Farhan Sadik Sium

This paper presents the design, fabrication, and characterization of a sub-milliwatt graphene-based micro thermal conductivity detector (µTCD) that utilizes a suspended multilayer graphene (MLG) bridge to sense volatile organic compounds (VOCs) in the gas phase based on their thermal transport properties. The graphene bridge is transferred onto a silicon chip with integrated microchannels using a photolithography-free process. By incorporating microchannel designs, this approach enables precise transfer of suspended MLG dimensions without conventional patterning steps. A key innovation of this work lies in the use of an ultra-low thermal mass suspended graphene architecture, which significantly increases temperature rise per unit input power, thereby enhancing sensitivity per unit power compared to conventional metal-based TCDs. The fabricated µTCD successfully produces chromatograms of multiple VOC species, closely matching those obtained using a standard flame ionization detector (FID). The device demonstrates an estimated limit of detection (LOD) of 190 ppm while consuming an average power of 151 µW under DC operation.

Monitoring dissolved carbon dioxide (CO₂) in aquatic and sediment systems is critical for understanding carbon cycling and climate feedback. This study develops and characterizes a compact, low-cost microbolometer-based attenuated total reflectance (ATR) mid-infrared sensor for direct dissolved CO₂ measurement in liquid and soil-water environments. The system integrates a ZnSe ATR crystal with custom suspended SiN membrane microbolometers and uses evanescent-wave absorption at 4.26 μm with a broadband LED source and computational spectral reconstruction, eliminating the need for an interferometer. Calibration shows excellent linearity (R² ≈ 0.99) over 50–1000 ppm CO₂, with a practical limit of detection (LOD) of ~26–35 ppm at 5–25 °C. UV-induced CO₂ generation from soil-water mixtures was investigated across UV wavelengths, revealing UV-C (254 nm) as optimal, producing net ΔCO₂ ≈ 339 ppm above ambient levels in 30 min. Environmental factors (temperature 5–35 °C, pH 5–11, pressure 1–1.5 ATM, dissolved organic carbon) were systematically evaluated, confirming robust sensor performance (accuracy >90%, correlation r > 0.98 with reference instrument). This sensor represents the first integration of MEMS microbolometer detectors with ATR evanescent-wave spectroscopy for liquid-phase dissolved CO₂, enabling real-time monitoring and rapid sediment organic carbon assessment in a field-deployable platform.

We present a new palladium-based nanogap hydrogen sensor designed for fast, low-concentration H₂ detection in environmental monitoring applications. The device consists of a long electrically-conductive low-stress palladium cantilever beam separated by a nanometer-sized air gap from a second cantilever beam of different length placed near the first beam tip thus forming a H₂-dependent capacitor. The cantilevers are anchored on top of a thermally-isolated SiN membrane with an integrated microheater that maintains the sensor at constant temperature. When exposed to H₂, the capacitor nanogap is reduced resulting in a change in capacitance with H₂ sensitivity $S_{\mathrm {H2}} =0.15$ fF/ppm. The sensor shows a consistent response up to $55~^{\circ }$ C with a thermal drift smaller than 0.22 aF/^oC and a minimum detectable signal of 10 ppm. The device repeatability was confirmed across 5 consecutive test cycles. Selectivity tests show negligible response to common interfering gases CO, CO₂, CH₄, water vapor and NH₃ ( $S_{i}\lt 5.2\times 1{0}^{-6}$ fF/ppm). The sensor exhibited a response time of approximately 390 ms upon exposure to H₂.[2025-0141]

This letter presents a uniform 5-nm Pd-Au nanogap sensor, fabricated using a batch-fabrication wafer-scale process, enabling sub-ppm hydrogen (H2) detection for operating at room temperature with ultra-low-power operation in an in-field-deployable sensor node. The nanogap sensor was integrated into a compact size of 58 × 52 mm 2 that combines a Pd-Au nanogap sensor, signal processing unit, and wireless communication unit, while all units being powered by a coin cell battery (220 Ah). The nanogap sensor unit operates in a normally off mode, consuming 12 pW in standby, while the complete system including processing and communication units consumes 51.40 µW. Upon detecting ≥0.46 ppm H2, the Pd layer expands to close the nanogap, switching the node on to an active the state to wirelessly transmit detection alerts. This wake-up operation reduces average power consumption to 67.84 µW with one detection event per week, supporting up to 59 weeks of operation. At 1 ppm of H2, the sensor shows a response time of ≤5 seconds for detection and a recovery time of ≤3 minutes for resetting, thus, multiple H2 leak measurements can be achieved. The development of this compact Pd-Au nanogap sensor node is expected to advance the safety of H2 transportation and storage infrastructure where even sub-ppm levels of highly flammable H2 gas leaks is concerned.