Near-infrared (NIR) fluorescence sensing is desirable for in vivo biological measurements. But the method is currently limited by the availability of NIR fluorescent markers as well as by their poor performance, such as self-aggregation and dim fluorescence, in a physiological environment. To address this issue, this paper describes a NIR fluorescent polymer dot (Pdot) that emits at 777 nm. This Pdot was comparable in size to a water-soluble NIR quantum dot that emits at 800 nm (ITK Qdot800) but was about 4 times brighter and with a narrower emission peak. We formed the NIR Pdot by doping the NIR dye, silicon 2,3-naphthalocyanine bis(trihexylsilyloxide) (NIR775), into the matrix of poly (9,9-dioctylfluorene-co-benzothiadiazole) (PFBT) as the Pdot formed using a nanoscale precipitation technique. Free molecules of NIR775 aggregate in aqueous solution but encapsulating them into the hydrophobic Pdot matrix effectively introduced them into aqueous solution for use in biological studies. Most importantly, the brightness of NIR775 was dramatically enhanced because of the excellent light harvesting ability of PFBT and the very efficient energy transfer from PFBT to NIR775. We anticipate this bright NIR Pdot will be useful in biological measurements and cellular imaging where strong NIR emission is beneficial.
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