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

Enhancement Factors: A Central Concept during 50 Years of Surface-Enhanced Raman Spectroscopy

In this Perspective, we provide an overview of the core concepts around surface-enhanced Raman spectroscopy (SERS) enhancement factors (EFs), including both theoretical and experimental considerations: EF definitions, the distinction between maximum and average EFs, EF distribution and hot-spot localization, EF measurement and its order of magnitude. We then highlight some of the current challenges in this field, focusing on a selection of topics that we feel are both topical and important: analyte-capture onto a SERS substrate, surface-enhanced resonant Raman scattering, orientation/tensorial effects, and nonradiative effects. We hope this Perspective can provide a platform to reflect on the past 50 years of SERS and its future.

'When is a hotspot a good nanospot' - review of analytical and hotspot-dominated surface enhanced Raman spectroscopy nanoplatforms

Substrate development in surface-enhanced Raman spectroscopy (SERS) continues to attract research interest. In order to determine performance metrics, researchers in foundational SERS studies use a variety of experimental means to characterize the nature of substrates. However, often this process would appear to be performed indiscriminately without consideration for the physical scale of the enhancement phenomena. Herein, we differentiate between SERS substrates whose primary enhancing structures are on the hundreds of nanometer scale (analytical SERS nanosubstrates) and those whose main mechanism derives from nanometric-sized gaps (hot-spot dominated SERS substrates), assessing the utility of various characterization methods for each substrate class. In this context, characterization approaches in white-light spectroscopy, electron beam methods, and scanning probe spectroscopies are reviewed. Tip-enhanced Raman spectroscopy, wavelength-scanned SERS studies, and the impact of surface hydrophobicity are also discussed. Conclusions are thus drawn on the applicability of each characterization technique regarding amenability for SERS experiments that have features at different length scales. For instance, while white light spectroscopy can provide an indication of the plasmon resonances associated with 10 s-100 s nm-scale structures, it may not reveal information about finer surface texturing on the true nm-scale, critical for SERS' sensitivity, and in need of investigation via scanning probe techniques.

Imaging with Raman photons: a novel use of mixed-mode spectroscopy

Surface enhanced Raman spectroscopy is today an established technique used for chemical fingerprinting. Here, we showcase an engineered hierarchical substrate, in which the plasmonically active regions, restricted to a micron scale, two dimensional hexagonal pattern are examined. Spatial variation of the enhanced Raman signal from any analyte, uniformly coating the substrate, consequently bears a high registry with the underlying pattern. This spatially contrasted enhancement allows optical imaging of the 2D pattern solely using the Raman scattered photons from the analyte. While the pattern brightness and contrast determine analyte identification and detection sensitivity, hyperspectral imaging can be exploited for increasing specificity. Proof of concept demonstration of the technique is carried out via the acquisition of Raman images with rhodamine and fluorescein dyes and then applied to detect glucose in 40 mM concentration. The large area optical imaging and the requirement of long-range uniformity in the detected patterns for positive analyte detection, is implemented using a machine learning based pattern recognition protocol which also increases the statistical confidence of detection. This simultaneous, large area signal detection sacrifices continuous spectral information at the cost of speed, reproducibility and minimising human error via automation of detection in the hyperspectral imaging technique presented here.

The effect of electron dose on positive polymethyl methacrylate resist for nanolithography of gold bowtie nanoantennas

Plasmonic structures, such as bowtie nanoantennas, may be used in Surface Enhanced Raman Spectroscopy (SERS). Nanoantennas can be employed to amplify the biomolecular and chemical reactions, which is useful for biomedical applications. The electric field created by nanoantennas are optimized when the resonant wavelength of the probed laser light closely matches the resonant wavelength of the plasmonic structure. In this work, we fabricated several bowtie nanoantennas with varying geometric spacing for use with a 532 nm wavelength laser line in Raman Spectroscopy. The fabrication utilized nanolithography by electron beam lithography on a Raith Voyager, development, deposition, and metal lift-off. This study explored a specific bowtie nanoantenna geometry of 270 nm equilateral sides triangle pairs with 3 varying gap sizes, 50 nm, 20 nm, and 10 nm, and the effect of varying electron beam doses on the final structure of the nanoantenna. The results presented here, will show that the working dose factor range is 6.5-10.3 (650-10,300 μC/cm2) for 120 nm thick polymethyl methacrylate (PMMA), and with a 44.78% increase in dose, the footprint area increases between 5.9% and 10.7%.

Surface-Enhanced Raman Spectroscopy for Rapid Screening of Cinnamon Essential Oils

Cinnamon essential oil is used in food flavoring, food preservation, and for complementary medicine. The most common types of cinnamon used in essential oils are true cinnamon (Cinnamomum verum) and cassia cinnamon (Cinnamomum cassia). True cinnamon is commonly adulterated with cassia cinnamon because it is cheaper. However, cassia cinnamon contains higher concentrations of coumarin which has been shown to have adverse health effects. There is a need to develop simple, nondestructive, rapid screening methods for quality control and food authentication and to identify adulteration of cinnamon essential oil. Currently, the most common methods to screen for coumarin in cinnamon include high-performance liquid chromatography (HPLC) and gas chromatography (GC). However, these methods require time-consuming sample preparation and detection. Vibrational spectroscopy methods are emerging as a promising alternative for rapid, nondestructive screening for food safety applications. In this study, a rapid screening method has been developed to examine cinnamon essential oils using surface-enhanced Raman spectroscopy (SERS). The experimental spectra were compared to theoretical calculations using the DFT method BP86/6-311++G(d,p) basis set. The limit of detection of coumarin was determined to be 1 × 10^-6 M or 1.46 mg/L using SERS with colloid paste substrates. Furthermore, 1:16 dilutions of cinnamaldehyde and 1:8 dilutions of eugenol were detected using SERS which can help determine if the cinnamon essential oil was made from bark or from leaves. Seven commercially available cinnamon essential oils were also analyzed and compared to reference solutions. SERS was able to discriminate between essential oils primarily composed of cinnamaldehyde and those composed of eugenol. Furthermore, the SERS method detected peaks that are attributed to coumarin in two of the commercially available samples. To date, this is the first time SERS has been used to rapidly screen cinnamon essential oils.

Surface-enhanced Raman spectroscopy: benefits, trade-offs and future developments

Surface-enhanced Raman spectroscopy (SERS) is a vibrational spectroscopy technique with sensitivity down to the single molecule level that provides fine molecular fingerprints, allowing for direct identification of target analytes. Extensive theoretical and experimental research, together with continuous development of nanotechnology, has significantly broadened the scope of SERS and made it a hot research field in chemistry, physics, materials, biomedicine, and so on. However, SERS has not been developed into a routine analytical technique, and continuous efforts have been made to address the problems preventing its real-world application. The present minireview focuses on analyzing current and potential strategies to tackle problems and realize the SERS performance necessary for translation to practical applications.

Interaction of ZnO nanorods with plasmonic metal nanoparticles and semiconductor quantum dots

We model the enhancement of near band edge emission from ZnO nanorods using plasmonic metal nanoparticles and compare it with emission enhancement from ZnO with semiconducting quantum dots. Selected CdSe quantum dots with absorption energies close to those of Ag and Au nanoparticles are chosen to construct model systems with ZnO to comprehend the role of ZnO's intrinsic defects and plasmonic excitation in realizing the spectrally selective luminescence enhancement. Excitation wavelength dependent photoluminescence spectra along with theoretical models quantifying the related transitions and plasmonic absorption reveal that a complex mechanism of charge transfer between the ZnO nanorods and metal nanoparticles or quantum dots is essential along with an optimal energy band alignment for realizing emission enhancement. The theoretical model presented also provides a direct method of quantifying the relative transition rate constants associated with various electronic transitions in ZnO and their change upon the incorporation of plasmonic nanoparticles. The results indicate that, while the presence of deep level defect states may facilitate the essential charge transfer process between ZnO and the plasmonic nanoparticles, their presence alone does not guarantee UV emission enhancement and strong plasmonic coupling between the two systems. The results offer clues to designing novel multicomponent systems with coupled plasmonic and charge transfer effects for applications in charge localization, energy harvesting, and luminescence enhancement, especially in electrically triggered nanophotonic applications.

A review on recent advances in the applications of surface-enhanced Raman scattering in analytical chemistry

This review is focused on recent developments of surface-enhanced Raman scattering (SERS) applications in Analytical Chemistry. The work covers advances in the fabrication methods of SERS substrates, including nanoparticles immobilization techniques and advanced nanopatterning with metallic features. Recent insights in quantitative and sampling methods for SERS implementation and the development of new SERS-based approaches for both qualitative and quantitative analysis are discussed. The advent of methods for pre-concentration and new approaches for single-molecule SERS quantification, such as the digital SERS procedure, has provided additional improvements in the analytical figures-of-merit for analysis and assays based on SERS. The use of metal nanostructures as SERS detection elements integrated in devices, such as microfluidic systems and optical fibers, provided new tools for SERS applications that expand beyond the laboratory environment, bringing new opportunities for real-time field tests and process monitoring based on SERS. Finally, selected examples of SERS applications in analytical and bioanalytical chemistry are discussed. The breadth of this work reflects the vast diversity of subjects and approaches that are inherent to the SERS field. The state of the field indicates the potential for a variety of new SERS-based methods and technologies that can be routinely applied in analytical laboratories.

Spiers Memorial Lecture : Introductory lecture: Hot-electron science and microscopic processes in plasmonics and catalysis

In these introductory remarks we discuss the generation of nonequilibrium electrons in metals, their properties, and how they can be utilized in two emerging applications: for extending the capabilities of photodetection (left), and for photocatalysis (right), lowering the barriers of chemical reactions.

Light Concentration by Metal-Dielectric Micro-Resonators for SERS Sensing

Metal-dielectric micro/nano-composites have surface plasmon resonances in visible and near-infrared domains. Excitation of coupled metal-dielectric resonances is also important. These different resonances can allow enhancement of the electromagnetic field at a subwavelength scale. Hybrid plasmonic structures act as optical antennae by concentrating large electromagnetic energy in micro- and nano-scales. Plasmonic structures are proposed for various applications such as optical filters, investigation of quantum electrodynamics effects, solar energy concentration, magnetic recording, nanolasing, medical imaging and biodetection, surface-enhanced Raman scattering (SERS), and optical super-resolution microscopy. We present the review of recent achievements in experimental and theoretical studies of metal-dielectric micro and nano antennae that are important for fundamental and applied research. The main impact is application of metal-dielectric optical antennae for the efficient SERS sensing.

Light Driven Design of Dynamical Thermosensitive Plasmonic Superstructures: A Bottom-Up Approach Using Silver Supercrystals

When narrowly distributed silver nanoparticles (NPs) are functionalized by dodecanethiol, they acquire the ability to self-organize in organic solvents into 3D supercrystals (SCs). The NP surface chemistry is shown to introduce a light-driven thermomigration effect, thermophoresis. Using a laser beam to heat the NPs and generate steep thermal gradients, the migration effect is triggered dynamically, leading to tailored structures with high density of plasmonic hot spots. This work describes how to manipulate the hot spots and monitor the effect by holography, thus providing a complete characterization of the migration process on a single object basis. Extensive single object tracking strategies are employed to measure the SCs trajectories, evaluate their size, drift velocity magnitude and direction, allowing the identification of the physical chemical origins of the migration. The phenomenon is shown to happen as a result of the combination of thermophoresis (at short length scales) and convection (long-range), and does not require a metallic substrate. This constitutes a fully optical method to dynamically generate plasmonic platforms in situ and on demand, without requiring substrate nanostructuration and with minimal interference on the chemistry of the system. The importance of the proof-of-concept herein described stems from the numerous potential applications, spanning over a variety of fields such as microfluidics and biosensing.

Nanoscale Optical Microscopy and Spectroscopy Using Near-Field Probes

Light-matter interactions can provide a wealth of detailed information about the structural, electronic, optical, and chemical properties of materials through various excitation and scattering processes that occur over different length, energy, and timescales. Unfortunately, the wavelike nature of light limits the achievable spatial resolution for interrogation and imaging of materials to roughly λ/2 because of diffraction. Scanning near-field optical microscopy (SNOM) breaks this diffraction limit by coupling light to nanostructures that are specifically designed to manipulate, enhance, and/or extract optical signals from very small regions of space. Progress in the SNOM field over the past 30 years has led to the development of many methods to optically characterize materials at lateral spatial resolutions well below 100 nm. We review these exciting developments and demonstrate how SNOM is truly extending optical imaging and spectroscopy to the nanoscale.