Detection of sub-ppm traces of aqueous heavy-metal ions using micro-electro-mechanical beam resonators

Capacitive silicon micro-mechanical resonators have been utilized in this work as ultra-sensitive mass sensors for the detection of trace amounts of copper ions in water samples. The approach is based on the reduction of aqueous metal ions by the silicon in a resonant structure and consequently deposition of a very thin metal layer on the resonator surface changing its resonant frequency. Measurements demonstrate successful detection of sub-ppm concentrations of copper(II) ions in water. Relatively large frequency shifts (hundreds of ppm) have been measured for resonators exposed to copper concentrations as low as 4 µM (0.26 ppm). An analytical model for the resonant frequency of the resulting complex beams has been derived and used to calculate the thickness of the deposited copper layer based on the measured frequency shifts. The model shows that the measured frequency shifts correspond to only a few atomic layers of copper (as thin as ∼7 Å) deposited on the resonator surfaces. This corresponds to a mass sensitivity of more than 4000 Hz µg−1 cm−2 which is much larger than the highest mass sensitivities measured for quartz crystal microbalances