Polymer sorbents able to selectively capture specific classes of analytes have attracted significant interest in the context of photonic sensors (particularly waveguide-enhanced Raman spectroscopy, or WERS) for chemical warfare agents and industrial gases. We have developed a method for using aminopropyl methylsiloxane-dimethylsiloxane copolymers as inexpensive starting materials for the synthesis of new sorbents for chemical and biochemical sensing. Conversion of the starting polymer to a product sorbent can be accomplished via a simple, single-step reductive amination reaction with an aldehyde. Preliminary tests of a sorbent in the context of refractive index-based sensing of energetic compounds (explosives) using silicon nitride micoring resonators is also discussed.
Detection of threat materials is an important capability for the military and homeland security to protect soldiers and civilians. Waveguide-enhanced Raman spectroscopy (WERS), a photonic integrated circuit sensing methodology, is being developed for field detection of materials related to chemical warfare agents, explosives, and narcotic threats. Low-fluorescence silicon nitride spiral waveguides with long path lengths are used to obtain high signal levels with nearinfrared excitation (785 nm and 1064 nm). Compact single-mode-fiber-coupled spectrometers with high sensitivity are being utilized for detection of the Raman scattered light. Thermoelectrically cooled charged coupled device (CCD) or InGaAs detectors (-15 °C) provide for low-noise and high-quantum-efficiency spectral measurement. Performance comparable to that obtained with large benchtop spectrometers is observed. The spiral waveguides are coated with functionalized polymer sorbents suitable for concentrating relevant classes of threat materials in the evanescent field of the waveguide. The sorbents are deposited using piezoelectric microdispensers to allow for controlled deposition of thin films without the need for spin-coating. Raman chemical imaging microscopy is used to characterize the uniformity of the sorbent polymers on the waveguides. Library spectral matching can be used in combination with the selectivity of the sorbent materials to provide discrimination of the materials absorbed by the polymer coatings. The ultimate objective is development of a prototype handheld WERS sensor system suitable for defense and security applications in the field. WERS development and spectral measurements will be presented.
Detection of antibodies to upper respiratory pathogens is critical to surveillance, assessment of the immune status of individuals, vaccine development, and basic biology. The urgent need for antibody detection tools has proven particularly acute in the COVID-19 era. Array-based tools are desirable as methods for assessing broader patterns of antigen-specific responses, as well as providing information on SARS-CoV-2 immunity in the context of pre-existing immunity to other viruses. Also, methods that rapidly and quantitatively detect antibody responses to SARS-CoV-2 antigens using small (fingerstick) quantities of blood are essential for monitoring immunity at a global scale. This talk will describe the development of two optical sensor platforms (Arrayed Imaging Reflectometry, and an integrated photonics platform fabricated at AIM Photonics) for quantifying antibodies to SARS-CoV-2 and other upper respiratory pathogens, and oriented towards the needs of multiplex detection and speed.
Waveguide-enhanced Raman spectroscopy (WERS) enables the detection and identification of trace concentrations of vapor-phase analytes using a functionalized chip-scale photonic circuit. Here, we show that WERS signal can be collected from part-per-billion levels of targeted analytes in a backscatter geometry, which, compared to forward-scatter, simplifies component integration and is more tolerant of waveguide loss and modal interference. In addition, we discuss our progress towards a compact Raman sensing system that incorporates a handheld spectrometer and chip-scale optical filters. We demonstrate that a handheld, thermo-electrically cooled spectrometer can be used for backscatter WERS with a comparable signal-to-noise to that of a liquid-nitrogen cooled benchtop spectrometer. Finally, we describe efforts to integrate the dichroic Raman filter on-chip using arrays of unbalanced Mach-Zehnder interferometers. Measurements show filter performance sufficient for integration with WERS: Transmission of >80% of the laser in the cross port and Stokes signal in the through port; and extinction of the laser by >20 dB in the though port and of Stokes signal by >8 dB in the cross port.
Waveguide-enhanced Raman spectroscopy (WERS) enables the detection and identification of trace concentrations of vapor-phase analytes using a chip-scale photonic circuit coated with a sorbent material. Previous demonstrations of WERS utilized a hydrogen-bond acidic hyperbranched carbosilane fluoroalcohol-based sorbent polymer and focused on detection limits for different nerve agent simulants. In this work, we examine the Raman spectra of a number of new sorbent materials obtained using WERS. By comparing the spectra pre-exposure to the modified spectra measured during analyte exposure, the effects of hydrogen-bonding on the sorbent and analyte molecules are observed. Changes to the Raman transition strength or frequency of individual lines due to analyte binding shed light on the partitioning of vapor-phase molecular agents into the sorbent, and can be used to design sorbent materials with even higher sensitivity. We examine two new types of sorbents: Fluorinated bisphenol-based materials that increase the steric bulk of the substituents ortho- to the hydroxyl group, designed to reduce self-binding; and carbosilane fluoroalcohol polymers synthesized with a novel hydrosilylation reaction. The WERS detection limits for these new sorbents are measured for nerve-agent simulants and compared to previous generation materials.
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