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

Abstract

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 ∼7Å) 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.

Sensors for Corrosion Detection: Measurement of Copper Ions in 3.5% Sodium Chloride Using Screen-Printed Platinum Electrodes

“…An interesting Cu 2+ ion sensor that negates the requirements for a reference electrode is the micro-electro-mechanical beam resonator described by Rahafrooz and Pourkamali [21]. Here, aqueous copper ions oxidize the surface of a micromachined silicon beam resulting in the deposition of a thin metallic layer with associated increase in beam mass and concomitant change to the resonant frequency, which can be measured as an indicator of the film thickness.…”

Fabrication and characterization of thermally actuated micromechanical resonators for airborne particle mass sensing: II. Device fabrication and characterization

“…One of the interesting and highly desirable characteristics of the resonator structures used in this work is that, as expected and discussed in part I, they maintain relatively high quality factors of the order of 2000-4500 in air. Quality factors of capacitive beam resonators with such low frequencies typically drop to 100 or less in air [19,[30][31][32]. This makes such resonators particularly suitable for environmental sensory applications.…”

Miniaturized electrochemical sensor with micropillar array working electrode for trace lead online measurement in tap water

“…Spectroscopic analytical techniques, including inductively coupled plasma mass spectrometry (ICP-MS) [13,14], atomic absorption spectrometry (AAS) [15,16] and inductively coupled plasma atomic emission spectrometry (ICP-AES) [17], possess high detection sensitivity and accuracy advantages, but need a long sample preparation time, detection is time-consuming and the equipment is complex and expensive [18]. Electrochemical methods, especially anodic stripping voltammetry (ASV), have been recognized as a successful trace heavy metal analysis technique [19][20][21] due to its easy operation, flexibility, portability, high precision and high selectivity [22,23]. Undoubtedly, ASV is an ideal method for On-site field, qualitative, and quantitative measurement [24].…”