Reduction of graphene oxide by natural products and production of reduced graphene oxide-hydroxyapatite composites

Beside the well-known 3-D allotropes of carbon; like graphite and diamond, recently 0-D fullerenes and 1-D carbon nanotubes have been discovered; and the studies are focused on these allotropes. Graphene has been experimentally studied for over 40 years. Novoselov and Geim, synthesized graphene by a Scotch-tape method which includes mechanical exfoliation of graphite to single-layer graphene; and single-layer graphene was identified by a basic microscope in 2004. The discovery of this 2-D allotrope of carbon was awarded by Nobel Prize in physics in 2010. Graphene is a honeycomb lattice structure of one atom thick layer and comprises σ bonds between sp2 carbon atoms. This two dimensional sp2 carbon network structure makes it the thinnest and strongest material among others. Graphene has unique properties such as extremely high surface area to mass ratio (theoretically 2,630 m2g-1), excellent thermal conductivity (~5,000 Wm-1K-1, 10 times better than copper), optical transparency (~ 97.7 %). On the other hand, hydroxyapatite [HA: Ca10(PO4)6(OH)2], one of the bioactive materials, is a basic calcium phospate mineral. It is also major inorganic constituent of bones and teeth tissues. It has applications in orthopedic, dental and maxillofacial fields due to its binding potential to bone tissue, promotion ability of bone repair and excellent biocompatibility. However, it shows poor mechanical properties like low fracture toughness and high elastic modulus; and these properties makes them unsuitable for major load bearing applications. In the present study, it was focused on developing mechanically stronger HA composites by using green reduced graphene oxide (RGO) reinforcement to strengthen the negative features of HA. To our knowledge, there are no previous studies reported in the literature about production of green RGO-HA composites. For this purpose, graphene oxide was synthesized by Hummer's method; and this synthesized graphene oxide was reduced by applying plant-based method. In this method, Achillea nobilis (yarrow), Laurus nobilis (daphne), Lavandula agnustifolia (lavandula), Rosae caninae (rose hip), Salvia officinalis (salvia) and Melissa officinalis (melisa) extracts were used due to their rich chemical content such as flavonoids, ascorbic acid, pectins acting as reducing agents. The plant-based method has certain advantages over chemical-based methods such as being non-toxic, economic and usable for bio-applications. Therefore, the plant extract that showed the highest reduction efficiency among others was determined and the reduced graphene oxide by this plant extract was used as the second material to HA. The RGO-HA composites were produced via precipitation method at different concentrations of RGO in HA, 0.25, 0.5, 1 and 2% (wt) while pure HA was also produced for comparison. The samples were characterized by Ultraviolet-Visible Spectroscopy, Fourier Transform-Infrared Spectroscopy, X-Ray Powder Diffraction, Scanning Electron Microscope, Transmission Electron Microscope and Thermogravimetric Analysis. The mechanical properties of the samples have been evaluated by Universal Testing Machine. As a result, Melissa officinalis extract showed the optimum reduction efficiency on graphene oxide with the extraction conditions by sonication at temperature of 80°C for 2 h. Therefore, the reduced graphene oxide by Melissa extract (MRGO) sample was used as second material to HA. Furthermore, the characterization results showed that pure HA and RGO-HA composites were produced successfully and the mechanical properties of pure HA were improved with RGO reinforcement. Compared to pure HA, the compressive strength values increased 77.44%, 156.05%, 220.25% and 231.51% for 0.25, 0.5, 1, 2% (wt) RGO-HA composites, respectively. Thus, the produced RGO-HA composites would be open up for various future biotechnological applications such as bone tissue engineering and etc.