Electrokinetic Manipulation of Silver and Platinum Nanoparticles and Their Stochastic Electrochemical Detection

On Practical Aspects of Single-Entity Electrochemical Measurements with Hot Microelectrodes

When a 10s-100s MHz frequency alternating current (ac) waveform is applied to a disk ultramicroelectrode (UME) in an electrochemical cell, one achieves what is known as a hot microelectrode, or a hot UME. The electrical energy generates heat in an electrolyte solution surrounding the electrode, and the heat transfer leads to formation of a hot zone with the size comparable to the electrode diameter. In addition to heating, ac electrokinetic phenomena generated by the waveform include dielectrophoresis (DEP) and electrothermal fluid flow (ETF). These phenomena can be harvested to manipulate the motion of analyte species and achieve significant improvements in their single-entity electrochemical (SEE) detection. This work evaluates various microscale forces observable with hot UMEs in relation to their utility to improve the sensitivity and specificity of the SEE analysis. Considering only mild heating (with a UME temperature increase not exceeding 10 K), the sensitivity of the SEE detection of metal nanoparticles and bacterial (Staph. aureus) species is shown to be strongly affected by the DEP and ETF phenomena. The conditions have been identified, such as the ac frequency and supporting electrolyte concentration, that can lead to orders-of-magnitude enhancement of the frequency of analyte collisions with a hot UME. In addition, even mild heating is expected to result in up to four times increase in the magnitude of blocking collisions' current steps, with similar outcomes expected for electrocatalytic collisional systems. The findings presented here are thought to provide guidance to researchers wishing to adopt hot UME technology for SEE analysis. With many possibilities still open, the future of such a combined approach is expected to be bright.

Single-Impact Electrochemistry in Paper-Based Microfluidics

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.

Catalytic Interruption Mitigates Edge Effects in the Characterization of Heterogeneous, Insulating Nanoparticles

Blocking electrochemistry, a subfield of nanochemistry, enables nondestructive, in situ measurement of the concentration, size, and size heterogeneity of highly dilute, nanometer-scale materials. This approach, in which the adsorptive impact of individual particles on a microelectrode prevents charge exchange with a freely diffusing electroactive redox mediator, has expanded the scope of electrochemistry to the study of redox-inert materials. A limitation, however, remains: inhomogeneous current fluxes associated with enhanced mass transfer occurring at the edges of planar microelectrodes confound the relationship between the size of the impacting particle and the signal it generates. These "edge effects" lead to the overestimation of size heterogeneity and, thus, poor sample characterization. In response, we demonstrate here the ability of catalytic current amplification (EC') to reduce this problem, an effect we term "electrocatalytic interruption". Specifically, we show that the increase in mass transport produced by a coupled chemical reaction significantly mitigates edge effects, returning estimated particle size distributions much closer to those observed using ex situ electron microscopy. In parallel, electrocatalytic interruption enhances the signal observed from individual particles, enabling the detection of particles significantly smaller than is possible via conventional blocking electrochemistry. Finite element simulations indicate that the rapid chemical kinetics created by this approach contributes to the amplification of the electronic signal to restore analytical precision and reliably detect and characterize the heterogeneity of nanoscale electro-inactive materials.

Hot-SWV: Square Wave Voltammetry with Hot Microelectrodes

A promising strategy to lowering detection limits in electrochemical analysis is the active modulation of the electrode temperature. Specifically, by tuning the electrode's surface temperature one can enhance detection limits due to improved electrode process kinetics and increased mass transfer rates, all without affecting the bulk solution. Motivated by this argument, here we report the development of a new electroanalytical technique based on electrode-temperature modulation, which we call hot square wave voltammetry (Hot-SWV). The technique utilizes the superposition of conventional SWV, already considered as one of the most sensitive voltammetric techniques, and a high frequency alternating current (ac) waveform to electrically polarize microelectrodes. By applying about 100 MHz ac frequencies (with varying V_rms amplitudes), our method generates an electrothermal fluid flow (ETF) in the electrolyte surrounding the electrode, thereby increasing the sensitivity of the SWV-based detection. We demonstrate this by investigating the oxidation of ferrocyanide and iron(II) ions, as well as the reduction of the coordination compound ruthenium(III) hexamine under various experimental conditions. We validate our experimental results against a theoretical model built using finite element analysis and observe agreement within ≤15% error at temperatures ≤39 °C. Using Hot-SWV, we observe at least one-order-of-magnitude improvement in the limit of detection of ferrocyanide ions relative to conventional, mm-size electrodes at 25 °C. In addition, we anticipate that Hot-SWV will be particularly useful for electroanalytical measurements of ultralow (≤pM) concentrations of analytes in environmental and biomedical applications.