Biogenic E. coli EPS-capped Ag/ZrO₂ nanocomposites (Ag/ZrO₂ NCs): synthesis, characterization, and multitarget...

This study reports the biogenic synthesis of Ag/ZrO₂ nanocomposites (NCs) using Escherichia coli exopolysaccharide (EPS03) as a reducing and capping agent, with full physicochemical characterization and biomedical evaluation. XRD resolved monoclinic ZrO₂ (JCPDS 37-1484) and FCC Ag (JCPDS 04-0783) as independent crystalline phases with no intermetallic formation. Characteristic reflections at 28.2° and 31.5° confirmed ZrO₂, while Ag peaks appeared at 38.1°, 44.3°, 64.4°, and 77.4°, with Scherrer crystallite sizes of 15.5 nm and 29.0 nm, respectively. FTIR confirmed retained EPS biomolecular groups at the particle surface through hydroxyl stretching at 3444.81 cm^-1, amide I/II bands at 1507-1636 cm^-1, and metal oxide fingerprint absorptions at 675 and 517 cm^-1 for Ag-O and Zr-O-Zr, respectively. XPS confirmed metallic Ag alongside a Zr⁴⁺ oxidation state at binding energies of 181.5 and 183.9 eV, with O 1s deconvolution resolving Zr-O and surface-adsorbed water contributions at 529.9 and 531.6 eV. HRTEM resolved lattice fringes with a 0.283 nm interplanar spacing, assigned to the (111) plane of monoclinic ZrO₂. DLS recorded a Z-average hydrodynamic diameter of 75.6 nm (PDI = 0.201) and a zeta potential of -33.54 mV, confirming colloidal stability. MTT screening of WI-38 fibroblasts maintained viability above 95% up to 125 µg mL^-1, with a cytotoxic IC₅₀ of 292.66 ± 1 µg mL^-1. DPPH and ABTS assays yielded IC₅₀ values of 7.14 ± 0.13 and 6.68 ± 0.12 µg mL^-1, respectively, approaching those of ascorbic acid. COX-1 and COX-2 inhibition IC₅₀ values were 7.43 ± 0.2 and 10.5 ± 0.1 µg mL^-1 relative to celecoxib, while BSA denaturation inhibition recorded an IC₅₀ of 2.46 ± 0.112 µg mL^-1, closely approaching diclofenac sodium at 1.72 ± 0.022 µg mL^-1. Antimicrobial screening by agar well diffusion showed inhibition zones of 34, 26, and 18 mm against Bacillus subtilis, Staphylococcus aureus, and MRSA, respectively, each exceeding those of gentamicin (29, 24, and 14 mm). Against Candida albicans, a zone of 30 mm was recorded against fluconazole at 29 mm. Among Gram-negative strains, the NCs matched gentamicin against E. coli at 27 mm and showed a marginal advantage against Salmonella typhi at 21 mm vs. 20 mm. Broth microdilution yielded the lowest MIC of 7.8 µg mL^-1 against both B. subtilis and C. albicans, with MBC/MIC and MFC/MIC ratios of 2 across most strains and a tightest bactericidal ratio of 1 against E. coli, keeping every strain within the ≤4 bactericidal/fungicidal cutoff. These results demonstrate that E. coli EPS03-mediated synthesis yields a physicochemically stable Ag/ZrO₂ NC system with a well-defined multifunctional biomedical profile across oxidative, inflammatory, and microbial targets.