Probing Local Potentials inside Metallic Nanopores with SERS and Bipolar Electrochemistry

Abstract

CommuniCation(1 of 6) 1600907 considerable attention. In addition to being used as electrodes, optical nanostructures for extraordinary light transmission, [8] fluorescence, [9] and surface-enhanced Raman spectroscopy (SERS) [10] have demonstrated additional detection and/or control capabilities. For these optical techniques, metal layers are usually unbiased, so-called floating electrode mode, whereas applying a bias voltage may induce a potential drop over the floating metallic nanopore because of its high resistance. In the field of macroscale fluidics, a sufficient DC voltage difference between the floating metal layer and the solution enables the activation of oxidation and reduction reactions. [11,12] However, on the nanoscale, the study on the local potential and electrochemical effects on floating metallic nanopores remains challenging.Local potential and related electrochemistry on floating metal electrodes has been termed "bipolar electrochemistry" [12,13] In an unbiased state, bipolar electrochemistry requires either highly confined electric fields or extremely high concentration-gradients adjacent to bipolar electrodes [14][15][16] to drive reactions and investigate optical properties. Such investigation techniques include electrochemiluminescence, [17] electrochemical corrosion microscopy, [18] epifluoresence, [19] and surface plasmon resonance spectroscopy, [20] which are ultimately restricted by the diffraction of light. Here, working toward subwavelength detection, we propose the use of SERS to study the local potentials. [21][22][23][24][25] To simultaneously align localized SERS hot spots with fluidic focus for high spatial resolution and high specificity, [26] our technique probes the local potential and characterizes nanoscale bipolar electrochemical effects on metallic nanopores for the first time. Figure 1a depicts a schematic of the experiments on the metallic nanopores. The nanopore chip was mounted into a custom-made flow cell, separating the electrolyte into two reservoirs. Next, a 785 nm laser was tightly focused at the nanopore cavity. The electrical potential was applied to the top side of the membrane as shown in Figure 1a, and the other side was connected to ground. This defines the applied bias direction for positive and negative values in the experiments. A doublesided gold (MM) nanopore is used for the demonstration of bipolar electrochemical SERS. To maximize the plasmonic enhancement effect and subsequent SERS signals at 785 nm, It is essential to understand the local potential distribution of solid-state nanopores in nanofluidic systems. However, applying gate voltage or adding external electrical probes tends to disturb the electric field and/or flow patterns. To solve this problem, an approach is described to monitor the local potential using electrochemical surface enhanced Raman spectroscopy (EC-SERS) in two types of nanocavity pores: doubled-sided gold nanopores (MM nanopores) and single-sided gold nanopores with a dielectric passivation layer on the backside (MD nanopor...

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