Relative contributions of Franck-Condon to Herzberg-Teller terms in charge transfer surface-enhanced Raman scattering spectroscopy.

We have theoretically modeled charge transfer (CT) surface enhanced raman scattering (SERS) spectroscopy using pyridine bound to a planar Ag6 metal nanocluster. CT states were determined by natural transition orbital hole-particle plots and CT distance DCT and the amount of charge transferred qCT indices. We first consider a resonance Raman (RR) model based on the Albrecht approach and calculate the ratio of the Herzberg-Teller (HT) B or C term to the Franck-Condon (FC) A term for a totally symmetric a1 vibrational mode exciting in the lowest energy CT state. Using a dimensionless upper limit to the displacement factor ∆ = 0.05 in the FC term based on the examination of overtones in experimental spectra and a calculated HT coupling constant hCT = 0.439 eV/Å(amu)1/2 in the HT term, we calculated the scattering ratio of the HT to FC intensities as 147. This example indicated that for totally symmetric modes, the scattering intensity would all come from HT scattering. To further verify this result, we used the general time-dependent-RR formulation of Baiardi, Bloino, and Barone with the adiabatic Hessian model to calculate the FC, the Frank-Condon and Herzberg-Teller (FCHT), and the HT terms for pyridine in the C2v Ag6-pyridine complexes. For all cases we studied with pyridine in two orientations either parallel or perpendicular to the planar Ag6 cluster, the HT terms, FCHT + HT, dominate the FC term in the CT RR spectrum. These results indicate that for CT SERS, the intensity of all the totally and non-totally symmetric vibrational modes should come from the HT effect.

Detection and Quantitation of Trace Fentanyl in Heroin by Surface-Enhanced Raman Spectroscopy.

The identification of fentanyl, a main culprit in opioid overdose deaths, has become critical. Whereas Raman spectroscopy is an effective tool for detecting illicit drugs, the weak intensity of Raman scattering can make it difficult to distinguish trace materials. This shortcoming is addressed by surface-enhanced Raman spectroscopy (SERS), which produces strong signal enhancements when target compounds are near metal nanoparticles. This work examines the use of a paper-based substrate impregnated with silver nanoparticles for the detection of trace quantities of fentanyl alone and as an adulterant in heroin. In addition, intensity ratios of diagnostic peaks associated with each substance were fitted to a Langmuir isotherm calibration model and used for the quantitative analysis of fentanyl in heroin mixtures. Linearity was observed at <6% fentanyl, a significant finding that is consistent with concentrations found in drugs seized during law enforcement efforts. In addition, swabbing with these paper-based SERS substrates facilitated the recovery of fentanyl from surfaces, showing this to be applicable for crime scene investigations. However, assessment using the calibration model proved difficult for swabbed samples. Overall, this work demonstrates a potentially simple and sensitive technique for the forensic analysis and quantitation of fentanyl in trace amounts.

Enhanced Raman Scattering with Dielectrics.

Dielectrics represent a new frontier for surface-enhanced Raman scattering. They can serve as either a complement or an alternative to conventional, metal-based SERS, offering key advantages in terms of low invasiveness, reproducibility, versatility, and recyclability. In comparison to metals, dielectric systems and, in particular, semiconductors are characterized by a much greater variety of parameters and properties that can be tailored to achieve enhanced Raman scattering or related effects. Light-trapping and subwavelength-focusing capabilities, morphology-dependent resonances, control of band gap and stoichiometry, size-dependent plasmons and excitons, and charge transfer from semiconductors to molecules and vice versa are a few examples of the manifold opportunities associated with the use of semiconductors as SERS-active materials. This review provides a broad analysis of SERS with dielectrics, encompassing different optical phenomena at the basis of the Raman scattering enhancement and introducing future challenges for light harvesting, vibrational spectroscopy, imaging, and sensing.

The theory of surface-enhanced Raman scattering on semiconductor nanoparticles; toward the optimization of SERS sensors.

We present an expression for the lowest order nonzero contribution to the surface-enhanced Raman spectrum obtained from a system of a molecule adsorbed on a semiconductor nanoparticle. Herzberg-Teller vibronic coupling of the zero-order Born-Oppenheimer states results in an expression which may be regarded as an extension of the Albrecht A-, B-, and C-terms to SERS substrates. We show that the SERS enhancement is caused by combinations of several types of resonances in the combined system, namely, surface, exciton, charge-transfer, and molecular resonances. These resonances are coupled by terms in the numerator, which provide selection rules that enable various tests of the theory and predict the relative intensities of the Raman lines. Furthermore, by considering interactions of the various contributions to the SERS enhancement, we are able to develop ways to optimize the enhancement factor by tailoring the semiconductor nanostructure, thereby adjusting the locations of the various contributing resonances. This provides a procedure by which molecular sensors can be constructed and optimized. We provide several experimental examples on substrates such as monolayer MoS2 and GaN nanorods.

Enhancement of surface phonon modes in the Raman spectrum of ZnSe nanoparticles on adsorption of 4-mercaptopyridine.

By chemically etching a thin film of crystalline ZnSe with acid, we observe a strong Raman enhancement of the surface phonon modes of ZnSe on adsorption of a molecule (4-mercaptopyridine). The surface is composed of oblate hemi-ellipsoids, which has a large surface-to-bulk ratio. The assignment of the observed modes (at 248 and 492 cm(-1)) to a fundamental and first overtone of the surface optical mode is consistent with observations from high-resolution electron energy loss spectroscopy as well as calculations.

Detection of organic colorants in historical painting layers using UV laser ablation surface-enhanced Raman microspectroscopy.

Surface-enhanced Raman spectroscopy (SERS) has been increasingly used in the study of works of art to identify organic pigments and dyes in paintings, which (depending on the material) are difficult or not possible to detect by other current methods. The application of SERS to the study of paintings has been limited, however, by the lack of a sampling approach with sufficient sensitivity and spatial resolution. We show that ultraviolet laser ablation (LA) sampling coupled with SERS detection can be successfully used to study paint layers. LA-SERS permitted the isolation of signals from colorants in individual thin paint layers in sample cross-sections, avoiding contamination from adjacent layers. These results expand the range of analytical applications of SERS demonstrating how the technique can be used to sensitively detect minor organic components in complex matrices. While this is fundamental for the study of cultural heritage, it is also relevant in other fields such as forensic analysis, food science, and pharmacology.

Laser ablation surface-enhanced Raman microspectroscopy.

Improved identification of trace organic compounds in complex matrixes is critical for a variety of fields such as material science, heritage science, and forensics. Surface-enhanced Raman scattering (SERS) is a vibrational spectroscopy technique that can attain single-molecule sensitivity and has been shown to complement mass spectrometry, but lacks widespread application without a robust method that utilizes the effect. We demonstrate a new, highly sensitive, and widely applicable approach to SERS analysis based on laser ablation in the presence of a tailored plasmonic substrate. We analyze several challenging compounds, including non-water-soluble pigments and dyed leather from an ancient Egyptian chariot, achieving sensitivity as high as 120 amol for a 1:1 signal-to-noise ratio and 5 µm spatial resolution. This represents orders of magnitude improvement in spatial resolution and sensitivity compared to those of other SERS approaches intended for widespread application, greatly increasing the applicability of SERS.

Highly efficient construction of oriented sandwich structures for surface-enhanced Raman scattering.

The purpose of this study is to solve the problem of low achievement in fabricating sandwich surface-enhanced Raman scattering (SERS) substrates. We demonstrated a highly efficient sandwich structure by the oriented assembly of metal nanoparticles (NPs) on a periodic hexagonal array of metal nanoprisms with 1,4-benzenedithiol (1,4-BDT) as linkers. The metal nanoprism array was prepared by vacuum deposition of metal on a close-packed polystyrene nanosphere pre-patterned substrate. The metal nanoprism array presents different surface properties from the pits left from the removal of polystyrene nanospheres, which causes linkers to selectively adsorb on the metal nanoprism array and sequentially leads to the oriented immobilization of the second-layer metal NPs, avoiding mismatched orientation. These sandwich SERS substrates were characterized by extinction spectroscopy and atomic force microscopy and their enhancement activity was evaluated under different excitation wavelengths. The sandwich structure greatly increases the achievement of 'hot spots' to almost 100% of all the metal nanoprisms and enables a large amplification of SERS signals by a factor of ten. This method has the advantages of simplicity, high efficiency, high throughput, controllability and high reproducibility. It has significance in both the study of SERS substrates and the development of plasmonic devices.

Sample treatment considerations in the analysis of organic colorants by surface-enhanced Raman scattering.

The introduction of surface-enhanced Raman spectroscopy (SERS) in the field of cultural heritage has significantly improved the analysis of the organic dyes and their complexes that have been used as textile dyes and pigments in paintings and other polychrome works of art since antiquity. Over the last five years, a number of different procedures have been developed by various research groups. In this Article, we evaluate the effect of pretreating samples by exposing them to hydrofluoric acid (HF) vapor prior to SERS analysis, a step designed to hydrolyze the dye-metal complexes and increase analyte adsorption on the nanosized metallic support, thus enhancing the SERS signal. Materials studied include pure colorants, commercial lake pigments, and fibers from dyed textiles, as well as actual aged samples, such as microscopic fragments of lakes on paper and ancient pigments and glazes from several works of art, covering a wide range of time, from the second century B.C. to the early 20th century. In each case, SERS spectra obtained with or without HF hydrolysis were critically evaluated. The pretreatment with HF vapor resulted in faster analysis and increased sensitivity in most cases, with the exception of dyed silk fibers, where silk protein hydrolyzates were found to interfere with SERS analysis. As a final point, a two-step procedure including SERS on untreated and treated samples is proposed as a standard approach: by analyzing a sample first without hydrolysis, and then, following removal of the colloid, upon HF treatment, the best and most reliable results for a great number of dyes and substrates are assured.

The theory of surface-enhanced Raman scattering.

By considering the molecule and metal to form a conjoined system, we derive an expression for the observed Raman spectrum in surface-enhanced Raman scattering. The metal levels are considered to consist of a continuum with levels filled up to the Fermi level, and empty above, while the molecule has discrete levels filled up to the highest occupied orbital, and empty above that. It is presumed that the Fermi level of the metal lies between the highest filled and the lowest unfilled level of the molecule. The molecule levels are then coupled to the metal continuum both in the filled and unfilled levels, and using the solutions to this problem provided by Fano, we derive an expression for the transition amplitude between the ground stationary state and some excited stationary state of the molecule-metal system. It is shown that three resonances contribute to the overall enhancement; namely, the surface plasmon resonance, the molecular resonances, as well as charge-transfer resonances between the molecule and metal. Furthermore, these resonances are linked by terms in the numerator, which result in SERS selection rules. These linked resonances cannot be separated, accounting for many of the observed SERS phenomena. The molecule-metal coupling is interpreted in terms of a deformation potential which is compared to the Herzberg-Teller vibronic coupling constant. We show that one term in the sum involves coupling between the surface plasmon transition dipole and the molecular transition dipole. They are coupled through the deformation potential connecting to charge-transfer states. Another term is shown to involve coupling between the charge-transfer transition and the molecular transition dipoles. These are coupled by the deformation potential connecting to plasmon resonance states. By applying the selection rules to the cases of dimer and trimer nanoparticles we show that the SERS spectrum can vary considerably with excitation wavelength, depending on which plasmon and/or charge-transfer resonance is excited.

The Theory of Surface-Enhanced Raman Spectroscopy on Organic Semiconductors: Graphene.

Drawing on a theoretical expression previously derived for general semiconductor substrates, we examine the surface-enhancement of the Raman signal (SERS) when the substrate is chosen to be monolayer graphene. The underlying theory involves vibronic coupling, originally proposed by Herzberg and Teller. Vibronic coupling of the allowed molecular transitions with the charge-transfer transitions between the molecule and the substrate has been shown to be responsible for the SERS enhancement in semiconductor substrates. We then examine such an expression for the Raman enhancement in monolayer graphene, which is dependent on the square of the derivative of the density of states of the graphene. On integration, we find that the discontinuity of the density-of-states function leads to a singularity in the SERS intensity. Knowledge of the location of this resonance allows us to maximize the Raman intensity by careful alignment of the doping level of the graphene substrate with the charge-transfer transition.

Nondestructive identification of natural and synthetic organic colorants in works of art by surface enhanced Raman scattering.

We present a new method based on surface-enhanced Raman scattering (SERS) for the nondestructive identification of organic colorants in objects whose value or function precludes sampling, such as drawings, prints, historic and archeological textiles, handwritten or printed documents, and forensic evidence. A bead of a polymer hydrogel loaded with a solution containing water, an organic solvent, and a chelating agent is used to extract minimal amounts of the colorants from the work of art for SERS analysis. Using a gel as a medium for the solvent mixture confines its action only to the areas of the work of art covered by the gel bead. The gel bead is then removed from the work of art, covered with a drop of Ag colloid, and examined with a Raman microscope. Transfer of the dye from the substrate to the gel does not require removing a sample from the work of art, therefore preserving the physical integrity of the object. Spectrophotometric color measurements confirm that color change is below the limit perceivable by a human observer. Finally, the size of the polymer bead can be reduced to a fraction of a millimeter in order to further minimize any impact on the work of art, without detriment to the effectiveness of the method. The technique has been successfully used for the analysis of a mordant dye on the 15th century Netherlandish tapestry, "The Hunt for the Unicorn", and of a synthetic lake pigment on a Meiji period Japanese woodblock print.

Identification of organic colorants in fibers, paints, and glazes by surface enhanced Raman spectroscopy.

Organic dyes extracted from plants, insects, and shellfish have been used for millennia in dyeing textiles and manufacturing colorants for painting. The economic push for dyes with high tinting strength, directly related to high extinction coefficients in the visible range, historically led to the selection of substances that could be used at low concentrations. But a desirable property for the colorist is a major problem for the analytical chemist; the identification of dyes in cultural heritage objects is extremely difficult. Techniques routinely used in the identification of inorganic pigments are generally not applicable to dyes: X-ray fluorescence because of the lack of an elemental signature, Raman spectroscopy because of the generally intense luminescence of dyes, and Fourier transform infrared spectroscopy because of the interference of binders and extenders. Traditionally, the identification of dyes has required relatively large samples (0.5-5 mm in diameter) for analysis by high-performance liquid chromatography. In this Account, we describe our efforts to develop practical approaches in identifying dyes in works of art from samples as small as 25 microm in diameter with surface-enhanced Raman scattering (SERS). In SERS, the Raman scattering signal is greatly enhanced when organic molecules with large delocalized electron systems are adsorbed on atomically rough metallic substrates; fluorescence is concomitantly quenched. Recent nanotechnological advances in preparing and manipulating metallic particles have afforded staggering enhancement factors of up to 10(14). SERS is thus an ideal technique for the analysis of dyes. Indeed, rhodamine 6G and crystal violet, two organic compounds used to demonstrate the sensitivity of SERS at the single-molecule level, were first synthesized as textile dyes in the second half of the 19th century. In this Account, we examine the practical application of SERS to cultural heritage studies, including the selection of appropriate substrates, the development of analytical protocols, and the building of SERS spectral databases. We also consider theoretical studies on dyes of artistic interest. Using SERS, we have successfully documented the earliest use of a madder lake pigment and the earliest occurrence of lac dye in European art. We have also found several examples of kermes and cochineal glazes, as well as madder, cochineal, methyl violet, and eosin lakes, from eras ranging from ancient Egypt to the 19th century. The ability to rapidly analyze very small samples with SERS makes it a particularly valuable tool in a museum context.

Quantum confinement effects on charge-transfer between PbS quantum dots and 4-mercaptopyridine.

We obtain the surface enhanced Raman spectra of 4-mercaptopyridine on lead sulfide (PbS) quantum dots as a function of nanoparticle size and excitation wavelength. The nanoparticle radii are selected to be less than the exciton Bohr radius of PbS, enabling the observation of quantum confinement effects on the spectrum. We utilize the variation of nontotally symmetric modes of both b(1) and b(2) symmetry as compared to the totally symmetric a(1) modes to measure the degree of charge-transfer between the molecule and quantum dot. We find both size dependent and wavelength dependent resonances in the range of these measurements, and attribute them to charge-transfer resonances which are responsible for the Raman enhancement.

DFT and TD-DFT Investigation of a Charge Transfer Surface Resonance Raman Model of N3 Dye Bound to a Small TiO2 Nanoparticle.

Raman spectroscopy is an important method for studying the configuration of Ru bipyridyl dyes on TiO2. We studied the [Ru(II)(4,4'-COOH-2,2'-bpy)2(NCS)2)] dye (N3) adsorbed on a (TiO2)5 nanoparticle using Density Functional Theory, DFT, to optimize the geometry of the complex and to simulate normal Raman scattering, NRS, for the isolated N3 and the N3-(TiO2)5 complex. Two configurations of N3 are found on the surface both anchored with a carboxylate bridging bidentate linkage but one with the two NCS ligands directed away from the surface and one with one NSC tilted away and the other NCS interacting with the surface. Both configurations also had another -COOH group hydrogen bonded to a Ti-O dangling bond. These configurations can be distinguished from each other by Raman bands at 2104 and 2165 cm-1. The former configuration has more intense Normal Raman Scattering, NRS, on TiO2 surfaces and was studied with Time-Dependent Density Functional Theory, TD-DFT, frequency-dependent Raman simulations. Pre-resonance Raman spectra were simulated for a Metal to Ligand Charge Transfer, MLCT, excited state and for a long-distance CT transition from N3 directly to (TiO2)5. Enhancement factors for the MLCT and long-distance CT processes are around 1 × 103 and 2 × 102, respectively. A Herzberg-Teller intensity borrowing mechanism is implicated in the latter and provides a possible mechanism for the photo-injection of electrons to titania surfaces.

Charge-Transfer Resonance and Electromagnetic Enhancement Synergistically Enabling MXenes with Excellent SERS Sensitivity for SARS-CoV-2 S Protein Detection.

The outbreak of coronavirus disease 2019 has seriously threatened human health. Rapidly and sensitively detecting SARS-CoV-2 viruses can help control the spread of viruses. However, it is an arduous challenge to apply semiconductor-based substrates for virus SERS detection due to their poor sensitivity. Therefore, it is worthwhile to search novel semiconductor-based substrates with excellent SERS sensitivity. Herein we report, for the first time, Nb2C and Ta2C MXenes exhibit a remarkable SERS enhancement, which is synergistically enabled by the charge transfer resonance enhancement and electromagnetic enhancement. Their SERS sensitivity is optimized to 3.0 × 106 and 1.4 × 106 under the optimal resonance excitation wavelength of 532 nm. Additionally, remarkable SERS sensitivity endows Ta2C MXenes with capability to sensitively detect and accurately identify the SARS-CoV-2 spike protein. Moreover, its detection limit is as low as 5 × 10-9 M, which is beneficial to achieve real-time monitoring and early warning of novel coronavirus. This research not only provides helpful theoretical guidance for exploring other novel SERS-active semiconductor-based materials but also provides a potential candidate for the practical applications of SERS technology.

Human ACE2-Functionalized Gold "Virus-Trap" Nanostructures for Accurate Capture of SARS-CoV-2 and Single-Virus SERS Detection.

The current COVID-19 pandemic urges the extremely sensitive and prompt detection of SARS-CoV-2 virus. Here, we present a Human Angiotensin-converting-enzyme 2 (ACE2)-functionalized gold "virus traps" nanostructure as an extremely sensitive SERS biosensor, to selectively capture and rapidly detect S-protein expressed coronavirus, such as the current SARS-CoV-2 in the contaminated water, down to the single-virus level. Such a SERS sensor features extraordinary 106-fold virus enrichment originating from high-affinity of ACE2 with S protein as well as "virus-traps" composed of oblique gold nanoneedles, and 109-fold enhancement of Raman signals originating from multi-component SERS effects. Furthermore, the identification standard of virus signals is established by machine-learning and identification techniques, resulting in an especially low detection limit of 80 copies mL-1 for the simulated contaminated water by SARS-CoV-2 virus with complex circumstance as short as 5 min, which is of great significance for achieving real-time monitoring and early warning of coronavirus. Moreover, here-developed method can be used to establish the identification standard for future unknown coronavirus, and immediately enable extremely sensitive and rapid detection of novel virus. Supplementary Information: The online version contains supplementary material available at 10.1007/s40820-021-00620-8.

A unified view of surface-enhanced Raman scattering.

In the late 1970s, signal intensity in Raman spectroscopy was found to be enormously enhanced, by a factor of 10(6) and more recently by as much as 10(14), when an analyte was placed in the vicinity of a metal nanoparticle (particularly Ag). The underlying source of this huge increase in signal in surface-enhanced Raman scattering (SERS) spectroscopy has since been characterized by considerable controversy. Three possible contributions to the enhancement factor have been identified: (i) the surface plasmon resonance in the metal nanoparticle, (ii) a charge-transfer resonance involving transfer of electrons between the molecule and the conduction band of the metal, and (iii) resonances within the molecule itself. These three components are often treated as independently contributing to the overall effect, with the implication that by properly choosing the experimental parameters, one or more can be ignored. Although varying experimental conditions can influence the relative degree to which each resonance influences the total enhancement, higher enhancements can often be obtained by combining two or more resonances. Each resonance has a somewhat different effect on the appearance of the resulting Raman spectrum, and it is necessary to invoke one or more of these resonances to completely describe a particular experiment. However, it is impossible to completely describe all observations of the SERS phenomenon without consideration of all three of these contributions. Furthermore, the relative enhancements of individual spectral lines, and therefore the appearance of the spectrum, depend crucially on the exact extent to which each resonance makes a contribution. In this Account, by examining breakdowns in the Born-Oppenheimer approximation, we have used Herzberg-Teller coupling to derive a single expression for SERS, which includes contributions from all three resonances. Moreover, we show that these three types of resonances are intimately linked by Herzberg-Teller vibronic coupling terms and cannot be considered separately. We also examine the differences between SERS and normal Raman spectra. Because of the various resonant contributions, SERS spectra vary with excitation wavelength considerably more than normal Raman spectra. The relative contributions of totally symmetric and non-totally symmetric lines are also quite different; these differences are due to several effects. The orientation of the molecule with respect to the surface and the inclusion of the metal Fermi level in the list of contributors to the accessible states of the molecule-metal system have a strong influence on the observed changes in the Raman spectrum.