Study On Application Of Gold Nanoparticles (Au NPs) In Chemical Sensors

Based on the distance-dependent surface plasmon resonance absorption of AuNPs, visual detection of targets can be realized by controlling the distances betweenAu NPs in a proper way. The aggregation of Au NPs will shorten the interparticledistances and result in a color change from red to purple. Furthermore, Au NPs arehighly-efficient fluorescence quenchers as they possess high extinction coefficient,which are3～5orders of magnitude higher than those of organic dyes, and a moresensitive detection can be achieved by using Au NPs as a platform for quenching. Inthis thesis, around13nm citrate-capped Au NPs were prepared by sodium citratereduction of hydrogen tetrachloroaurate (HAuCl4), and several chemical sensors weredesigned and studied based on the surface plasmon resonance absorption andhighly-efficient fluorescence quenching properties of Au NPs.Four chapters were involved in this thesis.In chapter one, the preparation, characterization, modification and opticalproperties of Au NPs were introduced, and the recent advances in Au NPs-basedchemical sensors such as colorimetric, fluoremetric, electrochemical,electroluminescence, chemiluminescence et al. were summarized. The purpose andmain content of the thesis were proposed at the end of this chapter.In chapter two, a simple and colorimetric pH sensor was designed based on a pHsensitive conformational switch of a C-rich ssDNA (i-motif DNA) and unmodified AuNPs. The conformational switch of i-motif DNA from pH7.0to5.0resulted in thecolor change of Au NPs from red to purple. The sensor was low cost, fast and can becarried out by naked eyes. It is promising to use in monitoring some life processwhich associated with pH variation.In chapter three, a fast and visual detection of melamine was developed by usingunmodified Au NPs as an indicator. Melamine can directly induce the aggregation ofAuNPs, accompanying a rapid color change from red to blue. A fast simplepretreatment method was constructed to remove the protein and fat in the milkproducts such as liquid milk and infant formula by trichloroacetic acid and chloroform,respectively. The detection of melamine in milk products can be accomplished within25min. The linear relationship between the absorbance ratio A650/A520and melamineconcentration was oberserved over the range of0.076～0.20ppm with the detection limit of0.029ppm,0~15.2ppm with the detection limit of0.15ppm and0~79.8ppmwith the detection limit of2.5ppm for the aqueous solution, liquid milk and infantfomula, respectively. In addition, the melamine concentration that could bedifferentiated by naked eyes were0.076,0.48and4.2ppm for the aqueous solution,liquid milk and infant formula, respectively.In chapter four, a fluorescence turn-on detection of melamine was proposed.Based on the high overlap between the absorption of Au NPs and the fluorescence offluorescein, the fluorescence was quenched via fluorescence resonance energy transferwhen fluorescein was adsorbed on Au NPs. By the addition of melamine, thefluorescence recovered since the melamine replaced fluorescein by a strongerinteraction with Au NPs. The fluorescence intensity was linearly increased over themelamine concentration of1.0×10-7mol/L~3.0×10-6mol/L and4.0×10-6mol/L~8.0×10-6mol/L with the limit of detection of1.1×10-9mol/L, it has great potential use inmelamine detection of milk products.