Long-term monitoring at Department of Defense (DoD) sites is currently done by collecting aqueous and soil samples that are then sent to a laboratory for analysis, which can be both time consuming and costly. Often, the analytical results are questionable due to sample handling procedures and biochemical interactions. New, long-term monitoring technologies are needed that will measure contaminants of concern in situ and minimize sampling time and costs.
The objective of this SERDP Exploratory Development (SEED) project was to develop a sensor that uses cationic-coated surface enhanced Raman spectroscopy (SERS) substrates to detect perchlorate, chromate, dichromate, and cyanide anions. The coating attracts the anions to the SERS substrate where they are identified and quantified by their characteristic Raman emission.
The advantages of SERS over other spectroscopic techniques are specificity and sensitivity. All polyatomic species will exhibit a characteristic Raman signature that can be used to both identify and quantify it. To protect the silver or gold SERS substrates from degradation, the surface is allowed to react with a thiol to form a self-assembled monolayer. Besides protecting the SERS surface, thiol coatings are selected to attract the analytes of interest. Therefore, the success of using SERS to detect perchlorate, chromate, dichromate, and cyanide anions was dependent on identifying a suitable thiol coating(s) to attract the anions. Once feasibility to detect these anions was demonstrated, means of improving selectivity and sensitivity were investigated.
In this effort, detection of polyatomic anions using SERS was demonstrated. The interaction between the polyatomic anions and the cationic coatings was instantaneous. The concentration response between the anions and cationic coatings was described by a Frumkin isotherm. In general, the cationic coatings used in this feasibility study interacted with multiple anionic species. The advantage is that, for anions exhibiting a Raman active mode, multiple anionic species can be detected simultaneously. However, quantification of these species requires knowing the values of the selectivity coefficients, just as is done when using ion-selective electrodes. The observed selectivities depend on the chemical properties of both the cationic thiol and the anion. For example, it was found that 4-(2-mercaptoethyl) pyridinium (MEP+) cation exhibited great selectivity for chromate. This selectivity was attributed to the hydrogen bonding capabilities of MEP+. Using conventional SERS substrates, detection limits for perchlorate, chromate, and cyanide were in the high ppb to low ppm concentration range. Much lower detection limits were possible using SERS-active capture matrices comprised of cationic-coated gold/silver colloidal particles immobilized on magnetic microspheres.
This technology provides an improved capability for in situ detection of perchlorate, chromate, and cyanide. Such a capability has the potential to replace the current sampling and laboratory analyses. In turn, this will result in savings both in time and costs. Furthermore, by changing the coating on the SERS substrate, additional species can be detected, including volatile organic compounds, metal ions, drugs, explosives, and chemical/biological warfare agents.