Green tea polyphenol–reduced graphene oxide: derivatisation, reduction efficiency, reduction mechanism and cytotoxicity

Abstract

This paper reports on the derivatisation, reduction efficiency, reduction mechanism and cytotoxicity of green tea polyphenol–reduced graphene oxide (GTP–RGO). The reduction of graphene oxide (GO) at 90 °C using a weight ratio (WR) of GTP/GO = 1 resulted in the production of a stable GTP–RGO dispersion in aqueous media, as indicated by the results of ultraviolet-visible (UV-Vis) spectroscopy, Fourier transform infrared (FTIR) spectroscopy, thermogravimetric analysis (TGA) and the measurement of zeta potential and electrophoretic mobility. In addition, the results from UV-Vis spectroscopy and X-ray photoelectron spectroscopy (XPS) analysis indicated the comparable reduction ability of GTP relative to the standard reducing agent, hydrazine (N₂H₄). The removal mechanism of epoxy groups from GO via reduction reaction with GTP was investigated by implementing the hybrid functional method of Becke-3-parameters-Lee–Yang–Parr (B3LYP) using Gaussian 09 software. The energy and frequency calculations showed that the GO reduction using GTP was more spontaneous and took place relatively faster than the reduction using N₂H₄, as evidenced by a higher entropy change (ΔS) (0.039 kcal mol⁻¹ K⁻¹) and lower Gibbs free energy (ΔG) barrier (58.880 kcal mol⁻¹). The cytotoxicities of the GO and GTP–RGO samples were evaluated against human colonic fibroblast cells (CCD-18Co). The GO sample was determined to be toxic even at low concentration (6.25 μg mL⁻¹), while the GTP–RGO sample possesses notably low toxicity at the same concentration. The cell culture experiments revealed that the incorporation of GTP led to a decrease in the toxicity of GTP–RGO samples.