Synergistic Mechanisms Underlying Optical, Antimicrobial, Anticancer, and Antioxidant Activities of Multifunctional Bioactive ZnO/CuO/Clay Nanocomposites

This study investigates the multifunctional potential of ZnO/CuO/Clay heterojunction nanocomposites (NCs) synthesized via the solution combustion method. Six NCs were prepared by varying ZnO/CuO ratios within two clay molar fractions (0.25 and 0.5 mol). Structural and compositional analyses (field emission scanning electron microscopy (FE-SEM), energy-dispersive X-ray spectroscopy (EDS), inductively coupled plasma optical emission spectroscopy (ICP-OES), dynamic light scattering (DLS), and Brunauer-Emmett-Teller (BET)) confirmed successful heterojunction formation, uniform elemental distribution, stable colloidal behavior, and a mesoporous nanostructure. Ultraviolet-visible diffuse reflectance spectroscopy (UV-vis DRS) revealed enhanced visible-light absorption with increasing CuO and decreasing clay content, thereby improving the NC's optical characteristics and resulting in enhanced photocatalytic performance. Band gap measurements revealed CuO's band gap narrowing effect, while ZnO and clay increased it. Antibacterial assays against <i>Escherichia coli</i> and <i>Staphylococcus aureus</i> showed significantly enhanced activity, with lower MIC values observed for NCs containing 0.25 mol clay. This behavior can be attributed to the smaller particle size, improved nanoparticle (NP) dispersion, reduced aggregation, increased porosity, and greater active surface area of these NCs compared to those with 0.5 mol clay. Transmission electron microscopy (TEM) imaging confirmed membrane disruption as a key antibacterial mechanism, supported by reactive oxygen species (ROS) generation, ion release, and synergistic interaction with clay nanosheets. Cytotoxicity tests on cancerous HT-29 cells demonstrated dose- and time-dependent behavior for the 0.75CuO/0.25Clay NC and dose-dependent behavior for the 0.75ZnO/0.25Clay NC, both with minimal toxicity to normal HFF cells. Antioxidant evaluation showed significant 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging activity (57%), comparable to ascorbic acid (60.2%). Overall, these results highlight solution combustion-synthesized ZnO/CuO/Clay NCs as promising bioactive materials for photocatalytic, antibacterial, anticancer, and antioxidant applications in medicine, food packaging, and environmental remediation.