Indocyanine green was encapsulated inside the pores of pegylated amino-functionalized MCM-41 Mesoporous Silica Nanoparticles (ICG-MSNs). In addition to a greater stability and a decrease of toxicity, the photoacoustic effect of ICG-MSNs increases by nearly 400% compared to free ICG due to fluorescence quenching and high photothermal conversion of the encapsulated dyes. Upon i.v. administration in tumor-bearing mice, an overall photoacoustic enhancement of ca. 25% was measured in the tumor region.