Alloyed Versus Physically Mixed Bimetallic Nanoparticle PLA Composites: Structure and Antimicrobial Performance

ABSTRACT In this work, polylactic acid (PLA) based nanocomposites containing bimetallic nanoparticle systems were made using the injection molding method to compare the antimicrobial behavior of alloyed nanoparticles and those physically mixed to be used in biomedical and environmental applications under similar processing and testing conditions. In particular, Ag–Cu alloy, Cu–Ni alloy, Ag–Cu mixture, and Cu–Ni mixture nanoparticles were introduced in PLA at a loading of 3 wt.% metal nanoparticles and 97 wt.% PLA. Scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X‐ray diffraction (XRD), thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC) were used to analyze structural, morphological, and thermal properties of the composites. The results of SEM/EDS and XRD analyses revealed more homogeneous dispersion of the nanoparticles and more phase‐separated crystalline structures in the mixture systems, while the alloy systems had higher agglomeration and a single intermediate diffraction peak corresponding to alloy formation; the thermal stability of the mixtures was quite retained with degradation starting from around 320°C–360°C, and the glass transition temperature between 56.5°C and 59.5°C. The time‐kill assay was used to determine antimicrobial activity against standard laboratory Gram‐positive and Gram‐negative bacterial strains, which cause healthcare‐associated infections. Time‐dependent antibacterial activity was observed for all composites. The four nanocomposites achieved mean log 10 reductions of 0.19 to 0.86 in the first 5 to 20 min, and 0.70 to 1.16 after 1 h, whereas the PLA control reduced by 0.29 log 10 after 1 h. PLA + Cu–Ni alloy showed the maximum strain‐specific activity against Enterococcus faecalis , reaching 1.99‐log 10 reduction (98.98%) after 1 h. All composite formulations resulted in near complete killing (> 99.9%) of four out of the six strains after 8 h and six out of six strains after 24 h. Alloyed systems showed an initial strong antibacterial activity, while the physically mixed systems showed similar long‐term bactericidal activity. These results suggest that the metallurgical integration is likely attributed to increased initial antimicrobial kinetics, and the sustained release of ions over time ensures long‐term killing, resulting in a shift from a kinetics‐dominated to a diffusion‐dominated antimicrobial response over time. The results show that these bimetallic nanoparticle PLA composites can be used as tunable antibacterial materials for healthcare, packaging, and other hygienic applications, but additional safety and durability testing is needed.