Development of fluorescent sensors based on a combination of PET (photo-induced electron transfer) and FRET (Förster resonance energy transfer) for detection of water
Fluorescent sensors DJ-1 and DJ-2 with large Stokes shift (SS) based on a combination of PET (photo-induced electron transfer) and FRET (Förster resonance energy transfer) have been developed for the detection of water in organic solvents. DJ-1 is composed of anthracene-(aminomethyl)phenylboronic acid ester as a PET-type donor fluorophore and BODIPY skeleton as an acceptor fluorophore in the FRET process. In contrast, DJ-2 is composed of anthracene skeleton as a donor fluorophore and BODIPY-(aminomethyl)phenylboronic acid ester skeleton as a PET-type acceptor fluorophore in the FRET process. In fact, the addition of water to organic solvents containing DJ-1 or DJ-2 caused both the PET suppression and the energy transfer from the donor fluorophore to the acceptor fluorophore through the FRET process, thus resulting in the enhancement of the fluorescence band originating from BODIPY skeleton. In addition, the pseudo-SS values of DJ-1 and DJ-2 between the photoabsorption maximum of the anthracene fluorophore and the fluorescence maximum of the BODIPY fluorophore are 7563 cm‒1 (141 nm) and 8017 cm‒1 (153 nm), respectively, which are significantly higher than those of a typical PET-based fluorescent sensors. It was found that the FRET efficiency for DJ-1 is quantitative, but that for DJ-2 is estimated to be ca. 50% based on time-resolved fluorescence lifetime measurements. Moreover, the detection limit of DJ-1 for water is superior to that of DJ-2. Based on the fluorescence sensing mechanism of DJ-1 and DJ-2 for water, we propose that a combination of a PET-type donor fluorophore and an acceptor fluorophore in the FRET process is one of the most promising molecular design to create an efficient fluorescent sensor for water in organic solvents.