A double-indole structure fluorescent probe for monitoring sulfur dioxide derivatives with distinct ratiometric fluorescence signals in mammalian cells

Small-molecule fluorogenic probes based on indole scaffold

Indoles are the most versatile organic N-heterocyclic compounds widely present in bioactive natural products and used in different fields such as coordination chemistry, pharmacy, dyes, and medicine, as well as in the biology and polymer industries. More recently, the indole scaffold has been widely used in analytical chemistry for the design and development of small-molecule fluorescent chemosensors in the fields of molecular recognition and molecular imaging. The indole-based chemosensor derivatives contain heteroatoms like N-, O-, and S-, through which they interact with analytes (cations, anions, and neutral species), producing measurable analytical signals that can be used for the fluorimetric and colorimetric detection of different analytes in biological, agricultural and environmental samples. This review focuses on indole-based small-molecule fluorimetric and colorimetric chemosensors for detecting cations, anions, and neutral species in a comprehensive manner. Furthermore, the recognition mechanisms are discussed in detail, which could help researchers design and develop more powerful and efficient fluorescent chemosensors in the near future.

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.

Mitochondria-targetable colorimetric and far-red fluorescent sensor for rapid detection of SO2 derivatives in food samples and living cells

Sulfur dioxide (SO₂) derivatives are intertwined with many physiological and pathological processes in living systems, and excess intake of them are associated with various diseases. Herein, we have rationally constructed a novel colorimetric and far-red fluorescent probe for HSO₃^- based on a rhodamine analogue skeleton bearing a 3-quinolinium carboxaldehyde moiety. The novel probe exhibited a significant far-red fluorescence "Turn-on" response to HSO₃^-, along with obvious color change from reddish to purple via the specific 1,4-nucleophilic addition reaction of HSO₃^- with the quinolinium moiety in 3-(4-(2-carboxyphenyl)-7-(diethylamino)chromenylium-2-yl)-1-methylquinolin-1-ium hypochlorite trifluoromethanesulfonate (AQCB). The AQCB had excellent water-solubility, and presented rapid response (<15 s),highsensibility(LOD = 49 nM) and selectivity toward HSO₃^-. In addition, the probe was able to detect the content of HSO₃^- in food samples with satisfactory results. Furthermore, the probe possessed good cell membrane and could be successfully applied for imaging HSO₃^- in the mitochondria of living cells.

A near-infrared emitted fluorescence probe for the detection of biosulfite in live zebrafish, mouse and real food samples

Bisulfite (HSO₃^-) has been widely used as an important food additive in daily life. Furthermore, a normal amount of HSO₃^- plays a significant role in biological systems. However, excessive intake of HSO₃^- will lead to a variety of diseases. Therefore, it is of great significance to develop an efficient fluorescent probe that can be used for detection of HSO₃^- in biological systems and food samples. In this work, a near-infrared (NIR) emitted fluorescent probe (SZY) based on hemicyanine dye was successfully synthesized and applied to detect HSO₃^- in several food samples and live animals. The proposed nucleophilic addition sensing mechanism of SZY towards HSO₃^- has been confirmed by ¹H NMR titration, high resolution mass spectrometry (HR-MS) and density functional theory (DFT) theoretical computation. The HSO₃^--induced nucleophilic reaction with α,β-unsaturated C=C binding of SZY results in the dramatic decline of the UV-vis absorption and remarkable quenching of the fluorescence emission. SZY features the advantages of near infrared emission (centered at 720 nm), high water solubility (in 98% aqueous solution), fast response time (50 s), large Stokes shift (244 nm) and low cytotoxicity. The probe SZY was successfully applied to image of HSO₃^- in live nude mouse and adult zebrafish. Semi-quantitatively analyzing the HSO₃^- level by "naked eye" in several food samples including canned fruit, white wine, white sugar and jasmine tea drinks has been realized by the colorimetric method.

A novel fluorescent probe based on triphenylamine for detecting sulfur dioxide derivatives

According to the nucleophilicity of sulfur dioxide derivatives, a reactive fluorescent probe was designed and synthesized by linking triphenylamine with benzoindole.

Carbazole-based near-infrared-emitting fluorescence probe for the detection of bisulfite in live animals and real food samples

A carbazole-based near-infrared (NIR)-emitting fluorescent probe (QPM) was successfully developed for the detection of HSO3− in live animals and in real food samples.

Mitochondria-targeted NIR fluorescent probe for sensing Hg2+/HSO3- and its intracellular applications

Mercury and sulfur dioxide (SO₂) are common pollutants in the ecological environment, which are important factors causing many diseases of organisms. The lack of appropriate analytical tools has limited the further understanding of the relationship between ionic mercury (Hg²⁺) and SO₂. Herein, a bifunctional fluorescent probe LJ was designed and explored to simultaneously detect Hg²⁺ and SO₂ via desulfurization reaction and Michael addition reaction, respectively. Probe LJ showed distinct fluorescence responses which a large near-infrared fluorescence enhancement towards Hg²⁺ at λ_em = 713 nm and a blue shift at λ_em = 450 nm towards SO₂ without any spectral cross interferences. To the best of our knowledge, this is the first fluorescent probe with dual fluorescent emission channels to detect Hg²⁺ and SO₂ with the detection limit of 187 nM and 354 nM, respectively. Moreover, cell fluorescent imaging experiments indicated that the probe was mitochondria targetable and provided evidence that SO₂ could be used as an antidote to attenuate the toxicity of Hg²⁺ in living cells.

Mitochondria-targeted reversible ratiometric fluorescent probe for monitoring SO2/HCHO in living cells

Sulfur dioxide (SO₂) maintains a certain steady state balance in the body, high concentration SO₂ will be harmful to human health. Seeking a suitable detection method to monitor sulfur dioxide in real time becomes an urgent requirement owing to the transient nature of sulfur dioxide in organisms. Here, a novel NIR ratiometric fluorescent probe for detection of SO₂ was developed based on a conjugation of coumarin and indol salt with excellent water solubility. The probe Mito-CI displayed highly sensitive (69 nM), fast response time (30 s), large Stokes shift (174 nm) and the NIR fluorescence emission wavelength (655 nm). In the reversibility process of the SO₃^--probe Mito-CI system induced by HCHO in vitro was also detected. Besides, cell imaging showed that Mito-CI possesses mitochondria-targeted ability. Particularly, Mito-CI was proved to reversibly detect SO₂/HCHO in living cells.