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Current water quality monitoring for heavy metal contaminants largely results in analytical snapshots at a particular time and place. Therefore, we have been interested in miniaturized and inexpensive sensors suitable for long-term, real-time monitoring of the drinking water distribution grid, industrial wastewater effluents, and even rivers and lakes. Among the biggest challenges for such sensors are the issues of in-field device calibration and sample pretreatment. Previously, we have demonstrated use of coulometric stripping analysis for calibration-free determination of copper and mercury. For more negatively reduced metals, O2 reduction interferes with stripping analysis; hence, most electroanalysis techniques rely on pretreatments to remove dissolved oxygen (DO). Current strategies for portable DO removal offer limited practicality, because of their complexity, and often cause inadvertent sample alterations. Therefore, we have designed an indirect in-line electrochemical DO removal device (EDOR), utilizing a silver cathode to reduce DO in a chamber that is fluidically isolated from the sample stream by an O2-permeable membrane. The resulting concentration gradient supports passive DO diffusion from the sample stream into the deoxygenation chamber. The DO levels in the sample stream were determined by cyclic voltammetry (CV) and amperometry at a custom thin-layer cell (TLC) detector. Results show removal of 98% of the DO in a test sample at flow rates approaching 50 μL/min and power consumption as low as 165 mW h L(-1) at steady state. Besides our specific stripping application, this device is well-suited for LOC applications where miniaturized DO removal and/or regulation are desirable.

Recent contamination events have emphasized the need for widespread heavy metal monitoring over extended periods of time. The customary and laborious random "grab sampling" for later analysis at central laboratories does not adequately satisfy this need. Our group is developing an electroanalytical approach that allows highly sensitive, calibration-less measurements to quantify heavy metals in microliter volumes. This approach offers promise for economical, miniaturized, remotely-deployable sensor networks. This study reports the application of microfabricated gold microelectrode arrays within a thin-layer cell and demonstrates sufficient sensitivity to detect sub-5-ppb arsenic concentrations in a microfluidic sample volume (< 5 µL).