Reaction Layer Imaging Using Fluorescence Electrochemical Microscopy

Abstract

The chemical confinement of a pH sensitive fluorophore to a thin-reaction layer adjacent to an electrode surface is explored as a potentially innovative route to improving the spatial resolution of fluorescence electrochemical microscopy. A thin layer opto-electrochemical cell is designed, facilitating the visualization of a carbon fiber (diameter 7.0 μm) electrochemical interface. Proton consumption is driven at the interface by the reduction of benzoquinone to hydroquinone and the resulting interfacial pH change is revealed using the fluorophore 8-hydoxypyrene-1,3,6-trisulfonic acid. It is demonstrated that the proton depletion zone may be constrained and controlled by the addition of a finite acid concentration to the system. Simulation of the resulting fluorescence intensity profiles is achieved on the basis of a finite difference model, with excellent agreement between the theoretical and experimental results.

A snapshot of the electrochemical reaction layer by using 3 dimensionally resolved fluorescence mapping

“…9 Fluorescence microscopy coupled to EC, proposed first by Engstrom and coworkers in the '90s, 10 , 11 is an efficient tool to image diffusion layers in either 2D (profile view or lateral cross section) or 3D (stacks of cross sections). 12 , 13 Fluorescence confocal laser scanning microscopy (FCLSM) allows recording of fluorescence in a micrometer-sized volume. In contrast to wide-field microscopy, the fluorescence signal is collected from an optical slice with very low field depth at a given focal plane.…”

Electrochemically Generated Interfacial pH Change: Application to Signal‐Triggered Molecule Release

“…[42] Fluorescence microscopy, particularly using a confocal microscope, has also been used to map interfacial pH at different distances from an electrode surface. [11,[43][44][45][46][47][48] Notably, the confocal fluorescence spectroscopy, when appropriate instrumentation is used, allows the in-operando monitoring of interfacial pH variation in real time under variable experimental conditions. [44] Figure 2A shows pH controlled fluorescence output of 5(6)-carboxynaphthofluorescein (CNF) dye demonstrating fluorescence decrease at 567 nm and increase at 667 nm upon pH increase measured in a bulk solution.…”

Visualising electrochemical reaction layers: mediated vs. direct oxidation

“…The first voltammetric peak at 0.46 V corresponds to the reversible oneelectron oxidation of the (HPTS) pyrene, from an overall charge of À3, to the deprotonated radical with an overall charge of À3. 50,51 The second peak at 0.92 V is result of a further electron transfer to leading to the formation of a cation, with overall charge of À2, which is proposed to undergo rapid hydrolysis to form a catechol moiety resulting in a third reductive peak near 0.0 V. 52 The oxidation is HPTS that of the phenol moiety, not the direct oxidation of the pyrene ring system. Accordingly, no oxidation is seen for PYTS in the electrochemical window (i.e.…”

“…Although local H + would be generated during the reaction, HPTS, is a 'photoacid' as such the protonated and deprotonated forms fluorescence at the same wavelength. Moreover, the protonated form has a higher extinction coefficient at 365 nm (as used experimentally) as such if there was a significant change in pH during the course of the experiment one would anticipate and increase in the fluorescence intensity 51 as opposed to the observed decrease. Consequently, the formation of H + is not likely relevant in the present situation where the buffer is expected to be sufficient to control the interfacial pH.…”