In 1928, C.V. Raman found that the scattering frequency of light changed when light passed through a transparent medium and interacted with molecules, now called Raman scattering. The incident photons interact with molecules and subsequently generate emitting photons under laser irradiation. In the process, most photons are elastically scattered without exchanging energy (Rayleigh scattering), while a small proportion of photons gain or lose energy, thus leading to a change in both the frequency and direction of the photon (Raman scattering). The intensity of Rayleigh scattering is only 10⁻³ of the incident light intensity, and the intensity of Raman scattering is only about 10⁻³ of the intensity of Rayleigh scattering, which is 10⁻⁶ of the incident light intensity. Typically, Raman scattering is weak, so some effective strategies need to be used to enhance the Raman signal for the detection of the molecules.

In 1974, Fleischmann and co-workers first reported that the Raman signal of pyridine when in close contact to a rough silver electrode was considerably enhanced. Subsequently, Jeanmaire and Van Duyne discovered that the Raman signal of adsorbed molecules on the surface of a roughened novel metal was significantly enhanced. Since then, the concept of surface-enhanced Raman scattering (SERS) has been proposed and attracted lots of attention. So far, the SERS enhancement theory has still been a controversial matter and has not been clearly explained. It is generally accepted that electromagnetic enhancement and chemical enhancement make major contributions to SERS enhancements.

Electromagnetic enhancement is the main contributor to the SERS effect and relies on the resonance between the electrons on the surface of a metallic nanostructure and the incident light, namely, the localized surface plasmon resonances (LSPR). It has been proven that the enhancement factor for electromagnetic fields is approximately proportional to the fourth power of the local electric field intensity generated by metal nanostructures. More importantly, the electromagnetic field around the nanostructures is not uniformly distributed but highly localized in a narrow space called “hot spots”. SERS enhancement declined nearly exponentially with the distance between the interested point and nanostructures, so only the Raman signal of molecules adsorbed on or very near the surface of the nanostructures can be enhanced. “Hot spots” usually occur in the gaps or sharp vertices of the nanostructures made by noble metals, semiconductors or metal–organic frameworks. Compared with traditional Raman, the SERS signal of molecules near the “hot spots” can be greatly enhanced with an enhancement factor of 10⁶~10⁸, and the density of a “hot spot” is proportional to the enhancement effect.

Chemical enhancement is attributed to the electronic-transfer processes between the metallic surface and the adsorbed molecules. The distance of the electronic-transfer effect is limited to within 10 nanometers. Once the incident light is matched with the electron transfer energy of the adsorbed molecules, resonance Raman enhancement can be achieved. This effect brings about a change in the molecular polarization, and the Raman signal of the analyte can be enhanced by two to three orders of magnitude. Compared to electromagnetic enhancement, chemical enhancement makes a lower contribution to the total enhancement of a Raman scattering signal. Unlike electromagnetic enhancement, chemical enhancement is closely related to the chemical structures of the molecules. The correlation between the molecular structures of different organothiols and their SERS enhancement factors can be estimated using a simple internal reference method.

Due to the merits of fast detection, high sensitivity, good selectivity, multiplexing and absence of interference in water, SERS has been widely used in the areas including food science, environmental analysis,material science, biosensing, catalysis, electrochemistry etc.