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Miniaturized complementary metal-oxide semiconductor (CMOS) capacitive sensors presented in this work are capable of providing sensitive detection in the sub-nM range for biosensing applications. A convenient post-CMOS fabrication process is proposed to make sub-μm interdigitated microelectrodes covered by inter-metal dielectric layer as the sensing interface. Capacitance change due to the electrode-analyte impedance is detected by the integrated continuous-time sensing circuit. For immunodetection of mouse IgG and anti-mouse IgG enhanced by 25% gold nanoparticles, the produced interface capacitance changes after specific bindings were -4, -7.4, and -17 fF for the 13-, 95-, and 925-fF microelectrode designs, respectively. Detection of the 200-nM neurotransmitter dopamine (DA) showed interface capacitance changes of 0.22,1.46, and 22 fF for the three designs. Based on the measured circuit noise of 14.2 μV/Hz1/2, the minimum detectable capacitance changes were 0.3, 2.9, and 22.5 aF, equivalent to detectable DA concentrations of 309, 394, and 203 pM, respectively.