NOVELTY - A nanoparticle comprises an organic photovoltaic semiconductor polymer and a phospholipid. The semiconductor polymer is near-infra red absorbing and generates a detectable photoacoustic signal and a fluorescent emission when irradiated by incident activation energy.    USE - Nanoparticle used for in vivo molecular imaging (claimed).    ADVANTAGE - The nanoparticle can serve as an efficient and stable nanoplatform to convert photons into ultrasound waves, permitting in vivo photoacoustic (PA) molecular imaging with fine spatial resolution at depths beyond the optical diffusion limit. As semiconducting Tr-conjugated polymers (SPs) typically used for solar cells were designed to have higher absorption in the near-infrared region relative to those for organic LEDs, these photovoltaic near infrared-absorbing SPs are evaluated as building blocks for development of PA molecular imaging probes, and it is unexpectedly found that dependent on molecular structure of SPs, SPNs can show better photo stability and generate higher PA signal output than single-wall carbon nanotubes (SWNTs) and gold nanorods (GNRs), two of the best current exogenous PA contrast nanoagents. The high PA brightness in the near-infrared region and favorable small nanoparticle size allow for PA lymph node mapping in living mice after a single intravenous administration at a low dosing level (50 mu g).    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for:    (1) a pharmaceutical composition comprising the nanoparticle; and    (2) a method of molecular imaging comprising delivering to a human or non-human subject a pharmaceutical composition comprising nanoparticles comprising an organic photovoltaic semiconductor polymer, and a phospholipid a reactive oxygen species (ROS)-inactivated fluorophore, where the semiconductor polymer is characterized as near-infra red absorbing and generating a first detectable photoacoustic signal spectrum when irradiated by an incident activation energy; irradiating the human or non-human subject with a first incident energy having a first wavelength and generating a first photoacoustic signal; irradiating the human or non-human subject with a second incident energy having a second wavelength and generating a second photoacoustic signal; determining the intensities of the first and the second photoacoustic signals; determining a first ratio of the intensities of the first and the second photoacoustic signals; comparing the first ratio with a second ratio determined from the nanoparticles before delivery to the human or non-human subject, where a difference in the first and second ratios indicates RONS degradation of the nanoparticles in the subject; and generating a ratiometric image indicating the difference in the first and second ratios relative to an image of the subject.