Reduction and modification of graphene derivatives with bioactive compounds

Green routes based on non-covalent functionalization approaches have been developed to prepare surface-functionalized graphene oxide (GO) nanosheets with bioactive compounds such as polyphenols. Amongst them, gallic acid (GA) and tannic acid (TA) are known to have strong antioxidant activity and can act as reducing and stabilizing agents of GO, resulting in reduced graphene oxide (rGO) derivatives with improved properties such as solubility in organic solvents, thermal stability and mechanical performance. Polyphenols act as crosslinkers between the GO  nanosheets, leading to higher thermal resistance and improved stiffness, strength and toughness. A synergistic effect of both GO and polyphenols on inhibiting bacterial growth has also been found. The Π-Π and H-bonding interactions are believed to be the reason for the improved properties. The combination of bioactive molecules and carbon nanomaterials is a useful method towards the development of green, low-cost, biodegradable nanocomposites to be used in biomedicine, energy storage devices or other enviromentally friendly applications. 

Methods
Scanning electron microscopy (SEM) analysis was performed with a Digital Scanning Microscope DSM-950 (Carl Zeiss, Germany) equipped with a tugnsten filament as electron source, working under vacuum at 20 kV. Prior to observation, samples were dried out and sputtered with a thin gold layer to avoid charging during electron irradiation. Examination of the micrographs was accomplished with the ImageJ software. Fourier-transformed infrared (FT-IR) spectra were acquired at 25 °C in the MIR region (500–4000 cm−1) with a resolution of 4 cm−1 using a Frontier infrared spectrophotometer (Spectrum Two, Waltham, MA, USA) working in ATR mode.

Raman spectra were acquired at RT using a high performance inVia confocal Raman instrument (Renishaw, Gloucestershire, UK) fitted with a Nd:YAG 532 nm laser. Five scans were at least recorded for each compound at a power of 500 mW. The acquisition of data was controlled with Windows®-based Raman Environment software v.2.3. Baseline correction method was applied to remove the baseline drift caused by fluorescence. Mechanical properties were tested using a Universal Testing Machine with a 100 N load cell at 1 mm/min loading rate and processed using Origing software version 8.5, Oringin Lab Corporation.