Gas sensing properties of single conducting polymer nanowires and the effect of temperature

Preparation and Characterization of PEDOT:PSS/TiO2 Micro/Nanofiber-Based Gas Sensors

In this study, we employed electrospinning technology and in situ polymerization to prepare wearable and highly sensitive PVP/PEDOT:PSS/TiO₂ micro/nanofiber gas sensors. PEDOT, PEDOT:PSS, and TiO₂ were prepared via in situ polymerization and tested for characteristic peaks using energy-dispersive X-ray spectroscopy (EDS) and Fourier transform infrared spectroscopy (FT-IR), then characterized using a scanning electron microscope (SEM), a four-point probe resistance measurement, and a gas sensor test system. The gas sensitivity was 3.46–12.06% when ethanol with a concentration between 12.5 ppm and 6250 ppm was measured; 625 ppm of ethanol was used in the gas sensitivity measurements for the PEDOT/composite conductive woven fabrics, PVP/PEDOT:PSS nanofiber membranes, and PVP/PEDOT:PSS/TiO₂ micro/nanofiber gas sensors. The latter exhibited the highest gas sensitivity, which was 5.52% and 2.35% greater than that of the PEDOT/composite conductive woven fabrics and PVP/PEDOT:PSS nanofiber membranes, respectively. In addition, the influence of relative humidity on the performance of the PVP/PEDOT:PSS/TiO₂ micro/nanofiber gas sensors was examined. The electrical sensitivity decreased with a decrease in ethanol concentration. The gas sensitivity exhibited a linear relationship with relative humidity lower than 75%; however, when the relative humidity was higher than 75%, the gas sensitivity showed a highly non-linear correlation. The test results indicated that the PVP/PEDOT:PSS/TiO₂ micro/nanofiber gas sensors were flexible and highly sensitive to gas, qualifying them for use as a wearable gas sensor platform at room temperature. The proposed gas sensors demonstrated vital functions and an innovative design for the development of a smart wearable device.

Nanowire-based sensor electronics for chemical and biological applications

Detection and recognition of chemical and biological species via sensor electronics are important not only for various sensing applications but also for fundamental scientific understanding. In the past two decades, sensor devices using one-dimensional (1D) nanowires have emerged as promising and powerful platforms for electrical detection of chemical species and biologically relevant molecules due to their superior sensing performance, long-term stability, and ultra-low power consumption. This paper presents a comprehensive overview of the recent progress and achievements in 1D nanowire synthesis, working principles of nanowire-based sensors, and the applications of nanowire-based sensor electronics in chemical and biological analytes detection and recognition. In addition, some critical issues that hinder the practical applications of 1D nanowire-based sensor electronics, including device reproducibility and selectivity, stability, and power consumption, will be highlighted. Finally, challenges, perspectives, and opportunities for developing advanced and innovative nanowire-based sensor electronics in chemical and biological applications are featured.

A chemiresistive sensor array from conductive polymer nanowires fabricated by nanoscale soft lithography

One-dimensional organic nanostructures are essential building blocks for high performance gas sensors. Constructing an e-nose type sensor array is the current golden standard in developing portable systems for the detection of gas mixtures. However, facile fabrication of nanoscale sensor arrays is still challenging due to the high cost of the conventional nanofabrication techniques. In this work, we fabricate a chemiresistive gas sensor array composed of well-ordered sub-100 nm wide conducting polymer nanowires using cost-effective nanoscale soft lithography. Poly(3,4-ethylene-dioxythiophene)-poly(styrene sulfonate) (PEDOT:PSS) nanowires functionalized with different self-assembled monolayers (SAMs) are capable of identifying volatile organic compounds (VOCs) at a low concentration range. The side chains and functional groups of the SAMs introduce different sensitivities and selectivities to the targeted analytes. The distinct response pattern of each chemical is subjected to pattern recognition protocols, which leads to a clear separation towards ten VOCs, including ketones, alcohols, alkanes, aromatics and amines. These results of the chemiresistive gas sensor array demonstrate that nanoscale soft lithography is a reliable approach for fabricating nanoscale devices and has the potential of mass producibility.

Selectively arranged single-wire based nanosensor array systems for gas monitoring

Gas nanosensors, comprised of arrays of nanoelectrodes with finger-widths of ∼100 nm developed by electron beam lithography and aerosol assisted chemical vapor deposited non-functionalized and Pt-functionalized tungsten oxide nanowires (<100 nm) subsequently integrated across the pairs of electrodes via the dielectrophoresis method, are developed in this work. The functionality of these devices is validated towards various concentrations of NO2 and C2H5OH. The results demonstrate reproducible and consistent responses with better sensitivity and partial selectivity for the non-functionalized systems to NO2, as opposed to the Pt-functionalized systems, which display better sensing properties towards C2H5OH with a loss of response to NO2. These results are explained on the basis of the additional chemical and electronic interactions at the Pt/tungsten oxide interface, which increase the pre-adsorption of oxygen species and make the functionalized surface rather more sensitive to C2H5OH than to NO2, in contrast to the non-functionalized surface.

Room Temperature Sensing Achieved by GaAs Nanowires and oCVD Polymer Coating

Novel structures comprised of GaAs nanowire arrays conformally coated with conducting polymers (poly(3,4-ethylenedioxythiophene) (PEDOT) or poly(3,4-ethylenedioxythiophene-co-3-thiophene acetic acid) display both sensitivity and selectivity to a variety of volatile organic chemicals. A key feature is room temperature operation, so that neither a heater nor the power it would consume, is required. It is a distinct difference from traditional metal oxide sensors, which typically require elevated operational temperature. The GaAs nanowires are prepared directly via self-seeded metal-organic chemical deposition, and conducting polymers are deposited on GaAs nanowires using oxidative chemical vapor deposition (oCVD). The range of thickness for the oCVD layer is between 100 and 200 nm, which is controlled by changing the deposition time. X-ray diffraction analysis indicates an edge-on alignment of the crystalline structure of the PEDOT coating layer on GaAs nanowires. In addition, the positive correlation between the improvement of sensitivity and the increasing nanowire density is demonstrated. Furthermore, the effect of different oCVD coating materials is studied. The sensing mechanism is also discussed with studies considering both nanowire density and polymer types. Overall, the novel structure exhibits good sensitivity and selectivity in gas sensing, and provides a promising platform for future sensor design.

Ultrasensitive QRS made by supramolecular assembly of functionalized cyclodextrins and graphene for the detection of lung cancer VOC biomarkers

A novel electronic nose system comprising functionalized β-cyclodextrin wrapped reduced graphene oxide (RGO) sensors with distinct ability of discrimination of a set of volatile organic compounds has been developed. Non-covalent modification of chemically functionalized cyclodextrin with RGO is carried out by using pyrene adamantane as a linker wherever necessary, in order to construct a supramolecular assembly. The chemical functionality on cyclodextrin is varied utilising the principle of selective chemical modification of cyclodextrin. In the present study, the combined benefits of the host-guest inclusion complex formation ability and tunable chemical functionality of cyclodextrin, as well as the high surface area and electrical conductivity of graphene, are utilized for the development of a set of highly selective quantum resistive chemical vapour sensors (QRS), which can be assembled in an electronic nose.

Probing the highly efficient room temperature ammonia gas sensing properties of a luminescent ZnO nanowire array prepared via an AAO-assisted template route

A highly ordered luminescent ZnO nanowire array was synthesized which has excellent sensitivity and fast response to NH₃ gas.

Self-gating effect induced large performance improvement of ZnO nanocomb gas sensors

Much greater surface-to-volume ratio of hierarchical nanostructures renders them with promising potential for high performance chemical sensing. In this work, crystalline nanocombs were synthesized via chemical vapor deposition and fabricated into resistive gas sensors. Particularly, NO2 sensing performance of these devices has been systematically characterized, showing higher sensitivity as compared to their nanobelt counterparts. Through device simulation, it was discovered that the teeth part of a nanocomb could serve as a "negative-potential gate" after accumulating electrons captured by surface adsorbed NO2 molecules. This self-gating effect eventually results in a greater reduction of nanocomb device channel conductance upon NO2 exposure, as compared to a nanobelt device, leading to a much higher NO2 detection sensitivity. This study not only sheds light on the mechanism of performance enhancement with hierarchical nanostructures, but also proposes a rational approach and a simulation platform to design nanostructure based chemical sensors with desirable performance.

Temperature-activated reverse sensing behavior of Pd nanowire hydrogen sensors

Hydrogen sensors built with individual palladium nanowires (Pd NWs) have been achieved by integrating Pd NWs across microelectromechanical system (MEMS) electrodes, followed by assembling and bonding them to a chip carrier platform. The sensing measurements reveal that the sensors with individual Pd NWs show reverse sensing behaviors between the temperature zones of (370-263 K) and (263-120 K).

Electrochemical synthesis and one step modification of PMProDot nanotubes and their enhanced electrochemical properties

Poly (3,4-(2-methylene)propylenedioxythiophene) (PMProDot) nanotubes were synthesized within the pores of polycarbonate and were further modified with styrene and vinylcarbazole by a one step electrochemical method through the methylene functional group. The enhanced electrochemical and electrochromic properties of composite nanotubes were investigated using FTIR, UV/Vis absorbance spectroscopy, and AFM.