Ghayathri Balasundaram,1,* Chris Jun Hui Ho,1,* Kai Li,2 Wouter Driessen,3 US Dinish,1 Chi Lok Wong,1 Vasilis Ntziachristos,3 Bin Liu,2 Malini Olivo1,41Bio-Optical Imaging Group, Singapore Bioimaging Consortium (SBIC), 2Institute of Materials Research and Engineering (IMRE), Agency for Science, Technology and Research (A*STAR), Singapore; 3Institute of Biological and Medical Imaging, Helmholtz Center Munich, Neuherberg, Germany; 4School of Physics, National University of Ireland, Galway, Ireland *These authors contributed equally to this work Abstract: Conjugated polymers (CPs) are upcoming optical contrast agents in view of their unique optical properties and versatile synthetic chemistry. Biofunctionalization of these polymer-based nanoparticles enables molecular imaging of biological processes. In this work, we propose the concept of using a biofunctionalized CP for noninvasive photoacoustic (PA) molecular imaging of breast cancer. In particular, after verifying the PA activity of a CP nanoparticle (CP dots) in phantoms and the targeting efficacy of a folate-functionalized version of the same (folate-CP dots) in vitro, we systemically administered the probe into a folate receptor-positive (FR+ve) MCF-7 breast cancer xenograft model to demonstrate the possible application of folate-CP dots for imaging FR+ve breast cancers in comparison to CP dots with no folate moieties. We observed a strong PA signal at the tumor site of folate-CP dots-administered mice as early as 1 hour after administration as a result of the active targeting of the folate-CP dots to the FR+ve tumor cells but a weak PA signal at the tumor site of CP-dots-administered mice as a result of the passive accumulation of the probe by enhanced permeability and retention effect. We also observed that folate-CP dots produced ~4-fold enhancement in the PA signal in the tumor, when compared to CP dots. These observations demonstrate the great potential of this active-targeting CP to be used as a contrast agent for molecular PA diagnostic imaging in various biomedical applications. Keywords: photoacoustic tomography, conjugated polymers, molecular imaging, breast cancer