Abstract
Plant extracts have been widely used to green synthesize zinc oxide nanoparticles (ZnO NPs); however, how the combination of ultrasound and coffee leaf extract (CLE) affects the structure characteristics and the yield of ZnO NPs remains unknown. In this study, we used CLE to green synthesize ZnO NPs with the help of ultrasound. The highest yield (43.59 ± 0.13%) of ZnO NPs was obtained under the optimal processing conditions of pH = 8.0, mass ratio of coffee leaves to C4H6O4Zn•2H2O = 1.71, ultrasound time = 10 min, ultrasound frequency = 28/40 kHz, ultrasound power = 180 W, and synthesis temperature = 30 °C. The as-synthesized ZnO NPs were characterized by UV-Vis, SEM, EDX, TEM, FTIR, XRD, and zeta potential analyses. SEM and TEM analyses revealed that ZnO NPs synthesized using ultrasound-assisted method were spherical with an average particle size of 8.29 ± 1.38 nm, which was smaller than ZnO NPs synthesized without ultrasound treatment (10.48 ± 1.57 nm) and the chemically synthesized ZnO NPs (17.15 ± 2.84 nm). HPLC analysis showed that the phenolic compounds in coffee leaves, especially 5-CQA, were the main reductants and chelating agents for ZnO NPs synthesis. The synthesized ZnO NPs were used to load mangiferin, which was control released under pH 7.4 over 132 h. Our study provides an easy and eco-friendly method using CLE assisted with ultrasound for green synthesis of ZnO NPs which can be used as nanocarriers to control release of mangiferin.