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.

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.

Zwitterionic Ferrocenes: An Approach for Redox Flow Battery (RFB) Catholytes.

Herein we present two new ferrocene compounds Fc3 and Fc4 with, respectively, propyl and butyl zwitterionic side chains. These compounds are highly soluble in water (0.66 M for Fc3 and 2.01 M for Fc4). When paired with anthraquinone-2,7-disulfonate as the anolyte, these zwitterionic ferrocenes exhibit excellent performance under neutral aqueous conditions. Voltage and energy efficiencies were ca. 88%, and the Coulombic efficiency was over 99% for both high-concentration redox flow batteries. We observed a difference in stability between the lengths of the zwitterionic chains, with Fc4 showing higher stability than Fc3, and the capacity decreased by ∼5% at the end of 20 cycles (∼1% per day). Density functional theory calculations revealed striking differences in the conformational properties between Fc3 and Fc4, with Fc4 retaining a linear structure of the side chain in solution, while Fc3 favored both linear and curved geometries.

Highly Soluble Imidazolium Ferrocene Bis(sulfonate) Salts for Redox Flow Battery Applications.

Redox flow batteries (RFBs) are scalable devices that employ solution-based redox active components for scalable energy storage. To maximize energy density, new highly soluble catholytes and anolytes need to be synthesized and evaluated for their electrochemical performance. To that end, we synthesized a series of imidazolium ferrocene bis(sulfonate) salts as highly soluble catholytes for RFB applications. Six salts with differing alkyl chain lengths on the imidazolium cation were synthesized, characterized, and electrochemically analyzed. While aqueous solubility was significantly improved, the reactivity of the imidazolium cation and the increased viscosities of the salt solutions in water (which increase with increasing imidazolium chain length) limit the applicability of these materials to RFB design.

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 Vrms 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.

Targeted Delivery of Anti-inflammatory and Imaging Agents to Microglial Cells with Polymeric Nanoparticles.

Insult to the central nervous system (CNS) results in an early inflammatory response, which can be exploited as an initial indicator of neurological dysfunction. Nanoparticle drug delivery systems provide a mechanism to increase the uptake of drugs into specific cell types in the CNS such as microglia, the resident macrophage responsible for innate immune response. In this study, we developed two nanoparticle-based carriers as potential theranostic systems for drug delivery to microglial cells. Poly(lactic-co-glycolic) acid (PLGA)- and l-tyrosine polyphosphate (LTP)-based nanoparticles were synthesized to encapsulate the magnetic resonance imaging (MRI) contrast agent, gadolinium-diethylenetriaminepentaacetic acid (Gd[DTPA]), or the anti-inflammatory drug, rolipram. Robust uptake of both polymer formulations by microglial cells was observed with no evidence of toxicity. In mixed glial cultures, we observed a preferential internalization of nanoparticles by microglia compared to that of astrocytes. Moreover, exposure of our nanoparticles to microglial cells did not induce the release of the proinflammatory cytokines, tumor necrosis factor α (TNF-α), interleukin-1 ß (IL-1ß), or interleukin-6 (IL-6). These studies provide a foundation for the development of LTP nanoparticles as a platform for the delivery of imaging agents and drugs to the sites of neuroinflammation.

Hot-Tip Scanning Electrochemical Microscopy: Theory and Experiments Under Positive and Negative Feedback Conditions.

Hot-tip scanning electrochemical microscopy (HT-SECM) is a novel surface characterization technique utilizing an alternating current (ac) polarized disk microelectrode as a probe. A high-frequency (∼100 MHz) ac waveform applied between the tip and a counter electrode causes the resistive heating of the surrounding electrolyte solution that leads also to the electrothermal fluid flow (ETF). The effects of the temperature and the convection driven by the ETF result in the increased rate of mass transfer of the redox species. In this paper, HT-SECM was studied in positive and negative feedback modes, for which approach curves and cyclic voltammograms were recorded. The experimental data showed that the use of a hot tip leads to a more pronounced feedback compared to that at room temperature. Numerical simulations performed in COMSOL Multiphysics supported the experimental findings. Additional analytical approximations were developed that could be used to predict the faradaic response in HT-SECM experiments. Finally, a possible contribution to the current from the Soret effect was studied theoretically. A good understanding of HT-SECM was achieved, both experimentally and theoretically, suggesting that this methodology could be applied to investigate electrode kinetics under the conditions of elevated temperature and increased rate of mass transfer.

Electrochemical detection of a single cytomegalovirus at an ultramicroelectrode and its antibody anchoring.

We report observations of stochastic collisions of murine cytomegalovirus (MCMV) on ultramicroelectrodes (UMEs), extending the observation of discrete collision events on UMEs to biologically relevant analytes. Adsorption of an antibody specific for a virion surface glycoprotein allowed differentiation of MCMV from MCMV bound by antibody from the collision frequency decrease and current magnitudes in the electrochemical collision experiments, which shows the efficacy of the method to size viral samples. To add selectivity to the technique, interactions between MCMV, a glycoprotein-specific primary antibody to MCMV, and polystyrene bead "anchors," which were functionalized with a secondary antibody specific to the Fc region of the primary antibody, were used to affect virus mobility. Bead aggregation was observed, and the extent of aggregation was measured using the electrochemical collision technique. Scanning electron microscopy and optical microscopy further supported aggregate shape and extent of aggregation with and without MCMV. This work extends the field of collisions to biologically relevant antigens and provides a novel foundation upon which qualitative sensor technology might be built for selective detection of viruses and other biologically relevant analytes.

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

Electrokinetic phenomena such as dielectrophoresis and electrothermal fluid flow are used to increase the rate of mass transfer of silver and platinum nanoparticles and improve their stochastic electrochemical detection. These phenomena are induced by applying a high frequency alternating current (ac) waveform between a counter electrode and a working disk microelectrode. By recording chronoamperograms at room temperature and various ac powers, it is shown that the ac heating leads to an increase in the collision frequency of studied nanoparticles with working electrode surface by a factor of ∼101-103 as well as the increase in the magnitude of the measured faradaic response. It is suggested that the developed methodology could be used in the future to improve the detection of ultralow concentrations of various important bioanalytes.

Time of first arrival in electrochemical collision experiments as a measure of ultralow concentrations of analytes in solution.

In electrochemical collision experiments, the frequency of collisions of nanoparticles (NPs) with an ultramicroelectrode (UME) is a measure of the solution concentration of NPs. The time of first arrival is evaluated as a measure of ultralow (sub-femtomolar) concentration of analytes in solution. This is the time from the beginning of the experiment until the moment of observation of the first electrochemically detectable collision event. Theoretical equations are developed relating the time of the first arrival and the concentration of analyte species in solution for the cases when the species is transferred by diffusion alone and with electrophoretic migration. These equations are supported by experimental data. According to analysis of the results, the time of first arrival can be used successfully to estimate the order of magnitude of the analyte concentration with the precision of analysis being affected by the inherent stochasticity of the analyte movement and its initial position near the electrode. The use of the multiplexed parallel detection based on simultaneous measurement of a series of time of first arrival values will allow both faster and more precise determination of ultralow concentrations of analytes in solution.

Characterizing emulsions by observation of single droplet collisions--attoliter electrochemical reactors.

We report an electrochemical study of the collisions of single droplets in an emulsion by two methods. In the first method, an electroactive redox species, for example, ferrocene, inside a toluene-in-water emulsion droplet (but not in the continuous phase) is measured by chronoamperometry during a collision with an ultramicroelectrode (UME). Here, a blip or spike type of collision signal is observed, representing electrolysis of the droplet contents. In the second method, electrochemical oxidation of an electroactive redox species in the continuous aqueous phase is hindered by a droplet blocking collision. In this case, a staircase current decrease is observed. From an analysis of single soft particle collision data, one can find the emulsion droplet size distribution and the droplet contents.

Electrophoretic migration and particle collisions in scanning electrochemical microscopy.

We report for the first time how electrophoretic migration of ions and charged nanoparticles (NPs) in low electrolyte concentration solutions affects positive feedback in scanning electrochemical microscopy (SECM). The strength of the electric field in the gap between either the tip and the substrate, or the tip and counter electrodes, is shown to increase proportionally to the decrease in gap size. This field affects the flux of the charged redox species as expected for dilute electrolyte solutions. However, the shape of the normalized approach curve is unaffected by the electrophoretic migration. We also report that the rate of collisions of charged insulating NPs with the tip electrode decreases as the tip is brought closer to the substrate electrode. This rather unexpected result (negative feedback) can be explained by the blocking of the particle flux with the glass insulating layer around the metal microwires. Observation of simultaneous changes in the faradaic current at the tip and substrate electrodes due to particle collisions with the tip confirms a high rate of mass transport between the two electrodes under the conditions of positive feedback SECM.

Electrochemistry of high concentration copper chloride complexes.

High concentrations of copper chloride solutions (in the molar range) are used in several industrial applications. In this work, we investigated the species distribution of copper chloride complexes and how to measure the copper concentration precisely at high concentrations using electrochemical methods, by including migrational effects. The latter, in fact, can be useful in determining the nature of the species in solution undergoing electron transfer at the electrode. The study indicates that the main species of Cu(II) complexes in high chloride concentration is CuCl4(2-) and the main species of Cu(I) complexes are CuCl2(-) and CuCl3(2-). However insoluble CuCl is an intermediate in the process and can deactivate the electrode surface. This can be ameliorated by increasing the temperature or Cl(-) concentration. Under these conditions, voltammetry with an ultramicroelectrode (UME) can measure copper concentration with good precision even at 1 M Cu(II) concentrations in a few molar chloride. The main charge of the species can be determined by fitting to a migration model.

Ultrahigh frequency voltammetry: effect of electrode material and frequency of alternating potential modulation on mass transport at hot-disk microelectrodes.

Ultrahigh frequency voltammetry involves low scan rate voltammetric measurements with microelectrodes polarized by high-frequency large-amplitude alternating potential. The method provides a simple means for studying electrothermal and dielectrophoretic effects, which are important in micro and nanofluidic systems. The method also allows for indirect measurements of electrode impedance at gigahertz frequencies. This increases the upper frequency limit in impedance measurements about 1000 times. In this work we demonstrated, for the first time, that the effect of dielectric relaxation of water can be observed in a simple voltammetric experiment. The paper focuses on the description of electrothermal convection at ac heated disk microelectrodes as a function of frequency and provides a comparison of numerical simulations with experimental results.

Effect of large-amplitude alternating current modulation on apparent reversibility of electrode processes.

We examined the effect of a large-amplitude high-frequency alternating potential modulation on direct currents associated with irreversible, quasi-reversible, and reversible electron-transfer processes occurring at microelectrodes under voltammetric conditions. All irreversible processes appear to be accelerated by the superimposed ac modulation, and under certain conditions this may even lead to an electrochemical etching of noble metal electrodes. In the case of electrode processes which are reversible on the time scale of a dc polarization, but quasi-reversible on the time scale of the ac modulation, the distortion of voltammograms caused by the ac modulation can provide useful information about the kinetics of fast electron-transfer processes. For completely reversible electrode processes the effect of the large-amplitude ac modulation is essentially trivial; the distortion of voltammetric curves causes broadening of analytical signals without providing any useful information.

Dielectrophoretic and electrothermal effects at alternating current heated disk microelectrodes.

When a disk microelectrode is polarized with an alternating potential of very high frequency (0.1-2 GHz) and a high amplitude (up to 2.8 V rms), the electrode is heated up, and at the same time, a very intense electric field is created around the electrode (>10(6) V/m for electrodes 1 microm in radius). This strong electric field gives rise to positive or negative dielectrophoretic effects. Positive dielectrophoretic effects can be used to assemble nanowires from nanoparticles at the electrode edge. On the other hand, a negative dielectrophoretic effect is probably responsible for "jet boiling" observed at overheated microelectrodes. In addition, a combination of a high temperature gradient and a high potential gradient generates an intense electrothermal flow of solution which very strongly enhances the mass transport and is responsible for intense convection in such systems. The electrothermal flow and dielectrophoretic forces can be generated directly on a microelectrode employed in electrochemical detection because the high frequency ac polarization of the electrode does not interfere with the acquisition of analytical signals.

Monitoring the electrophoretic migration and adsorption of single insulating nanoparticles at ultramicroelectrodes.

The individual adsorption events of sub-µm silica and polystyrene spheres (310-530 nm in diam.) were detected by monitoring the blocking of redox mediator diffusion to Pt ultramicroelectrode (UME) substrates by the adsorbing spheres. Under the diffusion limited oxidation of FcMeOH and at low supporting electrolyte concentrations, the negatively charged spheres arrive at the electrode by electrophoretic migration. Sphere adsorption monitoring experiments consisted of long-time (1000-5000 s) chronoamperograms recorded in solutions with fM concentrations of spheres and different concentrations of supporting electrolyte. Trends in the heights of the step features with time reflect changing surface coverage of spheres, and coupled step features in the chronoamperograms suggest dynamic rearrangement of spheres on the surface. Numerical simulations of diffusion blocking at electrodes by adsorbing particles as well as mass transport of particles under migration were also performed, and show good agreement with the experimental data collected.