Surface Engineered ZnO-Humic/Citrate Interfaces: Photoinduced Charge Carrier Dynamics and Potential Application for Smart and Sustained Delivery of Zn Micronutrient

Nanoscale science and nanotechnology are expected to bring about revolutionary advances in the agrochemical industry. Smart delivery of micronutrients enabling sustained release is one such innovative approach to combat the poor nutrient uptake efficiency of conventional micronutrient sources. Herein, we report a bottom-up surface fabrication strategy to modify nanosized zinc oxide (ZnO NPs) surfaces at an atomic scale for sustained release of Zn micronutrients. A large sized environmentally relevant macromolecule humic acid (HA) and a comparatively smaller biologically pertinent citrate ligand were used to functionalize ZnO NPs (30 nm), and the resultant nanohybrids were characterized using electron microscopy, diffraction, and spectroscopic techniques. Functionalization with citrate ligand turns the NP smaller than its original size. The resultant modification in the dissolution characteristics of ZnO NPs leads to sustained release of Zn over a broad pH range. Functionalization with HA causes minimum alteration of NP sizes. But dissolution patterns of NPs examined separately in the presence of HA depicts stabilization of colloidal suspensions. Additionally, picosecond-resolved lifetime measurements reveal efficient photoinduced charge transfer at the nanoheterojunction interfaces. To evaluate the impact of newly developed Zn delivery system on Zn bioavailability, their impacts on the growth of wheat plants (Triticum aestivum) was tested. Results reveal considerable growth enhancements of plants treated with functionalized NPs. The micronutrient nanoformulation requires no auxiliaries and is applicable to plant systems in aqueous dispersions. Our study reinstates the importance of "convergence thinking" or coming together of technologies as a key to creating a fundamental framework toward environmental sustainability.