Enhancing melatonin photostability and controlled release using pH-responsive mesoporous silica nanoparticles for agricultural applications

Abstract

pH-responsive and thiol-functionalized mesoporous silica nanoparticles (SH-MSNs) encapsulating the phytohormone melatonin were designed to release the hormone in a controlled manner while enhancing its photostability. Melatonin-loaded MSNs were prepared in weight ratios of 1:4, 1:2, 1:1, and 2:1, with the highest loading efficiency of 17.12 μg/mL observed at a 1:1 melatonin/SH-MSNs ratio. Detailed characterization of these MSNs confirmed their functionality: zeta potential and size analyses indicated enhanced stability post-functionalization and loading; FESEM confirmed spherical morphology; EDX and FTIR validated melatonin incorporation and disulfide linkage formation; and DSC and XRD analyses demonstrated the amorphous transformation of melatonin, enhancing dissolution rates. Reduced specific surface area and pore volume, as revealed by N₂ adsorption-desorption studies, confirmed melatonin entrapment. A distinct DSC peak at 118.82 °C reflected the crystalline nature of pure melatonin, contrasting with its amorphous state post-encapsulation. In vitro release studies demonstrated significant pH responsiveness, with 81 % melatonin release at pH 5.5 and 55% release at pH of 6.5 after 48 h due to disulfide bond reduction. Additionally, encapsulation improved melatonin photostability by 64.9% compared to free melatonin. This research suggests that MSNs grafted with redox-responsive gatekeepers could be used in crops to deliver melatonin in a controlled manner. Future research may explore the broader agricultural benefits of these MSNs for controlled release of other phytohormones under various environmental conditions.