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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.

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.