Microfluidic particle accumulation for visual quantitation of copper ions.

A portable biosensor has been developed based on microfluidic particle accumulation for visual quantification of copper ions. A copper-dependent DNAzyme is used to connect magnetic microparticles (MMPs) and polystyrene microparticles (PMPs), forming "MMPs-DNAzyme-PMPs." When copper ions are present, the DNAzyme is cleaved, allowing free PMPs to be released from the MMPs-DNAzyme-PMP complex. Using a capillary-flow-based microfluidic device, the MMPs-DNAzyme-PMPs are first separated by a magnetic chamber, allowing the free PMPs to continue flowing until being trapped at a particle dam with a narrowing nozzle. Therefore, as a thermometer-like display, the copper level can be visually quantified by the accumulation length of the free PMPs in the trapping microchannel. The limit of detection (LOD) is 33 nM determined by the linear range of 25-100 nM, which is 900 times lower than the prevalent standard (~30 μM) in Hong Kong. The system shows excellent selectivity (> 1000-folds) against other heavy metal ions and abilities to adapt to multiple water environmental conditions. Tests on tap water samples and three local natural water sources in Hong Kong manifest that the device can effectively monitor the quality of freshwater with >70% recovery and 26.16% RSD.