Operando Local pH Mapping of Electrochemical and Bioelectrochemical Reactions Occurring at an Electrode Surface: Effect of the Buffer Concentration

Abstract

In this study, we aim at demonstrating the possibility for operando quantification of the thickness of the boundary layer where pH changes are occurring upon electrochemical reactions accompanied with the production or consumption of H + cations. Confocal fluorescent microscopy (CFM) is combined with two different pH sensitive fluorescent dyes, namely 3,4'dihydroxy-3',5'-bis-(dimethylaminomethyl)flavone (FAM345) and rhodamine-6-aniline (R6H). This is the first attempt to quantify the effect of the buffer concentration (quantification of layer thickness) on local pH changes produced by ascorbic oxidation and oxygen reduction reaction directly occurring at nonmodified graphite electrodes. In addition, the effect of buffer concentration was studied also with respect to local pH variations produced by bioelectrochemical reactions.

Electrochemically produced local pH changes stimulating (bio)molecule release from pH-switchable electrode-immobilized avidin–biotin systems

“…The use of the pH-switchable reversible complex formation of the nitro-avidin-biotin or avidin-iminobiotin systems can further benefit from another research direction, which provides reversible pH changes near electrode surfaces upon performing some electrochemical reactions. 26 The local (interfacial) pH change has been already used to control electrochemically activity of surface-immobilized enzymes 27 and to release molecular loads bound to an electrode surface, 28 particularly with pH-degradable covalent bonds. 29 The present paper reports on the use of electrode-surface immobilized nitro-avidin or avidin in combination with biotinylated or iminobiotinylated substances, respectively, with the local pH changes produced electrochemically.…”

“…It has been already shown that the electrochemical reduction of O 2 results in pH increase near an electrode surface. 26,28 Indeed, the electrochemical oxygen reduction leading to H 2 O 2 or H 2 O production (depending on the potential applied and the electrode material) results in consumption of H + ions, thus, producing the H + depletion near the electrode surface and the pH increase. This results in the dissociation of the nitro-avidin-biotin complex and release of the biotinylated fluorescent dye, then detected in a bulk solution by fluorescence measurements.…”

“…Notably, the local pH change, which is a driving force for the release process strongly depends on the buffer concentration in the background solution. 26 When the buffer concentration is too high, the local pH change cannot proceed because its fast equilibration with the bulk solution, thus the release process is inhibited (Fig. ESM5 in the ESI †).…”

“…10, which is consistent with the local pH measurements performed with other pH dependent fluorescent dyes and a confocal microscope. 26 The total amount of the biotinylated dye released from the electrode surface was estimated, 18.6 pmoles, based on the calibration plot of the fluorescence vs. the dye concentration in standard solutions (Fig. ESM7 in the ESI †).…”

“…The ascorbate oxidation is accompanied with the H + ion release, thus acidifying a thin electrolyte layer near the electrode surface. 26 The avidin-iminobiotin-MP-11 complex immobilized at the electrode surface was dissociated in the acidic media and the iminobiotin-MP-11 conjugate was released to the solution. The bulk solution aliquots were collected at different time intervals and analyzed for the biocatalytic activity of MP-11 following the standard peroxidase assay with H 2 O 2 and ABTS (see the Experimental details in the ESI †).…”