Poly(Amic) Acid Copper Nanocubes as Biocide for <i>Staphylococcus epidermidis</i>

Herein, we demonstrate the growth pattern and mechanism of copper nanocubes (CuNCs) on the surface of biodegradable polyamic acid (PAA) film as a function of applied potential. The PAA solution was fabricated via a poly condensation reaction between 4,4'-oxidianiline (ODA) and pyromellitic dianhydride (PMDA) in dimethylacetamide (DMAC) at 25 °C. The resulting viscous PAA solution was drop-cast on a glassy carbon electrode (PAA|GCE) and dried at room temperature. The (PAA|GCE) electrode was incubated in a copper solution. The effect of applied potential was tested using chronoamperometry, which controls and drives the nucleation, growth, and shape evolution mechanism of copper nanocubes on the PAA surface. SEM, AFM, and TEM characterization confirmed 4.59 nm ± 0.20 sizes for the cubic shape of copper nanoparticles during the nucleation and growth phases. Effects of pH, polymer thickness, applied potential, and concentration of copper ions were investigated to achieve optimal growth of copper nanocubes on PAA polymer. The mechanism of copper nanocubes formation was studied using density functional theory (DFT) calculations, which revealed the specific binding site on PAA polymer for copper ion interaction. The DFT showed that the complexation of the Cu ions to the carboxyl group (171.9 meV) is lower in energy (i.e., more favorable) than the Cu ion complexed to the amine group of PAA. XPS analysis of PAA-Cu revealed that the Cu²⁺ is reduced to the ground state Cu⁰ in the presence of an applied potential. Antibacterial characterization (time killed assay) of PAA copper nanocubes on resistant <i>Staphylococcus epidermidis</i> was used to demonstrate a 100% inhibition via contact killing due to the high surface-to-volume ratio, active facets, and sharp edges of copper nanocubes. The shape and size effect of PAA-Cu on <i>S. epidermidis</i> exhibited 8.9 mm ± 0.54 and 15.2 mm ± 0.36 as the highest zones of inhibition respectively, compared to the performance of conventional CuNPs. The shape and size of nanoparticles play a role in antibacterial activity. This study demonstrates the role of the applied potential as the driving force for the nucleation and growth of copper nanocubes on a polymer substrate as a biocide against resistant microbes.