Facile, environmentally benign and scalable approach to produce pristine few layers graphene suitable for preparing biocompatible polymer nanocomposites

The success of developing graphene based biomaterials depends on its ease of synthesis, use of environmentally benign methods and low toxicity of the chemicals involved as well as biocompatibility of the final products/devices. We report, herein, a simple, scalable and safe method to produce defect free few layers graphene using naturally available phenolics i.e. curcumin/tetrahydrocurcumin/quercetin, as solid-phase exfoliating agents with a productivity of ∼45 g/batch (D/G ≤ 0.54 and D/D′ ≤ 1.23). The production method can also be employed in liquid-phase using a ball mill (20 g/batch, D/G ≤ 0.23 and D/D′ ≤ 1.12) and a sand grinder (10 g/batch, D/G ≤ 0.11 and D/D∼ ≤ 0.78). The combined effect of π-π interaction and charge transfer (from curcumin to graphene) is postulated to be the driving force for efficient exfoliation of graphite. The yielded graphene was mixed with the natural rubber (NR) latex to produce thin film nanocomposites, which show superior tensile strength with low modulus and no loss of % elongation at break. In-vitro and in-vivo investigations demonstrate that the prepared nanocomposite is biocompatible. This approach could be useful for the production of materials suitable in products (gloves/condoms/catheters), which come in contact with body parts/body fluids.


S1 Planetary ball milling of graphite with curcumin/tetrahydrocurcumin/quercetin
The planetary ball mill used was a Restch PM400 with 4 grinding bowl fasteners. In a typical procedure 20 g of graphite was milled with 60 g curcumin/tetrahydrocurcumin/quercetin (1:3 ratio) and 2.5 g Darvan I (stabilizer) for 1 h at 100 rpm (15 min. grinding followed by 15 min. pause to avoid excessive heat production). Zirconia containers and zirconia balls were used for the ball milling procedure. After grinding, the samples are dispersed in water and sonicated for 2 min. at 25 % amplitude using a 750 W sonicator.

S2.1 XRD of quercetin exfoliated graphite
The XRD of Graphite:Quercetin:Darvan (1:3:0.125 before and after grinding) sample and graphite is shown as Figure S1. Figure S1: Normalized XRD of graphite and graphene produced using quercetin as the exfoliating agent (ratios correspond to Graphite:Quercetin:Darvan).

S2.2 Raman spectra of curcumin/tetrahydrocurcumin/quercetin exfoliated graphite
The Raman spectra of Graphite:Quercetin:Darvan sample and graphite is shown as Figure S2.
The shape of 2D band 1:3:0.125 Graphite:Quercetin:Darvan shows that the produced graphenes are about 10 layers 1-3 thick. Figure S2: Raman spectra of graphene samples produced using quercetin as the exfoliating agent.
The 2D band of all Graphite:Curcumin:Darvan samples is shown as Figure S3. Here again, a clear shift of the 2D band to the lower wavelength is visible. The shape of 2D band in 1:3:0.125 Graphite:Curcumin:Darvan shows that the produced graphenes are few layers [1][2][3][4][5] Table S1.

S2.3 TEM analysis
The TEM images of 1:3:0.125 Graphite:Quercetin:Darvan sample is shown as Figure S6. The images clearly show that the produced graphene is multilayered (≥ 5) evidenced by the reduced transparency of the graphene sheets. The normalized XRD spectrum of graphite, curcumin & Graphite:Curcumin:Darvan samples with varying solvent ratios (acetone:water) is shown as Figure S7.

S3.2 Raman Spectra
The Raman spectra of Graphite:Curcumin:Darvan samples prepared using different solvent ratios (acetone:water) are shown as Figure S8. This demonstrates that in the case of Graphite

S4.2 Raman Spectra
The Raman spectra of graphite and graphene (Graphite:Curcumin:Darvan,1:3:0.125) produced using sand grinding process, along with the deconvolution of 2D band is shown as Figure S11 and the corresponding I D /I G ratios in table S3.

KMnO 4
Corrosive. 123   (c) Regulation of compound: The graphene-NR Latex was stabilized by addition of 1% ammonia and had a solids content of 48 ±3 %. The mixture was stirred slowly to ensure homogeneity and the optimum cure was checked using the following chloroform test.

S.6 Computational studies on interaction of graphene with curcumin
In a 50 mL beaker 5 mL of the above made NR latex was transferred. Then, 5 mL of chloroform was added to the same and stirred the mixture gently and continuously to obtain a coagulum. This mixture was then kept between two filter papers and pressed. This solid material was broken off gently and judged the nature of cure. All the nanocomposites were made from normal cure NR latex.
Graphene incorporated natural rubber nanocomposite thin films were produced using the following steps; (1) Pre-treatment of glass molds (2) Latex dipping (3) Vulcanization followed by stripping the thin film from mould using silica powder.
Prior to each dipping procedure cleaning of glass molds were done by brushing with detergent water followed by washing with hot water. Later, the molds were dried in hot air oven at 70 °C .
For lab scale production, we have used a semi-automatic type dipping machine supplied by PLASTOMEK Private Ltd. India. The thickness of the produced nanocomposite thin film samples can be controlled by varying the dipping speed.
Subsequent to each dipping, the skim and cream on the NR latex surface were removed using a cloth or sponge. The presence of any bubbles was also removed in order to avoid any weak spots on the thin films. The mold fitted on machine is then dipped slowly into the NR latex at a rate of speed of dipping from 1-1.5 cm/sec. After immersing mold up to a required length, the mold was withdrawn slowly and rotated the mold to ensure a uniform flow of the NR Latex. Then, the mold was kept in hot air oven for drying at 70 °C for 2-3 min. After drying and cooling, a second dipping was done and kept for drying again at 70 °C for 2-3 min. For vulcanization, the thin films on the mold are transferred to a hot air oven and heated the product for about 45 min. at a temperature of 80 ± 5 °C .

S8.1 TEM analysis
Morphology of the few layer graphene NR thin film nano-composites was analyzed using a JEOL JEM-2010TEM at 200 kV. Samples were prepared by cryo-microtoming at -70 ºC.

S8.1.1 TEM analysis of Graphite:Curcumin:Darvan-NR thin film nanocomposite
The TEM images of 1.5 phr Graphite:Curcumin:Darvan containing NR thin film nano-composite are shown as Figure S14. The samples show a network like structure.

S8.2 Stability of curcumin under the processing condition
1.5 phr few layer graphene -NR thin film nano-composite weighing about 1.11 g was Soxhlet extracted using200 mL of dichloromethane for 2 h. After completion of extraction, a greenish orange color extract was obtained which was then cooled, concentrated using rotavapor to get a greenish orange precipitate.

Characterization of curcumin (obtained after Soxhlet extraction) by High Performance
Liquid Chromatography (HPLC): The HPLC system consisting of Agilent 1260 series PDA detector was used for this study.
The mobile phase comprised of solvent A: water (1% acetic acid) which was adjusted to pH of