In this paper, a double-junction n+/n-/n+ polysilicon nanobelt selectively functionalized with platinum has been studied for hydrogen sensing application. The selective modification of the devices is performed by the combination of localized ablation of a resist and a lift-off process of e-beam evaporation of a catalyst material. The coverage of a Pt layer on the n- region is precisely controlled by adjusting Joule heating bias and pulse length. The Pt-functionalized devices show a rapid response to hydrogen with a limit of detection of only 5 ppm. The device with fully Ptcovered n- region is optimum for obtaining the best response to hydrogen.