Several Kinds Of Functionalized Gold Nanoclusters And Their Analytical Applications

Gold nanoclusters（AuNCs）,consisting of a few to hundreds of atoms,have emerged as a new class of inorganic nanomaterials.Their sizes usually are smaller than 3 nm,which bridging the gap between isolated gold atoms and large gold nanoparticles.They have gained a lot of attention due to their unique properties,including high catalytic activity,photoluminescence（PL）,and electrochemiluminescence（ECL）.The AuNCs typically have a core-shell structure that comprises of an inorganic metal core and an organic ligand shell.The outer ligand shell can regulate the physical and chemical properties of AuNCs and thus produce different functionalized AuNCs.In this work,we used N-acetyl-L-cysteine（NAC）,urease,and L-methionine（Met）as protective molecules to synthesized three different functionalized AuNCs.These newly-prepared AuNCs was characterized and further utilized to analytical applications.Using NAC-AuNCs as fluorescent nanoprobes,we constructed a analytical platform for the determination of urea,urease,urease inhibitor,glucose,and catalase.Based on urease-AuNCs,a fluorescent method was established to the determination of Hg²⁺.Using Met-AuNCs as fluorescent and ECL sensors,we established a novel method for the detection of Cu²⁺and dopamine,respectively.In addition,we converted water-insoluble AuNCs to water-soluble AuNCs via ligand-protein interaction.The interaction between protein and ligand was systematically investigated.Experimental results showed that the synthetic strategy demonstrated here has good general utility.Last,based on host-guest recognition,we introduced a novel method for improving the fluorescent intensity of AuNCs and the quantum yield of the resulting AuNCs was as high as 65%.From the characterizations and theoretical calculations,we concluded the origin of the fluorescence enhancement was ascribed to hydrogen-bonding interactions between 6-aza-2-thiothymine（ATT）and guanidine that makes the capping ligands of ATT rigid.The main contents and results were shown as follows:1.N-acetyl-L-cysteine-mediated formation of AuNCs and their applications for urea,urease and urease inhibitor detectionA facile synthetic process of AuNCs was realized by using NAC both as a reducing agent and as a protection ligand.The as-prepared NAC-AuNCs have intense red fluorescence,high colloidal stability,and a large Stoke’s shift.Furthermore,the NAC-AuNCs exhibited ultrasensitive pH-responsive properties and the linear range was 6.05-6.40.Based on the pH stimuli-responsive properties of the as-prepared AuNCs,we constructed a pH-sensing platform for the detection of urea,urease,and urease inhibitor.The constructed sensing platform was successfully utilized to assay the urea in human urine samples and H.pylori in human gastric tissue.2.Fluorescence detection of glucose and catalase activity based on the NAC-AuNCsH₂O₂ is reduced by Fe²⁺to form·OH,which is known as the Fenton reaction.We found that the NAC-AuNCs show high stability toward H₂O₂.However,the fluorescence of NAC-AuNCs can be quenched by the product from the Fenton reaction.The fluorescence quenching effect was ascribed to the strong oxidative power of·OH,which oxidizes the Au⁰ in AuNCs to Au⁺.On the basis of this phenomenon,we developed a“turn-off”biosensor for glucose and a“turn-on”biosensor for catalase.The proposed sensing methods were successfully applied for the detection of glucose in human serum and catalase in human saliva.3.Urease-templated fabrication of AuNCs and their application for Hg²⁺sensingAuNCs were prepared by using urease as a template.The as-synthesized urease-AuNCs have intense red fluorescence and high colloidal stability.It was found that the fluorescence of urease-AuNCs can be obviously quenched by Hg²⁺.Based on this effect,a fluorescent method was established to the determination of Hg²⁺.4.L-Methionine-directed fabrication of gold nanoclusters and their application for Cu²⁺sensingAuNCs were prepared by using Met both as a reductant and a stabilizer.The as-synthesized Met-AuNCs possess many attractive features including intense yellow fluorescence,a long fluorescence lifetime,high colloidal stability,and a large Stoke’s shift.Moreover,the Met-AuNCs can serve as an efficient fluorescent probe for simple,rapid and selective detection of Cu²⁺by fluorescence quenching.5.ECL sensor based on Met-AuNCs for highly sensitive determination of dopamine released by cellsThe ECL properties of the Met-AuNCs-modified glassy carbon electrode（Met-AuNC/GCE）were studied.The excellent cathodic ECL behavior has been observed with K₂S₂O₈ as a coreactant and the ECL mechanism has been discussed.Moreover,dopamine（DA）was taken as a model analyte to evaluate the potential application of the Met-AuNC-based ECL sensor.The practicality of the Met-AuNC/GCE was validated by determining the DA released from PC12 cells.6.Water-soluble gold nanoclusters prepared by protein-ligand interactionUsing 3-mercaptopropionic acid（MPA）as a model ligand,we converted water-insoluble AuNCs to water-soluble AuNCs via the introduction of BSA.It was concluded that MPA molecules,capped on the Au surface,guided the AuNCs to enter the cavity of BSA through hydrophobic interactions.After conjugation to AuNCs,the protein conformation of BSA was changed and moreα-helix structures were formed.The synthetic strategy demonstrated here has good general utility.By tuning the proteins and thiolate ligands,diverse water-soluble AuNCs could be produced with this new method.7.L-Arginine mediated emission enhancement of the AuNCs protected by6-aza-2-thiothymine and its mechanismThe AuNCs were synthesized with a facile one-pot approach using6-aza-2-thiothymine（ATT）as reducing-cum-protecting agent.The photoluminescence quantum yield（QY）of the as-synthesized ATT-AuNCs was 1.8%.The PL intensity of ATT-AuNCs was dramatically increased after the introduction of L-arginine（Arg）and a luminescence QY of 65%was obtained for Arg/ATT-AuNCs.Experimental results revealed that the guanidine group of Arg is crucial for the observation of ultrabright emission from Arg/ATT-AuNCs.The supramolecular host-guest assemblies formed between ATT capped on the gold core and guanidine group of Arg through intermolecular hydrogen-bonding interactions make the capping ligands of ATT rigid.Subsequently,the intramolecular vibration and rotation of ATT are greatly suppressed,which reduce the non-radiative relaxation of excited states and,as a result,predominantly raise the luminescence QY of ATT-AuNCs.Further experiments demonstrate that small change in guanidine substituents can arouse obvious changes in NCs photoluminescence features.