An effective colorimetric and ratiometric fluorescent probe for bisulfite in aqueous solution

We have developed the first two-photon colorimetric and ratiometric fluorescent probe, BICO, for the detection of bisulfite (HSO₃⁻) in aqueous solution. The probe contains coumarin and benzimidazole moieties and can detect HSO₃⁻ based on the Michael addition reaction with a limit of detection 5.3 × 10⁻⁸ M in phosphate-buffered saline solution. The probe was used to detect bisulfite in tap water, sugar and dry white wine. Moreover, test strips were made and used easily. We successfully applied the probe to image living cells, using one-photon fluorescence imaging. BICO overcomes the limitations in sensitivity of previously reported probes and the solvation effect of bisulfite, which demonstrates its excellent value in practical application.     

A Hybrid Coumarin–Thiazole Fluorescent Sensor for Selective Detection of Bisulfite Anions in Vivo and in Real Samples

A hybrid coumarin–thiazole compound was developed as a novel ratiometric and colorimetric sensor for bisulfite anions. Structure identification of the compound was confirmed by ¹H NMR, ¹³C NMR, ¹H,¹H COSY, heteronuclear single quantum coherence (HSQC), IR, and HRMS spectroscopy. The detection of bisulfite anions was performed through the Michael addition of the bisulfite anion toward the hybrid coumarin–thiazole sensor. The reaction between the sensor and bisulfite anion caused the fluorescence intensity to decrease at 600 nm and to increase at 450 nm and simultaneously yielded a visible color change from purplish red to colorless because the π conjugation between thiazole and coumarin was blocked. The sensor possessed high selectivity and sensitivity for bisulfite with respect to other common anions in aqueous solution. Moreover, the practical value of this sensor was confirmed by its application in the detection of bisulfite anion in human breast adenocarcinoma cells and granulated sugar.     

比率型SO2荧光探针的构建及细胞成像研究

本文构建了一种基于苯并吲哚季铵盐结构的荧光探针用于检测SO₂衍生物。该荧光探针能够快速、灵敏、高选择性地检测HSO₃^-和SO₃^2-,并显示出颜色和荧光变化双重响应。其比率荧光强度（I₄₆₂/I₅₈₈）与HSO₃^-的浓度（0～16 μmol/L）之间具有良好的线性关系,检测限低至12 nmol/L。¹HNMR表明该探针的响应机制为1,4-亲核加成反应。激光共聚焦荧光成像结果表明,CZBI具有良好的细胞膜通透性,并且可以通过比率荧光成像实现对细胞内SO₂衍生物的监测。     

A New Ratiometric Fluorescent Probe Based on BODIPY for Highly Selective Detection of Hydrogen Sulfide

Hydrogen sulfide (H₂S) as small molecular signal messenger plays key functions in numerous biological processes. The imaging detection of intracellular hydrogen sulfide is of great significance. In this work, a ratiometric fluorescent probe BH based on an asymmetric BODIPY dye for detection of H₂S was designed and synthesized. After the interaction with hydrogen sulfide, probe display colorimetric and ratiometric fluorescence response, with its maximum emission fluorescence wavelength red-shifted from 542 nm to 594 nm, which is attributed to the sequential nucleophilic reaction of H₂S leading to enhanced molecular conjugation after ring formation of the BODIPY skeleton. A special response mechanism has been fully investigated by NMR titration and MS, so that the probe has excellent detection selectivity. Furthermore, probe BH has low cytotoxicity and fluorescence imaging experiments indicate that it can be used to monitor hydrogen sulfide in living cells.     

A near-infrared large Stokes shift probe based enhanced ICT strategy for F- detection in real samples and cell imaging

In view of the few reports of the near-infrared emissive probe for fluorine ions, we herein designed and synthesized a new easy-to-get colorimetric and near-infrared emissive fluorescent probe (IS-NR-F) with a large Stokes shift (>127 nm). Based on specific F^? triggered desilylation reaction induced enhanced ICT strategy involving the donor phenolate anion and the acceptor malononitrile, the probe exhibited dual colorimetric and fluorescent turn-on responses, and provided excellent selectivity for fluoride ions. The fluorescent response at 665 nm displayed very good linear relationship in the wide concentration range and deduced a low detection limit of 0.09 ppm. The detection mechanism was confirmed by ¹H NMR, ESI-MS, and TLC calculation. Moreover, probe IS-NR-F has been successfully employed to detect F^? in tap water, toothpaste samples, and fluorescent imaging of F^? in HeLa cells.     

A two-photon fluorescent turn-on probe for imaging of SO2 derivatives in living cells and tissues

SO₂ and its derivatives (bisulfite/sulfite) play crucial roles in several physiological processes. Therefore, development of reliable analytical methods for monitoring SO₂ and its derivatives in biological systems is very significant. In this paper, a FRET-based two-photon fluorescent turn-on probe, A-HCy, was proposed for specific detection of SO₂ derivatives through the bisulfite/sulfite-promoted Michael addition reaction. In this FRET system, an acedan (2-acetyl-6-dialkylaminonaphthalene) moiety was selected as a two-photon donor and a hemicyanine derivative served as both the quencher and the recognition unit for bisulfite/sulfite. A-HCy exhibited excellent selectivity and rapid response to HSO₃⁻ with a detection limit of 0.24 μM. More importantly, probe A-HCy was first successfully applied in two-photon fluorescence imaging of biological SO₂ derivatives in living cells and tissues, suggesting its great potential for practical application in biological systems.     

A ratiometric fluorescent probe for iron(III) and its application for detection of iron(III) in human blood serum

A simple and versatile ratiometric fluorescent Fe³⁺ detecting system, probe 1, was rationally developed based on the Fe³⁺-mediated deprotection of acetal reaction. Notably, this reaction was firstly employed to design fluorescent Fe³⁺ probe. Upon treatment with Fe³⁺, probe 1 showed ratiometric response, with the fluorescence spectra displaying significant red shift (up to 132 nm) and the emission ratio value (I₅₂₂/I₃₉₀) exhibiting approximately 2362-fold enhancement. In addition, the probe is highly sensitive (with the detection limit of 0.12 μM) and highly selective to Fe³⁺ over other biologically relevant metal ions. The sensing reaction product of the probe with Fe³⁺ was confirmed by NMR spectra and mass spectrometry. TD-DFT calculation has demonstrated that the ratiometric response of probe 1 to Fe³⁺ is due to the regulation of intramolecular charge transfer (ICT) efficiency. Moreover, the practical utility in fluorescence detection of Fe³⁺ in human blood serum was also conducted, and probe 1 represents the first ratiometric fluorescent probe that can be used to monitor Fe³⁺ level in human blood serum. Finally, probe 1 was further employed in living cell imaging with pancreatic cancer cells, in which it displayed low cytotoxicity, satisfactory cell permeability, and selective ratiometric response to Fe³⁺.     

Ratiometric two-photon excited photoluminescence of quantum dots triggered by near-infrared-light for real-time detection of nitric oxide release in situ

Probe-donor integrated nanocomposites were developed from conjugating silica-coated Mn²⁺:ZnS quantum dots (QDs) with MoS₂ QDs and photosensitive nitric oxide (NO) donors (Fe₄S₃(NO)₇⁻, RBS). Under excitation with near-infrared (NIR) light at 808 nm, the Mn²⁺:ZnS@SiO₂/MoS₂-RBS nanocomposites showed the dual-emissive two-photon excited photoluminescence (TPEPL) that induced RBS photolysis to release NO in situ. NO caused TPEPL quenching of Mn²⁺:ZnS QDs, but it produced almost no impact on the TPEPL of MoS₂ QDs. Hence, the nanocomposites were developed as a novel QDs-based ratiometric TPEPL probe for real-time detection of NO release in situ. The ratiometric TPEPL intensity is nearly linear (R² = 0.9901) with NO concentration in the range of 0.01∼0.8 μM, which corresponds to the range of NO release time (0∼15 min). The detection limit was calculated to be approximately 4 nM of NO. Experimental results confirmed that this novel ratiometric TPEPL probe possessed high selectivity and sensitivity for the detection of NO against potential competitors, and especially showed high detection performance for NIR-light triggered NO release in tumor intracellular microenvironments. These results would promote the development of versatile probe-donor integrated systems, also providing a facile and efficient strategy to real-time detect the highly controllable drug release in situ, especially in physiological microenvironments.     

Ratiometric fluorescence-based dissolved carbon dioxide sensor for use in environmental monitoring applications

The focus of this work is on the development and characterisation of a fluorescence-based ratiometric sol–gel-derived dissolved carbon dioxide (dCO₂) sensor for use in environmental monitoring applications. Fluorescence-based dCO₂ sensors are attractive as they facilitate the development of portable and low-cost systems that can be easily deployed outside the laboratory environment. The sensor developed for this work exploits a pH fluorescent dye 1-hydroxypyrene-3,6,8-trisulfonic acid, ion-paired with cetyltrimethylammonium bromide (HPTS-IP), which has been entrapped in a hybrid sol–gel-based matrix derived from n-propyltriethoxysilane along with the liphophilic organic base. The sensor spot deposited on a cover slip has been interrogated with a robust, ratiometric optical probe that combines effective fluorescence excitation and detection and thus facilitates the production of a highly sensitive sensor system using low-cost optoelectronic components. The probe design involves the use of dual-LED excitation in order to facilitate ratiometric operation and uses a silicon PIN photodiode. HPTS-IP exhibits two pH-dependent changes in excitation bands, which allows for dual excitation ratiometric detection as an indirect measure of the dCO₂. Such measurements are insensitive to changes in dye concentration, leaching and photobleaching of the fluorophore and instrument fluctuations unlike unreferenced fluorescence intensity measurements. The performance of the sensor system is characterised by a high degree of repeatability, reversibility and stability. Calculated limit of detection for the sensor was 35 ppb. The sensor probe was used to monitor dCO₂ levels in a laboratory-based aquatic habitat, and the expected diurnal pattern was clearly visible. The influence of temperature, biofouling and photobleaching on sensor performance has been also investigated.     

Ratiometric fluorescent Zn chemosensor constructed by appending a pair of carboxamidoquinoline on 1,2-diaminocyclohexane scaffold

By appending a pair of carboxamidoquinoline pendants onto 1,2-diaminocyclohexane scaffold via N-alkylation, multifunctionalized ACAQ was designed and synthesized as a water soluble fluorescent ratiometric chemosensor for Zn²⁺. In 50% aqueous methanol buffer pH 7.4 solution, upon excitation at 316 nm, ACAQ (5 μM) displayed a selective ratiometric fluorescence changes with a shift from 410 to 490 nm in response to the interaction with Zn²⁺. After binding with 1 equiv of Zn²⁺, ACAQ exhibited a 12-fold enhancement in I₄₉₀/I₄₁₀ characterized by a clear isoemissive point at 440 nm. The metal sensor binding mode was established by Job’s plot and the combined fluorescence and ¹H NMR spectroscopic method. The selectivity of the probe toward biological relevant cations and transition metal ions was proven to be good. In addition, the interference caused by Cu²⁺ and Cd²⁺ in the quantitation of Zn²⁺ can be completely eliminated by the use of diethyldithiocarbamate as the screening agent. Exploitation of ACAQ as the sensing probe, ratiometric determination of Zn²⁺ with the limit of detection (LOD) at 28.3 nm can be realized. In addition, the unique responsive properties of the probe toward Fe³⁺ and Zn²⁺ were used to construct a fluorescent switch. The membrane permeability of ACAQ to living cells and bio-imaging of Zn²⁺ were demonstrated.     

A 1-Hydroxy-2,4-Diformylnaphthalene-Based Fluorescent Probe and Its Detection of Sulfites/Bisulfite

A novel 1-hydroxy-2,4-diformylnaphthalene-based fluorescent probe L was synthesized by a Knoevenagel reaction and exhibited excellent sensitivity and selectivity towards sulfite ions (SO₃²⁻) and bisulfite ions (HSO₃⁻). The detection limits of the probe L were 0.24 μM using UV-Vis spectroscopy and 9.93 nM using fluorescence spectroscopy, respectively. Furthermore, the fluorescent probe L could be utilized for detection in real water samples with satisfactory recoveries in the range 99.20%~104.30% in lake water and 100.00%~104.80% in tap water by UV-Vis absorption spectrometry, and in the range 100.50%~108.60% in lake water and 102.70%~103.80% in tap water by fluorescence spectrophotometry.     

A Native‐Chemical‐Ligation‐Mechanism‐Based Ratiometric Fluorescent Probe for Aminothiols

Thiol‐containing amino acids (aminothiols) such as cysteine (Cys) and homocysteine (Hcy) play a key role in various biological processes including maintaining the homeostasis of biological thiols. However, abnormal levels of aminothiols are associated with a variety of diseases. The native chemical ligation (NCL) reaction has attracted great attention in the fields of chemistry and biology. NCL of peptide segments involves cascade reactions between a peptide‐α‐thioester and an N‐terminal cysteine peptide. In this work, we employed the NCL reaction mechanism to formulate a Förster resonance energy transfer (FRET) strategy for the design of ratiometric fluorescent probes that were selective toward aminothiols. On the basis of this new strategy, the ratiometric fluorescent probe 1 for aminothiols was judiciously designed. The new probe is highly selective toward aminothiols over other thiols and exhibits a very large variation (up to 160‐fold) in its fluorescence ratio (I₄₅₈/I₆₀₃). The new fluorescent probe is capable of ratiometric detection of aminothiols in newborn calf and human serum samples and is also suitable for ratiometric fluorescent imaging of aminothiols in living cells.     

Colorimetric assay for on-the-spot alcoholic strength sensing in spirit samples based on dual-responsive lanthanide coordination polymer particles with ratiometric fluorescence

This study demonstrates a new strategy for colorimetric detection of alcoholic strength (AS) in spirit samples based on dual-responsive lanthanide infinite coordination polymer (Ln-ICP) particles with ratiometric fluorescence. The ICP used in this study are composed of two components: one is the supramolecular Ln-ICP network formed by the coordination between the ligand 2,2’-thiodiacetic acid (TDA) and central metal ion Eu³⁺; and the other is a fluorescent dye, i.e., coumarin 343 (C343), both as the cofactor ligand and as the sensitizer, doped into the Ln-ICP network through self-adaptive chemistry. Upon being excited at 300 nm, the red fluorescence of Ln-ICP network itself at 617 nm is highly enhanced due to the concomitant energy transfer from C343 to Eu³⁺, while the fluorescence of C343 at 495 nm is supressed. In pure ethanol solvent, the as-formed C343@Eu-TDA is well dispersed and quite stable. However, the addition of water into ethanolic dispersion of C343@Eu-TDA destructs Eu-TDA network structure, resulting in the release of C343 from ICP network into the solvent. Consequently, the fluorescence of Eu-TDA turns off and the fluorescence of C343 turns on, leading to the fluorescent color change of the dispersion from red to blue, which constitutes a new mechanism for colorimetric sensing of AS in commercial spirit samples. With the method developed here, we could clearly distinguish the AS of different spirit samples within a wide linear range from 10% vol to 100% vol directly by “naked eye” with the help of UV-lamp (365 nm). This study not only offers a new method for on-the-spot visible detection of AS, but also provides a strategy for dual-responsive sensing mode by rational designing the optical properties of the Ln-ICP network and the guest, respectively.     

Rational design of a ratiometric fluorescent probe with a large emission shift for the facile detection of Hg2+

A new ratiometric and colorimetric fluorescent probe for the highly selective, sensitive and facile detection of Hg(2+) has been rationally developed.     

Bithiophene triarylborane dyad: An efficient material for the selective detection of CN− and F− ions

A new fluorescent chemosensor based on bithiophene coupled dimesitylborane (BMB-1) was synthesized and characterized. BMB-1 was used for colorimetric and turn-on fluorescent sensing of cyanide (CN⁻) and fluoride (F⁻) ions, in the presence of other competitive anions in an aqueous (CH₃CN–H₂O) medium. BMB-1 showed a hypsochromic shift (blue shift) with addition of CN⁻ and F⁻ ions in absorption studies. The lower detection level of CN⁻ and F⁻ ions is 1.37 × 10⁻⁹ and 1.75 × 10⁻⁹ M, respectively. The BMB-1 binding mechanism is based on the nucleophilic addition of CN⁻ and F⁻ ions in the internal charge transfer transition of bithio moiety to the boranylmesitylene unit, and the color changes were observed under UV light. This result is further confirmed by Fourier transform infrared spectroscopy, mass spectrometry and density functional theory calculations. Also, the BMB-1 probe is found to be a good adsorbent for the removal of F⁻ ions in real water samples using the adsorption technique.     

A colorimetric,ratiometric and water-soluble fluorescent probe for simultaneously sensing glutathione and cysteine/homocysteine

A chlorinated coumarin-aldehyde was developed as a colorimetric and ratiometric fluorescent probe for distinguishing glutathione (GSH), cystenine (Cys) and homocysteine (Hcy). The GSH-induced substitution-cyclization and Cys/Hcy-induced substitution-rearrangement cascades lead to the corresponding thiol-coumarin-iminium cation and amino-coumarin-aldehyde with distinct photophysical properties. The probe can be used to simultaneously detect GSH and Cys/Hcy by visual determination based on distinct different colors – red and pale-yellow in PBS buffer solution by two reaction sites. From the linear relationship of fluorescence intensity and biothiols concentrations, it was determined that the limits of detection for GSH, Hcy and Cys are 0.08, 0.09 and 0.18 μM, respectively. Furthermore, the probe was successfully used in living cell imaging with low cell toxicity.     

A colorimetric and fluorometric dual-model assay for mercury ion by a molecule

The synthesis and sensing characteristics of a new class of colorimetric and fluorometric dual-modal probe for mercury ion are outlined. Judicious placement of two dithia-dioxa-aza macrocycles on the BODIPY chromophore generates this interesting molecule. A highly Hg2+-selective fluorescence enhancing property (>7-fold) in conjunction with a visible colorimetric change from purple to red-pink can be observed, leading to potential fabrication of both "naked-eye" and ratiometric fluorescent detection of Hg2+.     

Fluorescent carbazolyldithiane as a highly selective chemodosimeter via protection/deprotection functional groups: a ratiometric fluorescent probe for Cd(II)

A carbazole-based bis-dithiane (1) was rationally constructed in a straightforward manner for the selective and ratiometric fluorescent detection of Cd²⁺ with the concept of aldehyde group protection/deprotection. The probe showed a ratiometric fluorescent response to Cd²⁺ with a large emission wavelength shift (>50 nm) and displayed high selectivity for Cd²⁺ over other metal ions due to distinct deprotection conditions. In addition, a Cd²⁺-promoted dethioacetalization mechanism was proposed.     

A ratiometric fluorescent probe for fluoride ion employing the excited-state intramolecular proton transfer

A ratiometric fluorescent probe 1 for fluoride ion was developed based on modulation of the excited-state intramolecular proton transfer (ESIPT) process of 2-(2′-hydroxyphenyl)benzimidazole (HPBI) through the hydroxyl group protection/deprotection reaction. The probe 1 was readily prepared by the reaction of HPBI with tert-butyldimethylsilyl chloride (TBS-Cl) and shows only fluorescence emission maximum at 360 nm. Upon treatment with fluoride in aqueous DMF solution, the TBS protective group of probe 1 was removed readily and ESIPT of the probe was switched on, which resulted in a decrease of the emission band at 360 nm and an increase of a new fluorescence peak around 454 nm. The fluorescent intensity ratio at 454 and 360 nm (I₄₅₄/I₃₆₀) increases linearly with fluoride ion concentration in the range 0.3-8.0 μmol L⁻¹ and the detection limit is 0.19 μmol L⁻¹. The proposed probe shows excellent selectivity toward fluoride ion over other common anions. The method has been successfully applied to the fluoride determination in toothpaste and tap water samples.     

Ratiometric detection of alkaline phosphatase based on aggregation-induced emission enhancement

Alkaline phosphatase (ALP) is an important enzyme that is associated with many human diseases, so the quantitative detection of ALP is vital from a clinical perspective. Nevertheless, most fluorescent assays for monitoring ALP depend on aggregation-induced quenching (ACQ), single-signal modulation, or a “signal off” mode, which suffer from poor sensitivity, a “false positive” problem, and low signal output. In this work, we utilized the electrostatically driven self-assembly of glutathione-capped gold nanoclusters (GSH-AuNCs, which show aggregation-induced emission, AIE) and amino-modified silicon nanoparticles (SiNPs) to create a hybrid probe (SiNPs@GSH-AuNCs). This nanohybrid probe showed emission from the SiNPs at around 470 nm as well as aggregation-induced emission enhancement (AIEE) of the GSH-AuNCs at 580 nm. The AIEE of the GSH-AuNCs was quenched in the presence of KMnO₄, but the AIEE was recovered by adding ascorbic acid as an oxidation–reduction reaction occurred between KMnO₄ and the ascorbic acid. The fluorescence of the SiNPs remained constant whether the AIEE was quenched or not, meaning that the fluorescence of the SiNPs could be used as an internal reference. In a typical enzymatic reaction, ascorbic acid 2-phosphate is hydrolyzed by ALP to produce ascorbic acid. Therefore, the hybrid probe was shown to allow the ratiometric detection of ALP, with a linear range of 0.5–10 U L⁻¹ and a limit of detection (LOD) of 0.23 U L⁻¹. Finally, the proposed analytical strategy was successfully applied to detect ALP in human serum samples and to determine the concentration of an ALP inhibitor.

Graphical Abstract