Cold vapor integrated quartz crystal microbalance (CV-QCM) based detection of mercury ions with gold nanostructures

In this study, we developed a novel method of integrating the well-accepted cold vapor technique with gold nanostructured based quartz crystal microbalance (QCM) devices to selectively detect mercury ions (Hg2+). This method allows for the conversion of aqueous mercury ions into elemental mercury (Hg°) vapor form and thereon use the highly sensitive QCM based mercury vapor sensors to detect the evolved mercury. The method involves reducing mercury chloride (HgCl2) in contaminated water by mixing it with a 2% tin chloride (SnCl2) solution in order to evolve Hg° vapor from the liquid mixture. The selectivity and sensitivity performance of each gold nanostructure, namely, the control Au thin film (Au-ctrl), Au-nanospheres (Au-NS) and Au-nanourchins (Au-NU), towards mercury vapor was evaluated. It was found that Au-NS and Au-NU sensors displayed up to 79% and 243% higher response magnitudes than the Au-ctrl sensor for various concentrations of HgCl2, respectively. All three sensors exhibited repeatable sensing performance when reporting the concentrations from 5 sensing events involving 500 ppb HgCl2 solution with Au-ctrl, Au-NS and Au-NU having the coefficient of variance (CoV) values of ˜5.7, 2.9 and 3.8%, respectively. Moreover, the sensors were observed to operate in the linear region with the mercury ion concentration range calibrated and tested. Importantly, the sensors showed no cross-interference effects when tested toward Hg2+ ions with and without the presence of other metal ions such as lead, cadmium, manganese, iron, and zinc. The results indicate that the CV-QCM technique developed is feasible to be potentially used in real-world mercury monitoring applications