The objective of this project was to develop a cost-effective, miniaturized sensor for rapid field detection and monitoring of perchlorate (ClO4-), TNT, and RDX in contaminated groundwater or surface water via fiber-optic surface enhanced Raman spectroscopy (SERS) techniques. Miniaturized sensors have the potential to significantly reduce analytical costs for long term monitoring (LTM) and to provide real-time results during field assessment and remediation activities. Specific objectives were to (1) construct an integrated field-portable SERS-based Raman sensor; (2) design and develop sensitive and reproducible SERS substrates for energetics detection and analysis; (3) develop and optimize methodologies for detecting energetics in contaminated groundwater or surface water; and (4) evaluate the performance and cost effectiveness of the SERS technique for rapid field screening and monitoring of energetics at selected DoD sites.
This project built on recent advances and breakthroughs in the following three technologies: (1) development of a highly sensitive SERS substrate for detecting perchlorate at sub-ppb concentrations; (2) nanofabrication of highly ordered gold or silver SERS substrates; and (3) the availability of miniaturized or handheld Raman spectrometers that can be interfaced with a SERS probe. The combination of these technologies enables the construction of a field-portable Raman sensor. Success of this instrument, however, requires fabrication of sensitive and reproducible SERS substrates. Ordered, sensitive SERS substrates have been fabricated using both the template approach and electron beam lithography (EBL) based on nanofabrication techniques. These nanostructured SERS substrates are capable of detecting energetics with good sensitivity. Using molecular recognition techniques, the surface of gold nanoparticles were further modified with specific functional groups for improved sensitivity and selectivity. The resulting amplification of the SERS signal enables detection of energetics at low concentrations.
Through wet-chemical synthesis, sensitive gold nanoparticle-based SERS substrates were developed and their ability to detect ClO4-, TNT, and RDX at concentrations as low as 10-9 M (~ 0.1 µg/L), 10-8 M (~ 2.3 µg/L), and 5x10-7 M (~ 0.12 mg/L), respectively, was demonstrated. Combining nanofabrication steps of pattern definition by EBL, metal deposition, and reactive ion etching, researchers developed and demonstrated large SERS enhancement factors exceeding 1011 resulting from a new configuration of elevated gold bowtie arrays with controllable gap sizes to less than 8 nm. These new EBL-fabricated SERS substrates showed superior reproducibility and potential to be utilized for sensitive detection of energetics and other co-contaminants in groundwater and surface water. Researchers subsequently developed and constructed a miniaturized, field-deployable sensor by integrating the SERS with a portable Raman analyzer equipped with a 300-mW near infrared laser and a fiber-optic probe. The performance of the portable Raman sensor was evaluated for detection and analysis of energetics both in the laboratory and the field. The detection of ClO4-, TNT, and RDX at ppb to sub-ppm concentration levels in groundwater from a number of contaminated monitoring wells at several DoD military sites was shown. The performance and sensitivity of the new portable Raman sensor developed under this project is now being field-demonstrated, validated, and optimized under ESTCP project ER-201327.
The results of this project represent the first step in developing a SERS/Raman-based field sensor that combines a portable Raman spectrometer with novel elevated gold bowtie arrays. The technology shows the potential to provide a tool for rapid, in situ screening and analysis of energetics. Both labor and analytical costs associated with laboratory analysis may be eliminated, and a cost reduction of up to 50% may be realized by using SERS for LTM and analysis of multiple contaminants at DoD sites. The new technology also allows rapid turn-around of information to decision makers for site characterization and remediation.