Fluorescence Turn Research Articles

Facile Synthesis of Phthalazinone/Isoindolinone Substituted Fluorescence Active Indolo[1,2‐a]quinoxaline Derivatives via ANRORC and Fluorescence Turn Off Sensing of Fe2+

AbstractSynthesis of a series of indolo[1,2‐a]quinoxaline derivatives substituted with phthalazinones/isoindolinones/carbonyl benzoylesters at C‐4 position have been accomplished under open air rt/heating conditions following ANRORC (Addition of Nucleophile, Ring Opening and Ring Closure) mechanism. Initially condensation of 2‐(1‐indolyl)‐aniline and ninhydrin generates a spirocyclic intermediate 5′H‐spiro[indene‐2,6′‐indolo[1,2‐a]quinoxaline]‐1,3‐dione, which upon reaction with various nucleophiles like hydrazine/phenylhydrazine, amines and alcohols affords C‐4 substituted indolo[1,2‐a]quinoxalines in high yield (up to ~88 %) via ANRORC. The photophysical investigation shows that the quinoxaline derivatives possess significant fluorescence property with high quantum yield (QY~11.0–17.0). The N‐phenylphthalazinone substituted indolo[1,2‐a]quinoxaline, a molecule structurally similar to 2,2′‐bipyridine system, can efficiently and selectively detect Fe2+ through fluorescence turn off sensing.

An interesting aggregation induced red shifted emissive and ESIPT active hydroxycoumarin tagged symmetrical azine: Colorimetric and fluorescent turn on–off–on response towards Cu2+ and Cysteine, real sample analysis and logic gate application

We report a newly synthesized 7-diethylamino-4-hydroxycoumarin tagged symmetrical azine derivative (SHC), with an interesting color transformation from yellowish green to orange via aggregation induced red shifted emissive (117 nm) feature in THF-H2O mixture. Interestingly, the single crystal X-ray analysis of this molecule demonstrates that two hydroxycoumarin moieties were present in azine unit, among them one of the coumarin units was exist as enol form and another one transferred to keto form via ground state proton transfer reaction. The optical responses of the compound in different solvents exposed the observation of dual emissive bands which corresponds to the presence of ESIPT phenomenon in SHC molecule. Further, this characteristic was confirmed by absorption, emission, solid state structure and time resolved fluorescence decay measurements. Furthermore, the fluorophore, SHC was exploited as a colorimetric and turn on–off–on fluorescent probe for detection of Cu2+ ions and Cysteine (Cys). The 1:1 binding ratio of the probe with Cu2+ and Cys with SHC-Cu2+, was established via Job plot analysis, mass spectral technique and the DFT calculations. The probe, SHC was employed for the detection of copper ions in the environmental real water samples. Finally, the reversible fluorescent turn on–off–on character of the probe, SHC was established to construct the IMPLICATION logic gate application.

Rapid and ultra-sensitive trace water determination in organic solvents utilizing nitrobenzoxadiazole (NBD)-based fluorescent sensing system

The presence of minute quantities of water in organic solvents can affect the progress of many reactions and cause unnecessary losses and even safety accidents in the chemical industry, especially in the productions process of organic fine chemicals. Therefore, it is necessary to carry out high-performance strategies for trace water detections in commonly used organic solvents. In this work, a fluorescent sensing system based on competitive binding of protons has been developed, demonstrating remarkable responses by UV–vis absorption and fluorescence two-modes toward a trace amount of water in organic solvents including 1,4-dioxane (Diox), tetrahydrofuran (THF), acetonitrile (MeCN), acetone (ACE), dimethylsulfoxide (DMSO) and mixed organic solvents (THF: MeCN=1: 1). The key component of the sensing system is a newly designed fluorophore NBD-PMA, which can be deprotonated to form a dynamic non-luminescent adduct, namely NBD-PMA-F, by an organic fluoride salt tetrabutylammonium fluoride (TBAF). NBD-PMA-F can be reprotonated via using trace water, exhibiting fluorescence turn on of the system. The as-prepared sensing system shows superior sensitivity, low detection limits (v/v, 0.0007 %), quick response speed (≤1.2 s) and good reversibility. Moreover, naked-eye visual rapid detection has also been successfully realized at ambient temperature, which demonstrated their practical applications value for trace water determinations.

Green synthesis of copper nanoclusters using Cissus quadrangularis leaves extract and cysteine as cooperative ligands for the detection of Fe3+ ion and 2,6-dipicolinic acid in biofluids via fluorescence turn off–on mechanism

An off–on fluorescent sensor is devised by using greenish-blue light-emitting Cissusquadrangularis-cysteine-CuNCs (C. quadrangularis-Cys-CuNCs) for the rapid, precise, and recognition ofFe3+ ion and 2,6-dipicolinic acid (DPA). The structure, size, surface functionalities and oxidation state of the elements present in C. quadrangularis-Cys-CuNCs are confirmed by high-resolution transmission electron microscopy (HR-TEM), Fourier transform infrared (FT-IR), X-ray photoelectron spectroscopic and X-Ray diffraction (XRD) techniques. The as-synthesized C. quadrangularis-Cys-CuNCs exhibited λEm/Ex at491/370 nm. The fluorescence intensity ofC. quadrangularis-Cys-CuNCs was reduced selectively to 63 % by Fe3+, however it was restored with the addition of DPA. This method exhibited the limits of detection (LODs) of 0.125 and 0.38 μM for Fe3+ and DPA, respectively. The C. quadrangularis-Cys-CuNCs based fluorescence turn-off–on sensing platform showed remarkable performance for the analyses of Fe3+ ion and DPA in real samples, revealing great potentiality of the method for practical applications.

Synthesis of carbazole-based Schiff base and fluorescence turn on sensor sensitive to Cr3+ ion: Oxidative and electrochemical polymerization and investigation of electrochromic properties

In this study, a Schiff base monomer (ECBA) was initially synthesized from condensation reaction of 3-amino 9-ethyl carbazole (AEC) with 2,4-dihydroxy benzaldehyde (DHBA). Then, its poly(ECBA-1) polymer was synthesized via oxidative polycondensation in methanol/THF (v/v:4/1) medium by NaOCl oxidant. The structures of the obtained monomer and poly(ECBA-1) were confirmed by 1H NMR, 13C NMR and FT-IR. The optical and electrochemical properties of compounds were examined by UV–Vis, fluorescence spectroscopy and cyclic voltammetry (CV) measurements, respectively. The Highest occupied molecular orbital (HOMO), lowest unoccupied molecular orbital (LUMO) energy levels and electrochemical (E’g) and optical band gap (Eg) values were determined for monomer and polymer compounds by cyclic voltammetry and UV–Vis spectrophotometry. Thermal stabilities of the monomer and poly(ECBA-1) were determined from thermogravimetric-differential thermal analysis (TG-DTA) measurements. The molecular weight distribution and Tg temperature of the poly(ECBA-1) were determined by gel permeation chromatography (GPC) and differential scanning calorimetry (DSC) measurements, respectively. The surface morphologies of the synthesized polymers and copolymer were investigated by scanning electron microscopy (SEM). By fluorescence titration, ECBA was found to be selective towards Cr3+ ion among Ag+, K+, Mn2+, Hg2+, Co2+, Ca2+, Ni2+, Cu2+, Zn2+, Pb2+, Cd2+, Fe3+, Cr3+, Cr+6 ions. The selectivity of Cr3+ion was confirmed by the absence of interference from other competitive cationic species. The limit of detection (LOD) and binding constant (Ka) values for the fluorescent probe were calculated as 6.07 x 10-7 M and 1.25 x 104 M−1, respectively. In addition, homopolymer and copolymer synthesis was carried out on Pet ITO selected as working electrode via electropolymerization method by the synthesized ECBA monomer. Homopolymer synthesis was carried out by Cyclic Voltammetry (CV) method and copolymer synthesis was carried out by batch electrolysis method using EDOT with ECBA monomer. Spectroelectrochemical measurements were taken to investigate the electrochromic properties of the prepared homopolymer poly(ECBA-2) and copolymer (Poly(ECBA-co-EDOT)). The copolymer was found to have a more stable structure than the homopolymer.

Fluorescence turn off-on continuous response of dual lanthanide metal organic frameworks for selective detecting fluoroquinolone antibiotics

Eu3+ and Tb3+ as central ions and 1,4-phthalic acid (H2BDC) as ligand, a dual lanthanide metal-organic framework, Eu-Tb(BDC), was synthesized by co-precipitation method. The products performed stable crystal structure and adjustable fluorescence color. Eu0.15Tb2.85 (BDC) 3.0 was selected as an optimized probe to detect norfloxacin (NFX) in urine, and the fluorescence color was found to transfer from bright yellow to dark cyan and then to strong green with increasing NFX concentration. Mechanism analysis revealed that NFX with low concentration (10−6-10−3 M) stimulated light competition with BDC ligand due to internal filtration effect (IFE), resulting in the gradual fluorescence shutdown of Eu3+and Tb3+. While further increasing concentration of NFX to 10−3-10−2 M, the antibiotic molecules coordinated with lanthanide ions and transitioned their role from competitor to assistant. Where the energy transfer due to antenna effect overwhelmed IFE, the exciting light energy were retransferred to Eu3+ or Tb3+, and the probe's fluorescence restored. Generally, the Eu-Tb(BDC) probe performed fluorescence turn off-on response to norfloxacin with continuous concentration changing. Additionally, when detecting other fluoroquinolone antibiotics, the fluorescence of Eu0.15Tb2.85(BDC)3.0 changed from bright yellow to bright blue and eventually to dark red, which was attributed to the different coordination degrees of various antibiotic molecule with Tb3+ and Eu3+. It can be used to distinguish norfloxacin from other fluoroquinolone antibiotics.

Fluorophore-quencher complexes (Cu2+/Al3+) of coumarin Schiff bases as chemosensors for the detection of L-glutamic acid and L-arginine: in vitro and in vivo studies.

This study reports the development of new probes RR1 ((1E)-1-(1-(6-bromo-2-oxo-2H-chromen-3-yl)ethylidene)ethyl thiosemicarbazone) and RR2 ((1E)-1-(1-(6-bromo-2-oxo-2H-chromen-3-yl)ethylidene)phenyl thiosemicarbazone), which selectively showed fluorescence turn 'OFF' response towards Cu2+ and Al3+. Further, complexes of RR1-Cu2+ and RR2-Al3+ acted as chemosensors for the detection of L-amino acids. RR1-Cu2+ selectively detected L-arginine (fluorescence turn 'ON'), and RR2-Al3+ selectively detected L-glutamic acid (fluorescence turn 'ON'). The existence of the fluorophore-quencher complexes RR1-Cu2+ and RR2-Al3+ was confirmed by theoretical studies. Further, the chemosensors RR1-Cu2+ and RR2-Al3+ have three possible structural isomers (RR1-Cu2+-L-arginine - A, B and C) and (R2-Al3+-L-glutamic acid - D, E and F), as confirmed by theoretical studies. In vitro bio-imaging of the probes (RR1 and RR2), complexes (RR1-Cu2+ and RR2-Al3+) and complexes associated with L-arginine (RR1-Cu2+-L-arginine) and L-glutamic acid (R2-Al3+-L-glutamic acid) was performed in the MDA-MB-231 cell line using their IC50 concentrations. In addition, in vivo live cell imaging studies were conducted using C. elegans as the model organism.

Rapid detection of Hg2+ in an ON-OFF-ON process using N doped carbon dots.

Contamination of ground water with pollutants released from various anthropogenic activities is a major concern due to its adverse effects on the environment and human health. Rapid and efficient detection of such pollutants is the first step toward remediation of the problem. Herein we report a two-point fluorescence turn OFF-ON detection method for Hg2+ ions using nitrogen doped carbon dots (NCDs). The NCDs obtained through solvothermal treatment of ammonium citrate tribasic in DMF at 190 °C for four hours exhibited a quantum yield of 9.67%. Hg2+ detection is demonstrated in two steps, first the quenching of the fluorescence of NCDs by Hg2+ and second the fluorescence recovery upon addition of ascorbic acid from different sources. A rapid filter paper-based detection device is demonstrated based on the principles developed.

Fluorescent turn on detection of zinc(II) ion using a hydrazone derived from 4-tert-butyl-2,6-diformylphenol and 3-hydrazineylquinoxalin-2(1H)-one

A polydentate chelating ligand (LH) was synthesized from 4-tertbutyl-2,6-diformylphenol and 3-hydrazineylquinoxalin-2(1H)-one and characterized by mass spectrometry, IR and NMR spectroscopy. LH is weakly fluorescent in EtOH – HEPES buffer (5 mM, pH = 7.4, 8:2, v/v) but exhibited a turn on response with Zn2+ ion having a workable pH range of 6–12 and thus is compatible with biological pH. This sensor is selective for Zn2+ ion, over various metal ions and anions as evident from competition experiments. The probable mechanism of sensing by LH is restriction of ESIPT, PET and C = N isomerization processes and on-set of CHEF process upon coordination to Zn(II) ion. Reversibility was established using EDTA test and applicability in various real samples were evaluated. The calculated limit of detection for Zn2+ ion was 95 nM and Job plot as well as 1H NMR titration suggested a 2:1 binding ratio between Zn2+ ion and LH. From the reaction (in ethanol) of ZnCl2 with LH, in 2:1 ratio, crystalline solids of composition [Zn2(L)Cl3]⋅2H2O 2.5DMF was isolated and its molecular structure was established by single crystal XRD studies. DFT/TDDFT calculation supported the experimental electronic absorption spectrum.

A fluorescence turn “on-off” imaging probe for sequential detection of Al3+ and L-Cysteine in HeLa cells

At this “Aluminum Age”, exposure to aluminum (metallic or ionic form) is inevitable and inestimable. The presence of aluminum in biological systems is evident but more often aluminum toxicity is less understood. Therefore, the presence of biologically reactive aluminum needs to be identified and quantified. Alongside metals, L-cysteine, an essential amino acid, plays a pivotal role in the homeostasis of cellular oxidative and reductive stress. However, excess (<7g) could be lethal and can lead to death. Thus, in-situ selective detection of aluminum and L-cysteine is of larger interest. Here we report a fluorogenic probe (R) for the sequential selective detection and quantification of Al3+ and L-cysteine in a semi-aqueous medium (3:7; water: DMSO). The probe (R) was synthesized by a one-step acid-mediated condensation reaction between pyridine-3,4-diamine and 2-hydroxy-1-napthaldehyde. The synthesized probe was characterized using 1H and 13C NMR, and HR-Mass spectroscopic techniques. The probe (R) is non-emissive in nature, but on recognition of Al3+, the probe R showed “turn-on” emission (bright yellow colour) showing two emission maxima (522 nm and 547 nm), and no naked eye observable color change. Other competing cations do not show any noticeable fluorescence outcome. The R + Al3+ ensemble can specifically detect L-cysteine among all the essential amino acids by showing a fluorescence “turn-off” response. The sensing mechanism of Al3+ is obeying the chelation-enhanced fluorescence (CHEF) effect. The binding constant of R + Al3+ is 0.3 × 104 M−1. The limit of detection (LoD) for Al3+ and L-cysteine are 2.02 × 10-7 M and 0.5 × 10-5 M respectively. The probe (R) can show maximum efficiency within the pH range (7.0–10.0). The probe is found non-toxic (>80 % cell viability with 15 µM concentration) and employed for the in-vitro fluorescence imaging in the HeLa cell.

A triphenylamine-based near-infrared fluorescence turn off probe for nitroreductase imaging

Nitroreductase (NTR) is an essential biomarker for various diseases and widely used to evaluate the hypoxia status in tumor cells. In this work, we developed a near-infrared (NIR) fluorescence “on-off” probe PTP based on a triphenylamine scaffold, with an intrinsic long wavelength emission more than 700 nm. This probe showed typical aggregation-induced emission (AIE) properties with the increase of PBS fraction (fw), which might be attributed to the restriction of intramolecular motions (RIM) effect in high aqueous solution. The frontiers molecular orbitals (FMO) of the probe PTP and the resulting reduction compounds (PT-OH and TPA-OH) were calculated to determine their structure differences. The two nitro groups were reduced in response to NTR and then released the substance TPA-OH through the 1,6-rearrangement-elimination, resulting in simultaneous fluorescence decrease. This process was also demonstrated by the DFT/TD-DFT and docking calculations, indicating a favorable structural and spatial match between the probe and NTR. More importantly, the probe was successfully used to visualize the NTR distribution in living A431 cells with nontoxicity. It exhibited a time-dependent fluorescence quenching behavior at the cellular level, making it of great potential in NTR detection for other biosystems.

Application of surface nitrogen-doped graphene quantum dots in the sensing of ferric ions and glutathione: Spectroscopic investigations and DFT calculations

Developing a sensing platform that can quickly and accurately measure glutathione (GSH) is crucial for the early detection of various human diseases. GQDs have shown great potential in many technological and biological applications. This study focused on synthesizing nitrogen-doped GQDs (NGQDs) with stable blue fluorescence using a simple and easy hydrothermal method in one step. The bamboo fiber was used as the green source for this synthesis. The NGQDs had a tiny particle size of 4.7 nm and emitted light at 405 nm when excited. They displayed a remarkable quantum yield of 40.36 % and were effectively used as fluorescent probe to specifically detect Fe3+. The energy transfer mechanism led to the NGQDs' fluorescence being deactivated by Fe3+ ions (turn- “off”). However, with the addition of GSH to the system, the fluorescence intensity of NGQDs was reactivated (turn- “on”). Thus, a fluorescence turn “off–on” system was developed for the sensitive detection of Fe3+ and GSH. Using density functional theory (DFT), it was theoretically calculated that the surface of the fabricated NGQDs possess lone pairs of electrons on oxygens and doped nitrogen causing a photo-induced electron transfer (PET) process to occur. This PET process was suppressed previously owing to complex formation between oxygen atoms of modeled structure and ferric ions. The sensing platform displayed a sensitive response to Fe3+ in the 1–1000 μM range with LOD of 34 nM and GSH in the range of 1–50 μM, with a detection limit of 45 nM. Furthermore, the NGQDs exhibited high selectivity towards Fe3+ and GSH over other electrolytes and biomolecules. Additionally, the probe exhibited non-cytotoxicity and was practically applicable for the detection of GSH in HeLa cells.

A novel fluorescent turn on probe derived from Schiff base for highly selective and sensitive detection of Cu2+ ion

A fluorescent probe (L) of bis Schiff base secondary amine ligand was designed and synthesized from 2,6-pyridinedicarboxylic acid, ethylenediamine and salicylaldehyde by condensation and reduction reaction. Its structure was characterized by IR, UV–Vis, LIF, 1HNMR, 13C NMR, MS and elementary analysis. Investigation of binding test indicated that probe L could sensitively and selectively detect Cu2+ ion with striking fluorescent signaling responses in anhydrous ethanol solvent. Results from Job’s plot, fluorescent titration, and MS experiments indicated a 1:1 binding ratio between probe L and Cu2+, with a complexation constant of 1.28 × 108 M−1. According to MS, IR and molar conductivity analysis, the mechanism of the probes' detection of Cu2+ may due to CHEF mechanism by the lone electron pairs in the N atom, and the enhancement of the molecular rigidity caused by disruption of intermolecular hydrogen bonding force, which leading to the occurrence of chelation fluorescence enhancement. The detection limit of complexation constant was 2.69 × 10−8 M in the linear range of 0–40 μM, which provided an effective and convenient testing for trace copper in surface water and drinking water.

Active molecule-based theranostic agents for tumor vasculature normalization and antitumor efficacy

Tumor vascular normalization has emerged as a promising strategy for synergistic therapy recently. Based on the strategy of “fluorescence turn on-controllable release”, a novel bifunctional candidate was constructed based on previous developed vascular normalization inducer QDAU5, which could self-assemble to form functional enzyme infrared QDAU5 nanoparticles (FEIRQ NPs). Subsequently, biological evaluation demonstrated that the FEIRQ NPs could induce ferroptosis, endoplasmic reticulum stress, and antigen preconditioning and maturation of dendritic cells and CD8+ T cells, leading to excellent antitumor efficacy in the absence of cytotoxic drugs. Additionally, FEIRQ NPs show high fluorescence intensity upon exposure to the β-galactosidase (β-Gal) enzyme expressed in ovarian cancer, enabling real-time monitoring of therapeutic effects. Overall, our findings suggest a prospering strategy to early diagnosis and efficient therapy for ovarian cancer without cytotoxicity.

Anthraquinone Based Sensors for the Selective Detection of Cyanide Ion with Turn on Fluorescence with Logic Gate & Real Sample Applications.

In recent years, there is an increasing interest in finding better and more efficient ways to detect CN- ions. Most of the anthraquinone-based probes show less fluorescenceThis paper presents the design and synthesis of a new anthraquinone based imine probe with good colorimetric sensing property and fluorescent turn on behavior towardCN- ion. Herein, we report a receptor with both colorimetric and fluorescent enhancement of cyanide ion in DMSO medium is synthesized. The synthesized receptor shows an immediate color change from orange to pink when cyanide is added; and it can be readily observed visually due to the presence of diverse p-conjugated systems in the receptor. These studies wereconfirmed by UV-Visible, PL studies, DFT, HRMS and 1H NMR titration. Moreover, this receptor shows 1:1 stoichiometry and micromolar detection limit. Further the receptor wasapplied to areal sample in finger millet (Eleusine Coracana) to detect the presence of cyanide ion. Moreover, the receptor is applicable towardINHIBITION, IMPLICATION logic gates with two input systems.

A new aggregation-induced emission-based fluorescent probe for effective detection of Hg2+ in water, tea and seafood and its cell imaging

Mercury ion (Hg2+) is highly toxic to the environment and organisms, therefore, monitoring and quantitative detection of Hg2+ is of great significance. In this work, a new fluorescent probe TPEH that integrated tetraphenylethene (TPE) and 2-(2′-hydroxyphenyl)benzothiazole was designed and synthesised. In EtOH/HEPES (2:8, v/v, pH = 7.2) solution, TPEH displays fluorescence turn on detection of Hg2+ with the advantages of swift response (<100 s), high selectivity and strong disturbance-resistant, a large Stokes shift (220 nm), and low limit of detection (81.7 nM). The sensing mechanism was proved to be Hg2+-triggered releasing of the aggregation-induced emission (AIE)-active precursor compound 2. TPEH has been successfully applied to detect Hg2+ in seafood and tea, and image Hg2+ in living cells. In addition, TPEH can be used to detect the content of Hg2+ in actual water samples with the aid of smartphone.

Sensitive Fluorescent Determination of Silver Ion and Glutathione in Human Serum Using Polydopamine Nanodots as the Probe

Glutathione (GSH)-switched fluorescent assays have received attention due to their rapid signal changes of fluorescent nanoprobes. Small polydopamine nanodots (PDs) exhibit remarkable photoluminescent properties. In this work, fluorescent PDs were synthesized through oxidation using dopamine as precursor. The prepared PDs had rich surface functionalities and exhibited good optical properties with maximum emission using 330 nm excitation. The fluorescence was quenched by Ag+ through the interaction with catechols in the PDs. After the addition of GSH, Ag was bound with sulfhydryl groups in GSH and the fluorescence of the PDs was restored. Therefore, a fluorescence turn on-off-on strategy was constructed for Ag+ and GSH using the synthesized PDs as the probe. The PDs showed good performance for Ag+ from 0.01 to 1 μM, with a limit of detection (LOD) equal to 2.82 nM. The system also exhibited a linear relationship for GSH from 0.01 to 0.5 μM with a LOD equal to 1.93 nM. GSH was determined in human serum to demonstrate a practical application of the assay. This work expands the application of PDs in biological analysis.

Fluorescence turn off-on detection of Co2+ and enrofloxacin by N, Cl-carbon dots prepared in deep eutectic solvent and study of their photocatalytic antibacterial activity

Nitrogen, chloride doped carbon dots (N, Cl-CDs) were synthesized in deep eutectic solvent. TEM, XRD, FT-IR, XPS, EDAX, UV–Vis and fluorescence were used for characterization. Quantum yield and average size of N, Cl-CDs were 38.75 % and 2–3 nm, respectively. N, Cl-CDs fluorescence was turn off by cobalt ion and then turn on gradually after addition of enrofloxacin. Linear dynamic range and detection limit (DL) for Co2+ and enrofloxacin were (0.1–70 µM and 30 nM) and (0.05–50 µM and 25 nM), respectively. Enrofloxacin was detected in blood serum and water samples with the recovery of 96–103 %. Finally, Antibacterial activity of the carbon dots was also investigated.

Solid-state fluorescent pH sensors based on pyrimidine/purine-cored covalent organic framework

Fluorescent covalent organic frameworks have been increasingly used as chemical sensors due to their inherent outstanding nature. Here we report two cases of β-ketoenamine typed fluorescent COFs, pyrimidine-cored covalent organic framework (TpApy-COF) and purine-cored covalent organic framework (TpApu-COF). The emission intensities of TpApy-COF and TpApu-COF in aqueous dispersion descend gradually with the ascending of pH value in the range of from 1 to 13. The reversible fluorescence transformation along with the pH change indicates the favorable reusabilities of TpApy-COF and TpApu-COF. The successful assembly of fluorescent liquid glue and fluorescent crayon, as well as their fluorescent turn on and turn off sensing of HCl and NH3, suggest positive practicability and processability. The formation and disintegration of extra intramolecular H-bonding during protonation and deprotonation play crucial roles in the reversible fluorescence switching.

Tuning Energy Transfer in Metal‐Organic Frameworks for Fluorescence Turn‐on Sensing of Hg(II) Ions†

Comprehensive SummaryTuning Förster resonance energy transfer (FRET) in metal‐organic frameworks for fluorescence turn on sensing is rarely reported. Herein, we designed and synthesized several FRET‐based metal‐organic frameworks (MOFs) with different donor/acceptor ratios. Through the oxidation of the acceptor, the FRET process in MOFs can be efficiently modulated so as to turn‐on the fluorescence of the framework in a controlled manner. Interestingly, the MOF with an optimized donor/acceptor ratio of 7.0 was used as an efficient fluorescence turn on sensor for Hg(II) ions with good sensitivity and selectivity. We believe our present study will not only give a useful method to construct fluorescence turn‐on sensors but also trigger the construction of novel FRET‐based MOFs for the fluorescence turn‐on sensing of target analytes in the future.