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Amyloid fibrils are β-sheet rich protein aggregates that are strongly associated with various neurodegenerative diseases. Raman spectroscopy has been broadly utilized to investigate protein aggregation and amyloid fibril formation and has been shown to be capable of revealing changes in secondary and tertiary structures at all stages of fibrillation. When coupled with atomic force (AFM) and scanning electron (SEM) microscopies, Raman spectroscopy becomes a powerful spectroscopic approach that can investigate the structural organization of amyloid fibril polymorphs. In this review, we discuss the applications of Raman spectroscopy, a unique, label-free and non-destructive technique for the structural characterization of amyloidogenic proteins, prefibrilar oligomers, and mature fibrils. [ABSTRACT FROM AUTHOR]

Hair is one of the most common types of physical evidence found at a crime scene. Forensic examination may suggest a connection between a suspect and a crime scene or victim, or it may demonstrate an absence of such associations. Therefore, forensic analysis of hair evidence is invaluable to criminal investigations. Current hair forensic examinations are primarily based on a subjective microscopic comparison of hair found at the crime scene with a sample of suspect's hair. Since this is often inconclusive, the development of alternative and more-accurate hair analysis techniques is critical. In this study, we utilized surface-enhanced Raman spectroscopy (SERS) to demonstrate that artificial dyes can be directly detected on hair. This spectroscopic technique is capable of a confirmatory identification of analytes with single molecule resolution, requires minimal sample, and has the advantage of fluorescence quenching. Our study reveals that SERS can (1) identify whether hair was artificially dyed or not, (2) determine if a permanent or semipermanent colorants were used, and (3) distinguish the commercial brands that are utilized to dye hair. Such analysis is rapid, minimally destructive, and can be performed directly at the crime scene. This study provides a novel perspective of forensic investigations of hair evidence. [ABSTRACT FROM AUTHOR]

Localized surface plasmon resonance (LSPR) spectroscopy of metallic nanoparticles is a powerful technique for chemical and biological sensing experiments. Moreover, the LSPR is responsible for the electromagnetic-field enhancement that leads to surface-enhanced Ranian scattering (SERS) and other surface-enhanced spectroscopic processes. This review describes recent fundamental spectroscopic studies that reveal key relationships governing the LSPR spectral location and its sensitivity to the local environment, including nanoparticle shape and size. We also describe studies on the distance dependence of the enhanced electromagnetic field and the relationship between the plasmon resonance and the Raman excitation energy. Lastly, we introduce a new form of LSPR spectroscopy, involving the coupling between nanoparticle plasmon resonances and adsorbate molecular resonances. The results from these fundamental studies guide the design of new sensing experiments, illustrated through applications in which researchers use both LSPR wavelength-shift sensing and SERS to detect molecules of chemical and biological relevance. [ABSTRACT FROM AUTHOR]

The intense colors of noble metal nanoparticles have inspired artists and fascinated scientists for hundreds of years. In this review, we describe refractive index sensing platforms based on the tunability of the localized surface plasmon resonance (LSPR) of arrays of silver nanoparticles and of single nanoparticles. Specifically, the color associated with single nanoparticles and surface-confined nanoparticle arrays will be shown to be tunable and useful as platforms for chemical and biological sensing. Finally, the LSPR nanosensor will be compared to traditional, flat surface, propagating surface plasmon resonance sensors. [ABSTRACT FROM AUTHOR]

Reports on the optical properties of triangular silver nanoparticles. Sensitivity of the peak extinction wavelength of localized surface plasmon resonance (LSPR) spectrum to nanoparticle size, shape and local external dielectric environment; Characteristics and operational principles of LSPR nanobiosensors.

An introduction is presented in which the editors discuss various reports within the issue on topics including peptide mass fingerprinting (PMF) method for the identication of collagen materials, SERS approaches for organic pigments identification, and photoluminescence properties of zinc oxide.

We present a coupled wave semiclassical theory to describe plasmonic enhancement effects in surface-enhanced femtosecond stimulated Raman scattering (SE-FSRS). A key result is that the plasmon enhanced fields which drive the vibrational equation of motion for each normal mode results in dispersive lineshapes in the SE-FSRS spectrum. This result, which reproduces experimental lineshapes, demonstrates that plasmon-enhanced stimulated Raman methods provide unique sensitivity to a plasmonic response. Our derived SE-FSRS theory shows a plasmonic enhancement of |gpu|²Im { χR(ω)g² st/Im { χR(ω)}, where |gpu|² is the absolute square of the plasmonic enhancement from the Raman pump, χR(ω) is the Raman susceptibility, and gst is the plasmonic enhancement of the Stokes field in SE-FSRS. We conclude with a discussion on potential future experimental and theoretical directions for the field of plasmonically enhanced coherent Raman scattering. [ABSTRACT FROM AUTHOR]

A p-benzenedithiolate (BDT) molecule covalently bonded between two gold electrodes has become one of the model systems utilized for investigating molecular transport junctions. The plethora of papers published on the BDT system has led to varying conclusions with respect to both the mechanism and the magnitude of transport. Conductance variations have been attributed to difficulty in calculating charge transfer to the molecule, inability to locate the Fermi energy accurately, geometric dispersion, and stochastic switching. Here we compare results obtained using two transport codes, TRANSIESTA-C and HÜCKEL-IV, to show that upon Au–S bond lengthening, the calculated low bias conductance initially increases by up to a factor of 30. This increase in highest occupied molecular orbital (HOMO) mediated conductance is attributed to charging of the terminal sulfur atom and a corresponding decrease in the energy gap between the Fermi level and the HOMO. Addition of a single Au atom to each terminal of the extended BDT molecule is shown to add four molecular states near the Fermi energy, which may explain the varying results reported in the literature. [ABSTRACT FROM AUTHOR]

We present a discrete dipole approximation (DDA) method to determine extinction and Raman intensities for small metal particles of arbitrary shape. The Raman intensity calculation involves evaluation of surface electromagnetic fields, and thus is relevant to surface enhanced Raman scattering (SERS) intensities. We demonstrate convergence of the method by considering light absorption and scattering from an isolated spheroid, from an isolated tetrahedron, from two coupled spheroids, and from a spheroid on a flat surface. We also examine comparisons with traditional T-matrix methods. Extensions and simplifications of the method in studies of clusters and arrays of particles are presented. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

This article presents information on an emerging branch of photonics, called plasmonics. Plasmonic science aims to use nanostructured materials that support "surface plasmons.Plasmonics can potentially achieve highly complex miniaturized devices by controlling and manipulating light on the nanometer scale. When light interacts with a free-electron metal, such as a thin silver film, its surface electrons oscillate collectively and absorb, scatter, or re-radiate the incident photons. The resulting surface electromagnetic field propagates in the plane of the film.

Singlet fission (SF) converts a singlet exciton into two triplet excitons in two or more electronically coupled organic chromophores, which may then be used to increase solar cell efficiency. Many known SF chromophores are unsuitable for device applications due to chemical instability or low triplet state energies. The results described here show that efficient SF occurs in derivatives of 9,10-bis(phenylethynyl)anthracene (BPEA), which is a highly robust and tunable chromophore. Fluoro and methoxy substituents at the 4- and 4′-positions of the BPEA phenyl groups control the intermolecular packing in the crystal structure, which alters the interchromophore electronic coupling, while also changing the SF energetics. The lowest excited singlet state (S1) energy of 4,4′-difluoro-BPEA is higher than that of BPEA so that the increased thermodynamic favorability of SF results in a (16 ± 2 ps)−1 SF rate and a 180% ± 16% triplet yield, which is about an order of magnitude faster than BPEA with a comparable triplet yield. By contrast, 4-fluoro-4′-methoxy-BPEA and 4,4′-dimethoxy-BPEA have slower SF rates, (90 ± 20 ps)−1 and (120 ± 10 ps)−1, and lower triplet yields, (110 ± 4)% and (168 ± 7)%, respectively, than 4,4′-difluoro-BPEA. These differences are attributed to changes in the crystal structure controlling interchromophore electronic coupling as well as SF energetics in these polycrystalline solids. [ABSTRACT FROM AUTHOR]

Raman spectroscopic-based biosensing strategies are often complicated by low signal and the presence of multiple chemical species. While surface-enhanced Raman spectroscopy (SERS) nanostructured platforms are able to deliver high quality signals by focusing the electromagnetic field into a tight plasmonic hot-spot, it is not a generally applicable strategy as it often depends on the specific adsorption of the analyte of interest onto the SERS platform. This paper describes a strategy to address this challenge by using surface potential as a physical binding agent in the context of microneedle sensors. We show that the potential-dependent adsorption of different chemical species allows scrutinization of the contributions of different chemical species to the final spectrum, and that the ability to cyclically adsorb and desorb molecules from the surface enables efficient application of multivariate analysis methods. We demonstrate how the strategy can be used to mitigate potentially confounding phenomena, such as surface reactions, competitive adsorption and the presence of molecules with similar structures. In addition, this decomposition helps evaluate criteria to maximize the signal of one molecule with respect to others, offering new opportunities to enhance the measurement of analytes in the presence of interferants. [ABSTRACT FROM AUTHOR]

The incorporation of tetraoxolene radical bridging ligands into a microporous magnetic solid is demonstrated. Metalation of the redox-active bridging ligand 2,5-dichloro-3,6-dihydroxy-1,4-benzoquinone (LH2) with FeII atfords the solid (Me2NH2)2[Fe2L3]-2H20 6DM F . Analysis of X-ray diffraction, Raman spectra, and Mössbauer spectra confirm the presence of FeIII centers with mixed-valence ligands of the form (L3)8- that result from a spontaneous electron transfer from FeII to L2-. Upon removal of DMF and H2o solvent molecules, the compound undergoes a slight structural distortion to give the desolvated phase (Me2NH2)2- [Fe2L3], and a fit to N2 adsorption data of this activated compound gives a BET surface area of 885(105) m2/g. Dc magnetic susceptibility measurements reveal a spontaneous magnetization below 80 and 26 K for the solvated and the activated solids, respectively, with magnetic hysteresis up to 60 and 20 K. These results highlight the ability of redox-active tetraoxolene ligands to support the formation of a microporous magnet and provide the first example of a structurally characterized extended solid that contains tetraoxolene radical ligands. [ABSTRACT FROM AUTHOR]

A fundamental understanding of electrochemical processes at the nanoscale is crucial to solving problems in research areas as diverse as electrocatalysis, energy storage, biological electron transfer, and plasmon-driven chemistry. However, there is currently no technique capable of directly providing chemical information about molecules undergoing heterogeneous charge transfer at the nanoscale. Tip-enhanced Raman spectroscopy (TERS) uniquely offers subnanometer spatial resolution and single-molecule sensitivity, making it the ideal tool for studying nanoscale electrochemical processes with high chemical specificity. In this work, we demonstrate the first electrochemical TERS (EC-TERS) study of the nanoscale redox behavior of Nile Blue (NB), and compare these results with conventional cyclic voltammetry (CV). We successfully monitor the disappearance of the 591 cm-1 band of NB upon reduction and its reversible reappearance upon oxidation during the CV. Interestingly, we observe a negative shift of more than 100 mV in the onset of the potential response of the TERS intensity of the 591 cm-1 band, compared to the onset of faradaic current in the CV. We hypothesize that perturbation of the electrical double-layer by the TERS tip locally alters the effective potential experienced by NB molecules in the tip-sample junction. However, we demonstrate that the tip has no effect on the local charge transfer kinetics. Additionally, we observe step-like behavior in some TERS voltammograms corresponding to reduction and oxidation of single or few NB molecules. We also show that the coverage of NB is nonuniform across the ITO surface. We conclude with a discussion of methods to overcome the perturbation of the double-layer and general considerations for using TERS to study nanoscale electrochemical processes. [ABSTRACT FROM AUTHOR]

Abstract: The three solid-state indium/germanium selenides Cs4In8GeSe16, CsInSe2, and CsInGeSe4 have been synthesized at 1173K. The structure of Cs4In8GeSe16 is a three-dimensional framework whereas those of CsInSe2 and CsInGeSe4 comprise sheets separated by Cs cations. Both Cs4In8GeSe16 and CsInGeSe4 display In/Ge disorder. From optical absorption measurements these compounds have band gaps of 2.20 and 2.32eV, respectively. All three compounds are charge balanced. [Copyright &y& Elsevier]

Abstract: The compound Rb2Th7Se15 has been synthesized by the solid-state reaction at 1273K of Th, Rb2Se3, Se, and Ge, and its structure has been determined by single-crystal X-ray diffraction methods. Red crystals of Rb2Th7Se15 crystallize at 100(2)K with four formula units in a new structure type in the monoclinic space group −P21/c. The structure is three-dimensional and comprises Rb and Th atoms coordinated by Se atoms to form nine-, eight-, and seven-coordinate polyhedra. Infinite channels that contain Rb atoms are present. Crystals of Rb2Th7Se15 are red in color; an indirect band gap of 1.83eV was derived from single-crystal optical measurements. [Copyright &y& Elsevier]

The compounds ThOS, ThOSe, and ThOTe have been synthesized, and their structures have been determined by means of single-crystal X-ray diffraction methods. All three compounds adopt the PbFCl structure type in the tetragonal space group D4h7 - P4/nmm. More precise crystallographic data have been obtained for ThOS and ThOSe, which had previously only been known from X-ray powder diffraction data. ThOS, ThOSe, and ThOTe are yellow-, orange-, and black-colored, respectively. From single-crystal optical absorption measurements the band gaps are 2.22, 1.65, and 1.45 eV, respectively. Optical band gaps, ionic charges, and densities of states were calculated for the three compounds with the use of Density Functional methods. [ABSTRACT FROM AUTHOR]

Abstract: CsU2(PO4)3 was synthesized in highest yield by the reaction in a fused-silica tube of U, P, and Se in a CsCl flux at 1273K. It crystallizes with four formula units in space group P21/n of the monoclinic system in a new structure type. The structure of CsU2(PO4)3 is composed of U and Cs atoms coordinated by PO4 3− units in distorted octahedral arrangements. Each U atom corner shares with six PO4 3− units. Each Cs atom face shares with one, edge shares with two, and corner shares with three PO4 3− units. The structure shares some features with the sodium zirconium phosphate structure type. X-ray powder diffraction results demonstrate that the present CsU2(PO4)3 compound crystallizes in a structure different from the previously reported β′- and γ-CsU2(PO4)3 compounds. CsU2(PO4)3 is highly pleochroic, as demonstrated by single-crystal optical absorption measurements. [Copyright &y& Elsevier]

Resonance Raman (RR) spectroscopy has several advantages over the normal Raman spectroscopy (RS) widely used for in situcharacterization of solid catalysts and catalytic reactions. Compared with RS, RR can provide much higher sensitivity and selectivity in detecting catalytically-significant surface metal oxides. RR can potentially give useful information on the nature of excited states relevant to photocatalysis and on the anharmonic potential of the ground state. In this critical reviewa detailed discussion is presented on several types of RR experimental systems, three distinct sources of so-called Raman (fluorescence) background, detection limits for RR compared to other techniques (EXAFS, PM-IRAS, SFG), and three well-known methods to assign UV-vis absorption bands and a band-specific unified method that is derived mainly from RR results. In addition, the virtues and challenges of surface-enhanced Raman spectroscopy (SERS) are discussed for detecting molecular adsorbates at catalytically relevant interfaces. Tip-enhanced Raman spectroscopy (TERS), which is a combination of SERS and near-field scanning probe microscopy and has the capability of probing molecular adsorbates at specific catalytic sites with an enormous surface sensitivity and nanometre spatial resolution, is also reviewed (300 references). [ABSTRACT FROM AUTHOR]

Despite its high refractive index sensitivity, localized surface plasmon resonance (LSPR) spectrescopy has been generally restricted to large biological analytes. Sensing of smaller molecules is a compelling target for this technique; in particular, LSPR spectroscopy could be utilized to detect hazardous or toxic gases and manage industrial processes involving gaseous chemicals. Here, we report sensing of pure gases over Ag nanoparticles using ESPR spectroscopy, where the detected changes in bulk refractive index are <5 × 10-4 refractive index units (RIU). We further demonstrate a novel strategy for amplifying the sensing signal by coating the plasmonic substrate with a metal-organic framework (MOF) material. Cu3(BTC)2(H2O)3, BTC = benzenetricarboxylate, was grown on Ag nanoparticles using a layer-by-layer method in order to control the MOF thickness, which we show greatly affects the sensor response. Preferential concentration of CO2 within the MOF pores produces a 14-fold signal enhancement for CO2 sensing. In principle, MOFs can be tailored for sorbing different analytes, making them ideal materials for this amplification strategy. Because the sensing signal originates in the nanopartide extinction spectrum and not in the MOF itself, this comprises a generalizable sensing scheme applicable to any porous MOF and any analyte. [ABSTRACT FROM AUTHOR]

Molecular electronics is partly driven by the goal of producing active electronic elements that rival the performance of their solid-state counterparts, but on a much smaller size scale. We investigate what constitutes an ideal switch or molecular device, and how it can be designed, by analyzing transmission plots. The interference features in cross-conjugated molecules provide a large dynamic range in electron transmission probability, opening a new area for addressing electronic functionality in molecules. This large dynamic range is accessible through changes in electron density alone, enabling fast and stable switching. Using cross-conjugated molecules, we show how the width, depth, and energetic location of the interference features can be controlled. In an example of a single molecule transistor, we calculate a change in conductance of 8 orders of magnitude with an applied gate voltage. Using multiple interference features, we propose and calculate the current/voltage behavior of a molecular rectifier with a rectification ratio of >150 000. We calculate a purely electronic negative differential resistance behavior, suggesting that the large dynamic range in electron transmission probability caused by quantum interference could be exploited in future electronic devices. [ABSTRACT FROM AUTHOR]

Abstract: This paper summarizes our recent progress toward developing anthrax and glucose sensors based on surface-enhanced Raman spectroscopy (SERS). Ag film over nanosphere (AgFON) substrates was used as the SERS sensor platform in both cases. The AgFON substrates have been optimized for near-infrared (NIR) laser excitations by tuning the extinction maximum of their localized surface plasmon resonance (LSPR). Bacillus subtilis spores, harmless simulants for Bacillus anthracis, were studied using SERS. Calcium dipicolinate, a biomarker for bacillus spores, was efficiently extracted from spores and rapidly detected by SERS. A limit of detection (LOD) of ∼2.6×103 spores, below the anthrax infectious dose of 104 spores, has been achieved within 11min. For glucose detection, a mixed decanethiol (DT)/mercaptohexanol (MH) partition layer is used to bring glucose closer to the AgFON surface. Quantitative detection of glucose in bovine plasma, as well as complete and rapid partitioning and departitioning, was demonstrated. The root mean square error of prediction (RMSEP) is 83.16mg/dL (4.62mM) with 85% of the validation points falling within the A and B range of the Clarke error grid. [Copyright &y& Elsevier]

This article presents information on surface-enhanced Raman scattering (SERS). SERS can be exploited for sensitive and selective molecular identification. Recently, SERS has been used extensively as a signal transduction mechanism in biological and chemical sensing. The mechanism of SERS enhancement remains an active research topic. Since the discovery of the phenomenon, two major factions have disagreed on the relative significance of the proposed mechanisms, chemical enhancement, and electromagnetic enhancement. In the chemical mechanism, which is now thought to contribute an average enhancement factor of 100, a charge-transfer state is created between the metal and adsorbate molecules. In a manner analogous to that observed in resonance Raman spectroscopy, the existence of this charge-transfer state increases the probability of a Raman transition by providing a pathway for resonant excitation. This mechanism is site-specific and analyte-dependent. The molecule must be directly adsorbed to the roughened surface to experience the chemical enhancement. To understand the electromagnetic enhancement, one must consider the size, shape, and material of the nanoscale roughness features. These characteristics determine the resonant frequency of the conduction electrons in a metallic nanostructure.

A method for fabricating localized EC‐SERS probes based on nanopipettes and electrodeposition is described. Gold particles of fractal geometry with excellent SERS performance are produced, reliably and at low cost. By adapting the electrodeposition procedure, nanostructures of different sizes can be obtained, allowing the SERS platform to be tailored to many experimental configurations. In particular, by producing unique SERS platforms of dimensions comparable to the laser spot, quantitative comparison with electrochemical current is possible. By analyzing hundreds of samples, we thoroughly characterize the resulting geometry of the structures and their ability to enhance Raman signal, providing guidelines for the fabrication of optimized platforms. Control over the probes' surface potential also allows convenient modulation of surface‐analyte affinity and enable chemically unstable materials, such as Cu, to be reliably used. These are demonstrated by showing that Cu particles exposed to air can be easily re‐reduced, with no detriment in SERS performance. [ABSTRACT FROM AUTHOR]

Theory and experiment examining electron transfer through molecules bound to electrodes are increasingly focused on quantities that are conceptually far removed from current chemical understanding. This presents challenges both for the design of interesting molecules for these devices and for the interpretation of experimental data by traditional chemical mechanisms. Here, the concept of electronic coupling from theories of intramolecular electron transfer is extended and applied in the scattering theory (Landauer) formalism. This yields a simple sum over independent channels, that is then used to interpret and explain the unusual features of junction transport through cross-conjugated molecules and the differences among benzene rings substituted at the ortho, meta, or para positions. [ABSTRACT FROM AUTHOR]

We present theoretical and experimental studies of the infrared (IR) spectroscopy, normal Raman spectroscopy (NRS), surface-enhanced Raman spectroscopy (SERS), and surface-enhanced hyper-Raman (SEHRS) spectroscopy of trans-1,2-bis(4-pyridyl)ethylene (BPE). This centrosymmetric molecule is expected to have no common Raman and hyper-Raman lines provided that it is not strongly perturbed by adsorption on the surface. The measured SERS spectrum, obtained under electrochemical conditions on Ag film over nanosphere (AgFON) electrodes, shows a well defined C=C stretch band that is not seen in the SEHRS spectrum, but many of the other bands overlap closely. We use ab initio calculations for isolated BPE to assign the spectra, and find excellent agreement between the calculated and measured IR and SERS spectra, and good agreement between the calculated and measured SEHRS spectrum. We find that the apparently overlapping IR, SERS, and SEHRS bands are in fact due to modes that have similar vibrational characteristics but different symmetry. Our results indicate that SEHRS spectra are consistent with the expected (three photon) selection rules and intensities. This rules out an alternative mechanism in which the observed spectrum arises from surface second harmonic generation (SHG) followed by SERS excited at the second harmonic frequency. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Surface enhanced second harmonic generation (SESHG) has been observed for the first time from a Pt surface in a <10-3 Torr vacuum environment. Both ‘‘smooth,’’ mechanically polished Pt surfaces and rippled Pt microstructures prepared by laser microchemical etching in Cl2(g) were studied with the newly developed technique of SESHG imaging. The etching procedure and the behavior of the Pt surface under the SESHG imaging conditions is reported in detail. The rippled/smooth enhancement factor for SHG excited with 80 ps, 1064 nm pulses from a cw, mode-locked Nd-YAG laser focused to a 3.1 μm 1/e2 radius ranges from 4 to 17, in qualitative agreement with theoretical calculations for isolated Pt spheroids. [ABSTRACT FROM AUTHOR]

Ultrahigh vacuum tip-enhanced Raman spectroscopy (UHVTERS) is used to investigate adsorption of molecular oxygen (O2) on cobalt(II) phthalocyanine (CoPc) supported on Ag(111) single crystal surfaces, which is the initial step for the oxygen reduction reaction (ORR) using metal Pc catalysts. Two adsorption configurations are primarily observed, assigned as O2/CoPc/Ag(111) and O/CoPc/Ag(111) based on scanning tunneling microscopy (STM) imaging, TERS, isotopologue substitution, and density functional theory (DFT) calculations. Distinct vibrational features are observed for different adsorption configurations such as the 18O--18O stretching frequency at 1151 cm-1 for O2/CoPc/Ag(111), and Co--16O and Co--18O vibrational frequencies at 661 and 623 cm-1, respectively, for O/CoPc/Ag(111). DFT calculations show vibrational mode coupling of O--O and Co--O vibrations to the Pc ring, resulting in different symmetries of oxygen-related normal modes. This study establishes UHVTERS as a chemically sensitive tool for probing catalytic systems at the molecular scale. [ABSTRACT FROM AUTHOR]

We clarify mechanistic questions regarding plasmon-driven chemistry and nanoscale photocatalysis within optically confined near-field plasmonic systems. Using surfaceenhanced Raman scattering (SERS), we directly monitor the photoinduced reaction dynamics of 4,4'-bipyridine molecules, localized in plasmonic hot spots within individual gold nanosphere oligomers. Our experiment generates surface electrons from the gold nanoparticle using an intense offmolecular resonance continuous wave pump field and detects radical anion products via SERS. This is done by adopting a dualwavelength spectroscopic approach. Empirical evidence of plasmon-driven electron transfer is provided for the first time by direct detection of the 4,4'-bipyridine radical anion species localized in the plasmonic hot spots of individual gold nanosphere oligomers, corroborated by open-shell density functional theory calculations. An isotopologue approach using both protonated and deuterated 4,4'-bipyridine molecules demonstrates the single molecule response of plasmon-driven electron transfer occurring in single nanosphere oligomer systems with a 3% yield, a phenomenon unobserved in ensemble measurements under analogous experimental conditions. This mechanism has broad applicability to using nanoscale chemical reactors for surface redox reactions on the subnanometer scale. [ABSTRACT FROM AUTHOR]

Surface-enhanced Raman scattering (SERS) spectroscopy has evolved into a cross-disciplinary analytical technique by unveiling relevant chemical, biological, material, and structural information. The focus of this review is on two critical properties for successfully expanding applications of SERS spectroscopy: quality of the plasmonic substrate and molecule localization to the substrate. In this review, we discuss recent work on quantifying SERS distance dependence, key factors for substrate characterization and performance evaluation, expansion of SERS applications through substrate development for UV plasmonics and short-distance capture strategies for optimizing analyte-surface structures. After surveying the recent developments of these seemingly disparate fields, we suggest new research directions that may originate from a synergistic blend of all the herein discussed topics. Finally, we discuss major challenges and open questions related to the application of SERS for understanding of chemical processes at the nanoscale, with special interest on in situ catalysts and biosensing. [ABSTRACT FROM AUTHOR]

Femtosecond stimulated Raman spectroscopy (FSRS) is a vibrational spectroscopy technique that has been used in a wide variety of applications: from transient vibrational signature tracking to amplifying weak normal Raman scattering signals. Presented here is an application of FSRS to quantify the differential Raman scattering cross sections (DRSCs) of glucose. In using FSRS to determine the DRSCs of multiple glucose vibrational modes, we demonstrate the applicability of both stimulated Raman loss (SRL) spectroscopy and stimulated Raman gain (SRG) FSRS. Using the two analogous FSRS techniques, SRG and SRL, we determine that the DRSCs of glucose excited at 514.5 nm range from a low of 5.0 ± 1.1 × 10-30 to a high of 8.9 ± 0.9 × 10-30 cm2 molecule-1 sr-1. This work establishes both the compatibility of SRL for measuring DRSCs and values for the DRSC of multiple vibrational modes of glucose. [ABSTRACT FROM AUTHOR]

Single-molecule (SM) surface-enhanced Raman spectroscopy (SERS) and tip-enhanced Raman spectroscopy (TERS) have emerged as analytical techniques for characterizing molecular systems in nanoscale environments. SERS and TERS use plasmonically enhanced Raman scattering to characterize the chemical information on single molecules. Additionally, TERS can image single molecules with subnanometer spatial resolution. In this review, we cover the development and history of SERS and TERS, including the concept of SERS hot spots and the plasmonic nanostructures necessary for SM detection, the past and current methodologies for verifying SMSERS, and investigations into understanding the signal heterogeneities observed with SMSERS. Moving on to TERS, we cover tip fabrication and the physical origins of the subnanometer spatial resolution. Then, we highlight recent advances of SMSERS and TERS in fields such as electrochemistry, catalysis, and SM electronics, which all benefit from the vibrational characterization of single molecules. SMSERS and TERS provide new insights on molecular behavior that would otherwise be obscured in an ensemble-averaged measurement. [ABSTRACT FROM AUTHOR]

Molecule-surface interactions and processes are at the heart of many technologies, including heterogeneous catalysis, organic photovoltaics, and nano-electronics, yet they are rarely well understood at the molecular level. Given the inhomogeneous nature of surfaces, molecular properties often vary among individual surface sites, information that is lost in ensemble-averaged techniques. In order to access such site-resolved behavior, a technique must possess lateral resolution comparable to the size of surface sites under study, analytical power capable of examining chemical properties, and single-molecule sensitivity. Tip-enhanced Raman spectroscopy (TERS), wherein light is confined and amplified at the apex of a nanoscale plasmonic probe, meets these criteria. In ultrahigh vacuum (UHV), TERS can be performed in pristine environments, allowing for molecular-resolution imaging, low-temperature operation, minimized tip and molecular degradation, and improved stability in the presence of ultrafast irradiation. The aim of this review is to give an overview of TERS experiments performed in UHV environments and to discuss how recent reports will guide future endeavors. The advances made in the field thus far demonstrate the utility of TERS as an approach to interrogate single-molecule properties, reactions, and dynamics with spatial resolution below 1 nm. [ABSTRACT FROM AUTHOR]

Operando surface-enhanced Raman spectroscopy (SERS) was used to successfully identify hitherto unknown dimeric methylalumina surface species during atomic layer deposition (ALD) on a silver surface. Vibrational modes associated with the bridging moieties of both trimethylaluminum (TMA) and dimethylaluminum chloride (DMACl) surface species were found during ALD. The appropriate monomer vibrational modes were found to be absent as a result of the selective nature of SERS. Density functional theory (DFT) calculations were also performed to locate and identify the expected vibrational modes. An operando localized surface plasmon resonance (LSPR) spectrometer was utilized to account for changes in SER signal as a function of the number of ALD cycles. DMACl surface species were unable to be measured after multiple ALD cycles as a result of a loss in SERS enhancement and shift in LSPR. This work highlights how operando optical spectroscopy by SERS and LSPR scattering are useful for probing the identity and structure of the surface species involved in ALD and, ultimately, catalytic reactions on these support materials. [ABSTRACT FROM AUTHOR]

Electrochemical atomic force microscopy tip-enhanced Raman spectroscopy (EC-AFM-TERS) was employed for the first time to observe nanoscale spatial variations in the formal potential, E0', of a surface-bound redox couple. TERS cyclic voltammograms (TERS CVs) of single Nile Blue (NB) molecules were acquired at different locations spaced 5-10 nm apart on an indium tin oxide (ITO) electrode. Analysis of TERS CVs at different coverages was used to verify the observation of single-molecule electrochemistry. The resulting TERS CVs were fit to the Laviron model for surface-bound electroactive species to quantitatively extract the formal potential E0' at each spatial location. Histograms of single-molecule E0' at each coverage indicate that the electrochemical behavior of the cationic oxidized species is less sensitive to local environment than the neutral reduced species. This information is not accessible using purely electrochemical methods or ensemble spectroelectrochemical measurements. We anticipate that quantitative modeling and measurement of site-specific electrochemistry with EC-AFM-TERS will have a profound impact on our understanding of the role of nanoscale electrode heterogeneity in applications such as electrocatalysis, biological electron transfer, and energy production and storage. [ABSTRACT FROM AUTHOR]

We report here the first fabrication of aluminum film-over nanosphere (AlFON) substrates for UV surface-enhanced resonance Raman scattering (UVSERRS) at the deepest UV wavelength used to date (λex = 229 nm). We characterize the AlFONs fabricated with two different support microsphere sizes using localized surface plasmon resonance spectroscopy, electron microscopy, SERRS of adenine, tris(bipyridine)ruthenium(II), and trans-1,2-bis(4-pyridyl)-ethylene, SERS of 6-mercapto-1-hexanol (as a nonresonant molecule), and dielectric function analysis. We find that AlFONs fabricated with the 210 nm microspheres generate an enhancement factor of approximately 104-5, which combined with resonance enhancement of the adsorbates provides enhancement factors greater than 106. These experimental results are supported by theoretical analysis of the dielectric function. Hence our results demonstrate the advantages of using AlFON substrates for deep UVSERRS enhancement and contribute to broadening the SERS application range with tunable and affordable substrates. [ABSTRACT FROM AUTHOR]

Tip-enhanced Raman spectroscopy (TERS) combines the ability of scanning probe microscopy (SPM) to resolve atomic-scale surface features with the single-molecule chemical sensitivity of surface-enhanced Raman spectroscopy (SERS). Here, we report additional insights into the nature of the conformational dynamics of a free-base porphyrin at room temperature adsorbed on a metal surface. We have interrogated the conformational switch between two metastable surface-mediated isomers of meso-tetrakis(3,5-ditertiarybutylphenyl)-porphyrin (H2TBPP) on a Cu(111) surface. At room temperature, the barrier between the porphyrin ring buckled up/down conformations of the H2TBPP-Cu(111) system is easily overcome, and a 2.6 Å lateral resolution by simultaneous TERS and STM analysis is achieved under ultrahigh vacuum (UHV) conditions. This work demonstrates the first UHV-TERS on Cu(111) and shows TERS can unambiguously distinguish the conformational differences between neighboring molecules with Ångstrom-scale spatial resolution, thereby establishing it as a leading method for the study of metal-adsorbate interactions. [ABSTRACT FROM AUTHOR]

This paper demonstrates the direct sensing of glucose at physiologically relevant concentrations with surface-enhanced Raman spectroscopy (SERS) on gold film-over-nanosphere (AuFON) substrates functionalized with bisboronic acid receptors. The combination of selectivity in the bisboronic acid receptor and spectral resolution in the SERS data allow the sensors to resolve glucose in high backgrounds of fructose and, in combination with multivariate statistical analysis, detect glucose accurately in the 1–10 mM range. Computational modeling supports assignments of the normal modes and vibrational frequencies for the monoboronic acid base of our bisboronic acids, glucose and fructose. These results are promising for the use of bisboronic acids as receptors in SERS-based in vivo glucose monitoring sensors. [ABSTRACT FROM AUTHOR]

Conspectus: 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. [ABSTRACT FROM AUTHOR]

Tip-enhanced Raman spectroscopy (TERS) is a powerful technique that integrates the vibrational fingerprinting of Raman spectroscopy and the sub-nanometer resolution of scanning probe microscopy (SPM). As a result, TERS is capable of obtaining chemical maps of analyzed specimens with exceptional lateral resolution. This is extremely valuable for the study of interactions between molecules and substrates, in addition to structural characterization of biological objects, such as viruses and amyloid fibrils, 2D polymeric materials, and monitoring electrochemical and photo-catalytic processes. In this mini-review, we discuss the most significant advances of TERS, including: super high resolution chemical imaging, monitoring of catalytic processes, incorporation of pulsed-excitation techniques, single-site electrochemistry, biosensing, and art conservation. We begin with a short overview of TERS, comparing it with other surface analytical techniques, followed by an overview of recent developments and future applications in TERS. [ABSTRACT FROM AUTHOR]

The article discusses a study on the development of a quantum interference apart from the molecular electron transfer. This study addresses the consequence in the difference of acyclic cross-conjugated molecules synthesis. It explains the breakdown of the conventional molecular electron transfer between the physical properties of linearly conjugated counterparts.

We present a high-resolution distance dependence study of surface-enhanced Raman scattering (SERS) enabled by atomic layer deposition (ALD) at 55 and 100 °C. ALD is used to deposit monolayers of Al2O3 on bare silver film over nanospheres (AgFONs) and AgFONs functionalized with self-assembled monolayers. Operando SERS is used to measure the intensities of the Al-CH3 and C-H stretches from trimethylaluminum (TMA) as a function of distance from the AgFON surface. This study clearly demonstrates that SERS on AgFON substrates displays both a short- and long-range nanometer scale distance dependence. Excellent agreement is obtained between these experiments and theory that incorporates both short-range and long-range terms. This is a high-resolution operando SERS distance dependence study performed in one integrated experiment using ALD Al2O3 as the spacer layer and Raman label simultaneously. The long-range SERS distance dependence should make it possible to detect chemisorbed surface species located as far as ∼3 nm from the AgFON substrate and will provide new insight into the surface chemistry of ALD and catalytic reactions. [ABSTRACT FROM AUTHOR]

This perspective presents recent developments in the application of surface-enhanced Raman spectroscopy (SERS) to biosensing, with a focus on in vivo diagnostics. We describe the concepts and methodologies developed to date and the target analytes that can be detected. We also discuss how SERS has evolved from a "point-and-shoot" stand-alone technique in an analytical chemistry laboratory to an integrated quantitative analytical tool for multimodal imaging diagnostics. Finally, we offer a guide to the future of SERS in the context of clinical diagnostics. [ABSTRACT FROM AUTHOR]

Nanoscale chemical imaging of a dynamic molecular phase boundary has broad implications for a range of problems in catalysis, surface science, and molecular electronics. While scanning probe microscopy (SPM) is commonly used to study molecular phase boundaries, its information content can be severely compromised by surface diffusion, irregular packing, or three-dimensional adsorbate geometry. Here, we demonstrate the simultaneous chemical and structural analysis of N-N'-bis(2,6-diisopropylphenyl)-1,7-(4'-t-butylphenoxy)perylene-3,4:9,10-bis(dicarboximide) (PPDI) molecules by UHV tip-enhanced Raman spectroscopy. Both condensed and diffusing domains of PPDI coexist on Ag(100) at room temperature. Through comparison with time-dependent density functional theory simulations, we unravel the orientation of PPDI molecules at the dynamic molecular domain boundary with unprecedented ~4 nm spatial resolution. [ABSTRACT FROM AUTHOR]

We employ SrTiO 3 nanocuboid single crystals with well-defined (001) surfaces that are synthesized to have either a TiO 2 - or SrO-terminated surface to investigate the influence of surface termination on the morphology and the chemical property of supported metallic nanoparticles. Using such monodispersed STO nanocuboids allows for practical catalytic reaction studies as well as surface studies comparable to a single crystal model catalyst. Pd nanoparticles were grown by atomic layer deposition, which is able to control the effective coverage, chemical state, and the size of the Pd nanoparticles. The properties of Pd nanoparticles were examined by transmission electron microscopy, X-ray absorption spectroscopy, and X-ray photoemission spectroscopy. The morphology and growth pattern for the Pd nanoparticles supported on the SrTiO 3 nanocuboids are shown to depend on the surface termination. [ABSTRACT FROM AUTHOR]

We demonstrate that incorporation of a redox-active benzoquinoid ligand into a one-dimensional chain compound can give rise to a material that exhibits simultaneous solid-state redox switching of optical, magnetic, and electronic properties. Metalation of the ligand 4,5-bis(pyridine-2-carboxamido)-1,2-catechol (N,OLH4) with MnIII affords the chain compound Mn(N,OL)(DMSO). Structural and spectroscopic analysis of this compound show the presence of MnII centers bridged by N,OL2- ligands, resulting partially from a spontaneous ligand-to-metal electron transfer. Upon soaking in a solution of the reductant Cp2Co, Mn(N,OL)(DMSO) undergoes a ligand-centered solid-state reduction to [Mn(N,OL)]-, as revealed by a suite of techniques, including Raman and X-ray absorption spectroscopy. The ligand-based reduction engenders a dramatic modulation of the physical properties of the chain compound. An electrochromic response, evidenced by a color change from dark green to dark purple is accompanied by a nearly 40-fold increase in magnetic coupling strength, from J = -0.38(1) to -15.6(2) cm-1, and a 10,000-fold increase in electronic conductivity, from s = 2.33(1) × 10-12 S/cm (Ea = 0.64(1) eV) to 8.61(1) × 10-8 S/cm (Ea = 0.39(1) eV). Importantly, the chemical reduction is reversible: treatment of the reduced compound with [Cp2Fe]+ regenerates the oxidized chain. Taken together, these results highlight the ability of benzoquinoid ligands to facilitate solid-state ligand-based redox reactions in nonporous coordination solids, giving rise to reversible switching of optical properties, magnetic exchange interactions, and electronic conductivity. [ABSTRACT FROM AUTHOR]

Ultrafast surface-enhanced Raman spectroscopy (SERS) has the potential to study molecular dynamics near plasmonic surfaces to better understand plasmon-mediated chemical reactions such as plasmonically-enhanced photocatalytic or photovoltaic processes. This review discusses the combination of ultrafast Raman spectroscopic techniques with plasmonic substrates for high temporal resolution, high sensitivity, and high spatial resolution vibrational spectroscopy. First, we introduce background information relevant to ultrafast SERS: the mechanisms of surface enhancement in Raman scattering, the characterization of plasmonic materials with ultrafast techniques, and early complementary techniques to study molecule–plasmon interactions. We then discuss recent advances in surface-enhanced Raman spectroscopies with ultrafast pulses with a focus on the study of molecule–plasmon coupling and molecular dynamics with high sensitivity. We also highlight the challenges faced by this field by the potential damage caused by concentrated, highly energetic pulsed fields in plasmonic hotspots, and finally the potential for future ultrafast SERS studies. [ABSTRACT FROM AUTHOR]