Thermal annealing modulates the structural, optical, and antibacterial properties of Cu-based nanoparticles

Numerous studies have demonstrated the antiproliferative potential of copper-based nanoparticles (Cu-based NPs) in antibacterial and anticancer applications. This study investigates how thermal annealing influences the structural, optical, and antibacterial properties of Cu-based NPs. X-ray diffraction (XRD) analysis revealed a monoclinic Cu₄SO₄(OH)₆ phase for the as-prepared powder, and monoclinic CuO phase after annealing, alongside a notable increase in crystallite size from 8.20 nm to 30.20 nm. Optical characterization shows a reduction in the band gap, a decrease in Urbach energy and an increase in the steepness parameter as a result of annealing, confirming improved crystallinity. Interestingly, the as-synthesized Cu₄SO₄(OH)₆ phase exhibits higher antibacterial activity (MIC = 0.31 - 2.5 mg mL^-1) compared to the annealed particles, likely due to its higher specific surface area (114.16 m² g^-1 <i>vs.</i> 29.58 m² g^-1) and unique crystal morphology. Both materials exhibit dose-dependent anti-adhesive effects against MRSA (>70% inhibition at 5 mg mL^-1), and the intermediate phase shows enhanced efficacy at higher concentrations. These results underscore the critical role of thermal processing in tailoring material properties while revealing the unexpected antimicrobial potential of non-annealed copper based nanoparticles.