A novel ratiometric fluorescence nanoprobe based on aggregation-induced emission of silver nanoclusters for the label-free detection of biothiols
Graphical abstract
Introduction
Metal nanoclusters (NCs) as a new kind of luminescent material have fascinated scientists for decades owing to their unique molecule-like optical properties and outstanding physicochemical properties [1], [2], [3], [4]. The ultrasmall size, low toxicity, good photostability, and large Stokes shift of metal NCs have endowed them with great attentions in various applications, including optoelectronics, chemical sensors, bioimaging and nanomedicine [5], [6], [7], [8]. However, the luminescence of NCs, which is their most attractive feature, is still not sufficiently good in the practical applications. Under most circumstances, the luminescence quantum yields from metal NCs are still low when compared to those of quantum dots, posing a major hurdle for the development of their applications.
Recently, the aggregation-induced emission (AIE) behavior of NCs opens a door to resolve the above problem due to its abilities of optimizing and adjusting the emission properties of NCs [9], [10], [11]. The origin of NCs emission relates to the capping ligands on the surface of NCs [12], [13]. The aggregation of NCs is capable to restrain the intramolecular vibration and rotation of ligands, thus suppress ligand-related nonradiative relaxation of excited states and enhance the emission of metal NCs [14], [15]. For thiol-containing ligands-stabilized Au NCs and Cu NCs, the AIE property has been found and widely applied in the past years [14], [15], [16]. Nevertheless, little is known about this characteristic of Ag NCs and its applications in chemo/biosensing [11], [17]. Furthermore, the reported probes based on the AIE property of Ag NCs have been limited to single-signal detection [11], [17]. The target-triggered single-signal responsive analysis systems frequently suffer from the influence of probe concentration, excitation intensity and environment. Recent advance on the AIE property of Ag NCs illustrates that this property is very sensitive to ambient temperature and pH [11], [18]. Therefore, the false or unstable results caused by environmental effects, especially in the complex biological matrix, are easily to occur for these reported probes. Undoubtedly, further exploring the AIE property of Ag NCs and expanding its stable sensing mode are still important demand.
In this work, we proposed a novel and label-free nanoprobe based on the Au3+-regulated AIE of Ag NCs for the ratiometric fluorescence detection of biothiols. We found Au3+ had strong electrostatic and coordination interactions with the carboxylic anions in the glutathione ligands of Ag NCs. The aggregation of GSH-protected Ag NCs which accompanied with the luminescence enhancement could be triggered by Au3+ cross-link. On the other hand, the high-degree N-containing structure of the g-C3N4 nanosheets was ideal for grafting Au3+ on their surface [19], [20], [21]. In addition, the fluorescence quenching of g-C3N4 nanosheets could occur after ions binding because the redox potential of Au3+ lies between the conduction band and the valence band of g-C3N4, leading to photoinduced electron transfer (PET) from g-C3N4 nanosheets to complexed metal ions [21], [22], [23].
Inspired by these phenomenons, we employed Au3+ as linking bridge to combine g-C3N4 nanosheets with Ag NCs to design a ratiometric nanoprobe. After loading of Au3+ on the g-C3N4 nanosheets surface, GSH-protected Ag NCs were introduced into g-C3N4-Au3+ complexes solution to assemble with them and form aggregates. The detection of biothiols could be carried out by the remove of Au3+ from the nanoprobe via thiols coordination and the subsequently change of the fluorescence intensity ratio of g-C3N4 nanosheets to Ag NCs. The sensitive and accurate detection of biothiols is great important as they play key roles in biological systems and are associated with many diseases [24]. Ratiometric probes for biothiols are mostly based on thiol-sensitive organic fluorophores which usually undergo tedious synthetic and purification procedure, poor aqueous solubility, photobleaching [24], [25], [26]. To overcome these drawbacks, some ratiometric nanoprobes for biothiols based on Hg2+ [27], [28], [29] or MnO2-involved nanosystem [30], [31] have been reported, while they were subjected to high cytotoxicity or poor stability. Owing to the excellent properties of our nanomaterials and the unique design, our nanoprobe has clear advantages over those previous methods, including easy preparation, high photostability and water-dispersibility, low cost and cytotoxicity, and environmental friendly. This strategy not only can provide accurate measurement in complex human serum samples but also was a new way for the application of the AIE property of Ag NCs.
Section snippets
Chemicals and materials
Gold (III) chloride trihydrate (HAuCl4·3H2O), silver nitrate (AgNO3), L-glutathione reduced (GSH) were purchased from Sigma-Aldrich (St. Louis, MO, USA). L-cysteine (Cys), homocysteine (Hcy), ascorbic acid (AA), L-arginine (Arg), glycine (Gly), L-serine (Ser), bovine serum albumin (BSA), methionine (Met), DL-dithiothreitol (DTT) were obtained from Aladdin Industrial Corporation (Shanghai, China). The rest of the chemical reagents were purchased from Sinopharm Chemical Reagent Co., Ltd.
The principle of the nanoprobe
The cation-induced and solvent-induced aggregations are the two common ways for realizing the aggregation of metal NCs. To obtain good stability and flexibility, we chose cation-induced aggregation mode to design our ratiometric nanoprobe. Certain multivalent metal ions can link with the ligands of noble metal NCs and drive the aggregation of NCs, resulting aggregation induced emission enhancement of NCs. On the other side, as g-C3N4 nanosheets have abundant N-containing structure, they can
Conclusions
In conclusion, a novel and label-free ratiometric nanoprobe based on ion-regulated aggregation of Ag NCs was developed for the detection of biothiols. The nanoprobe was constructed by loading of Au3+ on g-C3N4 nanosheets surface and subsequently aggregating Ag NCs via ion binding. Au3+ in the nanoprobe had the ability to quench the emission of g-C3N4 nanosheets and simultaneously enhance the emission of GSH-protected Ag NCs. When biothiols were present, Au3+ was taken away from the nanoprobe
Acknowledgements
This work was supported by the National Natural Science Foundation of China (No. 21605043 and 11674085) and the Natural Science Foundation of Hubei Province (No. 2016CFB125).
Conflicts of interest
There are no conflicts of interest to declare.
References (40)
- et al.
In situ synthesized and embedded silver nanoclusters into poly vinyl alcohol-borax hydrogel as a novel dual mode “on and off” fluorescence sensor for Fe (III) and thiosulfate
Talanta
(2018) - et al.
A novel fluorometric and colorimetric sensor for iodide determination using DNA-templated gold/silver nanoclusters
Biosens. Bioelectron.
(2017) - et al.
Aggregation-induced emission from gold nanoclusters for use as a luminescence-enhanced nanosensor to detect trace amounts of silver ions
J. Colloid Interface Sci.
(2016) - et al.
Regulation on the aggregation-induced emission (AIE) of DNA-templated silver nanoclusters by BSA and its hydrolysates
J. Colloid Interface Sci.
(2017) - et al.
Two-dimensional graphitic carbon nitride nanosheets for biosensing applications
Biosens. Bioelectron.
(2017) - et al.
A mitochondria-targeted ratiometric two-photon fluorescent probe for detecting intracellular cysteine and homocysteine
Talanta
(2018) - et al.
A ratiometric two-photon fluorescent probe for cysteine and homocysteine in living cells
Sens. Actuators B
(2014) - et al.
7-Aminocoumarinyldisulfide as a ratiometric fluorescent probe for biothiols in water
Sens. Actuators B
(2013) Nanoelectrochemistry: metal nanoparticles, nanoelectrodes, and nanopores
Chem. Rev.
(2008)- et al.
Sonochemical synthesis of highly fluorescent Ag nanoclusters
ACS Nano
(2010)