Using simple and commonly available devices for signal reading has attracted much attention recently for DNA-based sensors. Most previous works focused on electrochemical or colorimetric signals. In this work, a portable device was structured by simple electronic and 3D-printed components to convert fluorescence signal to electric resistance with reading by a multimeter. To demonstrate its application, we took advantage of the slow ligand bind kinetics of Cr3+ to promote DNA adsorption on metal oxide nanoparticles (MONPs). Nanoceria (CeO2), screened from a total of 13 kinds of MONPs, possesses high specificity for Cr3+ from other metal ions. EDTA can mask other metal ions, while it cannot bind Cr3+ quickly so that Cr3+ promotes DNA adsorption and thus fluorescence quenching. The linear range is from 0 to 2 mu M Cr3+ with a LOD of 0.02 mu M. Using common electronic components and the 3D-printed miniaturized device, the sensor to transfer fluorescence to electric resistant signal was also measured by a digital multimeter. In this case, the LOD is about 1 mu M Cr3+. Such a strategy may provide a simple and general approach for fluorescent portable device fabrication and may find applications for other onsite sensing applications.