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Surface-Enhanced Raman Scattering Substrate Development


OBJECTIVE: To develop a metallic surface-enhanced Raman scattering (SERS) substrate to be utilized for augmentation of current/future Raman spectroscopic portable instrumentation for the detection of trace and residual chemical materials. The substrates should consist of nanostructured metals, preferably gold or silver, on a porous or non-porous material backing (such as filter paper, silicon, gallium nitride, etc.), with no less than 3 mm x 3 mm and no larger than 4 x 4 mm active SERS area providing the SERS enhancement, and be useable with minimally 633 nanometer (nm) or 785 nm excitation. DESCRIPTION: Raman spectroscopy has proven to be a reliable field deployable detection technique for assessing chemical threats, including chemical warfare agents, energetic materials, and illicit narcotics. Military and Homeland Security agencies commonly utilize various portable Raman systems in sensitive site exploitation, checkpoint scenarios, and to determine hazardous content on surfaces or containers. Enhanced Raman techniques, such as surface-enhanced Raman scattering (SERS) have been demonstrated to be a vibrant field of research that is growing significantly in scope and applicability while pushing at the ultimate limits of sensitivity. SERS occurs when nanometallic substrates locally amplify electromagnetic fields at or near particle surfaces providing enhancements over ‘normal’ Raman spectroscopy, typically over a million-fold. Along with other advantages such as reduction of interfering fluorescence, decreased detection times, and reduction of laser power required for analysis, SERS has been positioned to be an ideal technique for low-level, low-consumable detection schemes, while aiming towards miniaturization of instrumentation. The problem to date, however, is the lack of commercially available robust SERS active substrates that have an inherent low background signature which ultimately interferes with obtaining clean SERS spectra from low-level concentrations of threat analytes, while still having at least 104 SERS enhancement. The goal of this topic and the resulting research is to develop miniature metal-based surface-enhanced Raman spectroscopy substrates which could be manufactured at a large scale, while retaining both low-level baseline signatures (native background peaks are minimal) and low contaminant levels, to be utilized in various chemical and biological detection scenarios for augmentation of portable Raman technologies. PHASE I: Develop a conceptual design for the surface-enhanced Raman substrate detailing the technical feasibility of the proposed design and production of the substrate. Technical feasibility shall be demonstrated through modelling, production capability infrastructure, proposed optimal (633 nm or 785 nm) and non-optimal wavelength (< 400 nm or >800 nm) use, and theoretical shelf-life. This demonstration will elucidate the minimal SERS background spectral features when exposed to clean de-ionized water for a minimum of 10 minutes. The demonstration will also provide an estimated SERS enhancement value to be equal to or greater than 104. Use of 1,2-bis(4-pyridyl)-ethylene to determine the SERS enhancement value is encouraged. Of importance is a clean substrate with minimal production/manufacturing contamination present, so that the maximum potential exists for the binding of typically weakly bound analytes. Demonstration of technical feasibility in Phase I is required for consideration of a Phase II project award. PHASE II: Following technical feasibility demonstration of the Phase I requirements, the small business shall develop manufacturing protocols for the design and delivery of 500 substrates after 10 months, and 1000 SERS substrates after 24 months, meeting the goals of a 104 or better enhancement with native surface background Raman features (with no analyte present) not exceeding 3 times the background noise level with the same laser power and integration time with which a SERS Raman spectrum is obtained. The purposes of a low native surface background are both to reduce spectral interference and to maintain the maximum number of possible available binding sites for user introduced analytes. Also, spectral reproducibility characteristics of the substrates need to be within 30% for a measured analyte over 50 individual substrate measurements (analyte to be determined) obtained by comparison of peak areas across the measurements. The substrates will be tested by U.S. Army DEVCOM-CBC for requirement compliance. PHASE III: Following successful delivery of 1000 SERS substrates meeting the performance characteristics in Phase II, protocols for scale-up manufacturing will be developed in order to deliver thousands of substrates which can be utilized in various chemical and biological detection applications for the augmentation of field portable Raman spectroscopy systems. Methods for QA/QC will be developed to ensure standardization during mass production. In addition, packaging for shipment will be developed with the goal of protecting the substrates and minimizing additional contamination. PHASE III DUAL USE APPLICATIONS: In addition to use for the Department of Defense (DoD) low-level chemical detection scenarios, the designed SERS surfaces have commercialization activity for low-level explosive detection and biological detection for civilian uses by first responders and law enforcement personnel. DoD uses could include sensitive site exploitation, explosives detection, post decontamination survey and verification, and may serve as a technology upgrade for current and future portable Raman spectroscopic technologies. Civilian uses could include identification of illicit drugs and inspection of food products and/or hazardous waste containers. REFERENCES: 1. Emmons, E. D., Guicheteau, J. A., Fountain III, A. W., Tripathi, A. “Effect of Substituents on Surface Equilibria of Thiophenols and Isoquinolines on Gold Substrates Studied Using Surface-Enhanced Raman Spectroscopy”. Phys. Chem. Chem. Phys. 2020, 22, 15953-15965. 2. Tripathi, A., Emmons, E. D., Kline, N. D., Christesen, S. D., Fountain III, A. W., and Guicheteau, J. “Molecular Structure and Solvent Factors Influencing SERS on Planar Gold Substrates”, J. Phys. Chem. C. 2018, 122 (18), 10205–1021. 3. Guicheteau, J. A., Tripathi, A., Emmons, E. D., Christesen, S. D., Fountain III, A. W. “Reassessing SERS enhancement factors: using thermodynamics to drive substrate design”. Faraday Discuss. 2017, 205, 547-560. 4. Guicheteau, J. A, Farell, M. E., Christesen, S. D., Fountain III, A. W., Pelligrino, P. M., Emmons, E. D., Tripathi, A., Wilcox, P., Emge, D. “Surface-enhanced Raman Scattering (SERS) Evaluation Protocol for Nanometallic Surfaces”. Appl. Spec. 2013, 67, 4, 396-403. KEYWORDS: Surface-enhanced Raman Spectroscopy; SERS; Metallic Nanostructures; Chemical Detection
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