Chemosensors, Volume 2, Issue 1 (March 2014) – 7 articles , Pages 1-96

Four types of monolayer-protected gold nanoclusters (MPCs) were synthesized and characterized as active layers of vapor sensors. An interdigitated microelectrode (IDE) and quartz crystal microbalance (QCM) were used to measure the electrical resistance and mass loading changes of MPC films during vapor sorption. The vapor sensing selectivity was influenced by the ligand structure of the monolayer on the surface of gold nanoparticles. The responses of MPC-coated QCM were mainly determined according to the affinity between the vapors and surface ligands of MPCs. The responses to the resistance changes of the MPC films were due to the effectiveness of the swelling when vapor was absorbed. It was observed that resistive sensitivity to polar organics could be greatly enhanced when the MPC contained ligands that contain interior polar functional groups with exterior nonpolar groups. This finding reveals that reducing interparticle attraction by using non-polar exterior groups could increase effective swelling and therefore enhance the sensitivity of MPC-coated chemiresistors. Full article

High-temperature gas sensing requires the increase of sensitivity and reduction of cross-sensitivity. The use of TiO₂-Nanotubular layers as gas sensors has shown that the selectivity and sensitivity can be influenced by doping with trivalent elements and by optimization of morphological aspects such as pore diameter and nanotube length. In this work, focus has been given on the understanding of the effect of doping and properties of nano-tubular TiO₂-layers on sensing behavior and mechanism toward NO₂ by using equivalent circuit modeling achieved by impedance spectroscopic measurements. Full article

A chemical sensor (chemiresistor) was constructed from a xenon light bulb by coating it with a 3-D zinc oxide coated silica nanospring mat, where the xenon light bulb serves as the sensor heater. The sensor response to toluene as a function of xenon light bulb sensor temperature (T_LB) and vapor temperature (T_V) was observed and analyzed. The optimum operational parameters in terms of T_LB and T_V were determined to be 435 °C and 250 °C, respectively. The activation energy of toluene oxidation (E_d) on the ZnO surface was determined to be 87 kJ·mol⁻¹, while the activation energy of oxidation (E_a) of the depleted ZnO surface was determined to be 83 kJ·mol⁻¹. This study serves as proof of principle for integrating nanomaterials into an inexpensive sensor platform, which can also be used to characterize gas-solid, or vapor-solid, redox processes. Full article

The use of Ir(III) complexes in photo-responsive molecular devices for oxygen gas sensing is reviewed. Attention is focused on the immobilization of Ir(III) complexes in organic or inorganic host materials such as polymers, silica and clays in order to enhance robustness and reliability. Our recent works on constructing nanometer-thick films comprised of cyclometalated cationic Ir(III) complexes and clay minerals are described. The achievement of multi-emitting properties in response to oxygen pressure is demonstrated. Full article

Different grades of chemically functionalized carbon nanotubes (CNT) have been processed by spraying layer-by-layer (sLbL) to obtain an array of chemoresistive transducers for volatile organic compound (VOC) detection. The sLbL process led to random networks of CNT less conductive, but more sensitive to vapors than filtration under vacuum (bucky papers). Shorter CNT were also found to be more sensitive due to the less entangled and more easily disconnectable conducting networks they are making. Chemical functionalization of the CNT’ surface is changing their selectivity towards VOC, which makes it possible to easily discriminate methanol, chloroform and tetrahydrofuran (THF) from toluene vapors after the assembly of CNT transducers into an array to make an e-nose. Interestingly, the amplitude of the CNT transducers’ responses can be enhanced by a factor of five (methanol) to 100 (chloroform) by dispersing them into a polymer matrix, such as poly(styrene) (PS), poly(carbonate) (PC) or poly(methyl methacrylate) (PMMA). COOH functionalization of CNT was found to penalize their dispersion in polymers and to decrease the sensors’ sensitivity. The resulting conductive polymer nanocomposites (CPCs) not only allow for a more easy tuning of the sensors’ selectivity by changing the chemical nature of the matrix, but they also allow them to adjust their sensitivity by changing the average gap between CNT (acting on quantum tunneling in the CNT network). Quantum resistive sensors (QRSs) appear promising for environmental monitoring and anticipated disease diagnostics that are both based on VOC analysis. Full article

Nanowire networks have great potential in many industrial applications, including batteries, electrical circuits, solar cells, and sensors. In this paper we focus on a specific hydrogen gas nanosensor whose sensing element is a network of palladium nanowires. The nanosensor is modeled using a square, equilateral triangle, and hexagonal lattice. We provide the reliability behavior of this nanosensor when the electrical current is allowed to move in all directions. Our findings reveal an improvement in reliability compared to the scenario where the electrical current could not move from right to left. We show this improvement both analytically and through simulation. Full article

Phenylboronic acids are a class of compounds that bind glucose and other sugars. When polymerized into hydrogels, they provide a convenient nonenzymatic means for sensing glucose concentration, provided competing sugars are present at negligible concentrations. In this paper we provide a comprehensive study of swelling of hydrogels containing methacrylamidophenylboronic acid (MPBA), as a function of pH and concentration of either glucose or fructose. In one set of hydrogels, MPBA is substituted at 20 mol·% in a polyacrylamide hydrogel [p(MPBA-co-AAm)], while in a second set of hydrogels, 20 mol·% MPBA is supplemented with 20 mol·% of N-3-(dimethylaminopropyl methacrylamide) [p(MPBA-co-DMP-co-AAm)]. Swelling curves are markedly different for fructose and glucose, and for the two sets of hydrogels. While fructose alters swelling by binding and contributing to the ionization of MPBA, glucose does the same, but it also can form crosslinking bridges between separate chains, leading to hydrogel shrinkage. While the [p(MPBA-co-AAm)] hydrogels behaved as polyacids, swelling monotonically with increasing pH, the [p(MPBA-co-DMP-co-AAm)] hydrogels exhibited polyampholyte behavior, with swelling minima at intermediate pH values. Full article