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Naturally grown silver nanoparticle ensembles for 488nm in-situ SERS/SERDS-detection of PAHs in water

The detection of pollutant chemicals in water, from waste water up to drinking water, is of worldwide interest. Fast response chemical sensors based on Raman spectroscopy are well suited for a rapid identification and quantification of such substances. Because of the weak Raman scattering intensity surface-enhanced Raman scattering (SERS) was applied to achieve the high sensitivity necessary for trace detection. In the European Commission project SENSEnet, a SERS sensor based on a naturally grown Ag nanoparticle ensemble was developed and adapted for in-situ detection of polycyclic aromatic hydrocarbons (PAHs) in water. Silver nanoparticle ensembles with surface plasmon resonance (SPR) wavelengths around 488 nm were prepared under ultrahigh vacuum condition by Volmer-Weber growth on quartz plates. The laboratory set-up for Raman spectroscopy contains a microsystem frequency-doubled diode-laser which generates two emission wavelengths, 487.61 nm and 487.91 nm, thus the system was configured also for shifted excitation Raman difference spectroscopy (SERDS). The optical output power is up to 20 mW. The SERS substrate is located inside a flow-through cell which provides continuous flow conditions of an analyte solution. The SERS spectra were recorded using a laboratory spectrograph with a back-illuminated deep depletion CCD-detector. We present an atomic force microscopic image of the developed SERS substrates as well as results for the SERS activity and the limit of detection of selected PAHs, e.g. pyrene, in water with respect to the SPR wavelength. SERS/SERDS measurement of water samples containing mixture of several PAHs (e.g. pyrene and fluoranthene) down to the detection limit of 2 nmol/l will be discussed.