Photo-Induced Enhanced Raman spectroscopy (PIERS) for detection of ultra-trace levels of Explosives

Surface enhanced Raman spectroscopy SERS is a powerful technique for highly sensitive and selective detection and identification of trace levels of a wide range of important molecules. Strong SERS enhancement even for low... [ view full abstract ]

Surface enhanced Raman spectroscopy SERS is a powerful technique for highly sensitive and selective detection and identification of trace levels of a wide range of important molecules. Strong SERS enhancement even for low Raman cross-section materials, i.e. explosives can be achieved when their molecules adsorb on a roughened noble metal nano structures. There are two mechanisms contribute to the SERS enhancement, one is the electromagnetic enhancement (EM) where the electric filed modified due to the physisorpotion interaction between incident photons and nobel metal nanoparticles. The other mechanism is the chemical enhancement via polarizability and electron charge transfer as a chemisorption interaction occurs where there is a resonant charge transfer between adsorbent molecules and the metal particles [1, 2].
In this work, we propose a new Raman enhancement method titled ‘Photo-Induced Enhanced Raman Spectroscopy’ (PIERS), where photoactivited semiconducting metal oxide substrates (TiO2 or ZnO) combined with plasmonic nanoparticles (NPs) provide strong Raman signal enhancement and increase the sensitivity beyond normal SERS [3]. In PIERS, the substrate (TiO2 or ZnO) coated by either gold NPs or silver NPs were irradiated with 254 nm light, followed by deposition of an analyte and Raman measurement. We have observed strong enhancement for PIERS over typical SERS with no pre-irradiation. We propose a mechanism for PIERS whereby electrons migrate from the photo-excited semiconductor substrate to the metallic NPs, leading to increase electron density on the NPs, and as a consequence provide further enhancement, via increased chemical enhancement over and above the SERS enhancement. Increasing the electron density on the metallic particle was demonstrated by measures of localised surface plasmon resonance shifting and time dependant measures of PIERS enhancement.
References:
1. Avila, F., et al., Modelling the effect of the electrode potential on the metal-adsorbate surface states: relevant states in the charge transfer mechanism of SERS. Chem Commun (Camb), 2011. 47(14): p. 4210-2.
2. George C. Schatz, M.A.Y., and Richard P. Van Duyne, Electromagnetic Mechanism of SERS. Physics and Applications, 2009. 103: p. 19–46.
3. Ben-Jaber, S., et al., Photo-induced enhanced Raman spectroscopy for universal ultra-trace detection of explosives, pollutants and biomolecules. Nat Commun, 2016. 7.