Fluorescent effect obtained by encapsulation of vegetal extracts in appropriate silica-derived networks

The sol-gel method allows the physical trapping or the covalent bonding of many types of organic species inside inorganic or hybrid networks. The combination of organic and inorganic materials represents an attractive approach for purpose of creating some advanced optical hybrid materials with high performance and functionality. Sol-gel optics represents the most active field in the use of hybrid materials. Among the various inorganic/hybrid hosts, silica-derived matrices are preferred being indeed the best candidates for their superior mechanical, thermal and most important - optical properties. In fact, these host networks present a high flexibility in the final material property design (e.g. refractive index tunability) and a broad optical window (they are optically transparent glasses with excellent photonic media as a result of their low optical losses, have unparalleled optical properties and so on). In this context, the main advantage that may be fructified is derived from the possibility of embedding some organic chromophores in appropriate hosts, by coupling the hardness and optical transparency of silica-derived networks with optical properties and bioactivity of natural organic dyes, in order to make desired functional materials. Synthesis of some novel fluorescence nanomaterials loaded with bioactive polyphenols which are present in most plants with a high spectrum of biological activity, by replacing synthetic chemicals, may open new opportunities for optical and bio-medical applications. From this point of view, a driving force in this study was the possibility to create multifunctional materials with fluorescent properties by incorporating of some organic molecules with specific properties (e.g. vegetable extracts) in silica and hybrid silica-silsesquioxane matrices, by using a mild sol-gel encapsulation conditions which enables the reproducible and efficient confinement of selected molecules inside selected silica networks. The fluorescent hybrid nanomaterials are synthesised using a self-assembly approach in which the surfactants are assembled with inorganic and hybrid silica matrices into sophisticated nanostructures through favourable weak interactions. These weak interactions (e.g. hydrogen bonds) between organic molecules and different sol-gel matrices investigated by FT-IR and UV-VIS spectroscopy showed an important role in the optical properties of many natural extracts. As a result of vegetal extract entrappment, all synthesized hybrid materials manifest a meaningful increase of fluorescent signal (by more than twenty times), due to various factors: the physical adsorption of polyphenolic molecules from extract which does not affect the basic chemical structures; well known optical properties of selected silica-derived matrices that lead to enhancement of the transmission of optical signal within the matrix; tridimensional network of silica that assured an optimum conformational arrangement, without allowing a mobility of organic compounds and neither side reactions occurring from chemical point of view. The size distribution and morphology of the synthesised hybrid fluorescence materials were evaluated by dynamic light scattering and electronic microscopy analyses. For all entrapped samples, a narrow size distribution was observed, with good polydispersity indexes. According to TEM images, the average size of hybrid nanomaterials is of about hundred nanometers. As expected, the samples based on silsesquioxane exhibit a lower size, comparative to samples prepared by using an inorganic silica matrix. The results obtained clearly revealed the power of hybrid materials to generate complex systems that exhibited better fluorescent properties, the obtained hybrid materials being favorable potential optical systems for a variety of applications due to their outstanding properties based on the mixture of natural organic compounds and appropriate silica host matrices.