Andrew J. Wilson

Visualizing site-specific redox potentials on the surface of plasmonic nanoparticle aggregates with superlocalization SERS microscopy.

In this Letter, we demonstrate site-specific redox potentials for Nile Blue adsorbed to Ag nanoparticle electrodes using surface-enhanced Raman scattering (SERS) superlocalization microscopy. Nile Blue is electrochemically modulated between its oxidized and reduced form, which can be optically read out through a corresponding gain or loss in SERS intensity. SERS emission centroids are calculated by fitting the diffraction-limited SERS emission to a two-dimensional Gaussian to determine the approximate location of the emitter with 5-10 nm precision. With molecular coverage above the single molecule level, the SERS centroid trajectories shift reversibly with applied potential over multiple reduction and oxidation cycles. A mechanism is proposed to explain the centroid trajectories based on site-specific redox potentials on the nanoparticle electrode surface, where the first molecule reduced is the last to be oxidized, consistent with reversible electrochemical behavior of redox probes adsorbed to electrode surfaces.

Investigating Nanoscale Electrochemistry with Surface- and Tip-Enhanced Raman Spectroscopy

The chemical sensitivity of surface-enhanced Raman spectroscopy (SERS) methodologies allows for the investigation of heterogeneous chemical reactions with high sensitivity. Specifically, SERS methodologies are well-suited to study electron transfer (ET) reactions, which lie at the heart of numerous fundamental processes: electrocatalysis, solar energy conversion, energy storage in batteries, and biological events such as photosynthesis. Heterogeneous ET reactions are commonly monitored by electrochemical methods such as cyclic voltammetry, observing billions of electrochemical events per second. Since the first proof of detecting single molecules by redox cycling, there has been growing interest in examining electrochemistry at the nanoscale and single-molecule levels. Doing so unravels details that would otherwise be obscured by an ensemble experiment. The use of optical spectroscopies, such as SERS, to elucidate nanoscale electrochemical behavior is an attractive alternative to traditional approaches such as scanning electrochemical microscopy (SECM). While techniques such as single-molecule fluorescence or electrogenerated chemiluminescence have been used to optically monitor electrochemical events, SERS methodologies, in particular, have shown great promise for exploring electrochemistry at the nanoscale. SERS is ideally suited to study nanoscale electrochemistry because the Raman-enhancing metallic, nanoscale substrate duly serves as the working electrode material. Moreover, SERS has the ability to directly probe single molecules without redox cycling and can achieve nanoscale spatial resolution in combination with super-resolution or scanning probe microscopies. This Account summarizes the latest progress from the Van Duyne and Willets groups toward understanding nanoelectrochemistry using Raman spectroscopic methodologies. The first half of this Account highlights three techniques that have been recently used to probe few- or single-molecule electrochemical events: single-molecule SERS (SMSERS), superlocalization SERS imaging, and tip-enhanced Raman spectroscopy (TERS). While all of the studies we discuss probe model redox dye systems, the experiments described herein push the study of nanoscale electrochemistry toward the fundamental limit, in terms of both chemical sensitivity and spatial resolution. The second half of this Account discusses current experimental strategies for studying nanoelectrochemistry with SERS techniques, which includes relevant electrochemically and optically active molecules, substrates, and substrate functionalization methods. In particular, we highlight the wide variety of SERS-active substrates and optically active molecules that can be implemented for EC-SERS, as well as the need to carefully characterize both the electrochemistry and resultant EC-SERS response of each new redox-active molecule studied. Finally, we conclude this Account with our perspective on the future directions of studying nanoscale electrochemistry with SERS/TERS, which includes the integration of SECM with TERS and the use of theoretical methods to further describe the fundamental intricacies of single-molecule, single-site electrochemistry at the nanoscale.

Super-Resolution Imaging and Plasmonics

This review describes the growing partnership between super-resolution imaging and plasmonics, by describing the various ways in which the two topics mutually benefit one another to enhance our understanding of the nanoscale world. First, localization-based super-resolution imaging strategies, where molecules are modulated between emissive and nonemissive states and their emission localized, are applied to plasmonic nanoparticle substrates, revealing the hidden shape of the nanoparticles while also mapping local electromagnetic field enhancements and reactivity patterns on their surface. However, these results must be interpreted carefully due to localization errors induced by the interaction between metallic substrates and single fluorophores. Second, plasmonic nanoparticles are explored as image contrast agents for both superlocalization and super-resolution imaging, offering benefits such as high photostability, large signal-to-noise, and distance-dependent spectral features but presenting challenges for localizing individual nanoparticles within a diffraction-limited spot. Finally, the use of plasmon-tailored excitation fields to achieve subdiffraction-limited spatial resolution is discussed, using localized surface plasmons and surface plasmon polaritons to create confined excitation volumes or image magnification to enhance spatial resolution.

Imaging Electrogenerated Chemiluminescence at Single Gold Nanowire Electrodes.

We report electrogenerated chemiluminescence (ECL) generated at single gold nanowire electrodes supported on tin-doped indium oxide. Unlike other single nanoparticle electrochemical characterization techniques, ECL provides a massively parallel direct readout of electrochemical activity on individual nanoparticle electrodes without the need for extrinsic illumination or a scanning electrochemical probe. While ECL is not observed from as-purchased nanowires due to the surfactant layer, by removing the layer and coating the nanowires with a polymer blend, ECL from single nanowire electrodes is readily measured. With an increase in polymer thickness, an increase in ECL image quality and reproducibility over multiple redox cycles is observed. The polymer coating also provides a strategy for stabilizing gold nanoparticle electrodes against complete surface oxidation in aqueous environments.

Surface-enhanced Raman scattering imaging using noble metal nanoparticles.

Surface-enhanced Raman scattering (SERS) imaging is a powerful technique for studying biological systems both in vitro and in vivo. In SERS, Raman scattering from molecules located near the surface of either gold or silver nanoparticles is enhanced by 10(5) - 10(8). This review describes the basic enhancement mechanism of SERS and provides experimental details that must be considered when performing a SERS imaging experiment with a focus on cellular imaging. Specific examples highlighting the power of SERS for measuring chemical distributions in cells, signal multiplexing, and following dynamic motion of SERS probes in vivo are provided. Potential future directions in which SERS is combined with super-resolution imaging are also described.

Tunable electroresistance and electro-optic effects of transparent molecular ferroelectrics

A water-based approach grows molecular ferroelectric thin film with tunable electroresistance and electro-optic effect. Recent progress in molecular ferroelectrics (MOFEs) has been overshadowed by the lack of high-quality thin films for device integration. We report a water-based air-processable technique to prepare large-area MOFE thin films, controlled by supersaturation growth at the liquid-air interface under a temperature gradient and external water partial pressure. We used this technique to fabricate ImClO4 thin films and found a large, tunable room temperature electroresistance: a 20-fold resistance variation upon polarization switching. The as-grown films are transparent and consist of a bamboo-like structure of (2,1¯,0) and (1,0,2¯) structural variants of R3m symmetry with a reversible polarization of 6.7 μC/cm2. The resulting ferroelectric domain structure leads to a reversible electromechanical response of d33 = 38.8 pm/V. Polarization switching results in a change of the refractive index, n, of single domains, Δnn=0.3. The remarkable combination of these characteristics renders MOFEs a prime candidate material for new nanoelectronic devices. The information that we present in this work will open a new area of MOFE thin-film technologies.

Chemically Driven Interfacial Coupling in Charge-Transfer Mediated Functional Superstructures

Organic charge-transfer superstructures are enabling new interfacial electronics, such as organic thermoelectrics, spin-charge converters, and solar cells. These carbon-based materials could also play an important role in spin-based electronics due to their exceptionally long spin lifetime. However, to explore these potentials a coherent design strategy to control interfacial charge-transfer interaction is indispensable. Here we report that the control of organic crystallization and interfacial electron coupling are keys to dictate external stimuli responsive behaviors in organic charge-transfer superstructures. The integrated experimental and computational study reveals the importance of chemically driven interfacial coupling in organic charge-transfer superstructures. Such degree of engineering opens up a new route to develop a new generation of functional charge-transfer materials, enabling important advance in all organic interfacial electronics.

Multifunctional Charge-Transfer Single Crystals through Supramolecular Assembly

Centimeter-sized segregated stacking TTF-C60 single crystals are crystallized by a mass-transport approach combined with solvent-vapor evaporation for the first time. The intermolecular charge-transfer interaction in the long-range ordered superstructure enables the crystals to demonstrate external stimuli-controlled multifunctionalities and angle/electrical-potential-dependent luminescence.

CCDC 1449048: Experimental Crystal Structure Determination

Related Article: Beibei Xu, Zhipu Luo, Andrew J. Wilson, Ke Chen, Wenxiu Gao, Guoliang Yuan, Harsh Deep Chopra, Xing Chen, Katherine A. Willets, Zbigniew Dauter, Shenqiang Ren|2016|Adv.Mater.|28|5322|doi:10.1002/adma.201600383