Nanocomposites and nanomaterials

Properties of natural heteropolysaccharide xanthan in aqueous solutions and its compositions with cobalt nanoparticles
Permyakova N.M.1, Zheltonozhskaya T.B.1, Klepko V.V.1, Kozak N.V.1, Klymchuk D.O.2
1 Department of Polymer Physics, Institute of Macromolecular Chemistry of the NAS of Ukraine. 48 Kharkivske Shosse, Kyiv-02160, Ukraine.
E-mail: permyakova@ukr.net
2 Laboratory of Electron Microscopy, M.G. Kholodny Institute of Botany of the NAS of Ukraine, 2 Tereshchenkovska St., Kyiv-01601, Ukraine.

Natural polysaccharides are well known for their biocompatibility, non-toxicity and biodegradability. Among them, the heteropolysaccharide xanthan of microbial origin plays an important role in food and non-food industries as a matrix for tablets, nano- and microparticles, hydrogels. Due to its structure, xanthan can generate various types of interactions with other small molecules and macromolecules, forming interesting nano- and microstructures. In the present work, a xanthan sample produced by the bacterium Xanthomonas campestris was characterized in detail and used as a template for the in situ synthesis of cobalt nanoparticles (CoNPs) in an aqueous medium. 
The molecular weight of xanthan (1170 kDa) and its electrochemical properties, viscosity behavior and structure in aqueous and aqueous/Co-salt solutions were determined using viscometry, potentiometric titration, FTIR spectroscopy and TEM. A significant excess of carboxylate groups (10.24 mol∙kg-1) compared to carboxylic groups (1.29 mol∙kg-1) in xanthan macromolecules was established. The absence of a polyelectrolyte effect in aqueous solutions of xanthan in a wide range of concentrations was also revealed. In the presence of Co-salt, xanthan interacted with metal ions that led to the compaction of its macro coils.      
The process and products of the borohydride reduction of Co-salt in xanthan solutions were studied depending on the concentration of matrix and metal salt (and reducing agent) using photography, FTIR spectroscopy and TEM. An increase in the stability of black cobalt nanoparticles in reaction mixtures was established with an increase in the concentration of the Co-salt from 0.08 to 0.32 kgм-3 and the molar ratio of NaBH4/Co-ions from 1 to 5. The influence of the concentration of xanthan matrices on the stability of CoNPs with respect to their aggregation and sedimentation, as well as on the yield of metal nanoparticles, is also shown. The resulting composition consisted of swollen xanthan coils ~47-97 nm in size containing small amorphous CoNPs ~1.3-9.8 nm in diameter.