Abstract
The remarkable rise in antibiotic resistance seen in recent years has significantly reduced the effectiveness of most medications. This study aimed at evaluating the antibacterial and toxicological effects of gold nanoparticles (AuNPs), capped with a synthetic heterocyclic compound, 4-(4'-chlorophenyl)-2-imino-1,3-thiazino [2,3-b] benzimidazole (FDM29), and green-synthesized using a leaf extract from Baissea gracillima (B.gracillima-liana) plant. The synthesized AuNPs were characterized by ultraviolet-visible spectroscopy, dynamic light scattering, Fourier transform infrared, transmission electron microscopy, energy-dispersive and diffraction X-ray spectroscopy, supported by <^>1H and <^>13C nuclear magnetic resonance analysis of the FDM29 ligand. The antibacterial susceptibility of the AuNPs was investigated against Gram-negative (Escherichia coli, Klebsiella pneumoniae, and Proteus mirabilis) and Gram-positive bacteria (Bacillus subtilis, Mycobacterium smegmatis, and Staphylococcus aureus) using the broth microdilution method. The zebrafish embryo development toxicity test (ZFET) was employed to evaluate the toxicity of the synthesized AuNPs. Characterization indicated that AuNPs were successfully synthesized. The AuNPs were stable and spherical, with sizes ranging from 13 to 84 nm, which fall within the nanometer scale. In addition, the AuNPs effectively inhibited the growth of most bacterial strains, excluding P. mirabilis and M. smegmatis. The findings suggest that capping AuNPs enhances their antibacterial efficacy and reduces their toxicity compared to chemically synthesized uncapped AuNPs. The exposure of zebrafish embryos to the synthesized AuNPs revealed that the pristine AuNPs exhibited more significant toxicity at all concentrations compared to AuNPs-FDM29 and AuNPs-B. gracillima-liana. At 24 h post-fertilization, most eggs had coagulated, and by 48 hpf, the majority embryos showed absence of heartbeat and developmental retardation, thus being considered toxic. The mortality rate followed the order pristine AuNPs > AuNP-FDM29 > AuNP-B.gracillima-liana, at 81.6, 61.9, and 14.76%, respectively, at 0.25 mg/mL. These results indicate that biological functionalization of AuNPs significantly improves their biocompatibility and antibacterial properties. Notably, the green synthesis using B. gracillima-liana leaf extract yielded AuNPs with the lowest toxicity and substantial antibacterial efficacy, outperforming chemically synthesized counterparts. This study presents a novel, plant-based green synthesis approach for generating biocompatible AuNPs with dual antibacterial and low-toxicity profiles, offering a sustainable platform for developing next-generation antimicrobial agents with potential applications in nanobiotechnology, biomedicine, and pharmaceutical sciences.