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The current study describes the excellent sensing properties of zinc oxide samples with varying La doping concentrations (1, 3, and 5 %). The X-ray diffraction (XRD) study suggests that the produced ZnO samples exhibit hexagonal structure. Due to varying concentrations of La, a notable difference in the morphological image of the samples was observed using a Field Emission Scanning Electron Microscope (FESEM). The existence of La, Zn, and O elements in the sample was analyzed by Energy Dispersive X-Ray Analysis (EDX). The Fourier Transform Infrared Spectroscopy (FTIR) studies validate the sample's structure and functional group. The UV–Vis results showed that the maximum optical reflectance of 90 % was attained in the visible region and the obtained band gap values for the samples were in the range of 3.27–3.33 eV. At ambient temperature, the La-doped samples demonstrated high sensitivity towards ammonia (NH 3) and had the highest response value of 1940 % for the 250 ppm. According to the results, an excellent operating stability, and response/recovery time were obtained for the ZnO:La1% sample which suggests that the sample can serve as the most appropriate metal oxide for the detection of NH 3 at room temperature. [ABSTRACT FROM AUTHOR]

Beads free polyvinyl alcohol (PVA)/NiO nanofibers with an average diameter of 400 nm were successfully prepared through the electrospinning method. NiO nanograins were formed along the axis of the nanofiber due to the calcination of as-spun fibers for 24 h at 450 °C and their presence was confirmed by FESEM. NiO nanograins were characterized by XRD, XPS and FTIR. The characterization results showed the presence of NiO in nanograins and its polycrystalline nature with ionic states. The sensing studies of NiO nanograins were performed towards the pulmonary disease breath markers and they showed better response towards formaldehyde vapour at 350 °C. Calcined NiO grains showed a good response towards the 11–1145 ppm of formaldehyde vapour at the operating temperature of 350 °C. NiO nanograins also showed quick response time (37 s) and recovery time (14 s) towards 46 ppm of formaldehyde. A sensing mechanism was proposed for the formaldehyde vapour interaction at 350 °C with NiO nanograins. [ABSTRACT FROM AUTHOR]

Aromatic volatile organic compound (VOC) sensors are attracting growing interest as a response to the pressing market need for sensitive, fast response, low power consumption and stable sensors. Benzene and toluene detection is subject to several potential applications such as air monitoring in chemical industries or even biosensing of human breath. In this work, we report the fabrication of a room temperature toluene and benzene sensor based on multiwall carbon nanotubes (MWCNTs) decorated with gold nanoparticles and functionalised with a long-chain thiol self-assembled monolayer, 1-hexadecanethiol (HDT). High-resolution transmission electron microscopy (HRTEM) and Fourier transform infrared spectroscopy (FTIR) were performed to characterize the gold nanoparticle decoration and to examine the thiol monolayer bonding to the MWCNTs. The detection of aromatic vapours using Au-MWCNT and HDT/Au-MWCNT sensors down to the ppm range shows that the presence of the self-assembled layer increases the sensitivity (up to 17 times), selectivity and improves the response dynamics of the sensors.

A simple, low-cost, and quickly reversible sensor based on polymer cholesteric liquid crystal film (PCLCF) for detecting organic vapour is proposed in this work. PCLCF exhibits an obvious red shift of the reflective colour in the early stages of acetone vapour diffusion and has the ability to quickly recover as vapour is released. The red shift in the transmittance spectrum is attributed to pitch swelling of the PCLCs. The acetone-induced pitch change and response time of the PCLCs, which have been quantitatively characterised, increase with the amount of the adsorbed acetone vapour. This work also revealed a strong correlation between the morphology of the PCLCF and the pitch variation of the helical configuration when subjected to organic vapour. Based on the experimental results, the proposed sensor has the potential to be used for portable and real-time monitoring of organic vapour. [ABSTRACT FROM AUTHOR]

In this work, the development of a novel paper-supported ratiometric fluorescent sensing platform for amines detection and discrimination based on co-doped Tb3+ and Eu3+ silica film is reported. The sensor film was obtained by the sol-gel process, in a one-pot straightforward synthesis approach. After exposure to different amines, the resulting intensity ratio between Tb3+ and Eu3+ emission bands presents characteristic values that can be ascribed to the different amines. The higher emission intensity ratios were found for amines with the higher p K a values, as in the case of biogenic amines. The sensor can be stored for more than 120 days, keeping the capacity to detect and distinguish different amines. The prepared fluorescent probe was tested as a food freshness indicator in beef and tuna samples, where notable changes were observed during the food degradation. This proof-of-concept shows the capability of the developed sensor platform to be applied in the detection of multiple amines, and in daily and quality control operations by "naked-eye" inspection. [Display omitted] • Paper-supported co-doped Tb3+/Eu3+ silica film sensor for amine vapours detection. • Mechanism based on radiometric fluorescence of Tb3+ & Eu3+ emission intensities. • Different amine vapours produce distinguishable optical changes. • Biogenic amines lead to significant changes, useful for food freshness control. • Sensor platforms remain active and capable to discriminate amines after 120 days. [ABSTRACT FROM AUTHOR]

This paper reports on the preparation of cellulose/reduced graphene oxide (rGO) aerogels for use as chemical vapour sensors. Cellulose/rGO composite aerogels were prepared by dissolving cellulose and dispersing graphene oxide (GO) in aqueous NaOH/urea solution, followed by an in-situ reduction of GO to reduced GO (rGO) and lyophilisation. The vapour sensing properties of cellulose/rGO composite aerogels were investigated by measuring the change in electrical resistance during cyclic exposure to vapours with varying solubility parameters, namely water, methanol, ethanol, acetone, toluene, tetrahydrofuran (THF), and chloroform. The increase in resistance of aerogels on exposure to vapours is in the range of 7 to 40% with methanol giving the highest response. The sensing signal increases almost linearly with the vapour concentration, as tested for methanol. The resistance changes are caused by the destruction of the conductive filler network due to a combination of swelling of the cellulose matrix and adsorption of vapour molecules on the filler surfaces. This combined mechanism leads to an increased sensing response with increasing conductive filler content. Overall, fast reaction, good reproducibility, high sensitivity, and good differentiation ability between different vapours characterize the detection behaviour of the aerogels.

The vapour sensing performances of various surfactant–carbon nanotube (CNT) sensors fabricated by spray layer by layer (sLbL) towards various VOC including eleven lung cancer biomarkers have been investigated. It is found that changing the surfactants’ nature allows to efficiently tailor both the selectivity and the sensitivity of CNT vapour sensors to detect specifically some VOC biomarkers of lung cancer. An original boosting effect of chemo-resistive responses has also been evidenced and assumed to result from the combined expansion of surfactant molecules directly sheathing CNT at junctions and of those organised in micelles in the vicinity of CNT, upon analytes’ diffusion. The performances of surfactant–CNT sensors, hierarchically structured by sLbL with sodium deoxycholate (DOC), sodium dodecylbenzenesulfonate (SDBS), 1-hexadecyl trimethyl ammonium bromide (CTAB), benzalkonium chloride (BnzlkCl), and triton ® x-405 (TX405) were found to depend on the interactions of the surfactants with the analytes, on their supramolecular assembly with CNT but also on the initial resistance R 0 that can be adjusted by CNT content, surfactant:CNT ratio or the surfactant concentration over its CMC. CNT-DOC sensors were found sensitive to methanol and other alcohols but also water. TX405-CNT sensors were found sensitive to benzene, n -pentane and chloroform. SDBS-CNT sensors could detect ethanol, acetone, chloroform and water but not much biomarkers, excepted isopropanol. BnzlkCl-CNT sensors were sensitive to n -pentane, isoprene, acetone and ethanol. CTAB-CNT sensors were slightly sensitive to most VOC but did not show any strong selectivity, whereas pristine CNT sensors were found to be good to detect most aromatic VOC of the set. Moreover it is remarkable that the technique of fabrication used here is almost exclusively based on eco-friendly aqueous solutions of CNT. [ABSTRACT FROM AUTHOR]

InP/InGaP surface quantum dots represent an attractive material for optical chemical sensors since they show a remarkable near infra-red emission at room temperature, whose intensity increases rapidly and reversibly depending on the composition of the environmental atmosphere. We show here their emission properties by time resolved photoluminescence spectroscopy investigation. Photoluminescence transients with and without chemical solvent vapours (methanol, clorophorm, acetone and water) were fitted with a 3-exponential decay law with times of about 0.5 ns, 2 ns and 7 ns. The measurements revealed a weak effect on clorophorm, acetone and water, while the initial decay time of InP surface quantum dots increases (up to 15%) upon methanol vapour exposure, indicating that the organic molecules efficiently saturate QD non-radiative surface states. [ABSTRACT FROM AUTHOR]

Electrically conductive aerogels composed of carbon nanotubes (CNTs) and cellulose were used as vapour sensors for the first time. The vapour sensing behaviours of these aerogels were investigated by monitoring the electrical resistance change upon exposure to a series of volatile organic compound (VOC) vapours such as methanol, ethanol, toluene, and others. The effects of vapour types, CNT contents and vapour concentrations on relative electrical resistance change were analyzed. The results revealed that CNT–cellulose composite aerogels exhibit rapid response, high sensitivity and good reproducibility to both polar and nonpolar vapours. The dominant mechanism for these aerogels as vapour sensors is the resistance change from chemical molecule absorption by both CNT networks and cellulose matrix. This is mainly due to the unique three-dimensional (3D) porous structure of materials, which also broadens the scope of analytes to be detected. This work provides a novel concept for developing a new class of chemical sensors by combining the nature of CNTs with the 3D porous matrix. [ABSTRACT FROM AUTHOR]

Abstract: Vapour sensors based on single walled carbon nanotubes (SWCNT) networks offer a high sensitivity but poor selectivity. Different works were undertaken, with varying degrees of success, to overcome this drawback. In this context, we develop a new selective sensor architecture based on a SWCNT network covered with a mesoporous silica film elaborated by sol–gel process. The topological and chemical characteristics of this sensor are obtained by TEM, SEM, EDX, contact angle and electrical measurements. We demonstrate that highly polarised molecules are stopped by the silica layer while other molecules like NO2 can diffuse easily in the mesoporous silica layer to reach the SWCNT network without increasing the response time. [Copyright &y& Elsevier]

Abstract: Conductive biopolymer nanocomposites (CPC) have been prepared by dispersing multi-wall carbon nanotubes (CNT) “guest conducting filler” in biopolymer, poly(lactic acid) (PLA) “host matrix” via solution mixing to develop volatile organic compounds (VOC) sensors. CPC transducers were fabricated by spray layer by layer (sLbL) technique and the derived sensors chemo-resistive properties have been investigated by exposition to a set of organic vapours (chloroform, methanol, toluene and water) exhibiting different physical properties such as solubility, polarity and molecular size. The influence of both vapour nature and CNT content has been elucidated and explained on the basis of solubility parameters and percolation theory. The selectivity of PLA/CNT CPC towards vapours, was found to be well correlated to solubility parameters. Among all, chloroform was the vapour that led to the highest response of sensors. To determine the influence of crystallization on conductive network architecture and thus on chemo-resistive characteristics of PLA/CNT, a comparative study has been carried out before and after annealing of sensors. The modification induced by this thermal treatment on both surface morphology and bulk crystallinity of PLA/CNT CPC, has clearly evidenced by atomic force microscopy (AFM) and differential scanning calorimetry (DSC) are found to be responsible for important changes in vapour sensing behaviour. [Copyright &y& Elsevier]

Abstract: Thin films of polyaniline base, emeraldine base (EB), coating on the quartz crystal microbalance (QCM) electrode were used as a sensitive layer for the detection of a number of primary aliphatic alcohols such as ethanol, methanol, 2-propanol and 1-propanol vapours. The frequency shifts (Δf) of the QCM were increased due to the vapour adsorption into the EB film. Δf were found to be linearly correlated with the concentrations of alcohols vapour in part per million (ppm). The sensitivity of the sensor was found to be governed by the chemical structure of the alcohol. The sensor shows a good reproducibility and reversibility. The diffusions of different alcohols vapour were studied and the diffusion coefficients (D) were calculated. It is concluded that the diffusion of the vapours into the EB film follows Fickian kinetics. [Copyright &y& Elsevier]

Abstract: Inkjet-printed films of the conductive polymer polypyrrole have been used for vapour sensing at room temperature. The electrical properties, sensitivity, reproducibility and stability of the polymer chemiresistors are reported. A significant increase in conductivity was observed on the exposure of the films to the vapours of simple alcohols. The value of the fractional resistance change, ΔR/R, of the films increased linearly with increasing concentrations of both ethanol and methanol. A relatively high ΔR/R value, approximately 90%, was obtained on exposure to 5000 parts per million of methanol. The response time of the inkjet-printed sensors to polar vapours was generally shorter than that for non-polar compounds. It is suggested that the increase in conductivity may be related to a vapour-induced change in the transfer of charge carriers between the polymer chains. [Copyright &y& Elsevier]

Ion-beam sputtering deposited palladium-fluoropolymer (Pd-CFx) nano-structured composite films have been investigated as active layers for the detection of organic solvents vapours. The composites sensing properties have been studied and compared to those of homologous Au-CFx films by means of ellipsometric and quartz crystal microbalance (QCM) experiments. Differences have been shown between sensitivity, selectivity and response repeatability of Au-CFx and Pd-CFx sensors exposed to organic solvent vapours. X-ray photoelectron spectroscopy (XPS) analysis has revealed strong similarities between the two materials as to the polymer matrix structure, but higher concentration of metal oxides and fluorides in the case of palladium. The results of the analytical characterisation have been used to discuss the performance level of the sensors. [Copyright &y& Elsevier]

Chemically synthesised copper/polyaniline (PANi) nanocomposite has been utilised as a chloroform sensor for ppm level vapour concentration. The response in terms of increase in dc electric resistance on exposure to chloroform vapours has been observed. The FT–IR spectra of nanocomposite on exposure to chloroform show remarkable modifications in the far IR region indicating the interaction of chloroform with metal cluster. It is believed that the sensing mechanism mainly involves adsorption–desorption of chloroform at metal cluster surfaces. [Copyright &y& Elsevier]

This paper reports on the preparation of cellulose/reduced graphene oxide (rGO) aerogels for use as chemical vapour sensors. Cellulose/rGO composite aerogels were prepared by dissolving cellulose and dispersing graphene oxide (GO) in aqueous NaOH/urea solution, followed by an in-situ reduction of GO to reduced GO (rGO) and lyophilisation. The vapour sensing properties of cellulose/rGO composite aerogels were investigated by measuring the change in electrical resistance during cyclic exposure to vapours with varying solubility parameters, namely water, methanol, ethanol, acetone, toluene, tetrahydrofuran (THF), and chloroform. The increase in resistance of aerogels on exposure to vapours is in the range of 7 to 40% with methanol giving the highest response. The sensing signal increases almost linearly with the vapour concentration, as tested for methanol. The resistance changes are caused by the destruction of the conductive filler network due to a combination of swelling of the cellulose matrix and adsorption of vapour molecules on the filler surfaces. This combined mechanism leads to an increased sensing response with increasing conductive filler content. Overall, fast reaction, good reproducibility, high sensitivity, and good differentiation ability between different vapours characterize the detection behaviour of the aerogels. [ABSTRACT FROM AUTHOR]