Selection of Highly SERS-Active Nanostructures from a Size Gradient of Au Nanovoids on a Single Bipolar Electrode

High-Throughput Electrosynthesis of Gradient Polypyrrole Film Using a Single-Electrode Electrochemical System

As an effective approach for materials synthesis, bipolar electrochemistry has been earning a renewed interest nowadays thanks to its unique features compared to conventional electrochemistry. Indeed, the wireless mode of electrode reactions and the generation of a gradient potential distribution above the bipolar electrode are among the most appealing qualities of bipolar electrochemistry. In particular, the gradient potential distribution is a highly attractive characteristic for the fabrication of surfaces with gradients in their chemical properties or molecular functionalities. Herein, we report the high-throughput electrosynthesis of gradient polypyrrole films by means of a new electrochemical cell design named the single-electrode electrochemical system (SEES). SEESs are made by attaching an inert plastic board with holes onto an indium tin oxide electrode, constructing multiple microelectrochemical cells on the same electrode. This type of arrangement enables parallel electrochemical reactions to be carried out simultaneously and controlled in a contactless manner by a single electrode. Several experimental conditions for polypyrrole film growth were extensively investigated. Furthermore, the gradient property of the polymer films was evaluated by thickness determination, surface morphology analysis, and contact angle measurements. The use of SEES has been demonstrated as a convenient and cost-effective strategy for high-throughput electrosynthesis and electroanalytical applications and has opened up a new door for gradient film preparation via a rapid condition screening process.

High-Throughput Surface-Enhanced Raman Scattering for Screening Chemical Sensor Candidates Enabled by Bipolar Electrochemistry

A variety of hydrothermal or electrochemical methods have been explored to prepare noble metal nanostructures as surface-enhanced Raman scattering (SERS) substrates. However, most of those metallic nanoarrays are structurally homogeneous, which makes it laborious to select the high-performance substrates for particular Raman sensing purposes. Here, a high-throughput SERS imaging strategy is demonstrated for the first time for screening chemical sensors with sub-nanomolar sensitivities. Bipolar electrochemistry was applied to generate Au or Au-Ag gradient nanoarrays with diverse chemical compositions, morphologies, and particle dimensions ranging from several nanometers to micrometers. The selected "hot-spots" on the Au-Ag alloy nanoarray exhibited a 660-fold enhancement in SERS intensity compared to those on the pure Au gradient nanoarray. The SERS screening of 4-aminothiophenol, 4-nitrothiophenol, and 4-mercaptobenzoic acid was carried out that provided a limit of detection (LOD) between 1 and 5 pM. The distinctive LODs among three thiophenolic Raman probes are ascribed to the differences in the affinity of the probe to the alloy, orientation of the metal-ligand monolayer, or plasmonic environment of the nanoarray surface. As a continuous, rapid, and cost-effective manner to fabricate transitional nanostructures and screen out SERS responsive sites, this method not only facilitates controllable synthesis of noble metal nanoarrays but has the potential to provide an alternative tool for ultrasensitive chemical sensing on a wide range of bimetallic substrates.

Self-Motion of Water Droplets along a Spacing Gradient of Micropillar Arrays on Copper

Self-driven droplet transport along an open gradient surface is increasingly becoming popular for various microfluidics applications. In this work, a gradient copper oxide layer is formed on a copper sheet (as a bipolar electrode, BPE) in a KOH solution by bipolar electrochemistry. The deposits at different positions present a rich variety of colors, compositions, and microstructures along the longitudinal axis of the BPE. More than half the length of the anodic pole is covered by a Cu(OH)₂/CuO composite layer of several micrometers thick, which is composed of dense micropillars with a decreasing spacing gradient to the anodic direction. The micropillar arrays are superhydrophilic, and after modified with 1-dodecanethiol, the tops of the dense micropillars constitute a hydrophobic and microscopically discontinuous surface with a wettability gradient. On such a gradient surface water droplets can move spontaneously to more hydrophilic direction at a velocity of about 16 mm s^-1. The superhydrophobicity of the modified micropillar arrays is discussed through a comparison with the wax tubules on a lotus leaf. Theoretical analysis of the driving force reveals that the concave surface effect of water at the spacings between the micropillars is the critical factor for driving the rolling motion of the droplets along the gradient micropillar arrays.

Optimized Ag Nanovoid Structures for Probing Electrocatalytic Carbon Dioxide Reduction Using Operando Surface-Enhanced Raman Spectroscopy

Surface-enhanced Raman spectroscopy is a powerful analytical tool and a strongly surface structure-dependent process. Importantly, it can be coupled with electrochemistry to simultaneously record vibrational spectroscopic information during electrocatalytic reactions. Highest Raman enhancements are obtained using precisely tuned nanostructures. The fabrication and evaluation of a high number of different nanostructures with slightly different properties is time-consuming. We present a strategy to systematically determine optimal nanostructure properties of electrochemically generated Ag void structures in order to find the void size providing highest signal enhancement for Raman spectroscopy. Ag-coated Si wafers were decorated with a monolayer of differently sized polymer nanospheres using a Langmuir-Blodgett approach. Subsequently, bipolar electrochemistry was used to electrodeposit a gradient of differently sized void structures. The gradient structures were locally evaluated using Raman spectroscopy of a surface-adsorbed Raman probe, and the surface regions exhibiting the highest Raman enhancement were characterized by means of scanning electron microscopy. High-throughput scanning droplet cell experiments were utilized to determine suitable conditions for the electrodeposition of the found highly active structure in a three-electrode electrochemical cell. This structure was subsequently employed as the working electrode in operando surface-enhanced Raman measurements to verify its viability as the signal amplifier and to spectroscopically rationalize the complex electrochemical reduction of carbon dioxide.

Modulation of Wetting Gradients by Tuning the Interplay between Surface Structuration and Anisotropic Molecular Layers with Bipolar Electrochemistry

A new simple and versatile method for the preparation of surface-wetting gradients is proposed. It is based on the combination of electrode surface structuration introduced by a sacrificial template approach and the formation of a tunable molecular gradient by bipolar electrochemistry. The gradient involves the formation of a self-assembled monolayer on a gold surface by selecting an appropriate thiol molecule and subsequent reductive desorption by means of bipolar electrochemistry. Under these conditions, completion of the reductive desorption process evolves along the bipolar surface with a maximum strength localized at the cathodic edge and a decreasing driving force towards the middle of the surface. The remaining quantity of surface-immobilized thiol, therefore, varies as a function of the axial position, resulting in the formation of a molecular gradient. The surface of the bipolar electrode is characterized at each step of the modification by recording heterogeneous electron transfer. Also, the evolution of static contact angles measured with a water droplet deposited on the surface directly reveals the presence of the wetting gradient, which can be modulated by changing the properties of the thiol. This is exemplified with a long, hydrophobic alkane-thiol and a short, hydrophilic mercaptan.