NpFlamma® ROS 380 comprised of a physical assembly of H_​2O_​2 reactive fluorophore and catalytic additive into an amphiphilic stabilization colloid. This versatile reactive nanoparticle is hardly fluorescent in inactivated state yet displays a strong red-shifted emission upon oxidized by H_​2O_​2. The maxima of excitation/emission are at 380/450 nm. Since the emission is achieved by a chemical reaction, the interference between background fluorescence and the excitation light irradiation cannot occur. NpFlamma® ROS 380 allows a real-time imaging of low-level H_​2O_​2 involved in cellular processes along with oxidative stress. We offer NpFlamma® ROS 380 as a quantitative H_​2O_​2 selective fluorescence probe for biological and physiological analysis involving oxidative cell processes.