Morphology-controlled bandgap engineering and antifungal selectivity in Ca-doped CdFe2O4 nanoferrites synthesized by flash autocombustion

Cadmium ferrite (CdFe 2 O 4 ) and calcium-doped (Cd 0.5 Ca 0.5 Fe 2 O 4 ) nanoferrites were synthesized through a rapid flash autocombustion method followed by annealing at 600 °C. Their surface morphology, optical properties, and antifungal activities were systematically examined as components of a multifunctional nanomaterial platform. Two-dimensional atomic force microscopy revealed that calcium ion (Ca 2+ ) substitution modifies a relatively coarse, highly agglomerated surface into a finer and more uniform nanostructure, decreasing the mean particle size derived from AFM from 51.7 nm to 41.06 nm and increasing the surface factor, S, from 1.943 × 10 5 to 2.392 × 10 5 cm 2 g −1 . UV–Vis–NIR diffuse reflectance spectroscopy, analyzed with Tauc plots, revealed a notable blue shift in the fundamental absorption edge, accompanied by an increase in the direct bandgap from 3.0 eV in CdFe 2 O 4 to 3.3 eV in Cd 0.5 Ca 0.5 Fe 2 O 4 . This change aligns with the compositional tuning of the electronic structure in nanoferrites. Furthermore, substantial Urbach energies (1.36 eV for CdFe 2 O 4 and 1.14 eV for Cd 0.5 Ca 0.5 Fe 2 O 4 ) clearly indicate that low-energy absorption is predominantly governed by dense defect-related tail states rather than intrinsic band-edge transitions, highlighting an unusual defect landscape that significantly influences optical responses. The refractive index, derived from reflectance data, decreases slightly with Ca 2+ doping, offering additional avenues to manipulate optical constants relevant to nanophotonic and optoelectronic applications. Antifungal assays conducted against Aspergillus flavus and Candida albicans demonstrated that undoped CdFe 2 O 4 exhibits potent, broad-spectrum antifungal activity, with inhibition zones measuring 19 mm and 14 mm, respectively, and surpasses Amphotericin B against A. flavus. In contrast, Ca-doped Cd 0.5 Ca 0.5 Fe 2 O 4 maintains moderate activity exclusively against A. flavus and shows no activity against C. albicans . This composition-dependent antifungal specificity, coupled with morphology and defect-controlled optical properties, underscores the potential of calcium-doped CdFe 2 O 4 nanoferrites as a versatile nanoplatform. These findings hold promising implications for the development of nano-enabled antifungal surface coatings strictly for inanimate, high-risk industrial environments, contingent on minimizing cadmium leaching via polymeric encapsulation.