Visual quantitation of silver contamination in fresh water via accumulative length of microparticles in capillary-driven microfluidic devices

Silver is a heavy metal commonly used as bacteriostatic agents or disinfectants. However, excess amount of silver ion (Ag⁺) could lead to adverse biological effects on human health. To monitor silver ions in environmental samples, we report a visual quantitative method for analyzing the trace amount of Ag⁺. A sliver-specific RNA-cleaving DNAzyme Ag10C firstly makes the connection between magnetic microparticles (MMPs) and polystyrene microparticles (PMPs) forming a complex as “MMPs-Ag10C-PMPs”. When Ag⁺ is present, the Ag10C is cleaved, resulting in an increase of free PMPs. By dropping 3 μL of reacted particle solution to a capillary-driven microfluidic chip, MMPs and MMPs-Ag10C-PMPs are removed by a magnetic separator during the flow, while free PMPs can continue flowing until being trapped and accumulating at a particle dam with a narrow nozzle. The accumulation length of PMPs linearly increases with the increment of Ag⁺ concentrations in the range of 0–10 μM, and readable by the naked eye. We have achieved a limit of detection (LOD) down to 453.7 nM, which is significantly lower than the maximum contaminant level of 926 nM set by World Health Organization (WHO). More importantly, after validating the high selectivity against other metal ions and stable performance in different pH and water hardness, we demonstrate recovery rate >96.8% for tests of multiple fresh water sources, manifesting the feasibility in practical detection in real water samples.