Construction and Application of Phenothiazine-Based Fluorescent Probes for the Detection of Hypochlorous Acid

Yuehua Yuan; Chaoyi Yue; Mengqing Du; Feng Feng; Maozhong Tian

doi:10.7536/PC20250909

Progress in Chemistry ›› 2026, Vol. 38 ›› Issue (5) : 921-940 DOI: 10.7536/PC20250909

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Construction and Application of Phenothiazine-Based Fluorescent Probes for the Detection of Hypochlorous Acid

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Abstract

Hypochlorous acid/hypochlorite （HClO/ClO^-） is one of the prominent reactive oxygen species （ROS）， which has pivotal impacts in both daily life and complex biological processes. Excessive HClO/ClO^- can lead to a series of diseases， such as arthritis， cardiovascular diseases and cancer， etc. Therefore， it is of great significance to detect HClO/ClO^- in biological systems rapidly， sensitively and selectively. Phenothiazine （PTZ） and its derivatives with electron-rich properties are a class of significant fluorophores. At the main modification sites of a PTZ fluorophore， electron-donating or electron-withdrawing groups can be easily introduced to effectively regulate the photophysical properties of phenothiazine molecules. Moreover， they also have the advantages of non-planar butterfly-shaped curved structures that can fully inhibit the formation of molecular aggregates and environmental friendliness. Hence， they show obvious superiorities in the construction of HClO/ClO^- fluorescent probes. In this review， we systematically discuss the research progress of phenothiazine-based fluorescent probes for HClO/ClO^-. The sensing mechanism， relevant performance parameters and practical application of these fluorescent probes are highlighted， and the prospects for phenothiazine-based fluorescent probes for HClO/ClO^- are discussed.

Contents

1 Introduction

2 Fluorescent probes for HClO/ClO^- based on PET

3 Fluorescent probes for HClO/ClO^- based on ICT

4 Fluorescent probes for HClO/ClO^- based on other detection mechanism

5 Conclusion and outlook

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Keywords

phenothiazine / hypochlorous acid / hypochlorite / fluorescence probe / recognition mechanism / practical application

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Yuehua Yuan, Chaoyi Yue, Mengqing Du, Feng Feng, Maozhong Tian. Construction and Application of Phenothiazine-Based Fluorescent Probes for the Detection of Hypochlorous Acid. Progress in Chemistry, 2026, 38(5): 921-940 DOI:10.7536/PC20250909

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1 Introduction

Hypochlorous acid/hypochlorite (HClO/ClO^-) is one of the biologically important reactive oxygen species and is frequently used as a disinfectant and bleaching agent in our daily lives. In particular, it plays an important role in various physiological and pathological processes of living organisms. Endogenous hypochlorous acid is mainly produced in biological systems from hydrogen peroxide (H₂O₂) and chloride ions (Cl^-) catalyzed by myeloperoxidase (MPO)^[1-2]. On the one hand, HClO/ClO^- participates in various immune processes, including preventing microbial invasion, combating pathogens, activating transcription factors, and regulating apoptosis^[3-5]; on the other hand, excessive HClO/ClO^- is considered a major threat to human health, leading to cellular dysfunction and oxidative stress, thereby causing inflammatory diseases, lung injury, and even cancer^[6-11]. Therefore, developing reliable analytical methods for detecting HClO/ClO^- concentrations in the environment and living organisms is of great significance.

Currently, as an important analytical and imaging tool, fluorescent probes have been widely applied in fields such as environmental and food safety monitoring, biochemical analysis, and disease diagnosis.^[12-16]. Compared with detection methods such as electrochemical methods, UV-Vis spectroscopy, titration, and chemiluminescence, fluorescent probes offer advantages including low cost, high sensitivity, fast response, real-time detection, good selectivity, and in situ imaging. Furthermore, aided by advanced optical instruments, fluorescent probes can dynamically monitor the concentration of certain substances in biological systems in real time, providing authentic and reliable analytical and imaging experimental data for timely diagnosis and precise treatment of related diseases. Therefore, fluorescent probe analysis methods have attracted many researchers to actively carry out relevant studies on environmental and biological systems, achieving further integration of chemistry with other disciplines and improving both the quantity and quality of scientific research outputs.

In the design of fluorescent probes, as an important component of fluorescent probes, the selection of fluorophores is crucial for achieving optimal optical performance. Currently, many fluorophores have been selected to develop hypochlorous acid fluorescent probes^[17], such as BODIPY^[18-19], rhodamine^[20-21], fluorescein^[22], coumarin^[23], cyanine dyes^[24-25], and phenothiazine^[26], among others. These fluorophores all exhibit their own distinct excellent optical properties in constructing fluorescent probes. However, common fluorophores face some challenges, such as low fluorescence quantum yield, short emission wavelengths, self-quenching, and small Stokes shifts, which are unfavorable for in vitro and in vivo fluorescence imaging and still limit the development of fluorescent probes to a certain extent. Therefore, substantial work remains to be done to develop high-performance fluorescent probes capable of practical application in biological and environmental samples.

Due to its unique optical and electronic properties, potent pharmacological effects, and excellent bioactivity, combined with the advantages of low cost and easy functionalization, phenothiazine (PTZ) has received widespread attention in the fields of chemistry, biology, and medicine, becoming one of the most extensively studied aromatic compounds. In recent years, PTZ and its derivatives have provided a multifunctional platform for developing materials with broad applications.^[27-28]. The strong electron-donating ability of PTZ is attributed to the electron-rich nitrogen and sulfur heteroatoms within its molecule. These nitrogen and sulfur atoms not only enhance electron delocalization within the molecular structure, promoting effective light absorption and emission, but also impart rigidity to the aryl ring structure in the ground state, resulting in a non-planar "butterfly" configuration. This inhibits π-π stacking interactions and reduces non-radiative decay pathways, thereby facilitating fluorescence enhancement. The primary modification sites of the PTZ molecule are located at C-2, C-3, C-7, C-8, and the N atom positions, asFigure 1shows. Modification at these sites plays a key role in improving the photophysical properties of PTZ. Electron-donating groups (EDG) or electron-withdrawing groups (EWG) can be introduced separately to effectively regulate the energy levels of the Highest Occupied Molecular Orbital (HOMO) and the Lowest Unoccupied Molecular Orbital (LUMO). This enhances light absorption capabilities and achieves the goals of either increasing or decreasing PTZ molecular fluorescence. Such precise regulation of PTZ molecular function through rational multi-site modification holds significant theoretical value and broad application prospects.

In recent years, PTZ has attracted increasing attention in the construction of hypochlorous acid fluorescent probes with excellent optical properties. We have also recently carried out some active work on the design, synthesis, and bioimaging of PTZ-based hypochlorous acid fluorescent probes.^[29-32]. In view of the growing development in this field, this paper summarizes and compares PTZ-based fluorescent probes for detecting HClO/ClO^- in terms of excitation and emission wavelengths, detection limits, test conditions, and practical applications. Some of these probes are discussed in detail, including their sensing mechanisms, fluorescence response performance, and basic characteristics regarding biological and environmental applications. Finally, an outlook is provided on the potential challenges and future development trends of PTZ-based HClO/ClO^- fluorescent probes in research areas such as biological detection and environmental analysis.

2 HClO/ClO based on PET mechanism^-fluorescent probe

Photoinduced electron transfer (PET) is an important strategy for functional molecular design, commonly used to construct fluorescent probes operating in "off-on" or "on-off" modes. Since the development of the first PET-based fluorescent probe by de Silva et al. in 1985, it has attracted significant attention from researchers in chemistry, biology, and medicine due to its high signal-to-noise ratio. The structure of a typical PET probe consists of three parts—a fluorophore, a spacer, and a recognition group—connected via chemical bonds (Figure 2).

Within the PET system, intramolecular electron transfer occurs between the recognition group and the fluorophore, resulting in fluorescence quenching; generally, based on the difference in the direction of electron transfer between the fluorophore and the recognition group, PET can be divided into two different types, namely recognition group-excited PET (a-PET, Figure 3A) and donor-excited PET (d-PET, Figure 3B). In the a-PET process, the electron transfer path is from the recognition group to the fluorophore; this process occurs because the highest occupied molecular orbital (HOMO) of the recognition group has a higher energy level. In the d-PET mechanism, the direction of electron transfer is exactly opposite: after the fluorophore is excited, its excited-state electrons transfer to the lowest unoccupied molecular orbital (LUMO) of the recognition group. This electron transfer consumes the excited-state energy of the fluorophore, leading to probe fluorescence quenching. However, when the recognition site in the probe specifically binds to the target substance, the originally smooth a-PET or d-PET electron transfer process becomes restricted. At this point, the excited-state energy of the fluorophore is no longer consumed by the electron transfer process, allowing the probe's fluorescence signal to recover significantly.

To develop novel organic fluorophores with high performance, it is essential to prioritize the screening of molecular systems that combine excellent photophysical properties with efficient charge transfer behavior. In this context, cyano-based fluorescent dyes have attracted significant attention due to two key advantages: on one hand, they exhibit superior photophysical properties; on the other hand, ultrafast intramolecular charge transfer processes can be achieved within their molecules. This characteristic is crucial for enhancing the response speed and sensitivity of fluorophores in applications such as sensing and imaging. Precisely because of this, dicyanoisophorone (DCI) molecules have been propelled to the forefront of research into novel organic fluorophores. In 2025, Shen et al.^[32]designed and synthesized a new dual-responsive fluorescent probe P1 based on a PTZ-DCI binary system (Figure 4) for the simultaneous detection of viscosity and ClO^-both in vitro and in vivo. Due to the presence of twisted intramolecular charge transfer (TICT) and d-PET effects, this sensor emits weak fluorescence in aqueous solutions. However, in high-viscosity systems, the hydroxyl groups of glycerol can form abundant hydrogen bonds with the probe, improving its water solubility. Consequently, intramolecular random rotation and non-radiative deactivation are suppressed. Furthermore, excited-state geometric reorganization slows down, leading to a significant enhancement of fluorescence emission at 459 nm. After the reaction of probe P1 with ClO^-, the sulfur atom on PTZ is rapidly oxidized, blocking the d-PET process from PTZ to DCI, which results in a marked increase in fluorescence at 577 nm. The probe exhibits a limit of detection (LOD) for hypochlorous acid of 192.3 nmol/L and offers advantages such as good selectivity, pH insensitivity, and excellent photostability. Probe P1 has been successfully applied to the detection of ClO^-in onion and lotus root cells, as well as for the fluorescence imaging of viscosity and exogenous/endogenous hypochlorite in zebrafish.

In 2016, Liang et al.^[33]developed a fluorescent probe P2 (Figure 5) for HClO fluorescence imaging in INS-1 and RAW 264.7 cells, which was prepared by linking PTZ with acridine orange (AO) via a propyl group. The results indicated that the fluorescence of AO was quenched through an a-PET process due to the strong electron-donating nature of the PTZ moiety. The addition of HClO inhibited the a-PET process of the probe, leading to fluorescence enhancement. Probe P2 exhibited high selectivity and anti-interference capability. Its detection pH range was 5–11, with a response to HClO within 5 seconds. Upon the addition of 5 equivalents of HClO, the fluorescence intensity of the probe increased by more than 75-fold, and the limit of detection (LOD) for HClO was as low as 2.7 nmol/L. Importantly, under light excitation at 488 nm, probe P2 was successfully applied for the fluorescence imaging of exogenous and endogenous HClO in INS-1 and RAW 264.7 cells; therefore, this study provides an effective monitoring tool for the visualization of HClO in cells.

Boron-dipyrromethene (BODIPY) dyes possess advantages such as excellent photostability, high absorption coefficients and quantum yields, excitation/emission wavelengths in the visible or near-infrared regions, and ease of molecular functionalization, making them advantageous across various research fields and attracting widespread attention. In 2017, Soni et al.^[34]designed and synthesized an electron donor-acceptor bidentate fluorescent probe P3 based on PTZ and BODIPY (Figure 6), used for highly sensitive and selective detection of ClO^-. Upon excitation of the probe with 485 nm light, photoinduced electron transfer occurs from PTZ to the excited-state BODIPY within the system, resulting in fluorescence quenching of the probe. However, as the amount of added NaOCl increases, the fluorescence of probe P3 enhances, indicating that the a-PET process from PTZ to BODIPY is inhibited. The LOD for ClO^- is 0.72 μmol/L. Cytotoxicity studies indicate that the cell viability rate with this probe at a concentration of 5 μmol/L is higher than 85%.

Rhodamine dyes are widely used as fluorophores for developing fluorescent probes due to their advantages such as high molar absorption coefficient, broad fluorescence wavelength range in the visible region of the electromagnetic spectrum, high fluorescence quantum yield, and good photostability. In 2020, Zhao et al.^[35]designed and synthesized a novel electron donor-acceptor hypochlorous acid fluorescent probe P4 by fusing PTZ with rhodamine (Figure 7). The recognition mechanism of this probe is based on donor-excited photoinduced electron transfer (d-PET). After reacting with HClO, the fluorescence intensity of the probe increased 120-fold due to the inhibition of the d-PET process from the rhodamine moiety to PTZ. The reaction time is fast (approximately 20 s), and the limit of detection (LOD) for HClO was determined to be 17.8 nmol/L through fluorescence titration experiments. Probe P4 exhibits a good response to HClO within the pH range of 6–8. Due to these aforementioned advantages in responding to HClO, the probe has been successfully applied to the fluorescence imaging of HClO in live HepG2 cells.

Using the PTZ moiety as the recognition group for HClO and an electron-rich photoinduced electron transfer donor, and employing a cationic benzopyridinium moiety to enhance water solubility and mitochondrial targeting capability, in 2021, Bao et al.^[36]designed and synthesized the hypochlorous acid fluorescent probe P5 with a low oxidation potential (Figure 8). The d-PET effect from the PTZ to the benzopyridinium moiety causes the excited-state energy of the fluorophore within the probe to be consumed by the electron transfer process, leading to fluorescence quenching. This probe exhibits an ultrafast response speed to HClO in pure aqueous solution (<5 s), with a limit of detection (LOD) for HClO as low as 7.74 nmol/L. The optimal pH range for the probe's response to HClO is 4.5–8.5. The probe has been utilized to monitor concentration changes of HClO in live HeLa and RAW 264.7 cells as well as in zebrafish mitochondria, and flow cytometry was applied to achieve quantitative intracellular fluorescence detection of HClO. The quantitative results from flow cytometry showed good correlation with cell imaging results, demonstrating that probe P5 can be used for real-time detection of endogenous HClO generation in live cells. Furthermore, they also reported the use of probe P5 for real-time monitoring of HClO generation during ferroptosis.

Among numerous fluorophores, coumarin-based dyes have become the preferred tools for bioimaging due to their large Stokes shift, high fluorescence quantum yield, excellent biocompatibility, and membrane permeability. In 2023, Meng et al.^[37]synthesized a novel PTZ-coumarin-based fluorescent probe P6 (Figure 9) for detecting ClO^- ions. After the reaction between the probe and ClO^-, fluorescence enhancement of the probe (approximately 42-fold) was achieved by blocking the d-PET quenching pathway. Probe P6 exhibits high selectivity and rapid "fluorescence turn-on" functionality (within 3 s) for ClO^-, with a limit of detection (LOD) as low as 12.6 nmol/L. The fluorescent probe P6 was successfully utilized for fluorescence imaging of ClO^- in live HeLa and RAW 264.7 cells. Furthermore, the authors performed quantitative detection of ClO^- in actual water samples, achieving spike recovery rates of 98.3%~102.6%. These satisfactory results indicate that the P6 probe is suitable for sensitive and quantitative detection of ClO^- in actual water samples, including disinfectant and bleach solutions.^-

In 2023, Qiu et al.^[38]reported a PTZ-xanthene-based fluorescent probe P7 for trace analysis of HClO (Figure 10). In the presence of HClO, the sulfur atom on PTZ is rapidly oxidized to sulfoxide, inhibiting the d-PET quenching process and leading to significant fluorescence enhancement (approximately 100-fold). Furthermore, this probe offers advantages such as fast response (<10 s), ultra-high sensitivity (LOD for HClO is 6 nmol/L), strong specificity, and good photostability. Benefiting from these advantages, this unique HClO-responsive probe has been successfully applied to detect HClO in HeLa cells, RAW 264.7 cells, and environmental water samples. The spike recovery rates for the tested water samples ranged from 97.60% to 106.0%, demonstrating the effectiveness and reliability of the P7 probe for detecting HClO in environmental water samples.

To date, other scholars have also conducted many highly meaningful studies on PET mechanism-based PTZ hypochlorous acid/hypochlorite fluorescent probes^[39-51], and the structural formulas of the relevant probe molecules are shown inFigure 11. Important performance parameters of such fluorescent probes based on the PET mechanism are presented inTable 1.

3 HClO/ClO based on ICT mechanism^-fluorescent probe

When a fluorophore containing an electron-donating unit is conjugated to an electron-accepting unit, light excitation creates a "push-pull" π-electron conjugated system, leading to intramolecular charge transfer (ICT) from the donor to the acceptor. This mechanism has been widely used to construct novel fluorescent probes. When the electron-donating part interacts with the guest, the probe's electron-donating ability decreases, resulting in reduced conjugation of the system and a blue shift in the absorption spectrum. Conversely, when the guest interacts with the electron-accepting part, the ICT capability of the system increases, causing a red shift in the absorption spectrum (Figure 12). Furthermore, significant changes in fluorescence quantum yield and fluorescence lifetime can be observed in experiments; the core factors influencing these alterations in photophysical parameters are the size difference and charge magnitude of the guest.

Triphenylamine (TPA) has attracted significant attention in fluorescence probe and advanced material research due to its unique electronic structure, versatile donor-acceptor (D-A) configuration, ease of modification, and excellent optical properties. In 2021, Wu et al.^[52]rationally designed and developed a novel water-soluble NIR-II fluorescent probe I1 (Figure 13) for tracking and imaging HClO in rheumatoid arthritis (RA). Generally, if an alkyl substituent is present on the nitrogen atom of phenothiazine, the sulfur atom on phenothiazine can be oxidized by HClO to sulfoxide; based on this reaction mechanism, researchers have developed many phenothiazine-based HClO fluorescent probes. Interestingly, the authors found that when no substituent is present on the nitrogen atom of phenothiazine, the sulfur atom in the phenothiazine moiety is rapidly oxidized to a sulfonium salt upon treatment with HClO, generating a methylene blue derivative with strong electron-withdrawing capability. On the other hand, based on the D-A-D design strategy, the authors utilized functionalized triphenylamine and phenothiazine as strong electron donors (D-D-D structure) to construct an activatable NIR-II fluorescent probe. Upon reaction with HClO, phenothiazine converts to methylene blue, forming a typical D-A-D structure, which enhances the ICT ability of probe I1 and turns on NIR-II fluorescence. Furthermore, introducing polyethylene glycol (PEG, M_n = 2000) increased the water solubility of the probe, while it also possesses advantages such as high sensitivity (LOD = 55 nmol/L), high photostability, and rapid response to HClO (within 30 s). It enables sensitive and specific detection of exogenous and endogenous HClO in living cells within both visible and NIR-II windows. Moreover, by leveraging NIR-II fluorescence, the probe allows for sensitive and rapid observation of endogenous HClO in inflammatory RA mouse models. These results indicate that probe I1 holds broad application prospects in the diagnosis of HClO-mediated inflammatory diseases.

As two crucial types of highly reactive oxygen/nitrogen species, hypochlorite (ClO^-) and peroxynitrite (ONOO^-) interact directly with various pathophysiological processes; their real-time and accurate detection is of great significance for understanding mechanisms such as oxidative stress and signal transduction in living organisms. In 2022, Huang et al.^[53]connected a PTZ-based coumarin with an aniline derivative via a flexible alkyl chain to construct the dual-responsive ratiometric fluorescent probe I2 (Figure 14), used for the detection of ClO^-and ONOO^-. This probe employs the PTZ-based coumarin moiety as the ClO^--responsive fluorophore and the aniline derivative as the ONOO^-sensor. Upon selective and rapid oxidation of the sulfur atom in PTZ by ClO^-, the ICT performance of the probe is affected, thereby inducing its specific fluorescent response, shifting from red emission (640 nm) to green emission (520 nm); however, upon response to ONOO^-, the probe simultaneously emits red fluorescence (unchanged intensity, 640 nm) and blue fluorescence (enhanced, 450 nm). The probe exhibits high sensitivity: LOD = 17 nmol/L (ClO^-), LOD = 21 nmol/L (ONOO^-). Probe I2 can specifically image ClO^-and ONOO^-in live RAW 264.7 cells, zebrafish, and tissues with different fluorescent signals. Using this probe, the accumulation of endogenous ClO^-and ONOO^-in inflammatory RAW 264.7 cells and zebrafish can be monitored through two different emission channels, with a rapid response. Furthermore, its large fluorescence signal difference, high selectivity, and good biocompatibility may enable its application in deciphering the distribution and correlation of ClO^-and ONOO^-involved in biological activities.

To obtain a ratiometric AIE fluorescent probe for ClO^-response, in 2023, Liang et al.^[54]constructed four fluorescent probes based on the electron donor PTZ and the AIE-active acceptor quinoline-malononitrile. To improve the biocompatibility of the AIE probes, they encapsulated the probes with F-127 to prepare nanoprobe. In the presence of ClO^-, PTZ is oxidized, weakening its electron-donating ability; consequently, the ICT ability is reduced, leading to a blue shift in the probe's emission spectrum, thereby enabling ratiometric detection of ClO^-. Among them, I3 NPs exhibited the best ratiometric recognition of ClO^- (Figure 15). The probe showed that the LOD for near-infrared emission (680 nm) towards ClO^-was 76.7 nmol/L. It can not only successfully detect ClO^-fluctuations in living cells and even zebrafish, but also quantitatively distinguish the content of ClO^-in different types of cells using the ratiometric mode. Furthermore, I3 NPs confirmed that the concentration of ClO^-significantly increased in alcoholic liver cell injury models and alcoholic liver injury mouse models; therefore, ClO^-can serve as an effective biomarker for the diagnosis of alcoholic liver disease (ALD). The results indicate that this nanoprobe has the potential for ALD diagnosis and pharmacodynamic evaluation through ClO^-imaging.

Observing the spatiotemporal changes in ROS behavior during the progression of liver fibrosis (LF) contributes to the diagnosis of LF and antioxidant therapy. In 2023, Yuan et al.^[55]first reported an HClO-specific fluorescent probe I4 to monitor the process of LF (Figure 16). This probe features simple preparation, high selectivity, high sensitivity (LOD = 0.97 nmol/L), fast response (within seconds), and good membrane permeability. According to the report, it is speculated that the strong ICT and intense vibration of PTZ are the reasons for the probe's weak emission and low fluorescence lifetime; the oxidation of the sulfur atom weakens the ICT effect and suppresses vibration, leading to enhanced intensity and increased lifetime of probe I4. The probe can monitor minute changes in endogenous HClO concentration in cells and distinguish different cell types (normal or cancerous) based on their inherent HClO content. In particular, two LF rat models were constructed via chemical treatment (injection of CCl₄) and surgical procedure (bile duct ligation). For the first time, it was observed that HClO concentration in LF tissue increases with its developmental stage, whereas HClO concentration remains unchanged or decreases during acute liver injury or liver cirrhosis. The results fully demonstrate the unique potential of HClO as a biomarker for LF detection and even clinical diagnosis.

The uncontrollable distribution of antitumor drugs remains a major obstacle to achieving specific and efficient cancer therapy; therefore, constructing tumor-specific systems is highly necessary. In 2023, Zhang et al.^[56]proposed an overall design for tumor-stimulus-activated prodrugs using a series of structurally tunable triphenylamine derivatives, with PTZ, benzothiazine, and thiomorpholine serving as recognition groups for hypochlorous acid (HClO) (Figure 17). Specifically, the sulfur atom of PTZ in probe I5 is more readily oxidized by HClO into a sulfoxide group, converting it into an electron acceptor to form an effective push-pull electron system. This enhances the ICT effect, favoring absorption wavelength (Δλ=275 nm) and emission wavelength (Δλ=325 nm) ultra-long red shifts. Based on this, the probe utilizes overexpressed hypochlorous acid in the tumor microenvironment as a "molecular key." When this "key" interacts with the probe, it synchronously activates three "functional locks": near-infrared fluorescence, photothermal effects, and photoacoustic signals, thereby achieving multimodal diagnosis and synergistic therapy of tumors. In vitro and in vivo evaluation results clearly demonstrate that the probe can not only sensitively and rapidly distinguish between normal and tumor tissues via multimodal imaging but also simultaneously perform complete and precise tumor treatment using activable PTT. This work will promote the development of tumor microenvironment-activatable prodrugs to improve the safety of cancer therapy.

Macrocyclic confinement-induced near-infrared emission of guests is currently a research hotspot. In 2024, Wang et al.^[57]reported a cucurbituril (CB[7])-constrained PTZ-based near-infrared fluorescent probe I6 (Figure 18), achieving detection of HClO in living cells. This shows that the detection capability of the supramolecular probe I6, formed after encapsulation by cucurbituril, for HClO was improved to 24 nmol/L (LOD). This is due to the confinement of the macrocycle CB[7] increasing the fluorescence intensity by 103 times. Under excitation at 570 nm, the fluorescence emission wavelength red-shifts to 820 nm, enabling near-infrared lysosome imaging. Meanwhile, in RAW 264.7 cells and live animals, the supramolecular assembly probe I6 was also successfully used for highly sensitive sensing of exogenous HClO.

To effectively and timely intervene in cardiovascular diseases (CVDs), it is highly necessary to develop fluorescent probes capable of precisely identifying atherosclerotic plaques. In 2024, Wang et al.^[58]developed a powerful fluorescent probe I7 (Figure 19) by integrating bis(trifluoromethyl)benzyl and PTZ into a coumarin matrix. It achieves sensing detection for lipid droplets (LDs) and HClO sequentially through fluorescence turn-on and ratiometric readouts. The probe features a fast response speed (HClO recognition based on ICT within 80 s), high sensitivity (detection limits of 0.41 μg/mL for LDs and 23.38 nmol/L for HClO), and high selectivity. Probe I7 successfully realized, for the first time, real-time monitoring of the transformation of A7r5 cells into foam cells under high LDL/glucose conditions, further demonstrating its ability to precisely identify atherosclerotic plaques and clarifying its potential application value in elucidating the pathological mechanisms of atherosclerosis and in clinical diagnosis.

In recent years, emerging photoelectrochemical (PEC) analysis technology has attracted significant attention due to its ability to integrate with other analytical methods to construct sensing platforms with dual-mode signal output. In 2025, Chang et al.^[59]reported a strategy for developing a dual-signal sensing platform based on pyridine-anchored organic small-molecule fluorescent probes (Figure 20). This strategy achieves the intelligent integration of fluorescence analysis and photoelectrochemical (PEC) sensing, thereby enabling rapid and precise detection of hypochlorite. In this study, natural dye pigments were selected as fluorophores to generate fluorescence signals. Meanwhile, using PTZ as the specific recognition group and pyridine as the anchoring group, a novel organic small-molecule fluorescent probe I8 based on the ICT recognition mechanism was designed and synthesized. The resulting probe was used as a photosensitive material and immobilized on the TiO surface via N→Ti bonds to form a dual-signal detection platform based on the FTO/TiO₂ surface, forming an FTO/TiO₂/FPTZ-1 heterostructure dual-signal detection platform for hypochlorite detection. This sensing platform features high specificity, high sensitivity, and convenient preparation, enabling hypochlorite sensing through dual fluorescence and photoelectrochemical signals, with limits of detection of 0.288 μmol/L and 1.37 nmol/L, respectively. The strategy of utilizing pyridine-anchored organic small-molecule fluorescent probes to form dual-signal sensing platforms will provide new perspectives for the detection of various analytes. In complex water environments, spike recovery rates ranged from 97.3% to 101.9%, demonstrating that the I8 probe possesses good selectivity and accuracy in detecting ClO^- in actual water samples.

Hypochlorous acid exists as HClO under acidic conditions and as ClO^-under alkaline conditions. Due to their strong and very similar oxidative reactivity, their selective recognition poses a significant challenge. To address this challenge, in 2025, Che et al.^[60]proposed an asymmetric donor-acceptor-donor′ (D-A-D′) molecular structure and designed and prepared the fluorescent probe I9 (Figure 21), which exhibits extremely high specificity for HClO. By introducing a strong electron acceptor to modulate the reducing capability of the PTZ recognition site, it enables specific response to HClO. By adjusting the D-A molecular structure to alter the ICT effect, the fluorescence wavelength blue-shift during recognition is controlled, thereby improving the accuracy of in vivo ratiometric measurement signals. Through live-cell imaging and in vivo experiments using zebrafish and mouse models, the ability of I9 NPs to precisely recognize HClO in acidic environments was verified, achieving real-time monitoring of HClO. This two-pronged molecular design strategy, combining D-A interaction modulation with a D-A-D′ molecular structure, holds promise for revolutionizing the design of various biomolecular probes and promoting understanding of diseases associated with these analytes.

Cancer (malignant tumor) is a severe disease with a high mortality rate, and early diagnosis and treatment are crucial for cancer therapy. The average concentration of reactive oxygen species (ROS) in cancer cells is 10 times that of normal cells, and the overexpression of hypochlorous acid is closely related to cancer progression. In 2025, Liang et al.^[61]synthesized a novel near-infrared Golgi apparatus-targeting (GA) fluorescent probe I10 based on BODIPY and PTZ as the basic fluorophores and the ICT recognition mechanism for HClO monitoring (Figure 22). This probe offers advantages such as a low detection limit (35 nmol/L), short response time (within 13 s), and strong specific targeting capability. It exhibits excellent GA targeting ability and has been successfully applied to the real-time visualization of endogenous and exogenous HClO in cells and live zebrafish. Furthermore, the probe effectively distinguishes between tumor cells and normal cells, increasing the T/N ratio by 8 to 15 times. Using probe I10 to track HClO levels in tumor cells in real time enabled in situ imaging of subcutaneous HeLa tumors in mice. Additionally, its in situ imaging can accurately delineate the margins of tumor tissue in peritoneal metastasis models, providing precise and effective guidance for tumor resection. This offers a new method for in situ tumor diagnosis and accurate fluorescence-guided resection.

Despite significant progress in the design of fluorescent probes, developing ratiometric hypochlorous acid (HClO) probes with high specificity remains a challenge. In 2025, Xie et al.^[62]designed and synthesized a PTZ-derived coumarin-based fluorescent probe I11 (Figure 23) based on the ICT recognition mechanism, which can respond to HClO with high specificity. To address the obstacle posed by its weak fluorescence in aqueous media for ratiometric detection of HClO, they improved the emission behavior upon aggregation by gradually increasing the steric hindrance of the substituents at the 3-position of the probe fluorophore. Single-crystal analysis results indicated that weaker intermolecular π-π interactions and the higher rigidity of the phenyl group in the aggregated state are two key factors contributing to the high fluorescence quantum yield of this probe. This structural improvement enables a ratiometric response with high specificity and sensitivity (LOD: 43.02 nmol/L) toward HClO. Combined with its good photostability, pH tolerance, and excellent biocompatibility, it has demonstrated potential application prospects for ratiometric fluorescence imaging of excessive HClO generated during acetaminophen-induced liver injury progression. This work provides a potential solution to improve the weak fluorescence of ICT-based fluorescent probes in aqueous environments and offers a feasible molecular design strategy for developing ratiometric HClO fluorescent probes with high specificity.

Given that excessive levels of ClO^-in the environment and food occur from time to time, timely and effective monitoring of their concentration changes is very important. In 2025, Shen et al.^[31]designed and synthesized the ratiometric fluorescent probe I12 based on the ICT recognition mechanism by introducing an ester group onto PTZ (Figure 24), used for detecting ClO^-concentrations in environmental water samples and food. The probe exhibits advantages such as a rapid ratiometric response (within 10 s), high selectivity, and a very large Stokes shift (190 nm) towards ClO^-. Its detection limit for ClO^-is 0.47 μmol/L. Not only were test strips for efficient naked-eye detection of ClO^-successfully prepared, but their effective detection in natural water samples, beverages, and food was also achieved. Furthermore, probe I12 was successfully applied to the fluorescence imaging of ClO^-in onion epidermal cells. In actual water samples, the spike recovery rates were 99.1%~101.8%, with standard deviations of 0.59%~2.93%, demonstrating that probe I12 has potential application prospects for the rapid detection of ClO^-in complex water samples.^-

In recent years, research on PTZ hypochlorous acid/hypochlorite fluorescent probes based on the ICT mechanism has attracted increasing attention. The molecular structural formulas of other fluorescent probes of this type^{[29-30,51,63 -110]}are asshown in Figure 25andFigure 26. Important performance parameters of such ICT-based fluorescent probes are presented inTable 2.

4 HClO/ClO based on other detection mechanisms^-fluorescent probes

In addition to the two detection mechanisms introduced above, researchers are continuously exploring and developing HClO/ClO^-fluorescent probes (Table 3). In 2018, Ding et al.^[111]designed an energy transfer metal-organic nanoprobe O1 based on PTZ, which possesses excellent photostability, high brightness, good water solubility, and cellular internalization performance, used for ratiometric sensing of peroxynitrite (ONOO^-) and hypochlorite (ClO^-). The authors employed nano-sized metal-organic frameworks (NMOF) as energy donors and molecular probes as acceptors, introducing dextran to construct a Förster resonance energy transfer (FRET) nanoprobe (Figure 27). Due to efficient FRET, the bright blue fluorescence (410 nm) of the NMOF was quenched, while the red fluorescence emission (595 nm) from the acceptor was enhanced. After reaction with ONOO^-, the probe detached from the NMOF, and due to the interruption of FRET, the fluorescence of the NMOF was restored. When reacting with ClO^-, the PTZ moiety in the molecular probe was oxidized, leading to more efficient energy transfer, with the fluorescence changing from red to orange (570 nm). This nanoprobe was successfully applied to the detection of ONOO^-and ClO^-in living cells. The design strategy of this energy transfer nanoprobe can be extended to construct other ratiometric fluorescent nanoprobes suitable for various applications, representing a promising approach.

Most luminophores follow Kasha's rule, emitting typically from the lowest excited state regardless of excitation energy. In contrast, in certain cases, due to their ultrafast radiative rates or large S₂-S₁energy gaps, anomalous emission from high excited states occurs in very dilute solutions. This so-called anti-Kasha rule holds significant theoretical and experimental importance, as it avoids additional losses associated with internal transitions and other types of electronic internal conversion (IC) processes, thereby enhancing fluorescence quantum efficiency. In 2024, Wu et al.^[112]prepared the luminescent probe O2 based on PTZ (Figure 28), which exhibits anti-Kasha rule luminescence characteristics in the aggregated state. Studies on photophysical properties, single-crystal structures, femtosecond transient absorption (fs-TA), and theoretical calculations indicate that restriction of intramolecular motion (RIM) is the mechanism for multi-wavelength emission. The probe O2 used for ClO^-detection possesses excellent selectivity, sensitivity (LOD = 0.026 μmol/L), and stability, holding significant practical value. This work deepens the understanding of the impact of RIM on luminescence in the aggregated state and opens avenues for the design of other single-component multi-emission probes.

In 2021, Zhi et al.^[113]designed and synthesized a fluorescent probe O3 based on PTZ with a unique sensing mechanism (Figure 29), and applied it for the detection of HClO in living cells. Under the action of HClO, the fluorescence signal at 610 nm gradually increased, with a maximum enhancement of 15-fold; the fluorescence quantum yield improved from 0.01 to 0.12. Probe O3 features rapid response to HClO (5 s), large Stokes shift (148 nm), good selectivity, and high sensitivity (LOD = 0.676 μmol/L). The sensing mechanism of this probe for HClO involves the phenolic hydroxyl group and Cl⁺forming the corresponding chlorinated product; this recognition mechanism involving the formation of an oxygen-chlorine bond (O—Cl) has been confirmed by experiments such as UV-Vis absorption spectroscopy, HRMS, and isotope labeling. The results indicate that this method provides a unique, real-time, and biocompatible detection method for HClO. Finally, it was successfully used for fluorescence imaging of HClO in SGC7901 cells. The results fully demonstrate that this probe possesses good application potential in the biological field, providing strong support for subsequent related research and practical application development.

The sulfur atom in phenothiazine is prone to oxidation, leading to changes in the probe's fluorescence spectrum; however, the authors did not report on the sensing of O3 for high concentrations of HClO (>15 μmol/L).

In 2021, Zhao et al.^[114]designed and synthesized a novel hypochlorous acid fluorescent probe O4 based on PTZ-indanone conjugation (Figure 30). This probe achieved HClO detection under physiological conditions based on a unique electrophilic chlorination reaction mechanism triggered by chlorine cations. Experimental results showed that the probe exhibited a fluorescence "off-on" response to HClO, with the fluorescence intensity increasing more than 600-fold before and after detection. A good linear relationship was observed for HClO concentrations in the range of 0~6.0 μmol/L (R²=0.997), and the limit of detection reached 1.40 nmol/L. The probe demonstrated excellent performance; on one hand, it enabled naked-eye visual detection at concentrations as low as 0.25 μmol/L with an extremely fast response time of less than 3 s; on the other hand, its recognition mechanism for HClO was verified through multiple methods (mass spectrometry experiments, spectroscopic experiments, and theoretical calculations). Under physiological conditions, HOCl can dissociate to produce chlorine cations (Cl^＋), Cl^＋possesses strong electrophilicity and can undergo specific electrophilic chlorination reactions with aromatic systems, thereby enabling the detection of HClO by this probe. Furthermore, aided by fluorescence imaging technology, the probe successfully accomplished the detection of both endogenous and exogenous HClO within cells. These results indicate that probe O4 holds potential application value in the field of biology.

Interestingly, unlike the effect of HClO on the UV-Vis absorption spectrum of probe O3, the addition of HClO causes a blue shift in the UV-Vis absorption spectrum of probe O4. However, the authors also did not report the effect of high concentrations of HClO (>12 μmol/L) on the probe's spectrum.

5 Conclusion and Outlook

As an important fluorescent dye, phenothiazine fluorescent probes have achieved significant results in the research field of detecting hypochlorous acid/hypochlorite. Based on their rich electronic structure and unique optical properties, researchers have effectively improved probe performance through rational modification of their molecular structures, successfully developing various types of HClO/ClO^-fluorescent probes. These probes mainly utilize the high selective recognition ability of the sulfur atom in PTZ towards HClO/ClO^-, as well as mechanisms such as PET and ICT, to achieve highly sensitive and selective detection of HClO/ClO^-.

In terms of applications, PTZ fluorescent probes have been successfully applied to the detection of HClO/ClO^-in biological and environmental samples. In the field of biomedicine, imaging analysis of HClO/ClO^-in cells and living organisms has been achieved, providing a powerful tool for studying their roles in physiological and pathological processes; in the field of environmental monitoring, they can be used for the detection of HClO/ClO^-in environmental samples such as water and soil, offering new methods and technologies for environmental pollution monitoring.

This article elaborates on the basic design principles, recognition mechanisms, and performance of PTZ-based HClO/ClO^-fluorescent probes, as well as their applications in environmental and biological systems. We also hope this review can provide some new and valuable references for researchers interested in this field. Although PTZ fluorescent probes have made significant progress in HClO/ClO^-detection, several challenges and issues remain to be further addressed: First, develop simpler and more efficient synthesis methods to improve water solubility and enhance their application in biological systems and actual environmental water samples; Second, further optimize the probe structure to shift the emission wavelength to the near-infrared region (>700 nm), enabling deep tissue penetration and facilitating bioimaging; Third, explore new fluorescence mechanisms and signal transduction methods to develop ratiometric or multimodal fluorescent probes with higher sensitivity and specificity; Fourth, conduct in-depth research on the metabolic processes and biocompatibility of probes in living organisms to provide a theoretical basis for clinical applications; Fifth, combine PTZ fluorescent probes with other technologies, such as microfluidics and biosensor technologies, to achieve rapid and high-throughput detection of hypochlorous acid; Sixth, introduce specific targeting groups (such as triphenylphosphonium ions, quaternary ammonium cations, morpholine groups, etc.) into the design of these fluorescent probes to achieve more precise detection of HClO/ClO^-in subcellular organelles such as mitochondria and lysosomes, providing support for fully understanding and studying the essence of life activities. It is believed that with continuous research advancements, PTZ fluorescent probes in the field of HClO/ClO^-detection will demonstrate broader application prospects. In conclusion, we hope that the perspectives and case reviews presented in this article will provide valuable references for designing excellent PTZ-based fluorescent probes and offer technical support for achieving more direct and effective results in fields such as biomedical research and environmental monitoring.

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Funding

National Natural Science Foundation of China(22476118)

Basic Research Program of Shanxi Province， China （Free exploration）(20210302123479)

Basic Research Program of Shanxi Province， China （Free exploration）(202103021224312)

Applied Basic Research Project of Datong， China(2024064)

Research Topic of Postgraduate Education Reform of Shanxi Datong University(2024JG12)

Academic and Practical Innovation Project for Postgraduates of Shanxi Datong University(25XSCX18)

Academic and Practical Innovation Project for Postgraduates of Shanxi Datong University(25XSCX14)

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1 Introduction

2 HClO/ClO based on PET mechanism-fluorescent probe

3 HClO/ClO based on ICT mechanism-fluorescent probe

4 HClO/ClO based on other detection mechanisms-fluorescent probes

5 Conclusion and Outlook

References

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2 HClO/ClO based on PET mechanism^-fluorescent probe

3 HClO/ClO based on ICT mechanism^-fluorescent probe

4 HClO/ClO based on other detection mechanisms^-fluorescent probes