Ratiometric fluorescence and colorimetric detection for uric acid using bifunctional carbon dots

Iron, cobalt and nitrogen co-doped carbon dots (FeCo/N-CDs) were fabricated firstly by a one pot hydrothermal method in this study. As well as exhibiting peroxidase-like activity, the prepared FeCo/N-CDs emitted fluorescence at 475 nm upon the excitation of 360 nm. A dual-signal biosensor relied on ratiometric fluorescence and colorimetric method was developed for detecting uric acid (UA) based on FeCo/N-CDs-catalyzed oxidation of OPD. Uricase catalyzed the production of H2O2 from UA, and FeCo/N-CDs catalyzed H2O2 into the center dot OH radicals, contributing to the oxidation of 2,3-diaminophenazine (DAP) from o-phenylenediamine (OPD). The color from colorless to yellow was observed and DAP can quench the fluorescence of FeCo/N-CDs via the inner filter effect (IFE), accompanied with a significant fluorescence enhancement of DAP at 580 nm. The fluorescence emission and UV-vis absorption intensities were thus affected by sequential additions of UA. Quantitative analysis of UA was based on the fluorescence intensity of DAP/FeCo/N-CDs (I-5(80)/I-425) and color records. The calibration curve was linear within the range of 0.2-150 mu M and the limit of detection was low as 0.05 mu M. The biosensor was successfully applied to the detection of UA in serum and urine samples.