An HBT-Based Near-Infrared Fluorescent Probe for Colorimetric and Ratiometric Detection of Bisulfite and its Application in Living Cells

Theoretical study of excited state dynamics of a ratiometric fluorescent probe for detection of SO2 derivatives

Sulfur dioxide (SO2), a toxic air pollutant, can have harmful effects on human health when inhaled or when it forms bisulfite in the body. In the present work, a ratiometric fluorescent probe, 2-(2'-hydroxyphenyl)benzothiazole-3-ethyl-1,1,2-trimethyl-1H-benzo[e]indolium (HBT-EMBI), was selected to study the mechanism of SO2 derivatives detection. This study provides insights into the attributions of ratiometric fluorescence through hydrogen bond dynamics, electronic excitation properties, radiation rates, and excited state intramolecular proton transfer (ESIPT) processes using the density functional theory (DFT) and the time-dependent density functional theory (TDDFT) level. The results confirm that the large Stokes shifts and broad emission spectra of the HBT-EMBI probe are associated with its intramolecular charge transfer (ICT) characteristics and hydrogen bonding-driven ESIPT processes, respectively. After the addition reaction between the probe and HSO3-/SO32-, the conformational populations of HBT-EMBI-HSO3- transfer abnormally from enol configurations to more stable keto configurations, which leads to a distinguished change in the visible color and the ratiometric fluorescence signal, and is not due to the blockage of the ICT process of HBT-EMBI-HSO3-, as previously reported. This work provides a new perspective on the mechanism of detection of SO2 derivatives by ESIPT fluorescent probes.

A FRET-based ratiometric fluorescent probe for SO32- detection in Chinese medicine and living cells

Chinese herbal medicine is receiving more and more attention at home and abroad as a traditional Chinese clinical medicine. To make herbal medicines can be preserved for a long time, they are usually fumigated with sulfur. However, after the medicinal materials have been fumigated with sulfur, SO₂ residues will remain, which, when exposed to water, will create sulfites and bisulfites. Excessive sulfites can cause a variety of severe ailments and diminish the quality and effectiveness of therapeutic plants. Therefore, developing an effective SO₃^2-/HSO₃^- detection method is important. This study chose coumarin derivatives as fluorescent acceptors and pyridinium acrylonitrile structures as fluorescent donors to create a ratiometric fluorescent probe CPA using the fluorescence resonance energy transfer (FRET) effect. The probe CPA exhibited a fluorescence transition from red to green under excitation at 405 nm with an interval of 149 nm, a reaction time of less than 1 min, a low detection limit of 86 nM, and the probe CPA has good specific recognition of SO₃^2- and is resistant to interference. In addition, CPA has low in vitro cytotoxicity and can successfully detect endogenous sulfites in living cells.

Small-Molecule Fluorescent Probe for Detection of Sulfite

Sulfite is widely used as an antioxidant additive and preservative in food and beverages. Abnormal levels of sulfite in the body is related to a variety of diseases. There are strict rules for sulfite intake. Therefore, to monitor the sulfite level in physiological and pathological events, there is in urgent need to develop a rapid, accurate, sensitive, and non-invasive approach, which can also be of great significance for the improvement of the corresponding clinical diagnosis. With the development of fluorescent probes, many advantages of fluorescent probes for sulfite detection, such as real time imaging, simple operation, economy, fast response, non-invasive, and so on, have been gradually highlighted. In this review, we enumerated almost all the sulfite fluorescent probes over nearly a decade and summarized their respective characteristics, in order to provide a unified platform for their standardized evaluation. Meanwhile, we tried to systematically review the research progress of sulfite small-molecule fluorescent probes. Logically, we focused on the structures, reaction mechanisms, and applications of sulfite fluorescent probes. We hope that this review will be helpful for the investigators who are interested in sulfite-associated biological procedures.

A ratiometric and colorimetric fluorescent probe designed based on FRET for detecting SO32-/HSO3- in living cells and mice

Based on the principle of FRET, we have developed a ratiometric and colorimetric fluorescent probe TFBN, which can specifically recognize SO₂ derivatives (SO₃^2-/HSO₃^-), and exhibit a transition from red to green fluorescence under 405 nm excitation. The probe TFBN owns the advantages of short response time (<3 min), quantitative detection SO₂ derivatives in two linear ranges, extremely low detection limit (39 nM), large Stokes shift (239 nm) and wide emission window gap (140 nm). In addition, the NBC structure was used as a fluorescent donor for FRET probes for the first time, which expanded the diversity of donors. Importantly, with low toxicity and good biocompatibility, the probe TFBN successfully detects exogenous and endogenous sulfites in living cells. These characteristics endow the probe TFBN can be successfully used in living cells and mouse imaging.

The mechanisms of a bifunctional fluorescent probe for detecting fluoride and sulfite based on excited-state intramolecular proton transfer and intramolecular charge transfer

The mechanisms of 2-(Benzo[d]thiazol-2-yl)phenol-based bifunctional probe (HBT-FS) for detecting fluoride (F^-) and sulfite (SO₃ ^2-) based on excited-state intramolecular proton transfer (ESIPT) and intramolecular charge transfer (ICT) have been theoretically studied. Laplacian bond order of HBT-FS indicates that the F^- ion cleaves the Si-O bond and then forms Compound 2 possessing a six-membered ring with a hydrogen bond. Potential energy curves and dynamic simulations confirm that ESIPT in Compound 2 occurs along with this hydrogen bond and forms a keto structure with an emission at 623 nm, which agrees with the observed experimental value (634 nm) after adding F^-. Therefore, the fluorescence red-shift (from 498  to 634 nm) of HBT-FS observed in experiment after adding F^- is caused by ESIPT. The SO₃ ^2- ion is added to the C₅ site of HBT-FS, which is confirmed by orbital-weighted dual descriptor, and then forms Compound 3 with fluorescence located at 404 nm. The experimentally measured fluorescence at 371 nm after adding SO₃ ^2- is assigned to Compound 3. Charge transfer analyses indicate that the ICT extent of Compound 3 is relatively weak compared with that of HBT-FS because of the destruction of the conjugated structure by the addition reaction of SO₃ ^2-, which induces the blue-shift of the fluorescence of HBT-FS from 498 to 371 nm. The different fluorescence responses make HBT-FS a fluorescent probe to discriminatorily detect F^- and SO₃ ^2-.

Preparation of a Near-Infrared Fluorescent Probe Based on IR-780 for Highly Selective and Sensitive Detection of Bisulfite-Sulfite in Food, Living Cells, and Mice

A near-infrared fluorescent probe was designed and synthesized for the detection of bisulfite-sulfite in food, living HeLa cells, and mice. The probe is a stable hemicyanine skeleton based on IR-780, which produces a distinct color and fluorescence change before and after reaction with bisulfite and shows excellent analytical performance, such as high sensitivity with a detection limit of 0.37 μM and high selectivity toward bisulfite over other substances. More importantly, the probe determined bisulfite in real food samples (crystal sugar and red wine) with superior recoveries (96.7-106.1%) and has been successfully used to monitor bisulfite in living HeLa cells. Notably, the probe was applied to image bisulfite in BALB/c mice in vivo for the first time. Owing to its biocompatibility, this probe provides a desired method to study the metabolism of bisulfite in cells and shows substantial potential usage in other biosystems and in vivo imaging studies.

A Hybrid Coumarin-Semifluorescein-Based Fluorescent Probe for the Detection of Cysteine

A new type of turn-on fluorescent probe CF-AC for the detection of Cys was firstly reported. The probe exhibited an excellent response to Cys with high selectively and sensitivity. In the presence of Cys, two fluorescence emission peaks at 525 nm and 650 nm appeared accompanied by the fluorescence color change from blue to red. Morever, the probe had good biocompatibility and could be successfully used for fluorescence imaging of Cys in MCF-7 cells.