Controllable Synthesis Of Noble Metal Nanoparticles And Their Application In Colorimetric Detection Of Environmental Pollutants

Localized surface plasmonic resonance?LSPR?is a phenomenon in which the conducting electrons at the surface of metals are collectively oscillated when their frequency matches are of the incident electromagnetic radiation.As a consequence of this phenomenon,colloidal metal nanoparticles?NPs??e.g.Au and Ag NPs?show strong absorption within certain wavelength located in?near-?visible region.The LSPR is intimately related to NPs size,composition,morphology,inter-particle distances and refractive index of the surrounding media.Based on these properties,we designed two bimetallic nanoparticles,e.g.Au@AgI core-shell and Au@Ag@Au core-multishell NPs,for the sensing of sulfide and cyanide.The sensing mechanisms are based on the fact that analytes can induce the etching and transformation of the shell leading to the changes of dimensional core to shell ratio,which further induce observable color and spectrum changes.The developed methods are developed in the detection of sulfide and cyanide in natural water samples.In the first chapter,we mainly introduce the basic knowledge of optical sensors?especially colorimetric sensors?,and the basic properties,preparation and application of nanomaterials,including the principle of the sensor,mono-metal and core/shell NPs'synthesis,properties and colorimetric applications.In the second chapter,for S^2-testing,Au@AgI NPs were prepared by Tollens reaction between HCHO and[Ag?NH₃?₂]OH,with Au NPs as templates.Then Au@AgI core/shell nanoparticles were prepared with I₂ acting as oxidant and precipitant.When the probe was exposed to water samples containing S^2-,AgI shell reacts with S^2-to generate Ag₂S(Ksp=Ksp=2×10^-49),leading to changes in the composition and core/shell ratio and further inducing observable color and spectrum changes.As AgI shell of the probe is extremely stable and insoluble(K_sp,AgI=9.3×10^-17<K_sp,AgBr=5×10^-13<K_sp,AgCl=1.8×10^-10),it is difficult to destroy the structure by other interfering ions.However,S^2-is an exception,because it can convert the precipitation to form Ag₂S(Ksp=2×10^-49)which is more stable and insoluble.The standard equilibrium constant of the conversion reaction is as high as 4.4×10¹⁶.The probe has excellent sensitivity and selectivity to S^2-.By changing the seeds size of Au NPs,the amount of the silver precursor and I₂ as well as the ratio between the reducing agents,a kind of colorimetric probe with the best optical response to S^2-was screened out,and a colorimetric method was developed for the detection of S^2-with linear range from 0-80?mol L^-11 and a detection limit of 0.5?mol L^-1.This method was further applied to the determination of S^2-in lake and tap water with recoveries ranging from 111.1-117.35%.In the third chapter,Au@Ag@Au bimetallic core/multi-shell NPs were synthesized for CN^-sensing.Using Au NPs as a template,silver and gold precursor was successively reduced by ascorbic acid to produce Au@Ag@Au bimetallic core/multi-shell NPs.These NPs are more accessible for spectrum tuning in visible range due to their multi-shell properties.As these NPs are exposed to CN^-,CN^-can sequentially etch gold and silver shell due to its high complex ability towards these two metals leading to the color and spectrum changes.These changes are dependent on the CN^-concentration.The core shell size and silver thickness were extensively studied to obtain the best probe for the detection of CN^-.Parameters such as reaction temperature,time and pH was optimized to develop a quantitative method for the detection of CN^-,which exhibit linear range from 1.6-80?mol L^-1.The Au@Ag@Au bimetallic core/multi-shell NPs were embedded into agarose gels as test strips for the colorimetric detection of CN^-.