Aqueous Synthesis And Application Of Transition Metal Doped Quantum Dots And Their Interaction With Human Serum Albumin

Through introducing appropriate transition metal element or rare earth element into the host lattice of nanomaterials,doped quantum dots(QDs)could modify the optical,electrical and magnetic properties of QDs.It is reported that doped QDs possess excellent optical properties,with a relatively low biotoxicity.In the biological field,Mn-doped QDs are adopted for optical imaging and magnetic resonance imaging.The long lifetime of doped ions makes it ideal for reducing the background fluorescence caused by endogenic molecules.In the field of solar cells,the solar energy conversion efficiency could be improved by introducing appropriate doping element.There are three strategies to introduce dopant ions into host QDs:nucleation doping,growth doping and one-pot doping.In order to realize the introduction of different kinds of dopant ions,it is necessary to select appropriate doping strategy.For example,Cu doped QDs are usually prepared through growth doping method,and nucleation doping is adopted for preparing Mn doped QDs.In addition,the introduction of some metal ions(such as Pb2+ and Ag+)is not easy to realize.According to some literatures,the doping amount,location and distribution of dopant ions in QDs greatly affect the properties of doped QDs.Therefore,it is necessary to develop a new synthetic method to enrich species of doped QDs and realize the controllable synthesis of doped QDs(controlling doping amount,location and distribution of dopant ions).In this thesis,different sizes of Mn:ZnSe QDs were prepared at different pH using nucleation doping strategy.Moreover,Mn:ZnSe QDs(2.7 nm)were adopted as template in the two-step cation exchange reaction and a series of doped(Cd,Hg,Ag,Cu and Pb doped)ZnSe QDs were prepared,then,the reaction mechanism of two-step cation exchange was investigated.Based on the two-step cation exchange reaction,a novel QDs-based ion probe was developed for the detection of heavy metal ions Hg2+,Cd2+ and Pb2+.Finally,the interaction between ZnSe QDs and human serum albumin was studied,which could provide important information for the biosafety applications of QDs.This thesis consists of six chapters.Chapter 1:This chapter gave a comprehensive introduction of the properties,biomedical applications and progress in the synthesis of QDs.It also presented topic ideas as well as innovative ideas.Chapter 2:Three different sizes of Mn:ZnSe QDs were obtained by adjusting the pH values,the products were characterized by UV-vis absorption spectroscopy,fluorescence spectroscopy,transmission electron microscopy(TEM),Fourier transform infrared spectroscopy(FTIR)and inductively coupled plasma atomic emission spectrometry(ICP-AES),then,the best reaction parameters for preparing Mn:ZnSe QDs were investigated.Chapter 3:We developed a new two-step cation exchange strategy for preparing doped QDs.In the two-step cation exchange,Mn:ZnSe QDs(2.7 nm)were adopted as template.Under the mild conditions,the rapid synthesis of doped QDs was realized and UV-vis absorption spectra,fluorescence spectra,transmission electron microscopy(TEM),X-ray diffraction(XRD),inductively coupled plasma atomic emission spectrometry(ICP-AES)and X-ray photoelectron spectroscopy(XPS)were used to characterize the prepared doped QDs.The effects of reaction temperature,the amount of added cation and the kind of doping elements on the fluorescence properties of the product were discussed.Moreover,the mechanism of two-step cation exchange was investigated.The results show that the two-step cation exchange method is a general strategy for preparing doped QDs,and Mn2+ in Mn:ZnSe QDs could greatly promote the introduction of guest cations.The fluorescence peak of doped QDs can be tuned from 452 run to 700 nm,which is difficult to realize in the traditional method.At the same time,the room temperature doping of Cd2+ and Hg2+can be realized in 19 min and 70 min,respectively.The introduction of Ag+,Cu2+ and Pb2+ can only be achieved under heating conditions.The optimum temperature is 100 ?,and the quantum yield of the synthesized Ag:ZnSe quantum dots is up to 12.4%.Chapter 4:Based on the two-step cation exchange reaction,the detection of Hg2+was realized using Mn:ZnSe QDs(2.1 nm)as fluorescent probe.Inductively coupled plasma atomic emission spectrometry(ICP-AES)and X-ray photoelectron spectroscopy(XPS)were used to explore the detection mechanism.The results show that the fluorescence quenching agent(surface Mn2+)in Mn:ZnSe QDs was replaced by Hg2+ during Hg2+-Mn2+ cation exchange,while the inner Mn2+ could not be replaced by Hg2+ and gave rise to the Mn2+(4T1 ? 6A1)emission around 600 nm.The limit of detection for Hg2+ is 7 nM,which is lower than the mercury toxic level defined by the U.S.Environmental Protection Agency(10 nM).Different from traditional turn-off method for detecting Hg2+,we realize a turn-on strategy for the selective detection of Hg2+ by Hg2+-Mn2+ replacement.What's more,our new strategy implies that turn-on strategy for detecting Hg2+ can be realized by semiconductor nanocrystals through modifying the size and elemental composition of semiconductor nanocrystals.Chapter 5:The detection of heavy metal ions Pb2+ and Cd2+ in aquatic ecosystems was realized using Mn:ZnSe QDs(2.1 nm)as colorimetric probe.The results show that the absorption peak intensity of Mn:ZnSe QDs increased when Pb2+was added,and the absorption peak of Mn:ZnSe QDs shifted to longer wavelength when Cd2+ was added.The detection of Pb2+ and Cd2+ can be specifically detected by the different responses of Mn:ZnSe QDs to Pb2+ and Cd2+.The limit of detection for Pb2+ and Cd2+ are 130 nM and 1.5 nM,respectively.Chapter 6:Glutathione(GSH)modified ZnSe QDs were synthesized using aqueous synthesis method.ZnSe QDs were then characterized by fluorescence spectra,UV-Vis absorption spectra,transmission electron microscopy(TEM),Fourier transform infrared spectroscopy(FTIR)and X-ray diffraction(XRD).Subsequently,the interaction between GSH-ZnSe QDs and HSA was investigated by spectroscopy method(fluorescence spectroscopy,UV-Vis absorption spectroscopy and circular dichroism(CD)).The prepared GSH-ZnSe QDs have the same sphalerite crystal structure and approximate particle size(2.5 nm,2.8 nm and 3.0 nm).Fluorescence quenching and UV-Vis absorption spectra show that HSA could form GSH-ZnSe complex with larger QDs(3.0 nm),while smaller GSH-ZnSe QDs(2.5 nm and 2.8 nm)quenched the fluorescence of HSA through a dynamic quenching method.The different quenching strategies were caused by the different GSH-Zn complexes of GSH-ZnSe QDs.