Design of Fluorescent Nanocapsules as Ratiometric Nanothermometers

We have developed a novel design of optical nanothermometers that can measure the surrounding temperature in the range of 20–85 °C. The nanothermometers comprise two organic fluorophores encapsulated in a crosslinked polymethacrylate nanoshell. The role of the nanocapsule shell around the fluorophores is to form a well‐defined and stable microenvironment to prevent other factors besides temperature from affecting the dyes’ fluorescence. The two fluorophores feature different temperature‐dependent emission profiles; a fluorophore with relatively insensitive fluorescence (rhodamine 640) serves as a reference whereas a sensitive fluorophore (indocyanine green) serves as a sensor. The sensitivity of the nanothermometers depends on the type of nanocapsule‐forming lipid and is affected by the phase transition temperature. Both the fluorescence intensity and the fluorescence lifetime can be utilized to measure the temperature.