Surface engineering of semiconducting polymer nanoparticles for amplified photoacoustic imaging

Despite the deeper tissue penetration of photoacoustic (PA) imaging, its sensitivity is generally lower than optical imaging. This fact partially restricts the applications of PA imaging and greatly stimulates the development of sensitive PA imaging agents. We herein report that the surface coating of semiconducting polymer nanoparticles (SPNs) with the silica layer can simultaneously amplify fluorescence and PA brightness while maintaining their photothermal conversion efficiency nearly unchanged. As compared with the bare SPNs, the silica-coated SPNs (SPNs-SiO₂) have higher photothermal heating rate in the initial stage of laser irradiation due to the higher interfacial thermal conductance between the silica layer and water relative to that between the SP and water. Such an interfacial effect consequently results in sharp temperature increase and in turn amplified PA brightness for SPNs-SiO₂. By conjugating poly(ethylene glycol) (PEG) and cyclic-RGD onto SPNs-SiO₂, targeted PA imaging of tumor in living mice is demonstrated after systemic administration, showing a high signal to background ratio. Our study provides a surface engineering approach to amplify the PA signals of organic nanoparticles for molecular imaging.