Environmental classification of synthetic methods for zinc oxide nanoparticles: a comparative review of sustainable green and conventional approaches with their diverse applications.

Nanotechnology has significantly advanced the field of materials science, with zinc oxide nanoparticles (ZnO NPs) emerging as versatile materials owing to their unique physicochemical properties, biodegradability, and tunability. These features make them promising candidates for various biomedical and environmental applications. This review summarizes the literature on ZnO NPs, with a particular emphasis on environmental considerations. Conventional chemical synthesis methods, including sol-gel, emulsion, hydrothermal, solvothermal, precipitation, co-precipitation, and mechanochemical techniques, facilitate precise control over particle size and morphology; however, they frequently require the use of toxic reagents. Compared with green synthesis methods, these methodologies offer enhanced repeatability, crystallinity, and performance in photocatalytic and sensing applications; however, they impose a greater environmental burden. In contrast, the use of plant extracts and microorganisms for green synthesis offers a viable alternative for producing safer, more sustainable ZnO nanoparticles. Although bio-assisted reduction and the application of natural capping agents have improved biocompatibility and surface functionality, these methods continue to face significant challenges. Key issues include limited control over the particle size distribution, uniformity of morphology, and batch-to-batch reproducibility, all of which can negatively impact the consistency and functional performance of nanoparticles. Furthermore, characterization techniques are essential for understanding NP properties. XRD confirms crystallinity and estimates particle size (Scherrer equation), whereas SEM and TEM reveal morphology and shape variations, including spheres, cubes, rods, hexagonal flowers, and nanotubes. EDX can be used to determine the elemental composition, XPS can be used for surface chemical analysis, UV-Vis can be used to determine optical properties and band gap, and FT-IR can be used to detect functional groups and biomolecule capping in green synthesis. BET analysis is done to measure surface area, and the zeta potential is used to assess surface charge and stability. The applications of ZnO NPs encompass a wide range of fields, including photodegradation, photosensing, electrochemical catalysis, gas sensing, chemical detection, photocatalysis, agriculture, and biomedicine. Given the importance of both performance and sustainability, future research should prioritize hybrid approaches that integrate the precision of chemical synthesis with the ecological advantages of green methodologies, such as biosynthesis coupled with controlled thermal treatment.