This study reports on the use of thin films of polycrystalline diamond grown on doped silicon and titanium substrates for in vitro electrophysiological sensing devices. The electrical properties of the polycrystalline diamond surfaces were evaluated using electrochemical impedance spectroscopy, and electrical noise measurements. The polycrystalline diamond surface when immersed in the cell culture medium, establishes an electrical double-layer which in series with the bulk solution (cell culture medium) behaves as a classical Maxwell-Wagner relaxation with a relaxation frequency located at approximately 2 kHz. The low frequency (100 Hz) interfacial capacitance has a value of 10 µF/cm2. Furthermore, the interfacial resistance is low which minimizes the 1/f noise as well as the thermal noise, which is as low as 0.2 µV rms for a sensing electrode with an active area of 0.16 cm2. The high interfacial capacitance associated with the low thermal and 1/f noise makes these polycrystalline diamond coatings particularly suited to measure weak bioelectrical signals generated by non-electrogenic cells. Unlike neurons, non-electrogenic cells generated weak signals (micro-volts) in the mHz frequency range. The diamond coatings with different surface roughness were evaluated to record cell signals generated by the population of glial cells (C6 immortal cell line). Electrophysiological recordings were complemented with a detailed surface morphology analysis to gain insight into the best diamond morphology to minimize the low-frequency electrical noise and achieve bioelectrical recordings with a signal-to-noise ratio above 20 at frequencies as low as 1 Hz.