Synthesis and Characterization of Hydroxyapatite Assisted by Microwave-Ultrasound from Eggshells for Use as a Carrier of Forchlorfenuron and In Silico and In Vitro Evaluation

This study utilized eggshell biomass as a calcium precursor for synthesizing hydroxyapatite (Hap) through a co-precipitation method assisted by a combined microwave-ultrasound (Mu/Us) crystallization process. Different milling techniques (mortar, high-energy mill, and sieving) were employed to prepare the eggshell biomass and identify the most effective calcium precursor. The precursor derived from high-energy milling, followed by sieving and thermal treatment at 750 °C (designated as Sample Hap-H3 750), was selected due to its higher porosity, enhanced crystallinity, and smaller particle size than other synthesized materials. This sample was subsequently used as a carrier for the plant hormone forchlorfenuron (FCF), forming the composite Hap-FCF. Comprehensive characterization was conducted using X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), specific surface area analysis (BET method), zeta potential (ZP), scanning electron microscopy (SEM), and bright-field transmission electron microscopy (BFTEM), ensuring reliable and robust data. The in silico evaluation of the phytohormone FCF with two receptors, gibberellin (GA3Ox2) and auxin (IAA7), produced notable results. Docking and molecular dynamics (MD) simulations demonstrated that the gibberellin receptor was preferentially stimulated, as shown by the higher binding affinity and the receptor’s sustained stability during the MD simulations. These findings underscore the potential applications of this research, emphasizing its significance in materials science and biochemistry. Moreover, the in vitro assessment of Hap-H3 750, Hap-FCF, FCF, and the control (distilled water) on the germination and growth of butterhead lettuce seeds (Lactuca sativa) over 30 days revealed that Hap-H3 750 and Hap-FCF promoted plant growth by 275–330% relative to the control. This effect was attributed to the preferential stimulation of the gibberellin receptors responsible for stem and root elongation. These results suggest that HAP nanoparticles could facilitate the controlled delivery of FCF in the agricultural sector, promising to be an effective nanofertilizer.