This paper presents a novel implantable electro-chemical sensor with a built-in readout circuit, with an ultimate aim of measuring dopamine levels in human striatum. The chosen material as the ultra-micro electrode (UME) of the sensor is niobium wire around which a carbon nanospike layer is grown (Nb/CNS) by means of plasma-enhanced chemical vapor deposition. The main focus of this paper is the design of an ultra-low-power wide-range dual-slope analog-to-digital converter that is specifically designed for an Nb/CNS probe to measure Fast-Scan Cyclic Voltammetry (FSCV) current pulses within the range of +/- 2.56 mu A. The dual-slope ADC has a sampling rate of 10 kHz with a 10 MHz clock frequency while requiring an average power consumption of 1.57 mu W. The circuit has a root-mean-squared input noise of 30 pA, a resolution of 100 pA and a dynamic range of 58 dB with a 10-bit output.