Interdigitated microelectrodes integrated with CMOS sensing circuitry are presented in this work for detection of the neurotransmitter dopamine and as an immunosensor with sensitivity enhanced by gold nanoparticles. Microelectrodes covered by the silicon dioxide layer are fabricated through a maskless post-CMOS process. Charged biomolecules bind to the functionalized oxide surface after immobilization, producing a capacitive change for the underlying electrodes. Three electrode designs with gaps from 0.5 μm to 1 μm and capacitance values from 13 fF to 925 fF are tested. The capacitance increases with respect to the concentrations of nanoparticle and dopamine. The minimum detectable concentration of dopamine is 540 pM at a 1-Hz bandwidth.