Flexible sensor based on carbon nanofibers with multifunctional sensing features

Enhancement of Room Temperature Ethanol Sensing by Optimizing the Density of Vertically Aligned Carbon Nanofibers Decorated with Gold Nanoparticles

An ethanol gas sensor based on carbon nanofibers (CNFs) with various densities and nanoparticle functionalization was investigated. The CNFs were grown by means of a Plasma-Enhanced Chemical Vapor Deposition (PECVD), and the synthesis conditions were varied to obtain different number of fibers per unit area. The devices with a larger density of CNFs lead to higher responses, with a maximal responsivity of 10%. Furthermore, to simultaneously improve the sensitivity and selectivity, CNFs were decorated with gold nanoparticles by an impaction printing method. After metal decoration, the devices showed a response 300% higher than pristine devices toward 5 ppm of ethanol gas. The morphology and structure of the different samples deposited on a silicon substrate were characterized by TEM, EDX, SEM, and Raman spectroscopy, and the results confirmed the presence of CNF decorated with gold. The influence of operating temperature (OT) and humidity were studied on the sensing devices. In the case of decorated samples with a high density of nanofibers, a less-strong cross-sensitivity was observed toward a variation in humidity and temperature.

Ternary Nanocomposites Based on Oxidized Carbon Nanohorns as Sensing Layers for Room Temperature Resistive Humidity Sensing

This paper presents the relative humidity (RH) sensing response of a resistive sensor employing sensing layers based on a ternary nanocomposite comprising graphene oxide-oxidized carbon nanohorns-polyvinylpyrrolidone (GO-CNHox-PVP), at 1/1/1, 1/2/1, and 1/3/1 w/w/w mass ratios. The sensing structure is composed of a silicon substrate, a SiO2 layer, and interdigitated transducers (IDT) electrodes, on which the sensing layer is deposited via the drop-casting method. The morphology and the composition of the sensing layers are investigated through scanning electron microscopy (SEM) and RAMAN spectroscopy. The RH sensing capability of each carbonaceous nanocomposite-based thin film was analyzed by applying a current between the two electrodes and by measuring the voltage difference when varying the RH from 0% to 100% in humid nitrogen. The sensors have a room temperature response comparable to that of a commercial humidity sensor and are characterized by a rapid response, excellent linearity, good sensitivity, and recovery time. The manufactured sensing devices' transfer functions were established, and we extracted the response and recovery times. While the structures with GO/CNHox/PVP at 1/1/1 ratio (w/w/w) had the best performance in terms of relative sensibility, response time, and recovery time, the sensors employing the GO/CNHox/PVP nanocomposite at the 1/2/1 ratio (w/w/w) had the best linearity. Moreover, the ternary mixture proved to have much better sensing properties compared to CNHox and CNHox-PVP-based sensing layers in terms of sensitivity and linearity. Each component of the ternary nanocomposites' functional role is explained based on their physical and chemical properties. We analyzed the potential mechanism associated with the sensors' response; among these, the effect of the p-type semiconductor behavior of CNHox and GO, correlated with swelling of the PVP, was dominant and led to increased resistance of the sensing layer.

Recent Advances in Ammonia Gas Sensors Based on Carbon Nanomaterials

This review paper is devoted to an extended analysis of ammonia gas sensors based on carbon nanomaterials. It provides a detailed comparison of various types of active materials used for the detection of ammonia, e.g., carbon nanotubes, carbon nanofibers, graphene, graphene oxide, and related materials. Different parameters that can affect the performance of chemiresistive gas sensors are discussed. The paper also gives a comparison of the sensing characteristics (response, response time, recovery time, operating temperature) of gas sensors based on carbon nanomaterials. The results of our tests on ammonia gas sensors using various techniques are analyzed. The problems related to the recovery of sensors using various approaches are also considered. Finally, the impact of relative humidity on the sensing behavior of carbon nanomaterials of various different natures was estimated.

Synergistic CNFs/CoS2/MoS2 Flexible Films with Unprecedented Selectivity for NO Gas at Room Temperature

Recently, room-temperature flexible gas sensors have been widely studied because they can operate without being heated and create low-cost, low-power-consumption devices with long-term stability. Here, by designing the active material composition and structure, we report an electrospun carbon nanofiber (CNF) network grafted by two-dimensional MoS₂ nanosheets and embedded CoS₂ nanoparticles, which serves as a flexible gas sensor for various toxic or hazardous gases working at room temperature. In particular, the CNFs/CoS₂/MoS₂ hybrid films exhibit very high selectivity toward NO over other gases including NO₂ and CH₄, with selectivity coefficients (|S_NO/S_NO2| and |S_NO/S_CH4|) as high as 43 and 42 (defined as the ratio of responses between two gases). The sensor shows a linear relationship in the gas concentration range of 1-100 ppm and a stable response during repeated bending. Theoretical calculations suggest that MoS₂ can be selectively n-doped by NO, while CoS₂ can effectively capture NO molecules, leading to enhanced selectivity and sensitivity. Our large-area flexible sensors made by synergistic design have potential applications in biological and environmental areas for low-cost, selective detection of toxic or targeted gases.

Growth of MWCNTs on Plasma Ion-Bombarded Thin Gold Films and Their Enhancements of Ammonia-Sensing Properties Using Inkjet Printing

Multiwalled carbon nanotubes (MWCNTs) have been synthesized on thin gold (Au) films using thermal chemical vapor deposition (CVD). The films were evolved to catalytic Au nanoparticles (Au NPs) by plasma argon (Ar) ion bombardment with a direct current (DC) power of 216 W. The characteristics of the MWCNTs grown on Au catalysts are strongly dependent on the growth temperature in thermal CVD process. The MWCNTs were then purified by oxidation (550°C) and acid treatments (3 : 1 H2SO4/HNO3). After purifying the MWCNTs, they were dispersed in deionized water (DI water) under continuous sonication. The MWCNT solution was then ultrasonically dissolved in a conducting polymer mixture of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) to prepare for an electronic ink. The ink was deposited onto the flexible and transparent plastic substrates such as polyethylene terephthalate (PET) with fabricated silver interdigitated electrode using two methods such as drop-casting and inkjet printing to compare in the detection of ammonia (NH3) and other volatile organic compounds (VOCs) at room temperature. Based on the results, the gas response, sensitivity, and selectivity properties of MWCNT-PEDOT:PSS gas sensor for NH3 detection are significantly enhanced by using inkjet printing technique. The sensing mechanism of fabricated gas sensor exposed to NH3 has been also proposed based on the swelling behaviour of polymer due to the diffusion of NH3 molecules into the polymer matrix. For the MWCNTs, they were mentioned as the conductive pathways for the enhancement of gas-sensing signals.

Carbon-Based Materials for Humidity Sensing: A Short Review

Humidity sensors are widespread in many industrial applications, ranging from environmental and meteorological monitoring, soil water content determination in agriculture, air conditioning systems, food quality monitoring, and medical equipment to many other fields. Thus, an accurate and reliable measurement of water content in different environments and materials is of paramount importance. Due to their rich surface chemistry and structure designability, carbon materials have become interesting in humidity sensing. In addition, they can be easily miniaturized and applied in flexible electronics. Therefore, this short review aims at providing a survey of recent research dealing with carbonaceous materials used as capacitive and resistive humidity sensors. This work collects some successful examples of devices based on carbon nanotubes, graphene, carbon black, carbon fibers, carbon soot, and more recently, biochar produced from agricultural wastes. The pros and cons of the different sensors are also discussed in the present review.

Inkjet printing of silver nanowire networks

The development of printed electronics will require the ability to deposit a wide range of nanomaterials using printing techniques. Here we demonstrate the controlled deposition of networks of silver nanowires in well-defined patterns by inkjet printing from an optimized isopropyl alcohol-diethylene glycol dispersion. We find that great care must be taken while producing the ink and during solvent evaporation. The resultant networks have good electrical properties, displaying sheet resistances as low as 8 Ω/□ and conductivities as high as 10(5) S/m. Such optimized performances were achieved for line widths of 1-10 mm and network thicknesses of 0.5-2 μm deposited from ∼10-20 passes while using processing temperatures of no more than 110 °C. Thin networks are semitransparent with dc to optical conductivity ratios of ∼40.

Detection of volatile organic compounds using flexible gas sensing devices based on tungsten oxide nanostructures functionalized with Au and Pt nanoparticles

Flexible gas sensor devices are fabricated and optimized by integrating directly, via a single-step vapor-phase deposition method, highly crystalline tungsten oxide nanostructures functionalized with either gold or platinum nanoparticles. Gas tests of these devices show significant improvements with respect to flexible gas sensors based on non-functionalized structures, including greater responses to various volatile organic compounds (ethanol, acetone, methanol and toluene) and better selectivity towards ethanol and methanol, as demonstrate results for the sensors based on platinum-functionalized structures. The method presented here, which includes the fabrication of the whole flexible gas sensing device and the integration of functional nanostructures without the use of transfer methods, provides a simpler, faster and inexpensive method for the fabrication of highly functional flexible microsystems for gas sensing.

Flexible microfabricated film sensors for the in situ quantum dot-based voltammetric detection of DNA hybridization in microwells

A new flexible miniaturized integrated device was microfabricated for the in situ ultrasensitive voltammetric determination of DNA mutation in a microwell format, using quantum dots (QDs) labels. The integrated device consisted of thin Bi, Ag, and Pt films (serving as the working, reference, and counter electrode, respectively) deposited by sputtering on a flexible polyimide substrate. A DNA assay was employed in microwell format, where an immobilized complementary oligonucleotide probe hybridized with the biotinylated target oligonucleotide followed by reaction with streptavidin-conjugated PbS QDs. After the acidic dissolution of the QDs, the flexible sensor was rolled and inserted into the microwell and the Pb(II) released was determined in situ by anodic stripping voltammetry. Since the analysis took place directly in the microwell, the volume of the working solution was only 100 μL and the target DNA could be detected at a concentration down to 1.1 fmol L(-1). The proposed flexible microdevice addresses the restrictions of conventional rigid electrodes while it provides a low cost integrated transducer for the ultrasensitive detection of important biomolecules.

Printed and flexible biosensor for antioxidants using interdigitated ink-jetted electrodes and gravure-deposited active layer

Printing techniques have been extensively used in the fabrication of organic electronic devices, such as light-emitting diodes and display backplanes. These techniques, in particular inkjet printing, are being employed for the localized dispensing of solutions containing biological molecules and cells, leading to the fabrication of bio-functional microarrays and biosensors. Here, we report the fabrication of an all-printed and flexible biosensor for antioxidants. Gold (Au) interdigitated electrodes (IDEs) with sub-100 µm features were directly inkjet-printed on plastic substrates using a nanoparticle-based ink. Conductivities as high as 5×10(6) S/m (12% of bulk Au) were attained after sintering was conducted at plastic-compatible 200 °C for 6 h. The enzyme Tyrosinase (Tyr) was used in the active layer of the biosensors, being innovatively deposited by large-area rotogravure printing. A tailor-made ink was studied, and the residual activity of the enzyme was 85% after additives incorporation, and 15.5% after gravure printing. Au IDEs were coated with gravure films of the Tyr-containing ink, and the biosensor was encapsulated with a cellulose acetate dip-coating film to avoid dissolution. The biosensor impedance magnitude increases linearly with the concentration of a model antioxidant, allowing for the construction of a calibration curve. Control experiments demonstrated the molecular recognition characteristic inferred by the enzyme. We found that the biosensor sensitivity and the limit of detection were, respectively, 5.68 Ω/µm and 200 µM. In conclusion, a disposable, light-weight, all-printed and flexible biosensor for antioxidants was successfully fabricated using fast and large-area printing techniques. This opens the door for the fabrication of technological products using roll-to-roll processes.