Biogenic gold nanoparticles via Aloe vera waste: a sustainable catalyst for water treatment

Starting from the serious environmental problems associated with the growing increase of pollution, the present study investigates a sustainable and eco-friendly approach to water treatment by synthesizing green gold nanoparticles (AuNPs) starting from Aloe barbadensis Miller (Aloe vera) waste (AVW). Bioactive compounds naturally present in AVW were extracted in water at different temperatures (25-90 °C). The extract prepared at 90 °C was identified as the most suitable for stable AuNPs formation, containing higher amounts of reducing and stabilizing agents such as α- and β-glucose, sucrose and malic acid, as confirmed by ¹H-NMR studies. The appearance of a typical plasmonic band at 560 nm confirmed the success of AuNPs synthesis. Selected-area electron diffraction and transmission electron microscopy analyses demonstrated the crystalline nature of the synthesized AuNPs with an average diameter of 10 nm and with a zeta potential of -32.3 mV. Moreover, FTIR analysis revealed the presence of an organic layer around the AuNPs, also confirmed from UV-vis analysis. Methylene blue (MB), rhodamine b (ROb), methyl orange (MO) and 4-nitrophenol (4NP) were selected as water pollutant models to test their catalytic activity. The results obtained showed the high capacity of green AuNPs to speed up the reduction rate following a pseudo first order kinetic with k values observed for MB, ROb, MO and 4NP of 0.32, 0.15, 0.09 and 0.04 min^-1 respectively. This behavior remains highly efficient even when all pollutants are present simultaneously in a mixture, demonstrating their potential use also in complex matrices.