The overall goal of the project was to field-demonstrate, validate, and optimize the performance and sensitivity of a portable Raman sensor for rapid detection and analysis of the energetic compound perchlorate (ClO₄^-) in contaminated groundwater at multiple U.S. Department of Defense (DoD) military installations. Specific objectives were to: (1) construct and validate the performance of a portable Raman sensor for ClO₄^- detection in groundwater with varying characteristics, (2) optimize the performance and sensitivity of the sensor through nano-fabrication of surface-enhanced Raman scattering (SERS) substrates and the fiber-optic sensor probe, (3) optimize field-testing methodologies and establish testing protocols, (4) partner with a commercial vendor for large-scale production of SERS substrates via nanoimprinting, and (5) evaluate and document the cost effectiveness of the new sensing technology by comparing with conventional laboratory-based analytical protocols.

The technology used herein was SERS, million-fold or greater enhancements of the Raman signal of target analyte molecules adsorbed at or near nanostructured noble metal surfaces (SERS substrates) and thus allows the detection of analytes or pollutants at ultra-low concentrations in water. SERS also provides molecular signatures via vibrational frequency shifts of specific chemical bonds (e.g., ClO₄- at ~935 reciprocal centimeters [cm^-1], and nitrate [NO₃^-] at 1,050 cm^-1) so that the technique is selective to specific analyte molecules, potentially enabling simultaneous, multi-species detection and analysis. The key to successful application of SERS for chemical and environmental analyses was the fabrication of nanostructured substrates with small gap sizes that are reproducible, sensitive, and selective to target analyte molecules. This project developed and utilized a new type of ordered and elevated gold (Au) bowtie and ellipses nano-arrays with controllable gap sizes to <10 nanometers (nm). The substrate showed superior reproducibility and sensitivity with an enhancement factor (EF) on the order of 109–1011.

Field demonstration of the Raman sensor technology was conducted multiple times at two DoD sites: the Indian Head Division, Naval Surface Warfare Center (IHDIV) in Maryland, and Redstone Arsenal (Redstone) in Alabama, with varying groundwater geochemical characteristics. Major findings include the following:

1. A SERS sensor based on elevated Au ellipse dimer architectures was designed and developed for ClO₄^- with a detection limit of ~10^-6 molarity (M) (or 100 micrograms per liter [µg/L]). The performance of these sensors was evaluated and optimized through variation of their geometric characteristics (i.e., dimer aspect ratio, dimer separation, etc.).
2. Large-scale commercial production of SERS substrates via nanoimprinting technology was successfully demonstrated. This is a substantial step toward the commercialization of the SERS sensors and may potentially lead to significantly reduced fabrication costs of SERS substrates.
3. Commercially-produced SERS sensors were demonstrated to detect ClO₄^- at levels <10^‑6 M using a portable Raman analyzer. The performance of the commercial SERS sensors for ClO₄^- detection in the presence and absence of interferences was determined for a series of standard solutions.
4. Field demonstration of the portable Raman sensor with commercially-produced SERS substrates was completed twice at IHDIV and once at Redstone. Multiple wells were sampled at both sites, where a standard addition method was employed using the sensor to determine the ClO₄^- for each groundwater sample. Groundwater samples were also collected for method intercomparison with the standard ion chromatography (IC) approach. Results were generally comparable, although significant variations were observed in a small set of samples due to the presence of interference ions in the groundwater.

This is the first demonstration of a field portable SERS Raman sensor that combines a portable Raman analyzer with novel elevated Au ellipse nanostructural arrays. The technology shows the potential to provide a tool for rapid, in situ screening and analysis of ClO₄^- and possibly other energetics that are both important for environmental monitoring and of interest for national security.

As commonly observed with other analytical techniques, SERS technology is also prone to interferences due to its sensitivity and responses to other ionic species, such as NO₃^-, sulfate (SO₄^2-), and dissolved organics present in water, which could potentially mask the SERS signal of the target analyte (ClO₄^-). As such, SERS analysis could be subject to significant variations (e.g., ±20%). The biggest challenge is thus to reduce its variability due to the presence of various groundwater interferences so as to increase its detection limit. The reported ClO₄^- detection limit (~100 µg/L) and variability may not be suitable for routine quantitative analysis, particularly at low ClO₄^- concentrations. However, the portable Raman sensor developed in this project could be used as a rapid screening tool for ClO₄^- at concentrations >10^-6 M during site assessment work to aid more effective and timely decision-making during remediation projects. Future studies are warranted to further develop the technology and to optimize its performance, and eventually to bring the technology to market. With additional development and demonstration, the technology has the potential to reduce analytical costs by eliminating shipping and typical costs associated with laboratory analysis. A cost saving of 30–45% is estimated during a typical sampling event. The technology also allows rapid turnaround of information to decision makers for site characterization and remediation.