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 •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 (I580/I475) and color records. The calibration curve was linear within the range of 0.2–150 μM and the limit of detection was low as 0.05 μM. The biosensor was successfully applied to the detection of UA in serum and urine samples.