Optical Voltammetry of redox processes inside a nanohole with Opto-iontronic microscopy

TL;DR

The paper addresses the challenge of probing electrochemical reactions at nanoscopic, nonfluorescent interfaces by developing Opto-iontronic microscopy, which combines TIR-evanescent-field scattering with nanohole electrodes and lock-in detection to monitor EDL dynamics and redox processes in attoliter volumes. It demonstrates AC voltammetry inside a single nanohole using Fc(MeOH)2, correlating optical contrast with local redox-species concentrations and validating the observations with a Poisson-Nernst-Planck–Butler-Volmer (PNP-BV) model. The model provides spatially resolved ion concentration profiles and confirms that Fc/Fc+ dynamics chiefly govern the optical signal, while KCl ions mainly reflect EDL charging. The work offers a high-sensitivity, label-free method for nanoconfined electrochemistry with potential applications in nanocrystal growth monitoring and single-molecule electrochemistry, and outlines paths to further improve sensitivity and quantitative spectroscopy.

Abstract

Cyclic Voltammetry (CV) is the most commonly used method in electrochemistry to characterize electrochemical reactions, usually involving macroscopic electrodes. Here we demonstrate a novel optical CV technique called Opto-iontronic Microscopy, which is capable of monitoring electrochemical processes at the nanoscale. By integrating optical microscopy with nanohole electrodes, we enhance sensitivity in detecting redox reactions within volumes as small as an attoliter ($(100 \text{~nm})^{3}$). This technique uses total internal reflection (TIR) illumination, Electric-double-layer modulation, cyclic voltammetry, and lock-in detection, to probe ion dynamics inside nanoholes. We applied this method to study EDL (dis)charging coupled to ferrocenedimethanol (Fc(MeOH)$_2$) redox reactions. Experimental results were validated against a theoretical Poisson-Nernst-Planck-Butler-Volmer (PNP-BV) model, providing insights into ion concentration changes of reaction species that contribute to the optical contrast. This work opens up opportunities for high-sensitivity, label-free analysis of electrochemical reactions in nanoconfined environments, with potential applications in pure nanocrystal growth and monitoring, and potentially single-molecule electrochemistry.

Figures (5)

• Figure 1: (A) Schematic of the total--internal--reflection Opto--iontronic microscope. T: telescope for beam diameter adjustment; M1-M4: adjustable mirrors; L1, L2: beam focus lens; L3, L4: imaging lens; MM1, MM2: small prism mirrors; OBJ: microscope objective; BFP: back focal plane of the microscope objective; CyL: cylindrical lens; RE: reference electrode in the chemical cell; CE: counter electrode in the chemical cell; BS: beam splitter; PH: pinhole; PD: photodiode; sCMOS: scientific complementary metal--oxide--semiconductor camera; QPD: quadrant photodiode. (B) SEM image (top) and light scattering image (bottom) of the nanohole array. The light scattering image was taken by the sCMOS camera without an applied potential. (C) Schematic representation of the nanohole geometry, with a depth of $100$ nm and a diameter of $75$ nm.
• Figure 2: Potentiodynamic measurements of EDL charging/discharging in parallel with cyclic voltammetry inside a single nanohole. (A) Capacitive EDL charging/discharging of a single nanohole in contact with a grounded bulk KCl solution of concentration $C_{\mathrm{KCl}} = 0.1\text{~M}$. The potential waveform $V(t)$ (top panel) is applied to the nanohole electrode (yellow in the insets), the corresponding capacitive current $i(t)$ (middle panel) from the nanohole electrode and the intensity $I(t)$ (bottom) of the light scattered from a single nanohole normalized to the intensity at potential $V=0$ V. (B) Sketches of the ion arrangements at negative potential (left) and positive potential (right) on the inner surface of the nanohole, featuring the EDL of thickness $\lambda_d\simeq 1\text{~nm}$ (not to scale) that forms the capacitor. (C) Cyclic voltammetry measurement inside the nanohole with 1 mM Fc(MeOH)2 and 0.1 M KCl supporting electrolyte, where the potential is scanned back and forth between $-0.25$ V and $0.35$ V with a scanning speed of 600 mV/s. Top panel: The scan potential $V(t)$ (red) applied to the nanohole electrode and the corresponding normalized light scattering intensity $I(t)$ of the nanohole averaged over 100 cycles (green). Middle panel: The potential dependence of the total current $i(V(t))$, which changes sign upon entering the oxidation (dark gray) and reduction (light gray) windows due to Faradaic contributions that develop on top of the capacitive current. Bottom panel: The variation of the light scattering intensity with applied electrode potential, $dI/dV$, which drops below the constant capacitive baseline (dashed horizontal) in the two redox windows, signifying the optical measurement of the electrochemical redox processes in the nanohole. (D) Sketch of the ion and redox species arrangements in the nanohole in the oxidation window (positive potential).
• Figure 3: The AC voltammetry measurements. (A) The applied AC potential is the superposition of an AC sinusoidal modulation potential and a linear triangle offset potential. (B) Top panel: The applied modulation potential and quasi-DC scan potential(red curve) and the response currents from the nanohole electrode(blue curve). Bottom panel: The averaged amplitude and phase of the modulated optical signal over 8 cycles. Filled areas are the standard deviation of the averaged points. (C) Zoom-in on the applied potential and currents. The scan rate of the quasi-DC potential is $15$ mV/s from $-0.25$ V to $0.25$ V. The amplitude of the potential modulation is $50$ mV. Electrolyte solution is $10$ mM Fc(MeOH)2 in $0.1$ M KCl solution.
• Figure 4: (A) The amplitude of the modulated optical signal in the function of potential in different concentrations of Fc(MeOH)2 from 0.1 mM to 10 mM. The supporting electrolyte keeps at the same concentration at $0.1$ M KCl. The amplitude of the modulation potential is $V_{0}= 100$ mV, the frequency of the modulation potential is $f = 975$ Hz, the step of the quasi-DC potential scan is $V_{step}= 35$ mV, with the period of $t_{step}= 100$ s for each step. (B) The amplitude of the modulated optical signal as a function of AC potential in different concentrations of supporting electrolyte KCl from 0.15 M to 1.5 M. The concentration of Fc(MeOH)2 remains the same at $2$ mM. The amplitude of the modulation potential is $V_{0}=$ 100 mV, the frequency of the modulation potential is $f =$ 975 Hz, the quasi-DC potential linearly scans from $-250$ mV to $450$ mV and then scans back at a scan rate of $R_{scan}= 50$ mV/s. (C) The amplitude difference $\Delta A_{0}$ of the modulated optical signal as a function of the modulation frequency from 10 Hz to 975 Hz, the amplitude of the modulation potential is $V_{0}= 100$ mV, the concentration of Fc(MeOH)2 is $2$ mM.
• Figure 5: Results obtained from numerical solutions of the Poisson-Nernst-Planck and Butler-Volmer (PNP-BV) equations for diffusion and migration coupled to the redox reaction. (A) Schematics (not to scale) of the geometry of a single cylindrical nanohole electrode in contact with the grounded bulk electrolyte, with $d = 100$ nm, $h = 100$ nm, $w = 10~d$, and $l = 1000~h$. (B) Time-dependence of the slow triangular quasi-DC scanning potential (peak values $\pm250$ mV and scan rate $15\text{~mV/s}$) and its superimposed fast small-amplitude sinusoidal modulation (amplitude $50\text{~mV}$ and frequency $75\text{~Hz}$). (C) Comparison between the PNP-BV calculation (green) and experiment (blue) for time-dependent current at bottom point "H", shown for bulk concentrations $C_{\mathrm{Fc}}$ = $10$ mM and $C_{\mathrm{KCl}}$ = $0.1$ M. The calculated current density $j$ was multiplied by the total surface area of the electrode $A_{\mathrm{tot}}$. Time-evolution (black) of the nanohole-averaged concentration of (D) Fc+ and (E) Fc inside the nanohole for $C_{\mathrm{Fc}}$ = $10$ mM and $C_{\mathrm{KCl}}=0.1$ M, with their maxima (blue) and minima (orange). Their difference is the concentration modulation amplitude (red). (F) 2D heat map of the local Fc+ concentration modulation amplitude (mM) for $C_{\mathrm{Fc}}$ = $10$ mM and $C_{\mathrm{KCl}}$ = $0.1$ M. Sum of the concentration modulation amplitude of Fc and Fc+ as a function of the scanning potential, for various bulk concentrations of (G) Fc with $C_{\mathrm{KCl}}$ = $0.1$ M, and (H) KCl with $C_{\mathrm{Fc}} =10$ mM.