Platinum Ultramicroelectrode Research Articles

Localized Surface Modification during Alternating Current Scanning Electrochemical Microscopy: Origin and Mechanism

In the current study, we report for the first time the observation of unintended localized surface modification on commercially pure aluminum (Al) during an alternating current scanning electrochemical microscopy (AC-SECM) analysis, its origin, and the probable mechanism responsible for it. Application of an AC perturbation potential (∼100 mV amplitude at ∼100 kHz frequency) to the Platinum ultramicroelectrode (Pt UME), during AC-SECM in acidic, neutral chloride, tap water, and alkaline electrolytes was found to cause surface modification on the scanned region of Al. An increase in the local pH of the electrolyte between the UME and the Al substrate, irrespective of the electrolyte pH (3–11) and UME biasing conditions, led to the local surface modification. The reason for the enhancement of local pH is attributed to the occurrence of higher rates of cathodic reduction reactions than that of anodic oxidation reactions. The reduction of dissolved oxygen/protons/water in the electrolytes led to the generation or consumption of OH−/H+ ions, respectively, and thus increased the pH, whereas the oxidation of Pt UME/Al surfaces decreased the pH with the generation of H+ or consumption of OH− ions. These results contribute significantly to accurately analyzing Al and its alloys using the AC-SECM technique.

Cytochrome c Oxidase Detection in Human Fibroblasts Using Scanning Electrochemical Microscopy

Cytochrome C Oxidase Deficiency (COXD) is a disorder caused by the mutation in genes encoding for the cytochrome c oxidase enzyme (COX),1 a mitochondrial multimeric enzyme which acts as the terminal electron acceptor in the electron transport chain.2 COXD causes serious health issues in affected individuals, including severe muscle weakness, brain damage and disorders in organs such as heart, liver and kidneys.1 Diagnosis is mostly performed by muscle biopsy,3,4 which is invasive and time-consuming. Finding new effective and early diagnostic strategies is therefore crucial to minimize symptoms and long-term disabilities. This research proposes the use of scanning electrochemical microscopy (SECM) as a promising method to quantify COX activity in normal and COXD fibroblasts as cell models. We explore the interaction of the redox probe N,N,N’,N’-tetramethyl-p-phenylene diamine (TMPD) with COX as an indicator of enzymatic activity.5,6 A platinum ultramicroelectrode was used to monitor electrochemical currents at different scan speeds, which were later used to determine an apparent heterogeneous rate constant (k 0) for TMPD regeneration by each cell line using numerical modelling. A significant difference in k 0 values (p < 0.1) was observed when comparing normal and COXD fibroblasts. The further treatment of normal fibroblasts with a COX inhibitor, NaN3,7 induced significant decreases in k 0 (p < 0.01). The COX activity results determined using SECM were in good agreement with a standard spectrophotometric assay. Taken together, our findings confirm SECM as an effective and rapid method to detect COXD in living tissues and provides a foundation for the development of new assays to diagnose COXD in infants. References M. Brischigliaro and M. Zeviani, Biochimica et Biophysica Acta (BBA) - Bioenergetics, 1862, 148335 (2021).A. Timón-Gómez et al., Semin Cell Dev Biol, 76, 163–178 (2018).M. L. Simard, A. Mourier, L. C. Greaves, R. W. Taylor, and J. B. Stewart, Journal of Pathology, 245, 311–323 (2018).S. B. Wortmann, J. A. Mayr, J. M. Nuoffer, H. Prokisch, and W. Sperl, Neuropediatrics, 48, 309–314 (2017).S. Kuss, E. E. L. Tanner, M. Ordovas-Montanes, and R. G. Compton, Chem Sci, 8, 7682–7688 (2017).S. Thind et al., Proc Natl Acad Sci U S A (2023). In press.M. J. Fei et al., Acta Crystallogr D Biol Crystallogr, 56, 529–535 (2000).

Effect of High Local Diffusive Mass Transfer on Acidic Oxygen Reduction of Pt Catalysis

In this study, we utilize a platinum ultramicroelectrode as a model platform for platinum electrocatalysts in acidic electrolytes to study the effects of local mass transfer on the oxygen reduction reaction (ORR), which plays a significant role in fuel cells with reduced platinum loading. Finite element simulations show that the UME exhibits size-dependent ultrathin diffusion layers during the electrochemical process. Submicron-scale UMEs can achieve ultrahigh localized mass transfer, which is unattainable through other experimental techniques. By conducting catalytic experiments under various mass transfer conditions, we find that the mass transfer limiting current is significantly lower than the value predicted by the four-electron process equation. Additionally, the apparent electron transfer number (napp) decreases as the mass transfer coefficient (m0) increases. Furthermore, as m0 increases, the half-wave potential shifts toward more negative values, allowing for the evaluation of the intrinsic activity of the catalysts over a broader potential range. Due to the UME technique’s capability to conveniently control local mass transfer, we anticipate its potential application in understanding the effects of chemical microenvironments on complex electrochemical reactions, including ORR and other processes.

Localised Electrochemical Impedance Spectroscopy of Gold Nanoparticles Labelled Antibodies Probed by Platinum Microstructured Ultramicroelectrode.

This research is focused on enhancing the capabilities of scanning electrochemical impedance microscopy (SEIM) for detecting gold nanoparticle-labelled antibodies using electrochemically modified platinum ultramicroelectrode. The primary objective was to address the high resistance issue encountered in previous measurements with SEIM via the utilization of SEIM probes based on micro-electrodes modified by platinum microstructures, which improved the sensitivity and precision of the detection of targeted biomolecules. The modified probe resulted in a lowered charge transfer resistance by over ten times and a decrease in detection to around 100 fg/mL. We suggest potential applications in various biotechnological and biomedical fields, with future research expected to further refine this technique.

Can Single Cell Respiration be Measured by Scanning Electrochemical Microscopy (SECM)?

Ultramicroelectrode (UME), or, equivalently, microelectrode, probes are increasingly used for single-cell measurements of cellular properties and processes, including physiological activity, such as metabolic fluxes and respiration rates. Major challenges for the sensitivity of such measurements include: (i) the relative magnitude of cellular and UME fluxes (manifested in the current); and (ii) issues around the stability of the UME response over time. To explore the extent to which these factors impact the precision of electrochemical cellular measurements, we undertake a systematic analysis of measurement conditions and experimental parameters for determining single cell respiration rates via the oxygen consumption rate (OCR) in single HeLa cells. Using scanning electrochemical microscopy (SECM), with a platinum UME as the probe, we employ a self-referencing measurement protocol, rarely employed in SECM, whereby the UME is repeatedly approached from bulk solution to a cell, and a short pulse to oxygen reduction reaction (ORR) potential is performed near the cell and in bulk solution. This approach enables the periodic tracking of the bulk UME response to which the near-cell response is repeatedly compared (referenced) and also ensures that the ORR near the cell is performed only briefly, minimizing the effect of the electrochemical process on the cell. SECM experiments are combined with a finite element method (FEM) modeling framework to simulate oxygen diffusion and the UME response. Taking a realistic range of single cell OCR to be 1 × 10-18 to 1 × 10-16 mol s-1, results from the combination of FEM simulations and self-referencing SECM measurements show that these OCR values are at, or below, the present detection sensitivity of the technique. We provide a set of model-based suggestions for improving these measurements in the future but highlight that extraordinary improvements in the stability and precision of SECM measurements will be required if single cell OCR measurements are to be realized.

Determination of trimetazidine in urine by capillary electrophoresis with amperometric detection

In this contribution, nonaqueous capillary electrophoresis with end-column amperometric detection using a platinum ultramicroelectrode is presented along with several extraction procedures, both liquid–liquid extraction (LLE) and solid-phase extraction (SPE), for extracting trimetazidine from urine. Trimetazidine is an anti-ischemic drug, which changes hearth metabolism pathways and is being abused as a doping. Electrochemical detection in a nonaqueous environment provided a stable response with a relative standard deviation of only 3.6% (n = 10) in repeatability measurement at concentration of 50 µg cm−3. LOD and LOQ of the proposed method were determined as 0.054 µg cm−3 and 0.180 µg cm−3, respectively. From the point of view of LLE, the most efficient procedure was the double extraction with ethyl acetate as an extraction agent in combination with prior alkalinization of the sample by Na2CO3. Nevertheless, the extraction efficiency was only around 68%. The most efficient SPE procedure was based on the combination of HLB cartridge and elution with background electrolyte containing 20% of methanol. Its recovery reached up to 92% and 101% in case of 50 µg cm−3 and 5.0 µg cm−3 of trimetazidine in urine, respectively.Graphical abstract

Single liquid aerosol nano-impact electrochemistry: accessing the droplet|air interface

Fabrication of a dual-barrel platinum ultramicroelectrode allows for the stochastic detection of a single liquid aerosol droplet. The new measurement technique has significant potential for studying the microdroplet|air interface using electrochemistry.

Observation and Analysis of Staircase Response of Single Palladium Nanoparticle Collision on Gold Ultramicroelectrodes.

Collision (or impact) of single palladium nanoparticles (Pd NPs) on gold (Au), copper (Cu), nickel (Ni), and platinum (Pt) ultramicroelectrodes (UMEs) were investigated via electrocatalytic amplification method. Unlike the blip responses of previous Pd NP collision studies, the staircase current response was obtained with the Au UME. The current response, including collision frequency and peak magnitude, was analyzed depending on the material of the UME and the applied potential. Adsorption factors implying the interaction between the Pd NP and the UMEs are suggested based on the experimental results.

Brain-Implantable Multifunctional Probe for Simultaneous Detection of Glutamate and GABA Neurotransmitters: Optimization and In Vivo Studies.

Imbalances in levels of glutamate (GLU) and gamma-aminobutyric acid (GABA) and their sub-second signaling dynamics occur in several brain disorders including traumatic brain injury, epilepsy, and Alzheimer’s disease. The present work reports on the optimization and in vivo testing of a silicon (Si) multifunctional biosensor probe for sub-second simultaneous real-time detection of GLU and GABA. The Si probe features four surface-functionalized platinum ultramicroelectrodes (UMEs) for detection of GLU and GABA, a sentinel site, and integrated microfluidics for in-situ calibration. Optimal enzyme concentrations, size-exclusion phenylenediamine layer and micro spotting conditions were systematically investigated. The measured GLU sensitivity for the GLU and GABA sites were as high as 219 ± 8 nA μM−1 cm−2 (n = 3). The measured GABA sensitivity was as high as 10 ± 1 nA μM−1 cm−2 (n = 3). Baseline recordings (n = 18) in live rats demonstrated a useful probe life of at least 11 days with GLU and GABA concentrations changing at the levels of 100′s and 1000′s of μM and with expected periodic bursts or fluctuations during walking, teeth grinding and other activities and with a clear difference in the peak amplitude of the sensor fluctuations between rest (low) and activity (higher), or when the rat was surprised (a reaction with no movement). Importantly, the probe could improve methods for large-scale monitoring of neurochemical activity and network function in disease and injury, in live rodent brain.

Absence of CO2 electroreduction on copper, gold and silver electrodes without metal cations in solution

The electrocatalytic reduction of carbon dioxide is widely studied for the sustainable production of fuels and chemicals. Metal ions in the electrolyte influence the reaction performance, although their main role is under discussion. Here we studied CO2 reduction on gold electrodes through cyclic voltammetry and showed that, without a metal cation, the reaction does not take place in a pure 1 mM H2SO4 electrolyte. We further investigated the CO2 reduction with and without metal cations in solution using scanning electrochemical microscopy in the surface-generation tip-collection mode with a platinum ultramicroelectrode as a CO and H2 sensor. CO is only produced on gold, silver or copper if a metal cation is added to the electrolyte. Density functional theory simulations confirmed that partially desolvated metal cations stabilize the CO2– intermediate via a short-range electrostatic interaction, which enables its reduction. Overall, our results redefine the reaction mechanism and provide definitive evidence that positively charged species from the electrolyte are key to stabilize the crucial reaction intermediate.

Brain-implantable multifunctional probe for simultaneous detection of glutamate and GABA neurotransmitters

Glutamate (GLU) and gamma-aminobutyric acid (GABA) are neurotransmitters (NTs) with an essential role in signal transmission in the brain. Brain disorders, such as epilepsy, Alzheimer's and Parkinson's diseases, and traumatic brain injury can be linked to imbalances in the GLU-GABA homeostasis that occurs in sub-second to seconds time frames. Current measurement techniques can detect these two NT concentrations simultaneously only in vitro. The present work reports on the fabrication of a silicon multifunctional biosensor microarray probe for sub-second simultaneous GLU-GABA detection in real-time, with excellent analyte sensitivity and selectivity and in vivo capabilities. The novel Si probes feature four surface-functionalized platinum ultramicroelectrodes (UMEs) for simultaneous amperometric detection of GLU and GABA with a sentinel, and a built-in microfluidic channel for the introduction of neurochemicals in the proximity of the UMEs. The microchannel also allows functioning of an On-Demand In-situ Calibrator that runs in-situ biosensor calibration. The probe exhibited excellent robustness at insertion in agarose-gel brain-tissue-mimicking test, and remarkably high hydrogen peroxide sensitivity (a by-product of GLU-GABA enzyme biosensor) with values on the order of 5000 nA μM -1 cm -2 and maximum sensitivities of 204±15 nA μM -1 cm -2 and 37±7 nA μM -1 cm -2 for GLU and GABA, respectively. Furthermore, the limit of detection of the biosensors reached as low as 7 nM, 165 nM and 750 nM for H 2 O 2, GLU and GABA, respectively and a temporal resolution of hundreds of milliseconds during in vivo studies using freely moving rats.

Electrocatalytic Metallic Nanostructures Prepared By Electrorefining and Cathodic Corrosion

Metallic nanostructures (NS) due to their unique properties are often used as electrocatalytic electrode materials in energy conversion and storage as well as in sensing applications. Their common methods of preparation involve homogeneous synthesis and electrodeposition from solutions containing appropriate precursor and often reducing agent and capping agent (surfactant). The main drawbacks of these methods are restricted yield, waste production and often intrinsic contamination of the surface of obtained NS. Moreover, surface capping ligands affect electrocatalytic activity by hindrance of reactant access to the active sites.We propose a versatile method of preparation of bare (non-capped) mono- and multi-metallic (Au,1 Cu,2 Ag, Fe) NS by localized electrorefining of polycrystalline raw metals at high efficiency in a controlled manner. Catalytic activities of obtained NS towards oxygen reduction reaction (ORR) in alkaline media, hydrogen evolution (HER) and carbon dioxide reduction (CO2RR) to easily electrooxidizable fuels (formic acid and carbon monooxide) were analyzed by feedback mode scanning electrochemical microscopy (SECM). The morphology and specific activity of obtained NS depend on used support material (indium tin oxide (ITO) and glassy carbon) and other electrorefining parameters such as electrodeposition potential, source velocity and the composition of the electrolyte. The last parameter determines stoichiometry of Faradaic processes. Both gold and copper electrooxidation and electroreduction can follow one electron stoichiometry. ORR in alkaline media and CO2RR occur effectively on NS with highly developed surfaces, containing large number of defects, corners and edges, whereas HER occurs efficiently at crystallographic Cu facets. Bare gold NS also exhibit outstanding plasmonic properties. They were applied as support for surface-enhanced Raman spectroscopy (SERS) of single molecules providing enhancement factor >106.1 Another method of preparation of metallic nanostructures is cathodic corrosion. This approach for platinum was studied by Koper group.3 We used this procedure for platinum ultramicroelectrode treatment. Obtained Pt NS exhibit unique catalytic properties towards electrooxidation of glucose.

A Chemometric-Assisted Voltammetric Method for Simultaneous Determination of Four Antioxidants in Biodiesel Samples

A methodology for direct and simultaneous determination of four phenolic antioxidants—butylated hydroxyanisole (BHA), butylated hydroxytoluene (BHT), propyl gallate (PG), and tert-butylhydroquinone (TBHQ)—in biodiesel samples using differential pulse voltammetry associated with partial least squares (PLS) regression is presented. Measurements were taken directly in a biodiesel:ethanol 1:1 (v/v) medium using a platinum ultramicroelectrode as the working electrode. The voltammetric signals of the compounds overlapped significantly, and to facilitate the simultaneous determination of the analytes, a PLS chemometric tool was applied and approaches fitting one compound at a time (PLS-1) and multiple compounds (PLS-2) both allowed the quantification of the individual concentrations, overcoming the strongly overlapped voltammograms. Results show that the PLS-2 model provides more accurate predictions than PLS-1. For the PLS-2 model, root mean square errors of prediction of 3 × 10, 16, 24, and 3 × 10 mg L–1 have been obtained between nominal and predicted concentrations for BHA, BHT, PG, and TBHQ, respectively. Recovery percentages were between 83 and 104%.

Electrochemical detection of single attoliter aqueous droplets in electrolyte-free organic solvent via collision events

We report the electrochemical detection of single water droplets in an electrolyte-free organic solvent by collision events using a platinum ultramicroelectrode (Pt-UME). Water droplet emulsions were prepared by the ultrasonication of an organic solvent (e.g., dichloroethane) and an aqueous solution containing hydroquinone (H2Q) as the redox species and MgSO4 as an emulsion stabilizer as well as the supporting electrolyte. Under the appropriate potentials, we could observe single water droplet collision phenomena at the Pt-UME by amperometric current-time (I-t) measurements. Water droplets that established contact with the Pt-UME led to amperometric current spikes due to the instantaneous electrolysis of the constituent redox species. The concentration of H2Q in the emulsion was adjusted to confirm that the amperometric spike resulted from the electrochemical reaction in the water droplet. As the concentration of H2Q increased, the oxidation peak current increased. The size and contact area of the water droplets were also estimated. Based on these single droplet collision results, we could successfully establish a facile water droplet detection system in an organic solvent without the addition of a hydrophobic organic electrolyte.

Electrodeposition of Co and Pd Onto Carbon Fiber and Platinum Ultramicroelectrodes

The synthesis of Co and Pd structures with tuned magnetic properties have scarcely been investigated from the electrochemical point of view [1-7]. Probably, this is due that, these properties are highly dependent on their shape, size and composition. In this sense, the use of ultramicroelectrodes (UME) allow to form a limited number of nuclei onto its surface, which is useful to analyse the electrochemical variables that modify the shape, size and specific composition of these particles [8-11]. In order to analyze the influence of the electrochemical variables during the synthesis of Co and Pd materials employing ultramicroelectrodes; in this work we conducted voltamperometric and chronoamperometric studies from two aqueous solutions. The first one containing PdCl2 0.01 M + NH4Cl 0.1 M and the second one CoCl2 0.01 M + NH4Cl 0.1 M. The working electrodes were a platinum UME (Pt-UME) of 10 μm of diameter and a carbon fiber UME (CUME) of 11 μm of diameter. A graphite bar was used as a counterelectrode and all potentials are reported against an Ag/AgCl electrode. The electrochemical experiments were controlled by mean a bipotentiostat UNISCAN M370. Fig. 1a shows a set of transients obtained in the potential range [-0.8 – -0.95] V applying a potential pulse during 32 s from the system Pt-UME/CoCl2 0.01 M + NH4Cl 0.1 M, while in Fig. 1b are depicted the transients obtained from the system Pt-UME /PdCl2 0.01 M + NH4Cl 0.1 M in the potential range [-0.4 – -0.7] V. Figure 1c and 1d, shows the transients obtained onto CUME for the case of Co and Pd, respectively. All the transients reported in Fig. 1 were compared with the theoretical ones predicted by the mathematical model proposed by Correia et al [12], by means a nonlinear fitting employing the Levenberg-Marquardt algorithm. In all cases, the kinetic parameters such as the nucleation rate (A) and the number of active nucleation sites (N0) were potential dependent. Also, the N0 values in each potential allowed us to simulate the AFM image associated to each potential considering a random distribution on the UME surface. [1] E.O. Berlanga-Ramírez et al., Phys. Rev. B, 70 (2004) 014410. [2] T. Sondon and J. Guevara, J. Mag. Mag. Mat. 272-276 (2004) e1247. [3] S. R. Brankovic, X. Yang, T. J. Klemmer, M. Seigler, IEEE T. Magn., 42(2) (2006) 132. [4] S. Ouazi, S. Vlaic S. Rusponi, G. Moulas, P. Buluschek, K. Halleux, S. Bornemann, S. Mankovsky, J. Minar, J.B. Staunton, H. Ebert, H. Brune. Nature Communications, 3 (2012) 1313 [5] M. Rezaei, S. Hadi, D. Fatmehsari, Journal of Materials Chemistry A, 1 (2014) [6] C. Xu, Y. Liu, J. Wang, H. Geng, H. Qiu, ACS Appl. Mater. Interfaces, 12 (2011) 4626. [7] F. Nasirpouri, S. M. Peighambari, A. S. Samardak, A. V. Ognev, E. V. Sukovatitsina, E. B. Modin, L. A. Chebotkevich, S. V. Komogortsev, S. J. Bending. Journal of Applied Physics 117, (2015) 17E715 [8] M. Peña, R. Celdran, R. Duo, Journal of Electroanalytical Chemistry, 367, (1994) 85. [9] C. Barin, A. Correia, L. Avaca, S. Machado, Journal of the Brazilian Chemical Society, 11 (2000) 175. [10] A. Kandory, H. Cattey, L. Saviot, T. Gharbi, J. Vigneron, M. Fregnaux, A. Etcheberry, G. Herlem, The Journal of Physical Chemistry C, 121 (2017) 1129. [11] A. Correia, S. Machado y L. Avaca, Journal of the Brazilian Chemical Society, 5 (1994) 173. [12] A. N. Correia, S. A. Machado, J. C. Sampaio, L. A. Avaca, Journal of Electroanalytical Chemistry, 407, (1996), 37-43. [13] C. T. Rueden, J. Schindelin, M. C. Hiner, BMC Bioinformatics, 18: 529 (2017) Figure 1

Visualization of Inhomogeneuous Reactivity on Battery Material Using Scanning Electrochemical Cell Microscopy

To understand the metal oxide coating effect on battery performance, the following two techniques are required: 1) constructing a flat thin-film electrode surface to realize a well-defined interface and 2) analyzing the electrode/electrolyte interface reaction with nanoscale resolution. We previously studied flat LiCoO2 thin-film electrodes using in situ surface-sensitive X-ray absorption spectroscopy (XAS) and reported that Co reduction at the LiCoO2 surface resulting from electrolyte contact caused the initial degradation. We also showed that the ZrO2 layer successfully prevented physical contact between LiCoO2 and the electrolyte. And it confirmed that a thicker ZrO2 layer (above 2 nm) increased the diffusion resistance of the lithium ions in the ZrO2 layer. However, since XAS lacks in-plane resolution and provides only averaged information, it is impossible to analyze the ZrO2 morphology in detail. Recently, Taguchi et al. investigated a thin Li-Zr-layer (ca. 2 nm) on a LiCoO2 composite electrode by transmission electron microscopy (TEM). They suggested that this thin layer could improve the durability. However, it is difficult to analyze the electrochemical properties using TEM. To evaluate the intrinsic mechanism of the metal oxide coating effect, it is necessary to develop a novel in-situ method that can analyze the surface morphology with high spatial resolution and simultaneously determine the local electrochemical properties. Scanning electrochemical microscopy (SECM) is a powerful technique for linking the surface morphology of a sample to its electrochemical properties. For the battery materials, the SECM feedback mode is effective in monitoring solid electrolyte interphase formation. To directly and quantitatively investigate spatially resolved ionic processes, mercury-capped platinum ultramicroelectrodes were developed and employed for Li+ imaging based on Li stripping. However, it is difficult to visualize the Li+ flux in battery materials at the sub-micrometer scale by SECM. Scanning electrochemical cell microscopy (SECCM), which uses a nanopipette as a probe and forms a local electrochemical cell, is effective in characterizing surface reactivity. We recently applied SECCM for visualization of electrochemical activities on a lithium-ion battery cathode material at sub-micrometer resolution. The SECCM was applied to collect or provide Li in specified area confined by the nanopipette. Further, it collection visualized the electrochemical properties by scanning the nanopipette as an image. There are some strong advantages in SECCM for battery material research such as its high spatial resolution, small capacitive current, and isolated electrochemical cell. In this report, we applied SECCM to characterize a ZrO2-coated LiCoO2 thin-film electrode prepared by pulsed laser deposition. Local cyclic voltammetry (CV) and galvanostatic charge/discharge were performed to characterize the cycle durability and rate performance of ZrO2-coated LiCoO2 thin-film electrodes and to reveal the relationship between the ZrO2 morphology and thickness.

Fine-Tuning Porosity and Time-Resolved Observation of the Nucleation and Growth of Single Platinum Nanoparticles.

Porous metal nanoparticles (NPs) are important to a variety of applications; however, robust control over NP porosity is difficult to achieve. Here, we demonstrate control over NP porosity using nanodroplet-mediated electrodeposition by introducing glycerol into water droplets. Porosity approached 0 under viscous conditions (>6 cP), and intermediate viscosities allowed the fine-tuning of NP porosity between 0 and 15%. This method also allowed for control over average pore radius (1 to 5 nm) and pore density (2 to 6 × 1015 pores per square meter). Reduced mass transfer within water droplets was validated by studying single chloroplatinate-filled water droplet (droplet radius of ∼450 nm) collisions on a platinum ultramicroelectrode (UME, rUME = 5 μm). Collision transient lifetimes in the i- t response increased with increasing viscosity, and the total charge per event was conserved. The change in shape was consistent with the nucleation and growth of a platinum NP within the droplet, which was confirmed by fitting transients to classical nucleation and growth theory for single centers as a function of over-potential. This analysis allowed electrokinetic growth and diffusion-controlled growth to be distinguished and semi-quantified at the single NP level.

Direct Observation of C2O4•- and CO2•- by Oxidation of Oxalate within Nanogap of Scanning Electrochemical Microscope.

Oxalate oxidation in the presence of different oxidized luminophores leads to the emission of light and has been studied extensively in electrogenerated chemiluminescence (ECL). The proposed mechanism involves the initial formation of the oxalate radical anion, C2O4•-. The ensuing decomposition of C2O4•- produces a very strong reductant, CO2•-, which reacts with the oxidized luminophores to generate excited states that emit light. Although the mechanism has been proposed for decades, the experimental demonstration is still lacking, because of the complexity of the system and the short lifetimes of both radical anions. To address these issues, we studied oxalate oxidation at platinum ultramicroelectrodes (UMEs) in anhydrous N, N-dimethylformamide (DMF) solution by nanoscale scanning electrochemical microscopy (SECM) with the tip generation/substrate collection (TG/SC) mode. A Pt nanoelectrode was utilized as the SECM generator for oxalate oxidation, while another Pt UME served as the SECM collector and was used to capture the generated intermediates. We studied the influence of the gap distance, d, on the substrate current ( is). The results indicate that, when 73 nm < d < 500 nm, the species captured by the substrate were primarily CO2•-, while C2O4•- was the predominant intermediate measured when d was below 73 nm. A half-life of 1.3 μs for C2O4•- was obtained, which indicates a stepwise mechanism for oxalate oxidation. The relevance of these observations to the use of oxalate as the coreactant in ECL systems is also discussed.

Direct and simultaneous determination of four phenolic antioxidants in biodiesel using differential pulse voltammetry assisted by artificial neural networks and variable selection by decision trees

A new methodology using differential pulse voltammetry and artificial neural network (ANN) for simultaneous determination of butylated hydroxyanisole (BHA), butylated hydroxytoluene (BHT), propyl gallate (PG) and tert-butylhydroquinone (TBHQ) in biodiesel samples is proposed. A platinum ultramicroelectrode (ume) was used as working electrode and measures were taken directly in biodiesel:ethanol medium without previous preparation. On this condition, detection limits for the antioxidants separately are 20.5, 32.4, 35.5 and 26.5 mg L−1 for BHA, BHT, PG and TBHQ, respectively. The artificial neural network model allowed the quantification of the individual concentrations overcoming the strongly overlapped voltammograms obtained for the mixture of the four antioxidants. For the model construction, a variable selection step through decision trees (DT) led to a reduction of prediction errors by 17.5%. The optimized DT-ANN model presented high correlation (0.97474, 0.99995, 0.98246 and 0.98928 for BHA, BHT, PG and TBHQ, respectively) between real and predicted values. Recovery percentages found were between 82.6% and 106.7%, except for two samples whose values were 76.0% and 114.7%. From the accuracy found between nominal and estimated concentration, it is inferred that the proposed methodology is a good alternative to quantify phenolic antioxidants in biodiesel samples.

Mechanism studies of hydrazine electro-oxidation by a platinum ultramicroelectrode: Effects of supporting electrolytes

The effects of the supporting electrolyte on electro-catalytic reaction of hydrazine hydrate (HH) were studied by a platinum ultramicroelectrode. We found that the H+ electric field generated during the catalytic process can partly hinder the reactants from participating in the catalytic reaction by using low concentration supporting electrolytes. It results in the final steady current value is less than the theoretical value. On the other hand, supporting electrolyte has a regulatory effect on both concentrations of N2H4 and N2H5+ (protonated HH) involved in catalytic reaction, which in turn affects the number of steady state currents and the value of each steady state current generated in the reaction. In this case, the effect of neutral salts did not change with concentration variations. By contrast, when different concentrations of sulfuric acid or sodium hydroxide are used as the electrolytes, the number of steady state currents and the value of each steady state current are mainly determined by the original and the later produced H+ concentration around the electrode; while the number of steady state current and the value of each steady state current are co-determined by the thermodynamic equilibrium and the thermodynamics of catalytic reaction of HH, and the H+ absorbability of solution with different concentrations and types of sodium phosphate buffer. Moreover, this work can also provide useful information for the choice of supporting electrolyte and catalytic potential in determination of HH concentration by amperometric method and in catalytic reaction with HH as the redox medium.