Antioxidant, Antibacterial, and Antiparasitary Activities of Green Nanoparticles Synthesized Using Water-Soluble Melanins of Fruits

Up to date, fruit melanins have not been employed to synthesize nanoparticles (NPs). Crescentia alata, Randia echinocarpa, and Vitex mollis fruit are used in traditional medicine, and their soluble melanins (SMes) have high antioxidant activity. These SMes were used to prepare silver (green-AgNPs) and gold (green-AuNPs) NPs and evaluate their antioxidant capacity (DPPH and FRAP), activity against human pathogens (bacteria and the parasite Hymenolepis nana), and toxicity (Artemia salina assay). All SMes were useful to synthesize green-AgNPs but only V. mollis SMe for green-AuNPs. Infrared spectroscopy, dynamic light scattering, and transmission electron microscopy (TEM) showed that SMe is on the green-NPs surface. TEM showed 13–31-nm spherical green-AgNPs and 2–16-nm spherical and cylindrical green-AuNPs. Green-AgNPs showed higher antioxidant (FRAP = 3.4–725.4 µmol TE g−1; DPPH = 10.9–748.2 µmol TE g−1) and antibacterial (MIC = 1.85–15 µg mL−1; MBC = 3.7–30 µg mL−1) activities than the chemical-NPs. Moreover, the green-NPs (25 mg mL−1) were active against H. nana with the following times (min) (paralysis, death): green-AgNPs (10–40, 15–90) and V. mollis AuNPs (28, 40). The toxicity of the green-NPs (LC50 = 61.6 to > 1000 μg mL−1) depended on the SMe employed in the preparation. V. mollis NPs were the less toxic: minimal toxicity for the AgNPs (LC50 = 826.91 μg mL−1) and non-toxic for the AuNPs (LC50 > 1000 µg mL−1). Thus, the SMes are useful to obtain green-NPs of high stability that showed activities against human pathogens, suggesting their potential to be used as alternative health treatments.