Activationless Electron Transfer of Redox-DNA in Electrochemical Nanogaps.

Our recent discovery of decreased reorganization energy in electrode-tethered redox-DNA systems prompts inquiries into the origin of this phenomenon and suggests its potential use to lower the activation energy of electrochemical reactions. Here, we show that the confinement of the DNA chain in a nanogap amplifies this effect to an extent to which it nearly abolishes the intrinsic activation energy of electron transfer. Employing electrochemical atomic force microscopy (AFM-SECM), we create sub-10 nm nanogaps between a planar electrode surface bearing end-anchored ferrocenylated DNA chains and an incoming microelectrode tip. The redox cycling of the DNA's ferrocenyl (Fc) moiety between the surface and the tip generates a measurable current at the scale of ∼10 molecules. Our experimental findings are rigorously interpreted through theoretical modeling and original molecular dynamics simulations (Q-Biol code). Several intriguing findings emerge from our investigation: (i) The electron transport resulting from DNA dynamics is many times faster than predicted by simple diffusion considerations. (ii) The current in the nanogap is solely governed by the electron transfer rate at the electrodes. (iii) This rate rapidly saturates as overpotentials applied to the nanogap electrodes increase, implying near-complete suppression of the reorganization energy for the oxidation/reduction of the Fc heads within confined DNA. Furthermore, evidence is presented that this may constitute a general, previously unforeseen, behavior of redox polymer chains in electrochemical nanogaps.

Ballistic Brownian Motion of Nanoconfined DNA.

Theoretical treatments of polymer dynamics in liquid generally start with the basic assumption that motion at the smallest scale is heavily overdamped; therefore, inertia can be neglected. We report on the Brownian motion of tethered DNA under nanoconfinement, which was analyzed by molecular dynamics simulation and nanoelectrochemistry-based single-electron shuttle experiments. Our results show a transition into the ballistic Brownian motion regime for short DNA in sub-5 nm gaps, with quality coefficients as high as 2 for double-stranded DNA, an effect mainly attributed to a drastic increase in stiffness. The possibility for DNA to enter the underdamped regime could have profound implications on our understanding of the energetics of biomolecular engines such as the replication machinery, which operates in nanocavities that are a few nanometers wide.

Electrochemical Shot Noise of a Redox Monolayer.

Redox monolayers are the base for a wide variety of devices including high-frequency molecular diodes or biomolecular sensors. We introduce a formalism to describe the electrochemical shot noise of such a monolayer, confirmed experimentally at room temperature in liquid. The proposed method, carried out at equilibrium, avoids parasitic capacitance, increases the sensitivity, and allows us to obtain quantitative information such as the electronic coupling (or standard electron transfer rates), its dispersion, and the number of molecules. Unlike in solid-state physics, the homogeneity in energy levels and transfer rates in the monolayer yields a Lorentzian spectrum. This first step for shot noise studies in molecular electrochemical systems opens perspectives for quantum transport studies in a liquid environment at room temperature as well as highly sensitive measurements for bioelectrochemical sensors.

Nanoscale electrochemical charge transfer kinetics investigated by electrochemical scanning microwave microscopy.

We show how microwave microscopy can be used to probe local charge transfer reactions with unprecedented sensitivity, visualizing surface reactions with only a few hundred molecules involved. While microwaves are too fast under classical conditions to interact and sense electrochemical processes, this is different at the nanoscale, where our heterodyne microwave sensing method allows for highly sensitive local cyclic voltammetry (LCV) and local electrochemical impedance spectroscopy (LEIS). LCV and LEIS allow for precise measurement of the localized charge transfer kinetics, as illustrated in this study for a ferrocene self-assembled monolayer immersed in an electrolyte. The theoretical analysis presented here enables a consistent mapping of the faradaic kinetics and the parasitic contributions (nonfaradaic) to be spectrally resolved and subtracted. In particular, this methodology reveals an undistorted assessment of accessible redox site density of states associated with faradaic capacitance, fractional surface coverage and electron transfer kinetics at the nanoscale. The developed methodology opens a new perspective on comprehending electrochemical reactivity at the nanoscale.

Attoampere Nanoelectrochemistry.

Electrochemical microscopy techniques have extended the understanding of surface chemistry to the micrometer and even sub-micrometer level. However, fundamental questions related to charge transport at the solid-electrolyte interface, such as catalytic reactions or operation of individual ion channels, require improved spatial resolutions down to the nanoscale. A prerequisite for single-molecule electrochemical sensitivity is the reliable detection of a few electrons per second, that is, currents in the atto-Ampere (10-18 A) range, 1000 times below today's electrochemical microscopes. This work reports local cyclic voltammetry (CV) measurements at the solid-liquid interface on ferrocene self-assembled monolayer (SAM) with sub-atto-Ampere sensitivity and simultaneous spatial resolution < 80 nm. Such sensitivity is obtained through measurements of the charging of the local faradaic interface capacitance at GHz frequencies. Nanometer-scale details of different molecular organizations with a 19% packing density difference are resolved, with an extremely small dispersion of the molecular electrical properties. This is predicted previously based on weak electrostatic interactions between neighboring redox molecules in a SAM configuration. These results open new perspectives for nano-electrochemistry like the study of quantum mechanical resonance in complex molecules and a wide range of applications from electrochemical catalysis to biophysics.