Surface-enhanced Infrared Spectroscopy Research Articles

Basic and Advanced Logical Concept Derived from Surface Enhanced Infrared Spectroscopy (SEIRS) as Sensing Probe for Analysis of Chemical Species: A Brief Review

The worldwide concern for environmental pollution, climate change and health hazards caused by various pollutants has significantly increased in the recent past. Various techniques have so far been employed for sensing applications of such organic as well as inorganic pollutants. Amongst the different techniques, surface enhanced infrared spectroscopy (SEIRS) is a powerful tool which is utilized for label-free and unambiguous identification of molecular species. SEIRS overcomes the limitations of the conventional infrared spectroscopy and has emerged as a potential technique with high surface sensitivity by enhancing the signals by many folds and also facilitates new studies from the fundamental aspect to applied sciences. The current review is dedicated to a comprehension of the SEIRS technique to provide a critical overview of its application as sensing probe for analysis of chemical species. The major features of Fourier Transform Infrared spectroscopy and SEIRS have been critically discussed.

Probing the reactivity of protonated oxygen intermediate in aprotic media with in situ surface-enhanced infrared spectroscopy

A fundamental understanding of electrochemical reactions is essential to drive the practicality of batteries. The oxygen reduction reaction (ORR) that occurs on discharge in aprotic lithium-oxygen (Li-O2) batteries, invariably encounters interference from impurities (e.g., protons) in practical environments. It has been shown that moderate proton-mediated ORR can improve discharge capacity without altering the overall pathway; however, the reactivity of protonated oxygen intermediates formed towards aprotic electrolytes during ORR, remains controversial and unexplored. Herein, we interrogate the reactivity of protonated oxygen intermediates at the model Au | propylene carbonate and Au | trimethyl phosphate interfaces containing phenol as the moderate proton source, using in situ attenuated total reflection surface-enhanced infrared spectroscopy coupled with theoretical calculations. Direct spectroscopic evidence presents that the preferential reaction between superoxide and available protons to form protonated oxygen intermediates (e.g., HO2), can significantly mitigate superoxide anion (O2−)-induced solvent degradations while not triggering additional secondary parasitic reactions. Consequently, practical Li-O2 batteries containing phenol have also exhibited improved electrochemical performance and reversibility. We believe that the fundamental insights will provide important lessons for future practical design (e.g., protons control of electrolytes) of Li-O2 batteries and other related electrochemical devices.

Regulating Morphological Features of Nickel Single‐Atom Catalysts for Selective and Enhanced Electroreduction of CO2

Single-atom catalysts (SACs) are of interest for chemical transformations of significant energy and environmental relevance because of the envisioned efficient use of active sites and the flexibility in tuning their coordination environment. Future advancement in this vein hinges upon the ability to further increase the number and accessibility of active sites in addition to fine-tuning their chemical environment. In this work, a Ni SAC is reported with a unique hierarchical hollow structure (Ni/HH) that allows increased accessibility of the active sites. The successful obtainment of such a uniquely structured catalystwasenabled by the judiciously chosen solvent mixtures for the preparation of the precursor whose hierarchical feature is maintained during the subsequent pyrolysis and etching of the pyrolysis product. Comparative catalytic and mechanistic studies with reference to three closely related but more compact Ni SACs established the superior performance of Ni/HH for selective electroreduction of CO2 to CO. Experimental analyses by in situ attenuated total reflection surface-enhanced infrared spectroscopy reveal that it is the facilitated formation of the *COOH intermediate in the rate-determining step that leads to the enhanced reaction kinetics and the overall catalytic performance.

Dopamine as a bioinspired adhesion promoter for the metallization of multi-responsive phase change microcapsules

This work reports on an environmentally friendly method to produce encapsulated phase change material with a thin nickel coating, applicable for heat conversion, storage and thermal management of heat-sensitive components and suitable for active heating by electromagnetic radiation. A critical issue for the metallization is the adhesion between the polymer capsule shell and the metal layer. Based on previous studies using the bio-molecule dopamine as adhesion promoter in composites and for plastics metallization, commercial paraffin microcapsules were coated with an ultrathin polydopamine film via a simple wet chemical process. Subsequently, a thin, uniform and compact nickel layer was produced by electroless metallization. The successful deposition of both layers was verified with a broad range of imaging and spectroscopic techniques. For the first time, surface-enhanced IR spectroscopy was used to study the deposition of ultrathin PDA films. The combination of SEM and energy-dispersive X-ray spectroscopy allowed resolving the spatial distribution of the elements Ni, N, and O in the MC shell. Electrically conducting paths in the Ni shell were verified by conductive AFM. Thermal analysis revealed that the coated microcapsules show a phase change enthalpy of approx. 170 J/g, suitable for thermal storage and management. Additionally, the nickel layer enhanced the thermal diffusivity of the microcapsule powders and enables a fast heating of the PCM microcapsules by microwave radiation, demonstrating the applicability of the metallized MCs for controlled heating applications.Graphical abstract

Monitoring the Progression of Cell-Free Expression of Microbial Rhodopsins by Surface Enhanced IR Spectroscopy: Resolving a Branch Point for Successful/Unsuccessful Folding.

The translocon-unassisted folding process of transmembrane domains of the microbial rhodopsins sensory rhodopsin I (HsSRI) and II (HsSRII), channelrhodopsin II (CrChR2), and bacteriorhodopsin (HsBR) during cell-free expression has been investigated by Surface-Enhanced Infrared Absorption Spectroscopy (SEIRAS). Up to now, only a limited number of rhodopsins have been expressed and folded into the functional holoprotein in cell free expression systems, while other microbial rhodopsins fail to properly bind the chromophore all-trans retinal as indicated by the missing visible absorption. SEIRAS experiments suggest that all investigated rhodopsins lead to the production of polypeptides, which are co-translationally inserted into a solid-supported lipid bilayer during the first hour after the in-vitro expression is initiated. Secondary structure analysis of the IR spectra revealed that the polypeptides form a comparable amount of α-helical structure during the initial phase of insertion into the lipid bilayer. As the process progressed (>1 h), only HsBR exhibited a further increase and association of α-helices to form a compact tertiary structure, while the helical contents of the other rhodopsins stagnated. This result suggests that the molecular reason for the unsuccessful cell-free expression of the two sensory rhodopsins and of CrChR2 is not due to the translation process, but rather to the folding process during the post-translational period. Taking our previous observation into account that HsBR fails to form a tertiary structure in the absence of its retinal, we infer that the chromophore retinal is an integral component of the compaction of the polypeptide into its tertiary structure and the formation of a fully functional protein.

Ammonia oxidation on iridium electrode in alkaline media: An in situ ATR-SEIRAS study

• ATR-SEIRAS is extended to study the AOR on Ir electrode in basic media. • NH x ,ad species is attributed to the pivot intermediate. • N 2 H x+y ,ad species is identified as the key active intermediate to generate N 2. • NO 2 − and NO 3 − byproducts arise from NO ad oxidation. Mechanistic understanding of electrochemical ammonia oxidation reaction (AOR) is important for designing efficient AOR catalysts. In this work, in situ attenuated total reflection surface-enhanced infrared spectroscopy (ATR-SEIRAS) is employed to study the AOR at varied potentials on an Ir electrode in alkaline media. The AOR on Ir contains two sequential oxidation peaks in the anodic voltammograms. The in situ ATR-SEIRAS results indicate that the small Peak I at a lower potential is mainly due to dehydrogenation of interfacial ammonia and dimerization of as-generated NH x, ad species to N 2 H x+y, ad species. And the broad and intense Peak II at a higher potential mainly involves the N 2 production from N 2 H x+y, ad species, NO ad species generation from NH x ,ad species and further successive oxidation into the NO 2 − and NO 3 − by-products at higher potentials. It is thus inferred that NH x species are the pivotal intermediates for AOR on Ir electrode, and the N 2 H x+y ,ad and NO ad species the key intermediates for the production of N 2 , NO 2 − and NO 3 − , respectively.

Interaction of CO with Gold in an Electrochemical Environment

We present a joint theoretical–experimental study of CO coverage and facet selectivity on Au under electrochemical conditions. With in situ attenuated total reflection surface-enhanced IR spectroscopy, we investigate the CO binding in an electrochemical environment. At 0.2 V versus SHE, we detect a CO band that disappears upon facet-selective partial Pb underpotential deposition (UPD), suggesting that Pb blocks certain CO adsorption sites. With Pb UPD on single crystals and theoretical surface Pourbaix analysis, we eliminate (111) terraces as a possible adsorption site of CO. Ab initio molecular dynamics simulations of explicit water on the Au surface show the adsorption of CO on (211) steps to be significantly weakened relative to the (100) terrace due to competitive water adsorption. This result suggests that CO is more likely to bind to the (100) terrace than (211) steps in an electrochemical environment, even though Au steps under gas-phase conditions bind CO* more strongly. The competition between water and CO adsorption can result in different binding sites for CO* on Au in the gas phase and electrochemical environments.

Interfacial Acid-Base Equilibria and Electric Fields Concurrently Probed by In Situ Surface-Enhanced Infrared Spectroscopy.

Understanding how applied potentials and electrolyte solution conditions affect interfacial proton (charge) transfers at electrode surfaces is critical for electrochemical technologies. Herein, we examine mixed self-assembled monolayers (SAMs) of 4-mercaptobenzoic acid (4-MBA) and 4-mercaptobenzonitrile (4-MBN) on gold using in situ surface-enhanced infrared absorption spectroscopy (SEIRAS). Measurements as a function of the applied potential, the electrolyte pD, and the electrolyte concentration determined both the relative surface populations of acidic and basic forms of 4-MBA, as well as the local electric fields at the SAM-solution interface by following the Stark shifts of 4-MBN. The effective acidity of the SAM varied with the applied potential, requiring a 600 mV change to move the pKa by one unit. Since this is ca. 10× the Nernstian value of 59 mV/pKa, ∼90% of the applied potential dropped across the SAM layer. This emphasizes the importance of distinguishing applied potentials from the potential experienced at the interface. We use the measured interfacial electric fields to estimate the experienced potential at the SAM edge. The SAM pKa showed a roughly Nernstian dependence on this estimated experienced potential. An analysis of the combined acid-base equilibria and Stark shifts reveals that the interfacial charge density has significant contributions from both SAM carboxylate headgroups and electrolyte components. Ion pairing and ion penetration into the SAM also influence the observed surface acidity. To our knowledge, this study is the first concurrent examination of both effective acidity and electric fields, and highlights the relevance of experienced potentials and specific ion effects at functionalized electrode surfaces.

Adsorption of Halides on Au (111) Electrode in Aprotic Solvents: AC- Voltammetry, EIS, XPS and Surface-Enhanced Infrared Spectroscopy

Specific adsorption of ions on the electrode surface has a significant impact on the electric double layer (EDL) structure and the kinetics of electrode processes[1]. The ion adsorption at aqueous solution interfaces[2] on various metals has been examined intensively, but much fewer studies from non-aqueous solvents have been conducted[3]. Such studies are important because the metal-solvent [4] and ion-solvent interaction energy [5] depend on the chemical nature of the solvent.In this study, three organic solvents (propylene carbonate (PC), diglyme (DG), and dimethyl sulfoxide (DMSO)) were investigated for the adsorption of iodide and bromide ions on Au(111) single crystal electrode. This analysis was conducted by using differential capacity measurements, cyclic voltammetry (CV), Electrochemical Impedance Spectroscopy (EIS), X-ray photoelectron spectroscopy (XPS), and Surface-Enhanced Infrared Spectroscopy (SEIRAS).The magnitude of anion adsorption on Au(111) electrode increases in the series of halides I- ˃ Br- due to better solvation [6] and in the solvent order PC ˃ DG ˃ DMSO[7]. As well, the influence of water content on the adsorption rate of iodide on Au(111) electrode in PC and DMSO was studied. This effect, and the much higher adsorption rate in water is explained by the closer approach of the solvated halogen ion to the surface and the resulting stronger interaction of the transition state with the electrode. Quantification of the adsorbed iodide amount from XPS spectra confirms the coverage estimated from the adsorption charge in CVs. During SEIRAS measurements, a bipolar band is observed at around 1805 cm−1 which correlates to the C-O-vibration of PC as reported on silver electrodes from SERS measurements[8]. A stronger electrical field with decreasing potential is observed in absence of iodine leads to higher absorbance of PC molecules on the surface. Acknowledgments Financial support by the German Federal Ministry of Education and Research (BMBF) of the “LuCaMag” project (Fkz: 03EK3051A) under the framework of “Vom Material zur Innovation” program is acknowledged.

Mapping Adsorbates with Shell-Isolated Nanoparticle Enhanced Raman Spectroscopy to Understand Rapid Superconformal Filling of Cu Trenches

Additive adsorbates are widely used as surfactants to guide morphological and microstructural evolution during electrochemical deposition of thin films. Even though this strategy has been employed for several decades understanding of the mechanisms at play on the interface remain limited. Modern spectroelectrochemical methods, from surface-enhanced Raman spectroscopy (SERS) to surface-enhanced infrared spectroscopy (SEIRAS) and non-linear optical methods like sum frequency generation (SFG) can provide significant insight into the composition, structure, and dynamics of the surfactant adsorbates through their vibrational signatures. on nominally static surfaces. Each strategy is limited to specimen geometry; SERS is closely linked to the use of rough, plasmonically-active surfaces while SEIRAS measurements are constrained to thin, IR-transparent, metallic films. In the last decade, a significant advance was the introduction of nanoparticle reporters, based largely on silica coated Au nanoparticles, that enable Raman spectroscopy studies on well-defined, single-crystal surfaces whereby the nanoparticle serves to channel the plasmonic energy to the nanoparticle-substrate interface. The silica or alternative coating materials are designed to be inert with respect to the chemistry of the adsorbate species under study while remaining thin enough to allow effective focusing of the optical energy. This approach has been adopted by several researchers interested in understanding molecular adsorption on well-defined, single-crystal surfaces, the electrical double layer, and numerous aspects of reactivity related to catalysis, batteries, electrodeposition, etc. The present work demonstrates the use of Au@SiO2 (core-shell) nanoparticles, also known as SHINERS (Shell-Isolated Nanoparticle-Enhanced Raman Spectroscopy), to enable in situ vibrational spectroscopic measurements during electroplating of comparatively smooth, rapidly advancing, surfaces. In particular, the ability of the reporters to float on an advancing surface while responding to area change is used to reveal important, area-driven changes in adsorbate coverage. This study also opens a new avenue for exploring the intersection of analytical surface chemistry and composite electroplating. Figure 1

Is the Use of Surface-Enhanced Infrared Spectroscopy Justified in the Selection of Peptide Fragments That Play a Role in Substrate-Receptor Interactions? Adsorption of Amino Acids and Neurotransmitters on Colloidal Ag and Au Nanoparticles.

This paper describes an application of attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) and surface-enhanced infrared spectroscopy (SEIRA) to characterize the selective adsorption of four peptides present in body fluids such as neuromedin B (NMB), bombesin (BN), neurotensin (NT), and bradykinin (BK), which are known as markers for various human carcinomas. To perform a reliable analysis of the SERIA spectra of these peptides, curve fitting of these spectra in the spectral region above 1500 cm–1 and SEIRA measurements of sulfur-containing and aromatic amino acids were performed. On the basis of the analyses of the spectral profiles, specific conclusions were drawn regarding specific molecule–metal interactions and changes in the interaction during the substrate change from the surface of silver nanoparticles (AgNPs) to gold nanoparticles (AuNPs).

Confined hydration in nanometer-graded plasma polymer films: Insights from surface-enhanced infrared absorption spectroscopy

To shed light on recently explored long-range surface forces generated by subsurface-confined water, the structural characteristics of water molecules penetrating into nanoporous homogeneous and nanograded siloxane plasma polymer films (PPFs) over the time scale of 24 hours are studied by surface-enhanced IR spectroscopy (SEIRAS). Chemically graded PPFs, with embedded hydrophobic-to-hydrophilic gradient, are found to significantly change the average interfacial water orientation due to a unique nanoporous morphology and silanol group coordination. Diffusion of water through the hydrophobic SiO:CH matrix creates an evolution of the coordination of matrix silanol groups, which are eventually deprotonated as soon as the hydration network connects to the aqueous environment. This occurs after ~6 hours of water immersion and coincides with the change of average interfacial water orientation. Both effects are present on hydrophobic samples, but are significantly amplified by the presence of the subsurface vertical amphiphilic gradient (Vgrad), whereas enhanced water uptake in oxygen-plasma modified graded PPFs is covering such effects.

Perfect Absorption Efficiency Circular Nanodisk Array Integrated with a Reactive Impedance Surface with High Field Enhancement.

Infrared (IR) absorbers based on a metal–insulator–metal (MIM) have been widely investigated due to their high absorption performance and simple structure. However, MIM absorbers based on ultrathin spacers suffer from low field enhancement. In this study, we propose a new MIM absorber structure to overcome this drawback. The proposed absorber utilizes a reactive impedance surface (RIS) to boost field enhancement without an ultrathin spacer and maintains near-perfect absorption by impedance matching with the vacuum. The RIS is a metallic patch array on a grounded dielectric substrate that can change its surface impedance, unlike conventional metallic reflectors. The final circular nanodisk array mounted on the optimum RIS offers an electric field enhancement factor of 180 with nearly perfect absorption of 98% at 230 THz. The proposed absorber exhibits robust performance even with a change in polarization of the incident wave. The RIS-integrated MIM absorber can be used to enhance the sensitivity of a local surface plasmon resonance (LSPR) sensor and surface-enhanced IR spectroscopy.

Point mutation in the TGFBI gene: surface-enhanced infrared absorption spectroscopy (SEIRAS) as an analytical method

A novel method to detect DNA mutations based on gold nanoparticles is described. This bioconjugate was prepared by conjugation of the TGFBI gene on the surface of gold nanoparticles. The surface plasmon resonance band observed in the ultraviolet–visible spectrum of the gold nanoparticles showed a shift to a lower energy after the surface was covered with the TGFBI gene. Surface-enhanced infrared spectroscopy (SEIRAS) showed the absorption bands of the bioconjugate due to their proximity to the nanoparticles. The SEIRAS effect was able to detect small and specific changes in the sequence of the TGFBI gene using interactions of the bioconjugates. The study was performed on patients clinically diagnosed with lattice corneal dystrophy (LCD). To our knowledge, this is the first report of FTIR spectroscopy employed to analyse this gene. We used genotyping results previously obtained by next-generation DNA sequencing technology to ensure that the analysed samples had the mutation. Our results show highly reproducible signals, which could be used for analytical studies of mutations in the genome.

Surface-enhanced infrared detection of benzene in air using a porous metal-organic-frameworks film

Infrared (IR) spectroscopy is a powerful technique for observing organic molecules, as it combines sensitive vibrational excitations with a non-destructive probe. However, gaseous volatile compounds in the air are challenging to detect, as they are not easy to immobilize in a sensing device and give enough signal by themselves. In this study, we fabricated a thin nanocrystalline metal-organic framework (nMOF) film on a surface plasmon resonance (SPR) substrate to enhance the IR vibration signal of the gaseous volatile compounds captured within the nMOF pores. Specifically, we synthesized nanocrystalline HKUST-1 (nHKUST-1) particles of ca. 80 nm diameter and used a colloidal dispersion of these particles to fabricate nHKUST-1 films by a spin-coating process. After finding that benzene was readily adsorbed onto nHKUST-1, an nHKUST-1 film deposited on a plasmonic Au substrate was successfully applied to the IR detection of gaseous benzene in air using surface-enhanced IR spectroscopy.

Surface enhanced infra-red spectroscopy with modified silver nanoparticles (AgNPs) for detection of quaternary ammonium cationic surfactants

A novel surface enhanced infra-red spectroscopy (SEIRS) method with silver nanoparticles (AgNPs) assisted by single drop microextraction (SDME) was developed for detection of total mixed quaternary ammonium cationic surfactants (QACS) in water samples.

Superior activity of Pd nanoparticles confined in carbon nanotubes for hydrogen production from formic acid decomposition at ambient temperature

Designing highly efficient and low-cost catalysts is essential toward realizing the practical application of hydrogen generation by formic acid decomposition (FAD) under ambient conditions. Herein, we report the synthesis of a hybrid material of Pd nanoparticles encapsulated within carbon nanotubes (CNTs) (Pd-CNTs-in). Transmission electron microscopy images show that most Pd nanoparticles (mean diameter 4.2 ± 0.8 nm) are located inside the nanotubes. Temperature-programmed reduction studies of H2 reveal that the average reduction temperature of the Pd(II) species adsorbed on the interior wall of the CNTs is 12 °C lower than those adsorbed on the outer walls of the CNT. Moreover, the as-prepared Pd-CNTs-in catalysts show extremely high FAD activity and durability at ambient temperature. The turn over frequency (TOF) value is as high as 1135 h−1 for the initial 10 min and does not decay significantly during the consecutive 3-time recycling studies. X-Ray photoelectron spectroscopy (XPS), surface-enhanced infrared spectroscopy (SEIRAS), and gas chromatography (GC) studies indicate that CNT confinement induced electronic structure modulation of Pd could be the major reason for the enhancement of FAD catalysis on the Pd-CNTs-in surface. This work could provide promising strategies for the fabrication of cost-effective and high-active Pd-based catalysts for formic acid dehydrogenation.

Quantifying the Limits of Detection of Surface-Enhanced Infrared Spectroscopy with Grating Order-Coupled Nanogap Antennas.

Infrared spectroscopy is widely used for biomolecular studies, but struggles when investigating minute quantities of analytes due to the mismatch between vibrational cross sections and IR wavelengths. It is therefore beneficial to enhance absorption signals by confining the infrared light to deeply subwavelength volumes comparable in size to the biomolecules of interest. This can be achieved with surface-enhanced infrared absorption spectroscopy, for which plasmonic nanorod antennas represent the predominant implementation. However, unifying design guidelines for such systems are still lacking. Here, we introduce an experimentally verified framework for designing antenna-based molecular IR spectroscopy sensors. Specifically, we find that in order to maximize the sensing performance, it is essential to combine the signal enhancement originating from nanoscale gaps between the antenna elements with the enhancement obtained from coupling to the grating order modes of the unit cell. Using an optimized grating order-coupled nanogap design, our experiments and numerical simulations show a hotspot limit of detection of two proteins per nanogap. Furthermore, we introduce and analyze additional limit of detection parameters, specifically for deposited surface mass, in-solution concentration, and secondary structure determination. These limits of detection provide valuable reference points for performance metrics of surface-enhanced infrared absorption spectroscopy in practical applications, such as the characterization of biological samples in aqueous solution.

Applying Battery Tuning Method on Metal Oxide for Highly Selective CO2 Reduction

Engineering active grain boundaries (GBs) in oxide-derived (OD) electrocatalysts is critical to improve the selectivity in CO2 reduction reaction (CO2RR), which is becoming an increasingly important pathway for renewable energy storage and usage. Different from traditional in situ electrochemical reduction under CO2RR conditions, where some metal oxides are converted into active metallic phases but with decreased GB densities, here we introduce the Li electrochemical tuning (LiET) method to controllably reduce the oxide precursors into interconnected ultrasmall metal nanoparticles with enriched GBs. By using ZnO as a case study,1 we demonstrate that the LiET-Zn with freshly exposed GBs exhibits a CO2-to-CO partial current of ∼23 mA cm–2 at an overpotential of −948 mV, representing a 5-fold improvement from the OD-Zn with GBs eliminated during the in situ electro-reduction process. Another representative study is on Cu2O,2 we report the tuning of facet exposure on Cu foil through battery cycling which gives a 6-fold improvement in C2+ to C1 product ratio as compared to polished Cu foil with a maxima C2+ Faradaic efficiency over 60 % and H2 below 20 %. Detailed mechanism study is carried out using density functional theory calculations to shed light on the energetics of the initial C-C coupling steps, while dynamic catalyst surface structure evolution and relevant reactive intermediates are monitored by in situ spectroelectrochemical techniques including Raman scattering and surface enhanced IR spectroscopy.

Examination of Near-Electrode Concentration Gradients and Kinetic Impacts on the Electrochemical Reduction of CO2 using Surface-Enhanced Infrared Spectroscopy

Localized concentration gradients within the electrochemical double layer during various electrochemical processes can have wide-ranging impacts; however, experimental investigation to quantitatively correlate the rate of surface-mediated electrochemical reaction with the interfacial species concentrations has historically been lacking. In this work, we demonstrate a spectroscopic method for the in situ determination of the surface pH using the CO2 reduction reaction as a model system. Attenuated total reflectance surface-enhanced infrared absorption spectroscopy is employed to monitor the ratio of vibrational bands of carbonate and bicarbonate as a function of electrode potential. Integrated areas of vibrational bands are then compared with those obtained from calibration spectra collected in electrolytes with known pH values to determine near-electrode proton concentrations. Experimentally determined interfacial proton concentrations are then related to the resultant concentration overpotentials to exam...