Polymer films doped with fluorescent sensor for moisture and water droplet based on photo-induced electron transfer

Anthracene-(aminomethyl)phenylboronic acid pinacol ester (AminoMePhenylBPin) OF-2 acts as a PET (photo-induced electron transfer)-type fluorescent sensor for determination of a trace amount of water: the addition of water to organic solvents containing OF-2 causes a drastic and linear enhancement of fluorescence emission as a function of water content, which is attributed to the suppression of PET. Indeed, detection limits (DLs) for OF-2 were as low as 0.01–0.008 wt% of water in solvents, that is, the PET method makes it possible to visualize, detect, and determine a trace amount of water. Thus, in this work, in order to develop fluorescent polymeric materials for visualization and detection of water, we have achieved the preparation of various types of polymer films (polystyrene (PS), poly(4-vinylphenol) (PVP), polyvinyl alcohol (PVA), and polyethylene glycol (PEG)) which were doped with OF-2, and investigated the optical sensing properties of the OF-2-doped polymer films for water. As-prepared OF-2-doped polymer films initially exhibited green excimer emission in the PET active state, but blue monomer emission in the PET inactive state upon exposure to moisture or by water droplet. Moreover, it was found that the OF-2-doped polymer films show the reversible fluorescence properties in the dry–wet process. Herein we propose that polymer films doped with PET-type fluorescent sensors for water based on a fluorescence enhancement (turn-on) system are one of the most promising and convenient functional materials for visualizing moisture and water droplets.


Introduction
Needless to say, a novel coronavirus, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) that causes the Coronavirus Disease 2019 , dramatically changed the world to give people a sense of fear of death. Infectious viruses are generally released into the atmosphere through droplet spread from coughing and sneezing by an infected person. Thus, the infection route from an infected person to an uninfected person is predominately due to the droplet. Actually, face shields made of polyester or polycarbonate lms and partitions made of acrylic resin are commercially available for reducing the risk of droplet infection. Therefore, if we can visually conrm the droplet on the face shields and partitions, this allows us to accurately remove the viruses by wiping away the droplet. However, because the virus-containing droplet is generally 5 mm or more, it is practically difficult for us to visually conrm the droplet. Meanwhile, over 90% of the droplet is composed of water, and thus techniques and methods capable of visualizing water are undoubtedly useful for detecting the virus-containing droplet. Optical methods utilizing colorimetric and uorescent sensors for visualization as well as detection and quantication of water in samples and products, such as solutions, solids, and gases or water on substrate surfaces have been of considerable scientic and practical concern in recent years, because of not only fundamental studies in photochemistry, photophysics, and analytical chemistry, but also their potential applications to environmental and quality control monitoring systems and industry. 1,2 To date, various kinds of organic uorescent sensors and polymers for the determination of water content based on ICT (intramolecular charge transfer), 3-5 ESIPT (excited state intramolecular proton transfer), 6,7 PET (photo-induced electron transfer), 8-10 or solvatochromic properties 11 have been designed and synthesized. The optical sensing properties of these uorescent sensors for the detection and quantication of water content were investigated from the viewpoints of the relationship between ICT, ESIPT, or PET characteristics and the intermolecular interaction of the sensors with water molecules. As a result, it was found that most of the previous uorescent sensors for the water content determination, including uorescent conjugated polymers 12 and organic uorescent dyes with ICT and ESIPT characteristics, are based on a uorescence quenching (turn-off) system, that is, the uorescence intensity of the sensors decreases as a function of water content in organic solvents. However, this uorescence quenching system makes it difficult to detect a trace amount of water. In contrast, a uorescence enhancement (turn-on) system, where the uorescence intensity increases with an increase in water content in organic solvents, is useful for the visualization, detection, and quantication of a trace amount of water in organic solvents. In particular, the uorescence enhancement system based on PETtype uorescent sensors can detect reversible changes in their immediate environment, i.e., water content, through the reversible intermolecular interactions between the sensors and water molecules. Actually, during the past decade, we have designed and developed anthracene-(aminomethyl)phenylboronic acid pinacol esters (e.g., OM-1, OF-1, and OF-2) 9a,d as PET-type uorescent sensors for the determination of a trace amount of water (Fig. 1). 8-10 The PET takes place from the nitrogen atom of the amino moiety to the photoexcited uorophore (anthracene) skeleton in the absence of water, leading to uorescence quenching. The addition of water to organic solvents containing the PET-type uorescent sensors causes a drastic and linear enhancement of uorescence emission as a function of water content, which is attributed to the suppression of PET; that is, the nitrogen atom of the amino moiety is protonated or strongly interacts with water molecules, leading to the formation of the PET inactive species such as OF-2a. 9d Indeed, detection limits (DLs) for OF-2 were as low as 0.01-0.008 wt% of water in solvents. Thus, the PET method makes it possible to visualize, detect, and determine a trace amount of water. Consequently, uorescent sensors for water are one of the most promising functional materials contributing to not only post-COVID-19 society, but also Sustainable Development Goals (SDGs), which has been adopted by all United Nations Member States in 2015 and provides a shared blueprint for peace and prosperity for people and the planet now and in the future. In fact, some uorescent conjugated polymers 12 and ICTtype 5 or ESIPT-type 7 uorescent sensor-doped polymer lms for water that are based on a uorescence quenching (turn-off) system have been prepared. However, to the best of our knowledge, there are no report on PET-type uorescent polymers and PET-type uorescent sensor-doped polymer lms for water.
Thus, in this work, in order to develop uorescent polymeric materials for visualization and detection of water, we have achieved the preparation of various types of polymer lms (polystyrene (PS), poly(4-vinylphenol) (PVP), polyvinyl alcohol (PVA), and polyethylene glycol (PEG)) which were doped with PET-type uorescent sensor OF-2, and investigated the optical sensing properties of the OF-2-doped polymer lms for water. As-prepared OF-2-doped polymer lms initially exhibited green excimer emission in the PET active state, but blue monomer emission in the PET inactive state upon exposure to moisture or by water droplet. Moreover, it was found that the OF-2-doped polymer lms show the reversible uorescence properties in the dry-wet process. Herein we propose that polymer lms doped with uorescent sensors for water are convenient functional materials for visualizing the virus-containing droplet.

Results and discussion
First, in order to investigate the optical properties of OF-2 in the aggregate state, the spin-coated OF-2 lm was prepared on a glass substrate, and the photoabsorption and uorescence spectra of the spin-coated OF-2 lm before and aer exposure to moisture were repeatedly measured several times (Fig. 2). As  spectra (l ex ¼ 366 nm) of spin-coated OF-2 film before (in dry process) and after (in wet process) exposure to moisture. For photoabsorption spectra, baselinecorrection has been made to be the same absorbance at 425 nm for all the spectra. Inset in (b) shows reversible switching of fluorescence intensity at around 470 nm in dry process and at 415 nm in wet process of spin-coated OF-2 film. Fig. 3 (a) Photoabsorption and (b) fluorescence spectra (l ex ¼ 366 nm) of spin-coated PS films doped with various concentrations (20, 50, and 80 wt%) of OF-2 before (in dry process) and after (in wet process) exposure to moisture. For photoabsorption spectra, baselinecorrection has been made to be the same absorbance at 425 nm for all the spectra. Inset in (b) shows fluorescence peak intensity at 415 nm (l ex ¼ 366 nm) in wet process of OF-2-doped PS films as a function of the concentration of OF-2 in the PS films.
with the case of OF-2 in absolute acetonitrile (Fig. S1, ESI †), 9d the as-prepared spin-coated OF-2 lm (in dry process) shows a vibronically-structured photoabsorption band in the range of 300 nm to 400 nm originating from the anthracene skeleton ( Fig. 2a). The photoabsorption spectral shape of the spin-coated OF-2 lm did not undergo appreciable changes upon exposure to moisture, as with the case of OF-2 in acetonitrile that contained various concentrations of water, although a slight change in absorbance of the spin-coated lm during the repeated cycles was observed due to the disturbance of baseline in the photoabsorption spectra. In the corresponding uorescence spectra, OF-2 in absolute acetonitrile exhibits a feeble but vibronically-structured uorescence band with a uorescence maximum wavelength (l  max ) of 415 nm in the range of 400 nm to 500 nm in the PET active state, which is attributed to the monomer emission originating from the anthracene skeleton. The enhancement of the monomer emission band was observed upon addition of water to the acetonitrile solution of OF-2, which is due to the suppression of PET (the PET inactive state) (Fig. S1, ESI †). 9d On the other hand, the as-prepared spin-coated OF-2 lm shows a feeble and broad uorescence band in the range of 400 nm to 600 nm, which is assigned to the excimer emission originating from the anthracene skeleton in the PET active state (Fig. 2b). Interestingly, the spin-coated OF-2 lm underwent a change in the uorescence spectra upon exposure to moisture (in wet process), which caused the vibronicallystructured monomer emission (l  max ¼ 415 nm) arising from the PET inactive state. Moreover, it was found that when the spin-coated OF-2 lm aer exposure to moisture was dried in the atmosphere, the photoabsorption and uorescence spectra showed the original spectral shapes before exposure to moisture. Thus, for the spin-coated OF-2 lm, the reversibility of the uorescence intensity between the excimer and monomer emissions in the dry-wet (moisture) process was investigated Fig. 4 (a) Photoabsorption and (b) fluorescence spectra (l ex ¼ 366 nm) of spin-coated PS film with 50 wt% OF-2 before (in dry process) and after (in wet process) exposure to moisture. (c) Photoabsorption and (d) fluorescence spectra (l ex ¼ 366 nm) of spin-coated PVP film with 50 wt% OF-2 before (in dry process) and after (in wet process) exposure to moisture. (e) Photoabsorption and (f) fluorescence spectra (l ex ¼ 366 nm) of spin-coated PVA film with 50 wt% OF-2 before (in dry process) and after (in wet process) exposure to moisture. (g) Photoabsorption and (h) fluorescence spectra (l ex ¼ 366 nm) of spin-coated PEG film with 50 wt% OF-2 before (in dry process) and after (in wet process) exposure to moisture. For photoabsorption spectra, baseline-correction has been made to be the same absorbance at 425 nm for all the spectra.  ( Fig. 2b inset). The dry-wet cycle shows that the monomer emission was not observed in the third wet process. The poor reversibility of the uorescence intensity of the spin-coated OF-2 lm between the excimer and monomer emissions may be attributed to destruction of the lm during the dry-wet process.
Next, we prepared spin-coated PS lms doped with various concentrations (20, 50, and 80 wt%) of OF-2 and investigated the optical sensing properties of the OF-2-doped PS lms for water. As with the case of the spin-coated OF-2 lm, the asprepared OF-2-doped PS lms (in dry process) show a vibronically-structured photoabsorption band in the range of 300 nm to 400 nm and a feeble and broad uorescence band in the range of 400 nm to 600 nm attributable to the excimer emission originating from the anthracene skeleton in the PET active state (Fig. 3). With increasing the concentration of OF-2 in the PS lms, the absorbance increased, but the intensity of the excimer emission band changed little. When the OF-2-doped PS lms were exposed to moisture (in wet process), the photoabsorption spectral shape did not undergo appreciable changes, whereas the uorescence spectra underwent a change in the spectral shape to the vibronically-structured monomer emission (l  max ¼ 415 nm) arising from the PET inactive state, as with the case of the spin-coated OF-2 lm. In addition, for the OF-2-doped PS lms aer exposure to moisture, the intensity of the monomer emission band increased with the increase in the concentration of OF-2 in the PS lms, that is, the uorescence intensity showed a good linearity as a function of the concentration of OF-2 in the PS lms (Fig. 3b inset). Furthermore, we prepared various types of polymer lms (PS, PVP, PVA, and PEG) which are doped with OF-2 at 50 wt%, and the photoabsorption and uorescence spectra of the OF-2-doped polymer lms before and aer exposure to moisture were repeatedly measured several times (Fig. 4). For all the four OF-2-doped polymer lms during the repeated cycles, the photoabsorption spectral shape did not undergo appreciable changes, although a slight change in absorbance was observed due to the disturbance of baseline in the photoabsorption spectra. The corresponding uorescence spectra show a change in spectral shape from the feeble and broad excimer emission observed in the range of 400 nm to 600 nm to the vibronically-structured monomer emission (l  max ¼ 415 nm) before and aer exposure to moisture. In fact, one can see that an as-prepared OF-2-doped PS lm initially exhibits the green excimer emission in the PET active state, but the blue monomer emission in the PET inactive state upon exposure to moisture or by water droplet (Fig. 5a and b, see Movie S1 for water droplet, ESI †). The dry-wet cycles of the OF-2doped polymer lms show that a reversible switching in uorescent intensity between the excimer and monomer emissions was still observed in the third dry-wet process (Fig. 6). Thus, this result demonstrates that the reversibility of the uorescence intensity of the OF-2-doped polymer lms between the excimer and monomer emissions is superior to that of the spincoated OF-2 lm (Fig. 2b inset). On the other hand, a comparison of the reversibility of the uorescence intensity between the four types of polymer lms showed that the hydrophobic PS and PVP lms exhibit a good reversibility for moisture from the second dry-wet process, and the hydrophilic PVA and PEG lms produce a large change in the intensity between the excimer and monomer emissions during the dry-wet process. Consequently, it was found that PET-type uorescent sensor-doped polymer lms exhibit a reversible switching between the excimer and monomer emissions during the dry-wet process, that is, enable the visualization and detection of moisture and water droplet.

Conclusions
We have developed polymer lms doped with a PET-type uorescent sensor for water, and demonstrated that the sensordoped polymer lms exhibit a reversible switching in uorescent color between the green excimer emission in the PET active state under a drying process and the blue monomer emission in the PET inactive state upon exposure to moisture or by water droplet. Thus, we believe that polymer lms doped with PETtype uorescent sensors for water based on a uorescence enhancement (turn-on) system are one of the most promising and convenient functional materials for visualizing the viruscontaining droplet on the surfaces of face shields and partitions.

Experimental
General Photoabsorption spectra of solutions and lms were observed with a SHIMADZU UV-3150 spectrophotometer. Fluorescence spectra of solutions, lms, and solids were measured with a HITACHI F-4500 spectrouorometer.

Preparation of OF-2-doped polymer lms
Polystyrene (PS) (2-8 mg) was dissolved in a THF solution (1 mL) of OF-2 (2-8 mg) to form a 20 wt%, 50 wt%, or 80 wt% stock solution. Poly(4-vinylphenol) (PVP) or polyethylene glycol (PEG) (5 mg) was dissolved in a THF solution (1 mL) of OF-2 (5 mg) to form a 50 wt% stock solution. On the other hand, a THF solution (0.5 mL) of OF-2 (5 mg) was added to a polyvinyl alcohol (PVA) (5 mg) aqueous solution (0.5 mL) around 50 C to form a 50 wt% stock solution. To prepare a polymer lm, 300 mL of an OF-2-polymer solution was directly spin-coated (3000 rpm for 30 s) on a glass substrate (MIKASA MS-A-100 Opticoat Spincoater). The spin-coated lms were dried in air. The resulting OF-2doped polymer lms were exposed to moisture for 60 s using a humidier.

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