Study On Functionalized Gold Nanoparticles For Detection Of Heavy Metal Ions(Cr³⁺ And Cd²⁺) In Aqueous Solutions

With the rapid development of industry and agriculture, a large number of waste containing heavy metals, including waste water, waste gas and waste residue, is released into the environment, such as water, soil and the atmosphere, which exceeds the self-purification capacity of the environment, resulting in heavy metal pollution and threatening human beings’ health seriously. So it is very necessary to develop rapid and sensitive methods to detect heavy metals ions in the environment especially in water. Detection of heavy metals ions with traditional methods has many problems and new rapid detection methods still need to be improved. With unique optical properties of large molar extinction coefficient, high surface area and the dependence of aggregation degree on particle size and interparticle distance, and chemical properties of being surface functionalized easily, gold nanoparticles(AuNPs) have attracted much attention in the application of detection of heavy metal ions. Based on the principle of colorimetric detection of heavy metals using AuNPs sensor, the synthesized AuNPs were functionalized using small organic molecules and detections of Cr3+ and Cd2+ in aqueous solution using built colorimetric sensor were studied in the thesis. The specific contents are as follows.(1) Preparation of AuNPs by reduction of chloroauric acid(HAuCl4) with sodium citrate. AuNPs with average particle size in the range of 12~16 nm and concentration in the range of 2~5 nmol/L were synthesized, according to the different molar ratio of sodium citrate to HAu Cl4. Electron microscopy images and spectroscopic characterization showed that synthesized AuNPs were homogeneous and stable and were suitable for follow-up functionalization of Au NPs. Furthermore, the suitable concentration ranges of four kinds of small organic molecules(2-acetamido-5-mercapto-1,3,4-thiadiazole(AAMT),6-thioguanine(TG), 3-amino-5-mercapto-1,2,4-triazole(AMT), 2-amino-4-mercaptobutanoic acid(AMBA)) to functionalizing synthesized Au NPs were explored.(2) Colorimetric detection of Cr3+ based on the aggregation of AuNPs. A colorimetric method was developed to determine chromium ions(Cr3+) in water, using 13 nm diameter AuNPs synthesized by the above method and 3-amino-5-mercapto-1,2,4-triazole(AMT) as the transducer component for signaling color change and the recognition element for Cr3+, respectively. AMT was assembled onto the surface of AuNPs, and forming a stable water-soluble AMT-AuNPs compound. In the presence of Cr3+, the specific binding of AMT and Cr3+ in solution could lead to the aggregation of AMT-Au NPs and a red shift of the maximum absorption wavelength, simultaneously producing an obvious color change from red to blue-purple, which could be used for Cr3+ visual detection. Under optimized conditions(the concentration of AMT, 0.8 μmol/L; pH, 7), the detection limit(3σ) of Cr3+ could be as low as 100 nmol/L and there was almost no interference from other metal ions. The presented method has advantages of fast response(5 min), simplicity of preparation and manipulation, and no need of reading device.(3) Exploration of colorimetric detection of Cd2+ based on the aggregation of AuNPs. As above, 13 nm diameter AuNPs acted as colorimetric signal amplification component while the four kinds of small organic molecules(AAMT, TG, AMT and AMBA) were used to functionalize AuNPs, building colorimetric sensors to detect Cd2+ in aqueous solution. By chemical bonds or special functional groups in small molecules, the specific binding of molecules and Cd2+ could produce different intensities of colorimetric response, which could be used to identify the recognition ability of the four kinds of small molecules for Cd2+. The experimental results showed that AAMT-AuNPs and AMT-AuNPs had no ability to recognize Cd2+, AMBA-AuNPs and TG-Au NPs had weak abilities to recognize Cd2+, and that after mixing with cysteine, TG-AuNPs showed the enhanced recognition ability for Cd2+, further revealing that the recognition of Cd2+ may require more functional groups and specific spatial conformation of recognition molecules.