategic influence upon the crystal growth, crystal phase, or even, the direction of crystal growth. The technique is known to be simple and has the advantage that it inhibits the oxidation of MXenes. In addition, the acoustic cavitation and shock waves induced are conductive to gas removal. It has come to notice that the MXene QDs prepared without fluorinated agents acquire more biocompatibility. However, certain drawbacks which have limited the use of solvothermal/hydrothermal techniques are the long period, involvement with carcinogenic organic solvents, and decreased rate of reproducibility. [99]

Design Principles and Modification Strategies of MXene QDs for Diverse Forms of Applications
Investigations reveal that the size and surface characteristics of the MXene QDs lean on specific synthesis parameters such as the temperature, solvents, reaction time, and modifiers used. [1,25,73,74] The usage of HF as an etchant has been reported in great numbers for the synthesis of MXene QDs using MXene precursors. [81,84,85] HF is cheaper and provides control over the resultant product in terms of dimension and morphology, which in turn decides the property of the generated material. However, it is toxic and corrosive; and therefore, the use of alternative green strategies is appreciated. It has been noted that ultrasonic waves destroy structural integrity while the requirement of a sophisticated and expensive instrument is a necessity. MXene QDs prepared by exfoliation technique possess enhanced pore size and increased surface area. Chemical exfoliation however consumes large amounts of solvents and results in the generation of wastes harmful to the environment. It is noteworthy that the chemical-free method, www.advmatinterfaces.de acoustomicrofluidic synthesis reported by Hossein et. al. showcased lower oxygen content in the MXene QDs, with better fluorescent and electrochemical properties. [73] However, the higher oxygen content in MXene-derived TiO 2 was important for the chemisorption of lithium polysulfide in the application of Li-S batteries. In the solvothermal method, particularly, the hydrothermal method offers better controllability and is energy efficient; however, lacking repeatability and reproducibility. It is seen that lower hydrothermal temperatures of synthesis lead to MXene QDs with morphology similar to MXene precursor itself while the higher hydrothermal temperature of synthesis leads to the leaching of transition metal atoms, which can then be regarded more as carbon dots than MXene QDs. Furthermore, the PL characteristics of the MXene QDs rely on its sizedependent quantum confinement which in turn depends on the reaction temperature and surface composition of the QDs. [100] Therefore, an optimized synthesis temperature is required to obtain desired results in the MXene QDs.
Surface modification, doping with heteroatoms, and fabrication of composites with MXene QDs help in improving the properties of MXene QDs for desired applications. [1] They inherit several functional groups such as oxygen, hydroxyl, chlorine, or fluorine. Similarly, various organic/inorganic groups are also functionalized via electrostatic interactions and physical adsorption methods to enhance the stability, conductivity, yield, and PL property. [101][102][103] The currently produced pure MXene QDs are unsuitable for large scale practical applications; hence, it becomes important to address the possible surface modifications. [104][105][106] The above-mentioned functional groups assure strong electronegativity to the MXene QDs, which is advantageous to the QDs as it will passivate the active sites leading to the change of electronic structure and further resulting in surface defects. [19] Guan et al. reported MXene QDs with N, P-doping which displayed green fluorescence. Later, the DFT calculations reveal the important role of the doped N and P groups, as they involve in electron transfer, thereby enhancing  [97] Copyright 2020, Elsevier.
www.advmatinterfaces.de Figure 7. a) Schematic illustration of the preparation and application of Ti 3 C 2 . b) UV-vis absorption, fluorescence excitation, and emission spectra (the inset photograph is under UV light). c) Two-photon spectra with different laser excitation intensities from an 800 nm femtosecond pulse laser (the inset shows a photograph of a solution of Ti 3 C 2 MQDs with the 800 nm laser passing through). d) Relationship between the two-photon emission intensity and the square of laser excitation intensity has a linear slope of 1. e) TA spectra of Ti 3 C 2 MQDs at indicated delay times from 0.5 ps to 1.5 ns. f) Kinetic decay traces at 480 and 600 nm. Black solid lines are fitted curves. g) Results of global fitting with five exponential decay functions showing five decay-associated difference spectra (DADS). h) Percentage contributions of the three decay processes to the total dynamic within different wavelengths according to fitted DADS. i) Percentage contributions of the three decay processes to the total dynamic relating to surface state at different excitation energies. j) Percentage contributions of the three decay processes to the total dynamic relating to the core state at different excitation energies. Reproduced with permission. [98] Copyright 2019, Wiley. www.advmatinterfaces.de the fluorescence. [107] Heteroatom modifications have also been reported to improve the antioxidant capacity of the MXene QDs by retaining the intrinsic structure. We know, metal dopants allow the controllability over the energy level structure of the QDs; and therefore, enable us to increase the catalytic active sites. [91,106,108] Apart from metallic and non-metallic dopants, organic molecules having low toxicity and better biocompatibility have been reported for modifying the MXene QDs surface. It may/may not affect the catalytic activity but will refine the stability, dispersibility, bio-compatibility, and photophysical and fire-retardant properties. [109][110][111] Fabrication of MXene QDs-based heterostructures is a productive way to largely improve their conductivity and catalytic performance. Literature shows the development of various heterostructures of MXene QDs with different nanostructures for diverse forms of applications. For instance, pure CsPbBr 3 QDs-Ti 3 C 2 T x MXene QDs hetero-design had been prepared via ultrasonication for ion detection. The firm interaction of different functionalities of the MXene QDs with the Cs + at the heterointerface resulted in charge transfer from the metal ion to the QDs, leading to PL quenching which upon further addition of Cs + , resulted in the restoration of the PL intensity. [112] MXene QDs loaded with Au nanobones displayed higher electrochemiluminescence due to the surface plasmon resonance effect and superior electron transfer capacity of the Au nanobones, resulting in its utilization toward biomedical applications. [113] Heterointerface based on defect-rich MXene QDs/N-doped C-nanosheet nanocomposites was synthesized via the self-assembly method. Evenly dispersed QDs on the nanosheet were validated using TEM results, while the HRTEM revealed low crystal quality of the material due to the quantum size effect. The prepared QDs with immense edge defects and grain boundaries exhibited enhanced oxygen-and LiO 2 intermediate-adsorption ability when employed in Li-O 2 batteries. [114] Likewise, the incorporation of graphene, metal oxide/ C 3 N 4 nanosheets, g-C 3 N 4 nanofilms, and SiC into the MXene QDs was shown to boost its photocatalytic and electrocatalytic activities by improving the charge carrier system and transport channels. [115][116][117] Recent reports suggest that modifying MXene QDs with 3D g-C 3 N 4 furnishes plenteous interfacial active sites as well as increases its porosity and gas diffusion channels, thereby improving its photocatalytic activity toward H 2 O 2 generation. [62,63,118] Therefore, the literature reveals that the fabrication of appropriate heterostructures using suitable support materials can significantly enhance the stability, charge transference, and catalytic activity of such composites. [119][120][121][122][123][124][125] MXene QDs also serve as an efficient co-catalyst owing to their versatile surface chemistry, availability of abundant active sites to adsorb and activate different molecules, and good metallic conductivity. They provide a large active surface area with a tunable bandgap and showcase a strong quantum confinement effect. MXene QDs can control the band structure of materials and boost their charge separation properties and charge transfer kinetics. [19] DFT calculations paired with the experimental observations indicate the role of MXene QDs in Ti 3 C 2 MXene QDs/Cu 2 O nanowire as co-catalyst to lower the band edge bending and for enhancing the photogenerated carrier separation, resulting in higher light adsorption activity and carrier transport of Cu 2 O. The lower Fermi level of MXene QDs compared to Cu 2 O favors the flow of the photogenerated electrons from the conduction band of Cu 2 O to that of the QDs, preventing the recombination of the carriers and facilitating effectual redox reactions. This method was successfully applied for photocatalytic CO 2 conversion with high selectivity and yield of the products. [61] Similarly, efficient co-catalyst activity of MXene QDs has been recognized using a Z-scheme heterodesign based on WO 3 /Ti 3 C 2 QDs/In 2 S 3 toward the photocatalytic oxidation of bisphenol A and reduction of Cr(VI). [126] High metallic conductance and a wide photo-response range of the MXene QDs add to its catalytic performance and find application in photoelectrochemical water splitting. [46] Another report reveals the utilization of Ti 3 C 2 MXene QD/Ni-MOF in the photocatalytic nitrogen reduction reactions. The incorporation of MXene QDs increases the light absorption capacity and the charge transport activity at the interface, thereby improving the efficiency of the catalyst. The energy level consistency of the QDs with the Ni-MOF avoids the recombining of the photogenerated charge carriers, which results in a greater concentration of the electrons on the conduction band of the catalyst compared to pristine Ni-MOF, leading to increased NH 3 yield. [127] In addition, the co-catalyst activity of the MXene QDs has been widely explored in various energy-related applications such as full-spectrum solar energy conversion, photocatalytic hydrogen evolution, oxygen reduction, methanol oxidation, CO 2 reduction, H 2 O 2 generation, NH 3 generation, batteries, and supercapacitors. [19]

Applications of MXene QDs
MXene QDs integrate the 2D morphology, size effects, optical, mechanical, and electrical properties, along with the influence of the surface functionalities, which makes them a promising candidate for a wide range of applications in the fields of fluorescent detection, electrochemical detection, energy conversion and management, clinical diagnosis, photothermal therapy, and environmental applications (Figure 8). This paper explains the current progress in the use of MXene QDs for energy storage and conversion, detection of biomolecules, biomedical applications, detection and treatment of environmental pollutants, and other applications including photocatalysis, detection of ions, and analysis of natural products and dyes as well as laser applications.

MXene QDs for Energy Conversion and Storage Devices
The increasing environmental distress and climate changes push researchers toward fabricating eco-friendly and efficient energy conversion and storage methods. With the rising demand for global energy and the depletion of fossil fuels, it is important to have a green source of energy. For not so long, the promising applications of MXene QDs in this field have been extensively investigated. The appreciable performance of the MXene QDs benefitted from their high electrical conductivity, extremely small work function, conductor-insulator shift, reversibility, hydrophilicity, tunable band gap, and surface functionalities in comparison to that of their 2D equivalents. [20] In www.advmatinterfaces.de addition to the electronic properties, the MXene QDs exhibit high dispersibility in aqueous media, great photon absorption, superior electrocatalytic capacity, fast redox kinetics, and recyclability, broadening their utilizations in various applications such as electrocatalysis, energy management, batteries, supercapacitors, water splitting, and LEDs. [1]

MXene QDs for Water Splitting Reactions
MXene QDs are highly endorsed for their electrocatalytic activities in oxygen evolution reactions and hydrogen evolution reactions. Outstanding optical response, excellent surface influences, and good photostability enable the material to be used as a photoanode for photoelectrochemical water-splitting applications. A plasmonic nanostructure based on Ti 3 C 2 nanosheets functionalized with Ti 3 C 2 QDs and gold nanorods (Au NRs/ TDTS) was proposed for broad-spectrum solar simulated water splitting. Au seeds were dispersed on the Ti 3 C 2 nanosheets, which provided sites for the nucleation and growth of nanorods. The Au nanorods showed surface plasmon resonance in the NIR region and promoted the interfacial charge transfer from the nanorods to QDs-decorated nanosheets (TDTS). The TDTS showed photoelectrocatalysis in the UV region; and therefore; high activity of water splitting was noticed in the full spectrum of solar energy (300 to 1000 nm). The carrier density of Au/ TDTS was 4.37 × 10 21 cm −3 and the incident photon to current conversion efficiency (IPCE) of TDTS alone was 8% at 300 nm and 0% at 600 nm, while a 4% IPCE was seen for Au/TDTS from 700 to 1000 nm. This indicated an efficient charge separation and transfer at the Au-TDTS junction with a quantum efficiency of 16 ± 5%. The electrons were transmitted from the Au NRs/TDTS working electrode to the Pt counter electrode through the FTO substrate, where the H + in the water was reduced to produce H 2 . The remaining holes on the surface of the Au NRs/TDTS oxidized the OH − to produce O 2 . Under simulated solar irradiation of 100 mW cm −2 , the gas production rate increased in Au/TDTS by 3.4 times that of bare TDTS. [46] MXene QDs were incorporated as a co-catalyst to fabricate a Z-scheme BiVO 4 @ZnIn 2 S 4 /Ti 3 C 2 QDs (BV@ZIS/ TC QDs) heterojunction (Figure 9a). SEM images revealed the spherical structure and smooth surface of BiVO 4 , which on wrapping with ZnIn 2 S 4 , transformed into a hierarchical flowerlike structure (Figure 9b,c). The incorporation of Ti 3 C 2 QDs to the BiVO 4 @ZnIn 2 S 4 showed no QDs in the SEM images due to the very small size of the QDs (Figure 9d). Ti 3 C 2 QDs were used as co-catalyst owing to their good redox activity, high conductivity, brilliant photon absorption, and the formation of a Schottky barrier between the semiconductor interface. Various characterizations were performed to realize the active mechanism occurring there. The photoelectrons from the conduction band of BiVO 4 were transferred to the valence band of ZnIn 2 S 4 , and then, a much larger number of electrons to the conduction band of the MXene QDs ( Figure 9e). The MXene QDs expand the photo-response range from the visible to the NIR zone, generating abundant photoelectrons. The formation of a Schottky junction at the interface boosts the photoelectron capture due to the formation of a suitable energy band structure and facilitates efficient carrier separation. This remarkably improves the rate of H 2 and O 2 evolution (Figure 9f,g). The in situ growth of the ZnIn 2 S 4 nanosheet on the core of the BiVO 4 catalyst results in a solid-solid interface which makes electrons in the conduction band of the ORR catalyst recombine with the valence band of the HER catalyst to achieve effective water-splitting and photodegradation of the pollutants. The hydrogen and oxygen generated by the Z-scheme catalyst were 4.32 and 2.17 µmol h 1 , respectively (2:1) under visible light irradiation. The highest gas generation was found to be at 460 nm with a quantum efficiency of 2.9%. Bisphenol A was chosen for photocatalytic pollutant degradation experiments and its degradation using the Z-scheme catalyst showed 96% efficiency with the highest rate constant of 0.0185 min −1 . The catalyst was recyclable even after four consecutive cycles of degradation experiments. [128] A novel Janus-structured Cobalt nanoparticle-Ti 3 C 2 QDs hybrid material with superior optical response and high photostability was employed as a photoanode for photoelectrochemical water splitting. The incorporation of Co nanoparticles created a Schottky junction which produced surface plasmon resonance effects and superior visible light absorption capacity. This also increased the photogenerated charge carrier accumulation and carrier separation at the Schottky junction and enhanced the surface reaction rates. The conduction band and valence band of MXene QDs were located at 3.58 and 5.61 eV, while that of the Co nanoparticle was at 5 eV; therefore, the photoillumination of the catalyst transferred the electrons from the conduction band of the Co nanoparticle into the conduction band of MXene QDs. The holes generated in the MXene QDs were transferred to the Co and were then used for the water oxidation reaction. The bandgap of Co-MXene QDs using the Tauc function was calculated to be 1.96 eV. The electrostatic interactions of photogenerated electrons and holes along with the dipoles of MXene QDs and Co resulted in the shifting of emission peaks toward the shorter wavelength. The ratio of

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Co to MXene QDs could be varied depending on the thermal treatment and three such composites were reported Co-MXene QDs (26, 48, and 65). Co-MXene QDs-48 showed better efficiency, greater photocurrent density, larger bias potential, and the highest charge recombination inhibition of 95.6% (1.23 V vs RHE), which was found to be 146% higher than the synthesized MXene QDs. The photocatalysis and charge migration efficiency (87.56%) of the Co-MXene QDs were 194% and 236% ameliorated than those of MXene QDs. The carrier lifetime experimental results indicated that the surface trapping state of MXene QDs could be easily passivized and led to higher carrier migration by creating a Schottky junction at the Co/MXene QDs interface. This rapid decay suggested that the Co terminal coupling might effectively passivize the surface trap state of MXene QDs and prevent unexpected carrier recombination, thereby increasing the catalytic efficiency. [129] Graphitic carbon nitride nanosheets functionalized with Ti 3 C 2 quantum dots (g-C 3 N 4 @Ti 3 C 2 QDs) were synthesized via the self-assembly method for photocatalytic hydrogen generation (Figure 10a). The functionalization with Ti 3 C 2 MXene QDs improved the conductivity and charge transfer capacity of the g-C 3 N 4 , inhibiting the charge carrier recombination, and thereby increasing the lifetime of charge carriers. The MXene QDs as co-catalyst served as efficient electron acceptors and provided a more active surface for photocatalysis. The holes created in the valence band of g-C 3 N 4 were consumed by sacrificial reagents while the MXene QDs provided the sites for HER. The conduction band potential, valence band potential, and flat band potential of g-C 3 N 4 were calculated to be −0.82, 1.74, and −0.72 V (vs SHE), respectively. The lifetime of charge carriers was found to be 10.1242 µs for g-C 3 N 4 @Ti 3 C 2 QDs in contrast to 9.6841 µs for g-C 3 N 4 . Analysis of textural properties of g-C 3 N 4 @Ti 3 C 2 QDs indicated high mesoporosity (2-25 nm) and BET surface was of 40.149 m 2 g −1 for 100 mL of MXene QDs loaded on g-C 3 N 4 , signifying the effective photocatalysis due to more surface area and active adsorption sites. The pristine g-C 3 N 4 showed a high PL intensity caused by the rapid charge recombination; and therefore; exhibited poor catalytic efficiency. The addition of the QDs prolonged the carrier recombination of the g-C 3 N 4 owing to its superior electron acceptor ability, thereby quenching the steady state PL intensity (Figure 10b). Likewise, the excellent photogenerated electron capture ability of the QDs from the g-C 3 N 4 along with its co-catalytic functioning boosted the photocurrent intensity of the composite, compared to the g-C 3 N 4 (Figure 10c). The hydrogen evolution under simulated solar light was found to be enhanced by 26 times in g-C 3 N 4 NSs@MXene QDs with  [128] Copyright 2020, Elsevier.
www.advmatinterfaces.de the 100 mL of MXene QDs (5111.8 µmol g −1 h −1 ) to that of pristine g-C 3 N 4 (196.8 µmol g −1 h −1 ). The photocatalytic efficiency increased with a higher loading amount of MXene QDs till 100 mL, beyond which the catalytic efficiency decreased due to agglomeration of QDs and subsequent blockage of the active sites on g-C 3 N 4 . The apparent quantum efficiency of 3.654% was obtained for g-C 3 N 4 @Ti 3 C 2 QDs and no decrease in hydrogen production was noted even after 24 h of recycling. [60]

MXene QDs for Fuel Cells and Batteries
Research shows the application of MXene QDs as catalysts and co-catalysts to address energy-related needs as they find potential utilization in various crucial reactions such as gas evolution, oxygen reduction, and methanol oxidation. A novel multiwall carbon nanotube modified with MoS 2 QDs and Ti 3 C 2 QDs (MoS 2 QDs@Ti 3 C 2 T x QDs@MWCNTs) composite was employed as an effective bifunctional catalyst in the direct methanol fuel cell for methanol oxidation reaction (MOR) and oxygen reduction reaction (ORR) (Figure 11a-c). [130] Conventionally used platinum for the MOR and ORR was found to be less effective and unstable. The introduction of Ti 3 C 2 T x MXene QDs having a size of 2 nm notably increased the electrical conductivity and the electron transfer rate at the electrode surface, thereby fostering the electrocatalytic reactions. The SEM image of the composite revealed a homogeneous nanostructure with abundant functional groups. The composite was mixed with nafion and subsequently modified on a glassy carbon rotating disk electrode to form the working electrode. The composite gave a large half potential, E 1/2 of 0.75 V, that was comparable to that of Pt/C having a value of 0.80 V for ORR performance. The Tafel slope of the composite was found to be 90 mV dec −1 , which indicated fast electron transfer. The catalyst exhibited high electrochemical oxidation activity toward methanol with a maximum current density of 160 A·g −1 at 2.2 V. The mechanism involved the transfer of ≈four electrons for the reduction of one molecule of oxygen. The CV curves showed that the E onset potential for the composite was 1.5 V, which indicated greater catalytic ability toward the material MOR in KOH (1 m). The composite showed a wide embedded surface area, high electron transmission ability, low degradation rate, and low electrode poisoning, which could be attributed to the synergistic effects of the high electrocatalytic performance of the QDs and the outstanding electrochemical capacity of MWCNTs. [130] Ti n O 2n−1 QDs with the size of 3.3 nm derived from MXene were employed in stabilizing and increasing the life of Li-S batteries. The QDs were found to contain oxygen-vacant sites of 20% (OV-T n QDs). The lithium polysulfides (LiPSs) were strongly chemisorbed onto the OV-T n QDs which were dispersed evenly on the porous carbon nanosheets (PCN) to form OV-T n QDs@PCN. The T n QDs enhanced the PS conversion by catalytic redox reactions. The presence of oxygen-vacancy defects further lowered the PS adsorption energy and reduced the bond length, resulting in increased immobilization and conversion of PS (Figure 12a,b). The UV-vis spectral results revealed the high LiPS adsorption capacity of the OV-T n QDs@PCN compared to Ti 3 C 2 T x (Figure 12c). The combined effects of the miniature size effects of the T n QDs and its porous structure provided the electrode composite with high specific area and pore volume, Figure 10. a) Schematic photocatalytic mechanism of g-C 3 N 4 @Ti 3 C 2 QDs composites. b) Steady photoluminescence (PL). c) Photocurrent responses. Reproduced with permission. [60] Copyright 2019, American Chemical Society.
www.advmatinterfaces.de furnishing sufficient space for S accumulation and volume expansion (Figure 12d,e). PCN possessed a very high surface area and large volume, which resulted in high loading of sulfur (79.1 wt%), also giving physical polysulfide confinement and flexibility, blocking the aggregation of the MXene QDs. The specific surface area of OV-T n QDs@PCN was found to be 198.2 m 2 g −1 with micro and mesopores having 1.2 cm 3 g −1 pore volume. OV-T n QDs@PCN upon assembly with Li 2 S 4 showed a large redox current and low relative polarization (Figure 12f). At an S-mass loading of 2.2 mg.cm −1 , the OV-T n QDs@PCN/S had a high S loading of 79.1 wt%, a strong rate of 672 mA h.g −1 at 2 C as well as long-term cycling stability (88% capacity retention over 1000 cycles at 2 C), and an E/S ratio of 10 µLmg −1 . The discharge capacity was found to be 173.2 mA.h.g −1 (Figure 12g). Furthermore, the OV-T n QDs@PCN/S cathode exhibited good Li + storage at a high S-mass loading of 4.8 mg.cm −2 and a low electrolyte/S (E/S) ratio of 4.5 µL mg −1 , indicating its considerable potential for use in next-generation efficient Li-S batteries.
The low values of electrolyte resistance and charge transfer resistance depicted excellent charge transfer kinetics, while the diffusion coefficient of the OV-T n QDs@PCN/S cathode suggested better diffusion kinetics. [131] An article proposed the use of MXene QDs rivets for improving the electrochemical activity and stability of Ni-Co layered double hydroxides (LDH). The hydrothermal technique was employed to rivet the MXene QDs over the LDH. The uniform re-distribution of electrons around the metal atoms was observed at the surface of Ni-Co LDH@MXene QDs; hence, improving the surface activity. Electron transport and structural stability of the LDH were noticed to be improved in the case of MXene QD-coated LDH. In addition, the electrolyte ion adsorption was enhanced as the number of unpaired electrons increased owing to the cap of MXene QDs rivets. The CV area and discharging time results suggest an enhancement in the energy storage property after riveting the LDH with MXene QD. In a two-electrode device, Ni-Co LDH@MXene QDs demonstrated significant energy storage activity (87 mA h.g −1 at 1 A.g −1 ) and the gained energy was 60.7 W hkg −1 at a power density of 698 Wkg −1 . [132]

MXene QDs for LEDs and Laser
The Sulphur doped, Nitrogen-doped, and Sulphur/Nitrogendoped MXene quantum dots (SMXene QDs, NMXene QDs, and SNMXene QDs) were reported to show that the MXene QDs could emit over the blue to the orange color of the visible spectrum. Their quantum yields were found to be 7.78% (4.67 ns lifetime), 8.33% (5.81 ns lifetime), and 28.12% (7.74 ns lifetime), respectively. The QDs showed excitation-independent emission and were able to tolerate a wide range of pH without any significant changes in their emission properties. The MXene QDs had extensive hydrogen bonding in water, which led to increased lateral size and red shift in the emission spectra owing to the decrease in the band gap due to π-electron delocalization. However, the SMXene QDs showed no hydrogen bonding. When the MXene QDs were excited between 320 and 380 nm in the dry state, the fluorescence emission was found to be ≈440 nm and a small shift in emission was noted when dispersed in water with multicolor emission. The CO and CO bonds were captivated on the surface of the MXene QDs due to the formation of stalwart bridge-like hydrogen www.advmatinterfaces.de bonds which caused the enhancement in rigidity and fluorescence of the material. The method was applied for the construction of white LEDs. The MXene QDs were evenly blended with polyvinylpyrrolidone (PVP), irradiated with 365 nm, and a voltage of 3.0 V was applied to make the WLEDs. Another group of researchers reported bilayer Ti 3 C 2 MXene QDs with a particle size of 13.1 nm that showed highly stable two-photon white luminescence in EtOH under UV light. With silicon oil as a pressure-transmitting agent, white luminescence changed from cool to warm white under pressure (up to 2.53 GPa). The MXene QDs with oleylamine (OLA) were synthesized as a passivating agent. The white fluorescence mainly arose due to the combined effects of the bulk of MXene QD and surface OLA functionalities. The maximum excitation and emission were found to be 360 and 509 nm with a quantum yield of 9.36%. Hybrid nanocomposites were created by polymerizing MXene QDs in a polydimethylsiloxane (PDMS) solution as part of the application to produce white light LED. The devices were created by coating PDMS/Ti 3 C 2 MXene QDs on a prototype solid-state lighting unit with a 365 nm excitation lightemitting chip and polymerizing them in situ. [98] An article on N-doped V 2 C MXene QDs showed better luminescent properties than the undoped MXene QDs, which were then utilized for the fabrication of white lasers. The intensity of emission at 500 nm or longer wavelengths showed five times increase and the quantum yield was significantly enhanced by 15.88% (6.5 ns lifetime) in the doped MXene QDs in contrast with the undoped MXene QDs (2.5% with 4.99 ns). It was noted that in N-V 2 C MXene QDs, with the increase in excitation wavelength, the emission showed a shift toward a longer wavelength with decreased intensity attributed to the existence of different local electron states. The broadband optical feedback "cavity" was constructed using nonlinear scattering caused by solvent bubbles created during high-intensity excitations. The gain was boosted and a greater scattering was obtained when the excitation intensity was increased. When the excitation exceeded a threshold, the scattering induced an optical feedback "circuit" by the generation of the laser cavity. It was noted that the nonlinear scattering was enhanced with higher pulse energy at 0.5 mg mL −1 concentration, while keeping the absorption loss due to colloid concentration balance in mind. The pump size was 25 × 10 −2 cm −2 and the angle of incidence was maintained at 30°. The luminescence peaks at 545 and 613 nm became strong and intense with the influence of pump energy to show laser-like characteristics. Under irradiation using a xenon lamp and pulsed laser (intensity of 4.8 mJ cm −2 ) for 1 h, the variation of PL and laser intensities was only less than 2.5% and 3%, respectively, which suggested strong laser and PL stability of the V 2 C MXene QDs. [133]

MXene QDs for Biomedical Applications
Electro-chemiluminescent and fluorescent properties of MXene QDs have inspired scholars to widen their analytical applications toward fluorescent detection and bioimaging. Nowadays, MXene QDs are extensively explored for their applications in the detection of proteins, nucleic acids, and disease biomarkers. [35] High biocompatibility, water-solubility, dispersibility in an aqueous environment, and low cytotoxicity add to their practical applications for the detection of various biomolecules in real samples. These QDs, when employed for the detection of biomolecules, show high sensitivity, specificity, accuracy, fast analytical response, wide dynamic range, easy extensibility, low detection limits, and good reproducibility, which extend their use in clinical diagnostics, medicine, food, water analysis, and detection of biowarfare agents. [1,64] The detection effects are mostly observed by the fluorescence quenching and restoration of the sensor, which arise mainly due to the quantum confinement effects or the inner filter effects (IFE) of the QDs. By modifying the surface terminations, the detection characteristics of the QDs can be further improved to obtain better results. A detection platform based on amino-functionalized Ti 3 C 2 MXene QDs was proposed for the detection of histidine via a fluorescence turn-on mechanism. The sensor displayed bright blue fluorescence on account of its size and surface defects. The functionalization of the amino groups along with the doping of nitrogen atoms altered the properties of the QDs which produced an increase in the fluorescence intensity. The amino-functionalized sensor had a higher affinity toward the nickel ions. The binding of the nickel ions quenched the fluorescence of the sensor via the IFE (Figure 13a,b). In the presence of the amino acid histidine, the fluorescence was recovered due to the greater binding affinity of histidine with nickel ions (Figure 13c). The sensor displayed maximum emission at 410 nm when excited at 250 nm. The fluorescence lifetime of the sensor was 5.82 ns, which decreased to 1.8 ns with an increase in the Ni 2+ concentration. A low detection limit of 2.1 nm was obtained in the linear concentration range of 100−1000 nm for histidine. The method was found to be highly selective and less time-consuming; showed no significant interferences in the presence of alkali metal ions, alkaline earth ions, and a few transition ions; and was applied for the detection of histidine in spiked human serum samples with good recoveries. [93] A research paper on the highly selective and sensitive detection of cytochrome c and trypsin using ε-poly-L-lysine-protected Ti 3 C 2 quantum dots (PLL-Ti 3 C 2 QD) was published. The excitation at 330 nm resulted in a stronger emission at 415 nm with a quantum yield of 22%. The presence of cyt-c quenched the blue fluorescence of the sensor. The addition of trypsin led to the hydrolysis of cyt-c, converting Fe 3+ to Fe 2+ ions, due to which the fluorescence was restored. The quenching effect was mainly attributed to the IFE of cyt-c and the strong coordination of Fe 3+ metal ions present in cyt-c with the surface functional groups such as hydroxyl, amine, and carboxylic acid groups on PLL-Ti 3 C 2 QD. Low limits of detection for cyt-c and trypsin were 20.5 nm and 0.1 µg mL −1 , respectively with the range of detection of 0.2-40 µm and 0.5-80.0 µg mL −1 for cyt-c and trypsin, respectively. The probe when tested against various other biological molecules showed no significant response as with cyt-c and trypsin. However, a slight alteration in the quenching was noted with bovine serum albumin and lysozyme during the competitive experiments. The application to spiked real samples of serum showed recovery rates greater than 95%. [134] TiO 2 inverse opal photonic crystals (IOPCs)/Ti 3 C 2 QDs/Nafion electrode film with excellent stability was fabricated for the selective photoelectrochemical determination of glutathione in buffered www.advmatinterfaces.de solutions as well as Hela cell extracts. The type-II heterojunction created between the Ti 3 C 2 QDs and TiO 2 facilitated excellent carrier transport leading to a seven times lower charge transfer resistance (355 Ω) compared to TiO 2 (2430 Ω) (Figure 14a). The rapid, directional charge transfer between the QDs and the TiO 2 IOPCs with potent charge separation was further validated using the PL decay studies (Figure 14b), marked by a lifetime of 3.1 ns, and also confirmed using the Mott-Schottky (M-S) plots (Figure 14c). The UPS studies revealed the mechanism at the TiO 2 /Ti 3 C 2 QDs heterointerface based on their energy levels (Figure 14d,e). The matching energy band structure at the interface favors the formation of the type-II heterojunction, which on photoirradiation, allows a pool of both cold electrons from the QDs interband transfer as well as hot electrons for the QDs intraband interactions. The photoelectrons from the QDs can be rapidly transferred into the conduction band of the TiO 2 IOPCs with a back transfer of the holes into the conduction band of the QDs. This is followed by the shift of electrons from TiO 2 IOPCs to the FTO, and the holes into the valence band of the QDs, causing a high carrier separation, avoiding the electron-hole recombination. This results in a very high photovoltaic effect and superior redox properties for the oxidation of glutathione (GSH) to glutathione disulphide (GSSG) with a 26% power transformation at 350 nm. The photocurrent of the sensor progressively increases with the concentration of glutathione, which shows a low limit of detection of 9.0 nm in a linear dynamic range of 0.1-1000 µm. The method was applied to the detection of glutathione in Hela cell extracts, which displayed a limit of detection of 10 000 cells per mL and a linear range between 10 000 and 2 × 10 6 cells per mL with a detection limit of glutathione concentration up to 0.55 µm in a range of 1-200 µm at 800 nm. TiO 2 /Ti 3 C 2 /Nafion electrodes showed high sensitivity and anti-interference properties toward glutathione. [90] Fluorescent Ti 3 C 2 QDs were synthesized for alkaline phosphatase (ALP) activity evaluations as well as embryonic stem cell (ESC) recognition. ALP is an important enzyme that is responsible for the dephosphorylation reactions of various molecules and acts as a biomarker for many diseases as well as for ESC. ALP catalyzes the hydrolysis of the substrate, p-nitrophenyl phosphate to form p-nitrophenol. The prepared Ti 3 C 2 QDs had a diameter of ≈4.2 nm, high salt tolerance, good water dispersibility, and anti-photobleaching properties. Excellent overlap of the absorption band of p-nitrophenol with www.advmatinterfaces.de the excitation and emission bands of the QDs facilitated the fluorescence quenching of the MXene QDs. The quenching mechanism was based on the IFE. In addition to that, a small bathochromic shift observed in the PL emission of the QDs in the presence of p-nitrophenol can be ascribed to the formation of hydrogen bonding between the p-nitrophenol and the Ti 3 C 2 QDs, which may also contribute to the PL quenching. The method showed satisfactory results for the ALP assay and ALP was effectively and accurately detected with a low limit of detection of 0.02 UL −1 in a linear dynamic range of 0.1-2.0 UL −1 . [94] Recently, MXene QDs have been highly recommended by researchers for the extraordinary potentials of the bioconjugates of QDs for the bio-targeting, bio-imaging, and bio-labeling applications that can be accomplished by them. Excellent optical properties, including excellent NIR absorption characteristics of MXene QDs along with high biocompatibility, account for their direct applications in immunomodulation and photothermal therapy. In photothermal therapy, the photonto-heat conversion property of the MXene QDs is exploited, by which the temperature of the solution rises depending on the concentration of the QDs, laser power, and photon absorption. [94] The use of MXene QDs in photothermal therapy not only facilitates efficient target-specific photothermal treatment but also helps to overcome the toxicity and side effects of traditional physical and chemotherapy. However, only a limited number of research articles are currently available regarding the application of MXene QDs in disease diagnosis, immunomodulation, and photothermal therapy. Ti 3 C 2 QDs encapsulated liposomes having excellent photothermal activity were employed in the immunoassay for the analysis of prostatespecific antigen (PSA) (Figure 15). The liposomes using glutaraldehyde were conjugated with monoclonal antibodies for anti-Prostate Specific Antigen-173 (mAb2) and subsequently incubated onto the ELISA plates coated with mAb2. Functional liposomes conjugated with an antibody have been estimated to have a size of 205 nm using DLS and each liposome contained 403 Ti 3 C 2 QDs and 760 antibodies conjugated onto the surface. 403 Ti 3 C 2 QDs encapsulated in per liposome could assure the conversion of the one-to-one bio-recognition process into a one-to-many response signal and consequently enhance the signal amplification for the analyte detection. The encapsulated QDs showed weak fluorescence due to the quantum confinement effect, while the free QDs showed higher fluorescence under 365 nm UV light. The Triton X-100 was used to break the lyposomes and free the Ti 3 C 2 QDs, thereby increasing the fluorescence intensity. The QDs upon irradiation with the NIR laser of 808 nm (1.5 W cm −2 ), in the presence of PSA, converted the light to heat, and the temperature change was proportional to the concentration of the analyte. With PSA, a temperature rise of ≈18 °C with the PL emission peak was attained by the immunosensor upon 5 min of NIR irradiation, while no temperature rise and no PL response were observed in the absence Figure 14. a) EIS spectra of TiO 2 IOPCs, Ti 3 C 2 QDs, and TiO 2 /Ti 3 C 2 (0.4 mg mL −1 ) in 5 mm K 3 (FeCN) 6 solution. b) Photoluminescence decay of Ti 3 C 2 QDs and Ti 3 C 2 QDs in TiO 2 /Ti 3 C 2 . c) Mott-Schottky plots of TiO 2 IOPCs, Ti 3 C 2 QDs, and TiO 2 /Ti 3 C 2 composite film (SCE is saturated calomel electrode as reference electrode). d) PEC mechanism for TiO 2 /Ti 3 C 2 electrode with the illumination of standard solar lights. e) Ultraviolet photoelectron spectroscopy (UPS) spectra of TiO 2 IOPCs and Ti 3 C 2 QDs. Reproduced with permission. [90] Copyright 2019, Elsevier.
www.advmatinterfaces.de of PSA; thus, achieving good target specificity. The quantification was based on the binding ability of the liposome-tagged antibody with the antigen. A low detection limit of 0.4 ng mL −1 in a linear concentration range of 1.0-50 ng mL −1 and LOD was observed. Application of the method to human samples was conducted as a simple enzyme-free, comparatively inexpensive approach with no significant difference in the t-test for the above method and standard ELISA kit assay. Moreover, a portable, 3-min NIR imaging detection device for PSA, showing good accuracy and reproducibility, was fabricated. [74] Research work on N-Ti 3 C 2 QDs as the fluorescence assessors with Fe 3+ as fluorescence quencher based on non-radiative electron-hole annihilation was reported recently for the identification and fluorescence imaging of glutathione (GSH) in MCF-7 cells; thus finding application in the diagnosis of cancer (Figure 16a). When Fe 3+ approached the amino group of N-Ti 3 C 2 QDs, electron transport took place from the QDs to the half-filled d orbitals of the ferric ions, which resulted in the fluorescence quenching of the sensor. This N-Ti 3 C 2 QDs/Fe 3+ nanoprobe then responded to the plenteous GSH in the MCF-7 cancer cells, converting Fe 3+ to Fe 2+ , and thereby restoring the fluorescence of the detection device (Figure 16b). The PL intensity was found to increase with the concentration of GSH in a linear range between 0.5 and 100 µm (Figure 16c,d). Sensitive detection of GSH was achieved with a low limit of detection of 0.17 µm with no significant interference from other protein biomarkers. [22] A new heteronanomaterial with the combination of 2D bimetallic CoCu-zeolite imidazole framework and 0D Ti 3 C 2 T x MXene-derived carbon QDs (0D/2D CoCu-ZIF@CD) was fabricated for the electrochemical detection of cancer cells using EIS. The fluorescent QDs were incorporated into the 2D framework of non-fluorescent CoCu-zeolite imidazole and the resultant nanohybrid exhibited good label-free electrochemical and fluorescence activities, high biocompatibility, and superior endocytosis. CoCu-ZIF@CD was then deposited onto a silver electrode functionalized with the sgc-8 aptamer, which showed a distinct binding affinity with PTK7 receptors of B16-F10 cancer cells. The maximum emission of the probe was observed at 640 nm when excited at 428 nm. The bioimaging of the Cy3-Apt/CoCu-ZIF@CDs incubated cells revealed strong green fluorescence in the cytoplasm of the B16-F10 cells while the healthy L929 cells showed a slight fluorescence under confocal laser microscopy, indicating the applicability of the aptamer/CoCu-ZIF@CDs as an efficient cytosensor. A detection limit of 33 cells per mL in a linear detection range from 100 to 100 000 cells per mL was obtained from the EIS responses. [4] Non-oxidized Ti 3 C 2 T x MXene quantum dots (NMXene QDs-Ti 3 C 2 T x ) with 7.23 nm particle size, disc shape, and high biocompatibility, were employed for the cancer Figure 15. a) Schematic illustration of a near-infrared photothermal immunoassay for target PSA using Ti 3 C 2 QDs-encapsulated liposome as the amplifier and b) device of photothermal immunoassay by portable near-infrared imaging camera. Reproduced with permission. [74] Copyright 2019, Royal Society of Chemistry.

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therapy. The Ti 3+ of QDs in the presence of hydrogen peroxide generated free hydroxyl radicals that induced oxidative stress on the cells and enhanced microvascular permeability leading to hemorrhage and ultimately cell death. The concentration of H 2 O 2 in cancer cells is high compared to that in normal cells. The catalysis of MXene QDs for the production of hydroxyl radicals resulted in the change of Ti 3+ to Ti 4+ state, with the formation of H 2 TiO 4 (orange color) from H 2 O 2 . Laser irradiation of 488 nm onto QDs incubated cancer cells showed fluorescence in the lysosome and mitochondria (major locations for the generation of H 2 O 2 ). It also caused an upraise in the biomarkers of cell destruction. The incubation of the QDs with the HeLa cells, MCF-7 cells, and adipose-derived stem cells showed that HeLa cells obtained death in 24 h, MCF-7 cells in 48 h, and the stem cells showed a low rate of cell death. The GSH/GSSG ratio lowered significantly in the cancer cells as the hydroxyl radical activity increased. The quantum yield was estimated to be 9.62%. The extended studies on pathogen-free BALB/c nude mice containing the HeLa xenograft proved NMXene QDs-Ti 3 C 2 T x to be a good candidate for in vivo tumor therapeutic activity. [27] Excellent photothermal conversion ability of titanium nitride quantum dots (Ti 2 N QDs) with a size of ≈5 nm was demonstrated under laser irradiation, which showed absorption in the NIR-I/II bio-windows. Ti 2 N QDs exhibited superior biocompatibility, stability, photoacoustic results, aggregation, photothermal effectiveness, non-toxicity, and biodegradability. The absorption maximum was seen at 808 and 1064 nm for NIR I and II, respectively and laser irradiation for 5 min increased its temperature to 60 °C. The photothermal conversion efficiency was found to be 48.62% for 808 and 45.51% for 1064 nm. The technique, when extended to monitor breast tumors in mice, by the application of 20 mg kg −1 of Ti 2 N QDs with laser irradiation for 10 min, showed a reduction in the width and volume of the tumor. Photo-acoustic waves generated after laser irradiation were utilized for bioimaging. The imaging performed on mice with tumors showed strong acoustic signals in 4 h when injected with the QDs of 2 mg mL −1 , 200 µL. The outcome of these studies revealed the promising photoacoustic imagingcontrolled photothermal therapeutic abilities of Ti 2 N QDs for cancer. [28] Similarly, photoacoustic imaging and photothermal therapy applications for tumors using titanium carbide (Ti 3 C 2 ) MXene QDs were reported with a photothermal conversion Figure 16. a) Schematic illustration of the preparation of N-Ti 3 C 2 QDs/Fe 3+ probe and detection of glutathione. Reproduced with permission. b) Fluorescence spectra of the N-Ti 3 C 2 QDs, N-Ti 3 C 2 QDs/Fe 3+ , and N-Ti 3 C 2 QDs/Fe 3+ +GSH with the excitation wavelength at 350 nm. c) Fluorescence spectra of N-Ti 3 C 2 QDs/Fe 3+ in the presence of different concentrations of GSH (0-100 µm). d) Standard curve of fluorescence intensity F-F 0 versus GSH (0.5-100 µm) concentration. Reproduced with permission. [22] Copyright 2021, Elsevier. www.advmatinterfaces.de power of 52.2%. The QDs modified with ample amounts of Al oxoanions provided firm and wide absorption in the near-IR range. This helped in appropriate photoacoustic imaging of malignancy and the photoacoustic effect was largely boosted by the increase in the concentration of the QDs. The experiments on Balb/c nude mice HeLa cells revealed that the injection of 0.5 mg kg −1 of MXene QDs with the 808 nm laser irradiation for 5 min resulted in the elevation of temperature to ≈60 °C and the deterioration of the tumor. The method offered great advantages of green and fluorine-free synthesis, high biocompatibility, superior photothermal-conversion efficiency as well as non-toxicity. [29] A photothermal agent based on ultrathin Ti 3 C 2 MXene nanosheets (≈100 nm) enriched with Al 3+ was fabricated for cancer treatment (Figure 17). This showed excellent photothermal conversion power of ≈58.3% and higher singlet oxygen formation ( 1 O 2 ) when irradiated with a laser at 808 nm. Layer by layer approach using the anti-tumor doxorubicin and cancer-targeting hyaluronic acid was employed for the development of the multifunctional Ti 3 C 2 -DOX. Laser irradiation disturbed the electrostatic forces present in Ti 3 C 2 -DOX, which freed the DOX, and the presence of laser-caused hyaluronidase resulted in 70% drug release in less than 48 h in the acidic tumor conditions. This novel platform possessed high biocompatibility, drug loading ability up to 84.2%, target-determined accumulation, controlled multi-stimuli drug release, cooperative photothermal/photodynamic/chemo-therapy, and superior cancer ablation activity at 2 mg kg −1 packed with 1.6 mg kg −1 DOX. [30] Surface-modified 0D titanium carbide MXene quantum dots (Ti 3 C 2 T x MXene QDs) with a diameter of 4.58 nm were reported for their utilization in sub-cellular nanomedicine. The QDs functionalized with plentiful F, O, OH, NH, and COOH groups presented a wide active area, wonderful optical properties, rapid uptake, and stable localization into human endothelial cells within 24 h. The QDs absorbed into the cells also preserved their high autofluorescence at different excitation-emission wavelengths, which allowed the quantity determination and tracking. Ti 3 C 2 T x MXene QDs had a broad peak for absorption at 300 nm with a strong green fluorescence under the cytoplasmic region. The fluorescence properties of the QDs corresponded to commonly used DAPI, GFP, and Cy5. [31] Glutathione functionalized MXene quantum dots (GSH-Ti 3 C 2 MXene QDs) were reported for the detection of uric acid, a biomarker for various illnesses including oxidative stress, fatty liver, and failing kidney and heart. The functionalized sensor showed good fluorescence stability in a wide pH range with the same intensity of fluorescence at 430 nm when excited at 337 nm, along with a quantum yield of 21%. In this method, the enzyme uricase oxidized uric acid to allantoin and hydrogen peroxide, and the hydrogen peroxide formed in turn oxidized ortho-phenylenediamine to yellow-colored 2,3-diaminophenazine in the presence of horseradish peroxidase. The overlap of emission of GSH-Ti 3 C 2 MXene QDs and absorption spectra of 2,3-diaminophenazine enabled the FRET channel between them, resulting in a new absorption peak at 568 nm and blue fluorescence quenching. The limit of detection and linear range of estimation was 125 nm and 1.2-75 µm, respectively toward uric acid. The detection system when tested with Try, Cys, Urea, Na + , K + , and Cl − ions showed no interferences and was applied for the detection of uric acid in spiked serum and saliva. [32] Tantalum Carbide MXene QDs (Ta 4 C 3 T x MXene QDs) were explored for the therapy of allograft vasculopathy owing to its Figure 17. Schematic illustration of the preparation of Ti 3 C 2 -based nanoplatform, and the synergistic photodynamic/photothermal/chemotherapy of tumor. Reproduced with permission. [30] Copyright 2017, American Chemical Society.

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high anti-inflammatory, immunomodulatory, and antiapoptotic effects. The method followed a facile HF-free procedure and the prepared Ta 4 C 3 T x MXene QDs possess enhanced stability, high purity, and biocompatibility, and a large active area boosted with various bioactive functionalities. The endothelial cells rapidly absorbed the QDs which changed their receptor structure to diminish the allogeneic T-lymphocytes activation. Administration of 2 µg mL −1 of the material into HUVECs showed no cytotoxicity and the presence of numerous negatively charged surface functionalities of Ta 4 C 3 T x MXene QDs enabled its fast internalization, which further interacted with the cell proteins and altered the PD-L1 and CD86 expressions; thus, lowering the inflammatory activation against allograft rejection. The experiments using rat models further supported the effectiveness of the method. [33] Outstanding bioactive properties of Ti 3 C 2 MXene QDs were put to use for immunomodulation. The QDs functionalized chitosan hydrogel was reported to have promising applications in stem cell therapies by suppressing inflammatory cells, increasing in immune resistance of grafted cells, also by boosting stem cell persistence and proliferation. The QDs upon incubation with mononuclear cells from human blood, showed low cytotoxicity, good biocompatibility, and great cellular viability using flow cytometry. The negatively selected magnetic-activated cell sorting of naive CD4+ T-cells was activated to pro-inflammatory Th1 cells. The presence of QDs set off the stimuli by the inflammatory cells, leading to a significant decrease in the CD4 + IFN-γ + Th1 cell proliferation and cytokines such as tumor necrosis factor α (TNF-α) and interferon-γ (IFN-γ). The regulatory T-cells increased significantly. The QDs functionalized Chitosan hydrogel when employed as a bioscaffold to the cardiomyocyte; the signals emitted through the microelectrode array system were greater than the normal hydrogel. [33] In another experiment, single-layered titanium carbide (Ti 3 C 2 ) MXene QDs showed high water solubility due to the surface terminating groups; and were therefore, suitable for biological application. Tweaking the temperature resulted in MXene QDs with different morphology. MXene QDs prepared at 100 °C, 120 °C, and 150 °C had a particle size of 2.9, 3.7, and 6.2 nm, respectively. With increasing temperature, the Ti content decreased. Therefore, the MXene QDs synthesized at 100 °C showed an MXene-like structure while the MXene QDs at 150 °C may be regarded as close to carbon dots due to low Ti content. However, the MXene QDs@120 °C possessed TiO 2 as well as CTi at its surface and interior, respectively. Two peaks were observed for the excitation spectra at 250 and 320 nm and showed excitationdependent emission (340-440 nm). The quantum yield of MXene QDs-100, MXene QDs-120, and MXene QDs-150 were 9.9%, 8.7%, and 7.9%, respectively. The surface passivation due to surface groups such as NH groups was noted due to which there were no significant changes of emission with the change in pH. With MTT assay, MXene QDs-100 and MXene QDs-120 showed low cytotoxicity in RAW264.7 cells while the MXene QDs-150 had higher cytotoxicity. When the incubated cells (4 h with MXene QDs) were excited at 405, 488, and 543 nm, bright blue, green, and red colors were observed in the cell membrane and cytoplasmic region under confocal laser microscopy image. The nucleus was weakly fluorescent as the MXene QDs were unable to cross the nuclear membrane. This indicated the potential bioimaging of cells in the cytoplasmic regions with MXene QDs-100 and MXene QDs-120. As MXene QDs-150 was incompatible with cellular imaging, it was used to detect Zn 2+ ions in the solution. The ions significantly quenched the fluorescence of MXene QDs as a result of the interaction of the ions with the surface terminating groups and were therefore effective in quantitative detection. The range of detection was found to be from 5 × 10 −6 to 60 × 10 −6 m. [25] Niobium carbidederived quantum dots modified with thiol (Nb 2 C-SH QDs) were reported for the detection of the N-gene of SARS-CoV-2 based on surface plasmon resonance (SPR). The functionalized Nb 2 C-SH QDs were evenly incorporated on the gold chip of SPR by AuS bonds and self-assembly. N58 aptamer was functionalized via non-covalent interactions such as π-π* stacking, hydrogen bonding, and electrostatic forces. The interactions with the N-gene of SARS-CoV-2 resulted in the change in conformation of the aptamer strands, to distinctly bind to the N-gene forming the G-quadruplex. This caused an enhancement in the contact area between the chip and the aptamer, and such a change was noticed through the changes in SPR signals, leading to the detection of the virus at 633 nm of HeNe laser irradiation. The LOD was 4.9 pg mL −1 and the range of detection was 0.05 to 100 ng mL −1 in 1 ms. Nb 2 C-SH QDs showed great affinity toward N58 as well as produced amplified SPR signals. The sensor revealed feasible preparation, rapid detection, appreciable specificity, and stability, and was employed for the detection of the SARS-CoV gene in real samples such as human serum, seafood, and seawater with recoveries of more than 95%. However, the sensor exhibited limitations in repeatability and regenerability. [45] Functionalized Ti 3 C 2 MXene QDs were prepared and investigated for their activity toward intracellular pH detection, which could be used to examine cell function, health, and several diseases. The Ti 3 C 2 QDs displayed excitation-dependent intense blue emission owing to their size, electronic properties, and surface defects, along with quantum yields of 7.13%. The reversible pH-dependent responses of the Ti 3 C 2 QDs were due to the protonation and deprotonation of their surface functionalities combined with the changes in their zeta potential, according to variations in the pH of the medium. As the pH increases, the surface amino moieties of the QDs undergo deprotonation, which lowers the zeta potential of the system. Due to deprotonation, a part of the valence electrons was localized at the surface of the QDs, which was then unavailable for photoabsorption. This led to the partial transformation of the luminescent surface defects into non-radiative defect sites, resulting in a reduction of the absorption intensity of the QDs. Moreover, the pH-dependent Ti 3 C 2 QDs incorporated with pH-independent [Ru(dpp) 3 ]Cl 2 were constructed as a newfound fluorescent nanoprobe for measuring the intracellular pH in MCF-7 cells. The sensor offered the benefits of excellent biocompatibility, great water solubility and dispersibility, appreciable stability, and less cytotoxicity. [95] Hydroxyl radical is the most reactive free radical having the strongest one-electron reduction potential, which can cause oxidative degradation of organic contaminants, air purification, and soil remediation, and which shows disinfectant activity. [135] Nevertheless, its highly reactive nature also produces cytotoxic effects in organisms from oxidative damage of DNA and proteins to several acute and chronic diseases. [136,137] Thus, proper www.advmatinterfaces.de hydroxyl scavenging techniques to an extent help in addressing these health issues in living organisms. Chlorine and nitrogen co-doped Ti 3 C 2 QDs (Cl, N-Ti 3 C 2 QDs) were fabricated via an electrochemical method for hydroxyl radical scavenging activity. The Cl, N-Ti 3 C 2 QDs had a size of 3.45 nm with a high active surface area. Co-doping of the Ti 3 C 2 QDs using chlorine and nitrogen significantly improved the electrical properties of the QDs. A dye protection test was carried out to determine the scavenging activity of the QDs (Figure 18a-f). Hydroxyl radicals produced using TiO 2 precursor in the presence of UV light attacked and degraded the Rhodamine B (RhB), marked by zero absorption intensity of the solution. The addition of Cl, N-Ti 3 C 2 to the solution of RhB containing hydroxyl radicals showed little to no color change of RhB, implying good radical scavenging activity of the modified QDs. In addition to that, a sensitive fluorescence probe study using terephthalic acid as an indicator revealed the excellent radical scavenging performance of Cl, N-Ti 3 C 2 . To elucidate the mechanism of radical scavenging, an experiment on the reduction of KMnO 4 was done using the modified QDs and the changes in its valence state or composition were analyzed via XPS studies (Figure 18g-l). Upon incubation of the Cl, N-Ti 3 C 2 with KMnO 4 for 30 min, the characteristic absorption of KMnO 4 in the range of 450-600 nm vanished, and an optical color change from purple to yellow was noted indicating the change in the oxidation state of Mn +7 to Mn +4 . Thus, the radical scavenging ability was attributed to the strong reducing nature of the Cl, N-Ti 3 C 2 along with the high electron-donating ability of Cl and N. Owing to the high surface-to-volume ratio of the QDs, the hydroxyl radicals remained in close contact with its surface and oxidized the QDs beginning from the surface and edge defects. As a result, abundant OH functionalities were produced at the surface, which on dehydration, generated COC, COOH bonds along with TiO 2 , evident from the XPS spectrum. [36] Hydrogen peroxide (H 2 O 2 ) is another potent molecule that has various environmental, industrial, and medical applications owing to its redox properties. It has been found to exhibit a dual character in cancer, that is, increased concentrations of H 2 O 2 promote the growth of cancer, while it also causes H 2 O 2induced destruction of cancer cells compared to normal cells. Therefore, the detection of H 2 O 2 is important for a practical understanding of cellular dynamics in biomedical research. [138] A research work highlights the use of Nitrogen-doped Ti 3 C 2 quantum dots (N-Ti 3 C 2 QDs) for the reactive oxygen species scavenging as well as detection of H 2 O 2 with appreciable results. N atoms boosted the electrochemical interaction between the modified QDs and free radicals, making them more effective antioxidants. DFT results demonstrated the hydroxyl radical quenching activity as well as the increase in radical capture, particularly for fluorine and oxygen functionalities in N-Ti 3 C 2 QDs due to N doping. A detection limit of 1.2 nm and a range of detection from 5 nm to 5.5 µm was observed for H 2 O 2 . [69] Acoustomicrofluidic synthesis of Ti 3 C 2 T z MXene QDs was presented for electrochemical detection of H 2 O 2 . Conventional methods Figure 18. a) Absorption spectrum. b) Survival ratios of RhB with and without Cl, N-Ti 3 C 2 MQDs in solution after 1 h of UV illumination, absorption spectra of RhB without Cl, N-Ti 3 C 2 and MQDs before illumination were used as control. c) Photos of RhB dyes solution for fading degree before and after photodegradation reaction. d) PL emission spectrum of the experimental solution for different volume Cl, N-Ti 3 C 2 MQDs under light irradiation for 1 h. e) Absorption spectra of Cl, N-Ti 3 C 2 MQDs, and KMnO 4 reacted in the dark for 30 min, absorption spectra of KMnO 4 without Cl; N-Ti 3 C 2 MQDs were used as control. f) Photos of KMnO 4 reduction assay before and after photodegradation reaction. g) Schematic diagram of the scavenging mechanism toward • OH of Cl, N-Ti 3 C 2 MQDs. h) XPS spectra of Cl, N-Ti 3 C 2 MQDs after reaction with KMnO 4 and high-resolution XPS spectra of i) C1s, j) O1s, k) N1s, and l) Cl2p after the reaction. Reproduced with permission. [36] Copyright 2021, Elsevier.
www.advmatinterfaces.de of synthesis of MXene QDs using halo acid resulted in oxidized MXene QDs (TiO 2 or TiO 2 -xFx); therefore, changing the properties of the final material. In this method, MXene was converted into monolayer MXene QDs by taking advantage of the enormous mechanical force applied to the sample by nebulization with high-frequency acoustic waves. Acoustomicrofluidic synthesized MXene QDs after ten nebulization cycles contain low TiO 2 -xFx (14% compared to the quantity present in pristine MXene). MXene QDs prepared hydrothermally (HMXene QDs) have 84% of TiO 2 or TiO 2 -xFx. When MXene QDs and HMXene QDs were employed for electrochemical detection of H 2 O 2 as Naf/MXene QDs/GCE and Naf/HMXene QDs/GCE electrodes, the Naf/MXene QDs/GCE composite showed superior detection performance and the current response was five to six times greater than that of Naf/HMXene QDs/GCE electrode over the ranges 5 to 24 nm and 24 nm to 54.4 µm which was attributed to the lower TiO 2 or TiO 2 -xFx content. The LOD was found to be 5 nm. Furthermore, the CV results showed broad redox peaks at 100 mV s −1 , in contrast to the bare Nafion-coated electrode (Naf/GCE), which showed no redox peaks, confirming their electrochemical detection ability toward H 2 O 2 . [73]

MXene QDs for the Detection and Treatment of Environmental Pollutants
Environmental pollution is a major global crisis affecting habitat and human sustainability. The pollutants from fossil fuel combustion, automobiles, industries, power plants, and refineries, beyond a certain concentration, cause a serious threat to human life. The rising environmental pollution urges researchers to find efficient tools and techniques to monitor, control, and remove environmental pollutants. An ideal pollutant detection method should possess good sensitivity, selectivity, high conductivity, fast detection, very low detection limit, and a wide linear range of detection as well as good compatibility with the use in real samples. Though conventional methods such as spectrophotometry, GC, HPLC, and mass spectrometry provide acceptable results, many of them show up some limitations as well, such as low accuracy, complicated activity, and difficulty during in situ evaluation. [139] Lately, chemical, electrochemical, photochemical, and photocatalytic methods for the removal of pollutants have been extensively investigated, among which the photocatalytic routes gained much popularity, benefitting from their mild reaction parameters, simplicity of preparation, low cost, outstanding catalytic activity, high durability, and eco-friendly nature. However, the sluggish charge transfer, exciton recombination, and deficient photo-response of this method stand challenging. [4] MXene QDs, owing to their size and surface functionalities, excellent fluorescence, high optical absorption, strong emission characteristics, and optoelectronic properties, are an excellent candidate in the field of environmental pollutant detection and removal applications. [80] Sensitive and selective detection of 2,4,6-trinitrophenol using uric acid@Ti 3 C 2 quantum dots (UA@Ti 3 C 2 QDs) which produced a bright blue emission was reported. Uric acid molecules connected via hydrogen bonds efficiently encased Ti 3 C 2 QDs, and the resultant complex had enhanced properties such as fluorescence and resistance to photodamage. The quantum yield of the sensor was estimated to be 4.67%. The excitation and emission wavelengths were 360 and 420 nm. The vanishing of the CO, CH, and NH stretching frequencies and the broadening of the OH peak upon the introduction of TNP indicates the presence of various H-bonding interactions between the electrode and the analyte. The absorption of the TNP and emission and excitation of the uric acid-functionalized sensor constructively overlap, suggesting the chances of quenching due to the IFE or FRET mechanism. As the change in absorption intensity before and after the addition of the TNP is negligible, the possibility of transfer of electrons can be eliminated. Investigations on the influence of temperature on the relative PL intensity, which produced only a slight difference with the rise in temperature further confirms the IFE-induced quenching process. Furthermore, studies on the fluorescent lifetime and calculations of the Stern−Volmer quenching constant specify that type of quenching was static rather than dynamic. The detection limit was 9.58 nm in the range of detection of 0.01-40 µm. The quenching effect was observed only with TNP when tested against different ionic species. The method was applied for TNP detection in spiked water samples and also extended to the smartphone-based colorimetric detection method, wherein the filter paper-soaked sensor when added with different concentrations of TNP showed color change with 10 ng as the lowest detectable amount. [140] In a research work, Cr 6+ reduction using flower-like nickel nanoparticles functionalized with Ti 3 C 2 quantum dots nanolayers (Ni@MXene QDs) having a size of 35 nm, was detailed. The Ni@MXene QDs showed an absorption maximum at 290 nm with a wider spectrum due to the electron transfer between Ni and MXene QDs. The Cr 6+ showed an absorption peak at 350 nm which disappeared after 10.5 min of the addition of Ni@MXene QDs and the reducing agent, HCOOH. The formation of Cr 3+ was validated by the green solution of hexahydroxochromate(III) with the addition of NaOH. The Ni@MXene QDs interfacial interactions inhibited the Ni nanoparticles aggregation, lowered the activation energy of the reaction, fostered the reaction kinetics, and promoted the reduction reaction. The D-band center position and energy of Ni were higher in Ni@MXene QDs than in Ni alone; and therefore, more vacant antibonding orbitals were available, which improved the interaction between the catalyst and the adsorbates. As a result, Ni@MXene QDs outstanding catalytic activity stemmed from the Ni-MXene QDs interaction, which induced an up-shift of the D-band position of Ni sites, which resulted in increased HCOOH and H adsorption and a lower energy barrier for the catalytic reduction and could be applied to wastewater treatments. The mechanism mainly involved the in situ formation of CO 2 and H 2 from HCOOH, which then catalytically reduced Cr 6+ to Cr 3+ . Ni@MXene QDs displayed excellent catalytic activity, low activation energy, good conductivity, high rate constant, and short reaction time. The catalyst showed a 91% conversion rate even after the usage for five cycles and the decrease in the conversion rate was mainly due to the loss of the catalyst during the recycling. [141] Under visible light irradiation, the photocatalytic NO removal activity of a novel heterojunction composite based on titanium carbide MXene quantum dots with SiC (Ti3C2 QDs/SiC) was examined. This photocatalyst converted the NO pollutants into soluble NO form which could then be utilized for agricultural purposes. Ti3C2 QDs had www.advmatinterfaces.de a size of less than 8 nm, and the TQDs/SiC-20 had a substantially vast active area and a higher total pore volume than pure SiC. Ti3C2 QDs/SiC displayed good photostability, appreciable photocatalytic capability, enhanced superoxide radical generation, and improved oxidation activity. The influence of the QDs on the energy band structure of SiC narrowed the energy gap to 2.08 eV which favored the adsorption of a large amount of visible light. The studies using the Mott-Schottky plots showed that the prepared heterojunction composites were n-type semiconductors and that the electrical field produced at their interface appreciably increased the photoelectron transfer from SiC to QDs for the photocatalytic activity. The N 2 oxidation experiments substantiated a high oxidation ability of the valence band photogenerated holes of SiC in Ti 3 C 2 QDs/SiC-20 which could directly oxidize NO to NO 3− . The photoelectrons activated O 2 to generate · O 2− which could then oxidize NO to NO 3− . Similarly, the ·OH radicals produced also helped in the conversion of NO to NO 3− . Excellent carrier separation further enhanced the NO elimination process. All the prepared samples had fantastic removal efficiency, which immediately reached their maximum value in the first 6 min. Then, because of the inhibition of active sites by the reaction intermediates and products formed, the removal rate was reduced. TQDs/SiC-20 had the highest catalytic activity (74.6%), which was 3.1 and 3.7 times more efficient than SiC and Ti3C2 QDs, respectively. [117] Over the last several decades, there has been a huge rise in the concentration of heavy metals in the environment, mainly due to the growing industrial activities. These metals are mostly released into the water bodies which adds to environmental pollution and affects global sustainability. Heavy metals are nonbiodegradable, highly toxic, and are a major threat to living systems even in trace amounts. They can accumulate in the food chain and lead to various health hazards including several dis-orders, extreme harm due to oxidative stress, and cancer. Thus, efficient techniques for the detection and quantification of heavy metals in drinking water, edibles, biofluids, and the environment are highly needed. [142,143] A sensor based on fluorescent nitrogen-doped MXene quantum dots (N-Ti 3 C 2 MXene QDs) was employed for the detection of chromium ions (Cr 6+ ) based on the synergic IFE and static quenching (Figure 19). The sensor showed excellent excitation-dependent PL properties, good salt tolerance ability, and photobleaching resistance. The blue fluorescent intensity decreased with the addition of Cr 6+ ions. The overlap of the absorption band of Cr 6+ with the excitation/emission band of the QDs resulted in the fluorescence quenching. The fluorescence lifetime studies of the QDs confirmed that the fluorescence quenching was by the IFE mechanism and not by the FRET. In addition to IFE, the contribution of the static quenching process was further validated by the temperaturedependent studies. Then, the redox reaction between ascorbic acid and Cr 6+ caused the fluorescence turn-on. The addition of ascorbic acid reduced Cr 6+ to Cr 3+ , and this change in the oxidation state of chromium ions reduced the IFE, leading to fluorescence turn-on. The QDs showed a shift toward longer wavelengths with a maximum emission intensity at 440 nm when the excitation wavelength was 360 nm. The fluorescence lifetime of the sensor was 4.05 ns. A low limit of detection of 0.012 µm was obtained for Cr 6+ in a linear range of 0.1-500.0 µm. The method was also used to detect ascorbic acid with an LOD of 0.02 µm. The method demonstrated the practical application of Cr 6+ detection in spiked water samples that displayed recovery rates greater than 95% and a relative standard deviation of not more than 5%. However, the sensor showed significant interferences in the presence of various other heavy metal ions and MnO 4− ions, which could be minimized by the addition of chelating agents, masking agents, or Na 2 S 2 O 3 . [75] Figure 19. Schematic illustration of synthesizing highly fluorescent N-Ti 3 C 2 MXene QDs and fluorescence strategy for the determination of Cr (VI) and ascorbic acid. Reproduced with permission. [75] Copyright 2021, Elsevier.

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A research work highlighted the use of fluorescent MXene QDs for the detection of Fe 3+ in tap water samples with high selectivity and sensitivity. The quenching of the PL intensity induced by the metal ions was taken advantage of to detect the number of Fe 3+ ions present in the sample. The addition of Fe 3+ ions to the MXene QDs having a zeta potential of −10.9 mV solution induced precipitation, suggesting strong electrostatic interactions between the Fe 3+ ions and the MXene QDs. In addition, the highly positive Fe 3+ ions with superior oxidation capacity brought about the following redox process which led to the PL quenching of the QDs. What's more, the Fe 2+ ions also induced PL quenching due to the strong coordination between the metal and N 2 . The blue fluorescence turned-off QDs showed λ excitation and λ emission at 365 and 445 nm, respectively. The limit of detection of 1.4 µm and a linear range of detection of 0-0.8 mm were observed. The fluorescence quenching was observed only in the presence of the Fe 3+ in contrast to various monovalent, divalent, and trivalent cations. [104] Static fluorescence quenching of the N-doped Ti 3 C 2 QDs in the presence of Cu 2+ ions was investigated. The exfoliated MXene had a particle size of 600 nm, while N-doped MXene QDs had a smaller particle size of 6 nm with higher fluorescence emission characteristics and increased sensitivity and selectivity toward the determination of Cu 2+ (Figure 20a). Size-dependent PL characteristics with the emission peak of the sensor at 453 nm when excited at 330 nm, were observed. The strong coordination of the N atoms with the Cu 2+ during the ion detection process could change the energy states of the QDs leading to the quenching of the PL intensity (Figure 20b,c). The quenching of PL intensity was observed with both Cu 2+ and Fe 3+ , of which the quenching effect was greater for Cu 2+ (56%) while Fe 3+ showed a weak quenching effect (32%). This can be attributed to increased nitrogen atoms available for coordination with the Cu 2+ when compared to the oxygen coordination sites available for Fe 3+ . The PL quenching of the QDs in the presence of Fe 3+ ions was due to its coordination with the O atoms present at the surface of the QDs instead of the N atoms. A detection limit of 500 µm in a detection range of 0-2000 µm was attained. The PL lifetime before and after the quenching by the Cu 2+ was 7.81 and 3.79 ns, indicating a static quenching mechanism (Figure 20d). Quenching mechanism of N-MXene QDs by Cu 2+ is shown in Figure 20e,f. [97] A new biodegradable fluorescent detection material based on Nb 2 C QDs with the inherent crystallography of Nb 2 C MXene phases, remarkable photo-and chemical-stability, as well as anti-photobleaching properties, was proposed for the detection of Fe 3+ ions. The size-dependent fluorescence spectra showed absorption and emission at 370 and 485 nm. The sensor showed maximal fluorescence intensity from pH 5 to 11 with a fluorescence lifetime of 1.51 ns. Based on fluorescence decay, the quenching mechanism was static by the formation of a stable non-radiative complex between the sensor and the metal ion, involving the excited state partial electron shift to the metal d orbitals. The quenching resulted within 3 s of the addition of the metal ion. A low detection limit of 0.89 µm and a linear dynamic range of 0-300 µm were achieved. The competitive experiments revealed that the QDs showed high selectivity toward Fe 3+ ions even in the presence of 15 other metal ions. [64] A selective determination of ferric ions based on the fluorescence turn-off method using fluorescent Ti 3 C 2 MXene QDs was reported. The quantum size effect and surface states of the sensor allowed excitation wavelength-dependent emission Reproduced with permission. [97] Copyright 2019, Elsevier.

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profiles ranging from 370 to 715 nm. The maximum emission and excitation wavelength was found to be 410 and 320 nm, respectively. The presence of Fe 3+ considerably suppressed the fluorescence of the MXene QDs and the quenching mechanism was found to be based on the combined effects of the redox reaction between MXene QDs and Fe 3+ along with the IFE. The XPS peak shift of TiC and TiO from 457.9 to 458.7 eV and 464.3 to 465.2 eV demonstrated the characteristics of Ti 4+ while the XPS data of metal ions showed peaks corresponding to Fe 2+ ions. However, strong oxidizing agents such as KMnO 4 or K 2 Cr 2 O 7 quenched the fluorescence completely. A low detection limit of 310 nm in a 5-1000 µm detection range was obtained for Fe 3+ . The presence of amino acids, Vitamin C, or various metal ions such as Cr 3+ and Cu 2+ did not affect the detection ability of the sensor. The detection of the Fe 3+ was applied to serum and seawater samples which showed recovery ranges greater than 92% and 96%. [80] Synthesis of N-doped Ta 4 C 3 MXene QDs was done for the detection of Fe 3+ ions. Ethylenediamine was used for the generation of nitrogen and Ta 4 C 3 nanosheets were used as precursors. The produced N-MXene QDs had a size of 2.60 nm and showed high stability, outstanding blue PL characteristics, and a quantum yield of up to 23.4%. N-MXene QDs exhibited characteristic quenching of Fe 3+ with a low limit of detection of 2 µmol L −1 , indicating swift and sensitive detection of a heavy metal ion. The incorporation of N-MXene QDs into self-healing hydrogels may be utilized to create fluorescent hydrogels which find applications in mechanical detection assessment for remote force calculation [144] Similarly, Ti 3 C 2 MXene QDs were prepared in three different solvents namely, DMSO, ethanol, and DMF via a solvothermal process and their solvent-influenced size and optical properties were compared. Different size distribution was observed in the QDs synthesized from DMSO (D-MXene QDs), Ethanol (E-MXene QDs), and DMF (F-MXene QDs) which were 1.8 ± 0.1, 2.5 ± 0.2, and 3.3 ± 0.2 nm, respectively. The quantum yield of D-MXene QDs, E-MXene QDs, and F-MXene QDs were 4.1%, 6.9%, and 10.7%, respectively. The D-MXene QDs showed white luminescence while E-MXene QDs and F-MXene QDs showed blue emission under the 365 nm UV irradiation. The F-MXene QDs showed maximum absorbance at 271 nm and a shift in emission spectra based on solvent polarity. The luminescence of F-MXene QDs was quenched in the presence of Fe 3+ and thereby showed high selectivity toward Fe 3+ ions in comparison with various other divalent or trivalent metal ions. The variation in the PL properties was not observed in the D-MXene QDs and E-MXene QDs due to the low content of oxygen on the surface. The selectivity was mainly because of the interaction of Fe 3+ with the oxygen present as carbonyl groups on the F-MXene QDs. A low detection limit of 2.0 × 10 −6 m in wide linear concentration ranges from 5 to 470 × 10 −6 m and 510 to 750 × 10 −6 m was attained. Furthermore, fluorescent staining studies supported the use of MXene QDs in fluorescent chemical and biological imaging. [35] The nitrogen-doped Ti 3 C 2 MXene QDs synthesized via the hydrothermal method showed excitation-dependent blue fluorescence under UV light. The width of MXene QDs varied from 2 to 7 nm while the lateral size varied from 3.93, 3.7, and 5.76 nm due to different temperatures of 120 °C, 160 °C, and 200 °C, respectively. It was noted with the thickness, that the MXene QDs@120 °C was a monolayer, MXene QDs@160 °C was bilayered, and MXene QDs@200 °C was multilayered. The hydrothermal temperature also influenced their lifetimes. The maximum excitation and emission were at 360 and 447 nm, respectively. The quantum yield was highest for MXene QDs@160 °C with 18.7%. A wide range of pH (3 to 12) tolerance was noted. The QDs were employed in the detection of Fe 3+ ions which quenched the fluorescence significantly. The capacity of Fe 3+ for fluorescence quenching was credited to its electron/hole recombination annihilation activities via a different and potent charge transfer route. The detection limit was calculated to be 2 µm in the range of detection of 2 to 5000 µm, with a detection time of 0.5 min. The method was also used for the determination of H 2 O 2 , and it was seen that H 2 O 2 or Fe 2+ alone did not change the fluorescence of MXene QDs while the simultaneous addition of H 2 O 2 along with Fe 2+ resulted in a decrease in the fluorescence by 69.78%. [58] Sulfur and Nitrogen co-doped niobium carbide MXene QDs (S, N-Nb2C MXene QDs) were studied for the detection of trace amounts of Cu 2+ . The sensor showed maximum emission intensity at 520 nm when excited at 390 nm. The green fluorescence lifetime and quantum yield were estimated to be 7.12 ns and 17.25%, respectively. Addition of Cu 2+ ions increased the electron shift and improved the electron/ hole recombination annihilation, altering the surface morphology and electronic state of the QDs; and thus, changing the fluorescence intensity. The coordination of Cu 2+ with the surface of the QDs decreased the extent of surface oxidation and formed an alternative electron transport pathway, which facilitated a non-fluorescent electron/hole recombination along with the aggregation of the QDs into a complex structure. The size effects caused the vanishing of the excitations via photon scattering and the fluorescence of the QDs was quenched. Thus, the quenching mechanism was due to the IFE and aggregation of the QDs via its amino functionalities and its agglomeration with the Cu 2+ ions. A limit of detection of 2 µmol L −1 in the linear ranges of 2-1000 µmol L −1 and 1000-5000 µmol L −1 was found. Various mono-, di-, and trivalent cations when present as interference, did not affect the fluorescence quenching of the sensor significantly as that of Cu 3+ ions. Furthermore, Caco-2 cells showed viability for concentrations less than 10 mg mL −1 of the sensor; and therefore, could be used for bioimaging as well. [145] In a study, nitrogen and phosphorous-incorporated Ti 3 C 2 MXene QDs (N, P-MXene QDs) with the highest quantum yield of 20.01% were employed for the sensitive, ecofriendly, and label-free estimation of ultra-trace quantities of Cu 2+ ions. The PL lifetime of the sensor was found to be 8.54 ns while that of Ti 3 C 2 doped with nitrogen was 6.02 ns and the phosphorus-doped was 4.99 ns. The synergic effects of the high electron-withdrawing nature of the surface heteroatoms (N, P, O), along with the edge effects and the quantum confinement of the material contributed to the high yield and the stability of the excitons. The maximum fluorescence emission and excitation were observed at 560 and 480 nm, respectively. The amino groups of the sensor seized Cu 2+ ions and this nitrogen coordinated with the metal ions disrupting the electron/hole transfer, aggregation effect, and exciton loss, leading to fluorescence quenching due to the IFE. Cu 2+ was detected with a low detection limit of 2 µm in two linear ranges between 2 and 100 µm and between 250 and 5000 µm. [107] www.advmatinterfaces.de

MXene QDs for Auxiliary Applications
In addition to the above-discussed usages, the exceptional structural and mechanical properties, superior conductivity, and fluorescent activities of the MXene QDs make them promising candidates in several other applications including photocatalysis, qualitative, and quantitative analysis of ions, transparent conductors, analysis of natural products and dyes, monitoring radical scavenging activities, and catalytic conversion of nitrogen into useful products. Dual detection of curcumin and hypochlorite was achieved using Ti 3 C 2 MXene QDs which when excited at 330 nm showed maximum emission intensity at 430 nm. The significant overlap of the emission of Ti 3 C 2 MXene QDs with the UV absorption of curcumin activated the FRET between the two, which quenched the fluorescence intensity of the sensor at 430 nm and increased the intensity of fluorescence at 530 nm. The oxidation of curcumin to quinone in the presence of oxidizing reagents such as hypochlorite restored the fluorescence of the sensor. The fluorescence change could be observed optically by the color change from blue to yellow and back to blue under UV light of 360 nm. Ti 3 C 2 MXene QDs exhibited high stability and high selectivity toward curcumin as well as hypochlorite. The low limit of detection of 20 nm and 5 µm in a wide linear range of 0.05-10 µm and 25-275 µm was obtained for curcumin and hypochlorite ions, respectively. [37] N and P-doped Ti 3 C 2 MXene QDs (N, P-Ti 3 C 2 MXene QDs) were developed for the colorimetric and fluorometric dual-modal analysis of NO 2 − . The fluorescence of the modified QDs was quenched in the presence of 1,10-phenanthroline-Fe(II) complex through the IFE, marked by the enhancement of orange color. The addition of NO 2 − ions restored the fluorescence of the sensor due to the oxidation-reduction reactions between the NO 2 − and Fe 2+ ions, leading to a change from an orange color to colorless. Furthermore, paper-based N, P-Ti 3 C 2 MXene QDs/Phen-Fe 2+ test trips were prepared for NO 2 − determination, which exhibited rapid response with the aid of a smartphone. [146] Nitrogen-doped MXene quantum dots (N-MXene QDs) were fabricated for the highly selective analysis of Alizarin red following the fluorescence turn-off mechanism. The possibility of the static quenching mechanism was eliminated as there was no change in the absorption spectra after the interaction of the sensor and red dye. The fluorescence lifetimes of the sensor were constant, both in the presence and absence of the alizarin red and the detection mechanism followed Parker equations backing up the IFE. The fluorescence emission intensity at 580 nm decreased continually upon the addition of gradual amounts of alizarin red with excitation at 420 nm. The proposed method exhibited high selectivity toward the analyte detection in alizarin spiked water samples even in the presence of various metal ions. The detection limit of alizarin red dye was up to 1.21 µm in the dynamic range of 0-80 µm. [21] Investigations revealed the recognition of Ti 3 C 2 T x MXene quantum dots (Ti 3 C 2 T x QDs) as efficient electrocatalysts for the nitrogen reduction reaction. This helped to overcome the harsh conditions needed to reduce nitrogen to a useful form-like ammonia due to its very high bond energy. The adsorption sites for the N 2 were provided by the exposed Ti present on the edges of the QDs. It was noted that the material showed better electrocatalytic performance when hydroxyl groups were present instead of F groups. Ti 3 C 2 T x MXene suspension was exposed to alkalization with shearing in NaOH solution to produce Ti 3 C 2 OH. As an electrocatalyst, Ti 3 C 2 T x QDs provided an NH 3 yield of 62.94 µgh −1 mg −1 cat with a Faradaic efficiency of 13.30% at −0.50 V versus RHE. The NH 3 yield and Faradaic efficiency showed minimal variations during the recycling tests conducted six times at the optimal potential of 0.50 V, which indicated good electrochemical stability. [53] A heterostructure composite based on Ti 3 C 2 QDs incorporated Cu 2 O nanowires Ti 3 C 2 QDs/ Cu 2 O NWs/Cu was employed for the photochemical conversion of CO 2 to methanol. The incorporation of Ti 3 C 2 QDs not only helped to overcome the limitations caused by the charge carriers that tend to oxidize or reduce the Cu 2 O nanomaterial during the process but also enhanced the stability, photocatalytic activity, photon absorption, charge density, and charge transfer. Cu 2 O possesses more negative redox potential than CO 2 along with a suitable bandgap that facilitates a broad range of light absorption necessary for the reaction and the excellent conductivity of Ti 3 C 2 QDs favored electron transport. Electrons were photo-excited from the VB of Cu 2 O to its CB under simulated solar light irradiation. The photoexcited electrons were transferred to the QDs and they accumulated rather than being recombined within Cu 2 O due to the high conductivity and less negative Fermi level of Ti 3 C 2 QDs compared to the CB of Cu 2 O. The accumulated electrons accelerated the reduction process because of the more negative Fermi level of Ti 3 C 2 QD compared to the redox potential of CO 2 to methanol. Furthermore, Ti 3 C 2 QDs showed more chemisorptivity toward CO 2 rather than H 2 O, making CO 2 reduction easier by promoting H + attachment and rapidly giving electrons. The highest photoreduction yield was 153.38 ppm cm −2 and the production rate was 25.77 ppm cm −2 h −1 . The methanolic yield was 8.25 times greater than that with Cu 2 O Nanowire/Cu. The yield was ≈89% of that obtained from the 1st run, after six cycles. The flat band potential of the composite was ≈0.52 V (vs RHE), while the carrier density was found to be 2.75 × 10 19 cm −3 . [61] Table 1 summarizes some of the current articles on MXene quantum dots and their synthetic approaches along with their applications. Some of the most commonly reported synthesis methods include the etching process, solvothermal, hydrothermal, ultrasonic, and chemical/thermal exfoliation. The resultant MXene QDs have different sizes, structural features, and surface functional groups depending upon the synthesis method and the reaction conditions. Diverse applications of the obtained MXene QDs are discussed which comprise energy conversion and management applications, biomedical applications, environmental applications, and other applications such as analysis of ions, natural products, and dyes, monitoring radical scavenging activities, intracellular pH monitoring, and other photocatalytic usages. Various analytical parameters are also highlighted.

Conclusion and Future Perspectives
Owing to the synergic action of the side effects, optical, mechanical, and electrical properties along with the influence of the surface functionalities, MXene QDs stand as an excellent choice for a wide range of practical applications. Their areas www.advmatinterfaces.de  Glutathione detection [90] N-Ti 3 C 2 Hydrothermal method 4 -LOD: 0.17 µm GSH LRD: 0.5−100 µm GSH Glutathione detection [22] Ti 3 C 2 Fluorine-free method-exfoliation 4.9 and 1.2 --Cancer therapy [29] www.advmatinterfaces.de Ti 3 C 2 T x -2-3 --N 2 reduction [53] of utilization include catalysis, energy conversion and storage, electrochemical and fluorescent detection, pollutant removal, bioimaging, early disease diagnosis, and photothermal therapy. This article reviews the properties, various synthesis strategies as well as diverse applications of different MXene QDs for gaining essential information and knowledge for synthesizing novel MXene frameworks to satisfy the requirements of novel applications.

Effect of Synthesis Parameters on the Development of MXene QDs
It has been established that the size and surface characteristics of the MXene QDs rely on specific synthesis parameters such as the temperature, solvents, reaction time, and modifiers used.
The literature reveals the challenges in the large-scale synthesis of high-purity MXene QDs monolayers along with the advantages and limitations of different methods. Various reports of synthesis of MXene QDs have been published making use of HF as an etchant due to it being cheap and having the ability to control the morphological and dimensional properties of resultant MXene QDs. As the already existing methodologies are resulting in the formation of F-MXene QD which are not suitable for electrochemical applications, the development of novel methods excluding the utilization of F-free solvents should be explored. Therefore, the focus was later shifted toward the utilization of other strategies including ultrasonic waves, chemical exfoliation, and solvothermal methods. These methods were also proved to be less functional as they were affecting the structural integrity of the product formed and were not environmentally benign. Presently, the acoustomicrofluidic based synthesis method is being explored and has been giving positive responses with enhanced electrochemical and fluorescent properties. As metal oxidation occurring at the MXene QD surface during the preparation procedure is a major problem, novel synthesis methods for obtaining MXene QDs having better purity need to be conceptualized.
The colloidal stability of MXene QDs is another important aspect that is greatly influenced by the choice of synthesis route. MXene QDs produced via the electrochemical method followed by fluorination surpass degradation and showcase improved colloidal stability whereas those obtained by HFbased etching result in lower colloidal stability due to the presence of a higher number of defective sites. Both ethanol refluxing and the hydrothermal method have been proven to increase colloidal stability when compared to the microwave heating method.

Surface-Property Relationships
Various design principles including surface modification, doping with heteroatoms, and fabrication of composites with MXene QDs have been proposed for ameliorating the desirable properties of MXene QDs. Heteroatom doping has been proven to improve the detection and catalytic properties of MXene QDs. The size and surface modifications, quantum confinement effect, and edge effects allow the QDs to be explored for suitable bioimaging applications and ultrasensitive detection of molecules. More studies need to be conducted for understanding the effect of incorporating surface defects on their PL activities. In addition, the amount of migration of photoelectrons depends on the number of heterojunctions present within the system. Similarly, the interfacial properties among MXene QDs and different surface functional groups, gas molecules, and hybrid phases require further exploration.
Although the incomparable structural, electronic, and optical properties of MXene quantum dots have been meticulously established, many of their theoretical attributes need to be further examined. Furthermore, the tunability of the surface functionalities of the MXene QDs to alter and control their properties needs to be investigated in detail. Despite the fact that MXene QDs possess high stability and monodispersity, strategies for improving their recyclability could be analyzed in-depth.

MXene QDs in Energy-Related Applications
MXene QDs in recent times have elicited interest in the field of photocatalysis. Even though the research based on MXene QDs as photocatalysts is still in its infancy, much effort for the search in the next generation MXene QDs for applications including HER, CO 2 reduction, and N 2 reduction is still of great interest. The QDs of MXene have been shown to boost the visible light absorption of materials and therefore can be used as an interesting candidate for photocatalytic reactions. Understanding the underlying reaction mechanism associated with photocatalysis could excavate solutions for improving the poor oxygen resistance of MXene-based QDs. In addition, studying the effect of various reaction parameters on the ability of MXene QDs in photocatalytic systems at the molecular level can be considered to be of high importance. Studies on the influence of morphology and properties of MXene QDs on their catalytic activity are necessary for fabricating affordable, stable, and high-performance catalysts for diverse forms of applications. Detailed evaluation of the catalytic reaction sites and reaction mechanisms involving MXene QDs along with the changes in their active centers after the catalytic reaction can give new insights into the current field of application. The inclusion and utilization of computational models will also be of immense applicability in gaining a better understanding of invaluable mechanisms associated with MXenes-QD-based photocatalysis. In-depth modeling and analysis could help in bridging the correlation between the MXenes-QD-based functional materials and photocatalytic activities.
The MXene QDs because of their annihilation property, charge carrier separation, and suitable energy band structure are being widely employed as next-generation co-catalysts for water splitting and solar energy applications. The enlarged specific surface area, broad spectral response, mechanical stability, and ability to form a Schottky barrier based on the conductivity of MXene QDs have led to their improved utilization as co-catalyst while the functional groups act as active sites. The unique chemical and physical characteristics of MXene QDs can be further analyzed and utilized for making them a cocatalyst in both fuel conversion and photocatalytic applications. MXene QDs as a support material for different applications are of major importance; and therefore, studying the chemical interaction of MXene QDs with different functional groups and their interfacial properties is highly significant from an electrochemical performance perspective. Blessed with abundant surface functional groups, MXene QDs can be directly used as anchoring sites thereby limiting the utilization of other modification strategies.

Biomedical and Environmental Applications of MXene QDs
The chemiluminescence, chemical stability, water solubility, and good biocompatibility of MXene QDs have attracted much attention in the biomedical fields majorly as fluorescent probes. The detection of the biomarkers as well as photothermal therapeutic applications using MXene QDs open new scopes for researchers to further examine the potency of this material in terms of its biocompatibility, specificity, photostability, cytotoxicity, and biodistribution.
The usage of Ti, Nb-based MXene QDs has been studied vastly; however, Ta-based MXene QDs are less cited in the literature while research related to other transition metal-based MXene QDs can be an explorative field of study. The cytotoxic studies carried out suggest that Ti 3 C 2 and Nb 2 C induce apoptosis by upregulating various biomarkers. Therefore, it becomes pertinent with the usage, handling, and disposal of such MXene QDs when utilized at a larger scale. Further studies on the potential applications of sensors in vivo are needed as their applications in vivo show weak fluorescence and limited regions for surface engineering. The development of nextgeneration-based wearable sensors operating at the molecular level functioning beyond metabolites toward a wide range of biomarkers at very low concentrations can be a potential direction for future research. The optical nonlinearities, as well as applications of MXene QDs in photonics, need to be thoroughly understood as a tool to develop the material for many novel applications.