Synthesis of metallic silver nanoparticles and silver organometallic nanodisks mediated by extracts of Capsicum annuum var. aviculare (piquin) fruits
Silver-based nanostructures were prepared through reduction/oxidation reactions of aqueous silver nitrate solutions mediated by extracts of red fruits of the piquin pepper (Capsicum annuum var. aviculare) at room temperature. Detailed morphological and microstructural studies using X-ray diffraction, conventional and high-resolution transmission electron microscopy and selected area electron diffraction revealed that the product was constituted by three kinds of nanoparticles. One of them was composed of twinned metallic silver nanoparticles with a size of few nanometers. Other kind of particles was ultrafine disk-like single crystals of silver 4,4′-dimethyldiazoaminobenzene, being in our best knowledge the first time that this compound is reported in the form of nanoparticles. Both kinds of nanoparticles experienced processes of self-assembly and subsequent grain growth to form the third kind of nanoparticles. Such resulting nanostructures are monocrystalline and flattened metallic silver nanoparticles that have diameters around tens of nanometers, the  direction perpendicular to the particle plane, and are coated by a surface organometallic layer and residues of biomolecules. The ultraviolet-visible spectrum of the biosynthesized product showed a surface plasmon resonance (SPR) extinction band with an absorbance maximum at around 400 nm, thereby confirming the presence of fine Ag particles. Studies carried out by Fourier transform infrared spectroscopy indicated that the principal active compounds responsible of the reduction of the Ag ions are proteins and capsaicin (through the amino groups) and phenolic compounds (through hydroxyl groups).