Fully printed high performance humidity sensors based on two-dimensional materials

Wearable Sensors for Breath Monitoring Based on Water-Based Hexagonal Boron Nitride Inks Made with Supramolecular Functionalization

Wearable humidity sensors are attracting strong attention as they allow for real-time and continuous monitoring of important physiological information by enabling activity tracking as well as air quality assessment. Amongst 2Dimensional (2D) materials, graphene oxide (GO) is very attractive for humidity sensing due to its tuneable surface chemistry, high surface area, processability in water, and easy integration onto flexible substrates. However, strong hysteresis, low sensitivity, and cross-sensitivity issues limit the use of GO in practical applications, where continuous monitoring is preferred. Herein, a wearable and wireless impedance-based humidity sensor made with pyrene-functionalized hexagonal boron nitride (h-BN) nanosheets is demonstrated. The device shows enhanced sensitivity towards relative humidity (RH) (>1010 Ohms/%RH in the range from 5% to 100% RH), fast response (0.1 ms), no appreciable hysteresis, and no cross-sensitivity with temperature in the range of 25-60 °C. The h-BN-based sensor is able to monitor the whole breathing cycle process of exhaling and inhaling, hence enabling to record in real-time the subtlest changes of respiratory signals associated with different daily activities as well as various symptoms of flu, without requiring any direct contact with the individual.

Nanochitin/MXene Composite Coated on Quartz Crystal Microbalance for Humidity Sensing

MXenes, as a typical graphene-like material, excels in the realm of humidity sensing owing to its two-dimensional layer structure, high electrical conductivity, tunable chemical properties, hydrophilicity, and large specific surface area. This study proposed a quartz crystal microbalance (QCM) humidity sensor using a nanochitin/Ti3C2Tx MXene composite as a humidity-sensing material. The morphology, nanostructure, and elemental composition of nanochitin, Ti3C2Tx MXene, and nanochitin/Ti3C2Tx MXene composite materials were characterized using transmission electron microscopy, Fourier transform infrared spectroscopy, and X-ray diffraction. Compared to the pure Ti3C2Tx MXene-coated QCM humidity sensor, the nanochitin/Ti3C2Tx MXene-coated QCM humidity sensor exhibited a higher sensitivity (20.54 Hz/%RH) in the humidity range of 11.3% to 97.3%. The nanochitin/Ti3C2Tx Mxene-coated QCM humidity sensor also demonstrated low humidity hysteresis (2.12%RH), very fast response/recovery times (4.4/4.1 s), a high quality factor (37 k), and excellent repeatability and sustained stability over time. Eventually, a bimodal exponential kinetics adsorption model was utilized for the analysis of the response mechanism of the nanochitin/Ti3C2Tx MXene composite material-based QCM humidity sensor. This study provides new ideas for optimizing the moisture-sensitive performance of MXene-based QCM humidity sensors.

An Electroactive and Self-Assembling Bio-Ink, based on Protein-Stabilized Nanoclusters and Graphene, for the Manufacture of Fully Inkjet-Printed Paper-Based Analytical Devices

Hundreds of new electrochemical sensors are reported in literature every year. However, only a few of them makes it to the market. Manufacturability, or rather the lack of it, is the parameter that dictates if new sensing technologies will remain forever in the laboratory in which they are conceived. Inkjet printing is a low-cost and versatile technique that can facilitate the transfer of nanomaterial-based sensors to the market. Herein, an electroactive and self-assembling inkjet-printable ink based on protein-nanomaterial composites and exfoliated graphene is reported. The consensus tetratricopeptide proteins (CTPRs), used to formulate this ink, are engineered to template and coordinate electroactive metallic nanoclusters (NCs), and to self-assemble upon drying, forming stable films. The authors demonstrate that, by incorporating graphene in the ink formulation, it is possible to dramatically improve the electrocatalytic properties of the ink, obtaining an efficient hybrid material for hydrogen peroxide (H2 O2 ) detection. Using this bio-ink, the authors manufactured disposable and environmentally sustainable electrochemical paper-based analytical devices (ePADs) to detect H2 O2 , outperforming commercial screen-printed platforms. Furthermore, it is demonstrated that oxidoreductase enzymes can be included in the formulation, to fully inkjet-print enzymatic amperometric biosensors ready to use.

Torsional deformation modulation of the electronic structure and optical properties of molybdenum ditelluride systems doped with halogen atoms X (X = F, Cl, Br, I): a first-principles study

CONTEXT

Using a first-principles plane-wave pseudopotential technique within the context of density-functional theory, the electronic structure and optical properties of the molybdenum ditelluride system doped with halogen atoms X (X = F, Cl, Br, I) were investigated. The electronic structure, density of states, charge transfer, and optical properties of halogen atom X doped on MoTe2 monolayer are systematically calculated and analyzed. It shows that the Fermi energy level is shifted upward after doping with halogen atoms. With F-MoTe2 doping, the geometrical distortion is the most pronounced, the charge transfer number is the highest, and the semiconductor shifts from a direct band gap to an indirect band gap. When the torsional deformation is between 1° and 5°, the F-doped MoTe2 system stays an indirect band gap semiconductor and transitions to quasi-metal at 6°. It is shown that the torsional deformation can modulate the electronic properties of the doped structure and realize the semiconductor-metal transition.

OPTICAL PROPERTIES

The F-doped system has a strong absorption peak reflection peak after torsion, and with the increase of torsion angle, the absorption peak is red-shifted, and the reflection peak is blue-shifted. Moreover, the absorption and reflection peaks start to decrease with the rise of the torsion angle.

METHODS

We apply the generalized gradient approximation plane-wave pseudopotential technique based on Perdew-Burke-Ernzerhof (PBE) generalized functions, under the first principles of the density-functional theory framework. The overall optimization of the intrinsic molybdenum ditelluride structure and the halogen atom X-doped molybdenum ditelluride structure was carried out. Then, the F-doped molybdenum ditelluride system was selected for torsional deformation with torsion angles from 1° to 6° for computational analysis.

SPECIFIC METHOD

To make the presentation more accessible, the atoms in the F-doped molybdenum ditelluride system were colored differently. The pink chain edge atoms were first reversed by θ°. Then, the blue chain edge atoms were reversed by θ° in the other direction. The middle row of atoms was adjusted accordingly to the different twisting angles of the two sides by doing the corresponding torsion with the torsion angle θ°/2 and fixing the individual atoms. The calculation employs the Monkhorst-Pack particular K-point sampling method. The 3 × 3 × 1 inverted-space K-point grid is utilized for material structure optimization calculations in each model, and the 9 × 9 × 1 K-point grid is used for material electronic structure calculations. A 15 Å vacuum layer is put on the crystal surface of vertical monolayer molybdenum ditelluride supercells to avoid interactions with adjoining cells.

A Review on Progress, Challenges, and Prospects of Material Jetting of Copper and Tungsten

Copper (Cu) and tungsten (W) possess exceptional electrical and thermal conductivity properties, making them suitable candidates for applications such as interconnects and thermal conductivity enhancements. Solution-based additive manufacturing (SBAM) offers unique advantages, including patterning capabilities, cost-effectiveness, and scalability among the various methods for manufacturing Cu and W-based films and structures. In particular, SBAM material jetting techniques, such as inkjet printing (IJP), direct ink writing (DIW), and aerosol jet printing (AJP), present a promising approach for design freedom, low material wastes, and versatility as either stand-alone printers or integrated with powder bed-based metal additive manufacturing (MAM). Thus, this review summarizes recent advancements in solution-processed Cu and W, focusing on IJP, DIW, and AJP techniques. The discussion encompasses general aspects, current status, challenges, and recent research highlights. Furthermore, this paper addresses integrating material jetting techniques with powder bed-based MAM to fabricate functional alloys and multi-material structures. Finally, the factors influencing large-scale fabrication and potential prospects in this area are explored.

Ultra-Sensitive and Fast Humidity Sensors Based on Direct Laser-Scribed Graphene Oxide/Carbon Nanotubes Composites

In this paper, the relative humidity sensor properties of graphene oxide (GO) and graphene oxide/multiwalled nanotubes (GO/MWNTs) composites have been investigated. Composite sensors were fabricated by direct laser scribing and characterized using UV-vis-NIR, Raman, Fourier transform infrared, and X-ray photoemission spectroscopies, electron scanning microscopy coupled with energy-dispersive X-ray analysis, and impedance spectroscopy (IS). These methods confirm the composite homogeneity and laser reduction of GO/MWNT with dominant GO characteristics, while ISresults analysis reveals the circuit model for rGO-GO-rGO structure and the effect of MWNT on the sensor properties. Although direct laser scribing of GO-based humidity sensor shows an outstanding response (|ΔZ|/|Z| up to 638,800%), a lack of stability and repeatability has been observed. GO/MWNT-based humidity sensors are more conductive than GO sensors and relatively less sensitive (|ΔZ|/|Z| = 163,000%). However, they are more stable in harsh humid conditions, repeatable, and reproducible even after several years of shelf-life. In addition, they have fast response/recovery times of 10.7 s and 9.3 s and an ultra-fast response time of 61 ms when abrupt humidification/dehumidification is applied by respiration. All carbon-based sensors' overall properties confirm the advantage of introducing the GO/MWNT hybrid and laser direct writing to produce stable structures and sensors.

A Flexible Humidity Sensor with Wide Range, High Linearity, and Fast Response Based on Ultralong Na2Ti3O7 Nanowires

A flexible humidity sensor with wide sensing range, superior sensitivity, high linearity, and advanced response/recovery capabilities is extremely desirable for practical applications in human body-related (HBR) monitoring and human-machine interaction (HMI). However, the practical sensor lacks a versatile nanomaterial integrated with sensing capabilities and mechanical flexibility to meet the criteria. Herein, a comprehensive flexible humidity sensor with ultralong Na₂Ti₃O₇ nanowires (>120 μm) is subtly constructed for the first time. Owing to the distinguish nanowires network structure, the sensor exhibits wide sensing range (11-95% RH), high sensitivity (>10³), high linearity (R² > 0.98), and fast response/recovery capability (8.9/2.1 s), as well as excellent respiratory stability (>5000 s). In addition, the Na₂Ti₃O₇-based humidity sensor demonstrates superior flexibility and antibacteria capabilities, and exhibits diverse applications in respiration monitoring, noncontact detection, as well as dynamic interactive display. This work provides a multifunctional humidity sensor with excellent practicability, suggesting the great potential in next-generation human-related flexible/wearable devices.

Optical fiber humidity sensor based on a hollow core fiber filled with BPQDs-PVA

An optical fiber Fabry-Perot (FP) interferometric humidity sensor based on black phosphorus quantum dots (BPQDs) and polyvinyl alcohol (PVA) is proposed for the first time, to the best of our knowledge, and experimentally verified. The sensor is constructed by splicing a brief hollow core fiber (HCF) with a single-mode fiber (SMF) and filling the BPQDs-PVA compound into the HCF. When the proposed humidity sensor is placed in a humidity environment, BPQDs-PVA adsorbs water molecules in the air with increasing humidity, which changes the length of the FP cavity, as well as the refractive index of BPQDs-PVA, resulting in a spectral blueshift. The influence of the mixing ratio on humidity response properties has been experimentally investigated. A linear enhanced sensitivity of -0.7525n m/% R H within the humidity range of 45-75 %RH has been achieved. The maximum instability is 0.07 %RH in a long-term stability test, whereas the response and recovery times are 1.44 and 1.48 s, respectively.

Conduction Mechanism in Graphene Oxide Membranes with Varied Water Content: From Proton Hopping Dominant to Ion Diffusion Dominant

Proton conductors, particularly hydrated solid membranes, have various applications in sensors, fuel cells, and cellular biological systems. Unraveling the intrinsic proton transfer mechanism is critical for establishing the foundation of proton conduction. Two scenarios on electrical conduction, the Grotthuss and the vehicle mechanisms, have been reported by experiments and simulations. But separating and quantifying the contributions of these two components from experiments is difficult. Here, we present the conductive behavior of a two-dimensional layered proton conductor, graphene oxide membrane (GOM), and find that proton hopping is dominant at low water content, while ion diffusion prevails with increasing water content. This change in the conduction mechanism is attributable to the layers of water molecules in GOM nanosheets. The overall conductivity is greatly improved by forming one layer of water molecules. It reaches the maximum with two layers of water molecules, resulting from creating a complete hydrogen-bond network within GOM. When more than two layers of water molecules enter the GOM nanosheets, inducing the breakage of the ordered lamellar structure, protons spread in both in-plane and out-of-plane directions inside the GOM. Our results validate the existence of two conduction mechanisms and show their distinct contributions to the overall conductivity. Furthermore, these findings provide an optimization strategy for the design of realizing the fast proton transfer in materials with water participation.

Multifunctional Flexible Humidity Sensor Systems Towards Noncontact Wearable Electronics

In the past decade, the global industry and research attentions on intelligent skin-like electronics have boosted their applications in diverse fields including human healthcare, Internet of Things, human-machine interfaces, artificial intelligence and soft robotics. Among them, flexible humidity sensors play a vital role in noncontact measurements relying on the unique property of rapid response to humidity change. This work presents an overview of recent advances in flexible humidity sensors using various active functional materials for contactless monitoring. Four categories of humidity sensors are highlighted based on resistive, capacitive, impedance-type and voltage-type working mechanisms. Furthermore, typical strategies including chemical doping, structural design and Joule heating are introduced to enhance the performance of humidity sensors. Drawing on the noncontact perception capability, human/plant healthcare management, human-machine interactions as well as integrated humidity sensor-based feedback systems are presented. The burgeoning innovations in this research field will benefit human society, especially during the COVID-19 epidemic, where cross-infection should be averted and contactless sensation is highly desired.

Fast Lead-Free Humidity Sensor Based on Hybrid Halide Perovskite

An environmentally friendly analog of the prominent methylammonium lead halide perovskite, methylammonium bismuth bromide (MA3Bi2Br9), was prepared and investigated in the form of powder, single crystals and nanowires. Complete characterization via synchrotron X-ray diffraction data showed that the bulk crystal does not incorporate water into the structure. At the same time, water is absorbed on the surface of the crystal, and this modification leads to the changes in the resistivity of the material, thus making MA3Bi2Br9 an excellent candidate for use as a humidity sensor. The novel sensor was prepared from powder-pressed pellets with attached carbon electrodes and was characterized by being able to detect relative humidity over the full range (0.7–96% RH) at ambient temperature. Compared to commercial and literature values, the response and recovery times are very fast (down to 1.5 s/1.5 s).

High-precision tabletop microplotter for flexible on-demand material deposition in printed electronics and device functionalization

Microstructuring, in particular, the additive functionalization of surfaces with, e.g., conductive or bioactive materials plays a crucial role in many applications in sensing or printed electronics. Mostly, the lithography steps are made prior to assembling functionalized surfaces into the desired places of use within a bigger device as a microfluidic channel or an electronic casing. However, when this is not possible, most lithography techniques struggle with access to recessed or inclined/vertical surfaces for geometrical reasons. In particular, for "on-the-fly" printing aiming to add microstructures to already existing devices on demand and maybe even for one-time trials, e.g., in prototyping, a flexible "micropencil" allowing for direct write under direct manual control and on arbitrarily positioned surfaces would be highly desirable. Here, we present a highly flexible, micromanipulator-based setup for capillary printing of conductive and biomaterial ink formulations that can address a wide range of geometries as exemplified on vertical, recessed surfaces and stacked 3D scaffolds as models for hard to access surfaces. A wide range of feature sizes from tens to hundreds of micrometer can be obtained by the choice of capillary sizes and the on-demand in situ writing capabilities are demonstrated with completion of a circuit structure by gold line interconnects deposited with the setup.

2D Molybdenum Carbide MXenes for Enhanced Selective Detection of Humidity in Air

2D transition metal carbides and nitrides (MXenes) open up novel opportunities in gas sensing with high sensitivity at room temperature. Herein, 2D Mo₂ CT_x flakes with high aspect ratio are successfully synthesized. The chemiresistive effect in a sub-µm MXene multilayer for different organic vapors and humidity at 10¹ -10⁴  ppm in dry air is studied. Reasonably, the low-noise resistance signal allows the detection of H₂ O down to 10 ppm. Moreover, humidity suppresses the response of Mo₂ CT_x to organic analytes due to the blocking of adsorption active sites. By measuring the impedance of MXene layers as a function of ac frequency in the 10^-2 -10⁶  Hz range, it is shown that operation principle of the sensor is dominated by resistance change rather than capacitance variations. The sensor transfer function allows to conclude that the Mo₂ CT_x chemiresistance is mainly originating from electron transport through interflake potential barriers with heights up to 0.2 eV. Density functional theory calculations, elucidating the Mo₂ C surface interaction with organic analytes and H₂ O, explain the experimental data as an energy shift of the density of states under the analyte's adsorption which induces increasing electrical resistance.

Recent Advances on 2D Materials towards 3D Printing

In recent years, 2D materials have been implemented in several applications due to their unique and unprecedented properties. Several examples can be named, from the very first, graphene, to transition-metal dichalcogenides (TMDs, e.g., MoS2), two-dimensional inorganic compounds (MXenes), hexagonal boron nitride (h-BN), or black phosphorus (BP). On the other hand, the accessible and low-cost 3D printers and design software converted the 3D printing methods into affordable fabrication tools worldwide. The implementation of this technique for the preparation of new composites based on 2D materials provides an excellent platform for next-generation technologies. This review focuses on the recent advances of 3D printing of the 2D materials family and its applications; the newly created printed materials demonstrated significant advances in sensors, biomedical, and electrical applications.

Mesoporous Solid and Yolk-Shell Titania Microspheres as Touchless Colorimetric Sensors with High Responsivity and Ultrashort Response Times

Touchless user interfaces offer an attractive pathway toward hygienic, remote, and interactive control over devices. Exploiting the humidity generated from fingers or human speech is a viable avenue for realizing such technology. Herein, titania microspheres including solid and yolk-shell structures with varying microstructural characteristics were demonstrated as high-performance, ultrafast, and stable optical humidity sensors aimed for touchless control. When water molecules enter the microporous network of the microspheres, the effective refractive index of the microsphere increases, causing a detectable change in the light scattering behavior. The microstructural properties of the microspheres, namely, the pore characteristics, crystallinity, and particle size, were examined in relation to the humidity-sensing performance, establishing optimum structural conditions for realizing humidity-responsive wavelength shifts above 100 nm, near full-scale relative humidity (RH) responsivity, ultrashort response times below 30 ms, and prolonged lifetimes. These optimized microspheres were used to demonstrate a colorimetric touchless sensor that responds to humidity from a finger and a microcontroller-based detector that translates the moisture pattern from human speech to electrical signals in real time. These results provide practical strategies for enabling humidity-based touchless user interfaces.

Noble metal (Ag, Au, Pd and Pt) doped TaS2 monolayer for gas sensing: a first-principles investigation

Two-dimensional (2D) layered nanomaterials have attracted increasing attention in gas sensing due to their graphene-like properties. Although the gas sensing performances of 2D layered semiconductor transition metal dichalcogenides (TMDs), including MoS₂, WS₂, MoSe₂ and WSe₂, have been extensively studied, it has remained a grand challenge to develop a high-performance gas sensing material that can meet practical applications. Tantalum disulfide (TaS₂), as a metallic TMD with low resistance and high current signal, has great promise in high-performance gas sensing. In stark contrast with Mo and W, Ta has a stronger positive charge, which contributes to a higher surface energy to capture gas molecules. Herein, through calculating the adsorption energy, charge transfer, electronic structure, and work function of the adsorption system with first-principles calculations, we first systematically studied the performance of noble metal atom substitution doping on a TaS₂ monolayer for toxic nitrogen-containing gas (NH₃, NO and NO₂) sensing. We found that the TaS₂ monolayer exhibits excellent NO sensing performance with an adsorption energy of 0.49 eV and a charge transfer of 0.17 e. However, it has a considerable adsorption energy (-0.22 and -0.39 eV) to NH₃ and NO₂ molecules, but a low charge transfer (-0.03 and 0.04 e) between the gas molecules and the TaS₂ monolayer. To further enhance the gas-sensing performance of the TaS₂ monolayer, noble metal atoms (Ag, Au, Pd and Pt) were substitutionally doped into the lattice of the TaS₂ monolayer. The results showed that the values of adsorption energy and charge transfer can be significantly improved, and the electronic structure and work function of the doping system has also greatly changed, which makes it much easier to detect the changes in electrical signal due to gas adsorption. Our work indicates that the intrinsic as well as the noble metal doped TaS₂ monolayers are promising candidates for high-performance gas sensors.

Liquid-Exfoliated 2D Materials for Optoelectronic Applications

Two-dimensional (2D) materials have attracted tremendous research attention in recent days due to their extraordinary and unique properties upon exfoliation from the bulk form, which are useful for many applications such as electronics, optoelectronics, catalysis, etc. Liquid exfoliation method of 2D materials offers a facile and low-cost route to produce large quantities of mono- and few-layer 2D nanosheets in a commercially viable way. Optoelectronic devices such as photodetectors fabricated from percolating networks of liquid-exfoliated 2D materials offer advantages compared to conventional devices, including low cost, less complicated process, and higher flexibility, making them more suitable for the next generation wearable devices. This review summarizes the recent progress on metal-semiconductor-metal (MSM) photodetectors fabricated from percolating network of 2D nanosheets obtained from liquid exfoliation methods. In addition, hybrids and mixtures with other photosensitive materials, such as quantum dots, nanowires, nanorods, etc. are also discussed. First, the various methods of liquid exfoliation of 2D materials, size selection methods, and photodetection mechanisms that are responsible for light detection in networks of 2D nanosheets are briefly reviewed. At the end, some potential strategies to further improve the performance the devices are proposed.

Humidity sensor based on Gallium Nitride for real time monitoring applications

Gallium Nitride (GaN) remarkably shows high electron mobility, wide energy band gap, biocompatibility, and chemical stability. Wurtzite structure makes topmost Gallium atoms electropositive, hence high ligand binding ability especially to anions, making it usable as humidity sensor due to water self-ionization phenomenon. In this work, thin-film GaN based humidity sensor is fabricated through pulse modulated DC magnetron sputtering. Interdigitated electrodes (IDEs) with 100 μm width and spacing were inkjet printed on top of GaN sensing layer to further enhance sensor sensitivity. Impedance, capacitance, and current response were recorded for humidity and bio-sensing applications. The sensor shows approximate linear impedance response between 0 and 100% humidity range, the sensitivity of 8.53 nF/RH% and 79 kΩ/RH% for capacitance and impedance, and fast response (Tres) and recovery (Trec) time of 3.5 s and 9 s, respectively. The sensor shows little hysteresis of < 3.53% with stable and wide variations for accurate measurements. Especially, it demonstrates temperature invariance for thermal stability. Experimental results demonstrate fabricated sensor effectively evaluates plant transpiration cycle through water level monitoring by direct attachment onto leaves without causing any damage as well as freshness level of meat loaf. These properties of the proposed sensor make it a suitable candidate for future electronics providing a low-cost platform for real time monitoring applications.

Reversing the Humidity Response of MoS2- and WS2-Based Sensors Using Transition-Metal Salts

Two-dimensional materials, such as transition-metal dichalcogenides (TMDs), are attractive candidates for sensing applications due to their high surface-to-volume ratio, chemically active edges, and good electrical properties. However, their electrical response to humidity is still under debate and experimental reports remain inconclusive. For instance, in different studies, the impedance of MoS₂-based sensors has been found to either decrease or increase with increasing humidity, compromising the use of MoS₂ for humidity sensing. In this work, we focus on understanding the interaction between water and TMDs. We fabricated and studied humidity sensors based on MoS₂ and WS₂ coated with copper chloride and silver nitrate. The devices exhibited high chemical stability and excellent humidity sensing performance in relative humidity between 4 and 80%, with response and recovery times of 2 and 40 s, respectively. We have systematically investigated the humidity response of the materials as a function of the type and amount of induced metal salt and observed the reverse action of sensing mechanisms. This phenomenon is explained based on a detailed structural analysis of the samples considering the Grotthuss mechanism in the presence of charge trapping, which was represented by an appropriate lumped-element model. Our findings open up a possibility to tune the electrical response in a facile manner and without compromising the high performance of the sensor. They offer an insight into the time-dependent performance and aging of the TMD-based sensing devices.

Sniffing Bacteria with a Carbon-Dot Artificial Nose

Novel artificial nose based upon electrode-deposited carbon dots (C-dots). Significant selectivity and sensitivity determined by "polarity matching" between the C-dots and gas molecules. The C-dot artificial nose facilitates, for the first time, real-time, continuous monitoring of bacterial proliferation and discrimination among bacterial species, both between Gram-positive and Gram-negative bacteria and between specific strains. Machine learning algorithm furnishes excellent predictability both in the case of individual gases and for complex gas mixtures. Continuous, real-time monitoring and identification of bacteria through detection of microbially emitted volatile molecules are highly sought albeit elusive goals. We introduce an artificial nose for sensing and distinguishing vapor molecules, based upon recording the capacitance of interdigitated electrodes (IDEs) coated with carbon dots (C-dots) exhibiting different polarities. Exposure of the C-dot-IDEs to volatile molecules induced rapid capacitance changes that were intimately dependent upon the polarities of both gas molecules and the electrode-deposited C-dots. We deciphered the mechanism of capacitance transformations, specifically substitution of electrode-adsorbed water by gas molecules, with concomitant changes in capacitance related to both the polarity and dielectric constants of the vapor molecules tested. The C-dot-IDE gas sensor exhibited excellent selectivity, aided by application of machine learning algorithms. The capacitive C-dot-IDE sensor was employed to continuously monitor microbial proliferation, discriminating among bacteria through detection of distinctive "volatile compound fingerprint" for each bacterial species. The C-dot-IDE platform is robust, reusable, readily assembled from inexpensive building blocks and constitutes a versatile and powerful vehicle for gas sensing in general, bacterial monitoring in particular.

Computational insights into modulating the performance of MXene based electrode materials for rechargeable batteries

MXene, a still-growing large family of two-dimensional (2D) materials, has aroused enormous attention in the scientific community. Owing to their high specific surface area, good electronic conductivity, stability, and hydrophilicity, MXene has found a wide application involving electromagnetic interference shielding, sensors, catalysis, and energy storage, etc. In the field of energy storage, MXenes are promising electrode materials for various metal-ion batteries and they are also effective anchoring materials for Li-S batteries. One of the most unique features of MXene is its abundant compositions, which renders us large room to modulate its properties. Besides, other effective approaches applicable to traditional 2D materials can also be used to optimize the performance of MXene. Theoretical calculations have played a significant role in predicting and screening high-performance MXene based electrode materials. So far, theoretical researchers have made much progress in optimizing the performance of MXene as electrode materials for various rechargeable batteries. In the present review, started by a brief introduction of the involved mechanism and basic calculation methods, we comprehensively overview the latest theoretical studies of modulating the performance of MXene based electrode materials for rechargeable batteries.

Highly stable Pd/HNb3O8-based flexible humidity sensor for perdurable wireless wearable applications

Real-time, daily health monitoring can provide large amounts of patient data, which may greatly improve the likelihood of diagnosing health conditions at an early stage. One potential sensor is a flexible humidity sensor to monitor moisture and humidity information such as dehydration. However, achieving a durable functional nanomaterial-based flexible humidity sensor remains a challenge due to partial desorption of water molecules during the recovery process, especially at high humidities. In this work, we demonstrate a highly stable resistive-type Pd/HNb₃O₈ humidity sensor, which exhibits a perdurable performance for over 100 h of cycle tests under a 90% relative humidity (RH) without significant performance degradation. One notable advantage of the Pd/HNb₃O₈ humidity sensor is its ability to regulate hydroniums due to the strong reducibility of H atoms dissociated on the Pd surface. This feature realizes a high stability even at a high humidity (99.9% RH). Using this superior performance, the Pd/HNb₃O₈ humidity sensor realizes wireless monitoring of the changes in the fingertip humidity of an adult under different physiological states, demonstrating a facile and reliable path for dehydration diagnosis.

High-Performance Humidity Sensor Based on the Graphene Flower/Zinc Oxide Composite

Performance of an electronic device relies heavily on the availability of a suitable functional material. One of the simple, easy, and cost-effective ways to obtain novel functional materials with improved properties for desired applications is to make composites of selected materials. In this work, a novel composite of transparent n-type zinc oxide (ZnO) with a wide bandgap and a unique structure of graphene in the form of a graphene flower (GrF) is synthesized and used as the functional layer of a humidity sensor. The (GrF/ZnO) composite was synthesized by a simple sol–gel method. Morphological, elemental, and structural characterizations of GrF/ZnO composite were performed by a field emission scanning electron microscope (FESEM), energy-dispersive spectroscopy (EDS), and an x-ray diffractometer (XRD), respectively, to fully understand the properties of this newly synthesized functional material. The proposed humidity sensor was tested in the relative humidity (RH) range of 15% RH% to 86% RH%. The demonstrated sensor illustrated a highly sensitive response to humidity with an average current change of 7.77 μA/RH%. Other prominent characteristics shown by this device include but were not limited to high stability, repeatable results, fast response, and quick recovery time. The proposed humidity sensor was highly sensitive to human breathing, thus making it a promising candidate for various applications related to health monitoring.

Survey of Saliva Components and Virus Sensors for Prevention of COVID-19 and Infectious Diseases

The United States Centers for Disease Control and Prevention considers saliva contact the lead transmission means of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), which causes the coronavirus disease 2019 (COVID-19). Saliva droplets or aerosols expelled by heavy breathing, talking, sneezing, and coughing may carry this virus. People in close distance may be exposed directly or indirectly to these droplets, especially those droplets that fall on surrounding surfaces and people may end up contracting COVID-19 after touching the mucosa tissue on their faces. It is of great interest to quickly and effectively detect the presence of SARS-CoV-2 in an environment, but the existing methods only work in laboratory settings, to the best of our knowledge. However, it may be possible to detect the presence of saliva in the environment and proceed with prevention measures. However, detecting saliva itself has not been documented in the literature. On the other hand, many sensors that detect different organic components in saliva to monitor a person's health and diagnose different diseases that range from diabetes to dental health have been proposed and they may be used to detect the presence of saliva. This paper surveys sensors that detect organic and inorganic components of human saliva. Humidity sensors are also considered in the detection of saliva because a large portion of saliva is water. Moreover, sensors that detect infectious viruses are also included as they may also be embedded into saliva sensors for a confirmation of the virus' presence. A classification of sensors by their working principle and the substance they detect is presented. This comparison lists their specifications, sample size, and sensitivity. Indications of which sensors are portable and suitable for field application are presented. This paper also discusses future research and challenges that must be resolved to realize practical saliva sensors. Such sensors may help minimize the spread of not only COVID-19 but also other infectious diseases.

Integration of Conductive Materials with Textile Structures, an Overview

In the last three decades, the development of new kinds of textiles, so-called smart and interactive textiles, has continued unabated. Smart textile materials and their applications are set to drastically boom as the demand for these textiles has been increasing by the emergence of new fibers, new fabrics, and innovative processing technologies. Moreover, people are eagerly demanding washable, flexible, lightweight, and robust e-textiles. These features depend on the properties of the starting material, the post-treatment, and the integration techniques. In this work, a comprehensive review has been conducted on the integration techniques of conductive materials in and onto a textile structure. The review showed that an e-textile can be developed by applying a conductive component on the surface of a textile substrate via plating, printing, coating, and other surface techniques, or by producing a textile substrate from metals and inherently conductive polymers via the creation of fibers and construction of yarns and fabrics with these. In addition, conductive filament fibers or yarns can be also integrated into conventional textile substrates during the fabrication like braiding, weaving, and knitting or as a post-fabrication of the textile fabric via embroidering. Additionally, layer-by-layer 3D printing of the entire smart textile components is possible, and the concept of 4D could play a significant role in advancing the status of smart textiles to a new level.

Inkjet-Printed Ultrathin MoS2-Based Electrodes for Flexible In-Plane Microsupercapacitors

Flexible and wearable energy storage microdevice systems with high performance and safety are promising candidates for the electronics of on-chip integration. Herein, we demonstrate inkjet-printed ultrathin electrodes based on molybdenum disulfide (MoS₂) nanosheets for flexible and all-solid-state in-plane microsupercapacitors (MSCs) with high capacitance. The MoS₂ nanosheets were uniformly dispersed in the low-boiling point and nontoxic solvent isopropanol to form highly concentrated inks suitable for inkjet printing. The MSCs were assembled by printing the highly concentrated MoS₂ inks on a polyimide substrate with appropriate surface tension using a simple and low-cost desktop inkjet printer. Because of the two-dimensional structure of MoS₂ nanosheets, the as-assembled planar MSCs have high loadings of active materials per unit area, resulting in more flexibility and thinness than the capacitors with a traditional sandwich structure. These planar MSCs can not only possess any collapsible shape through the computer design but also exhibit excellent electrochemical performance (with a maximum energy density of 0.215 mW h cm^-3 and a high-power energy density of 0.079 W cm^-3), outstanding mechanical flexibility (almost no degradation of capacitance at different bending radii), good cycle stability (85.6% capacitance retention even after 10,000 charge-discharge cycles), and easy scale-up. Moreover, a blue light-emitting diode can be powered using five MSCs connected in series. The in-plane and low-cost MSCs with high energy densities have great application potential for integrated energy storage systems including wearable planar solar cells and other electronics.

Characterizing carrier transport in nanostructured materials by force-resolved microprobing

The advent of novel nanostructured materials has enabled wearable and 3D electronics. Unfortunately, their characterization represents new challenges that are not encountered in conventional electronic materials, such as limited mechanical strength, complex morphology and variability of properties. We here demonstrate that force-resolved measurements can overcome these issues and open up routes for new applications. First, the contact resistance to 2D materials was found to be sensitively depending on the contact force and, by optimizing this parameter, reliable contacts could be repeatably formed without damage to the fragile material. Moreover, resistance of three-dimensional surfaces could be investigated with high accuracy in spatial position and signal through a force-feedback scheme. This force-feedback approach furthermore permitted large-scale statistical characterization of mobility and doping of 2D materials in a desktop-sized automatic probing system that fits into glove boxes and vacuum enclosures using easily available and low-cost components. Finally, force-sensitive measurements enable characterization of complex electronic properties with high lateral resolution. To illustrate this ability, the spatial variation of a surface’s electrochemical response was investigated by scanning a single electrolyte drop across the sample.

Inter-dependency between surface morphology and sensitive low RH detection: exploration of an intricate mechanism to extend the lower detection limit

The water vapor molecular dynamics inside a pore structure control both molecular adsorption and desorption processes and the limit of minimum detection (LOD). Pore morphology design, and a higher concentration of electrolyte-driven anions, in accordance with the kinetics of water vapor molecules, is reported here, as the ultimate answer to extremely low relative humidity (RH) detection. In this report, a series of samples were prepared by anodization in different voltage windows, related to specific electrolyte solutions. The sensing attributes comprised: (i) a LOD of ∼3 RH%, (ii) excellent response time (6 s) and recovery time (54 s), and (iii) a hysteresis loss of ∼0.36%, with sustained stability over the period of one year; all these occurring in a sample with a pore diameter ∼5 nm ±3 nm. Interestingly, the LOD extend towards a lower RH% with a decrease in pore diameter; and a suitable explanation is given for the entire range of humidity level, in terms of the molecular mean free path, loss of kinetic energy due to scattering inside the pores, and subsequent overall loss of Brownian energy of the molecules. It is inferred from the sensing response characteristics that pore morphology and lower detection limit are interrelated; therefore, a further extension in LOD from extremely low RH% to trace levels, needs careful engineering of the pore morphology and parameters related to molecular kinetics.

Using chaotic advection for facile high-throughput fabrication of ordered multilayer micro- and nanostructures: continuous chaotic printing

This paper introduces the concept of continuous chaotic printing, i.e. the use of chaotic flows for deterministic and continuous extrusion of fibers with internal multilayered micro- or nanostructures. Two free-flowing materials are coextruded through a printhead containing a miniaturized Kenics static mixer (KSM) composed of multiple helicoidal elements. This produces a fiber with a well-defined internal multilayer microarchitecture at high-throughput (>1.0 m min^-1). The number of mixing elements and the printhead diameter determine the number and thickness of the internal lamellae, which are generated according to successive bifurcations that yield a vast amount of inter-material surface area (∼10² cm² cm^-3) at high resolution (∼10 µm). This creates structures with extremely high surface area to volume ratio (SAV). Comparison of experimental and computational results demonstrates that continuous chaotic 3D printing is a robust process with predictable output. In an exciting new development, we demonstrate a method for scaling down these microstructures by 3 orders of magnitude, to the nanoscale level (∼150 nm), by feeding the output of a continuous chaotic 3D printhead into an electrospinner. The simplicity and high resolution of continuous chaotic printing strongly supports its potential use in novel applications, including-but not limited to-bioprinting of multi-scale layered biological structures such as bacterial communities, living tissues composed of organized multiple mammalian cell types, and fabrication of smart multi-material and multilayered constructs for biomedical applications.

Strain Engineering on the Electronic and Optical Properties of WSSe Bilayer

Controllable optical properties are important for optoelectronic applications. Based on the unique properties and potential applications of two-dimensional Janus WSSe, we systematically investigate the strain-modulated electronic and optical properties of WSSe bilayer through the first-principle calculations. The preferred stacking configurations and chalcogen orders are determined by the binding energies. The bandgap of all the stable structures are found sensitive to the external stress and could be tailored from semiconductor to metallicity under appropriate compressive strains. Atomic orbital projected energy bands reveal a positive correlation between the degeneracy and the structural symmetry, which explains the bandgap evolutions. Dipole transition preference is tuned by the biaxial strain. A controllable transformation between anisotropic and isotropic optical properties is achieved under an around - 6%~- 4% critical strain. The strain controllable electronic and optical properties of the WSSe bilayer may open up an important path for exploring next-generation optoelectronic applications.

Printable Highly Stable and Superfast Humidity Sensor Based on Two Dimensional Molybdenum Diselenide

Transition metal dichalcogenides (TMDCs) are promising materials for sensing applications, due to their exceptional high performance in nano-electronics. Inherentely, the chemical and thermal responses of TMDCs are highly stable, hence, they pave way for real time sensor applications. This article proposes inceptively a stable and superfast humidity sensor using two-dimensional (2D) Molybdenum diselenide (MoSe₂) through printed technlogies. The 2D MoSe₂ ink is synthesized through wet grinding to achieve few-layered nano-flakes. Inter digital electrodes (IDEs) are fabricated via screen-printing on Polyethylene terephthalate (PET) substrate and thin film of MoSe₂ nano-flakes is fabricated through spin coating. The impedance and capacitance response are recorded at 1 kHz between temperature levels ranging from 20-30 °C. The impedance and capacitance hysteresis results are recorded <1.98% and <2.36%, respectively, ensuring very good repeatability during humidification and dehumidification. The stability of impedance and capacitance response are recorded with maximum error rate of ~ 0.162% and ~ 0.183%, respectively. The proposed sensor shows fast impedance response time (T_res) of ~ 0.96 s, and recovery time (T_rec) of ~ 1.03 s, which has T_res of ~ 1.87 s, and T_rec of ~ 2.13 s for capacitance. It is aimed to develop a high performance and stable humidity sensor for various monitoring applications.

Rapid response flexible humidity sensor for respiration monitoring using nano-confined strategy

Development of wearable devices for continuous respiration monitoring is of great importance for evaluating human health. Here, we propose a new strategy to achieve rapid respiration response by confining conductive polymers into 1D nanowires which facilitates the water molecules absorption/desorption and maximizes the sensor response to moisture. The nanowires arrays were fabricated through a low-cost nanoscale printing approach on flexible substrate. The nanoscale humidity sensor shows a high sensitivity (5.46%) and ultrafast response (0.63 s) when changing humidity between 0% and 13% and can tolerate 1000 repetitions of bending to a curvature radius of 10 mm without influencing its performance. Benefited by its fast response and low power assumption, the humidity sensor was demonstrated to monitor human respiration in real time. Different respiration patterns including normal, fast and deep respiration can be distinguished accurately.

Ultrafast Self-Assembly MoS2/Cu(OH)2 Nanowires for Highly Sensitive Gamut Humidity Detection with an Enhanced Self-Recovery Ability

Humidity sensors have attracted intense interest in various fields because of the importance of humidity detection. Different methods have been adopted to enhance sensing performances of humidity sensors, while it is challenging for researchers to avoid the invalidation of the sensors after being wet. Here, we, for the first time, introduce self-assembly MoS₂/Cu(OH)₂ nanowires fabricated by liquid self-spreading-coating-evaporating as sensing materials and present MoS₂/Cu(OH)₂ nanowire-based quartz crystal microbalance gamut humidity sensors with superior sensitivity and self-recovery ability. The sensors deliver a remarkable sensitivity (60.8 Hz/% RH) under a wide range (0-97% RH) with fast response (1.9 s)/recovery time (3.8 s) and upgrade self-recovery ability that can maintain their original performances even after being wet, frozen, and heated or immersed in water. The sensors are also employed to monitor water counting, dew alarming, and human breathing (within 4 s), further showing their ultrahigh sensitivity for water molecules. The underlying humidity-sensing mechanism is interpreted by density functional theory calculations and in-situ Fourier transform infrared spectra experiments adequately, revealing that the high sensing performances are attributed to abundant adsorption sites and physisorption of water molecules. Our work proposes a strategy for transferring materials to arbitrary nanostructures swiftly and demonstrates new perspectives for highly sensitive humidity detection as well as self-recovery ability.

Elemental Substitution of Two-Dimensional Transition Metal Dichalcogenides (MoSe2 and MoTe2): Implications for Enhanced Gas Sensing

The quest for a suitable material with the potential of capturing toxic nitrogen-containing gases (NH₃, NO, and NO₂) has motivated us to explore the structural, electronic, and gas-sensing properties of transition metal dichalcogenides (TMDs); MoSe₂ and MoTe₂. Spin-polarized density functional theory (DFT) calculations demonstrate weak binding of nitrogen-containing gases (NCGs) with the pristine TMDs, which limits the use of the latter as efficient sensing materials. However, suitable elemental substitutions improve the binding mechanism enormously. Our dispersion-corrected DFT calculations revealed that Se (Te) substitution with Ge (Sb) in MoSe₂ (MoTe₂) not only enhances the binding energies but also causes a significant variation in the electronic properties and work functions. A charge-transfer mechanism based on Bader analysis indicates that transfer of charges from MoSe₂-Ge (MoTe₂-Sb) to the NCGs is responsible for the improvement in the binding characteristics. Based on our findings, it is evident that 2.08% of elemental substitutional makes both MoSe₂ and MoTe₂ promising materials for NH₃, NO, and NO₂ gas sensing.

Screen-Printing of a Highly Conductive Graphene Ink for Flexible Printed Electronics

Conductive inks for the future printed electronics should have the following merits: high conductivity, flexibility, low cost, and compatibility with wide range of substrates. However, the state-of-the-art conductive inks based on metal nanoparticles are high in cost and poor in flexibility. Herein, we reported a highly conductive, low cost, and super flexible ink based on graphene nanoplatelets. The graphene ink has been screen-printed on plastic and paper substrates. Combined with postprinting treatments including thermal annealing and compression rolling, the printed graphene pattern shows a high conductivity of 8.81 × 10⁴ S m^-1 and good flexibility without significant conductivity loss after 1000 bending cycles. We further demonstrate that the printed highly conductive graphene patterns can act as current collectors for supercapacitors. The supercapacitor with the printed graphene pattern as the current collector and printed activated carbon as the active material shows a good rate capability of up to 200 mV s^-1. This work potentially provides a promising route toward the large-scale fabrication of low cost yet flexible printed electronic devices.

Ultrahigh Performance of Nanoengineered Graphene-Based Natural Jute Fiber Composites

Natural fibers composites are considered as a sustainable alternative to synthetic composites due to their environmental and economic benefits. However, they suffer from poor mechanical and interfacial properties due to a random fiber orientation and weak fiber-matrix interface. Here we report nanoengineered graphene-based natural jute fiber preforms with a new fiber architecture (NFA) which significantly improves their mechanical properties and performances. Our graphene-based NFA of jute fiber preform enhances the Young modulus of jute-epoxy composites by ∼324% and tensile strength by ∼110% more than untreated jute fiber composites, by arranging fibers in a parallel direction through individualization and nanosurface engineering with graphene derivatives. This could potentially lead to manufacturing of high-performance natural alternatives to synthetic composites in various stiffness-driven applications.

Highly conductive carbon-based aqueous inks toward electroluminescent devices, printed capacitive sensors and flexible wearable electronics

Carbon-based conductive inks are one of the most important materials in the field of printing electronics. However, most carbon-based conductive inks with small electrical resistance are expensive, such as graphene. It limits the commercial use of carbon inks in the fields of flexible electronics and printed electronics. Here, we propose a low-cost and environmentally friendly formula based on dihydroxyphenyl-functionalized multi-walled carbon nanotubes (MWNT-f-OH)/carbon black/graphite as conductive fillers and waterborne acrylic resins as binders for preparing highly conductive carbon-based aqueous inks (HCCA-inks). Our study showed that when the mass fraction of carbon black, graphite and MWNT-f-OH was 3.0%, 10.2% and 4.1%, respectively, on a thickness of 40 μm, optimal conductivity (sheet resistance up to 29 Ω sq-1) was achieved, and the printed HCCA-inks on a paper could withstand extremely high folding cycles (>2000 cycles) while the resistance value of the flexible circuit only increased by 11%. The carbon-based aqueous inks showed high electrical conductivity and excellent mechanical stability, which makes it possible for them to be used as flexible wearable electronics, electroluminescent (EL) devices and printed capacitive sensors.

The critical role of hydroxyl groups in water vapor sensing of graphene oxide

Excellent adsorption of water vapor on the surface of graphene oxide (GO), which contains several inherited functional groups, leads to the development of improved humidity monitoring systems that can urgently meet the high industrial demand. In this study, we fabricated a GO-based humidity sensor and investigated the influence of hydroxyl group concentration on its performance. The sensor exhibited excellent humidity sensing performance in terms of sensitivity (sensor response ∼40 for 90% RH), selectivity, stability (both long-term and short-term) and reaction time (τ _res = 8.5 s and τ _rec = 13 s). Additionally, this sensor does not require external power consumption for heating; thus, the aforementioned performance (recorded at room temperature) with an applied voltage of 0.1 V can significantly reduce the power/energy consumption to about ∼1.314 × 10^-4 kW h per year. In the future, this type of sensor can be integrated into smart humidity monitoring systems to not only monitor but also control the humidity levels on a specific application area. Based on complementary characterization techniques, such as XRD, AFM, Raman and electrical measurement, here, we propose a physical-chemical sensing model to elucidate the aforementioned sensor characteristics.

Black Phosphorus-New Nanostructured Material for Humidity Sensors: Achievements and Limitations

The prospects of using nanostructured black phosphorus for the development of humidity sensors are considered. It was shown that black phosphorus has a set of parameters that distinguish it from other two-dimensional (2D) materials such as graphene, silicone, and dichalcogenides. At the same time, an analysis of shortcomings, limiting the use of black phosphorus as a humidity sensitive material in devices aimed for market of humidity sensors, was also conducted.

Advanced electronic skin devices for healthcare applications

Electronic skin, a kind of flexible electronic device and system inspired by human skin, has emerged as a promising candidate for wearable personal healthcare applications. Wearable electronic devices with skin-like properties will provide platforms for continuous and real-time monitoring of human physiological signals such as tissue pressure, body motion, temperature, metabolites, electrolyte balance, and disease-related biomarkers. Transdermal drug delivery devices can also be integrated into electronic skin to enhance its non-invasive, real-time dynamic therapy functions. This review summarizes the recent progress in electronic skin devices for applications in human health monitoring and therapy systems as well as several potential mass production technologies such as inkjet printing and 3D printing. The opportunities and challenges in broadening the applications of electronic skin devices in practical healthcare are also discussed.

Functional inks and printing of two-dimensional materials

Graphene and related two-dimensional materials provide an ideal platform for next generation disruptive technologies and applications. Exploiting these solution-processed two-dimensional materials in printing can accelerate this development by allowing additive patterning on both rigid and conformable substrates for flexible device design and large-scale, high-speed, cost-effective manufacturing. In this review, we summarise the current progress on ink formulation of two-dimensional materials and the printable applications enabled by them. We also present our perspectives on their research and technological future prospects.

Precise and label-free tumour cell recognition based on a black phosphorus nanoquenching platform

Duplexed recognition of label-free breast cancer cells: a duplexed assay platform based on a BP nanoquencher allows simultaneous detection of two tumor markers within one run.