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Hybrid diamond/graphite nanostructures were synthesized in CH4/H2 mixture gas using microwave plasma enhanced chemical vapor deposition (MPCVD) at a power of 10 kW. The microstructure and the composition of the films were characterized by scanning electron microscope (SEM), tran mission electron microscope (TEM), Raman spectroscopy and X-ray diffraction (XRD). Special attention has been paid to the influence of the methane concentrations on the microstructures of diamond films, which shows a gradual transition from nanocrystalline to microcrystalline films, and finally displays a hybrid diamond-graphite nanostructure with the length of a few micrometers. A single crystalline diamond core being about 6.5 nm in diameter and graphitic shells with the thickness of a few nan meters has been revealed by TEM analysis. The formation mechanism of diamond nanostructures enveloped with graphite phase by MPCVD without introducing nitrogen in plasmas was discussed. The as-deposited diamond films at high methane level in hydrogen plasma show good conductivity and excellent electrochemical activity, owing to the co-existence of diamond and graphite phases in the films. The largest electrochemical potential window of the hybrid diamond/graphite films is determined to be 3.1 V, which is comparable with the B-doped diamond. The films exhibit quasi-reversible, mass controlled electrode reactions in both aqueous and organic solutions. In the application of trace heavy metal ion detection, the hybrid diamond/graphite electrodes present low background currents and detection limits (S/N ≥ 3): ~1.5 μA/cm2 and 5.8 ppb for Ag+, ~4.7 μA/cm2 and 5.6 ppb for Cu2+. The diamond/graphite electrodes also possess good linearity over a wide concentration range from 10 ppb to 1 ppm. In addition, the simultaneous determination of silver and copper ions was also successful. Hence, the hybrid diamond/graphite films are promising for electrochemical applications such as trace heavy metal ions detection because of its wide potential window, lower background current and high sensitivity.