(733c) Continuous in Situ Sensing of Marine Pollutants Using Surface Enhanced Raman Spectroscopy (SERS)

Naturally occurring low concentrations of nitrate and phosphate in water bodies play an essential role in the nutrient cycle. At excess levels caused by human activity, however, damaging effects such as algae blooms, ecosystem disruption, and human health risks can develop. In addition, economic factors can be impacted as commercial activity is hampered. To avoid these negative effects, early detection of nitrate and phosphate concentration changes is essential. Early in situ detection of over‑nutrification is required for rapid response and mitigation plans. Commercial nitrate and phosphate sensors utilize ultraviolet-visible (UV-vis) spectroscopy as well as various colorimetric methods. Current detection methods are either incapable of measuring at the detection limits set by the Environmental Protection Agency (EPA), or require the handling of potentially harmful chemical reagents on a research vessel. Thus, there is an urgent need for new approaches for in situ monitoring. Surface enhanced Raman spectroscopy (SERS) is a technique theoretically capable of single molecule detection. Unlike UV‑Vis spectroscopy, SERS shows minimal interference with water, and therefore is a promising approach for nitrate and phosphate detection in coastal environments. Recent developments have led to the availability of compact and portable Raman spectrometers that could overcome the size constraints of research laboratories on marine vessels. These devices could even fit in buoys and floaters to enable real time in situ monitoring. However, there are clear challenges as SERS sensing is negatively affected by interference in complex media and reduced accuracy in solution. It is because of these challenges, in part, why much of the data reported in the literature are taken for purified samples that are then dried on a SERS substrate. Our goal is to address the engineering challenges for a SERS in situ seawater nutrient concentration measurement system. We designed and tested batch and flow-through devices that incorporate commercially available, nanostructured gold SERS substrates. By benchmarking against 4‑nitrobenzenethiol/ethanol solutions as well as nitrate and phosphate/water solutions and conducting long-term studies in water, our results show that SERS devices can be used as a development platform for a seawater nutrient sensor. They also depict the challenges associated with complex media and provide insight into surface modification strategies needed to selectively detect nitrate and phosphate. This work provides a framework for the use of surface-modified nanostructured gold substrates for detection and monitoring of nitrate and phosphate in the marine environment.