Sol–gel-derived mullite nanoparticles: Structural and antibacterial insights

Mullite (3Al₂O₃·2SiO₂) is synthesized via a controlled calcination process at 950 °C and comprehensively characterized using multiple advanced techniques to establish its structural and functional properties. AFM analysis revealed densely packed grains with irregular morphology, significant surface roughness (Z-axis variation ≈ 767 nm), and an average grain size of ~85 nm, while FESEM confirmed the presence of nanoscale particles uniformly distributed alongside larger agglomerates, yielding a moderately broad size distribution (17–167 nm, average ~100 nm). TEM micrographs provided direct evidence of discrete nano crystallites with a narrow size distribution (17–49 nm, average ~30 nm), highlighting the distinction between primary crystallites and surface grains. The complementary nature of AFM, FESEM, and TEM results demonstrated the multi-scale particle organization in mullite, ranging from nano crystallites to secondary aggregates. Antibacterial activity is systematically evaluated against Escherichia coli and Staphylococcus aureus using both agar diffusion method (ADM) and spread plate method (SPM). The results revealed complete inhibition of bacterial growth (0 colonies) in SPM and the absence of inhibition zones in ADM, indicating the strong antibacterial efficacy of 950 °C-fabricated mullite pellets. These findings emphasize the potential of mullite as a multifunctional material, combining nanoscale structural control with promising antimicrobial activity for biomedical and environmental applications.