SERS Application in Environmental and Medical Science
Surface-enhanced Raman spectroscopy (SERS) offers both a molecularly-specific and sensitive analysis of low molecular weight substances even in complex matrices. It is used for environmental issues such as the concentration of antibiotics in bodies of water and for the determination of metabolites in bodily fluids.
By: Dana Cialla-May // Olga Zukovskaja // Sophie Patze // Izabella Jahn // Karina Weber // Jürgen Popp
Surface-enhanced Raman spectroscopy (SERS) is an excellent analytical method that combines the molecularly-specific information from Raman spectroscopy with increased sensitivity based on the use of plasmonically-active metal nanostructures. This makes it possible to determine low molecular weight substances even in complex matrices when the higher affinity of the analyte molecules to the metal surface can be exploited.
For example, antibiotic residues can be found in drinking water samples and surface water, which is due to the high use of these drugs combined with inadequate treatment of wastewater. For this reason, a cartridge system was developed which allows the use of structured metal surfaces as SERS-active substrates. Based on the example of the antibiotic sulfamethoxazole (SMX), a semi-quantitative determination was carried out in an aqueous environment. Under laboratory conditions, a concentration of 2.2·10-10 mol/L (in drinking water samples: 2.2·10-9 mol/L) can be specified as the detection limit. The legally permitted concentration of SMX in drinking water worldwide is approximately 2·10-7 mol/L. By using a microfluidic setup and embedding the corresponding SERS-active structures, measurement conditions can be applied which are very stable and thus allow a high comparability. Furthermore, this approach shows high potential for automated sample feeding and the recording of many SERS spectra in a short time. Moreover, it was possible to obtain information on the orientation of the molecule towards the metal surface. For low concentrations in the sub-nanomolar range, the benzamine ring of the molecule is oriented parallel to the metal surface. For higher concentrations, the ring is rotated perpendicular to the metal surface due to the changed space requirement. Furthermore, statements can be made about the interaction between neighboring molecules. In summary, it can be concluded that SERS in a cartridge system possesses high potential for environmental issues and is suitable for the analysis of antibiotics in water samples.
In recent years, it has become clear that the combination of SERS with a lab-on-a-chip system is very promising for medical issues. One medical issue included the detection of pyocyanine (PYO), a metabolite of Pseudomonas aeruginosa, in human bodily fluids. The presence of PYO specifically suggests an acute or chronic infection with P. aeruginosa. If the infection is detected quickly, antibiotic treatment can be rapidly initiated. Our work has shown that the presence of the analyte leads to an aggregation of the SERS-active nanoparticles and that, therefore, small concentrations can be detected. The PYO analyte has been successfully detected in an aqueous solution in the clinically relevant range extending from 7 to 130 µM. The linear range is about 0.5 to 15 µM, and the detection limit can be specified at <0.5 µM. Furthermore, saliva samples from volunteers were used as a biological matrix into which PYO was spiked in different concentrations. In the case of two saliva samples, PYO could be detected up to a concentration of 10 µM and for the third saliva sample up to a concentration of 25 µM. The results show that PYO can be detected in complex biological matrices.
Funded by: BMBF