Herein, a novel enzyme-free ratiometric system was proposed for fluorescence sensing of uric acid (UA). A positively charged polyethyleneimine modified graphene quantum dots (PEI@GQDs) and negatively charged eosin Y were rationally admixed. The former acts as a fluorescence signal reporter while the latter represents an internal reference output. Addition of IO4−, an oxidizing agent, quenched the fluorescence of PEI@GQDs due to the isomerization of surface CN groups, which can be chemically reduced by the addition of UA. Simultaneously, the fluorescence emission signal of eosin Y remained unchanged. Under the optimum conditions, the fluorescence ratio (F460/F543) of PEI@GQDs/IO4− system was linearly increased with increasing the concentration of UA in the range of 0.05–180 µM with a detection limit (S/N) of 0.006 µM. The presence of high concentrations of reducing biological compounds e.g. ascorbic acid (AA) and glutathione (GSH)/cysteine (CYS) can affect the performance of the PEI@GQDs/IO4− system. The pretreatment steps were sufficient to oxidize AA, while the addition of maleimide masked the interference from common GSH/CYS. The system imposes some advantages such as simplicity, reproducibility, reliability, selectivity, and sensitivity. The PEI@GQDs/IO4− system was applied to detect UA in human serum and saliva samples. The results were compared with the results obtained from commercial UA kits, which well agreed, confirming the accuracy of the proposed system for the detection of UA. This strategy represents a new approach for the use of non-selective QDs as ideal probes for the detection of reducing substances with acceptable levels of selectivity and reliability.
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