Volatile organic compounds (VOCs) are analytes of increasing interest due to the risks they pose to human health in terms of cardiovascular and carcinogenic complications. Current state of the art VOC detectors includes MEMS and semiconductor type sensors which suffer from cross sensitivity, drift, and poor selectivity between VOCs. Furthermore, these devices often require complex and costly fabrication and readout techniques. Holographic sensors based on various types of diffractive structures have been demonstrated previously as highly sensitive, robust, and versatile sensing devices for numerous analytes including temperature, humidity and VOCs, with simple and low-cost fabrication and operation characteristics. It has also been demonstrated that the sensitivity of the holographic structures can be enhanced through coupling into an optomechanical configuration, however this coupling was only investigated for unslanted volume transmission gratings. In this work, the sensitivity of a range of structures fabricated using holographic recording techniques is investigated. The response of volume transmission holographic gratings is studied in three different configurations – fixed gratings on glass slides, optomechanical holographic cantilever and optomechanical holographic membrane. The samples are tested in a vacuum chamber for response to air with the view to optimizing the most sensitive platform as a highly accurate VOC detector. The results further validate that the coupling of volume transmission holographic gratings with a mechanical transducing method enhances sensitivity. Furthermore, the study demonstrates that the membrane configuration has the potential to provide the most sensitive, stable, and repeatable sensing platform of the three presented.
Detecting volatile organic compounds (VOCs) is important, their presence in modern indoor environments being associated to health risks including respiratory diseases and cancers. State-of-the-art VOCs sensors as MEMS and semiconductor devices achieve high sensitivity but exhibit poor selectivity and high cross-sensitivity with other environmental analytes including temperature and humidity. Such sensors often require complex and costly fabrication/operation processes and/or expensive readout equipment. Here, a novel optomechanical sensing platform, based on the combination of a holographic diffractive element and a static deflection bilayer cantilever, is presented. Its operation principle is based on the differential response of the cantilever layers to target analytes, and was verified using COMSOL Multiphysics. The cantilever deflection due to analyte presence was visually measured. As the sensitive layer is a photopolymer, a transmission volume holographic diffraction grating was recorded enabling a second, more sensitive, detection mode based on the variations in the diffracted beam intensity as the cantilever deflection angle changes. We compared the sensitivity of the optomechanical holographic sensor configuration to that of a holographic diffraction grating in a photopolymer layer coated on a glass slide. Selectivity and sensitivity of both configurations was increased by doping the photopolymer matrix with zeolite nanoparticles. The initial tests monitored the diffraction efficiency changes during the 5 minutes exposure time to 1000 ppm ethanol. The TOS presented changes of 1–4% in diffraction efficiency depending on the dopant concentration and photopolymer layer thickness, while the optomechanical sensor exhibited 7–14% change in diffraction efficiency.
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