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

The Nature of Plasmonically Assisted Hot-Electron Transfer in a Donor–Bridge–Acceptor Complex

This work provides a mechanistic understanding of hot-electron-based catalysis on Au nanoparticles (NPs) induced under plasmonic excitation. Plasmon excitation-induced hot-electron transfer from an Au NP (donor) to a ferricyanide anion (acceptor) was studied as a function of the donor–acceptor distance set by a thiolate-based self-assembled monolayer (SAM). Hot-electron-transfer rates and activation barrier heights were measured as a function of the donor–acceptor distance, up to 20 Å. Hot-electron transfer was found to be longer range than anticipated. The distance-dependent kinetics reveal that the hot-electron transfer takes place via multistep hopping in a “wire-like” manner across the insulating ligands, quite unlike the tunneling-dominated electron transfer known to take place across SAMs in the absence of plasmonic excitation. Field-assisted electron hopping may play a crucial role in hot-electron extraction and catalysis involving plasmon-excited NPs

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

Super-resolution surface-enhanced Raman scattering (SERS) is used to investigate local surface potentials on plasmonic gold and silver colloidal aggregates using the redox-active 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

Plasmonic Control of Multi-Electron Transfer and C–C Coupling in Visible-Light-Driven CO₂ Reduction on Au Nanoparticles

Artificial photosynthesis relies on the availability of synthetic photocatalysts that can drive CO₂ reduction in the presence of water and light. From the standpoint of solar fuel production, it is desirable that these photocatalysts perform under visible light and produce energy-rich hydrocarbons from CO₂ reduction. However, the multistep nature of CO₂-to-hydrocarbon conversion poses a significant kinetic bottleneck when compared to CO production and H₂ evolution. Here, we show that plasmonic Au nanoparticle photocatalysts can harvest visible light for multielectron, multiproton reduction of CO₂ to yield C₁ (methane) and C₂ (ethane) hydrocarbons. The light-excitation attributes influence the C₂ and C₁ selectivity. The observed trends in activity and selectivity follow Poisson statistics of electron harvesting. Higher photon energies and flux favor simultaneous harvesting of more than one electron from the photocharged Au nanoparticle catalyst, inducing the C–C coupling required for C₂ production. These findings elucidate the nature of plasmonic photocatalysis, which involves strong light-matter coupling, and set the stage for the controlled chemical bond formation by light excitation

Side-Chain Supramolecular Polymers Employing Conformer Independent Triple Hydrogen Bonding Arrays

Derivatives of thymine have been extensively used to promote supramolecular materials assembly. Such derivatives can be synthetically challenging to access and may be susceptible to degradation. The current article uses a conformer-independent acceptor–donor–acceptor array (ureidopyrimidine) which forms moderate affinity interactions with diamidopyridine derivatives to effect supramolecular blend formation between polystyrene and poly­(methyl methacrylate) polymers obtained by RAFT which have been functionalized with the hydrogen bonding motifs

Covalent Cross-Linking within Supramolecular Peptide Structures

β-Sheet peptide nanostructures (e.g., amyloid fibrils) are recognized as important entities in biological systems and as functional materials in their own right. Their unique physical properties and architectural complexity, however, present a challenge for structure determination at atomic resolution. Covalent cross-linking and mass spectrometry are appealing methods for this endeavor because, potentially, a large amount of information can be extracted from a small sample in a single experiment. Previously, we described preliminary studies on the use of a photoreactive diazirine-containing amino acid to cross-link peptide monomers in nanostructures, together with the integrated separation and analysis of the products using ion mobility spectrometry coupled to conventional mass spectrometry. Here, a pH-switchable system (<b>Aβ_16–22</b>, a sequence from the amyloid-β peptide) was used to examine cross-linking chemistry in morphologically distinct supramolecular structures containing, or entirely composed of, diazirine-functionalized peptides. We examine the relationship between cross-linker chemistry, covalent cross-links (identified using chemical derivatization and tandem mass spectrometry), and noncovalent structure, and report differences in the site of cross-linking that can only be explained by supramolecular templating. The results demonstrate the applicability of the approach for obtaining structural restraints in ordered supramolecular assemblies, provided that a considered evaluation of the cross-linked products is undertaken

Analysis of Amyloid Nanostructures Using Photo-cross-linking: <i>In Situ</i> Comparison of Three Widely Used Photo-cross-linkers

Photoinduced cross-linking (PIC) has become a powerful tool in chemical biology for the identification and mapping of stable or transient interactions between biomacromolecules and their (unknown) ligands. However, the value of PIC for <i>in vitro</i> and <i>in vivo</i> structural proteomics can be realized only if cross-linking reports accurately on biomacromolecule secondary, tertiary, and quaternary structures with residue-specific resolution. Progress in this area requires rigorous and comparative studies of PIC reagents, but despite widespread use of PIC, these have rarely been performed. The use of PIC to report reliably on noncovalent structure is therefore limited, and its potentials have yet to be fully realized. In the present study, we compared the abilities of three probes, phenyl trifluoromethyldiazirine (TFMD), benzophenone (BP), and phenylazide (PA), to record structural information within a biomolecular complex. For this purpose, we employed a self-assembled amyloid-like peptide nanostructure as a tightly and specifically packed model environment in which to photolyze the reagents. Information about PIC products was gathered using mass spectrometry and ion mobility spectrometry, and the data were interpreted using a mechanism-oriented approach. While all three PIC groups appeared to generate information within the packed peptide environment, the data highlight technical limitations of BP and PA. On the other hand, TFMD displayed accuracy and generated straightforward results. Thus TFMD, with its robust and rapid photochemistry, was shown to be an ideal probe for cross-linking of peptide nanostructures. The implications of our findings for detailed analyses of complex systems, including those that are transiently populated, are discussed

Occupancy of the top 3 parallel clusters.

<p>Color coded by starting conformation during the final 17 ns of the simulation. Inter-conversion between clusters #1 and #2 is observed by conformation 4 and 10.</p

Time evolution of parallel (left) and anti-parallel (right) Phe-Nap-ⁱPr arylamide conformations.

<p>a) the number of intermolecular pairs of atoms between <i>h</i>DM2 and the ligand that are within 3.5 Å for parallel conformations; b) the number of intermolecular pairs of atoms between <i>h</i>DM2 and the ligand that are within 3.5 Å for anti-parallel conformations; c) difference in protein-arylamide center of mass distance (Å) from a minimized parallel starting conformation; and d) difference in protein-arylamide center of mass distance (Å) from a minimized anti-parallel starting conformation. The number of contacts tends to increase, indicating increasing stability of the bound arylamide, although there is wider variation in the number of contacts compared to the results presented in Figure S9 (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0043253#pone.0043253.s001" target="_blank">Text S1</a>). The hypothesis that an increasing number of contacts indicates increasing stability is supported by the decrease in the distance between the centers of mass of arylamide and <i>h</i>DM2 as the simulations progress, indicating a closer fit. It should be noted that decrease in the distance between centers of mass of ligand and protein is not always indicative of greater ligand burial therefore a system specific decision must be made before employing this analysis technique.</p

Orientation and positioning of arylamide compounds in the <i>h</i>DM2 binding pocket at 10 ps intervals.

<p>Four of five simulations sample in the same region of space, whereas simulation c does not. Ether oxygens from anti-parallel conformations of Phe-Nap-ⁱPr are projected onto a plane defined by Cα atoms from Tyrosine 56, Methione 62 and Valine 93. Data points are color-coded depending on which ether oxygen they belong to: R₁ (Blue); R₂ (Green); and R₃ (Red). Data was plotted at 10 ps intervals starting after 4 ns of data collection. Values at t = 0 ps are plotted with diamonds. Graphs show images of starting conformation relative to the high affinity p53 helix and data from: a) conformation 1; b) conformation 2; c) conformation 3; d) conformation 7; and e) conformation 8.</p