Stable Sensor Research Articles

A highly sensitive and anti-freezing conductive strain sensor based on polypyrrole/cellulose nanofiber crosslinked polyvinyl alcohol hydrogel for human motion detection

Electro-conductive hydrogels emerge as a stretchable conductive materials with diverse applications in the synthesis of flexible strain sensors. However, the high-water content and low cross-links density cause them to be mechanically destroyed and freeze at subzero temperatures, limiting their practical applications. Herein, we report a one-pot strategy by co-incorporating cellulose nanofiber (CNF), Poly pyrrole (PPy) and glycerol with polyvinyl alcohol (PVA) to prepare hydrogel. The addition of PPy endowed the hydrogel with good conductivity (∼0.034 S/m) compared to the no PPy@CNF group (∼0.0095 S/m), the conductivity was increased by 257.9 %. The hydrogel exhibits comparable ionic conductivity at −18 °C as it does at room temperature. It's attributed to the glycerol as a cryoprotectant and the formation of hydrated [Zn(H2O)n]2+ ions via strong interaction between Zn2+ and water molecules. Moreover, the cellulose nanofiber intrinsically assembled into unique hierarchical structures allow for strong hydrogen bonds between adjacent cellulose and PPy polymer chains, greatly improve the mechanical strength (stress∼0.65 MPa, strain∼301 %) and excellent viscoelasticity (G'max ∼ 82.7 KPa). This novel PPy@CNF-PVA hydrogel exhibits extremely high Gauge factor (GF) of 2.84 and shows excellent sensitivity, repeatability and stability. Therefore, the hydrogel can serve as reliable and stable strain sensor which shows excellent responsiveness in human activities monitoration.

Trace level sensitivity and selectivity of room temperature NH3 gas sensors based on RGO/ZnO@SiNWs heterostructure

Ammonia (NH3) is one of the most common gaseous pollutants in various environmental and industrial processes which is accountable for various health issues, even for life-threatening situations. Due to this, high-precision detection of NH3 has become a top priority for various sectors so that healthy and safe environment could be ensured. Therefore, this work aims to develop a simple, reliable, and effective sensor with spatially distributed monitoring of NH3 gas wherein reduced graphene oxide (RGO) and Zinc oxide (ZnO) nanocomposite structure prepared using refluxing method were chosen to interact with NH3 gas. The vertically aligned silicon nanowires (SiNWs) were developed by using metal-assisted chemical etching (MACE) method. The RGO/ZnO nanostructure was then deposited on SiNWs to develop a selective and stable sensor for the trace level detection of NH3 gas with varying concentration from 0.01 ppm to 5 ppm at Room temperature (∼30 °C). The prepared materials were characterized using scanning electron microscope (SEM), transmission electron microscope (TEM), energy dispersive spectroscopy (EDS), X-ray diffractometer (XRD), Raman spectrometer, X-Ray Photoelectron Spectrometer (XPS) and Ultraviolet–visible spectrophotometer which provide a thorough explanation about the developed nanostructures. The sensitivity of the sensor was observed to lie between 21 % at 0.01 ppm and 176 % at 5 ppm concentration of NH3 wherein the response/recovery time was measured to be 3 s/6 s and 5 s/12 s respectively. The fast response of the sensor for NH3 gas at low limit of detection (LOD) could be attributed due to the precise interaction of the analyte at the interface of RGO/ZnO@SiNWs Schottky heterostructure. Hence, the reported sensor can continuously be utilized to monitor the target analyte that is NH3 gas at low concentrations and reduces the chance of an unfortunate incident.

Humidity activated ultra-selective room temperature gas sensor based on W doped MoS2/RGO composites for trace level ammonia detection

The lack of highly efficient, cost effective and stable ammonia gas sensors functionable at room temperature even in extreme humid environments poses significant challenge for the future generation gas sensors. The prime factors that impede the development of such next generation gas sensors are the strong interference of humidity and sluggish selectivity. Herein, we fabricated tungsten doped molybdenum disulphide/reduced graphene oxide composite by an in-situ hydrothermal method to exploit the adsorption, dissolution (solubility), ionization and transmission process of ammonia and thereby to effectuate its trace level detection even in indispensable humid environments. The protype based on 5 at.% Tungsten doped MoS2/RGO (W5) gas sensor exhibited 3.8-fold increment in its response to 50 ppm of ammonia when the relative humidity varied from 20 % to 70 % with ultra-high selectivity at room temperature. The as prepared gas sensor revealed a practical detection limit down to 1 ppm with a substantial response and rapid recovery time. Furthermore, W5 gas sensor exhibited a 42-fold increment in response to 50 ppm of ammonia relative to its pristine (MoS2/RGO) MG composite with a RH of 70 %. The proton hopping mechanism accountable for such an enormous enhancement in ammonia sensing and its potential for breath sensor are briefly annotated.

Highly sensitive and stable yolk-shell Bi2MoO6 gas sensor for ppb-level isopropanol detection

Metal oxide semiconductor used for the fabrication of volatile organic gas sensors has gained significant popularity in recent years. This paper discusses the synthesis of yolk-shell-structure Bi2MoO6 (YS-BMO) for isopropanol detection through quasi-molecular imprinting-based solvent-thermal methods. Various qualitative characterization techniques, gas sensing performance tests, and theoretical calculations collectively indicate that YS-BMO with molecular imprinting is a potential material for isopropanol sensing. YS-BMO with an average diameter of approximately 1.5 µm (15 h of solvothermal), demonstrates impressive isopropanol detection capabilities, a practical detection limit of 100 ppb, along with suitable response and recovery times. Density functional theory (DFT) calculations provide a deeper understanding and analysis of the response mechanism of Bi2MoO6 to isopropanol, considering aspects such as band structure, electron transfer, and adsorption energy.

Oligohexamethylene Guanidine Derivative as a Means to Prevent Biological Fouling of a Polymer-Based Composite Optical Oxygen Sensor.

The use of biocidal agents is a common practice for protection against biofouling in biomass-rich environments. In this paper, oligohexamethyleneguanidine (OHMG) polymer, known for its biocidal properties, was further modified with para-aminosalicylic acid (PAS) to enhance its properties against microorganisms coated with a lipid membrane. The structure of the product was confirmed by 1H NMR, 13C NMR, and FTIR spectroscopy. The values of the minimum inhibitory concentration (MIC) against Mycobacterium smegmatis ATCC 607 and Pseudomonas chlororaphis 449 were found to be 1.40 and 1.05 μg/mL, respectively. The synthesized substance was used as an additive to the polymer matrix of the composite optical oxygen sensor material. A series of samples with different contents of OHMG-PAS was prepared using a co-dissolution method implying the fabrication of a coating from a solution containing both polymers. It turned out that the mutual influence of the components significantly affects the distribution of the indicator in the matrix, surface morphology, and contact angle. The optimal polymer content turned out to be wt.3%, at which point the water contact angle reaches almost 122°, and the fouling rate decreases by almost five times, which is confirmed by both the respiratory MTT assay and confocal microscopy with staining. This opens up prospects for creating stable and biofouling-resistant sensor elements for use in air tanks or seawater.

Highly sensitive Ti3C2Tx MXenes-RGO humidity sensor for human non-contact respiratory monitoring

In this study, a highly sensitive and stable Ti3C2Tx MXenes-RGO humidity sensor was developed for real-time respiratory monitoring. Ti3C2Tx-RGO composite material increases the interlayer spacing of RGO, further expanding the channels for water molecule transport. Introduction of Ti3C2Tx promotes the generation of more -O, -OH, -F groups on the composite material surface, which can serve as adsorption sites for water molecules to improve response, and they form weak physical adsorption with water molecules, shortening the response and recovery time of the sensor. Ti3C2Tx-RGO composite material increases the interlayer spacing of RGO, further expanding the channels for water molecule transport. Density functional theory (DFT) calculation results indicate that Ti3C2Tx surface -F has higher adsorption energy for water molecules, which is beneficial for enhancing the sensitivity of the humidity sensor. Ti3C2Tx-RGO humidity sensor exhibits high sensitivity (22,117), low humidity hysteresis (1%), relatively short response/recovery time (7/16 s), and good stability and repeatability in the range of 11%− 95% RH. The sensor can also monitor changes in respiratory intensity and frequency, achieving real-time analysis of respiratory patterns, making it demonstrate strong application potential in non-contact human respiratory detection. This study provides new research ideas and methods for preparing high-performance humidity sensors.

Biocompatible, Antibacterial, and Stable Deep Eutectic Solvent-Based Ionic Gel Multimodal Sensors for Healthcare Applications.

In this study, we investigated a novel approach to fabricate multifunctional ionic gel sensors by using deep eutectic solvents (DESs) as replacements for water. When two distinct DESs were combined, customizable mechanical and conductive properties were created, resulting in improved performance compared with traditional hydrogel-based strain sensors. DES ionic gels possess superior mechanical properties, transparency, biocompatibility, and antimicrobial properties, making them suitable for a wide range of applications such as flexible electronics, soft robotics, and healthcare. We conducted a comprehensive evaluation of the DES ionic gels, evaluating their performance under extreme temperature conditions (-70 to 80 °C), impressive optical transparency (94%), and biocompatibility. Furthermore, a series of tests were conducted to evaluate the antibacterial performance (Escherichia coli) of the DES ionic gels. Their wide strain (1-400%) and temperature (15-50 °C)-sensing ranges demonstrate the versatility and adaptability of DES ionic gels for diverse sensing requirements. The resulting DES ionic gels were successfully applied in human activity and vital sign monitoring, demonstrating their potential for biointegrated sensing devices and healthcare applications. This study offers valuable insights into the development and optimization of hydrogel sensors, particularly for applications that require environmental stability, biocompatibility, and antibacterial performance, thereby paving the way for future advancements in this field.

Microscale, Electrochemical, Aptamer-Based Sensors for Enhanced Small-Molecule Detection at Millisecond Time Scales.

Microscale electrodes offer the advantages of increased mass transport rates, high sensitivity, and rapid measurement capabilities. Fabricating electrochemical aptamer-based (E-AB) sensors on these electrode platforms opens new applications to chemical and biological sensing but has remained challenging due to low signal-to-noise ratios and monolayer instability. In this article, we report the development and characterization of E-AB sensors on a gold microelectrode platform (∼500 nm radius). To overcome the small current response, we modified the electrodes by growing nanostructures via electrodeposition. We interrogated the sensors with two different electroanalytical techniques, square wave voltammetry (SWV) and intermittent pulse voltammetry (IPA), to measure the representative response of an ATP sensor and determine aptamer-target binding and dissociation time scales. We find robust and stable sensor performance with an increased response rate over sensors fabricated on macroscale electrodes. These results demonstrate that sensors developed on this microelectrode platform can be employed for enhanced spatiotemporal resolution measurements in chemical and biological environments.

Room temperature sensing of ammonia and formaldehyde gases through novel anisotype heterojunction of p-Co3O4/n-Gd0.1Ce0.9O2-δ as highly responsive and stable sensors

The development of the room temperature (RT) gas sensors is absolute need. Hydrothermally and sol-gel derived Co3O4 and Gd0.1Ce0.9O2-δ (GDC) were used to fabricate the novel heterostructures in the ratio of 1:1 Co3O4/GDC, 2:1 Co3O4/GDC and 1:2 Co3O4/GDC. These samples were scientifically analysed through X-ray diffraction (XRD), Field emission scanning electron microscope (FESEM), Energy dispersive X-ray (EDX) spectra, X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), Raman, UV–Vis-Diffuse reflectance spectra (DRS) and photoluminescence (PL). XRD, XPS together with FTIR and Raman spectra are the testimonial of the formation of Co3O4, GDC and their heterostructures. Interestingly, the performed gas sensing studies conclusively disclose that these sensors have adequate sensitivity to detect the tested gases at RT. Furthermore, O2− species played crucial role than O2− and O− in chemisorption and gas sensing mechanism. Additionally, 2:1 Co3O4/GDC and 1:2 Co3O4/GDC exhibited an enormous selectivity and highly responsive towards the detection of formaldehyde (CH2O) and ammonia (NH3), respectively. 2:1 Co3O4/GDC/1:2 Co3O4/GDC offered the high response (S) of 106.59/83.52 for CH2O/NH3 gas. In fact, the formation of anisotype p-n/n-p rather than isotype p-p/n-n junctions in 2:1 Co3O4/GDC and 1:2 Co3O4/GDC endowed these two heterostructures to exhibit quick response even at low ppm of selected gases. The outcome of this work might provide strategy for the design of novel sensing materials to detect multiple gases.

Highly Stable, Stretchable, and Sensitive Strain Sensor Based on a New Type of Elastic Fiber

Highly Stable, Stretchable, and Sensitive Strain Sensor Based on a New Type of Elastic Fiber

Structural Design and DLP 3D Printing Preparation of High Strain Stable Flexible Pressure Sensors.

Flexible pressure sensors are crucial force-sensitive devices in wearable electronics, robotics, and other fields due to their stretchability, high sensitivity, and easy integration. However, a limitation of existing pressure sensors is their reduced sensing accuracy when subjected to stretching. This study addresses this issue by adopting finite element simulation optimization, using digital light processing (DLP) 3D printing technology to design and fabricate the force-sensitive structure of flexible pressure sensors. This is the first systematic study of how force-sensitive structures enhance tensile strain stability of flexible resistive pressure sensors. 18 types of force-sensitive structures have been investigated by finite element design, simultaneously, the modulus of the force-sensitive structure is also a critical consideration as it exerts a significant influence on the overall tensile stability of the sensor. Based on simulation results, a well-designed and highly stretch-stable flexible resistive pressure sensor has been fabricated which exhibits a resistance change rate of 0.76% and pressure sensitivity change rate of 0.22% when subjected to strains ranging from no tensile strain to 20% tensile strain, demonstrating extremely low stretching response characteristics. This study presents innovative solutions for designing and fabricating flexible resistive pressure sensors that maintain stable sensing performance even under stretch conditions.

A braided construction of ionogels as a wearable sensor for human motion monitoring

A highly stable flexible sensor from the braided organization of linear ionogel (IG-BPS) was reported in this work to monitor the human physiological signals and daily movements. The compression effect in the normal direction arisen from the longitudinal stretching is prone to sensor distortion and energy dissipation. This braid construction can effectively reduce the interference of stretching on the stress sensing accuracy. The piezoresistive variations in the longitude and latitude directions of the braided structure can be mutually compensated during sensor deformation, simultaneously generating ample conductive pathway to attenuate hysteresis behaviors significantly. The IG-BPS features great flexibility with broad pressure applications (0–30kPa), high sensing behavior (S=0.137 kPa−1, GF=0.029) and stability (>1000 cycles), which can be fabricated in a simple but efficient strategy. This sensor is capable to accurately detect normal movement and subtle epidermal pulse waves at various locations of human body (heart beating and pulse bounding), which is crucial for real-time tracking the changes in overall wellness status especially the cardiovascular health went through an intensive exercise process. It is expected that this braided organization sensor would act as an exciting part in sports-level tracking, healthcare assessment as well as human-machine interactions development.

High-Temperature resistant ethanol sensing enhanced by ZnO Nanoparticles/SiC nanowire heterojunctions

Metal-oxide (MO) semiconductors are important materials for developing sensors to detect toxic gases and chemicals. However, achieving robust thermal and chemical stability under harsh working conditions, such as high temperatures, remains a significant challenge. Here, we present the construction of a high-temperature resistant ethanol sensor based on rationally designed heterojunctions formed by ZnO nanoparticles and β-SiC single-crystalline nanowires (ZnO-NPs/β-SiC-NWs). The formation of ZnO-NPs/β-SiC-NWs heterojunctions is achieved through a simple hydrothermal process, allowing the anchoring of ZnO NPs on the surface of SiC NWs. As a result, the assembled sensor demonstrates significantly improved performance in sensing ethanol at 465 °C. It exhibits a high response (S = 25.4 at 100 ppm), compared to pure ZnO NPs (10.4) and β-SiC NWs (1.2), along with fast detection (response/recovery time = 19/49 s), excellent selectivity, and a detection limit down to ppb levels. The archived response value surpasses the previously reported MO-based analogues at the similar temperatures and suggests promising applications for stable sensors operating under harsh working conditions.

Thermally stable piezoelectric sensors for quantitative pressure sensing based on linear piezoelectric zinc oxide thin films

Linear piezoelectric materials offer new opportunities for next-generation multifunctional micro/nano systems on chip aside from ferroelectrics. However, the advantage of quantitative pressure sensing of linear piezoelectric materials over ferroelectrics has not been demonstrated, while the thermal stability of linear piezoelectric zinc oxide (ZnO) is yet to be evaluated. In this work, pressure sensors based on linear piezoelectric undoped ZnO and ferroelectric vanadium doped ZnO (V-ZnO) films are studied side by side. Both films possess similar fiber textures oriented along the polar ZnO [002] direction, yet each of the V-ZnO grains consists of strip-like domains observed by SEM. Applying triangular-wave pressures, the linear piezoelectric ZnO sensor generates triangular-wave voltage signals that are one-to-one linearly correlated to the applied pressure, showing decisive advantages for quantitative pressure sensing compared to the ‘sail-like’ voltage signals of the ferroelectric V-ZnO sensor that is many-to-one correlated to the applied pressure. Superior thermal stability is demonstrated in the linear piezoelectric ZnO sensor showing stable pressure sensing properties up to 450 °C compared to the degraded sensing properties of the ferroelectric V-ZnO sensor at 150 °C. The thermally stable piezoelectric sensors for quantitative pressure sensing based on linear piezoelectric ZnO films serve as candidates for high-performance multifunctional micro/nano systems.

Highly efficient and stable NiSe2-rGO composite-based room temperature hydrogen gas sensor

The widespread consumption of hydrogen in commercial and industrial applications has resulted in the development of different hydrogen gas sensors. However, their practical application is restricted due to their non-cost efficiency, low sensitivity, poor selectivity and high temperature sensing properties. Thus, the development of a sensor capable of overcoming the aforementioned limitations is critical for commercial deployment. As a result, a NiSe2-rGO composite-based H2 gas sensor has been prepared for the first time using a simple hydrothermal technique. The developed material has been systematically characterized for its morphological properties and thoroughly investigated for H2 gas sensing applications. The findings show that the developed NiSe2-rGO composite exhibited enhanced H2 gas sensing properties, besides possessing appreciable selectivity, repeatability as well as long-term stability. Furthermore, the results demonstrate a stable sensor response of 254% within 43 s and 13 s of response and recovery time respectively at a hydrogen concentration of 500 ppm which is 8 and 500 times higher than that of its individual counterparts (NiSe2 and rGO) over a wide relative humidity range, indicating the material's potential for application in industrial environments. Thus, this work paves the way for the development of stable, effective, fast-responding, and selective hydrogen gas sensors that are functional at room temperature.

Publisher's Note: “Highly efficient and stable near-infrared photo sensor based on multilayer MoS2/p-Si integrated with plasmonic gold nanoparticles” [Appl. Phys. Lett. 123, 061104 (2023)

Publisher's Note: “Highly efficient and stable near-infrared photo sensor based on multilayer MoS2/p-Si integrated with plasmonic gold nanoparticles” [Appl. Phys. Lett. <b>123</b>, 061104 (2023)

Highly Stable Fluorescent-Traffic-Light Sensor for Point-of-Care Detection of Tetracycline.

Fluorescent point-of-care (POC) sensors have found great utility in fields like clinical diagnosis, food testing, and environmental monitoring. Herein, we developed a highly stable POC sensor that enabled the visual detection of tetracycline (TC) in a distinct fluorescent-traffic-light manner. In the sensor, a composite material of copper nanoclusters and metal-organic framework (CuNCs@MOF-5) prepared with a facile one-pot synthetic strategy was employed as the core element for target recognition and signal transduction. As evidenced by experiments, the as-prepared CuNCs@MOF-5 exhibited significantly improved fluorescence properties in terms of emission enhancement (about 28-fold) and stability improvement (over 110 days) compared to the CuNCs without confining and protection by MOF-5. More importantly, it was found that TC could uniquely interact with Zn(II) to trigger the disassembly of CuNCs@MOF-5, resulting in green fluorescence emission from the TC-Zn(II) complex and red fluorescence weakening of CuNCs. On the basis of this finding, a simple and stable sensor was proposed for POC detection of TC, which demonstrated high sensitivity, selectivity, and reproducibility. In addition to homogeneous visual detection in a 96-well plate, a CuNCs@MOF-5-contained agarose gel array was easily fabricated to achieve direct detection of TC in milk without any pretreatment, thanks to the size-sieving effect of the gel. Moreover, a test paper array was also put forward for low-cost TC detection, which indicates the extensibility and practicability of this sensing strategy.

Preparation of a Mesoporous Biosensor for Human Lactate Dehydrogenase for Potential Anticancer Inhibitor Screening.

Cancer is the second leading cause of death worldwide, with a dramatic impact due to the acquired resistance of cancers to used chemotherapeutic drugs and treatments. The enzyme lactate dehydrogenase (LDH-A) is responsible for cancer cell proliferation. Recently the development of selective LDH-A inhibitors as drugs for cancer treatment has been reported to be an efficient strategy aiming to decrease cancer cell proliferation and increase the sensitivity to traditional chemotherapeutics. This study aims to obtain a stable and active biocatalyst that can be utilized for such drug screening purposes. It is conceived by adopting human LDH-A enzyme (hLDH-A) and investigating different immobilization techniques on porous supports to achieve a stable and reproducible biosensor for anticancer drugs. The hLDH-A enzyme is covalently immobilized on mesoporous silica (MCM-41) functionalized with amino and aldehyde groups following two different methods. The mesoporous support is characterized by complementary techniques to evaluate the surface chemistry and the porous structure. Fluorescence microscopy analysis confirms the presence of the enzyme on the support surface. The tested immobilizations achieve yields of ≥80%, and the best retained activity of the enzyme is as high as 24.2%. The optimal pH and temperature of the best immobilized hLDH-A are pH 5 and 45 °C for the reduction of pyruvate into lactate, while those for the free enzyme are pH 8 and 45 °C. The stability test carried out at 45 °C on the immobilized enzyme shows a residual activity close to 40% for an extended time. The inhibition caused by NHI-2 is similar for free and immobilized hLDH-A, 48% and 47%, respectively. These findings are significant for those interested in immobilizing enzymes through covalent attachment on inorganic porous supports and pave the way to develop stable and active biocatalyst-based sensors for drug screenings that are useful to propose drug-based cancer treatments.

UiO-66/Carboxylated Multiwalled Carbon Nanotube Composites for Highly Efficient and Stable Voltammetric Sensors for Gatifloxacin

UiO-66/Carboxylated Multiwalled Carbon Nanotube Composites for Highly Efficient and Stable Voltammetric Sensors for Gatifloxacin

Anti-freezing conductive zwitterionic composite hydrogels for stable multifunctional sensors

Zwitterionic conductive hydrogels have shown potential application in wearable strain and pressure sensors. However, there are still fundamental challenges to achieve zwitterionic hydrogels with excellent mechanical properties, able to keep flexibility at sub-zero temperatures. To overcome these limitations, a zwitterionic conductive hydrogel was fabricated in this work by in-situ polymerization of aniline (ANI) monomer in a copolymer of sulfobetaine methacrylate (SBMA) and acrylic acid (AA) matrix. The obtained hydrogel possesses outstanding anti-freezing performance (without obvious loss of stretchability at −18 °C) and water-retaining properties, due to the introduction of LiCl on the zwitterionic polymer matrix. The synergy of chemical and physical crosslinking between poly (SBMA-co-AA) and polyaniline (PANI) networks enhance the mechanical performance of the zwitterionic hydrogel, that exhibits a fracture tensile strength of 470 kPa, and a fracture strain up to 600 %. Additionally, the integration of PANI confers stable conductivity (2.23 S m−1, maintained at 1.89 S m−1 even at −18 °C), high sensitivity (GF = 1.74), and short response and recovery times (223 ms and 191 ms, respectively). The hydrogel can be applied as a flexible sensor to accurately detect various human motions. This work provides a feasible strategy for developing wearable multifunctional sensors in a wide working temperature range.