In-situ synthesis of soft, non-aggregated and well-dispersed Ag0, Au0 and Ag0-coated-Au0 nanoparticles using cold-macerated bark extract of Combretum glutinosum: Stability, DNA-binding, and antibacterial potential

Natural resource-derived nanoparticles have continued to gain popularity due to their potential application in the biomedical and other domains. This study presents an in-situ synthesis method that uses cold-macerated aqueous extract generated from Combretum glutinosum’s stem bark tissue to synthesize soft, non-aggregated, and well-dispersed Ag.⁰, Au.⁰, and Ag.⁰-coated-Au.⁰ nanoparticles. Their TEM and FESEM images demonstrated that they are soft, non-aggregated, well-dispersed, and spherically shaped particles with mean diameters of 13.10 nm and 20.00 nm for CgAgNPs and CgAg⁰-Au⁰NPs, respectively, whereas, particles of CgAuNPs exhibited the same characteristics but had variable shapes including spherical, cylindrical, triangular, pentagonal, and hexagonal with a mean diameter of 29.48 nm. Their mean hydrodynamic sizes (from DLS), core sizes (from TEM), and crystallite sizes (from PXRD) appeared in the range of 4.0–45.5 nm, which follows a trend of crystallite sizes ˂ core sized ˂ hydrodynamic sizes. Besides the crystallite sizes, PXRD analysis exposed their purity and crystallinity with average crystallite diameters (nm) of 4.49 for CgAgNPs, 9.23 for CgAuNPs, and 7.9 for CgAg⁰-Au⁰NPs. These nanoparticles are highly stable as revealed by their zeta potential values which were further supported by their hardness (դ) values obtained from the Tauc plot for the bandgap energies that followed a trend of դ= 2.495 (–29.5 mV) ˃ դ= 2.315 (–27.4 mV) ˃ դ= 1.495 (–13.2 mV) for CgAg⁰-Au⁰NPs, CgAgNPs, and CgAuNPs, respectively. The robust stability preserved their physicochemical behaviour over time and equipped them with promising biological effects. The binding potential of these nanoparticles to C.t.DNA revealed effective interaction with binding constant (K_b) values of 2.467 × 10³ (CgAgNPs), 2.438 × 10³ (CgAuNPs), and 2.199 × 10³ (CgAg⁰-Au⁰NP), suggesting their usage as antimicrobial agents. Following this, they were investigated for antibacterial activities and the outcomes unveiled strong activity with potent IC₅₀ of 0.05 mg/ml(50 µg./ml) over Staphylococcus aureus and 0.1 mg/ml(100 µg/ml) over Pseudomonas aeruginosa for CgAgNPs; 0.1 mg/ml(100 µg/ml) over both organisms for CgAuNPs; and 0.05 mg/ml(50 µg/ml) over both organisms for CgAg⁰-Au⁰NPs. Their stability, softness, non-aggregation, and well-dispersed nature equipped them with incredible biological effects which are credited to the synergistic potential contributed by each component.