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.
We describe the electrochemical detection of single nanoparticle (NP) attachment on a nanoelectrode by the increase in the active electrode area. The attachment of gold NP-decorated single wall carbon nanotubes (Au-SWCNTs) was observed by their current-time transients for ferrocenemethanol (FcMeOH) oxidation. Since the attached Au-SWCNT increases the electroactive area available for FcMeOH oxidation, the current increases after attachment of the particle. The "staircase" shape of the current response establishes that the particles do not become deactivated for the outer-sphere electron transfer reaction after attachment. Au-SWCNTs migrate to and are held at the nanoelectrode by an electric field. However, SWCNTs that are not decorated with a gold NP produce only a sharp transient ("blip") response.
The collision events
of single Lactococcus lactis bacteria
at Pt disk ultramicroelectrodes (UMEs) were characterized
using electrochemistry with correlated microscopy. A finite element model was developed which applied coupled
simulations of concentration and solution velocity to elucidate the
influence of electroosmotic flow on transport of bacteria near the
electrode. The model established that, in stochastic collision experiments
with steady-state oxidation at disk UMEs in low ionic strength solutions,
electroosmotic flow occurring at the glass insulation of the electrode
produces significant convection in the vicinity of the electrode disk
(velocities >50 μm/s). For L. lactis, the particle velocity due to convection driven by electroosmotic
flow exceeded that of electrophoresis at locations radial to the electrode
disk, leading to transport away from the electrode. Correlated microscopy
of collision experiments of L. lactis using a 5 μm radius Pt disk UME in 2 mM ferrocenemethanol
(FcM) with either 0.035 or 0.1 mM KCl confirmed that L. lactis experienced transport by convection due
to electroosmotic flow. Velocities of L. lactis extracted from correlated microscopy movies collected at the two
KCl concentrations agreed with the finite elements model. Current–time
(i–t) curves recorded during
the collision experiments showed transients that occurred when colliding L. lactis reduced transport of FcM to the electrode.
The current transients had step shapes when L. lactis collided and adsorbed and spike shapes when they collided and then
moved away from the electrode. By comparing the microscopy to simulations,
we concluded that the driving mechanism for the spike-shaped transients
was convection due to electroosmotic flow. Moreover, these findings
suggest that electroosmotic flow is significant for particle transport
in stochastic collision experiments in solutions of low ionic strength,
regardless of the analyte.
Self-assembled monolayers of 5,10,15,20-tetra(4-pyridyl)porphyrin (TPyP) were formed by equilibrium adsorption from a perchloric acid solution onto Au(111) using an interposed adlayer of bromide. The passivating bromide adlayer was generated by addition of 150 microM KBr to the electrochemical cell and allowed monolayer ordering at positive potentials where a disordered TPyP monolayer would be found on a bare Au(111) surface. The TPyP monolayers were characterized in situ with electrochemical scanning tunneling microscopy (EC-STM) and cyclic voltammetry. They were successfully observed at working electrode potentials between 0.0 and +1.3 V vs Ag/AgCl. This wide potential window of usability for the bromide adlayer extends to potentials more positive than what has been achieved for similar observations using iodide-modified Au(111). Within the TPyP monolayers, isolated domains with differing geometries could be distinguished, suggesting dynamic monolayer rearrangements. These results demonstrate that the presence of a passivating bromide adlayer is conducive to the formation of highly ordered organic monolayers. Indeed, bromide is not only one of the few anions that are suitable for this purpose, but it may be superior to the more frequently used iodide.
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