Green Synthesis of Highly Monodisperse and Spherical Ag Nanoparticles by a Combination of Teucrium ramosissimum Desf. (Lamiaceae) Extracts with Emphasis on the Stabilizing and Capping Biomolecules

Here, the aqueous (Taquo) and hydroethanolic (Thydro) extracts of the Teucrium ramosissimum Desf. are used for the synthesis of one of the most spherical and monodisperse Ag nanoparticles (Ag NPs) reported by green chemistry. Several parameters, such as pH, amount of extract, reaction time, and reaction temperature, are investigated. The optimized Ag NPs are obtained at a pH value of ca. 10, using 1 mL of extract, a reaction time of 4 h, at 60 °C, and allowing an incubation period at room temperature. The average particle size ranged between 18 and 22 nm in all cases, and no significant differences were observed between the Taquo or the Thydro extracts. Apart from that, the principal bioactive molecules responsible for the reduction process were identified by nuclear magnetic resonance spectroscopy, and the molecules incorporated on the Ag surface were determined by headspace-solid phase microextraction (HS-SPME), followed by gas chromatography/quadrupole-mass spectrometry (GC–qMS). In both cases, we found that phenolic acids, phenylpropanoids, citric acid, and malic acid are molecules involved in the reduction process, and some of them are found on the Ag NPs surface in their oxidized form. Moreover, a stabilization study as a function of pH is also presented that confirms the high stability of our fabricated Ag NPs. UV–vis analysis confirmed the presence of the Ag plasmon band as well as the particle stability, and transmission electron microscopy images demonstrate that our synthesized Ag NPs are some of the most spherical and monodisperse biobased Ag nanosystems presented in the literature. Ag NPs were also analyzed by scanning electron microscopy, energy-dispersive X-ray spectroscopy, selected area electron diffraction, and Fourier-transform infrared spectroscopy.