We introduce a method to monitor the integrity of micellar nanocarriers using a novel fluorescent dye, IR-780-PDMS and Förster resonance energy transfer (FRET) as a readout. In addition, these dye-loaded nanocarriers can be used as a phototoxic agent in vitro. Mainly, a nanocarrier was designed, based on a previously described amphiphilic ABA-copolymer (Pip-PMOXA-PDMS-PMOXA-Pip) scaffold that incorporates the fluorescent FRET dye partners IR-780-PDMS (donor) and IR-780 (acceptor). The confirmation of FRET (that only occurs when donor and acceptor are in the required close proximity of less than ∼10 nm) in the nanocarriers is used to prove that the acceptor dye (IR-780) is still contained in its hydrophobic core. We measured such FRET signals of the nanocarriers also upon cellular uptake into HeLa cells using fluorescence-lifetime imaging microscopy (FLIM). Confocal laser scanning microscopy after incubation with nanocarriers demonstrated the intracellular uptake of the particles and their localization in an intracellular granular pattern. To demonstrate the intactness of the nanocarriers by detection of FRET we measured the fluorescence lifetime (FLIM) of the donor dye. FLIM showed that both types of lifetimes, that of the quenched donor, and that of the unquenched donor were present, in a granular pattern and homogeneously in the cytosol, respectively, indicating the presence of intracellular intact and disintegrated micellar nanocarriers. These data show that the herewith-described FRET method allows monitoring the intactness of nanocarriers while en route to the target, and also that the cargo is delivered and released within a potential target cell. In addition, near-infrared (NIR) irradiation of IR-780-loaded micellar nanocarriers leads to photocytotoxicity, which we demonstrated in in vitro experiments. Our findings open potential avenues in photodynamic therapy (PDT) of cancer.