Volatile organic compounds (VOCs), which originate from painting, oil refining and vehicle exhaust emissions, are hazardous gases that have significant effects on air quality and human health. The detection of VOCs is of special importance to environmental safety. Among the various detection methods, chemoresistive semiconductor metal oxide gas sensors are considered to be the most promising technique due to their easy production, low cost and good portability. Sensitivity is an important parameter of gas sensors and is greatly affected by the microstructure, defects, catalyst, heterojunction and humidity. By adjusting the aforementioned factors, the sensitivity of gas sensors can be improved further. In this review, attention will be focused on how to improve the sensitivity of chemoresistive gas sensors towards certain common VOCs with respect to the five factors mentioned above.

Semiconductor oxide chemoresistive gas sensors are widely used for detecting deleterious gases due to low cost, simple preparation, rapid response and high sensitivity. The performance of gas sensor is greatly affected by the morphology of the semiconductor oxide. There are many semiconductor oxide morphologies, including zero-dimensional, one-dimensional, two-dimensional and three-dimensional ones. The semiconductor oxides with different morphologies significantly enhance the gas-sensing performance. Among the various morphologies, hollow nanostructures and core-shell nanostructures are always the focus of research in the field of gas sensors due to their distinctive structural characteristics and superior performance. Herein the morphologies of semiconductor oxides and their gas-sensing properties are reviewed. This review also proposes a potential strategy for the enhancement of gas-sensing performance in the future.

Fluorescent molecular switches have great application potential in sensing and data storage. Materials that exhibit fluorescence switching properties both in solution and in solid state cannot be easily synthesized because of the contrasting ACQ and AIE effects. Herein, a dyad of pyrene-decorated spiropyran was designed via single bond linkage, and the material exhibited solvent polarity-correlated blue to green emission in solution and blue emission in solid state, although neither pyrene nor spiropyran emit in the visible light region. Acidic conditions could trigger the isomerization of spiropyran moiety to the protonated merocyanine (MCH) form accompanied by the disappearance of emission in solution. Crystalline Py–MCH exhibited red/dark “on/off” fluorescent switch upon grinding. Moreover, acid sensing and reversible fluorescent display were demonstrated based on Py–Sp-doped polymer nanofibers and films utilizing stimuli-responsive emission switching property.

Small-loop transient electromagnetic (TEM) method can be used for urban underground space detection. Given the mutual inductance between transmitting and receiving coils, the early secondary field signals are contaminated by primary field signals. To compensate for the primary field, bucking coil technology has been utilized in the airborne TEM system due to attractive features of the technology, which include a compensation effect and a strong coupling strength between the receiving coil and the underground body. However, many problems arise when the bucking coil technology is used directly in a ground small-loop TEM system. One is that the size of the receiving coil is limited, resulting in insufficient bandwidth; the other is that the bucking coil brings the near-field effect, which causes the loss of shallow information. To solve these problems, in this article, we propose a new TEM structure with noncoplanar bucking compensation. Compared with the traditional design, the new structure overcomes not only the near-field effect but also reduces the constraint of the receiving coil size. Furthermore, the approach can suppress the primary field coupling more effectively, and has a better fault tolerance for the installation accuracy. The simulation and field experiment results verify the effectiveness and practicability of the new structure.

This paper reviews recent progress on the wearable devices based on piezoelectric sensors. In recent years, many researches have turned eyes to the studies of wearable devices based on piezoelectric sensors because of the sensors' high sensitivity and efficiency. In addition, on account of their numerous advantages, the piezoelectric sensors have made great progress in medical monitoring domain. In this paper, the latest development for the use of piezoelectric sensors in medical devices will be reviewed briefly. The working principle of the piezoelectric sensors will be given. And then, the body parts that can be used for medical monitoring will be illustrated. After that, wearable medical devices made of different materials will be discussed, and their strengths and weaknesses will be analyzed. Finally, all the work listed in this article will be summarized and the future of piezoelectric sensors in the field of medical equipment will be expected.

Summary: The technique of surface nuclear magnetic resonance (SNMR) has been widely used for hydrological investigations in recent years. Unfortunately, the detected SNMR signals are limited to tens of nanovolts and are thus susceptible to environmental noise. While pre-polarization pulses to enhance the detected signal amplitudes are common in laboratory applications, SNMR field testing has only utilized excitation pulses until now. In conducting measurements in China, we demonstrate that adding a pre-polarization field to the SNMR pulse sequence is feasible and allows for the reliable detection of SNMR signals in noisy scenarios that otherwise prohibit signal detection. We introduce a forward modelling for pre-polarization using SNMR and present a three-layer model obtained from inverse modelling that satisfies the observed data from the field experiment. We expect this development to open up new applications for SNMR technology, especially in high-noise level places, such as active mines.