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.

Figure 1

Fluorescence intensity (red) and current response (black) over the course of potential sweeping (green) from 0 to −0.7 V at the scan rate of 50 mV/s in 50 mM phosphate buffer (pH 7.4) containing 50 μM resazurin. WE, carbon film electrode (20 nm thick, deposited on a glass slide); RE, Ag/AgCl wire; CE, Pt wire.

Figure 2

(a) Surface height profile of a thin mesoporous silica film on the ITO surface. (b) SEM image of mesoporous silica film on the ITO surface. (c) TEM image of mesoporous silica film (top view) deposited on a carbon-coated TEM grid. Inset: high-magnification top view. (d) CVs of 1 mM Ru(NH₃)₆³⁺ in 1 M KCl on the bare ITO (black dashed) and the mesoporous silica-ITO electrode before (red dotted) and after (blue solid) the removal of surfactant templates. Scan rate: 50 mV/s.

Figure 3

Distribution of τ_on of single resorufin molecules on the mesoporous silica coated ITO surface. Red line is a single-exponential decay fit with a time constant of 0.064 ± 0.003 s. Inset: (left) the fluorescence trajectory from one fluorescent spot; (right) histogram of the fluorescence trajectory over a 13 × 13 μm² area for 40 s.

Figure 4

(a) A series of fluorescence images captured over the potential sweep of 1 nM resorufin in 50 mM phosphate buffer (pH 7.4) and 1 M KCl solution on the silica-ITO electrode. The potential was cycled from 0 to −1 V versus an Ag/AgCl reference electrode at the scan of 50 mV/s. Image size is 16 μm × 16 μm. (b) The number of single molecular spots detected on each frame over three continuous potential sweeps (black solid). The potential waveform was shown in dashed line. (c) The plot of the number of single molecular spots detected on each frame (black) and averaged over three potential sweeps (red), and the time derivative (blue dashed) as a function of applied potential from 0 to −1 V.

Figure 5

(a) The number of single molecular spots detected on each frame over three continuous potential sweeps with the scan rate of 20 mV/s, 50 mV/s, 100 mV/s, and 200 mV/s in 1 nM resorufin solution containing 50 mM phosphate buffer (pH 7.4) and 1 M KCl on the silica-ITO electrode. (b) The time derivative of the number of single molecular spots as a function of applied potential from 0 to −1 V at different scan rates of 20 mV/s (orange), 50 mV/s (blue), 100 mV/s (magenta), and 200 mV/s (black). (c) The scan rate dependence of the peak values (black square) of the time derivative traces in (b). Red lines are the linear fit.

Figure 6

(a) Measured k_obs, the time constant of single-exponential decay of τ_on distribution, and (b) the average number of photons per fluorescent spot at the potentials from 0 to −1 V with 0.1 V step. The error bars indicate the standard deviation.

Scheme 1. (a) Schematic Illustration of Single-Molecule Electrochemistry on a Porous Silica-Modified ITO Electrode^a and (b) Reaction Scheme for Irreversible Reduction from Resazurin (S) to Resorufin (P), and Reversible Redox Reaction between Resorufin (P) and Dihydroresorufin (PH₂)

^aNot drawn to scale.