Synthesis Of Novel Water-soluble Quantum Dots And The Study On Their Application In Chiral Sensing Field

Fluorescent nanomaterials have drawn tremendous attention due to their variety of potential applications in optical bioimaging,drug delivery,biochemical detection,disease diagnosis and optical devices.Semiconductor quantum dots(QDs),a new type of nanomaterial developed in recent years have become one of the most promising nanomaterials due to their high photo-stability,large molar extinction coefficients,high photoluminescence quantum yields(PL QYs),tunable emission and broad excitation.However,these Cd-based QDs have raised concerns over serious toxicity in real application.Thus,further reducing their toxicity,improving their biocompatibility,synthesizing excellent functional QDs to meet the requirements of biochemical detection in future are of great importance.In this paper,two types of high-quality emission-tunable ternary water-soluble QDs were synthesized by adopting one-pot hydrothermal route and the carbon dots(C-dots)with excellent biocompatibility was synthesized through pyrolysis method.Various synthetic conditions have been optimized and the morphology,structure,composition have been characterized.Specially,we carefully studied their formation and PL mechanisms.Finally,chiral N-acetyl-L-cysteine(L-NAC)capped CdSe/CdS QDs were selected to realize the study of enantioselective recognition of Tyrosine(Tyr)enantiomers,and the possible chiral recognition mechanism has been proposed.The main works are summarized as follow:1.High-quality water-soluble ZnCdSe QDs were synthesized via optimized one-step hydrothermal method with a new thiol as ligand,within a short time of 65 min.The emission wavelength of prepared QDs is tunable in the range of 420–540 nm by merely controlling the molar ratio of Cd: Zn or Se: Zn,and the quantum yield reaches 37%.More importantly,the maximum Cd: Zn molar ratio has been reduced to 0.04: 1.0,much lower than that reported in the literature(0.5: 1.0),resulting in excellent biological compatibility of prepared QDs and thus their promising applications in biological fields.Moreover,the transmission electron microscopy(TEM)was employed to examine the effect of Cd: Zn ratio on the size of prepared ZnCdSe QDs,and the elemental constitutions of which were characterized by x-ray photoelectron spectroscopy(XPS)and electron diffraction spectroscopy(EDS).The results indicate that changing the value of n(Cd)/1(Zn)leads to the variation of composition of the obtained QDs without significant change in particle size.2.High-quality water-soluble Mn2+ doped CdTe QDs have been synthesized through hydrothermal route with NAC as capping reagent,allowing a rapid preparation time(< 1 h),tunable emitting peaks(from 530 to 646 nm)and excellent QYs(approximately 50%).The influences of various experimental variables,including Mn-to-Cd ratio,Te-to-Cd ratio,pH value,and reaction time on the growth rate and luminescent properties of the obtained QDs have been systematically investigated.And the optimum reaction conditions(Cd: Mn: NAC: Te = 1.0: 1.0: 2.4: 0.2,pH = 9.5,35 min,200?)are found out.The optical features and structure of the obtained CdMnTe QDs have been characterized through fluorescence spectrum,UV absorption spectrum and TEM.In particular,we realized qualitative,semi-quantitative and quantitative studies on the doping of Mn to CdTe QDs through XPS,EDS,and atomic absorption spectrum.The actual molar ratio of Mn to Cd in CdMn Te QDs(551 nm)is 1.166: 1.00,very close to the feed ratios 1: 1.3.A green pyrolysis approach to prepare C-dots(QY reaches 55.7%)with precursors of citric acid(CA)as carbon source and NAC as N and S dopant has been explored.The optical properties and the origin of PL have been investigated at length.The excitation independence character(200 ? ?ex ? 380 nm)and the excitation dependence feature(?ex > 380 nm)along with the biexponential behavior of lifetime of prepared C-dots all indicate the existence of two different emissive sites,that is,the surface state caused by the introducing of N and S on carbon nucleus' surfaces and the molecular state induced by the fluorophores covalently bonded onto C-dots surfaces.More importantly,through comparing the PL performances of various types of N,S-C-dots,N-C-dots and S-C-dots synthesized under same condition,we report the reactant,which could form small fluorophore molecules with high PL during the synthesis and thus exhibits favorable impact on PL performances of resulting C-dots.Our results would be valuable aids in selecting efficient reactants for preparing highly fluorescent CA-derived C-dots.4.Tyr enantiomers were successfully recognized by adopting chiral L-NAC-CdSe/CdS core-shell QDs as fluorescent probes.It exhibits extraordinary capacity for the chiral recognition of Tyr enantiomers via an enantioselective fluorescence quenching response,L-Tyr quenches the fluorescence of QDs dramatically,while D-Tyr displays no effect.The linear decrease of the fluorescence of chiral QDs caused by the addition of L-Tyr indicates the possibility of quantification of L-Tyr in Tyr enantiomers in the whole ranges of ee values.To our delight,exceptional selectivity for Tyr enantiomers in comparison to other important interfering substances is observed.Furthermore,the vital factor of chiral recognition capacity of QDs/ Tyr system is found to be pH,and the impact of pH has been systematically investigated,while the best enantioselective recognition effect occurs at pH 8.5.Specially,the molecular recognition mechanism is studied via UV-visible absorption spectra and CD spectra,which verifies the attachment of L-Tyr to the capping agent L-NAC of QDs,offering valuable aids in obtaining a better understanding of the possible mechanism of enantioselective recognition.Our constructed chiral sensing system is indicative of the fact that one could potentially find similarly selective system for other chiral molecules.