The synthesis of monodisperse silver nanoparticles with plant extracts.

Plant extracts are known for their antihyperglycemic, antioxidant, antimutagenic, antifungal, anti-inflammatory, antiviral and antibacterial properties. These biological properties make plant extracts interesting surface modifiers of nanoparticles (NPs), which are also known for their unique features. Plant extracts can play a multifunctional role in the synthesis of NPs (i.e. can act as a reducing agent, stabiliser and bioactive compound), which gives additional properties to the final hybrid material. The combination of an extract of natural origin with NPs results in bioconjugates with unique final properties that often may not be obvious. The properties of a bioconjugate depend on both the plant extract (chemical composition, amount, a method of conjugation to NP surface, etc.) and the NPs (type, size, shape, polydispersity, etc.). Syntheses of NPs with plant extract usually lead to polydisperse particles with heterogeneous properties that are difficult to control from a biological point of view. In this paper, we present a synthesis protocol to obtain monodisperse silver nanoparticles (AgNPs) with plant extract. Cacao beans and grape seed extracts were selected as natural sources of polyphenols having biomedical importance (e.g. catechin, tannic acid, epicatechin gallate) and used to synthesise using a chemical reduction method. Syntheses were carried out with different molar ratios of reagents to find the best conditions for obtaining AgNPs that are homogeneous in size and shape. The colloids obtained were characterised with ultraviolet-visible (UV-Vis) spectroscopy, scanning transmission electron microscopy (STEM) and dynamic light scattering (DLS). It was found that syntheses carried out only with plant extract resulted in unstable colloids containing polydisperse nanoparticles. Stable colloids containing spherical monomodal particles were obtained by the incorporation of sodium citrate as an additional reagent in the synthesis mixture. The results obtained clearly indicate the crucial role of sodium citrate in the synthesis of spherical AgNPs of controlled size using plant extracts for biological applications.