Thesis (Ph.D.)--Chulalongkorn University, 2012

A novel method has been proposed to realize the visual and colorimetric detection of Hg(II) ions based on direct reduction of Au(III) solution to gold nanoparticle (AuNPs). Three dithia-diaza ligands: 3-AEPE, 4-AEBE and 5-AEPE, were synthesized and used as a stabilizer for preventing the aggregation of gold nanoparticles in the absence of Hg(II) ion, showing a rose-red color. In the presence of Hg(II) ion, gold nanoparticle stabilized by dithia-diaza ligand solution became purple and blue immediately when addition of reducing agent NaBH4. For spectrometric and naked eye detection, the suitable concentration of Au(III) solution, 3-AEPE, NaBH4 and Triton X-100 was 125 M, 0.3 mM, 0.6 mM and 0.1%(v/v), respectively. The suitable pH of 3-AEPE-stabilized gold nanoparticles was 1.4. By comparing with other metal ions, only Hg(II) ion can induce the aggregation of gold nanoparticles, resulting solution turns to blue rapidly. By measuring at a wavelength of 680 nm, absorbance values increased linearly as three levels of concentration of Hg(II) ion range of 0 - 1.25 M (low), 1.25 - 5.25 M (medium) and 2.25 - 10.0 M (high). The limit of detection was estimated to be 35 nM, with a relative standard deviation of 1.3% (n = 11). The acceptable recoveries of spiked samples were found in the range of 90.7 - 106.7%. Similarly, 4-AEBE and 5-AEPE ligands were able to stabilize gold nanoparticles from aggregation, but some conditions were slightly different, such as the concentration of 4-AEBE, 5-AEPE and NaBH4. However, among three dithi-diaza ligands, 3-AEPE provided the highest sensitivity and was more easily observed by naked eye. This method offers advantages of simplicity, rapidity, cost effectiveness and no requirement of any sophisticated instruments and it is alternatively possible method for Hg(II) monitoring in bottled drinking water. Moreover, this method has several potential advantages as optical sensor, especially no as-prepared AuNPs synthesis and shorter observation time.