Fluorescence Turn-on Mechanism Research Articles

Tuning the sensing properties of isophthalic dihydrazide-based receptor via formation of organic–inorganic hybrid and metal complex: An enhanced sensing activity for Hg2+ and CN− detection in semi-aqueous medium

A Schiff base receptor H2L has been synthesized through a condensation reaction between 2-hydroxy-3-methoxybenzaldehyde and isophthalic dihydrazide. The compound has been characterized by various characterization techniques such as 1H, FT-IR, CHNS and single crystal X-ray crystallography. Despite its limited solubility in water, a strategy was employed to enhance its effectiveness in aqueous environment by forming fluorescent nanoaggregates of H2L through a re-precipitation method. The fluorescent nanoaggregates of H2L show strong recognition ability towards Hg2+ ions through a fluorescence turn-on mechanism in H2O–DMSO (90 %, v/v). The study has been extended by developing a hybrid nano-assembly of H2L as Ag@H2L which exhibits higher sensitivity for Hg2+ ions with an improved detection limit 9.3 × 10−9 M as compared to that of organic nanoaggregates having 7.9 × 10−8 M in H2O–DMSO (90 %, v/v). Further Hg2+ complex of H2L has also been synthesized and characterized using elemental analysis, mass spectrometry and IR, UV–Vis spectroscopic techniques. The findings have been complimented by density functional theory (DFT) computations. The synthesized H2L-Hg complex is able to detect CN− ion using UV–Vis spectroscopy in H2O–DMSO (80 %, v/v) with a detection limits of 1.04 × 10−6 M.

TDDFT Study on the ESIPT Properties of 2-(2'-Hydroxyphenyl)-Benzothiazole and Sensing Mechanism of a Derived Fluorescent Probe for Fluoride Ion.

The level of fluoride ions (F-) in the human body is closely related to various pathological and physiological states, and the rapid detection of F- is important for studying physiological processes and the early diagnosis of diseases. In this study, the detailed sensing mechanism of a novel high-efficiency probe (PBT) based on 2-(2'-hydroxyphenyl)-benzothiazole derivatives towards F- has been fully investigated based on density functional theory (DFT) and time-dependent density functional theory (TDDFT) methods. F- attacks the O-P bond of PBT to cleavage the dimethylphosphinothionyl group, and the potential products were evaluated by Gibbs free energy and spectroscopic analyses, which ultimately identified the product as HBT-Enol1 with an intramolecular hydrogen bond. Bond parameters, infrared vibrational spectroscopy and charge analysis indicate that the hydrogen bond is enhanced at the excited state (S1), favoring excited state intramolecular proton transfer (ESIPT). The mild energy barrier further evidences the occurrence of ESIPT. Combined with frontier molecular orbital (FMO) analysis, the fluorescence quenching of PBT was attributed to the photoinduced electron transfer (PET) mechanism and the fluorescence turn-on mechanism of the product was attributed to the ESIPT process of HBT-Enol1.

Visualizing Drug Release from a Stimuli-Responsive Soft Material Based on Amine-Thiol Displacement.

In this research, we developed a photoluminescent platform using amine-coupled fluorophores, generated from a single conjugate acceptor containing bis-vinylogous thioesters. Based on the experimental and computational results, the fluorescence turn-on mechanism was proposed to be charge separated induced energy radiative transition for the amine-coupled fluorophore, while the sulfur-containing precursor was not fluorescent since the energy internal conversion occurred through vibrational 2RS- (R represents alkyl groups) as energy acceptor(s). Further utilizing the conjugate acceptor, we establish a new fluorogenic approach via a highly cross-linked soft material to selectively detect cysteine under neutral aqueous conditions. Turn-on fluorescence emission and macroscopic degradation occurred in the presence of cysteine as the stimuli, which can be visually tracked due to the generation of an optical indicator and the cleavage of linkers within the matrix. Furthermore, a novel drug delivery system was constructed, achieving controlled release of sulfhydryl drug (6-mercaptopurine) which was tracked by photoluminescence and high-performance liquid chromatography. The photoluminescent molecules developed herein are suitable for visualizing polymeric degradation, making them suitable for additional "smart" material applications.

BODIPY-Based Fluorescent Probes for Selective Visualization of Endogenous Hypochlorous Acid in Living Cells via Triazolopyridine Formation.

In this work, the two pyridylhydrazone-tethered BODIPY compounds (2 and 3) were synthesized. These compounds aimed to detect hypochlorous acid (HOCl) species via cyclic triazolopyridine formation. The open forms and the resulting cyclic forms of BODIPYs (2, 3, 4, and 5) were fully characterized by nuclear magnetic resonance, mass spectrometry, infrared spectroscopy, and single-crystal X-ray diffraction. These two probes can selectively detect HOCl through a fluorescence turn-on mechanism with the limit of detections of 0.21 µM and 0.77 µM for compounds 2 and 3, respectively. This fluorescence enhancement phenomenon could be the effect from C = N isomerization inhibition due to HOCl-triggered triazolopyridine formation. In cell imaging experiments, these compounds showed excellent biocompatibility toward RAW 264.7 murine live macrophage cells and greatly visualized endogenous HOCl in living cells stimulated with lipopolysaccharide.

Dipicolylamine-Based Fluorescent Probes and Their Potential for the Quantification of Fe3+ in Aqueous Solutions.

We have synthesized two ligand systems, N(SO2)(R1)dpa (L1) and N(SO2)(R2)dpa (L2), where R1 = biphenyl and R2 = azobenzene, which are sulfonamide derivatives of the NNN-donor chelating dipicolylamine. Both L1 and L2 can be used as sensors for detecting Fe3+ and are highly sensitive and selective over a wide range of common cations. Time-dependent density functional theory (TDDFT) calculations confirmed that the key excitations of L2 and the [Fe(L2)(H2O)3]3+ model complex involve −R2-unit-based π and π* charge transfer. L2 demonstrates a relatively high photostability, a fluorescence turn-on mechanism, and a detection limit of 0.018 μM with 1.00 μM L2 concentration, whereas L1 has a detection limit of 0.67 μM. Thus, both ligands have the potential to be used as fluorosensors for the detection of Fe3+ in aqueous solutions.

MIL-101(Cr)-induced nano-optical sensor for ultra-sensitive detection of enrofloxacin in aquatic products using a fluorescence turn-on mechanism via upconversion nanoparticles

In this study, an ultra-sensitive and selective biosensor for the detection of enrofloxacin (ENR) in complex matrices has been established. The study intelligently converges the spectral properties of core-shell upconverted nanoparticles (UCNPs) and the excellent adsorption satiating aptitudes of metal organic frameworks (MOFs). The principle was based on the fluorescence resonance energy transfer (FRET) between aptamer-modified UCNPs as the donors and MOFs MIL-101(Cr) as the effective receptors. The spectral overlap between UCNPs fluorescence emission and MIL-101(Cr) absorption generated the FRET process under the action of π-π stacking interactions. The presence of ENR induced conformational changes in the aptamer via preferential binding, resulting in a decline in the nucleobase exposure on the surface of MIL-101(Cr) and eventually in the fluorescence recovery. The fluorescence intensity (FL) was enhanced as the ENR concentration was increased. The detection limit was 0.034 ng⋅mL−1 and the linear calibration curve between logarithmic ENR concentration and FL was achieved in the range of 0.1–1000 ng⋅mL−1. The applicability of the platform for ENR recognition was confirmed in real fish and shrimp samples and further validated by a standard HPLC technique with no significant difference (p > 0.05). The current UCNPs-MIL-101(Cr) aptasensor proposes a fast, sensitive and quantitative detection approach for ENR in aquatic products.

Selective detection of Fe (III) via fluorescence turn-on mechanism with Rhodamine tethered poly(vinyl amine) microbeads

Selective detection of Fe (III) via fluorescence turn-on mechanism with Rhodamine tethered poly(vinyl amine) microbeads

Design of fluorescence turn-on sensors with novel response mechanism based on C[dbnd]N isomerization

A series of new type naphthalimide-based fluorescent molecules linking aniline-like groups via a coupling chain with –CN– structure were developed as novel fluorescence chemosensors with high sensitivity and selectivity toward H+ in solution and acidic atmosphere as well as Al3+ in the presence of other cations. The fluorescence turn-on mechanism was studied and proved to be based on the CN isomerization. For these fluorescent molecules with donor-acceptor structure, their luminescent wavelength and photophysical properties can be dominated by adjusting the structure of aniline-like groups and the chain length connecting with naphthalimide. All of these fluorescent molecules emitted strong solid-state fluorescence due to their inherent twisted geometries, and exhibited excellent thermal stability, which is beneficial for their application at high temperature.

Detection of the pesticide dodine using a cucurbit[10]uril-based fluorescent probe

In this contribution, a new fluorescent probe is presented which is capable of the selective recognition and determination of dodine (DD) from amongst 14 different pesticides. The system utilizes a simple guest-replacement mechanism, which allows for fluorescence turn-on. The probe is constructed from cucurbit[10]uril (Q[10]) and protonated acridine (MeAD), and makes use of the strong fluorescence ability of MeAD in aqueous solution versus the lack of fluorescence for the homoternary inclusion complex MeAD2@Q[10]. The addition of pesticide DD to an aqueous solution of MeAD2@Q[10] resulted in MeAD displacement and subsequent recovery of the strong fluorescence. Interestingly, the similar addition of a number of other common pesticides (13 others were tested herein) failed to result in the recovery of the fluorescence. 1H NMR spectroscopic studies were employed to investigate the guest-replacement fluorescence turn-on mechanism.

Detecting Pesticide Dodine by Displacement of Fluorescent Acridine from Cucurbit[10]uril Macrocycle.

According to a simple guest-replacement fluorescence turn-on mechanism, we constructed a fluorescent probe system based on cucurbit[10]uril (Q[10]) and protonated acridine (AD) to detect the pesticide dodine (DD). Formation of a homoternary inclusion complex AD2@Q[10] in both aqueous solution and solid state was studied by means of 1H NMR spectroscopy and X-ray crystallography. Although AD can emit strong fluorescence in aqueous solution, the homoternary inclusion complex AD2@Q[10] does not exhibit any fluorescence. Upon the addition of the pesticide DD into the aqueous solution of AD2@Q[10], the AD molecules in the Q[10] cavity are displaced by the pesticide DD, and strong fluorescence recovers. The fluorescent probe system based on Q[10] and AD provided a wide determination of DD from 0 to 4.0 × 10-5 mol·L-1 with a low limit of detection of 1.827 × 10-6 mol·L-1. The guest-replacement fluorescence turn-on mechanism is also confirmed by 1H NMR spectroscopy. Further, the fluorescent probe can directly detect DD residues in real agricultural products, and obvious fluorescence signal was observed under UV irradiation.

A multiresponsive luminescent probe of antibiotics, pesticides, Fe3+ and ascorbic acid with a Cadmium(II) metal-organic framework

A novel two-dimensional (2D) cadmium-based metal-organic framework [Cd2(HDDB)(bimpy)(NMP)(H2O)]·3H2O (1) (H5DDB = 3,5-di(2′,4′-dicarboxylphenyl)benozoic acid and bimpy = 3,5-bis(1-imidazolyl)pyridine) has been constructed by solvothermal method, which exhibits good water stability, pH stability and excellent photoluminescence properties. Complex 1 reveals a fluorescence turn-off behaviour towards nitrofuran/nitroimidazole antibiotics and DCN pesticide due to PET (photoinduced electron transfer) and FRET (fluorescence resonance energy transfer). Interestingly, complex 1 can be designed as an “on-off-on” fluorescence probe to detect Fe3+ ion and ascorbic acid (AA). The fluorescence of complex 1 can be quenched by Fe3+ ion to form a Fe3+@1 hybrid system, which can be ascribed to the FRET process and the week interaction between Fe3+ and 1. Then, after gradually adding AA into Fe3+@1 system, the fluorescence intensity of Fe3+@1 fluorescence-silent system was recovered (turn on). The possible fluorescence turn-on mechanism is the elimination of FRET process between Fe3+ and complex 1. This work represents the first example of recognition of nitrofuran/nitroimidazole antibiotics, DCN pesticide, Fe3+ ion and AA based on a MOF material in aqueous solution.

Selective Detection of Fe3+, F-, and Cysteine by a Novel Triazole-Linked Decaamine Derivative of Pillar[5]arene and Its Metal Ion Complex in Water.

Appropriately functionalized pillar[n]arenes are elegant supramolecular hosts for ion and molecule sensing. A water-soluble decaamine derivative of pillar[5]arene (APA) bearing triazole and amide moieties is synthesized. The ion and molecular recognition properties of APA are studied by fluorescence, UV–visible, and 1H nuclear magnetic resonance (NMR) spectroscopy. The APA selectively detects Fe3+ among 11 studied ions, which are important in several biological processes. Moreover, the in situ prepared Fe3+ complex of APA (FeAPA) exhibits the highest responsiveness toward F– (∼12-fold) among 11 anions and cysteine (∼120-fold) among the 20 naturally occurring amino acids by a fluorescence turn-on mechanism.

Basic Orange 21: A molecular rotor probe for fluorescence turn-on sensing of amyloid fibrils

Developing label-free fluorescence sensor for amyloid fibrils is a very important and active field of research. Currently, there is a surge in efforts towards developing fluorescence sensor probes for amyloid fibrils which operates via fluorescence turn-on mechanism. Herein, we have identified a new molecular rotor probe, Basic Orange 21 (BO21), which displays a very large fluorescence turn-on emission selectively in response to insulin fibrils in the green region of the visible spectrum. The ground-state absorption, steady–state and time-resolved emission measurements have been extensively employed to characterize the interaction of BO21 with the insulin fibril matrix. The ionic strength dependent results indicate that BO21 binds to the fibril mostly through hydrophobic interactions which has been further corroborated by molecular docking calculations. Moreover, BO21 can also monitor the kinetic progression of the fibrillation process of insulin protein without interfering with the fibrillation process itself. Compared to extensively employed amyloid marker, Thioflavin-T, this new probe, BO21, works with comparatively longer excitation and emission wavelength which is desired for an amyloid marker, and additionally, the sensing performance of BO21 is immune to the ionic strength of the medium which is expected to significantly fuel the prospect of this new probe in various amyloid investigations.

Fluorescence turn-on chemosensors based on surface-functionalized MoS2 quantum dots

The multiple sensing capabilities of molybdenum disulfide quantum dots (MoS2 QDs) towards metal ions were scrutinized by tuning their surface functional groups. The MoS2 QDs surface was individually modified with thiol-containing capping agents to form carboxylic-, amine- and thiol-functionalized MoS2 QDs (MoS2/COOH, MoS2/NH2 and MoS2/SH) by the facile hydrothermal method. Each as-prepared QDs exhibits strong excitation wavelength dependent fluorescence behavior. The design of MoS2 QDs based metal ion sensor was implemented based on the fluorescence turn-on mechanism. These MoS2/COOH, MoS2/NH2 and MoS2/SH QDs sensors exhibit superior performance towards the highly selective detection of Co2+, Cd2+ and Pb2+ ions, respectively, due to the varied association of each functional group towards metal ions. The resultant detection limit of Co2+, Cd2+ and Pb2+ was evaluated to be 54.5, 99.6 and 0.84 nM, respectively, and the related fluorescence turn-on mechanism is verified unambiguously. The binding energies were calculated for QDs with metal ions pairs and the results lent support to the determined sensitivity. The as-prepared QDs were also successfully demonstrated to detect the above metal ions in real water samples. While becoming potential candidates in the chemosensors based on the fluorescence probe, these surface modified MoS2 QDs can offer an excellent sensing capability for specific metal ions with extremely high selectivity.

Glutathione sensing mechanism of a fluorescent probe: Excited state intramolecular proton transfer and photoinduced electron transfer

In this work, the fluorescence turn-on mechanism of glutathione probe azido-substituted 2-(2′-hydroxyphenyl)benzoxazole derivative (AHBO) has been thoroughly studied based on the density functional theory and time-dependent density functional theory methods. The constructed potential energy curves demonstrate that the proton transfer (PT) processes of the probe AHBO and the final product AHBOG after the glutathione-azide reaction are more likely to occur in the first excited state than in the ground state. Results of frontier molecular orbital analyses show that the S1 state of AHBO is a complete charge-separation state, and the non-radiative acceptor-excited photoinduced electron transfer (a-PET, fluorophore as the electron acceptor) from the excited azide group to the 2-(2′-hydroxyphenyl)benzoxazole (HBO) would take place upon photoexcitation, which is responsible for the fluorescence quenching of the probe AHBO. Whereas, without the electron-rich azide group, the product AHBOG undergoes the excited state intramolecular proton transfer (ESIPT) in conjunction with the weak intramolecular charge transfer (ICT) process in the S1 state. The absence of the a-PET and the two processes mentioned above provide explanations for the fluorescent enhancement observed with the product AHBOG.

Monitoring underlying epoxy-coated St-37 corrosion via 8-hydroxyquinoline as a fluorescent indicator

In the present study, successful performance of 8-hydroxyquinoline (8-HQ) as a ferric ion sensitive indicator is described. 8-HQ was used in epoxy coating because of its desirable properties. It doesn’t exhibit premature fluorescence when mixed with coating precursors. Additionally it shows fluorescence turn-on mechanism upon chelate formation with Fe2+/Fe3+ ions produced during anodic reaction. The effect of different concentrations of 8-HQ (0.05, 0.1, 0.5 and 1 wt.%) incorporated in the epoxy coating on corrosion detection as well as optical and electrochemical behavior of the applied coating were studied. The fluorescence property of 8-HQ/Fe3+ solutions was evaluated by using fluorometer. The UV–Visible spectroscopy was used to investigate the effect of 8-HQ presence in the coating on transparency of the free films of the samples. The corrosion detection was performed by fluorescence microscope and the anti-corrosion performance of coated samples containing different concentrations of 8-HQ was studied using salt spray standard test and electrochemical impedance spectroscopy (EIS). The results of UV–Visible spectroscopy demonstrated that increasing 8-HQ concentration causes a slight decrease in coating transparency. According to the results of electrochemical impedance spectroscopy (EIS) measurements, the polarization resistance of the coated St-37 sample containing 0.1 wt.% 8-HQ was about 109 Ohm cm2 after 6 weeks immersion in corrosive electrolyte, while St-37 plates coated with other 8-HQ concentrations showed decreased resistance levels of about 106 Ohm cm2, during the same immersion period. Based on fluorescence microscopic investigation, as a result of incorporating 8-HQ into the epoxy matrix, fluorescence could be observed in regions where Fe2+/Fe3+ ions were produced through anodic reactions. This method is capable of detecting corrosion in situ at early stages before the metal surface suffers serious damages.

Target-activated streptavidin-biotin controlled binding probe.

Target-activated chemical probes are important tools in basic biological research and medical diagnosis for monitoring enzyme activities and reactive small molecules. Based on the fluorescence turn-on mechanism, they can be divided into two classes: dye-based fluorescent probes and caged-luciferin. In this paper, we introduce a new type of chemical probe in which the fluorescence turn-on is based on controlled streptavidin-biotin binding. Compared to conventional probes, the streptavidin-biotin controlled binding probe has several advantages, such as minimal background at its "OFF" state, multiple signal amplification steps, and unlimited selection of the optimal dyes for detection. To expand the scope, a new synthetic method was developed, through which a wider range of analyte recognition groups can be easily introduced to construct the binding probe. This probe design was successfully applied to image and study secreted peroxynitrite (ONOO-) at the cell surface of macrophages where information on ONOO- is difficult to obtain. As the signals are generated upon the binding of streptavidin to the biotin probe, this highly versatile design can not only be used in fluorescence detection but can also be applied in various other detection modes, such as electrochemical and enzyme-amplified luminescence detection.

One-Pot Synthesis of Carbon Nanodots in an Organic Medium with Aggregation-Induced Emission Enhancement (AIEE): A Rationale for “Enzyme-Free” Detection of Cholesterol

Addressing the limitations associated with the detection of cholesterol, we present a one-pot synthesis of a carbon nanodot (CD) in an organic medium (CDorg) from a novel bile acid hydrazone-based organogel. Interestingly, CDorg possesses the aggregation-induced emission enhancement (AIEE) phenomenon, which rationally aids in the “enzyme-free” detection of cholesterol through a fluorescence turn-on mechanism. On dilution of the THF/water mixture of CDorg with its poor solvent (water), a 9.8-fold enhancement in its photoluminescence (PL) emission is witnessed. Such an enhancement in PL emission is credited to the occurrence of molecular restrictions due to the formation of nanoaggregates of CDorg, thereby initiating a radiative pathway for exciton decay. Excitingly on adding cholesterol to CDorg, we observed a similar enhancement in its PL emission without the use of any cholesterol oxidase (ChOx) enzyme. The limit of detection and limit of quantification of cholesterol is found to be as low as 1.09 and 3.64 μM, respectively. Hence, this contribution highlights the enzyme-free fluorescence turn-on detection of cholesterol by a novel CD rationally designed to extend its applicability in an organic medium, where it is still considered a major restraint.

Oligo(p-phenylene ethynylene) Electrolytes: A Novel Molecular Scaffold for Optical Tracking of Amyloids.

Finding new optical probes to detect and track amyloid protein aggregates is key to understanding and defeating the myriad of neurodegenerative and other diseases associated with these misfolded proteins. Herein we report that a series of fluorescent, soluble oligo(p-phenylene ethynylene)s (OPEs) are able to detect amyloids in vitro by massive binding-activated superluminescence, with low micromolar affinity and high selectivity for the amyloid conformer. The OPEs track the kinetics of amyloid fibril formation from native hen egg white lysozyme (HEWL) similarly to thioflavin T (ThT), and the dependence of binding affinity on OPE length supports the theory of a linear binding groove. We hypothesize, based on spectral properties, induced circular dichroism, and previous work in analogous systems, that the fluorescence turn-on mechanism is a combination of the reduction of static solvent-mediated quenching at the ethyl ester end groups of the phenylene ethynylene fluorophore and the formation of chiral J-type aggregates templated on the amyloid fibril surface.

PH-sensitive fluorescent salicylaldehyde derivative for selective imaging of hydrogen sulfide in living cells

The salicylaldehyde derivative, 9-formyl-8-hydroxyjulolidine was developed as a fluorescent probe to detect weak acid ions. With the salicylaldehyde moiety as the chromophore, the probe exhibits pH sensitive detection of hydrogen sulfide and silicate in aqueous solution. It displays an excellent selectivity for hydrogen sulfide and silicate over other weak acid salts. Time-dependent density functional theory calculations confirmed that the fluorescence turn-on mechanism involved blocking intramolecular charge transfer. The probe detects S2− in live cells, providing a powerful method to study H2S chemistry in biological systems.