Humidity Sensitive Material Research Articles

Double D-shaped optical fiber temperature and humidity sensor based on ethanol and polyvinyl alcohol

In the paper, a double D-shaped optical fiber temperature and humidity sensor based on surface plasmon resonance (SPR) is reported. The two flat surfaces of the double D-shaped optical fiber are respectively coated with ethanol as a temperature-sensitive material and polyvinyl alcohol (PVA) as a humidity-sensitive material. The sensing properties of the structure were numerically simulated by the finite element method (FEM). The influence of the structural parameters on the sensing performance was investigated. The results show that the temperature sensitivity reaches 2.9 nm/℃ in the range of 10–70 ℃, and the humidity sensitivity reaches 11.6 nm/%RH in the range of 20−80%RH. A high linearity of 0.9933 and 0.9998 are obtained, respectively. This structure will promote the development of high-performance dual-parameter sensors.

A capacitive humidity sensor based on all-protein embedded with gold nanoparticles @ carbon composite for human respiration detection

Wool and silk fiber are the most extensive resources of protein fibers and have been used in the textile field for many years. The extracted biocompatible proteins are more and more widely used in flexible devices, sensors, tissue engineering, etc. Here, a fully biomaterial based flexible humidity sensor has been successfully fabricated for the first time. Interdigital electrodes of humidity sensor are printed on a transparent sensor substrate made of silk protein by inkjet printing. The humidity sensitive material is gold nanoparticles hosted nitrogen doped carbon (AuNPs@NC), which is fabricated by in situ dispersion of gold nanoparticles in a wool keratin assisted porous carbon precursor. The best treatment condition of the sensitive materials is obtained by comparing the sensitivity of humidity response. Moreover, the as-prepared biocompatible flexible sensor was successfully used to detect human respiration.

Temperature and humidity sensor based on a double D-shaped optical fiber with incorporated toluene and polyethylene

A double D-shaped optical fiber for simultaneous temperature and relative humidity (RH) sensing is described. The flat surfaces of the D-shaped optical fiber are coated with toluene as a thermally sensitive material and polyethylene as a humidity-sensitive material. Using the finite element method, the effects of the metal film thickness and polished depth of the D-shaped optical fiber on the sensing performance were analyzed and the optimal structural parameters were obtained. The temperature and humidity sensing properties were studied and show that temperature and humidity sensitivities of 1.02 nm/°C and 0.79 nm/% RH were obtained from 25 °C to 65 °C and 30% RH and 70% RH with maximum linearities of 0.996 and 0.997. The proposed optical fiber sensor demonstrates potential applications for dual-parameter detection.

PVDF/ZnO hybrid nanocomposite applied as a resistive humidity sensor

Polyvinyledene fluoride (PVDF), an aliphatic polymer with massive hydrophobic groups, has been developed as humidity sensitive material (hydrophilic) using an additive of ZnO nanoparticles as resistant transduction layers, an PVDF/ZnO nanocomposite bas been prepared as humidity sensor through a simple hydrothermal approach. PVDF/ZnO nanocomposite have been fabricated by spin coating method and characterized by XRD, FE-SEM, TEM, BET, UV-Vis spectra and PL spectra. Owing to its 2D replicated structure with rod-like nanocomposite, the PVDF/ZnO provide more water molecules adsorption and desorption across the humidity sensor surface compare than pure ZnO nanoparticles. The humidity sensing properties was established by using saturated salt solutions and the relative humidity range of 5-95% were achieved at 35°C. The experimental results confirmed that the sensitivity factor (Sf) of PVDF/ZnO nanocomposite humidity sensor exhibit 3 order of magnitude variation than that of pure PVDF and good linearity compare than pure ZnO nanoparticles. The response and recovery times of the nanocomposite based humidity sensor are to be 30 s and 51 s, respectively. The performance of PVDF/ZnO nanocomposite sensor can be improved by PVDF modifier on ZnO nanoparticles, because the PVDF/ZnO nanocomposite have more surface oxygen vacancy defects was synthesized by hydrothermal method. The experimental analysis indicates that the enhanced humidity sensitivity was probably due to BET surface area and provides more water adsorption sites in hybrid nanocomposite. These results open the door for a new idea for high performance and promising 2D nanocomposite materials in the development of sensitive RH% sensor for potential in the manufacture of humidity sensors.

3D printed castle style Fabry-Perot microcavity on optical fiber tip as a highly sensitive humidity sensor

A 3D printed castle style Fabry-Perot (FP) microcavity on optical fiber tip for humidity sensing is developed and demonstrated experimentally. The castle style FP microcavity, which has periodically arranged square holes on its side wall, is directly fabricated on an optical fiber tip through two-photon polymerization. Because of the existence of square holes in the castle style FP microcavity, Polyvinyl Alcohol (PVA) can be filled into the FP microcavity and a 360-degree humidity response sensitive area is formed. The contact area between water molecules and humidity sensitive materials is significantly increased, which will result in a high uniformity and a faster response of humidity sensing. Experimental results show that, the sensitivity of proposed castle style FP microcavity humidity sensor is up to 248.9 pm/%RH with relative humidity changing from 46 %RH to 75 %RH. In addition, repeated experiments indicate that the 3D printed castle style FP microcavity humidity sensors are repeatable and stable. Such excellent sensing performance with compact structure, highly sensitive, faster response and good stability makes the proposed sensing structure a highly promising candidate for humidity sensing applications.

Sensitive and renewable quartz crystal microbalance humidity sensor based on nitrocellulose nanocrystals

High-sensitivity and inexpensive humidity sensors with rapid response are crucial for humidity detection. In this study, a quartz crystal microbalance (QCM) sensor based on nitro-modified cellulose nanocrystals (NCNCs) films were developed for rapid and sensitive detection of humidity. The NCNCs films with good hydrophilicity along with fast water adsorption and dehydration were used as the humidity sensitive material. Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and X-ray diffraction were used to examine the chemical and crystalline properties of NCNCs. The morphology, roughness, and thickness of NCNCs deposited on the QCM silver electrode were characterized by atomic force microscopy and field emission scanning electron microscopy. The results confirmed that the humidity sensor with NCNCs loading of 2.67 μg (QCM-4) exhibited high sensitivity (25.6 Hz/% RH) and extremely small response/recovery times (18 s / 10 s). In addition, the sensor showed excellent reliability and logarithmic linearity in 11−84 % relative humidity (RH). The adsorption of water on the surfaces of NCNCs and the sensing mechanism was examined by density functional theory (DFT) simulation. From the perspective of a sustainable and renewable material, low-cost NCNCs with high humidity sensitivity seem to be the ideal material for developing high-performance sensors with multiple potential applications.

Highly sensitive open-ended coaxial cable-based microwave resonator for humidity sensing

A novel portable open-ended hollow coaxial cable resonator (OE-HCCR) is proposed and demonstrated for direct relative humidity (RH) measurements without the assistance of any humidity-sensitive material. The sensor is fabricated from a homemade hollow coaxial cable platform that provides ease of modification and improved robustness. The microwave cavity resonator is a metal post-coupled open-ended coaxial resonator, which includes two highly-reflective reflectors, i.e., the metal post and the metal plate-backed open end of the coaxial line. The gap distance between the metal plate and the open end is defined by a circular spacer. By simple one-step adjustments of the thickness of the spacer, the RH measurement sensitivity of the OE-HCCR can be greatly enhanced. The device physics is discussed; numerical calculations and experimental investigations are conducted to verify the sensitivity enhancement of the OE-HCCR for RH sensing. Due to the increased sensitivity, this is the first report of an open-ended coaxial line that is able to directly sense gaseous analyte, to the best of our knowledge. The proposed OE-HCCR with the sensitivity enhancement has the potential for highly sensitive chemical sensing combined with functional materials.

Fabricating Flexibly Resistive Humidity Sensors with Ultra‐high Sensitivity Using Carbonized Lignin and Sodium Alginate

AbstractLignin, an abundant and environmentally friendly renewable polymer, was used to attain the carbonized lignin in current study. Then, a highly humidity sensitive composite composite film was prepared by carbonized lignin (CL) applied as the conductive particles and sodium alginate (SA). The repeatability, hysteresis, stability and response‐recovery of the CL/SA composite film sensor were investigated in the range of 11 to 97 % relative humidity at room temperature. The results showed the maximum responsivity of CL/SA composite film is 502,895.40 % under 97 % relative humidity. In addition, CL/SA composite film can be used as humidity sensors with ultrahigh sensitivity, low hysteresis, and stable repeatability in a wide relative humidity range of 11–97 %. Finally, the humidity mechanism of the CL/SA sensor and its monitoring application in human respiration were analyzed. The research revealed that CL/SA composite is an excellent humidity sensitive material.

Cellulose Nanofiber/Carbon Nanotube Dual Network-Enabled Humidity Sensor with High Sensitivity and Durability.

Humidity sensors have been widely used for humidity monitoring in industrial fields, while the unsatisfactory flexibility, time consumption, and expensive integration process of conventional inorganic sensors significantly limit their application in wearable electronics. Using paper-based humidity sensors is considered a feasible method to overcome these drawbacks because of their good flexibility and roll-to-roll manufacturability, while they still face problems such as poor durability and low sensitivity. In this study, we report a high-performance paper-based humidity sensor based on a rationally designed bilayered structure consisting of a nanoporous cellulose nanofiber/carbon nanotube (CNF/CNT) sensitive layer and a microporous paper substrate. The vast number of hydrophilic hydroxyl groups on the surface of CNF and paper fibers enables fast water molecule exchange between the humidity-sensitive material and the external environment via hydrogen bonding, endowing the paper-based sensor with an excellent humidity responsive property. The obtained sensor displays a maximum response value of 65.0% (ΔI/I0) at 95% relative humidity. Furthermore, the mechanical interlocking structure formed between the CNF/CNT layer and the paper layer provides the sensor with strong interlayer adhesion. Benefiting from the unique structure, the sensor also exhibits outstanding bending (with a maximum curvature of 22.2 cm-1) and folding durability (up to 50 times). Finally, as a proof of concept, a simple humidity-measuring device is assembled, which demonstrates an excellent responsive property toward human breath and the change of air humidity, indicating a great potential of our paper-based humidity sensor toward practical applications.

One-step and large-scale fabrication of flexible and wearable humidity sensor based on laser-induced graphene for real-time tracking of plant transpiration at bio-interface

The rapidly growing demand for humidity sensing in various applications such as noninvasive epidermal sensing, water status tracking of plants, and environmental monitoring has triggered the development of high-performance humidity sensors. In particular, timely communication with plants to understand their physiological status may facilitate preventing negative influence of environmental stress and enhancing agricultural output. In addition, precise humidity sensing at bio-interface requires the sensor to be both flexible and stable. However, challenges still exist for the realization of efficient and large-scale production of flexible humidity sensors for bio-interface applications. Here, a convenient, effective, and robust method for massive production of flexible and wearable humidity sensor is proposed, using laser direct writing technology to produce laser-induced graphene interdigital electrode (LIG-IDE). Compared to previous methods, this strategy abandons the complicated and costly procedures for traditional IDE preparation. Using graphene oxide (GO) as the humidity-sensitive material, a flexible capacitive-type GO-based humidity sensor with low hysteresis, high sensitivity (3215.25 pF/% RH), and long-term stability (variation less than ± 1%) is obtained. These superior properties enable the sensor with multifunctional applications such as noncontact humidity sensing and human breath monitoring. In addition, this flexible humidity sensor can be directly attached onto the plant leaves for real-time and long-term tracking transpiration from the stomata, without causing any damage to plants, making it a promising candidate for next-generation electronics for intelligent agriculture.

An optical fiber sensor based on carboxymethyl cellulose/carbon nanotubes composite film for simultaneous measurement of relative humidity and temperature

In this paper, a carboxymethyl cellulose (CMC)/carbon nanotubes (CNTs) composite film was manufactured and used as a good humidity sensitive material to design a humidity and temperature simultaneous measurement sensor with simple structure, high sensitivity and fast response time. The sensor is constructed by a fiber Bragg grating (FBG) and then connected a hollow-core fiber (HCF) on the end face of which coated a CMC/CNTs hydrogel composite film. Through contrast experimental tests, the results show that the humidity sensitivity of the sensor with CMC film is 170.55 pm/%RH, while the sensitivity of the sensor with CMC/CNTs composite film is 230.95 pm/%RH. The humidity-insensitive FBG can be used to obtain accurate temperature information, and the temperature sensitivity is 26.35 pm/°C. This paper not only proves that CMC is an excellent humidity sensitive material for optical fiber sensors but also proves that carbon nanotubes can increase the humidity sensitivity of the hydrogel film effectively. This temperature and humidity simultaneous measurement sensor has a good application prospect.

Utilization of poly (4-styrenesulfonic acid) doped polyaniline as a hole transport layer of organic solar cell for indoor applications

The demand for low dimensional, micro powered and wireless indoor electronic devices has been increasing. To power-up those devices, organic photovoltaic (OPV) cells are being employed. The OPV cells exhibit good spectral matching and mechanical flexibility, and can harvest artificial indoor light energy efficiently. Hole transport layer (HTL) is an important component of an OPV cell. Water stable, low temperature processable poly (3, 4-ethylene dioxythiophene): poly (4-styrenesulfonic acid) (PEDOT:PSS) based HTL is commonly used in the indoor OPV cells. However, strongly acidic, highly hydrophilic and expensive PEDOT:PSS resulted in the development of cheaper, mildly acidic and less humidity sensitive alternative hole transport material (HTM) for the indoor OPV cells. Here, we utilized an economical and low acidic, water-stable PSS doped polyaniline (PANI) as HTM for a poly (3-hexylthiophene): [6, 6]-indene-C60 bisadduct (P3HT:ICBA) active material-based indoor OPV cell. The film formed by chemically synthesized PANI:PSS exhibited over 90% transmittance and approximately 5.15 eV work function value. The OPV device exhibited higher shunt resistance and greater power conversion efficiency (PCE) during the operation in an indoor environment. For 1000 Lux white LED light, the device showed around 10% PCE. Furthermore, the device exhibited better stability than PEDOT:PSS HTL based indoor OPV.

Large core-offset based in-fiber Michelson interferometer for humidity sensing

A humidity sensor based on Michelson interferometer (MI) is proposed and demonstrated. The sensor achieves intermodal interference by a large core-offset between two single mode fibers (SMFs) through arc fusion splicing. Experimental results show the sensor with a length of 25 mm and core-offset of 50 μm exhibits a linear humidity sensitivity of −0.083 dB/%RH in the humidity range of 35–95 %RH, which is comparable to that of intensity-modulated humidity sensors with coating humidity sensitive materials. The temperature response and stability is also investigated, a linear temperature sensitivity of 9.5 pm/°C in the temperature range of 20–180 °C is obtained. The low manufacturing cost, easy fabrication and good stability make the sensor has great potential in many applications.

Evaluation of the Food Contact Suitability of Aged Bio-Nanocomposite Materials Dedicated to Food Packaging Applications

Nanocomposite materials based on bio-polyesters (PBSA and PHBV) have been evaluated for their suitability for food contact according to the recommendations defined for non-biodegradable plastic materials, and subsequently, according to accelerated aging treatment. On the basis of the limited number of material/migrant/food simulant combinations studied here, the test for migration, using food simulants, appeared directly applicable to testing such materials which are not considered humidity-sensitive materials. Considering the only compliance criterion that must be met by the materials in contact, the materials submitted to the aging processing are not of safety concern and the incorporation of nanoclays in aged biodegradable materials does not interfere with their inertial properties in a dramatic way. At the molecular scale, the UV irradiation proved to induce an increase in the degree of crystallinity, resulting in a modification of transport properties of both packaging materials. The values of overall migration and specific migration were reduced without decreasing the diffusion coefficients of the target additives. The UV treatment and the addition of nanoparticles, therefore, seem to jointly promote the retention of organic compounds in the materials by increasing their affinity for packaging material.

High sensitivity humidity sensor based on gelatin coated side-polished in-fiber directional coupler

We demonstrate the first experimental realization of in-fiber directional coupler-based relative humidity (RH) sensor. A segment of single eccentric hole-assistant dual-core fiber (SEHADCF) is directly spliced between two single mode fibers (SMFs) to realize an in-fiber directional coupler (IFDC), and the suspended core of the SEHADCF is exposed to the external environment by using the side-polishing process. Gelatin is covered on the exposed suspended core to form the cladding of the suspended core and acts as a humidity sensitive material. Compared to the previous literatures, we report the highest sensitivity of the RH sensor to our best knowledge. In the RH range of 70 %–90 %, the average and maximum RH sensitivities are −7.005 nm/%RH and −18.94 nm/%RH, respectively. In addition, the temperature response and stability of the IFDC is also experimentally measured. Due to the super-high sensitivity and low temperature crosstalk, the proposed sensor will be expected to be a promising candidate for highly sensitive RH sensing applications in high humidity environment.

An optical fiber relative humidity sensor based on hollow-core fiber and hydroxypropyl methylcellulose hydrogel film

Hydroxypropyl methylcellulose (HPMC) is a kind of water-soluble polymer material. The hydrogel formed by HPMC swelling in water is a polymer network system with a netlike cross-linked structure and it can absorb lots of water if it exposed to the water environment. Therefore, an optical fiber Fabry-Perot interferometer (FPI) relative humidity (RH) sensor based on hollow-core fiber (HCF) and HPMC hydrogel film is proposed and demonstrated. The experimental results show that the proposed sensor has a binomial humidity response and 2.25 s response time in the range of 40%–99%RH. The HPMC hydrogel with hydrophilicity, natural grid-like nanostructure and biocompatibility is an effective humidity-sensitive material for optical fiber RH sensor and has great application potential in various fields.

Optical Fiber Humidity Sensor Based on Water Absorption Peak Near 2-μm Waveband

We demonstrate the absorption-based fiber humidity sensors working near 1950 nm wavelength which is a strong water absorption peak. No additional coating of humidity-sensitive material is needed for the proposed relative humidity (RH) fiber sensors. Two sensor structures including a taper fiber and a microfiber knot resonator are fabricated. For the taper fiber sensor, upto 0.18 mW/% RH sensitivity has been achieved via direct power measurement. For the fiber knot sensor, the RH variations induce both extinction ratio change and wavelength shift. The optical spectra of the fiber knot resonator under different RH are measured, and the extinction ratio and wavelength shift sensitivity are measured to be 0.034 dB/% RH and 10 pm/% RH, respectively. The response time for the tapered fiber is 1.13 s for RH increasing from 28.6% to 53.4% and 2.27 s for the reverse operation. The microfiber knot sensor can respond in 0.8 and 1.55 s accordingly. The sensor performance for the two structures is also investigated under different temperatures from 25 °C to 45 °C and the results indicate negligible impact from the temperature variations. We believe the simple fabrication and low cost of the sensor will make it potential and practical for the RH measurement.

Flexible and Highly Sensitive Humidity Sensor Based on Cellulose Nanofibers and Carbon Nanotube Composite Film.

Flexible and highly sensitive humidity sensors are crucial for humidity detection. In this study, a flexible cellulose nanofiber/carbon nanotube (NFC/CNT) humidity sensor with high sensitivity performance was developed via fast vacuum filtration. CNTs were well dispersed in water by using 2,2,6,6-tetramethylpiperidinyl-1-oxyl (TEMPO)-oxidized NFC as a dispersant. More importantly, NFC also acted as a humidity sensitive material, achieving superior performance of NFC/CNT humidity sensors. The obtained NFC/CNT humidity sensor with 5 wt % CNT loading exhibits outstanding sensitive performance, and its response value reaches a maximum of 69.9% (Δ I/ I0) at 95% relative humidity (RH). It also displays good bending resistance and long-term stability. In addition, the NFC/CNT humidity sensor was employed to monitor human breath. Therefore, we believe that the flexible, highly sensitive, and simply designed NFC/CNT humidity sensor is a promising candidate for various applications in the field of humidity measurement.

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.

Flexible photoluminescent humidity sensing material based on electrospun PVA nanofibers comprising surface-carboxylated QDs

Flexible humidity-sensitive materials have drawn increasing attention due to their vital function in monitoring ambient humidity for tactile sensors, allowing them to approach the functionality of human skin. In this work, an electrospun polyvinyl alcohol (PVA)-based nanofiber film comprising surface-carboxylated CdSe/CdxZn1-xS quantum dots (QDs) is fabricated as a high-performance flexible humidity sensing material, based on the relationship between its photoluminescence (PL) and the humidity. The as-prepared film shows excellent PL-humidity linearity, a wide sensing range from 10% to 90% relative humidity, a relatively fast response time, good recoverability, and flexibility. The humidity sensing mechanism can be attributed to the breaking and reforming of hydrogen bonds and proton transfer occurring at the PVA-QD interfaces. This flexible humidity-sensitive material is believed to be a good candidate for humidity-sensitive components in electronic skin.