Plasmon-driven selective reductions revealed by tip-enhanced Raman spectroscopy

We successfully realized in-situ monitoring plasmon-driven selective reduction of 2,4-dinitrobenzenethiol to 2,2′-diamino-dimercaptoazobenzene, revealed by high vacuum tip-enhanced Raman spectroscopy (HV-TERS). The HV-TER spectra revealed that the 2-nitro and the 4-nitro of 2,4-DNBT were selectively reduced to the 2-amine and the –N = N– bond of 2,2′-diamino-dimercaptoazobenzene (2,2′DA-DMAB). Raman-active and IR-active modes as well as Fermi resonance were simultaneously observed in HV-TERS, which demonstrated the advantages of HV-TERS over SERS, since only Raman-active modes were observed in SERS. The intensities of molecular IR-active modes can be manipulated by the distance between tip and substrate in the near field, due to different dependences of the plasmon gradient and plasmon intensity over the distance of nano gap. Our results in HV-TERS are in-situ “complete-vibration modes” spectral analysis, which significantly extend the application of HV-TERS in the field of ultrasensitive spectral analysis on the nano scale.