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Three-dimensional ring electrode arrays can provide means for highly-parallelized nanoparticle-impact electrochemistry within bulk solution.

Single-entity electrochemistry is a powerful technique to study the interactions of nanoparticles at the liquid-solid interface. In this work, we exploit Faradaic (background) processes in electrolytes of moderate ionic strength to evoke electrokinetic transport and study its influence on nanoparticle impacts. We implemented an electrode array comprising a macroscopic electrode that surrounds a set of 62 spatially distributed microelectrodes. This configuration allowed us to alter the global electrokinetic transport characteristics by adjusting the potential at the macroscopic electrode, while we concomitantly recorded silver nanoparticle impacts at the microscopic detection electrodes. By focusing on temporal changes of the impact rates, we were able to reveal alterations in the macroscopic particle transport. Our findings indicate a potential-dependent micropumping effect. The highest impact rates were obtained for strongly negative macroelectrode potentials and alkaline solutions, albeit also positive potentials lead to an increase in particle impacts. We explain this finding by reversal of the pumping direction. Variations in the electrolyte composition were shown to play a critical role as the macroelectrode processes can lead to depletion of ions, which influences both the particle oxidation and the reactions that drive the transport. Our study highlights that controlled on-chip micropumping is possible, yet its optimization is not straightforward. Nevertheless, the utilization of electro- and diffusiokinetic transport phenomena might be an appealing strategy to enhance the performance in future impact-based sensing applications.

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.

During the recent SARS-CoV-2 pandemic, lateral flow assays (LFAs) have written an indescribable story of success, as they provide means for decentralized, low-cost, and easy-to-use testing. However, LFAs in their most common form support only qualitative results and their sensitivity and limit of detection is significantly limited by the colorimetric readout method. In contrast, single-impact electrochemistry offers the possibility to quantify species beyond picomolar concentrations by recording individual species collisions with a biased microelectrode. Within this work, we investigate the integration of stochastic sensing into a LFA-architecture by combining a wax-patterned microchannel with a microelectrode array in order to detect silver nanoparticles by their oxidative dissolution. Here, we demonstrate the possibility to resolve individual nanoparticle collisions in a paper-based microchannel using a simplified reference-on-chip setup. Furthermore, we simulated a lateral-flow sensor, by flushing previously dried silver nanoparticles towards the electrode array, where the particles are subsequently detected. This proof-of-principle illustrates that single-impact electrochemistry might be a promising technique to extend the capability of LFAs. Especially, the integration of functionalized nanoparticle labels could enable the rapid and on-site detection of very dilute species with exceptional sensitivity.