Antimicrobial activity of CuFe2O4 and CuFe2O4/ZnO: Squaring colorimetric and traditional microbiology assays with atomic force- and holotomography-microscopies

Abstract

Intensive research efforts seek the discovery of efficient antimicrobial compounds to fight microbial resistance. Magnetic nanoparticles represent a promising option due to their multifunctional antimicrobial properties, as well as their versatility, stability, and the limited microbial resistance they generate. In this study, magnetic nanomaterials (MNMs) are synthesized using a microwave-activated solvothermal method. Characterization data indicate the crystalline nature (cubic spinel CuFe2O4 NMs), superparamagnetic behavior (MCF = 52 emu/g and MCF-ZnO = 29 emu/g) nanometric and regular size (spherical particles with a mean size of 7.4 ± 1.3 nm) of the MNMs. The antimicrobial activity of the as-prepared (naked) MNMs is moderate against bacteria, yeast, and fungal strains due to the aggregation of the MNMs in the exposure media. However, citrate-functionalized MNMs (CuFe2O4-Cit, CF-Cit) show enhanced antimicrobial activity, further increasing upon activation under a high-frequency magnetic field (magnetic hyperthermia). CF-Cit exerts stronger antibacterial activity towards S. aureus (MIC <25 μg/mL) than E. coli (MIC <50 μg/mL In addition, CuFe2O4-ZnO composites were synthesized and fully characterized. The MNMs under study exhibit efficient antifungal and anti-biofilm activity. Working towards the sustainable development of multifunctional (CuFe2O4 and CuFe2O4-ZnO) MNMs, their compatibility and bioactivity was evaluated using traditional assays and advanced microscopy techniques (Atomic Force Microscopy-AFM and Holotomographic Microscopy -HTM). Because of their chemical composition, the MNMs under study lixiviate transition metal ions to the exposure media, interfering with traditional colorimetric assays used to evaluate the bioactivity of the MNMs (antimicrobial, biocompatibility). Thus, AFM and HTM studies render complementary information during the interpretation of the bioactivity mechanisms exhibited by the MNMs.