Multidentate europium chelates as luminoionophores for anion recognition: impact of ligand design on sensitivity and selectivity, and applicability to enzymatic assays

An overview on the development of different optical sensing platforms for adenosine triphosphate (ATP) recognition

Adenosine triphosphate (ATP), one of the biological anions, plays a crucial role in several biological processes including energy transduction, cellular respiration, enzyme catalysis and signaling. ATP is a bioactive phosphate molecule, recognized as an important extracellular signaling agent. Apart from serving as a universal energy currency for various cellular events, ATP is also considered a factor responsible for numerous physiological activities. It regulates cellular metabolism by breaking phosphoanhydride bonds. Several diseases have been reported widely based on the levels and behavior of ATP. The variation of ATP concentration usually causes a foreseeable impact on mitochondrial physiological function. Mitochondrial dysfunction is responsible for the occurrence of many severe diseases such as angiocardiopathy, malignant tumors and Parkinson's disease. Therefore, there is high demand for developing a sensitive, fast-responsive, nontoxic and versatile detection platform for the detection of ATP. To this end, considerable efforts have been employed by several research groups throughout the world to develop specific and sensitive detection platforms to recognize ATP. Although a repertoire of optical chemosensors (both colorimetric and fluorescent) for ATP has been developed, many of them are not arrayed appropriately. Therefore, in this present review, we focused on the design and sensing strategy of some chemosensors including metal-free, metal-based, sequential sensors, aptamer-based sensors, nanoparticle-based sensors etc. for ATP recognition via diverse binding mechanisms.

Luminescent Lanthanide Probes for Inorganic and Organic Phosphates

Inorganic and organic phosphates-including orthophosphate, nucleotides, and DNA-are some of the most fundamental anions in cellular biology, regulating numerous processes of both medical and environmental significance. The characteristic long lifetimes of emitting lanthanides, including the brighter europium(III) and terbium(III), make them ideally suited for the development of molecular probes for the detection of phosphates directly in complex aqueous media. Moreover, given their high oxophilicity and the exquisite sensitivity of their quantum yields to their hydration number, those luminescent lanthanides are perfect for the detection of phosphates. Herein we discuss the principles that have guided the recent developments of molecular probes selective for inorganic or organic phosphates and how these lanthanide complexes facilitate the study of numerous biological processes.

Lanthanide-Aromatic Iminodiacetate Frameworks with Helical Tubes: Structure, Properties, and Low-Temperature Heat Capacity

A series of lanthanide coordination polymers [LnL(H₂O)₂] _n [Ln = Pr (1), Nd (2), Sm (3), Eu (4), and Gd (5), H₃L = N-(4-carboxy-benzyl)iminodiacetic acid] was hydrothermally prepared and structurally characterized. All the five compounds have been confirmed as 3D Ln-CPs with one-dimensional helical tunnels composed of four helical chains, although there are different coordination geometries around Ln³⁺. Enantiomeric helixes in 1-3, and absolute left-handed and right-handed helical chains in 4 and 5, respectively, lead to different tunnel spaces. Their conformations can also be featured by different space groups and unit cell dimensions. Photoluminescence measurement on 3 and 4 show characteristic emission peaks of Sm³⁺ and Eu³⁺ ions, respectively. The low-temperature heat capacity of 1-4 has been investigated in the temperature range of 1.9-300 K. Their heat capacity values are nearly equal below 10 K and display a crossover with the value order C _p,m(2) > C _p,m(1) ≈ C _p,m(4) > C _p,m(3) above 10 K. The measured heat capacities have been fitted, and the corresponding thermodynamic functions were consequently calculated based on the fitting parameters. The standard molar entropies at 298.15 K have been determined to be (415.71 ± 4.16), (451.32 ± 4.51), (308.53 ± 3.09), and (407.62 ± 4.08) J·mol^-1·K^-1 for 1, 2, 3, and 4, respectively.

Advances in anion binding and sensing using luminescent lanthanide complexes

Luminescent lanthanide complexes have been actively studied as selective anion receptors for the past two decades. Ln(iii) complexes, particularly of europium(iii) and terbium(iii), offer unique photophysical properties that are very valuable for anion sensing in biological media, including long luminescence lifetimes (milliseconds) that enable time-gating methods to eliminate background autofluorescence from biomolecules, and line-like emission spectra that allow ratiometric measurements. By careful design of the organic ligand, stable Ln(iii) complexes can be devised for rapid and reversible anion binding, providing a luminescence response that is fast and sensitive, offering the high spatial resolution required for biological imaging applications. This review focuses on recent progress in the development of Ln(iii) receptors that exhibit sufficiently high anion selectivity to be utilised in biological or environmental sensing applications. We evaluate the mechanisms of anion binding and sensing, and the strategies employed to tune anion affinity and selectivity, through variations in the structure and geometry of the ligand. We highlight examples of luminescent Ln(iii) receptors that have been utilised to detect and quantify specific anions in biological media (e.g. human serum), monitor enzyme reactions in real-time, and visualise target anions with high sensitivity in living cells.

Pyridinedicarboxylate-Tb(III) Complex-Based Luminescent Probes for ATP Monitoring

The pyridinedicarboxylate-Tb(III) complexes, TbPDC and Tb(PDC)₃, as luminescent probes for ATP monitoring have been conveniently prepared and characterized by FT-IR, ¹H-NMR, ESI-MS, UV-Vis, excitation, and emission spectroscopy. Interestingly, these two Tb(III) complexes were quenched by ATP by a similar mechanism via π-π stacking interaction between the chelating ligand and adenine moiety. The ability of luminescent probes applied for the determination of ATP in aqueous solution has been investigated. The dynamic ranges for the quantification of ATP are within 10-90 μM and 10-100 μM with detection limits of 7.62 and 11.20 μM for TbPDC and Tb(PDC)₃, respectively. The results demonstrated that these luminescent probes would be a potential candidate assay for ATP monitoring in hygiene assessment.

A pentadentate member of the picolinate family for Mn(ii) complexation and an amphiphilic derivative

We report a pentadentate ligand containing a 2,2'-azanediyldiacetic acid moiety functionalized with a picolinate group at the nitrogen atom (H3paada), as well as a lipophylic derivative functionalized with a dodecyloxy group at position 4 of the pyridyl ring (H3C12Opaada). The protonation constants of the paada3- ligand and the stability constant of the Mn(ii) complex were determined using a combination of potentiometric and spectrophotometric titrations (25 °C, 0.15 M NaCl). A detailed relaxometric characterisation was accomplished by recording 1H Nuclear Magnetic Relaxation Dispersion (NMRD) profiles and 17O chemical shifts and relaxation rates. These studies provide detailed information on the microscopic parameters that control their efficiency as relaxation agents in vitro. For the sake of completeness and to facilitate comparison, we also characterised the related [Mn(nta)]- complex (nta = nitrilotriacetate). Both the [Mn(paada)]- and [Mn(nta)]- complexes turned out to contain two inner-sphere water molecules in aqueous solution. The exchange rate of these coordinated water molecules was slower in [Mn(paada)]- (k298ex = 90 × 107 s-1) than in [Mn(nta)]- (k298ex = 280 × 107 s-1). The complexes were also characterised using both DFT (TPSSh/def2-TZVP) and ab initio CAS(5,5) calculations. The lipophylic [Mn(C12Opaada)]- complex forms micelles in solution characterised by a critical micellar concentration (cmc) of 0.31 ± 0.01 mM. This complex also forms a rather strong adduct with Bovine Serum Albumin (BSA) with an association constant of 5.5 × 104 M-1 at 25 °C. The enthalpy and entropy changes obtained for the formation of the adduct indicate that the binding event is driven by hydrophobic interactions.

Molecular Receptors for Recognition and Sensing of Nucleotides

Nucleotides are constituents of nucleic acids and they have a variety of functions in cellular metabolism. Synthetic receptors and sensors are required to reveal the role of nucleotides in living organisms and mechanisms of signal transduction events. In recent years, a large number of nucleotide-selective synthetic receptors have been devised, which utilize different molecular designs and sensing mechanisms. This Minireview presents recent progress in the design of synthetic molecular receptors for selective recognition of nucleotides in aqueous solution. The binding properties of receptors and the origins of their selectivity for a particular nucleotide are discussed.

A turn-on luminescent europium probe for cyanide detection in water

A luminescent europium probe that responds to cyanide directly in water with a large nine-fold turn-on of the EuIII centered time-gated luminescence is presented. Unlike other CN- probes reported, the mechanism of action of EuIII-Lys-HOPO does not rely on reaction of CN- with the probe, but on direct coordination of CN- to the EuIII ion concomitant with displacement of three inner-sphere water molecules. This unusual coordination of CN- with a lanthanide ion in aqueous solution was confirmed by luminescence lifetime measurements.

Cationic Europium Complexes for Visualizing Fluctuations in Mitochondrial ATP Levels in Living Cells

The ability to study cellular metabolism and enzymatic processes involving adenosine triphosphate (ATP) is impeded by the lack of imaging probes capable of signalling the concentration and distribution of intracellular ATP rapidly, with high sensitivity. We report here the first example of a luminescent lanthanide complex capable of visualizing changes in the concentration of ATP in the mitochondria of living cells. Four cationic europium(III) complexes [Eu.1-4]+ have been synthesized and their binding capabilities towards nucleoside polyphosphate anions examined in aqueous solution at physiological pH. Complexes [Eu.1]+ and [Eu.3]+ bearing hydrogen bond donor groups in the pendant arms showed excellent discrimination between ATP, ADP and monophosphate species. Complex [Eu.3]+ showed relatively strong binding to ATP (logKa =5.8), providing a rapid, long-lived luminescent signal that enabled its detection in a highly competitive aqueous medium containing biologically relevant concentrations of Mg2+ , ADP, GTP, UTP and human serum albumin. This EuIII complex responds linearly to ATP within the physiological concentration range (1-5 mm), and was used to continuously monitor the apyrase-catalyzed hydrolysis of ATP to ADP in vitro. We demonstrate that [Eu.3]+ can permeate mammalian (NIH-3T3) cells efficiently and localize to the mitochondria selectively, permitting real-time visualization of elevated mitochondrial ATP levels following treatment with a broad spectrum kinase inhibitor, staurosporine, as well as depleted ATP levels upon treatment with potassium cyanide under glucose starvation conditions.

Application of lanthanide luminescence in probing enzyme activity

Enzymes play critical roles in the regulation of cellular function and are implicated in numerous disease conditions. Reliable and practicable assays are required to study enzyme activity, to facilitate the discovery of inhibitors and activators of enzymes related to disease. In recent years, a variety of enzyme assays have been devised that utilise luminescent lanthanide(iii) complexes, taking advantage of their high detection sensitivities, long luminescence lifetimes, and line-like emission spectra that permit ratiometric and time-resolved analyses. In this Feature article, we focus on recent progress in the development of enzyme activity assays based on lanthanide(iii) luminescence, covering a variety of strategies including Ln(iii)-labelled antibodies and proteins, Ln(iii) ion encapsulation within defined peptide sequences, reactivity-based Ln(iii) probes, and discrete Ln(iii) complexes. Emerging approaches for monitoring enzyme activity are discussed, including the use of anion responsive lanthanide(iii) complexes, capable of molecular recognition and luminescence signalling of polyphosphate anions.

A continuous luminescence assay for monitoring kinase activity: signalling the ADP/ATP ratio using a discrete europium complex

We report the application of a stable cationic europium complex [Eu.1]+ in a continuous-read luminescence assay for kinase activity. [Eu.1]+ binds reversibly to ATP and ADP in water, at neutral pH, in the presence of Mg²⁺ ions, providing distinctive luminescence responses that permits the kinase-catalysed conversion of ATP to ADP to be monitored in real-time.

Responsive, Water-Soluble Europium(III) Luminescent Probes

The design principles, mechanism of action and performance of europium(III) complexes that serve as strongly emissive and responsive molecular probes in water are critically discussed. Examples of systems designed to assess pH, selected metal ions and anions, including chiral species, as well as selected small molecules and biopolymers are considered, and prospects evaluated for improved performance in more complex biological media such as in bio-fluids and within living cells. Modulation of the emission spectral form, lifetime and degree of circular polarisation can be used to quantify the spectral response and permit calibration.

Guest dependent structure and acetone sensing properties of a 2D Eu3+ coordination polymer

A new 2D Eu³⁺ coordination polymer 1 has been constructed and investigated with its acetone sensing properties. In addition, the desolvated sample 1a reveals enhanced sensing sensitivity compared with that of 1 owing to its flexible structure.

Macroscopic and Fluorescent Discrimination of Adenosine Triphosphate via Selective Metallo-hydrogel Formation: A Visual, Practical, and Reliable Rehearsal toward Cellular Imaging

With use of simple terpyridine zinc nitrate complexes, intriguing visual recognition of adenosine triphosphate (ATP) via selective coordination assembly leading to two-component metallo-hydrogel formation has been realized. With intensive fluorescent study and density functional theory calculations, it may be inferred, besides the selective metal-ligand interaction between Zn center and phosphate groups, the intramolecular π-stacking between the planar nucleobases of ATP and the metal-hybrid aromatic ring of pincer complex strongly affected the geometry of the coordinated adducts and possible molecular self-assembly process, which constitute a completely new sensing strategy in comparison with the conventional approaches. Furthermore, in light of extreme sensitivity of pincer zinc complexes toward ATP at micromolar scale (1.85 μM) and remarkable fluorescent enhancement (ca. 44-fold) upon ATP addition, the feasibility of the low cytotoxicity pincer zinc complexes in monitoring ATP in HeLa cells has been fulfilled with confocal fluorescence microscopy.

Selective Sensing of Tyrosine Phosphorylation in Peptides Using Terbium(III) Complexes

Phosphorylation of tyrosine residues in proteins, as well as their dephosphorylation, is closely related to various diseases. However, this phosphorylation is usually accompanied by more abundant phosphorylation of serine and threonine residues in the proteins and covers only 0.05% of the total phosphorylation. Accordingly, highly selective detection of phosphorylated tyrosine in proteins is an urgent subject. In this review, recent developments in this field are described. Monomeric and binuclear Tb(III) complexes, which emit notable luminescence only in the presence of phosphotyrosine (pTyr), have been developed. There, the benzene ring of pTyr functions as an antenna and transfers its photoexcitation energy to the Tb(III) ion as the emission center. Even in the coexistence of phosphoserine (pSer) and phosphothreonine (pThr), pTyr can be efficintly detected with high selectivity. Simply by adding these Tb(III) complexes to the solutions, phosphorylation of tyrosine in peptides by protein tyrosine kinases and dephosphorylation by protein tyrosine phosphatases can be successfully visualized in a real-time fashion. Furthermore, the activities of various inhibitors on these enzymes are quantitatively evaluated, indicating a strong potential of the method for efficient screening of eminent inhibitors from a number of candidates.

Luminescent Alkyne-Bearing Terbium(III) Complexes and Their Application to Bioorthogonal Protein Labeling

Two new bifunctional macrocyclic chelate ligands that form luminescent terbium(III) complexes featuring an alkyne group for conjugation to (bio)molecules via the Cu(I)-catalyzed "click" reaction were synthesized. Upon ligation, the complexes exhibit a significant luminescent enhancement when excited at the λ(max) of the "clicked" products. To demonstrate the utility of the complexes for luminescent labeling, they were conjugated in vitro to E. coli aspartate/glutamate-binding protein incorporating a genetically encoded p-azido-L-phenylalanine or p-(azidomethyl)-L-phenylalanine residue. The complexes may prove useful for time-gated assay applications.

A postsynthetically modified MOF hybrid as a ratiometric fluorescent sensor for anion recognition and detection

MIL-125(Ti)-AM-Eu, which is derive from MIL-125(Ti)-NH₂ by covalent postsynthetic modification, is synthesized and exhibit excellent photoluminescence features. It have been designed to be a ratiometric fluorescent sensor (I_L/I_Eu) for sensitively detecting phosphate and oxalate with a proposed dual-readout orthogonal identification pattern.

A FRET based pH probe with a broad working range applicable to referenced ratiometric dual wavelength and luminescence lifetime read out

A luminescent probe for determination of pH was designed based on a Förster resonance energy transfer (FRET) system, combining a europium chelate as the donor and carboxynaphtho-fluorescein as a pH sensitive acceptor. The FRET system enables referenced pH detection in an exceptional broad dynamic range from pH 3 to 9.

A terbium(III)-complex-based on-off fluorescent chemosensor for phosphate anions in aqueous solution and its application in molecular logic gates

A new Tb(III) complex based on a tripodal carboxylate ligand has been synthesized for the selective fluorescent recognition of phosphate anions, including inorganic phosphates and nucleoside phosphates (e.g., ATP), in Tris buffer solution. The resulting L · Tb complex shows the characteristic emission bands centered at about 495 and 550 nm from the Tb(III)-centered (5)D4 excited state to (7)FJ transitions with J = 6 and 5, where the chelating ligand acts only as an "antenna". Upon the addition of phosphate anions to the aqueous solution of Tb(III) complex, significant "on-off" fluorescence changes were observed, which were attributed to the inhibition of the "antenna" effect between the ligand and Tb(III) after the incorporation of phosphate anions. Furthermore, this unique Tb(III) complex has been successfully utilized to detect phosphate anions with filter papers and hydrogels. Notably, the Tb(III) complex also can be used for the construction of molecular logic gates with TRANSFER and INHIBIT logic functions by using the above fluorescence changes.