Abstract <p>Spinel CoFe₂O₄ nanoparticles (NPs) and Lanthanum (La) doped CoFe₂O₄ NPs was achieved through simple sol–gel method and analyzed their structural, morphological, and optical properties by various characterization techniques including X-ray diffraction (XRD), Fourier transform infrared (FT-IR), field emission scanning electron microscopy (FE-SEM), energy dispersive X-ray (EDX), UV–Vis diffuse reflectance spectroscopy (UV-DRS), thermo gravimetric (TGA) and photoluminescence (PL) studies. XRD spectra confirmed the spinel structure with crystalline nature of products. The average crystallite size (nm) of the samples were calculated by Sherrer formula and found 19.6 nm for CoFe₂O₄ and 16.1 nm for La doped CoFe₂O₄ respectively. FT-IR spectra showed stretching band for pure CoFe₂O₄ at 543 cm^–1 and 485 cm^–1, whereas, for La-CoFe₂O₄ at 547 cm^–1 and 411 cm^–1 corresponding to the Fe-O and Co-O bond, respectively. The bandgap value was calculated using UV-DRS and obtained 1.45 and 1.64 eV for pure CoFe₂O₄ and La-CoFe₂O₄ NPs, respectively. PL spectra of pure CoFe₂O₄ and La doped CoFe₂O₄ NPs are excited at 465&#xa0;and 467 nm, respectively, which are related to electron and hole recombination in the valence and conduction bands. TGA characterization revealed 0.5% weight loss for the pure CoFe₂O₄ NPs, whereas 0.4%&#xa0;weight loss for La-CoFe₂O₄ NPs, which confirms that the samples are thermal stability. FE-SEM characterization exhibited well-defined spherical shaped grains with minimal agglomeration. In photocatalytic dye degradation (PCD) applications, pure CoFe₂O₄ exhibited 86%, whereas, La doped CoFe₂O₄ exhibited 96% efficiency of degradation of methylene blue (MB) dye within 240 minutes. La doped CoFe₂O₄ exhibit higher antimicrobial activity against <i>Bacillus subtilis, Streptococcus pyogenes, Pseudomonas aeruginosa, Escherichia coli</i> and <i>Candida albicans</i> than pure CoFe₂O₄.</p>