Fluorescent sensor for water based on photo-induced electron transfer and Förster resonance energy transfer: anthracene-(aminomethyl)phenylboronic acid ester-BODIPY structure

An anthracene-(aminomethyl)phenylboronic acid ester-BODIPY (DJ-1) was designed and developed as a fluorescent sensor based on photo-induced electron transfer (PET) and Förster resonance energy transfer (FRET) for the detection of a trace amount of water in solvents, where the anthracene skeleton and BODIPY skeleton are the donor fluorophore and the acceptor fluorophore in the FRET process, respectively. It was found that the addition of water to organic solvents containing DJ-1 causes both the suppression of PET in the anthracene-(aminomethyl)phenylboronic acid ester as the PET-type fluorescent sensor skeleton and the energy transfer from the anthracene skeleton to the BODIPY skeleton through a FRET process, thus resulting in the enhancement of the fluorescence band originating from the BODIPY skeleton. This work demonstrates that the PET/FRET-based fluorescent dye composed of the donor fluorophore possessing PET characteristics and the acceptor fluorophore in the FRET process can act as a fluorescent sensor with a large SS for the detection of a trace amount of water in solvents.


Introduction
Development of uorescent sensors for the visualization as well as detection and quantication of water in samples and products, such as solutions, solids, and gas or water on the surface of a substrate, has been of considerable concern in recent years for not only fundamental study in analytical chemistry, photochemistry, and photophysics, but also their potential applications to environmental and quality control monitoring systems and industry. 1 To date, some kinds of uorescent sensor for water based on ICT (intramolecular charge transfer), 2,3 PET (photo-induced electron transfer), 4,5 or ESIP (excited state intramolecular proton transfer) 6 have been designed and synthesized, and the optical sensing properties of these uorescent sensors for the detection and quantication of water have been investigated from the viewpoints of the relationship between ICT, PET, or ESIP characteristics and the intermolecular interaction of the sensor with water molecules. Among them, in particular, the PET-type uorescent sensor based on the uorescence enhancement system is useful for the detection and quantication of a trace amount of water in organic solvents because the uorescence intensity of the sensor increases as a function of water content in organic solvents, which is attributed to the suppression of PET from the electron donor part to the photo-excited uorophore due to the intermolecular interaction between the uorescent sensors and water molecules. Actually, in our previous work, anthracene-(aminomethyl)phenylboronic acid pinacol esters (OM-1, OF-1 and OF-2) was designed and synthesized as PET-type uorescent sensors for a trace amount of water (Fig. 1a). 5a The PET takes place from the nitrogen atom of amino moiety to the photoexcited uorophore skeleton in the absence of water, leading to the uorescence quenching. The addition of water to organic solvents containing PET-type uorescent sensor causes a drastic enhancement of uorescence, which is attributed to the suppression of PET. The nitrogen atom of amino moiety is protonated or strongly interacts with water molecule, leading to the formation of PET inactive species such as OM-1a. Thus, the uorescence enhancement system based on the PET method is useful for the detection and quantication of a trace amount of water in organic solvents. However, the PET-type uorescent sensor such as OM-1 usually has the disadvantage of very small Stokes shi (SS), leading to serious self-quenching and uorescence detection errors due to photoexcitation and scattering lights from the excitation source; SS is the difference in wavelength or frequency units between the maximum of the rst photoabsorption band and the maximum of uorescence band. On the other hand, Förster resonance energy transfer (FRET)type sensor is useful for applications in biochemistry and environmental research such as nucleic acid and ion analysis, signal transduction, and light harvesting, as well as for designing ratiometric uorescent sensors. 7,8 FRET is well described as an energy transfer process between an excited-state donor uorophore and a ground-state acceptor uorophore linked together by a non-conjugated spacer, and as the result, the emission spectrum from acceptor uorophore is observed. For an effective FRET, a strong spectral overlap between the donor emission and the acceptor absorption is required. Consequently, the pseudo-SS between the maximum of donor absorption band and the maximum of acceptor uorescence band of FRET-type sensors is larger than the SS of either the donor or acceptor uorophores, leading to an effective avoidance of the self-quenching and uorescence detection errors due to photoexcitation and scattering lights from the excitation source. However, to the best of our knowledge there are no reports for the PET/FRET-based uorescent sensor composed of the donor uorophore possessing PET characteristics and the acceptor uorophore in FRET process. It is expected that the development of PET/FRET-based uorescent sensor can allow creation of uorescent sensor system with large SS for the detection of a trace amount of water in solvents, that is, the PET/ FRET-based uorescent sensor has the advantage over PET-type uorescent sensor 4,5 which usually has the disadvantage of very small SS.
Thus, in this work, in order to develop a uorescent sensor possessing large SS for a trace amount of water in solvents, we have designed and synthesized an anthracene-(aminomethyl) phenylboronic acid ester-BODIPY (DJ-1) as a uorescent sensor based on PET and FRET characteristics, where the anthracene skeleton and BODIPY skeleton are the donor uorophore and the acceptor uorophore in the FRET process, respectively (Fig. 1b). That is, it is expected that the addition of water to organic solvents containing DJ-1 causes both the suppression of PET in the anthracene-(aminomethyl)phenylboronic acid ester as the PET-type uorescent sensor skeleton and the energy transfer from anthracene skeleton to BODIPY skeleton through a FRET process, and thus resulting in the enhancement of the uorescence band originating from BODIPY skeleton. Herein, we report the optical sensing property of DJ-1 for the detection of water in solvents based on the PET and FRET characteristics of the anthracene-(aminomethyl)phenylboronic acid ester-BODIPY.
The photoabsorption and uorescence spectra of OM-1, B-1 and DJ-1 in acetonitrile are shown in Fig. 2. OM-1 shows a photoabsorption band in the ranges of 300 nm to 400 nm originating from the anthracene skeleton, and B-1 shows a photoabsorption band in the ranges of 420 nm to 520 nm originating from the BODIPY skeleton. In addition, for B-1 a feeble and broad photoabsorption band was observed in the ranges of 300 nm to 400 nm. The molar extinction coefficient (3 max ) for the photoabsorption maximum (l abs max ¼ 498 nm) of B-1 is 72 600 M À1 cm À1 , which is signicantly higher than that (l abs On the other hand, DJ-1 shows two photoabsorption bands in the ranges of 300 nm to 400 nm (l abs max ¼ 367 nm, 3 max ¼ 14 200 M À1 cm À1 ) and 420 nm to 520 nm (l abs max ¼ 498 nm, 3 max ¼ 72 200 M À1 cm À1 ), which are assigned to the anthracene skeleton and the BODIPY skeleton, respectively (Fig. 2a). For the corresponding uorescence spectra, OM-1 and B-1 exhibit a uorescence maximum (l  max ) at 412 nm and 507 nm, by the photoexcitation (l ex ) at 366 nm and 367 nm, respectively (Fig. 2b). It is worth mentioning here that the edge for the uorescence band of OM-1 reached 500 nm, that is, the photoabsorption spectrum originating from the BODIPY skeleton of B-1 has spectral overlap with the uorescence spectrum originating from the anthracene skeleton of OM-1. The fact suggests that for DJ-1 the FRET from the anthracene skeleton as the donor uorophore to the BODIPY skeleton as the acceptor uorophore occurs by the Scheme 1 Synthesis of DJ-1.
photoexcitation of the anthracene skeleton. In fact, DJ-1 exhibits an only uorescence band with the l  max at 508 nm in the ranges of 480 nm to 600 nm originating from the BODIPY skeleton by the photoexcitation (l ex ¼ 367 nm) of the anthracene skeleton, as well as the photoexcitation (l ex ¼ 472 nm) of the BODIPY skeleton (Fig. S7, ESI †). In addition, the pseudo-SS value of DJ-1 between the l abs max of the anthracene skeleton and the l  max of the BODIPY skeleton is 7563 cm À1 (141 nm), which is signicantly higher than that (395 cm À1 ) of OM-1 and that (356 cm À1 ) of B-1. Therefore, it is expected that the addition of water to organic solvents containing DJ-1 causes both the suppression of PET in the anthracene-(aminomethyl)phenylboronic acid ester and the energy transfer from anthracene skeleton to BODIPY skeleton through a FRET process, and thus resulting in the enhancement of the uorescence band originating from BODIPY skeleton.
Thus, in order to investigate the optical sensing ability of DJ-1 for water in a solvent, the photoabsorption and uorescence spectra of OM-1 and B-1 as well as DJ-1 were measured in acetonitrile that contained various concentrations of water (Fig. 3). The photoabsorption spectra of DJ-1 show unnoticeable changes upon addition of water to the acetonitrile solution (Fig. 3a). On the other hand, DJ-1 exhibits an enhancement of uorescence band at 508 nm originating from the BODIPY skeleton by the photoexcitation (l ex ¼ 367 nm) of the anthracene skeleton upon addition of water to the acetonitrile solution (Fig. 3b). The enhancement of the uorescence band levels off when the water content becomes 5.0 wt%. This result indicates that the enhancement of uorescence band originating from the BODIPY skeleton is attributed to both the suppression of PET in the anthracene-(aminomethyl)phenylboronic acid ester and the occurrence of FRET from the excited-state anthracene uorophore to the ground-state BODIPY uorophore upon addition of water to the acetonitrile solution. As more evidence for the FRET process in DJ-1, the uorescence spectra of DJ-1 by the photoexcitation (l ex ¼ 472 nm) of the BODIPY skeleton did not undergo appreciable changes in intensity and shape of the uorescence band originating from BODIPY skeleton upon addition of water to the acetonitrile solution (Fig. S7, ESI †). On the other hand, the photoabsorption spectra of OM-1 did not undergo appreciable changes upon addition of water to the acetonitrile solution as with the case of DJ-1 (Fig. 3c), but the uorescence spectra of OM-1 underwent an increase in intensity at around 415 nm with the increase in the water content, which is attributed to the uorescence emission originating from the anthracene skeleton due to the suppression of PET (Fig. 3d). Moreover, it is worth mentioning here that the photoabsorption and uorescence spectra of B-1 did not undergo appreciable changes upon the addition of water to the acetonitrile solution (Fig. 3e, f, and S8, ESI †). Consequently, as shown in Fig, 4, these facts strongly indicate that for DJ-1 the enhancement of the uorescence band upon addition of water to the solvent is due to both the suppression of PET in the donor uorophore (anthracene-(aminomethyl)phenylboronic acid ester) and the occurrence of FRET from the excited-state donor uorophore and the ground-state acceptor uorophore (BODIPY skeleton), that is, DJ-1 can act as a uorescent sensor for water based on PET and FRET characteristics (Fig. 4).
In order to estimate the sensitivity and accuracy of DJ-1 as a PET/FRET-type uorescent sensor for the detection of water in solvent, the changes in uorescence intensity were plotted against the water fraction in acetonitrile (Fig. 5a). The plot for  the low water content region below 1.0 wt% demonstrated that the uorescence peak intensity at 508 nm increased linearly as a function of the water content (Fig. 5a inset). Indeed, the correlation coefficient (R 2 ) value for the calibration curve is 0.96, which indicates the good linearity. The enhancement of the uorescence peak intensity levels off in the water content region greater than 5.0 wt%, which is similar to the case of OM-1 (Fig. 5b). In addition, we performed the measurement of uorescence quantum yield (F  ) for DJ-1 in the acetonitrile solution with various water content. Indeed, these F  values are in good agreement with the intensity of the uorescence spectra (Fig. 6). These facts also indicate that the uorescence sensing mechanism of DJ-1 for water is based on the suppression of PET and occurrence of FRET by water molecules. Thus, the detection limit (DL) was determined from the plot of the uorescence intensity at 508 nm versus the water fraction in the low water content region below 1.0 wt% (DL ¼ 3.3s/m s , where s is the standard deviation of the blank sample and m s is the slope of the calibration curve). The m s and DL values of DJ-1 are 13 and 0.25 wt%, which are inferior to those (m s ¼ 67, DL ¼ 0.04 wt%) of the PET-type uorescent sensor OM-1 (Fig. 5b). 5a The m s and DL values of PET/FRET-type uorescent sensor DJ-1 may be dependent on the non-conjugated spacer between the donor uorophore and the acceptor uorophore, that is, the substituent on the phenylboronic acid pinacol ester. In fact, the m s value (55) and DL value (0.06 wt%) of OF-1 having a methoxy group as an electron-donating substituent is inferior to that of OM-1, but the m s value (382) and DL value (0.009 wt%) of OF-2 having a cyano group as an electron-withdrawing substituent is superior to those of OM-1 and OF-1. 5d Therefore, it is expected that the m s value and DL values of a PET/FRET-type uorescent sensor for the detection of water can be improved by modifying the non-conjugated spacer between the donor uorophore and the acceptor uorophore.

Conclusions
We have designed and developed an anthracene-(aminomethyl) phenylboronic acid ester-BODIPY (DJ-1) as a uorescent sensor possessing large SS based on PET and FRET characteristics for the detection of a trace amount of water in solvents. It was found that the enhancement of uorescence band originating from the BODIPY skeleton upon addition of water to the acetonitrile solution is due to both the suppression of PET in the donor uorophore (anthracene-(aminomethyl)phenylboronic acid ester) and the occurrence of FRET from the excitedstate donor uorophore to the ground-state acceptor uorophore (BODIPY skeleton). Thus, this work demonstrates that the PET/FRET-based uorescent dye composed of the donor uorophore possessing PET characteristics and the acceptor uorophore in FRET process can act as a uorescent sensor with large SS for the detection of a trace amount of water in solvents.

Conflicts of interest
There are no conicts to declare.