Study On The Synthesis And Characterization Of Ag Nanoclusters And Construction Of Fluorescent Probe

Compared with traditional methods,fluorescence methods are powerful for the determination of metal ions,due to their highly sensitive and selective,cheap,and easy to perform,as well as allow real-time and online analysis.Therefore,fluorescent molecular probe,fluorescent molecular sensor,fluorescent molecular switch and other scientific technology in various fields have been attracted more and more attention and unprecedented applications,and they have good prospect for development.Recently,fluorescent probes are based on fluorescence molecules,including metal nanoclusters,carbon nanodots,quantum dots and organics with chromophore.Among them,Metal（e.g.,Au,Ag,Pt,and Cu）nanoclusters are a new kind of luminescent nanomaterials that exhibit ultra-small size,good biocompatibility,excellent photostability,and strong photoluminescence.In this paper,silver nanoclusters（Ag NCs）with red fluorescence were synthesized that were used as fluorescent probes for the detection of melamine,iodide and trivalent chromium ions.Specifically,it includes the following three parts:1.Fluorescence detection of melamine based on inhibiting Cu²⁺-induced disaggregation of red-emitting silver nanoclustersA facile method was designed to synthesize red-emitting,water-soluble Ag NCs employing dihydrolipoic acid（DHLA）as the stabilizing agent and sodium borohydride（NaBH₄）as the reductant.The optical properties and morphology of Ag NCs were studied by ultraviolet visible spectra,fluorescence spectra,Fourier transform infrared spectra,X-ray photoelectron spectra and transmission electron microscopy.The Ag NCs show aggregation-induced emission property,monodispersity,and have good stability.Cu²⁺can disperse the aggregated Ag NCs,accompanied by the quenching of fluorescence.However,the formed Cu²⁺-melamine complex by the coordination chemistry between free copper ion and melamine is able to effectively avoid the quenching process of Cu²⁺to the Ag NCs.By controlling the concentration of copper ions unchanged,the concentration of melamine has a linear relationship with the fluorescence intensity of the mixture.Hence,the Ag NCs can be applied to design a novel fluorescent probe based on this property to detect melamine in real samples and satisfactory results were obtained.2.Facile method for iodide ion detection via the fluorescence decrease of dihydrolipoic acid/beta-cyclodextrin protected Ag nanoclustersThe DHLA and beta-cyclodextrin（β-CD）as the stabilizing agent and NaBH₄ as the reductant were used to synthesize Ag NCs with red emission.With the assistance ofβ-CD,the solubility and stability in water of Ag NCs are enhanced.The size and fluorescence intensity of Ag NCs can be controlled by changing the concentration ofβ-CD.The obtained DHLA andβ-CD-protected Ag NCs（DHLA/β-CD-Ag NCs）are spherical and display a dispersed state.In the process of synthesis,DHLA andβ-CD are combined with Ag atoms by the formation of Ag-S bonds and hydrophobic interaction,respectively.However,the DHLA/β-CD-Ag NCs are aggregated in the presence of I^-,accompanied by the decrease in their fluorescence intensity.The reason of this phenomenon is that the integrity ofβ-CD cavities is retained on the surface of DHLA/β-CD-Ag NCs,which preserves their capability for I^-host-guest recognition.In addition,other ions,including halide ions,cause negligible influence on the fluorescence of DHLA/β-CD-Ag NCs.Hence,this fluorescent probe not only distinguishes I^-with the existence of other halide ions,but also sensitively and selectively determines the I^-level.3.Fabrication of silver nanoclusters with enhanced fluorescence triggered by ethanol solvent:A selective fluorescent probe for Cr³⁺detectionA facile method of synthesizing Ag NCs was developed using DHLA as the stabilizing agent and NaBH₄ as the reductant.The Ag NCs were uniformly distributed in the ethanol aqueous solution and has red fluorescence under ultraviolet light.By comparing the optical properties of Ag NCs synthesized in aqueous solution or different alcohol aqueous solutions,it is found that ethanol solvent endow Ag NCs with dramatically enhanced fluorescence.Hence,the Ag NCs are synthesized in ethanol/water solution and the specific trivalent chromium ions(Cr³⁺)recognition capability of the Ag NCs can thus be obtained based on its fluorescence quenching.The mechanisms for fluorescence enhancement and quenching are respectively investigated in detail.The fluorescence intensity of Ag NCs has a good linear relationship with the concentration of Cr³⁺but not affected by other metal ions and anions.Hence,this fluorescent probe based on the Ag NCs shows enough sensitivity and good selectivity toward Cr³⁺.