Development of Lysosome-Targeted Fluorescent Probes for Cys by Regulating the Boron-dipyrromethene (BODIPY) Molecular Structure

AIE-active large Stokes-shift BODIPY Functionalized with Carbazolyl for Lysosome-Targeted Imaging in Living Cells

A large number of studies have shown that lysosomal microcircumstances changes can affect many physiological and pathological processes at the cellular level. However, the visual detection of lysosomal microcircumstances is relatively difficult due to low pH (4.5-6.0) value in lysosomal that require the probe not only stable under acidic condition but also has a good localization effect to lysosomal. Obviously, novel fluorescent which possessed both acidic stability and lysosomal-target property together with lysosomal viscosity active is highly demanded. Herein, a novel BODIPY molecular CarBDP based on carbazole group was rationally designed and synthesized for the lysosomal imaging. CarBDP exhibited AIE feature with a large Stokes shift of up to 157 nm. More importantly, co-localization assay of the CarBDP-treated MCF-7 cells indicated that CarBDP has a good localization effect on lysosomal (Rr = 0.7109) due to the carbazole group while the normal BODIPY that without carbazole group (PhBDP) shows poor localization performance, this was the first time that a small molecule can locate lysosomes only based on carbazole group. CarBDP exhibits strong solid emission with long fluorescence decay lifetime (τ = 44.54 ns) and was stable under acid condition.The probe CarBDP assembled with carbazole group was successfully utilized for lysosomal localization and mapping lysosomal viscosity in live cells, which provides a novel candidate tool for the determination of lysosomal microcircumstances.

An extra-large Stokes shift near-infrared fluorescent probe for specific detection and imaging of cysteine

Cysteine (Cys) plays a crucial role in numerous physiological and pathological processes. Therefore, it is imperative to design a highly selective and sensitive near-infrared (NIR) fluorescent probe to monitor Cys. In this study, we have developed a novel NIR fluorescent probe XA based on Xanthene hybrid tetrahydro-acridine salt dye for specifically tracking of Cys, where a chlorine-substituted tetrahydro-acridine acts as a high Cys-reactive site and water-soluble group. Probe XA exhibits a remarkable "turn-on" NIR emission (830 nm) with an extra-large Stokes shift (305 nm) for monitoring Cys. It also has a high selectivity, rapid response time (6 min) and high sensitivity (LOD as 0.5 μM). We fully characterized and discussed the sensing mechanism of XA toward Cys using HPLC and MS spectrums, as well as quantum theory calculations. Furthermore, the excellent properties of NIR fluorescent detection allow this novel probe to successfully monitor fluctuations of exogenous and endogenous Cys concentration levels in living cells and in vivo.

The two-steps reaction fluorescent probe for the selective detection of cysteine and its applications

We reported the specific fluorescent probe (MC-BOD-XDS) with two-steps reaction based on monomercapto-coumarin-BODIPY for selective detection of cysteine，high activty mercapto-coumarin as the multiple reaction group instead of a group internal standard fluorophore. The reaction mechanism of MC-BOD-XDS for detecting cysteine was different from the reported probes about the nucleophilic  aromatic substitution reaction (SNAr) of chlorinated BODIPY. The fluorescent color of MC-BOD-XDS changed from yellow to red, and then to orange. The linear calibration diagram showed that it can potentially be used for quantitatively detection of Cys. Its potential applications were demonstrated by employing it for detection of Cys in artificial urine and in fluorescent imaging in HeLa cells.

A highly selective quinolizinium-based fluorescent probe for cysteine detection

A novel fluorescent quinolizinium-based turn-off probe has been developed for selective detection of cysteine. The probe showed high selectivity and sensitivity towards cysteine over other amino acids including the similarly structured homocysteine and glutathione with a detection limit of 0.18 μM (S/N = 3). It was successfully applied to cysteine detection in living cells with low cytotoxicity and quantitative analysis of spiked mouse serum samples with moderate to good recovery (96-109%).

Direct Quantification and Visualization of Homocysteine, Cysteine, and Glutathione in Alzheimer's and Parkinson's Disease Model Tissues

Alzheimer's disease (AD) and Parkinson's disease (PD) are chronic neurodegenerative diseases with high morbidity and mortality. Homocysteine (Hcy), cysteine (Cys), and glutathione (GSH) are closely related to AD and PD. However, the dynamics of Hcy, Cys, and GSH in the brain tissues and the potential pathogenesis between Cys/Hcy/GSH with AD and PD remain unclear. Herein, a novel fluorescent probe 1 with multiple binding sites was rationally designed and exploited for the direct quantification of serum total Hcy and Cys along with superior optical properties. Importantly, differentiation and simultaneity fluorescence imaging of Cys, Hcy, and GSH dynamics were achieved in living cells, tissues, and mouse models of AD and PD with this probe, providing direct evidences for the relationship between Hcy/Cys/GSH and AD/PD for the first time. In addition, pathogenesis studies demonstrated that elevated Hcy and Cys levels are closely related to imbalanced redox homeostasis, increased amyloid aggregates, and nerve cell cytotoxicity. These findings will greatly promote the understanding of the functions of Hcy/Cys/GSH in Alzheimer's and Parkinson's diseases, demonstrating clinical promise for the early diagnosis and prevention of AD and PD.

Fluorescent Probes for Live Cell Thiol Detection

Thiols play vital and irreplaceable roles in the biological system. Abnormality of thiol levels has been linked with various diseases and biological disorders. Thiols are known to distribute unevenly and change dynamically in the biological system. Methods that can determine thiols' concentration and distribution in live cells are in high demand. In the last two decades, fluorescent probes have emerged as a powerful tool for achieving that goal for the simplicity, high sensitivity, and capability of visualizing the analytes in live cells in a non-invasive way. They also enable the determination of intracellular distribution and dynamitic movement of thiols in the intact native environments. This review focuses on some of the major strategies/mechanisms being used for detecting GSH, Cys/Hcy, and other thiols in live cells via fluorescent probes, and how they are applied at the cellular and subcellular levels. The sensing mechanisms (for GSH and Cys/Hcy) and bio-applications of the probes are illustrated followed by a summary of probes for selectively detecting cellular and subcellular thiols.

A turn-on homodimer fluorescent probe based on homo-FRET for the sensing of biothiols in lysosome: a trial of a new turn-on strategy

Fluorescence resonance energy transfer (FRET) is often applied to construct fluorescent probes for acquiring high selectivity and sensitivity. According to the FRET theory, a homodimer composed of two identical fluorophores with a small Stokes shift has only weak fluorescence due to homo-FRET between fluorophores, and the fluorescence could be recovered after the destruction of the homodimer. In this study, we designed and synthesized a homodimer fluorescent probe, namely 1,3,5,7-tetramethyl-8-(4'-phenylthiophenol)-boron difluoride-dipyrrole methane dimer (D-TMSPB), based on this turn-on strategy. In D-TMSPB, the disulfide moiety was selected as the response moiety of biothiols, and BODIPY fluorophore was chosen as both donor and acceptor in FRET due to the ultra-small Stokes shifts and obvious overlap of its excitation/emission peak. D-TMSPB exhibited only weak fluorescence. After selective reaction with biothiols, FRET was destroyed and the derivative exhibited strong fluorescence at 514 nm with the limit of detection of about 0.15 μM for GSH. Notably, the derivative of biothiols shows remarkable fluorescence only in acidic conditions, which accords with the internal environment of lysosome. Thus, D-TMSPB was applied to image the biothiols of lysosome in living cells. The turn-on fluorescence of D-TMSPB indicated that homo-FRET is a practical strategy to design turn-on fluorescent probes, particularly for the sensing mechanism based on leaving groups.

Fluorescent Probe for Simultaneous Discrimination of GSH, Cys, and SO2 Derivatives

Biothiols and SO₂ derivatives, as essential reactive sulfur species (RSS), play vital roles in various physiological processes and have a close network of generation and metabolic pathways among them. To clarify their complex correlations, fluorescent probes to simultaneously detect GSH, Cys, and SO₂ derivatives are highly desirable. Herein, we develop the first fluorescent probe (BO-HEM) to simultaneously discriminate GSH, Cys, and SO₂ derivatives. The fluorescent probe is designed by integration of hemicyanine and BODIPY fluorophores through an ether bond. The ether bond of the probe is rapidly replaced by thiolates through nucleophilic aromatic substitution (S_NAr) to generate hemicyanine with NIR fluorescence and sulfur-BODIPY. The amino groups of Cys but not GSH then further replace the thiolate to form amino-BODIPY. As for SO₃^2-, nucleophilic addition to the double bond of BO-HEM generates adduct O-BODIPY with green fluorescence. To further improve the sensing performance, the nanoprobe with increased reactivity and biocompatibility is constructed by encapsulation of BO-HEM into the polymeric micelle. More importantly, by taking advantage of the hydrophilicity of the reaction products, the spectral discrimination was further enhanced to avoid signal interference. The nanoprobe is applied to discriminate biothiols and SO₂ derivatives in living cells through three-color fluorescence imaging.

Selective Detection and Visualization of Exogenous/endogenous Hypochlorous Acid in Living Cells using a BODIPY-based Red-emitting Fluorescent Probe

Herein, a red-emitting fluorescent probe DM-BDP-OCl containing a para-DMTC benzyl pyridinium moiety at the meso position of BODIPY as self-immolative portion for the detection of HOCl was designed and synthesized. DM-BDP-OCl exhibited excellent specificity and a fast response for HOCl beyond other ROS/RNS. It was used for the accurately measurable detection of HOCl with a linear range from 0 μM to 50 μM, and the detection limit for HOCl reached 60 nM. Moreover, the probe could directly monitor fluctuations of exogenous and endogenous HOCl in living HeLa and RAW 264.7 cells. This work provided a powerful and convenient imaging tool for probing pathological and physiological actions of HOCl.

BODIPY-Based Fluorescent Probes for Biothiols

Fluorescent probes for biothiols have aroused increasing interest owing to their potential to enable better understanding of the diverse physiological and pathological processes related to the biothiol species. BODIPY fluorophores exhibit excellent optical properties, which can be readily tailored by introducing diverse functional units at various positions of the BODIPY core. In the present review, the development of fluorescent probes based on BODIPYs for the detection of biothiols are systematically summarized, with emphasis on the preferable detection of individual biothiols, as well as simultaneous discrimination among cysteine (Cys), homocysteine (Hcy), reduced glutathione (GSH). In addition, organelle-targeting probes for biothiols are also highlighted. The general design principles, various recognition mechanisms, and biological applications are elaboratively discussed, which could provide a useful reference to researchers worldwide interested in this area.

A Reaction-Based Fluorescent Probe for Imaging of Native Hypochlorous Acid

Hypochlorous acid (HOCl), one of the reactive oxygen species (ROS), is highly reactive and short-lived. It is a challenge to dynamic monitor HOCl activity in living systems. Hence, we synthesized a new fluoresce nt probe RF1 based on protection of the hydroxyl group by N,N-dimethylthiocarbamate recognition group, which reached a low fluorescence background signal and highly sensitive property. On account of the electrophilic addition of Cl⁺ to the sulfide of thiocarbamate moiety, probe RF1 was converted to resorufin and triggered emitting bright. RF1 showed not only the highly sensitive and selective response to HOCl in vitro, but also can be applied in environmental water samples and detected HOCl by test strips. Besides, the ability of RF1 monitoring HOCl in HeLa cells by exogenous simulation and tracing native HOCl in macrophages cells were also explored.