The authors describe a fluorometric and colorimetric nanoprobe for HO. The detection scheme is based on the in-situ formation of silver(I) ions from a composite consisting of nitrogen-doped carbon quantum dots (N-CQDs) and silver nanoparticles (AgNPs). A drastic change occurs both in fluorescence and color of the solution of the N-CQD/AgNPs composite. The fluorescence of composite (with excitation/emission peaking at 320/384 nm) is enhanced on increasing the concentration of HO due to the oxidation of silver metal in the N-CQD/AgNPs to form Ag(I) ions. The latter undergo strong coordination with the nitrogen atoms of the N-CQDs. In-situ formation of Ag(I) ions further results in a change in color of the solution from pale yellow (with a peak at 408 nm) to colorless. Under optimized conditions, the probe gives a fluorometric and colorimetric response in the 10 to 50 μM HO concentration range with a 4.7 μM limit of detection. The probe is highly selective over several potentially interfering ions and agents. It was successfully applied to the determination of HO in spiked samples without prior treatment. Graphical abstract Graphical presentation for specific detection of HO based on the in-situ formation of Ag(I) ions from a composite consisting of silver nanoparticles and nitrogen-doped carbon quantum dots.