Nowadays, the development of real-time water quality monitoring sensors is critical. However, traditional water monitoring technologies, such as enzyme-linked immunosorbent assay (ELISA), liquid chromatography, mass spectroscopy, luminescence screening, surface plasma resonance (SPR), and analysis of living bioindicators, are either time consuming or require expensive equipment and special laboratories. Because of the low cost, self-sustainability, direct current output and real-time response, microbial fuel cells (MFCs) have been implemented as biosensors for water toxicity monitoring. In this paper, we report a microscale MFC biosensor to study the dose-response curve of exoelectrogen to toxic compounds in water. The microscale MFC biosensor has an anode chamber volume of 200 μL, which requires less sample consumption for water toxicity monitoring compared with macroscale or mesoscale MFC biosensors. For the first time, the MFC biosensor is exposed to a large formaldehyde concentration range of more than 3 orders of magnitudes, from a low concentration of 1 × 10 g/L to a high concentration of 3 × 10 g/L in water, while prior studies investigated limited formaldehyde concentration ranges, such as a small concentration range of 1 × 10 g/L to 2 × 10 g/L or only one high concentration of 0.1 g/L. As a result, for the first time, a sigmoid dose-response relationship of normalized dose-response versus formaldehyde concentration in water is observed, in agreement with traditional toxicology dose-response curve obtained by other measurement techniques. The biosensor has potential applications in determining dose-response curves for toxic compounds and detecting toxic compounds in water.