Selection, characterisation and mapping of complex electrochemical processes at individual single-walled carbon nanotubes: the case of serotonin oxidation.

The electrochemical (EC) oxidation of the neurotransmitter, serotonin, at individual single-walled carbon nanotubes (SWNTs) is investigated at high resolution using a novel platform that combines flow-aligned SWNTs with atomic force microscopy, Raman microscopy, electronic conductance measurements, individual SWNT electrochemistry and high-resolution scanning electrochemical cell microscopy (SECCM). SECCM has been used to visualise the EC activity along side-wall sections of metallic SWNTs to assess the extent to which side-walls promote the electrochemistry of this complex multi-step process. Uniform and high EC activity is observed that is consistent with significant reaction at the side-wall, rather than electrochemistry being driven by defects alone. By scanning forward and reverse (trace and retrace) over the same region of a SWNT, it is also possible to assess any blocking of EC activity by serotonin oxidation reaction products. At a physiologically relevant concentration (5 µM), there is no detectable blocking of SWNTs, which can be attributed, at least in part, to the high diffusion rate to an individual, isolated SWNT in the SECCM format. At higher serotonin concentration (2 mM), oligomer formation from oxidation products is much more significant and major blocking of the EC process is observed from line profiles recorded as the SECCM meniscus moves over an SWNT. The SECCM line profile morphology is shown to be highly diagnostic of whether blocking occurs during EC processes. The studies herein add to a growing body of evidence that various EC processes at SWNTs, from simple outer sphere redox reactions to complex multi-step processes, occur readily at pristine SWNTs. The platform described is of general applicability to various types of nanostructures and nanowires.

A new approach for the fabrication of microscale lipid bilayers at glass pipets: application to quantitative passive permeation visualization.

A new method of planar bilayer lipid membrane (BLM) formation is presented that allows stable, solvent-free lipid bilayers exhibiting high seal resistances to be formed rapidly, easily and reproducibly. Using these bilayers the passive permeation of a series of carboxylic acids is investigated, to determine quantitatively the trend in permeability with lipophilicity of the acid. BLMs are formed at the tip openings of pulled theta pipets, and the rate of permeation of each carboxylic acid across the bilayer, from within the pipet into the bulk solution is determined. This is achieved through spatially-resolved measurements of the pH change that occurs upon the permeation of the weak acid, visualized using a pH-sensitive fluorophore with a confocal laser scanning microscope. The extracted fluorescence profiles are matched to finite element method (FEM) simulations, to allow the associated permeation coefficient for each weak acid to be determined with high accuracy, since this is the only adjustable parameter used to fit the experimental data. For bilayers formed in this way, the weak acids show increasing permeability with lipophilicity. Furthermore, the arrangement allows the effect of a trans-membrane electric field on permeation to be explored. For both propanoic and hexanoic acid it is found that an applied electric field enhances molecular transport, which is attributed to the formation of pores within the membrane.

High resolution mapping of oxygen reduction reaction kinetics at polycrystalline platinum electrodes.

The scanning droplet-based technique, scanning electrochemical cell microscopy (SECCM), combined with electron backscatter diffraction (EBSD), is demonstrated as a powerful approach for visualizing surface structure effects on the rate of the oxygen reduction reaction (ORR) at polycrystalline platinum electrodes. Elucidating the effect of electrode structure on the ORR is of major interest in connection to electrocatalysis for energy-related applications. The attributes of the approach herein stem from: (i) the ease with which the polycrystalline substrate electrode can be prepared; (ii) the wide range of surface character open to study; (iii) the possibility of mapping reactivity within a particular facet (or grain), in a pseudo-single-crystal approach, and acquiring a high volume of data as a consequence; (iv) the ready ability to measure the activity at grain boundaries; and (v) an experimental arrangement (SECCM) that mimics the three-phase boundary in low temperature fuel cells. The kinetics of the ORR was analyzed and a finite element method model was developed to explore the effect of the three-phase boundary, in particular to examine pH variations in the droplet and the differential transport rates of the reactants and products. We have found a significant variation of activity across the platinum substrate, inherently linked to the crystallographic orientation, but do not detect any enhanced activity at grain boundaries. Grains with (111) and (100) contributions exhibit considerably higher activity than those with (110) and (100) contributions. These results, which can be explained by reference to previous single-crystal measurements, enhance our understanding of ORR structure-activity relationships on complex high-index platinum surfaces, and further demonstrate the power of high resolution flux imaging techniques to visualize and understand complex electrocatalyst materials.

Mapping nanoscale electrochemistry of individual single-walled carbon nanotubes.

We introduce a multiprobe platform for the investigation of single-walled carbon nanotubes (SWNTs) that allows the electrochemical response of an individual SWNT to be mapped at high spatial resolution and correlated directly with the intrinsic electronic and structural properties. With this approach, we develop a detailed picture of the factors controlling electrochemistry at SWNTs and propose a definitive model that has major implications for future architectures of SWNT electrode devices.

Electrochemical mapping reveals direct correlation between heterogeneous electron-transfer kinetics and local density of states in diamond electrodes.

Conducting carbon materials: a multi-microscopy approach shows that local heterogeneous electron-transfer rates at conducting diamond electrodes correlate with the local density of electronic states. This model of electroactivity is of considerable value for the rational design of conducting diamond electrochemical technologies, and also provides key general insights on electrode structure controls in electrochemical kinetics.

Trace voltammetric detection of serotonin at carbon electrodes: comparison of glassy carbon, boron doped diamond and carbon nanotube network electrodes.

The characteristics of three different carbon electrodes, glassy carbon (GC), oxygen-terminated polycrystalline boron-doped diamond (pBDD) and "pristine" carbon nanotube networks (CNTN) as voltammetric sensors for detection of the neurotransmitter serotonin have been investigated. For each electrode, detection sensitivity was determined using cyclic voltammetry (CV), a technique often used to provide information on chemical identity in electrochemical assays. The CNTN electrodes were found to exhibit background current densities ca. two orders of magnitude smaller than the GC electrode and ca. twenty times smaller than pBDD, as a consequence of their "pristine" low capacitance and low surface coverage. This was a major factor in determining serotonin detection limits from CV, of 10 nM for the CNTN electrode, 500 nM for pBDD and 2 microM for GC. The two most sensitive electrodes (CNTN and pBDD) were further investigated in terms of resistance to electrode fouling. CV analysis showed that fouling was less on the pBDD electrode compared to the CNTN and, furthermore, for the case of pBDD could be significantly minimised by careful selection of the CV potential limits, in particular by scanning the electrode potential to suitably cathodic values after oxidation of the serotonin.