Tunable green graphene-silk biomaterials: Mechanism of protein-based nanocomposites Fang Wang a,b,1 , S.S. Jyothirmayee Aravind a,c,1 , Hao Wu d , Joseph Forys a , Venkat Venkataraman d , Kandalam Ramanujachary c , Xiao Hu a,e,f, ⁎ a Department of Physics and Astronomy, Rowan University, Glassboro, NJ 08028, USA b Center of Analysis and Testing, Nanjing Normal University, Nanjing 210023, China c Department of Chemistry and Biochemistry, Rowan University, Glassboro, NJ 08028, USA d Department of Cell Biology, Rowan University School of Osteopathic Medicine, Stratford, NJ 08084, USA e Department of Biomedical and Translational Sciences, Rowan University, Glassboro, NJ 08028, USA f Department of Biomedical Engineering, Rowan University, Glassboro, NJ 08028, USA abstract article info Article history: Received 30 March 2017 Received in revised form 12 May 2017 Accepted 16 May 2017 Available online 17 May 2017 Green graphene materials prepared by photoreduction of graphite oxide were first time blended with aqueous- based silk fibroin proteins to improve the mechanical and thermal properties of silk biomaterials, and their nano- composite interaction mechanism was illustrated. Powder X-ray diffraction (XRD) analysis confirmed the com- plete exfoliation of graphite oxide to graphene in presence of focused pulses of solar radiation. By varying the concentration of graphene (0.1 wt% to 10 wt%), a series of free standing graphene-silk films were prepared and were systematically characterized by Fourier transform infrared spectroscopy (FTIR), scanning electron mi- croscopy (SEM) and nanoindentation measurements. The homogeneity of graphene in silk as well as the thermal stability of the composite films was demonstrated by thermal gravimetric analysis (TGA) and temperature-mod- ulated differential scanning calorimetry (TMDSC). Surprisingly, silk composite film containing only 0.5 wt% of graphene gives the highest Young's modulus of 1.65 GPa (about 5.8 times higher than the pure silk's modulus), indicating a nano-composite to micro-composite transition of silk-graphene structure occurred around this mixing ratio. This finding provided an easy approach to improve the elastic modulus and other physical proper- ties of silk materials by adding a tiny amount of graphene sheets. Fibroblast cells studies also proved that these graphene-silk materials can significantly improve cell adhesion, growth and proliferation. This protein nanocom- posite study provided a useful model to understand how to manipulate the hydrophobic-hydrophobic and polar- polar interactions between high-surface-area inorganic nanomaterials and amphiphilic protein materials, which has many emerging applications in the material science and engineering, such as bio-device fabrication, drug storage and release, and tissue regeneration. © 2017 Published by Elsevier B.V. Keywords: Silk Graphene Elastic modulus Nanocomposite 1. Introduction Hybrid and inorganic materials have remarkable properties for de- livery [1–3], therapy [4,5] and sensor applications [6–9]. Recently, two-dimensional graphene materials with only an atom thickness have paved novel ways for the development of various functional mate- rials. Several studies have demonstrated fascinating attributes of graphene such as superior electrical and thermal transport properties as well as its high mechanical strength [10–12]. Furthermore, the high surface area of graphene makes it an ideal conducting matrix for anchoring nanomaterials [13]. Different thermal [14], chemical [15], ep- itaxial growth [16] and photo reduction techniques [17–18] have been reported to produce single-layer graphene sheets and manipulate its properties for a variety of device applications. Graphene has also been investigated for varied applications in nanoelectronic and energy stor- age devices, biomedical sensors and was used as a critical component for nanocomposite materials recently [19–22]. There has been a great interest in developing graphene-polymer composites to completely exploit the superior properties of graphene in a flexible material. However, there are still many challenges associat- ed with the development of graphene-polymer composites. For exam- ple, delicate morphological organization, fine interface control and uniform dispersion of graphene into polymers are crucial factors on determining the performance of the resulting composite. These requirements arise largely from the surface properties of graphene, Materials Science and Engineering C 79 (2017) 728–739 ⁎ Corresponding author at: Department of Physics and Astronomy, Rowan University, Glassboro, NJ 08028, USA. E-mail address: hu@rowan.edu (X. Hu). 1 Fang Wang and SS Jyothirmayee Aravind contributed equally to this manuscript. http://dx.doi.org/10.1016/j.msec.2017.05.120 0928-4931/© 2017 Published by Elsevier B.V. Contents lists available at ScienceDirect Materials Science and Engineering C journal homepage: www.elsevier.com/locate/msec