Surface-Enhanced Femtosecond Stimulated Raman Spectroscopy

Impact of Surface Enhanced Raman Spectroscopy in Catalysis

Catalysis stands as an indispensable cornerstone of modern society, underpinning the production of over 80% of manufactured goods and driving over 90% of industrial chemical processes. As the demand for more efficient and sustainable processes grows, better catalysts are needed. Understanding the working principles of catalysts is key, and over the last 50 years, surface-enhanced Raman Spectroscopy (SERS) has become essential. Discovered in 1974, SERS has evolved into a mature and powerful analytical tool, transforming the way in which we detect molecules across disciplines. In catalysis, SERS has enabled insights into dynamic surface phenomena, facilitating the monitoring of the catalyst structure, adsorbate interactions, and reaction kinetics at very high spatial and temporal resolutions. This review explores the achievements as well as the future potential of SERS in the field of catalysis and energy conversion, thereby highlighting its role in advancing these critical areas of research.

Exploring Excited State Landscapes with Surface Enhanced Hyper-Raman Spectroscopy

In this Perspective, we provide a historical overview of the surface-enhanced hyper-Raman scattering (SEHRS) effect, describe its essential components, highlight the close connection between theory and experiment in several vignettes, and discuss recent analytical applications. SEHRS, the two-photon analog of surface-enhanced Raman scattering (SERS), is a spontaneous nonlinear scattering exhibited by molecules in a plasmonic field. Hyper Raman provides distinctive information on the molecular vibrations and electronic excited states of analytes. A 40-year old mystery surrounding the SEHRS spectra of R6G is used to illustrate the power of SEHRS to explore excited electronic states, revealing how non-Condon effects can influence the two-photon molecular properties. The exceptionally large enhancement factors (>1013) obtained from SEHRS enable the analysis of single molecules and molecules at very low concentrations. This high sensitivity is further augmented by an increased sensitivity to chemical effects, allowing SEHRS to probe changes in the local environment and the orientation of surface ligands. As most SEHRS experiments employ near-infrared (NIR) and short-wave infrared (SWIR) light, it also holds promise for bioimaging studies. Before concluding, we discuss future directions and challenges for the field as it moves forward.

Sixty years of electrochemical optical spectroscopy: a retrospective

Sixty years ago, Reddy, Devanatan, and Bockris performed the first in situ electrochemical ellipsometry experiment, which ushered in a new era in the study of electrochemistry, using optical spectroscopy. After six decades of development, electrochemical optical spectroscopy, particularly electrochemical vibrational spectroscopy, has advanced from a phase of immaturity with few methods and limited applications to a phase of maturity with excellent substrate generality and significantly improved resolutions. Here, we divide the development of electrochemical optical spectroscopy into four phases, focusing on the proof-of-concept of different electrochemical optical spectroscopy studies, the emergence of plasmonic enhancement-based electrochemical optical spectroscopic (in particular vibrational spectroscopic) methods, the realization of electrochemical vibrational spectroscopy on well-defined surfaces, and the efforts to achieve operando spectroelectrochemical applications. Finally, we discuss the future development trend of electrochemical optical spectroscopy, as well as examples of new methodology and research paradigms for operando spectroelectrochemistry.

Quantifying the ultrafast and steady-state molecular reduction potential of a plasmonic photocatalyst

Plasmonic materials are promising photocatalysts as they are well suited to convert light into hot carriers and heat. Hot electron transfer is suggested as the driving force in many plasmon-driven reactions. However, to date, there are no direct molecular measures of the rate and yield of plasmon-to-molecule electron transfer or energy of these electrons on the timescale of plasmon decay. Here, we use ultrafast and spectroelectrochemical surface-enhanced Raman spectroscopy to quantify electron transfer from a plasmonic substrate to adsorbed methyl viologen molecules. We observe a reduction yield of 2.4 to 3.5% on the picosecond timescale, with plasmon-induced potentials ranging from [Formula: see text]3.1 to [Formula: see text]4.5 mV. Excitingly, some of these reduced species are stabilized and persist for tens of minutes. This work provides concrete metrics toward optimizing material-molecule interactions for efficient plasmon-driven photocatalysis.

Factor analysis of the time series of SERS spectra reveals water arrangement and surface plasmon changes in Ag nanoparticle systems

The enhancement of Raman signals of molecules localized in the vicinity of plasmonic nanoparticles, known as surface-enhanced Raman scattering (SERS) effect, is strongly influenced by the selected excitation wavelength. The optimal excitation wavelength in SERS measurements is given by the position of the surface plasmon extinction (SPE) band of the studied system. Even a small change of the SPE band intensity, position and/or shape during the measurement may influence the SERS signal significantly. In this work, we prepared several systems of Ag nanoparticles, which were used for the demonstration how the information about SPE changes can be obtained by multivariate statistical analysis (factor analysis; FA) from SERS spectral sets, and employed in more precise and more comprehensive interpretation of the results. In non-aggregated Ag colloidal systems measured at the excitation wavelength of 445 nm, SPE band changes could be monitored by the analysis of water stretching vibration together with the vibrations in the fingerprint region. The FA of the water stretching band region was shown to provide unique information on both arrangement and disarrangement of water molecules in the vicinity of Ag NPs during the time evolution of these SERS active systems. In addition, the FA of the fingerprint region helped to monitor a rapid metalation of meso-tetrakis(N-methyl-4-pyridyl)porphine in etched SERS systems with Ag⁺ ions released from the NPs surface. In aggregated Ag colloidal systems measured at the excitation wavelength of 785 nm, the FA of SERS spectral sets enabled us to reveal the contribution of the 2nd electromagnetic enhancement to the overall SERS signal. The reliability of our conclusions was verified by comparing the results obtained from FA of SERS spectral sets with the data obtained from the parallel SPE measurements of the studied systems.

Photoexcited Plasmon-Driven Ultrafast Dynamics of the Adsorbate Probed by Femtosecond Time-Resolved Surface-Enhanced Time-Domain Raman Spectroscopy

Metal nanoparticles have high potential in light-harvesting applications by transferring absorbed photon energy to the adsorbates. However, photoexcited plasmon-driven ultrafast dynamics of the adsorbate on metal nanoparticles have not been clearly understood. We studied ultrafast plasmon-driven processes of trans-1,2-bis(4-pyridyl)ethylene (BPE) adsorbed on gold nanoparticle assemblies (GNAs) using time-resolved surface-enhanced impulsive stimulated Raman spectroscopy (TR-SE-ISRS). After photoexciting the localized surface plasmon resonance (LSPR) band of the GNAs, we measured femtosecond time-resolved surface-enhanced Raman spectra of the adsorbate, which exhibited transient bleach in the Raman signal and following biphasic recovery that proceeds on the time scale of a few tens of picoseconds. The TR-SE-ISRS data were analyzed with singular value decomposition, and the obtained species-associated Raman spectra indicated that photoexcitation of the LSPR band alters chemical interaction between BPE and the GNAs on an ultrafast time scale; initial steady-state BPE is recovered through a precursor state that has weaker interaction with the GNAs.

Dimensional Design for Surface-Enhanced Raman Spectroscopy

Surface-enhanced Raman spectroscopy (SERS) is a vibrational spectroscopy technique that enables specific identification of target analytes with sensitivity down to the single-molecule level by harnessing metal nanoparticles and nanostructures. Excitation of localized surface plasmon resonance of a nanostructured surface and the associated huge local electric field enhancement lie at the heart of SERS, and things will become better if strong chemical enhancement is also available simultaneously. Thus, the precise control of surface characteristics of enhancing substrates plays a key role in broadening the scope of SERS for scientific purposes and developing SERS into a routine analytical tool. In this review, the development of SERS substrates is outlined with some milestones in the nearly half-century history of SERS. In particular, these substrates are classified into zero-dimensional, one-dimensional, two-dimensional, and three-dimensional substrates according to their geometric dimension. We show that, in each category of SERS substrates, design upon the geometric and composite configuration can be made to achieve an optimized enhancement factor for the Raman signal. We also show that the temporal dimension can be incorporated into SERS by applying femtosecond pulse laser technology, so that the SERS technique can be used not only to identify the chemical structure of molecules but also to uncover the ultrafast dynamics of molecular structural changes. By adopting SERS substrates with the power of four-dimensional spatiotemporal control and design, the ultimate goal of probing the single-molecule chemical structural changes in the femtosecond time scale, watching the chemical reactions in four dimensions, and visualizing the elementary reaction steps in chemistry might be realized in the near future.

Enhancement of Molecular Coherent Anti-Stokes Raman Scattering with Silicon Nanoantennas

Surface-enhanced coherent anti-Stokes Raman scattering (SE-CARS) takes advantage of surface plasmon resonances supported on metallic nanostructures to amplify the coherent Raman response of target molecules. While these metallic antennas have found significant success in SE-CARS studies, photoinduced morphological changes to the nanoantenna under ultrafast excitation introduce significant hurdles in terms of stability and reproducilibty. These hurdles need to be overcome in order to establish SE-CARS as a reliable tool for rapid biomolecular sensing. Here, we address this challenge by performing molecular CARS measurements enhanced by nanoantennas made from high-index dielectric particles with more favorable thermal properties. We present the first experimental demonstration of enhanced molecular CARS signals observed at Si nanoantennas, which offer much improved thermal stability compared to their metallic counterparts.

Fabrication of Stabilized Gold Nanoparticle Oligomers for Surface-Enhanced Spectroscopies

Various time-resolved spectroscopies that take advantage of surface-enhancement have been developed. Only the most robust substrates can withstand the high-intensity laser pulses used by time-resolved methods. We present a simple and reliable stabilization procedure that uses polyvinyl alcohol for the formation of robust gold nanoparticle oligomers that can withstand different hydration and temperature levels. This procedure can be used to produce oligomers with varying and reproducible plasmon resonance conditions. Results show that gold nanoparticle oligomers stabilized in this way are sufficiently sturdy to be used in 3D printing, opening the door for easy production and integration of plasmonic substrates.

Vibrational line shape effects in plasmon-enhanced stimulated Raman spectroscopies

A density matrix treatment of plasmon-enhanced (PE) stimulated Raman spectroscopies is developed. Specifically, PE stimulated Raman Gain/Loss (PE-SRG/L) and coherent anti-Stokes Raman scattering (PE-CARS) due to monochromatic excitation and PE femtosecond stimulated Raman spectroscopy (PE-FSRS) are considered. A Lorentz oscillator model is used to explicitly describe the time dependence of plasmon-enhanced optical fields. These temporal characteristics are required for a density matrix based description of all plasmon-enhanced nonlinear molecular spectroscopies. Dispersive vibrational line shapes in PE-SRG/L and PE-FSRS spectra are shown to result primarily from terms proportional to the square of the complex optical field enhancement factor. The dependence on the plasmon resonance, picosecond and femtosecond pulse characteristics, and molecular vibrational properties are evident in the density matrix derived PE-FSRS intensity expression. The difference in signal detection mechanisms accounts for the lack of dispersive line shapes in PE spontaneous Raman spectroscopy. This density matrix treatment of PE-FSRS line shapes is compared with prior coupled wave results.

Advances of surface-enhanced Raman and IR spectroscopies: from nano/microstructures to macro-optical design

Raman and infrared (IR) spectroscopy are powerful analytical techniques, but have intrinsically low detection sensitivity. There have been three major steps (i) to advance the optical system of the light excitation, collection, and detection since 1920s, (ii) to utilize nanostructure-based surface-enhanced Raman scattering (SERS) and surface-enhanced infrared absorption (SEIRA) since 1990s, and (iii) to rationally couple (i) and (ii) for maximizing the total detection sensitivity since 2010s. After surveying the history of SERS and SEIRA, we outline the principle of plasmonics and the different mechanisms of SERS and SEIRA. We describe various interactions of light with nano/microstructures, localized surface plasmon, surface plasmon polariton, and lightning-rod effect. Their coupling effects can significantly increase the surface sensitivity by designing nanoparticle-nanoparticle and nanoparticle-substrate configuration. As the nano/microstructures have specific optical near-field and far-field behaviors, we focus on how to systematically design the macro-optical systems to maximize the excitation efficiency and detection sensitivity. We enumerate the key optical designs in particular ATR-based operation modes of directional excitation and emission from visible to IR spectral region. We also present some latest advancements on scanning-probe microscopy-based nanoscale spectroscopy. Finally, prospects and further developments of this field are given with emphasis on emerging techniques and methodologies.

Decoding Chemical and Physical Processes Driving Plasmonic Photocatalysis Using Surface-Enhanced Raman Spectroscopies

In order to mitigate the advancing effects of environmental pollution and climate change, immediate action is needed on social, political, and industrial fronts. One segment of industry that contributes significantly to this current crisis is bulk chemical production, where fossil fuels are primarily used to drive reactions at high temperatures and pressures. Toward mitigating the environmental impact of these processes, solar energy has shown promise as a clean and renewable alternative for the photocatalytic synthesis of chemicals. In recent decades, plasmonic materials have emerged as candidates for making this a reality. Because of their unique and tunable interactions with light, plasmonic materials can be used to create energy-rich nanoscale environments. In fact, there is a growing library of chemical reactions that can utilize this plasmonic energy to drive industrially relevant chemistries under standard ambient conditions. However, the efficiency of these reactions is typically low, and a lack of mechanistic understanding of how energy is transferred from plasmons to molecules hinders reaction optimization for use on large scales.To decode the complex chemical and physical processes involved in plasmon-driven photocatalytic reactions, we use surface-enhanced Raman spectroscopy (SERS). In this Account, we detail SERS techniques that we have used and are developing to study molecular transformations, charge transfer, and plasmonic heating in dynamic plasmon-molecule systems on time scales ranging from seconds to femtoseconds. SERS is an ideal analytical tool for understanding plasmon-molecule interactions, as it gives highly specific information about molecular vibrations with high sensitivity, down to the single-molecule level. Importantly, SERS allows for simultaneous pumping of a plasmonic resonance and probing of the enhanced Raman signal from nearby molecules. We have already used these techniques to study a plasmon-driven methyl migration with nanoscale spatial specificity and to understand the charge transfer mechanism and role of heating in the plasmon-mediated dimerization of 4-nitrobenzenethiol. Importantly, from this work we conclude that direct charge transfer, not heating, may play a significant role in driving many plasmon-driven reactions. Despite these recent insights, more work is needed in order to obtain a comprehensive understanding of the broad range of chemistries accessible in plasmon-molecule systems. In the future, our continued development of these SERS-based techniques shows promise in answering questions regarding direct charge transfer, resonance energy transfer, and excitation conditions in plasmon-mediated chemistries.

Gold-induced photothermal background in on-chip surface enhanced stimulated Raman spectroscopy

Surface enhanced Raman spectroscopy (SERS) and stimulated Raman spectroscopy (SRS) are well established techniques capable of boosting the strength of Raman scattering. The combination of both techniques (surface enhanced stimulated Raman spectroscopy, or SE-SRS) has been reported using plasmonic nanoparticles. In parallel, waveguide enhanced Raman spectroscopy has been developed using nanophotonic and nanoplasmonic waveguides. Here, we explore SE-SRS in nanoplasmonic waveguides. We demonstrate that a combined photothermal and thermo-optic effect in the gold material induces a strong background signal that limits the detection limit for the analyte. The experimental results are in line with theoretical estimates. We propose several methods to reduce or counteract this background.

Spontaneous versus Stimulated Surface-Enhanced Raman Scattering of Liquid Water

We have observed for the first time the surface-enhanced (SE) signal of water in an aqueous dispersion of silver nanoparticles in spontaneous (SERS) and femtosecond stimulated Raman (SE-FSRS) processes with different wavelengths of the Raman pump (515, 715, and 755 nm). By estimating the fraction of water molecules that interact with the metal surface, we have calculated enhancement factors (EF): 4.8 × 10⁶ for SERS and (3.6-3.7) × 10⁶ for SE-FSRS. Furthermore, we have tested the role of simultaneous plasmon resonance and Raman resonance conditions for the aν ₁ + bν₃ overtone mode of water (755 nm) in SE-FSRS signal amplification. When the wavelength of the Raman pump is within the plasmon resonance of the metal nanoparticles, the Raman resonance has a negligible effect on the EF. However, the Raman resonance with the aν ₁ + bν₃ mode strongly enhances the signal of the fundamental OH stretching mode of water.

Plasmon-enhanced coherent anti-stokes Raman scattering vs plasmon-enhanced stimulated Raman scattering: Comparison of line shape and enhancement factor

Plasmon-enhanced coherent Raman scattering microscopy has reached single-molecule detection sensitivity. Due to the different driven fields, there are significant differences between a coherent Raman scattering process and its plasmon-enhanced derivative. The commonly accepted line shapes for coherent anti-Stokes Raman scattering and stimulated Raman scattering do not hold for the plasmon-enhanced condition. Here, we present a theoretical model that describes the spectral line shapes in plasmon-enhanced coherent anti-Stokes Raman scattering (PECARS). Experimentally, we measured PECARS and plasmon-enhanced stimulated Raman scattering (PESRS) spectra of 4-mercaptopyridine adsorbed on the self-assembled Au nanoparticle (NP) substrate and aggregated Au NP colloids. The PECARS spectra show a nondispersive line shape, while the PESRS spectra exhibit a dispersive line shape. PECARS shows a higher signal to noise ratio and a larger enhancement factor than PESRS from the same specimen. It is verified that the nonresonant background in PECARS originates from the photoluminescence of nanostructures. The decoupling of background and the vibrational resonance component results in the nondispersive line shape in PECARS. More local electric field enhancements are involved in the PECARS process than in PESRS, which results in a higher enhancement factor in PECARS. The current work provides new insight into the mechanism of plasmon-enhanced coherent Raman scattering and helps to optimize the experimental design for ultrasensitive chemical imaging.

Quantitative Drug Dynamics Visualized by Alkyne-Tagged Plasmonic-Enhanced Raman Microscopy

Visualizing live-cell uptake of small-molecule drugs is paramount for drug development and pharmaceutical sciences. Bioorthogonal imaging with click chemistry has made significant contributions to the field, visualizing small molecules in cells. Furthermore, recent developments in Raman microscopy, including stimulated Raman scattering (SRS) microscopy, have realized direct visualization of alkyne-tagged small-molecule drugs in live cells. However, Raman and SRS microscopy still suffer from limited detection sensitivity with low concentration molecules for observing temporal dynamics of drug uptake. Here, we demonstrate the combination of alkyne-tag and surface-enhanced Raman scattering (SERS) microscopy for the real-time monitoring of drug uptake in live cells. Gold nanoparticles are introduced into lysosomes of live cells by endocytosis and work as SERS probes. Raman signals of alkynes can be boosted by enhanced electric fields generated by plasmon resonance of gold nanoparticles when alkyne-tagged small molecules are colocalized with the nanoparticles. With time-lapse 3D SERS imaging, this technique allows us to investigate drug uptake by live cells with different chemical and physical conditions. We also perform quantitative evaluation of the uptake speed at the single-cell level using digital SERS counting under different quantities of drug molecules and temperature conditions. Our results illustrate that alkyne-tag SERS microscopy has a potential to be an alternative bioorthogonal imaging technique to investigate temporal dynamics of small-molecule uptake of live cells for pharmaceutical research.

Tip-enhanced Raman spectroscopy for nanoscale probing of dynamic chemical systems

Dynamics are fundamental to all aspects of chemistry and play a central role in the mechanism and product distribution of a chemical reaction. All dynamic processes are influenced by the local environment, so it is of fundamental and practical value to understand the structure of the environment and the dynamics with nanoscale resolution. Most techniques for measuring dynamic processes have microscopic spatial resolution and can only measure the average behavior of a large ensemble of sites within their sampling volumes. Tip-enhanced Raman spectroscopy (TERS) is a powerful tool for overcoming this limitation due to its combination of high chemical specificity and spatial resolution that is on the nanometer scale. Adapting it for the study of dynamic systems remains a work in progress, but the increasing sophistication of TERS is making such studies more routine, and there are now growing efforts to use TERS to examine more complex processes. This Perspective aims to promote development in this area of research by highlighting recent progress in using TERS to understand reacting and dynamic systems, ranging from simple model reactions to complex processes with practical applications. We discuss the unique challenges and opportunities that TERS presents for future studies.

Mammalian cell and tissue imaging using Raman and coherent Raman microscopy

Direct imaging of metabolism in cells or multicellular organisms is important for understanding many biological processes. Raman scattering (RS) microscopy, particularly, coherent Raman scattering (CRS) such as coherent anti-Stokes Raman scattering (CARS) and stimulated Raman scattering (SRS), has emerged as a powerful platform for cellular imaging due to its high chemical selectivity, sensitivity, and imaging speed. RS microscopy has been extensively used for the identification of subcellular structures, metabolic observation, and phenotypic characterization. Conjugating RS modalities with other techniques such as fluorescence or infrared (IR) spectroscopy, flow cytometry, and RNA-sequencing can further extend the applications of RS imaging in microbiology, system biology, neurology, tumor biology and more. Here we overview RS modalities and techniques for mammalian cell and tissue imaging, with a focus on the advances and applications of CARS and SRS microscopy, for a better understanding of the metabolism and dynamics of lipids, protein, glucose, and nucleic acids in mammalian cells and tissues. This article is categorized under: Laboratory Methods and Technologies > Imaging Biological Mechanisms > Metabolism Analytical and Computational Methods > Analytical Methods.

Time-Domain Observation of Surface-Enhanced Coherent Raman Scattering with 105-106 Enhancement

Combining surface-enhanced Raman scattering (SERS) with the coherent nonlinear Raman technique is a promising route for achieving higher sensitivity and time-resolved SERS measurements, yet such attempts have just been started. Here, we report time-domain Raman measurements of trans-1,2-bis(4-pyridyl)ethylene (BPE) adsorbed on gold nanoparticle assemblies (GNAs), which were carried out with impulsive stimulated Raman spectroscopy using sub-8 fs pulses. We observe coherent nuclear wavepacket motion of BPE on GNAs with drastic enhancement through the surface plasmon resonance, which provides information on the Raman-active vibrations in the time domain. Through Fourier transform of the measured time-domain Raman data, we obtained SERS spectra of BPE on GNAs with enhancement factors as high as 10⁵-10⁶. The present study not only demonstrates applicability of time-domain nonlinear Raman techniques in SERS, i.e., surface-enhanced impulsive stimulated Raman spectroscopy (SE-ISRS), but also provides a technical basis for femtosecond time-resolved SE-ISRS experiments to track ultrafast dynamics of the adsorbates.

Absolute Cross Sections of Liquids from Broadband Stimulated Raman Scattering with Femtosecond and Picosecond Pulses

Broadband stimulated Raman scattering (SRS) is often observed in applications that use nonlinear spectroscopy to probe the composition or dynamics of complex systems. Whether the SRS response is measured intentionally or unintentionally, as a background signal, the relative scattering intensities provide a quantitative measure of the population profile of target molecules. Solvent scattering is a valuable internal reference for determining absolute concentrations in these applications, but accurate cross sections have been reported for only a limited number of transitions in select solvents and were measured using spontaneous Raman scattering with narrowband continuous wave or nanosecond light sources. This work reports the measurement and analysis of absolute Raman scattering cross sections spanning the frequency range of 500-4000 cm^-1 for cyclohexane, DMSO, acetonitrile, methanol, water, benzene, and toluene using broadband SRS with femtosecond and picosecond Raman pump pulses at 488 nm. Varying the duration of the Raman pump pulses from ∼80 fs to >1 ps confirms that the cross sections are independent of the spectral bandwidth across the range of ∼250 to <20 cm^-1. The cross sections and depolarization ratios measured using broadband SRS agree with the limited number of previously reported values, after accounting for overlapping transitions in the lower-resolution femtosecond and picosecond spectra. The SRS cross sections reported here can be used with confidence as internal reference standards for a wide range of applications, including nonlinear spectroscopy and coherent microscopy measurements using ultrafast lasers.

Unveiling the molecule-plasmon interactions in surface-enhanced infrared absorption spectroscopy

Nanostructure-based surface-enhanced infrared absorption (SEIRA) spectroscopy has attracted tremendous interest as an ultrasensitive detection tool that supplies chemical-fingerprint information. The interactions between molecular vibrations and plasmons lead to not only the enhancement of spectral intensity, but also the distortion of spectral Lorentzian lineshapes into asymmetric Fano-type or more complicated lineshapes in the SEIRA spectra; this effect hampers the correct readout of vibrational frequencies and intensities for an accurate interpretation of the measured spectra and quantitative analysis. In this work, we investigate the Fano interference between molecular vibrations and plasmons based on exact electrodynamic simulations and theoretical models. We report that, even if the molecular vibrational energy is equal to the plasmon resonant energy, the molecule–nanostructure distance-dependent dipole–dipole interactions, the plasmon-mediated coherent intermolecular interactions and the decay rates of plasmons have a significant impact on the SEIRA lineshapes. This study paves the way for controllable Fano interference at the nanoscale and more studies on plasmon-dressed molecular electronic or vibrational excited states.

Raman Scattering: From Structural Biology to Medical Applications

This is a review of relevant Raman spectroscopy (RS) techniques and their use in structural biology, biophysics, cells, and tissues imaging towards development of various medical diagnostic tools, drug design, and other medical applications. Classical and contemporary structural studies of different water-soluble and membrane proteins, DNA, RNA, and their interactions and behavior in different systems were analyzed in terms of applicability of RS techniques and their complementarity to other corresponding methods. We show that RS is a powerful method that links the fundamental structural biology and its medical applications in cancer, cardiovascular, neurodegenerative, atherosclerotic, and other diseases. In particular, the key roles of RS in modern technologies of structure-based drug design are the detection and imaging of membrane protein microcrystals with the help of coherent anti-Stokes Raman scattering (CARS), which would help to further the development of protein structural crystallography and would result in a number of novel high-resolution structures of membrane proteins—drug targets; and, structural studies of photoactive membrane proteins (rhodopsins, photoreceptors, etc.) for the development of new optogenetic tools. Physical background and biomedical applications of spontaneous, stimulated, resonant, and surface- and tip-enhanced RS are also discussed. All of these techniques have been extensively developed during recent several decades. A number of interesting applications of CARS, resonant, and surface-enhanced Raman spectroscopy methods are also discussed.

Plasmon-enhanced stimulated Raman scattering microscopy with single-molecule detection sensitivity

Stimulated Raman scattering (SRS) microscopy allows for high-speed label-free chemical imaging of biomedical systems. The imaging sensitivity of SRS microscopy is limited to ~10 mM for endogenous biomolecules. Electronic pre-resonant SRS allows detection of sub-micromolar chromophores. However, label-free SRS detection of single biomolecules having extremely small Raman cross-sections (~10^-30 cm² sr^-1) remains unreachable. Here, we demonstrate plasmon-enhanced stimulated Raman scattering (PESRS) microscopy with single-molecule detection sensitivity. Incorporating pico-Joule laser excitation, background subtraction, and a denoising algorithm, we obtain robust single-pixel SRS spectra exhibiting single-molecule events, verified by using two isotopologues of adenine and further confirmed by digital blinking and bleaching in the temporal domain. To demonstrate the capability of PESRS for biological applications, we utilize PESRS to map adenine released from bacteria due to starvation stress. PESRS microscopy holds the promise for ultrasensitive detection and rapid mapping of molecular events in chemical and biomedical systems.

Spectral Reshaping of Single Dye Molecules Coupled to Single Plasmonic Nanoparticles

Fluorescent molecules are highly susceptible to their local environment. Thus, a fluorescent molecule near a plasmonic nanoparticle can experience changes in local electric field and local density of states that reshape its intrinsic emission spectrum. By avoiding ensemble averaging while simultaneously measuring the super-resolved position of the fluorophore and its emission spectrum, single-molecule hyperspectral imaging is uniquely suited to differentiate changes in the spectrum from heterogeneous ensemble effects. Thus, we uncover for the first time single-molecule fluorescence emission spectrum reshaping upon near-field coupling to individual gold nanoparticles using hyperspectral super-resolution fluorescence imaging, and we resolve this spectral reshaping as a function of the nanoparticle/dye spectral overlap and separation distance. We find that dyes bluer than the plasmon resonance maximum are red-shifted and redder dyes are blue-shifted. The primary vibronic peak transition probabilities shift to favor secondary vibronic peaks, leading to effective emission maxima shifts in excess of 50 nm, and we understand these light-matter interactions by combining super-resolution hyperspectral imaging and full-field electromagnetic simulations.

Biological imaging of chemical bonds by stimulated Raman scattering microscopy

All molecules consist of chemical bonds, and much can be learned from mapping the spatiotemporal dynamics of these bonds. Since its invention a decade ago, stimulated Raman scattering (SRS) microscopy has become a powerful modality for imaging chemical bonds with high sensitivity, resolution, speed and specificity. We introduce the fundamentals of SRS microscopy and review innovations in SRS microscopes and imaging probes. We highlight examples of exciting biological applications, and share our vision for potential future breakthroughs for this technology.

Continuous-Wave Coherent Raman Spectroscopy via Plasmonic Enhancement

In this paper, we report a successful combination of stimulated Raman spectroscopy (SRS) and surface-enhanced Raman scattering (SERS) using cw laser sources and gold/silica nanoparticles with embedded reporter molecules. We describe the preparation method for our gold/silica nanoparticles as well as the effect of probe wavelength, pump and probe power, polarization and sample concentration on the cwSESRS signal. Altogether, a stable ~12 orders of magnitude enhancement in the stimulated Raman signal is achieved because of the amplification of both pump and probe beams, leading to the detection of pico-molar nanoparticle concentrations, comparable to those of SERS. The coherent Raman spectra matches the incoherent conventional Raman spectra of the reporter molecules. Unlike conventional incoherent SERS this approach generates a coherent stimulated signal of microwatt intensities, opening the field to applications requiring a coherent beam, such as Molecular Holography.

A Review on Surface-Enhanced Raman Scattering

Surface-enhanced Raman scattering (SERS) has become a powerful tool in chemical, material and life sciences, owing to its intrinsic features (i.e., fingerprint recognition capabilities and high sensitivity) and to the technological advancements that have lowered the cost of the instruments and improved their sensitivity and user-friendliness. We provide an overview of the most significant aspects of SERS. First, the phenomena at the basis of the SERS amplification are described. Then, the measurement of the enhancement and the key factors that determine it (the materials, the hot spots, and the analyte-surface distance) are discussed. A section is dedicated to the analysis of the relevant factors for the choice of the excitation wavelength in a SERS experiment. Several types of substrates and fabrication methods are illustrated, along with some examples of the coupling of SERS with separation and capturing techniques. Finally, a representative selection of applications in the biomedical field, with direct and indirect protocols, is provided. We intentionally avoided using a highly technical language and, whenever possible, intuitive explanations of the involved phenomena are provided, in order to make this review suitable to scientists with different degrees of specialization in this field.

First demonstration of surface enhanced-stimulated Raman spectroscopy (SE-SRS) using low-power CW sources

Using commercially available nanoparticles, continuous wave Surface-Enhanced Stimulated Raman spectroscopy (CW SE-SRS) is demonstrated for the first time using two Ti:Sapphire lasers producing a pump beam (785 nm, 100 mW) and appropriately varying probe/Stokes beams (860-870 nm, 120 mW). The Ti-Sapphire lasers are co-pumped by a 10 W low noise 532 nm Spectra Physics Millennia laser. Pulsed SE-SRS is also demonstrated using a Coherent Chameleon Ultra laser for the Stokes/probe (863-871 nm) beam and a Coherent Ultra II as the pump laser (785 nm). In both cases lock-in techniques are used to extract the small signal (1 in 10⁹) successfully. These experiments convincingly demonstrate that SRS with CW sources is possible using appropriate nanoparticles, and this realization creates opportunities for a wider range of stimulated Raman spectroscopy applications.

Spiers Memorial Lecture. Surface-enhanced Raman spectroscopy: from single particle/molecule spectroscopy to ångstrom-scale spatial resolution and femtosecond time resolution

Four decades on, surface-enhanced Raman spectroscopy (SERS) continues to be a vibrant field of research that is growing (approximately) exponentially in scope and applicability while pushing at the ultimate limits of sensitivity, spatial resolution, and time resolution. This introductory paper discusses some aspects related to all four of the themes for this Faraday Discussion. First, the wavelength-scanned SERS excitation spectroscopy (WS-SERES) of single nanosphere oligomers (viz., dimers, trimers, etc.), the distance dependence of SERS, the magnitude of the chemical enhancement mechanism, and the progress toward developing surface-enhanced femtosecond stimulated Raman spectroscopy (SE-FSRS) are discussed. Second, our efforts to develop a continuous, minimally invasive, in vivo glucose sensor based on SERS are highlighted. Third, some aspects of our recent work in single molecule SERS and the translation of that effort to ångstrom-scale spatial resolution in ultrahigh vacuum tip-enhanced Raman spectroscopy (UHV-TERS) and single molecule electrochemistry using electrochemical (EC)-TERS will be presented. Finally, we provide an overview of analytical SERS with our viewpoints on SERS substrates, approaches to address the analyte generality problem (i.e. target molecules that do not spontaneously adsorb and/or have Raman cross sections <10^-29 cm² sr^-1), SERS for catalysis, and deep UV-SERS.

Label-Free Optical Single-Molecule Micro- and Nanosensors

Label-free optical sensor systems have emerged that exhibit extraordinary sensitivity for detecting physical, chemical, and biological entities at the micro/nanoscale. Particularly exciting is the detection and analysis of molecules, on miniature optical devices that have many possible applications in health, environment, and security. These micro- and nanosensors have now reached a sensitivity level that allows for the detection and analysis of even single molecules. Their small size enables an exceedingly high sensitivity, and the application of quantum optical measurement techniques can allow the classical limits of detection to be approached or surpassed. The new class of label-free micro- and nanosensors allows dynamic processes at the single-molecule level to be observed directly with light. By virtue of their small interaction length, these micro- and nanosensors probe light-matter interactions over a dynamic range often inaccessible by other optical techniques. For researchers entering this rapidly advancing field of single-molecule micro- and nanosensors, there is an urgent need for a timely review that covers the most recent developments and that identifies the most exciting opportunities. The focus here is to provide a summary of the recent techniques that have either demonstrated label-free single-molecule detection or claim single-molecule sensitivity.

Junction Plasmon Driven Population Inversion of Molecular Vibrations: A Picosecond Surface-Enhanced Raman Spectroscopy Study

Molecular surface-enhanced Raman spectra recorded at single plasmonic nanojunctions using a 7 ps pulse train exhibit vibrational up-pumping and population inversion. The process is assigned to plasmon-driven, dark, impulsive electron-vibration (e-v) excitation. Both optical (Raman) pumping and hot-electron mediated excitation can be rejected by the characteristic spectra, which allow the simultaneous measurement of vibrational temperature of the molecules and electronic temperature of the metal. Vibrational populations are determined from anti-Stokes to Stokes intensity ratios, while the electron temperature is obtained from the anti-Stokes branch of the electronic Raman scattering continuum. Population inversion survives in high-frequency vibrations that effectively decouple from the metal.

High repetition-rate femtosecond stimulated Raman spectroscopy with fast acquisition

Time-resolved femtosecond stimulated Raman spectroscopy (FSRS) is a powerful tool for investigating ultrafast structural and vibrational dynamics in light absorbing systems. However, the technique generally requires exposing a sample to high laser pulse fluences and long acquisition times to achieve adequate signal-to-noise ratios. Here, we describe a time-resolved FSRS instrument built around a Yb ultrafast amplifier operating at 200 kHz, and address some of the unique challenges that arise at high repetition-rates. The setup includes detection with a 9 kHz CMOS camera and an improved dual-chopping scheme to reject scattering artifacts that occur in the 3-pulse configuration. The instrument demonstrates good signal-to-noise performance while simultaneously achieving a 3-6 fold reduction in pulse energy and a 5-10 fold reduction in acquisition time relative to comparable 1 kHz instruments.

Transient SHG Imaging on Ultrafast Carrier Dynamics of MoS2 Nanosheets

Understanding the collaborative behaviors of the excitons and phonons that result from light-matter interactions is important for interpreting and optimizing the underlying fundamental physics at work in devices made from atomically thin materials. In this study, the generation of exciton-coupled phonon vibration from molybdenum disulfide (MoS₂ ) nanosheets in a pre-excitonic resonance condition is reported. A strong rise-to-decay profile for the transient second-harmonic generation (TSHG) of the probe pulse is achieved by applying substantial (20%) beam polarization normal to the nanosheet plane, and tuning the wavelength of the pump beam to the absorption of the A-exciton. The time-dependent TSHG signals clearly exhibit acoustic phonon generation at vibration modes below 10 cm^-1 (close to the Γ point) after the photoinduced energy is transferred from exciton to phonon in a nonradiative fashion. Interestingly, by observing the TSHG signal oscillation period from MoS₂ samples of varying thicknesses, the speed of the supersonic waves generated in the out-of-plane direction (Mach 8.6) is generated. Additionally, TSHG microscopy reveals critical information about the phase and amplitude of the acoustic phonons from different edge chiralities (armchair and zigzag) of the MoS₂ monolayers. This suggests that the technique could be used more broadly to study ultrafast physics and chemistry in low-dimensional materials and their hybrids with ultrahigh fidelity.

Novel Biosensors for the Rapid Detection of Toxicants in Foods

The modern environmental and food analysis requires sensitive, accurate, and rapid methods. The growing field of biosensors represents an answer to this demand. Unfortunately, most biosensor systems have been tested only on distilled water or buffered solutions, although applications to real samples are increasingly appearing in recent years. In this context, biosensors for potential food applications continue to show advances in areas such as genetic modification of enzymes and microorganisms, improvement of recognition element immobilization, and sensor interfaces. This chapter investigates the progress in the development of biosensors for the rapid detection of food toxicants for online applications. Recent progress in nanotechnology has produced affordable, mass-produced devices, and to integrate these into components and systems (including portable ones) for mass market applications for food toxicants monitoring. Sensing includes chemical and microbiological food toxicants, such as toxins, insecticides, pesticides, herbicides, microorganisms, bacteria, viruses and other microorganisms, phenolic compounds, allergens, genetically modified foods, hormones, dioxins, etc. Therefore, the state of the art of recent advances and future targets in the development of biosensors for food monitoring is summarized as follows: biosensors for food analysis will be highly sensitive, selective, rapidly responding, real time, massively parallel, with no or minimum sample preparation, and platform suited to portable and handheld nanosensors for the rapid detection of food toxicants for online uses even by nonskilled personnel.

Surface-enhanced Raman spectroscopy: bottlenecks and future directions

This feature article discusses developmental bottleneck issues in surface Raman spectroscopy in its early stages and surface-enhanced Raman spectroscopy (SERS) in the past four decades and future perspectives.

A paper-based SERS test strip for quantitative detection of Mucin-1 in whole blood

A paper-based SERS test strip combining strengths of paper chip and Raman active substrate was demonstrated to overcome challenges in spectroscopic sensing of complicated samples and realize quantitative detection of disease markers in whole blood. The precisely controlled Au NPs were not only capable of generating condensed hot spots on the fibers, but also enhanced the size exclusion effect of paper, resulting in the novel performance on both SERS detection and sample pretreatment. A biosensor for Mucin-1 is developed by equipping the Au NPs with aptamer. Combining all these merits, this small, cheap and portable test strip might find wide application in clinical diagnosis and health evaluation.

Studying Stimulated Raman Activity in Surface-Enhanced Femtosecond Stimulated Raman Spectroscopy by Varying the Excitation Wavelength

We present the first multiwavelength surface-enhanced femtosecond stimulated Raman spectroscopy (SE-FSRS) study, as well as the first observation of anti-Stokes vibrational features in SE-FSRS spectra. We compare stimulated Raman loss (SRL) and stimulated Raman gain (SRG) signals at three pump wavelengths chosen to sample different portions of nanoparticle aggregate localized surface plasmon resonances. The SE-FSRS signals exhibit similar signal magnitudes in the SRL or SRG regions of the spectra regardless of Raman pump or probe wavelength. The spectral lineshapes, however, differ dramatically with excitation wavelengths. The observed trends in spectral line shape show a strong dependence on the relative position of the excitation fields with respect to the plasmon resonance but do not match predictions from any existing SE-FSRS theory. These results suggest the need for further theoretical efforts with complementary experimental studies of individual aggregates to remove the effects of inherent ensemble averaging.

Two-Dimensional Fano Lineshapes in Ultrafast Vibrational Spectroscopy of Thin Molecular Layers on Plasmonic Arrays

Two-dimensional femtosecond infrared (2DIR) spectroscopy routinely provides insights into molecular structure and ultrafast dynamics in 1-100 μm thick bulk samples. Confinement of molecules to surfaces, gaps, crevices, and other topographic features, frequently encountered on the nanometer length scale, significantly alters their structure and dynamics, affecting physical and chemical properties. Amplification of 2DIR signals by the plasmon-enhanced fields around metal nanostructures can permit structural and dynamics measurements of the confined molecules. Fano resonances, induced by the interaction between laser pulses, plasmon, and vibrational modes significantly distort 2D lineshapes. For different detuning from plasmon resonance, the interference between multiple signal components leads to different line shape asymmetry, which we demonstrate on a set of linear absorption, transient absorption, and 2DIR spectra. An intuitive model used to describe experimental data points to the interference's origin. Our results will facilitate the application of surface-enhanced 2DIR spectroscopy for studies of molecular structure and dynamics in a nanoconfined environment.

Surface enhanced hyper Raman scattering (SEHRS) and its applications

Surface enhanced hyper Raman scattering (SEHRS) is the spontaneous, two-photon excited Raman scattering that occurs for molecules residing in high local optical fields of plasmonic nanostructures. Being regarded as a non-linear analogue of surface enhanced Raman scattering (SERS), SEHRS shares most of its properties, but also has additional characteristics. They include complementary spectroscopic information resulting from different selection rules and a stronger enhancement due to the non-linearity in excitation. In practical spectroscopy, this can translate to advantages, which include a high selectivity when probing molecule-surface interactions, the possibility of probing molecules at low concentrations due to the strong enhancement, and the advantages that come with excitation in the near-infrared. In this review, we give examples of the wealth of vibrational spectroscopic information that can be obtained by SEHRS and discuss work that has contributed to understanding the effect and that therefore provides directions for SEHRS spectroscopy. Future applications could range from biophotonics to materials research.

Coupled wave equations theory of surface-enhanced femtosecond stimulated Raman scattering

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|(2)ImχR(ω)gst (2)/ImχR(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.

Nano-Raman Scattering Microscopy: Resolution and Enhancement

Raman scattering microscopy is becoming one of the hot topics in analytical microscopy as a tool for analyzing advanced nanomaterials, such as biomolecules in a live cell for the study of cellular dynamics, semiconductor devices for characterizing strain distribution and contamination, and nanocarbons and nano-2D materials. In this paper, we review the recent progress in the development of Raman scattering microscopy from the viewpoint of spatial resolution and scattering efficiency. To overcome the extremely small cross section of Raman scattering, we discuss three approaches for the enhancement of scattering efficiency and show that the scattering enhancement synergistically increases the spatial resolution. We discuss the mechanisms of tip-enhanced Raman scattering, deep-UV resonant Raman scattering, and coherent nonlinear Raman scattering for micro- and nanoscope applications. The combinations of these three approaches are also shown as nanometer-resolution Raman scattering microscopy. The critical issues of the structures, materials, and reproducibility of tips and three-dimensionality for TERS; photodegradation for resonant Raman scattering; and laser availability for coherent nonlinear Raman scattering are also discussed.

Electromagnetic theories of surface-enhanced Raman spectroscopy

A fundamental theoretical understanding of SERS, and SERS hotspots, leads to new design principles for SERS substrates and new applications in nanomaterials and chemical analysis.

Stimulated Raman Scattering: From Bulk to Nano

Stimulated Raman scattering (SRS) describes a family of techniques first discovered and developed in the 1960s. Whereas the nascent history of the technique is parallel to that of laser light sources, recent advances have spurred a resurgence in its use and development that has spanned across scientific fields and spatial scales. SRS is a nonlinear technique that probes the same vibrational modes of molecules that are seen in spontaneous Raman scattering. While spontaneous Raman scattering is an incoherent technique, SRS is a coherent process, and this fact provides several advantages over conventional Raman techniques, among which are much stronger signals and the ability to time-resolve the vibrational motions. Technological improvements in pulse generation and detection strategies have allowed SRS to probe increasingly smaller volumes and shorter time scales. This has enabled SRS research to move from its original domain, of probing bulk media, to imaging biological tissues and single cells at the micro scale, and, ultimately, to characterizing samples with subdiffraction resolution at the nanoscale. In this Review, we give an overview of the history of the technique, outline its basic properties, and present historical and current uses at multiple length scales to underline the utility of SRS to the molecular sciences.