Determination of Thiols by Fluorescence using Au@Ag Nanoclusters as Probes

Gold Nanocluster-Based Ratiometric Probe with Surface Structure Regulation-Triggered Sensing of Hydrogen Sulfide in Living Organisms

The development of reliable probes for in vivo detection of hydrogen sulfide (H₂S) with high sensitivity and selectivity is of great significance due to its key roles in many pathological and physiological processes. Herein, it was found that H₂S could finely regulate surface structure of gold nanoclusters (AuNCs) through reduction of surface Au(I)-ligand motifs and further quench their fluorescence by a two-stage kinetic reaction process. Stage I showed the H₂S-assisted surface Au(I)-ligand reduction and Au(0) core growth with a rapid fluorescence decrease; stage II showed the surface structure optimization and reconstruction with a relatively slow fluorescence quenching. By virtue of the excellent fluorescence response of AuNCs to H₂S, a novel ratiometric fluorescence probe (RBDA) for sensing H₂S was designed through electrostatic attraction-induced fluorescence resonance energy transfer (FRET) between AuNCs and rhodamine B. The probe was facilely prepared, showing a straightforward, rapid ratiometric fluorescence response to H₂S with built-in self-calibration. It presented the high detection sensitivity with a detection limit (LOD) of 56 nM and an excellent sensing selectivity for H₂S over various other biological species. The probe was demonstrated to possess high biostability, low cytotoxicity, good cell and issue penetrability, and favorable biocompatibility. It realizes successful monitoring of both exogenous and endogenous H₂S levels in living cells and zebrafish.

Use of metal nanoparticles for preconcentration and analysis of biological thiols

Metal nanoparticles (NPs) exhibit several unique physicochemical properties, including redox activity, surface plasmon resonance, ability to quench fluorescence, biocompatibility, or a high surface-to-volume ratio. They are being increasingly used in analysis and preconcentration of thiol containing compounds, because they are able to spontaneously form a stable Au/Ag/Cu-S dative bond. They thus find wide application in environmental and particularly in medical science, especially in the analysis of biological thiols, the endogenous compounds that play a significant role in many biological systems. In this review article, we provide an overview of various types of NPs that have been applied in analysis and preconcentration of biological thiols, mainly in human biological fluids. We first discuss shortly the types of NPs and their synthesis, properties, and their ability to interact with thiol compounds. Then we outline the sample preconcentration and analysis methods that were used for this purpose with special emphasis on optical, electrochemical, and separation techniques.

Turn-on fluorescence detection of cysteine with glutathione protected silver nanoclusters

In this work, we report a sensitive and selective turn-on fluorescence detection of cysteine with glutathione protected silver nanoclusters (GSH-Ag NCs). The glutathione stabilized silver nanoclusters were synthesized by the boiling water method. When excited at 380 nm, the GSH-Ag NCs exhibited a weak emission at about 680 nm, which could be enhanced by cysteine. The proposed method allows evaluation of cysteine in the range of 2-3000 μM with a detection limit of 0.51 μM. The recoveries were found to be 95.07%-101.38% when detecting cysteine contents in fetal bovine serum samples. In addition, we also discussed the possible mechanism for the fluorescence enhancement of GSH-Ag NCs by addition of cysteine. It might be the formation of cysteine and glutathione co-capped Ag NCs. This work reported a fluorimetric method for the assay of cysteine and provided a strategy for the synthesis of dual ligand-protected Ag nanoclusters.

“Turn-On” Fluorescent Assay of Biothiols Based on Nitrogen-Rich Polymer Carbon Nanostrips and Its Application in Cell Imaging

In this work, a sensitive and selective turn-on fluorimetric method has been developed for the determination of biothiols based on blocking Ag+-induced fluorescence quenching of nitrogen-rich polymer carbon nanostrips (NRPCNSs). Ag+ion can induce the fluorescence quenching of NRPCNSs due to the formation of nonfluorescent coordination complexes via robust Ag-N interaction. Once addition of biothiols, such as cysteine (Cys) and glutathione (GSH), Ag+ions prefer to interact with biothiols rather than NRPCNSs, which could be attribute to the formation of Ag-S bond, thus leading to effective fluorescent recovery of NRPCNSs. Under the optimized conditions, excellent linear relationships existed between the recovery degree of the NRPCNSs and the concentrations of Cys and GSH in the range of 0.05 μM to 10 μM and 0.2 μM to 30 μM, respectively. And, the limits of detection (LODs) for Cys and GSH are 16.5 nM and 65.1 nM, respectively. The detection system also shows high selectivity against other non-thiol amino acids. Moreover, the potential in practical applications of this proposed method has been demonstrated by detecting biothiols in human serum and fluorescence imaging of biothiols in living cells.

Dually emitting gold-silver nanoclusters as viable ratiometric fluorescent probes for cysteine and arginine

Glutathione coated gold and silver nanoclusters (GSH-Au/AgNCs) were synthesized by one-pot reduction methods and are found to be viable fluorescent nanoprobes for cysteine (Cys) and arginine (Arg), with good selectivity over other amino acids. The GSH-Au/AgNCs have two emissions at 616 nm and 412 nm when excited at 360 nm. With the increased concentration of Cys, the ratio of the emission intensities (I₆₁₆/I₄₁₂) linearly decreases with Cys in concentration ranging from 0.05 to 10 μM and from 10 to 50 μM, respectively. With increased concentrations of Arg, the ratio of I₆₁₆/I₄₁₂ linearly decreases with Arg concentration ranging from 0 to 50 μM and from 50 to 100 μM, respectively. The probe was applied to the determination of Cys and Arg in spiked samples of serum and urine where it gave good recoveries. Graphical abstract Glutathione-coated gold and silver nanoclusters (GSH-Au/AgNCs) were synthesized by one-pot reduction and are found to be viable fluorescent nanoprobes for cysteine (Cys) and arginine (Arg).