Single-Molecule Electrochemistry on a Porous Silica-Coated Electrode

Abstract

Here we report the direct observation and quantitative analysis of single redox events on a modified indium–tin oxide (ITO) electrode. The key in the observation of single redox events are the use of a fluorogenic redox species and the nanoconfinement and hindered redox diffusion inside 3-nm-diameter silica nanochannels. A simple electrochemical process was used to grow an ultrathin silica film (~100 nm) consisting of highly ordered parallel nanochannels exposing the electrode surface from the bottom. The electrode-supported 3-nm-diameter nanochannels temporally trap fluorescent resorufin molecules resulting in hindered molecular diffusion in the vicinity of the electrode surface. Adsorption, desorption, and heterogeneous redox events of individual resorufin molecules can be studied using total-internal reflection fluorescence (TIRF). The rate constants of adsorption and desorption processes of resorufin were characterized from single-molecule analysis to be (1.73 ± 0.08) × 10−4 cm·s−1 and 15.71 ± 0.76 s−1, respectively. The redox events of resorufin to the non-fluorescent dihydroresorufin were investigated by analyzing the change in surface population of single resorufin molecules with applied potential. The scan-rate-dependent molecular counting results (single-molecule fluorescence voltammetry) indicated a surface-controlled electrochemical kinetics of the resorufin reduction on the modified ITO electrode. This study demonstrates the great potential of mesoporous silica as a useful modification scheme for studying single redox events on a variety of transparent substrates such as ITO electrodes and gold or carbon film coated glass electrodes. The ability to electrochemically grow and transfer mesoporous silica films onto other substrates makes them an attractive material for future studies in spatial heterogeneity of electrocatalytic surfaces.

Nanoscale Electrochemical Mapping

“…229 Adsorption, desorption, and heterogeneous redox events of individual resorufin molecules were investigated on ITO electrodes modified with mesoporous silica using totalinternal reflection fluorescence microscopy. 230 The 0.7 zL silica channels (3 nm diameter and Super-resolution fluorescence microscopy can also be applied to follow dynamic processes of NP collision and oxidation in a one-dimensional space. For example, in a recent study, 76 an innovative nanochannel cell configuration was developed to optically monitor the dynamic collision and oxidation of AgNPs in situ using single-particle fluorescence microscopy.…”

Single-molecule fluorescence microscopy review: shedding new light on old problems

“…Thus, the group of Bo Zhang discovered that mesoporous silica reduces the rate of diffusion of fluorogenic redox molecules, enabling observation of single redox events. The study was carried out on fluorescent resorufin allowing analysis of adsorption, desorption and redox events by TIRF molecules on transparent ITO electrodes coated with mesoporous silica [ 110 ].…”

Observing the Reversible Single Molecule Electrochemistry of Alexa Fluor 647 Dyes by Total Internal Reflection Fluorescence Microscopy

“…There are experimental restrictions encountered by electrochemical modulation of fluorescence that are not faced by optical methods. Firstly, the surface fluorescent species must be maintained close to the electrode surface for oxidation and reduction to occur . This often means the fluorescent species are immobilized .…”

“…[7] Through repeated cycles,t he fluorophores can then be collected to reconstruct an overall image.T his is the principle behind single molecule localization microscopy (SMLM) techniques such as photoactivated light microscopy (PALM) and stochastic optical reconstruction microscopy (STORM). [15] This often means the fluorescent species are immobilized. Electrochemistry may provide an alternative to achieve reversible redox switching of fluorescence between highly fluorescent and weakly fluorescent states as these are typically redox processes.…”

“…[10][11][12][13][14] There are experimental restrictions encountered by electrochemical modulation of fluorescence that are not faced by optical methods.F irstly,t he surface fluorescent species must be maintained close to the electrode surface for oxidation and reduction to occur. [15] This often means the fluorescent species are immobilized. [16,17] This immobilisation does allow total internal reflection fluorescence (TIRF)based experiments to be performed, which provides single molecule sensitivity such that single molecule electrochemical processes can be explored.…”