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The uorescence properties, e.g., uorescence intensity, of uorescent sensors can change due to covalent derivatization or noncovalent complexation with a target chemical species (i.e., molecules and ions) or by variations in circumstantial physical parameters (e.g., temperature and viscosity). The internal charge transfer (ICT) character and photoinduced electron transfer (PET) ef ciency can be used to tune the uorescence switching mechanism, facilitating the development of new uorescent sensors. In addition, the utilization of an environment─ sensitive (i.e., polarity─ and hydrogen bonding─ sensitive) uorophore in stimulus─ responsive macromolecules to design novel uorescent sensors has been proposed. Based on this concept, highly sensitive uorescent polymeric thermometers and (extremely sensitive) digital uorescent pH sensors have been developed. These thermometers are being used to measure the temperature of live cells in biological and medical studies. This concept has also allowed nanoscale proton mapping near membranes, which exempli es the downsizing of targets for uorescent sensing from a micrometer─ scale to a nanometer─ scale. Figure 1. Fluorescent ‘off─ on’ derivatization reagents having a benzofurazan structure. (a) Chemical structure of benzofurazan. R 1 and R 2 are substituents. (b) MTBD─ SH for carboxylic acids with the control of an ICT character [ref. 4]. (c) NBD─ Bz─ p ─ NMe 2 for peroxy acid with the control of a PET process [ref. 5]. The indicated Φ f values were determined in acetonitrile. ( 64 ) J. Synth. Org. Chem., Jpn. 1116 cent pH sensors with a benzofurazan core can be utilized for mapping protons in nanoscale architectures. Section 4 addresses this challenge of uorescent sensors, namely, the downsizing of their subjects from micrometer─ scale (e.g., within biological cells) to nanometer─ scale subjects (e.g., in the vicinity of membranes and nucleic acids). 2. Fluorescent Polymeric Thermometers Functioning in Aqueous Solution To the best of our knowledge, the development of uorescent polymeric thermometers is the most celebrated of our achievements reported to date. Sensitive uorescent thermometers are currently of great importance in the eld of uorescent sensors, and the rst such example was reported by us in 2003. 6 Luckily, an exciting application of uorescent thermometers, i.e., intracellular thermometry (temperature measurements within a single live cell), was established with our uorescent polymeric thermometers. The details of uorescent thermometers have been summarized in a number of textbooks, review articles, and accounts in scienti c magazines by us 7 and others. 8 This section is an abridged and updated version of the review article published in Chem. Commun. in 2017. 7e Intracellular thermometry has also been summarized in the literature. 9 2.1 Background Temperature is one of the most important physical parameters in our daily lives. In science, the temperature of the surrounding environment affects all chemical and biochemical reactions and vice versa. Fluorescent thermometers, which are the smallest thermometers available, are organic molecule─ based thermometers that show temperature─ dependent uorescence properties (i.e., uorescence intensity, wavelength, and uorescence lifetime). Fluorescent thermometers function best in small spaces (on the scale of a micrometer or less) because of the small size of the environmentally sensitive molecules. Our focus on the development of highly sensitive uorescent polymeric thermometers started in 2002, and we were inspired by the following factors. (1) Synthetic uorescent probes for various kinds of ions and molecules 2 have been successfully applied for monitoring intracellular environments. 10 In contrast, there were few uorescent sensors to monitor intracellular physical parameters. (2) Some organic molecules and inorganic complexes show temperature─ dependent emission. 11 However, their sensitivities to temperature variations are not suf cient for distinguishing differences of 1 °C. Thus, improving the sensitivity of uorescent thermometers needed to be addressed. (3) Striking properties of thermoresponsive poly (N ─ isopropylacrylamide) (PNIPAM) had been reported. 12 PNIPAM is soluble in water at temperatures below 32 °C (this temperature is de ned as the lower critical solution temperature, LCST) by hydration of the amido groups in PNIPAM with solvent water molecules, whereas the solution undergoes a phase transition and is colloidal when the temperature exceeds 32 °C. In the latter state, the hydrogen bonds between PNIPAM and water are broken, and PNIPAM assumes a globular form due to hydrophobic interactions involving the isopropyl side groups of PNIPAM. 2.2 Establishment of a Design and Functional Mechanism for Sensitive Fluorescent Polymeric Thermometers Factors (2) and (3) described in Subsection 2.1 inspired us to design sensitive uorescent polymeric thermometers based on a new concept. Because the sensitivity of uorophores to temperature variations is generally low (Figure 2a), we decided to separate the requisite roles in uorescent thermometers, i.e., sensing the temperature variation and signaling the variation. As shown in Figure 2b, our uorescent polymeric thermometers consist of thermoresponsive units and environment─ sensitive uorescent units. The unit ratio of environment─ sensitive uorescent units to thermoresponsive units should be kept low (normally 1:1,000 to 1:100) so as to not interfere with the original sensitivity of the thermoresponsive units. Benzofurazan was selected as an environment─ sensitive uorophore due to its high sensitivity. While nitrobenzofurazans (NBDs) 13 are more common as uorophores, N,N ─ dimethylaminosulfonylbenzofurazans (DBDs) 14 were chosen for our studies. The uorescence properties of DBD─ amino derivatives are more easily in uenced by the environments (e.g., solvent) than NBD─ amino derivatives, indicating that the DBD structure is more suitable for use as an environment─ sensitive uorophore in uorescent polymeric thermometers. Thus, we prepared uorescent polymeric thermometer 1, 6 which consisted of thermoresponsive NIPAM units and uorescent DBD─ AE (4─ N ─ (2─ acryloyloxyethyl)─ N ─ methylamino─ 7─ N,N ─ dimethylaminosulfonyl─ 2,1,3─ benzoxadiazole) units (Figure 3a). The procedure for synthesizing our uorescent polymeric thermometers is generally simple: (1) the monomers and a radical initiator (AIBN, 2,2’─ azobisisobutyronitrile) are mixed in an adequate solvent (e.g., 1,4─ dioxane), (2) dissolved oxygen was removed by bubbling argon (or nitrogen) gas, (3) the solution was left to stand at 60 °C for 4─ 16 hours, and (4) the uorescent polymeric thermometers were puri ed by reprecipitation or dialysis. The actual composition of the thermometers (i.e., unit ratio) can be determined by 1 H NMR and UV─ Vis spectroscopy. The actual compositions of the uorescent polymeric thermometers were approximately the same as those of the corresponding monomers in the feed. Figure 3b shows the uorescence spectra of 1 in water with increasing temperature. As expected, the uorescence intensity of 1 dramatically increased at approximately 32 °C, which is Figure 2. Design of uorescent polymeric thermometers. (a) Temperature has a relatively small effect on the relaxation processes of a uorophore. (b) Therefore, in addition to the uorophore, a thermoresponsive moiety (e.g., PNIPAM) is included in uorescent thermometers to sense temperature variations. When the thermoresponsive moiety encounters a temperature change, the microenvironment near the thermoresponsive moiety varies with the three─ dimensional structural change in this group. Finally, the environment─ sensitive uorophore in the uorescent polymeric thermometer outputs a signal by sensing the environmental variation (e.g., the variation in polarity). Vol.77 No.11 2019 ( 65 ) 1117 the phase transition temperature of PNIPAM (Figure 3c). The proposed functional mechanism was con rmed from the temperature─ dependent uorescence decay curves of the uorophore in a later study. 14 The average variation in the uorescence intensity of 1 from 29 to 37 °C was 38%/°C, which was much higher than that of the most sensitive conventional luminescent thermometer (2.4%/°C) at that time. In addition to its high sensitivity, uorescent polymeric thermometer 1 had several advantages: (1) it was functional in aqueous solutions, (2) it had a stronger uorescence signal at higher temperatures (opposite to the typical behavior caused by thermal quenching) and (3) its functional temperature range could be modulated by replacing the thermoresponsive units. 2.3 Modulation of the Functional Temperature Ranges of Fluorescent Polymeric Thermometers Although PNIPAM is the most successful thermoresponsive polymer, other acrylamide─ or acrylic ester─ derived polymers also show a phase transition involving a three─ dimensional structural change in water. For instance, PNNPAM (poly(N ─ n ─ propylacrylamide)) undergoes a phase transition at 21 °C in water. Thus, we prepared uorescent polymeric thermometer 2 with a functional temperature range of approximately 21 °C by replacing the NIPAM units with NNPAM units (Figure 4a). 6 Moreover, the functional temperature range of a uorescent polymeric thermometer can be more precisely tuned by using two kinds of thermoresponsive units. When we adopted a mixture of NNPAM and NIPAM units (the molar ratio of NNPAM and NIPAM in the feed was 1:1) as the thermoresponsive units, the functional temperature range shifted to approximately the average of those of uorescent polymeric thermometers 1 and 2 (Figure 4b). 15 2.4 Introduction of Ionic Units to Fluorescence Polymeric Thermometers to Improve Solubility and Accuracy As previously shown in Figure 3c, thermometer 1 exhibited hysteresis in heating and cooling cycles. In practical settings, this hysteresis signi cantly decreases the accuracy of the thermometry because the uorescence signals of t

yūki gōsei kagaku kyōkaishi/yuuki gousei kagaku kyoukaishi

Fluorescent Sensors Based on a Novel Functional Design: Combination of an Environment-sensitive Fluorophore with Polymeric and Self-assembled Architectures