Structural properties and sensing performance of CeTiO3 ceramic films as a solid-state pH sensor

In this paper, we have studied the impact of postannealing treatment on the structural properties and sensing characteristics of CeTiO₃ ceramic membranes deposited on Si substrate by sputtering for solid-state electrolyte-insulator-semiconductor (EIS) pH sensors. X-ray photoelectron spectroscopy, Auger electron spectroscopy, X-ray diffraction, and atomic force microscopy were used to study the chemical compositions, elemental depth profiles, film structures, and surface morphologies of CeTiO₃ ceramic membranes treated at three rapid thermal annealing (RTA) temperatures of 700, 800 and 900?°C. The sensing performance of the CeTiO₃ ceramic membranes annealed at three different RTA temperatures is strongly correlated to their structural properties. The CeTiO₃ EIS device after RTA at 800?°C exhibited the best sensing characteristics (pH sensitivity, hysteresis voltage and drift rate) among these RTA temperatures. We attribute this behavior to the optimal RTA temperature enhancing the Ce³⁺/Ce⁴⁺ ratio of CeTiO₃ ceramic membrane, reducing an interfacial layer at the CeTiO₃-Si interface, and increasing its surface roughness.     

Analytical modelling of monolayer graphene-based ion-sensitive FET to pH changes

Graphene has attracted great interest because of unique properties such as high sensitivity, high mobility, and biocompatibility. It is also known as a superior candidate for pH sensing. Graphene-based ion-sensitive field-effect transistor (ISFET) is currently getting much attention as a novel material with organic nature and ionic liquid gate that is intrinsically sensitive to pH changes. pH is an important factor in enzyme stabilities which can affect the enzymatic reaction and broaden the number of enzyme applications. More accurate and consistent results of enzymes must be optimized to realize their full potential as catalysts accordingly. In this paper, a monolayer graphene-based ISFET pH sensor is studied by simulating its electrical measurement of buffer solutions for different pH values. Electrical detection model of each pH value is suggested by conductance modelling of monolayer graphene. Hydrogen ion (H⁺) concentration as a function of carrier concentration is proposed, and the control parameter (Ƥ) is defined based on the electro-active ions absorbed by the surface of the graphene with different pH values. Finally, the proposed new analytical model is compared with experimental data and shows good overall agreement.     

Effect of pH on the properties of protonated polyaniline-based films for pH sensing applications

The chemical and physical properties of protonated polyaniline (PANI)-based films, including PANI-emeraldine salt (ES) and PANI/polyvinyl acetate (PVAc) films, before and after pH treatments are characterized and compared. Protonated PANI-based films are prepared by spin coating. The effects of pH value and immersion time on the film properties are investigated to gain a better understanding of their performance in pH sensing. The films are characterized based on color, morphology, chemical structure, phase state, and protonation state. Protonated PANI-based films exhibit a color change from green to dark blue as deprotonation occurs in solutions with higher pH. The highly porous structure of PANI/PVAc films is slightly affected by the pH of the solution. However, the globular structure of the PANI-ES films forms cracks with increasing immersion time. PANI/PVAc films exhibit better stability in acidic and neutral solutions than in alkaline solutions because of the hydrolysis of PVAc. Compared with the negligible differences in the PANI/PVAc film in buffers with different pH, PANI-ES exhibits noticeable changes. Therefore, PVAc improves the stability and performance of PANI for pH-sensing applications.     

Covalently cross-linked multilayer thin films composed of diazoresin and brilliant yellow for an optical pH sensor

The multilayered thin films composed diazoresin (DR) and brilliant yellow (BY) were fabricated by a layer-by-layer (LbL) deposition technique, and the ionic bonds between diazonium ion in DR and sulfonate residues in BY were converted to covalent bonds by UV light irradiation. The cross-linking between BY and DR prevented BY from desorption even in a pH 13 solution. The cross-linked (DR/BY)₁₀DR film exhibited pH-dependent absorption spectra in pH 9-13, and the response was repeatable and quick. The pH response time of the films was within a few seconds upon change from pH 9.0 to 13.0, and ca. 40 s upon change from pH 13.0 to 9.0.     

Investigation of extended-gate field-effect transistor pH sensors based on different-temperature-annealed bi-layer MWCNTs-In2O3 films

In this paper, indium (In) films were deposited on glass substrates using DC sputtering method. Multiwalled carbon nanotubes (MWCNTs) and dispersant were dissolved in alcohol, and the mixed solution was deposited on the In films using the spray method. The bi-layer MWCNTs-In₂O₃ films were annealed at different temperatures (from room temperature to 500°C) in O₂ atmosphere. The influences of annealing temperature on the characteristics of the bi-layer MWCNTs-In₂O₃ films were investigated by scanning electron microscopy, X-ray diffraction pattern, Fourier transform infrared (FT-IR) spectroscopy, and Raman spectroscopy. A separative extended-gate field-effect transistor (EGFET) device combined with a bi-layer MWCNTs-In₂O₃ film was constructed as a pH sensor. The influences of different annealing temperatures on the performances of the EGFET-based pH sensors were investigated. We would show that the pH sensitivity was dependent on the thermal oxygenation temperature of the bi-layer MWCNTs-In₂O₃ films.     

A pH sensor based on electric properties of nanotubes on a glass substrate

We fabricated a pH-sensitive device on a glass substrate based on properties of carbon nanotubes. Nanotubes were immobilized specifically on chemically modified areas on a substrate followed by deposition of metallic source and drain electrodes on the area. Some nanotubes connected the source and drain electrodes. A top gate electrode was fabricated on an insulating layer of silane coupling agent on the nanotube. The device showed properties of an n-type field effect transistor when a potential was applied to the nanotube from the top gate electrode. Before fabrication of the insulating layer, the device showed that the p-type field effect transistor and the current through the source and drain electrodes depend on the buffer pH. The current increases with decreasing pH of the CNT solution. This device, which can detect pH, is applicable for use as a biosensor through modification of the CNT surface.     

Effect of pH on the physicochemical properties of starch films

We demonstrate a simple approach of using highly alkaline starch solutions to obtain films with high flexibility and improved water resistance. Extensive studies have been done to develop films from starch, primarily for food applications. However, films developed from starch are brittle and generally chemical modifications or plasticizers are used to improve the flexibility and other properties of starch films. Such modifications make starch expensive, decrease biodegradability, and affect the morphology and subsequent processing of starch. In this research, we have prepared films using starch solutions at pH between 3 and 11. Starch solutions having different pHs were made into films and the tensile, thermal properties, and resistance to water were studied. It was found that preparing starch with pH 11 solution imparts high flexibility without any apparent physical damage. Films prepared under strong acidic and alkaline conditions also had considerably reduced hydrophilicity, which is required for food packaging and other applications. The films prepared at alkaline pH show nearly 50% reduction in water sorption and an increase in elongation. The films obtained from alkaline pH are flexible and water resistant and can potentially be used to develop various bioproducts. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48563.     

Effect of BaTiO3 on the sensing properties of PVDF composite-based capacitive humidity sensors

Capacitive humidity sensors consisting of materials such as polymers, ceramics, and piezoelectrics are widely used to monitor relative humidity levels. The effect of barium titanate (BaTiO₃) nanoparticles on the humidity sensing properties, dielectric response, thermal stability, and hydrophilicity of the polyvinylidene fluoride (PVDF)-BaTiO₃ composite films is investigated. Hydrophilicity and surface morphology of the PVDF-BaTiO₃ composite films are modified for the development of a good humidity sensor. The nanocomposite solutions are prepared by mixing an optimized concentration (2.5 wt%) of PVDF with different concentrations (0.5, 1, and 2 wt%) of BaTiO₃ nanoparticles. X-ray diffraction, thermogravimetric analysis, field emission scanning electron microscopy, and contact angle measurements are used to characterize the structure, morphology, thermal stability, and hydrophilicity of the spin-coated sensing films. The dielectric study of PVDF-BaTiO₃ composite film shows that as the concentration of BaTiO₃ particles increase, the dielectric constant of the composite films increases as well. PVDF-BaTiO₃ (2.5 wt%-1 wt%) based capacitive sensors show stable capacitive response and low hysteresis as compared to the other concentrations of the PVDF-BaTiO₃ composites. The maximum hysteresis of the capacitive PVDF-BaTiO₃ (2.5 wt%- 1 wt%) humidity sensor is found to be ~2.5%. The response and recovery times of the PVDF-BaTiO₃ (2.5 wt%-1 wt%) based capacitive sensors are determined as 40 s and 25 s, respectively, which are significantly lower than those reported for the other PVDF composite based sensors.     

Spectroscopic study of dyes for pH and methanol sensing

The spectral response of three dyes, in both pH and methanol in strongly acidic environments as examples of the inner media of a proton exchange membrane such as Nafion, was investigated. Picric acid responded predominantly to pH while C.I. Acid Orange 5 and C.I. Basic Green 4 each gave a combined response to both pH and methanol concentration. The spectroscopic results obtained show the feasibility of using a single dye for measuring pH without significant interference from methanol; the simultaneous determination of pH and methanol concentration using a binary combination of dyes is also feasible. These characteristics of the dyes allow their use as primary transducers for a fuel concentration sensor in a direct methanol fuel cell.     

Swelling and morphological properties of poly(vinyl alcohol) (PVA) and poly(acrylic acid) (PAA) hydrogels in solution with high salt concentration

Understanding the swelling properties of hydrogels and how they affect the hydrogel's morphology is of fundamental importance in the development of hydrogel-based artificial muscles, bio-actuators, sensors and other devices. In this paper, the swelling behavior of PVA-PAA hydrogel films in saline water and in buffer solutions of different pH values was investigated. It was observed that the swelling factor of the hydrogel decreases when the ionic strength of the solvent solution increases. Scanning Electron Microscopy (SEM) revealed structures with different pore shapes and sizes depending on the type of solution used for hydration. In saline water, Energy Dispersive X-Ray (EDS) analysis indicated the formation of NaCl crystals within the polymeric network. Finally, the PVA-PAA hydrogel was used as an actuator to strain a fiber Bragg grating sensor, thus providing an indirect measurement of the pH value of the surrounding solution.     

Controlling properties of ceramic formulations for porcelain robocasting

Porcelain pastes (PlotPastes) were formulated to be used on an additive manufacturing (AM) process (material extrusion) process, primarily robocasting (R3D) technique. The material morphological and thermal characteristics were evaluated by scanning electron microscopy (SEM), differential thermal analysis (DTA) and thermogravimetric analysis (TGA). The rheology and the electrical potential of the ceramic particles were also studied to select and adequate the porcelain paste properties to the R3D AM technique. It was found that shifting the pH values to acidic, the surface charge of the particles changes and increases the pastes viscosity due to agglomeration effects. This behaviour was exploited to optimize the paste rheological behaviour which resulted in the optimum pH at 1.94 (PlotPaste 5). This paste was used in the study of R3D operating parameters. It was found that small variations in pressure and speed affects the dimensional accuracy of the printed models. The results showed the disruptive potential of porcelain R3D in the production of customized ceramic products.     

Atmospheric flame vapor deposition of WO3 thin films for hydrogen detection with enhanced sensing characteristics

Nowadays, with the increasing demand for hydrogen, sensors that can detect low concentrations of this gas are essential for its safe use. In this paper, Pd/WO₃ film hydrogen sensors are developed using a solid-feed flame vapor deposition (SF-FVD), as an atmospheric, economical, and fast film fabrication method. The crystal structure and morphology of the samples were characterized by different means. The performance of the obtained sensors was investigated for different hydrogen concentrations (1–2500 ppm) and at different operating temperatures (100–250 °C). We attempted to determine the optimum deposition conditions, including feed and substrate to flame nozzle distances. In most of the sensing conditions, the response and recovery times were measured in the order of 20 to 30 s. The layer with a more open morphology showed sensitivity at ppb hydrogen level, good stability, and selectivity. The response behavior of the samples was explained according to the power-law in the metal oxide semiconductor (MOS) gas sensors.     

Constructing bilayer sensors with Co-MOF-derived Co3O4 porous sensing films and SnO2 catalytic overlayers to enhance room-temperature triethylamine sensing performance

Gas sensors with good repeatability and controllable fabrication method are extremely desired for practical applications. Co-MOF-derived Co₃O₄ is a promising gas-sensing material candidate because of its large surface area, ultrahigh porosities, and abundant oxygen defects. However, the advantages of Co-MOF precursor were limited by the traditional sensor fabrication methods. Moreover, the high resistance and poor surface activity of Co₃O₄ resulted in low gas-sensing performance at room temperature (RT). To overcome these challenges, in-situ sensors based on Co₃O₄ porous films with controlled nanoscale thickness were directly prepared on ceramic substrates by using Co-MOF films as precursors. To further improve the conductivity, SnO₂ catalytic overlayers were introduced on top of Co₃O₄ sensors to construct SnO₂/Co₃O₄ bilayer sensors, which were promising for triethylamine (TEA) detection at RT. As a result, the optimized SnO₂/Co₃O₄ sensor exhibited a fast response/recovery rate (11 s/16 s), high selectivity, and a satisfactory sensitivity (150%) to TEA at RT. The enhanced gas-sensing performance could be attributed to the unique bilayer structures, improved conductivity, and synergistic effects of the SnO₂ catalytic overlayers and Co₃O₄ sensing layers.     

Preparation and characterization of nanostructured and high transparent hydrogel films with pH sensitivity and application

Novel nanostructured, high transparent, and pH sensitive poly(2‐hydroxyethyl methacrylate‐co‐methacryliac acid)/poly(vinyl alcohol) (P(HEMA‐co‐MA)/PVA) interpenetrating polymer network (IPN) hydrogel films were prepared by precipitation copolymerization of aqueous phase and sequential IPN technology. The first P(HEMA‐co‐MA) network was synthesized in aqueous solution of PVA, then followed by aldol condensation reaction, it formed multiple IPN nanostructured hydrogel film. The film samples were characterized by IR, SEM, DSC, and UV‐vis spectrum. The transmittance arrived at 93%. Swelling and deswelling behaviors showed the multiple IPN nanostuctured film had rapid response. The mechanical properties of all the IPN films improved than that of PVA film. Using crystal violet as a model drug, the release behaviors of the films were studied. The results showed that compared with PVA, which had low drug loading and exhibited high and burst release, the three IPN films had high drug loading and exhibited sustained release. Besides, the release followed different release mechanism at pH = 4.0 and pH = 7.4, respectively. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Dopants dependent ion sensitivity of polypyrrole‐modified electrode: Case of pH sensing

The polypyrrole (PPY)‐modified electrode made through electropolymerization of pyrrole monomer is found sensitive to different cations depending on the dopants present within microstructure of polymeric domain. When p‐toluenesulfonate (PTS) anion is doped, the modified electrode is sensitive to proton whereas, under similar conditions, in the presence of tetraphenylborate (TPB) anion, the same is sensitive to zinc ion. A comparative study on ion sensing behavior of PPY‐modified electrodes is reported in this communication. The electro‐polymerization of PTS‐doped PPY‐modified electrode, made under both potentiodynamic and potentiostatic conditions, are studied. The modified electrode is characterized by scanning electron microscopy and cyclic voltammetry and compared with those recorded for TPB‐doped PPY‐modified electrode made under similar conditions. The results based on scanning electron microscopy and cyclic voltammetry suggested; (i) dependence of microstructure based on mode of electropolymerization; (ii) dopant‐dependant ion sensitivity of PPY‐modified electrode, (iii) variation in the redox behavior of the PPY‐modified electrode based on dopants and mode of electropolymerization. Typical results on pH sensing together stability of PPY‐based pH sensor are reported and compared with those recorded for polyaniline (PANI)‐based pH sensor. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Influence of pH on the crystal structure of NiMoO4 nanomaterials and their supercapacitor performances

Three types of NiMoO₄ nanomaterials with different morphologies and crystal structures were synthesized by regulating the pH value of the precursor solutions. The corresponding electrochemical performances were also systematically measured. Very interestingly, with increasing of the pH value, the morphologies of NiMoO₄ nanomaterials changed from nanorods to nanosheets, and eventually became to nanoparticles. The NiO/MoO₃ (NiMo-7) sample, prepared at the pH of 7 in the precursor solution, exhibited highly interconnected porous structure, demonstrating much higher specific surface area and enhanced specific capacitance (1516 F/g) at 1 A/g. The assembled asymmetric supercapacitor NiMo-7//AC also delivered a high energy density of 50.13 Wh/kg at 749.9 W/kg and good cyclic stability with a capacity retention of 76% after 5000 cycles at 10 A/g. These results indicated that the NiMo-7 electrode has a very promising application prospect in electrochemical energy storage.     

Development of W/WO3 Sensors for the Measurement of pH in An Emulsion Polymerization System

This paper describes the development of a rugged pH sensor that can be used to monitor hydrogen ion activity in an emulsion polymerization system under demanding reaction conditions. To inhibit deposition of the latex onto the sensor surface, a sensor was chosen to have the same surface charge as that on the latex particles at the pH of the emulsion system. An analysis of pzc (pH of zero charge) of the emulsion latex, as inferred from the zeta potential, the expected pH of the latex, and the known pzc values for various metal oxides, indicated that the W/WO₃ electrode should be a viable pH sensor for the particular latex system of interest. It was concluded that the pzc values for both the latex and the pH sensor would lie below the expected pH of the latex, rendering the surfaces of both systems negatively charged. Coulombic repulsion should therefore inhibit the deposition of latex particles onto the pH sensor surface, thereby avoiding possible interference with the pH-determining reactions. Measurement of the W/WO₃ sensor potential against a Ag/AgCl, KCl (sat.) reference electrode has shown that the sensor potential varies linearly with pH over a wide pH range, as established by various borate buffers, at temperatures of 25 °C, 70 °C and 130 °C. The slope of the potential versus pH correlation at each temperature is slightly less than the theoretical value indicated by the Nernst equation for a system at electrochemical equilibrium. This deviation, which has been noted in previous work on the W/WO₃ sensor in homogeneous aqueous systems, is attributed to the fact that the sensor is not strictly at equilibrium, but instead displays a mixed potential. The W/WO₃ sensor can be accurately calibrated over the conditions that exist in the latex system explored in this work and, while no independently measured pH data are available for the latex at elevated temperatures, the value determined by the W/WO₃ sensor is consistent with expectations. To our knowledge, this work represents the first measurement of proton activity in a latex system under polymerization reaction conditions.     

纳米卟啉材料的制备及其传感性能研究

在水/二甲基亚砜(DMSO)混合溶剂中,制备了5,10,15,20-四苯基卟啉铟(InTPP)和5,10,15,20-四苯基卟啉锰(MnTPP)两种功能化分子的自组装纳米材料。采用UV-Vis法考察了两种功能化卟啉纳米材料MnTPP和InTPP对沙林类似物甲基膦酸二甲酯(DMMP)的检测效果,得到检测限分别为10-9和10-10 L/L。通过密度泛函理论(DFT)构建并优化两种卟啉分子与DMMP的反应模型,计算结果显示,InTPP更易实现对DMMP分子的检测,与UV-Vis的检测结果一致。     

A novel approach for evaluation of polymeric interface materials for chemical sensors using alternate indirect methods

The heart of a chemical sensor based on bulk or surface acoustic wave devices is a polymer‐coated piezoelectric substrate that selectively sorbs and concentrates the target analyte vapors. The development of such sensors often necessitates the screening and evaluation of suitable polymeric interface materials meeting the specified sensitivity and selectivity toward the analytes of interest. The magnitude and dynamics of sorption–desorption of the vapors in the polymer and the extent of polymer–vapor interactions largely determine the performance of a sensor. The standard protocol used for the purpose is rather tedious, involving the generation and calibration of individual analyte vapors, with stringent control on temperature, humidity, and test parameters. This article outlines four different alternative techniques based on mass uptake of the analyte vapors, on its partitioning in polymers, or both, which in combination can determine the characteristics of an interface material used for coating a piezoelectric substrate in acoustic wave‐based chemical sensors. These methods were applied to poly(ethylene maleate), a representative interface material. The analytes ranged from volatile organic chemicals to sarin—a chemical warfare agent—and its simulant, dimethyl methylphosphonate. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 91: 3428–3432, 2004     

Structural,optical and methanol sensing properties of sprayed In2O3 nanoparticle thin films

Indium oxide (In₂O₃) nanoparticle thin films were grown on cleaned glass substrates by the chemical spray pyrolysis technique using the precursor solution of indium nitrate (In (NO₃)₃). The XRD studies confirm that the films are polycrystalline In₂O₃, possessing cubic structure with lattice parameters, a = b = c = 10.17 Å. The optical studies show a direct optical band gap of 3.32 eV and an indirect band gap of 2.6 eV in the prepared films. The films exhibit high optical transparency >80% in the visible region, reaching a maximum of 85% at 684 nm wavelength. Further, the gas sensing properties of the films have been investigated for various concentrations of methanol in air at different operating temperatures. At 300 °C the film exhibits a very high response 99% to methanol vapor at a concentration of 40 ppm in air, which is ideal to be used as a methanol sensor. The film shows fast response and recovery to methanol vapor at higher operating temperatures. A possible methanol sensing mechanism has been proposed.