Formation-free multi-level resistive switching characteristics by using 10 nm-thick polycrystalline GeO x film in a simple W/GeO x/W structure and understanding of switching mechanism through redox reaction in H 2O 2/sarcosine sensing (or changing Ge°/Ge 4+ oxidation states under external bias) have been reported for the first time. Oxidation states of Ge 0/Ge 4+ are confirmed by both XPS and H 2O 2 sensing of GeO x membrane in electrolyte-insulator-semiconductor structure. Highly repeatable 1000 dc cycles and stable program/erase (P/E) endurance of >10 6 cycles at a small pulse width of 100 ns are achieved at a low operation current of 0.1 µA. The thickness of GeO x layer is found to be increased to 12.5 nm with the reduction of polycrystalline grain size of <7 nm after P/E of 10 6 cycles, which is observed by high-resolution TEM. The switching mechanism is explored through redox reaction in GeO x membrane by sensing 1 nM H 2O 2, which is owing to the change of oxidation states from Ge 0 to Ge 4+ because of the enhanced O 2− ions migration in memory device under external bias. In addition, sarcosine as a prostate cancer biomarker with low concentration of 50 pM to 10 µM is also detected.