Aliaksei Boika

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.

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.

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

Significance The need for rapid, dependable, and sensitive detection of biological threats is ever increasing. Relatively arduous techniques, with varying degrees of sensitivity, exist for the detection of pathogens, including ELISA, electrogenerated chemiluminescence methods, sensitive PCR techniques, culturing, and microscopy. Here, we extend the observation of particle collisions on ultramicroelectrodes to murine cytomegalovirus. We further present an electrochemical technique for the specific detection of low concentrations of a virus by observing the effect of virus and antibody-specific polystyrene bead binding. This work, in principle, provides a framework for the detection of any biologically relevant antigen. 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.

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.

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.

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.

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.

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.

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.