Carbon dots (CDs) with efficient excitation and emission in deep-red/near-infrared (NIR) spectral range are important for bioimaging applications. Herein, we develop a simple and effective method to significantly enhance both the absorption and emission of CDs in deep-red/NIR by suppressing nonradiative charge recombination via deprotonation of the CD surface. As compared to aqueous solutions at room temperature, NIR emission of CDs in N,N-dimethylformamide and glycerol experience a 50- and 70-fold increase at -20 °C, respectively, due to enhanced deprotonation ability and viscosity. On the basis of the adjustable NIR fluorescence intensity of CDs, multilevel data encryption in the NIR region is realized by controlling the humidity and the temperature of a CD-ink stamped paper.