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Single-particle electrochemistry has become an important area of research with the potential to determine the rules of electrochemical reactivity at the nanoscale. These techniques involve addressing one entity at the time, as opposed to the conventional electrochemical experiment where a large number of molecules interact with an electrode surface. These experiments have been made feasible  through the utilization of ultramicroelectrode (UMEs), i.e., electrodes with at least one dimension, e.g., diameter of 30 μm or less. This paper provides a theoretical and practical introduction to single entity electrochemistry (SEE), with emphasis on collision experiments between suspended NPs and UMEs to introduce concepts and techniques that are used in several SEE experimental modes. We discuss the intrinsically small currents, below 1 nA, that result from the electroactive area of single entities in the nanometer scale. Individual nanoparticles can be detected using the difference in electrochemical reactivity between a substrate and a nanoparticle (NP). These experiments show steady-state behavior of single NPs that result in discrete current changes or steps. Likewise, the NP can have transient interactions with the substrate electrode that result in current blips. We review the effect of diffusion, the main mass transport process that limits NP/electrode interactions. Also, we pointed out the implications of aggregation and tunneling in the experiments. Finally, we provid a perspective on the possible applications of single-element electrochemistry of electrocatalyst. 

Because of its large electrochemical window, acetonitrile (MeCN) is one of the most widely used solvents in electrochemistry. It is a suitable solvent for nonaqueous electrolytes that allows studies of cathodic and anodic processes, but electrolyte purification remains challenging. As received, the high-performance liquid chromatography (HPLC) grade is unsuitable for most electroanalytical applications. We present an approach to optimize the purification of HPLC-grade acetonitrile to yield a tetrabutylammonium perchlorate (TBAP)/MeCN electrolyte for experiments in nonaqueous media. We used cyclic voltammetry (CV) to show the background due to impurities and to guide the experimental design to a background current acceptable for CVs of a 1 mM typical concentration of a redox-active molecule. We use 3A molecular sieves, followed by distillation over CaH2 with a final treatment with Al2O3. The optimized procedure yields CH3CN with small background currents, increasing the signal-to-noise ratio and minimizing chemical complications over a wide potential window. Our approach includes discriminating between impurities in the solvent and electrolyte salts; for TBAP, we recrystallize from ethyl acetate and 95 % ethanol. The process and theoretical guidelines apply to other nonaqueous electrolytes dealing with electroactive impurities, including organic molecules, oxygen, and water. 

Temperature dependence studies of electrochemical parameters provide insight into electron transfer processes. In cases where adding excess electrolyte causes experimental complications, e.g., colloidal systems, organic or biological samples, it is preferable to deal with the high resistivity of the medium. We validate the use of unsupported and weakly-supported solutions in thermoelectrochemical experiments. The temperature dependence of the diffusion coefficient allows calibration of the steady-state current to measure changes when a continuous-wave (CW) ultraviolet laser,λ= 325 nm, illuminates an ultramicroelectrode (UME) from the front. Calibrating the steady-state current ratios, before and after heating with a thermostatic bath, allows temperature measurements within an accuracy of 0.6 K. The solutions are without supporting electrolytes in methanol, a volatile solvent, and we use a model that accurately describes the viscosity and temperature dependence of the solvent. We calculated the temperature and derived an equation to estimate the error in the temperature measurement. A numeric method yields satisfactory results, considering the changes for both diffusion coefficients and viscosity explicitly, and predicts the thermostatic temperature bath, agreeing with the theoretical model’s error. In unsupported solutions, the ferrocene diffusion coefficient and the iodide apparent diffusion coefficient follow the expected increase with temperature. Under CW laser illumination, the UME temperature increase is: ΔT = 4 ± 1 K. 

We demonstrate that colloidal quantum dots of CdSe and CdSe/ZnS are detected during the photooxidation of MeOH, under broad spectrum illumination (250 mW/cm 2 ). The stepwise photocurrent vs. time response corresponds to single entities adsorbing to the Pt electrode surface irreversibly. The adsorption/desorption of the QDs and the nature of the single entities is discussed. In suspensions, the QDs behave differently depending on the solvent used to suspend the materials. For MeOH, CdSe is not as stable as CdSe/ZnS under constant illumination. The photocurrent expected for single QDs is discussed. The value of the observed photocurrents, > 1 pA is due to the formation of agglomerates consistent with the collision frequency and suspension stability. The observed frequency of collisions for the stepwise photocurrents is smaller than the diffusion-limited cases expected for single QDs colliding with the electrode surface. Dynamic light scattering and scanning electron microscopy studies support the detection of aggregates. The results indicate that the ZnS layer on the CdSe/ZnS material facilitates the detection of single entities by increasing the stability of the nanomaterial. The rate of hole transfer from the QD aggregates to MeOH outcompetes the dissolution of the CdSe core under certain conditions of electron injection to the Pt electrode and in colloidal suspensions of CdSe/ZnS. 

We present the analysis of formaldehyde (HCHO) in anhydrous methanol (CH 3 OH) as a case study to quantify HCHO in non-aqueous samples. At higher concentrations (C > 0.07 M), we detect a product of HCHO, methoxy methanol (MM, CH 3 OCH 2 OH), by Fourier transform infrared spectroscopy, FTIR. Formaldehyde reacts with CH 3 OH, CD 3 OH, and CD 3 OD as shown by FTIR with a characteristic spectral feature around 1,195 cm −1 for CH 3 OH used for the qualitative detection of MM, a formaldehyde derivative in neat methanol. Ab initio calculations support this assignment. The extinction coefficient for 1,195 cm −1 is in the order of 1.4 × 10 2  M −1 cm −1 , which makes the detection limit by FTIR in the order of 0.07 M. For lower concentrations, we performed the quantitative analysis of non-aqueous samples by derivatization with dinitrophenylhydrazine (DNPH). The derivatization uses an aqueous H 2 SO 4 solution to yield the formaldehyde derivatized hydrazone. Ba(OH) 2 removes sulfate ions from the derivatized samples and a final extraction with isobutyl acetate to yield a 1:1 methanol: isobutyl acetate solvent for injection for electrospray ionization (ESI). The ESI analysis gave a linear calibration curve for concentrations from 10 to 200 µM with a time-of-flight analyzer (TOF). The detection and quantification limits are 7.8 and 26 μM, respectively, for a linear correlation with R 2 > 0.99. We propose that the formaldehyde in CH 3 OH is in equilibrium with the MM species, without evidence of HCHO in solution. In the presence of water, the peaks for MM become less resolved, as expected from the well-known equilibria of HCHO that favors the formation of methylene glycol and polymeric species. Our results show that HCHO, in methanol does not exist in the aldehyde form as the main chemical species. Still, HCHO is in equilibrium between the production of MM and the formation of hydrated species in the presence of water. We demonstrate the ESI-MS analysis of HCHO from a non-aqueous TiO 2 suspension in methanol. Detection of HCHO after illumination of the colloid indicates that methanol photooxidation yields formaldehyde in equilibrium with the solvent. 

We present evidence of the generation of radical ion formation during the oxidation of iodide on a fluorine doped tin oxide (FTO) electrode in acetonitrile. The cyclic voltammograms for the oxidation of iodide and triiodide on FTO are significantly different as in the case of the oxidation of Pt electrode.  These differences are assigned to kinetic differences on the FTO surface that require significant over potentials to drive the oxidation of iodide and triiodide. We propose that at the highly positive potentials the iodine radical intermediate, I·, becomes thermodynamically stable at FTO. The radical nature of the intermediate was verified by the formation of radicals of the usual traps of 5,5-dimethyl-1-pyrroline N-oxide (DMPO) and 2,2,5,5 tetramethyl-1-pyrroline N-oxide (TMPO) when these were added to an electrolyzed solution. Irradiation of an iodine solution causes the homolytic cleavage of I2 and yields the same radical intermediate with TMPO as in the electrolysis experiment. Similar results were obtained from the electrolysis of bromide solutions upon addition of TMPO. Short term electrolysis (< 1 h) gives triiodide as a final product while long-term electrolysis (> 17 h) yields additional byproducts. Byproducts were determined to be organoiodines by gas chromatography-mass spectrometry (GC-MS). Overall, our results are consistent with iodine atoms reacting with the electrolyte during electrolysis at the FTO electrode and with a sequential two-electron transfer process.