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An experimental study of the photoinduced molecular reorientation of dyed liquid crystals for a set of guest-host combinations is reported. We find large variations in the magnitude of the effect for different dyes but also for different hosts, with polar hosts resulting often significantly more effective than nonpolar ones. The data are interpreted in terms of a kinetic mean-field model for the dye molecule rotational dynamics and interaction with the liquid crystal host. The results point to a significant variation of guest-host intermolecular forces upon photoinduced electronic excitation of dye molecules. This force variation is reflected in a variation of dye molecule physical parameters such as the rotational friction coefficient and the orientational mean-field energy.

The crystal structure of graphene flakes is expected to significantly affect their sensing properties. Here we report an experimental investigation on the crystalline structure of graphene aimed at exploring the effects on the gas sensing properties. The morphology of graphene, prepared via Chemical Vapor Deposition (CVD), Liquid Phase Exfoliation (LPE) and Mechanical Exfoliation (ME), is inspected through Raman spectroscopy, Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM). CVD and LPE-graphene structures are found to be more defective with respect to ME-graphene. The defects are due to the jagged morphology of the films rather than originating from intrinsic disorder. The flatness of ME-graphene flakes, instead, explains the absence of defects. Chemiresistors based on the three different graphene preparation methods are subsequently exposed to NO in the concentration range 0.1-1.5 ppm (parts per million). The device performance is demonstrated to be strongly and unambiguously affected by the material structure: the less defective the material is, the higher the response rate is. In terms of signal variation, at 1.5 ppm, for instance, ME-graphene shows the highest value (5%) among the three materials. This study, comparing simultaneously graphene and sensors prepared via different routes, provides the first experimental evidence of the role played by the graphene level of defectiveness in the interaction with analytes. Moreover, these findings can pave the path for tailoring the sensor behavior as a function of graphene morphology.

The concerns related to particulate matter’s health effects alongside the increasing demands from citizens for more participatory, timely, and diffused air quality monitoring actions have resulted in increasing scientific and industrial interest in low-cost particulate matter sensors (LCPMS). In the present paper, we discuss 50 LCPMS models, a number that is particularly meaningful when compared to the much smaller number of models described in other recent reviews on the same topic. After illustrating the basic definitions related to particulate matter (PM) and its measurements according to international regulations, the device’s operating principle is presented, focusing on a discussion of the several characterization methodologies proposed by various research groups, both in the lab and in the field, along with their possible limitations. We present an extensive review of the LCPMS currently available on the market, their electronic characteristics, and their applications in published literature and from specific tests. Most of the reviewed LCPMS can accurately monitor PM changes in the environment and exhibit good performances with accuracy that, in some conditions, can reach R2 values up to 0.99. However, such results strongly depend on whether the device is calibrated or not (using a reference method) in the operative environment; if not, R2 values lower than 0.5 are observed.

Here, we present a room temperature operating chemi-sensor based on a graphene film that shows sensitivity to NO2 up to a 50 parts-per-billion (ppb) with extremely limited interference from relative humidity and can be also calibrated in a sub-parts-per-million (ppm) range with a response and recovery time of few seconds. The device has been fabricated using as active material, a solution of graphene nanosheets suspended in N-methyl-pyrrolidone drop casted on an alumina substrate with gold interdigitated electrodes. The derivative of the device response is found to be univocally correlated to NO2 concentrations from 100 ppb up to 1000 ppb and the sensor can therefore be calibrated in this same range.