Antimicrobial Activity of Morphology-Controlled Cu2O Nanoparticles: Oxidation Stability under Humid and Thermal Conditions

Abstract

Metal oxides can be used as antimicrobial agents, especially since they can be fabricated into various forms such as films, masks, and filters. In particular, the durability of antimicrobial agents and the duration of their antimicrobial activity are important factors that determine their suitability for a specific purpose. These factors are related to the morphology and size of particles. The metal oxide Cu₂O is often oxidized to CuO in various conditions, which reduces its antimicrobial activity. This study focused on the oxidation of nanoparticles of Cu₂O with three morphologies, namely, spherical, octahedral, and cubic morphologies, in excessively humid and excessive-thermal environments for a specific duration and the antimicrobial activity of the NPs. Cu₂O nanoparticles were prepared using the chemical reduction method, and their morphology could be varied by adjusting the molar ratio of OH^- to Cu²⁺ and changing the reducing agent. It was found that cubic Cu₂O was the most stable against oxidation and had the smallest reduction in antimicrobial activity. This study examined the antimicrobial activity and the oxidation stability of Cu₂O NPs with different morphologies but similar particle sizes.

Keywords: antimicrobial activity; cuprous oxide; morphology; nanoparticle; oxidation stability.

Figure 1

Flowchart and a schematic of the synthesis process of Cu₂O NPs (A: Spherical Cu₂O, B: Octahedral Cu₂O, C: Cubic Cu₂O).

Figure 2

SEM images of Cu₂O NPs with (a) spherical, (b) octahedral, and (c) cubic morphologies showing the effect of zero-, two-, four-, and eight-week exposure to humid conditions (temperature of 20 ± 5 °C and RH of 85 ± 5%).

Figure 3

SEM images of as-synthesized (a1) octahedral and (b1) cubic Cu₂O; HRTEM images of as-synthesized (a2,a3) octahedral and (b2,b3) cubic Cu₂O.

Figure 4

(a) BET specific surface area of Cu₂O NPs obtained from conditions of not being exposed to humid conditions and (b) bactericidal rate (%) of Cu₂O NPs with different morphologies exposed to humid conditions for 0 (raw), 2, 4, and 8 weeks.

Figure 5

XRD patterns of Cu₂O NPs: (a) 0W samples (●: Cu₂O) and (b) spherical Cu₂O, (c) octahedral Cu₂O, and (d) cubic Cu₂O exposed to humid conditions for 0, 4, and 8 weeks (●: Cu₂O; ○: CuO; ▲: Cu(OH)₂).

Figure 6

(a) Cu 2p region of the XPS spectra of samples and high-resolution XPS spectrum of the Cu 2p peak: (b) spherical Cu₂O, (c) octahedral Cu₂O, and (d) cubic Cu₂O in humid conditions for zero and four weeks; (A: Spherical Cu₂O, B: Octahedral Cu₂O, C: Cubic Cu₂O).

Figure 7

(a) TGA-DSC curves at 250 °C for 2 h, (b) bactericidal rate of Cu₂O NPs in raw samples and following TGA, and (c) XRD patterns of Cu₂O NPs (●: Cu₂O; ○: CuO); (A: Spherical Cu₂O, B: Octahedral Cu₂O, C: Cubic Cu₂O).