Deconvoluting Surface-Bound Quinone Proton Coupled Electron Transfer in Unbuffered Solutions: Toward a Universal Voltammetric pH Electrode

Abstract

While quinone proton coupled electron transfer (PCET) under buffered conditions is well understood, the situation is more complicated in unbuffered aqueous solutions. With a view to producing a quinone-based voltammetric pH electrode that can function universally in both buffered and unbuffered solutions by following a twoelectron (2e − )/two-proton (2H + ) Nernstian pathway over a wide pH range, the voltammetric response of strongly electronically coupled surface-bound quinones, directly integrated into a boron-doped diamond (BDD) electrode, is investigated. A laser ablation process enables integration of quinones into the BDD electrode surface with a high pK a1 (first protonation state) and with controllable, very low surface coverages (as low as 2 orders of magnitude below monolayer coverage). Under buffered conditions, one wave results for all pH values, and the 2e − /2H + pathway is followed across the entire pH range. The measured ET rate constant values, from Laviron analysis, are also high, indicative of fast ET pathways. Under unbuffered conditions, one wave is again observed for all pH values; however, deviations from the buffered 2e − /2H + behavior are seen in the neutral region (pH 6−8). While 2e − /2H + transfer is maintained at all times, we attribute the observed deviation to local pH changes caused by the consumption and generation of protons at the electrode surface during the redox electrochemistry of the quinone. The associated proton fluxes generated at such sparse surface coverages are thought to be sufficiently high enough to prevent ET from occurring exclusively via a proton-independent route. By reducing surface coverage (down to ∼4 × 10 −12 mol cm −2 ; the limit of our laser ablation process) local pH changes can be reduced but are not eradicated completely. By moving to a pulsed voltammetric technique, where for each potential step protons consumed at the electrode are immediately replaced, it is possible, provided the surface coverage is low enough, to obtain a Nernstian 2e − /2H + response across a wide pH range in unbuffered solution.

Assessment of acid and thermal oxidation treatments for removing sp2 bonded carbon from the surface of boron doped diamond

“…Briefly, ESI3 shows that the graphite clusters remain on the BDD surface, supporting the electrochemical data. Whilst the robustness of this remaining layer towards complete removal by acid oxidation is somewhat surprising, it is also extremely useful for electrochemical applications which make use of this hybrid sp 2 -sp 3 bonded material, [13,33,35] especially in the more challenging solution environments. [38] Thermal oxidation of laser ablated BDD Thermal oxidation was next investigated as a means for sp 2 removal from the ns-laser micromachined BDD surface.…”

“…one ns-pulse per 0.01 s, with a fluence of 15 J cm -2 , at a speed of 0.3 mm s -1 ; parameters typical of those used in the laser micromachining of BDD electrodes. [33,35] For electrochemical tests, the pits were machined in quintuplicate (×5), and spaced such that they were individually addressable. For electrochemical studies it was necessary to produce a reliable titanium carbide ohmic contact to the BDD.…”

“…[34] sp 2 bonded carbon also possesses a wider range of surface functional groups, compared to diamond, some of which, e.g. quinones, are electroactive and can be used for voltammetric pH sensing [13,35]. sp 2 bonded carbon can also be introduced during the CVD growth process and is especially prevalent during thin film growth (e.g.…”

“…This contrasts with ns-laser micromachining where sp 2 bonded carbon structures can be written in the surface with a spatial resolution limited only by the laser optics, typically on the micron scale. For example, ns-laser techniques have been used to create arrays of micro-spots of sp 2 bonded carbon on a BDD electrode in order to increase the electrochemical sensitivity of the electrode towards pH, [13,35] dissolved oxygen [33] and hypochlorite [38]. sp 2 bonded carbon creation is also possible using ion implantation via the creation of defects in diamond, which when appropriately annealed, graphitise.…”

Persistent Radical Pairs Between N-Substituted Naphthalimide and Carbanion Exhibit pKa-Dependent UV-vis Absorption

“…Knowing the pK a value of a deprotonatable carbon atom attached to a functional group (CH X R Y , X>0, a carbon acid, CA) is critical in organic chemistry since it foretells the reactivity and selectivity of reactions participated by the -carbon. Over the past century, a plethora of experimental techniques, including potentiometry, 1,2 spectrometry, [3][4][5] conductometry, 6,7 electrophoresis, 8,9 nuclear magnetic resonance (NMR), 10,11 voltammetry, 12,13 high performance liquid chromatography (HPLC), 14,15 fluorometry, [16][17][18] and more recently, theoretical calculations, 19,20 have been employed to measure or estimate pK a . Among these techniques, the UV-vis spectrometry method, similarly to pH indicators, is particularly favoured due to its wide accessibility, high throughput, and easy operation for nonexperts.…”