Optical readouts of electrochemistry on plasmonic nanoparticle electrodes
MetadataShow full item record
This body of work uses optical signatures of molecules to read out electrochemical events on plasmonic nanoparticle electrodes to report on nanoscale electrochemistry. Using optical imaging we are able to probe electrochemical processes with high spatial resolution and sensitivities from single nanoparticles to single molecules. First, we monitor the change in surface-enhanced Raman scattering (SERS) from Nile Blue as it is oxidized and reduced to serve as an optical readout for electrochemistry on diffraction-limited plasmonic nanoparticle electrodes. Superlocalization imaging of SERS is used to show that the position of a molecule on the surface of a plasmonic nanoparticle aggregate electrode influences the potential at which it is oxidized and reduced. To probe the role of plasmon excitation in electrochemical redox events, a block copolymer lithography (BCPL) method is developed to provide tunable, monodisperse plasmonic nanoparticle electrodes over a large area. Differential pulse voltammetry and SERS show that the E₁/₂ of Nile Blue depends on the nanoparticle electrode size. Further, we demonstrate that laser illumination, which leads to plasmon excitation, shifts the E₁/₂ and the onset potentials of Nile Blue to more positive values. We rule out plasmonic heating as a possible mechanism and provide insight into the role plasmon-generated hot carriers and the plasmoelectric effect may have on plasmon-assisted electrochemistry. Lastly, we develop a strategy to image electrogenerated chemiluminescence (ECL) at single gold nanowires to probe the electrochemical activity of many plasmonic nanoparticle electrodes in tandem. ECL preserves the high sensitivity of an optical readout without the use of an external illumination source which has been revealed to alter electrochemical potentials through plasmon excitation. Removal of the stabilizing surfactant was found to be critical in electrochemically addressing individual gold nanowire electrodes. Additionally, we use a transparent, conductive polymer blend to coat the nanowires in order to prevent electrode passivation by a gold oxide monolayer. An increase in polymer thickness is shown to increase the ECL image quality and reproducibility, enabling an inexpensive, high throughput testing strategy for correlating nanoparticle electrode size, shape, and composition with electrochemical activity.