We report a novel real-time method to monitor the reaction of an organic dye, 5(6)-carboxynaphthofluorescein succinimidyl ester and a CdSe/ZnS quantum dot (QD) using fluorescence spectra of microdroplets formed in a microfluidic device. Dual fluorescence peaks (at λ = 605 and 670 nm) were observed via a fluorescence energy transfer (FRET) process from the CdSe/ZnS QD to the organic dye; the rate of the chemical reaction of the two reagents was estimated by the ratio of the two fluorescence peaks, defined as the fluorescence intensity ratio. In addition, well-controlled microdroplets were continuously generated (with a diameter of ∼150 μm) in the microfluidic device and the reaction time was determined as the length from the mixing point to the sampling point using fluorescence spectroscopy. The fluorescence intensity ratio increased with increasing reaction time after mixing of the two reagents indicating that the chemical reaction was observed directly in the microdroplet. Therefore, the real-time monitoring of the chemical reaction was successfully achieved by measuring the fluorescence intensity ratio as a function of the reaction time. When compared with a conventional batch process, a fast reaction time of <1 s and a high fluorescence intensity ratio was achieved in the microdroplets formed in the microfluidic device.
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