Unlimiting ionic conduction: manipulating hydration dynamics through vibrational strong coupling of water.

The energy states of molecules and the vacuum electromagnetic field can be hybridized to form a strong coupling state. In particular, it has been demonstrated that vibrational strong coupling can be used to modify the chemical dynamics of molecules. Here, we propose that ion dynamics can be altered through modifications of the dynamic hydration structure in a cavity vacuum field. We investigated the effect of different electrolyte species on ionic conductivity. Infrared spectroscopy of aqueous electrolyte solutions within the cavity confirmed the formation of vibrational ultrastrong coupling of water molecules, even in the presence of electrolytes. Interestingly, we observed significant enhancements in ionic conductivity, for specific alkali cations, particularly those classified as structure-breaking cations. These enhancements cannot be explained within the current theoretical framework for liquid electrolytes. Our analysis suggests that the vibrational strong coupling modifies the local dielectric friction experienced by hydrated ions. In addition, we propose the enthalpic and entropic modification of ionic conductivity through the systematic investigation of the hydration properties of different electrolytes. This study unveils the potential role of polaritons for exploring uncharted spaces in the design of materials with enhanced ionic conduction. Harnessing the unique properties of strong coupling and its influence on hydration dynamics could lead to the development of novel electrolytes and advancements in the field of ionic conduction.

Unveiling the Hidden Energy Profiles of the Oxygen Evolution Reaction via Machine Learning Analyses.

The oxygen evolution reaction (OER) is a crucial electrochemical process for hydrogen production in water electrolysis. However, due to the involvement of multiple proton-coupled electron transfer steps, it is challenging to identify the specific elementary reaction that limits the rate of the OER. Here we employed a machine-learning-based approach to extract the reaction pathway exhaustively from experimental data. Genetic algorithms were applied to search for thermodynamic and kinetic parameters using the current-electrochemical potential relationship of the OER. Interestingly, analysis of the datasets revealed the energy state distributions of reaction intermediates, which likely originated in the interactions among intermediates or the distribution of multiple sites. Through our exhaustive analyses, we successfully uncovered the hidden energy profiles of the OER. This approach can reveal the reaction pathway to activate for efficient hydrogen production, which facilitates the design of catalysts.

Understanding Spatial Distributions of Dye Molecules Coupled to the Surface Lattice Resonance Mode through Electrochemical Reaction Control.

The strong coupling, which is the light-matter interaction, leads to changes in the energy landscape of the chemical dynamics, resulting in the modulation of the reaction pathways. In this study, we achieved strong coupling between dye molecules dispersed in the polymer films and the surface lattice resonance mode, which is excited on plasmonic lattice arrays. In addition, we successfully tuned the coupling strength by introducing the electrochemical potential control method. Reversible decreases and increases in the coupling strength were observed as a result of the reversible electrochemical redox reactions of dye molecules. It is important that the spatial distribution of the molecules coupled to the lattice resonance mode was clarified by using various polymer film thicknesses. Our present electrochemical method for controlling strong coupling states represents a promising method for tuning the light-absorption properties of systems.

Inherent Promotion of Ionic Conductivity via Collective Vibrational Strong Coupling of Water with the Vacuum Electromagnetic Field.

Hydrogen bonding interactions among water molecules play a critical role in chemical reactivity, dynamic proton mobility, static dielectric behavior, and the thermodynamic properties of water. In this study, we demonstrate the modification of ionic conductivity of water through hybridization with a vacuum electromagnetic field by strongly coupling the O─H stretching mode of H2O to a Fabry-Perot cavity mode. The hybridization generates collective vibro-polaritonic states, thereby enhancing the proton conductivity by an order of magnitude at resonance. In addition, the dielectric constants increase at resonance in the coupled state. The findings presented herein reveal how a vacuum electromagnetic environment can be engineered to control the ground-state properties of water.

Rapid detection of donor-dependent photocatalytic hydrogen evolution by NMR spectroscopy.

Understanding molecular processes at nanoparticle surfaces is essential for designing active photocatalytic materials. Here, we utilize nuclear magnetic resonance (NMR) spectroscopy to track photocatalytic hydrogen evolution using donor molecules and water isotopologues. Pt-TiO2 catalysts were prepared and used for isotopic hydrogen evolution reactions using alcohols as electron donors. 1H NMR monitoring revealed that evolution of the H2 and HD species is accompanied by the oxidation of donor molecules. The isotopic selectivity in the hydrogen evolution reaction gives rise to formal overpotential. Based on a comparison of the rates of hydrogen evolution and donor oxidation, we propose the use of ethanol as an efficient electron donor for the hydrogen evolution reaction without re-oxidation of radical intermediates.

Generation of Ultralong Liposome Tubes by Membrane Fusion beneath a Laser-Induced Microbubble on Gold Surfaces.

Membrane fusion (MF) is one of the most important and ubiquitous processes in living organisms. In this study, we developed a novel method for MF of liposomes. Our method is based on laser-induced bubble generation on gold surfaces (a plasmonic nanostructure or a flat film). It is a simple and quick process that takes about 1 min. Upon bubble generation, liposomes not only collect and become trapped but also fuse to form long tubes beneath the bubble. Moreover, during laser irradiation, these long tubes remain stable and move with a waving motion while continuing to grow, resulting in the creation of ultralong tubes with lengths of about 50 µm. It should be noted that the morphology of these ultralong tubes is analogous to that of a sea anemone. The behavior of the tubes was also monitored by fluorescence microscopy. The generation of these ultralong tubes is discussed on the basis of Marangoni convection and thermophoresis.

Unique Electronic Excitations at Highly Localized Plasmonic Field.

ConspectusUnder visible light illuminations, noble metal nanostructures can condense photon energy into the nanoscale region. By precisely tuning the metal nanostructures, the ultimate confinement of photoenergy at the molecular scale can be obtained. At such a confined photon energy field, various unique photoresponses of molecules, such as efficient visible light energy conversion processes or efficient multielectron transfer reactions, can be observed. Light-matter interactions also increase with the condensation of photons with nanoscale regions, leading to efficient light energy utilizations. Moreover, the strong field confinement can often modulate electronic excitations beyond normal selection rules. Such unique electronic excitations could realize innovative photoenergy conversion systems. On the other hand, such interactions lead to changes in the optical absorption property of the system via the formation of hybridized electronic energy states. This hybridized state is expected to have the potential to modulate the chemical reaction pathways. Taking these facts into consideration, a probe for the molecular absorption process with high sensitivity allows us to find novel ways for further precise tuning of light-matter interactions. In this Account, we review phenomena of unique electronic excitations from the perspective of our previous investigations using surface-enhanced Raman scattering (SERS) spectroscopy at electrified interfaces. Because the enhancement mechanism of Raman scattering at interfaces is deeply correlated with the photon absorption process accompanied by the electronic excitations between molecules and electrode surfaces, the detailed SERS investigations of the well-defined system can provide information on the electronic excitation processes. Through SERS observations of single-molecule junctions at electrodes or well-defined low-dimensional carbon materials, we have observed the characteristic Raman bands containing additional polarization tensors, indicating the occurrence of electronic polarization induced by electronic excitations based on a distinct selection rule. The origins for the observed facts were attributed to the highly condensed electric field producing the huge intensity gradient at the nano scale. The electrochemical potential control of the system would be valuable for the control of the excitation process. Additionally, from Raman spectra of dye molecules coupled to the plasmonic field, the changes in the Raman scattering intensity depending on the strength of interactions suggested the modulation of the absorption characteristics of the system. In addition, we have proved that the electrochemical potential control method can be a powerful tool for the active tuning of the light-matter interaction, leading to the change in the light absorption property. The molecular behaviors of dyes in the strong-coupling regime were reversibly tuned to show intense SERS. The current descriptions provide novel insights for these unique electronic excitations, realized by the plasmon excitation, that lead to advanced photoenergy conversions beyond the limits of present systems.

Room-Temperature Molecular Manipulation via Plasmonic Trapping at Electrified Interfaces.

For the motion control of individual molecules at room temperature, optical tweezers could be one of the best approaches to realize desirable selectivity with high resolution in time and space. Because of physical limitations due to the thermal fluctuation, optical manipulation of small molecules at room temperature is still a challenging subject. The difficulty of the manipulation also emerged from the variation of molecular polarizability depending on the choice of molecules as well as the molecular orientation to the optical field. In this article, we have demonstrated plasmonic optical trapping of small size molecules with less than 1 nm at the gap of a single metal nanodimer immersed in an electrolyte solution. In situ electrochemical surface-enhanced Raman scattering measurements prove that a plasmonic structure under electrochemical potential control realizes not only the selective molecular condensation but also the formation of unique mixed molecular phases which is distinct from those under a thermodynamic equilibrium. Through detailed analyses of optical trapping behavior, we established the methodology of plasmonic optical trapping to create the novel adsorption isotherm under applying an optical force at electrified interfaces.

Tuning Electrogenerated Chemiluminescence Intensity Enhancement Using Hexagonal Lattice Arrays of Gold Nanodisks.

Electrogenerated chemiluminescence (ECL) microscopy shows promise as a technique for mapping chemical reactions on single nanoparticles. The technique's spatial resolution is limited by the quantum yield of the emission and the diffusive nature of the ECL process. To improve signal intensity, ECL dyes have been coupled with plasmonic nanoparticles, which act as nanoantennas. Here, we characterize the optical properties of hexagonal arrays of gold nanodisks and how they impact the enhancement of ECL from the coreaction of tris(2,2'-bipyridyl)dichlororuthenium(II) hexahydrate and tripropylamine. We find that varying the lattice spacing results in a 23-fold enhancement of ECL intensity because of increased dye-array near-field coupling as modeled using finite element method simulations.

Spatial distribution of active sites for plasmon-induced chemical reactions triggered by well-defined plasmon modes.

Plasmon-induced chemical reactions triggered by near-infrared light illumination might enable efficient photo energy conversion. Here, electrochemical oxidative polymerization of a conductive polymer was conducted on plasmonic photoconversion electrodes. The absolute electrochemical potential of the generated holes was estimated from the redox potentials of the monomers. In addition, well-defined plasmonic structures were examined to better understand the relationship between the excited plasmon mode and spatial distribution of reaction active sites. Rod structures with various lengths had distinct spatial distributions of reaction active sites that depended on the higher plasmon modes, as visualized by Raman measurements.

Ultra-fine electrochemical tuning of hybridized plasmon modes for ultimate light confinement.

Highly reproducible control of metal plasmonic nanostructures has been achieved via precise tuning of the electrochemical Au dissolution reaction that occurs at the surfaces of well-defined bridged nanodisk dimer structures on an atomic scale. It was found that the scattering intensity is strongly suppressed during the transition from the conductive mode to the gap mode of the localized surface plasmon resonance during the period when the gap is formed and increased between Au nanodisks. The characteristic shift of the plasmon mode during this suppression of the scattering intensity verifies the excitation of the bonding quadrupolar mode, which appears only at sub-nanometer gap distances (d < 1 nm). Electrochemical potential control demonstrates that the scattering suppression states with estimated gap distances of less than 1 nm can be maintained for more than 100 s under ambient conditions. The method and phenomena presented here will be useful in the preparation of plasmonic structures for ultimate light confinement applications.

Potential energy shift of the Fermi level at plasmonic structures for light-energy conversion determined by graphene-based Raman measurements.

Single layer graphene was used to determine the electrochemical potential of plasmonic nano-structures for photoelectrochemical energy conversions. From electrochemical Raman measurements of the graphene layer under near-infrared light, illumination has revealed the relationship between the photoenergy conversion ability and the Fermi level of the plasmonic structure. The determination is based on in situ monitoring of G and 2D Raman bands of the graphene layer on plasmonic structures. The correlation plots of G and 2D bands show the dependence on the photoconversion ability. The present electrochemical Raman measurements provide detailed understanding of the plasmon-induced charge transfer process for further developments on the ability.

Plasmonic Manipulation of DNA using a Combination of Optical and Thermophoretic Forces: Separation of Different-Sized DNA from Mixture Solution.

We demonstrate the size-dependent separation and permanent immobilization of DNA on plasmonic substrates by means of plasmonic optical tweezers. We found that a gold nanopyramidal dimer array enhanced the optical force exerted on the DNA, leading to permanent immobilization of the DNA on the plasmonic substrate. The immobilization was realized by a combination of the plasmon-enhanced optical force and the thermophoretic force induced by a photothermal effect of the plasmons. In this study, we applied this phenomenon to the separation and fixation of size-different DNA. During plasmon excitation, DNA strands of different sizes became permanently immobilized on the plasmonic substrate forming micro-rings of DNA. The diameter of the ring was larger for longer DNA (in base pairs). When we used plasmonic optical tweezers to trap DNA of two different lengths dissolved in solution (φx DNA (5.4 kbp) and λ-DNA (48.5 kbp), or φx DNA and T4 DNA (166 kbp)), the DNA were immobilized, creating a double micro-ring pattern. The DNA were optically separated and immobilized in the double ring, with the shorter sized DNA and the larger one forming the smaller and larger rings, respectively. This phenomenon can be quantitatively explained as being due to a combination of the plasmon-enhanced optical force and the thermophoretic force. Our plasmonic optical tweezers open up a new avenue for the separation and immobilization of DNA, foreshadowing the emergence of optical separation and fixation of biomolecules such as proteins and other ncuelic acids.

Present and Future of Surface-Enhanced Raman Scattering.

The discovery of the enhancement of Raman scattering by molecules adsorbed on nanostructured metal surfaces is a landmark in the history of spectroscopic and analytical techniques. Significant experimental and theoretical effort has been directed toward understanding the surface-enhanced Raman scattering (SERS) effect and demonstrating its potential in various types of ultrasensitive sensing applications in a wide variety of fields. In the 45 years since its discovery, SERS has blossomed into a rich area of research and technology, but additional efforts are still needed before it can be routinely used analytically and in commercial products. In this Review, prominent authors from around the world joined together to summarize the state of the art in understanding and using SERS and to predict what can be expected in the near future in terms of research, applications, and technological development. This Review is dedicated to SERS pioneer and our coauthor, the late Prof. Richard Van Duyne, whom we lost during the preparation of this article.

Nonzero Wavevector Excitation of Graphene by Localized Surface Plasmons.

Electrochemical surface-enhanced Raman scattering measurements of single layer graphene provide unique information on resonant excitation induced by localized surface plasmons under controlled electron or hole doping. The highly confined electromagnetic field from the LSPs of the Au nanodimer structures prepared on defect-free graphene can generate holes and electrons of the electrochemical potentials beyond the limit of far-field light illumination. The electrochemical in situ SERS spectra prove nonzero wavevector excitation through the observation of normally forbidden Raman bands in graphene. The present findings point to a novel approach to breaking the limit of optoelectronic interactions and photochemical reactions of graphene and other semiconductors.

Photoelectrochemical Formation of Polysulfide at PbS QD-Sensitized Plasmonic Electrodes.

Effective electron-hole separation is a key to enhance photoenergy conversion of semiconductor quantum dot (QD)-sensitized plasmonic solar cells. However, in contrast to intense studies on electron transfer, hole transfer from QDs and consequent chemical reactions with donors in electrolytes remain unclear. Herein, in situ electrochemical surface-enhanced Raman scattering (SERS) measurement on a PbS QD-sensitized TiO2/Au/TiO2 photoelectrode indicated formation of cyclo-octasulfur (α-S8) via tuning the electrochemical potential. A photocurrent density of 100 nA/cm2 was recorded simultaneously even with an extremely low QD loading. Two-dimensional correlation analysis of the SERS revealed subsequent formation of S8- and S42- at -1.1 to -0.1 V (vs Ag/AgCl), S8 from -0.3 V, and S52- and S62- at ≥0.2 V via complex disproportionation reactions. The sensitive detection is attributed to the enhanced electromagnetic field of localized surface plasmon resonance, which provides a better understanding of charge separation processes in QD-sensitized solar cells.

Revealing High Oxygen Evolution Catalytic Activity of Fluorine-Doped Carbon in Alkaline Media.

Oxygen evolution reactions (OER) are important reactions for energy conversion. Metal-free carbon-based catalysts potentially contribute to the catalytic materials for OER. However, it has been difficult to understand the intrinsic catalytic activity of carbon materials, due to catalyst decomposition over the course of long-term reactions. Here, we report high oxygen evolution reaction catalytic activity of F-doped carbon in alkaline media. Intrinsic OER activity was evaluated from a combination of measurements using a rotating disk electrode and O2 sensor. The F-doped carbon catalyst is a highly active catalyst, comparable to state-of-the-art precious-metal-based catalysts such as RuO2.

Plasmonically enhanced electromotive force of narrow bandgap PbS QD-based photovoltaics.

Electromotive force of photovoltaics is a key to define the output power density of photovoltaics. Multiple exciton generation (MEG) exhibited by semiconductor quantum dots (QDs) has great potential to enhance photovoltaic performance owing to the ability to generate more than one electron-hole pairs when absorbing a single photon. However, even in MEG-based photovoltaics, limitation of modifying the electromotive force exists due to the intrinsic electrochemical potential of the conduction band-edges of QDs. Here we report a pronouncedly improved photovoltaic performance by constructing a PbS QD-sensitized electrode that comprises plasmon-active Au nanoparticles embedded in a titanium dioxide thin film. Significant enhancement on electromotive force is characterized by the onset potential of photocurrent generation using MEG-effective PbS QDs with a narrow bandgap energy (Eg = 0.9 eV). By coupling with localized surface plasmon resonance (LSPR), such QDs exhibit improved photoresponses and the highest output power density over the other QDs with larger bandgap energies (Eg = 1.1 and 1.7 eV) under visible light irradiation. The wavelength-dependent onset potential and the output power density suggest effective electron injection owing to the enhanced density of electrons excited by energy overlapping between MEG and LSPR.

Electrochemical surface-enhanced Raman scattering measurement on ligand capped PbS quantum dots at gap of Au nanodimer.

The vibrational characteristics of ligand-capped lead sulfide (PbS) quantum dots (QDs) were clarified via electrochemical surface-enhanced Raman spectroscopy (EC-SERS) using a hybridized system of gold (Au) nanodimers and PbS QDs under electrochemical potential control. Enhanced electromagnetic field caused by the coupling of QDs with plasmonic Au nanodimers allowed the characteristic behavior of the ligand oleic acid (OA) on the PbS QD surface to be detected under electrochemical potential control. Binding modes between the QDs and OA molecules were characterized using synchronous two-dimensional correlation spectra at distinct electrochemical potentials, confirming that the bidentate bridging mode was probably the most stable mode even under relatively negative potential polarization. Changes in binding modes and molecular orientations resulted in fluctuations in EC-SERS spectra. The present observations strongly recommend the validity of the QD-plasmonic nanostructure coupled system for sensitive molecular detection via EC-SERS.

Sensitive Raman Probe of Electronic Interactions between Monolayer Graphene and Substrate under Electrochemical Potential Control.

In situ electrochemical Raman spectroscopic measurements of defect-free monolayer graphene on various substrates were performed under electrochemical potential control. The G and 2D Raman band wavenumbers (ωG, ω2D) of graphene were found to depend upon the electrochemical potential, i.e., the charge density of graphene. The values of ωG and ω2D also varied depending on the choice of substrates. On metal substrates where graphene was synthesized by chemical vapor deposition, a strong blue shift of ω2D was induced, which could not account for the strain and charge doping. We attributed the blue shift of ω2D to a change in the electronic properties of graphene induced by distinct electronic interactions with the metal substrates. To explain the unique characteristics in the Raman spectrum of graphene on various substrates, a novel mechanism is proposed considering reduction of the Fermi velocity in graphene owing to dielectric screening from the metal substrates.