Toward the Detection Limit of Electrochemistry: Studying Anodic Processes with a Fluorogenic Reporting Reaction.

Recently, shot noise has been shown to be an inherent part of all charge-transfer processes, leading to a practical limit of quantification of 2100 electrons (≈0.34 fC) [ Curr. Opin. Electrochem. 2020, 22, 170-177]. Attainable limits of quantification are made much larger by greater background currents and insufficient instrumentation, which restricts progress in sensing and single-entity applications. This limitation can be overcome by converting electrochemical charges into photons, which can be detected with much greater sensitivity, even down to a single-photon level. In this work, we demonstrate the use of fluorescence, induced through a closed bipolar setup, to monitor charge-transfer processes below the detection limit of electrochemical workstations. During this process, the oxidation of ferrocenemethanol (FcMeOH) in one cell is used to concurrently drive the oxidation of Amplex Red (AR), a fluorogenic redox molecule, in another cell. The spectroelectrochemistry of AR is investigated and new insights on the commonplace practice of using deprotonated glucose to limit AR photooxidation are presented. The closed bipolar setup is used to produce fluorescence signals corresponding to the steady-state voltammetry of FcMeOH on a microelectrode. Chronopotentiometry is then used to show a linear relationship between the charge passed through FcMeOH oxidation and the integrated AR fluorescence signal. The sensitivity of the measurements obtained at different timescales varies between 2200 and 500 electrons per detected photon. The electrochemical detection limit is approached using a diluted FcMeOH solution in which no faradaic current signal is observed. Nevertheless, a fluorescence signal corresponding to FcMeOH oxidation is still seen, and the detection of charges down to 300 fC is demonstrated.

The in situ electrochemical detection of microbubble oscillations during motion through a channel.

Bubble oscillation has many applications, from driving local fluid motion to cleaning. However, in order to exploit their action, a full understanding of this motion, particularly in confined spaces (such as crevices etc. which are important in ultrasonic decontamination) is important. To this end, here we show how a Coulter counter can be used to characterize microbubbles produced through the ultrasonication of electrolytes. These microbubbles are shown to exist in relatively high concentrations while bubble activity is driven by ultrasound. Detection of these microbubbles, and their oscillatory behaviour, is achieved via translocation through a cylindrical glass microchannel (GMC). The microbubbles oscillate within the 40 µm channel employed and this behaviour is observed to change over the translocation period. This is attributed to the acoustic environment present or changes to the physical conditions in the interior of the chamber compared to the exterior. High-speed imaging confirms the presence of microbubbles as they move or 'skate' across the surface of the structures present before translocating through the channel. The observations are useful as they show that microbubble oscillation occurs within small structures, is preceded by surface confined bubbles and could be enhanced through pressure driven flow through a structure.

An Analytical Differential Resistance Pulse System Relying on a Time Shift Signal Analysis-Applications in Coulter Counting.

Improving the sensitivity and ultimately the range of particle sizes that can be detected with a single pore extends the versatility of the Coulter counting technique. Here, to enable a pore to have greater sensitivity, we have developed and tested a novel differential resistive pulse sensing (DiS) system for sizing particles. To do this, the response was generated through a time shift approach utilizing a "self-servoing regime" to enable the final signal to operate with a zero background in the absence of particle translocation. The detection and characterization of a series of polystyrene particles, forced to translocate through a cylindrical glass microchannel (GMC) by a suitable static pressure difference using this approach, is demonstrated. An analytical response, which scales with the size of the particles employed, was verified. Parasitic capacitive effects are discussed; however, translocations on the millisecond time scale can be detected with high sensitivity and accuracy using the approach described.