Electrochemical piezoelectric sensors for trace ionic contaminants

Industrial processes, such as fossil fuel combustion and nuclear materials processing, have resulted in heavy metal contamination of soils and potentially of the surrounding groundwater. In particular, mercury contamination of groundwater is a serious threat to the ecosystem, cumulating in serious health problems for humans as well as wildlife. Monitoring of mercury contamination in groundwater requires a method of long-term verification. Sensors with lifetimes of months to years of operation without operator intervention are required. One sensor geometry, which is capable of detecting relevant concentrations of aqueous ionic contaminants, such as mercury, while withstanding typical environmental conditions, is the shear horizontal acoustic plate mode (SHAPM) sensor. This sensor protects the electronics from the potentially corrosive aqueous fluid environment while providing a significant interaction with the fluid. Gold films are employed to accumulate the mercury via surface amalgamation. The added mass is measured as a change in the resonant frequency of the piezoelectric sensing element. Electrochemical techniques are employed to impart reversibility and to accelerate response kinetics. Results indicate a sensitivity of approximately 2.4 ng/mL, which approaches the 2.0 ng/mL limit imposed by the Safe Drinking Water Act (SDWA).