Picric acid (PA) is an organic substance widely used in industry and military, which poses a great threat to the environment and security due to its unstable, toxic, and explosive properties. Trace detection of PA is also a challenging task because of its highly acidic and anionic character. In this work, silver nanoparticles (AgNPs)-decorated porous silicon photonic crystals (PS PCs) were controllably prepared as surface-enhanced Raman scattering (SERS) substrates using the immersion plating solution. PA and Rhodamine 6G dye (R6G) were used as the analyte to explore the detection performance. As compared with single layer porous silicon, the enhancement factor of PS PCs substrates is increased to 3.58 times at the concentration of 10−6 mol/L (R6G). This additional enhancement was greatly beneficial to the trace-amount-detection of target molecules. Under the optimized assay condition, the platform shows a distinguished sensitivity with the limit of detection of PA as low as 10−8 mol/L, the linear range from 10−4 to 10−7 mol/L, and a decent reproducibility with a relative standard deviation (RSD) of ca. 8%. These results show that the AgNPs-modified PS PCs substrates could also find further applications in biomedical and environmental sensing.

Inspired by the enhanced gas-sensing performance by the one-dimensional hierarchical structure, one-dimensional hierarchical polyaniline/multi-walled carbon nanotubes (PANI/CNT) fibers were prepared. Interestingly, the simple heating changed the sensing characteristics of PANI from p-type to n-type and n-type PANI and p-type CNTs form p–n hetero junctions at the core–shell interface of hierarchical PANI/CNT composites. The p-type PANI/CNT (p-PANI/CNT) and n-type PANI/CNT (n-PANI/CNT) performed the higher sensitivity to NO2 and NH3, respectively. The response times of p-PANI/CNT and n-PANI/CNT to 50 ppm of NO2 and NH3 are only 5.2 and 1.8 s, respectively, showing the real-time response. The estimated limit of detection for NO2 and NH3 is as low as to 16.7 and 6.4 ppb, respectively. After three months, the responses of p-PANI/CNT and n-PANI/CNT decreased by 19.1% and 11.3%, respectively. It was found that one-dimensional hierarchical structures and the deeper charge depletion layer enhanced by structural changes of PANI contributed to the sensitive and fast responses to NH3 and NO2. The formation process of the hierarchical PANI/CNT fibers, p–n transition, and the enhanced gas-sensing performance were systematically analyzed. This work also predicts the development prospects of cost-effective, high-performance PANI/CNT-based sensors.

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Sycamore villus fibers were used to prepare hollow and porous carbon microtubes (CMTs) and the ZnO/CMT composite with heterojunctions by simple carbonization for the first time. Because the hollow and porous structure provided more channels to facilitate the adsorption and desorption of gas molecules, both CMTs and ZnO/CMT exhibited higher sensitivity and quicker response (<16 s) to and recovery (<2 s) from multiple target analytes. Furthermore, ZnO nanoparticles were uniformly dispersed on the CMTs by zinc acetate-assisted carbonization, which avoided the agglomeration of ZnO and formed a large number of heterojunctions, greatly improving the sensitivity of ZnO/CMT. In comparison to the pure CMTs and ZnO, the response of ZnO/CMT to the four target gases increased by 1.4∼4.3 and 9.9∼18.1 times, respectively. Their limit of detection for NH 3 was calculated as 62.5 and 8.8 ppb, respectively. After 30 days, the responses of CMTs and ZnO/CMTs to 500 ppm NH 3 decreased by 9.4 and 6.5%, respectively. This indicated that CMTs and ZnO/CMT had high sensitivity and good long-term stability. This study provides a feasible way for the gas-sensing application of biomass carbon materials with heterojunction structures.