Local pH Value Research Articles

An Ordinary Differential Equation Model for Simulating Local-pH Change at Electrochemical Interfaces

The local pH value at an electrochemical interface (pHs) inevitably changes during redox reactions involving the transfer of H+ or OH− ions. It is important to quantitatively estimate pHs during polarization, as this parameter has a significant impact on the activity and selectivity of electrochemical reactions. Numerical simulation is an effective means of estimating pHs because it is not subject to experimental constraints. As demonstrated in a number of studies, pHs can be estimated by solving partial differential equations that describe diffusion process. In the present work, we propose a method to consider the process by using ordinary differential equations (ODEs), which can significantly reduce the computational resources required for estimating pHs values. In the ODE-based model, the description of the diffusion process was achieved by considering the reaction plane in the diffusion layer over which the H+ and OH− concentrations are balanced while assuming that the concentration profiles in the layer are in a steady state. The resulting model successfully reproduces experimental voltammograms characterized by local pH changes in association with the hydrogen evolution and hydrogen peroxide reduction reactions.

The Interplay of Nanoconfinement and pH from the Perspective of a Dye-Reporter Molecule

A novel thiazol-based ratiometric dye for the detection of local pH values is synthesized, and its properties are characterized by a combination of optical spectroscopy, solid-state NMR and DNP (dynamic nuclear polarization)-enhanced solid-state NMR. This novel dye covers a completely different sensitivity range with its acidic pKa value of 3.5 compared to other established dyes for ratiometric pH detection, such as SNARF. The dye is grafted to the surfaces of mesoporous silica materials, which enables, for the first time, direct in situ measurements of the local pH values in silica mesopores by a simple UV-vis spectroscopy method. The obtained results, which are in good agreement with previous indirect techniques, indicate a background electrolyte-dependent pKa shift of at least one pH unit under nanoconfined conditions compared to the pKa of the dye in bulk solution.

Responsive Colloidosomes with Triple Function for Anticorrosion.

Strategies for corrosion protection are required to prolong the life span of metallic structures used by the construction, aerospace, and transport industries. Currently, there are no coatings that can provide at the same time information about the corrosion status of the coated metal and protect the metal against corrosive species and mechanical damage. Herein, triple-functional microcarriers with functions of corrosion sensing, self-healing, and corrosion inhibition are produced and embedded in coatings to prolong the lifetime of metals and enhance the anticorrosion performance of coatings. The microcarriers are prepared by creating Pickering droplets loaded with a corrosion inhibitor and a healing agent and stabilized by silica nanocapsules containing thymol blue as corrosion sensor. The microcarriers are then embedded in a water-based polymer matrix coated on metal substrates. When the coating or metal is mechanically damaged, the healing agent is released from the droplets to hinder further corrosion of the metal. When the local pH value near the metal surface is changing by the generation of hydroxide ion due to the corrosion process, a change of color is detected as well as a release of corrosion inhibitor, leading to a significant decrease of corrosion rate of the coated metal.

Clarifying the influence of albumin on the initial stages of magnesium corrosion in Hank's balanced salt solution

Abstract The corrosion behavior of Mg, a promising biodegradable metallic material, in protein-containing pseudophysiological environment is still not fully understood. In this work, the influence of albumin on the corrosion behavior of commercially pure magnesium (CP Mg) is investigated during short-term tests in Hank's Balanced Salt Solution (HBSS). This work focuses on the reactions at the Mg/medium interface from the perspective of the interactions among albumin, media components and substrate. Hydrogen evolution tests demonstrate that the physiological amount of albumin (40 g L−1) accelerates Mg corrosion in HBSS during the first few hours but slows down the degradation afterwards. The presence of albumin decreases the concentration of free Ca2+ in HBSS and delays formation of protective co-precipitation products on Mg surface. The evolution of local pH (differs from typically monitored bulk pH) near Mg/medium interface in albumin-containing HBSS is reported for the first time. Comparison of local and bulk pH values elucidates the pH buffering effect of albumin during the immersion period. Based on these results, adsorption, chelating and pH buffering effects are summarized as three important aspects of albumin influence on Mg corrosion. Additionally, constant replenishment of medium components is shown to be an influential factor during the Mg corrosion tests.

Three Birds with One Stone: Injectable CaC2 Nanobombs with Triple Effects for Minimally Invasive Tumor Chemical Ablation.

Percutaneous chemical ablation (PCA) is the oldest and most established technique for treating small solid tumors in organs. It has been widely used in clinics even on an outpatient basis. However, compared with the emerging microwave or magnetic hyperthermal ablation, PCA is faced with relatively poor necrosis results and needs to repeat multiple sessions. Inspired by the three effects in the bomb's explosive process, we herein expect to combine calcium carbide (CaC2) nanoparticles into the PCA technique to generate local explosion within tumor tissues, leading to three killing effects against tumors to further improve the ablation efficacy of PCA. Through an efficient wet milling procedure with poly(ethylene glycol), three kinds of nanobombs including CaC2, calcium oxide (CaO), and calcium hydroxide (Ca(OH)2) were fabricated, and they all exhibited desirable suspension stability. Among these nanobombs, in particular CaC2 nanobombs showed a synergistic effect that the generation of ethyne gas bubbles could facilitate the most rapid diffusion of hyperthermia. Also, CaC2 nanobombs offered the powerful ability to cause the sudden rise of local high temperature and pH value. According to the in vivo mice tumor excision trial, the tumors of 75% of cases that received CaC2 treatment were destroyed and eradicated, exhibiting the excellent ablation ability of CaC2 nanobombs against small solid tumors planted in mice.

Assessing the Corrosion Pattern of Magnesium Alloy Implants in Simulated Salivary Fluids at Different pH

Magnesium (Mg) alloy implants are formidable structural metals for biomedical applications, they are limited due to Mg chemical reactivity and its susceptible to corrosion. Efficient mitigation control toward Mg corrosion can only be proposed if its degradation kinetics are properly understood in various media. In the biomedical field, the auto-controlled degradation mechanism of orthopedic Mg implants eliminates repeated surgeries while also minimizing the risks associated with these medical procedures. However, their rapid degradation kinetics between living tissues contribute to sudden changes in local pH value, accumulation of unwanted hydrogen gas in bone cavities and subsequent mechanical failure. In this work, we have investigated the degradation pattern of Mg alloy implant in chloride-enriched simulated salivary fluids at different pH at room temperature. The implant demonstrated varying rates of corrosion and passivation within a range of pH of corrosion media. The effects of solution treatment on implant corrosion behavior was vividly studied and the trend of increasing corrosion was low pH > saline pH > high pH. Localized corrosion behavior has been observed by scanning electron microscopy evidence while electrochemical impedance spectroscopy and potentiodynamic polarization techniques revealed increased anodic polarization and charge transfer, respectively. The chemistry and diffraction phases of adhering corrosion products as well as their surface morphologies was also been identified. The implant must have uniformly corroded within the salivary fluids over an extended duration; however, the early corrosion stages reveal localized passivation of oxide films at grain boundaries at low pH.Most metallic implants made of corrosion-resistant alloys (e.g., cobalt, titanium and nickel) are excellent candidates for oral surgeries. However, most of them cannot serve as bone cements to stabilize dental implants during post-surgery tissue healing; a function that is possible with magnesium-based bone cement [1,2]. The use of these metal-type temporary implants in binding bone structures normally will require a secondary surgery operation to remove them, and this might lead to potential infections. This study features the analyses of the corrosion patterns of Mg alloy implants deployed in orthopedic applications. The salivary pH changes induce corrosion toward septic loosening prior to Mg implant degradation and dental implant infections.

Nanosized Iron Oxyhydroxide: Properties, Application, Preparation

Nanoparticles are increasingly in demand in such rapidly developing types of human activity as microelectronics, catalysis, biosensors, etc. The mild neutralization process is desirable to obtain a high-quality product due to forming a great amount of nucleation centers. The advantage of carbamide application is its capability to dissolve in the reaction volume without interacting with it, and evenly distribute in volume, slowly hydrolyzing with ammonium ions formation. At the same time, a very mild neutralization regime is realized without local pH value excess. Method of iron oxyhydroxide obtaining by carbamide precipitation is illustrated below. The aim of investigation was to obtain nanoscale precipitate, to study the properties of the obtained material and to describe the precipitation process. The study of the iron (III) oxyhydroxide precipitation kinetics from the iron (III) sulphate solution, and a mathematical model of the process are presented in the article. A mathematical model of the precipitation process allows to take into account the influence of the substances concentration and the process temperature to the rate of precipitate production. The precipitate was characterized by X-ray and DTA analysis, as well as by Raman spectroscopy and scanning electron microscopy methods.

Kinetic coupling of the respiratory chain with ATP synthase, but not proton gradients, drives ATP production in cristae membranes

Mitochondria have a characteristic ultrastructure with invaginations of the inner membrane called cristae that contain the protein complexes of the oxidative phosphorylation system. How this particular morphology of the respiratory membrane impacts energy conversion is currently unknown. One proposed role of cristae formation is to facilitate the establishment of local proton gradients to fuel ATP synthesis. Here, we determined the local pH values at defined sublocations within mitochondria of respiring yeast cells by fusing a pH-sensitive GFP to proteins residing in different mitochondrial subcompartments. Only a small proton gradient was detected over the inner membrane in wild type or cristae-lacking cells. Conversely, the obtained pH values did barely permit ATP synthesis in a reconstituted system containing purified yeast F1F0 ATP synthase, although, thermodynamically, a sufficiently high driving force was applied. At higher driving forces, where robust ATP synthesis was observed, a P-side pH value of 6 increased the ATP synthesis rate 3-fold compared to pH 7. In contrast, when ATP synthase was coreconstituted with an active proton-translocating cytochrome oxidase, ATP synthesis readily occurred at the measured, physiological pH values. Our study thus reveals that the morphology of the inner membrane does not influence the subcompartmental pH values and is not necessary for robust oxidative phosphorylation in mitochondria. Instead, it is likely that the dense packing of the oxidative phosphorylation complexes in the cristae membranes assists kinetic coupling between proton pumping and ATP synthesis.

Revealing the Local pH Value Changes of Acidic Aqueous Zinc Ion Batteries with a Manganese Dioxide Electrode during Cycling

The research on aqueous zinc ion batteries (AZIB) is getting more attention as the energy transition continues to develop and the need for inexpensive and safe stationary storage batteries is growing. As the detailed reaction mechanisms are not conclusively revealed, we want to take an alternative approach to investigate the importance of pH value changes during cycling. By adding a pH-indicator to the electrolyte (2 M ZnSO4 + 0.1 M MnSO4), the local pH-value change during operation is visualized in operando. The overall pH value was found to increase during cycling whereas a major temporary pH drop in close proximity of the manganese dioxide electrode surface occurs. Additionally, this pH value change was quantified locally by in operando measurements with a pH micro electrode. Different electrolyte compositions with additives (sodium dodecyl sulfate (SDS), sulfuric acid (H2SO4)) and operation voltages were tested. The pH-potential-diagrams of manganese and zinc reveal pH value and potential limits, leading to active material dissolution at lower pH values and oxygen gas evolution at higher potentials >1.7 V. The procedure of combining a pH indicator, pH microelectrode measurements and pH-potential diagrams can be seen as an appropriate method to determine the recommendable working window of aqueous batteries.

Effect of anion exchange membrane capacity loss on pH and electric conductivity of saline solution during neutralization dialysis

Neutralization dialysis (ND) is a relatively simple and easy-to-use method of water desalination, which does not require large investments, like reverse osmosis or electrodialysis. One of the problems encountered when using this method is a relatively rapid deterioration of the effective exchange capacity of the anion-exchange membrane caused by its contact with the alkali compartment. We report the results of experimental study of this method using a fresh Neosepta® AMX membrane (Astom, Japan) and the same membrane after its ND operation for about 400 h. As well, we describe a new non-steady state mathematical model of the ND process in batch mode. The model is based on the Nernst-Planck equations and considers a cell containing an acid, a saline and an alkali compartments separated by a cation- and an anion-exchange membrane. The incomplete dissociation degree of secondary and tertiary amino groups present in the used AMX membrane is taken into account as a function of the local pH value. It is accepted that the pristine AMX contains only the quaternary amino groups, positively charged at all pH. When assuming that the fraction of the quaternary groups in the used AMX is 35% of the initial value, we find a very good agreement between calculated and experimental time-dependences of the conductivity and pH of the feed saline solution. Simulations show that the fraction of weakly basic functional groups in anion-exchange membrane has a strong effect on the course of ND process. Deprotonation of weakly basic functional groups in the AMX causes a decrease in the counterion flux through this membrane and an increase in the co-ion leakage, which reduces the rate of desalination and causes acidification of the saline solution. The time needed to decrease the conductivity of the initial saline solution by 70% in the case of the used AMX membrane is twice as long as that of the fresh membrane. The pH of the product solution is 7 in the case of the fresh AMX and only 3.5 in the case of the used one.

Visual detection of the prostate specific antigen via a sandwich immunoassay and by using a superwettable chip coated with pH-responsive silica nanoparticles.

A pH-responsive superwettable chip is described whose surface can switch between superhydrophobic and superhydrophilic. It can be used for the visual detection of the prostate specific antigen (PSA) based on contact angle readout. Magnetic beads were modified with primary antibody against PSA. After immunobinding, gold nanoparticles loaded with secondary antibody labeled with glucose oxidase is added. On addition of glucose, gluconic acid is formed which causes a drop in the local pH value. This results in a wettability switch of the pH-responsive superwettable chip from hydrophobic to hydrophilic. Under the optimized conditions, PSA can be quantified with a 3.2pgmL-1 limit of detection by analyzing the contact angle and the related color that changes from blue via orange to red. The method is applicable to PSA detection in serum samples and for visual classification by cancer patients and healthy persons. It is also suitable for color-blind and color-weak individuals. Conceivably, this kind of assay can be transferred to the determination of various kinds of other bioanalytes including nucleotide, proteins, and even of ions and small organic molecules, and thus is has a wide scope. Graphical abstract Schematic presentation of a pH-responsive superwettable chip coated with silica nanoparticles for the visual detection of prostate specific antigen (PSA) by reading the contact angle. The superwettable chip achieves reliable clinical detection of serum PSA from prostate cancer patients.

Light-harvesting metal-organic framework nanoprobes for ratiometric fluorescence energy transfer-based determination of pH values and temperature.

Light-harvesting nanoprobes were developed by self-assembly of nanoscale metal-organic frameworks (NMOFs) and stimuli-responsive polymers for fluorometric sensing of pH values and temperature. Two kinds of fluorescent NMOFs (acting as the energy donor) and stimuli-responsive polymers conjugated to fluorophores (acting as energy acceptors) were prepared and characterized. The NMOFs include zirconium(IV) and π-conjugated dicarboxylate ligands. The fluorophores inclued cyaine dyes and aBodipy dye. The energy donor and energy acceptor form a Förster resonance energy transfer (FRET) nanosystem. In the light-harvesting system, the chain lengths of the stimuli-responsive polymers vary when the local pH value or temperature change. Ratiometric sensing of pH and temperature was accomplished by monitoring fluorescence. pH values were can be sensed between 3.0 and 8.0 under 420nm excitation and by ratioing the emission peaks at 645 and 530nm. Temperature can be sensed in the range from 25 to 50°C under 550nm excitation and by ratioing the emission peaks at 810 and 695nm. The nanoprobes display excellent water dispersibility and cell membrane permeability. They were applied to image pH values and temperature in HeLa cells. Graphical abstract Schematic presentation of an effective strategy to fabricate light-harvesting nanoprobes by self-assembly of MOFs and stimuli-responsive polymers for ratiometric pH and temperature sensing. The distance as the polymer length between energy donor and acceptor is crucial for energy transfer efficiency.

Metal-ion-induced DNAzyme on magnetic beads for detection of lead(II) by using rolling circle amplification, glucose oxidase, and readout of pH changes.

This work reports on a method for determination of lead(II) ion in environmental water samples. A Pb2+-specific DNAzyme immobilized on magnetic beads was coupled to rolling circle amplification (RCA) and pH meter-based readout. On addition of Pb2+ ion, it induces partial cleavage of the DNAzyme on the magnetic beads. The single-stranded DNA remaining on the magnetic beads is used as the primer to trigger the RCA reaction with the assistance of a circular DNA template, polymerase and dNTPs. This results in the formation of numerous oligonucleotide repeats on the magnetic bead. Subsequently, these repeats hybridize with glucose oxidase-labeled single-stranded DNA (GOx-ssDNA) to form a long concatamer containing tens to hundreds of GOx-ssDNA tandem repeats. The concatenated GOx molecules oxidize glucose, and this is accompanied by a drop in the local pH value. The pH values (vs. background signal) drop linearly when Pb2+ concentrations increase from 1.0-100nM, and the detection limit is 0.91nM. The method displays good reproducibility, high specificity and acceptable accuracy. It was applied to the analysis of spiked water samples, and results compared favorably with those obtained by ICP-MS. Graphical abstract Schematic presentation of metal-ion-induced DNAzyme on magnetic bead (MB) with rolling circle amplification (RCA) for pH meter-based detection of lead(II) ion andby using aglucose oxidase (GOx)-labeled probe.

Electrostatically controlled fluorometric assay for differently charged biotargets based on the use of silver/copper bimetallic nanoclusters modified with polyethyleneimine and graphene oxide.

An electrostatically controlled fluorometric assay is described that is based on the use of silver/copper bimetallic nanoclusters. The nanoclusters were coated with polyethyleneimine (PEI-Ag/CuNCs). At pH7.4, these particles are positively charged. Their blue fluorescence (with excitation/emission peaks at 341/464nm) depends on local pH values and temperature. If graphene oxide (which is negatively charged at pH7.4) is introduced, the fluorescence of the PEI-Ag/CuNCs is quenched. Based on various electrostatic interactions, three kinds of biomacromolecules were detected by fluorometry. These include (negatively charged) heparin, (positively charged) protamine, and (virtually uncharged) trypsin. Heparin is detected by using GO/PEI-Ag/CuNCs, protamine by using GO/heparin/PEI-Ag/CuNCs, and trypsin by using GO/protamine/heparin/PEI-Ag/CuNC. The detection limits and linear ranges are 4.8nM and 10-450nM for heparin, 0.09μg·mL-1 and 0.25-5μg·mL-1 for protamine, and 0.03μg·mL-1 and 0.05-1μg·mL-1 for trypsin. Zeta potentials of the various substances in the system were determined to elucidate the detection mechanism. Comceivably, the method provides a widely applicable approach for electrostatically controlled biomolecular assays. Graphical abstract Schematic presentation of electrostatically controlled fluorometric assay for the detection of heparin, protamine, and trypsin based on the silver/copper bimetallic nanoclusters modified with polyethyleneimine and graphene oxide.

A lysosome-targeting nanosensor for simultaneous fluorometric imaging of intracellular pH values and temperature.

Lysosomal pH and temperature are two crucial physiological parameters that are involved in regulating intracellular homeostasis, and their precise measurements are extremely important in understanding this process and diseases diagnosis. A lysosome-targeting nanosensor has been designed for simultaneous imaging of pH values and temperature in HeLa cells. Three dyes were covalently immobilized either inside or on silica nanoparticles. The nanosensors have an average diameter of 95nm. The large surface area of these nanomaterials provides abundant sites for multi-functionality. The surface of nanosensors has been modified with positively-charged amino groups in order to facilitate endocytosis and targeting lysosome. Fluorescein is used as the indicator probe for pH measurement, rhodamine B is the probe for temperature, and a europium complex acts as the reference dye. The dual nanosensor responds to pH values in the range from 3.0 to 9.0, and to temperature in the range from 20 to 60°C. Owing to its good biocompatibility and good sensitivity, the dual nanosensor has been used to monitor changes in local pH values and temperature in the lysosome of HeLa cells. Graphical abstract A dual nanosensor for simultaneously imaging ofpH values and temperature inside the lysosome of HeLa cellswas constructed by labelling three luminophores in/on silica nanoparticles. It shows high sensitivity and selectivity, good photostability, and good biocompatibility.

PH-Dependent Adsorption Release of Doxorubicin on MamC-Biomimetic Magnetite Nanoparticles.

New biomimetic magnetite nanoparticles (hereafter BMNPs) with sizes larger than most common superparamagnetic nanoparticles were produced in the presence of the recombinant MamC protein from Magnetococcus marinus MC-1 and functionalized with doxorubicin (DOXO) intended as potential drug nanocarriers. Unlike inorganic magnetite nanoparticles, in BMNPs the MamC protein controls their size and morphology, providing them with magnetic properties consistent with a large magnetic moment per particle; moreover, it provides the nanoparticles with novel surface properties. BMNPs display the isoelectric point at pH 4.4, being strongly negatively charged at physiological pH (pH 7.4). This allows both (i) their functionalization with DOXO, which is positively charged at pH 7.4, and (ii) the stability of the DOXO-surface bond and DOXO release to be pH dependent and governed by electrostatic interactions. DOXO adsorption follows a Langmuir-Freundlich model, and the coupling of DOXO to BMNPs (binary biomimetic nanoparticles) is very stable at physiological pH (maximum release of 5% of the drug adsorbed). Conversely, when pH decreases, these electrostatic interactions weaken, and at pH 5, DOXO is released up to ∼35% of the amount initially adsorbed. The DOXO-BMNPs display cytotoxicity on the GTL-16 human gastric carcinoma cell line in a dose-dependent manner, reaching about ∼70% of mortality at the maximum amount tested, while the nonloaded BMNPs are fully cytocompatible. The present data suggest that BMNPs could be useful as potential drug nanocarriers with a drug adsorption-release governed by changes in local pH values.

Effects of Electrolyte Anions on the Reduction of Carbon Dioxide to Ethylene and Ethanol on Copper (100) and (111) Surfaces.

The CO2 electroreduction reaction has been investigated on Cu(100) and Cu(111) surfaces in 0.1 m aqueous solutions of KClO4 , KCl, KBr, and KI electrolyte. The formation of ethylene and ethanol on these surfaces generally increased as the electrolyte anion was changed from ClO4- →Cl- →Br- →I- . For example, on Cu(100) at -1.23 V versus RHE, as the electrolyte anion changed from ClO4- to I- , the faradaic efficiency (FE) of ethylene formation increased from 31 to 50 %, FEethanol increased from 7 to 16 %, and the associated current densities increased five- and sevenfold, respectively. A remarkable total FE of up to 74 % for C2 and C3 products was obtained in the presence of KI. Despite surface roughening in the presence of the electrolytes, the Cu(100) electrode still enhanced the formation of C2 compounds better than Cu(111). The favorable reduction of CO2 to C2 products in KI electrolyte was correlated with a higher *CO population on the surface, as shown using linear sweep voltammetry. In situ Raman spectroscopy indicated that the coordination environment of *CO was altered by the used electrolyte anion. Thus, apart from affecting the morphology of the electrode and local pH value, we propose that the anion plays a critical role in enhancing the formation of C2 products by tuning the coordination environment of adsorbed *CO, which gives rise to more efficient C-C coupling.

Molecular weight dependence of chain conformation of strong polyelectrolytes.

Using sodium polystyrene sulfonate (NaPSS) and quarternized poly 4-vinylpyridine (QP4VP) as model systems, the chain conformation of polyelectrolytes under finite salt concentrations is investigated at a single molecular level. By fluorescence correlation spectroscopy (FCS), the hydrodynamic radius (R h) of the samples with the molecular weight ranging more than one order of magnitude was measured. The variations of R h as a function of molecular weight reveal the molecular weight dependence: under moderate salt concentrations (such as 10-4 and 0.1M), the shorter chains of both NaPSS and QP4VP take the rod-like conformation, while the longer chains take the coiled conformation (random coil or swelled random coil conformation, respectively). At high enough salt levels, both the charged chains take the coiled conformations. Photon counting histogram (PCH) measurements of the local pH value at the vicinity of the NaPSS chain expose the higher extent of counterion adsorption for longer chains as well as higher salt concentrations, telling that the charge regularization process is the major governing factor.

Quantification of Interfacial pH Variation at Molecular Length Scales Using a Concurrent Non-Faradaic Reaction.

We quantified changes in interfacial pH local to the electrochemical double layer during electrocatalysis by using a concurrent non-faradaic probe reaction. In the absence of electrocatalysis, nanostructured Pt/C surfaces mediate the reaction of H2 with cis-2-butene-1,4-diol to form a mixture of 1,4-butanediol and n-butanol with selectivity that is linearly dependent on the bulk solution pH value. We show that kinetic branching occurs from a common surface-bound intermediate, ensuring that this probe reaction is uniquely sensitive to the interfacial pH value within molecular length scales of the surface. We used the pH-dependent selectivity of this reaction to track changes in interfacial pH during concurrent hydrogen oxidation electrocatalysis and found that the local pH value can vary dramatically (>3 units) relative to the bulk value even at modest current densities in well-buffered electrolytes. This study highlights the key role of interfacial pH variation in modulating inner-sphere electrocatalysis.

Quantification of Interfacial pH Variation at Molecular Length Scales Using a Concurrent Non‐Faradaic Reaction

AbstractWe quantified changes in interfacial pH local to the electrochemical double layer during electrocatalysis by using a concurrent non‐faradaic probe reaction. In the absence of electrocatalysis, nanostructured Pt/C surfaces mediate the reaction of H2 with cis‐2‐butene‐1,4‐diol to form a mixture of 1,4‐butanediol and n‐butanol with selectivity that is linearly dependent on the bulk solution pH value. We show that kinetic branching occurs from a common surface‐bound intermediate, ensuring that this probe reaction is uniquely sensitive to the interfacial pH value within molecular length scales of the surface. We used the pH‐dependent selectivity of this reaction to track changes in interfacial pH during concurrent hydrogen oxidation electrocatalysis and found that the local pH value can vary dramatically (>3 units) relative to the bulk value even at modest current densities in well‐buffered electrolytes. This study highlights the key role of interfacial pH variation in modulating inner‐sphere electrocatalysis.