Zinc oxide (ZnO) nanomaterials have emerged as promising candidates for gas sensing applications due to their high sensitivity, fast response–recovery cycles, thermal and chemical stability, and low fabrication cost. However, the performance of pristine ZnO remains limited by high operating temperatures, poor selectivity, and suboptimal detection at low gas concentrations. To address these limitations, significant research efforts have focused on dopant incorporation and polymer hybridization. This review summarizes recent advances in dopant engineering using elements such as Al, Ga, Mg, In, Sn, and transition metals (Co, Ni, Cu), which modulate ZnO’s crystal structure, defect density, carrier concentration, and surface activity—resulting in enhanced gas adsorption and electron transport. Furthermore, ZnO–polymer nanocomposites (e.g., with polyaniline, polypyrrole, PEG, and chitosan) exhibit improved flexibility, surface functionality, and room-temperature responsiveness due to the presence of active functional groups and tunable porosity. The synergistic combination of dopants and polymers facilitates enhanced charge transfer, increased surface area, and stronger gas–molecule interactions. Where applicable, sol–gel-based studies are explicitly highlighted and contrasted with non-sol–gel routes to show how synthesis controls defect chemistry, morphology, and sensing metrics. This review provides a comprehensive understanding of the structure–function relationships in doped ZnO and ZnO–polymer hybrids and offers guidelines for the rational design of next-generation, low-power, and selective gas sensors for environmental and industrial applications.

The sol–gel method is a powerful route for producing ceramics, glasses, and hybrid nanostructures; however, conventional sol–gel processing remains limited by long reaction times, high-temperature treatments, and the use of toxic organic solvents. This review evaluates the hypothesis that combining eco-efficient strategies with accelerated sol–gel routes can significantly improve sustainability and synthesis efficiency while maintaining or enhancing material performance. Reported studies show that eco-efficient formulations can reduce solvent-related hazards and VOC emissions, in some cases approaching reductions of 70–90% depending on the solvent system and precursor chemistry. Processing times have also been shortened substantially, with several reports documenting the reduction of gelation or drying steps from hours to minutes under optimized microwave or ultrasound activation. These quantitative trends are summarized in the revised Table 2 and detailed in the Supplementary Information. These rapid routes also produce hybrid nanomaterials with 20–40% smaller particle sizes, higher homogeneity, and improved optical and catalytic behavior. Emphasis is placed on inorganic–polymer, bioactive glass–biopolymer, and carbon-based hybrid systems, which exhibit enhanced mechanical stability, biocompatibility, and functional versatility. Comparative analysis with conventional sol–gel processes highlight the key advantages, existing gaps, and application relevance in energy, environmental remediation, and biomedical fields. This review provides a concise and up-to-date reference for researchers seeking sustainable, energy-efficient, and high-performance sol–gel strategies for advanced hybrid nanomaterial design. Graphical Abstract

Maghemite (γ-Fe₂O₃) nanoparticles have attracted considerable interest for electronic applications due to their tunable electrical properties. Doping strategies offer an effective way to optimize their resistive behavior for use in electronic devices. In this study, cobalt (Co) was incorporated into γ-Fe₂O₃ to enhance its resistive properties. X-ray diffraction (XRD) confirmed the retention of the cubic P4₃32 phase, with Co doping inducing subtle lattice distortions due to ionic substitution. Scanning and transmission electron microscopy (SEM/TEM) revealed morphological changes, where Co incorporation influenced particle shape and size distribution. Electrical conductivity analysis demonstrated a decrease in both AC and DC conductivity with the increase in Co content, indicating enhanced resistive behavior. The increase in activation energy suggests a reduction in charge carrier mobility, leading to higher resistivity. Impedance spectroscopy further confirmed increased real and imaginary impedance values, reinforcing the role of Co in suppressing charge transport. These results position cobalt-doped maghemite as a promising material for electronic resistive devices, such as tunable resistors and negative temperature coefficient (NTC) thermistors, where controlled conductivity and stable resistive behavior are essential.

The purpose of the presented study is to examine the impact of zinc oxide nanoparticles (ZnO NPs) on the spectrum features of poly [2-methoxy-5-(3′,7′-dimethyloctyloxy)-1, 4-phenylenevinylene] (MDMO-PPV). The characteristics of the MDMO-PPV and doped ZnO NPS samples were assessed using several techniques. A set of solutions of MDMO-PPV in toluene that were doped with different ratio percentages of ZnO NPs was prepared to obtain thin films. Pristine and composite solutions were spin-coated on glass substrates. It was observed that MDMO-PPV had two distinct absorbance bands at $310$ and $500 \mathrm{nm}$ in its absorption spectrum. The UV-Vis spectrum was dramatically changed when $5\%$ of ZnO NPs were added. The result showed a significant reduction in absorption of the band $500 \mathrm{nm}$, while $310 \mathrm{nm}$ absorption increased rapidly and became more pronounced. Upon adding (10%) ZnONPs to the sample, no noticeable change was observed in the 500 nm band. However, the 310 nm band shifted towards the blue region. There is a dominant peak in the PL spectrum of MDMO-PPV in its pristine form around $575 \mathrm{nm}$ and a smaller hump around $600 \mathrm{nm}$ of the spectrum. The spectral profile at 600 nm and the intensity of both bands are improved by raising the ZnO NP concentration. These bands feature two vibronic transitions identified as (0-0) and (0-1). When the dopant concentration increased to the maximum dopant percentage ($10\%$), the energy band gap values increased by $0.21 \mathrm{eV}$ compared to the pristine MDMO-PPV. In addition, the refractive index (n) decreased to its lowest value of $2.30$ with the presence of concentrations of ZnO NPs.