Study On The Preparation And Photoluminescence Properties Of Water-soluble ZnO:S,Eu Quantum Dots For Biomarker

Owing to the nontoxicity,pollution-free,excellent luminescent properties and photostability of Zinc oxide(ZnO),ZnO,as a direct wideband gap semiconductor material with a wide bandgap(3.37ev)at room temperature,has been found application in many fields such as optoelectronic devices,biological markers and photocatalysis.The photoluminescence(PL)and water-dispersible properties of ZnO quantum dots(QDs)can be improved by doping and modification methods and then make it more widely used.The luminescence property and biocompatibility of ZnO QDs are the most important characteristics in the application of biomarker.The investigation on the luminescent properties of doped ZnO is significative for both scientific attention and practical applications.Therefore,the preparation,photoluminescence properties,surface modification and cytotoxicity properties of ZnO:S,Eu QDs were researched in this work.In this work,the following work has been finished:1.ZnO nanoparticles with difference size were prepared by a hydrothermal method,precipitation method,and microwave hydrothermal method,respectively.The results of XRD and TEM showed that all ZnO samples had a hexagonal wurtzite structure with a particle size of 43.2nm.8.3nm and 8.5nm,respectively.Among them,ZnO QDs prepared by microwave hydrothermal method had the strongest PL intensity in the near ultraviolet region.2.ZnO:S,Eu QDs were synthesized by a microwave hydrothermal method.The effects of Eu3+ and S2-contents on the PL intensity of ZnO QDs were researched in detail.Compared with undoped ZnO sample,the PL intensity of ZnO QDs could be remarkable increased by doping with S or Eu,and the optimal molar ratio was 0.05(S/Zn)and 3/97(Eu/Zn)for doping with S and Eu,respectively.Moreover,the best PL intensity of ZnO can be obtained by co-doping with 5%S and 3%Eu(under 325 nm excitation)and 5%S and 7%Eu(under 464 nm excitation),respectively.For ZnO:S QDs,S2-doping could increase the carrier concentration of the sample,improved the chance of radiative recombination in ZnO:S QDs and thus enhanced the PL intensity of ZnO.In ZnO:Eu QDs,Eu3+ could provide a new luminescence center and transfer energy to ZnO host,which should improve the PL intensity of the sample.For ZnO:S,Eu QDs,due to the synergetic effect of S2-and Eu3+,the luminescence intensity of QDs in the visible region was not only improved,but also broadened the range of the light absorption.3.L-cysteine capped ZnO:S,Eu QDs were prepared by two steps.The effects of L-cysteine(L-cys)on the crystal structure,band gap and PL properties were studied.It was found that the surface modification of L-cys on ZnO:S,Eu QDs not only did not change the crystal structure but also could adjust the band gap value of QDs.The results of First-principles calculations indicated that the conduction band minimum(CBM)of L-cys-ZnO(112?)surface was higher than that of ZnO,and the VBM of ZnO was lower than that of L-cys-ZnO(112?)surface.Therefore,it is not surprising that the L-cys-ZnO(112?)surface and ZnO can form heterojunction,which helps the separation and transfer of photogenerated electrons and holes.Hence,the photogenerated electrons in the CB of L-cys-ZnO(112?)surface should transfer to the CB of ZnO and the photogenerated holes in the VB of ZnO to move to the VB of L-cys-ZnO(112?)surface,which resulted in the accumulation of electrons at the CB of ZnO and holes at the VB of Lcys-ZnO(112?).This process not only suppresses the non-radiative recombination of photogenerated electrons and holes in the surface,but also increases the radiative recombination of electron-hole in the ZnO core,and then the PL intensity is strengthened.The result also showed that L-cys can passivate the surface of ZnO QDs and then decreases the number of the surface dangling bonds and defects,which resulted in an increase in PL intensity.4.Folic acid(FA)modified L-cys@ZnO:S,Eu QDs were synthesized by two-step method.The experimental results showed that all samples had hexagonal wurtzite ZnO crystal structure,and FA modification could further improve the water-solubility and PL intensity of QDs.Biological experimental results showed that L-cys and FAmodification were nontoxic and maintained remarkable cell viability and morphology up to the concentration of QDs was 50 ug/m L for L929 cells.Moreover,FA-modified L-cys @ ZnO: S,Eu QDs can enter and accumulate in the liver of mice,which will not affect the life of mice.This work suggested that FA capped L-cys @ ZnO: S,Eu QDs had favorable biocompatibility and may be used for in vitro cell imaging of L929 cells.