Yong-Hyok Kwon

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.

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.

Surface-enhanced in-situ Raman-sensor applied in the arctic area for analyses of water and sediment

Investigations on the seafloor in the arctic area are of great scientific interest as well as of progressive economic importance. Therefore, measurements in the water column and of sediments were carried out by applying different analytical methods. In JCR 253 arctic cruise a microsystem diode laser with reflection Bragg grating emitting at 671 nm was introduced and integrated into an optode housing which was laboratory pressure tested up to 200 bar. The connection to the mobile spectrometer is realized through an optical fiber. All performed measurements were carried out on the James-Clark-Ross research vessel during a three week experiment in August 2011. Conventional Raman spectra and SERS spectra of arctic surface water and sediment acquired from locations around 78° N and 9° E will be presented. Selected SERS substrates developed for SERS measurements in sea-water were tested for their capability to detect different substances in the water down to very small (pmol/l) concentrations. Additionally, the applicability of shifted excitation Raman difference spectroscopy (SERDS) and a combination of SERS with SERDS for analytical applications during sea-trials for in-situ analyses of sea-water and sediments will be discussed.

Noble metal nanoparticles on quartz supports as SERS substrates excited by a diode laser system for SERDS

In this contribution we present surface enhanced Raman scattering (SERS) measurements of pyrene as a function of the surface plasmon resonance position of noble metal nanoparticle ensembles, which served as SERS substrates. The noble metal nanoparticle ensembles were prepared under ultrahigh vacuum (UHV) conditions by Volmer-Weber growth on quartz substrates. For the SERS measurements, the substrates were mounted in a flow-through cell as part of the optical Raman set-up. A diode laser microsystem with an emission wavelength of 488 nm was used. The system generates two slightly different emission wavelengths (Δλ ≈ 0.3 nm) with a spectral width of ≈ 10 pm and an optical power of ≈ 10 mW. With this set-up SERS as well as shifted excitation Raman difference spectroscopy (SERDS) can be carried out. For trace analysis of pyrene in water SERS/SERDS experiments were accomplished as a function of molecule concentration and spectral position of the plasmon resonance. The best results with a limit of detection of 2 nmol of pyrene were obtained with a nanoparticle ensemble with a plasmon resonance in the vicinity of the excitation wavelength of λ = 488 nm. If the plasmon resonance frequency is slightly off-resonance the detection limit is significantly lower. The latter results are discussed in more detail and we will demonstrate that the morphology and the optical properties of the SERS substrates crucially influence the LOD.