Surface enhanced Raman spectroscopy (SERS)
Surface enhanced Raman scattering (SERS) is a surface plasmon resonace (SPR) based nano-photonic phenomenon. It can provide non-destructive and ultra-sensitive molecular fingerprint vibrational information down to single molecular level. However, only a few ‘free-electron-like’ metals (mainly Au, Ag and Cu) can provide a large SERS effect under the condition that the metal surface must be roughened or nanostructured. Lack of substrate (materials) and surface (morphology) generalities has severely limited the breadth of practical applications of SERS to other materials widely used in energy, life and medical sciences and industries.
Focusing on how to break SERS limitations, we have developed several pioneered methodologies step by step as below:
1. Electrodeposition of transition metals onto roughened Au or Ag surfaces.
2. Electrochemical roughening of pure transition metal surfaces.
3. Chemical deposition of ultra-thin transition metal layers onto Au nanoparticles.
4. Shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS)
1. SERS on Transition Metal Surfaces
Three approaches have been developed to obtain SERS-active substrates of transition metals (VIII B): 1) Electrodeposition of transition metals onto roughened Au or Ag surfaces. The obtained substrates display high SERS activities, but the ultra-thin coating layers are non-uniform with pinholes. 2) Electrochemical roughening of pure transition metal surfaces. Raman signals can be directly obtain on pure transition metal electrodes for the first time, but the enhancement is not sufficient enough to investigate some important interfacial phenomena, such as water adsorption. 3) Chemical deposition of ultra-thin (ca. 1-10 atomic layers) transition metals onto Au nanoparticles. The substrates exhibit high SERS activity with uniform morphology and pinhole-free, but it is quite difficult to apply to many other materials, and impossible to work on the smooth surface.
In general, the total surface enhancement factors of transition metals can be substantially boosted up to 104–105, for Pt, Pd, Ru, Rh, Ni and Co, respectively. The obtained good-quality SERS spectra allow us to investigate some crucial interfacial phenomena for the first time, such as the adsorption of water or hydrogen with very small Raman cross-section, on several transition metals (e.g., Pt, Pd and Rh).
In 2010, we have developed a new operation mode, Shelled-Isolated Nanoparticle-Enhanced Raman Spectroscopy (SHINERS), where Au or Ag nanoparticles are coated with ultra-thin shells (about 1 to 5 nm) of chemically inert SiO2, Al2O3 or MnO2 shells. This mode is different from the contact mode of conventional SERS and the non-contact mode of tip-enhanced Raman spectroscopy (TERS). The Au or Ag cores provides a large enhancement to the probed molecules nearby, while the ultra-thin inert shells isolate the Au or Ag nanoparticles from the ambient environment. The main virtue of such “smart dusts” is the easy preparation and application over surfaces of any material and any morphology, which has been demonstrated with the high quality Raman spectra of probed molecules obtained at various single-crystal surfaces, semiconductors and living cells.
SHINERS method has avoided long-standing limitations (inaccessibility or difficulty) of SERS for the precise characterization of various materials, surfaces and biological samples. The concept of shell-isolated-nanoparticle enhancement may also be applicable to a wider range of spectroscopies, such as infrared spectroscopy, sum frequency generation and fluorescence, etc..
3. Applications of SERS
SERS has a broad and diverse range of chemical applications from electrochemistry and catalysis, single-molecule detection, sensing and trapping, to solid-phase synthesis, to bioanalytical applications. Very recently, the emerging of portable Raman instruments open a new window for SERS applications: on-site fast detection with high sensitivity towards food &medical safety, environmental protection, and social & national security. We mainly focus on the application of SERS to food & medical safety, such as the pesticides in fresh foods or teas, the illegal additives in processed foods and healthy products.