This paper presents a low-power electrochemical sensor readout IC with a bi-directional potentiostat for accommodating a large dynamic sensing current range. The proposed chopper stabilization potentiostat, suppressing low-frequency noise at the front end and providing stable positive and negative potentials between the working and reference electrodes for bi-directional current sensing through a switched push-pull readout stage. The digital implementation of an oscillator-based current to frequency converter and frequency counter transforms the sensing current directly into digital codes. The design is realized in a 0.18μm CMOS process and occupies an area of 1.3mm2, including all passive components and pads. The proposed readout IC achieves a 0.9pA minimum detectable current in a current range of ± 18μA, an R2 linearity of 0.999, and a dynamic detectable current range of 152dB. The static power consumption of the proposed readout IC is 2.9μW from a 1.5V supply.