Graphene-based multifunctional humidity sensors with an ultrahigh current response

Prospective composites, based on graphene (G) and hexagonal boron nitride (h-BN) nanoparticles, synthesized using a plasma jet and conducting polymer PEDOT:PSS, were used to create and study a set of sensors in the current study. The composites used were G:PEDOT:PSS (GPP) and G:h-BN:PEDOT:PSS (GBNPP). The PEDOT:PSS content in the composites was 10-3 wt%, and the ratio of G : h-BN was 1 : 1 in GBNPP. The development of these new highly conductive graphene-based composites makes it possible to create an active sensor layer with an ultra-low thickness of several nanometers. The ultra-high sensitivity of the current response, S, was ((2.0-3.3) × 106)% for GPP and GBNPP (2-3 printing layers) for a humidity range of 20-80%. The sensor response in the form of current pulses associated with human breathing has a range of ∼2-3 orders of magnitude. Two different processes are assumed to determine the form of the current pulse: the first is a fast process with a rise time of less than 1-4 seconds; the second is a relatively slow process with a front time of several tens of seconds. When touching with a finger (useful, for instance, for a flexible touchpad), a current response was observed as pulses of ∼2-3 orders of magnitude. We hypothesize that skin sweat is likely to play a critical role in the sensory response. Thus, this work presents an effective approach to creating a highly sensitive humidity sensor based on composite 2D materials. Moreover, the ultra-high sensitivity of the studied sensors is accompanied by their low cost and ease of manufacturing by 2D-printing.

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.

Metal–Oxide Nanowire Molecular Sensors and Their Promises

During the past two decades, one–dimensional (1D) metal–oxide nanowire (NW)-based molecular sensors have been witnessed as promising candidates to electrically detect volatile organic compounds (VOCs) due to their high surface to volume ratio, single crystallinity, and well-defined crystal orientations. Furthermore, these unique physical/chemical features allow the integrated sensor electronics to work with a long-term stability, ultra-low power consumption, and miniature device size, which promote the fast development of “trillion sensor electronics” for Internet of things (IoT) applications. This review gives a comprehensive overview of the recent studies and achievements in 1D metal–oxide nanowire synthesis, sensor device fabrication, sensing material functionalization, and sensing mechanisms. In addition, some critical issues that impede the practical application of the 1D metal–oxide nanowire-based sensor electronics, including selectivity, long-term stability, and low power consumption, will be highlighted. Finally, we give a prospective account of the remaining issues toward the laboratory-to-market transformation of the 1D nanostructure-based sensor electronics.

Cu/graphene interdigitated electrodes with various copper thicknesses for UV-illumination-enhanced gas sensors at room temperature

Effective detection of NO2 and NH3 gases at room temperature (RT) is critical for environmental monitoring and protection. Here, graphene-based gas sensors (Cu/Gr device) of single layer graphene decorated by 6, 8 and 10 nm thick Cu layers with graphene instead of conventional metal as interdigital electrodes are designed and fabricated. The RT performance for both NO2 and NH3 detection can be greatly enhanced by UV light illumination which is closely related to the thickness of Cu layers in which the device with 8 nm thickness (8 nm Cu/Gr device) exhibits the best performances. Analysis of XPS reveals that Cu is partly oxidized to Cu+ and Cu2+ for 6 nm with extra Cuδ+ (1 < δ < 2) for 8 and 10 nm. The contents and distributions of copper oxides and copper in Cu layers influence the catalytic effects and the heterojunction barrier and thus the performances. The RT responses of -30.9% and -8.1% for 5 and 0.3 ppm NO2, and of +29.1% and +5.9% for 105 and 10 ppm NH3 are achieved for the 8 nm Cu/Gr device, respectively. The limits of detection (LODs) for NO2 and NH3 are 12 ppb and 17 ppb, respectively. The sensing mechanisms are discussed in terms of density functional theory (DFT) calculations and energy band diagrams. The study demonstrates an effective solution of improving the device performance by modifying the device configuration and incorporating combined oxides naturally oxidized, which provides the novel design alternatives for high performance sensors.

Qualitative Detection Toward Military and Improvised Explosive Vapors by a Facile TiO2 Nanosheet-Based Chemiresistive Sensor Array

A facile TiO₂ nanosheets-based chemiresistive gas sensor array was prepared to identify 11 kinds of military and improvised explosive vapors at room temperature. The morphology of TiO₂ nanosheets was well-controlled by adjusting the concentration of HF applied during the preparation. Owing to the morphology difference, the TiO₂ nanosheet-based sensors show different response values toward 11 kinds of explosives, which is the basis of the successful discriminative identification. This method owes lots of advantages over other detection techniques, such as the facile preparation procedure, high response value (115.6% for TNT and 830% for PNT) at room temperature, rapid identifying properties (within 30 s for 9 explosives), simple operation, high anti-interference property, and low probability of misinforming, and consequently has a huge potential application in the qualitative detection of explosives.

Ultrafast and Ultrasensitive Gas Sensors Derived from a Large Fermi-Level Shift in the Schottky Junction with Sieve-Layer Modulation

Gas sensors play an important role in numerous fields, covering a wide range of applications, including intelligent systems and detection of harmful and toxic gases. Even though they have attracted much attention, the response time on the order of seconds to minutes is still very slow. To circumvent the existing problems, here, we provide a seminal attempt with the integration of graphene, semiconductor, and an addition sieve layer forming a nanocomposite gas sensor with ultrahigh sensitivity and ultrafast response. The designed sieve layer has a suitable band structure that can serve as a blocking layer to prevent transfer of the charges induced by adsorbed gas molecules into the underlying semiconductor layer. We found that the sensitivity can be reduced to the parts per million level, and the ultrafast response of around 60 ms is unprecedented compared with published graphene-based gas sensors. The achieved high performance can be interpreted well by the large change of the Fermi level of graphene due to its inherent nature of the low density of states and blocking of the sieve layer to prevent charge transfer from graphene to the underlying semiconductor layer. Accordingly, our work is very useful and timely for the development of gas sensors with high performance for practical applications.

Ultrasensitive, Real-time and Discriminative Detection of Improvised Explosives by Chemiresistive Thin-film Sensory Array of Mn(2+) Tailored Hierarchical ZnS

A simple method combing Mn²⁺ doping with a hierarchical structure was developed for the improvement of thin-film sensors and efficient detection of the explosives relevant to improvised explosive devices (IEDs). ZnS hierarchical nanospheres (HNs) were prepared via a solution-based route and their sensing performances were manipulated by Mn²⁺ doping. The responses of the sensors based on ZnS HNs towards 8 explosives generally increase firstly and then decrease with the increase of the doped Mn²⁺ concentration, reaching the climate at 5% Mn²⁺. Furthermore, the sensory array based on ZnS HNs with different doping levels achieved the sensitive and discriminative detection of 6 analytes relevant to IEDs and 2 military explosives in less than 5 s at room temperature. Importantly, the superior sensing performances make ZnS HNs material interesting in the field of chemiresistive sensors, and this simple method could be a very promising strategy to put the sensors based on thin-films of one-dimensional (1D) nanostructures into practical IEDs detection.

High efficiency room temperature detection of NO2gas based on ultrathin metal/graphene devices

High efficiency detection of NO₂by ultrathin metal decorated graphene sensors at room temperature.

A Review on Graphene-Based Gas/Vapor Sensors with Unique Properties and Potential Applications

Graphene-based gas/vapor sensors have attracted much attention in recent years due to their variety of structures, unique sensing performances, room-temperature working conditions, and tremendous application prospects, etc. Herein, we summarize recent advantages in graphene preparation, sensor construction, and sensing properties of various graphene-based gas/vapor sensors, such as NH3, NO2, H2, CO, SO2, H2S, as well as vapor of volatile organic compounds. The detection mechanisms pertaining to various gases are also discussed. In conclusion part, some existing problems which may hinder the sensor applications are presented. Several possible methods to solve these problems are proposed, for example, conceived solutions, hybrid nanostructures, multiple sensor arrays, and new recognition algorithm.

An AuNPs-functionalized AlGaN/GaN high electron mobility transistor sensor for ultrasensitive detection of TNT

In this work, we developed a gold nanoparticle functionalized AlGaN/GaN HEMT sensor for fast and ultrasensitive detection of TNT. This HEMT sensor showed attractive properties for TNT detection in terms of speed, sensitivity and miniaturization.

Nanostructure-based optoelectronic sensing of vapor phase explosives--a promising but challenging method

Optoelectronic sensing of gas phase hazardous chemicals is a newly explored field, which shows great advantages towards low concentration sensing when compared to normal gas sensing in the dark. Here, based on the recent progress on nanostructured vapor phase explosive gas sensors operated in dark conditions, the attractiveness of developing optoelectronic sensors for vapor phase explosive detection was highlighted. Furthermore, we try to propose some new insights to enhance optoelectronic sensing of vapor phase explosives. We suggest employing photocatalysis principles to enhance the sensitivity and employing a molecular imprinting technique (MIT) to enhance the selectivity.