Room-temperature ionic liquids (ILs) have been proposed as alternative solvents for organic synthesis, separations, and electrochemical applications. Here, we report studies that probe the electrochemical and solvation properties of a tetraalkylammonium (methyltributylammonium bis(trifluoromethylsulfon)imide, M3BNIm) and an imidazolium (1-butyl-3-methylimidazolium hexafluorophosphate, BMIPF6) based ionic liquid. It is demonstrated that despite impurities, the cathodic limit at a Pt electrode is enhanced for the tetraalkylammonium-based IL. Electrogenerated chemiluminescence of tris(2,2‘-bipyrindinyl)ruthenium (Ru(bpy)3 2+) was observed in both ionic liquids, and differences in the response were interpreted in terms of the solvent reactivity and polarity. As ILs have been proposed as alternatives to organic solvents in extraction processes, an understanding of the relative lipophilicity of the IL ions and the equilibrium potential difference established across the IL/water interface is of fundamental relevance. Here, electrochemical measurements at a conventionally polarized liquid−liquid interface (water/1,2-dichloroethane) were used to determine the relative lipophilicity of the IL constituent ions. From formal ion transfer potential values (   ) obtained, the standard ionic partition coefficients could be estimated. The polarizability of the neat ionic liquid/water interface was investigated. From these studies, it can be seen that BMIPF6 is hydrophilic while M3BNIm is moderately hydrophobic. The significance of the potential difference established across the IL/water interface is discussed.

The dynamics of photoresponses associated with heterogeneous quenching of zinc tetrakis(carboxphenyl)porphyrin (ZnTPPC) and ferrocene derivatives at the water/1,2-dichloroethane interface were studied by intensity modulated photocurrent spectroscopy (IMPS). The contribution arising from the electron injection, recombinationÈproduct separation competition and the attenuation associated with the uncompensated resistance and interfacial capacitance (RC) time constant of the cell were deconvoluted in the frequency domain. The Ñux of electron injection was described in terms of a competition between the relaxation of the porphyrin excited state and the electron transfer step. Experimental results in the presence of ferrocene and diferrocenylethane conÐrmed that as the Galvani potential di †erence is increased, the phenomenological electron transfer rate constant increases and the ZnTPPC coverage at the liquid/liquid junction decreases. Furthermore, the recombination rate constant decreases with increasing potentials, while the product separation rate constant did not show a clear potential dependence. Photocurrent studies were extended to the electron donors dimethylferrocene and trianisylamine, as well as to the electron acceptor tetracyanoquinodimethane. The results obtained clearly indicate that the Gibbs energy of activation for the charge transfer process is a †ected by the Galvani potential di †erence. It is suggested that the electron transfer dynamics are dependent on the local electric Ðeld generation by the speciÐcally adsorbed ZnTPPC. The general expressions for the frequency dependent photocurrents at liquid/liquid interfaces are also introduced.

Scanning electrochemical microscopy (SECM) was used to investigate the kinetics of heterogeneous electron transfer (ET) as a function of driving force at the interface between two immiscible electrolyte solutions. At high driving force, experimental rate constants decreased with increasing overpotential, deviating from predictions based on Butler-Volmer kinetics. This decrease in ET rate with increasing driving force is consistent with Marcus theory inverted region behavior. At low driving force, the potential dependence of the forward and reverse ET rate constants followed Butler-Volmer theory. SECM is also demonstrated to be a useful means of studying the effect of high ionic strength on the kinetics of heterogeneous ET.

The electrochemical kinetics for the oxidation of ferrocenemethanol (FcCH 2 OH) over the whole composition range of dimethyl sulfoxide (DMSO)-water solutions of different viscosities (η) containing 50.0 mM (CH 3 ) 4 NClO 4 (TMAP) at a Pt microelectrode was studied using scanning electrochemical microscopy (SECM). The measured diffusion coefficient, D FcCH 2 OH , as well as the standard rate constant of the heterogeneous electron transfer, k 0 , as a function of solution composition, showed a minimum at about a DMSO molar fraction (x DMSO ) of 0.33, corresponding to the mixture with the maximum solution viscosity. The largest k 0 value found, 2.06 ( 0.31 cm s -1 in pure water (electrolyte) medium, was about 15 times larger than that obtained in the solution of x DMSO ) 0.33 (0.14 ( 0.02 cm s -1 ). A good linear correlation between ln k 0 and ln η was observed within the solution composition range of 0.10 e x DMSO e 0.60. An excellent linear correlation between ln k 0 and ln τ L , the longitudinal relaxation time, was also obtained with a slope equal to 1.0 when x DMSO ) 0-0.60. Unusually small rate constants found in the solutions of x DMSO g 0.70 were attributed to adsorption effects at the tip and the substrate electrode. The k 0 obtained for the present system was generally found to be inversely proportional to the viscosity of the solution and directly proportional to the diffusion coefficient of the electroactive species.

A technique that combines scanning electrochemical microscopy (SECM) and scanning optical microscopy (OM) was developed. Simultaneous scanning electrochemical/optical microscopy (SECM/OM) was performed by a special probe tip, which consists of an optical fiber core for light passage, surrounded by a gold ring electrode, and an outermost electrophoretic insulating sheath, with the tip attached to a tuning fork. To regulate the tip-substrate distance, either the shear force or the SECM tip current was employed as the feedback signal. The application of a quartz crystal tuning fork (32.768 kHz) for sensing shear force allowed simultaneous topographic, along with SECM and optical imaging in a constant-force mode. The capability of this technique was confirmed by obtaining simultaneously, for the first time, topographic, electrochemical, and optical images of an interdigitated array electrode. Current feedback from SECM also provided simultaneous electrochemical and optical images of relatively soft samples, such as a polycarbonate membrane filter and living diatoms in a constant-current mode. This mode should be useful in mapping the biochemical activity of a living cell.

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In vivo oxygen evolution above single stomata in Brassica juncea has been used to investigate, for the first time, the effect of Cd-induced stress as imaged by scanning electrochemical microscopy (SECM). SECM images showed a clear stomatal structure-a pore, whose aperture is modulated by two guard cells, serving as the conduit for the oxygen produced. Lower stomatal density and larger stoma size were found in plants treated with 0.2 mM CdCl2 compared with control plants. Either the introduction of Cd caused a slower cell replication in the plane of the epidermis, hence fewer stomata, and/or the number of open stomata was reduced when plants were under Cd-stress. Oxygen evolution above individual stomatal complexes in Cd-treated plants was lower than that from control plants, as determined from the electrochemical current above the middle of each stoma. All guard cells under illumination were swollen, indicating that the stomata were open in both control and treated plants. Thus, decreased oxygen evolution in response to Cd cannot be attributed to simple closing of the stomata, but to a lower photosynthetic yield. SECM provides an excellent tool for monitoring the effects of Cd on photosynthetic activity at the scale of individual stomata.

The interfacial photoreactivity of water soluble porphyrin ion pairs was explored by photocurrent measurements at the water/1,2-dichloroethane interface. The anionic zinc meso-tetrakis(p-sulfonatophenyl)porphyrin and cationic zinc tetrakis(N-methylpyridyl)porphyrin undergo spontaneous association in solution, leading to electrically neutral surface active heterodimer species. Heterogeneous quenching of the porphyrin ion pair was observed in the presence of the electron donor decamethylferrocene and the electron acceptor tetracyanoquinodimethane. No substantial photoresponses were detected in the presence of only one of the porphyrin species. Fundamental aspects on the dimer structure and photoreactivity were extracted from the photocurrent dependence on the concentration of both porphyrins. Photocurrent spectra as well as analysis of the association constant indicate that the porphyrin species are intimately associated as inner sphere ion pairs. Relevant issues in connection to back electron transfer, heterodimer adsorption, and orientation at the interface are also exposed.