This paper describes the theory and results for a new class of low-cost chemoresistive gas sensors designed for selective hydrocarbon gas detection. The sensors utilize a multiwalled carbon nanotube (MWCNT) backbone functionalized with metal oxide nanocrystals. Specifically, nanoparticles were grown on the surface of the MWCNTs using atomic layer deposition. The crystallinity of the ZnO-MWCNTs’ heterostructure was examined by using a high-resolution transmission electron microscope. The structure of the ZnO/MWCNTs was analyzed using a scanning electron microscope and energy dispersive x ray. The Hall effect measurement shows p-type characteristics of the MWCNTs, supporting the typical PN junction formation with n-type ZnO nanocrystals. The electron-donating ability of ZnO provided a strong response to the ppm levels of toluene at room temperature (25 °C) and showed strong selectivity with other volatile organic compound gases such as benzene, methane, and formaldehyde.

Photonic Crystals (PhCs) are periodically structured dielectric materials that have attracted significant research interest in the last two decades for their ability to slow down group velocity of a propagating pulse envelope with promising sensing applications. This review paper discusses the properties of two-dimensional (2D) PhCs including the slow-light phenomenon, band-gap generation and application of these properties for gas and liquid sensing. Waveguide generation by introducing defects with light guiding and confinement is discussed. Additionally, for 2D PhC line waveguides, a comprehensive review on slow-light principle and phenomenon of slow-light enhanced sensing for gases and liquids is discussed. One-dimensional (1D) and three-dimensional (3D) PhCs are also reviewed for band-gap generation and defects in PhCs along with present fabrication challenges and future trends. Our study highlights an increase in the detection capabilities of PhC based sensors paving way for high sensitivity detectors with applications in ubiquitous monitoring of gases and liquids.

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Photonic Crystals (PhC) are periodically structured dielectric materials that have been subject to extensive research efforts over the last two decades. PhC are known for the slow-light phenomenon, which increases the interaction time between light and the target gas, thereby enhancing sensitivity when applied to sensors. Slow light is best realized using three-dimensional (3D) PhC with a complete photonic band-gap (PBG). However, they are inherently difficult to fabricate with planar microfabrication techniques. In this work, we present two-photon polymerized (2PP) stereolithographically fabricated 3D PhC targeting midinfrared (MIR) spectroscopic range. Finite Element Analysis (FEA) was performed on a two-dimensional (2D) PhC waveguide (PhCW) to analyze the significance of PBG and slow light properties. Additional FEA was conducted and validated experimentally using fabricated 3D PhC. The ability to tune PhC to a desired wavelength along with their repeatability and feasibility is experimentally demonstrated. This paper presents the first, to our knowledge, 3D PhC fabricated using 2PP stereolithography operating at this wavelength.

Exposure to respirable coal dust and diesel exhaust in underground coal mines can cause detrimental airway diseases such as coal worker's pneumoconiosis (CWP), silicosis, and lung cancer. In this paper, we present the design, fabrication, and experimental evaluation of a low-cost wearable respirable dust monitor (WEARDM) which uses a dual-resonator gravimetric sensing approach for real-time measurement of respirable airborne particulate matter (PM) concentrations. The sensor selects for the ISO respirable dust fraction using a miniature virtual impactor and removes moisture from the collected dust to ensure accurate mass measurement. WEARDM uses a novel dual-resonator mass sensor which is composed of a quartz crystal microbalance (QCM) and a film bulk acoustic resonator (FBAR). The QCM measures the mass concentration of particles generated from coal mining operations (typically >2.5 µm A.D.), separated using inertial impaction. Thermophoretic precipitation is used to deposit the fine and ultrafine particles, such as emitted from diesel sources (typically <0.1 µm A.D.) on FBAR. This allows the WEARDM system to maintain large dynamic range and uniform collection efficiency across the entire respirable fraction. The WEARDM system is optimized for a low flow rate of 250 ml/min which results in low power usage and a small form factor, and is an order of magnitude smaller and less expensive than currently available devices.