S. G. Lemay, S. Kang, K. Mathwig and P. S. Singh
Single-Molecule Electrochemistry: Present Status and Outlook
Accounts of Chemical Research 46 (2013) 369.
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The development of methods for detecting and manipulating matter at the level of individual macromolecules represents one of the key scientific advancements of recent decades. These techniques allow us to get information that is largely unobtainable otherwise, such as the magnitudes of microscopic forces, mechanistic details of catalytic processes, macromolecular population heterogeneities, and time-resolved, step-by-step observation of complex kinetics. Methods based on optical, mechanical, and ionic-conductance signal transduction are particularly developed. However, there is scope for new approaches that can broaden the range of molecular systems that we can study at this ultimate level of sensitivity and for developing new analytical methods relying on single-molecule detection. Approaches based on purely electrical detection are particularly appealing in the latter context, since they can be easily combined with microelectronics or fluidic devices on a single microchip to create large parallel assays at relatively low cost.
A form of electrical signal transduction that has so far remained relatively underdeveloped at the single-molecule level is the direct detection of the charge transferred in electrochemical processes. The reason for this is simple: only a few electrons are transferred per molecule in a typical faradaic reaction, a heterogeneous charge-transfer reaction that occurs at the electrode’s surface. Detecting this tiny amount of charge is impossible using conventional electrochemical instrumentation. A workaround is to use redox cycling, in which the charge transferred is amplified by repeatedly reducing and oxidizing analyte molecules as they randomly diffuse between a pair of electrodes. For this process to be sufficiently efficient, the electrodes must be positioned within less than 100 nm of each other, and the analyte must remain between the electrodes long enough for the measurement to take place. Early efforts focused on tip-based nanoelectrodes, descended from scanning electrochemical microscopy, to create suitable geometries. However, it has been challenging to apply these technologies broadly.
In this Account, we describe our alternative approach based on electrodes embedded in microfabricated nanochannels, so-called nanogap transducers. Microfabrication techniques grant a high level of reproducibility and control over the geometry of the devices, permitting systematic development and characterization. We have employed these devices to demonstrate single-molecule sensitivity. This method shows good agreement with theoretical analysis based on the Brownian motion of discrete molecules, but only once the finite time resolution of the experimental apparatus is taken into account. These results highlight both the random nature of single-molecule signals and the complications that it can introduce in data interpretation. We conclude this Account with a discussion on how scientists can overcome this limitation in the future to create a new experimental platform that can be generally useful for both fundamental studies and analytical applications.
Redox cycling between two electrodes separated by a narrow gap allows dramatic amplification of the faradaic current. Unlike conventional electrochemistry at a single electrode, however, the mass-transport-limited current is controlled by the diffusion coefficient of both the reduced and oxidized forms of the redox-active species being detected and, counter-intuitively, by the redox state of molecules in the bulk solution outside the gap itself. Using a combination of finite-element simulations, analytical theory and experimental validation, we elucidate the interplay between these interrelated factors. In so doing we generalize previous results obtained in the context of scanning electrochemical microscopy and obtain simple analytical results that are generally applicable to experimental situations where efficient redox cycling takes place.
Klaus Mathwig 