Preparation Of Cationic Gold Nanoclusters And Study Of Their Chemiluminescent Catalytic Activity

Chemiluminescence?CL?is the emission of light from a chemical reaction that the intermediate chemical compound emits photons while it returns to ground state from the excited electronic state.Chemiluminescence reactions are generally multi-step oxidation reactions with fast reaction kinetics,and it possesses extensive applications in clinical chemistry,bioanalysis and environmental analysis because of numerous advantages including wide linear range,high sensitivity,relatively low cost,and simple operation.Luminol-H₂O₂ chemiluminescence system is one of the most commonly used CL system.Common catalysts for luminol-H₂O₂ CL system are metal ions,natural enzymes,biological proteins,and so on.With the development of nanotechnology,nanomaterials have gradually become a research hotspot due to their unique properties.Numerous nanomaterials have been confirmed to possess the peroxidase-like activity.Gold nanoclusters?Au NCs?are a new type of luminescent nanomaterial with size smaller than2 nm.Compared with gold nanoparticles,Au NCs possess unique properties such as high surface-to-volume ratio and fluorescence properties because of their ultra-small size.It was reported that Bovine serum albumin?BSA?-stabilized Au NCs have highly intrinsic peroxidase-like activity and it could enhance the CL emission of the luminol-H₂O₂ system.However,the catalytic activity of Au NCs was low which limited their applications.Hence,we would like to investigate how the surface charge of Au NCs affects their catalytic activity,and to develop Au NCs with high catalytic activity for the luminol-H₂O₂ CL reaction.In the second chapter,the cationic Au NCs were synthesized by conjugating ethanediamine on the surface of BSA-stabilized Au NCs.The cationic Au NCs possessed higher catalytic activity to luminol-H₂O₂ CL system compared with unmodified Au NCs.And the LOD for cationic Au NCs improved nearly 10 times using cationic Au NCs as a catalyst in luminol-H₂O₂ reaction.Mechanism research showed that the improved catalytic activity of cationic Au NCs may be attributed to the generation of ¹O₂ on the surface of cationic Au NCs and the positive surface charge for better affinity to luminol.The cationic Au NCs also showed good catalytic activity on ABTS reaction which is a substrate commonly used to evaluate the catalytic activity of peroxidase.In addition,the cationic Au NC as a peroxidase mimics has much better stability than HRP over a wide range of temperature.We believe that the cationic Au NCs may be useful as artificial peroxidase for a wide range of potential applications in CL and bioanalysis.In the third chapter,we chose PEI as template for the synthesis of cationic Au NCs.The PEI-Au NCs were red-emitting with size distribution below 2 nm.CL analysis showed that the PEI-Au NCs could catalyze the luminol-H₂O₂ CL system.The CL signal was enhanced about 38-fold by 125?M PEI-Au NCs indicating good catalytic activity of PEI-Au NCs on the luminol-H₂O₂ CL reaction.The CL spectra of luminol-H₂O₂-PEI-Au NCs system showed that the maximum emission wavelengths was near 425 nm,indicating that 3-APA*was still the luminophor of this case.Based on the radical quenching results,we speculated the possible mechanism of luminol-H₂O₂-cationic Au-NCs CL system was related to the formation of ¹O₂.Abundant of ¹O₂ was generated on(or around the surface of PEI-Au NCs,and the negatively charged luminol anion tended to move closer to the surface of cationic Au NCs,benefiting the interaction between ¹O₂ and luminol anion,and the generation of 3-APA*.Finally,higher CL emission would be obtained.In addition,the peroxidase-like activity of the PEI-Au NCs was examined by using ABTS as a chromogenic substrate as well.As expected,the PEI-Au NCs had higher affinity to negatively charged ABTS than the natural enzyme HRP.