The simultaneous coupling and reduction of graphene oxide (GO) with diatom silica (Amphora sp., Navicula ramossisira and Skeletonema sp.) were demonstrated in this work. Binding of GO with diatom silica via direct esterification reaction at 100 degrees C was observed as well as the reduction of GO. The Raman spectra of GO-diatom silica revealed the typical peaks for reduced graphene oxide at 1350 cm(-1) (D band) and 1585 cm(-1) (G band). Infrared spectroscopy also showed the presence of a unique peak at 1260-1300 cm(-1) indicative of Si-O-C=O bond formation. This confirms the successful functionalization of GO with silica. Scanning electron microscopy showed the presence of GO on the diatom. For the pennate diatoms, Amphora sp. and N. ramossisira, their pores were closed demonstrating that GO was able to cover the surface of the diatom via the Si-O-C bond formation. For the centric diatom, Skeletonema sp., GO was found to be on its rib cage-like body structure and on its centric top. Electrochemical measurements by cyclic voltammetry using a redox probe, K-3[Fe(CN)(6)], showed that GO-Amphora and GO-Navicula had more surface negative charge compared with bare GO or bare diatom silica. Furthermore, they demonstrated similar surface charge characteristics as the chemically reduced GO (by hydrazine hydrate). This implies that the composite (reduced GO-diatom) can possibly replace chemically reduced GO (by exposure to hydrazine vapor) and it could probably function as an electrode in sensing cationic biomolecules.