Detection of Contaminants in Water Using Surface Enhanced Raman Spectroscopy

Abstract: Due to deteriorating water quality and the world’s increasing demand for clean water, the need for cheap, easy and portable techniques to characterize and quantify pollutants in waters is urgent. Hence, surface-enhanced Raman spectroscopy (SERS) have gained considerable attention in this field. Atrazine and bentazon are two of the most occurring pesticides causing pollution in Sweden, and where therefore examined in this study, along with 4-mercaptopyridine (mpy) as a reference molecule. In this project, silver and gold nanoparticles where synthesised and used as SERS substrates for detection of contaminants in water by using a handheld Raman device provided by Serstech AB. Sodium chloride (NaCl) and magnesium sulfate (MgSO4) where used as aggregation agents allowing the nanoparticles to form hot spots. Mpy was detected to 0.5 nM and an enhancement factor of 108 using silver nanoparticles aggregated with NaCl was obtained. No Raman signal was obtained from atrazine nor bentazon using the handheld Raman device with silver nanoparticles aggregated with NaCl. Therefore the Raman cross-section of the probe molecules where investigated using the handheld Raman device and a conventional Raman device. Bentazon was not detectable using the handheld Raman device but detectable using a conventional Raman device. Atrazine was detectable at high concentrations i.e. atrazine powder using the handheld Raman device and detectable at 100 nM using a conventional Raman device. Since bentazon was not detectable with the handheld Raman device, more focus was put on getting a detectable signal from atrazine using the handheld Raman device. Investigation of the adsorption of atrazine and bentazon to the silver nanoparticle surface was performed. Due to the weaker adsorption to the nanoparticle surface, MgSO4 was used aggregation agent instead of NaCl with mpy, atrazine and bentazon. Mpy was detectable using MgSO4 as aggregation agent, atrazine and bentazon was not. Measurements of mpy, atrazine and bentazon without any salt was performed. For these measurements, no detectable signal from neither molecule was obtained, indicating that the formation of hot spots is necessary to obtained a detectable Raman signal. Measurements of mpy and atrazine with gold nanostars where performed. Enhancement factor using the gold nanostars was calculated to 107, and a detectrable signal from mpy was obtained, not from atrazine. Measurements of atrazine and mpy simultaneously was performed, where mpy peaks was observed but no atrazine peaks. The affinity of the probe molecule and the nanoparticle is crucial to obtain a detectable signal. This study inducates that both the chemical enhancement and electromagnetic enhancement are needed to obtain a detectable signal. For that, strongly binding species is necessary. Considering the simplicity of this method and the limited optimization efforts, there is plenty of room for improvements, including different probe molecules and different SERS substrates. With the right conditions, the evaluated technique reveals a promising and accessible method using a commercially available handheld Raman spectrometer for detection and quantification of contaminants in water.