A switch-on luminescent europium(iii) probe for selective and time-resolved detection of adenosine diphosphate (ADP)

Adenosine diphosphate (ADP) is a key product of two essential classes of biological reactions, catalysed by ATPases and kinases. This makes ADP a highly appealing target for supramolecular detection. However, doing so selectively is exceedingly difficult due to ADP's lower overall charge and similar structure to ATP and the need for compatibility with biological media. Overcoming this challenge, here we present a water-soluble, ADP-selective, luminescent europium(iii) probe suitable for use in vitro and in cellular microscopy. This negatively charged Eu(iii) complex binds ADP reversibly and responds by switching on its luminescence, whilst showing minimal interference from ATP, pyrophosphate and a wide range of biological anions. The probe is equipped with two π-conjugated quinolyl-phenoxyacetate antennae, facilitating excitation at 355 nm in fluorescence microscopy. The ancillary carboxylate groups ensure high water solubility and suppress non-specific binding to albumin protein. Our novel probe demonstrates a level of sensing selectivity for ADP that is unrivaled, producing a linear emission response across the physiologically relevant concentration range (10-400 μM), even in the presence of excess millimolar ATP. We demonstrate that this amphiphilic Eu(iii) probe permeates mammalian cells and localises within the mitochondria and lysosomes. The low background emission of the probe combined with its excellent ADP selectivity and long-lived luminescence makes it a promising tool for visualising ADP levels in living cells.

Luminescent Metal-Organic Framework with Negative Electrostatic Pores for Highly Selective GDP Sensing

Electrostatic potential (ESP) plays an essential role in studying interactions among molecules. Developing probe materials capable of selectively detecting analytes by aligning their molecular ESP with the electrostatic interaction of the host probe material is critically important for identifying analogous analytes; however, relevant research is extremely lacking. In this work, we synthesized a luminescent metal-organic framework (LMOF, Cd-DBDP) featuring negative electrostatic pore environments achieved by incorporating numerous electronegative oxygen atoms and N-containing aromatic rings from organic linkers. The molecular ESP distributions of Cd-DBDP and RNA-related nucleotides were calculated and employed to predict the sensing results. Fluorescence tests demonstrated that Cd-DBDP represents the first example of an MOF-based sensor for guanosine diphosphate (GDP) sensing, and the experimental observations were highly consistent with the theoretical prediction. The sensing mechanism for GDP was thoroughly studied through Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy/energy-dispersive spectroscopy (SEM/EDS), X-ray photoelectron spectroscopy (XPS), and theoretical calculations. These findings provide valuable insights into understanding the interplay between the molecular ESP distribution condition and the sensing results. This study offers a theoretical guide for future sensory research and provides effective means for the design and synthesis of highly efficient sensing MOFs, lending a solid groundwork for further exploration in this field.

Luminescent terbium(III) probes containing an aromatic amino acid-based antenna for discrimination between adenine and guanine nucleotides

Herein we present a series of luminescent Tb(III)-probes ([Tb-Ltrp], [Tb-Ltyr], and [Tb-Lphe]) for sensing and discriminating purine nucleoside polyphosphates (NPP) based on a modified DTTA chelator appended to aromatic amino acids (Laa). The optically most effective luminescent [Tb-Ltrp] probe preferentially discriminates the guanine-NPPs over the adenine-NPPs via PeT-based modulation of Tb(III) luminescence within the biological concentration range.

Transition Metal Complexes with Appended Benzimidazole Groups for Sensing Dihydrogenphosphate

Four new complexes [Ru(bpy)2(bbib)](PF6)2, [Ru(phen)2(bbib)](PF6)2, [Re(CO)3(bbib)(py)](PF6) and [Ir(ppy)2(bbib)](PF6) [where bbib=4,4'-bis(benzimidazol-2-yl)-2,2'-bipyridine] have been prepared and their photophysical properties determined. Their behaviour has been studied with a variety of anions in acetonitrile, DMSO and 10 % aquated DMSO. Acetate and dihydrogenphosphate demonstrate a redshift in the bbib ligand associated absorptions suggesting that the ligand is strongly interacting with these anions. The 3MLCT emissive state is sensitive to the introduction of small quantities of anion (sub-stoichiometric quantities) and significant quenching is typically observed with acetate, although this is less pronounced in the presence of water. The emissive behaviour with dihydrogenphosphate is variable, showing systematic changes as anion concentration increases with several distinct interactions evident. 1H- and 31P-NMR titrations in a 10 % D2O-DMSO-D6 mixture suggest that with dihydrogenphosphate, the imidazole group is able to act as both a proton acceptor and donor. It appears that all four complexes can form a {[complex]2-H2PO4} "dimer", a one-to-one species (which the X-ray crystallography study suggests is dimeric in the solid-state), and a complex with a combined bis(dihydrogenphosphate) complex anion. The speciation relies on complex equilibria dependent on several factors including the complex charge, the hydrophobicity of the associated ligands, and the solvent.

Probing Phoretic Transport of Oxidative Enzyme-Bound Zn(II)-Metallomicelle in Adenosine Triphosphate Gradient via a Spatially Relocated Biocatalytic Zone

Although cellular transport machinery is mostly ATP-driven and ATPase-dependent, there has been a recent surge in understanding colloidal transport processes relying on a nonspecific physical interaction with biologically significant small molecules. Herein, we probe the phoretic behavior of a biocolloid [composed of a Zn(II)-coordinated metallomicelle and enzymes horseradish peroxidase (HRP) and glucose oxidase (GOx)] when exposed to a concentration gradient of ATP under microfluidic conditions. Simultaneously, we demonstrate that an ATP-independent oxidative biocatalytic product formation zone can be modulated in the presence of a (glucose + ATP) gradient. We report that both directionality and extent of transport can be tuned by changing the concentration of the ATP gradient. This diffusiophoretic mobility of a submicrometer biocolloidal object for the spatial transposition of a biocatalytic zone signifies the ATP-mediated functional transportation without the involvement of ATPase. Additionally, the ability to analyze colloidal transport in microfluidic channels using an enzymatic fluorescent product-forming reaction could be a new nanobiotechnological tool for understanding transport and spatial catalytic patterning processes. We believe that this result will inspire further studies for the realization of elusive biological transport processes and target-specific delivery vehicles, considering the omnipresence of the ATP-gradient across the cell.

Macrocyclic Conformational Switch Coupled with Pyridinium-Induced PET for Fluorescence Detection of Adenosine Triphosphate

The binding of nucleotides is crucial for signal transduction as it induces conformational protein changes, leading to downstream cellular responses. Synthetic receptors that bind nucleotides and transduce the binding event into global conformational rearrangements are highly challenging to design, especially those that operate in an aqueous solution. Much work is focused on evaluating functionalized dyes to detect nucleotides, whereas coupling of a nucleotide-induced conformational switching to a sensing event has not been reported to date. We disclose synthetic receptors that undergo a global conformational rearrangement upon nucleotide binding. Integrating naphthalimide and the pyridinium ion into the structure enables stabilization of the folded conformation and efficient fluorescence quenching. The binding of a nucleotide rearranges the receptor conformation and alters the strong fluorescence enhancement. The methylpyridinium-containing receptor demonstrated high sensing selectivity for adenosine 5'-triphosphate (ATP) and a record 160-fold fluorescence enhancement. It can detect fluctuations of ATP in HeLa cells and possesses low cytotoxicity. The developed systems present an attractive approach for designing ATP-responsive artificial molecular switches that operate in water and integrate a strong fluorescence response.

Zn(II)-Dipicolylamine-Attached Amphiphilic Polythiophene for Quantitative Pattern Recognition of Oxyanions in Mixtures

Herein, we propose a novel amphiphilic polythiophene-based chemosensor functionalized with a Zn(II)-dipicolylamine side chain (1poly  ⋅ Zn) for the pattern recognition of oxyanions. Optical changes in amphiphilic 1poly  ⋅ Zn can be induced by the formation of a random coil from a backbone-planarized structure upon the addition of target oxyanions, which results in blueshifts in the UV-vis absorption spectra and turn-on-type fluorescence responses. Dynamic behavior in a polythiophene wire and/or among wires could be a driving force for obtaining visible color changes, while the molecular wire effect is dominant in obtaining fluorescence sensor responses. Notably, the magnitude of optical changes in 1poly  ⋅ Zn has depended on differences in properties of oxyanions, such as their binding affinity, hydrophilicity, and molecular geometry. Thus, various colorimetric and fluorescence response patterns of 1poly  ⋅ Zn to oxyanions were obtained, albeit using a single chemosensor. A constructed information-rich dataset was applied to pattern recognition for the simultaneous group categorization of phosphate and carboxylate groups and the prediction of similar structural oxyanions at a different order of concentrations in their mixture solutions.

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.

Converting pH probes into "turn-on" fluorescent receptors for anions

Recognition of anions by synthetic receptors is an integral part of supramolecular chemistry continuing to expand and find new application areas in our daily life. Many applications require visualization of anion recognition events, and the generated analytical signal is used to quantify anions in solution. Transferring a binding event to a measured signal is a challenging task. The design of a synthetic receptor must involve not only the perfectly positioned binding sites with complementary noncovalent interactions for a guest but should also realize the sensing mechanism that generates a strong analytical response upon guest binding. This feature article outlines the design concept for the construction of "turn-on" fluorescent receptors for anions involving fluorescent pH probes. Applications of this concept for the construction of synthetic fluorescent receptors for inorganic anions and nucleotides are described. Features of the obtained receptors and possible competing binding and sensing processes in solution are analyzed to understand the scope and limitations of the approach.

Colorimetric detection of ATP by inhibiting the Peroxidase-like activity of carbon dots

Adenosine triphosphate (ATP) is the main energy currency for cells and an important biomolecule involved in cellular reactions, whose abnormal levels are closely related to physical disease, thus it is extremely important to establish a convenient, fast and simple ATP monitoring method. Toward this end, we developed a facile method for colorimetric detection of ATP on the basis of the inhibiting effect of ATP on the peroxidase-like activity of carbon dots (CDs). The detection principle of this method was utilizing the peroxidase-like activity of CDs, which catalyze the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) by H₂O₂ to generate blue products. However, the introduction of ATP in the system can inhibit the generation of blue products, so ATP can be colorimetric detected. This method exhibited high sensitivity with a detection limit of 34 nM and a wide linear range (0.050-2.0 μM). The as-proposed colorimetric ATP sensor was capable of detecting ATP in real samples accurately.

Molecular Probes, Chemosensors, and Nanosensors for Optical Detection of Biorelevant Molecules and Ions in Aqueous Media and Biofluids

Synthetic molecular probes, chemosensors, and nanosensors used in combination with innovative assay protocols hold great potential for the development of robust, low-cost, and fast-responding sensors that are applicable in biofluids (urine, blood, and saliva). Particularly, the development of sensors for metabolites, neurotransmitters, drugs, and inorganic ions is highly desirable due to a lack of suitable biosensors. In addition, the monitoring and analysis of metabolic and signaling networks in cells and organisms by optical probes and chemosensors is becoming increasingly important in molecular biology and medicine. Thus, new perspectives for personalized diagnostics, theranostics, and biochemical/medical research will be unlocked when standing limitations of artificial binders and receptors are overcome. In this review, we survey synthetic sensing systems that have promising (future) application potential for the detection of small molecules, cations, and anions in aqueous media and biofluids. Special attention was given to sensing systems that provide a readily measurable optical signal through dynamic covalent chemistry, supramolecular host-guest interactions, or nanoparticles featuring plasmonic effects. This review shall also enable the reader to evaluate the current performance of molecular probes, chemosensors, and nanosensors in terms of sensitivity and selectivity with respect to practical requirement, and thereby inspiring new ideas for the development of further advanced systems.

Multi-Oxyanion Detection by an Organic Field-Effect Transistor with Pattern Recognition Techniques and Its Application to Quantitative Phosphate Sensing in Human Blood Serum

We herein report an organic field-effect transistor (OFET) based chemical sensor for multi-oxyanion detection with pattern recognition techniques. The oxyanions ubiquitously play versatile roles in biological systems, and accessing the chemical information they provide would potentially facilitate fundamental research in diagnosis and pharmacology. In this regard, phosphates in human blood serum would be a promising indicator for early case detection of significant diseases. Thus, the development of an easy-to-use chemical sensor for qualitative and quantitative detection of oxyanions is required in real-world scenarios. To this end, an extended-gate-type OFET has been functionalized with a metal complex consisting of 2,2'-dipicolylamine and a copper(II) ion (CuII-dpa), allowing a compact chemical sensor for oxyanion detection. The OFET combined with a uniform CuII-dpa-based self-assembled monolayer (SAM) on the extended-gate gold electrode shows a cross-reactive response, which suggests a discriminatory power for pattern recognition. Indeed, the qualitative detection of 13 oxyanions (i.e., hydrogen monophosphate, pyrophosphate, adenosine monophosphate, adenosine diphosphate, adenosine triphosphate, terephthalate, phthalate, isophthalate, malonate, oxalate, lactate, benzoate, and acetate) has been demonstrated by only using a single OFET-based sensor with linear discriminant analysis, which has shown 100% correct classification. The OFET has been further applied to the quantification of hydrogen monophosphate in human blood serum using a support vector machine (SVM). The multiple predictions of hydrogen monophosphate at 49 and 89 μM have been successfully realized with low errors, which indicates that the OFET-based sensor with pattern recognition techniques would be a practical sensing platform for medical assays. We believe that a combination of the OFET functionalized with the SAM-based recognition scaffold and powerful pattern recognition methods can achieve multi-analyte detection from just a single sensor.

Sterically demanding macrocyclic Eu(iii) complexes for selective recognition of phosphate and real-time monitoring of enzymatically generated adenosine monophosphate

The design of molecular receptors that bind and sense anions in biologically relevant aqueous solutions is a key challenge in supramolecular chemistry. The recognition of inorganic phosphate is particularly challenging because of its high hydration energy and pH dependent speciation. Adenosine monophosphate (AMP) represents a valuable but elusive target for supramolecular detection because of its structural similarity to the more negatively charged anions, ATP and ADP. We report two new macrocyclic Eu(iii) receptors capable of selectively sensing inorganic phosphate and AMP in water. The receptors contain a sterically demanding 8-(benzyloxy)quinoline pendant arm that coordinates to the metal centre, creating a binding pocket suitable for phosphate and AMP, whilst excluding potentially interfering chelating anions, in particular ATP, bicarbonate and lactate. The sensing selectivity of our Eu(iii) receptors follows the order AMP > ADP > ATP, which represents a reversal of the order of selectivity observed for most reported nucleoside phosphate receptors. We have exploited the unique host–guest induced changes in emission intensity and lifetime for the detection of inorganic phosphate in human serum samples, and for monitoring the enzymatic production of AMP in real-time.

We present two new europium-based anion receptors that selectively bind to inorganic phosphate and AMP in aqueous media. Their sensing selectivity follows the order AMP > ADP > ATP, representing a reversal of the selectivity order observed for most nucleoside phosphate receptors.

Mono- and dinuclear zinc complexes bearing identical bis(thiosemicarbazone) ligand that exhibit alkaline phosphatase-like catalytic reactivity

Mono- and dinuclear zinc(II) complexes bearing bis(thiosemicarbazone) (bTSC) ligand were employed in the cleavage of phosphoester bonds. Comparative kinetic studies combined with theory suggested that the P-O bond cleavage is much accelerated by dinuclear zinc(II) complex in the presence of base. Based on the DFT-optimized structures of the proposed intermediates, it is plausible that (1) the removal of sulfur atoms of bTSC ligand from the zinc center provides two vacant sites for the binding of water (or hydroxide ion) and phosphoester and (2) the H-bonding between water (or hydroxide ion) and phosphoester, through several water molecules, may also assist the P-O bond cleavage and facilitate the nucleophilic attack. The kinetic and catalytic studies on the hydrolysis of phosphoester by dinuclear zinc complex showed a much-enhanced reactivity under basic reaction conditions, reaching over 95% conversion yield within 4 h. The currently presented compounds are arguably one of the faster synthetic Zn-based model performing phosphatase-like activity presented so far.

Mitochondrial Metabolism Targeted Nanoplatform for Efficient Triple-Negative Breast Cancer Combination Therapy

Tumor reprogram pathway of mitochondrial metabolism is an emerging approach for malignant tumor treatment, such as triple-negative breast cancer. In this study, a tumor/mitochondria cascaded targeting, adenosine-triphosphate (ATP) responsive nanocarrier of zeolitic imidazolate framework-90 (ZIF-90) for breast cancer combination therapy is reported. Atovaquone (AVO) and hemin are loaded into ZIF-90, then a peptide iRGD with tumor-targeting ability is modified on the ZIF-90 nanoplatform. Hemin can specifically degrade BTB and CNC homology1 (BACH1), resulting in the changes of mitochondrial metabolism, and AVO acts as the inhibitor of the electron transport chain (ETC). The degradation of BACH1 using hemin can effectively improve the anti-tumor efficiency of mitochondrial metabolism inhibitor AVO, by increasing dependency on mitochondrial respiration. This nanoplatform displays both tumor-targeting and mitochondria-targeting capacity with high level of ATP responsive drug release behavior. The specific characteristic of mitochondria-targeting ability of this nanoplatform can increase the accumulation of AVO in the mitochondria, and in turn, can effectively improve the inhibition of the ETC. Both in vitro and in vivo results reveal that this composite nanocarrier has excellent tumor inhibition ability with limited side effects. Accordingly, this study provides an attractive strategy in the mitochondrial metabolism for cancer targeted therapy.

A Fluorescence Sensor Array Based on Zinc(II)-Carboxyamidoquinolines: Toward Quantitative Detection of ATP*

The newly prepared fluorescent carboxyamidoquinolines (1-3) and their Zn(II) complexes (Zn@1-Zn@3) were used to bind and sense various phosphate anions utilizing a relay mechanism, in which the Zn(II) ion migrates from the Zn@1-Zn@3 complexes to the phosphate, namely adenosine 5'-triphosphate (ATP) and pyrophosphate (PPi), a process accompanied by a dramatic change in fluorescence. Zn@1-Zn@3 assemblies interact with adenine nucleotide phosphates while displaying an analyte-specific response. This process was investigated using UV-vis, fluorescence, and NMR spectroscopy. It is shown that the different binding selectivity and the corresponding fluorescence response enable differentiation of adenosine 5'-triphosphate (ATP), adenosine 5'-diphosphate (ADP), pyrophosphate (PPi), and phosphate (Pi). The cross-reactive nature of the carboxyamidoquinolines-Zn(II) sensors in conjunction with linear discriminant analysis (LDA) was utilized in a simple fluorescence chemosensor array that allows for the identification of ATP, ADP, PPi, and Pi from 8 other anions including adenosine 5'-monophosphate (AMP) with 100 % correct classification. Furthermore, the support vector machine algorithm, a machine learning method, allowed for highly accurate quantitation of ATP in the range of 5-100 μM concentration in unknown samples with error <2.5 %.

Aptamer-based strategies for recognizing adenine, adenosine, ATP and related compounds

Adenine is a key nucleobase, adenosine is an endogenous regulator of the immune system, while adenosine triphosphate (ATP) is the energy source of many biological reactions. Selective detection of these molecules is useful for understanding biological processes, biochemical reactions and signaling. Since 1993, various aptamers have been reported to bind to adenine and its derivatives. In addition, the adenine riboswitch was later discovered. This review summarizes the efforts for the selection of RNA and DNA aptamers for adenine derivatives, and we pay particular attention to the specificity of binding. In addition, other molecular recognition strategies based on rational sequence design are also introduced. Most of the work in the field was performed on the classic DNA aptamer for adenosine and ATP reported by the Szostak group. Based on this aptamer, some representative applications such as the design of fluorescent, colorimetric and electrochemical biosensors, intracellular imaging, and ATP-responsive materials are also described. In addition, we critically review the limit of the reported aptamers and also important problems in the field, which can give future research opportunities.

Selective sensing of adenosine monophosphate (AMP) over adenosine diphosphate (ADP), adenosine triphosphate (ATP), and inorganic phosphates with zinc(II)-dipicolylamine-containing gold nanoparticles

Mixed monolayer-protected gold nanoparticles containing surface-bound triethylene glycol and dipicolylamine groups aggregated in water/methanol, 1 : 2 (v/v) in the presence of nucleotides, if the solution also contained zinc(ii) nitrate to convert the dipicolylamine units into the corresponding zinc complexes. Nanoparticle aggregation could be followed with the naked eye by the colour change of the solution from red to purple followed by nanoparticle precipitation. The sensitivity was highest for adenosine triphosphate (ATP), which could be detected at concentrations >10 μM, and decreased over adenosine diphosphate (ADP) to adenosine monophosphate (AMP), consistent with the typically higher affinity of zinc(ii)-dipicolylamine-derived receptors for higher charged nucleotides. Inorganic sodium diphosphate and triphosphate interfered in the assay by also inducing nanoparticle aggregation. However, while the nucleotide-induced aggregates persisted even at higher analyte concentrations, the nanoparticles that were precipitated with inorganic salts redissolved again when the salt concentration was increased. The thus resulting solutions retained their ability to respond to nucleotides, but they now preferentially responded to AMP. Accordingly, AMP could be sensed selectively at concentrations ≥50 μM in an aqueous environment, even in the presence of other nucleotides and inorganic anions. This work thus introduces a novel approach for the sensing of a nucleotide that is often the most difficult analyte to detect with other assays.

Ultra-sensitive detection of ATP in serum and lysates based on nitrogen-doped carbon dots

In this work, novel types of nitrogen-doped carbon dots (N-CDs) were prepared from citric acid and glycine (GLY) as precursors through a simple pyrolysis method. The GLY-CDs showed strong fluorescence with a fluorescence quantum yield as high as 33.34% and good water solubility. The fluorescence of GLY-CDs could be selectively quenched by iron(III) ion (Fe³⁺ ) resulting in the non-fluorescent complex. Due to the high affinity of Fe³⁺ to adenosine-5'-triphosphate (ATP), the fluorescence of the GLY-CDs in GLY-CDs-Fe³⁺ could be recovered by ATP. Thereby, quantitatively fluorescent turn-on detection of ATP could be achieved. The fluorescence recovery ratio was linearly proportional to the concentration of ATP with a detection limit as low as 15.0 nM, indicating the CDs have high sensitivity. The GLY-CDs were successfully employed in the detection of ATP in serum and cell lysates.

Novel perylene probe-encapsulated metal-organic framework nanocomposites for ratiometric fluorescence detection of ATP

Adenosine triphosphate (ATP) plays an important role in various biological processes and the ATP level is closely associated with many diseases. Herein, a novel ratiometric fluorescence assay for ATP was developed based on the excimer-monomer transfer of a perylene probe. By encapsulating a perylene probe, N,N'-bis(6-caproic acid)-3,4:9,10-perylenediimide (PDI), into zeolitic imidazolate framework-8 (ZIF-8) nanocrystals, fluorescent nanocomposites (PDI@ZIF-8) with significant excimer emission of the perylene probe were prepared for the first time. The presence of ATP will trigger the decomposition of PDI@ZIF-8 due to much stronger coordination between ATP and Zn2+ than that of 2-methylimidazole and Zn2+. As a result, the encapsulated PDI probes were released, leading to significantly increased monomer emission accompanying the decrease in the excimer emission. The excimer-monomer transition signal was utilized for ratiometric ATP sensing and its potential application for detecting ATP in cell lysates was also successfully demonstrated.

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.

Protonation of cyclen-based chelating agents containing fluorescent moieties

The fluorescence emission properties of 1,4,7,10-tetraazacyclododecane-based receptors with appended heteroaromatic fluorophores are tuned by photoinduced electron and proton transfer processes.

An ATP responsive fluorescent supramolecular assembly based on a polyelectrolyte and an AIE active tetraphenylethylene derivative

Aggregation induced emission (AIE) active probes have attracted enormous attention due to their wide-spread and ever increasing number of applications in the sensing of chemically and biologically important molecules. AIE probes undergo drastic modulation in their photophysical features from a monomeric to aggregated state. In the current work, we report the aggregation of tetra-anionic Su-TPE (AIE active probe) in the presence of a cationic polyelectrolyte, poly(allylaminehydrochloride) (PAH). A supramolecular assembly is formed by the electrostatic interaction between cationic PAH and anionic Su-TPE molecules, which leads to drastic modulations in the spectral features of anionic Su-TPE upon addition of cationic PAH. The Su-TPE-PAH aggregate assembly has been investigated using various photophysical techniques, such as, ground-state absorption, steady-state and time-resolved emission spectroscopic techniques along with 1H NMR measurements. The Su-TPE-PAH aggregate assembly is found to be responsive towards the ionic strength of the medium and temperature which results in drastic modulations of the spectral features of the emissive supramolecular aggregate assembly. Finally, the specific recognition of an important bioanalyte, ATP, has been achieved using the formed Su-TPE-PAH supramolecular aggregate assembly as a sensing platform which displays good selectivity and high sensitivity towards ATP. Importantly, the developed sensor platform could also function in the human serum matrix, hence, demonstrating the potential of the established sensor platform for real-life applications in near future.

Optical detection of di- and triphosphate anions with mixed monolayer-protected gold nanoparticles containing zinc(II)-dipicolylamine complexes

Gold nanoparticles covered with a mixture of ligands of which one type contains solubilizing triethylene glycol residues and the other peripheral zinc(II)–dipicolylamine (DPA) complexes allowed the optical detection of hydrogenphosphate, diphosphate, and triphosphate anions in water/methanol 1:2 (v/v). These anions caused the bright red solutions of the nanoparticles to change their color because of nanoparticle aggregation followed by precipitation, whereas halides or oxoanions such as sulfate, nitrate, or carbonate produced no effect. The sensitivity of phosphate sensing depended on the nature of the anion, with diphosphate and triphosphate inducing visual changes at significantly lower concentrations than hydrogenphosphate. In addition, the sensing sensitivity was also affected by the ratio of the ligands on the nanoparticle surface, decreasing as the number of immobilized zinc(II)–dipicolylamine groups increased. A nanoparticle containing a 9:1 ratio of the solubilizing and the anion-binding ligand showed a color change at diphosphate and triphosphate concentrations as low as 10 μmol/L, for example, and precipitated at slightly higher concentrations. Hydrogenphosphate induced a nanoparticle precipitation only at a concentration of ca. 400 μmol/L, at which the precipitates formed in the presence of diphosphates and triphosphates redissolved. A nanoparticle containing fewer binding sites was more sensitive, while increasing the relative number of zinc(II)–dipicolylamine complexes beyond 25% had a negative impact on the limit of detection and the optical response. Transmission electron microscopy provided evidence that the changes of the nanoparticle properties observed in the presence of the phosphates were due to a nanoparticle crosslinking, consistent with the preferred binding mode of zinc(II)–dipicolylamine complexes with phosphate anions which involves binding of the anion between two metal centers. This work thus provided information on how the behavior of mixed monolayer-protected gold nanoparticles is affected by multivalent interactions, at the same time introducing a method to assess whether certain biologically relevant anions are present in an aqueous solution within a specific concentration range.

Selective Coordination of Cu2+ and Subsequent Anion Detection Based on a Naphthalimide-Triazine-(DPA)2 Chemosensor

A new fluorescent chemosensor for copper (II) and subsequent anion sensing was designed and fully characterized. The sensor consisted of a 1,8-naphthalimide core, bearing two terminal dipicolylamine (DPA) receptor units for binding metal cations, and an ethoxyethanol moiety for enhanced water solubility. The DPA units are connected to position 4 of the fluorophore via a triazine-ethylenediamine spacer. Fluorescence titration studies of the chemosensor revealed a high selectivity for Cu²⁺ over other divalent ions, the emissions were strongly quenched upon binding, and a stability constant of 5.52 log units was obtained. Given the distance from DPA chelating units and the fluorophore, quenching from the Cu²⁺ complexation suggests an electron transfer or an electronic energy transfer mechanism. Furthermore, the Cu²⁺-sensor complex proved to be capable of sensing anionic phosphate derivatives through the displacement of the Cu²⁺ cation, which translated into a full recovery of the luminescence from the naphthalimide. Super-resolution fluorescence microscopy studies performed in HeLa cells showed there was a high intracellular uptake of the chemosensor. Incubation in Cu²⁺ spiked media revealed a strong fluorescent signal from mitochondria and cell membranes, which is consistent with a high concentration of ATP at these intracellular sites.

Functional Nanochannels for Sensing Tyrosine Phosphorylation

Tyrosine phosphorylation (pTyr), much of which occurred on localized multiple sites, initiates cellular signaling, governs cellular functions, and its dysregulation is implicated in many diseases, especially cancers. pTyr-specific sensing is of great significance for understanding disease states and developing targeted anticancer drugs, however, it is very challenging due to the slight difference from serine (pSer) or threonine phosphorylation (pThr). Here we present polyethylenimine-g-phenylguanidine (PEI-PG)-modified nanochannels that can address the challenge. Rich guanidinium groups enabled PEI-PG to form multiple interactions with phosphorylated residues, especially pTyr residue, which triggered the conformational change of PEI-PG. By taking advantage of the "OFF-ON" change of the ion flux arising from the conformational shrinkage of the grafted PEI-PG, the nanochannels could distinguish phosphorylated peptide (PP) from nonmodified peptide, recognize PPs with pSer, pThr, or pTyr residue and PPs with different numbers of identical residues, and importantly could sense pTyr peptides in a biosample. Benefiting from the strong interaction between the guanidinium group and the pTyr side-chain, the specific sensing of pTyr peptide was achieved by performing a simple logic operation based on PEI-PG-modified nanochannels when Ca²⁺ was introduced as an interferent. The excellent pTyr sensing capacity makes the nanochannels available for real-time monitoring of the pTyr process by c-Abl kinase on a peptide substrate, even under complicated conditions, and the proof-of-concept study of monitoring the kinase activity demonstrates its potential in kinase inhibitor screening.

Molecular recognition of bisphosphonate-based drugs by di-zinc receptors in aqueous solution and on gold nanoparticles

Metal-based anion receptors have several important applications in sensing, separation and transport of negatively charged species. Amongst these receptors, di-zinc(ii) complexes are of particular interest for the recognition of oxoanions, in particular phosphate derivatives. Herein we report the synthesis of a di-zinc(ii) receptor and show that it has high affinity and selectivity for bisphosphonates such as alendronate and etidronate - which are used to treat a number of skeletal disorders as well as showing interesting anticancer properties. The binding mode of the di-zinc(ii) receptor with alendronate and etidronate has been unambiguously established by single crystal X-ray crystallography. In addition, by modifying the backbone of the receptor, we show that the drug-loaded receptor can be attached onto gold nanoparticles as potential drug-delivery vehicles.

Chemosensing of Guanosine Triphosphate Based on a Fluorescent Dinuclear Zn(II)-Dipicolylamine Complex in Water

Guanosine triphosphate (GTP) is a key biomarker of multiple cellular processes and human diseases. The new fluorescent dinuclear complex [Zn₂(L)(S)][OTf]₄, 1 (asymmetric ligand, L = 5,8-Bis{[bis(2-pyridylmethyl)amino] methyl}quinoline, S = solvent, and OTf = triflate anion) was synthesized and studied in-depth as a chemosensor for nucleoside polyphosphates and inorganic anions in pure water. Additions at neutral pH of nucleoside triphosphates, guanosine diphosphate, guanosine monophosphate, and pyrophosphate (PPi) to 1 quench its blue emission (λ_em = 410 nm) with a pronounced selectivity toward GTP over other anions, including adenosine triphosphate (ATP), uridine triphosphate (UTP), and cytidine triphosphate (CTP). The efficient quenching response by the addition of GTP was observed in the presence of coexisting species in blood plasma and urine with a detection limit of 9.2 μmol L^-1. GTP also shows much tighter binding to the receptor 1 on a submicromolar level. On the basis of multiple spectroscopic tools (¹H, ³¹P NMR, UV-vis, and fluorescence) and DFT calculations, the binding mode is proposed through three-point recognition involving the simultaneous coordination of the N⁷ atom of the guanosine motif and two phosphate groups to the two Zn(II) atoms. Spectroscopic studies, MS-ESI, and DFT suggested that GTP bound to 1 in 1:1 and 2:2 models with high overall binding constants of log β_1 (1:1) = 6.05 ± 0.01 and log β₂ = 10.91 ± 0.03, respectively. The optical change and selectivity are attributed to the efficient binding of GTP to 1 by the combination of a strong electrostatic contribution and synergic effects of coordination bonds. Such GTP selectivity of an asymmetric metal-based receptor in water is still rare.

Tuning the anion binding properties of lanthanide receptors to discriminate nucleoside phosphates in a sensing array

The development of synthetic receptors for the selective binding and discrimination of anions in water requires an understanding of how anions interact with these synthetic receptors. Molecules designed to differentiate nucleoside phosphate anions (e.g. ATP, ADP, GTP, GDP, UDP) under physiological conditions could underpin exciting new sensing tools for biomedical research and drug discovery, but it is very challenging due to the similarities in anion structure, size and charge. We present a series of lanthanide-based anion receptors and establish key structural elements that impact on nucleoside phosphate anion binding and sensing. Structural evidence of anion binding using X-ray crystallographic and NMR data, supported by DFT calculations indicate the binding modes between the lanthanide complexes and certain phosphoanions, revealing a bidentate (α-, γ-) binding mode to ATP. We further use four of the receptors to allow discrimination of eight nucleoside phosphate anions in the first array-based assay using lanthanide complexes, taking advantage of the multiple emission bands and long emission lifetimes associated with luminescent lanthanide complexes.

Selective sensing of ATP by hydroxide-bridged dizinc(ii) complexes offering a hydrogen bonding cavity

This work illustrates the highly selective fluorescence detection of ATP in the presence of other competing anions, such as AMP, ADP, PPi and other phosphates by using a set of hydroxide-bridged dizinc(ii) complexes offering a cavity lined with hydrogen bonds and other interactive forces. ATP, as a whole, was recognized by the synergic combination of Zn-phosphate bonding, ππ stacking between the adenine ring of ATP and the pyridine ring of the dizinc complex and hydrogen bonding interactions that modulate the cavity structure of the dizinc complexes.

A supramolecule based fluorescence turn-on and ratiometric sensor for ATP in aqueous solution

Considering the biological relevance of adenosine triphosphate (ATP) as an "energy currency" in all organisms and significance of its detection in various diseased conditions, enormous efforts have been made to develop selective and sensitive fluorescent sensors for the detection of ATP. However, these developed sensor probes frequently involve technically challenging and time-consuming synthetic protocols for the production of sensor molecules and often suffer from poor solubility in aqueous medium. Another major disadvantage of these developed sensor systems is their single wavelength based operation which makes their performance susceptible to minute changes in experimental conditions. Herein, we report a fluorescence turn-on ratiometric sensor for the detection of ATP which operates by the dissociation of Thioflavin-T-sulphated-β-cyclodextrin supramolecular assembly by Zn²⁺ followed by ATP induced reassociation of the same. This modulation of the monomer/aggregate equilibrium of the supramolecular assembly followed by subsequent interactions with Zn²⁺ and ATP acts as an optimal scheme for the ratiometric detection of ATP. Overall this supramolecular ensemble based sensing platform provides a simple, sensitive, selective and label free detection approach for ATP in aqueous solution. Importantly, our sensor platform responds to ATP in the biologically complex media of serum samples suggesting its potential for possible applications in real-life scenarios.

Zinc(ii) and copper(ii) complexes as tools to monitor/inhibit protein phosphorylation events

A perspective on the advance of copper(ii) and zinc(ii) complexes of varied ligand architectures as binders of phosphorylated peptides/proteins and as sensors of phosphorylation reactions is presented.

Ternary Zn(II) Complexes of Fluorescent Zinc Probes Zinpyr-1 and Zinbo-5 with the Low Molecular Weight Component of Exchangeable Cellular Zinc Pool

The intracellular exchangeable Zn(II) is usually measured with synthetic fluorescent zinc sensors. 4',5'-Bis[bis(2-pyridylmethyl)aminomethyl]-2',7'-dichlorofluorescein (Zinpyr-1) is a sensor containing the fluorescein platform and a duplicated chelating unit. Its advantages include brightness and a relatively high affinity for Zn(II), K_d = 0.7 nM. 2-(4,5-Dimethoxy-2-hydroxyphenyl)-4-(2-pyridylmethyl)aminomethylbenzoxazole (Zinbo-5) is a member of a growing family of ratiometric synthetic Zn(II) probes, offering a possibility to determine Zn(II) concentration independently of the sensor concentration. Cells, however, contain high, millimolar or nearly millimolar concentrations of low molecular weight ligands (LMWLs) capable of binding Zn(II) ions. Previously, we demonstrated that such LMWLs can perturb the performance of some fluorescent zinc sensors by competition and formation of ternary Zn(sensor) (LMWL) complexes. Here we tested Zinpyr-1 and Zinbo-5 in this respect. Despite structural differences, both sensors formed such ternary complexes. We determined their stability constants ^CK_tern and performed numerical simulations of Zn(II) distributions at physiological concentrations of selected LMWLs. Glutamic acid was found to provide the strongest ternary complexes with either of the studied sensors. Zn(Zinpyr-1)(Glu) was an absolutely dominant Zn(II)/Zinpyr-1 species (more than 96% of the exchangeable Zn(II)), and Zn(Zinbo-5)(Glu) was the most abundant one (more than 40%) in these simulations. Our results indicate that under cellular conditions these sensors are able to report Zn(II) complexed to LMWLs rather than free Zn²⁺ ions. On the other hand, the specific affinity of Zn(Zinpyr-1) and Zn(Zinbo-5) for Glu creates interesting opportunities for determining glutamic acid in biological samples.

Chemical Sensing Platforms Based on Organic Thin-Film Transistors Functionalized with Artificial Receptors

Organic thin-film transistors (OTFTs) have attracted intense attention as promising electronic devices owing to their various applications such as rollable active-matrix displays, flexible nonvolatile memories, and radiofrequency identification (RFID) tags. To further broaden the scope of the application of OTFTs, we focus on the host-guest chemistry combined with the electronic devices. Extended-gate types of OTFTs functionalized with artificial receptors were fabricated to achieve chemical sensing of targets in complete aqueous media. Organic and inorganic ions (cations and anions), neutral molecules, and proteins, which are regarded as target analytes in the field of host-guest chemistry, were electrically detected by artificial receptors. Molecular recognition phenomena on the extended-gate electrode were evaluated by several analytical methods such as photoemission yield spectroscopy in the air, contact angle goniometry, and X-ray photoelectron spectroscopy. Interestingly, the electrical responses of the OTFTs were highly sensitive to the chemical structures of the guests. Thus, the OTFTs will facilitate the selective sensing of target analytes and the understanding of chemical conversions in biological and environmental systems. Furthermore, such cross-reactive responses observed in our studies will provide some important insights into next-generation sensing systems such as OTFT arrays. We strongly believe that our approach will enable the development of new intriguing sensor platforms in the field of host-guest chemistry, analytical chemistry, and organic electronics.

A copper(ii)-dipicolylamine-coumarin sensor for maltosyltransferase assay

A Cu(ii)-[di(2-methylpyridyl)methylamino]coumarin fluorescence turn-on sensor (Cu-1b) is designed to detect phosphate ions with K_ass = 1.4 × 10⁵ M^-1 in HEPES buffer. Cu-1b is applied to probe the GlgE-catalyzed maltose-transfer reaction of α-maltose-1-phosphate to α-1,4-glucan with concomitant release of phosphate ions in Mycobacterium tuberculosis.

A simple, robust, universal assay for real-time enzyme monitoring by signalling changes in nucleoside phosphate anion concentration using a europium(iii)-based anion receptor

Enzymes that consume and produce nucleoside polyphosphate (NPP) anions represent major targets in drug discovery. For example, protein kinases are one of the largest classes of drug targets in the fight against cancer. The accurate determination of enzyme kinetics and mechanisms is a critical aspect of drug discovery research. To increase confidence in the selection of lead drug compounds it is crucial that pharmaceutical researchers have robust, affordable assays to measure enzyme activity accurately. We present a simple, sensitive microplate assay for real-time monitoring of a range of pharmaceutically important enzyme reactions that generate NPP anions, including kinases and glycosyltransferases. Our assay utilises a single, stable europium(iii) complex that binds reversibly to NPP anions, signalling the dynamic changes in NPP product/substrate ratio during an enzyme reaction using time-resolved luminescence. This supramolecular approach to enzyme monitoring overcomes significant limitations in existing assays, obviating the need for expensive antibodies or equipment, chemically labelled substrates or products and isolation or purification steps. Our label and antibody-free method enables rapid and quantitative analysis of enzyme activities and inhibition, offering a potentially powerful tool for use in drug discovery, suitable for high-throughput screening of inhibitors and accurate measurements of enzyme kinetic parameters.

Protein-Catalyzed Capture Agents

Protein-catalyzed capture agents (PCCs) are synthetic and modular peptide-based affinity agents that are developed through the use of single-generation in situ click chemistry screens against large peptide libraries. In such screens, the target protein, or a synthetic epitope fragment of that protein, provides a template for selectively promoting the noncopper catalyzed azide-alkyne dipolar cycloaddition click reaction between either a library peptide and a known ligand or a library peptide and the synthetic epitope. The development of epitope-targeted PCCs was motivated by the desire to fully generalize pioneering work from the Sharpless and Finn groups in which in situ click screens were used to develop potent, divalent enzymatic inhibitors. In fact, a large degree of generality has now been achieved. Various PCCs have demonstrated utility for selective protein detection, as allosteric or direct inhibitors, as modulators of protein folding, and as tools for in vivo tumor imaging. We provide a historical context for PCCs and place them within the broader scope of biological and synthetic aptamers. The development of PCCs is presented as (i) Generation I PCCs, which are branched ligands engineered through an iterative, nonepitope-targeted process, and (ii) Generation II PCCs, which are typically developed from macrocyclic peptide libraries and are precisely epitope-targeted. We provide statistical comparisons of Generation II PCCs relative to monoclonal antibodies in which the protein target is the same. Finally, we discuss current challenges and future opportunities of PCCs.

A new tetraphenylethylene based AIE sensor with light-up and tunable measuring range for adenosine triphosphate in aqueous solution and in living cells

An AIE based tetraphenylethylene derivative (TPPTPE) was synthesized for light-up sensing of ATP in aqueous solution. The measuring range for ATP can be tuned by varying the concentration of the TPPTPE. A one-step straightforward quantitative analysis of the ATP level in cell lysates can be realized using the TPPTPE. Moreover, the TPPTPE can be used for monitoring apyrase activity in aqueous solution and detecting ATP both in living cancer cell lines and in living normal cell lines.