Surface plasmons are coherent and collective electron oscillations confined at the dielectric-metal interface. Benefitting from the inherent subwavelength nature of spatial profile, surface plasmons can greatly accumulate the optical field and energy on the nanoscale and dramatically enhance various light-matter interactions. The properties of surface plasmons are strongly related to materials and structures, so that metals, semiconductors and two-dimensional materials with various morphologies and structures can have alternating plasmonic wavelengths ranging from ultraviolet, visible, near infrared to far infrared. Because the electric field can be enhanced by orders of magnitude within plasmonic structures, various light-matter interaction processes including fluorescence, Raman scattering, heat generation, photoacoustic effects, photocatalysis, nonlinear optical conversion, and solar energy conversion, can be significantly enhanced and these have been confirmed by both theoretical, computational and experimental studies. In this review, we present a concise introduction and discussion of various plasmon-enhanced light-matter interaction processes. We discuss the physical and chemical principles, influencing factors, computational and theoretical methods, and practical applications of these plasmon-enhanced processes and phenomena, with a hope to deliver guidelines for constructing future high-performance plasmonic devices and technologies.

Recent achievements in semiconductor surface‐enhanced Raman scattering (SERS) substrates have greatly expanded the application of SERS technique in various fields. However, exploring novel ultra‐sensitive semiconductor SERS materials is a high‐priority task. Here, a new semiconductor SERS‐active substrate, Ta 2 O 5 , is developed and an important strategy, the “coupled resonance” effect, is presented, to optimize the SERS performance of semiconductor materials by energy band engineering. The optimized Mo‐doped Ta 2 O 5 substrate exhibits a remarkable SERS sensitivity with an enhancement factor of 2.2 × 10 7 and a very low detection limit of 9 × 10 −9 m for methyl violet (MV) molecules, demonstrating one of the highest sensitivities among those reported for semiconductor SERS substrates. This remarkable enhancement can be attributed to the synergistic resonance enhancement of three components under 532 nm laser excitation: i) MV molecular resonance, ii) photoinduced charge transfer resonance between MV molecules and Ta 2 O 5 nanorods, and iii) electromagnetic enhancement around the “gap” and “tip” of anisotropic Ta 2 O 5 nanorods. Furthermore, it is discovered that the concomitant photoinduced degradation of the probed molecules in the time‐scale of SERS detection is a non‐negligible factor that limits the SERS performance of semiconductors with photocatalytic activity.

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.

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.

The current COVID-19 pandemic urges us to develop ultra-sensitive surface-enhanced Raman scattering (SERS) substrates to identify the infectiousness of SARS-CoV-2 virions in actual environments. Here, a micrometer-sized spherical SnS 2 structure with the hierarchical nanostructure of "nano-canyon” morphology was developed as semiconductor-based SERS substrate, and it exhibited an extremely low limit of detection of 10 −13 M for methylene blue, which is one of the highest sensitivities among the reported pure semiconductor-based SERS substrates. Such ultra-high SERS sensitivity originated from the synergistic enhancements of the molecular enrichment caused by capillary effect and the charge transfer chemical enhancement boosted by the lattice strain and sulfur vacancies. The novel two-step SERS diagnostic route based on the ultra-sensitive SnS 2 substrate was presented to diagnose the infectiousness of SARS-CoV-2 through the identification standard of SERS signals for SARS-CoV-2 S protein and RNA, which could accurately identify non-infectious lysed SARS-CoV-2 virions in actual environments, whereas the current PCR methods cannot.

Hydrogenation was discovered to be an effective method to improve the surfaced-enhanced Raman scattering (SERS) performance of semiconductor TiO2 and enhance its enhancement factor (EF) by at least 3 orders of magnitude. The TiO2 substrate hydrogenated for 3 h showed the most remarkable SERS activity with a detection limit of 1 × 10–7 M for R6G and an EF of 1.20 × 106, which can be comparable to the Ag substrate. The remarkable SERS activities can be attributed to the chemical enhancement mechanism dominated by the enhanced photoinduced charge transfer (PICT) process between R6G and the oxygen vacancy-containing partly amorphous black TiO2 NWs substrate, as well as the electromagnetic enhancement (EM) derived from the metal-like local surface plasma resonance (LSPR) of the hydrogenated randomly oriented TiO2 nanowires. The first principle based on the density functional theory has been applied to demonstrate the appearance of tailed electron energy state produced by hydrogenation and provide the reasonable explanation for an easier PICT process, a stronger light absorption, and the enhanced SERS performance of our hydrogenated TiO2 substrates. Another impressive fact was that the photodegradation capability of TiO2 was also evidently improved. After 14 cycles of detection-and-degradation of R6G molecules, the substrates can still maintain regenerative and remarkable SERS activity. Ultrasensitive SERS activity and self-cleaning performance were successfully integrated on the black TiO2 NWs substrate by hydrogenation. Moreover, our substrate exhibited the excellent signal reproducibility and the outstanding stability of antioxidation in atmosphere thanks to the protection of the surface amorphous layer.

Highlights Ag/BP-NS exhibit remarkable surface-enhanced Raman scattering performance with single-molecule detection ability. This remarkable enhancement can be attributed to the synergistic resonance enhancement of R6G molecular resonance, photo-induced charge transfer resonance and electromagnetic resonance. A new polarization-mapping method was proposed, which can quickly screen out single-molecule signals and prove that the obtained spectra are emitted by single molecule. The recognition of different tumor exosomes can be realized combining the method of machine learning. Abstract Single-molecule detection and imaging are of great value in chemical analysis, biomarker identification and other trace detection fields. However, the localization and visualization of single molecule are still quite a challenge. Here, we report a special-engineered nanostructure of Ag nanoparticles embedded in multi-layer black phosphorus nanosheets (Ag/BP-NS) synthesized by a unique photoreduction method as a surface-enhanced Raman scattering (SERS) sensor. Such a SERS substrate features the lowest detection limit of 10 –20 mol L −1 for R6G, which is due to the three synergistic resonance enhancement of molecular resonance, photo-induced charge transfer resonance and electromagnetic resonance. We propose a polarization-mapping strategy to realize the detection and visualization of single molecule. In addition, combined with machine learning, Ag/BP-NS substrates are capable of recognition of different tumor exosomes, which is meaningful for monitoring and early warning of the cancer. This work provides a reliable strategy for the detection of single molecule and a potential candidate for the practical bio-application of SERS technology. Supplementary Information The online version contains supplementary material available at 10.1007/s40820-022-00803-x.

Recent advances in MXenes with high carrier mobility show great application prospects in the surface-enhanced Raman scattering (SERS) field. However, challenges remain regarding the improvement of the SERS sensitivity. Herein, an effective strategy considering charge-transfer resonance for semiconductor-based substrates is presented to optimize the SERS sensitivity with the guidance of the density functional theory calculation. The theoretical calculation predicted that the excellent SERS enhancement for methylene blue (MeB) on Ti 3 C 2 MXene can be excited by both 633 and 785 nm lasers, and the Raman enhanced effect is mainly originated from the charge-transfer resonance enhancement. In this work, the Ti 3 C 2 MXenes exhibit an excellent SERS sensitivity with an enhancement factor of 2.9 × 10 6 and a low detection limit of 10 –7 M for MeB molecules. Furthermore, the SERS enhancement of Ti 3 C 2 and Au–Ti 3 C 2 substrates exhibit higher selectivity on different molecules, which contributes to the detection of target molecules in complex solution environments. This work can provide some theoretical and experimental basis for the research on SERS activity of other MXene materials.