Proton transfer across single-layer graphene proceeds with large computed energy barriers and is therefore thought to be unfavourable at room temperature unless nanoscale holes or dopants are introduced, or a potential bias is applied. Here we subject single-layer graphene supported on fused silica to cycles of high and low pH, and show that protons transfer reversibly from the aqueous phase through the graphene to the other side where they undergo acid–base chemistry with the silica hydroxyl groups. After ruling out diffusion through macroscopic pinholes, the protons are found to transfer through rare, naturally occurring atomic defects. Computer simulations reveal low energy barriers of 0.61–0.75 eV for aqueous proton transfer across hydroxyl-terminated atomic defects that participate in a Grotthuss-type relay, while pyrylium-like ether terminations shut down proton exchange. Unfavourable energy barriers to helium and hydrogen transfer indicate the process is selective for aqueous protons.

Potential-dependent activation energies are calculated quantum mechanically, using a local reaction center model, for the hydrogen reduction−oxidation reaction over platinum by the Volmer−Heyrovsky mechanism, Pt−H + H+ + e-(U) ↔ Pt + H2 (i), modeled by Pt−H···H+(OH2)(OH2)2 + e-(U) ↔ Pt···H−H···OH2(OH2)2 (ii). A contribution from the electrolyte to the potential of the reaction centers in ii is included in the ab initio Hamiltonian. The reversible potential predicted for i based on model ii is 0.04 V, close to the standard hydrogen electrode value of 0 V, and the predicted activation energy at the predicted reversible potential is 0.076 eV, close to the literature value of 0.12 eV for the apparent activation energy. The theoretical results validate the possibility of the Volmer−Heyrovsky mechanism being followed on platinum.

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Quantum mechanically determined electrode potential dependent activation energies for hydronium ion discharge over Pt-H (Heyrovsky reaction) and the reverse reaction have been used to predict Tafel plots. The calculated Tafel plot for H 2 oxidation is similar in shape to an experimental plot from the literature for a Pt(100) electrode and will overlap it when an appropriate preexponential factor is chosen in the Arrhenius expression. This provides strong theoretical support for the first electron-transfer step being rate limiting during H 2 oxidation over the potential range 0 to 0.15 V, and the second electron-transfer step being rate limiting during H 2 evolution over this electrode. The exchange current density is determined from the calculated oxidation and reduction currents and is found to overestimate experiment primarily because the predicted activation energy at the reversible potential underestimates the experimental value. This study illustrates that curvature in nonlinear Tafel plots may stem from the potential dependence of the activation energies or transfer coefficients as well as diffusion and concentration gradient effects. The observed current density and its increase, leveling off, and then decrease at potentials greater than the activation energy-controlled region are attributed to removal of under potential deposited H, passing through the double layer region, and then site blocking by water and its oxidation product OH(ads).

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Two redox reactions on platinum electrodes in base, the formation of underpotential deposited hydrogen, forming a Pt-H bond, and the electro-oxidation of water, forming a Pt-OH bond, were studied by two methods. The first applies a linear relationship between reaction energy in solution and standard reversible potential, an approach recently used in this lab to predict the formation potential of the surface-bonded species. This method depends on the availability of accurate surface adsorption bond strengths from measurement or theory and can be applied in two formats, the empirical model and the linear correlation model. The second method treats the reaction within the so-called double-layer model where reactants and products on the surface are well defined and are experiencing the influence of the electrolyte. When this approach is used, two coordination shells of hydrogen bonded water molecules are found necessary to sufficiently stabilize the hydroxide ion in this model, unlike acid for which past work showed only one shell around the hydronium ion is needed. The calculated reversible potentials for both reactions by the empirical and linear correlation models are in good agreement with the experimental onset potentials observed in cyclic voltammetry measurements for Pt(111) surface electrodes when empirical or accurately calculated H, OH, and H(2)O adsorption energies are used. The double layer models for these reactions also yield satisfactory results, and it is concluded that the models should be useful for studying electron-transfer reactions in base, as has already been done for forming Pt-H and Pt-OH in acid solution.

Serum albumin is commonly used as a blocking agent to reduce nonspecific protein adsorption in bioassays and biodevices; however, the details of this process remain poorly understood. Using single molecule techniques, we investigated the dynamics of human serum albumin (HSA) on four model surfaces as a function of protein concentration. By constructing super-resolution maps, identifying anomalously strong adsorption sites, and quantifying surface heterogeneity, we found that the concentration required for site blocking varied dramatically with surface chemistry. When expressed in terms of protein surface coverage, however, a more consistent picture emerged, where a significant fraction of strong sites were passivated at a fractional coverage of 10(-4). On fused silica (FS), "non-fouling" oligo (ethylene glycol) functionalized FS, and hydrophobically modified FS, a modest additional site blocking effect continued at higher coverage. However, on amine-functionalized surfaces, the surface heterogeneity exhibited a minimum at a coverage of ∼10(-4). Using intermolecular Förster resonance energy transfer (FRET), we determined that new anomalous strong sites were created at higher coverage on amine surfaces and that adsorption to these sites was associated with protein-protein interactions, i.e., surface-induced aggregation.

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