A tetraphenylimidazole-based fluorescent probe for the detection of hydrogen sulfide and its application in living cells

Activity-Based Fluorescent Probes for Hydrogen Sulfide and Related Reactive Sulfur Species

Hydrogen sulfide (H2S) is not only a well-established toxic gas but also an important small molecule bioregulator in all kingdoms of life. In contemporary biology, H2S is often classified as a "gasotransmitter," meaning that it is an endogenously produced membrane permeable gas that carries out essential cellular processes. Fluorescent probes for H2S and related reactive sulfur species (RSS) detection provide an important cornerstone for investigating the multifaceted roles of these important small molecules in complex biological systems. A now common approach to develop such tools is to develop "activity-based probes" that couple a specific H2S-mediated chemical reaction to a fluorescent output. This Review covers the different types of such probes and also highlights the chemical mechanisms by which each probe type is activated by specific RSS. Common examples include reduction of oxidized nitrogen motifs, disulfide exchange, electrophilic reactions, metal precipitation, and metal coordination. In addition, we also outline complementary activity-based probes for imaging reductant-labile and sulfane sulfur species, including persulfides and polysulfides. For probes highlighted in this Review, we focus on small molecule systems with demonstrated compatibility in cellular systems or related applications. Building from breadth of reported activity-based strategies and application, we also highlight key unmet challenges and future opportunities for advancing activity-based probes for H2S and related RSS.

A novel coumarin-fluorescein-based fluorescent probe for ultrafast and visual detection of H2S in a Parkinson's disease model

Hydrogen sulfide (H2S) has a crucial impact on diverse biological processes and has been shown to be related to various diseases. Many probes have been developed to detect intracellular H2S by fluorescent imaging. However, the development of rapid, highly selective and sensitive H2S probes remains a challenge. Herein, two fluorogenic probes, CNS and FCS, are designed and synthesized for the ultrafast detection of H2S with fluorescein and coumarin fluorophores. The results show that both probes can be applied to monitor and image endogenous H2S in cervical cancer HeLa cells and live zebrafish, and FCS shows a higher sensitivity, selectivity and fluorescence intensity. We then further applied FCS in a Parkinson's disease Drosophila model, and the results show that FCS can precisely indicate the level of H2S in the Parkinson's disease model. Thus, FCS will likely to be applied for the early diagnosis of Parkinson's disease.

Dinitrobenzene ether reactive turn-on fluorescence probes for the selective detection of H2S

Two novel fluorescent probes, namely, 3-(2,4-dinitrophenoxy)-2-(4-(diphenylamino)phenyl)-4H-chromen-4-one (P1) and 3-(2,4-dinitrophenoxy)-2-(pyren-1-yl)-4H-chromen-4-one (P2), were designed and synthesized here. The probes (P1 and P2) were found to be highly selective and sensitive toward hydrogen sulfide (H₂S) in the presence of a wide range of anions. The new probes (P1 and P2) were fully characterized by analytical, NMR spectroscopy (¹H and ¹³C), and ESI mass spectrometry. The sensing capability of chemodosimeters (P1 and P2) toward H₂S was confirmed by fluorescence studies. The 'turn-on' fluorescence was used to calculate the detection limit of probes (LOD), which were found to be 2.4 and 1.2 μM for P1 and P2, respectively. Moreover, the probes were tested for their cytotoxicity against HeLa cells using the MTT assay and found to be non-cytotoxic in nature; hence, the probes P1 and P2 were successfully utilized to visualize H₂S in the living cells.

Nanomolar determination of nitrofurans in water via excited-state inter-ligand proton transfer

Qualification and quantification of trace organic contaminants necessitates development of highly efficient sensing system, where excited-state inter-ligand proton transfer (ESILPT) provides a feasible pathway to construct efficient chemo-sensors. Herein, a strategically synthesized lanthanide complex, Eu(DBM)₃(MeOH)₃ (briefly as Eu-DBM-MeOH; DBM = dibenzoylmethane), features two-step ESILPT processes, along with modification on molecular structure and energy band. As a result, Eu-DBM-MeOH exhibits excellent photophysical properties with characteristic luminescence of Eu³⁺ ion. Benefiting from these merits, the Eu-DBM-MeOH complex acts as ultra-sensitive chemo-sensor toward nanomolar-level nitrofuran antibiotics (nitrofurazone and nitrofurantoin) in water, by disrupting ESILPT processes. Combining the advantages on photophysical property and luminescent sensitivity, ESILPT-active compounds are expected to widen and deepen the research on complex-based luminophores, being potentially useful in trace detection and biological imaging.

Smartphone as a simple device for visual and on-site detection of fluoride in groundwater

Developing a portable device for visual and on-site detection of fluoride in groundwater is highly anticipated. In this paper, 2-(tert-butyl-diphenylsilanyloxy)-5-nitro-1H-benzoimidazole (1) has been rationally designed via a silanization reaction for self-calibration detection of fluoride, and the detection limit was calculated as 0.11 μM. The contact of 1 with fluoride would induce the cleavage of Si-O bond and trigger the emergence of excited state intramolecular proton transfer (ESIPT) process, and then the enol-like emission at 437 nm decreased accompanying with the increase of keto-like tautomerism emission at 550 nm. More importantly, considering the demand of field detection for fluoride in groundwater and combining the function of smartphone to obtain the chroma of photos. The chroma value of the fluorescence color changes from blue to yellow could be conveniently determined through a color recognizer application installed in smartphone. The device can accurately reflect the concentration of fluoride by analyzing the chroma value. The test in actual water samples confirmed that the simple device based on smartphone could be used efficiently for visual, on-site and accurate detection of fluoride in groundwater.

Sensing mechanism of a new fluorescent probe for hydrogen sulfide: photoinduced electron transfer and invalidity of excited-state intramolecular proton transfer

It is of great significance for biological research to develop efficient detection methods of hydrogen sulfide (H₂S). When DFAN reacts with H₂S, 2,4-dinitrophenyl ether group acting as an electron acceptor generates a hydroxyl-substituted 2,4-dinitrophenyl ether group, resulting in the disappearance of photoinduced electron transfer (PET), and the new formed DFAH can be observed, while being accompanied by a significant fluorescence. In the present study, the PET sensing mechanism of probe DFAN and the excited state intramolecular proton transfer (ESIPT) process of DFAH have been explored in detail based on the density functional theory (DFT) and time-dependent density functional theory (TD-DFT) methods. Our theoretical results show that the fluorescence quenching of DFAN is caused by the PET mechanism, and the result of ESIPT mechanism is not due to the large Stokes shift fluorescence emission of DFAH. We also optimized the geometric structure of the transition state of DFAH. The frontier molecular orbitals and potential barrier show that the ESIPT process does not easy occur easily for DFAH. The enol structure of DFAH is more stable than that of the keto structure. The absence of the PET process resulted in the enol structure emitting strong fluorescence, which is consistent with the single fluorescence in the experiment. Above all, our calculations are sufficient to verify the sensing mechanism of H₂S using DFAN.

Our theoretical results show that the fluorescence quenching of DFAN is caused by PET mechanism and the large Stokes shift fluorescence emission of DFAH is not due to the ESIPT mechanism. Due to the absence of PET process, enol structure emits strong fluorescence, which is inconsistent with the fluorescence mechanism in the experiment.

A novel "turn-on" fluorescent probe based on hydroxy functionalized naphthalimide as a logic platform for visual recognition of H2S in environment and living cells

A novel hydroxy functionalized naphthalimide-based fluorescent probe (denoted SP2)was successfully designed and synthesized for monitoring of H₂S in living cells and environmental. In particular, SP2 can detect the H₂S without assistance of organic solvent or surfactant. When H₂S is present, the azide group in SP2 was reduced to amine group, resulting in a turn-on fluorescence signal. This remarkable properties of SP2 enable its applications in monitoring ex/endogenous H₂S in HepG-2 cells and hydrogen sulfide release in laboratories or chemical plants through visual recognition by optical color change. The probe displays highly selective and sensitive recognition to H₂S, with a low detection limit of 50.8 nM. Futhermore, this work presents the possibility of using naphthalimide-based "logic gate" platform for monitoring H₂S in biological and environmental samples.

A photostable fluorescent probe based on PET off for the detection of hydrogen sulfide and its application in bioimaging

As an important small gas signal molecule, H₂S has become an important research hotspot for both chemists and biologists.

Aggregation-induced emission fluorescent probe for monitoring endogenous alkaline phosphatase in living cells

Alkaline phosphatase (ALP) is a non-specific phosphate monoesterase and often regarded as an important biomarker of hypothyroidism and hepatobiliary diseases in medical diagnosis. In-situ detection of endogenous ALP and exploration of the distribution of ALP in cells are of great importance for the diagnosis of diseases associated with ALP. In this work, we designed and synthesized an aggregation-induced emission (AIE) fluorescent probe, (E)-2-(((9H-fluoren-9-ylidene) hydrazono)methyl)phenyl dihydrogen phosphate (FAS-P), that can respond to ALP with a remarkable large Stokes shift (>200 nm) based on excited state intramolecular proton transfer (ESIPT) mechanism. The probe FAS-P has high selectivity and sensitivity to the detection of ALP. And there is a linear relationship between the fluorescence intensity of FAS-P and ALP activity in the range of 1-100 U L^-1, the limit of detection (LOD) is as low as 0.6 U L^-1. More importantly, we successfully applied FAS-P to detect ALP in living cells and the monitoring of ALP in real time.

A fluorescent probe based on tetrahydro[5]helicene derivative with large Stokes shift for rapid and highly selective recognition of hydrogen sulfide

In this work, we have designed and synthesized a dinitrobenzene-sulfonate tetrahydro[5]helicene (H-DNP) as an effective fluorescent probe for detection of hydrogen sulfide (H₂S). Upon the addition of H₂S, a significant fluorescence enhancement (75-fold) at 495 nm can be observed with a distinct color change from colorless to yellow. Additionally, H-DNP shows low background spectroscopic signal, large Stokes Shift up to ~140 nm, good sensitivity, rapid response time less than 2 min, low detection limit (48 nM) and high selectivity toward common bio-thiols (Cysteine, Homocysteine and Glutathione). Compared with the previous dinitrophenoxy tetrahydro[5]helicene, this probe has shorter response time and lower detection limit. Most importantly, this probe H-DNP has low toxicity to cells and excellent cell permeability, which can be applied to visualize H₂S in living cells.

Superb-selective chemodosimetric signaling of sulfide in the absence and in the presence of CT-DNA and imaging in living cells by a plant alkaloid berberine analogue

New berberine analogue (BER-S), as a colorimetric probe in the absence of DNA and turn-on fluorometric probe in the presence of DNA towards S²⁻ detection is reported.

A fluorescent probe based on tetrahydro[5]helicene for highly selective recognition of hydrogen sulfide

Endogenous hydrogen sulfide plays an important role in various physiological and pathological processes and the convenient and selective recognition of hydrogen sulfide has become a research hotspot. We designed and synthesized a tetrahydro[5]helicene and 2,4-dinitrobenzene conjugate (HD-DNP) as an effective fluorescent probe for selective detection of H₂S. The selective deprotection of 2,4-dinitrophenyl ether group of HD-DNP by H₂S led to a dramatic fluorescent enhancement (101-fold) at 500 nm and colorimetric change in DMSO-PBS solution. HD-DNP displays many advantages including low background without any self-fluorescence, as well as high selectivity towards common bio-thiols such as Cysteine, Homocysteine and Glutathione. The detection limit of this probe for H₂S was found to be about 2.4 μM with a wide linear range (10-70 μM). The response mechanism of the probe with HS^- is confirmed to be thiolysis of the dinitrophenyl ether induced by HS^- through ¹H NMR comparison investigations.

A NIR fluorescent probe for the detection and visualization of hydrogen sulfide using the aldehyde group assisted thiolysis of dinitrophenyl ether strategy

A NIR fluorescent probe exploiting the aldehyde group assisted thiolysis of dinitrophenyl ether strategy for H₂S imaging in cells, tissues and mice.

A novel isophorone-based red-emitting fluorescent probe for selective detection of sulfide anions in water for in vivo imaging

A new red-emitting fluorescent probe for detection of sulfide anions in living cells (MCF-7 cells) and zebrafish was developed.

Light-Induced Direct Arylation in the Solid Crystalline State as a Strategy Towards π-Expanded Imidazoles

π-Expanded imidazoles bearing the 2-iodophenyl substituent at position 2 undergo direct photoinduced intramolecular arylation in the solid, crystalline state leading to large non-planar heterocycles. An analogous reaction employing 2-bromophenyl and 2-chlorophenyl substituents is considerably slower. Such processes have never before been demonstrated to occur in crystals and have allowed the efficient synthesizes of structurally unique compounds containing either the phenanthro[9',10':4,5]imidazo[1,2-f]phenanthridine moiety or structurally related skeletons. The reaction occurs in the thin crystalline layers irradiated with UV photons in an almost quantitative manner over 48-72 h. Several previously unknown architectures have been prepared using this methodology. Furthermore, the optical properties of these π-expanded imidazoles can be altered with the addition of heteroatoms and/or electron-donating groups.

A new fluorescence turn-on probe for biothiols based on photoinduced electron transfer and its application in living cells

A new biothiol-selective fluorescent probe 1 based on photoinduced electron transfer (PET) mechanism was designed and synthesized. The UV-Vis absorption and fluorescent emission properties of probe 1 towards various analytes were studied in detail. The probe exhibited a large stokes shift (~200nm) after reacted with biothiols and could selectively detect cysteine (Cys) in dimethyl sulfoxide (DMSO)/H2O solution (9:1, v/v, 10mM phosphate buffer saline, pH3.5) over glutathione (GSH), homocysteine (Hcy) and other analytes with a detection limit of 0.117μM. In addition, probe 1 responded well to GSH, Hcy and Cys in the same above solution with pH5.5 and got the detection limits of 0.151μM, 0.128μM and 0.037μM, respectively. Probe 1 was of very low cytotoxicity and successfully applied for imaging of thiols in living cells.

How To Reach Intense Luminescence for Compounds Capable of Excited-State Intramolecular Proton Transfer?

Photoinduced intramolecular direct arylation allows structurally unique compounds containing phenanthro[9',10':4,5]imidazo[1,2-f]phenanthridine and imidazo[1,2-f]phenanthridine skeletons, which mediate excited-state intramolecular proton transfer (ESIPT), to be efficiently synthesized. The developed polycyclic aromatics demonstrate that the combination of five-membered ring structures with a rigid arrangement between a proton donor and a proton acceptor provides a means for attaining large fluorescence quantum yields, exceeding 0.5, even in protic solvents. Steady-state and time-resolved UV/Vis spectroscopy reveals that, upon photoexcitation, the prepared protic heteroaromatics undergo ESIPT, converting them efficiently into their excited-state keto tautomers, which have lifetimes ranging from about 5 to 10 ns. The rigidity of their structures, which suppresses nonradiative decay pathways, is believed to be the underlying reason for the nanosecond lifetimes of these singlet excited states and the observed high fluorescence quantum yields. Hydrogen bonding with protic solvents does not interfere with the excited-state dynamics and, as a result, there is no difference between the occurrences of ESIPT processes in MeOH versus cyclohexane. Acidic media has a more dramatic effect on suppressing ESIPT by protonating the proton acceptor. As a result, in the presence of an acid, a larger proportion of the fluorescence of ESIPT-capable compounds originates from their enol excited states.

A TICT-based fluorescent probe for rapid and specific detection of hydrogen sulfide and its bio-imaging applications

Through rational design, the first TICT based fluorescent H₂S probe was developed, which exhibited ideal properties such as fast response, good selectivity, high sensitivity, and it was suitable for visualization of exogenous and endogenous H₂S in living cells.

A selective and sensitive phthalimide-based fluorescent probe for hydrogen sulfide with a large Stokes shift

A phthalimide-based fluorescent probe for H₂S with a large Stokes shift has been developed. This probe displayed good selectivity and high sensitivity. Imaging intracellular H₂S by using this probe was successfully achieved in living cells.