A self-powered sensor for toxic materials can be the most desirable and promising prototype of the future monitoring systems for environmental protection/detection. Wiring power-source to sensors that are placed in every corner of our surroundings not only causes a vast amount of labours, resources and budget, but also potential contamination to be produced by the batteries. Although solar energy is most attractive, it depends on the weather, season and has day and night difference. However, regardless of day and night, mechanical energy is one of the most abundant/sustainable sources practically to be utilized through piezoelectric materials. ZnO nanowires (NWs) not only have piezoelectricity, but also are environmental friendly and biological compatible, which make it a strong candidate for energy-harvesting for environmental sensors. Here, we have demonstrated a fully stand-alone, self-powered environmental sensor driven by nanogenerators with harvesting vibration energy. Such a system is made of ZnO nanowire-based nanogenerator, a rectification circuit, a capacitor for charge storage, a signal transmission LED light and a carbon nanotube-based Hg²⁺ ion sensor. The circuit lights up the LED indicator when it detects mercury ions in water solution. It is the first demonstration of nanomaterial-based, self-powered sensor system for detecting a toxic polluter.