A liquid-solid contact triboelectric nanogenerator (TENG) based on poly(tetrafluoroethylene) (PTFE) film, a copper electrode, and a glass substrate for harvesting energy in oil/water multiphases is reported. There are two distinctive signals being generated, one is from the contact electrification and electrostatic induction between the liquid (water/oil) and the PTFE film (V-TENG and I-TENG); and the other is from the electrostatic induction in the copper electrode by the oil/water interfacial charges (Delta V-interface and I-interface), which is generated only when the liquid-solid contact TENG is inserted across the oil/water interface. The two signals show interesting opposite changing trends that the V-TENG and I-TENG decrease while the oil/water interfacial signals of Delta V-interface and I-interface increase after coating a layer of polydopamine on the surfaces of PTFE and glass via self-polymerization. As an application of the observed phenomena, both the values of I-TENG and I-interface have a good linear relationship versus the natural logarithm of the concentration of the dopamine. Based on this, the first self-powered dual-signal detection of dopamine using TENG is demonstrated.