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Microfluidic paper-based analytical devices (μPADs) have experienced an unprecedented story of success. In particular, as of today, most people have likely come into contact with one of their two most famous examples─the pregnancy or the SARS-CoV-2 antigen test. However, their sensing performance is constrained by the optical readout of nanoparticle agglomeration, which typically allows only qualitative measurements. In contrast, single-impact electrochemistry offers the possibility to quantify species concentrations beyond the pM range by resolving collisions of individual species on a microelectrode. Within this work, we investigate the integration of stochastic sensing into a μPAD design by combining a wax-patterned microchannel with a microelectrode array to detect silver nanoparticles (AgNPs) by their oxidative dissolution. In doing so, we demonstrate the possibility to resolve individual nanoparticle collisions in a reference-on-chip configuration. To simulate a lateral flow architecture, we flush previously dried AgNPs along a microchannel toward the electrode array, where we are able to record nanoparticle impacts. Consequently, single-impact electrochemistry poses a promising candidate to extend the limits of lateral flow-based sensors beyond current applications toward a fast and reliable detection of very dilute species on site.

This work demonstrates a lateral flow assay concept on the basis of stochastic-impact electrochemistry. To this end, we first elucidate requirements to employ silver nanoparticles as redox-active labels. Then, we present a prototype that utilizes nanoimpacts from biotinylated silver nanoparticles as readouts to detect free biotin in solution based on competitive binding. The detection is performed in a membrane-based microfluidic system, where free biotin and biotinylated particles compete for streptavidin immobilized on embedded latex beads. Excess nanoparticles are then registered downstream at an array of detection electrodes. In this way, we establish a proof of concept that serves as a blueprint for future "digital" lateral flow sensors.