Since the discovery of surface enhanced Raman scattering (SERS) in the 1970’s, the sensitivity of Raman detection has been improved by over a millionfold benefiting from the introduction of noble-metal substrates. Conquering the inherent disadvantage of weak signals tangling with normal Raman measurement, SERS detection has gained wide application in the fields varied from food safety, environmental monitoring to life science, and soon became one of the most sensitive in-situ spectral detection techniques for surface species. Unfortunately, SERS experiments have been essentially dominated by adsorbates on rough metallic surfaces, especially limited to noble metals such as Au, Ag and Cu. In addition, noble metals typically show poor stability and biocompatibility in practical use, and therefore the search for other alternative materials as highly active SERS substrate becomes an urgent task.
Recently, Professor Zhao Zhigang’s group of Suzhou Institute of Nanotech and Nanobionics (SINANO), Chinese Academy of Sciences and Professor Geng Fengxia’s group from Soochow University together carried on a detailed and in-depth research, and made a breakthrough progress in the study of semiconductor-based SERS substrates. A new concept was put into practice that Raman signals associated with surface species adsorbed on none or weak SERS-active substrates (e.g. transition metal oxide) can be dramatically enhanced via proper modulation of the stoichiometric composition, particularly surface oxygen vacancies, of the substrates. Under the guidance of this strategy, employing vacancy-containing W18O49 sea urchin-like nanoparticles as the substrate material, we achieved greatly enhanced SERS effect for the first time on function-rich tungsten oxide material, and the enhancement was further improved by creating surface deficiencies, which gave a detection limit as low as 10-7 M and the maximum enhancement factor (EF) of 3.4×105, in the rank of the highest sensitivity, to our best knowledge, among semiconducting materials, even comparable to noble metals without ‘hot spots’.
This work reveals that with a suitable modulation of the oxygen vacancy density in semiconductor oxides, the SERS activity of which can be dramatically promoted. These findings break through the limitation of noble metal substrates in common SERS applications, and provide important clues in the future design strategy for highly-efficient semiconducting SERS substrates.
This work was supported by the National Natural Science Foundation of China (51372266, 51402204). Relative results have been published in Nature Communication (Volume 6, Article number: 7800, July 17, 2015).
The outstanding performance of oxygen vacancy-rich W18O49 nanoparticles as SERS substrate for R6G detection(Image by Zhao Zhigang’s group and Geng Fengxia’s group)
