Electrochemistry at the single entity level (such as individual nanoparticles, molecules, etc.,) is continuously gaining interest because of its ability to reveal heterogeneous behavior in the system that is not possible to obtain using the traditional ensemble approach. In the past few years, the electrochemical version of a nanophotonic platform called electrochemical zero-mode waveguides (E-ZMWs) or nanopore electrode arrays (NEAs) is gaining much attention as a versatile platform to study the behavior of single entities using a correlated electrochemical-optical approach. ZMWs are cylindrical or conical nanopores in a thin metal film (typically 100 – 200 nm) with a bottom pore diameter smaller than the wavelength of visible light, capable of confining excitation radiation within the nanopore with the intensity decaying exponentially along the axial direction. E-ZMWs structures provide a bifunctional capability, in which the optical cladding layer serves both to confine radiation and as an electrochemical working electrode. We have used E-ZMWs to successfully probe the change in proton concentration (pH) in an attoliter volume scale down to single proton changes. Additionally, we have used it to sequester single nanoparticles and enzymes to probe both their optical and electrochemical properties. Apart from the fundamental studies, this substrate can also be transformed into an ultra-sensitive sensing platform.

Single entity electrochemistry in nanopore electrode arrays

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