Plant-Assisted Synthesis of Nd-Doped ZnO Nanoparticles—Characterization and In Vitro Antimicrobial Assay

Green-synthesized pure and Neodymium-doped zinc oxide nanoparticles (ZnO NPs and Nd-doped ZnO NPs) were prepared using Vaccinium vitis-idaea L. (Ericaceae) leaf extract via a hydrothermal route. The structural, morphological, and magnetic characteristics of the nanoparticles were studied by powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS) and UV–Vis diffuse reflectance spectroscopy (DRS). The increasing of the Nd content leads to a slightly reduced band gap width value and increased crystallite sizes. The electron paramagnetic resonance (EPR) analysis demonstrated that Nd3+ incorporation significantly modifies the defect structure of ZnO lattice. The incorporation of Nd3+ ions results in a decreased concentration of intrinsic defects, including oxygen vacancies (g = 2.000), shallow donor/Zn-related centers (g = 1.953), Zn interstitials (g = 1.906), and Zn vacancy-related centers (g ≈ 2.11–2.127), accompanied by the formation of Nd–defect complexes. This fact indicates partial defect passivation and stabilization of the ZnO lattice. The antimicrobial activity of undoped and Nd-doped ZnO NPs was evaluated against five bacterial (Escherichia coli, Klebsiella pneumoniae, Salmonella enterica, Bacillus subtilis, Bacillus cereus) and two fungal (Candida albicans, Aspergillus niger) strains, using the Resazurin Microtiter Assay (REMA). Nd-doped ZnO NPs demonstrated enhanced antimicrobial activity compared to undoped samples, with the degree of potency depending on the Nd concentration. The strongest activity was observed for 2%Nd-doped ZnO NPs (MICs 0.15 and 0.2 mg/mL for B. cereus and E. coli, respectively). Overall, the effect was less pronounced in fungi (MICs 5 and 10 mg/mL for C. albicans and A. niger, respectively). Nd doping is proven to be an effective approach for targeted modification of the physicochemical and antimicrobial properties of green-synthesized ZnO NPs.