Synthesis Of Functionalized Quantum Dots And Their Applications On Heavy Metal Analysis

In recent years, quantum dots (QDs) have become an area of growing interestbecause of their attractive optical properties, including broad excitation and narrowemission spectra, size-tunable emission profiles, high photoluminescence quantumyields, and excellent photochemical stability. These unique features of QDs showconsiderable advantages over traditional organic fluorophores in fluorescentapplications, resulting in the increased use of QDs as fluorescent probe in analyticalchemistry.In this dissertation, the new and green method is developed to synthesizehigh-quality QDs. Through the functionalization of chelating reagents on QDs, theunique photoluminescence properties of QDs are combined with the specific and strongmetal ions affinity of chelating reagents to construct sensitive and selective fluorescentprobes. These fluorescent probes are applied to environmental samples analysis withsatisfactory results.The major contents in this dissertation are described as follows.In Chapter One, a brief review is given on the principle mechanisms,photoluminescence properties, synthesis process and surface-modification of QDs.Their applications and development trends are also presented. The research topics ofthis dissertation are thus proposed.In Chapter Two, a new and convenient route is developed to synthesize CdSe andcore shell CdSe/CdS QDs in aqueous solution. CdSe QDs are prepared by introducingH2Se gas into the aqueous medium containing Cd2+ions. The synthesized CdSe QDs arefurther capped with CdS to form core shell CdSe/CdS QDs by reacting with H2S gas.The gaseous precursors, H2Se and H2S, are generated on-line by reducing SeO32withNaBH4and the reaction between Na2S and H2SO4, and introduced sequentially into thesolution to form CdSe and CdSe/CdS QDs, respectively. The synthesized water-solubleCdSe and CdSe/CdS QDs possess high quantum yield (3%and20%) and narrow full-width-at-half-maximum (43nm and38nm). The synthesis process is easilyreproducible with simple apparatus and low-toxic chemicals. The relatively standarddeviation of maxima fluorescence intensity is only2.1%(n=7) for CdSe and3.6%(n=7) for CdSe/CdS QDs. This developed route is simple, environmentally friendly and canbe readily extended to the large-scale aqueous synthesis of QDs.In Chapter Three, a new and green route is developed to synthesize CdSe andcore shell CdSe/CdS QDs in organic phase. The core CdSe QDs are synthesized basedon the use of CdO and Se powder as precursor. Then a new route using H2S as theprecursor of sulfur is performed to synthesize high-quality core shell CdSe/CdS QDs.The synthesized CdSe and CdSe/CdS QDs possess high quantum yield (21%and53%)and narrow full-width-at-half-maximum of about30nm, which are much better thanthat of QDs synthesized in aqueous solution. This developed route is facile andenvironmentally friendly with simple apparatus and low-toxic chemicals, which can bereadily applied to the synthesis of QDs for chemical analysis and biological labeling.In Chapter Four, an ultrasensitive fluorescent probe is developed for thedetermination of lead ion by utilizing dithizone (Dz) functionalized CdSe/CdS QDs.Dithizone was bound to the QDs via a surface coordinating reaction to form QDs Dzconjugates and quench fluorescence of the QDs by fluorescence resonance energytransfer (FRET) mechanism. Upon the addition of Pb2+, a dramatic enhancement of thefluorescence intensity was observed, which resulted from the FRET pathway shuttingoff, and hence the fluorescence of the QDs was recovered. Two successive linear rangesof0.01~1000nmol L1and1~20μmol L1allow a very wide determination of Pb2+concentration from0.01nmol L1to20μmol L1, with a detection limit of0.006nmolL1. The fluorescent probe was successfully applied to the determination of lead inenvironmental samples with satisfactory results.In Chapter Five, a new turn-on fluorescent probe based on xylenol orange (XO)functionalized CdSe/CdS QDs is developed for the determination of lead ion. CdSe/CdSQDs were first modified by mercaptoacetic acid (MAA). The MAA-modified QDs werethen capped with the natural biopolymer chitosan and the negatively charged XO. TheXO-functionalized QDs were formed via the layer-by-layer self-assembly reaction. Thefluorescence of the QDs was quenched by electron transfer mechanism after XO wasbound to the QDs. Upon the addition of Pb2+, a dramatic enhancement of thefluorescence intensity was observed, which resulted from the coordination between Pb2+ and XO on QDs surface and the disruption of the electron transfer mechanism. Hencethe fluorescence of the QDs was recovered. The recovery of the fluorescence intensityshowed a good linear relationship with the concentration of Pb2+added from0.05to6μmol L-1. A detection limit of0.02μmol L-1was achieved. This method wassuccessfully applied to the determination of lead in real samples with satisfactoryresults.In Chapter Six, a new ultrasensitive copper ion fluorescent probe based onCdSe/CdS QDs capped with dimercaprol (BAL) is described. BAL was bound to theQDs via a surface-ligand exchange to form BAL-capped QDs and the fluorescence ofthe BAL-capped QDs was quenched after coordination with Cu2+. Compared with thereported methods using QDs as fluorescent probe for Cu2+detection, the proposedmethod exhibited excellent sensitivity and selectivity due to the specific and strongaffinity of BAL with Cu2+and the unique photoluminescence properties of QDs. Thefluorescent probe based on BAL-capped QDs showed a very good linear response rangeto Cu2+from0.1to50μg L1with the detection limit of0.087μg L1. The possiblequenching mechanism is discussed. This method was successfully applied to thedetermination of ultratrace copper in real samples with satisfactory results.