Provided are nanoparticles comprising an organic photovoltaic semiconductor polymer and a phospholipid, the semiconductor polymer being near-infra red absorbing and generating a detectable photoacoustic signal and a fluorescent emission when irradiated by an incident activation energy. Also provided are methods of molecular imaging, comprising delivering to a subject a plurality of said nanoparticles, irradiating the subject with a first incident energy to generate a first photoacoustic signal, irradiating the subject with a second incident energy to generate a second photoacoustic signal, determining a ratio of the intensities of the two photoacoustic signals, comparing this first ratio with a second ratio determined from the nanoparticles before delivery to the subject, whereby a difference in said first and second ratios indicates ROS degradation of the nanoparticles in the subject and generating a ratiometric image indicating the difference in said first and second ratios relative to an image of the subject.
