Kaiping Yuan

Tunable diode laser absorption spectroscopy (TDLAS) has been widely applied in industrial gas detection, but traditional calibration methods face challenges including high standard gas consumption, exorbitant costs, and heavy calibration workloads—particularly under wide temperature and pressure ranges, where variations in laser parameters can introduce significant errors. This study proposes a simplified calibration method for peak-locking TDLAS systems based on wavelength modulation spectroscopy (WMS). First, a coupling relationship model between the modulation index (m) and amplitude modulation (AM) is established, and an iterative calibration method is developed to synchronously adjust the laser operating current and temperature, enabling both m = 2.2 and AM = 0.1 to converge to their target values simultaneously, thereby eliminating the need for per-device temperature–pressure–concentration ternary calibration. Subsequently, the effects of quadratic nonlinearity in the laser power–current (P-I) and wavenumber–current (v-I) curves on detection accuracy are systematically analyzed for both 2f/1f and 4f/1f WMS representation methods. The simulation results obtained within a temperature range of 300 K ± 100 K and a pressure range of 1 atm ± 0.2 atm show that, for 2f/1f-WMS, a v-I nonlinearity rate of ±5% yields a maximum detection error of no more than 0.11% (with the error peak occurring in the 40–65% concentration range and no zero offset), while a P-I nonlinearity rate of ±5% results in a maximum error of no more than 0.82%, nearly independent of concentration. By comparison, the 4f/1f-WMS scheme effectively suppresses the zero offset induced by P-I nonlinearity, achieves lower detection errors, and requires only span calibration for each device. The proposed method significantly reduces the calibration workload and cost of TDLAS systems while ensuring detection accuracy.

The application of oxygen sensors based on Tunable Diode Laser Absorption Spectroscopy (TDLAS) in the industrial field has received extensive attention. However, most of the existing studies construct detection systems using discrete devices, making it difficult to apply them in the industrial field. In this work, through the optimization of the sensor circuit, the size of the core components of the sensor is reduced to 7.8 × 7.8 × 11.8 cm³, integrating the laser, photodetector, and system control circuit. A novel integrated optical path design is proposed for the optical mechanical structure, which enhances the structural integration and long-term optical path stability while reducing the system assembly complexity. The interlocking design of the laser-driven digital-to-analog converter (DAC) and photocurrent acquisition analog-to-digital converter (ADC) reduces the requirements of the harmonic signal extraction for the system hardware. By adopting a high-precision ADC and a high-resolution pulse-width modulation (PWM), the peak-to-peak value of the laser temperature control noise is reduced to 2 m°C, thereby reducing the detection noise of the sensor. This oxygen detection system has a minimum response time of 0.1 s. Under the condition of a 0.5 m detection optical path, the Allan variance shows that when the integration time is 5.6 s, the detection limit reaches 53.4 ppm, which is ahead of the detection accuracy of similar equipment under the very small system size.

Flexible room-temperature gas sensors with good adaptability to various surfaces and low energy consumption hold a great promise for different applications such as environmental monitoring and human exhalation detection. However, the development of flexible sensing devices with stable and superior monitoring capability are still restricted by the insufficiently selectivity or sensitivity at low temperatures. In this study, a simultaneous etching and oxidation polymerization strategy is developed toward the synthesis of porous polypyrrole (PPy)/Ti₃C₂T_x MXene nanocomposites. In the nanocomposites, PPy nanosheets align vertically on the surface of Ti₃C₂T_x, forming numerous open and porous channels. As a result, the PPy/Ti₃C₂T_x-based flexible sensor shows highly exposed active sites and superior ammonia sensing performances at room temperature with good selectivity, high sensitivity, fast response rate and a low detection limit of 375 ppb. The sensor can exhibit stable and reliable sensitivity under different bending angles and even after 100 bending cycles. Moreover, the PPy/Ti₃C₂T_x nanocomposites can be integrated into a portable flexible wireless sensor for remote and real-time ammonia monitoring, exhibiting great potentials in various applications

The emission of harmful gases has seriously exceeded relative standards with the rapid development of modern industry, which has shown various negative impacts on human health and the natural environment. Recently, metal–organic frameworks (MOFs)-based materials have been widely used as chemiresistive gas sensing materials for the sensitive detection and monitoring of harmful gases such as NO_x, H₂S, and many volatile organic compounds (VOCs). In particular, the derivatives of MOFs, which are usually semiconducting metal oxides and oxide–carbon composites, hold great potential to prompt the surface reactions with analytes and thus output amplified resistance changing signals of the chemiresistors, due to their high specific surface areas, versatile structural tunability, diversified surface architectures, as well as their superior selectivity. In this review, we introduce the recent progress in applying sophisticated MOFs-derived materials for chemiresistive gas sensors, with specific emphasis placed on the synthesis and structural regulation of the MOF derivatives, and the promoted surface reaction mechanisms between MOF derivatives and gas analytes. Furthermore, the practical application of MOF derivatives for chemiresistive sensing of NO₂, H₂S, and typical VOCs (e.g., acetone and ethanol) has been discussed in detail.

Dynamic coassembly of block copolymers (BCPs) with Keggin-type polyoxometalates (POMs) is developed to synthesize heteroatom-doped tungsten oxide with controllable nanostructures, including hollow hemispheres, nanoparticles, and nanowires. The versatile coassembly in dual n-hexane/THF solvent solution enables the fomation of poly(ethylene oxide)-b-polystyrene (PEO-b-PS)/POMs (e.g., silicotungstic acid, H₄SiW₁₂O₄₀) nanocomposites with different morphologies such as spherical vesicles, inverse spherical micelles, and inverse cylindrical micelles, which can be readily converted into diverse nanostructured metal oxides with high surface area and unique properties via in situ thermal-induced structural evolution. For example, uniform silicon-doped WO₃ (Si-WO₃) hollow hemispheres derived from coassembly of PEO-b-PS with H₄SiW₁₂O₄₀ were utilized to fabricate gas sensing devices which exhibit superior gas sensing performance toward acetone, thanks to the selective gas-solid interface catalytic reaction that induces resistance changes of the devices due to the high specific surface areas, abundant oxygen vacancies, and the Si-doping induced metastable ε-phase of WO₃. Furthermore, density functional theory (DFT) calculation reveals the mechanism about the high sensitivity and selectivity of the gas sensors. On the basis of the as-fabricated devices, an integrated gas sensor module was constructed, which is capable of real-time monitoring the environmental acetone concentration and displaying relevant sensing results on a smart phone via Bluetooth communication.

Transition metal oxides/noble metal (TMOs/NM) nanocomposites are one kind of important material for semiconductor gas sensors. The controllable construction of a highly connected mesoporous structure and easily accessible active sites is essential for gas sensing performance but remains a great challenge. Herein, a soluble mercapto phenolic resin polymer mediated co‐assembly approach is proposed for the construction of ordered dual mesoporous structure and the simultaneous loading of highly dispersed noble metal nanoparticles. The home‐made soluble mercapto phenolic resin polymer enabled the co‐assembly of transition metal precursors, noble metal precursors, and poly(ethylene oxide)‐block‐polystyrene (PEO‐b‐PS) micelles, resulting in a straightforward synthesis of ordered dual‐mesoporous TMOs/NM nanocomposites (e.g., WO₃/Au, TiO₂/Au, NbO_x/AuPd). As proof of the concept, the synthesized dual‐mesoporous WO₃/Au materials are applied for sensing of 3‐hydroxy‐2‐butanone, a biomarker of food‐borne pathogenic bacteria Listeria monocytogenes. The sensors exhibit high sensitivity (R_a/R_g = 18.8 to 2.5 ppm) and high selectivity based on their noble metal sensitization and superior mesopore connectivity for gas diffusion. Furthermore, the synthesized gas sensors are integrated into a wireless sensing module connected to a smartphone, providing a rapid and convenient real‐time detection of 3‐hydroxy‐2‐butanone.