A Fully Integrated, Ready-to-Use Distance-Based Chemosensor for Visual Quantification of Multiple Heavy Metal Ions

Point-of-care devices offering quantitative results with simple steps would allow great useability for untrained end-users. Here, we report a ready-to-use chemosensor integrating automatic sample metering, on-chip reaction, gravitational-magnetic separation, and a distance-based readout for visual quantification of multiple heavy metal ions. Deoxyribozymes (DNAzymes), probe-modified magnetic microparticles (MMPs), and polystyrene microparticles (PMPs) are preloaded into a microfluidic chip and freeze-dried. After the water sample is collected with automatic volume metering, the particles are resuspended, and the MMPs and PMPs hybridize with DNAzyme at its two termini, forming the "MMPs-DNAzyme-PMPs" structure. When target metal ions are present, the DNAzymes are cleaved, yielding an increased number of free PMPs. All on-chip reactions are controlled by stopping the liquid flow at capillary valves and bursting it with hand-controlled tilting. Using the chip with a gravitational-magnetic separator, the free PMPs are separated from "MMPs-DNAzyme-PMPs" and accumulate into the trapping channel with a nozzle, forming a visual bar with growing distances proportional to the concentration of target metal ions. The achieved limit of detection (LOD) values for Cu²⁺ (103.1 nM), Pb²⁺ (69.5 nM), and Ag⁺ (793.6 nM) are below the maximum contamination levels. High selectivity of 100-fold, 200-fold, and 20-fold against interference is obtained. Moreover, by integrating three identical channels in parallel, simultaneous detection of the above-mentioned heavy metal ions in fresh and tap water samples is also achieved with high accuracy. Together, this fully integrated and easily operated platform embodies excellent potential for rapid, on-site sensing by unskilled users.