Portable microfluidic device with thermometer-like display for real-time visual quantitation of Cadmium(II) contamination in drinking water

Cadmium (Cd2+) is a toxic metal ion widely existing in water, soil and food. Conventional water quality control heavily relies on expensive, bulky and sophisticated instrument such as spectrometry, which is time-consuming and incompatible with on-site, real-time detection. Here, a portable microfluidic device with thermometer-like visual readouts is developed for real-time quantitation of cadmium (II) contamination in drinking water. We use Cd2+-dependent DNAzyme (Cd16), which is cleaved when Cd2+ is present, creating a single strand DNA which triggers catalytic hairpin assembly (CHA) with two hairpins H1 and H2 as the building blocks. Plenty of H1H2 complex, the product after the Cd2+-mediated CHA, are generated, which can connect magnetic microparticles (MMPs) and polystyrene microparticles (PMPs), forming "MMPs-H1H2-PMPs" sandwich structure. To provide visual readout to quantitate the particle connection, the particle solution is loaded into a portable microfluidic chip. A magnetic separator first removes MMPs and the connected PMPs, while free PMPs can continue flowing until accumulating into a bar at the particle dam. Shown as a thermometer-like display, the accumulating length is inversely proportional to the concentration of Cd2+, enabling quantitative detection of Cd2+ by the naked eye. The proposed device exhibits a limit of detection of 11.3 nM of Cd2+, selectivity >200-fold against other metal ions, high tolerance to the interferents present in drinking water and high recovery rate in tap water. With high analytical performance without any sample preparation step, this portable device is highly promising in real-time monitoring in urban drinking water at sites.