Characterizing the Spatial Dependence of Redox Chemistry on Plasmonic Nanoparticle Electrodes Using Correlated Super-Resolution Surface-Enhanced Raman Scattering Imaging and Electron Microscopy

Abstract

Super-resolution surface-enhanced Raman scattering (SERS) is used to investigate local surface potentials on plasmonic gold and silver colloidal aggregates using the redoxactive reporter molecule, Nile Blue A. This molecule is electrochemically modulated between an oxidized emissive state and a reduced dark state. The diffraction-limited SERS emission from Nile Blue on the surface of a single plasmonic nanoparticle aggregate is fit to a two-dimensional Gaussian to track the position of the emission centroid as a function of applied potential. Potential-dependent centroid positions are observed, consistent with molecules experiencing site-specific oxidation and reduction potentials on the nanoparticle electrode surface. Correlated structural analysis performed with scanning electron microscopy reveals that molecules residing in nanoparticle junction regions, or SERS hot spots, appear to be reduced and oxidized at the most negative applied potentials as the potential is cycled. F rom light harvesting 1,2 to trace chemical sensing 3−10 and electrocatalysis, 11−13 plasmonic nanoparticles represent a promising material with a diverse array of applications that are highly dependent upon nanoparticle geometry. Although ensemble studies can elucidate information about a population of nanoparticles, these studies often overlook how subtle differences in nanoparticle structure can influence their properties. To develop a more complete nanoscale understanding of the effect of nanoparticle geometry upon optical or other behaviors, our group and others have used correlated structural and super-resolution optical studies to study molecular interactions with plasmonic substrates. 14−28 In recent work, we used this approach to study the redox reaction of Nile Blue A (NB) at the surface of aggregated silver nanoparticles using surface-enhanced Raman scattering (SERS) as a reporter of the redox state of the molecule. 19 Although the oxidized form of NB is resonant with 642 nm laser excitation and produces strong SERS signal, the reduced form produces little to no SERS. 11,29−31 Thus, NB acts as a simple on/off probe, and potential cycling allows us to move from the ensemble to single-molecule regime with ease. Our earlier studies using super-resolution imaging to localize the SERS emission as the applied potential was modulated suggested that NB molecules in different locations on an aggregated nanoparticle structure experienced different local potentials, indicating a relationship between local redox chemistry and nanoparticle structure. 19

Unforeseen distance-dependent SERS spectroelectrochemistry from surface-tethered Nile Blue: the role of molecular orientation

“…However, in subsequent work from our lab, we immobilized the NB on spherical Au nanoparticle aggregates via carbodiimide coupling to a carboxylic acid-terminated alkanethiol self-assembled monolayer (SAM) to prevent molecule diffusion and desorption. 8 In contrast to the physisorption case, Fig. 1B shows that in the anodic scan, the SERS intensity initially rises, as expected, then falls to a minimum intensity value at the most positive potential (red dashed lines in Fig.…”

“…[1][2][3] The redox dye Nile Blue (NB) has emerged as an excellent probe for probing fundamental electrochemistry with SERS. [4][5][6][7][8][9][10] A key feature of NB is that resonant excitation (e.g. 642 nm) leads to a strong SERS signal when the molecule is in its oxidized form and a weak SERS signal when it is in its reduced form, providing a spectral readout for reduction and oxidation events (e.g.…”

“…In addition to its redox-dependent spectral signatures, which are detectable down to the single molecule level, 4 NB is attractive because it can be immobilized on noble metal electrodes, allowing the possibility of structure-activity correlation. 8,11 Molecules tethered to an electrode, however, will have electrochemical behaviors different from their solvated analogues, where diffusion or convection control the current response. Furthermore, the nature of the tether (e.g.…”

“…1 highlights a previous example from our group in which tethering the NB molecules to a nanoelectrode altered the SERS spectroelectrochemical response. 8 Fig. 1A shows the integrated SERS intensity from NB physisorbed onto Ag colloidal aggregates as a function of applied potential.…”

Single-Particle Plasmon Voltammetry (spPV) for Detecting Anion Adsorption

“…3,7,8,14 Pt wires formed a quasi-reference electrode and counter-electrode. 53 Cells were filled with 0.1 M Na 2 SO 4 electrolyte, allowing the bare nanoparticles and conductive substrates to be directly accessed by ions in the electrolyte solution.…”