Conductive and air-stable Cu patterns were directly made on ordinary photo paper using a roller pen filled with nano-Cu ink, which was mainly composed of metallic Cu nanoparticles (NPs) capped with poly(N-vinylpyrrolidone) (PVP). The nano-Cu NPs were obtained via the reduction of Cu2+ ions by using an excess of hydrazine and PVP. The low sintering temperature (160 degrees C) in Ar atmosphere played an important role for the preparation of air-stable Cu patterns. The conductivity of a radio-frequency identification antenna made from nano-Cu ink was tested by a lamp, and its resistivity achieved 13.4 +/- 0.4 mu Omega cm. The Cu NPs were confirmed by means of X-ray powder diffraction and X-ray photo-electron spectra, and the Cu patterns were characterized by scanning electron microscopy and energy dispersive X-ray spectrometry. A mechanism for the high conductivity of the Cu pattern made from Cu NPs is proposed. (C) 2014 Elsevier B.V. All rights reserved.