Blue luminescent carbon dots (CDs) with a high photoluminescence (PL) quantum yield (48.3 +/- 5.3%) were prepared by the one-pot hydrothermal reaction of citric acid with poly(ethylenimine) (PEI). The CDs display bright PL, narrow emission spectra, pH-dependent PL intensity, high photostability, and up-converted luminescence. The CDs exhibit a quenching of both down- and up-conversion PL in the presence of morin and thus serve as useful probes for morin detection. Both down- and up-conversion measurements allow the quantification of concentrations from 0 to 300 mu mol/L with a detection limit of 0.6,mu mol/L, and this dual-mode detection increases the reliability of the measurement. The proposed method of determination is simple, sensitive, and cost-effective, with potential applications in clinical and biochemical assays.