Raman and infrared spectroscopies are important tools for chemical analysis at the molecular level, providing key information such as the chemical bonding of a system to be measured. However, Raman and IR spectroscopies often face the bottleneck issues of low sensitivity when applied to trace chemical analysis of materials and biological surfaces. Over the past four decades, significant efforts have been made to break this bottleneck and to promote the application and industrialization of related technologies.
In a new paper published in Light Science & Application, a team of scientists, led by Professor Zhong-Qun Tian from State Key Laboratory of Physical Chemistry of Solid Surfaces, Collaborative Innovation Center of Chemistry for Energy Materials, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China, have presented the development of surface-enhanced Raman and infrared absorption spectroscopies in three stages: (1) developing surface-enhanced Raman scattering (SERS) effect and surface-enhanced infrared absorption (SEIRA) effect to improve sensitivity, (2) designing optical excitation and collection optical systems to improve sensitivity, and (3) effective coupling between (1) and (2) to maximize the sensitivity. Starting with a brief history of SERS and SEIRA, this review outlines three physical mechanisms for enhancing spectral sensitivity by enhancing the local electromagnetic field, namely surface plasmons, the lightning rod effect and coulpling effect. Secondly, this review discussed the coupling mechanisms between nanoparticle-particle, particle-substrate, and probe-substrate. Thirdly, new strategies were proposed on how to optimize multiscale design of macro optics to micro/nanophotonics for achieving high excitation and collection efficiency in SERS and SEIRA , which significantly will improve the spectral sensitivity. Finally, the possibility of extending the efficient coupling between macro optics and micro-nano substrates to the atomic scale is discussed, and more multi-scale optical coupling strategies have prospected.
Some features can be seen from milestone timeline in Raman and IR spectra. (1) From 1800 to 1974, the researches were focused on Raman and IR instruments, and the establishment of Raman and IR and their derivative spectroscopies from scratch. (2) From 1974 to 2010, SERS and SEIRA effect observed. (3) Since 1997, the research objective of SERS and SEIRA has begun to shift to the single-molecule. The sensitivity of Raman and IR increasingly could not satisfy the demand for samples with weak Raman/IR response, even to the level of single moleculars. It is more urgent than ever to improve the sensitivity of Raman and IR spectra.
In SERS and SEIRA, the key of the electromagnetic (EM) theory is enhancing the EM field in a narrow region on the micro/nanostructures with the interaction between light and micro/nanostructures of Au or Ag, etc. The narrow region is also called ‘hotspot’. The light absorption and scattering cross-sections of molecules in the hotspot are significantly improved by the enhanced EM field in a hotspot. For a long time, the central issue blocking the development of SERS and SEIRA is how to improve the intensity of EM field in hotspots by micro/nano optical design. Localized surface plasmon polaritons, propagating surface plasmon polaritons, and lightning-rod effect are three basic physical processes to enhance the EM field in the hotspot. The SERS and SEIRA substrates could be classified as non-coupling substrates and coupling substrates. Non-coupling substrate, such as a single nanoparticle, metal film, and metal tip on a dielectric substrate, only supports one of the three basic physical effects. The EM enhancement factor of the non-coupling substrate is always lower than 5 orders of magnitude. But non-coupling substrate is the perfect model to research the basic physical process for SERS and SEIRA. The coupling substrate supports two or more physical effects which always have stronger EM in the hotspot. Raman scattering or IR absorption of molecules will be improved significantly in the hotspot of a coupling substrate, especially these with nano-structures like nanogap or nanotip. The sensitivity on these substrates even reaches a single-molecule level. Typical coupling substrates include nanoparticle-nanoparticle dimer, oligomeric structures, array structures, bow-tie structures, and gold (or silver) nanotip-gold (or silver) substrate coupling structures, etc.
However, ultra-sensitive SERS and SEIRA mainly focus on a few molecular systems with large scattering or absorption cross-sections, and these substrate structures also face many difficulties in practical applications. How can we make the regular SERS or SEIRA substrates gain the ultrahigh sensitivity, so that effective signal can be obtained even for molecules with small scattering or absorption cross-section on the substrates such as single SHINERS particle, TERS probe, single SEIRA rod, and nanoIR probe? In recent years, research on the design of macroscopic optical systems for specific micro-nano substrates has shown great potential. A few macro optics, such as ATR optics, waveguiding optics, fiber optics etc., has been proved effectively improving the sensitivity of SERS and SEIRA substrates. The multiscale coupling efficiency have been focused between macro optical systems and micro-nano substrates. In the next 3-5 years, we believe that even the commonly and widely used spherical nanoparticles have the opportunity to achieve a further order of magnitude improvement in the sensitivity.
Furthermore, in addition to the coupling design of macro and micro/nano-optics, the combination of lightning-rod effect and surface plasmonics at atomic scale has achieved a series of breakthroughs in recent years, such as the imaging of single-molecule and even a single chemical bond with TERS. However, detectable molecular systems are still limited to a small number of molecular species. Therefore, it is necessary to improve the coupling efficiency not only from macroscopic optics to micro-nano optics but also from micro-nano optics to atomic optics. The solution to this problem is very important not only for TERS but also for nanoIR.
In practical applications, the environmental universality of SERS and SEIRA is also an important factor. Especially in TERS and nanoIR, great significance has been foreseen to develop optical structure design suitable for a specific application such as electrochemical multiphase interface systems and life science in liquid environment.