Molecular Optomechanics Approach to Surface-Enhanced Raman Scattering

A Statistical Route to Robust SERS Quantitation Beyond the Single-Molecule Level

Single-molecule surface-enhanced Raman spectroscopy (SM-SERS) holds great potential to revolutionize ultratrace quantitative analysis. However, achieving quantitative SM-SERS is challenging because of strong intensity fluctuation and blinking characteristics. In this study, we reveal the relation P = 1 - e-α between the statistical SERS probability P and the microscopic average molecule number α in SERS spectra, which lays the physical foundation for a statistical route to implement SM-SERS quantitation. Utilizing SERS probability calibration, we achieve quantitative SERS analysis with batch-to-batch robustness, extremely wide detection range of concentration covering 9 orders of magnitude, and ultralow detection limit far below the single-molecule level. These results indicate the physical feasibility of robust SERS quantitation through statistical route and certainly open a new avenue for implementing SERS as a practical analysis tool in various application scenarios.

Advancing SERS as a quantitative technique: challenges, considerations, and correlative approaches to aid validation

Surface-enhanced Raman scattering (SERS) remains a significant area of research since it's discovery 50 years ago. The surface-based technique has been used in a wide variety of fields, most prominently in chemical detection, cellular imaging and medical diagnostics, offering high sensitivity and specificity when probing and quantifying a chosen analyte or monitoring nanoparticle uptake and accumulation. However, despite its promise, SERS is mostly confined to academic laboratories and is not recognised as a gold standard analytical technique. This is due to the variations that are observed in SERS measurements, mainly caused by poorly characterised SERS substrates, lack of universal calibration methods and uncorrelated results. To convince the wider scientific community that SERS should be a routinely used analytical technique, the field is now focusing on methods that will increase the reproducibility of the SERS signals and how to validate the results with more well-established techniques. This review explores the difficulties experienced by SERS users, the methods adopted to reduce variation and suggestions of best practices and strategies that should be adopted if one is to achieve absolute quantification.

SERS Platform for Integrated Enrichment, Isolation, and Identification of Multiple Respiratory Viruses in a Single Assay Using 3D Stereoscopic SERS Tags and Flocked Swabs

Influenza (flu) and severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) exhibit similar clinical symptoms, complicating the diagnosis and clinical management of these critical respiratory infections. Thus, there is an urgent need for rapid on-site detection technologies that can simultaneously detect SARS-CoV-2 and influenza A viruses. Here, we have developed the first platform that combines in situ sampling with immune swabs and multichannel surface-enhanced Raman spectroscopy (SERS) for simultaneous screening of these two respiratory viruses in a single assay. A seed-mediated growth method was used to assemble a number of silver spheres on the surface of Fe3O4@SiO2 spheres, which not only creates extensive Raman hotspots but also provides numerous sites for Raman signaling molecules, enhancing the sensing sensitivity. Integrating two specific Raman signaling molecules into the nanospheres allows for the parallel detection of both viruses, improving the efficiency of SERS signal read-out. Rapid quantitative screening of both SARS-CoV-2 and H1N1 is achievable within 15 min, with detection limits of 7.76, and 8.13 pg·mL-1 for their respective target proteins. The platform demonstrated excellent performance in testing and analyzing 98 clinical samples (SARS-CoV-2:50; influenza A:48), achieving sensitivities of 88.00, and 95.83% for SARS-CoV-2 and influenza A, respectively. Pearson's correlation analysis revealed a significant correlation with the clinical CT values (P < 0.0001), underscoring the great potential of this platform for the early, rapid, and simultaneous diagnostic discrimination of multiple pathogens.

Recent advances in microfluidic-based spectroscopic approaches for pathogen detection

Rapid identification of pathogens with higher sensitivity and specificity plays a significant role in maintaining public health, environmental monitoring, controlling food quality, and clinical diagnostics. Different methods have been widely used in food testing laboratories, quality control departments in food companies, hospitals, and clinical settings to identify pathogens. Some limitations in current pathogens detection methods are time-consuming, expensive, and laborious sample preparation, making it unsuitable for rapid detection. Microfluidics has emerged as a promising technology for biosensing applications due to its ability to precisely manipulate small volumes of fluids. Microfluidics platforms combined with spectroscopic techniques are capable of developing miniaturized devices that can detect and quantify pathogenic samples. The review focuses on the advancements in microfluidic devices integrated with spectroscopic methods for detecting bacterial microbes over the past five years. The review is based on several spectroscopic techniques, including fluorescence detection, surface-enhanced Raman scattering, and dynamic light scattering methods coupled with microfluidic platforms. The key detection principles of different approaches were discussed and summarized. Finally, the future possible directions and challenges in microfluidic-based spectroscopy for isolating and detecting pathogens using the latest innovations were also discussed.

Amplifying Frequency Up-Converted Infrared Signals with a Molecular Optomechanical Cavity

Frequency up-conversion, enabled by molecular optomechanical coupling, has recently emerged as a promising approach for converting infrared signals into the visible range through quantum coherent conversion of signals. However, detecting these converted signals poses a significant challenge due to their inherently weak signal intensity. In this work, we propose an amplification mechanism capable of enhancing the signal intensity by a factor of 1000 or more for the frequency up-converted infrared signal in a molecular optomechanical system. The mechanism takes advantage of the strong coupling enhancement with molecular collective mode and the Stokes sideband pump. This work demonstrates a feasible approach for up-converting infrared signals to the visible range.

Coherent Electron-Vibron Interactions in Surface-Enhanced Raman Scattering (SERS)

In this Letter we identify coherent electron-vibron interactions between near-resonant and nonresonant electronic levels that contribute beyond standard optomechanical models for off-resonant or resonance surface-enhanced Raman scattering (SERS). By developing an open-system quantum model using first molecular interaction principles, we show how the Raman interference of both resonant and nonresonant contributions can provide several orders of magnitude modifications of the SERS peaks with respect to former optomechanical models and over the fluorescence backgrounds. This cooperative optomechanical mechanism allows for generating an enhancement of nonclassical photon pair correlations between Stokes and anti-Stokes photons, which can be detected by photon-counting measurements. Our results demonstrate Raman enhancements and suppressions of coherent nature that significantly impact the standard estimations of the optomechanical contribution from SERS spectra and their quantum mechanical observable effects.

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

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

Hybrid cavity-antenna architecture for strong and tunable sideband-selective molecular Raman scattering enhancement

Plasmon resonances at the surface of metallic antennas allow for extreme enhancement of Raman scattering. Intrinsic to plasmonics, however, is that extreme field confinement lacks precise spectral control, which would hold great promise in shaping the optomechanical interaction between light and molecular vibrations. We demonstrate an experimental platform composed of a plasmonic nanocube-on-mirror antenna coupled to an open, tunable Fabry-Perot microcavity for selective addressing of individual vibrational lines of molecules with strong Raman scattering enhancement. Multiple narrow and intense optical resonances arising from the hybridization of the cavity modes and the plasmonic broad resonance are used to simultaneously enhance the laser pump and the local density of optical states, and are characterized using rigorous modal analysis. The versatile bottom-up fabrication approach permits quantitative comparison with the bare nanocube-on-mirror system, both theoretically and experimentally. This shows that the hybrid system allows for similar SERS enhancement ratios with narrow optical modes, paving the way for dynamical backaction effects in molecular optomechanics.

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

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

A path towards single molecule vibrational strong coupling in a Fabry-Pérot microcavity

Interaction between light and molecular vibrations leads to hybrid light-matter states called vibrational polaritons. Even though many intriguing phenomena have been predicted for single-molecule vibrational strong coupling (VSC), several studies suggest that these effects tend to be diminished in the many-molecule regime due to the presence of dark states. Achieving single or few-molecule vibrational polaritons has been constrained by the need for fabricating extremely small mode volume infrared cavities. In this theoretical work, we propose an alternative strategy to achieve single-molecule VSC in a cavity-enhanced Raman spectroscopy (CERS) setup, based on the physics of cavity optomechanics. We then present a scheme harnessing few-molecule VSC to thermodynamically couple two reactions, such that a spontaneous electron transfer can now fuel a thermodynamically uphill reaction that was non-spontaneous outside the cavity.

Mode-Specific Coupling of Nanoparticle-on-Mirror Cavities with Cylindrical Vector Beams

Nanocavities formed by ultrathin metallic gaps permit the reproducible engineering and enhancement of light-matter interaction, with mode volumes reaching the smallest values allowed by quantum mechanics. While the enhanced vacuum field in metallic nanogaps has been firmly evidenced, fewer experimental reports have examined the far-field to near-field input coupling under strongly focused laser beam. Here, we experimentally demonstrate selective excitation of nanocavity modes controlled by the polarization and frequency of the laser beam. We reveal mode selectivity by recording confocal maps of Raman scattering excited by cylindrical vector beams, which are compared to the known excitation near-field patterns. Our measurements reveal the transverse vs longitudinal polarization of the excited antenna mode and how the input coupling rate depends on laser wavelength. The method introduced here is easily applicable to other experimental scenarios, and our results help connect far-field with near-field parameters in quantitative models of nanocavity-enhanced phenomena.

Detecting nanoparticles by "listening"

In the macroscopic world, we can obtain some important information through the vibration of objects, that is, listening to the sound. Likewise, we can also get some information of the nanoparticles that we want to know by the means of "listening" in the microscopic world. In this review, we will introduce two sensing methods (cavity optomechanical sensing and surface-enhanced Raman scattering sensing) which can be used to detect the nanoparticles. The cavity optomechanical systems are mainly used to detect sub-gigahertz nanoparticle or cavity vibrations, while surface-enhanced Raman scattering is a well-known technique to detect molecular vibrations whose frequency generally exceeds terahertz. Therefore, the vibrational information of nanoparticles from low-frequency to high-frequency could be obtained by these two methods. The size of the viruses is at the nanoscale and we can regard it as a kind of nanoparticles. Rapid and ultrasensitive detection of the viruses is the key strategies to break the spread of the viruses in the community. Cavity optomechanical sensing enables rapid, ultrasensitive detection of nanoparticles through the interaction of light and mechanical oscillators and surface-enhanced Raman scattering is an attractive qualitatively analytical technique for chemical sensing and biomedical applications, which has been used to detect the SARS-CoV-2 infected. Hence, investigation in these two fields is of vital importance in preventing the spread of the virus from affecting human's life and health.

Trends of Biosensing: Plasmonics through Miniaturization and Quantum Sensing

Despite being extremely old concepts, plasmonics and surface plasmon resonance-based biosensors have been increasingly popular in the recent two decades due to the growing interest in nanooptics and are now of relevant significance in regards to applications associated with human health. Plasmonics integration into point-of-care devices for health surveillance has enabled significant levels of sensitivity and limit of detection to be achieved and has encouraged the expansion of the fields of study and market niches devoted to the creation of quick and incredibly sensitive label-free detection. The trend reflects in wearable plasmonic sensor development as well as point-of-care applications for widespread applications, demonstrating the potential impact of the new generation of plasmonic biosensors on human well-being through the concepts of personalized medicine and global health. In this context, the aim here is to discuss the potential, limitations, and opportunities for improvement that have arisen as a result of the integration of plasmonics into microsystems and lab-on-chip over the past five years. Recent applications of plasmonic biosensors in microsystems and sensor performance are analyzed. The final analysis focuses on the integration of microfluidics and lab-on-a-chip with quantum plasmonics technology prospecting it as a promising solution for chemical and biological sensing. Here it is underlined how the research in the field of quantum plasmonic sensing for biological applications has flourished over the past decade with the aim to overcome the limits given by quantum fluctuations and noise. The significant advances in nanophotonics, plasmonics and microsystems used to create increasingly effective biosensors would continue to benefit this field if harnessed properly.

A Novel Bio-Inspired Ag/3D-TiO2/Si SERS Substrate with Ordered Moth-like Structure

This paper reports a novel method to fabricate a bio-inspired SERS substrate with low reflectivity, ultra-sensitivity, excellent uniformity, and recyclability. First, double layers of polystyrene spheres with different particle sizes were assembled on the surface of a silicon wafer to act as a moth-like template. Second, through the template sacrifice method, the TiO₂ film with a three-dimensional moth-like eye structure was induced by the double-layer polystyrene spheres in the previous step, and its microscopic morphology showed a high degree of order. Finally, Ag nanoparticles were assembled on the TiO₂ film to form a bio-inspired SERS substrate. This ordered bio-inspired structure can not only reduce reflection, but also reinforce the uniformity of hotspot density, which helps to improve the sensitivity and uniformity of the Raman signal. This bio-inspired SERS substrate can detect R6G molecules at a concentration as low as 1.0 × 10^-10 mol/L, and its enhancement factor (EF) can reach 6.56 × 10⁶. In addition, the composite of Ag and TiO₂ can realize the photocatalytic degradation of R6G and then realize the recyclability of the SERS substrate.