There has been growing interest in preparing silver nanoparticles (AgNPs) under mild conditions in short time and using AgNPs to detect hydrogen peroxide. Here, we report the facile preparation of AgNPs by co-incubating AgNO3 and ionic liquid (IL) 1-carboxymethyl-3-methylimidazolium nitrate (HO2CMIMNO3) modified carbon dots (NO3-CDs) under mild circumstance in 10 s without extra reductant. Mechanism investigations indicate that the strong electronegativity and steric effect of the anionic moiety of IL endow the CDs high affinity toward Ag(+)and facilitates the fast fabrication of AgNPs. The resultant NO3-CD/AgNP hybrid exhibits absorbance fading behavior toward H2O2 and the decrease in absorbance at 415 nm provides accurate detection within 0.1-10 mu mol L-1, along with a detection limit of 0.087 mu mol L-1. The colorimetric method is successfully applied to H2O2 detection in urine and lake water sample with satisfactory recoveries. Moreover, the fluorescence "off-on" phenomena of NO3-CD/AgNP hybrid in the presence of H2O2, offers successful monitor on the fluctuation of H2O2 in human breast cancer (MCF-7) cells. This work provides a fast approach to CD/AgNP hybrid, and demonstrates its potential as sensing probe for H2O2 in lake water, urine, and living cells by making full use of the absorbance and fluorescence of the hybrid. [GRAPHICS] .