High sensitivity calixarene SERS substrates for the continuous in-situ detection of PAHs in seawater

In-situ monitoring of pollutant chemicals in sea-water is of worldwide interest. For that purpose, fast response sensors based on Raman spectroscopy are suitable for a rapid identification and quantification of these substances. Surface-enhanced Raman scattering (SERS) was applied to achieve the high sensitivity necessary for trace detection. In the project SENSEnet, funded by the European Commission, a SERS sensor based on calixarene-functionalized silver nanoparticles embedded in a sol-gel matrix was developed and adapted for the in-situ detection of polycyclic aromatic hydrocarbons (PAHs). The laboratory set-up contains a microsystem Raman diode laser with two slightly different emission wavelengths (670.8 nm and 671.3 nm) suitable also for shifted excitation Raman difference spectroscopy (SERDS). The output power at each of both wavelengths is up to 200 mW. For the detection of the SERS spectra integration times of typically 1 - 10 seconds were chosen. The SERS substrate is located inside a flow-through cell which provides continuous flow conditions of the analyte. The spectra were recorded using a laboratory spectrograph with a back-illuminated deep depletion CCD-detector. We present scanning electron microscope images of the developed calixarene-functionalized Ag colloid based SERS substrates as well as results for the SERS adsorption properties of major PAHs (pyrene, fluoranthene, and anthracene) in artificial sea-water and their limits of detection (e. g. 0.1 nM for pyrene). The suitability of the presented device as an in-situ SERS sensor for application on a mooring or buoy will be discussed.