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. 2022 Jan 28;2(3):219–223. doi: 10.1021/acsmeasuresciau.1c00055

Simultaneous Detection of Hypochlorite and Singlet Oxygen by a Thiocoumarin-Based Ratiometric Fluorescent Probe

Moonyeon Cho 1, Van-Nghia Nguyen 1,*, Juyoung Yoon 1,*
PMCID: PMC9838813  PMID: 36785865

Abstract

graphic file with name tg1c00055_0007.jpg

The development of fluorescent probes derived from thiocarbonyl compounds for reactive oxygen species has been actively pursued in recent years. However, a better understanding of the optical response behaviors of thiocarbonyl compounds toward reactive oxygen species remains a challenge. Along with this, further studies to overcome the limitation of a single emission channel and aggregation-caused quenching features of thiocarbonyl-based fluorescent probes are highly desirable. Due to the important role of hypochlorite and singlet oxygen in biological processes and their common coexistence in living systems with frequent intertransformations, the design of a fluorescent probe that can recognize both hypochlorite and singlet oxygen is of great interest. Herein, a thiocarbonyl-based ratiometric fluorescent probe (Fcoum-S) for simultaneous detection of hypochlorite and singlet oxygen in aqueous solution and living cells was designed and synthesized. Upon the addition of hypochlorite in Fcoum-S solution (phosphate-buffered saline, 10 mM, pH 7.4, 10% acetonitrile), a ratiometric fluorescence response was observed via a specific hypochlorite-promoted desulfurization reaction with a good linear relationship between the ratio of fluorescence intensities at 526 and 602 nm (I526nm/I602nm) and the hypochlorite concentrations (a low detection limit of 0.15 μM). Furthermore, upon green light irradiation, Fcoum-S was efficiently desulfurized to its oxo analogue (Fcoum-O) by in situ generated singlet oxygen, leading to a significant change in fluorescence. Fcoum-S could work well in an aqueous medium owing to the high reactivity of the thiocarbonyl group and the aggregation-induced emission characteristics. More importantly, Fcoum-S could target mitochondria and was successfully utilized for fluorescence imaging of mitochondrial hypochlorite/singlet oxygen in live cells. This work provides a molecular design guideline for further exploring thioketone derivatives as fluorescent probes.

Keywords: ratiometric fluorescent probes, thiocoumarin, hypochlorite, singlet oxygen, imaging

Reactive oxygen species (ROS) are chemically reactive molecules that are generated in living organisms as a normal byproduct of the metabolism of oxygen.1−3 Hypochlorite (ClO–) is a principal member of the ROS family and plays a pivotal role in host innate immunity and maintaining intracellular redox homeostasis.4−7 Singlet oxygen (1O2) is another intracellular ROS which participates in cell signaling and induction of gene expression.8,9 Moreover, 1O2 is a key species in photodynamic therapy (PDT), a well-known therapy for cancer.10−12 In particular, ClO– and 1O2 usually coexist in living systems, mainly in mitochondria, and they play essentially interconnected roles in various physiological processes.8,9,13,14 Therefore, the development of rapid and effective methods to simultaneously monitor ClO– and 1O2 is in high demand. Due to the high sensitivity, technical simplicity, and fast response times, fluorescent probes have attracted considerable attention for ROS detection.15−17 However, most of these probes suffer from some problems, especially single-channel output, which probably hinders their applications in biological imaging because the change of fluorescence intensity in a single channel is susceptible to various interferences from instrumental parameters or environmental factors.18 Fortunately, ratiometric fluorescent probes, which have two emission bands, can effectively alleviate the interferences owing to their self-calibration function. The design of ratiometric fluorescent probes is a promising approach as the ratio of intensities at two wavelengths directly correlates with the concentration of the target analyte, and as a result, these probes possess effective internal referencing systems that ensure reliability, increase sensitivity, and improve quantification.19−21

In recent years, sulfur-substituted biocompatible carbonyl fluorophores have received considerable attention as heavy-atom-free photosensitizers22−24 and fluorescent probes,25−27 especially for ROS. For example, some thiocarbonyl-based probes have been used for monitoring hypochlorite in aqueous solutions and cancer cells with high selectivity and sensitivity.28−32 As an example of singlet oxygen fluorescent probes, Tang and co-workers found that the thio-caged fluorophores such as thio-Nile Red (SNile Red), thiophthalimide (SDMAP), thiocoumarin (SCou), and thioacridone (SACD) could be readily converted to their corresponding fluorescent oxo forms upon visible light irradiation in the presence of oxygen.32 While SACD, SDMAP, and SNile Red exhibited distinct intracellular fluorescence during in situ irradiation of the cells, SCou did not show any intracellular fluorescence. Although the reason has been provided, further studies are still required for clarification of this phenomenon. To date, most existing thiocarbonyl compounds have exhibited “off–on” fluorescence signals, which suffer from the drawbacks of normal fluorescent probes with single emission features.33 Thus, further investigations on the photophysical properties of sulfur-substituted carbonyl fluorophores are crucial for enhanced clinical applications. Recently, we developed a novel thiocoumarin derivative that exhibited a unique phenomenon, aggregation-induced emission in the red region, as well as mitochondrial-targeting ability in cancer cells by introducing a trifluoromethyl (CF3) group at the 7-position of the coumarin skeleton and replacement of a single oxygen atom by sulfur.33 With its unique properties, we expect that this thiocoumarin may work as a potential fluorescent probe for ROS.

Taking the above-mentioned factors into account, we decided to prepare and investigate the photophysical response behaviors of this novel thiocoumarin derivative (Fcoum-S) to ROS for further biological applications. Promisingly, Fcoum-S could sensitively detect not only ClO– but also 1O2 in the aqueous medium. Importantly, the Fcoum-S was successfully applied as a fluorescent probe for monitoring and imaging of exogenous and endogenous hypochlorite/in situ-generated singlet oxygen in the mitochondria of living cells. To the best of our knowledge, this is the first example of a thiocarbonyl-based ratiometric fluorescent probe for simultaneous monitoring and imaging of ClO– and 1O2 in mitochondria.

Results and Discussion

Design and Synthesis of Fcoum-S

Due to the high reactivity of the thioketone group, the replacement of oxygen by sulfur in classical carbonyl fluorophores is a simple and effective approach to constructing ROS fluorescent probes. The current thiocarbonyl compounds are usually nonfluorescent due to the promoted intersystem crossing (ISC) and a nonradiative relaxation pathway by intramolecular rotation, which hinder their applications in biological imaging.33 A suitable thiocarbonyl-based fluorescent probe should exhibit aggregation-induced emission (AIE) properties. Furthermore, the ROS probe should target mitochondria because intracellular ROS are mainly produced in mitochondria.13,14 We proposed that the introduction of CF3 into the coumarin scaffold could not only improve the mitochondria targeting of thiocoumarin but also enhance the reactivity of the thiocarbonyl group to detect both ClO– and 1O2 (Figure 1a). Keeping these aspects in mind, we prepared Fcomn-S according to reported procedures,33 and its photophysical responses toward ROS were investigated.

Figure 1.

Figure 1

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(a) Proposed mechanism of Fcoum-S in response to ClO–. (b,c) UV–vis absorption and fluorescence spectra of Fcoum-S (10 μM, λex = 435 nm) upon the addition of ClO– in PBS (1 mM, pH 7.4, containing 10% ACN). (d) Linear relationship between the fluorescence intensity and ClO– concentrations. (e) Fluorescence spectra of Fcoum-S (10.0 μM) in response to various species (100.0 μM) in PBS, including Mn+ (Ca2+, Co2+, Cu2+, Pb2+, Mg2+, Na+); other ROS (ONOO–, H2O2, O2•–, •OH). Inset: Photographs of the Fcoum-S solution under UV illumination (365 nm) in the presence of various analytes.

Optical Responses to ClO– and 1O2

With Fcoum-S in hand, we investigated the optical responses of Fcoum-S to ClO– and 1O2 in phosphate-buffered saline (PBS) (10.0 mM, pH 7.4, containing 10% acetonitrile (ACN)) using ultraviolet–visible (UV–vis) absorption and photoluminescence (PL) spectra. In the absence of ClO–, Fcoum-S displayed moderate absorption and fluorescence bands, with maximum wavelengths at 494 and 602 nm, respectively. Upon the incremental addition of ClO–, the absorption peaks around 494 nm gradually decreased and a new absorption band appeared at 406 nm with a distinct isosbestic point at 437 nm (Figure 1b). Meanwhile, the fluorescence in the red channel was slightly increased, and simultaneously, a new fluorescence centered at 526 nm significantly increased (Figure 1c). The UV–vis and PL spectra of the resulting product and corresponding Fcoum-O were well-matched. The PL titration curve revealed that the fluorescence emission intensity ratio at 526 and 602 nm (I526nm/I602nm) showed a linear relationship with ClO– in the concentration range of 0–30 μM (Figure 1d). The detection limit of Fcoum-S to ClO– was determined to be 0.15 μM. The above observation was associated with ClO–-induced oxidative desulfurization to form the corresponding oxo analogue. Notably, no significant change in fluorescence intensity was observed when adding metal ions (Mn+) and other ROS (Figure 1e).

Inspired by the previous work by Xiao and co-workers,32 we investigated the effects of dissolved oxygen and light on the photoactivation of Fcoum-S in PBS (10 mM, pH 7.4, 10% ACN). As depicted in Figure 2, after irradiation with green light (520 nm) for 12 min, the fluorescence intensities at 526 and 602 nm increased sharply and slightly, respectively, whereas no fluorescence changes were observed in the absence of light. This indicated that Fcoum-S could be suitable for singlet oxygen detection in biological systems. Notably, the Coum-S also showed a gradual increase in fluorescence intensity upon blue light irradiation in PBS (Figure S2). The increase in fluorescence intensity of thiocoumarin upon light irradiation was ascribed to the self-generation of singlet oxygen, which could oxidize the thiocarbonyl group within thiocoumarin to form the corresponding oxo analogue.32

Figure 2.

Figure 2

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(a,b) UV–vis absorption and fluorescence spectra of Fcoum-S upon green LED irradiation (10 μM, λex = 442 nm, PBS, 10 mM, pH = 7.4, 10% ACN) under air conditions.

Fluorescence Imaging of ClO– in Living Cells

Before exploring the imaging ability in living cells, the cytotoxicity of Fcoum-S toward HeLa cells was evaluated by the traditional MTT assay. HeLa cells incubated with Fcoum-S remained over 90% viable even at a high dose of 25 μM for 24 h, indicating negligible dark cytotoxicity (Figure S10). Importantly, Fcoum-S could rapidly and selectively accumulate into mitochondria with red emission, as mentioned in a previous report.33 Motivated by these results, Fcoum-S was then applied for monitoring and imaging of exogenous ClO– in mitochondria of HeLa cells. As shown in Figure 3, while the HeLa cells incubated with Fcoum-S only displayed moderate fluorescence in a red channel, bright fluorescence emission in both green and red channels was observed in the cells that were further treated with ClO–. Notably, the green fluorescence signal was progressively increased by increasing the concentration of ClO–, whereas only a slight increase in the red fluorescence signal was observed. In addition, Fcoum-S could also monitor endogenous ClO– in RAW 264.7 macrophage cells (Figure S7). These results indicated that Fcoum-S was suitable to detect and image ClO– in living cells.

Figure 3.

Figure 3

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Confocal fluorescence imaging of HeLa cells incubated with Fcoum-S (5 μM) for 1 h and then treated with ClO– at various concentrations (0.0, 5.0, 10, and 20 μM) for another 30 min. Fluorescence images were acquired at λex = 405 nm and λem = 575–675 nm (red channel); 535–565 nm (green channel). Scale bar: 20 μm.

Fluorescence Imaging of Photoirradiation-Induced 1O2 in Living Cells

To investigate the fluorescence response of Fcoum-S toward self-generated 1O2 in living cells, HeLa cells were incubated with Fcoum-S (5 μM) followed by green LED irradiation (520 nm, 20.0 mW/cm2). Cells were illuminated for different times (0, 2.0, 4.0, and 8.0 min) and visualized by confocal fluorescence microscopy. As shown in Figure 4, with the increase in irradiation time, the fluorescence intensity in the green channel increased significantly, whereas the fluorescence intensity in the red channel increased slightly, which allowed visualization of 1O2 within the mitochondria in cells. These results demonstrated the important role of the CF3 group in the photoactivated properties of Fcoum-S in living cells, in which the strong electron-withdrawing effect of the CF3 group may be responsible for the enhanced reactivity of C=S bond in the ground state.

Figure 4.

Figure 4

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Confocal fluorescence imaging of HeLa cells incubated with Fcoum-S (5 μM) after continuous irradiation with green LED light (520 nm, 20.0 mW/cm2). Fluorescence images were acquired at λex = 405 nm and λem = 575–675 nm (red channel); 535–565 nm (green channel). Scale bar: 20 μm.

Mechanism Studies

To further characterize the ClO–-triggered desulfurization process in Fcoum-S, we measured 1H NMR and ESI-MS spectral changes. Upon the addition of ClO– ions, protons in the aromatic region of Fcoum-S showed an upfield shift from 7.55, 7.52, 7.10, 6.85, 6.82, and 6.72 ppm to 7.50, 7.47, 6.76, 6.73, 6.60, and 6.36 ppm, respectively. After the addition of 3 equiv of ClO–, the 1H NMR spectrum of the final product showed good agreement with that of the corresponding Fcoum-O (Figure 5). The ClO–-triggered product was also characterized by ESI mass spectrometry. The observed mass spectrometry confirmed the regeneration of commercially available Fcoum-O (Figure 6). The formation of corresponding Fcoum-O under green light irradiation of Fcoum-S solution was also confirmed by ESI mass spectrometry (Figure S3).

Figure 5.

Figure 5

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Partial 1H NMR spectra of Fcoum-S, Fcoum-S + ClO–, and corresponding Fcoum-O in CD3CN.

Figure 6.

Figure 6

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Mass spectrum of Fcoum-S after reaction with 3.0 equiv of ClO–.

Conclusions

In summary, we provided the first example of a thiocarbonyl-based ratiometric fluorescent probe for simultaneous monitoring and imaging of ClO– and 1O2. Upon treatment with ClO– or self-generated 1O2, Fcoum-S displayed significant fluorescence enhancement in the green region (526 nm) and slight fluorescence enhancement in the red region (602 nm) in an aqueous solution. The highly sensitive response of Fcoum-S to ClO– and 1O2 was due to the high reactivity of the thiocarbonyl group and the AIE characteristics. More importantly, Fcoum-S was specifically accumulated into mitochondria and was successfully applied for imaging exogenous and endogenous ClO– and self-generated 1O2 in mitochondria of living cells.

Acknowledgments

This study was supported by grants from the National Research Foundation of Korea (NRF) funded by the Korean government (MSIP) (No. 2012R1A3A2048814 for J.Y.). The high-resolution mass spectrometer analysis was performed on the Synapt G2-HDMS mass spectrometer (Waters, Manchester, U.K.), which was operated on the MassLynx 4.1 software at KBSI (Korea Basic Science Institute, Ochang, Center of Research Equipment).

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsmeasuresciau.1c00055.

  • Synthesis, characterization, spectroscopic data, MTT assay, and confocal microscopic images (PDF)

Author Contributions

The manuscript was written through the contributions of all authors. All authors have approved the final version of the manuscript.

The authors declare no competing financial interest.

Supplementary Material

tg1c00055_si_001.pdf (1.2MB, pdf)

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Associated Data

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Supplementary Materials

tg1c00055_si_001.pdf (1.2MB, pdf)

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