Mn-doped ZnS Quantum Dots For Detection Of Ions In The Environment

Molecular recognition is an important area of supramolecular chemistry, which includes sensing of neutral molecular, cations and anions. The recognition and sensing of anions and cations have received considerable attention for their important roles in biological, industrial and environmental process. As a new type of fluorescent sensors, quantum dot (QDs) offer a number of attractive features, including high photobleaching threshold, good chemical stability, narrow and symmetric luminescence bands, which are expected to remedy the deficiencies of the organic dyes. In the thesis, several highly selective and sensitive sensors for anions or cations were constructed. The sensors (modified QDs) were characterized, and the assays for sensing sulfide anion, cadium (Ⅱ) ion, cupper (Ⅱ) ion and silver (Ⅰ) ion were introduced.In Chapter 1, the concept, the nature and the preparation methods development of quantum dots were briefly reviewed. Then the special functional and chemical sensors are emphatically introduced.In Chapter 2, water-soluble Mn2+-doped ZnS quantum dots (QDs) were prepared using mercaptoacetic acid as the stabilizer. The optical properties and structure features were characterized by X-Ray diffraction, absorption spectrum, IR spectrum and fluorescence spectrum. In pH 7.8 Tris-HCl buffer, the QDs emitted strong fluorescence peaked at 590 nm with excitation wavelength at 300 nm. The presence of sulfide anion resulted in the quenching of fluorescence and the intensity decrease was proportional to the S2- concentration. The linear range was from 2.5×10-6 to 3.8×10-5 mol L-1. Most anions such as F-, Cl-, Br-, I-, CH3CO2-, ClO4-, CO32-, NO2-, NO3-, S2O32-, SO32- and SO42- did not interfere with the determination. A highly selective assay was introduced and applied to determine S2- in discharged water with recovery of ca.103%.In Chapter 3, ZnS:Mn2+quantum dots with good dispersity and fluorescent characteristics were synthesized in aqueous solution with thioglycolic acid as a stabilizing agent, which were proposed for the determination of cadmium ions (Ⅱ). In the Tris-HCl buffer with pH 7.7, it was found that the fluorescent intensity of thiol-capped ZnS:Mn2+ was enhanced by the presence of Cd2+. The fluorescence intensity of ZnS:Mn2+ increased linearly with the concentration of Cd2+ in the rang of 5.0×10-7-8.9×10-5 mol·L-1. The detection limit was 3.08×10-8 mol·L-1. This method was used to the determination of Cd2+ in discharged water, and the recovery was 93.7-97.4%. The properties of above ZnS:Mn2+ quantum dots were studied by the fluorescence spectra, UV-vis absorption spectra and X-ray diffraction(XRD). The possible mechanism was discussed as well.In Chapter 4, ZnS:Mn2+ quantum dots with peculiar spectral properties were synthesized in an aqueous solution using thioglycolic acid as a stabilizing agent. To improve their fluorescence properities in water solution, bovine serum albumin (BSA) was absorbed on their surface. Under the optimal conditions, the fluorescence intensity of the ZnS:Mn2+ decreased linearly with the concentration of copper(II) ions in the rang of 2×10-6-7.8×10-5 mol·L-1. The liner calibration equation was following: Fo/F= 1.01+0.06 CCu2+ and the detection limit was 2.87×10-7 mol×L-1. The proposed method was successfully applied to determine copper (Ⅱ) ions in tap water samples with recovery of ca.98.8%.In Chapter 5, the probe consisted of fluorescent ZnS:Mn2+/ZnS core/shell quantum dots (QDs) were properly conjugated to iminodiacetic acid through CS2-assisted zero-length covalent coupling. The functionalized nanoparticles not only improve water-soluble, but also exhibit a strong fluorescent emission at ca.590 nm. Their optical and structural properties have been characterized by UV-vis spectroscopy, fluorescent spectroscopy and 1H NMR spectroscopy. In pH 7.3 phosphate buffer, the calibration plot between intensity ratio (Fo/F) and the concentration of Ag+ was linear in the range of 1.5-10.5×10-6 mol·L-1 with correlation coefficient of 0.992. The limit of detection was 2.65×10-7 mol·L-1. The effect of the other ions (including alkali metal ions, alkali earth metal ions, Pb2+, Cd2+, Zn2+, Ba2+ Al3+, Ca2+, Mg2+, Na+, K+ and anions such as NO3-, SO42-, CO32- was negligible even at a very high concentration. The possible mechanism was proposed.