Synthesis and Application of Titanium Carbide (Ti3C2)-Cobalt Sulfide (Co3S4) Nanocomposites in Amino Acid Biosensing

Background The fabrication of titanium carbide (Ti3C2)-cobalt sulfide (Co3S4)-based biosensors with high sensitivity and selectivity can change the biosensor manufacturing industry completely. Molecular and clinical diagnostics, disease progression monitoring, and drug discovery could utilize these sensors for early biomarker detection. MXene (Ti3C2) is a two-dimensional material with exceptional electrical conductivity, hydrophilicity, great thermal stability, large interlayer spacing, and a high surface area. Ti3C2's remarkable characteristics make it well-suited for biomolecule immobilization and target analyte detection. Co3S4 is a transition metal chalcogenide that has shown great potential in biosensors. Co3S4 nanoparticles (NPs) can potentially enhance Ti3C2 electrocatalytic activity, particularly in amino acid detection. L-arginine is a semi-essential amino acid, and the body frequently uses it to support healthy circulation and plays a crucial role in protein synthesis. We fabricated the Ti3C2-Co3S4 biosensor for L-arginine detection. Aim This study aims to synthesize and apply Ti3C2-Co3S4 nanocomposites in amino acid biosensing. Materials and methods The Ti3C2 nanosheets were synthesized by the selective removal of an aluminum (Al) layer from the precursor (Ti3AlC2) using hydrofluoric acid (HF). The resulting mixture serves as an etchant, especially targeting the Al layers on Ti3AlC2 while protecting the desired MXene layers at room temperature. Cobalt nitrate hexahydrate was dissolved in deionized water. Sodium hydroxide was added to the cobalt solution and stirred. Thioacetamide was added to the above solution and stirred (Solution B). A mixture of Solution A and Solution B was stirred for 30 minutes. The mixture is transferred to a hydrothermal reactor and maintained at a temperature of 180°C for 12 hours. Once the reaction completes, we cool the resultant mixture to room temperature and then filter it using the washing technique. The sample underwent a 12-hour drying process at 80°C. Results This study investigated the use of a biosensor that employed Ti3C2-Co3S4 NPs to detect the concentration of L-arginine. The X-ray diffraction (XRD) shows clear and distinct peaks, which means that the synthesized Ti3C2-Co3S4 nanostructures have a crystalline structure. Scanning electron microscopy (SEM) analysis revealed that the sheetlike structure of synthesized Ti3C2-Co3S4 nanostructures revealed the crystalline morphology. The results of this study show that the Ti3C2-Co3S4 NP-based biosensor can be used to detect L-arginine in a sensitive and selective way. Conclusion This study investigated the synthesis of Ti3C2-Co3S4 NPs and their ability to detect L-arginine levels and show a distinct correlation between the L-arginine concentration and the fluorescence intensity, demonstrating the biosensor's effectiveness in detecting L-arginine levels.


Introduction
Amino acids are important elements of the human body that are fundamental, integral, and essential to health.Amino acids are utilized as biocompatible materials for an array of applications, including drug delivery, diagnostics, human health, nutrition, and imaging.They are also significant components of body metabolism [1,2].L-Arginine is a semi-essential amino acid crucial for regulating various metabolic processes and boasts a diverse range of biological functions.It serves as a precursor to several important compounds, such as L-ornithine, γ-amino butyric acid, and spermine.Notably, L-arginine is one of the most highly charged positively polarized amino acids.This amino acid is vital for our metabolic processes and plastic metabolism.It is also responsible for the synthesis of nitric oxide, certain hormones, polyamines, and creatine.Additionally, it acts as a precursor for the formation of other amino acids such as proline, glutamate, and glutamine.L-Arginine is capable of endogenous synthesis in the body; however, its synthesis decreases considerably with age-related changes and in some pathological conditions [3,4].
L-arginine is a chemical that has great importance in clinical and quality control applications.It is mostly located at the active sites of many proteins.The structure of the enzyme is conducive to facilitating the attachment of phosphate anions, which in turn enables the catalysis of phosphorylation processes.Additionally, arginine is crucial in regulating the electrical charge of several proteins.Arginine is degraded into urea and ornithine during nitrogen metabolism by the activity of arginase.Arginine also assists in the process of removing ammonia from the body as well as in the release of hormones and the maintenance of the immune system.According to reports, the process of converting arginine into nitric oxide through nitric oxide synthase can potentially aid in the treatment of various physiological conditions including cardiovascular diseases, peripheral vascular disease, erectile dysfunction, atherosclerosis, vascular headaches, and chest pain.This is achieved by improving vasodilation, or the widening of blood vessels.Arginine stimulates protein synthesis, prevents tissue degradation, and promotes spermatogenesis [5,6].
L-Arginine has a diverse array of effects on the functioning of different systems, making it a potential therapy choice for a range of disorders.These disorders include illnesses related to cardiology, diseases of the central nervous system and liver, and even respiratory disorders.There has been an increase in the occurrence of counterfeit medications, despite the availability of high-quality treatments.These counterfeit medicines typically contain incorrect proportions of active chemicals or may contain no active compounds at all.Hence, regulating the content of arginine in medicines is of the utmost importance.It is necessary to measure the concentration of L-arginine in biological fluids.To determine the concentration of L-arginine in solutions, various methods, including spectrophotometry, ion-exchange chromatography, and highperformance liquid chromatography (HPLC), are used.Most of the existing physicochemical and chemical techniques have drawbacks, including limited selectivity and sensitivity, as well as the need for expensive and complex equipment.To address these issues and reduce costs, biosensors represent a highly promising alternative [7].
An important demand for the field of biosensors is the development of sophisticated sensors that possess exceptional sensitivity and selectivity.Several nanomaterials have been developed to build biosensors that possess exceptional sensitivity and selectivity, fulfilling the required parameters.MXene (Ti 3 C 2 ) is a highly regarded nanomaterial that is garnering attention for its excellent characteristics, resulting in a perfect choice for designing biosensors.Ti 3 C 2 is a two-dimensional (2D) material, is a transition metal carbon, nitride and carbonitrides.It has gained significant interest in various fields, including supercapacitors, Liion batteries, catalysts, transparent conductors, biomaterials, lubricants, field-effect transistors, sensors, drug carriers, dual-responsive surfaces, EMI shielding materials, purifiers, hybrid nanocomposites, dye substrates, and cancer treatment [8].This is because Ti 3 C 2 possesses distinct properties such as hydrophilicity, biocompatibility, conductivity, and stability.Ti 3 C 2 are produced by selectively etching the A layer from the Ti 3 AlC 2 MAX phase using a chemical etchant.In this process, M represents the transition metal materials, A represents the elements from the III A and Ⅵ A columns, and X represents carbon and/or nitrogen elements.The layers consist of C atoms arranged between the octahedral sites, and Ti n+1 C n layers are alternating with layers of Al atoms.The Ti-C bond exhibits a strong bonding of covalent/metallic/ionic character, while the Ti-Al layers are weakly bonded and contain a pure metallic nature.As a result, the Ti-Al bond has a tendency to break down at elevated temperatures, forming Ti n+1 C n compounds.This process promotes recrystallization and the creation of a three-dimensional (3D) rocksalt-like structure of Ti n+1 C n .
Ti 3 C 2 is the most extensively researched MXene because of its ease of synthesis and high stability.sheets.The 2D layer nanostructure has a substantial surface area capable of accommodating biological components.The surface of the Ti 3 C 2 nanocomposite contains active functional groups that immobilize biological components, hence altering the electrocatalytic properties and resulting in a linear response.This has been achieved by the alteration of Ti 3 C 2 characteristics to optimize them for particular types of biosensors or by integrating them with other nanomaterials [9][10][11][12].
Cobalt sulfide (Co 3 S 4 ) is an important class of transition metal chalcogenides that have many uses in many energy storage and generating technologies, including batteries, supercapacitors, and photovoltaic solar cells, as well as enzyme-free glucose sensors.The distinctive arranged morphological properties of 2D Co 3 S 4 not only enhance the penetration of electrolyte but also offer a more reactive surface when assessed as an electrode material for biosensors.Additionally, it enables the spontaneous arrangement of thiolated compounds on its surfaces, which presents a benefit in effortless surface modification.Nevertheless, Co 3 S 4 is unsuitable for usage in electrode materials due to its semiconductor nature and low conductivity.Therefore, it is extremely advantageous to create a composite of Co 3 S 4 with materials that are electrically conductive in order to enhance the transfer of charges for applications in electrochemical biosensing [13].Co 3 S 4 has excellent biocompatibility, high electrocatalytic activity, and significant adsorption capacity, making it highly desirable for sensing fabrications.Despite its potential, Co 3 S 4 is hindered by its low electrical conductivity, which restricts its capacity to exhibit high sensitivity, reliability, and rapid response times [14].
A proposed method to combat these issues is to mold the Co 3 S 4 into a Ti 3 C 2 , which would highlight the advantageous characteristics of both materials and reduce the drawbacks they have.The synergistic effect of the two existing materials will eliminate the problem of low sensitivity in Ti 3 C 2 and the poor selectivity in

Synthesis of Ti 3 C 2 MXene
Ti 3 C 2 MXene was synthesized using the Ti 3 AlC 2 MAX phase, which was obtained commercially, as the initial material.The 2.5 g of Ti 3 AlC 2 black powder was immersed in 60 mL of 40% HF for 24 h.A magnetic stirring process was used to maintain heating (40°C) and stirring throughout the whole etching process, with 24 h of etching time and increased reaction efficiency.An amount of about 10 cm 3 of HF per 1 g of the initial material was used.Small amounts of Ti 3 AlC 2 were introduced into HF during the reaction.A highly exothermic reaction and significant hydrogen evolution were observed.The resultant mixture functions as an etchant, specifically targeting the aluminum layers on Ti 3 AlC 2 while protecting the desirable MXene layers.To neutralize the compound's acidity, the material was thoroughly washed until its pH level reached 8 (Solution A).

Synthesis of Ti 3 C 2 -Co 3 S 4
A total of 20 mmol of Co(NO 3 ) 2 •6H 2 O were dissolved in 50 mL of deionized water in a separate beaker, and 6 mmol of NaOH were dissolved in 50 mL of deionized water in a separate beaker.NaOH solution was slowly introduced into Co solution and stirred slowly for 30 min.After the suspension was created, about 4 mmol of thioacetamide was dissolved in the solution while vigorously stirring for 30 min (Solution B).Solution B was gradually introduced into Solution A, which contained Ti 3 C 2 MXene.A mixture of Solution A and Solution B was stirred for 30 min.The mixture is transferred to a hydrothermal reactor and maintained at a temperature of 180°C for 12 h.Once the reaction is complete, we cool the resultant mixture to room temperature and then filter it using the washing technique.The sample underwent a 12 h drying process at 80°C.

X-ray diffraction (XRD)
XRD is a rapid analytical technique used to determine the crystal structure of materials.The XRD spectra of Ti 3 C 2 -Co 3 S 4 were determined by measuring the range of 2θ from 10° to 90°. Figure 1    being 100%, demonstrates its purity and the absence of impurities in the synthesized compound.EDS analysis confirms that no additional materials are present, indicating that the Ti 3 C 2 -Co 3 S 4 is impurityfree (Table 1).

FIGURE 3: EDS analysis of Ti3C2-Co3S4
EDS: Energy dispersive X-ray spectroscopy; Ti   Various binding mechanisms, such as electrostatic attraction, hydrogen bonding, or covalent binding, might promote this interaction and cause the fluorescent probe to undergo a conformational shift.Confocal fluorescence microscopy is used to examine the biosensor's fluorescence response.After positioning the substrate on the microscope stage, the fluorescent probe is excited at its excitation wavelength using a laser.In order to capture high-resolution fluorescence pictures, the emission is detected using a confocal pinhole.It is possible to determine the concentration of L-arginine in a sample by measuring the intensity of its fluorescence.To determine the amount of an unknown material, calibration curves are made using known Larginine concentrations.A sensitive and quantitative detection approach is provided by plotting the change in fluorescence intensity (quenching or amplification) against the L-arginine concentration.The Ti 3 C 2 -Co 3 S 4 heterostructure guarantees great sensitivity and selectivity by providing a large surface area for probe attachment and an effective interaction with L-arginine.immobilizing tyrosinase, a model enzyme.This was achieved by creating a mediator-free biosensor capable of detecting phenol with high sensitivity and speed.The results have demonstrated that a surface-controlled electrochemical mechanism may readily accomplish the direct transfer of electrons between tyrosinase and the electrode.The tyrosinase biosensor that has been produced demonstrates excellent sensitivity, reproducibility, and stability.It also has a low detection limit and a large linear range.The suggested biosensing technique exhibited excellent repeatability, reproducibility, long-term stability, and high recovery when detecting phenol in actual water samples.The exceptional performances of Ti 3 C 2 MXene, which possesses a graphene-like structure, demonstrate its durability and versatility as an electrochemical biosensing platform for enzyme-based biosensors and biocatalysis.This material has significant promise for many applications in biomedical detection and environmental analysis [18].Additional enhancements have been accomplished by the application of metal sulfide NPs to the surface of Ti 3 C 2 MXenes.This process effectively augments the surface area and conductivity of the electrodes while optimizing the enzyme loading.2D transition metal chalcogenides, including copper sulfide, Co 3 S 4 , tungsten sulfide, and molybdenum sulfide, have garnered considerable interest due to their numerous benefits for basic and technological study in several disciplines, such as energy storage, sensing, and catalysis [21,22].They consist of alternating layers of metal and S that are held together by weak van der Waals' forces.This type of material is anticipated to work exceptionally well due to the 2D electron-electron correlations between metal atoms, which can enhance planar electric conductivity [23].

Biosensing of L-arginine
The CoS/AuNPs/GCE exhibited a low baseline current, excellent conductivity, and a substantial electroactive surface area.A novel electrochemical aptasensor was subsequently developed for the detection of 17βestradiol.The aptasensor utilized methylene blue (MB) as an indicator and cDNA, which has a high concentration of guanine, as a signal amplifier.The cDNA exhibited sequence complementarity to the 17βestradiol aptamer, suggesting that it might potentially compete with 17β-estradiol for binding to the immobilized aptamer on the electrode surface.When 17β-estradiol forms a hybrid with an aptamer, it will reduce the extent to which cDNA binds to the aptamer, hence lowering the signal produced by MB attached to the cDNA.Conversely, the hybridization of cDNA will improve the specificity of the aptasensor [13].
The distinctive layered morphology of 2D Co 3 S 4 enables easy penetration of electrolyte and enhances its surface activity as an electrode material for biosensors.Additionally, it allows for the self-assembly of thiolated compounds on its surfaces, providing an advantage in simple surface modification.Co 3 S 4 is inefficient for usage in electrode materials due to its semiconductor nature and very poor conductivity.Therefore, it is extremely advantageous to produce a composite of Co 3 S 4 with materials that have the ability to conduct electricity in order to enhance the transfer of charges for the purpose of electrochemical biosensing applications [24].Ti 3 C 2 MXene are highly recommended due to their ability to significantly enhance the current response of electrochemical sensors.This is attributed to their excellent conductivity and the ability to immobilize biomolecules.Fabrication of the Ti 3 C 2 -Co 3 S 4 would not only significantly enhance its conductivity but also establish a sensing platform with amplified signal for target molecules [25].When Co 3 S 4 is used alone, it can act as a semiconductor and have a poor conductivity.However, when it is combined with MXene, these limitations can be reduced.The resulting material has improved capabilities for transferring charges and increased sensitivity to specific substances being analyzed.The synthesized Ti 3 C 2 -Co 3 S 4 demonstrated favorable performance attributes, such as enhanced sensitivity, exceptional selectivity, and low detection thresholds, rendering them appropriate for amino acid biosensing [26].

Limitation
The practical implementation of these biosensors is hindered by several hurdles, including the repeatability of their results and the feasibility of mass production.The lack of published research on the in vitro and in vivo cytocompatibility of MXene-based materials poses a potential risk to their medical applications.Conventional methods of analysis only capture a single time point in samples, making it difficult to achieve real-time biosensing in vivo or intracellular using S-containing NPs.

Conclusions
This study investigated the synthesis of Ti

Ti 3 C 2 have
been used in the development of various advanced biosensors, including electrochemical, fluorescent/optical, and surface-enhanced Raman spectroscopy (SERS) biosensors.Ti 3 C 2 biosensor utilizes the distinctive electrocatalytic properties of the Ti 3 C 2 sheets in response to the concentration of the target signals.The electrical properties and current signal are modified by biological targets that bind to the Ti 3 C 2

Co 3 S 4 .
Co 3 S 4 nanoparticles (NPs) have the ability to facilitate redox reactions that are crucial in electrochemical biosensing.On the other hand, Ti 3 C 2 can improve the catalytic efficiency as a result of their large surface area and conductivity.The combination of these materials may result in enhanced catalytic activity and accelerated response times in biosensors, due to their synergistic impact.Ti 3 C 2 -Co 3 S 4 improve the overall conductivity of the sensor.Enhancing the efficiency of electron transfer processes in biomolecular contacts can result in improved sensitivity and biosensing applications.This study focuses on the synthesis of a Ti 3 C 2 -Co 3 S 4 , which involves combining Ti 3 C 2 and Co 3 S 4 materials.Confocal fluorescence microscopy is used to further investigate possible uses of this nanomaterial in L-arginine sensing.
3 C 2 and Co 3 S 4 to form a fluorescent probe.To get a measurable fluorescence signal and prevent quenching effects, it is important to adjust the concentration of the probe.Create a calibration curve to establish a correlation between the intensity of fluorescence and the concentration of L-arginine.This entails creating a sequence of standardized solutions with predetermined L-arginine concentrations and quantifying their fluorescence intensity.Quantify the brightness of your samples by using a fluorescence spectrometer to measure the intensity of fluorescence.The customary procedure is to stimulate the fluorescent probe with light of the same wavelength as its excitation.Quantify the fluorescence emitted at the specific emission wavelength.Examine the fluorescence data by contrasting the fluorescence intensity of your sample with the calibration curve to ascertain the concentration of L-arginine.Create L-arginine standard solutions with different concentrations, such as 0.1, 0.25, 0.50, 0.75, and 1 mg/mL.Introduce a predetermined quantity of the fluorescent probe solution into each standard solution.Quantify the level of fluorescence intensity for each standard solution.Probes based on coumarin are highly effective.Coumarin derivatives are well-known for their remarkable sensitivity and selectivity.These compounds can be modified to show fluorescence changes in the presence of specific amino acids, like L-arginine.
displays the XRD pattern of Ti 3 C 2 -Co 3 S 4 .The Co 3 S 4 exhibits prominent diffraction peaks at 26.6° and 31.3°,which are assigned to the (220) and (311) lattice planes of the standard (JCPDS No. 42-1448).This confirms the existence of single-phase hexagonal Co 3 S 4 .The distinct and well-defined peaks indicate that the synthesized Co 3 S 4 exhibits a good crystalline structure.The XRD analysis reveals that the peaks of Ti 3 C 2 align with the crystal plane of (004) at an angle of 18.4° (JCPDS file No. 52-0875).Simultaneously, the distinct peak associated with the (104) lattice planes of Ti 3 AlC 2 disappears, indicating the successful elimination of the interlayer of Al atoms in Ti 3 AlC 2 and the transformation of Ti 3 AlC 2 into Ti 3 C 2 .

FIGURE 2 :
FIGURE 2: FESEM analysis of Ti3C2-Co3S4 FESEM: Field emission scanning electron microscopy; Ti 3 C 2 -Co 3 S 4 : titanium carbide-cobalt sulfide Developing arginine-based fluorescent biosensors entails converting arginine into ammonia via a two-step process.The protonation of the pH-sensitive indicator (rhodamine 6G) occurs during ammonium ion production; deprotonation alters the indicator's fluorescence spectra.Confocal fluorescence microscopy is an effective imaging technique used for the biosensing of biomolecules.The technique has several benefits for biosensing, including high spatial resolution, the capability to optically segment samples, and the ability to observe dynamic processes in real time.The graph illustrates the correlation between the concentration of L-arginine and the intensity of fluorescence in a biosensor that contains Ti 3 C 2 -Co 3 S 4 NPs.It was examined using confocal fluorescence microscopy.The Ti 3 C 2 -Co 3 S 4 biosensor, as seen in Figure4, is capable of detecting L-arginine even at low concentrations.At a concentration of 0.50 mg/mL, the fluorescence intensity continues to increase compared to 0.25 mg/mL.At a concentration of 1.00 mg/mL, the fluorescence intensity increases further compared to 0.75 mg/mL.As the concentration of L-arginine increases, there is a noticeable increase in fluorescence intensity.The results demonstrate a concentration-dependent correlation between L-arginine concentration and fluorescence intensity in the L-arginine biosensor utilizing Ti 3 C 2 -Co 3 S 4 NPs.An L-arginine-specific fluorescent probe is functionalized into the Ti 3 C 2 -Co 3 S 4 heterostructure.Adsorption or covalent attachment of a fluorophore that changes its fluorescence intensity upon binding to L-arginine may accomplish this.Common fluorophores include rhodamine.A glass substrate that is appropriate for confocal fluorescence microscopy is used to immobilize the functionalized Ti 3 C 2 -Co 3 S 4 heterostructure.One way to do this is to pour the heterostructure dispersion onto the substrate using a dropcasting technique and then let it cure.The fluorescence characteristics of the connected probe are altered when L-arginine interacts with the Ti 3 C 2 -Co 3 S 4 heterostructure.Fluorescence amplification was seen upon L-arginine binding to several probes, which varied in nature and interaction mechanism.Larginine attaches to the fluorescent probe-functionalized Ti 3 C 2 -Co 3 S 4 heterostructure at specified places.

FIGURE 4 :
FIGURE 4: Detection of L-arginine using Ti3C2-Co3S4 biosensor with different concentrations Ti 3 C 2 -Co 3 S 4 : Titanium carbide-cobalt sulfide An enzymatic glucose detection biosensor platform utilizing a nanocomposite of gold (Au)/MXene is reported by Rakhi et al., which exhibits high sensitivity.The biosensor utilizes distinctive electrocatalytic characteristics and synergistic interactions between AuNPs and Ti 3 C 2 MXene sheets.A glucose biosensor is produced by immobilizing the glucose oxidase (GOx) enzyme on a Nafion-solubilized Au/Ti 3 C 2 MXene nanocomposite on a glassy carbon electrode (GCE).The biomediated AuNPs serve a crucial role in enhancing the electron transfer between the electroactive core of GOx and the electrode.The biosensor electrode consisting of GOx/Au/Ti 3 C 2 MXene/Nafion/GCE exhibited a linear amperometric response while measuring glucose concentrations.Furthermore, the biosensor demonstrated exceptional stability, reproducibility, and repetition.Hence, the Au/ Ti 3 C 2 MXene nanocomposite discussed in that study has promise as an electrochemical transducer for use in electrochemical biosensors[19].Wang et al. described a biosensor that does not require a mediator to detect H 2 O 2 .The biosensor works by immobilizing hemoglobin (Hb) on an electrode that has been treated with Ti 3 C 2 MXene.A nanocomposite of TiO 2 -Ti 3 C 2 , where TiO 2 NPs are transformed into an organ like Ti 3 C 2 MXene, was produced.TiO 2 -Ti 3 C 2 nanocomposite was then utilized to trap Hb in order to create a biosensor without the need for a mediator.The spectroscopic and electrochemical data indicate that the TiO 2 -Ti 3 C 2 nanocomposite serves as a very effective immobilization matrix for redox proteins, exhibiting outstanding biocompatibility and promoting both protein bioactivity and stability.The TiO 2 -Ti 3 C 2 hybrid structure, which resembles an organ, promotes the direct transfer of electrons from Hb.The biosensors that were made exhibited excellent performance in detecting H 2 O 2 , with a broad linear range for H 2 O 2 detection.Specifically, a large number of TiO 2 NPs with exceptional biocompatibility coat the nanocomposite surface.This coating creates a protective milieu for Hb, resulting in improved long-term biosensor stability.The TiO 2 -Ti 3 C 2 nanocomposite has great potential as a matrix for creating biosensors without the need for a mediator.It might have a wide range of applications in environmental monitoring and biological detection[20].

3 C 2 -
Co 3 S 4 NPs and their detection of amino acids.It revealed that these NPs are efficient biosensors for detecting levels of L-arginine.XRD analysis shows clear and well-defined peaks, which show that the synthesized Ti 3 C 2 -Co 3 S 4 nanostructures have a crystalline structure.This crystalline structure is crucial for the biosensor's stability and performance.A FESEM study reveals that synthesized Ti 3 C 2 -Co 3 S 4 has a sheetlike structure with a highly organized arrangement.EDS analysis indicates the existence of elements such as Co, Ti, S, and C. The results show a clear relationship between the concentration of L-arginine and the fluorescence intensity, confirming the biosensor's efficiency in detecting L-arginine levels.This study illustrates the feasibility and efficiency of the Ti 3 C 2 -Co 3 S 4 NP-based biosensor for sensitive and selective detection of L-arginine.