Biological synthesis of gold nanoparticles by endophytic bacterium Priestia megaterium (CBMAI 2841) and their application as a catalyst in Knoevenagel reaction

Gold nanoparticles (AuNPs) have found widespread applications across chemistry, physics, biology, and medicine. Microorganisms can be used for the biological synthesis of nanoparticles under controlled conditions. In this study, the endophytic bacterium Priestia megaterium (CBMAI 2841), isolated from Lavandin inflorescences and cultured in a liquid Nutrient Broth medium, was employed for the extracellular biological synthesis of metallic AuNPs (Bio-AuNPs) without the use of toxic solvents. Highlighting the experimental novelty, the utilization of a bacterium sourced from a specific plant origin distinguishes this work from studies employing microorganisms from broader environmental niches or established laboratory strains. The bacterial biomass was separated by centrifugation, and the resulting growth supernatant (20 mL) was reacted with a 1 mmol L−1 aqueous solution of tetrachloroauric(III) acid trihydrate ( ). The bioreduction process was conducted at 32 °C and 130 rpm for 72 h. The formation of Bio-AuNPs was observed as early as 3 h of reaction, a finding confirmed by UV–Vis spectroscopy. Transmission electron microscopy (TEM) revealed that the biosynthesized Bio-AuNPs were formed through self-assembly and coalescence of smaller primary nanoparticles, resulting in an average final size of 45 nm. The aqueous suspension of Bio-AuNPs exhibited good colloidal stability, characterized by a zeta potential of approximately −18 mV and a hydrodynamic size of around 70 nm. The Bio-AuNPs were successfully applied as catalysts (0.4 mg solid Bio-AuNPs, 50 °C, 30 min, 5 mL ethanol) in the Knoevenagel condensation reaction using malononitrile and a series of aromatic aldehydes (benzaldehyde, 4-bromobenzaldehyde, 4-ethylbenzaldehyde, 4-chlorobenzaldehyde, 4-fluorobenzaldehyde, 3-nitrobenzaldehyde, 4-nitrobenzaldehyde, 3,4,5-trimethoxybenzaldehyde, 3-pyridinecarboxaldehyde, furan-2-carbaldehyde, and 2-thiophenecarboxaldehyde). Notably, high yields (70%–99%) of the Knoevenagel adducts were achieved with a low catalyst mass loading within a short reaction time, using a common solvent (ethanol) at a mild temperature, suggesting enhanced catalytic efficiency compared to some reported Bio-AuNP catalysts. The bacterium P. megaterium (CBMAI 2841) demonstrated efficiency in the extracellular biological synthesis of AuNPs. The rapid synthesis combined with promising catalytic activity underscores the potential of this specific bio-inspired approach.