Photo-induced enhanced Raman spectroscopy for universal ultra-trace detection of explosives, pollutants and biomolecules

Surface-enhanced Raman spectroscopy is one of the most sensitive spectroscopic techniques available, with single-molecule detection possible on a range of noble-metal substrates. It is widely used to detect molecules that have a strong Raman response at very low concentrations. Here we present photo-induced-enhanced Raman spectroscopy, where the combination of plasmonic nanoparticles with a photo-activated substrate gives rise to large signal enhancement (an order of magnitude) for a wide range of small molecules, even those with a typically low Raman cross-section. We show that the induced chemical enhancement is due to increased electron density at the noble-metal nanoparticles, and demonstrate the universality of this system with explosives, biomolecules and organic dyes, at trace levels. Our substrates are also easy to fabricate, self-cleaning and reusable.


Supplementary
. PIERS without nanoparticles. Raman spectra of DNT on non-irradiated (blue) and pre-irradiated (green) TiO 2 without metallic nanoparticles on the surface. It is notable that there is low spectral intensity and no difference between conditions. Supplementary Figure 7. Spectral shifting with Ag nanoparticles. UV-visible spectrum of AgNPs on TiO 2 showing blue shift on UV pre-irradiation. The initial LSPR of the AgNPs is shifted from their solution value (428 nm) due to the change in refractive index. This is similar to the gold case highlighted in Fig. 3b in the main text.
Supplementary Figure 8. PIERS with Ag nanoparticles. PIERS (green) vs SERS (blue) of 10 -9 M DNT on TiO 2 (R) with ~ 60 nm AgNPs. Black lines are the powder spectra maxima. The enhancement effect is similar to that achieved by AuNPs as the Ag work function is also below the energy of the TiO 2 conduction band. Figure 9. Ultra-trace PIERS. PIERS spectra of DNT at ultra-trace levels (a) 10 -12 M and (b) 10 -15 M. Black lines are the powder spectra, blue the normal SERS spectrum and green the PIERS spectra. Similar peaks are visible, but in the latter they are more obscured by spectral noise.  Where I SERS and I RS are the intensities of SERS and Raman bands respectively, (I PIERS uses for the intensities of PIERS bands), and C RS , and C SERS are the corresponding analyte concentrations in the Raman and SERS measurements, respectively, and C PIERS in PIERS bands. The analyte concentration were 10 -7 M for R6G, 10 -9 M for DNT and glucose, 10 -5 M for TNT, RDX, and PETN.

Supplementary Note 2. Estimation of TNT vapour concentration.
TNT concentration was calculated from the vapour pressure at 25 o C using the vapour pressure of 7.3 x 10 -4 Pa as follows: .

(Supplementary Eqn. 2)
This could then be converted into a rough molar concentration using the mass of TNT (227.13 g mol -1 ) and 1 ppm = 1 mg L -1 .

Supplementary Methods
Titanium tetrachloride (TiCl 4 , 99%) and ethyl acetate (C 4 H 8 O 2 , 99.8%), both from Sigma-Aldrich, were used as metal and oxygen sources, respectively. All the components of the CVD apparatus were kept at high temperature (200 ºC). The precursors were heated independently in stainless steel bubblers and carried under controlled flows using pre-heated nitrogen gas (supplied by BOC). The precursors were mixed in a stainless steel chamber (250 ºC) before accessing the CVD reactor and then plain nitrogen flow dragged the gas precursors' mixture through a triple baffle manifold to generate a wide laminar flow. The cold-wall CVD reactor consists of a 320 mm-long heating graphite block accommodated in a quartz tube, with three inserted Whatman heater cartridges. The temperature of the entire system was controlled by Pt-Rh thermocouples.
In a typical deposition, bubbler temperatures and gas flows of the precursors were set to 1.2 L min -1 /70 C and 0.25 L min -1 /40 C for TiCl 4 and C 4 H 8 O 2 , respectively. The TiO 2 film was deposited (growth rate, 0.45 -0.5 m min -1 ) on quartz slides (25x25 mm, Multi-Lab) at 500 C and then annealed to 900 C for 10 h. Due to temperature limitation of the CVD (chemical vapour deposition) rig, pure rutile films were obtained after heat treatment (1050 °C, 10 h) of an anatase film deposited on quartz slides. X-ray diffraction and Raman spectroscopy confirmed the presence of pure rutile; no traces of anatase were detected.
Comparative SiO 2 coated glass was obtained from Pilkington NSG in the form of commercial barrier glass.
AuNPs were produced using the standard Turkevich-Frens procedure to give an average size of 26.6 nm, with standard deviation (s.d.) of 5 nm. HAuCl 4 (120 mg) was dissolved in 250 mL of boiling water. A 1% (w/w) sodium citrate solution (25 mL) was added and the reaction kept boiling for 1 hour. AuNPs were precipitated by ultra-centrifugation at 12000 x g, and re-suspended in MeOH at an approximate concentration of 4.2 x 10 -10 M (by UV-vis). The solution was drop cast onto the substrates, followed by air drying, to give a rough coverage of 250 AuNPs m -2 .
AgNPs were produced by the standard Turkevich method with a single modification in the pH values along the reaction. Initially, NaOH (0.1 M) was used to adjust the pH to 7.7 of an aqueous solution of tri-sodium citrate solution (7 mM). This solution was heated until it started boiling and then 1 mL of 0.1 M aqueous silver nitrate was added. After stirring the reaction solution for 5 minutes the pH was adjusted to 6.1 by addition of HNO 3 in order to slow down the reaction and get better shape and size distribution. The reaction was complete after 30 minutes, giving rise to particles of 58nm with s.d. of 14 nm.
UV-visible (UV-Vis) spectra were collected on Perkin Elmer Lambda 25 and 950 systems in absorbance or reflectance mode, and a Shimadzu UV-2550 instrument. XRD patterns were collected between 10° and 65° with a Bruker-Axs D8 System, with Cu Kα source (1.54 Å). The incident beam angle was 1 o . X-ray Photoelectron Spectroscopy (XPS) measurements were performed with a Thermo monochromated aluminium k-alfa photoelectron spectrometer, using monochromic Al-Kα radiation (1486.7 eV). Survey scans were collected in the range of 0 -1200 eV. Highresolution peaks were used for the principal peaks of Ti (2p), O (1s), C (1s) and Au (4f). The peaks were modelled using sensitivity factors to calculate the film composition. The area underneath these bands is an indication of the concentration of element within the region of analysis (spot size 400 µm). Data was analysed with CasaXPS software. Nanoparticles and films were imaged on a Jeol 6700F FEG SEM operating at 5 kV and a Jeol 2100 TEM operating at 200 kV with a Gatan Orius digital camera. Particle sizing was performed using ImageJ software.