Abstract
Alcoholic and phenolic hydrogen-bond (HB) acidic absorbents activated by fluorine chemistries have been previously developed at the U.S. Naval Research Laboratory and elsewhere to augment sorbent HB acidity, reduce sorbent HB basicity and target complementary HB basicity of a wide range of hazardous target chemicals. A significant limitation of HB sorbents developed to date has been the propensity for sorbent self-association. This intermolecular bonding between sorbent molecules hinders sorbate access to active HB acid sites in the sorbent, limiting its overall efficacy. Sorbent self-association is evidenced by broadened hydroxyl peaks in the mid-infrared (MIR) region, which can obscure important absorption frequencies that appear upon absorption of an analyte into the sorbent. In this current work our aim is to develop improved HB acidic sorbent compounds which minimize undesired sorbent self-association in order to apply these sorbents to infrared- and Raman-based sensing devices for chemical threat detection.
A series of new HB acidic sorbents have been synthesized and the subsequent sorbent-sorbate interactions have been characterized by a number of methods. MIR spectroscopy has been used to help elucidate sorbent-analyte vapor interactions. The newly synthesized sorbents have been challenged with various analyte vapors, including toxic industrial chemicals, chemical warfare agent simulants, and background interferents. A particular focus of these characterization efforts has been to observe the spectral changes that occur in the hydroxyl region of the MIR upon exposure of a sorbent material to an analyte vapor. Analyte binding of an HB base occurs principally at the sorbent hydroxyl site. The resulting redshift of the hydroxyl stretching frequency is characteristic of the basicity of the analyte.
These sorbent materials are specifically designed to be selective toward hazardous chemicals through complementary hydrogen-bonding interactions between sorbent and analyte molecules. Generally, common interferents, such as hydrocarbons, have little or no hydrogen-bond basicity, while hazardous chemicals of interest have moderate to high basicity. More strongly HB basic analytes trigger larger redshifts of the hydroxyl absorption frequency. Benchtop FTIR characterization has confirmed that these newly designed sorbent materials are responsive to threat chemicals of interest at low concentrations and largely unresponsive to interferent chemicals, even at relatively high concentrations.
Based on the strong affinity of these sorbents to threat chemicals of interest and the significant spectral changes that occur in the MIR upon formation of the hydrogen-bonded complex, these sorbent materials make useful candidates for MIR sensing applications. A frequency shift of the hydroxyl stretch indicates a sorbate has formed a HB with the sorbent. The magnitude of the frequency shift correlates with the basicity of the analyte, which is indicative of the class of compound to which the newly bound chemical belongs. In a sensing application, this feature can provide an alert that a hazardous chemical is present, even if it is an unknown threat. While the hydroxyl region allows for class specificity of unknown compounds, the fingerprint region complexity may facilitate specific analyte recognition. At present, this work has been focused on analysis of the hydroxyl region and distinction of different classes of compounds, but future efforts will turn to the fingerprint region to provide an avenue for specific chemical identification.
Raman spectroscopy has also been used to characterize these sorbent materials. Specifically, a technique known as waveguide-enhanced Raman spectroscopy (WERS) has been used, which features the use of highly evanescent, low-loss waveguides with the sorbent material as a top cladding.2 WERS can be achieved using an incredibly small footprint with a sorbent-functionalized nanophotonic waveguide that is approximately a few centimeters long. Using WERS, the differential Raman spectra of the sorbent material interacting with different chemical warfare agent simulants has been measured at parts-per-billion detection levels. The spectra exhibit extrapolated three-sigma detection limits as low as 3 ppb. Continuing efforts are focused on adapting this technique to photonic integrated circuit-based fabrication and chip-scale Raman spectroscopy for trace chemical vapor detection.
This presentation will highlight the design of these next-generation sorbents as a tool to facilitate MIR- and Raman-based sensing of threat chemicals. It will focus on analyzing sorbent-analyte spectral interactions and discuss how to exploit these features to develop MIR- and Raman-based sensors.
References:
1. Roberts, C. A.; McGill, R. A. Bisphenol hypersorbents for enhanced detection of, or protection from, hazardous chemicals. U.S. Patent Application 2019/0134601 A1, 2019.
2. Tyndall, N. F.; Stievater, T. H.; Kozak, D. A.; Koo, K.; McGill, R. A.; Pruessner, M. W.; Rabinovich, W. S.; Holmstrom, S. A. Optics Letters, 2018, 43, 4803-4806.