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This review describes work done in scanning electrochemical microscopy (SECM) since 2000 with an emphasis on new applications and important trends, such as nanometer-sized tips. SECM has been adapted to investigate charge transport across liquid/liquid interfaces and to probe charge transport in thin films and membranes. It has been used in biological systems like single cells to study ion transport in channels, as well as cellular and enzyme activity. It is also a powerful and useful tool for the evaluation of the electrocatalytic activities of different materials for useful reactions, such as oxygen reduction and hydrogen oxidation. SECM has also been used as an electrochemical tool for studies of the local properties and reactivity of a wide variety of materials, including metals, insulators, and semiconductors. Finally, SECM has been combined with several other nonelectrochemical techniques, such as atomic force microscopy, to enhance and complement the information available from SECM alone.

Highly oriented pyrolytic graphite (HOPG) is an important electrode material as a structural model of graphitic nanocarbons such as graphene and carbon nanotubes. Here, we apply scanning electrochemical microscopy (SECM) to demonstrate quantitatively that the electroactivity of the HOPG basal surface can be significantly lowered by the adsorption of adventitious organic impurities from both ultrapure water and ambient air. An SECM approach curve of (ferrocenylmethyl)trimethylammonium (FcTMA(+)) shows the higher electrochemical reactivity of the HOPG surface as the aqueous concentration of organic impurities, i.e., total organic carbon (TOC), is decreased from ∼20 to ∼1 ppb. SECM-based nanogap voltammetry in ∼1 ppb-TOC water yields unprecedentedly high standard electron-transfer rate constants, k(0), of ≥17 and ≥13 cm/s for the oxidation and reduction of the FcTMA(2+/+) couple, respectively, at the respective tip-HOPG distances of 36 and 45 nm. Anomalously, k(0) values and nanogap widths are different between the oxidation and reduction of the same redox couple at the same tip position, which is ascribed to the presence of an airborne contaminant layer on the HOPG surface in the noncontaminating water. This hydrophobic layer is more permeable to FcTMA(+) with less charge than its oxidized form so that the oxidation of FcTMA(+) at the HOPG surface results in the higher tip current and, subsequently, apparently narrower gap and higher k(0). Mechanistically, we propose that HOPG adsorbs organic impurities mainly from ambient air and then additionally from ∼20 ppb-TOC water. The latter tightens a monolayer of airborne contaminants to yield lower permeability.

A numerical model is developed for the SECM feedback mode for the case of irreversible electron transfer (ET) processes at the interface between two immiscible electrolyte solutions (ITIES). In this application, a redox-active species is electrogenerated by the reduction/oxidation of the oxidized/reduced form of a couple at an ultramicroelectrode (UME) tip located in one liquid (phase 1). The tip is positioned close to the interface with a second immiscible liquid (phase 2), that contains the oxidized/reduced half of another redox couple. If ET occurs between the tip-generated species in phase 1 and the redox-active species in phase 2, then the original species in phase 1 is regenerated at the interface and undergoes positive feedback at the tip, enhancing the steady-state current. The feedback current, for a given separation between the tip and the interface, is shown to depend on the ratio of the concentrations of the redox-active species in the two phases, their relative diffusion coefficients, and the rate constant for the redox reaction. The results of the model are used to identify the conditions under which (i) diffusion in phase 2 has to be considered and; (ii) a simpler limiting (constant composition) model for phase 2, employed to analyze earlier SECM experiments, can be used. In addition to diversifying the range of conditions under which redox reactions at ITIES can be studied, the results of the model demonstrate that there are considerable advantages to lifting the constant composition restriction on phase 2 for the accurate characterization of rapid redox reactions. The theoretical predictions are examined through experimental studies of electron transfer between the electrogenerated, oxidized form of zinc-21H, 23H-tetraphenylporphine (ZnPor) in benzene or benzonitrile and the reductants Fe(CN)6 4-, Ru(CN)6 4-, Mo(CN)8 4-, or FeEDTA2- (where EDTA denotes ethylenediaminetetraacetic acid) in an aqueous solution. Bimolecular rate constants for each of these systems are reported, with the potential across the ITIES biased with either perchlorate or tetrafluoroborate ions in each phase.

A highly sensitive analytical method is required for the assessment of nanomolar perchlorate contamination in drinking water as an emerging environmental problem. We developed the novel approach based on a voltammetric ion-selective electrode to enable the electrochemical detection of "redox-inactive" perchlorate at a nanomolar level without its electrolysis. The perchlorate-selective electrode is based on the submicrometer-thick plasticized poly(vinyl chloride) membrane spin-coated on the poly(3-octylthiophene)-modified gold electrode. The liquid membrane serves as the first thin-layer cell for ion-transfer stripping voltammetry to give low detection limits of 0.2-0.5 nM perchlorate in deionized water, commercial bottled water, and tap water under a rotating electrode configuration. The detection limits are not only much lower than the action limit (approximately 246 nM) set by the U.S. Environmental Protection Agency but also are comparable to the detection limits of the most sensitive analytical methods for detecting perchlorate, that is, ion chromatography coupled with a suppressed conductivity detector (0.55 nM) or electrospray ionization mass spectrometry (0.20-0.25 nM). The mass transfer of perchlorate in the thin-layer liquid membrane and aqueous sample as well as its transfer at the interface between the two phases were studied experimentally and theoretically to achieve the low detection limits. The advantages of ion-transfer stripping voltammetry with a thin-layer liquid membrane against traditional ion-selective potentiometry are demonstrated in terms of a detection limit, a response time, and selectivity.