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The design of optical nanosensors based on carbon nanozymes has gained considerable interest in analyte detection. In this study, a simple and rapid fluorescence/colorimetry probe, NH 2 GQDs/o-phenylenediamine (OPD), was constructed by combining amino-functionalized graphene quantum dots (NH 2 GQDs) and o-phenylenediamine. In the presence of NH 2 GQDs/H 2 O 2 , OPD was oxidized to 2,3-diaminophenazine (DAP), giving fluorescent and colorimetric signals, which can be used to quantitatively measure H 2 O 2. Furthermore, NH 2 GQDs/OPD/ H 2 O 2 exhibited high selectivity and sensitivity for detecting ascorbic acid (AA) with a detection limit of 0.062 μM using the fluorescent method. Its selectivity for AA was ascribed to the formation of the condensation compound of DAP with dehydroascorbic acid rather than the quinoxaline path for dopamine and glucose. The colorimetric sensing system NH 2 GQDs/OPD displayed yellow in the presence of H 2 O 2 , while it changed to green upon adding AA. This sensing system was successfully applied for imaging H 2 O 2 and AA in human cervical cancer cells and onion tissues. Additionally, the flexible sensing system can be used to fabricate the logic gate "AND." The results provide a valuable method for its potential application in the biosensing, clinical medicine, and food industries. [Display omitted] • A dual optical sensing system was constructed by combining amino-functionalized graphene quantum dots and o-phenylenediamine. • The amino-functionalized graphene quantum dots exhibit high peroxidase activities. • The sensing system was used to cascade detection of H 2 O 2 and ascorbic acid in living cells. • The fluorescent mechanism is based on IFE and the formation of new fluorescent species. [ABSTRACT FROM AUTHOR]