English

• 1. Introduction

  Innovations and breakthroughs in experimental technology have vigorously promoted the research process of biochemistry and molecular biology [1]. Extensive evidence shows that diverse bioactive species are critical for cell proliferation, differentiation, aging and apoptosis, and many diseases are closely related to the abnormal expression of these species [2]. Among these bioactive species, reactive oxygen species (ROS) and reactive sulfur species (RSS) have attracted increasing interest because they are associated with the intracellular redox homeostasis and many biological events; in addition, some enzymes are widely distributed in subcellular organelles and play vital and different roles in biosystems [3]. Therefore, accurately monitoring their concentrations is beneficial for understanding the diverse physiological and pathological processes and for exploring novel strategies of regulating biological phenomena and improving diagnosis, treatment and prognosis of different diseases.

  Among the diverse detection strategies, fluorescence probes represent a simple, sensitive and non-invasive technique, which enables real-time, dynamic and selective imaging of these targets in living cells [4]. As a response to the analytes, fluorescent probes can give a specific signal output generally based on such response mechanisms as fluorescence resonance energy transfer (FRET), photo-induced electron transfer (PET), intramolecular charge transfer (ICT) and excited-state intramolecular proton transfer (ESIPT) [5–7]. Of course, dual-channel signal readout under one excitation wavelength is more favorite because it could provide diverse information about the levels of the target analyte. FRET and ESIPT-based probes all satisfy the above requirements, however, the development of FRET-based probes are limited to the rare donor-acceptor pair matched in energy [8]. In comparison, ESIPT-based probes are easy to construct and modify. Therefore, increasing number of ESIPT-based probes have been developed for biological imaging [9].

  In general, ESIPT-based fluorophores incorporate an intramolecular hydrogen bonding interaction between proton donor (OH/NH₂ “OH” is common) and proton acceptor (C=N and C=O). The common ESIPT fluorophores shown in Fig. 1, exist as enol form on the ground state. Upon photoexcitation, an extremely fast enol to keto phototautomerization event occurs to them along with intramolecular proton transfer, leading to dual emission from enol form (E^*) and keto form (K^*). Consequently, ESIPT-based fluorophores feature dual emission under one excitation wavelength, and are characterized by large Stokes shift which helps avoiding unwanted self-reabsorption and inner-filter effects. Therefore, ESIPT-based fluorophores have great attraction for biological imaging [10].

  Figure 1

  

  Figure 1.  The common ESIPT-based fluorophores.

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  Since ESIPT-based emission (keto form K^*) usually arises from the intramolecular proton transfer from OH to the nitrogen/oxygen atom participating in double bonds, protection-deprotection of hydroxyl group as proton donor could regulate the on/off switch of ESIPT process by intervening the occurrence of proton-transfer (Fig. 2). More, it has been as the widely used strategy of designing ESIPT-based probes for which the key issue is selecting an appropriate protective group that can specifically leave in the presence of the target analyte. In the last decade, significant achievements have been made in developing ESIPT-based probes specific for target analytes by masking the hydroxyl group [11].

  Figure 2

  

  Figure 2.  The design of ESIPT-based probe by masking the hydroxyl group.

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  This review mainly focuses on the specific protecting groups (sites) and deprotection mechanisms of ESIPT-based probes for bioactive species (mainly including RSS/ROS and a few common enzymes, etc.), and analyzes the advantages and disadvantages of different protection mechanisms from some aspects including probe stability, selectivity, response rate and assay system, etc. Moreover, the current challenges and the potential future directions in this field are included as well.

  2. ESIPT-based sensors for RSS

  RSS including biothiols, hydrogen sulfide (H₂S), sulfur dioxide (SO₂), hydrogen polysulfides (H₂S_n) and other sulfur-containing agents in living organisms, play important roles in maintaining cell health. Biothiols including glutathione (GSH), homocysteine (Hcy) and cysteine (Cys), as the main part of RSS, act as antioxidants and free-radical scavengers and function in maintaining redox homeostasis [12]. Other RSS (H₂S, H₂S_n, and SO₂) play vital roles in signal transduction and information transmission. They function independently but have a certain connection in their production, transportation, and downstream functions (Fig. 3) [13]. For example, a shortage of Cys-perhaps caused absolutely slow synthesis rate and low concentration of erythrocyte GSH for the human immunodeficiency virus (HIV) infected patients. Therefore, it is of great significance to specifically monitor the levels of each analyte for better understanding their roles in physiological and pathological processes [14].

  Figure 3

  

  Figure 3.  The transformation process of different RSS.

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  2.1 H₂S

  H₂S, is a key gaseous transmitter involving vascular tension mediation in blood vessels and neuroregulation in the brain [15]. In recent years, focusing on strong reducibility and nucleophilicity of hydrogen sulfide, some ESIPT-based probes have been conducted based on protection and deprotection of hydroxyl group.

  Based on the recognition that azido group could act as specific reaction site for H₂S [16], Lin et al. creatively developed probe 1 by employing 2-(azidomethyl)benzoic acid masking the hydroxyl group in 2-(benzo[d]thiazol-2-yl)phenol (HBT) (Fig. 4) [17]. For probe 1, ESIPT-based emission was quenched due to hydroxyl group being masked, but in the presence of hydrogen sulfide, the azide group was reduced to an amino group, which spontaneously cyclized with intramolecular ester unit to release the fluorophore HBT, and the quenched emission was restormed. As a result, probe 1 exhibited high selectivity and a large signal change (400-fold), except for a slow response. In view of the physiological features of H₂S including low concentration, short lifetime and high reactivity, it was difficult for probe 1 to detect the real-time levels of intracellular H₂S. Hence, it was very necessary to develop response-rapid probes for hydrogen sulfide.

  Figure 4

  

  Figure 4.  The design and synthesis of probe 1 for H₂S.

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  Since H₂S has smaller pK_a values and a stronger nucleophilicity than thiols at physiological condition (H₂S: 7.0; biothiols ≥ 8.5), the S-S bond could act as a specific reaction site for H₂S [18]. For example, in 2012, Qian et al. developed a rapid ratiometric probe 2 based on successive nucleophilic substitution, where the hydroxyl group was protected by 2-(pyridin-2-yl-disulfanyl)benzoic acid (Fig. 5) [19]. As the disulfide was reduced by H₂S, a new disulfide was formed where the electron-poor carbonyl carbon atom was subsequently attacked by the nucleophilic sulfhydryl group forming a self-immolative benzoic acid-disulfide structure cleaved, resulting in the recovery ESIPT-based emission.

  Figure 5

  

  Figure 5.  The design and synthesis of probes 2 and 3 for H₂S.

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  Concerning that the reaction rate of diselenides with H₂S was much faster than that of disulfides, in 2019, Guo et al. reported a diselenides-contained probe 3, which could generate a 47-fold fluorescence enhancement immediately after the addition of H₂S [20]. The nucleophilic attack by H₂S at diselenides bond was more favorable both kinetically and thermodynamically than at disulfide bond. Although probe 3 could sensitively detect the H₂S levels in cells or in vivo, the large masking groups were released to the biological system as organic waste, raising a concern on toxicity. As a response, Emrullahoğlu et al. reported an excellent probe 4 with a cyanate (O−CN) unit as a mask of flavonoid hydroxyl in 2016 (Fig. 6) [21].

  Figure 6

  

  Figure 6.  The design and synthesis of probe 4 for H₂S.

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  Due to the stronger nucleophilicity of H₂S than that of cysteine and glutathione, H₂S more readily added to the electrophilic carbon atom in cyanate unit to form a thiocarbamate derivative which could rapidly hydrolyse in physical condition to release the highly emissive fluophore. Probe 4 was confirmed to have the exceptional selectivity and rapid response to H₂S, also successfully employed to detect the levels of introcellular H₂S.

  HBT-based fluorophores as the classic ESIPT-based groups, usually are excited by short-wavelength light which tended to cause cellular autofluorescence, photodamage and artifactual generation of ROS. Based on the fact that two photon excitation fluorescence (TPEF) could be excited by long-wavelength light with minor damage to living organism and that HBT unit possessed TP cross-sections for TPEF imaging, Yoon et al. developed TP fluorescent probe 5 with obvious aggregation-induced emission (AIE) effect, which was prepared through a cascade type Baylis-Hillman and followed intramolecular Michael addition reaction (Fig. 7) [22].

  Figure 7

  

  Figure 7.  The design and synthesis of probe 5 for H₂S.

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  The nucleophilic addition response of H₂S to probe 5 would lead to the opening of chromene ring thereby the formation of ESIPT-based fluorophore. Experiments results illustrated that probe 5 could detect intracellular H₂S levels under the excitation of two near-infrared (NIR) photons (740 nm) with negligible light damage and background interference by using two-photonmicroscopy (TPM).

  According to that the electron-withdrawing 2,4-dinitrophenyl ether was easily cleaved by H₂S over biothiols, quite a few ESIPT-based probes were developed based on the recognition site, including probe 6 for which the relative response mechanism was shown in Fig. 8 [23].

  Figure 8

  

  Figure 8.  The design and synthesis of probe 6 for H₂S.

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  2.2 H₂S₂

  H₂S_n (n > 1) as an important signal molecullar, performs the function of activating ion channels, transcription factors and tumor suppressors, which could be endogenously generated by H₂S under the oxidation of ROS like HClO [24]. Based on aromatic nucleophilic substitution reaction and reduction reaction, some probes for H₂S_n have been developed. For example, Chen and co-workers selected the 2-fluoro-5-nitro-benzoic moiety as the mask of hydroxyl group to construct ESIPT-based probe 7 (Fig. 9), where the nucleophilic reaction occured in presence of H₂S₂ then the spontaneous intermolecular cyclization reaction released the dye resulted in an on-off emmision [25]. The related experiments results indicated that probe 7 could sense H₂S₂ by a remarkable fluorescence enhancement (328-fold) at 534 nm with a low detection limit of 26 nmol/L.

  Figure 9

  

  Figure 9.  The design and synthesis of probe 7 for H₂S₂.

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  2.3 Thiols

  Biothiols including GSH, Hcy and Cys have similar structures and properties, which independently function in the biological system and metabolically related. It has been confirmed that Cys and Hcy can metabolize to produce GSH, and GSH as the most abundant small molecule thiol, is responsible for maintaining intracellular redox homeostasis [26]. For Hcy, its content in plasma is about 15 µmol/L, and its abnormal increase can easily lead to thrombosis. As for Cys, normal cells do not have a high demand for its concentration, and its abnormally elevated levels can easily change iron homeostasis further lead to mitochondrial decline [27]. Therefore, it is of great significance for prevention and diagnosis of related diseases and understanding of related physiological and pathological processes to develop fluorescent probes that can specifically detect these thiol molecules.

  Although it was hard that these thiols were selectively recognized due to their similarity in structure and property, a big breakthrough came in 2011 when Strongin's group designed first ESIPT-based probe 8 by masking hydroxy group in 2-(2′–hydroxy-3′-methoxyphenyl)benzothia-zole (HMBT) with α,β-unsaturated carbonyl moiety, which could discriminate Cys from Hcy based on their different reaction rates (Fig. 10) [28].

  Figure 10

  

  Figure 10.  The design and synthesis of probes 8 and 8′ for Cys.

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  The sulfhydryl group of Cys is nucleophilically added to the C=C double bonds of the acrylate to generate thioethers, which undergoes subsequent intramolecular cyclization to release the dye. Hcy similarly could generate thioether, but subsequent intramolecular cyclization reaction to form eight-membered ring would not be kinetically favored compared to the formation of seven-membered ring resulted from Cys. Therefore, based on different circularization rates, the designed fluorescent probe 8 could distinguish Cys from Hcy.

  Based on similar strategy, bromopropionyl or chloropropionyl groups were also used as the masker of hydroxy group to design ESIPT-based fluorescent probes for Cys. Also, the bromopropionyl group is expected to show a faster nucleophilic substitution rate with Cys than chloropropionyl as it incorporated a better leaving group. Churchill et al. report a new HBT-based probe 9 involving bromopropionyl group [29]. Probe 9 underwent nucleophilic substitution reaction with Cys to form the intermediate product similar to the first-step reaction product of probe 8 with Cys, then underwent intramolecular cyclization to release the fluorophore (Fig. 11).

  Figure 11

  

  Figure 11.  The design and synthesis of probe 9 for Cys.

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  However, the response rate of probe 9 to Cys was slower than that of probe 8 possibly because the sulfhydryl group of Cys was nucleophilically added to the C=C double bonds of the acrylate to generate thioethers more easily than it nucleophilically substituted bromine atom of bromopropionyl group. Therefore, acrylate group is more attractive as specific recognition site for Cys than bromopropionyl group. In 2020, using acrylate group as reaction site we also reported a NIR Cys probe 8′ by coupling dicyanoisophorone with HMBT, which turned on the 686 nm emission at the presense of Cys when excited by 423 nm light [30].

  Relative to probes 8 and 9, probe 8′ well performed Cys detection in vivo, because visible light as excitation and NIR emission caused less damage to the tissues and less interference from biological autofluorescence than short-wavelength excitation and emission.

  Due to slightly stronger acidity of Cys than Hcy and GSH, Cys was more nucleophilic thereby ready to react with related probes at physical condition. Therefore, many thiol probes could generate a strong response to Cys/Hcy over GSH, as a result, developing GSH-specific probes especially based on ESIPT mechanism remains a challenge [31]. It is well known that cetyltrimethylammonium bromide (CTAB) micelles can not only enrich hydrophobic probes but also tend to electrostatically combine with GSH which is more negatively charged than Cys or Hcy due to its contained two carboxyl groups. In 2016, Chen et al. employed dinitrophenyl ether as masker of hydroxy group to construct probe 10 which could selectively detect GSH catalyzed by CTAB micelles (Fig. 12) [32].

  Figure 12

  

  Figure 12.  The reaction of probe 10 with GSH.

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  The excellent selectivity of probe 10 to GSH over Cys and Hcy is mainly attributed to one more carboxyl groups in GSH. Probe 10 was readily aggregated in positively charged CTAB micelles through hydrophobic and hydrogen bonding interaction. GSH with two carboxyl groups easily generates negative charges thereby shows a stronger affinity to CTAB micelles than Cys/Hcy. Therefore, GSH could likely attack the probe inside the micellar aggregates, resulting in the release of fluorophores.

  In addition, a general strategy to discriminate Cys/Hcy from GSH was two-step tandem reaction permitted only by Cys/Hcy which contained nucleophilic substitution of sulfhydryl groups and S and N-acyl intromolecular transfer [33]. Combining this strategy with “quinone-phenol” transduction of rhodol dye related to ESIPT process in HBT unit, in 2014, Strongin's group reported a rhodol thioester probe 11 discriminating Cys/Hcy from GSH (Fig. 13) [34]. Probe 11 reacted with Cys/Hcy through a series of tandem (nucleophilic substitution of sulfhydryl groups, S and N-acyl intromolecular transfer and spirocyclization) to form the corresponding deconjugated spirolactam, which converted the structure of rhodol dye from quinone type to phenol type thereby turned on the ESIPT-based emission at 454 nm. For the tripeptide GSH, only the nucleophilic substitution reaction of the sulfhydryl group occurred to probe 11, arising a large enhancement of rhodol-based emission at 587 nm due to the removement of the PET process caused by 4-nitrobenzene. In the above strategy, the process of S and N-acyl intramolecular transfer was the key to discriminate Cys/Hcy with GSH by probe 11.

  Figure 13

  

  Figure 13.  The response of probe 11 to thiols.

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  2.4 SO₂

  Many studies claim that SO₂ can be produced endogenously by sulfur-containing amino acids in mammals, which as an active small molecule play a vital role in regulating cardiovascular functions [35]. In addition, epidemiological studies have illustrated that sulfur dioxide is also related to many diseases, including cardiovascular diseases, neurological disorders respiratory diseases even lung cancer [36]. Lin et al. developed first generation AIE + ESIPT ratiometric SO₂ probe 12 by introducing a thiazole group and a levulinate group into the tetraphenylethene scaffold (Fig. 14) [37]. In the pure aqueous solution, probe 13 only exhibited a strong enol-form emission at 422 nm due to the protection of the hydroxyl group by levulinate moiety, and upon the deprotection of levulinate by SO₂, the strong keto-form emission at 565 nm was turned on and the enol-form emission disappeared. Relative to pure organic solution, probe 12 could ratiometricly monitor SO₂ better due to AIE effect in pure aqueous solution.

  Figure 14

  

  Figure 14.  The design and synthesis of probe 12 for SO₂.

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  3. ROS

  ROS are endogenous oxidants with vital functions such as signal transduction and host defense, and are associated with stem cell differentiation, aging and cancer [38]. ROS are mainly composed of H₂O₂/HClO/HNO/ONOO⁻ and free radicals OH/O₂⁻/NO^., and they are transformed into each other in the body (Fig. 15) [39]. These transient species may endanger human health when their levels are higher than normal. To understand their effects and impede oxidative damage, developing fluorescent probes for ROS is necessary [40].

  Figure 15

  

  Figure 15.  The transformation relationship among different reactive oxygen species.

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  3.1 H₂O₂

  H₂O₂ is one of the most studied ROS, which maintains the redox homeostasis in the organism together with GSH [41]. The monitoring of hydrogen peroxide level is helpful for the assessment of human health. Chang and his colleagues pioneered the development of a borate-based hydrogen peroxide sensor and applied it to the detection of hydrogen peroxide in biological systems [42]. On this basis, some hydrogen peroxide probes were developed based on ESIPT mechanism through masking hydroxyl groups with p-phenylborate benzyl bromide. For example, probe 13 reported by Tang's group, could respond to hydrogen peroxide according to the mechanism proposed in Fig. 16 [43]. For probe 13, phenylboronate as the sensing group and ESIPT blocking group, was oxidized by H₂O₂ and a 1,6-rearrangement elimination reaction was followed to free the phenolic OH group and recover the ESIPT process. It exhibited high selectivity toward H₂O₂ over other ROS/RNS and biological species with a NIR emission. Similar H₂O₂ probes were also reported including that in Fig. 17 [44].

  Figure 16

  

  Figure 16.  The design and synthesis of probe 13 for H₂O₂.

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  Figure 17

  

  Figure 17.  The structures of other probes 14–17 for H₂O₂.

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  3.2 Peroxynitrite (ONOO⁻)

  ONOO⁻ as a highly reactive nitrogen species, is more commonly known for its deleterious effects including causing irreversible damage to lipids, proteins and DNA [45]. However, it also acts as a signaling molecule in vivo for a number of pathways. Therefore, the development of powerful tools for the real-time detection of ONOO⁻ is essencial to understand the role of ONOO⁻ in different biological system and pathway process [46]. Utilizing the stronger oxidability of ONOO⁻, aryl borate would be oxydized by ONOO⁻ orders of magnitude faster than by H₂O₂. Based on this, Sedgwick et al. reported an ESIPT-based ratiometric fluorescence probe 17, which was able to selectively detect low concentrations of ONOO (limit of detection: 21.4 nmol/L) within a few seconds [47]. In addition, Shen et al. developed a simple AIE-ESIPT fluorescent probe 18 for monitoring ONOO⁻ using diphenylphosphinate group as the recognition site and salicylaldehyde azine as the fluorophore, which displayed weakly emissive in aqueous solution because of protection of OH group in salicylaldehyde azine by diphenylphosphinate group (Fig. 18) [48]. Oxidized by ONOO⁻, probe 18 split diphenylphosphinate group away to produce salicylaldehyde azine, resulted in an enhanced emission in aqueous solution induced by a combination of the AIE-ESIPT mechanism.

  Figure 18

  

  Figure 18.  The design and synthesis of probe 18 for ONOO⁻.

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  3.3 KO₂

  O₂^•− as both an anion and a free radical, has been identified as potential messenger to regulate the cell-signaling network. Abnormal concentrations of O₂^•− ultimately correlate with the aetiology of disease [49]. Owing to its short half-life, extremely low concentration, and high reactivity, it is challenged to develope the probes that could detect O₂^•− in vitro or in vivo. Churchill et al. firstly presented an ESIPT-based superoxide sensor 19 with phophinates [P(O)Ph₂] as the site of O₂^•− and the mask of the hydroxyl group in HBT [50]. Superoxide permits an addition elimination reaction of phophinates and hydrolysis (Fig. 19) to give free HBT.

  Figure 19

  

  Figure 19.  The design and synthesis of probe 19 for O₂^•−.

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  3.4 Hypochlorous acid (HOCl)

  HOCl is a biologically important ROS, which partially dissociates to form its hypochlorite anion (ClO⁻) under physiological conditions. In biological systems, myeloperoxidase as an enzyme found in leukocytes, produces HOCl/ClO by catalysing the reaction between Cl and H₂O₂. This vital ROS functions in immune defence systems due to its microbicidal properties. However, excessive production of HOCl/ClO can lead to the damage of a range of biological targets such as amino acids, proteins, carbohydrates and lipids.

  Sedgwick et al. reported the ESIPT-based fluorescence probe 20 by using a dimethylthiocarbamate protecting group [51], which could detect HClO/ClO within 10 s and have an excellent selectivity towards other ROS/RNS and amino acids (Fig. 20).

  Figure 20

  

  Figure 20.  The design and synthesis of probe 20 for HOCl.

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  4. Enzymes

  Enzymes are a class of important substances for life, and their abnormal levels are associated with many diseases. In recent years, some ESIPT-based enzymes-related fluorescent probes have been developed, including nitroreductase (NTR).

  4.1 NTR

  NTR can reduce nitro group into the corresponding amines or hydroxyl amines in the presence of reduced nicotinamide adenine dinucleotide (NADH), and therefore, its recognition moiety often contains the nitro group as an essential part. 4-Nitrobenzyl alcohol is another commonly used recognition moiety for NTR. Shao et al. developed probe 21 by incorporating 4-nitrobenzyl unit on HBT-contained derivatives (Fig. 21) [52]. The chromophore was yielded along with the reduction, rearrangement and elimination of 4-nitrobenzyl unit. Probe 21 displayed a wide linear range (0.1–1.5 mg/mL) and low detection limit (2.8 ng/mL) response to NTR.

  Figure 21

  

  Figure 21.  The design and synthesis of probe 21 for NTR.

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  4.2 Esterases

  Esterases widely existed in tissue cells of various organisms, and catalyzed the hydrolysis of various esters, and mediated the metabolism of various tissues and cells. More importantly, esterase plays a significant role in cell viability and cytotoxicity assays. Therefore the quantitation of esterase activity could be used to evaluate cellular status. Tong et al. developed the esterase probe 22 with the hydroxyl group covered by acetoxy [53], as shown in Fig. 22. It could sense the esterase quantitatively in the range of 0.01–0.15 U/mL.

  Figure 22

  

  Figure 22.  The design and synthesis of probe 22 for esterases.

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  In view of that esterase was active in live cells but deactivated in dead cells, Lin’s group developed an fluorescent probe 23 by acetylization of 3-hydroxyflavone [54], which can be hydrolyzed into 3-hydroxyflavone by active esterase with ESIPT process recovered (Fig. 23). Therefore, it displays blue emission in dead cells and orange one in live cells, discriminating live cells from dead cells clearly in two emission colors.

  Figure 23

  

  Figure 23.  The design and synthesis of probe 23 for esterases.

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  4.3 Alkaline phosphatase (ALP)

  ALP is widely distributed in human bone, intestine, liver, placenta and other tissues, which is a crucial biomarker for the diagnosis of hepatobiliary and skeletal diseases. The abnormal level of ALP is also associated with some other diseases, such as extrahepatic biliary obstruction, intrahepatic space occupying lesions, rickets and cancers. In recent years, to understand the roles of ALP in these diseases, various fluorescent probes have been developed to detect ALP activity in serum, and image ALP in cells and tumor tissues.

  Yang et al. presented an ALP probe 24 (4-benzamio-2-(benzo[d] thiazol-2-yl)phenyl dihydrogen phosphate by transforming the hydroxyl group of ESIPT-based fluorophore into the corresponding phosphate group [55]. In the presence of ALP, probe 24 was hydrolyzed and the fluorophore was released, which exhibited large emission spectral red-shift (120 nm) because of the recovery of ESIPT process (Fig. 24).

  Figure 24

  

  Figure 24.  The design and synthesis of probe 24 for ALP.

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  4.4 β-Galactosidase

  β-Galactosidase is overexpressed in primary ovarian cancers and abnormally accumulated in senescent cells, that is an important biomarker for senescence and ovarian cancers.

  The sensitive detection of β-galactosidase is thus of great importance. Tong et al. reported a fluorescent probe 25 with light-up response to β-galactosidase (Fig. 25), which could be well retrained in living cells and emit strong fluorescence [56]. Probe 25 had a Stokes shift of 190 nm, while Pan et al. developed a probe combining a similar reaction with an ICT mechanism, which only exhibits a Stokes shift of around 30 nm [57]. The combination of hydroxyl protection strategy and ESIPT mechanism in constructing probes fully exemplifies their unique advantages.

  Figure 25

  

  Figure 25.  The design and synthesis of probe 25 for β-galactosidase.

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  4.5 Caspases

  Caspases as aspartate-specific cysteine proteases, play vital roles in apoptosis and inflammation. Their inactivation inevitably causes some diseases including neurodegenerative diseases. Kuang et al. constructed a novel peptide probe 26 for peptidases through the Michael addition reaction between an acryloylated ESIPT fluorophore and a cysteine-appended peptide substrate [58]. The peptide bond in the probe was broken under the catalysis of intracellular peptidase, accordingly, the substrate peptide was released and the ESIPT fluorophore was recovered along with intramolecular cyclization (Fig. 26). The design strategy of probe 26 was generally applicable, which enabled the development of a variety of probes for peptidases. The performance parameters of the above probes were cited in detail and summarized in Table S1 (Supporting information).

  Figure 26

  

  Figure 26.  The design and synthesis of probe 26 for caspases.

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  5. Conclusion and outlook

  The design and synthesis of probes based on regulating ESIPT through hydroxyl protection and deprotection are relatively simple and mature. Many related achievements have been made in developing fluorescent probes for RSS, ROS and enzymes in recent years. Unfortunately, most of these probes belong to esters, amides or ethers, which are easy to hydrolyze and sensitive to pH. Moreover, the solubility of these probes is poor, and the detection is carried out in systems containing dimethylsulfoxide or acetonitrile. All of these limit the application of such probes in complex biological systems. Considering that ESIPT emission could be regulated by electronic effects and intermolecular hydrogen bonding, therefore, we put forward the following outlook: (1) Based the specific chemical reaction, ESIPT process is switched through analyte-induced electronic effect, realizing the detection of the analyte; (2) The ESIPT switch can also be regulated by the new-born intromolecular hydrogen bonding from the adduct of probe with analyte, thus possible for constructing multiple- sites probes.

  Declaration of competing interest

  The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  CRediT authorship contribution statement

  Haixian Ren: Writing – original draft. Yuting Du: Software. Xiaojing Yang: Data curation. Fangjun Huo: Validation. Le Zhang: Formal analysis. Caixia Yin: Project administration.

  Acknowledgments

  We thank the National Natural Science Foundation of China (Nos. 22277104, 22325703, 22074084), the Natural Science Foundation of Shanxi Province (No. 202203021212184), Research Project supported by Shanxi Scholarship Council of China (No. 2022–002), the Basic Research Program of Shanxi Province (Free Exploration) (No. 202203021221009), 2022 Lvliang City science and technology plan project (Nos. 2022SHFZ51, 2022GXYF15) and Scientific Instrument Center of Shanxi University (No. 201512).

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2024.109867.

  
  
• 1. [1]

     V.N. Vakharia, J.S. Duncan, J.A. Witt, et al., Ann. Neurol. 83 (2018) 676–690. doi: 10.1002/ana.25205

  2. [2]

     L.L. Wu, A.C. Sedgwick, X.L. Sun, et al., Acc. Chem. Res. 52 (2019) 2582–2597. doi: 10.1021/acs.accounts.9b00302

  3. [3]

     N. Kwon, D. Kim, K.M. Swamy, J. Yoon, Coord. Chem. Rev. 427 (2021) 213581. doi: 10.1016/j.ccr.2020.213581

  4. [4]

     H.W. Liu, L. Chen, C. Xu, et al., Chem. Soc. Rev. 47 (2018) 7140–7180. doi: 10.1039/c7cs00862g

  5. [5]

     D.P. Li, X.J. Han, Z.Q. Yan, et al., Dyes Pigm. 151 (2018) 95–101. doi: 10.1016/j.dyepig.2017.12.056

  6. [6]

     J. Ding, R. Xiao, A. Bi, et al., Chin. Chem. Lett. 34 (2023) 108273. doi: 10.1016/j.cclet.2023.108273

  7. [7]

     S. Shen, W. Xu, J. Lu, et al., Chin. Chem. Lett. 35 (2024) 108360. doi: 10.1016/j.cclet.2023.108360

  8. [8]

     X. He, F. Ding, W. Xu, et al., Anal. Chim. Acta 1127 (2020) 29–38. doi: 10.3390/healthcare8010029

  9. [9]

     K. Wang, D. Xi, C. Liu, et al., Chin. Chem. Lett. 31 (2020) 2955–2959. doi: 10.1016/j.cclet.2020.03.064

  10. [10]

      J. Li, Y. Chen, T. Chen, et al., Sens. Actuator. B: Chem. 268 (2018) 446–455. doi: 10.1016/j.snb.2018.04.130

  11. [11]

      X. Wei, Q. Wu, Y. Feng, et al., Sens. Actuator. B: Chem. 304 (2020) 127242. doi: 10.1016/j.snb.2019.127242

  12. [12]

      L.Y. Niu, Y.Z. Chen, H.R. Zheng, et al., Chem. Soc. Rev. 44 (2015) 6143–6160. doi: 10.1039/C5CS00152H

  13. [13]

      H. Kimura, Neurochem. Int. 126 (2019) 118–125. doi: 10.1016/j.neuint.2019.01.027

  14. [14]

      Z.G. Zhu, L.L. Zhang, Q.H. Chen, et al., Biochem. Biophys. Res. Commun. 524 (2020) 916–922. doi: 10.1016/j.bbrc.2020.02.013

  15. [15]

      C.W. Leffler, H. Parfenova, S. Basuroy, et al., Am. J. Physiol. Heart Circ. Physiol. 300 (2010) H440–H447.

  16. [16]

      L. Sun, Y. Wu, J. Chen, J. Zhong, F. Zeng, S. Wu, Theranostics 9 (2019) 77–89. doi: 10.7150/thno.30080

  17. [17]

      B. Deng, M. Ren, X. Kong, K. Zhou, W. Lin, RSC Adv. 6 (2016) 62406–62410. doi: 10.1039/C6RA12127F

  18. [18]

      J. Wang, Y. Wen, F. Huo, C. Yin, Sens. Actuator. B: Chem. 297 (2019) 126773. doi: 10.1016/j.snb.2019.126773

  19. [19]

      Z. Xu, L. Xu, J. Zhou, et al., Chem. Commun. 48 (2012) 10871–10873. doi: 10.1039/c2cc36141h

  20. [20]

      H. Guan, A. Zhang, P. Li, L. Xia, F. Guo, ACS Omega 4 (2019) 9113–9119. doi: 10.1021/acsomega.9b00934

  21. [21]

      E. Karakuş, M. Üçüncü, M. Emrullahoğlu, Anal. Chem. 88 (2016) 1039–1043. doi: 10.1021/acs.analchem.5b04163

  22. [22]

      L. Chen, D. Wu, C.S. Lim, et al., Chem. Commun. 53 (2017) 4791–4794. doi: 10.1039/C7CC01695F

  23. [23]

      C. Wu, X. Hu, G. Biao, et al., Anal. Methods 10 (2018) 604–610. doi: 10.1039/c7ay02492d

  24. [24]

      R. Miyamoto, S. Koike, Y. Takano, et al., Sci. Rep. 7 (2017) 45995. doi: 10.1038/srep45995

  25. [25]

      P. Hou, J. Wang, S. Fu, L. Liu, S. Chen, Spectrochim. Acta A: Mol. Biomol. Spectrosc. 213 (2019) 342–346. doi: 10.1016/j.saa.2019.01.081

  26. [26]

      D.M. Townsend, K.D. Tew, H. Tapiero, Biomed. Pharmacother. 57 (2003) 145–155. doi: 10.1016/S0753-3322(03)00043-X

  27. [27]

      B.D. Paul, J.I. Sbodio, R.S. Xu, et al., Nature 509 (2014) 96–100. doi: 10.1038/nature13136

  28. [28]

      X. Yang, Y. Guo, R.M. Strongin, Angew. Chem. Int. Ed. 50 (2011) 10690–10693. doi: 10.1002/anie.201103759

  29. [29]

      Y. Kim, M. Choi, S. Seo, et al., RSC Adv. 4 (2014) 64183–64186. doi: 10.1039/C4RA12981D

  30. [30]

      H. Ren, F. Huo, Y. Zhang, S. Zhao, C. Yin, Sens. Actuator. B: Chem. 319 (2020) 128248. doi: 10.1016/j.snb.2020.128248

  31. [31]

      Y. Xu, R.X. Li, X.J. Zhou, et al., Talanta 205 (2019) 120–125.

  32. [32]

      X. Ren, F. Wang, J. Lv, et al., Dyes Pigm. 129 (2016) 156–162. doi: 10.1016/j.dyepig.2016.02.027

  33. [33]

      G.P. Xu, Y.H. Tang, W.Y. Lin, New J. Chem. 42 (2018) 12615–12620. doi: 10.1039/c8nj01793j

  34. [34]

      X.F. Yang, Q. Huang, Y. Zhong, et al., Chem. Sci. 5 (2014) 2177–2183. doi: 10.1039/c4sc00308j

  35. [35]

      K. Li, L.L. Li, Q. Zhou, et al., Coord. Chem. Rev. 388 (2019) 310–333. doi: 10.1016/j.ccr.2019.03.001

  36. [36]

      D.O. Johns, W.S. Linn, Inhal. Toxicol. 23 (2011) 33–43. doi: 10.3109/08958378.2010.539290

  37. [37]

      Y. Liu, J. Nie, W. Wang, W. Lin, J. Mater. Chem. B 6 (2018) 1973–1983. doi: 10.1039/C8TB00075A

  38. [38]

      B.C. Dickinson, C.J. Chang, Nat. Chem. Biol. 7 (2011) 504–511. doi: 10.1038/nchembio.607

  39. [39]

      J.T. Hou, M. Zhang, Y. Liu, et al., Coord. Chem. Rev. 421 (2020) 213457. doi: 10.1016/j.ccr.2020.213457

  40. [40]

      K.J. Barnham, C.L. Masters, A.I. Bush, Nat. Rev. 3 (2004) 205–214. doi: 10.1038/nrd1330

  41. [41]

      P.D. Ray, B.W. Huang, Y. Tsuji, Cell Signal 24 (2012) 981–990. doi: 10.1016/j.cellsig.2012.01.008

  42. [42]

      A.R. Lippert, G.C. Van De Bittner, C.J. Chang, Acc. Chem. Res. 44 (2011) 793–804. doi: 10.1021/ar200126t

  43. [43]

      L. Tang, M. Tian, H. Chen, et al., Dyes Pigm. 158 (2018) 482–489. doi: 10.1016/j.dyepig.2017.12.028

  44. [44]

      Y. Wu, Z. Li, Y. Shen, ACS Omega 4 (2019) 16242–16246. doi: 10.1021/acsomega.9b02594

  45. [45]

      P. Pacher, J.S. Beckman, L. Liaudet, Physiol. Rev. 87 (2007) 315–424. doi: 10.1152/physrev.00029.2006

  46. [46]

      X. Chen, F. Wang, J.Y. Hyun, et al., Chem. Soc. Rev. 45 (2016) 2976–3016. doi: 10.1039/C6CS00192K

  47. [47]

      L. Wu, Y. Wang, M. Weber, et al., Chem. Commun. 54 (2018) 9953–9956. doi: 10.1039/c8cc04919j

  48. [48]

      Y. Shen, M. Li, M. Yang, et al., Spectrochim. Acta A: Mol. Biomol. Spectrosc. 222 (2019) 117230. doi: 10.1016/j.saa.2019.117230

  49. [49]

      S. Chaudhury, P.K. Sarkar, Biochim. Biophys. Acta 763 (1983) 93–98. doi: 10.1016/0167-4889(83)90030-7

  50. [50]

      D.P. Murale, H. Kim, W.S. Choi, D.G. Churchill, Org. Lett. 15 (2013) 3946–3949. doi: 10.1021/ol4017222

  51. [51]

      L. Wu, Q. Yang, L. Liu, et al., Chem. Commun. 54 (2018) 8522–8525. doi: 10.1039/c8cc03717e

  52. [52]

      Q. Yang, S. Wang, D. Li, J. Yuan, J. Xu, S. Shao, Anal. Chim. Acta 1103 (2020) 202–211. doi: 10.1016/j.aca.2019.12.063

  53. [53]

      L. Peng, S. Xu, X. Zheng, et al., Anal. Chem. 89 (2017) 3162–3168. doi: 10.1021/acs.analchem.6b04974

  54. [54]

      M. Tian, J. Sun, Y. Tang, et al., Anal. Chem. 90 (2018) 998–1005. doi: 10.1021/acs.analchem.7b04252

  55. [55]

      Y. Yang, C. Zhang, R. Pan, et al., Chin. Chem. Lett. 31 (2020) 125–128. doi: 10.1016/j.cclet.2019.04.037

  56. [56]

      L. Peng, M. Gao, X. Cai, et al., J. Mater. Chem. B 3 (2015) 9168–9172. doi: 10.1039/C5TB01938A

  57. [57]

      H. Pan, X. Chai, J. Zhang, Chin. Chem. Lett. 34 (2023) 108321. doi: 10.1016/j.cclet.2023.108321

  58. [58]

      W. Liu, S. Liu, Y. Kuang, et al., Anal. Chem. 88 (2016) 7867–7872. doi: 10.1021/acs.analchem.6b02174

• 

  Figure 1  The common ESIPT-based fluorophores.

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  Figure 2  The design of ESIPT-based probe by masking the hydroxyl group.

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  Figure 3  The transformation process of different RSS.

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  Figure 4  The design and synthesis of probe 1 for H₂S.

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  Figure 5  The design and synthesis of probes 2 and 3 for H₂S.

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  Figure 6  The design and synthesis of probe 4 for H₂S.

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  Figure 7  The design and synthesis of probe 5 for H₂S.

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  Figure 8  The design and synthesis of probe 6 for H₂S.

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  Figure 9  The design and synthesis of probe 7 for H₂S₂.

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  Figure 10  The design and synthesis of probes 8 and 8′ for Cys.

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  Figure 11  The design and synthesis of probe 9 for Cys.

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  Figure 12  The reaction of probe 10 with GSH.

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  Figure 13  The response of probe 11 to thiols.

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  Figure 14  The design and synthesis of probe 12 for SO₂.

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  Figure 15  The transformation relationship among different reactive oxygen species.

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  Figure 16  The design and synthesis of probe 13 for H₂O₂.

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  Figure 17  The structures of other probes 14–17 for H₂O₂.

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  Figure 18  The design and synthesis of probe 18 for ONOO⁻.

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  Figure 19  The design and synthesis of probe 19 for O₂^•−.

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  Figure 20  The design and synthesis of probe 20 for HOCl.

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  Figure 21  The design and synthesis of probe 21 for NTR.

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  Figure 22  The design and synthesis of probe 22 for esterases.

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  Figure 23  The design and synthesis of probe 23 for esterases.

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  Figure 24  The design and synthesis of probe 24 for ALP.

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  Figure 25  The design and synthesis of probe 25 for β-galactosidase.

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  Figure 26  The design and synthesis of probe 26 for caspases.

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•