The transition from insulator to electro-responsive has been successfully achieved by earlier studies for some inorganic materials by applying external stimuli that modify their 3D and/or electronic structures. In the case of insulating polymers, this transition is frequently accomplished by mixing them with other electroactive materials, even though a few physical treatments that induce suitable chemical modifications have also been reported. In this work, a smart approach based on the application of an electro-thermal reorientation process followed by a charged gas activation treatment has been developed for transforming insulating 3D printed polymers into electro-responsive materials. First, the developed procedure has been exhaustively investigated for 3D printed poly(lactic acid) (PLA) and subsequently has been extended to 3D printed polypropylene (PP) and poly(ethylene terephthalate glycol) (PETG) specimens. FTIR and Raman spectroscopies, X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, and water contact angle measurements confirmed that, while the electro-thermal reorientation mainly promotes the crystallinity of the samples, the charged gas activation oxidizes the C–O bonds at the surface and consequently modifies the surface morphology and wettability. Furthermore, cyclic voltammetry assays demonstrated that treated PLA, PP, and PETG are electro-responsive, even though the electrochemical activity was much higher for oxygen-containing polymers (PLA and PETG) than for the oxygen-free one (PP). Finally, as a proof of concept, treated 3D printed PLA specimens have been used as electrochemical sensors to detect dopamine (DA), an important neurotransmitter, in a concentration interval ranging from 50 to 1000 µM. The peak associated with the oxidation from DA to dopaminoquinone and the linearity of the calibration plot, which was constructed using the anodic peak current, proved that treated PLA is not only electro-responsive but also able to electrocatalyze the oxidation of DA.