The rising of microscopic electrochemistry: “watching” the local electron transfer optically

Abstract

Determination of the electron transfer rate plays a central role in electrochemistry. In a typical single-electron electro-oxidation reaction (Red e = Ox), a reactant molecule, Red, loses one electron to generate a product molecule, Ox. Traditionally, electrical signal measured by ammeter or potentiostat has been the only read-out signal that was utilized to quantify the electron transfer rate, i.e., the current, and to study its dependence with the electrode potential. Although powerful, electrical read-out faces two emerging challenges. Firstly, electrical signal is lack of spatial resolution and is not suitable for the wide-field electrochemical imaging. When measuring the electrode current, all the electrons merge together and contribute to a total signal. It is almost impossible to trace the original locations where each electron is transferred, placing a significant barrier for studying the heterogeneous interfaces. Secondly, electrical measurement is sometimes lack of sensitivity, especially when electron transfer occurs at extremely small interfaces such as a nano-sized electrode and the surface of a single nanoparticle. That is because the electrochemical current is a function of electrode surface area and it rapidly decreases with the square of the electrode dimension. When the electrode dimension decreases from 1 mm (regular electrode) to 10 nm (nano-electrode), the electrode current could decrease by 10 orders of magnitudes, resulting in a typical electrochemical current in the range of fA–pA. This value is in the marginal detection zone of even the mostly advanced current amplifiers that are commercially available. Since the past decade, optical imaging of electrochemical current has become a rapidly growing field. Typically, an optical microscopy was utilized to monitor the dynamic process of electrochemical reactions occurring on certain substrate–solution interface. By analyzing these optical images, one was able to indirectly map the interfacial electron transfer. Although multiple optical microscopy techniques have been utilized, the general philosophy is simple. If we go back to the classical electrochemical equation Red e = Ox, instead of directly measuring the electrons (e), these optical techniques measure the molecules (Red and/or Ox), which usually exhibit some kinds of optical properties. For example, from the consumption rate of the reactant or the generation rate of the product, one can equivalently resolve the electron transfer rate. Because of the excellent spatial resolution as well as the superior capability to image single nanoparticles and even single molecules, optical microscopy is able to map the local electron transfer with high spatial resolution and sensitivity, especially when heterogeneous and nano-sized interfaces are involved. Based on the philosophy above, Tao and co-workers [1] pioneered a novel electrochemical imaging method in 2010, termed plasmonic-based electrochemical microscopy, P-ECM. P-ECM is a concept utilizing surface plasmon resonance microscopy (SPRM) to achieve electrochemical imaging. SPRM is an optical microscopy that can map the spatial distribution of refractive index within a near-field close to a gold-coated coverslip, on which planar surface plasmon resonance is excited [2, 3]. By virtue of measuring the tiny difference in the refractive index between reactant and product molecules, P-ECM was able to map the local electrochemical current with a high spatial resolution close to the optical diffraction limit. Similarly, P-ECM was subsequently expanded to study the nonW. Wang (&) State Key Laboratory of Analytical Chemistry for Life Science, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, China e-mail: wei.wang@nju.edu.cn