Function of thin film nanocrystalline diamond–protein SGFET independent of grain size

We employ nanocrystalline diamond (NCD) thin films with average grain sizes of 250 and 80 nm to resolve influence of grain boundaries and sp2 phase on protein adsorption and electronic function of solution-gated field-effect transistors (SGFET). Microscopic (20 μm × 60 μm) SGFETs are fabricated without gate oxides based on hydrogen-terminated NCD films on glass substrates. We show that NCD with grain sizes down to 80 nm and film thickness of 100 nm is fully operational as SGFET. We find that inherent hysteresis of SGFET transfer characteristics, their reaction time, their negative shift after protein adsorption and cell culturing process, low gate leakage currents and no influence of UV sterilization or rinsing are all independent of the NCD grain size. Thus we propose a microscopic model where the function of NCD SGFET is determined by the H-terminated surface of diamond nanocrystals and its interaction with proteins, not by grain boundaries or sp2 phase in general.