{"paper_id":"06e08c4e-b151-439f-b9fd-b0938bd56c7f","body_text":"Frontal Polymerization Synthesis and Humidity Sensing Properties of Pd-Doped Polyacrylamide with Theoretical Validation using DFT | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Frontal Polymerization Synthesis and Humidity Sensing Properties of Pd-Doped Polyacrylamide with Theoretical Validation using DFT Lava Kumar Gupta, Bal Chandra Yadav, Shripal Sharma, Utkarsh Kumar, and 1 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-7606983/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted 17 You are reading this latest preprint version Abstract In this paper, the frontal polymerization synthesis of Pd-doped polyacrylamide has been reported. A thin film based on this material is fabricated for characterization purposes as well as for humidity sensing. For material characterization, Field Emission Scanning Electron Microscopy (FESEM), Energy Dispersive X-ray (EDX) spectroscopy, Particle Size Analyzer, UV-visible spectroscopy, X-ray diffraction (XRD), and Fourier Transform Infrared (FTIR) spectroscopy techniques are used. FESEM shows that the synthesized polymer is porous. The EDX spectrum confirms the presence of constituent elements in the metallopolymer. The particle size analyzer shows that the synthesized polymer has a particle size in the nano range. Pd-doped polyacrylamide has a 5.3 eV optical band gap of energy. After characterization, the polymer is employed for humidity sensing, and the sensor shows 2.01 MΩ/%RH, 96.52% and 5 and 40 s sensitivity, repeatability response and recovery times, respectively. A theoretical framework was established to elucidate the humidity-sensing mechanism of Pd-doped polyacrylamide. Density functional theory (DFT) calculations provided detailed insights into the structural and electronic modifications induced by H 2 O adsorption. The simulated electronic responses, including band gap modulation and charge redistribution, exhibit strong agreement with experimental observations, thereby validating the proposed sensing mechanism and confirming the reliability of the theoretical model. Palladium Doped Polyacrylamide Moisture Frontal polymerization DFT Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 1. Introduction Metal centres in metallopolymers exhibit diversity ranging from group metals to lanthanides and transition metals. In polymers, metals are positioned either in the side group structure or in the main chain. Furthermore, metallopolymers can be highly branched, linear, or dendritic. Over the past decades, groups of researchers worldwide have tackled many difficulties in the synthesis of soluble and higher molecular weight metallopolymers [ 1 – 3 ]. Numerous protocols for the preparation of metallopolymers have been established, involving the development of new techniques compatible with the presence of metal centers [ 4 – 8 ]. Researchers have devised processes to synthesize selected metallopolymers using methods such as polycondensation, chain-growth condensation polymerization, electro-polymerization, ring-opening polymerization, and controlled radical polymerization. In addition to these techniques, a new and advanced technique known as frontal polymerization has garnered attention due to its simplicity, ease, and cost-effectiveness. This technique allows for the synthesis of various metallopolymers with diverse structures, enabling the study of their properties and a wide range of advanced sustainable applications [ 9 – 13 ]. Humidity, one of the essential elements of the environment, is crucial for various applications, including environmental monitoring in greenhouses, medical facilities, clean rooms, industrial process control, and heating ventilation air conditioning (HVAC) systems [ 14 – 21 ]. Humidity sensors can be classified into several types based on sensing mechanisms and readout signals, including impedance-based, capacitance-based, resistance-based, surface acoustic wave (SAW), optical, and field-effect transistor sensors. Moreover, a variety of materials such as nanostructured metals, nanocomposites, carbon nanotubes, graphene, and polymers are utilized as sensing materials [ 22 – 33 ]. Many research papers focusing on the synthesis of polymer/inorganic materials using electrochemical and polymerization techniques have been published for humidity and gas sensing applications [ 34 ]. In a review article, the improvement in humidity sensing characteristics based on thin films of polymers has been demonstrated by Y. Sakai et al. [ 35 ]. Researchers have also published several reports on functional humidity/gas sensing nanomaterials [ 36 , 37 ]. A humidity sensor for respiration monitoring based on Graphene/PVA/SiO 2 -based surface acoustic wave (SAW) was developed by Y. Su et al. [ 38 ]. Previously, metallopolymers-based fluorescent/phosphorescent dual-emissive polymer dots were reported [ 39 – 41 ]. The frontal polymerization method has been utilized by S. Singh et al. for the preparation of nanostructured ZnS and PbS with acrylamide complex in sensing applications [ 42 ]. Recently, using the frontal polymerization technique, rhodium acrylamide SnO 2 nanocomposite polymer was prepared for humidity sensing [ 43 ]. Among various polymers and metals, metallopolymers have attracted researchers and scientists due to their altered electrical, magnetic, and optical properties compared to pure polymers and metals [ 44 ]. The nanostructured complex of acrylamide with metal oxides may prove to be fruitful for sensing purposes. In the present work, an attempt has been made to synthesize a nanostructured composite of Palladium-doped polyacrylamide using the frontal polymerization route, and the sensing element was fabricated using the spin coating technique. 2. Experimental procedure 2.1 Synthesis Method In the synthesis process, the revised method for the preparation of acrylamide complexes of transition metal nitrates was used accordingly [ 45 ]. Acrylamide and palladium nitrate were assorted in a molar ratio of 5:1 in the inert atmospheric condition, we got a paste, which was washed with dry diethyl ether, and dried in a vacuum (0.2 Torr) up to constant weight. For frontal polymerization purposes, the PdAAm were hard-pressed into pellets (having a diameter of 0.5–0.8 cm, along with a height of 1.2–1.5 cm, and also a density of 1.45 ± 0.02 g/cm 3 ) and were put into a glass ampoule. After placing the ampoule bottom (2–3 mm) into a porcelain crucible having a heat transfer medium at 130–140 ºC for 8–20 s, the polymerization process was initiated. From the motion of the coloured zone front, the reaction rate was predicted. The hybrid composite or powdery polymer products were reserved from the ampoule, ground, and washed with ethanol and diethyl ether, after that it was dried in a vacuum at room temperature. The finally prepared sample was recognized as PdAAm i.e. polymer. 2.2 Fabrication of the thin films The quality of the coated films can be maintained by adjusting the preparation parameters, such as solution concentration, spinning velocity, and heating temperature. In this study, the spin coating technique [ 20 ] was employed to deposit uniform thin films of polymeric material onto flat borosilicate glass substrates measuring 1.0×1.0 cm². Prior to deposition, the substrates underwent a series of cleaning steps in an ultrasonic bath (WUC-AO2H), including immersion in lab detergent (Extran), de-ionized water, isopropyl alcohol, and acetone for 10 minutes each. Subsequently, heat treatment at 100°C for 15 minutes was applied to evaporate organic impurities. The prepared substrates were then used for film deposition. The thin films were deposited at a spinning speed of 1000 RPM and dried for 40 minutes at 55°C. These films were later utilized as sensing elements for humidity detection. The schematic diagram for film fabrication is depicted in Fig. 1 . 2.3 Characterizations The film's surface morphology and elemental analysis were conducted usingField emission scanning electron microscopy (FESEM) (JEOL, JSEM-6490LV) coupled with an energy dispersive X-ray (EDX) analysis detector. Particle size in aqueous and dilute solutions was assessed using the Zeta nanosizer (Malvern, Nano-ZS90). The UV-Visible absorption spectrum of the thin film within the 150–250 nm wavelength range was recorded using the UV-Vis spectrophotometer (Evolution 201). X-ray diffraction (XRD) patterns were obtained using a PAN analytical X-ray diffractometer at a scan rate of 0.1°/s employing Cu Kα (λ = 1.540598 Å). Fourier-transform infrared spectroscopy (FTIR) spectra of the material were captured using the FTIR instrument (Nicolet TM 6700) within the 400–4000 cm − 1 wave number range. This characterization technique is utilized for studying the functional groups, bonding, stretching, and vibrations present in the synthesized polymer. 2.4 Theoretical calculations Theoretical calculations were performed using the Gaussian 16 software package to investigate the structural and electronic properties of Pd-doped polyacrylamide before and after H 2 O adsorption. The molecular structures were constructed and visualized in GaussView 6.1, followed by full geometry optimizations without symmetry constraints. All computations employed density functional theory (DFT) with the B3LYP exchange–correlation functional and the LANL2DZ basis set to accurately describe the Pd center and light elements. Implicit water solvation effects were incorporated using the polarizable continuum model (PCM) to simulate realistic sensing conditions. Frontier molecular orbitals, HOMO–LUMO energy gaps, and reactivity descriptors including ionization potential, electron affinity, electronegativity, hardness, softness, and dipole moment were extracted from the optimized geometries to correlate electronic structure variations with humidity sensing performance. 3. Results and Discussions The XRD pattern of the Pd-doped polyacrylamide thin film is shown in Fig. 2 . It can be observed from the figure that there are portions containing noise, and few peaks are present. The observed peaks are located at 2θ positions of 39, 46, 66, and 80, corresponding to the (111), (200), (220), and (311) planes, respectively, which are well matched with JCPDS # 00-005-0681. These peaks indicate the crystalline nature of the Pd metal present in the complex, while the noise in the data confirms the amorphous nature of the polymer. For morphological analysis of the synthesized materials, FESEM is used. The FESEM micrographs of the frontal polymerized material are shown in Fig. 3 (a-d). Various shapes and sizes of nanomaterials are revealed in these figures. The metal nanoparticles are uniformly distributed throughout the polymeric material. These images indicate that the synthesized material is highly porous and may be suitable for moisture sensing applications. The EDX spectrum of the synthesized polymer is shown in Fig. 4 (a). The presence of all elements along with their concentrations is shown in this figure. The measurement of the particle size distribution of the nanomaterials relies on the Dynamic Light Scattering (DLS) technique. In a dilute solution, the size distribution of the materials is estimated by determining the zig-zag motion of the particles. This technique considers the virtual sphere with a diameter of particles. A dilute solution of the synthesized polymer was prepared, and particle size distribution analysis was performed. The observed data is plotted in Fig. 4 (b), showing that the polymer has a particle size ranging from 254 to 825 nm. The average size of the material observed here is 459 nm. Photons with energy ranging from 1.6 to 6.5 eV were used to study the optical properties of the synthesized polymer. Within the wavelength range of 190–250 nm, the polymer-based thin film exhibits maximum absorption, resulting in the determination of the optical bandgap of the polymeric material. The plot of absorption versus wavelength and absorption versus photon energy (inset of the figure) is shown in Fig. 4 (c). Electron excitations occur from the lower energy range (valence band) to the higher energy range (conduction band), aiding in the estimation of the optical band gap. Using extrapolation, the band gap of the material was estimated to be 5.3 eV, as shown in the Tauc plot inset of the figure. The wide bandgap is attributed to the small size of the polymeric material and the basic quantum confinement in nano-dimensions. The observed optical bandgap holds promise for humidity sensing applications. The FTIR spectrum of the synthesized polymer was recorded from 400 to 4000 cm⁻¹ wavenumbers, and the plot is shown in Fig. 4 (d). In this figure, absorption bands are observed at wavenumbers 617, 780, 1109, 1380, 1573, 2200, 2920, and 3320 cm⁻¹, respectively. The valleys present at 617, 1109, 1380, 2200, and 3420 cm⁻¹ correspond to –CH, –CO, –C = C, C ≡ C, and –OH stretching vibrations, respectively, in the polymer. The presence of hydroxyl, carbonyl, and unsaturated bonds in the material, as well as hydrophilic groups, suggests potential for humidity sensing applications. 4. Moisture detection application Based on the material characterization, the synthesized material was employed for moisture detection. Humidity detection was carried out in a lab-made humidity sensing setup [ 43 ]. All data related to humidity sensing were recorded, and the consulted data has been plotted in Fig. 5 (a-d). The humidity sensing mechanism based on metallopolymers has been reported in our recently published research article [ 44 ]. In Fig. 5 (a), the impedance of the sensing element was recorded with increasing and decreasing humidity in a humidity-controlled chamber. As humidity increased, the impedance of the sensing element decreased. The sensitivity of the humidity sensor was calculated using the formula S = ΔR/Δ%RH [ 45 ]. All other sensor parameters have already been defined in our previous articles [ 46 ]. The sensitivity of the humidity sensor was found to be 2.01 MΩ/%RH. In order to study other parameters of the sensors such as response-recovery times, repeatability, and aging effect, the corresponding data have been plotted in Fig. 5 (b), (c), and (d) respectively. The sensor is considered good if it has lower values of response and recovery time. In our humidity sensor, the response and recovery times were found to be 5 and 40 s respectively, as represented in Fig. 5 (b). It exhibits lower values of response and recovery time compared to previously developed sensors. The sensor demonstrates a 96.52% repeatability nature, as shown in Fig. 5 (c). The long-term stability of the sensor was also studied after 7, 21, and 35 days, and the corresponding plot is illustrated in Fig. 5 (d). In Table 1 , a review compares our research work with other studies, focusing on materials used for humidity measurement. Table 1 Comparison of humidity sensors based on metallopolymers. S. No. Material Humidity Range Sensitivity Response/Recovery time (s) Reference 1. RhAAm/SnO 2 polymer 10–95%RH 1.74 MΩ/%RH 66 s/164 s [ 46 ] 2. RhAAm/SnO 2 monomer 10–95%RH 1.86 MΩ/%RH 12 s/160 s [ 46 ] 3. Polyimide-coated fiber Bragg grating (FBG) 90 − 65%RH 0.0035 nm/%RH -/- [ 47 ] 4. Metal-polyimide 20–90%RH 0.90 pF/%RH -/- [ 48 ] 5. MWCNTs/polyelectrolyte 35–60%RH 4.20 pF/%RH 60 /90 [ 49 ] 6. PANI/TiO 2 25–95%RH 84.21% 60 /100 [ 50 ] 7. PDDA/rGO 0–97%RH 1.36 147 /133 [ 51 ] 8. Scandium nitrate PAAm 10–99%RH 1.85 MΩ/ %RH 24/101 [ 52 ] 9. PdAAm 10–95 2.01 MΩ/%RH 5/40 s Current Work Figure 6 illustrates the optimized molecular geometries of Pd-doped polyacrylamide in its pristine form (Fig. 6 a) and after H 2 O adsorption (Fig. 6 b), calculated using DFT at the B3LYP/LANL2DZ level. In the pristine configuration (a), the Pd atom (depicted in teal) is coordinated to the polymer backbone through nitrogen and oxygen donor atoms, forming a stable metal–ligand complex. This arrangement provides well-defined active sites for molecular adsorption and facilitates charge transfer during sensing. Upon interaction with an H 2 O molecule (b), a hydrogen-bonding interaction is established between the oxygen atom of water and the Pd center, as indicated by the dotted line. This adsorption geometry suggests a chemisorption-dominated binding mechanism, where the lone-pair electrons on the water oxygen coordinate with the Pd d-orbitals. This interaction not only stabilizes the adsorbed water molecule but also induces electronic structure perturbations in the Pd–polyacrylamide framework, which manifest as a narrowing of the HOMO–LUMO band gap (as seen in the FMO analysis). Structurally, the Pd coordination sphere experiences a slight distortion upon H 2 O binding, which is indicative of electron density redistribution and bond polarization effects. These changes can enhance charge carrier density and mobility within the sensing matrix, thereby increasing its electrical response to H 2 O. Such a molecular-level interaction is crucial in explaining the experimentally observed sensitivity enhancement in Pd-based polymeric gas sensors under humid conditions. The presented FMO diagrams illustrate the electronic structure modulation of Pd-doped polyacrylamide before and after H 2 O adsorption. In the pristine state (Fig. 7 a), the HOMO is located at − 4.30 eV and the LUMO at − 0.76 eV, yielding an electronic band gap (E g ) of 3.54 eV. This relatively wide band gap corresponds to a lower intrinsic conductivity, characteristic of the undisturbed polymer–metal complex. Upon interaction with a water molecule (Fig. 7 b), a notable redistribution of the HOMO and LUMO densities occurs, predominantly localized around the Pd coordination site and the adsorbed H 2 O. The HOMO energy shifts to − 4.61 eV, while the LUMO energy decreases to − 1.32 eV, resulting in a narrowed band gap of 3.31 eV. The reduction in band gap can be ascribed to orbital hybridization between the Pd d-orbitals and the lone-pair electrons on the oxygen atom of the water molecule, which introduces new electronic states closer to the Fermi level. This hybridization promotes partial charge transfer from the water molecule to the sensing matrix, enhancing local carrier density and thus facilitating electronic transitions under lower excitation energies. From a sensing perspective, this interaction is consistent with a chemisorption-driven mechanism, wherein the H 2 O molecule binds strongly to the Pd active site via coordination bonds, leading to substantial electronic perturbation. Such chemisorption not only modifies the density of states but also affects the charge transport pathways within the polymer matrix, causing a measurable change in conductivity. In real sensing conditions, the narrowed band gap implies faster response and recovery kinetics, as the reduced excitation energy enables rapid carrier generation upon H 2 O exposure. The observed orbital redistribution also suggests that the sensing signal is dominated by modulation of the conduction band edge, which is particularly advantageous for high-sensitivity detection in humid environments. Table 2 Variation in the theoretical parameters of Pd-doped polyacrylamide before and after interaction with H 2 O Theoretical Parameters Before interaction After Interaction with H 2 O Ionization Potential (eV) 4.31 4.61 Electron Affinity (eV) 0.76 1.32 HOMO-LUMO Gap (eV) 3.54 3.31 Electronegativity (eV) 2.54 2.97 Hardness (eV) 0.56 0.61 Softness (eV) 1.77 1.65 Dipole moment (Debye) 4.51 8.33 The Table 2 summarizes the variation in key theoretical parameters of Pd-doped polyacrylamide before and after interaction with H 2 O, as obtained from density functional theory (DFT) calculations. A noticeable increase in the ionization potential (from 4.31 eV to 4.61 eV) is observed upon H 2 O adsorption, indicating that the removal of an electron from the system requires slightly more energy after interaction. This suggests enhanced electronic stability due to the coordination between the water molecule and the Pd center. Similarly, the electron affinity rises from 0.76 eV to 1.32 eV, implying that the system becomes more favorable for electron acceptance, which can facilitate charge transfer processes during sensing. The HOMO–LUMO energy gap decreases from 3.54 eV to 3.31 eV after adsorption, reflecting band gap narrowing caused by orbital hybridization between the lone-pair electrons of the water molecule and the Pd d-orbitals. This reduction in band gap enhances electrical conductivity, supporting the proposed sensing mechanism. The electronegativity also increases from 2.54 eV to 2.97 eV, signifying a higher tendency to attract electrons after H₂O binding, which correlates with increased charge carrier density. In terms of chemical reactivity descriptors, the hardness increases slightly from 0.56 eV to 0.61 eV, indicating a marginally more resistant system to deformation in electron density, while the softness decreases from 1.77 eV to 1.65 eV, reflecting a minor reduction in polarizability. The dipole moment shows a substantial increase from 4.51 Debye to 8.33 Debye after adsorption, which can be attributed to significant charge redistribution within the molecule upon water interaction. This marked change in dipole moment is a strong indicator of increased polarity, which is highly beneficial for enhancing the sensor’s electrical response under humid conditions. 5. Conclusion The Pd-doped polyacrylamide nanocomposites-based polymer was successfully synthesized via the frontal polymerization technique. The polymer was identified as a highly porous material in the FESEM analysis. The synthesized polymer exhibits a particle size ranging from 254 to 825 nm, with an average particle size of 459 nm. The observed peaks in the XRD pattern are attributed to the presence of Pd metal in the polymers. The optical energy bandgap of the Pd-doped polyacrylamide has been estimated to be 5.3 eV. The polymer-based humidity sensor demonstrates sensitivity, repeatability, response, and recovery times of 2.01 MΩ/%RH, 96.52%, 5, and 40 s, respectively. Additionally, the sensor exhibited long-term stability and DFT analysis validated the experimental data. Declarations Acknowledgement: Principal author acknowledges to Dr. Kuldeep Kumar for his support. Also, authors acknowledge to USIC, BBAU, Lucknow for SEM, XRD and FTIR facilities. Declaration of Competing Interest Authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Funding Declaration Authors declare that there was no funding for this research work. Ethics, Consent to Participate, and Consent to Publish declarations : The datasets generated during and/or analysed during the current study are available from the corresponding author on reasonable request. Also, the results/data/figures in this manuscript have not been published elsewhere, nor are they under consideration by another publisher. Author Contributions Lava Kumar Gupta : Writing – original draft, Investigation, Manuscript preparation, Methodology, experimentation, Bal Chandra Yadav: Supervision, Review, Resource management, Validation. : Writing - Review and editing, Shripal Sharma : Review and editing, Utkarsh Kumar: Theoretical investigation and DFT analysis, Rajeev Kumar : Review and Resource management. References G.V. Shultz, L. N. Zakarahov and D. R. Tyler, Transition-Metal-Containing Polymers by ADMET: Polymerization of cis-Mo (CO) 4 (Ph 2 P(CH 2 ) 3 CHCH 2 ) 2 . Macromolecules 41 5555 (2008). I. Dragutan, V. Dragutan and H. Fischer, Synthesis of metal-containing polymers via ring opening metathesis polymerization (ROMP). Part II: Polymers containing transition metals. J. Inorg. Organomet. Polym.,18, 311 (2008). V. Dragutan, I. Dragutan and H. Fischer, Synthesis of metal-containing polymers via ring opening metathesis polymerization (ROMP). Part I. Polymers containing main group metals, J. Inorg. 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5\",\"display\":\"\",\"copyAsset\":false,\"role\":\"figure\",\"size\":274521,\"visible\":true,\"origin\":\"\",\"legend\":\"\\u003cp\\u003e\\u003cstrong\\u003e(a):\\u003c/strong\\u003e \\u0026nbsp;Humidity sensing characteristics curves of Pd-doped polyacrylamide\\u003cstrong\\u003e (b)\\u003c/strong\\u003e Response and recovery times of Pd-doped polyacrylamide\\u003cstrong\\u003e (c)\\u003c/strong\\u003eReproducibility curves of Pd-doped polyacrylamide\\u003cstrong\\u003e (d)\\u003c/strong\\u003e Ageing effect of Pd-doped polyacrylamide\\u003c/p\\u003e\",\"description\":\"\",\"filename\":\"floatimage5.png\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-7606983/v1/aa6574b022899f7a242b0cdf.png\"},{\"id\":94122766,\"identity\":\"38aac251-6da5-49a0-a70b-84348b1c7c62\",\"added_by\":\"auto\",\"created_at\":\"2025-10-22 15:28:19\",\"extension\":\"png\",\"order_by\":6,\"title\":\"Figure 6\",\"display\":\"\",\"copyAsset\":false,\"role\":\"figure\",\"size\":204687,\"visible\":true,\"origin\":\"\",\"legend\":\"\\u003cp\\u003eMolecular structure of (a) Pd- doped poly acrylamide (b) Pd- doped poly acrylamide with H\\u003csub\\u003e2\\u003c/sub\\u003eO\\u003c/p\\u003e\",\"description\":\"\",\"filename\":\"floatimage6.png\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-7606983/v1/7f6357c59288e2c9ee8c44d1.png\"},{\"id\":94122997,\"identity\":\"c75b9352-23cb-4884-8665-153da10cf215\",\"added_by\":\"auto\",\"created_at\":\"2025-10-22 15:36:19\",\"extension\":\"png\",\"order_by\":7,\"title\":\"Figure 7\",\"display\":\"\",\"copyAsset\":false,\"role\":\"figure\",\"size\":88870,\"visible\":true,\"origin\":\"\",\"legend\":\"\\u003cp\\u003eVariation in HOMO-LUMO levels of Pd-doped polyacrylamide \\u003cstrong\\u003e(a)\\u003c/strong\\u003e before \\u003cstrong\\u003e(b) \\u003c/strong\\u003eafter interaction with H\\u003csub\\u003e2\\u003c/sub\\u003eO molecules\\u003c/p\\u003e\",\"description\":\"\",\"filename\":\"floatimage7.png\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-7606983/v1/8cf5461292c5e65ddeeb9e6b.png\"},{\"id\":94123967,\"identity\":\"a3fa4cc3-4e92-45a2-803c-27d41a5b7c98\",\"added_by\":\"auto\",\"created_at\":\"2025-10-22 15:44:19\",\"extension\":\"pdf\",\"order_by\":0,\"title\":\"\",\"display\":\"\",\"copyAsset\":false,\"role\":\"manuscript-pdf\",\"size\":2149118,\"visible\":true,\"origin\":\"\",\"legend\":\"\",\"description\":\"\",\"filename\":\"manuscript.pdf\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-7606983/v1/b4981b31-9dd7-4fb0-a854-93377a52dfba.pdf\"}],\"financialInterests\":\"No competing interests reported.\",\"formattedTitle\":\"Frontal Polymerization Synthesis and Humidity Sensing Properties of Pd-Doped Polyacrylamide with Theoretical Validation using DFT\",\"fulltext\":[{\"header\":\"1. Introduction\",\"content\":\"\\u003cp\\u003eMetal centres in metallopolymers exhibit diversity ranging from group metals to lanthanides and transition metals. In polymers, metals are positioned either in the side group structure or in the main chain. Furthermore, metallopolymers can be highly branched, linear, or dendritic. Over the past decades, groups of researchers worldwide have tackled many difficulties in the synthesis of soluble and higher molecular weight metallopolymers [\\u003cspan additionalcitationids=\\\"CR2\\\" citationid=\\\"CR1\\\" class=\\\"CitationRef\\\"\\u003e1\\u003c/span\\u003e\\u0026ndash;\\u003cspan citationid=\\\"CR3\\\" class=\\\"CitationRef\\\"\\u003e3\\u003c/span\\u003e]. Numerous protocols for the preparation of metallopolymers have been established, involving the development of new techniques compatible with the presence of metal centers [\\u003cspan additionalcitationids=\\\"CR5 CR6 CR7\\\" citationid=\\\"CR4\\\" class=\\\"CitationRef\\\"\\u003e4\\u003c/span\\u003e\\u0026ndash;\\u003cspan citationid=\\\"CR8\\\" class=\\\"CitationRef\\\"\\u003e8\\u003c/span\\u003e]. Researchers have devised processes to synthesize selected metallopolymers using methods such as polycondensation, chain-growth condensation polymerization, electro-polymerization, ring-opening polymerization, and controlled radical polymerization. In addition to these techniques, a new and advanced technique known as frontal polymerization has garnered attention due to its simplicity, ease, and cost-effectiveness. This technique allows for the synthesis of various metallopolymers with diverse structures, enabling the study of their properties and a wide range of advanced sustainable applications [\\u003cspan additionalcitationids=\\\"CR10 CR11 CR12\\\" citationid=\\\"CR9\\\" class=\\\"CitationRef\\\"\\u003e9\\u003c/span\\u003e\\u0026ndash;\\u003cspan citationid=\\\"CR13\\\" class=\\\"CitationRef\\\"\\u003e13\\u003c/span\\u003e].\\u003c/p\\u003e\\u003cp\\u003eHumidity, one of the essential elements of the environment, is crucial for various applications, including environmental monitoring in greenhouses, medical facilities, clean rooms, industrial process control, and heating ventilation air conditioning (HVAC) systems [\\u003cspan additionalcitationids=\\\"CR15 CR16 CR17 CR18 CR19 CR20\\\" citationid=\\\"CR14\\\" class=\\\"CitationRef\\\"\\u003e14\\u003c/span\\u003e\\u0026ndash;\\u003cspan citationid=\\\"CR21\\\" class=\\\"CitationRef\\\"\\u003e21\\u003c/span\\u003e]. Humidity sensors can be classified into several types based on sensing mechanisms and readout signals, including impedance-based, capacitance-based, resistance-based, surface acoustic wave (SAW), optical, and field-effect transistor sensors. Moreover, a variety of materials such as nanostructured metals, nanocomposites, carbon nanotubes, graphene, and polymers are utilized as sensing materials [\\u003cspan additionalcitationids=\\\"CR23 CR24 CR25 CR26 CR27 CR28 CR29 CR30 CR31 CR32\\\" citationid=\\\"CR22\\\" class=\\\"CitationRef\\\"\\u003e22\\u003c/span\\u003e\\u0026ndash;\\u003cspan citationid=\\\"CR33\\\" class=\\\"CitationRef\\\"\\u003e33\\u003c/span\\u003e].\\u003c/p\\u003e\\u003cp\\u003eMany research papers focusing on the synthesis of polymer/inorganic materials using electrochemical and polymerization techniques have been published for humidity and gas sensing applications [\\u003cspan citationid=\\\"CR34\\\" class=\\\"CitationRef\\\"\\u003e34\\u003c/span\\u003e]. In a review article, the improvement in humidity sensing characteristics based on thin films of polymers has been demonstrated by Y. Sakai et al. [\\u003cspan citationid=\\\"CR35\\\" class=\\\"CitationRef\\\"\\u003e35\\u003c/span\\u003e]. Researchers have also published several reports on functional humidity/gas sensing nanomaterials [\\u003cspan citationid=\\\"CR36\\\" class=\\\"CitationRef\\\"\\u003e36\\u003c/span\\u003e, \\u003cspan citationid=\\\"CR37\\\" class=\\\"CitationRef\\\"\\u003e37\\u003c/span\\u003e]. A humidity sensor for respiration monitoring based on Graphene/PVA/SiO\\u003csub\\u003e2\\u003c/sub\\u003e-based surface acoustic wave (SAW) was developed by Y. Su et al. [\\u003cspan citationid=\\\"CR38\\\" class=\\\"CitationRef\\\"\\u003e38\\u003c/span\\u003e].\\u003c/p\\u003e\\u003cp\\u003ePreviously, metallopolymers-based fluorescent/phosphorescent dual-emissive polymer dots were reported [\\u003cspan additionalcitationids=\\\"CR40\\\" citationid=\\\"CR39\\\" class=\\\"CitationRef\\\"\\u003e39\\u003c/span\\u003e\\u0026ndash;\\u003cspan citationid=\\\"CR41\\\" class=\\\"CitationRef\\\"\\u003e41\\u003c/span\\u003e]. The frontal polymerization method has been utilized by S. Singh et al. for the preparation of nanostructured ZnS and PbS with acrylamide complex in sensing applications [\\u003cspan citationid=\\\"CR42\\\" class=\\\"CitationRef\\\"\\u003e42\\u003c/span\\u003e]. Recently, using the frontal polymerization technique, rhodium acrylamide SnO\\u003csub\\u003e2\\u003c/sub\\u003e nanocomposite polymer was prepared for humidity sensing [\\u003cspan citationid=\\\"CR43\\\" class=\\\"CitationRef\\\"\\u003e43\\u003c/span\\u003e]. Among various polymers and metals, metallopolymers have attracted researchers and scientists due to their altered electrical, magnetic, and optical properties compared to pure polymers and metals [\\u003cspan citationid=\\\"CR44\\\" class=\\\"CitationRef\\\"\\u003e44\\u003c/span\\u003e]. The nanostructured complex of acrylamide with metal oxides may prove to be fruitful for sensing purposes. In the present work, an attempt has been made to synthesize a nanostructured composite of Palladium-doped polyacrylamide using the frontal polymerization route, and the sensing element was fabricated using the spin coating technique.\\u003c/p\\u003e\"},{\"header\":\"2. Experimental procedure\",\"content\":\"\\u003cdiv id=\\\"Sec3\\\" class=\\\"Section2\\\"\\u003e\\u003ch2\\u003e2.1 Synthesis Method\\u003c/h2\\u003e\\u003cp\\u003eIn the synthesis process, the revised method for the preparation of acrylamide complexes of transition metal nitrates was used accordingly [\\u003cspan citationid=\\\"CR45\\\" class=\\\"CitationRef\\\"\\u003e45\\u003c/span\\u003e]. Acrylamide and palladium nitrate were assorted in a molar ratio of 5:1 in the inert atmospheric condition, we got a paste, which was washed with dry diethyl ether, and dried in a vacuum (0.2 Torr) up to constant weight.\\u003c/p\\u003e\\u003cp\\u003eFor frontal polymerization purposes, the PdAAm were hard-pressed into pellets (having a diameter of 0.5\\u0026ndash;0.8 cm, along with a height of 1.2\\u0026ndash;1.5 cm, and also a density of 1.45\\u0026thinsp;\\u0026plusmn;\\u0026thinsp;0.02 g/cm\\u003csup\\u003e3\\u003c/sup\\u003e) and were put into a glass ampoule. After placing the ampoule bottom (2\\u0026ndash;3 mm) into a porcelain crucible having a heat transfer medium at 130\\u0026ndash;140 \\u0026ordm;C for 8\\u0026ndash;20 s, the polymerization process was initiated. From the motion of the coloured zone front, the reaction rate was predicted. The hybrid composite or powdery polymer products were reserved from the ampoule, ground, and washed with ethanol and diethyl ether, after that it was dried in a vacuum at room temperature. The finally prepared sample was recognized as PdAAm i.e. polymer.\\u003c/p\\u003e\\u003c/div\\u003e\\u003cdiv id=\\\"Sec4\\\" class=\\\"Section2\\\"\\u003e\\u003ch2\\u003e2.2 Fabrication of the thin films\\u003c/h2\\u003e\\u003cp\\u003eThe quality of the coated films can be maintained by adjusting the preparation parameters, such as solution concentration, spinning velocity, and heating temperature. In this study, the spin coating technique [\\u003cspan citationid=\\\"CR20\\\" class=\\\"CitationRef\\\"\\u003e20\\u003c/span\\u003e] was employed to deposit uniform thin films of polymeric material onto flat borosilicate glass substrates measuring 1.0\\u0026times;1.0 cm\\u0026sup2;. Prior to deposition, the substrates underwent a series of cleaning steps in an ultrasonic bath (WUC-AO2H), including immersion in lab detergent (Extran), de-ionized water, isopropyl alcohol, and acetone for 10 minutes each. Subsequently, heat treatment at 100\\u0026deg;C for 15 minutes was applied to evaporate organic impurities. The prepared substrates were then used for film deposition. The thin films were deposited at a spinning speed of 1000 RPM and dried for 40 minutes at 55\\u0026deg;C. These films were later utilized as sensing elements for humidity detection. The schematic diagram for film fabrication is depicted in Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig1\\\" class=\\\"InternalRef\\\"\\u003e1\\u003c/span\\u003e.\\u003c/p\\u003e\\u003cp\\u003e\\u003c/p\\u003e\\u003c/div\\u003e\\u003cdiv id=\\\"Sec5\\\" class=\\\"Section2\\\"\\u003e\\u003ch2\\u003e2.3 Characterizations\\u003c/h2\\u003e\\u003cp\\u003eThe film's surface morphology and elemental analysis were conducted usingField emission scanning electron microscopy (FESEM) (JEOL, JSEM-6490LV) coupled with an energy dispersive X-ray (EDX) analysis detector. Particle size in aqueous and dilute solutions was assessed using the Zeta nanosizer (Malvern, Nano-ZS90). The UV-Visible absorption spectrum of the thin film within the 150\\u0026ndash;250 nm wavelength range was recorded using the UV-Vis spectrophotometer (Evolution 201). X-ray diffraction (XRD) patterns were obtained using a PAN analytical X-ray diffractometer at a scan rate of 0.1\\u0026deg;/s employing Cu Kα (λ\\u0026thinsp;=\\u0026thinsp;1.540598 \\u0026Aring;). Fourier-transform infrared spectroscopy (FTIR) spectra of the material were captured using the FTIR instrument (Nicolet TM 6700) within the 400\\u0026ndash;4000 cm\\u003csup\\u003e\\u0026minus;\\u0026thinsp;1\\u003c/sup\\u003e wave number range. This characterization technique is utilized for studying the functional groups, bonding, stretching, and vibrations present in the synthesized polymer.\\u003c/p\\u003e\\u003c/div\\u003e\\u003cdiv id=\\\"Sec6\\\" class=\\\"Section2\\\"\\u003e\\u003ch2\\u003e2.4 Theoretical calculations\\u003c/h2\\u003e\\u003cp\\u003eTheoretical calculations were performed using the Gaussian 16 software package to investigate the structural and electronic properties of Pd-doped polyacrylamide before and after H\\u003csub\\u003e2\\u003c/sub\\u003eO adsorption. The molecular structures were constructed and visualized in GaussView 6.1, followed by full geometry optimizations without symmetry constraints. All computations employed density functional theory (DFT) with the B3LYP exchange\\u0026ndash;correlation functional and the LANL2DZ basis set to accurately describe the Pd center and light elements. Implicit water solvation effects were incorporated using the polarizable continuum model (PCM) to simulate realistic sensing conditions. Frontier molecular orbitals, HOMO\\u0026ndash;LUMO energy gaps, and reactivity descriptors including ionization potential, electron affinity, electronegativity, hardness, softness, and dipole moment were extracted from the optimized geometries to correlate electronic structure variations with humidity sensing performance.\\u003c/p\\u003e\\u003c/div\\u003e\"},{\"header\":\"3. Results and Discussions\",\"content\":\"\\u003cp\\u003e\\u003c/p\\u003e\\u003cp\\u003eThe XRD pattern of the Pd-doped polyacrylamide thin film is shown in Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig2\\\" class=\\\"InternalRef\\\"\\u003e2\\u003c/span\\u003e. It can be observed from the figure that there are portions containing noise, and few peaks are present. The observed peaks are located at 2θ positions of 39, 46, 66, and 80, corresponding to the (111), (200), (220), and (311) planes, respectively, which are well matched with JCPDS # 00-005-0681. These peaks indicate the crystalline nature of the Pd metal present in the complex, while the noise in the data confirms the amorphous nature of the polymer.\\u003c/p\\u003e\\u003cp\\u003e\\u003c/p\\u003e\\u003cp\\u003eFor morphological analysis of the synthesized materials, FESEM is used. The FESEM micrographs of the frontal polymerized material are shown in Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig3\\\" class=\\\"InternalRef\\\"\\u003e3\\u003c/span\\u003e(a-d). Various shapes and sizes of nanomaterials are revealed in these figures. The metal nanoparticles are uniformly distributed throughout the polymeric material. These images indicate that the synthesized material is highly porous and may be suitable for moisture sensing applications.\\u003c/p\\u003e\\u003cp\\u003e\\u003c/p\\u003e\\u003cp\\u003eThe EDX spectrum of the synthesized polymer is shown in Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig4\\\" class=\\\"InternalRef\\\"\\u003e4\\u003c/span\\u003e (a). The presence of all elements along with their concentrations is shown in this figure. The measurement of the particle size distribution of the nanomaterials relies on the Dynamic Light Scattering (DLS) technique. In a dilute solution, the size distribution of the materials is estimated by determining the zig-zag motion of the particles. This technique considers the virtual sphere with a diameter of particles. A dilute solution of the synthesized polymer was prepared, and particle size distribution analysis was performed. The observed data is plotted in Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig4\\\" class=\\\"InternalRef\\\"\\u003e4\\u003c/span\\u003e (b), showing that the polymer has a particle size ranging from 254 to 825 nm. The average size of the material observed here is 459 nm.\\u003c/p\\u003e\\u003cp\\u003ePhotons with energy ranging from 1.6 to 6.5 eV were used to study the optical properties of the synthesized polymer. Within the wavelength range of 190\\u0026ndash;250 nm, the polymer-based thin film exhibits maximum absorption, resulting in the determination of the optical bandgap of the polymeric material. The plot of absorption versus wavelength and absorption versus photon energy (inset of the figure) is shown in Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig4\\\" class=\\\"InternalRef\\\"\\u003e4\\u003c/span\\u003e (c). Electron excitations occur from the lower energy range (valence band) to the higher energy range (conduction band), aiding in the estimation of the optical band gap.\\u003c/p\\u003e\\u003cp\\u003eUsing extrapolation, the band gap of the material was estimated to be 5.3 eV, as shown in the Tauc plot inset of the figure. The wide bandgap is attributed to the small size of the polymeric material and the basic quantum confinement in nano-dimensions. The observed optical bandgap holds promise for humidity sensing applications.\\u003c/p\\u003e\\u003cp\\u003eThe FTIR spectrum of the synthesized polymer was recorded from 400 to 4000 cm⁻\\u0026sup1; wavenumbers, and the plot is shown in Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig4\\\" class=\\\"InternalRef\\\"\\u003e4\\u003c/span\\u003e (d). In this figure, absorption bands are observed at wavenumbers 617, 780, 1109, 1380, 1573, 2200, 2920, and 3320 cm⁻\\u0026sup1;, respectively. The valleys present at 617, 1109, 1380, 2200, and 3420 cm⁻\\u0026sup1; correspond to \\u0026ndash;CH, \\u0026ndash;CO, \\u0026ndash;C\\u0026thinsp;=\\u0026thinsp;C, C\\u0026thinsp;\\u0026equiv;\\u0026thinsp;C, and \\u0026ndash;OH stretching vibrations, respectively, in the polymer. The presence of hydroxyl, carbonyl, and unsaturated bonds in the material, as well as hydrophilic groups, suggests potential for humidity sensing applications.\\u003c/p\\u003e\"},{\"header\":\"4. Moisture detection application\",\"content\":\"\\u003cp\\u003eBased on the material characterization, the synthesized material was employed for moisture detection. Humidity detection was carried out in a lab-made humidity sensing setup [\\u003cspan citationid=\\\"CR43\\\" class=\\\"CitationRef\\\"\\u003e43\\u003c/span\\u003e]. All data related to humidity sensing were recorded, and the consulted data has been plotted in Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig5\\\" class=\\\"InternalRef\\\"\\u003e5\\u003c/span\\u003e(a-d). The humidity sensing mechanism based on metallopolymers has been reported in our recently published research article [\\u003cspan citationid=\\\"CR44\\\" class=\\\"CitationRef\\\"\\u003e44\\u003c/span\\u003e].\\u003c/p\\u003e\\u003cp\\u003eIn Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig5\\\" class=\\\"InternalRef\\\"\\u003e5\\u003c/span\\u003e(a), the impedance of the sensing element was recorded with increasing and decreasing humidity in a humidity-controlled chamber. As humidity increased, the impedance of the sensing element decreased. The sensitivity of the humidity sensor was calculated using the formula S\\u0026thinsp;=\\u0026thinsp;ΔR/Δ%RH [\\u003cspan citationid=\\\"CR45\\\" class=\\\"CitationRef\\\"\\u003e45\\u003c/span\\u003e]. All other sensor parameters have already been defined in our previous articles [\\u003cspan citationid=\\\"CR46\\\" class=\\\"CitationRef\\\"\\u003e46\\u003c/span\\u003e]. The sensitivity of the humidity sensor was found to be 2.01 MΩ/%RH.\\u003c/p\\u003e\\u003cp\\u003eIn order to study other parameters of the sensors such as response-recovery times, repeatability, and aging effect, the corresponding data have been plotted in Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig5\\\" class=\\\"InternalRef\\\"\\u003e5\\u003c/span\\u003e (b), (c), and (d) respectively. The sensor is considered good if it has lower values of response and recovery time. In our humidity sensor, the response and recovery times were found to be 5 and 40 s respectively, as represented in Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig5\\\" class=\\\"InternalRef\\\"\\u003e5\\u003c/span\\u003e (b). It exhibits lower values of response and recovery time compared to previously developed sensors. The sensor demonstrates a 96.52% repeatability nature, as shown in Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig5\\\" class=\\\"InternalRef\\\"\\u003e5\\u003c/span\\u003e (c). The long-term stability of the sensor was also studied after 7, 21, and 35 days, and the corresponding plot is illustrated in Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig5\\\" class=\\\"InternalRef\\\"\\u003e5\\u003c/span\\u003e (d).\\u003c/p\\u003e\\u003cp\\u003e\\u003c/p\\u003e\\u003cp\\u003eIn Table\\u0026nbsp;\\u003cspan refid=\\\"Tab1\\\" class=\\\"InternalRef\\\"\\u003e1\\u003c/span\\u003e, a review compares our research work with other studies, focusing on materials used for humidity measurement.\\u003c/p\\u003e\\u003cp\\u003e\\u003cdiv class=\\\"gridtable\\\"\\u003e\\u003ctable float=\\\"Yes\\\" id=\\\"Tab1\\\" border=\\\"1\\\"\\u003e\\u003ccaption language=\\\"En\\\"\\u003e\\u003cdiv class=\\\"CaptionNumber\\\"\\u003eTable 1\\u003c/div\\u003e\\u003cdiv class=\\\"CaptionContent\\\"\\u003e\\u003cp\\u003eComparison of humidity sensors based on metallopolymers.\\u003c/p\\u003e\\u003c/div\\u003e\\u003c/caption\\u003e\\u003ccolgroup cols=\\\"6\\\"\\u003e\\u003cdiv align=\\\"left\\\" class=\\\"colspec\\\" colname=\\\"c1\\\" colnum=\\\"1\\\"\\u003e\\u003c/div\\u003e\\u003cdiv align=\\\"left\\\" class=\\\"colspec\\\" colname=\\\"c2\\\" colnum=\\\"2\\\"\\u003e\\u003c/div\\u003e\\u003cdiv align=\\\"left\\\" class=\\\"colspec\\\" colname=\\\"c3\\\" colnum=\\\"3\\\"\\u003e\\u003c/div\\u003e\\u003cdiv align=\\\"left\\\" class=\\\"colspec\\\" colname=\\\"c4\\\" colnum=\\\"4\\\"\\u003e\\u003c/div\\u003e\\u003cdiv align=\\\"left\\\" class=\\\"colspec\\\" colname=\\\"c5\\\" colnum=\\\"5\\\"\\u003e\\u003c/div\\u003e\\u003cdiv align=\\\"left\\\" class=\\\"colspec\\\" colname=\\\"c6\\\" colnum=\\\"6\\\"\\u003e\\u003c/div\\u003e\\u003cthead\\u003e\\u003ctr\\u003e\\u003cth align=\\\"left\\\" colname=\\\"c1\\\"\\u003e\\u003cp\\u003eS. No.\\u003c/p\\u003e\\u003c/th\\u003e\\u003cth align=\\\"left\\\" colname=\\\"c2\\\"\\u003e\\u003cp\\u003eMaterial\\u003c/p\\u003e\\u003c/th\\u003e\\u003cth align=\\\"left\\\" colname=\\\"c3\\\"\\u003e\\u003cp\\u003eHumidity Range\\u003c/p\\u003e\\u003c/th\\u003e\\u003cth align=\\\"left\\\" colname=\\\"c4\\\"\\u003e\\u003cp\\u003eSensitivity\\u003c/p\\u003e\\u003c/th\\u003e\\u003cth align=\\\"left\\\" colname=\\\"c5\\\"\\u003e\\u003cp\\u003eResponse/Recovery time (s)\\u003c/p\\u003e\\u003c/th\\u003e\\u003cth align=\\\"left\\\" colname=\\\"c6\\\"\\u003e\\u003cp\\u003eReference\\u003c/p\\u003e\\u003c/th\\u003e\\u003c/tr\\u003e\\u003c/thead\\u003e\\u003ctbody\\u003e\\u003ctr\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c1\\\"\\u003e\\u003cp\\u003e1.\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c2\\\"\\u003e\\u003cp\\u003eRhAAm/SnO\\u003csub\\u003e2\\u003c/sub\\u003e polymer\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c3\\\"\\u003e\\u003cp\\u003e10\\u0026ndash;95%RH\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c4\\\"\\u003e\\u003cp\\u003e1.74 MΩ/%RH\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c5\\\"\\u003e\\u003cp\\u003e66 s/164 s\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c6\\\"\\u003e\\u003cp\\u003e[\\u003cspan citationid=\\\"CR46\\\" class=\\\"CitationRef\\\"\\u003e46\\u003c/span\\u003e]\\u003c/p\\u003e\\u003c/td\\u003e\\u003c/tr\\u003e\\u003ctr\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c1\\\"\\u003e\\u003cp\\u003e2.\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c2\\\"\\u003e\\u003cp\\u003eRhAAm/SnO\\u003csub\\u003e2\\u003c/sub\\u003e monomer\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c3\\\"\\u003e\\u003cp\\u003e10\\u0026ndash;95%RH\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c4\\\"\\u003e\\u003cp\\u003e1.86 MΩ/%RH\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c5\\\"\\u003e\\u003cp\\u003e12 s/160 s\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c6\\\"\\u003e\\u003cp\\u003e[\\u003cspan citationid=\\\"CR46\\\" class=\\\"CitationRef\\\"\\u003e46\\u003c/span\\u003e]\\u003c/p\\u003e\\u003c/td\\u003e\\u003c/tr\\u003e\\u003ctr\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c1\\\"\\u003e\\u003cp\\u003e3.\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c2\\\"\\u003e\\u003cp\\u003ePolyimide-coated fiber Bragg grating (FBG)\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c3\\\"\\u003e\\u003cp\\u003e90\\u0026thinsp;\\u0026minus;\\u0026thinsp;65%RH\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c4\\\"\\u003e\\u003cp\\u003e0.0035 nm/%RH\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c5\\\"\\u003e\\u003cp\\u003e-/-\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c6\\\"\\u003e\\u003cp\\u003e[\\u003cspan citationid=\\\"CR47\\\" class=\\\"CitationRef\\\"\\u003e47\\u003c/span\\u003e]\\u003c/p\\u003e\\u003c/td\\u003e\\u003c/tr\\u003e\\u003ctr\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c1\\\"\\u003e\\u003cp\\u003e4.\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c2\\\"\\u003e\\u003cp\\u003eMetal-polyimide\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c3\\\"\\u003e\\u003cp\\u003e20\\u0026ndash;90%RH\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c4\\\"\\u003e\\u003cp\\u003e0.90\\u0026thinsp;pF/%RH\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c5\\\"\\u003e\\u003cp\\u003e-/-\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c6\\\"\\u003e\\u003cp\\u003e[\\u003cspan citationid=\\\"CR48\\\" class=\\\"CitationRef\\\"\\u003e48\\u003c/span\\u003e]\\u003c/p\\u003e\\u003c/td\\u003e\\u003c/tr\\u003e\\u003ctr\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c1\\\"\\u003e\\u003cp\\u003e5.\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c2\\\"\\u003e\\u003cp\\u003eMWCNTs/polyelectrolyte\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c3\\\"\\u003e\\u003cp\\u003e35\\u0026ndash;60%RH\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c4\\\"\\u003e\\u003cp\\u003e4.20 pF/%RH\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c5\\\"\\u003e\\u003cp\\u003e60 /90\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c6\\\"\\u003e\\u003cp\\u003e[\\u003cspan citationid=\\\"CR49\\\" class=\\\"CitationRef\\\"\\u003e49\\u003c/span\\u003e]\\u003c/p\\u003e\\u003c/td\\u003e\\u003c/tr\\u003e\\u003ctr\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c1\\\"\\u003e\\u003cp\\u003e6.\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c2\\\"\\u003e\\u003cp\\u003ePANI/TiO\\u003csub\\u003e2\\u003c/sub\\u003e\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c3\\\"\\u003e\\u003cp\\u003e25\\u0026ndash;95%RH\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c4\\\"\\u003e\\u003cp\\u003e84.21%\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c5\\\"\\u003e\\u003cp\\u003e60 /100\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c6\\\"\\u003e\\u003cp\\u003e[\\u003cspan citationid=\\\"CR50\\\" class=\\\"CitationRef\\\"\\u003e50\\u003c/span\\u003e]\\u003c/p\\u003e\\u003c/td\\u003e\\u003c/tr\\u003e\\u003ctr\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c1\\\"\\u003e\\u003cp\\u003e7.\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c2\\\"\\u003e\\u003cp\\u003ePDDA/rGO\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c3\\\"\\u003e\\u003cp\\u003e0\\u0026ndash;97%RH\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c4\\\"\\u003e\\u003cp\\u003e1.36\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c5\\\"\\u003e\\u003cp\\u003e147 /133\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c6\\\"\\u003e\\u003cp\\u003e[\\u003cspan citationid=\\\"CR51\\\" class=\\\"CitationRef\\\"\\u003e51\\u003c/span\\u003e]\\u003c/p\\u003e\\u003c/td\\u003e\\u003c/tr\\u003e\\u003ctr\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c1\\\"\\u003e\\u003cp\\u003e8.\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c2\\\"\\u003e\\u003cp\\u003eScandium nitrate PAAm\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c3\\\"\\u003e\\u003cp\\u003e10\\u0026ndash;99%RH\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c4\\\"\\u003e\\u003cp\\u003e1.85 MΩ/ %RH\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c5\\\"\\u003e\\u003cp\\u003e24/101\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c6\\\"\\u003e\\u003cp\\u003e[\\u003cspan citationid=\\\"CR52\\\" class=\\\"CitationRef\\\"\\u003e52\\u003c/span\\u003e]\\u003c/p\\u003e\\u003c/td\\u003e\\u003c/tr\\u003e\\u003ctr\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c1\\\"\\u003e\\u003cp\\u003e9.\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c2\\\"\\u003e\\u003cp\\u003ePdAAm\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c3\\\"\\u003e\\u003cp\\u003e10\\u0026ndash;95\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c4\\\"\\u003e\\u003cp\\u003e2.01 MΩ/%RH\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c5\\\"\\u003e\\u003cp\\u003e5/40 s\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c6\\\"\\u003e\\u003cp\\u003eCurrent Work\\u003c/p\\u003e\\u003c/td\\u003e\\u003c/tr\\u003e\\u003c/tbody\\u003e\\u003c/colgroup\\u003e\\u003c/table\\u003e\\u003c/div\\u003e\\u003c/p\\u003e\\u003cp\\u003e\\u003c/p\\u003e\\u003cp\\u003eFigure\\u0026nbsp;\\u003cspan refid=\\\"Fig6\\\" class=\\\"InternalRef\\\"\\u003e6\\u003c/span\\u003e illustrates the optimized molecular geometries of Pd-doped polyacrylamide in its pristine form (Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig6\\\" class=\\\"InternalRef\\\"\\u003e6\\u003c/span\\u003ea) and after H\\u003csub\\u003e2\\u003c/sub\\u003eO adsorption (Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig6\\\" class=\\\"InternalRef\\\"\\u003e6\\u003c/span\\u003eb), calculated using DFT at the B3LYP/LANL2DZ level. In the pristine configuration (a), the Pd atom (depicted in teal) is coordinated to the polymer backbone through nitrogen and oxygen donor atoms, forming a stable metal\\u0026ndash;ligand complex. This arrangement provides well-defined active sites for molecular adsorption and facilitates charge transfer during sensing. Upon interaction with an H\\u003csub\\u003e2\\u003c/sub\\u003eO molecule (b), a hydrogen-bonding interaction is established between the oxygen atom of water and the Pd center, as indicated by the dotted line. This adsorption geometry suggests a chemisorption-dominated binding mechanism, where the lone-pair electrons on the water oxygen coordinate with the Pd d-orbitals. This interaction not only stabilizes the adsorbed water molecule but also induces electronic structure perturbations in the Pd\\u0026ndash;polyacrylamide framework, which manifest as a narrowing of the HOMO\\u0026ndash;LUMO band gap (as seen in the FMO analysis).\\u003c/p\\u003e\\u003cp\\u003eStructurally, the Pd coordination sphere experiences a slight distortion upon H\\u003csub\\u003e2\\u003c/sub\\u003eO binding, which is indicative of electron density redistribution and bond polarization effects. These changes can enhance charge carrier density and mobility within the sensing matrix, thereby increasing its electrical response to H\\u003csub\\u003e2\\u003c/sub\\u003eO. Such a molecular-level interaction is crucial in explaining the experimentally observed sensitivity enhancement in Pd-based polymeric gas sensors under humid conditions.\\u003c/p\\u003e\\u003cp\\u003e\\u003c/p\\u003e\\u003cp\\u003eThe presented FMO diagrams illustrate the electronic structure modulation of Pd-doped polyacrylamide before and after H\\u003csub\\u003e2\\u003c/sub\\u003eO adsorption. In the pristine state (Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig7\\\" class=\\\"InternalRef\\\"\\u003e7\\u003c/span\\u003ea), the HOMO is located at \\u0026minus;\\u0026thinsp;4.30 eV and the LUMO at \\u0026minus;\\u0026thinsp;0.76 eV, yielding an electronic band gap (E\\u003csub\\u003eg\\u003c/sub\\u003e) of 3.54 eV. This relatively wide band gap corresponds to a lower intrinsic conductivity, characteristic of the undisturbed polymer\\u0026ndash;metal complex. Upon interaction with a water molecule (Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig7\\\" class=\\\"InternalRef\\\"\\u003e7\\u003c/span\\u003eb), a notable redistribution of the HOMO and LUMO densities occurs, predominantly localized around the Pd coordination site and the adsorbed H\\u003csub\\u003e2\\u003c/sub\\u003eO. The HOMO energy shifts to \\u0026minus;\\u0026thinsp;4.61 eV, while the LUMO energy decreases to \\u0026minus;\\u0026thinsp;1.32 eV, resulting in a narrowed band gap of 3.31 eV.\\u003c/p\\u003e\\u003cp\\u003eThe reduction in band gap can be ascribed to orbital hybridization between the Pd d-orbitals and the lone-pair electrons on the oxygen atom of the water molecule, which introduces new electronic states closer to the Fermi level. This hybridization promotes partial charge transfer from the water molecule to the sensing matrix, enhancing local carrier density and thus facilitating electronic transitions under lower excitation energies. From a sensing perspective, this interaction is consistent with a chemisorption-driven mechanism, wherein the H\\u003csub\\u003e2\\u003c/sub\\u003eO molecule binds strongly to the Pd active site via coordination bonds, leading to substantial electronic perturbation. Such chemisorption not only modifies the density of states but also affects the charge transport pathways within the polymer matrix, causing a measurable change in conductivity. In real sensing conditions, the narrowed band gap implies faster response and recovery kinetics, as the reduced excitation energy enables rapid carrier generation upon H\\u003csub\\u003e2\\u003c/sub\\u003eO exposure. The observed orbital redistribution also suggests that the sensing signal is dominated by modulation of the conduction band edge, which is particularly advantageous for high-sensitivity detection in humid environments.\\u003c/p\\u003e\\u003cp\\u003e\\u003cdiv class=\\\"gridtable\\\"\\u003e\\u003ctable float=\\\"Yes\\\" id=\\\"Tab2\\\" border=\\\"1\\\"\\u003e\\u003ccaption language=\\\"En\\\"\\u003e\\u003cdiv class=\\\"CaptionNumber\\\"\\u003eTable 2\\u003c/div\\u003e\\u003cdiv class=\\\"CaptionContent\\\"\\u003e\\u003cp\\u003eVariation in the theoretical parameters of Pd-doped polyacrylamide before and after interaction with H\\u003csub\\u003e2\\u003c/sub\\u003eO\\u003c/p\\u003e\\u003c/div\\u003e\\u003c/caption\\u003e\\u003ccolgroup cols=\\\"3\\\"\\u003e\\u003cdiv align=\\\"left\\\" class=\\\"colspec\\\" colname=\\\"c1\\\" colnum=\\\"1\\\"\\u003e\\u003c/div\\u003e\\u003cdiv align=\\\"char\\\" char=\\\".\\\" class=\\\"colspec\\\" colname=\\\"c2\\\" colnum=\\\"2\\\"\\u003e\\u003c/div\\u003e\\u003cdiv align=\\\"char\\\" char=\\\".\\\" class=\\\"colspec\\\" colname=\\\"c3\\\" colnum=\\\"3\\\"\\u003e\\u003c/div\\u003e\\u003cthead\\u003e\\u003ctr\\u003e\\u003cth align=\\\"left\\\" colname=\\\"c1\\\"\\u003e\\u003cp\\u003eTheoretical Parameters\\u003c/p\\u003e\\u003c/th\\u003e\\u003cth align=\\\"left\\\" colname=\\\"c2\\\"\\u003e\\u003cp\\u003eBefore interaction\\u003c/p\\u003e\\u003c/th\\u003e\\u003cth align=\\\"left\\\" colname=\\\"c3\\\"\\u003e\\u003cp\\u003eAfter Interaction with H\\u003csub\\u003e2\\u003c/sub\\u003eO\\u003c/p\\u003e\\u003c/th\\u003e\\u003c/tr\\u003e\\u003c/thead\\u003e\\u003ctbody\\u003e\\u003ctr\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c1\\\"\\u003e\\u003cp\\u003e\\u003cb\\u003eIonization Potential (eV)\\u003c/b\\u003e\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c2\\\"\\u003e\\u003cp\\u003e4.31\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c3\\\"\\u003e\\u003cp\\u003e4.61\\u003c/p\\u003e\\u003c/td\\u003e\\u003c/tr\\u003e\\u003ctr\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c1\\\"\\u003e\\u003cp\\u003e\\u003cb\\u003eElectron Affinity (eV)\\u003c/b\\u003e\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c2\\\"\\u003e\\u003cp\\u003e0.76\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c3\\\"\\u003e\\u003cp\\u003e1.32\\u003c/p\\u003e\\u003c/td\\u003e\\u003c/tr\\u003e\\u003ctr\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c1\\\"\\u003e\\u003cp\\u003e\\u003cb\\u003eHOMO-LUMO Gap (eV)\\u003c/b\\u003e\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c2\\\"\\u003e\\u003cp\\u003e3.54\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c3\\\"\\u003e\\u003cp\\u003e3.31\\u003c/p\\u003e\\u003c/td\\u003e\\u003c/tr\\u003e\\u003ctr\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c1\\\"\\u003e\\u003cp\\u003e\\u003cb\\u003eElectronegativity (eV)\\u003c/b\\u003e\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c2\\\"\\u003e\\u003cp\\u003e2.54\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c3\\\"\\u003e\\u003cp\\u003e2.97\\u003c/p\\u003e\\u003c/td\\u003e\\u003c/tr\\u003e\\u003ctr\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c1\\\"\\u003e\\u003cp\\u003e\\u003cb\\u003eHardness (eV)\\u003c/b\\u003e\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c2\\\"\\u003e\\u003cp\\u003e0.56\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c3\\\"\\u003e\\u003cp\\u003e0.61\\u003c/p\\u003e\\u003c/td\\u003e\\u003c/tr\\u003e\\u003ctr\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c1\\\"\\u003e\\u003cp\\u003e\\u003cb\\u003eSoftness (eV)\\u003c/b\\u003e\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c2\\\"\\u003e\\u003cp\\u003e1.77\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c3\\\"\\u003e\\u003cp\\u003e1.65\\u003c/p\\u003e\\u003c/td\\u003e\\u003c/tr\\u003e\\u003ctr\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c1\\\"\\u003e\\u003cp\\u003e\\u003cb\\u003eDipole moment (Debye)\\u003c/b\\u003e\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c2\\\"\\u003e\\u003cp\\u003e4.51\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c3\\\"\\u003e\\u003cp\\u003e8.33\\u003c/p\\u003e\\u003c/td\\u003e\\u003c/tr\\u003e\\u003c/tbody\\u003e\\u003c/colgroup\\u003e\\u003c/table\\u003e\\u003c/div\\u003e\\u003c/p\\u003e\\u003cp\\u003eThe Table\\u0026nbsp;\\u003cspan refid=\\\"Tab2\\\" class=\\\"InternalRef\\\"\\u003e2\\u003c/span\\u003e summarizes the variation in key theoretical parameters of Pd-doped polyacrylamide before and after interaction with H\\u003csub\\u003e2\\u003c/sub\\u003eO, as obtained from density functional theory (DFT) calculations. A noticeable increase in the ionization potential (from 4.31 eV to 4.61 eV) is observed upon H\\u003csub\\u003e2\\u003c/sub\\u003eO adsorption, indicating that the removal of an electron from the system requires slightly more energy after interaction. This suggests enhanced electronic stability due to the coordination between the water molecule and the Pd center. Similarly, the electron affinity rises from 0.76 eV to 1.32 eV, implying that the system becomes more favorable for electron acceptance, which can facilitate charge transfer processes during sensing.\\u003c/p\\u003e\\u003cp\\u003eThe HOMO\\u0026ndash;LUMO energy gap decreases from 3.54 eV to 3.31 eV after adsorption, reflecting band gap narrowing caused by orbital hybridization between the lone-pair electrons of the water molecule and the Pd d-orbitals. This reduction in band gap enhances electrical conductivity, supporting the proposed sensing mechanism. The electronegativity also increases from 2.54 eV to 2.97 eV, signifying a higher tendency to attract electrons after H₂O binding, which correlates with increased charge carrier density.\\u003c/p\\u003e\\u003cp\\u003eIn terms of chemical reactivity descriptors, the hardness increases slightly from 0.56 eV to 0.61 eV, indicating a marginally more resistant system to deformation in electron density, while the softness decreases from 1.77 eV to 1.65 eV, reflecting a minor reduction in polarizability. The dipole moment shows a substantial increase from 4.51 Debye to 8.33 Debye after adsorption, which can be attributed to significant charge redistribution within the molecule upon water interaction. This marked change in dipole moment is a strong indicator of increased polarity, which is highly beneficial for enhancing the sensor\\u0026rsquo;s electrical response under humid conditions.\\u003c/p\\u003e\"},{\"header\":\"5. Conclusion\",\"content\":\"\\u003cp\\u003eThe Pd-doped polyacrylamide nanocomposites-based polymer was successfully synthesized via the frontal polymerization technique. The polymer was identified as a highly porous material in the FESEM analysis. The synthesized polymer exhibits a particle size ranging from 254 to 825 nm, with an average particle size of 459 nm. The observed peaks in the XRD pattern are attributed to the presence of Pd metal in the polymers. The optical energy bandgap of the Pd-doped polyacrylamide has been estimated to be 5.3 eV. The polymer-based humidity sensor demonstrates sensitivity, repeatability, response, and recovery times of 2.01 MΩ/%RH, 96.52%, 5, and 40 s, respectively. Additionally, the sensor exhibited long-term stability and DFT analysis validated the experimental data.\\u003c/p\\u003e\"},{\"header\":\"Declarations\",\"content\":\"\\u003cp\\u003e\\u003cstrong\\u003eAcknowledgement:\\u003c/strong\\u003e\\u003c/p\\u003e\\n\\u003cp\\u003e\\u003cstrong\\u003e\\u0026nbsp;\\u003c/strong\\u003ePrincipal author acknowledges to Dr. Kuldeep Kumar for his support. Also, authors acknowledge to USIC, BBAU, Lucknow for SEM, XRD and FTIR facilities.\\u003c/p\\u003e\\n\\u003cp\\u003e\\u0026nbsp;\\u003cstrong\\u003eDeclaration of Competing Interest\\u0026nbsp;\\u003c/strong\\u003e\\u003c/p\\u003e\\n\\u003cp\\u003eAuthors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.\\u003c/p\\u003e\\n\\u003cp\\u003e\\u003cstrong\\u003eFunding Declaration\\u003c/strong\\u003e\\u003c/p\\u003e\\n\\u003cp\\u003eAuthors declare that there was no funding for this research work.\\u003c/p\\u003e\\n\\u003cp\\u003e\\u003cstrong\\u003eEthics, Consent to Participate, and Consent to Publish declarations\\u003c/strong\\u003e:\\u0026nbsp;\\u003c/p\\u003e\\n\\u003cp\\u003eThe datasets generated during and/or analysed during the current study are available from the corresponding author on reasonable request.\\u003cbr\\u003e\\u0026nbsp;Also, the results/data/figures in this manuscript have not been published elsewhere, nor are they under consideration \\u0026nbsp;by another publisher.\\u003c/p\\u003e\\n\\u003cp\\u003e\\u003cstrong\\u003eAuthor Contributions\\u003c/strong\\u003e\\u003c/p\\u003e\\n\\u003cp\\u003e\\u003cstrong\\u003eLava Kumar Gupta\\u003c/strong\\u003e\\u003cstrong\\u003e:\\u0026nbsp;\\u003c/strong\\u003eWriting \\u0026ndash; original draft, Investigation, Manuscript preparation, Methodology, experimentation,\\u0026nbsp;\\u003cstrong\\u003eBal Chandra Yadav:\\u003c/strong\\u003e Supervision, Review, Resource management, Validation.\\u003cstrong\\u003e:\\u0026nbsp;\\u003c/strong\\u003eWriting - Review and editing,\\u0026nbsp;\\u003cstrong\\u003eShripal\\u003c/strong\\u003e\\u003cstrong\\u003e\\u0026nbsp;Sharma\\u003c/strong\\u003e\\u003cstrong\\u003e\\u0026nbsp;\\u003c/strong\\u003e\\u003cstrong\\u003e:\\u003c/strong\\u003e Review and editing,\\u0026nbsp;\\u003cstrong\\u003eUtkarsh Kumar:\\u003c/strong\\u003e\\u003cstrong\\u003e\\u0026nbsp;\\u003c/strong\\u003eTheoretical investigation and DFT analysis, \\u003cstrong\\u003eRajeev Kumar\\u003c/strong\\u003e\\u003cstrong\\u003e\\u0026nbsp;\\u003c/strong\\u003e\\u003cstrong\\u003e:\\u003c/strong\\u003e Review and Resource management.\\u0026nbsp;\\u003c/p\\u003e\"},{\"header\":\"References\",\"content\":\"\\u003col\\u003e\\n\\u003cli\\u003eG.V. 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Sensors and Actuators B: Chem. 304, 127138 (2020).\\u003c/li\\u003e\\n\\u003c/ol\\u003e\"}],\"fulltextSource\":\"\",\"fullText\":\"\",\"funders\":[],\"hasAdminPriorityOnWorkflow\":false,\"hasManuscriptDocX\":true,\"hasOptedInToPreprint\":true,\"hasPassedJournalQc\":\"\",\"hasAnyPriority\":false,\"hideJournal\":false,\"highlight\":\"\",\"institution\":\"\",\"isAcceptedByJournal\":true,\"isAuthorSuppliedPdf\":false,\"isDeskRejected\":\"\",\"isHiddenFromSearch\":false,\"isInQc\":false,\"isInWorkflow\":false,\"isPdf\":false,\"isPdfUpToDate\":true,\"isWithdrawnOrRetracted\":false,\"journal\":{\"display\":true,\"email\":\"info@researchsquare.com\",\"identity\":\"discover-sensors\",\"isNatureJournal\":false,\"hasQc\":true,\"allowDirectSubmit\":false,\"externalIdentity\":\"\",\"sideBox\":\"Learn more about [Discover Sensors](https://link.springer.com/journal/44397)\",\"snPcode\":\"44397\",\"submissionUrl\":\"https://submission.nature.com/new-submission/44397/3\",\"title\":\"Discover Sensors\",\"twitterHandle\":\"\",\"acdcEnabled\":true,\"dfaEnabled\":true,\"editorialSystem\":\"stoa\",\"reportingPortfolio\":\"Discover Series\",\"inReviewEnabled\":true,\"inReviewRevisionsEnabled\":true},\"keywords\":\"Palladium Doped Polyacrylamide, Moisture, Frontal polymerization, DFT\",\"lastPublishedDoi\":\"10.21203/rs.3.rs-7606983/v1\",\"lastPublishedDoiUrl\":\"https://doi.org/10.21203/rs.3.rs-7606983/v1\",\"license\":{\"name\":\"CC BY 4.0\",\"url\":\"https://creativecommons.org/licenses/by/4.0/\"},\"manuscriptAbstract\":\"\\u003cp\\u003eIn this paper, the frontal polymerization synthesis of Pd-doped polyacrylamide has been reported. A thin film based on this material is fabricated for characterization purposes as well as for humidity sensing. For material characterization, Field Emission Scanning Electron Microscopy (FESEM), Energy Dispersive X-ray (EDX) spectroscopy, Particle Size Analyzer, UV-visible spectroscopy, X-ray diffraction (XRD), and Fourier Transform Infrared (FTIR) spectroscopy techniques are used. FESEM shows that the synthesized polymer is porous. The EDX spectrum confirms the presence of constituent elements in the metallopolymer. The particle size analyzer shows that the synthesized polymer has a particle size in the nano range. Pd-doped polyacrylamide has a 5.3 eV optical band gap of energy. After characterization, the polymer is employed for humidity sensing, and the sensor shows 2.01 MΩ/%RH, 96.52% and 5 and 40 s sensitivity, repeatability response and recovery times, respectively. A theoretical framework was established to elucidate the humidity-sensing mechanism of Pd-doped polyacrylamide. Density functional theory (DFT) calculations provided detailed insights into the structural and electronic modifications induced by H\\u003csub\\u003e2\\u003c/sub\\u003eO adsorption. 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