Developing novel holographic optomechanical sensing platform for detection of volatile organic compounds

Abstract

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.

Developing novel holographic optomechanical sensing platform for application in volatile organic compounds detection

“…intensity of the beam diffracted by the volume grating recorded in the active layer of the cantilever. The principle of operation of the cantilever sensing configuration has been previously described [20], [21], [27]. In this work, a membrane sensor is presented.…”

“…The cantilever and fixed grating sensor fabrication is as previously described [21] All holographic sensors presented in this work have in common a photopolymer layer of thickness of 60 μm, 2.5% doping with zeolite nanoparticles and a spatial frequency of 800 lines/mm. In previous investigations, 2.5 % doping was found to be most effective and minimize scattering.…”

“…The sensors were tested in a controlled gas testing chamber as previously described [21]. In this work, the response of the sensors going from a vacuum in the chamber to air was tested in 5 minute cycles.…”

“…The intensity of the diffracted beam was measured in the case of the fixed grating and membrane samples, the cantilever sensitivity was measured through monitoring of the zeroorder intensity. The cantilever and fixed grating testing procedure has been detailed [21], the membrane testing process is similar. The first order intensity is first allowed to stabilize under a vacuum condition, the probe beam angle is adjusted so that the intensity diffracted to the first order is approximately 50% of the maximum, this is to allow the diffracted beam intensity to remain in the linear portion of the Bragg selectivity curve.…”

“…It has been demonstrated that coupling optical detectors, including holographic sensors with a mechanical transducer to form an optomechanical sensor enhances the sensitivity of both methods of transducing when comparing each of the methods in isolation [18] Previously, an optomechanical holographic sensor has been presented in the form of a bilayer cantilever sensor with demonstrated sensitivity for relative humidity [19], [20]. Recently, this device has undergone initial testing for sensitivity to VOCs, with the cantilever configuration demonstrating enhanced sensitivity to both air and ethanol when compared with a holographic grating recorded in a polymer layer coated a on glass slide (fixed grating) [21] However, challenges remain with the cantilever configuration presenting issues in terms of stability, repeatability and cross-sensitivity to other environmental analytes. In this work, a membrane volume holographic sensor is proposed.…”