A Highly Selective and Sensitive Ratiometric Fluorescent Probe for pH Measurement Based on Fluorescence Resonance Energy Transfer

Fluorescein-rhodamine 6G(Flu-Rh) was synthesized and used as the fluorescence probe for pH measurement based on fluorescence resonance energy transfer(FRET). In the probe, fluorescein fluorophore and pH-sensitive rhodamine 6G hydrazide were used as FRET donor and acceptor, respectively. The values of acidity constant(pK_a) and fluorescence quantum yield(Φ) of Flu-Rh were 3.71 and 0.72, respectively. The fluorescence efficiency of Flu-Rh remains almost constant when the pH value of the sample solution changed 10 times in a range of 4.78-7.03 continuously. The present probe is simple and easy-to-use for the pH measurement in acidic media.     

Developing a novel ratiometric fluorescent probe based on ESIPT for the detection of pH changes in living cells

As an important parameter of intracellular metabolism, pH plays important roles in maintaining normal physiological processes. The abnormal pH could cause disorder of cell function which may cause neurological diseases. Herein, we present two novel ratiometric fluorescent probes to detect pH changes. The probes employed 2-(2′-hydroxyphenyl)benzothiazole as fluorescent platform, and displayed desirable fluorescence response to pH on the basis of excited state intramolecular proton transfer (ESIPT) process. The probe BtyC-1 showed green fluorescence at 546 nm under acidic conditions, while it displayed strong blue fluorescence at 473 nm and weak green fluorescence at 546 nm under alkaline conditions. Biological experiments demonstrated that the probe BtyC-1 could be successfully applied for the ratiometric imaging of cellular pH and the NH₄Cl-induced pH changes in living cells.     

Highly Fe selective ratiometric fluorescent probe based on imine-linked benzimidazole

We synthesized a novel benzimidazole-based fluorescent receptor bearing imine linkages with two sets of sp² nitrogens, and investigated its binding properties toward various metal ions. The receptor exhibited a shift in emission band upon binding with Fe³⁺ ions, and no such significant response was noticed in other metal ions. The receptor shows a property of selective ratiometric fluorescent probe of Fe³⁺ ions without interferences of the background metal ions.     

Investigation of the hybrid molecular probe for intracellular studies

Monitoring gene expression in vivo is essential to the advancement of biological studies, medical diagnostics, and drug discovery. Adding to major efforts in developing molecular probes for mRNA monitoring, we have recently developed an alternative tool, the hybrid molecular probe (HMP). To optimize the probe, a series of experiments were performed to study the properties of HMP hybridization kinetics and stability. The results demonstrated the potential of the HMP as a prospective tool for use in both hybridization studies and in vitro and in vivo analyses. The HMP has shown no tendency to produce false positive signals, which is a major concern for living cell studies. Moreover, HMP has shown the ability to detect the mRNA expression of different genes inside single cells from both basal and stimulated genes. As an effective alternative to conventional molecular probes, the proven sensitivity, simplicity, and stability of HMPs show promise for their use in monitoring mRNA expression in living cells. Figure Hybrid molecular probe (HMP). HMPs consist of two single strands of DNA (green) and a polyethylene glycol (PEG, purple) linker that is used to tether these two sequences together. When a target (orange strand) containing the complementary sequences to both probes at adjacent positions is added, each strand binds to its corresponding target sequence, thus bringing the two fluorophores into close proximity, which allows energy transfer to occur     

Design of quinoline-based fluorescent probe for the ratiometric detection of cadmium in aqueous media

A new fluorescent probe,DQCd2,based on 4-piperidinoquinoline has been synthesized as a fluorescent probe for Cd²⁺.It can ratiometrically respond to Cd²⁺ in PBS buffer by a remarkable emission intensity enhancement（88-fold） and wavelength shift（70 nm）.     

Highly selective sensing of cyanide by a benzochromene-based ratiometric fluorescence probe

Enone-functionalized benzochromene chemodosimeter (1) was prepared through the Baylis-Hillman condensation reaction and was utilized as a ratiometric fluorescence probe for cyanide anions in aqueous buffer. The probe has shown a selective and sensitive response to cyanides over other various anions through the Michael addition and a subsequent [1,3]-sigmatropic rearrangement reaction. When cyanide anions were added, a prominent ratiometric fluorescence change of 1 was observed thus allowing to detect the micromolar concentration of cyanides by the naked eye.     

Monitoring intracellular pH fluctuation with an excited-state intramolecular proton transfer-based ratiometric fluorescent sensor

Intracellular pH is a key parameter related to various biological and pathological processes. In this study, a ratiometric pH fluorescent sensor ABTT was developed harnessing the amino-type excited-state intramolecular proton transfer (ESIPT) process. Relying on whether the ESIPT proceeds normally or not, ABTT exhibited the yellow fluorescence in acidic media, or cyan fluorescence in basic condition. According to the variation, ABTT behaved as a promising sensor which possessed fast and reversible response to pH change without interference from the biological substances, and exported a steady ratiometric signal (I₄₇₈/I₅₄₆). Moreover, due to the ESIPT effect, large Stokes shift and high quantum yield were also exhibited in ABTT. Furthermore, ABTT was applied for monitoring the pH changes in living cells and visualizing the pH fluctuations under oxidative stress successfully. These results elucidated great potential of ABTT in understanding pH-dependent physiological and pathological processes.     

A ratiometric approach for pH optosensing with a single fluorophore indicator

A new fiber-optic prototype of luminometer has been designed in order to perform ratiometric-based measurements for optical sensing purposes. The coupling of a pH-selective sensing phase to the fiber-optic prototype has been evaluated for robust pH optosensing in drinking water. The pH-sensitive material has been synthesized by entrapping a pH-sensitive luminescent indicator (mercurochrome) in a sol-gel inorganic matrix. The pH optosensing is based on the detection of pH-induced reversible changes in the mercurochrome fluorescent emission and in the light reflected by the sensing phase.The instrument has been constructed using low-cost and simple optoelectronic components. The active phase was excited by means of a visible 470 nm high intensity light emitting diode (LED). The radiant power of the LED was modulated using a sinusoidal function so that scattered light due to light sources of different frequency than the modulating signal (e.g. sunlight) can be easily removed by adequate electronic filtering of the emission signal. Both the fluorescence emission from the dye and the sensing phase reflected light were collected in a bifurcated fiber-optic to allow the ratiometric measurement.Two different ratiometric approaches have been evaluated. The analytical performance of the pH optrode using both measurement methods have been compared, between them and with simple fluorescence intensity measurements, in terms of sensitivity, measurement range, response time, repeatability and insensitivity to changes in excitation light intensity.The applicability of the developed pH optrode and methods has been tested for pH analysis in tap and bottled still mineral water samples. The results obtained showed good agreement with the corresponding pH values provided by a commercial glass electrode.In this work, pH was selected as a model analyte to evaluate the performance of the proposed methodology, although other optical sensors for different applications/analytes could benefit of this approach.     

A chloroacetate based ratiometric fluorescent probe for cysteine detection in biosystems

The specific detection of cysteine (Cys) over homocysteine (Hcy), glutathione (GSH) and other amino acids is of great significance for studying its biological functions as well as for the diagnosis of related diseases. Chloroacetyl group was often used as a reaction site for cysteine fluorescent probes for its sensitivity and selectivity. However, high background fluorescence and low stability are common problems encountered by such probes. Here, four chloroacetyl group based fluorescent probes (C1, C2, C3, and H4) was synthesized for a comparative study. We found that the inefficient quenching ability of chloroacetyl group turned into an advantage when connected with a ratiometric fluorophore. With the modification of chloroacetyl group, probe H4 displayed excellent ratiometric property and great selectivity for Cys, the stability was also improved. Additionally, the probe was successfully applied for quantitative detection of Cys in fetal bovine serum and real-time imaging in living HeLa cells with low toxicity.     

A ratiometric fluorescent probe for palladium detection based on an allyl carbonate group functionalized hemicyanine dye

A novel ratiometric fluorescent probe for palladium species was synthesized based on an allyl carbonate group, a novel reaction site, and a hemicyanine dye. The probe displays relatively rapid response, high selectivity, and anti-disturbance toward palladium in HEPES buffers without additional reagents. The detection mechanism, palladium triggers the cleavage of allyl carbonate in the probe and then decarboxylation of the product to induce the ratiometric fluorescence response, was verified using UV–vis and mass spectrometry analysis. The probe was successfully applied for ratiometric fluorescent detection of palladium in tap water, river water, and in fetal bovine serum.     

Development of ratiometric fluorescent probe for zinc ion based on indole fluorophore

A water-soluble ratiometric fluorescent probe ZID-1 has been developed on the basis of an internal charge transfer (ICT) mechanism. Upon complexation with Zn²⁺ under physiological conditions, ZID-1 exhibits a significant blue shift of 77 nm in the emission spectrum. The fluorescent behavior of ZID-1 suggests that the pyridyl group incorporated into the fluorophore coordinates the metal ion as the fourth ligand and affords an appropriate binding affinity (K_d = 17.1 nM) for the intracellular imaging of Zn²⁺.     

Thiazole sulfonamide based ratiometric fluorescent chemosensor with a large spectral shift for zinc sensing

A new thiazole sulfonamide (TTP, 1) based Zn²⁺ selective intrinsic chemosensor has been synthesized and investigated. The chemosensor shows a selective fluorescence enhancement (3.0 fold) with Zn²⁺ over biologically relevant cations (Ca²⁺, Mg²⁺, Na⁺, and K⁺) and biologically non-relevant cations (Cd²⁺) in an aqueous ethanol system. It produces an increase in the quantum yield and a longer emission wavelength shift (64 nm) on Zn²⁺ binding with the potential of a ratiometric assay.     

A mitochondria-targeted fluorescent probe for ratiometric detection of hypochlorite in living cells

A new fl uorescent probe 1 was designed for mitochondrial localization and ratiometric detection of hypochlorite in living cells. It is noteworthy that a high Pearson’s co-localization coeffi cient (Rr) we have obtained was calculated to be 0.97.     

A novel ratiometric fluorescent Fe sensor based on a phenanthroimidazole chromophore

Phenanthroimidazole derivative 1 has been developed as a rare example of ratiometric fluorescent sensors for Fe³⁺. Interestingly, upon treatment with Fe³⁺, the sensor displayed a ratiometric fluorescent response with an enhancement of the ratios of emission intensities at 440 and 500 nm from 0.36 to 3.24. The detection range of the sensor for Fe³⁺ is in the 1.0 × 10⁻⁵-1.5 × 10⁻⁴ M concentration range and the detection limit is 5.26 × 10⁻⁶ M. In addition, the sensor showed good selectivity to Fe³⁺ with the selectivity coefficients (KFe3+=SFe3+/S₀) of Fe³⁺ over other metal ions tested in the range of 5-68.     

A ratiometric fluorescent probe for gasotransmitter hydrogen sulfide based on a coumarin-benzopyrylium platform

A ratiometric fluorescent probe for H₂S was developed based on a coumarin– benzopyrylium platform. The ratiometric sensing is realized by a selective conversion of acyl azide to the corresponding amide, which subsequently undergoes an intramolecular spirocyclization to alter the large π-conjugated system of CB fluorophore. Compared with the traditional azide-based H₂S probes, the proposed probe utilizes the acyl azide as the recognition moiety and exhibits a rapid response (∼1 min) towards H₂S, which is superior to most of the azide-based H₂S probes. Preliminary fluorescence imaging experiments show that probe 1 has potential to track H₂S in living cells.     

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 ratiometric and exclusively selective Cu fluorescent probe based on internal charge transfer (ICT)

A new ratiometric and exclusively selective fluorescent probe N-butyl-4,5-di[N-(phenyl)-2-(amino)-acetamino]-1,8-naphthalimide (1) was designed and synthesized on the basis of the mechanism of internal charge transfer (ICT). The probe 1 showed exclusively selectivity for Cu^II in the presence of a variety of other metal ions in aqueous ethanol solutions and the binding mode of probe 1 with Cu^II was 1:1 metal-ligand complex. Fluorescent emission spectra of probe 1 in the presence of Cu^II showed a 50 nm blue shift, which is from 521 nm to 471 nm. Furthermore, probe 1 shows the same fluorescent change with the Cu^II in living cells.     

A ratiometric fluorescent probe for alkaline phosphatase with high sensitivity

Alkaline phosphatase(ALP)is one of essential biomarkers in mammalian tissue.Here we report a ratiometric probe for ALP,which is rationally designed and synthesized by employing ESIPT fluorophore N-(3-(benzo[d]thiazol-2-yl)-4-hydroxyphenyl)benzamide(BTHPB).The enzymatic dephosphorylation converts the probe to BTHPB,which exhibits a large spectral red-shift(120 nm),allowing extremely high sensitivity of ALP sensing at 0.004 mU/mL.The probe also shows excellent biocompatibility and has been applied for monitoring the endogenic ALP in living cells.     

A boronate-based ratiometric fluorescent probe for fast selective detection of peroxynitrite

Given that peroxynitrite (ONOO^?) is profoundly associated with health and diseases, a new fluorescent probe ABT was designed and synthesized for detection of ONOO^?. ABT manifested not only ratiometric fluorescence signals simultaneously in response to concentrations of ONOO^? (within 10?s), but high selectivity and sensitivity towards ONOO^? over other physiological relevant species (detection limit?=?26.3?nM). Moreover, ABT worked in a broad pH range with biological relevance. Thus, ABT could be used to quantitative detection of ONOO^? concentration and has the potential to efficiently monitor ONOO^? in living organisms.     

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.