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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.

The adsorption events and movements of individual E. coli and B. subtilis bacteria were detected in real time using ultramicroelectrodes(UMEs). Electrochemical detection of single bacteria was enabled by their inhibition of diffusive flux of a redox species when near the UME. With the UME biased for steady state oxidation of the redox species, each bacteria–electrode collision event at the UME resulted in a transient in the oxidation current. The two bacteria species show differences in their collision transient shapes, with E. coli showing “step” shaped transients and B. subtilis showing predominantly “blip” (spike) shaped transients. Simultaneous fluorescence microscopy of the UME surface during bacteria detection experiments allowed each current transient in the electrochemical data to be correlated with a specific collision event of fluorescently labeled bacteria. It was established that step-shaped transients observed for E. coli were due to adsorption of bacteria to the UME surface, while blip-shaped transients observed for B. subtilis were due to short-lived bounce events of the bacteria. The frequency of recorded transients varied linearly over bacteria concentrations of 1–10 fM for E. coli and 15–60 fM for B. subtilis, establishing the blocking collision method as a route for in situ determinations of bacteria concentration. This work demonstrates for the first time differences in blocking collision transient shapes between two bacterial analytes, and suggests applications of the method for selective electrochemical bacteria sensing.

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.

Stabilization of the ( 3 × 22 ) reconstruction of Au(1 1 1) in halide solutions by adsorbed, uncharged organic molecules was investigated using cyclic voltammetry. In aqueous solutions containing either iodide, bromide, or chloride, the presence of α-cyclodextrin (α-CD), β-cyclodextrin (β-CD), or a ferrocene inclusion complex of the latter (Fc·β-CD) was found to shift the potential of the ( 3 × 22 ) to (1 × 1) transition positive by several tens of mV. This shift is explained by the inhibition of halide adsorption due to the need to displace adsorbed cyclodextrin species. The size of the shift follows the trend in the strength of interaction between the three halides and the Au surface. Changes to the CV shapes suggest an influence of the cyclodextrins on both the thermodynamics and kinetics of the reconstruction lifting event. Differences observed in CVs recorded with the three cyclodextrin adsorbates also demonstrate the utility of the potential dependent surface reconstruction processes of Au(1 1 1) as a probe of adsorbed species. Each halide/cyclodextrin combination was further examined by perturbing the electrode surface with potential cycling and subsequently recording CVs to monitor the recovery of the surface to an equilibrium state.

Self-assembled monolayers (SAMs) of 5,10,15,20-tetra(4-pyridyl)porphyrin (TPyP) and cobalt 5,10,15,20-tetra(4-pyridyl)-21H,23H-porphine (CoTPyP) were formed by in situ equilibrium adsorption onto Au(1 1 1) electrodes passivated by an interposed adlayer of iodine. Adsorption of the porphyrins onto Au(1 1 1) was confirmed by cyclic voltammetry, and each of the SAMs was imaged using electrochemical scanning tunneling microscopy (EC-STM). The TPyP and CoTPyP SAMs were found to contain highly ordered domains on I–Au(1 1 1) at positive potentials where a disordered SAM would be expected for bare Au(1 1 1). Cyclic voltammetry was also used to observe in situ the assembly of binary SAMs containing TPyP and CoTPyP as they formed.

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 report preparation of silver nanoparticles in the 10 nm size range by exhastive reduction of a well-known and well-characterized AgBr nanosol. These silver nanoparticles exhibit a second order hyperpolarizability tensor, β=(100±10) x 10 -30 esu per atom. The nanoparticles chmisorb a monolayer of iodide ions with concomitant bleaching of the surface plasmon absorption but no attenuation of Hyper-Rayleigh (second harmonic light) Scattering (HRS). Further addition of iodide leads to significant enhancement of HRS, which we attribute so surface-enhanced second harmonic scattering (SESHS) from iodide ion physisorbed on the AgI adlayer. A merocyanine dye, which does not exhibit HRS in solution, chemisorbs as a monolayer to the nanosilver, with formation of J-aggregates. These aggregates, like the free dye, are not active in second harmonic scattering. Further addition of dye leads to HRS enhancement, which we interpret as physisorption of dye onto the J-aggregate monolayer; the non-centrosymmetric surface environment allows observation of SESHS. A lower-limit estimate of the second order hyperpolarizability of the adsorbed dye is 200 X 10 -30 esu per molecule.