Surface-enhanced Raman scattering (SERS) is a powerful spectroscopic tool, characterized by rapid, nondestructive, ultra-sensitive and fingerprint diagnostics, and thus has a wide spectrum of promising applications in chemical and biological analysis at trace levels. With the indispensable use of substrate materials, SERS can give dramatically enhanced Raman signals for analytes with small Raman scattering cross-sections, which are usually difficult to detect by traditional Raman spectroscopy. Although striking enhancement factors have been achieved with noble-metal based substrate, constructing SERS substrates with excellent reproducibility and renewability remains a great challenge until now.
Recently, Professor ZHAO Zhigang’s group from Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences has succeeded in finding an attractive strategy to ensure the reproducibility and renewability of SERS substrates. That is electrochromism, a typical phenomenon that transitional-metal oxides reversibly alter their optical outlook between transparent and colored states upon ion/electron insertion and extraction driven by bias potentials. Electrochromic technique has been actively researched for displays, adjustable mirrors, “smart” windows, and other cutting-edge applications, but never been proposed to overcome the critical problems in the field of SERS.
This color-changing strategy is based on a unique quantitative relationship between the SERS signal amplification and the coloration degree within a certain range, in which the information on the SERS activity of the substrate can be effectively inferred by judging the degree of color change. In practice, the colored substrate shows a strikingly increased SERS enhancement by a factor of 28 relative to the pristine uncolored film, with a record low batch-to-batch relative standard deviation (RSD) of 6.79% and run-to-run RSD of 2.52%. This indicates the excellent reproducibility and renewability of our electrochromic SERS substrates.
The unique colorimetric enhancement in SERS is assumed to be derived from the quantitative control of d-orbital electron filling by the electrochromic process, which thus shows a linear voltage dependence in the density of states near Fermi level, eventually dramatically affecting the Raman scattering polarizability of analytes adsorbed onto the colored SERS substrates. The results may provide a first step toward the rational design of electrochromic SERS substrates with a high sensitivity, reproducibility, and renewability, which are both scientifically and technically important due to the great need for quantitative analysis, standardized production and low cost in SERS.
Prof.ZHAO Zhigang, Suzhou institute of Nano-Tech and Nano-Bionic, Chinese Academy of Sciences.