Calcium carbonate casein-based composite microspheres with mycogenic copper nanoparticles: synthesis, characterization and plant growth promotion

Abstract
Nanotechnology offers a promising approach by enabling slower, controlled release of fertilizer, improving nutrient uptake, and reducing the overall quantity of fertilizer needed, compared to conventional chemical fertilizers, which are often inefficient, require intensive application, and often lead to nutrient leaching into the soil. Copper nanoparticles (CuNPs) were synthesized using Fusarium graminearum autolysate, then complexed with casein micelles (CuMC), and subsequently CuMC were encapsulated within calcium carbonate composite microspheres (CuMS). Maize was treated with Hoagland solution containing a copper source substituted with CuNPs, CuMC, or CuMS. After 30 days, the plantlets were harvested, their length and fresh and dry weight were measured, and copper content and leaf chlorophyll content were assessed. 
CuNPs were spherical, small, negatively charged, showing a cubic phase structure, with biomolecules on their surface, and exhibited promising antimicrobial activity. CuMC were spherical with an average diameter of 133.65 nm, while CuMS were mesoporous with a mean size of 1.09 µm. The treatment of maize with CuMC increased the fresh weight of stems and roots by 15-20%, while CuMS increased the fresh weight of leaves, stems, and roots, as well as the dry weight of leaves by 14-32%. Application of CuNPs, CuMC, and CuMS resulted in higher chlorophyll content (9-24%), whereas CuNPs treatment increased copper concentration (15-42%). Casein micelles and calcium carbonate composite microspheres serve as effective carrier materials that encapsulate CuNPs and slow their degradation rate. CuNPs-based composites enhance maize growth and increase chlorophyll content, demonstrating great potential as environmentally friendly nanofertilizers for agricultural applications.