Near-infrared temperature-switchable fluorescence nanoparticles.

BACKGROUND: Near infrared (NIR) environment-sensitive fluorophores are highly desired for many biomedical applications because of its non-invasive operation, high sensitivity and specificity, non-ionizing radiation and deep penetration in biological tissue. When the fluorophores are appropriately encapsulated in or conjugated with some thermal-sensitive polymers, they could work as excellent temperature-sensing probes. METHODS: In this study, we synthesized and characterized a series of NIR temperature-switchable nanoparticles based on two series of NIR fluorophores aza-BODIPY (ADP is used for abbreviation in this work) and Zinc phthalocyanine (ZnPc) and four pluronic polymers (F127, F98, F68 and F38). Encapsulating the fluorophores in the polymers by sonication, we synthesized the nanoparticles that showed switch-like functions of the fluorescence intensity (and/or lifetime) as the temperature, with high switch on-to-off ratio. We also investigated various factors that might change the temperature thresholds (Tth) of the switch functions, in order to control Tth during synthesis. RESULTS: These nanoparticles showed excellent temperature-switchable properties of fluorescence intensity and/or lifetime. Meanwhile, some factors (i.e., pluronic categories and nanoparticles' concentration) significantly affected the nanoparticles' Tths while other (i.e., fluorophore categories) that weakly affected Tths. CONCLUSIONS: By selecting appropriate pluronic categories and adjusting the nanoparticle's concentration, we can synthesize the nanoparticles with a wide range of Tths. These temperature-switchable fluorescence nanoparticles can be used for biomedical imaging and in vivo tissue temperature sensing/imaging.