Nanocomposites and nanomaterials 

Bioactive nanocomposites based on silica and plant extracts 
R.V. Ivannikov1, I.V. Laguta2, O.N. Stavinskaya2, O.A. Kazakova2, V.M. Anishchenko3, O.I. Dzjuba1, O.V. Severinovska2
1 M.M. Gryshko National Botanic Garden, Natl. Acad. of Sci. of Ukraine, Timiryazevska Street, 1, Kiev-01014, Ukraine. 
E-mail: icvmtt34@gmail.com
2 Chuiko Institute of Surface Chemistry, Natl. Acad. of Sci. of Ukraine, General Naumov Street, 17, Kiev-03164, Ukraine.
3 L.M. Litvinenko Institute of Physical-Organic and Coal Chemistry, Natl. Acad. of Sci. of Ukraine, Kharkivs'ke Hwy, 50, Kiev-02160, Ukraine.

Bioactive extracts were prepared from the leaves of the plants of Orchidaceae Juss. Composition of the extracts was investigated by means of HPLC and MALDI-MS methods. Phenolic acids and flavonoids were the main groups of bioactive compounds of the extracts. By means of adsorption of the extracts on fumed silica, nanocomposites with high antioxidant activity were prepared. The release of the phenols from silica surface into water and ethanol was studied. 
UV-Vis spectra of the desorbed compounds and reflectance spectra of the nanocomposites before and after desorption were compared with the spectra of the original extracts of the appropriate concentration. The distinctions in the spectra show that some components of the extracts are rapidly released into solution while other ones retain on the silica surface for the longer time. To explain these distinctions, the interaction between various components of the extracts and silica surface was studied using quantum chemical method. 
The adsorption of phenol compounds on silica surface was found to occur mainly due to the formation of hydrogen bonds between hydroxyl groups of the phenols and silanol groups of the surface. The smallest Gibbs free energy values of 5-6 kJ/mol were obtained for interaction of silica with such compounds as ferullic and gallic acids while the adsorption of caffeic acid, rhamnetin and quercetin were characterized by Gibbs energy of 40-45 kJ/mol. The common feature of the latter compounds is the presence of two hydroxyl groups in the phenol ring of the molecules. The oxygen/hydrogen atoms of these groups form the hydrogen bonds with the hydrogen/oxygen atoms of silanol groups, which are much shorter (0.17 nm) than the hydrogen bonds formed by other hydroxyl groups (up to 0.24 nm). This seems to be the main reason for much stronger interaction of such phenols with silica surface.