Development of a Zinc Ion-Selective Luminescent Lanthanide Chemosensor for Biological Applications

Aza-BODIPY-based logic gate chemosensors and their applications

Dimethylaniline-substituted aza-BODIPY dyes (DA, DM, DP) were designed and synthesized aiming for ion detection. The Zn2+ recognition ability was found in all compounds and the binding mechanism was possibly via dimethylaniline sites linked to the aza-BODIPY core. Upon Zn2+ addition, the new absorption band and the color change occurred due to the altered charge transfer of the adducts. The custom-made colorimeter was successfully integrated into the dye's application, demonstrating a good linear relationship between resistance values and Zn2+ concentration. The chromophore test strips were fabricated and exhibited distinct color changes upon aqueous Zn2+ exposure. The compound DA also exhibits logical behavior with DA-Zn2+-Cu2+ system. In terms of environmental hazards, the compounds exhibited no adverse effect on Pseudomonas putida at the concentration level of 0.2 mg/mL. These findings indicated that all synthesized aza-BODIPYs might be suitable for chemosensor probes for Zn2+ detection with possibly low environmental risk.

Pyranine Interaction with Amines in Micelles

The fluorescence behavior of pyranine in anionic micellar system of sodium dodecyl sulphate was studied in the presence of selected amines. The amines included cyclopropylamine (CPA), ethylenediamine (EDA), benzylamine (BA), dibutylamine (DBA), cyclohexylamine (CHA), and polyethylenediamine (PEDA). All the studied amines quenched the intensity of pyranine. Study was performed in 0.05 M and 0.1 M SDS. The thermodynamic parameters were determined in order to understand the quenching of pyranine by the studied amines. Change in Gibbs free energy and quenching was found higher in 0.05 M SDS concentration and was found lower when SDS concentration was increased to 0.1 M SDS. Pyranine quenching by the amines studied were treated with an extended Stern-Volmer equation that produced the Stern-Volmer constant ([Formula: see text]). Binding constant (Kb), number of binding stoichiometry (n) and Gibbs free energy change (ΔGbinding) were found higher for lower surfactant concentration as compare to higher surfactant concentration. More negative (-ve) the Gibbs free energies more will be the quenching, higher will be the sensitivity and vice versa. The Gibbs free energies for all the studied amines were found in the order as cyclopropylamine > ethylenediamine > benzylamine > dibutylamine > cyclohexylamine > polyethylenediamine. Fluorescence quenching of pyranine by amines in aqueous SDS is reproducible and is useful for the determination of amines in environmental samples.

Synthesis, DFT studies on a series of tunable quinoline derivatives

The synthesis, Density Functional Theory (DFT) calculations, and photo physical characteristics of a range of quinoline derivatives have been described in the present work. Initially, the innovative derivatives are synthesized through the cyclization of 2-amino-5-nitrobenzophenone with either acetyl acetone or ethyl acetoacetate, followed by reducing the nitro group to an amine. Subsequently, these compounds undergo an acid-amine cross-coupling reaction. The investigation shows the DFT and photo physical properties of these substances. It is noteworthy that compound 6z exhibits the most remarkable Stokes shift among the fluorophores investigated. Furthermore, the research also provides insights into the electrophilicity index, Electronegativity, chemical potential, chemical hardness and softness properties. These properties are determined by utilizing Density Functional Theory (DFT) calculations and evaluating electron potential efficiency and using computational methods Time-Dependent Density Functional Theory (TD-DFT) to predict absorption spectra in molecules at the B3LYP/6-31G'(d,p) level/basis.

A pyrene-induced PET-based chemosensor for biologically important Zn(II) ions: application in test strips and live cell imaging studies

Cation and anion sensing is vital owing to their universal dispersion in ecosystems and biological functions. It has been shown that fluorescent receptors based on organic platforms are efficient for detecting a number of ions and have many advantages such as low cost, superior sensitivity and simplicity in installation. This study demonstrates the design and synthesis of a novel receptor (E)-3-[(3,5-di-tert-butyl-2-hydroxybenzylidene)amino]-2-(pyren-1-yl)-2,3-dihydroquinazolin-4(1H)-one (DTQ) for the rapid recognition of Zn(II) ions. DTQ exhibited a significant fluorometric "turn-on" characteristic towards Zn(II) at λmax 444 nm in aqueous acetonitrile by inhibiting the photo-induced electron transfer (PET) and -CN- process. The ESI-MS analysis and Job's plot experimental results confirmed stoichiometric 1 : 1 complex formation between DTQ and Zn(II). Fluorometric investigations revealed the detection limit and association constant of DTQ towards Zn(II), which were found to be 13.4 nM and 1.47 × 105 M-1, respectively. DTQ was employed to sense Zn(II) on low-cost test strips. The present research findings imply that DTQ can function as an effective sensor for Zn(II).

Computational insights of excited state intramolecular proton transfer (ESIPT) based fluorescent detection and imaging of γ-glutamytranspeptidase activity

γ-Glutamytranspeptidase (GGT) is an important tumor biomarker that widely appears in the tumor cells. Therefore, accurate imaging and detection of GGT activity in live cells, serum and pathological cells grasp great importance for the diagnosis, management, and treatment of cancer. Herein, 2-(2-hydroxyl-phenyl)-6-chloro-4-(3H)-quinazolinone (HPQ) is considered as the fluorophore probe for the detection of GGT activity, which is known for the typical mechanism of excited-state intramolecular proton transfer (ESIPT). All the simulations adopted to evaluate the sensing mechanism were carried out via DFT and TDDFT calculations at CAM-B3LYP/TZVP level of theory. The emission properties of HPQ and HPQ-TD are thoroughly studied to understand the photoinduced electron transfer (PET) and excited state intramolecular proton transfer (ESIPT) process. The results reveal that the fluorescence quenching of HPQ (enol form) is assigned to the PET process, whereas the large Stokes shift in fluorescence emission of HPQ (keto form) is related with ESIPT mechanism. The obtained results are further cross validated by frontier molecular orbital (FMO) analysis, geometric analysis, and potential energy curve (PEC) scanning. Our calculations provide powerful evidence for the ESIPT based sensing mechanism of HPQ (keto-enol form) for GGT activity.

Comparison of Photophysical Properties of Lanthanide(III) Complexes of DTTA- or DO3A-Appended Aryl-2,2'-Bipyridines

New Tb(III) and Eu(III) complexes based on aryl-2,2'-bipyridine ligands with a cyclic DO3A chelating unit appended in the alpha position of the bipyridine core were synthesized. The photophysical properties of these complexes were compared with those of complexes of ligands with identical aryl-2,2'-bipyridine chromophores, but with an acyclic DTTA residue as an additional chelating site in the alpha position of the bipyridine core. The nature of the polyaminocarboxylic acid fragments was found to have a significant influence on the luminescence. For some of the Eu(III) complexes, upon the transition from acyclic DTTA- to the cyclic DO3A-appended ligands, a noticeable increase in the intensity of Eu(III) luminescence was observed, with an increase in the quantum yield of up to 2.55 times. In contrast, for most of the Tb(III) complexes, a similar transition resulted in a noticeable decrease in the luminescence intensity of the Tb(III) cation.

Theoretical Investigations on the Sensing Mechanism of Phenanthroimidazole Fluorescent Probes for the Detection of Selenocysteine

The level of selenocysteine (Sec) in the human body is closely related to a variety of pathophysiological states, so it is important to study its fluorescence sensing mechanism for designing efficient fluorescent probes. Herein, we used time-dependent density functional theory to investigate the fluorescence sensing mechanism of phenanthroimidazole derivates A4 and B4 for the detection of Sec, which are proposed to be designed based on excited state intramolecular proton transfer (ESIPT) and intramolecular charge transfer (ICT) mechanisms. The calculation results show that the fluorescence quenching mechanism of A4 and B4 is due to the photo-induced electron transfer (PET) process with the sulfonate group acts as the electron acceptor. Subsequently, A4 and B4 react with Sec, the sulfonate group is substituted by hydroxyl groups, PET is turned off, and significant fluorescence enhancement of the formed A3 and B3 is observed. The theoretical results suggest that the fluorescence enhancement mechanism of B3 is not based on ICT mechanism, and the charge transfer phenomenon was not observed by calculating the frontier molecular orbitals, and proved to be a local excitation mode. The reason for the fluorescence enhancement of A3 based on ESIPT is also explained by the calculated potential energy curves.

A quinazolin-based Schiff-base chemosensor for colorimetric detection of Ni2+ and Zn2+ ions and 'turn-on' fluorometric detection of Zn2+ ion

Herein, we have reported a novel quinazolin-based Schiff base chemosensor (E)-2-benzamido-N'-(1-(pyridin-2-yl)ethylidene)benzohydrazide (L). L has been designed, synthesised and characterised by 1H-NMR, IR spectroscopy, ESI-MS spectrometry and theoretical studies. The receptor showed appreciable colorimetric λ max shift for both Ni2+ and Zn2+ ions and fluorometric "turn on" response in presence of only Zn2+ ion. The Jobs plot analysis revealed that receptor forms 2 : 1 complex with both the ions Ni2+ and Zn2+, further confirmed by ESI-MS analysis. The single crystal structure of L-Ni2+ complex (1) has also been determined. The colorimetric detection limits were calculated to 7.9 nM and 7.5 nM respectively for Ni2+ and Zn2+ in methanol-Tris-HCl buffer medium (10 mM, pH 7.2, 1 : 1 v/v). The chemosensor L can be applied for the recovery of contaminated water samples.

Exploring the influence of intermolecular hydrogen bonding on the fluorescence properties of HQCT and HQPH fluorescent chemosensors

Recently, two trace water detection probes, 8-hydroxyquinoline-2-carboxaldehyde thiosemicarbazone(HQCT) and 8-hydroxyquinoline-2-carboxaldehyde (pyridine-2-carbonyl)-hydrazine(HQPH) have been successfully designed in the experiment. The original intramolecular proton transfer can be prevented by the water molecules, leading to fluorescence quenching. In order to investigate the fluorescence quenching mechanism, the effect of water molecules on the excited state proton transfer process will be studied in detail. In this contribution, the six models have been optimized and the related analysis have been carried out. When water molecules are involved in the proton transfer process, the energy barrier decreases significantly and the conversion of the enol structure to the keto structure is accelerated. Moreover, the intermolecular hydrogen bonding, not participating in the proton transfer process, can facilitate the proton transfer process by affecting the distribution of the electrostatic potential within the molecule, which in turn lowers the energy barrier for proton transfer.

An ESIPT-based fluorescent turn-on probe with isothiocyanate for detecting hydrogen sulfide in environmental and biological systems

A probe with an isothiocyanate group was synthesized and evaluated for its H₂S sensing ability. Upon addition of H₂S, the probe exhibited ratiometric properties during absorption with a red-shift. The probe exhibited fluorescent off-on responses towards H₂S via the ESIPT process, due to the conversion of isocyanate into amine. UV-vis, fluorescence, and ¹H NMR spectroscopic analyses were performed to investigate the sensing mechanism. The probe has a large Stokes shift, short response time, and low detection limit. It can be used to estimate H₂S levels within the range of 0-36 nM. The practical applicability of the probe was demonstrated using water samples and living cells.

A Zinc (II) Complex Comprising Aminoethyl-Nitropyridine Derived N,N,O-Donor Schiff Base Ligand Serves as an Efficient ON-OFF Probe for Cu (II)

A new fluorescent zinc (II) complex based probe 1 encompassing a Schiff base (E)-2-methoxy-6-((2-(5-nitropyridin-2-ylamino)ethylimino)methyl)phenol (HL) has been designed, synthesized and used for the highly selective detection of Cu²⁺ . Ligand HL and complex 1 have been characterized by various spectroscopic techniques such as ¹ H, ¹³ C-NMR, FT-IR, HR-MS, UV/vis and fluorescence studies. Ligand HL did not exhibit any considerable change in fluorescence in presence of various cations. Notably, its Zn (II)-complex 1 exhibited highly selective 'Turn-OFF' fluorescence signalling toward Cu²⁺ which remains uninterrupted with competing analytes. Probe 1 interacts with Cu²⁺ in 1:2 (1: Cu²⁺ ) stoichiometry as estimated through Job's plot. Moreover, selectivity of 1 was further confirmed through interaction of 1+ Cu²⁺ complex with some possible interfering metal ions inducing insignificant response. Additionally, association and quenching constant have been determined to be 3.30 × 10⁴ M^-1 and 0.21× 10⁵ M^-1 through Benesi-Hildebrand method and Stern-Volmer plot, respectively.

A Review on Metal Ion Sensors Derived from Chalcone Precursor

Disclosure of new molecular probes as chromogenic and fluorogenic cation sensors is scientifically exigent work. Recently chalcone derivatives gained more attention because of their structural variability. A suitable donor and acceptor groups separated by delocalized π-orbitals display excellent chromogenic and fluorogenic properties because of intramolecular charge transfer (ICT). These designed molecular frameworks provide the coordination sites to the incoming metal ions results in small changes in the optical properties. In a typical sensing behavior, coordination leads to a large conjugation plane with the probe resulted in hypo/hyperchromic shifts or red/blue shifts. In this review, we tried to converge the reported chalcone-derived sensors and explored the design, synthesis, metal ion sensing mechanism, and practical application of the probes. We expect that this review gives a basic outline for researchers to explore the field of chalcone-based sensors further.

Investigating the Influence of Electronic Effects of Functional Groups on the Fluorescence Mechanism of Probes in Water Samples

This study investigates the fluorescence quenching mechanism of formaldehyde detection probe Naph1 and its contrast probe Naph3 in water samples and discussed the effect of the electron-donating group and electron-withdrawing group on fluorescence characteristics based on density functional theory (DFT) and time-dependent density functional theory (TD-DFT). We optimized the structures of the four probes Naph1, Naph1-S, Naph3, and Naph3-S (Scheme 1) and calculated the absorption and emission spectra, which were in good agreement with the experiment. Frontier molecular orbitals (FMOs) were used to analyze the charge arrangement in the excited state. To investigate the intramolecular proton transfer (ESIPT) phenomenon, a potential energy curve was constructed. The amount of fragment charge transfer was analyzed by the IFCT method, and then it was determined whether there was an intramolecular charge transfer (ICT) process. It was found that there was an ICT process in Naph3. The electronic effect of the functional groups did not determine the ICT characteristics and the fluorescence characteristics of the substance. Furthermore, the spin-orbit coupling  (SOC) constant based on the intersystem crossing (ISC) was supplemented, which showed that the fluorescence quenching of Naph1 and Naph3 was caused by the ISC and the corresponding quenching of Naph3-S was caused by charge transfer (CT) in the excited state.

A Cd(ii) and Zn(ii) selective naphthyl based [2]rotaxane acts as an exclusive Zn(ii) sensor upon further functionalization with pyrene

A new multi-functional [2]rotaxane, ROTX, has been synthesized via a Cu(i) catalysed azide-alkyne cycloaddition reaction between Ni(ii) templated azide terminated pseudorotaxane composed of a naphthalene based heteroditopic wheel, NaphMC, and an alkyne terminated stopper. Subsequently, ROTX has been functionalized with pyrene moieties to develop a bifluorophoric [2]rotaxane, PYROTX, having naphthalene and pyrene moieties. Detailed characterization of these two rotaxanes is performed by utilizing several techniques such as ESI-MS, (1D and 2D) NMR, UV/Vis and PL studies. Comparative metal ion sensing studies of NaphMC (a fluorophoric cyclic receptor), ROTX ([2]rotaxane with a naphthyl fluorophore) and PYROTX ([2]rotaxane having naphthyl and pyrene fluorophores) have been performed to determine the effect of dimensionality/functionalization on the metal ion selectivity. Although NaphMC fails to discriminate between metal ions, ROTX serves as a selective sensor for Zn(ii) and Cd(ii). Importantly, PYROTX shows exclusive selectivity towards Zn(ii) over various transition, alkali and alkaline earth metal ions including Cd(ii).

Imaging Tissue Physiology In Vivo by Use of Metal Ion-Responsive MRI Contrast Agents

Paramagnetic metal ion complexes, mostly based on gadolinium (Gd3+), have been used for over 30 years as magnetic resonance imaging (MRI) contrast agents. Gd3+-based contrast agents have a strong influence on T1 relaxation times and are consequently the most commonly used agents in both the clinical and research environments. Zinc is an essential element involved with over 3000 different cellular proteins, and disturbances in tissue levels of zinc have been linked to a wide range of pathologies, including Alzheimer's disease, prostate cancer, and diabetes mellitus. MR contrast agents that respond to the presence of Zn2+ in vivo offer the possibility of imaging changes in Zn2+ levels in real-time with the superior spatial resolution offered by MRI. Such responsive agents, often referred to as smart agents, are typically composed of a paramagnetic metal ion with a ligand encapsulating it and one or more chelating units that selectively bind with the analyte of interest. Translation of these agents into clinical radiology is the next goal. In this review, we discuss Gd3+-based MR contrast agents that respond to a change in local Zn2+ concentration.

Applications of fluorescent materials in the detection of alkaline phosphatase activity

Alkaline phosphatase (ALP) is important in the diagnosis of many diseases. Because ALP is used to detect biomarkers for many diseases, many researchers conduct investigations to develop ALP detection strategies. The use of fluorescent material has attracted attention because of the technique's high sensitivity and the low sample volume required. Herein, we review and discuss the working mechanisms and advantages of four main categories:DNA fluorescent probes, molecular fluorescent probes, chemical coordination-based probes, and nanoparticle probes. Development prospects and trends are also discussed.

Responsive Metal Complex Probes for Time-Gated Luminescence Biosensing and Imaging

The development of reliable bioanalytical probes for selective and sensitive detection of particular analytes in biological systems is essential for better understanding the roles of the analytes in their native contexts. In the last two decades, luminescent metal complexes have greatly contributed to the development of such probes for biosensing and imaging due to their unique spectral and temporal properties, controllable cell membrane permeability, and cytotoxicity. Conjugating an analyte-activatable moiety to the metal complex luminophores allows the production of responsive metal complex probes for this analyte detection. Owing to their long-lifetime emissions, the responsive metal complex probes are accessible to the technique of time-gated luminescence (TGL) detection and imaging. With a delay time after pulsed excitation, the TGL technique allows for collection of only long-lived luminescence from responsive metal complex probes, while filtering out short-lived background autofluorescence, providing a background-free approach for the detection and imaging of the analyte at subcellular and/or molecular levels. Responsive metal complex probes, therefore, have emerged as complementary sensing and imaging tools of organic dye-based fluorescent probes for the in situ detection of analytes in complicated biological environments.In this Account, we describe the advances in the development of metal complex probes and their applications for TGL bioassays with particular focus on our efforts made in this field. We first introduce the photophysical/-chemical properties of luminescent metal complexes, including lanthanide (europium and terbium) and transition metal (ruthenium and iridium) complexes. The luminescence lifetimes (τ) of lanthanide and transition metal complexes are at micro/millisecond (μs/ms) and hundreds/thousands nanosecond (ns) levels, respectively. The emission lifetimes are significantly longer than the autofluorescence lifetime (τ < 10 ns) of biological samples. Such long-lived luminescence of these metal complexes enables our research on demonstrating responsive probes for background-free TGL detection of some reactive biomolecules, such as reactive oxygen/nitrogen species (ROS/RNS) and biothiols.We conclude this Account by outlining the future directions to further develop new generation responsive TGL probes for promoting their practical applications. The responsive TGL probes are expected to be translated for biomedical and/or (pre)clinical investigations of biomolecules in situ. Reversibility, lower toxicity, ability of excitation at longer wavelength, and potential to be translated are key criteria for the development of next-generation probes. We also anticipate that further development of responsive TGL probes will contribute to the bioassay in more challenging biological systems, such as plants that have significant higher background autofluorescence than animals.

A Chemodosimetric Approach for Fluorimetric Detection of Hg2+ Ions by Trinuclear Zn(II)/Cd(II) Schiff Base Complex: First Case of Intermediate Trapping in a Chemodosimetric Approach

The present work discloses the application of two fluorescent zinc and cadmium complexes (1 and 2) for sensing of Hg(II) ions through a chemodosimetric approach. The ligand under consideration in this work is a N₂O donor Schiff base ligand (E)-4-bromo-2-(((2-morpholinoethyl)imino)methyl)phenol (HL), which has been harnessed to generate complexes [Zn₃L₂(OAc)₄] (1) and [Cd₃L₂(OAc)₄] (2). X-ray single crystal diffraction studies unveil the trinuclear skeleton of complexes 1 and 2. Both complexes have been found to be highly fluorescent in nature. However, the quantum efficiency of Zn(II) complex (1) dominates over the Cd(II) analogue (2). The absorption and emission spectroscopic properties of the complexes have been investigated by density functional theory. Complexes 1 and 2 can detect Hg²⁺ ions selectively by fluorescence quenching, and it is noteworthy to mention that the mechanism of sensing is unique as well as interesting. In the presence of Hg²⁺ ions, complexes 1 and 2 are transformed to mononuclear mercuric intermediate complex (3) and finally to a trinuclear mercuric complex (4) by hydrolysis. We have successfully trapped the intermediate complex 3, and we characterized it with the aid of X-ray crystallography. Transformation of complexes 1 and 2 to intermediate complex 3 and finally to 4 has been established by UV-vis spectroscopy, fluorescence spectroscopy, ESI-MS spectroscopy, ¹H NMR spectroscopy, and X-ray crystallography. The spontaneity of the above conversion is well supported by thermodynamic aspects as reflected from density functional theoretical calculations.

Design and synthesis of two new terbium and europium complex-based luminescent probes for the selective detection of zinc ions

Zinc plays a key role in many physiological processes and has implications for the environment. Consequently, detection of chelatable zinc ion (Zn²⁺ ) has attracted widespread interest from the research community. Lanthanide-based luminescent probes offer particular advantages, such as high water solubility, long luminescence lifetimes and a large Stokes' shift, over common organic dye-based fluorescent sensors. Here, we report the synthesis of terbium and europium complex-based probes, Tb-1 and Eu-1, for sensitive and selective detection of Zn²⁺ in water. These probes featured the incorporation of bis(2-pyridylmethyl)]amine (DPA) receptor for Zn²⁺ chelation and the 1,4,7-tris(carboxymethyl)-1,4,7,10-tetraazacyclododecane (DO3A) ring to chelate lanthanide (Ln³⁺ ). Tb-1 and Eu-1 displayed high selectivity for Zn²⁺ ions over a wide range of competing ions, with limits of detection of 0.50 ± 0.1 μM and 1.5 ± 0.01 μM, respectively. Density functional theory simulations were in good agreement with experimental observations, displaying high Zn²⁺ selectivity compared with most competing ions. In the competing ions experiments, the luminescence response of Tb-1 and Eu-1 was moderately quenched by some ions such as Cu²⁺ , this was linked to the comparable binding abilities of these ions for the receptor of the probe.

Quinoline-based fluorescent small molecules for live cell imaging

Small molecule probes are essential tools for biomedical applications, with utility as cellular stains, labels for biomolecules, environmental indicators, and biosensors. However, a fluorophore's characteristics are difficult to predict solely through calculations or rational design, making the development of a core scaffold that is amenable to late stage functionalization particularly desirable. In this chapter, we describe the synthesis and application of a tunable quinoline scaffold that can be readily functionalized and optimized for a variety of imaging applications. We present a facile synthesis that results in three functional domains that influence the compound's photophysical properties, structural diversity, and polarization. We demonstrate a method with which to study the scaffold's tunable photophysical properties as a result of its structure and environment, and finally exhibit its utility in pH sensitive, live-cell imaging.

Harnessing chemical exchange: 19F magnetic resonance OFF/ON zinc sensing with a Tm(iii) complex

A fluorinated, thulium(iii) complex (Tm-PFZ-1) serves as an off-on ¹⁹F magnetic resonance probe for Zn(ii). Rapid exchange among different conformations combined with paramagnetic relaxation and chemical shift effects of Tm(iii) effectively eliminate the ¹⁹F NMR/MRI signal in Tm-PFZ-1. Chelation of Zn(ii) induces increased structural rigidity and reduces exchange rate, affording a robust ¹⁹F NMR/MRI signal. Tm-PFZ-1 represents a first-in-class paramagnetic ¹⁹F MR agent that exploits a novel sensing mechanism for Zn(ii) and is the first ¹⁹F MR-based scaffold to provide an "off-on" response to Zn(ii) in aqueous solution.

Fluorescence Quenching of the Probes L-Tryptophan and Indole by Anions in Aqueous System

A fluorescence quenching study of the fluorescent probes L-tryptophan and indole by anions (NO3-, Cl-, SO42-) was carried out in an aqueous system. The ions NO3-, Cl- and SO42- showed very good quenching of both the probes. Quenching of L-tryptophan by all the anions studied was higher as compare to the quenching of indole. The data was fitted with the Stern-Volmer equation. Stern-Volmer constants were observed in the order NO3- > Cl- > SO42-. Stern-Volmer constants reflect the sensitivity of the method for the studied anions. Limit of detection (LOD) was calculated as three times the standard deviation of the blank for n = 10 (3 × SD) while the limit of quantification (LOQ) was calculated as ten times the standard deviation of the blank for n = 10 (10 × SD). In the case of L-tryptophan LOD and LOQ varied from 4.08 × 10-5 - 4.56 × 10-4 mol L-1, while in the case of indole the values ranged from 3.87 × 10-5 - 6.59 × 10-4 respectively. Fluorescence quenching of L-tryptophan and indole by the studied anions showed good reproducibility and the method could be very effective for the determination of anions.

Europium(III) Macrocyclic Chelates Appended with Tyrosine-based Chromophores and Di-(2-picolyl)amine-based Receptors: Turn-On Luminescent Chemosensors Selective to Zinc(II) Ions

Zinc ions play an important role in many biological processes in the human body. To selectively detect Zn²⁺ , two EuDO3A-based complexes (DO3A=1,4,7,10-tetraazacyclododecane-1,4,7-tricarboxylic acid) appended with tyrosine as a chromophore and di-(2-picolyl)amine (DPA) as the Zn²⁺ recognition moiety were developed as suitable luminescent sensors. Their luminescence intensity is affected by the photoinduced electron transfer mechanism. Upon addition of Zn²⁺ , both probes display an up to sevenfold enhancement in Eu³⁺ emission. Competition experiments demonstrated their specificity toward Zn²⁺ over other metal ions, while also revealing the nonspecificity of the derivatives lacking the DPA-moiety, thus confirming the essential role of the DPA for the recognition of Zn²⁺ . The induced emission changes of Eu³⁺ allow for precise quantitative analysis of Zn²⁺ , establishing these lanthanide-based complexes as viable chemosensors for biological applications.

A fluorescent sensor for intracellular Zn2+ based on cylindrical molecular brushes of poly(2-oxazoline) through ion-induced emission

Poly(2-oxazoline) molecular brushes bearing enaminitrile receptors in the side chain ends exhibit good biocompatibility, excellent fluorescent selectivity for Zn²⁺, and possibility in detecting intracellular Zn²⁺.

Unveiling the Relationship between Energy Transfer and the Triplet Energy Level by Tuning Diarylethene within Europium(III) Complexes

Luminescence performance and photoisomerization control of sensitized energy transfer in a series of Eu(acac)₃De complexes that contain photochromic diarylethene (De) as the ligand are studied by theoretical methods. Both the open-ring and closed-ring isomers exhibit a consistent coordination mode between the Eu^III ion and De. An unneglected weak interaction originating from electrostatic attraction is found in the region of the coordinate bond Eu-N. The open-ring isomer has higher triplet energy levels than ⁵D₁ and ⁵D₀ of the Eu^III ion, which facilitates forward energy transfer from De to the Eu^III ion. The closed-ring isomer, for the extended conjugated system formed in cyclization, has a much lower triplet energy level than ⁵D₀ of the Eu^III ion. The energy-gap deficit makes energy transfer unavailable. By utilization of this phenomenon, regulation of energy transfer and reversible on/off luminescence switching of the europium(III) complex can be achieved. The forward and backward energy-transfer rates in different channels are also calculated for the series of complexes. A statistics diagram is obtained to exhibit the change trend of energy-transfer rates in the forward and backward directions as a function of the triplet energy level, which indicates the contribution of different channels to energy transfer in each level region and figures out that the optimal triplet energy level should be in the range of 21740-19532 cm^-1.

The Marriage of Protein and Lanthanide: Unveiling a Time-Resolved Fluorescence Sensor Array Regulated by pH toward High-Throughput Assay of Metal Ions in Biofluids

A protein/lanthanide complex (BSA/Tb³⁺)-based sensor array in two different pH buffers has been designed for high-throughput recognition and time-resolved fluorescence (TRF) detection of metal ions in biofluids. BSA, which acted as an antenna ligand, can sensitize the fluorescence of Tb³⁺ (i.e., antenna effect), while the presence of metal ions would lead to the corresponding conformational change of BSA for altering the antenna effect accompanied by a substantial TRF performance of Tb³⁺. This principle has also been fully proved by both experimental characterizations and coarse-grained molecular dynamics (CG-MD) studies. By using Tris-HCl buffer with different pHs (at 7.4 and 8.5), 17 metal ions have been well-distinguished by using our proposed BSA/Tb³⁺ sensor array. Moreover, the sensor array has the potential to discriminate different concentrations of the same metal ions and a mixture of metal ions. Remarkably, the detection of metal ions in biofluids can be realized by utilizing the presented sensor array, verifying its practical applications. The platform avoids the synthesis of multiplex sensing receptors, providing a new method for the construction of convenient and feasible lanthanide complex-based TRF sensing arrays.

Full-color emission of a Eu3+-based mesoporous hybrid material modulated by Zn2+ ions: emission color changes for Zn2+ sensing via an ion exchange approach

We reported on the fabrication of a multicolor fluorescent sensor for Zn2+ ions based on the Eu complex functionalized mesoporous hybrid material Eu(TTA-MSN)2L (TTA: 2-thenoyltrifluoroacetone, HL: 4'-(4-carboxy-methyleneoxyphenyl)-2,2':6',2''-terpyridinel). The material was characterized by Fourier transform infrared spectroscopy, X-ray diffraction, transmission electron microscopy, and N2 adsorption measurements. The results revealed that the material preserves mesoscopically ordered structures and exhibited highly uniform pore size distributions. Spectroscopic analyses showed that the mesoporous hybrid material Eu(TTA-MSN)2L exhibited the characteristic emission of Eu3+ ions, reasonably long lifetimes (0.62 ms), and high quantum yields (18.4%). Moreover, the material displayed a highly selective and sensitive function toward Zn2+ ions in aqueous solutions. This sensing function could be attributed to the cation exchange of Eu3+ in the parent material Eu(TTA-MSN)2L with Zn2+. More importantly, by only modulating the concentration of Zn2+ ions in the water solution of Eu(TTA-MSN)2L, a tunable full-color emission (including white-light emission) can be generated under single-wavelength excitation. To our knowledge, this effort is the first example of a multicolor emissive sensor for Zn2+ ions in water solutions based on lanthanide mesoporous hybrid material.

A novel coumarin-based fluorescent sensor for Ca2+ and sequential detection of F- and its live cell imaging

A novel fluorescent sensor CPM for relay detecting Ca²⁺ and F^- based on coumarin has conveniently synthesized and characterized. The sensor CPM showed highly fluorescence enhancement to Ca²⁺ over other metal ions, and the CPM-Ca²⁺ complex could selectively recognize F^- among other anions. The limits of detection for Ca²⁺ and F^- were 5.81 × 10^-7 M and 4.28 × 10^-7 M in aqueous solution (DMF/HEPES buffer 1:1 v/v, 10 mM, pH = 7.2), respectively. Their sensing mode had been testified by Job's plots, UV-vis titration, ¹H NMR titrations, ESI-mass, fluorescence and DFT calculations. The fluorescence imaging indicated that CPM was cell-permeable and could be used to effectively detect Ca²⁺ and F^- within living cells.