PANI-PVA conductive hydrogel based chemiresistive gas sensor

Abstract

Abstract TITLE: PANI-PVA conductive hydrogel based chemiresistive gas sensor Polyaniline / Polyvinyl Alcohol Conductive Hydrogel for Chemiresistive Ultra-Sensitive Ammonia Gas Sensor and a method of its fabrication utilizing micro girder (µG) printing technology employing repetitive freeze-thaw (FT) cycled Polyaniline / Polyvinyl Alcohol(PANI / PVA) hydrogel as the sensing material is provided for the detection of ammonia (NH3) gas down to 100 ppm at room temperature. The sensor shows an improved sensitivity of 94.7% for detecting 100 ppm of NH3 with a limit of detection (LOD) of 1 ppm. The repetitive FT cycles promotes the formation of well-defined microstructures of the hydrogels, which has been shown by detailed structural and morphological analysis. This hydrogel-based chemiresistive sensor demonstrated a good response to NH3 gas by monitoring changes in resistance, offering a cost-effective and practical alternative for NH3 detection. The sensor exhibited high sensitivity to NH3 with a rapid response and recovery time of 25 s and 10 s, respectively, making it suitable for real-time applications. Figure 6 (a)

Description

Description:Field of InventionThe present invention provides for Polyaniline / Polyvinyl Alcohol Conductive Hydrogel for Chemiresistive Ultra-Sensitive Ammonia Gas Sensor and a method of its fabrication utilizing micro girder (µG) printing technology employing repetitive freeze-thaw (FT) cycled Polyaniline / Polyvinyl Alcohol (PANI / PVA) hydrogel as the sensing material for the detection of ammonia (NH3) gas down to 100 ppm at room temperature. The sensor shows an improved sensitivity of 94.7% for detecting 100 ppm of NH3 with a limit of detection (LOD) of 1 ppm. The repetitive FT cycles promotes the formation of well-defined microstructures of the hydrogels, which has been shown by detailed structural and morphological analysis. This hydrogel-based chemiresistive sensor demonstrated a good response to NH3 gas by monitoring changes in resistance, offering a cost-effective and practical alternative for NH3 detection. The sensor exhibited high sensitivity to NH3 with a rapid response and recovery time of 25 s and 10 s, respectively, making it suitable for real-time applications. Additionally, the present invention explored the hydrogel sensor’s cross-sensitivity to NH3, Nitrogen dioxide (NO2) and Carbon dioxide (CO2). The sensitivity of the sensor towards some of the breath volatile organic compounds (VOCs) are also analyzed. The sensor's behavior is found to adhere to the Langmuir adsorption / desorption model, further confirming its efficacy in gas detection. The excellent sensing ability with a high cross-sensitivity of the developed sensor is attributed to the formation of well-developed cross-linked network structures during FT cycles. The stability of hydrogel-based sensor over a three-month period further supports its potential for practical deployment in various environmental and healthcare monitoring scenarios, aiming to provide a straightforward strategy for detecting exhaled breath ammonia.Keywords: Conducting polymer hydrogel, PANI / PVA, supramolecular interaction, Ammonia Gas sensor, Microfabrication, Chemiresistive.Background ArtPrinting technology is an emerging microfabrication technique being explored as a potential alternative to traditional semiconductor technology. This innovative approach allows for high precision, ease of fabrication, integration of multiple incompatible materials and scalability, leading to the production of low-cost and high-performance devices. Ammonia is a natural waste byproduct produced in the body, especially by liver, kidneys and intestines. It plays a crucial role in various physiological processes within the human body and holds promise as a potential biomarker for the detection of certain diseases1. Within the human physiological context, ammonia predominantly is originated from the metabolic breakdown of proteins. This intricate process results in its conversion into urea, subsequently being eliminated through urine2,3. Interestingly, ammonia maintains a baseline presence not only within urine but also within the bloodstream, or non-invasively in fecal matter, saliva, sweat and exhaled breath, and even the subtle emissions from the skin, albeit in minimal concentrations4–6. Owing to its volatile characteristics, ammonia exists in the form of gaseous NH3 at the air around the alveolar interface within the lungs. Consequently, there is a prevailing contention that, similar to other volatile biomarkers7, it should be feasible to identify concentrations of NH3 in exhaled breath, indicative of the ammonia content in the bloodstream8. Over the past few decades, significant endeavors have been directed towards the detection of exhaled breath ammonia (eNH3) in pursuit of an alternative to blood or urine testing 9.To date, a number of techniques have been developed to measure ammonia in breath at physiologically relevant concentrations in real (humidified) breath samples. These have mostly been instrumental techniques including chemical ionization, gas chromatography-mass spectrometry (GC-MS) 10–13, laser spectroscopy and photonic crystal spectroscopy 6,14. However, most of the developed instrumental techniques are typically complex, expensive and not suitable for diagnostic application. In addition, a number of such methods contribute to pre-analytical errors due to issues, such as, sample transportation and handling. Despite the development of various sensor-based approaches, including quartz crystal microbalance (QCM) 15,16,chemical and optical sensors17–19, many encounter difficulties related to detection limits or operational efficacy with real humidified breath samples20. Consequently, these challenges render them unsuitable for physiological applications. However, some of the sensors have now demonstrated effective application in human clinical studies 6.Although, the development of a smart material is crucial for overcoming the drawbacks associated with commercially available ammonia gas sensors based on metal oxide semiconductors, carbon materials, and conducting polymers. These drawbacks include the necessity for high temperatures during sensing processes, operational complexities, intricate device fabrication, and the rigid nature with a lack of deformability in conducting polymers 21–23.Compared with traditional materials, conducting hydrogel-based (CH) sensors operate satisfactorily at room temperature24,25. Moreover, CH displays excellent stretchability and fast response time7,26–28. Zhi et al.29 proposed a self-responsive double network (DN) PVA-carrageenan (PCVA) hydrogel-based gas sensor to detect NO2 and NH3.They reported the three types of supramolecular interactions-hydrogel bonding, electrostatic interaction and crystallization with fast and reversible response of the sensing device. Further, the structural design exhibited impressive tensile strength and recovery features. Jin Wu et al.10 reported the fabrication of ultra stretchable gas sensors based on an ionic conductive polyacrylamide / carrageenan double-network (DN) hydrogels exhibiting high sensitivity (78.5ppm-1) to NO2 and NH3 sensors at room temperature. This ionic hydrogel provides unique advantages of extreme mechanical deformability and high transparency without a negative effect on the gas sensing performance. Further, they have also reported the high performance at low concentration (500 and 100 ppb) of NH3 and NO2 detection at room temperature based on vitamin C (VC) modified reduced graphene oxide hydrogel (V-RGOH) synthesized via a green and facile self-assembly. The V-RGOH sensor exhibits remarkable selectivity and linearity and a wide detection range revealing the remarkable effects of chemical modification with biodegradable molecules and three dimensional (3D) structure design on improving the gas sensing performance30. Therefore it is a longfelt need in the art to overcome the drawbacks as stated above by considering the consequent current emphasis given to advanced smart gas sensing materials that can integrate multiple features, coupled with innovative sensor fabrication techniques, to meet the future demands. The semiconductor printing method represents a paradigm shift in electronic device fabrication, offering cost-effective, flexible, and environmentally sustainable alternatives to traditional techniques.References: (1) Chan, M. J.; Li, Y. J.; Wu, C. C.; Lee, Y. C.; Zan, H. W.; Meng, H. F.; Hsieh, M. H.; Lai, C. S.; Tian, Y. C. Breath Ammonia Is a Useful Biomarker Predicting Kidney Function in Chronic Kidney Disease Patients. Biomedicines2020, 8 (11), 1–13. https: / / doi.org / 10.3390 / biomedicines8110468; (2) Martin, T. M. P.; Esculier, F.; Levavasseur, F.; Houot, S. Human Urine-Based Fertilizers: A Review. Crit. Rev. Environ. Sci. 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Sensors and Diagnostics2023, 2 (5), 1256–1266. https: / / doi.org / 10.1039 / d3sd00067b.Objects of the InventionIt is thus the primary object of the present invention to provide for PANI / PVA hydrogel based smart gas sensing material with notable advantages including room temperature sensing operability giving significant sensing response that would emphasize the potential of the PANI / PVA hydrogel as an efficient and cost-effective alternative for gas sensing including NH3 detection to suit breath ammonia sensor at point of care sensing, especially in comparison to complex instrumental techniques.It is another object of the present invention to provide for said PANI / PVA hydrogel based smart gas sensing material that would not only show quick response but would also have quick recovery time.It is still another object of the present invention to provide for said PANI / PVA hydrogel based smart gas sensing material that would demonstrate excellent stability of at least three-month period, making it suitable for practical and long-term applications. It is yet another object of the present invention to provide for said PANI / PVA hydrogel based smart gas sensing material that would be able to show high sensitivity and selectivity towards NH3 even in presence of volatile organic compounds.It is another object of the present invention to provide for said PANI / PVA hydrogel based smart gas sensing material that would be fabricated in a facile manner despite the challenges presented by the sticky PANI / PVA substrate by way of micro girder printing technology with meticulous control.Summary of the InventionThus according to the basic aspect of the present invention there is provided PANI-PVA hydrogel based gas sensor comprising freeze thawed in-situ polymerized Polyaniline / Polyvinyl Alcohol (PANI / PVA) microstructure with enhanced pore size in the range of 2.36-3.42 nm diameter and surface area in the range of 39.48-78.12 m2 / g facilitating absorption and diffusion of gas molecule and sensor sensivity.Preferably said PANI-PVA hydrogel based gas sensor is provided as chemiresistive ultra-sensitive gas sensor preferably ammonia gas sensor comprising Polyaniline / Polyvinyl Alcohol (PANI / PVA) based said in-situ polymerized conductive hydrogel having repetitive freeze-thaw (FT) cycle promoted morphology based microstructure enabling enhanced gas sensing response. wherein said freeze-thawed (FT) in-situ polymerized PANI / PVA hydrogel in repetitive cycles favours said microstructural features including improved crystallinity, morphology, surface area and improved hydrophilicity enabling said gas sensing response for hydrophilic gases including NH3, said four freeze-thaw (FT) cycle promoted morphology of PANI / PVA hydrogel enabling most significant swelling ratio of 750.63% within 72 hours,said four freeze-thaw (FT) cycle promoted morphology of PANI / PVA hydrogel as PPH4 exhibits largest specific surface area (78.12 m2 / g), pore volume (0.067 cm3 / g) and pore diameter (3.42 nm) signifying well developed and more defined pores,said freeze-thaw (FT) cycle promoted morphology of PANI / PVA hydrogel from initial contact angle observed at 0s at 92° over the course of 5, 10, 15 and 20s freeze thaw cycles progressively decreases to 90.5°, 87°, 82.5° and 82° respectively indicative of emerging hierarchical structure for improved absorption of water molecules, improved hydrophilic gas sensing activity,said four freeze-thaw (FT) cycle promoted morphology of PANI / PVA hydrogel enables cross-sensitivity to sense 100 ppm CO2 and NO2 along with NH3 devoid of significant change in sensitivity compared to sensitivity of NH3 alone.According to another preferred aspect of the present invention there is provided said PANI-PVA hydrogel based gas sensor wherein said sensor is able to sense and detect ammonia (NH3) gas down to 100 ppm at room temperature giving improved sensitivity of 88-95% for detecting 100 ppm of NH3 with a limit of detection (LOD) of 1 ppm by monitoring changes in resistance in having four-five freeze-thaw (FT) cycle promoted morphology thereby offering a cost-effective and practical alternative for NH3 detection. More preferably said PANI-PVA hydrogel based gas sensor is provided wherein said sensor exhibits high sensitivity to NH3 with a rapid response and recovery time of 25 s and 10 s, respectively suitable for real-time applications.According to another aspect of the present invention there is provided a process for the fabrication of the PANI-PVA hydrogel based gas sensor comprising Polyaniline / Polyvinyl Alcohol (PANI / PVA) based conductive hydrogel comprising the steps of Providing polyvinyl alcohol solution and adding aniline together with an oxidizing agent that is drop casted on a substrate followed by freeze thawing in cycles and obtaining therefrom repetitive freeze-thaw (FT) cycle promoted morphology of Polyaniline / Polyvinyl Alcohol (PANI / PVA) based conductive in-situ polymerized hydrogel adapted for desired gas sensing response.Preferably a process for fabrication PANI-PVA hydrogel based gas sensor is provided including the following sub-steps:(a) dissolving 5wt% PVA in DI water at 90°C to provide for PVA solution that was cooled for 24 h to remove any trapped air bubbles,(b) adding 0.4 M aniline including protonic acids to the solution of step (a) to which APS (Ammonium persulfate-oxidizing agent) including protonic acid was slowly added by maintaining oxidant-to-monomer molar ratio of 1.25 that was preferably magnetically stirred at 650 rpm for 5 h while maintaining ice bath temperature;(c) dropcasting the solution of step (b) on an ozone cleaned processed substrate for coating including on glass substrate followed by placing in freezer at -22°C for 3 days and subsequently thawed at room temperature for 4 h for one cycle, that was repeated for two cycles, three cycles, four cycles, five cycles and six cycles to obtain therefrom PPH1, PPH2, PPH3, PPH4, PPH5, and PPH6 as freeze-thaw (FT) cycle promoted morphology of Polyaniline / Polyvinyl Alcohol (PANI / PVA) based conductive in-situ polymerized hydrogel on said substrate inlcuding glass substrate.More preferably a process for fabrication PANI-PVA hydrogel based gas sensor is provided wherein said hydrogels were thoroughly washed with 0.1M HCl and DI (deionized) water several times followed by freeze-drying to remove the inherent water.According to yet another preferred aspect of the present invention there is provided a process for fabrication PANI-PVA hydrogel based gas sensor as fabricated gas sensing device including NH3 gas sensing device wherein silver nanoparticles (AgNP) as electrodes at preferably 50 μm channel gap on said freeze-thaw (FT) cycle promoted morphology of Polyaniline / Polyvinyl Alcohol (PANI / PVA) based conductive in-situ polymerized hydrogel on said substrate, are fabricated of Micro-Girder (µG) based printing with 30 mins at 45% humidity at room temperature whereby two different spots of AgNP ink were drop-casted on said substrate with a spacing of roughly 1 mm of the µG tip above the base substrate with the µG tip precisely controlled to minimize the gap between the spots involving dragging the AgNP ink closer to the counter spot, reducing the separation by roughly 80 μm with similar strategy employed to further minimize the gap to 50 μm to the next spot,annealing the thus fabricated AgNP electrodes at 50°C for 10 minutes and obtaining therefrom silver nanoparticles (AgNP) coated substrate as AgNP / (PANI / PVA) / AgNP, metal-semiconductor-metal (MSM) framework based gas sensing device.Preferably in said process for fabrication PANI-PVA hydrogel based gas sensor wherein said protonic acids include HCl, H2SO4, HCSA (camphor sulfonic acid) and HNO3.More preferably in said process for fabrication PANI-PVA hydrogel based gas sensor wherein said fabricated gas sensing device is stable for a period for over 3 months time and is suitable for breath ammonia sensor at point of care sensing applications.Brief description of FiguresFigure 1. illustrates schematic diagram for PANI / PVA hydrogel preparation;Figure 2. illustrates (a) Device fabrication steps (b) Proposed device structure (c) Optical image of the device, (d) FESEM image of the device (e) Schematic diagram of gas sensing set up;Figure 3. illustrates FESEM images of PANI / PVA hydrogels-PPH1, PPH2, PPH3, PPH4, PPH5 and PPH6 (a) Top view (b) cross sectional view (c) EDAX of PANI / PVA hydrogels;Figure 4. illustrates PANI / PVA hydrogels- PPH1, PPH2, PPH3, PPH4, PPH5 and PPH6 (a) XRD patterns (b) FTIR Spectra;Figure 5. illustrates repetitive PANI / PVA hydrogels: (a) Water content (b) Swelling Studies (c) TGA thermogram (d) Tg vs number of FT cycles;Figure 6. illustrates (a) Schematic depicting the formation of hydrogen bonds between NH3and function groups (b) Probable NH3 Gas Sensing Mechanism;Figure 7. illustrates (a) Sensing response of all PANI / PVA hydrogels sensors at 100 ppm NH3 gas (b) I-V characteristics of PPH4 Sensor (c) Sensitivity and AER for different concentration of NH3of PPH4 sensor;Figure 8. illustrates (a) Transient response of the sensor (PPH4) towards different concentration of NH3 (b) XRD pattern and (c) FTIR spectra of G0 and G1 Samples of PPH4 sensor (d) Langmuir Adsorption model (polynomial fitted) with experimental data (e) desorption model (polynomial fitted) with experimental data;Figure 9. illustrates (a) Sensitivity of Sensor towards other components present in breath (b) Temperature response of the sensor (c) Humidity response of the sensor (d) Stability of sensor; PPH4;Figure 10. illustrates DSC thermogram of repetitive FT cycles PANI / PVA hydrogels: PPH1,PPH2,PPH3,PPH4,PPH5 and PPH6;Figure 11. illustrates Water Contact Angle based hydrophilicity measurement (water contact angle measurement of PANI / PVA hydrogels): PPH1, PPH2, PPH3, PPH4, PPH5 and PPH6.Detailed description of the inventionAs discussed hereinbefore, the present invention provides for Polyaniline / Polyvinyl Alcohol Conductive Hydrogel for Chemiresistive Ultra-Sensitive gas sensor including Ammonia Gas Sensor and a method of its fabrication utilizing micro girder (µG) printing technology employing repetitive freeze-thaw (FT) cycled Polyaniline / Polyvinyl Alcohol (PANI / PVA) hydrogel as the sensing material for the detection of ammonia (NH3) gas down to 100 ppm at room temperature.The fabrication and investigation of a chemiresistive gas sensor through printing technology, is being explored as a potential alternative for traditional semiconductor technology. This technique offers the greatest potential for integrating incompatible materials on a variety of substrates through a more efficient bottom-up process compared to conventional etching and lift-off processes. The sensor is printed using inorganic conductive ink (silver, Ag), widely favored in printed electronics for its adaptability and cost-effectiveness in ambient environments. Additionally, PANI / PVA hydrogel film prepared through repetitive freeze-thaw (FT) process, is employed as a sensing material for the detection of NH3 gas at room temperature. This repetitive FT process may enhance the hydrogel features such as crystallinity, morphology, surface area and improved hydrophilicity. Through the fabrication of a series of sensors labeled PPH1 through PPH6, based on repetitive FT PANI / PVA hydrogels, PPH4 is identified as the best material exhibiting notable sensitivity. EXAMPLES1. Experimental Section2.1. MaterialsPolyvinyl alcohol, PVA crystal (molecular weight ~60, 000 gmol-1 and ~98% hydrolysis), aniline (molecular weight 93.13 gmol-1, purity 99%), ammonium persulfate (APS) (99%) and hydrochloric acid (HCl) (grade ~37%) was used without further purification. PVA was procured from Sigma-Aldrich. Aniline was purchased from Merck, India. Ammonium persulfate (APS) was obtained from Sisco Research Laboratories. Hydrochloric acid (HCl) was acquired from Fisher Scientific. Glass slide, acetone, methanol, ethanol and propanol were procured from Sigma-Aldrich, India. The Milli-Q grade water was used in all the experiments.2.2. Preparation of PANI / PVA hydrogelPANI / PVA hydrogel was synthesized in a cryogenic environment adopting the protocol mentioned in our previous work 31. To prepare the hydrogel, PVA solution (soln) was first made by dissolving 5wt% PVA in DI water at 90°C. The solution was then cooled down for 24 h to remove any trapped air bubbles. In the following step, 0.4 M aniline was mixed in the solution, and then APS (oxidizing agent) was slowly added maintaining oxidant-to-monomer molar ratio of 1.25. This mixture was stirred magnetically at 650 rpm for 5 h while maintaining an ice bath temperature. The resulting solution was immediately drop-casted on an ozone cleaned processed glass substrate. The coated substrate was then placed at freezer at -22°C for 3 days and subsequently thawed at room temperature for 4 h for one cycle. This process was repeated for multiple cycles, including two cycles, three cycles, four cycles, five cycles and six cycles. The hydrogels obtained after one, two, three, four, five and six freeze-thaw cycles are referred to as PPH1, PPH2, PPH3, PPH4, PPH5, and PPH6, respectively. The schematic diagram of material preparation is presented in Figure 1. After freeze-thaw cycles, the samples were thoroughly washed with 0.1M HCl and DI water several times. Subsequently, the hydrogel samples underwent freeze-drying to remove the inherent water.2.3. Fabrication of PANI / PVA hydrogel based NH3 gas sensing deviceThe sensor was fabricated utilizing PANI / PVA hydrogel as the primary sensing material, developed on a glass substrate measuring 1 cm×1 cm, which underwent standard cleaning procedures. The fabrication process for the PANI / PVA hydrogel-based NH3 gas sensor is depicted in Figure 2a.After cleaning, the substrates were treated with UV ozone for 15 minutes to enhance the adhesion of the PANI / PVA solution. Typically, 200 µl of PANI / PVA solution was drop-casted onto the ozone-cleaned substrate and subjected to multiple freeze-thaw cycles as described earlier in section 2.2. Using silver nanoparticles (AgNP) as the coated substrate, AgNP / (PANI / PVA) / AgNP,metal-semiconductor-metal (MSM) framework was developed, featuring a 50 μm channel gap. The fabrication of AgNP electrodes was achieved using a Micro-Girder (µG) based printing system from K-Fab Tech Pvt. Ltd. The printing process for electrode fabrication was conducted under approximately 45% humidity at room temperature. Initially, two different spots of AgNP ink were drop-casted with a spacing of roughly 1mm above the base substrate. Despite the challenging nature of the coated PANI / PVA substrate due to its stickiness, the µG tip was precisely controlled to minimize the gap between the spots. This involved dragging the AgNP ink closer to the counter spot, reducing the separation by roughly 80 μm. The same strategy was employed to further minimize the gap to 50 μm to the next spot. The electrodes were then gently annealed at 50°C for 10 minutes. The ability to fabricate electrodes within thirty minutes allows for rapid prototyping of the device with minimal material wastage, resulting in significant savings on cost and time. The successful fabrication of the device, despite the challenges posed by the sticky nature of the PANI / PVA substrate, underscores the importance of meticulous control. The printing process was facilitated using specialized software called K-Fab software, which provided precise control over the printer. It should be noted that establishing metal contacts through other semiconductor processes would be challenging due to the unique properties of the PANI / PVA substrate. The fabricated device is shown in the Figure 2 (b, c, d).2.4. Characterizations2.4.1. Material CharacterizationThe as-prepared samples were dried using lyophilizer (INNOVA, model INOPD-10P) before undergoing further characterization. Microscopic images of the freezed dried hydrogels were investigated using a field emission scanning electron microscope, (FESEM)(JEOL, JSM-7610F) with an accelerating voltage of 5K. The elemental composition and mapping of the samples were analyzed using energy dispersive X-ray, EDAX (Zeiss, Gemini 300). Attenuated total Reflectance-Fourier transform infrared spectroscopy (ATR-FTIR) (Shimadzu, IRAffinity) was employed to investigate the functional groups in the PANI / PVA hydrogels within the frequency range of 400-4000 cm-1. X-ray diffraction (XRD) (Rigaku Technologies, JAPAN, Smartlab) patterns were recorded using CuKα radiation (λ=1.54Å, 40kV, 40mA) at a scan rate of 4°min-1within the 2θ range of 10°-90°. The surface area and pore volume of the hydrogels were determined using Brunauer-Emmett-Teller (BET) (Quantachrome Instruments, Autosorb, IQ MP). The water content and swelling properties were measured by weighing the samples in both their swollen and dry state using analytical microbalance (METTLER TOLEDO, ME204). Thecontact angle (CA) of 5µl water droplets placed on the drop-casted freeze dried hydrogel (PPH sensors) was measured using a contact angle goniometer (Holmarc, HO IAD-CAM-01B) to quantify the hydrophilic nature of the sensors.2.4.2 Device Sensing CharacterizationThe sensing experiment was conducted using a gas sensing system integrated with a parametric analyzer with low-noise triaxial cables.The gas sensing chamber is equipped with a bubbler and an MFC panel which comprise of three Mass Flow Controllers (MFCs)as described elsewhere 32. The schematic diagram of the NH3 gas sensing set up is shown in Figure2e.The sensing chamber is initially evacuated three times by purging nitrogen gas to avoid contamination. The sensing experiment is accomplished intwo different states, (a) in nitrogen ambient and (b) in differentconcentrations of NH3.The base reading of the sensorswas taken in the nitrogen ambient, below 1% humidity andnearly in atmospheric pressure.The I-V characteristic of thedevices for sensing experiments are analyzed by sweeping avoltage from 0 to 5 V through Keithley 4200A-SCS parametric analyzer. All the devices show almost a linear response.The sensing response, evaluated at a concentration of 100 ppm, indicates that the PPH4 sensor exhibits the highest sensing response among all the devices considered. By altering the NH3 gas concentrationfrom 1 to 100 ppm and obtaining the I-V characteristics, aseries of investigations are carried out to evaluate the device’s response and the significant impact of the sensing material.3. Results and Discussion3.1. Morphological and Structural AnalysisFigure3 illustrates FESEM images exhibiting the microstructures of hydrogels that have undergone repetitive freeze-thaw cycles. There is a drastic shift in microstructure across multiple freeze-thaw cycles, as shown in both the top view (Figure 3a) and cross-sectional view (Figure 3b). This may be due to polymer chain interaction between PVA and PANI chains. The samples show evidence of the formation of physically cross-linked interlacing networks. The pore size of the hydrogels and interlacing network have been increased dramatically as the number of freeze-thaw cycles was increased, which may be attributed to the hydrogen bonding33. The interlaced network become a scale-like leaf with increasing the freeze-thaw cycles. The EDAX data (as shown in Figure3c), confirms the presence of C, O, N, S, and Cl and the elemental mapping (Figure S2, Supporting Information) shows that PANI nanorods are evenly distributed over the PVA matrixThe XRD patterns of PANI / PVA hydrogels subjected to repetitive freeze-thaw cycles are depicted in Figure 4(a). The figure exhibits a prominent peak at 2θ=19.6° and a broad peak at 2θ=40.4°, corresponding to (101) and (400) plane of PVA crystal, respectively. The (101) diffraction is due to the intermolecular interference between the PVA chain in the direction of the intermolecular hydrogen bonding 34.The peak at 2θ=40.4° confirms the semicrystalline nature of the PVA,which is originated from the intra- and inter-molecular hydrogen bonding between hydroxyl groups 35. Additionally, it is observed that as the number of freeze-thaw cycleincreases, there is an increase in the peak intensity at 2θ=19.6°. This can be attributed to the enhanced proximity of PVA chain expelled by the ice crystals during freezing steps, forming PVA-rich phase, facilitating a strong hydrogen bonding as well as crystallization. All the PVA chains are not fully involved in the gelation in the first cycle due to the slow movement of polymer chains.Therefore, repetitive cycle ensures the participation of larger content of PVA into the gelation structure increases the PVA crystallinity, yielding stronger and more stable gel 36 which is also confirmed from the increasing pore size of samples as depicted in FESEM images (Figure 3).Moreover, a blunt peak at 2θ = 25°,corresponding to (122) plane of PANI(ES) 37is consistently observed in the samples indicating the presence of PANI (ES) in the hydrogel samples which is also clearly confirmed from the EDAX data (Figure 3c) This signifies that PANI (ES) is evenly distributed over the entired surface of PVA matrix.Figure 4(b) illustrates the FTIR spectra of all the repetitive freeze-thaw cycles of PANI / PVA hydrogels. Peak positions obtained in the range of 4000-400 cm-1 are presented in Table 1. The peak positioncorresponding between –OH group of PVA and NH group of PANI(ES)may vary due to molecular interaction through hydrogen bonding (H-bonding)38. The intensity of the bandwidth corresponding to OH:NH group decreases with increasing repetitive freeze-thaw cycle, which indicates that the free hydroxyl groups were significantly decreased. This may be attributed to the higher inter and intramolecular hydrogen bonding between PANI and PVA and involvement of large PVA chains during phase separation.The FTIR results align well with the FESEM images (Figure 3), showcasing an augmentation in pore size with each successive FT cycle. Moreover, the increased peak intensity observed in the XRD data (Figure 4a) corroborates the presence of strong hydrogen bonding.Table 1.FTIR spectral assignments of PANI / PVA hydrogelsSample O-H:N-Hcm-1 C-H Stretchingcm-1 C=O stretching of acetate groupcm-1 benzenoid C=C stretching band Wagging of CH2cm-1 Wagging of CHcm-1 C-O stretchingcm-1 C-C stretchingcm-1PPH1 3210 2928 1656 1431 1319 1056 828PPH2 3249 2943 1656 1412 1319 1087 828PPH3 3251 2945 1656 1410 1319 1087 828PPH4 3257 2947 1656 1432 1319 1087 828PPH5 3289 2939 1656 1429 1319 1087 828PPH6 3290 2923 1656 1431 1320 1087 8283.2. Water Content, Swelling and Contact Angle StudyComprehensive measurements of water content and swelling ratios for each PPH material are shown in Figure 5.Among the PPH materials, PPH4, PPH5 and PPH6 clearly exhibit the slightly highest water content (as shown in Figure5a). However, all of them are above ~ 81 wt % which is significantly higher than that of the hydrogel reported by Zhang et at. 39, which might be attributed to alteration of network structure due to hydrogen bonding. This signifies the material possesses an excellent mechanical strength which was also reported in our previous work31. It is also noteworthy that all materials reach a point of equilibrium swelling ratio within 72 hours. The most significant swelling ratio of 750.63%is observed in PPH4 (Figure 5b).Moreover, the hydrophilicity of the PPH material was investigated by measuring their dynamic water contact angle within 20s as demonstrated in the supporting information section(Figure S3 (a-f)).Taking thePPH1 material as an illustrative example, the initial contact angle observed at 0s was 92°. Subsequently, over the course of 5,10,15 and 20s, the contact angle progressively decreased to 90.5°,87°,82.5° and 82° respectively. As the freeze-thaw cycles were increased, there was a gradual reduction in the contact angle, indicative of the emergence of a hierarchical structure. This structure possesses the capacity to absorb a higher quantity of water molecules during freeze-thaw cycles, resulting in an enhanced hydrophilic characteristic of the material surface. It is significate to note that all PPH materialsexhibited this hydrophilic behavior consistently.Thus, it can be concluded that our prepared materials provide an ideal condition for detecting and sensing hydrophilic gas molecules due to the possess of hydrophilic environment coupled with the increased swelling ratios due to the enhanced proximity of PVA chain during FT cycles.3.3. Thermal AnalysisThe thermogravimetric analysis (TGA) studies were conducted in a nitrogen atmosphere 20-600°C at a heating rate of 10°C / min to investigate the effect of FT cycles on the thermal stability of PANI / PVA hydrogel. The TGA thermogram of all the PANI / PVA hydrogel samples are depicted in Figure 5c. PANI / PVA hydrogel shows a three step degradation process. The first stage of weight loss (~100°C) for the samples was mainly due to evaporation of water molecules and loss of adhered moisture 31. The second stage decomposition of the samples wasdue to polymer backbone40with a maximum degradation temperature at 192°C,140°C, 197°C, 201°C,190°C, 184°C and third stage due to the continuance of polymer backbone residue31,40 at 439°C, 435°C,430°C, 434°C, 438°C, 444°C for PPH1, PPH2, PPH3, PPH4, PPH5 and PPH6, respectively.At 600°C, a residue of 19.87%, 34.7%, 36.44%, 25.16 %, 22.53%,15.81% were left for the samples corresponding to PPH1 to PPH6 respectively. This show that at 1st FT cycle, gelation not completely done due to poor involvement of PVA chain. Consequently, the cross-link between PANI and PVA is not significant which is evident from the FTIR spectra (Figure 4b). With increasing FT cycles, the physical cross-linking between PANI and PVA become distinct due to the increased involvement of PVA chain and high PANI consumption.However, after 4th FT cycle,the amount of residue started decreasing due to the change in orientation of PVA chains arising from stretching of inter and intramolecular hydrogens bond with the non-bonded hydroxyl groups of PVA.This change in H-bonding is also confirmed from the broader peak intensity corresponding to OH-NH group from FTIR spectra (Figure 4b). PPH1showed higher degradation rate as compared to PPH6, thereby, it can be concluded that as repetitive FT cycle increases the thermal stability enhances.To further understand the changes in the thermal properties of PANI-PVA hydrogels with respect to repetitive freeze-thaw (FT) cycles, we conducted a differential scanning calorimetry (DSC) study. The relationship between the glass transition temperature (Tg) and the number of FT cycles, as derived from the DSC thermogram of the PANI-PVA hydrogel (detailed in Supporting Information,Figure S4), is illustrated in the Figure 5d. The Tg curve for the repetitive FT cycles of PANI-PVA hydrogels lies between the glass transition temperatures of PANI and PVAhydrogel (Figure. 5d, inset), following a distinct trend. This trend indicates that the incorporation of PANI into the PVA matrix restricts the rearrangement of PVA chains40 as the number of FT cycles increases. This observation is clearly supported by the FTIR spectra(Figure 4b), which correlate with the changes in Tg. 3.4. Surface area AnalysisThe BET method is a crucial tool for investigating both the surface and internal variations in the physical structure of materials. Determining the surface area and porosity of materials is essential for understanding the significant characteristics relevant to gas sensing applications.Table2 shows the porosity characteristics of all the hydrogels. PPH4 exhibited the largest specific surface area(78.12 m2 / g), pore volume(0.067 cm3 / g)and pore diameter(3.42 nm)signifying well developed and more definedpores as displayed in FESEM images (Figure 3) and swelling study (Figure 5b).This may be attributed to theincreased interfacial interaction between PVA chain and PANI nanorods31with increased number of freeze-thaw cycles.Conversely, in PPH6, the surface area and pore volume were observed to decrease with increasing pore diameter which is also depicted in the FESEM images(Figure 3).This phenomenon may be attributed to the close contact and accumulation of molecular chains during the formation of ice crystalsin the hydrogel freezing process41.Therefore, as the number of freeze-thaw cycles increases, re-organization of polymer chains takes place, leading to an increased pore size, while surface area decreases. Table 2. Surface area, pore volume and pore diameter of hydrogelsSamples Surface aream2 / g Pore volumecc / g Pore diameternmPPH1 39.38 0.023 2.36PPH2 47.10 0.039 3.33PPH3 63.55 0.048 3.02PPH4 78.12 0.067 3.42PPH5 72.76 0.032 1.74PPH6 23.31 0.019 3.393.5. Gas Sensing Characteristics3.5.1 Sensing MechanismPANI is commonly doped with protonic acids (HA) such as HCl, H2SO4, HCSA (camphor sulfonic acid)and HNO3. PANI-based sensors for detecting ammonia(NH3) primarily operate by de-protonating acid-doped PANI, which is relianton the presence of the NH342.When PANI is exposed to ammonia, NH3 molecules may accept protons from the PANI backbone, generating more energetically favorable NH4+ ions in the process. In other word, when PANI is exposed to NH3, the basic nature of NH3 results in the transfer of the electrons to PANI. This electron transfer is believed to induce de-protonation, resulting to the transformation of emeraldine salt (conducting in nature) form to emeraldine base form (insulating in nature). As the supply of NH3 is depleted,reversible decomposition of NH4+ ions to NH3 occurs accompanied by the conversion of non-conducting PANI to conducting PANIH+. Therefore, the majority of PANI-based sensors operate on the principle of doping and de-doping, or protonation–de-protonation 43.This mechanism, underpinned by the interplay of electron dynamics, enables PANI sensors to respond sensitivelyto NH3. The anticipated interaction between PANI and NH3 is expected to involve the following reaction, with PANI and PANIH+ representing the undoped and doped forms of PANI, respectively: (1) (2)The schematic diagram of sensing mechanism is illustrated in Figure 6.When PANI / PVA hydrogel sensors are exposed to 100 ppm NH3 gas (as shown in Figure 7a), the PPH4 sensor exhibits the strongest response, which may be due to the larger specific surface area as illustrated by BET analysis (Table 2). Furthermore, the formation of the 3D porous structure of PANI / PVA hydrogel and physically cross-linked interlacing networks with increased freeze-thaw cycles enhances the surface area and pore size of the hydrogels ((as depicted from FESEM image, Figure 4)). Consequently, the increased surface area and pore size facilitate better absorption and diffusion of gas molecules, improving sensor sensitivity.Thus, we have selectively studied the PPH4 sensor for different concentrations of NH3 gas for better understanding of its mechanism. Uponexposure to different concentration of NH3 gas,current reduces and thus resistance of the sensor increases(Figure 7b) which is attributed to the interaction between NH3 and the sensor matrix.The sensitivity vs.average electrical resistance (AER) response curve for different NH3 concentrations for PPH4 sensor is illustrated in Figure 7c, where the sensitivity was calculated based on the following equation44 denoting Ro and Rg as sensor resistance in the air and the target gas (3) The response increases with increasing NH3 concentrations. The enhanced response of the sensor may be attributed to the formation of hydrogen bonds47,48between NH3and oxygenated functional groups (OH and NH) of the PANI / PVA hydrogel matrix upon exposure to varying concentrations of NH3 gas, as illustrated in Figure 6a. Consequently, the absorption of NH3gas into the hydrogel matrix hinders ionic mobility (as depicted in Figure 6b), thereby blocking ionic transport. This results in increased resistivity. The sensing mechanism of the hydrogel differs from protonation and deprotonation process in PANI43 even though the hydrogel contains PANI. This mechanism is also differ from the traditional charge-transfer mechanism for carbon materials and the electron depletion principle for metal oxides used in electron-conducting gas-responsive materials45,46. However, it is highly effective for NH3 sensing. Thus, the interaction of PANI / PVA hierarchical structure withNH3increases the resistance and enhanced the response of the sensor. The sensor shows a limit of detection (LOD) of 1 ppm, calculated using the equation given below: (4) where is the average electrical resistance for the various analyte gas concentrations and σ is the average of the standard deviation of the base reading.3.5.2 Transient responseTransient response of the hydrogel sensor is shown in Figure 8a. This study helps to understand the response and recovery time of the sensor in detailed. The sensor was flushed continuously to four pulses of NH3 gas with various concentrations ranging from 1ppm, 10 ppm, 30 ppm and 50 ppm accompanied by mild evacuation of the gas in the sensing chamber under a constant voltage of 5V. The gas in condition begins during the gas sensing chamber pressurized to the atmospheric pressure with a blend of NH3 and N2 gas and gas out condition starts when the flow of the analyte gas has halted and the chamber was evacuated. The increase in the amplitude of resistance change corresponds directly to the rising gas concentration, underscoring the intricate interplay of electron dynamics that empowers PANI sensors to respond effectively. The response and recovery times were calculated according to the change in resistance curves. The response and recovery time are described as the length of time required for the resistance amplitude to increase and decrease from 10%to 90% and 90% to 10% of the resistance value, respectively. The transient response gives a good response time ranging from 25 to 45 s for different NH3 concentrations with excellent recovery time of 10s. No baseline shift was observed when sensor was shifted back and forth between mild vacuum and test gas environment. Additionally, the phase composition analysis from XRD pattern of the PANI / PVA hydrogel sample, PPH4, after subjecting to 100 ppm of NH3 for 24 hours, (G1) and as prepared PANI / PVA hydrogel, PPH4 (G0) (Figure 8b)revealed no significant alterations in peak diffraction. Similarly, to identify the changes of chemical composition and functional groups on the surface of G1 and G0 samples, we recorded FTIR spectra for both the samples. FTIR spectra (Figure 8c) comparisons indicated minimal differences in the PANI-PVA hydrogel-based sensor when exposed to the same NH3 concentration. These findings validate the sensor's effective recovery process post-exposure to NH3.3.5.3 Adsorption and desorption Kinetics AnalysisThe interaction between NH3 molecules and the synthesized PANI / PVA hydrogel occurs by the following equation49: (5)where K+ and K-are the adsorption and desorption rate constants. Here, we have assumed that single layer of NH3 gas gets deposited on the PANI-PVA hydrogel by the adsorption / desorption model (Langmuir model). The response and recovery time of the sensing system can also be obtained by fitting the conductance data in the adsorption / desorption model 49. At constant time t, assuming Langmuir isotherm adsorption kinetics for a single adsorption site, the transient conductance for response process and the recovery process are given by the following equations50: for response(6) for recovery(7)Where , and are base conductance, saturated conductance in air and saturated conductance with test gas of the sensing material and τ+ and τ-is the is the response and recovery time, respectively. The aforementioned two parameters can be expressed as (8) (9)where C represents the concentration of NH3. However, the exponential curves (corresponding to equations 8 and 9) can be converted to polynomial fitting by taking natural logarithm of both the sides and where G and G change in base conductance and saturated conductance. Hence, the equations are converted as follows: (10) (11)Figure 8(d,e) shows the plot of the Langmuir adsorption model with experimental data and polynomial fitin equation 10 and 11; and R2 was found to be0.98. R2 value shows our sensor follows the Langmuir adsorption model. 3.5.4 Cross sensitivityThe selectivity of the sensor to CO2, NO2, and various volatile organic compounds (VOCs), including acetone, methanol, isopropyl alcohol (IPA), and ethanol, each at a concentration of 100 ppm, is presented in Figure 9a.These targeted VOC concentrations were achieved by diluting the compounds with deionized (DI) water and subsequently injecting the resulting mixture into the chamber using a bubbler. It is observed that the sensor's selectivity for these gases and VOCs is noticeably lower compared to its selectivity for NH3 gas at equivalent concentrations.To assess cross-sensitivity, two separate experiments were conducted. In one experiment, the sensor was exposed to a 100 ppm mixture of NO2 and NH3 in nitrogen. In another experiment, the sensor was exposed to a 100 ppm mixture of CO2 and NH3 in nitrogen, in the sensing chamber. Cross-sensitivity analysis reveals that the presence of 100 ppm CO2 and NO2 along with NH3 does not cause a significant change in sensitivity compared to the sensitivity to NH3 alone (as depicted in Figure 9a). Since the gas sensing measurements were performed at room temperature, we have also analyzed the hydrogel sensor towards temperature variation in a tightly sealed chamber. Initially, the chamber is maintained at 20oC and heated to 120oC for analyzing the sensor characteristics with respect to the variation of the resistance. The variation of resistance with temperature is displayed in Figure 9b. It has beenobserved that the resistance decreases gradually in the temperature range from 30° to 50 °C and drastically reduces the resistance beyond 50 °C. Therefore, it can be concluded that with increasing temperature, the resistance of the device reduces due to ion migration (in thermally activated process)26,51. The effect of relative humidity (RH) on the hydrogel sensor was tested in the above chamber at 50 oC. For achieving the greater levels of RH,a fixed quantity of DI water was injected inside the chamber at 50 oC. The RH response curve reveals the continuous decrease in the resistance with the increase in RH from 15 to 90% (as shown in Figure 9c), which is due to the dissociative ionization of adsorbed water molecules on the sensitive surface 52,53. The adsorption of a water molecule on sensitive film causes the auto-ionization reaction of H2O to form H+ ions and OH- ions in which H+ ions are protonated with another H2O molecule to form H3O+ ions, which facilitates the protonic conduction 53 along with lowering the resistance. The result intimate that fabricated hydrogel sensor is sensitive to moisture at 50 oC. Hence, it may be concluded that moisture is an external factor that can impact performance of the hydrogel sensor. In order to find the stability of the sensor, the base current of the sensor was measured over a period of 3 months with a 2-week interval. The sensor was reasonably stable for the entire period of study, and the stability study is shown in Figure 9d. The present sensor exhibits the fastest response time of 25 s and recovery time of 10 s with varying NH3 detection range at room temperature as compared to other NH3 gas sensors (Table S3)Table S3. Comparison between recent technologies for NH3 detection based on chemiresistive sensorSamples NH3 concentration Temp (oC) Sensitivity LOD Response time Recovery time Ref.PANI mesh / PET 2.5 ppb to 100 ppm - 75%(100ppm) 2.5ppb - - 1PANI NF / Glass substrate (1-100) ppm - 62%(100ppm) 1ppm 24s(100 ppm) 72s(100 ppm) 2CFN / PANI-PVA film (20-100) ppm RT 84%(100ppm) - 46s(60 ppm) 62s(60 ppm) 3PANI / Paper / Ag 200 ppb to 3.15 ppm RT 20.5% / ppm 180ppb - - 4PET-NH2-MWCNTs / PANI (33 -100) ppm RT 159%(100 ppm) - - - 5CA-PVA hydrogel (5– 40)ppm RT 31.60% for 40 ppm 140.85 ppb 46 s(40 ppm) 29 s( 40 ppm) 6SnO2 / ppy / PVA hydrogel film 50 ppb - 500 ppm RT - 50ppb 124s(50 ppb) 68s(50 ppb) 7PANI-PVA hydrogel (1-100) ppm RT 94.7%(100 ppm) 1ppm 25 s(50ppm) 10s(50ppm) Present studyIt is thus possible for the present invention to provide for conducting PANI / PVA hydrogel exhibits varied properties and applications depending on its composition and preparation methods. In the present paper, the PANI / PVA hydrogel used for NH3 gas sensors is prepared by repetitive freezing-thawing (FT) in-situ polymerization method. The microstructure of freeze-dried repetitive FT PANI / PVA hydrogels (denoted as PPH1…PPH6 based on number of cycles) were investigated by FESEM, which illustrated the variations in pore size and interlacing of network due to polymer chain interactions. The sensing characteristics of PPH1 through PPH6 fabricated sensors demonstrated notable advantages, including room temperature operability, a significant response, emphasizing the potential of the PANI / PVA hydrogel as an efficient and cost-effective alternative for NH3 detection, especially in comparison to complex instrumental techniques. Among PPH sensors, PPH4 showed the highest response of 94.7% with LOD 1 ppm and 25s and 10s response and recovery time respectively. The hydrogel sensor exhibited excellent stability over a three-month period, making it suitable for practical and long-term applications. Additionally, its cross-sensitivity to various volatile organic compounds (VOCs) was evaluated, revealing a high selectivity for NH3. Despite the challenges presented by the sticky PANI / PVA substrate, the micro girder, µG printing system allowed for precise electrode fabrication within thirty minutes, highlighting the importance of meticulous control. Overall, the PANI / PVA hydrogel sensor presented in this study holds promise for a breath ammonia sensor at point of care sensing. Its combination of sensitivity, selectivity, and practical attributes positions it as a valuable tool for real-time NH3 detection in diverse settings. Further development and optimization of this hydrogel-based sensing approach could lead to advancements in wearable gas sensing devices and contribute to the growing field of environmental and health monitoring technologies.

Claims

, Claims:We Claim:

1. PANI-PVA hydrogel based gas sensor comprising freeze thawed in-situ polymerized Polyaniline / Polyvinyl Alcohol (PANI / PVA) microstructure with enhanced pore size in the range of 2.36-3.42 nm diameter and surface area in the range of 39.48-78.12 m2 / g facilitating absorption and diffusion of gas molecule and sensor sensivity.

2. The PANI-PVA hydrogel based gas sensor as claimed in claim 1 as chemiresistive ultra-sensitive gas sensor preferably ammonia gas sensor comprising Polyaniline / Polyvinyl Alcohol (PANI / PVA) based said in-situ polymerized conductive hydrogel having repetitive freeze-thaw (FT) cycle promoted morphology based microstructure enabling enhanced gas sensing response. wherein said freeze-thawed (FT) in-situ polymerized PANI / PVA hydrogel in repetitive cycles favours said microstructural features including improved crystallinity, morphology, surface area and improved hydrophilicity enabling said gas sensing response for hydrophilic gases including NH3, said four freeze-thaw (FT) cycle promoted morphology of PANI / PVA hydrogel enabling most significant swelling ratio of 750.63% within 72 hours,said four freeze-thaw (FT) cycle promoted morphology of PANI / PVA hydrogel as PPH4 exhibits largest specific surface area (78.12 m2 / g), pore volume (0.067 cm3 / g) and pore diameter (3.42 nm) signifying well developed and more defined pores,said freeze-thaw (FT) cycle promoted morphology of PANI / PVA hydrogel from initial contact angle observed at 0s at 92° over the course of 5, 10, 15 and 20s freeze thaw cycles progressively decreases to 90.5°, 87°, 82.5° and 82° respectively indicative of emerging hierarchical structure for improved absorption of water molecules, improved hydrophilic gas sensing activity,said four freeze-thaw (FT) cycle promoted morphology of PANI / PVA hydrogel enables cross-sensitivity to sense 100 ppm CO2 and NO2 along with NH3 devoid of significant change in sensitivity compared to sensitivity of NH3 alone.

3. The PANI-PVA hydrogel based gas sensor as claimed in claim 1 or 2 wherein said sensor is able to sense and detect ammonia (NH3) gas down to 100 ppm at room temperature giving improved sensitivity of 88-95% for detecting 100 ppm of NH3 with a limit of detection (LOD) of 1 ppm by monitoring changes in resistance in having four-five freeze-thaw (FT) cycle promoted morphology thereby offering a cost-effective and practical alternative for NH3 detection.

4. The PANI-PVA hydrogel based gas sensor as claimed in claims 1-3 wherein said sensor exhibits high sensitivity to NH3 with a rapid response and recovery time of 25 s and 10 s, respectively suitable for real-time applications.

5. A process for the fabrication of the PANI-PVA hydrogel based gas sensor comprising Polyaniline / Polyvinyl Alcohol (PANI / PVA) based conductive hydrogel as claimed in claims 1-4 comprising the steps of Providing polyvinyl alcohol solution and adding aniline together with an oxidizing agent that is drop casted on a substrate followed by freeze thawing in cycles and obtaining therefrom repetitive freeze-thaw (FT) cycle promoted morphology of Polyaniline / Polyvinyl Alcohol (PANI / PVA) based conductive in-situ polymerized hydrogel adapted for desired gas sensing response.

6. The process for fabrication PANI-PVA hydrogel based gas sensor as claimed in claim 5 including the following sub-steps:(a) dissolving 5wt% PVA in DI water at 90°C to provide for PVA solution that was cooled for 24 h to remove any trapped air bubbles,(b) adding 0.4 M aniline including protonic acids to the solution of step (a) to which APS (Ammonium persulfate-oxidizing agent) including protonic acid was slowly added by maintaining oxidant-to-monomer molar ratio of 1.25 that was preferably magnetically stirred at 650 rpm for 5 h while maintaining ice bath temperature;(c) dropcasting the solution of step (b) on an ozone cleaned processed substrate for coating including on glass substrate followed by placing in freezer at -22°C for 3 days and subsequently thawed at room temperature for 4 h for one cycle, that was repeated for two cycles, three cycles, four cycles, five cycles and six cycles to obtain therefrom PPH1, PPH2, PPH3, PPH4, PPH5, and PPH6 as freeze-thaw (FT) cycle promoted morphology of Polyaniline / Polyvinyl Alcohol (PANI / PVA) based conductive in-situ polymerized hydrogel on said substrate inlcuding glass substrate.

7. The process for fabrication PANI-PVA hydrogel based gas sensor as claimed in claims 5 or 6 wherein said hydrogels were thoroughly washed with 0.1M HCl and DI (deionized) water several times followed by freeze-drying to remove the inherent water.

8. The process for fabrication PANI-PVA hydrogel based gas sensor as claimed in claims 5-7 as fabricated gas sensing device including NH3 gas sensing device wherein silver nanoparticles (AgNP) as electrodes at preferably 50 μm channel gap on said freeze-thaw (FT) cycle promoted morphology of Polyaniline / Polyvinyl Alcohol (PANI / PVA) based conductive in-situ polymerized hydrogel on said substrate, are fabricated of Micro-Girder (µG) based printing with 30 mins at 45% humidity at room temperature whereby two different spots of AgNP ink were drop-casted on said substrate with a spacing of roughly 1 mm of the µG tip above the base substrate with the µG tip precisely controlled to minimize the gap between the spots involving dragging the AgNP ink closer to the counter spot, reducing the separation by roughly 80 μm with similar strategy employed to further minimize the gap to 50 μm to the next spot,annealing the thus fabricated AgNP electrodes at 50°C for 10 minutes and obtaining therefrom silver nanoparticles (AgNP) coated substrate as AgNP / (PANI / PVA) / AgNP, metal-semiconductor-metal (MSM) framework based gas sensing device.

9. The process for fabrication PANI-PVA hydrogel based gas sensor as claimed in claims 5-8 wherein said protonic acids include HCl, H2SO4, HCSA (camphor sulfonic acid) and HNO3.

10. The process for fabrication PANI-PVA hydrogel based gas sensor as claimed in claims 5-9 wherein said fabricated gas sensing device is stable for a period for over 3 months time and is suitable for breath ammonia sensor at point of care sensing applications.Dated this the 11th day of December, 2024 Anjan Sen Applicants Agent & Advocate IN / PA 199