A conducting polymer based composite material of poly(3,4-ethylenedioxythiophene) (PEDOT) doped with pure insoluble ionic liquid (IL), 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, was prepared through a cost-effective electrodeposition method. A very tiny amount of insoluble IL (less than 15 μL), containing 0.1 M conducting polymer monomer, was drop-coated on an electrode surface and then used for electropolymerization using a homemade electrochemical cell filled with aqueous supporting electrolyte. With this design, the consumption of expensive IL for the preparation of the PEDOT/IL composite was greatly reduced, and thus effectively reduced the cost. The prepared PEDOT/IL composite was found to be highly conductive and stable, and it exhibited a highly nanoporous microstructure and excellent electrocatalytic activity toward the oxidation of dopamine (DA). Based on the excellent stability and electrocatalytic activity of the PEDOT/IL nanocomposite, a highly stable, sensitive and selective DA sensor with a detection limit of about 51 nM was developed.