Colorimetric Detection Of Heavy Metal Ions Based On Gold Nanoparticles

The contamination of heavy metals is different from these of other pollutants since it is difficult to be eliminated and can be accumulated in biological organisms. Therefore, it is of significance to develop the real time and on-site methods for detecting heavy metal ions in water sample. Currently, detecting heavy metal ions is mainly operated by the expensive instruments. However, these methods suffered from some shortcomings, such as complex processing steps, expensive and timeconsuming. Due to the unique optical properties of noble metal nanoparticles, the changes of state of the disaggregation/aggregation,size and morphology of nanoparticles have the influence on the surface plasmon resonance absorption. In this work, the unique optical properties of gold nanoparticles were employed to sensitively and rapidly detect Hg²⁺, Pb²⁺ and Cr³⁺ ions. The main research contents includes as follows:（1） Mercaptosuccinic acid（MSA） modified gold nanoparticles was employed to selectively and rapidly detect Cr³⁺ for the first time. The process is indicated by a red shift of the UV-vis absorption and a dramatic color change of solution from red to blue. Under optimized conditions, this technique exhibits the high selectivity to Cr³⁺ ions over other tested heavy metal ions. The ratio between the absorbance at 650 nm and 520 nm of wavelength(A₆₅₀/A₅₂₀) was linearly proportional to Cr³⁺ ions concentration in the range from 0.60-1.4 μM. The limit of detection was estimated as low as 0.040 μM by UV-vis absorption spectroscopy and 0.6 μM by naked eyes, respectively, resultingin that naked eye-detection of Cr³⁺ ions becoming to be feasible. This simple approach was employed to detect trace Cr³⁺ ions in real water samples by recovery experiments and showed recoveries of 93-107%.（2） Glutathione functionalized gold nanoparticles（GSH-AuNPs） for the rapid detection of Pb²⁺ were established. Two size of AuNPs with diameters of ca. 13 nm and 45 nm were synthesized, respectively. The core-shell structure was achieved by the introduction of Pb²⁺ between the 13 nm AuNPs and 45 nm AuNPs. Pb²⁺ ions was detected due to the formation of coordinated complexation between GSH-AuNPs and Pb²⁺. Under the optimized experimental conditions, a good linear relationship between absorbance ratio of the solution at 700 nm and 520 nm of wavelength（A700/A520） and Pb²⁺ ions concentration is observed and the LOD is 80 nM. Furthermore, the method was employed to detect Pb²⁺ ions in real water samples, and the result revealed the recoveries of 96-120 %.（3） A novel and simple colorimetric assay to detect Hg²⁺ ions in aqueous solution was developed. The proposed method is based on the anti-aggregation of AuNPs induced by 6-thioguanine（6-TG） and Hg²⁺ ions. In the absence of Hg²⁺ ions, 6-TG could bind to the AuNPs by Au-S bond, resulting in the aggregation of AuNPs solution, accompanied a visible color change from red to blue. However, in the presence of the Hg²⁺ ions, both thiol group and nitrogen atom from6-TG preferentially bind to Hg²⁺ ions, thus 6-TG lost the capability which inducing the aggregation of AuNPs. Meanwhile, the color of the AuNPs solution still kept red, which was observed via the naked eye, as well. It is shown that the absorbance ratio（A520/A720） was linearly proportional to the concentration of Hg²⁺ ions in a range from 400-1000 nM, with a LOD of 8 nM.These established methods were successfully employed in the detection of target metal ions in water samples. The results reveal that the new developed colorimetric sensors might open a new avenue for the fast and on-site recognition of Cr³⁺, Pb²⁺ and Hg²⁺ ions.