Chameleon-Inspired Colorimetric Sensors for Real-Time Detections with Humidity.

In recent decades, vapor sensors have gained substantial attention for their crucial roles in environmental monitoring and pharmaceutical applications. Herein, we introduce a chameleon-inspired colorimetric (CIC) sensor, detailing its design, fabrication, and versatile applications. The sensor seamlessly combines a PEDOT:PSS vapor sensor with a colorimetric display, using thermochromic liquid crystal (TLC). We further explore the electrical characteristics of the CIC sensor when doped with ethylene glycol (EG) and polyvinyl alcohol (PVA). Comparative analyses of resistance change rates for different weight ratios of EG and PVA provide insights into fine-tuning the sensor's responsiveness to varying humidity levels. The CIC sensor's proficiency in measuring ambient humidity is investigated under a voltage input as small as 2.6 V, capturing resistance change rates and colorimetric shifts at relative humidity (RH) levels ranging from 20% to 90%. Notably, the sensor exhibits distinct resistance sensitivities of 9.7 mΩ (0.02% ∆R/R0)/%RH, 0.5 Ω (0.86% ∆R/R0)/%RH, and 5.7 Ω (9.68% ∆R/R0)/%RH at RH 20% to 30%, RH 30% to 80%, and RH 80% to 90%, respectively. Additionally, a linear temperature change is observed with a sensitivity of -0.04 °C/%RH. The sensor also demonstrates a colorimetric temperature sensitivity of -82,036 K/%RH at RH 20% to 30% and -514 K/%RH at RH 30% to 90%, per captured image. Furthermore, real-time measurements of ethanol vapor with varying concentrations showcase the sensor's applicability in gas sensing applications. Overall, we present a comprehensive exploration of the CIC sensor, emphasizing its design flexibility, electrical characteristics, and diverse sensing capabilities. The sensor's potential applications extend to real-time environmental monitoring, highlighting its promising role in various gas sensing fields.

Directional Activated Exciton Highway via Fractal Electric Field Modulation for Ultrasensitive Carbon Nanotube-Based Sensors.

The electrical vapor sensor based on carbon nanotubes (CNTs) has attracted wide attention due to its excellent conductivity, stable interfacial structure, and low dimensional quantum effects. However, the conductivity and contact interface activity were still limited by the random distribution of coated CNTs, which led to limited performance. We developed a new strategy to unify the CNT directions with image fractal designing of the electrode system. In such a system, directional aligned CNTs were gained under a well-modulated electric field, leading to microscale CNT exciton highways and molecule-scale host-guest site activation. The carrier mobility of the aligned CNT device is 20-fold higher than that of the random network CNT device. With excellent electrical properties, such modulated CNT devices based on fractal electrodes behave as an ultrasensitive vapor sensor for methylphenethylamine, a mimic of illicit drug methamphetamine. The detection limit reached as low as 0.998 ppq, 6 orders of magnitude sensitive than the reported 5 ppb record based on interdigital electrodes with random distributed CNTs. Since the device is easily fabricated in wafer-level and compatible with the CMOS process, such a fractal design strategy for aligned CNT preparation will be widely applied in a variety of wafer-level electrical functional devices.

Hydrogen-Terminated Two-Dimensional Germanane/Silicane Alloys as Self-Powered Photodetectors and Sensors.

2D monoelemental materials, particularly germanene and silicene (the single layer of germanium and silicon), which are the base materials for modern electronic devices demonstrated tremendous attraction for their 2D layer structure along with the tuneable electronics and optical band gap. The major shortcoming of synthesized thermodynamically very unstable layered germanene and silicene with their inclination toward oxidation was overcome by topochemical deintercalation of a Zintl phase (CaGe2, CaGe1.5Si0.5, and CaGeSi) in a protic environment. The exfoliated Ge-H, Ge0.75Si0.25H, and Ge0.5Si0.5H were successfully synthesized and employed as the active layer for photoelectrochemical photodetectors, which showed broad response (420-940 nm), unprecedented responsivity, and detectivity on the order of 168 µA W-1 and 3.45 × 108 cm Hz1/2 W-1, respectively. The sensing capability of exfoliated germanane and silicane composites was explored using electrochemical impedance spectroscopy with ultrafast response and recovery time of less than 1 s. These positive findings serve as the application of exfoliated germanene and silicene composites and can pave a new path to practical applications in efficient future devices.

Zn and Co ferrite nanoparticles: towards the applications of sensing and adsorption studies.

An important deliberation of this current work is the impending applications of bivalent transition metals doped with nano ferrites and to study their emerging properties of magnetically active ferrites, which constitute oxides of iron (different conformers most demanding γ-Fe2O3) and transition metal complexes of bivalent metal oxides like cobalt (Co(II)) and magnesium (Mg(II)). Fe3+ ions occupy tetrahedral sites; the rest of Fe3+ and the Co2+ ions occupy octahedral sites. For the synthesis, a self-propagating method of combustion at lower temperature was used. Zinc and cobalt nano ferrites are synthesized from the chemical coprecipitation method of 20 to 90 nm in average size, characterized thoroughly employing FTIR and PXRD and surface morphology studied using SEM. These results explain the existence of ferrite nanoparticles in cubic spinel. Magnetically active metal oxide nanoparticles are now commonly employed in main studies of sensing, absorption, and other properties. All studies showed the interesting results.

2D Few-Layered PdPS: Toward High-Efficient Self-Powered Broadband Photodetector and Sensors.

Photodetectors and sensors have a prominent role in our lives and cover a wide range of applications, including intelligent systems and the detection of harmful and toxic elements. Although there have been several studies in this direction, their practical applications have been hindered by slow response and low responsiveness. To overcome these problems, we have presented here a self-powered (photoelectrochemical, PEC), ultrasensitive, and ultrafast photodetector platform. For this purpose, a novel few-layered palladium-phosphorus-sulfur (PdPS) was fabricated by shear exfoliation for effective photodetection as a practical assessment. The characterization of this self-powered broadband photodetector demonstrated superior responsivity and specific detectivity in the order of 33 mA W-1 and 9.87 × 1010 cm Hz1/2 W-1, respectively. The PEC photodetector also exhibits a broadband photodetection capability ranging from UV to IR spectrum, with the ultrafast response (∼40 ms) and recovery time (∼50 ms). In addition, the novel few-layered PdPS showed superior sensing ability to organic vapors with ultrafast response and a recovery time of less than 1 s. Finally, the photocatalytic activity in the form of hydrogen evolution reaction was explored due to the suitable band alignment and pronounced light absorption capability. The self-powered sensing platforms and superior catalytic activity will pave the way for practical applications in efficient future devices.

Bio-inspired polymer array vapor sensor with dual signals of fluorescence intensity and wavelength shift.

Organic vapor sensors based on polymer owing to their tunable molecular structures and designable functions have attracted considerable research interest. However, detecting multiple organic vapors with high accuracy and a low detection limit is still challenging. Herein, inspired by the mammalian olfactory recognition system, organic vapor sensors based on one-dimensional microfilament array structures with a wide range of sensing gases are demonstrated. By introducing aggregation-induced emission (AIE) molecules, sensors possess dual-optical sensing mechanisms of variation in fluorescence intensity and wavelength. By virtue of the synergistic effects of dual signals, superb accuracy and incredibly low detection limit are achieved for identifying analytes. In particular, the polymer/AIE microfilament array can detect acetone vapor down to 0.03% of saturated vapor pressure. In the saturated vapor of acetone, the fluorescence intensity of the sensor arrays was reduced by 53.7%, while the fluorescence wavelength was red-shifted by 21 nm. Combined with the principal component analysis (PCA) algorithm, the polymer/AIE molecular sensor arrays accomplished the classification and identification of acetone, ethanol, methylene chloride, toluene, and benzene. This bioinspired approach with dual sensing signals may broaden practical applications to high-performance gas sensors for precise molecular detection.

Wearable Motion Smartsensors Self-Powered by Core-Shell Au@Pt Methanol Fuel Cells.

A wearable self-powered sensor is a promising frontier in recent flexible electronic devices. In this work, a wearable fuel cell (FC)-type self-powering motion smartsensor has been fabricated, particularly in choosing methanol vapor as a target fuel for the first time. The core-shell structure of Pt@Au/N-rGO and the porous carbon network act as methanol oxidation and oxygen reduction reaction catalysts, with a highly conductive alkaline hydrogel as a solid-state electrolyte. As a result, a wearable FC for a self-powered sensing system demonstrates excellent sensing performance toward 2-20% (v/v) methanol vapor with a maximum power density of 2.26 µW cm-1 and good mechanical behaviors during the bending or twisting process. Significantly, this wearable FC device could power strain sensors of human motion, and real-time signals can be easily remotely detected via a cellphone. With attractive biocompatibility and self-powering performance, wearable FCs for a self-powering system would provide new opportunities for next-generation flexible smartsensing electronics and initiate a developed self-powering platform in future practical application of wearable smart monitoring.

Self-Powered Broadband Photodetector and Sensor Based on Novel Few-Layered Pd3(PS4)2 Nanosheets.

Optoelectronics and sensing devices are of enormous importance in our modern lives, which has propelled the scientific community to explore new two-dimensional (2D) nanomaterials to meet the requirements of future devices. Herein, we present the exfoliation of palladium thiophosphate (Pd3(PS4)2) by mechanical shear force exfoliation. The Pd3(PS4)2-based photoelectrochemical (PEC) device demonstrated self-powered broadband photodetection in the range of 385-940 nm with an unprecedented responsivity of 2 A W-1 and a specific detectivity of about 8.67 × 1011 cm Hz1/2 W-1 under the illumination of 420 nm LED light. The crucial parameters such as photoresponsivity, response, and recovery time of the device can be controlled by an externally applied voltage and the analyte concentration. Moreover, Pd3(PS4)2-based vapor-sensing devices exhibited frequency-dependent selective acetone sensing in the presence of other organic vapors with an ultrafast response and a recovery time of less than 1 s. Finally, the photocatalytic activity of Pd3(PS4)2 was revealed, which can be attributed to the presence of an appropriate band alignment with the catalytic activity of Pd. This novel material with the aforementioned fascinating phenomenon will pave the way toward practical future applications in optoelectronics and sensing.

Effect of temperature and humidity on the sensing performance of TiO2nanowire-based ethanol vapor sensors.

In this paper, we study the influence of two key factors, temperature, and humidity, on gas sensors based on titanium dioxide nanowires synthesized at 4 different temperatures and with different morphology. The samples' structure are investigated using SEM, XRD and FTIR analysis. The effects of humidity and temperature are studied by measuring the resistance and gas response when exposed to ethanol. At room temperature, we observed a 15% sensitivity response to 100 ppm of ethanol vapor and by increasing the operating temperature up to 180 °C, the response is enhanced by two orders of magnitude. The best operating temperature for the highest gas response is found to be around 180 °C. Also, it was observed that every nanowire morphology has its own optimum operating temperature. The resistance of sensors is increased at higher Relative Humidity (RH). Besides, the response to ethanol vapor experiences a gradual increase when the RH rises from 10% to 60%. On the other hand, from 60% to 90% RH the gas response decreases gradually due to different mechanisms of interaction of the TiO2with H2O and ethanol molecules.

Aggregation-Induced Emission Molecule Microwire-Based Specific Organic Vapor Detector through Structural Modification.

An optical organic vapor sensor array based on colorimetric or fluorescence changes quantified by spectroscopy provides an efficient method for realizing rapid identification and detection of organic vapor, but improving the sensitivity of the optical organic vapor sensor is challenging. Here, AIE/polymer (AIE, ggregation-induced emission) composites into microwires arrays are fabricated as organic vapor sensors with specific recognition and high sensitivity for different vapors using the capillary-bridge-mediated assembly method. Such organic vapor sensor successfully detects organic vapor relying on a swelling-induced fluorescence change of the AIE/polymer composites, combating the unique property of AIE molecules and vapor absorption-induced polymer swelling. A series of AIE/polymer composites into microwires arrays with four different groups on the AIE molecule and four different side chains on the polymer is fabricated to detect four different organic vapors. The mechanism for improved sensitivity of the AIE/polymer composites microwires arrays sensors is the same because of the similar polarity between the group of AIE molecules and the vapor molecules. Molecular design of the side chains of the polymer and the groups of AIE molecules based on the polarity of the targeted vapor molecule can enhance the sensitivity of the sensors to the subparts per million level.

Pressure Sensitive Adhesive Tape: A Versatile Material Platform for Optical Sensors.

Pressure sensitive adhesive (PSA) tapes are a versatile, safe and easy-to-use solution for fastening, sealing, masking, or joining. They are widely employed in daily life, from domestic use to industrial applications in sectors such as construction and the automotive industry. In recent years, PSA tapes have found a place in the field of micro- and nanotechnology, particularly in contact transfer techniques where they can be used as either sacrificial layers or flexible substrates. As a consequence, various optical sensing configurations based on PSA tapes have been developed. In this paper, recent achievements related to the use of PSA tapes as functional and integral parts of optical sensors are reviewed. These include refractive index sensors, optomechanical sensors and vapor sensors.

Scotch Tape Optical Vapor Sensor for Ethanol-Methanol Mixtures.

A simple optical vapor sensor based on conventional Scotch adhesive tape, for analyzing ethanol-methanol mixtures, is proposed and demonstrated. The sensing signal relies on the variation of optical power transmitted through the tape, resulting from the response of the adhesive material to vapor sorption. The optical sensor exhibits high selectivity for ethanol vapor over methanol vapor. When exposed to vapors from ethanol-methanol liquid mixtures, the sensor shows a linear detection range of 0-100 vol%, and detection limits of 8.8 vol% ethanol and 17.6 vol% methanol. Repeatability, reproducibility, reversibility, and sensitivity to other volatile organic compounds are also studied.

Reversible Multiphase Transition in a BioMOF and Its Distinctive Luminescence Turn-On in Alcohol Vapor.

Solvothermal reaction of zinc meso-tetra(4-carboxyphenyl)porphyrin and 2,6-diaminopurine with zinc salt in DMF affords a three-dimensional bioMOF (1-α). Its infinite rod-shaped building block features an alternation of octahedral Zn4O and paddle-wheel Zn2 clusters bridged by 2,6-diaminopurines. The paddle-wheel Zn2 cluster undergoes reversible transformation with half into quasi-paddle-wheel Zn2 cluster and the other half into two tetrahedral mononuclear clusters upon release/uptake of guest molecules, resulting in a new phase 1-ß. This single-crystal to single-crystal transformation is accompanied by luminescence on/off switching, possibly associated with the structural conversion between the porphyrin-ligand-based photoactive 1-α and the porphyrin-stacking-caused non-photoactive 1-ß. Interestingly, 1-ß exhibits quick luminescence turn-on in alcohol vapor instead of other volatile organic solvents by transforming into an intermediate phase 1-γ, which shows a partial luminescence enhancing likely due to the intermittent porphyrin π-π stacking. In view of experimental results and theoretical calculations, this distinctive alcohol-vapor-induced luminescence turn-on is attributed to the coordination ability to porphyrin-bound zinc ion, molecular size, and vapor pressure, in which methanol and ethanol are particularly favored.

Scalable Arrays of Chemical Vapor Sensors Based on DNA-Decorated Graphene.

Arrays of DNA-functionalized graphene field-effect transistors (gFETs) hold great promise for high-performance vapor sensing. In this chapter, we describe methods for the scalable production of gFET-based vapor sensors with high sensitivity and efficiency in size, cost, and time. Large-area graphene sheets were prepared via chemical vapor deposition (CVD); a standard photolithographic processing for large-area graphene was used to fabricate gFETs with high mobility and low doping level under ambient conditions. The gFETs were functionalized by single-stranded DNA (ssDNA), which binds to the graphene channels through π-π stacking interaction and provides affinity to a wide range of chemical vapors. The resulting sensing arrays demonstrate detection of target vapor molecules down to parts-per-million concentrations with high selectivity among analytes with high chemical similarity including a series of carboxylic acids and structural isomers of carboxylic acids and pinene.

Superhydrophilic, Underwater Superoleophobic, and Highly Stretchable Humidity and Chemical Vapor Sensors for Human Breath Detection.

Humidity and chemical vapor sensors have promising applications in the field of environment protection, human healthcare, and so forth. It is still challenging to develop sensor materials that can serve as both humidity and chemical vapor sensors with high sensitivity, low detection limit, and excellent stretchability, repeatability, and reliability. In this study, a flexible, stretchable, and conductive nanofiber composite (CNC) with superhydrophilicity and underwater superoleophobicity is prepared by acidified carbon nanotube (ACNT) decoration onto the thermoplastic polyurethane (PU) nanofiber surface. ACNT introduction increases both the Young's modulus and tensile strength and almost maintains the superelasticity of the PU nanofibrous membrane. The as-obtained CNC could be used to detect both moisture and chemical vapors. When used as the humidity sensor, ACNTs can absorb surrounding water molecules and thus increase their resistance. On the other hand, the PU can be swollen by different chemical vapors, which can, to a different extent, damage the conductive network inside the composite and cause the increase of the composite resistance. The CNC can be integrated with a mask for real-time detection of human respiration. The CNC-based chemical vapor sensor possesses low detection limit, quick response, good selectivity, and excellent recyclability (even in a high humid environment) and has potential applications in monitoring biomarker gases from human breath.

Environmental Energy Harvesting Adapting to Different Weather Conditions and Self-Powered Vapor Sensor Based on Humidity-Responsive Triboelectric Nanogenerators.

Triboelectric nanogenerators (TENGs) have been widely applied for energy harvesting and self-powered sensing, whereas smart deformable materials can be combined with the TENG to acquire a more intelligent and self-adaptive system. Here, based on the vapor-driven actuation material of a perfluorosulfonic acid ionomer (PFSA), we propose a type of humidity-responsive TENG. The integrated TENG array can automatically bend to the desired angles in response to different humidity conditions, and thus, it can effectively collect energy from both wind and rain drops, where the power density can reach 1.6 W m-2 at a wind speed of 25 m s-1 and 230 mW m-2 under rainy conditions. Meanwhile, this TENG array can fully lay down in dry weather, using the reflective surface to reflect sunlight and heat radiation. The vapor absorption process of the PSFA film can also result in the charge accumulation process. Accordingly, relying on the strong absorption capability of PFSA, a TENG-based vapor sensor with high sensitivity has been developed for monitoring chemical vapor leakage and humidity change. This work opens up a promising approach for the application of the humidity-responsive materials in the field of energy harvesting and self-powered sensors. It can also promote the development of TENG toward a more intelligent direction.

Multipurpose sensing applications of biocompatible radish-derived carbon dots as Cu2+ and acetic acid vapor sensors.

A recent trend in the preparation of carbon dots, optically unique nanomaterials, revolves around the use of readily-available, low-cost natural resources as precursors and their multipurpose applications. In this work, a hydrothermal method for preparing biocompatible carbon dots from radish was developed. The carbon dots were then tested for sensing of Cu2+ and acetic acid vapor. The carbon dots exhibited blue emission under UV illumination with, a quantum yield of 15%. The fluorescence emission was selectively quenched when Cu2+ ions were added, giving a detection limit of 0.16 µM. A paper-based fluorescent sensor was fabricated and shown to sense Cu2+ with a limit of detection of 6.8 µM. The carbon dots were able to determine the Cu2+ concentration in real water samples, with excellent recovery and reliability. The carbon dots were also used as the sensing material in an optical electronic nose, and tested for real-time detection of acetic acid vapor. Using principal component analysis, different ratios of acetic acid to methanol in solution were successfully identified with a detection limit of 15.5%. The acetic acid concentration in a real vinegar sample was also accurately determined. Our results demonstrated that label-free carbon dots derived from readily available radish can be simply used as versatile probes, giving them potential uses in multipurpose sensing applications.

Polystyrene Opals Responsive to Methanol Vapors.

Photonic crystals (PCs) show reflectance spectra depending on the geometrical structure of the crystal, the refractive index (neff), and the light incident angle, according to the Bragg-Snell law. Three-dimensional photonic crystals (3D-PCs) composed of polymeric sub-micrometer spheres, are arranged in an ordered face cubic centered (fcc) lattice and are good candidates for vapor sensing by exploiting changes of the reflectance spectra. We synthesized high quality polystyrene (PS) 3D-PCs, commonly called opals, with a filling factor f near to the ideal value of 0.74 and tested their optical response in the presence of different concentrations of methanol (MeOH) vapor. When methanol was present in the voids of the photonic crystals, the reflectance spectra experienced energy shifts. The concentration of methyl alcohol vapor can be inferred, due to a linear dependence of the reflectance band maximum wavelength as a function of the vapor concentration. We tested the reversibility of the process and the time stability of the system. A limit of detection (LOD) equal to 5% (v/v0), where v was the volume of methanol and v0 was the total volume of the solution (methanol and water), was estimated. A model related to capillary condensation for intermediate and high methanol concentrations was discussed. Moreover, a swelling process of the PS spheres was invoked to fully understand the unexpected energy shift found for very high methanol content.

Implications of Thermal Annealing on the Benzene Vapor Sensing Behavior of PEVA-Graphene Nanocomposite Threads.

The effect of thermal treatments, on the benzene vapor sensitivity of polyethylene (co-)vinylacetate (PEVA)/graphene nanocomposite threads, used as chemiresistive sensors, was investigated using DC resistance measurements, differential scanning calorimetry (DSC), and scanning electron microscopy (SEM). These flexible threads are being developed as low-cost, easy-to-measure chemical sensors that can be incorporated into smart clothing or disposable sensing patches. Chemiresistive threads were solution-cast or extruded from PEVA and <10% graphene nanoplatelets (by mass) in toluene. Threads were annealed at various temperatures and showed up to 2 orders of magnitude decrease in resistance with successive anneals. Threads heated to ≥80 °C showed improved limits of detection, resulting from improved signal-noise, when exposed to benzene vapor in dry air. In addition, annealing increased the speed of response and recovery upon exposure to and removal of benzene vapor. DSC results showed that the presence of graphene raises the freezing point, and may allow greater crystallinity, in the nanocomposite after annealing. SEM images confirm increased surface roughness/area, which may account for the increase response speed after annealing. Benzene vapor detection at 5 ppm is demonstrated with limits of detection estimated to be as low as 1.5 ppm, reflecting an order of magnitude improvement over unannealed threads.

Vapor sensing with color-tunable multilayered coatings of cellulose nanocrystals.

Colloidal cellulose nanocrystals were LBL deposited to form firmly-stacked optical coatings in which the nanorods regulated their head-to-tail association and aligned in the axial-centrifuged direction. The periodically transition from blue to orange of reflected colors was tunable via deposition layer adjustment. While the sensing coating was exposed to vapors of NH3.H2O, H2O, HCl and HAc, respectively, the color variation in the response process was irreversible at room temperature and highly dependent on vapor diffusion and chemical interface interaction. Consequently, HAc vapor presented the longest sensing transition of wavelength, whereas the alkaline NH3.H2O displays a less recovery ratio than HAc and H2O at room temperature. Under heating at 50°C, the sensed coatings could mostly be restored to their original state except HCl-etched one. Therefore, the naked-eyed qualitative detectability of vapors by nanocellulose could be realized by the divergence in color shift which is of great importance in chemical sensors.