Abstract
Water-soluble fluorescent Ag nanoclusters (Ag NCs) with distinct pH-switchable agglomeration and spectral signal responses are prepared using a facile etching method. Increased and decreased pH cause the Ag NCs to switch between agglomeration and dispersion, accompanied by decreases in and recoveries of fluorescence intensity and absorbance. The pH switchable behavior of the Ag NCs is attributed to carboxyl groups on the nanocluster surface that are rich in the citrate and amido functional groups of ligands (glutathione), creating an easily formed, weak molecular interaction among Ag NCs (for example, hydrogen bonding) and maintaining a balance in the colloidal solution, whereas a change in pH will disrupt the balance, leading to the reversible agglomeration of Ag NCs and the switchable spectral signal response. In addition, because urea and glucose can change the pH of a solution by producing NH3 and gluconic acid in enzyme-catalyzed reactions, the pH-switchable behavior of the Ag NCs is used to develop them as an optical probe to establish a regenerated biosensing platform for the sensitive and selective detection of urea and glucose, and the test results are satisfactory. Fluorescent silver nanoclusters that reversibly form or break agglomerates on varying the pH are promising for sensing biological molecules. The ability to cause fluorescent nanoclusters to agglomerate and break up enables their fluorescence properties to be altered, but most systems studied to date do so only irreversibly. Now, Nian Bing Li and Hong Qun Luo of Southwest University in China and co-workers have produced water-soluble silver nanoclusters that agglomerate in basic and neutral solutions and disperse in acidic solutions in a reversible manner. The fluorescence intensity of the nanoclusters drops with increasing pH because aggregation quenches fluorescence. These nanoclusters are fabricated by straightforward etching of silver nanocrystals coated in citrate. The team demonstrated the potential of the nanoclusters for biosensing by using them to detect the biologically important molecules urea and glucose. Novel fluorescent Ag nanoclusters are prepared based on an etching method with citrate and GSH as the stabilizer of Ag nanocrystals and the etchant. The as-prepared Ag nanoclusters show good pH-switchable agglomeration and dispersion accompanied with the reversible agglomeration induced signal quenching. The pH-regulated behavior mechanism has been discussed, and the Ag nanoclusters is introduced as a probe to establish regenerable biosensing platform for urea and glucose detection.
Highlights
Metal nanoparticles on an ~ 2 nm scale are defined as metal nanoclusters, and they show physicochemical properties that are significantly different from those of their bulk form
Many fluorescent metal nanoclusters, especially nanoclusters of Au and Ag, among noble metals, have been reported, and the ligands that stabilize these small nanoparticles have a key role in their fundamental properties: (i) ligands can influence optical properties by transferring their charge to the metal cores or by directly donating delocalized electrons contained on them to the metal cores;[3,4] (ii) surface ligands can determine the physical properties of nanoclusters, and amphiphilic metal nanoclusters can be produced by skillfully controlling the type and amount of ligands;[5] (iii) the functional groups of the surface ligands determine the chemical properties of the metal nanoclusters; fluorescent nanoclusters can be more widely used as probes for detecting various targets.[6,7]
The citrate had the role of stabilizer to protect the prepared Ag nanocrystals from aggregation and precipitation, and the groups of the ligands (GSH) was used as an etchant to obtain fluorescent Ag nanoclusters (Ag NCs)
Summary
Metal nanoparticles on an ~ 2 nm scale are defined as metal nanoclusters, and they show physicochemical properties that are significantly different from those of their bulk form. The reported etching methods to obtain Au and Ag NCs always involve toxic organic solvents, rigorous reaction conditions (for example, oxygen-free and high temperature) and complicated procedures.[13,14,15]
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