In Situ Measurement of the Size Distribution and Concentration of Insulating Particles by Electrochemical Collision on Hemispherical Ultramicroelectrodes

Electrochemical Behaviors of Ultramicro Triangular Pipettes

Ultramicro pipettes with circular orifices have practically become common probes in exploring the microscopic world, yet the versatility of differently shaped pipettes is undermined in the pore family. Herein, ultramicro triangular pipettes with a pseudotriangular-shaped orifice were fabricated by laser-pulling triangular quartz capillaries and characterized by microscopic and electrochemical methods. Then, the differences in the electrochemical behaviors of triangular and circular pores were revealed through experiments and simulations. A liquid/liquid interface was supported first at a triangular pipette, and the facilitated potassium ion transfer reactions exhibited steady-state voltammetric responses. An empirical equation for triangular pores between the diffusion-limited current and the side length (a) of the orifice was evaluated, and the corresponding mass transfer coefficient of ion ingress was estimated as mTri = 8.05D/a. As for ion egress transfer, the tip angle and pore shape would affect the diffusion regime and the ion distribution, where the mass transfer would be enhanced at the corners of a triangular pipette with a large tip angle. Triangular submicropores and nanopores were employed to detect particle translocations at a high capture rate, though the events had broader distributions and lower charges than those of circular pores. These findings demonstrate the shape effect in triangular pores that promotes the mass transfer rate of interfacial ion transfer reactions and the capture rate of ionic blockades. The universal laser-pulling method could make ultramicro pores of arbitrary shapes accessible in fundamental studies and applicable as chemical sensors.

Computer-Assisted Processing of Current Step Signals in Single Blocking Impact Electrochemistry

Current step signals related to single-entity collisions in blocking impact electrochemistry were analyzed by computer-assisted processing for estimating the size distributions of various particles. In this work, three different types of entities were studied by single blocking impact electrochemistry: polystyrene nanospheres (350 nm diameter) and microspheres (1 μm diameter), phospholipid liposomes (300 nm diameter) and two different strains of Gram-negative bacillus bacteria (Escherichia coli and Shewanella oneidensis). The size estimations of these different entities from the current step signal analysis were compared and discussed according to the shape and size of each entity. From the magnitude of the current step transient, the size distribution of each entity was calculated by a new computer program assisting in the detection and analysis of single impact events in chronoamperometry measurements. The data processing showed that the size distributions obtained from the electrochemical data agreed with the dynamic light scattering and atomic force microscopy data for nanospheres and liposomes. In contrast, the size estimation calculated from the electrochemical data was underestimated for microspheres and bacteria. We demonstrated that our computer program was efficient for detecting and analyzing the collision events in single blocking impact electrochemistry for various entities from spherical hard nanoparticles to micrometer-sized rod-shaped living bacteria.

Analyzing bacterial detection and transport using redox impact electrochemistry

Understanding bacterial transport dynamics, particularly at the single-particle level, is crucial across diverse fields from environmental science to biomedical research. In recent times, the emerging impact electrochemistry method offers a transformative approach for detection of bacteria at the single-particle level. The method employs the principle of single-entity electrochemistry to scrutinize electrochemical processes during interaction with the working electrode. In this study, we utilized redox impact electrochemistry to detect bacteria and analyze their transport processes towards the working electrode. Stochastic detection using redox reactions at the ultramicroelectrode enabled the detection of individual bacteria, with collision resulting in a current spike signal due to charge transfer. Notably, the detection of bacteria was demonstrated at an exceptionally low concentration (100 CFU/mL), with recorded current spikes reaching approximately 8.1 nA. Analysis of integrated areas under these spikes unveiled a diverse distribution of charge transfer at the ultramicroelectrode during redox reactions, implying variations in bacterial sizes, collision positions on the electrode surface, and redox activity among bacteria. Remarkably, the average charge transfer per bacterium between E. coli and the electrode was found to be (244 ± 24) pC, underscoring the intrinsic redox activity of the bacteria, equivalent to (2.52 ± 0.25) × 10-15 mol. Additionally, our investigation explored the effects of cell transport mechanisms, including diffusion, migration, convection, and settlement on stochastic interactions of the bacteria at the ultramicroelectrode. Through the collision frequency calculations, we found that migration is the primary factor shaping bacterial transport, with gravitational cell settlement also exerting a significant influence.

Polymer bead size revealed via neural network analysis of single-entity electrochemical data

Single-entity electrochemistry methods for detecting polymer microbeads offer a promising approach to analyzing microplastics. However, conventional methods for determining microparticle size face challenges due to non-uniform current distribution across the surface of a sensing disk microelectrode. In this study, we demonstrate the utility of neural network (NN) analysis for extracting the size information from single-entity electrochemical data (current steps). We developed fully connected regression NN models capable of predicting microparticle radii based on experimental parameters and current-time data. Once trained, the models provide near-real-time predictions with good accuracy for microparticles of the same size, as well as the average size of two different-sized microparticles in solution. Potential future applications include analyzing various bioparticles, such as viruses and bacteria of different sizes and shapes.

Average collision velocity of single yeast cells during electrochemically induced impacts

We recorded current-time (i-t) profiles for oxidizing ferrocyanide (FCN) while spherical yeast cells of radius (rc ≈ 2 μm) collided with disk ultramicroelectrodes (UMEs) of increasing radius (re ≈ 12-45 μm). Collision signals appear as minority steps and majority blips of decreased current overlayed on the i-t baseline when cells block ferrocyanide flux (JFCN). We assigned steps to adsorption events and blips to bouncing collisions or contactless passages. Yeast cells exhibit impact signals of long duration (Δt ≈ 15-40 s) likely due to sedimentation. We assume cells travel a threshold distance (T) to generate collision signals of duration Δt. Thus, T represents a distance from the UME surface, at which cell perturbations on JFCN blend in with the UME noise level. To determine T, we simulated the UME current, while placing the cell at increasing distal points from the UME surface until matching the bare UME current. T-Values at 90°, 45°, and 0° from the UME edge and normal to the center were determined to map out T-regions in different experimental conditions. We estimated average collision velocities using the formula T/Δt, and mimicked cells entering and leaving T-regions at the same angle. Despite such oversimplification, our analysis yields average velocities compatible with rigorous transport models and matches experimental current steps and blips. We propose that single-cells encode collision dynamics into i-t signals only when cells move inside the sensitive T-region, because outside, perturbations of JFCN fall within the noise level set by JFCN and rc/re (experimentally established). If true, this notion will enable selecting conditions to maximize sensitivity in stochastic blocking electrochemistry. We also exploited the long Δt recorded here for yeast cells, which was undetectable for the fast microbeads used in early pioneering work. Because Δt depends on transport, it provides another analytical parameter besides current for characterizing slow-moving cells like yeast.

The role of applied potential on particle sizing precision in single-entity blocking electrochemistry

Blocking electrochemistry, a subfield of single-entity electrochemistry, enables in-situ sizing of redox-inactive particles. This method exploits the adsorptive impact of individual insulating particles on a microelectrode, which decreases the electrochemically active surface area of the electrode. Against the background of an electroactive redox reaction in solution, each individual impacting particle results in a discrete current drop, with the magnitude of the drop corresponding to the size of the blocking particle. One significant limitation of this technique is "edge effects", resulting from the inhomogeneous flux of the redox species' diffusion due to increased mass transport to the edge of the disk electrode surface. "Edge effects" cause increased errors in size detection, resulting in poor analytical precision. Here, we use computational simulations to demonstrate that inhomogeneous diffusional edge flux of quasi-reversible redox species is mitigated at lowered overpotentials. This phenomenon is further illustrated experimentally by lowering the applied potential such that the system is operating under a kinetically-controlled regime instead of a diffusion-limited regime, which mitigates edge effects and increases particle sizing precision significantly. In addition, we found this method to be generalizable, as the precision enhancement is not limited to geometrically spherical particles but also occurs for cubic particles. This work presents a simple, novel methodology for edge effect mitigation with general applicability across different particle types.

When microplastics meet electroanalysis: future analytical trends for an emerging threat

Microplastics are a major modern challenge that must be addressed to protect the environment, particularly the marine environment. Microplastics, defined as particles ≤5 mm, are ubiquitous in the environment. Their small size for a relatively large surface area, high persistence and easy distribution in water, soil and air require the development of new analytical methods to monitor their presence. At present, the availability of analytical techniques that are easy to use, automated, inexpensive and based on new approaches to improve detection remains an open challenge. This review aims to outline the evolution and novelties of classical and advanced methods, in particular the recently reported electroanalytical detectors, methods and devices. Among all the studies reviewed here, we highlight the great advantages of electroanalytical tools over spectroscopic and thermal analysis, especially for the rapid and accurate detection of microplastics in the sub-micron range. Finally, the challenges faced in the development of automated analytical methods are discussed, highlighting recent trends in artificial intelligence (AI) in microplastics analysis.

Observing Discrete Blocking Events at a Polarized Micro- or Submicro-Liquid/Liquid Interface

Single-entity collisional electrochemistry (SECE), a subfield of single-entity electrochemistry, enables directly characterizing entities and particles in the electrolyte solution at the single-entity resolution. Blockade SECE at the traditional solid ultramicroelectrode (UME)/electrolyte interface suffers from a limitation: only redox-inactive particles can be studied. The wide application of the classical Coulter counter is restricted by the rapid translocation of entities through the orifice, which results in a remarkable proportion of undetected signals. In response, the blocking effect of single charged conductive or insulating nanoparticles (NPs) at low concentrations for ion transfer (IT) at a miniaturized polarized liquid/liquid interface was successfully observed. Since the particles are adsorbed at the liquid/liquid interface, our method also solves the problem of the Coulter counter having a too-fast orifice translocation rate. The decreasing quantal staircase/step current transients are from landings (controlled by electromigration) of either conductive or insulating NPs onto the interface. This interfacial NP assembly shields the IT flux. The size of each NP can be calculated by the step height. The particle size measured by dynamic light scattering (DLS) is used for comparison with that calculated from electrochemical blocking events, which is in fairly good agreement. In short, the blocking effect of IT by single entities at micro- or submicro-liquid/liquid interface has been proven experimentally and is of great reference in single-entity detection.

Utilizing the Oxygen Reduction Reaction in Particle Impact Electrochemistry: A Step toward Mediator-Free Digital Electrochemical Sensors

The current blockade particle impact method opens a route toward highly parallelized single-entity electrochemical assays. An important limitation is, however, that a redox mediator must be present in the sample, which can detrimentally interfere with molecular recognition processes. Dissolved O2 that is naturally present in aqueous solutions under ambient conditions can in principle serve as a suitable mediator via the oxygen reduction reaction (ORR). Here, we demonstrate the validity of this concept by performing current blockade experiments to capture and detect individual microparticles at Pt microelectrodes using solely the ORR. The readout modality is independent of the absolute O2 concentration, allowing operation under varying conditions. We further determine how the trajectories of individual microparticles are influenced by the combination of electrophoresis and electroosmotic flows and how these can be utilized to provide continuous detection of cationic particles in water for environmental monitoring.

Crystalline silica particle functionalized by PEG for its collision-enhanced detection at ultramicroelectrode

Detecting individual particulate matter is highly significant in many areas, such as mine safety, environment, and human health. The analytical method based on single entity electrochemistry (SEE) has shown great potential in detecting, counting, and measuring individual particles, especially conductive metals or carbon particles, based on their unique charge transfer reactions at an ultramicroelectrode (UME). In this study, we report an innovative SEE method for improving the sensitivity of the detection of electrochemical inert crystalline silica particles by functionalizing silica particles with polyethylene glycol (PEG) molecules. The PEG surface functionalization of the silica was characterized by Fourier-transform infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS) techniques. The morphology of silica particles was characterized by a scanning electron microscope (SEM), and a transmission electron microscope (TEM) was employed to calibrate size distribution and determine the elemental composition of silica particles. The surface charges of silica particles were measured by dynamic light scattering techniques. The collision behaviors of crystalline silica particles with UME were investigated by cyclic voltammetric experiments, which are rarely reported in the literature. The crystalline silica particles were detected based on electrochemically blocking the flux of the redox mediator at the surface of UME, which showed significant signal amplification in the proposed method. Our method was demonstrated for detecting crystalline silica functionalized with or without PEG, acquiring the limit of quantification (LOQ) values of 0.391 μM (23.45 μg/L) and 0.824 μM (49.45 μg/L), respectively, which confirmed that a more than two times improvement in LOQ could be achieved over the PEG functionalized silica particles. We further presented a theoretical model using finite element simulations with COMSOL Multiphysics. We deduced a quantitative relation between the distribution of the current step size and the size distribution of silica particles. Therefore, the reported method here provides a paradigm for SEE-based detection of electrochemically inert crystalline silica particles, which extends the previous report substantially concerning particle detection.

Mass Transport and Electron Transfer at the Electrochemical-Confined Interface

Single entity measurements based on the stochastic collision electrochemistry provide a promising and versatile means to study single molecules, single particles, single droplets, etc. Conceptually, mass transport and electron transfer are the two main processes at the electrochemically confined interface that underpin the most transient electrochemical responses resulting from the stochastic and discrete behaviors of single entities at the microscopic scale. This perspective demonstrates how to achieve controllable stochastic collision electrochemistry by effectively altering the two processes. Future challenges and opportunities for stochastic collision electrochemistry are also highlighted.

Trends in single-impact electrochemistry for bacteria analysis

Single-impact electrochemistry for the analysis of bacteria is a powerful technique for biosensing applications at the single-cell scale. The sensitivity of this electro-analytical method has been widely demonstrated based on chronoamperometric measurements at an ultramicroelectrode polarized at the appropriate potential of redox species in solution. Furthermore, the most recent studies display a continuous improvement in the ability of this sensitive electrochemical method to identify different bacterial strains with better selectivity. To achieve this, several strategies, such as the presence of a redox mediator, have been investigated for detecting and identifying the bacterial cell through its own electrochemical behavior. Both the blocking electrochemical impacts method and electrochemical collisions of single bacteria with a redox mediator are reported in this review and discussed through relevant examples. An original sensing strategy for virulence factors originating from pathogenic bacteria is also presented, based on a recent proof of concept dealing with redox liposome single-impact electrochemistry. The limitations, applications, perspectives, and challenges of single-impact electrochemistry for bacteria analysis are briefly discussed, based on the most significant published data.

Ring Ultramicroelectrodes for Current-Blockade Particle-Impact Electrochemistry

In current-blockade impact electrochemistry, insulating particles are detected amperometrically as they impinge upon a micro- or nanoelectrode via a decrease in the faradaic current caused by a redox mediator. A limit of the method is that analytes of a given size yield a broad distribution of response amplitudes due to the inhomogeneities of the mediator flux at the electrode surface. Here, we overcome this limitation by introducing microfabricated ring-shaped electrodes with a width that is significantly smaller than the size of the target particles. We show that the relative step size is somewhat larger and exhibits a narrower distribution than at a conventional ultramicroelectrode of equal diameter.

Transparent Ultramicroelectrodes for Studying Interfacial Charge-Transfer Kinetics of Photoelectrochemical Water Oxidation at TiO2 Nanorods with Scanning Electrochemical Microscopy

Scanning electrochemical microscopy (SECM) has been extensively applied to the electrochemical analysis of the surfaces and interfaces of a photoelectrochemical (PEC) system. A semiconductor photoelectrode with a well-defined geometry and active surface area comparable to SECM's tip is highly desired for accurately quantifying interfacial charge-transfer activities and photoelectrochemically generated redox species, where the broadening effects due to the mass transfer gradient and nonlocal electron transfer at a planar semiconductor surface can be minimized. Here, we present a newly developed platform as a SECM substrate for investigating semiconductor PEC activities, which is based on a transparent ultramicroelectrode (UME) fabricated by using two-step photolithographic patterning and ion milling methods. This transparent UME with a 25 μm recessed disk shape is fully characterized with SECM for quantifying the interfacial charge-transfer rates of IrCl₆^2-/IrCl₆^3- by comparing with theoretical results from finite element simulations in COMSOL Multiphysics. When coated with TiO₂ nanorods as a model semiconductor material, the transparent UME can be used to quantify the catalytic PEC water oxidation in a feedback mode of SECM by sampling tip and substrate current signals simultaneously. This transparent UME-SECM study provides insights into the potential-dependent PEC water oxidation reaction mechanism and the quantitative analysis of photocurrent contributions from water oxidation and the SECM tip-generated redox mediator. The transparent UME-SECM method can be potentially expanded to other SECM operation modes such as surface interrogation for understanding the dynamics of the electrode surfaces and interfaces of a PEC system.

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.

Detection and Characterization of Single Particles by Electrochemical Impedance Spectroscopy

We present an electrochemical impedance spectroscopy (EIS) technique that can detect and characterize single particles as they collide with an electrode in solution. This extension of single-particle electrochemistry offers more information than typical amperometric single-entity measurements, as EIS can isolate concurrent capacitive, resistive, and diffusional processes on the basis of their time scales. Using a simple model system, we show that time-resolved EIS can detect individual polystyrene particles that stochastically collide with an electrode. Discrete changes are observed in various equivalent circuit elements, corresponding to the physical properties of the single particles. The advantages of EIS are leveraged to separate kinetic and diffusional processes, enabling enhanced precision in measurements of the size of the particles. In a broader context, the frequency analysis and single-object resolution afforded by this technique can provide valuable insights into single pseudocapacitive microparticles, electrocatalysts, and other energy-relevant materials.

Stochastic Particle Approach Electrochemistry (SPAE): Estimating Size, Drift Velocity, and Electric Force of Insulating Particles

Stochastic particle impact electrochemistry (SPIE) is considered one of the most important electro-analytical methods to understand the physicochemical properties of single entities. SPIE of individual insulating particles (IPs) has been particularly crucial for analyses of bioparticles. In this article, we introduce stochastic particle approach electrochemistry (SPAE) for electrochemical analyses of IPs, which is the advanced version of SPIE; SPAE is analogous to SPIE but focuses on deciphering a sudden current drop (SCD) by an IP-approach toward the edge of an ultramicroelectrode (UME). Polystyrene particles (PSPs) with and without different surface functionalities (-COOH and - NH₃) as well as fixed human platelets (F-HPs) were used as model IPs. From theory based on finite element analysis, a sudden current drop (SCD) induced by an IP during electro-oxidation (or reduction) of a redox mediator on a UME can represent the rapid approach of an IP toward an edge of a UME, where a strong electric field is generated. It is also found that the amount of current drop, i_drop, of an SCD depends strongly on both the size of an IP and the concentration of redox electrolyte. From simulations based on the SPAE model that fit the experimentally obtained SCDs of three types of PSPs or F-HP dispersed in solutions with two redox electrolytes, their size distribution histograms are estimated, from which their average radii determined by SPAE are compared to those from scanning electron microscopic images. In addition, the drift velocity and corresponding electric force of the PSPs and F-HPs during their approach toward an edge of a Pt UME are estimated, which cannot be addressed currently with SPIE. We further learned that the estimated drift velocity and the corresponding electric force could provide a relative order of the number of excess surface charges on the IPs.

Electrochemical Collisions of Individual Graphene Oxide Sheets: An Analytical and Fundamental Study

We propose an analytical method based on electrochemical collisions to detect individual graphene oxide (GO) sheets in an aqueous suspension. The collision rate is found to exhibit a complex dependence on redox mediator and supporting electrolyte concentrations. The analysis of multiple collision events in conjunction with numerical simulations allows quantitative information to be extracted, such as the molar concentration of GO sheets in suspension and an estimate of the size of individual sheets. We also evidence by numerical simulation the existence of edge effects on a 2D blocking object.

Stochastic Electrochemical Cytometry of Human Platelets via a Particle Collision Approach

The quantitative analysis of human platelets is important for the diagnosis of various hematologic and cardiovascular diseases. In this article, we present a stochastic particle impact electrochemical (SPIE) approach for human platelets with fixation (F-HPs). Carboxylate-functionalized polystyrene particles (PSPs) are studied as well as a standard platform of SPIE-F-HPs. For SPIE-PSPs (or F-HPs), [Fe(CN)₆]^4- was used as the redox mediator, and electro-oxidation of [Fe(CN)₆]^4- to [Fe(CN)₆]^3- was conducted on a Pt ultramicroelectrode (UME) by applying a constant potential, where the corresponding oxidation current is mass-transfer-controlled. When PSPs (or F-HPs) are introduced into aqueous solution with [Fe(CN)₆]^4-, sudden current drops (SCDs) were observed, which resulted from the partial blockage of a Pt UME by collision of an individual PSP (or F-HP). For SPIE-PSPs (or F-HPs), we found that it is essential to enhance the migration of PSPs (F-HPs) toward a Pt UME by maximizing the steady state current associated with electro-oxidation of [Fe(CN)₆]^4-. This was accomplished by increasing its concentration to the solubility limit. We successfully measured the concentration of F-HPs dispersed in aqueous solution containing [Fe(CN)₆]^4- with a minimum detectable concentration of 0.1 fM, and the size distribution of F-HPs was also estimated from the obtained i_drop distribution based on the SPIE analysis, where i_drop stands for the magnitude of the current drop of each SCD. Lastly, we revealed that HPs without the fixation process (WF-HPs) are difficult to quantitatively analyze by SPIE because of their transient activation process, which results in changes from their spherical shape. The observed difficulty was also confirmed by finite element analysis, which shows that i_drop can be significantly increased, as an elongated WF-HP is adsorbed on the edge of an UME.

Viologen-Bromide Dual-Redox Ionic Solid Complexes: Understanding Their Electrochemical Formation and Proton-Accompanied Redox Chemistry

The inhibition of self-discharge in a redox-enhanced electrochemical capacitor (Redox-EC) is crucial for excellent energy retention. Heptyl viologen dibromide (HVBr₂) was chosen as a strong candidate of a dual-redox species in Redox-EC due to its solid complexations during the charging process, at which HV²⁺ is electrochemically reduced to HV^+• and form a solid complex, [HV^+•·Br^-], on an anode while Br^- is electro-oxidized to Br₃^- and renders [HV²⁺·2Br₃^-] on a cathode. The solid complexes could not transfer across the separator, resulting in significant diminution of the self-discharge. In this Article, we present detailed electrochemical studies of formation of [HV²⁺·2Br₃^-] and [HV^+•·Br^-], their redox features, and galvanic exchange reactions between the two types of dual-redox ionic solids on a Pt ultra-microelectrode (UME) in neutral (0.33 M Na₂SO₄) and acidic (1 M H₂SO₄) solutions. Most importantly, through voltammetric and particle-impact electrochemical analyses, we found that the redox and galvanic exchange reactions of the two dual-redox ionic solid complexes involve H⁺ transfer, which is the key process to limit the overall kinetics of the electrochemical reactions. We also rationalize the proton-accompanied galvanic exchange reaction based on computational simulation.