Mesoporous supraparticles with a tailored solid-liquid-gas interface for visual indication of H2 gas and NH3 vapours.

Dual-gasochromic supraparticles that undergo rapid eye-readable and gas-specific colour changes upon reaction with hydrogen or ammonia are reported. This functionality is achieved by tailoring the solid-liquid-gas interface within the mesoporous framework of supraparticles via spray-drying.

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.

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.

Micromolar Affinity and Higher: Synthetic Host-Guest Complexes with High Stabilities.

The design of high-affinity synthetic host-guest complexes is of paramount importance because they are key elements in constructing unprecedented supramolecular assemblies, functional materials, molecular probes, artificial signal transduction events, and interfaces with the biological world. The present review article collects recent achievements in the design of 1 : 1 host-guest complexes with outstanding stabilities, i.e., exceeding 106  M-1 . The relationships between the measured thermodynamic constants and the structural parameters of the interacting species are analyzed. The design features of high-affinity hosts are discussed in light of their binding properties. Different solvents and different types of noncovalent interactions are considered for the stabilization of the complexes. Finally, some hints are provided for the design of future synthetic receptors displaying high affinity and selectivity.

Three-state fluorescence hydrochromism of a fluorophore-spacer-receptor system with variations in relative humidity.

The uptake of atmospheric moisture by hygroscopic materials can have marked effects on a material's physical and chemical properties. This is true of materials that go on to incorporate waters of hydration in their molecular structural lattice, forming stable hydrates with fluctuations in relative humidity (RH). Nevertheless, RH remains relatively uncontrolled for a variable that can fluctuate widely depending on geographical climate, weather fluctuations, and building HVAC system stability. Herein, we report a processable 1,8-napthalimide-based fluorophore-spacer-receptor system that unexpectedly exhibited reversible three-state fluorescence hydrochromism with changes in RH due to RH-induced solid state molecular rearrangement. Care should be taken to evaluate the impact of variations in RH when characterising the solid state emission properties of charged fluorescent materials.

Acenaphthenoannulation Induced by the Dual Lewis Acidity of Alumina.

We have discovered a dual (i. e., soft and hard) Lewis acidity of alumina that enables rapid one-pot π-extension through the activation of terminal alkynes followed by C-F activation. The tandem reaction introduces an acenaphthene fragment - an essential moiety of geodesic polyarenes. This reaction provides quick access to elusive non-alternant polyarenes such as π-extended buckybowls and helicenes through three-point annulation of the 1-(2-ethynyl-6-fluorophenyl)naphthalene moiety. The versatility of the developed method was demonstrated by the synthesis of unprecedented structural fragments of elusive geodesic graphene nanoribbons.

Dye-functionalized phosphate-binding macrocycles: from nucleotide to G-quadruplex recognition and "turn-on" fluorescence sensing.

A novel strategy to design "turn-on" fluorescent receptors for G-quadruplexes of DNA is presented, which relies on the connection of phosphate binding macrocycles (PBM) with naphthalimide dyes. A new PBM-dye family was synthesized and evaluated in terms of binding and detection of nucleotides and DNA G-quadruplexes of different topologies.

Binding and Sensing Properties of a Hybrid Naphthalimide-Pyrene Aza-Cyclophane towards Nucleotides in an Aqueous Solution.

Selective recognition of nucleotides with synthetic receptors is an emerging direction to solve a series of nucleic acid-related challenges in biochemistry. Towards this goal, a new aza-cyclophane with two different dyes, naphthalimide and pyrene, connected through a triamine linker has been synthesized and studied for the ability to bind and detect nucleoside triphosphates in an aqueous solution. The receptor shows Foerster resonance energy transfer (FRET) in fluorescence spectra upon excitation in DMSO, which is diminished dramatically in the presence of water. According to binding studies, the receptor has a preference to bind ATP (adenosine triphosphate) and CTP (cytidine triphosphate) with a "turn-on" fluorescence response. Two separate emission bands of dyes allow one to detect nucleotides in a ratiometric manner in a broad concentration range of 10-5-10-3 M. Spectroscopic measurements and quantum chemical calculations suggest the formation of receptor-nucleotide complexes, which are stabilized by dispersion interactions between a nucleobase and dyes, while hydrogen bonding interactions of nucleobases with the amine linkers are responsible for selectivity.

Control of Photoisomerization of an Azoazacryptand by Anion Binding and Cucurbit[8]uril Encapsulation in an Aqueous Solution.

Control of isomerization of a receptor bearing multiple light-switchable subunits in a confined space is critical for the design of synthetic molecular machines. Toward this goal, a new azacryptand containing three azobenzene subunits has been developed, and its photoisomerization in an aqueous solution has been studied depending on anion coordination and recognition by a larger host-cucurbit[8]uril (CB[8]). The cryptand in its hexaprotonated form shows considerable affinity for fluoride and perchlorate, which in turn affects the isomer distribution of the receptor under UV-light irradiation, stabilizing the isomers of the cryptand with Z-configurations. CB[8] was found to be able to encapsulate the isomers of the cryptand by forming a Matryoshka-type complex. The irradiation of a 10:1 CB[8]-cryptand mixture has led to a selective conversion of the cryptand to the E,E,Z isomer inside CB[8]. It has been demonstrated that the addition of fluoride to the resulted complex induces the release of the cryptand as a major E,E,E isomer, while other studied anions were ineffective in this reaction. To our knowledge, this work presents a first example of a host-controlled photoisomerization of an anion receptor bearing multiple switching azobenzenes that model the function of naturally occurring chaperones.

Ratiometric Detection of ATP by Fluorescent Cyclophanes with Bellows-Type Sensing Mechanism.

Pyrene-based cyclophanes have been synthesized with the aim to realize a bellows-type sensing mechanism for the ratiometric detection of nucleotide concentrations in a buffered aqueous solution. The sensing mechanism involves the encapsulation of a nucleobase between two pyrene rings, which affects the monomer-excimer equilibrium of the receptor in the excited state. The nature of the spacer and its connection pattern to pyrene rings have been varied to achieve high selectivity for ATP. The 1,8-substituted pyrene-based cyclophane with the 2,2'-diaminodiethylamine spacer demonstrates the best selectivity for ATP showing a 50-fold increase in the monomer-excimer emission ratio upon saturation with the nucleotide. The receptor can detect ATP within the biological concentrations range over a wide pH range. NMR and spectroscopic studies have revealed the importance of hydrogen bonding and stacking interactions for achieving a required receptor selectivity. The probe has been successfully applied for the real-time monitoring of creatine kinase activity.

Anthracene-Based Amido-Amine Cage Receptor for Anion Recognition under Neutral Aqueous Conditions.

Invited for this month's cover picture is the group of Evgeny A. Kataev at the Technical University Chemnitz. The cover picture shows the authors' association of fluorescence anion sensing with pearl hunting - the activity of recovering pearls from wild molluscs. In the presented work, the group of Kataev has developed a new water-soluble amido-amine azacryptand bearing a fluorescence anthracene dicarboxamide fragment. With the help of the fluorescent receptors (a hand) one can catch the phosphate anion (a glowing pearl) and visualize this binding event. The recognition of phosphate and oxalate has led to a fluorescent enhancement in a selective manner. Read the full text of their Communication at https://doi.org/10.1002/open.201900309.

Anthracene-Based Amido-Amine Cage Receptor for Anion Recognition under Neutral Aqueous Conditions.

A new amido-amine cage receptor, which combines 1,8-anthracene diacarboxamide subunit and a polyammonium azamacrocycle, is reported. Bearing both the hydrogen bond donor and the acceptor binding sites, the receptor is able to bind phosphate selectively under neutral (pH 7.2) aqueous conditions. The recognition events for phosphate and dicarboxylates are accomplished by a fluorescence enhancement in the anthracene emission. As revealed by experimental and theoretical studies, phosphate and oxalate show different recognition modes. Phosphate demonstrates hydrogen bond acceptor properties, while the coordination of oxalate favours the protonation of the receptor.

Anthracene-Based Receptors with a Turn-on Fluorescence Response for Nitrate.

Herein we describe the design, synthesis, and anion binding properties of bicyclic receptors with two or three anthracenes, which show a turn-on fluorescence response in the presence of nitrate and chaotropic anions in a buffered aqueous solution. The receptor with two anthracenes binds nitrate with 103 M-1 affinity and stabilizes it in the inner cavity though electrostatic, hydrogen bonding, and anion-π interactions.

Conformational Selection in Anion Recognition: cGMP-Selective Binding by a Naphthalimide-Functionalized Amido-Amine Macrocycle.

Amido-amine macrocycles with two and four naphthalimide dyes were designed to bind nucleoside monophosphates and oligonucleotides in an aqueous buffered solution. Anion-templated synthesis was used to direct the macrocyclization reaction to the [2+2] product, while high dilution conditions favored the formation of the [4+4] macrocycle with an unprecedented geometry, as revealed from the X-ray analysis. The [2+2] product was found to exhibit a remarkable binding strength and fluorescence response for cyclic guanosine monophosphate (cGMP) in an aqueous solution. To our knowledge, this is the first synthetic receptor for cGMP, which also demonstrates a high preference to bind guanine-rich sequences accomplished by a strong fluorescence quenching. The receptor conformation is very sensitive to the guest structure in an aqueous solution, thus modeling the adaptive behavior of proteins. The study of synthetic systems with a detectable conformational equilibrium represents a great potential for understanding highly specific and tightly regulated interactions in biological systems.

Anthracene-Based Cyclophanes with Selective Fluorescent Responses for TTP and GTP: Insights into Recognition and Sensing Mechanisms.

Three anthracene-based cyclophanes were synthesized and their binding properties towards nucleoside triphosphates were studied. A new polycyclic amine derived from dearomatized anthracene was identified as a major side product in the cyclization reaction between 9,10-anthracenedicarboxaldehyde and diethylenetriamine. Its structure was determined by single-crystal X-ray analysis. The cyclophanes were found to form 1:1 complexes with all nucleoside triphosphates as well as with pyrophosphate in a buffered aqueous solution at pH 6.2. A turn-on fluorescence response was observed for all nucleotides except for GTP, which demonstrated strong fluorescence quenching. The strongest turn-on fluorescence was observed for the largest receptor 3 in the presence of thymidine triphosphate (TTP). Based on the NMR and fluorescence experiments, two major binding modes for nucleotide complexes were identified.

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.

Cation Molecular Exchanger Based on a Conformational Hinge.

A cation molecular exchanger has been developed that consists of the bipyridine and crown ether receptor subunits. It has been shown that binding of the zinc(II) cation to the bipyridine subunit induces the conformational switching of the crown ether subunits, which results in a release of the potassium cation. Two conformational states of the cation exchanger have been supported by the results from solution- and solid-state studies. It has been demonstrated that the cation exchanger is able to communicate with a cation sensor induced by a chemical stimulus.

Finding a receptor design for selective recognition of perrhenate and pertechnetate: hydrogen vs. halogen bonding.

Receptors bearing hydrogen and halogen bond donors for recognition of perrhenate and pertechnetate were designed and studied. Acyclic hosts with N-H and C-H binding sites showed the best selectivity for TcO4- and ReO4- over spherical and more basic tetrahedral anions.

Hybrid Macrocycles for Selective Binding and Sensing of Fluoride in Aqueous Solution.

Synthesis and anion binding properties of hybrid macrocycles containing ammonium and hydrogen bond donor groups are reported. Receptor properties were studied in a 10 mM MES buffer solution at pH 6.2, at which the receptors carry two positive charges at the secondary amine groups. Receptor 1 was found to bind fluoride with the highest affinity (105 M-1) and selectivity among the synthesized receptors. It was the only receptor that demonstrated fluorescence increase upon addition of fluoride. Other titration experiments with halides and oxyanions led to an anion-induced aggregation and fluorescence quenching. The mechanism of the particular turn-on fluorescence for fluoride was explained by the ability of receptor 1 to encapsulate several fluoride anions. Multiple anion coordination resulted in the protonation of the tertiary amine group and subsequent hindering of the PET process. 1H and 19F NMR titrations, single-crystal X-ray structure of chloride complex, and DFT calculation suggest that 1 can perfectly accommodate two fluoride anions in the inner cavity but only one chloride, keeping the second chloride in the outer coordination sphere. Thus, the importance of size selectivity, which is reflected in a collective behavior of molecules in an aqueous solution, represents a new strategy for the design of highly selective probes for fluoride functioning in an aqueous solution.

Straightforward Design of Fluorescent Receptors for Sulfate: Study of Non-Covalent Interactions Contributing to Host-Guest Formation.

A straightforward design of receptors for binding and sensing of sulfate in aqueous medium was developed. The design involves the connection of two naphthalimide-based pH probes through a hydrogen-bonding motif. The structure of the receptor-sulfate complex, predicted by DFT calculations, was unambiguously confirmed by NMR measurements. There are three major interactions stabilizing the host-guest complex: electrostatic interactions, hydrogen bonding, and stacking interactions of the dyes. Study of two control receptors containing either one dye or methyl amide groups instead of amides, revealed that electrostatic and hydrogen bonding interactions contribute the most to affinity and selectivity of receptors. The receptors can detect sulfate in a 1:1 THF-buffer mixture in pH window 3.6-4.5 demonstrating up to 7-fold fluorescence enhancement. To the best of our knowledge, the reported PET (photoinduced electron transfer) anion probes possess the largest response for sulfate in aqueous solution yet described.