Electromagnetic enhancement spectra of one-dimensional plasmonic hotspots along silver nanowire dimer derived via surface-enhanced fluorescence.

We developed a spectroscopic method for directly obtaining the spectra of electromagnetic (EM) enhancement of plasmonic hotspots (HSs). The method was applied to one-dimensional (1D) HSs generated between silver nanowire (NW) dimers. The EM enhancement spectra were derived by dividing the spectra of surface-enhanced fluorescence (SEF) from single NW dimers with SEF obtained from large nanoparticle aggregates, where aggregate-by-aggregate variations in the SEF spectra were averaged out. Some NW dimers were found to exhibit EM enhancement spectra that deviated from the plasmon resonance Rayleigh scattering spectra, indicating that their EM enhancement was not generated by superradiant plasmons. These experimental results were examined by numerical calculation based on the EM mechanism by varying the morphology of NW dimers. The calculations reproduced the spectral deviations as the NW diameter dependence of EM enhancement. Phase analysis of the enhanced EM near-fields along the 1D HSs revealed that the dipole-quadrupole coupled plasmon, which is a subradiant mode, mainly generates EM enhancement for dimers with NW diameters larger than ∼80 nm, which was consistent with scanning electron microscopic measurements.

Demonstration of electromagnetic enhancement correlated to optical absorption of single plasmonic system coupled with molecular excitons using ultrafast surface-enhanced fluorescence.

The relationship between the electromagnetic (EM) enhancement of the optical responses of molecules and plasmon resonance has been investigated using Rayleigh scattering or the extinction spectra of plasmonic systems coupled with molecular excitons. However, quantum optics predicts that the EM enhancement of such optical responses, e.g., fluorescence, Raman, and their nonlinear counterparts, is related directly to optical absorption and indirectly to Rayleigh scattering and extinction. To demonstrate this prediction, a micro-spectroscopic method for obtaining Rayleigh scattering, extinction, absorption, and EM enhancement is developed using single-coupled plasmonic systems composed of silver nanoparticle dimers and dye molecules. The EM enhancement is derived from ultrafast surface-enhanced fluorescence. An evaluation of the spectral relationships demonstrates that the EM enhancement can be reproduced better by absorption than by Rayleigh scattering or extinction. This reproduction is phenomenologically confirmed by numerical calculations based on classical electromagnetism, indicating the importance of absorption spectroscopy in coupled plasmonic systems for evaluating EM enhancement.

Correlated Polarization Dependences between Surface-Enhanced Resonant Raman Scattering and Plasmon Resonance Elastic Scattering Showing Spectral Uncorrelation to Each Other.

We investigated the origin of the identical polarization angle dependence between surface-enhanced resonant Raman scattering (SERRS) and plasmon resonance elastic scattering (PRES) for two types of single silver nanoparticle aggregates. The first type (Type I), in which the SERRS spectral envelopes are similar to the PRES spectra, shows the identical polarization dependence between the SERRS and PRES. The second type (Type II), in which the SERRS envelopes largely deviate from the PRES spectra, also exhibits identical polarization dependence. Scanning electron microscopy observations indicated that the aggregates were dimers. This unintuitive result was examined by calculating the electromagnetic enhancement by changing the morphology of the dimers. The calculations revealed that the Type I dimer generates SERRS directly by superradiant plasmons. The Type II dimer generates SERRS indirectly via subradiant plasmons, which receive light energy from superradiant plasmons. This indirect SERRS process clarifies that the interaction between the superradiant and subradiant plasmons results in an identical polarization dependence between SERRS and PRES for Type II dimers.

Toward a New Era of SERS and TERS at the Nanometer Scale: From Fundamentals to Innovative Applications.

Surface-enhanced Raman scattering (SERS) and tip-enhanced Raman scattering (TERS) have opened a variety of exciting research fields. However, although a vast number of applications have been proposed since the two techniques were first reported, none has been applied to real practical use. This calls for an update in the recent fundamental and application studies of SERS and TERS. Thus, the goals and scope of this review are to report new directions and perspectives of SERS and TERS, mainly from the viewpoint of combining their mechanism and application studies. Regarding the recent progress in SERS and TERS, this review discusses four main topics: (1) nanometer to subnanometer plasmonic hotspots for SERS; (2) Ångström resolved TERS; (3) chemical mechanisms, i.e., charge-transfer mechanism of SERS and semiconductor-enhanced Raman scattering; and (4) the creation of a strong bridge between the mechanism studies and applications.

All-dielectric chiral-field-enhanced Raman optical activity.

Raman optical activity (ROA) is effective for studying the conformational structure and behavior of chiral molecules in aqueous solutions and is advantageous over X-ray crystallography and nuclear magnetic resonance spectroscopy in sample preparation and cost performance. However, ROA signals are inherently minuscule; 3-5 orders of magnitude weaker than spontaneous Raman scattering due to the weak chiral light-matter interaction. Localized surface plasmon resonance on metallic nanoparticles has been employed to enhance ROA signals, but suffers from detrimental spectral artifacts due to its photothermal heat generation and inability to efficiently transfer and enhance optical chirality from the far field to the near field. Here we demonstrate all-dielectric chiral-field-enhanced ROA by devising a silicon nanodisk array and exploiting its dark mode to overcome these limitations. Specifically, we use it with pairs of chemical and biological enantiomers to show >100x enhanced chiral light-molecule interaction with negligible artifacts for ROA measurements.

Between plasmonics and surface-enhanced resonant Raman spectroscopy: toward single-molecule strong coupling at a hotspot.

The purpose of this minireview is to build a bridge between two research fields: surface-enhanced resonant Raman spectroscopy (SERRS) under near-single-molecule conditions and the branch of plasmonics treating strong coupling between plasmons and molecular excitons. SERRS enables single-molecule spectroscopy owing to its significant enhancement at SERRS hotspots (HSs), localized at gaps or junctions between plasmonic nanoparticle aggregates. SERRS is SERS (surface enhanced Raman spectroscopy) under a resonant Raman excitation condition. The origin of the Raman enhancement in SERRS is electromagnetic coupling between plasmons and molecular excitons at HSs. It has been reported that the coupling energy at HSs reaches the strong coupling region, meaning that they are potential platforms for applications of single molecular excitons modified by strong coupling. In this review, we discuss recent progress related to electronic strong coupling in near-single-molecule SERRS: collective (e.g., vibrational) strong coupling is out of the scope of this minireview. First, we explain the relationship between the electromagnetic enhancement factor and coupling energy. Second, we introduce three theoretical methods for obtaining evidence of strong coupling at HSs. Third, we discuss a method for reproducing enhanced and modified molecular Raman and fluorescence spectra at HSs using the coupling energy. Finally, we propose the use of two experimental methods of absorption spectroscopy at HSs for modifying molecular electronic dynamics by strong coupling and comment on future applications of SERRS HSs to photophysics and photochemistry.

Porous carbon nanowire array for surface-enhanced Raman spectroscopy.

Surface-enhanced Raman spectroscopy (SERS) is a powerful tool for vibrational spectroscopy as it provides several orders of magnitude higher sensitivity than inherently weak spontaneous Raman scattering by exciting localized surface plasmon resonance (LSPR) on metal substrates. However, SERS can be unreliable for biomedical use since it sacrifices reproducibility, uniformity, biocompatibility, and durability due to its strong dependence on "hot spots", large photothermal heat generation, and easy oxidization. Here, we demonstrate the design, fabrication, and use of a metal-free (i.e., LSPR-free), topologically tailored nanostructure composed of porous carbon nanowires in an array as a SERS substrate to overcome all these problems. Specifically, it offers not only high signal enhancement (~106) due to its strong broadband charge-transfer resonance, but also extraordinarily high reproducibility due to the absence of hot spots, high durability due to no oxidization, and high compatibility to biomolecules due to its fluorescence quenching capability.

Distinguishing Enantiomers by Tip-Enhanced Raman Scattering: Chemically Modified Silver Tip with an Asymmetric Atomic Arrangement.

Discrimination between enantiomers is achieved by tip-enhanced Raman scattering (TERS) using a silver tip that is chemically modified by an achiral para-mercaptopyridine (pMPY) probe molecule. Differences in the relative intensities of the pMPY spectra were monitored for three pairs of enantiomers containing hydroxy (-OH) and/or amino (-NH2 ) groups. The N: or N+ -H functionality of the pMPY-modified tip participates in hydrogen-bond interactions with a particular molecular orientation of each chiral isomer. The asymmetric arrangement of silver atoms at the apex of the tip induces an asymmetric electric field, which causes the tip to become a chiral center. Differences in the charge-transfer (CT) states of the metal-achiral probe system in conjunction with the asymmetric electric field produce different enhancements in the Raman signals of the two enantiomers. The near-field effect of the asymmetric electric field, which depends on the number of analyte functional groups capable of hydrogen-bond formation, improves the degree of discrimination.

Anti-crossing property of strong coupling system of silver nanoparticle dimers coated with thin dye molecular films analyzed by electromagnetism.

Evidence of strong coupling between plasmons and molecular excitons for plasmonic nanoparticle (NP) dimers exhibiting ultra-sensitive surface enhanced resonant Raman scattering is the observation of anti-crossing in the coupled resonance. However, experimentally tuning the plasmon resonance of such dimers for the observation is difficult. In this work, we calculate the anti-crossing property of dimers coated with thin dye films according to the classical electromagnetism. This property is quantitatively evaluated according to the coupled oscillator model composed of a plasmon and a molecular exciton representing the molecular multi-level system. A comparison of the film thickness dependences of dimer spectral changes with those of silver ellipsoidal NPs indicates that the dipole plasmons localized in the dimer gap are coupled with molecular excitons of the film much stronger than the dipole plasmons of ellipsoidal NPs. Furthermore, the anti-crossing behavior of coupled resonances is investigated while tuning plasmon resonance by changing the morphology and refractive index of the surrounding medium. The spectral changes observed for ellipsoidal NPs clearly exhibit anti-crossing property; however, the anti-crossing behavior of dimers is more complex due to the strong coupling of dipoles and higher-order plasmons with multiple molecular excitons. We find that the anti-crossing for dimers is clearly confirmed by the refractive index dependence of coupled resonance.

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.

Rapid detection of synthetic cannabinoids in herbal highs using surface-enhanced Raman scattering produced by gold nanoparticle co-aggregation in a wet system.

Synthetic cannabinoids (SCs) are a major category of new psychoactive substances that are frequently distributed after addition to plants. To date, various SCs with small differences in their chemical structures have prevailed in the illegal drug market. Thus, the development of a method for rapid detection with high discrimination capability is critically important for the forensic field. Vibrational spectroscopy is a possible analytical technique for this purpose because it can sensitively reflect differences among chemical structures. In this study, we applied surface-enhanced Raman scattering (SERS) with gold nanoparticle co-aggregation in a wet system to plant samples containing SCs. The experimental protocol used was simple and involved only mixing of the sample with several other solutions. It was possible to detect SERS spectra from various stock solutions of SCs by this method. The method was then applied to street samples containing SCs. Some of the plant samples containing SCs did not produce significant SERS signals even though stock solutions of the same SCs did produce SERS spectra. We investigated the reason for this discrepancy and speculated that the solubility in aqueous solutions was a factor determining whether a significant SERS signal could be detected or not. According to this hypothesis, minimal sample pre-treatment methods were applied. This allowed for the detection of SERS spectra from the examined plant samples. The developed approach is a powerful method for screening analysis of SCs in plant fragments.

Rapid detection of hypnotics using surface-enhanced Raman scattering based on gold nanoparticle co-aggregation in a wet system.

Sensitive detection of drugs using a method with high qualification capability is important for forensic drug analysis. Vibrational spectroscopy is a powerful screening technique because it can provide detailed structural information of the compounds included in samples with simple experimental protocols. Among various spectroscopic techniques, surface enhanced Raman scattering (SERS) spectroscopy has attracted enormous attention owing to its ultra-high sensitivity. In this study, we developed a method for rapid detection of hypnotics using SERS with gold nanoparticle co-aggregation in a wet system. The developed method required a simple analytical protocol. This enabled rapid analysis with high stability and repeatability. We analyzed various hypnotics (19 types including benzodiazepines and nonbenzodiazepines) to investigate the structure-spectrum relationship. As a proof of concept for application to real crime samples, simulated spiked beverages containing one hypnotic (etizolam, flunitrazepam, zolpidem, or zopiclone) were analyzed. Diluting the beverage samples decreased the matrix effect and allowed for detection of these hypnotics. Except for flunitrazepam, strong signals were observed for all hypnotics, and the estimated lower limit of detection was 50 ppm in apple drink. The developed approach is a rapid method for screening analysis of hypnotics with low sample requirements.

Calculated shape dependence of electromagnetic field in tip-enhanced Raman scattering by using a monopole antenna model.

To evaluate the shape of an Ag tip with regard to tip-enhanced Raman scattering (TERS) signal, the enhanced electromagnetic (EM) field and scattering spectrum, arising from surface plasmon resonance at the apex of the tip, were calculated using a finite-difference time domain (FDTD) method. In the calculated forward scattering spectra from the smooth Ag tip, the band appeared within the visible region, similar to the experimental results and calculation for a corrugated Ag cone. In the FDTD calculation of TERS, the Ag tip acting as a monopole antenna was adopted by insertion of a perfect electric conductor between the root of the tip and a top boundary surface of the calculation space. As a result, the EM field was only enhanced at the apex. The shape dependence i.e. the EM field calculated at the apex with various curvatures on the different tapered tips, obtained using the monopole antenna model, was different from that simulated using a conventional dipole antenna model.

Reproduction of surface-enhanced resonant Raman scattering and fluorescence spectra of a strong coupling system composed of a single silver nanoparticle dimer and a few dye molecules.

The spectral changes in surface-enhanced resonant Raman scattering (SERRS) and surface enhanced fluorescence (SEF) of single silver nanoparticle dimers adsorbed by near-single dye molecules are reproduced under strong coupling regimes. For the reproduction, the enhancement and quenching factors in SERRS and SEF are derived from the Purcell factors including both radiative and nonradiative plasmon modes. The Purcell factors are estimated using the coupling energies obtained by analyzing the spectral changes in plasmon resonance during SERRS and SEF decay processes on the basis of a classical hybridization model. The model is composed of a plasmon and a molecular exciton with phonon replicas accurately representing the molecular multi-level system. The reproduced SERRS spectral changes are consistent with the experimental ones. Furthermore, the calculated SEF spectral changes can reproduce the experimental ones by phenomenologically assuming transitions from ultra-fast SEF to conventional SEF with decreasing coupling energies.

Measurement of pH-dependent surface-enhanced hyper-Raman scattering at desired positions on yeast cells via optical trapping.

Surface-enhanced hyper-Raman scattering (SEHRS) spectra were obtained at desired positions on yeast by focusing a continuous wave near-infrared laser beam while silver nanoparticles (AgNPs) were simultaneously optically trapped. However, the optically trapped colloidal AgNP suspension bubbled up at the focusing point, preventing spectral measurement. In the case of optically trapped AgNPs functionalized with 4-mercaptobenzoic acid (p-MBA), surface-enhanced hyper-Rayleigh scattering was considerably strong, indicating the suppression of the photothermal conversion to form the bubble. Interestingly, the SEHRS peaks that are attributed not only to p-MBA, but also to other species, were very occasionally observed. They may be partly assigned to the ß1,3 glucan and protein amide II band. The SEHRS peak at 1366 cm-1 was barely visible in the measurements of conventional baker's yeast even in the suspension (pH 9) despite the effects of high pH on p-MBA. In contrast, the SEHRS peak in the measurements of yeast for biological applications was occasionally observed at 1366 cm-1. This suggests that acidity is correlated with fermentation efficiency. At different positions on single yeast cells, the intensity of the SEHRS peak at 1366 cm-1 varied. This result represents the pH distribution on yeast.

Plasmon-enhanced spectroscopy of absorption and spontaneous emissions explained using cavity quantum optics.

The purpose of this tutorial review is to provide a comprehensive explanation of plasmon-enhanced spectroscopies, such as plasmon-enhanced Raman scattering, fluorescence, absorption, Rayleigh scattering, and hyper Raman scattering. Plasmon-enhanced spectroscopy implies the spectroscopy of enhanced optical responses of molecules in close proximity to plasmonic nanostructures, resulting in a strong enhancement in sensitivity. In this review, we explain the enhancement in plasmon-enhanced spectroscopy as an optical response of a molecule interacting with an optical resonator, which represents a plasmonic nanostructure, in analogy to cavity quantum optics to easily understand all types of plasmon-enhanced spectroscopy in the same manner. The keys to understanding the enhancement factor of each plasmon-enhanced spectroscopy are a quality factor and a mode volume of plasmonic resonators, which are well-known parameters in the Purcell effect of standard optical cavity resonators.

Recent topics on single-molecule fluctuation analysis using blinking in surface-enhanced resonance Raman scattering: clarification by the electromagnetic mechanism.

Surface-enhanced Raman scattering (SERS) spectroscopy has become an ultrasensitive tool for clarifying molecular functions on plasmonic metal nanoparticles (NPs). SERS has been used for in situ probing of detailed behaviors of few or single molecules (SMs) at plasmonic NP junctions. SM SERS signals are commonly observed with temporal and spectral changes known as "blinking", which are related to various physical and chemical interactions between molecules and NP junctions. These temporal and spectral changes simultaneously take place, therefore resulting in serious complexities in interpretations of the SM SERS results. Dual contributions of Raman enhancement mechanisms in SERS (i.e., electromagnetic (EM) and chemical enhancements) also make interpretations more difficult. To resolve these issues and reduce the degree of complexities in SM SERS analyses, the present review is focused on the recent studies of probing SM behaviors using SERS exclusively within the framework of the EM mechanism. The EM mechanism is briefly introduced, and several recent topics on SM SERS blinking analysis are discussed in light of the EM mechanism. This review will provide a basis for clarification of complex SERS fluctuations of various molecules.

Different behaviour of molecules in dark SERS state on colloidal Ag nanoparticles estimated by truncated power law analysis of blinking SERS.

For single colloidal Ag nanoaggregates, covered with either large or small amounts of citrate anions, blinking surface-enhanced Raman scattering (SERS) of anionic thiacyanine was measured and analyzed by a truncated power law. The power law without and with an exponential function reproduces a probability distribution for bright and dark SERS events versus their duration times, respectively. On the Ag surface, except for junctions of the nanoaggregate with a large or small amount of the citrate anions, two-dimensional fast or one-dimensional slow random walk of the anionic thiacyanine, respectively, was estimated by the exponents and the truncation times in the power law for the dark SERS events. In addition, the power law exponents for the bright SERS events were derived to be of similar values, indicating a similar molecular random walk near the junction, which may be dominated evenly by a surface-plasmon-enhanced electromagnetic field on the same-sized Ag nanoaggregate. Thus, not only the bright SERS, but also the dark SERS molecular behaviour on the Ag surface was investigated by the truncated power law analysis.

Tip-enhanced Raman spectroscopic measurement of stress change in the local domain of epitaxial graphene on the carbon face of 4H-SiC(000-1).

We develop a bulk silver tip for tip-enhanced Raman scattering (TERS) and obtain TERS spectra of epitaxial graphene on the carbon face of 4H-SiC(000-1) with a high signal-to-noise ratio. Thanks to the high quality of TERS spectra we firstly find that the G band in the TERS spectra exhibits position-by-position variations in both lower wavenumber shifts and spectral broadening. The analysis of the variations reveals that the shifts and broadenings have a linear correlation between each other, indicating that the variations are induced by the position dependent local stress on graphene based on a uniaxial strain model.

A study on the interaction of single-walled carbon nanotubes (SWCNTs) and polystyrene (PS) at the interface in SWCNT-PS nanocomposites using tip-enhanced Raman spectroscopy.

Normal Raman and tip-enhanced Raman scattering (TERS) spectra were recorded for single-walled carbon nanotube (SWCNT)-polystyrene (PS) nanocomposites to investigate the distribution of SWCNTs in the nanocomposites and local interactions at an interface between SWCNTs and PS. The normal Raman spectra do not show an evident point-to-point variation. In contrast, in the TERS spectra, Raman bands of SWCNTs show obvious point-to-point shifts; the shifts depend on the points. The shift of the G' band which has high sensitivity to the mechanical compression reflects its distribution at the surface of composites. The shift of the G band arises from two reasons: the disentanglement of SWCNTs in the SWCNT-PS system due to the penetration of PS chains into SWCNT bundles during melt mixing and the mechanical compression distribution from the PS matrix. Moreover, the relative intensity of the G' band and the Raman band of PS at 3058 cm(-1) changes with the points in the TERS spectra of the nanocomposites, which further reflects the dispersion state of SWCNTs in the polymer matrix. This study demonstrates that TERS has great potential to be applied to polymer nanocomposite materials for local structure and function studies.