A real-time ratiometric fluorescent probe for imaging of SO2 derivatives in mitochondria of living cells

A real-time ratiometric fluorescent probe (IN-CZ) for highly selective detection of sulfite was designed and synthesized, which is based on modulating the intramolecular charge transfer (ICT) of the hemicyanine dye platform. The mechanism of using the probe is mainly through the Michael addition that occurs between IN-CZ and sulfite with a detection limit of 2.99 × 10⁻⁵ M. IN-CZ displays a fast response (within 1 minute) and is highly selective for SO₃²⁻/HSO₃⁻ over ROS, biologically relevant ions, biological mercaptans and other reactive species. More importantly, IN-CZ was suitable for ratiometric fluorescence imaging in living cells, by real-time monitoring of SO₃²⁻/HSO₃⁻ changes in mitochondria targeted in living cells.

A real-time ratiometric fluorescent probe (IN-CZ) for highly selective detection of sulfite was designed and synthesized, which is based on modulating the intramolecular charge transfer of the hemicyanine dye platform.     

A mitochondrion-targeted dual-site fluorescent probe for the discriminative detection of SO32− and HSO3− in living HepG-2 cells

Sulfur dioxide, known as an environmental pollutant, produced during industrial productions is also a common food additive that is permitted worldwide. In living organisms, sulfur dioxide forms hydrates of sulfite (SO₂·H₂O), bisulfite (HSO₃⁻) and sulfite (SO₃²⁻) under physiological pH conditions; these three exist in a dynamic balance and play a role in maintaining redox balance, further participating in a wide range of physiological and pathological processes. On the basis of the differences in nucleophilicity between SO₃²⁻ and HSO₃⁻, for the first time, we built a mitochondrion-targeted dual-site fluorescent probe (Mito-CDTH-CHO) based on benzopyran for the highly specific detection of SO₃²⁻ and HSO₃⁻ with two diverse emission channels. Mito-CDTH-CHO can discriminatively respond to the levels of HSO₃⁻ and SO₃²⁻. Besides, its advantages of low cytotoxicity, superior biocompatibility and excellent mitochondrial enrichment ability contribute to the detection and observation of the distribution of sulfur dioxide derivatives in living organisms as well as allowing further studies on the physiological functions of sulfur dioxide.

Rational design and sensing mechanism of a dual-site fluorescence probe for HSO₃⁻ and SO₃²⁻.     

Phenothiazine and semi-cyanine based colorimetric and fluorescent probes for detection of sulfites in solutions and in living cells

Four hemicyanine probes for selectively detecting sulfites (HSO₃⁻/SO₃²⁻) have been constructed by the condensation reaction of 7-substituted (CN, Br, H and OH) phenothiazine aldehyde with 1-ethyl-2,3,3-trimethylindolium iodide. All four probes show a fast and sensitive response to HSO₃⁻/SO₃²⁻via a Michael addition, with a detection limit lower than 40 nM based on monitoring their UV/vis absorption changes. Although all four probes display an increase in fluorescence when responding to HSO₃⁻/SO₃²⁻, the increment is larger for the probe with an electron-withdrawing group than the probe with an electron-donating group, except for Br. Thus, among four probes the 7-cyano probe (PI-CN) possesses the largest fluorescent response to HSO₃⁻/SO₃²⁻, and the lowest detection limit (7.5 nM). More expediently and easily, a film and a test paper with PI-CN have been prepared to detect HSO₃⁻/SO₃²⁻ in a sample aqueous solution selectively. Finally, the detection of HSO₃⁻/SO₃²⁻ by PI-CN in biological environments has been demonstrated by cell imaging.

Four 7-substituted phenothiazine hemicyanines display a substituent effect on the fluorescence response toward sulfites. The CN-substituted probe exhibits the best sensing behavior.     

A dual responsive fluorescent probe for selective detection of cysteine and bisulfite and its application in bioimaging

A coumarin-based dual responsive fluorescent probe with a simple structure was developed for the detection of Cys and HSO₃⁻. Under simulated physiological conditions, Cou-F displayed an on–off fluorescence response to Cys at 521 nm and an off–on fluorescence response to HSO₃⁻ at 500 nm. Furthermore, Cou-F had the advantages of high sensitivity, strong specificity and rapid response. The detection limits of Cou-F toward Cys and HSO₃⁻ were 0.54 μM and 0.65 μM, respectively. Cou-F enabled high selective responses to Cys and HSO₃⁻ over other biologically related species. The response times of Cou-F toward Cys and HSO₃⁻ were 80 s and 100 s. The fluorescence imaging of Cys and HSO₃⁻ was achieved in living RAW246.7 cells.

A coumarin-based dual responsive fluorescent probe with a simple structure was developed for the detection of Cys and HSO₃⁻.     

A ratiometric fluorescent probe for detection of exogenous mitochondrial SO2 based on a FRET mechanism

A novel imidazo[1,5-a]pyridine-hemicyanine based ratiometric fluorescent probe for detection of mitochondrial SO₂ was designed and synthesized. The probe is based on a fluorescence resonance energy transfer (FRET) mechanism. It exhibits high selectivity and sensitivity towards SO₃²⁻ with a fast response time (3 min) and detection limit of 0.13 μM. Further, it showed low cytotoxicity and was successfully applied to image exogenous mitochondrial SO₂ in cells.

A novel imidazo[1,5-a]pyridine-hemicyanine based ratiometric fluorescent probe for detection of mitochondrial SO₂ was designed and synthesized.     

A highly selective fluorescent probe for detection of Cd2+ and HSO3− based on photochromic diarylethene with a triazole-bridged coumarin-quinoline group

A novel photochromic diarylethene containing a quinoline-linked 3-aminocoumarin Schiff base unit (1O) was synthesized and used for the selective detection of Cd²⁺ and HSO₃⁻. The synthesized probe exhibited a straightforward response for the selective detection of Cd²⁺. Its fluorescence emission red-shifted ∼126 nm and was enhanced 24.9 fold in the presence of Cd²⁺. Meanwhile, the fluorescence color of 1O changed from dark cyan to golden yellow. The binding stoichiometry between 1O and Cd²⁺ was determined to be 1 : 1. A molecular logic circuit with three inputs and one output was successfully constructed with its light and metal-responsive behaviors. In addition, 1O was able to selectively recognize HSO₃⁻ with a 135-fold enhanced fluorescence emission and a notable fluorescence color change from dark cyan to bright cyan. The ¹H NMR and mass spectrometry analyses suggest that the HSO₃⁻ sensing of 1O is based on the hydrolysis of the Schiff base group of 1O.

A novel photochromic diarylethene containing a quinoline-linked 3-aminocoumarin Schiff base unit (1O) was synthesized and used for the selective detection of Cd²⁺ and HSO₃⁻.     

A new class of fluorogenic thiazolo[2,3-b]quinazolinone receptor: selective detection towards mercury and hydrogen bisulfate ions in aqueous medium

A series of fluorophoric and structurally diverse thiazoloquinazoline derivatives were synthesized in a one-pot multicomponent cascade reaction using a microwave irradiation technique. The unique structural arrangement of the synthesized compounds encouraged us to design a new type of bioactive molecular receptor. This receptor interacts with HSO₄⁻ in 1 : 1 and Hg²⁺ in 1 : 2 binding stoichiometric ratios resulting in a change in fluorescence as well as absorption spectra in aqueous medium. The ion bonded receptor complex possibly enhances the fluorescence signal of the receptor via H-bonded complex formation with HSO₄⁻ ions and co-ordinate complex formation with Hg²⁺ ions.

Fluorophoric thiazoloquinazoline derivatives were synthesized under microwave assisted one-pot three-component cascade reaction. Owing to their unique structural arrangement, a new bioactive molecular receptor was developed for HSO⁴⁻ and Hg²⁺ ions.     

A theoretical study on the formation and oxidation mechanism of hydroxyalkylsulfonate in the atmospheric aqueous phase

Hydroxymethanesulfonate (HMS) is an important organosulfur compound in the atmosphere. In this work, we studied the formation mechanism of HMS via the reaction of formaldehyde with dissolved SO₂ using the quantum chemistry calculations. The results show that the barrier (9.7 kcal mol⁻¹) of the HCHO + HSO₃⁻ reaction is higher than that (1.6 kcal mol⁻¹) of the HCHO + SO₃²⁻ reaction, indicating that the HCHO + SO₃²⁻ reaction is easier to occur. For comparison, the reaction of acetaldehyde with dissolved SO₂ also was discussed. The barriers for the CH₃CHO + HSO₃⁻ reaction and CH₃CHO + SO₃²⁻ reaction are 16.6 kcal mol⁻¹, 2.5 kcal mol⁻¹, respectively. This result suggests that the reactivity of HCHO with dissolved SO₂ is higher than that of CH₃CHO. The further oxidation of CH₂(OH)SO₃⁻ and CH₃CH(OH)SO₃⁻ by an OH radical and O₂ shows that the SO₅˙⁻ radical can be produced.

We report the formation of an important organosulfur compound HMS and its oxidation using theoretical calculation.     

A chemosensor with a paddle structure based on a BODIPY chromophore for sequential recognition of Cu2+ and HSO3−

In this study, a highly selective chemosensor ML based on a BODIPY fluorescent chromophore was synthesized for sequential recognition of Cu²⁺ and HSO₃⁻ in a CH₃OH/H₂O (99 : 1 v/v) system, which contained three recognition sites and its structure characterized by ¹H NMR, ¹³C NMR and ESI-HR-MS. The sensor ML showed an obvious “on–off” fluorescence quenching response toward Cu²⁺ and the ML-Cu²⁺ complex showed an “off–on” fluorescence enhancement response toward HSO₃⁻. The detection limit of the sensor ML was 0.36 μM to Cu²⁺ and 1.4 μM to HSO₃⁻. In addition, the sensor ML showed a 1 : 3 binding stoichiometry to Cu²⁺ and the recovery rate of ML-Cu²⁺ complex identifying HSO₃⁻ could be over 70%. Sensor ML showed remarkable detection ability in a pH range of 4–8.

A highly selective chemosensor based on a BODIPY chromophore for sequential recognition of Cu²⁺ and HSO₃⁻.     

A simple and highly selective 1,2,4,5-tetrazine-based colorimetric probe for HSO3− ion recognition in food

A series of 3,6-bis-substituted-1,2,4,5-tetrazine-based colorimetric probes has been developed in good yields that exhibits highly selective and sensitive colorimetric recognition of HSO₃⁻ in aqueous solution. The color of the solution containing colorimetric probes changed markedly from orange to colorless, upon the addition of HSO₃⁻. Quantification of the absorption titration analysis shows that the detection limit of 3,6-bis(2-aminoethylamino)-1,2,4,5-tetrazine (2a) for HSO₃⁻ was 3.8 μM. It was noted that 3,6-bis-substituted-1,2,4,5-tetrazine-based colorimetric probes have a specific response toward HSO₃⁻ without interference from other 17 different anions and 16 common cations. A plausible mechanism was proposed by high-resolution mass spectroscopy analysis and NMR spectrometry analysis, involving the nucleophilic reaction of a bisulfite anion with the tetrazine ring and free radical rearrangement of ·SO₃H. Moreover, probes 2a–2c possessed applicability for sensing bisulfite in actual food samples. Therefore, the present work established a novel strategy for investigating bisulfite in food or other products.

3,6-Bis-substituted-1,2,4,5-tetrazine-based colorimetric probes, which have good water solubility, could detect bisulfate with high sensitivity and selectivity in food samples.     

Theoretical study of the oxidation reactions of sulfurous acid/sulfite with ozone to produce sulfuric acid/sulfate with atmospheric implications

Herein, theoretical studies were performed on the atmospheric oxidation of sulfurous acid (H₂SO₃) and sulfite ions (HSO₃⁻) by ozone (O₃) to produce sulfuric acid and hydrosulfate ions. The most favorable path for the H₂SO₃ + O₃ reaction has been found to be initiated from concerted H-abstraction and oxygen addition, with an overall energy barrier of 18.3 kcal mol⁻¹. On the other hand, the most favorable path for the HSO₃⁻ + O₃ reaction is initiated from oxygen addition, with an overall energy barrier of only 0.3 kcal mol⁻¹. Kinetic simulations were performed to estimate the significance of these reactions in the formation of atmospheric sulfate and destruction of the ozone layer. The results provide new insight into the missing source of atmospheric sulfate and particulate matter.

Herein, theoretical studies were performed on the atmospheric oxidation of sulfurous acid (H₂SO₃) and sulfite ions (HSO₃⁻) by ozone (O₃) to produce sulfuric acid and hydrosulfate ions.     

Novel dual-site fluorescent probe for monitoring cysteine and sulfite in living cells

Fluorescent probes have been considered to be efficient tools for the visualization of physiological and pathological processes. Herein, a dual-site fluorescence probe denoted as LC-1 was developed for the detection of cysteine (Cys) and its metabolite SO₃²⁻. The probe was shown to be highly sensitive to Cys and SO₃²⁻ with a turn-on mode fluorescence signal through two emission channels under excitations at wavelengths of 320 nm and 440 nm. Notably, the LC-1 probe was also observed to be satisfactorily sensitive to Cys and SO₃²⁻ in the presence of other amino acids and reactive oxygen species (ROS). Meanwhile, LC-1 was shown to have low cytotoxicity and was successfully applied for imaging the metabolism of Cys in living cells.

A dual-site fluorescence probe LC-1 was developed for the detection of Cys and SO₃²⁻.     

A dual emission metal–organic framework for rapid ratiometric fluorescence detection of CO32− in seawater

A dual emission metal–organic framework (IRMOF-10-Eu) was prepared and used as a ratiometric fluorescent sensor for CO₃²⁻ detection. IRMOF-10-Eu had good stability and excellent luminescence in aqueous solution. IRMOF-10-Eu showed dual fluorescence emission from the ligand and Eu³⁺ with single excitation. Upon treatment with CO₃²⁻, the fluorescence ratio (I₆₂₄/I₃₅₈) of the probe displayed significant change. The relative fluorescence intensity ratio (I₆₂₄/I₃₅₈) and CO₃²⁻ concentration had a linear relationship in 50–300 μM range with a low detection limit of 9.58 μM. And the luminescence probe of CO₃²⁻ showed a fast detection time. The possible mechanism was investigated. CO₃²⁻ changed the structure of IRMOF-10-Eu and interrupted the energy transfer process. Thus, the fluorescence emission intensity of the ligand was increased and Eu³⁺ was decreased with the addition of CO₃²⁻. IRMOF-10-Eu was used to detect CO₃²⁻ in seawater, which showed good prospect in practical application. Subsequently, a highly selective and sensitive probe, IRMOF-10-Eu, may pave an efficient way for CO₃²⁻ detection in seawater.

A dual emission metal–organic framework (IRMOF-10-Eu) was prepared and used as a ratiometric fluorescent sensor for CO₃²⁻ detection.     

Ratiometric fluorescent sensors for nitrite detection in the environment based on carbon dot/Rhodamine B systems

A novel carbon dot/Rhodamine B-based ratiometric fluorescent probe was developed for a highly sensitivity and selective detection of nitrite (NO₂⁻). The probe showed colour changes from blue to orange under ultraviolet light in response to NO₂⁻ with a detection limit as low as 67 nM in the range of 0 to 40 μM. A ratiometric fluorescent test paper was successfully prepared using the probe solution, which demonstrated its feasibility towards a rapid and semi-quantitative detection of NO₂⁻ in real samples.

A visual ratiometric fluorescent sensor based on blue carbon dot/Rhodamine B is used to selectively detect NO₂⁻ in the environment.     

A ratiometric fluorescence sensor based on enzymatically activatable micellization of TPE derivatives for quantitative detection of alkaline phosphatase activity in serum

A novel ratiometric fluorescence assay via enzymatically activatable micellization in aqueous solution was devised for quantitative detection of alkaline phosphatase (ALP) activity. We demonstrated that the dephosphorylation of the water-soluble, phosphate-functionalized, fluorophore monomer P-TPE-TG, induced by an enzymatic reaction of ALP, leads to micelle formation in aqueous solution because its water-soluble functionality is reduced. The dephosphorylation-induced micellization of P-TPE-TG exhibited a ratiometric sensing response for various ALP concentrations (10–200 mU mL⁻¹) and provided a suitable sensing platform for naked eye detection with increased fluorescence quantum yield (Φ = 3.2%), even compared to a typical TPE-based sensor (Φ = 1.0%), where ALP can be sensed with a detection limit of 0.034 mU mL⁻¹. In addition, P-TPE-TG displayed excellent sensing performance at concentrations from 0 to 50 mU mL⁻¹ in diluted human serum (10%), which offers the capability to exploit ratiometric responses for bioactive substances under practical conditions.

A novel ratiometric fluorescence assay via enzymatically activatable micellization in aqueous solution was devised for quantitative detection of alkaline phosphatase (ALP) activity.     

A sensitive and selective BINOL based ratiometric fluorescence sensor for the detection of cyanide ions

A highly selective, novel BINOL based sensor BBCN has been developed for the fluorescent ratiometric detection of cyanide ions (CN⁻). The optical study revealed that BBCN exhibited unique spectral changes only with cyanide ions in the presence of other competing ions. Besides, an apparent fluorescent colour change from green to blue was observed. A clear linear relationship was observed between the fluorescence ratiometric ratio of BBCN and the concentration of CN⁻ with a reasonably low detection limit (LOD) of 189 nM (507 ppb). The optical response was due to the nucleophilic addition of CN⁻ to the dicyanovinyl group of the sensor, which compromises the probe''s intramolecular charge transfer. This mechanism was well confirmed by Job''s plot, ¹H-NMR and ESI-MS studies. BBCN showed immediate spectral response towards (1 second) CN⁻ and detection could be realized in a broad pH window. Furthermore, the practical utility of BBCN was studied by test paper-based analysis and the detection of CN⁻ in various water resources.

A highly selective, novel BINOL based sensor BBCN has been developed for the fluorescent ratiometric detection of cyanide ions (CN⁻).     

Impact of the acidic group on the hydrolysis of 2-dinitromethylene-5,5-dinitropyrimidine-4,6-dione

The hydrolysis mechanism and the kinetics of using 2-dinitromethylene-5,5-dinitropyrimidine-4,6-dione (NMP) to prepare the representative insensitive energetic material 1,1-diamino-2,2-dinitroethylene (FOX-7) in a nitric–sulfuric acid system are systematically investigated via a density functional theory (DFT) method. The impact of the co-existing acidic group of HSO₄⁻ as well as the solvent effects of the mixed acids on the hydrolysis of NMP are elucidated and discerned, and the proposed catalysis and promotion of the hydrolysis of NMP with HSO₄⁻ are verified. The HSO₄⁻-catalyzed hydrolysis pathway is more favorable than the direct pathway as well as the H₂O-catalyzed hydrolysis, indicating that HSO₄⁻ may be a promising catalyst for the preparation of FOX-7 in a mixed acid system. The present study is expected to provide a better understanding of the hydrolysis of NMP, and will significantly help with better preparation of FOX-7 and other nitro-energetic materials.

Unique and incredible catalysis of the titled hydrolysis using HSO₄⁻ is proposed and verified in the gas and solvent phases.     

One-step synthesis of green emission carbon dots for selective and sensitive detection of nitrite ions and cellular imaging application

In recent years, carbon dot (CD)-based fluorescent sensors for selective ions or small biomolecules have drawn great attention. In this work, highly fluorescent CDs (QY = 21%) were prepared from 2,3-diamino pyridine as the precursor through a facile solvothermal process. The CDs showed high stability and a green emission in aqueous, and the optimal emission wavelength of CDs is 508 nm under the excitation wavelength of 438 nm. Interestingly, a CDs-based nanoprobe was developed for a selective and sensitive fluorescence quenching response to NO₂⁻ in water, and the quenching mechanism was investigated in the work. Besides, the recovery rates of NO₂⁻ in the range of 98–103.5% were found to be acceptable, indicating that the proposed CDs could be act as potential candidates for determination of nitrite ions in real samples. Meanwhile, the nanoprobe was also successfully employed in a visualization biosensing platform for determination of NO₂⁻ in living cells due to its eminent biocompatibility.

Schematic route of the carbon dots and their applications for the nitrite detection.     

Acridinedione as selective flouride ion chemosensor: a detailed spectroscopic and quantum mechanical investigation

The use of small molecules as chemosensors for ion detection is rapidly gaining popularity by virtue of the advantages it offers over traditional ion sensing methods. Herein we have synthesized a series of acridine(1,8)diones (7a–7l) and explored them for their potential to act as chemosensors for the detection of various anions such as fluoride (F⁻), acetate (OAc⁻), bromide (Br⁻), iodide (I⁻), bisulfate (HSO₄⁻), chlorate (ClO₃⁻), perchlorate (ClO₄⁻), cyanide (CN⁻), and thiocyanate (SCN⁻). Acridinediones were found to be highly selective chemosensors for fluoride ions only. To investigate in detail the mechanism of selective fluoride ion sensing, detailed spectroscopic studies were carried out using UV-visible, fluorescence and ¹H NMR spectroscopy. Fluoride mediated (NH) proton abstraction of acridinedione was found to be responsible for the observed selective fluoride ion sensing. Quantum mechanical computational studies, using time dependent density functional theory (TDDFT) were also carried out, whereupon comparison of acridinedione interaction with fluoride and acetate ions explained the acridinedione selectivity for the detection of fluoride anions. Our results provide ample evidence and rationale for further modulation and exploration of acridinediones as non-invasive chemosensors for fluoride ion detection in a variety of sample types.

The use of small molecules as chemosensors for ion detection is rapidly gaining popularity by virtue of the advantages it offers over traditional ion sensing methods.     

CdTe QD-based inhibition and reactivation assay of acetylcholinesterase for the detection of organophosphorus pesticides

An enzyme immobilized glutathione (GSH)-capped CdTe quantum dot (QD)-based fluorescence assay has been developed for monitoring organophosphate pesticides. In principle, GSH-capped CdTe QDs exhibit higher sensitivity towards H₂O₂ produced from the active enzymatic reaction of acetylcholinesterase (AChE) and choline oxidase (CHOx), which results in the fluorescence (FL) “turn-off” of the GSH-capped CdTe QDs. A “turn-on” FL of the CdTe QDs at 520 nm was recovered in the presence of organophosphate (OP). The FL changes of the GSH-capped CdTe QD/AChE/CHOx biosensor reasonably correspond to the amount of OP pesticides. The detection limit of the CdTe/AChE/CHOx biosensor towards paraoxon, dichlorvos, malathion and triazophos was 1.62 × 10⁻¹⁵ M, 75.3 × 10⁻¹⁵ M, 0.23 × 10⁻⁹ M and 10.6 × 10⁻¹² M, respectively. The GSH-capped CdTe QDs/AChE/CHOx biosensor was applied as a FL nanoprobe for assaying the enzymatic activity of AChE. The inhibited AChE was reactivated up to 94% using pyridine oximate (2-PyOx⁻), and functionalized pyridinium oximates (4-C₁₂PyOx⁻ and 4-C₁₈PyOx⁻) of varying chain lengths. It was found that the reactivation potency of the tested oximes varied with the chain length of the oximes. This biosensing system offers the promising benefit for the determination of the OP pesticides in food, water and environmental samples.

An enzyme immobilized glutathione (GSH)-capped CdTe quantum dot (QD)-based fluorescence assay has been developed for monitoring organophosphate pesticides.