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.