Utilizing a pH-Sensitive Dye in the Selective Fluorescent Recognition of Sulfate

Anion-templated synthesis of a switchable fluorescent [2]catenane with sulfate sensing capability

Anion templation strategies have facilitated the synthesis of various catenane and rotaxane hosts capable of strong and selective binding of anions in competitive solvents. However, this approach has primarily relied on positively charged precursors, limiting the structural diversity and the range of potential applications of the anion-templated mechanically interlocked molecules. Here we demonstrate the synthesis of a rare electroneutral [2]catenane using a powerful, doubly charged sulfate template and a complementary diamidocarbazole-based hydrogen bonding precursor. Owing to the unique three-dimensional hydrogen bonding cavity and the embedded carbazole fluorophores, the resulting catenane receptor functions as a sensitive fluorescent turn-ON sensor for the highly hydrophilic sulfate, even in the presence of a large excess of water. Importantly, the [2]catenane exhibits enhanced binding affinity and selectivity for sulfate over its parent macrocycle and other acyclic diamidocarbazole-based receptors. We demonstrate also, for the first time, that the co-conformation of the catenane may be controlled by reversible acid/base induced protonation and deprotonation of the anionic template, SO42-. This approach pioneers a new strategy to induce molecular motion of interlocked components using switchable anionic templates.

Highly sensitive and selective off-on fluorescent platform for tricresyl phosphate flame retardant based on twisted intramolecular charge transfer probe

BACKGROUND

Tricresyl phosphate (TCP), a typical organic phosphorus flame retardant (OPFR), is an emerging pollutant that causes great concern in recent years due to its high neurotoxicity and reproductive toxicity, etc. Conventional analysis methods for TCP such as gas chromatography and liquid chromatography-tandem mass spectrometry exhibit high sensitivity and accuracy. However, these techniques generally suffer from certain limitations, such as high cost, bulky equipment, time-consuming and operator-dependent properties. Therefore, the establishment of fast and efficient analytical methods for TCP still remains a great challenge.

RESULT

A "turn on" fluorescence sensing strategy for the efficient detection of TCP was established, based on a unique molecular rotor probe of 9-(2,2-dicyanovinyl)-julolidine (DCVJ). The introduction of TCP led to a significant enhancement of the fluorescence intensity of DCVJ. The results show that twisted intramolecular charge transfer (TICT) process might play an important role for this enhancement of fluorescence response via dynamic light scattering measurements and fluorescence lifetime analysis. Further investigations demonstrate that the hydrophobic interaction and conjugation effect between DCVJ and TCP constrain the molecular rotation and vibration of DCVJ, thereby regulating the TICT process, which contribute to this intriguing "turn on" behavior. In view of this, a new sensing platform with excellent performance for TCP was established, which offers quick response time, high selectivity, wide linear range (20-1200 ng mL-1, 1600-8000 ng mL-1), and low detection limit (4.82 ng mL-1).

SIGNIFICANCE

The established new sensing platform for TCP demonstrates the advantages of simplicity, high efficiency, excellent sensitivity and selectivity. The obtained results are also superior to some other previously reported fluorescence methods. This work opens up a new perspective for the efficient detection of emerging OPFRs pollutants.

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.

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.

Selective turn-on fluorescence sensing of sulfate in aqueous-organic mixtures by an uncharged bis(diamidocarbazole) receptor

A linear, uncharged, hydrogen bonding receptor A with two carbazole-based binding domains was synthesised and evaluated for its anion binding properties in DMSO/H₂O mixtures. ¹H NMR titrations revealed that, in DMSO/H₂O 0.5%, A forms both 1 : 1 and 1 : 2 complexes with SO₄^2-, H₂PO₄^-, PhCOO^- and Cl^-. In 1 : 1 complexes the receptor encloses the tetrahedral anions tightly, forming a helical structure, while Cl^- binds with a single carbazole unit only. In the presence of 10% of water the 1 : 2 complexes with SO₄^2- and PhCOO^- disappear, and the respective 1 : 1 binding constants decrease sufficiently to be quantified by UV-Vis titration. In this highly competitive medium, A binds sulfate with K_1:1 = 10^5.47 M^-1, i.e., it binds approx. 30, 360 and >1000 times more strongly than H₂PO₄^-, PhCOO^- and Cl^-, respectively. Furthermore, the association with sulfate is over 50 times stronger than that for a model diamidocarbazole 1 under identical conditions, suggesting a very strong chelating effect due to the diglycoyl linker. Increasing the amount of water to 25% (the solubility limit of A) lowers the 1 : 1 binding constant with SO₄^2- to 10^3.73 M^-1. Receptor A was shown to act as a selective turn-on fluorescent sensor for sulfate, able to sense sulfate in sulfate-rich mineral water.

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.

Naphthalimide-Based Polyammonium Chemosensors for Anions: Study of Binding Properties and Sensing Mechanisms

New naphthalimide-based receptors for anions have been synthesized. Efficient synthetic routes have been discovered to functionalize the naphthalimide core with branched polyamines. Binding and sensing properties of the receptors were studied by potentiometric, NMR and fluorescence titrations. The receptors bind selectively to the pyrophosphate anion in buffered aqueous solutions. The receptors with more than six amine groups in the structure demonstrated the highest affinities for pyrophosphate. The fluorescence response towards anions was found to be dependent on the position of the amine groups relative to the naphthalimide core, and on the pH of the buffered solution. Three sensing mechanisms have been found that explain fluorescence responses of receptors towards anions in an aqueous solution.

Pyrrole N-H Anion Complexes

Synthetic pyrrole-based anion receptors date back to the 1990s. They have been extensively developed in the context of macrocyclic systems as expanded porphyrins and calixpyrroles, and related systems. The chemistry of open-chain pyrrolic systems is, in many respects, no less venerable. It also has more direct analogy to naturally occurring pyrrole-based anion binding motifs. However, it has not been the subject of a comprehensive review. Presented herein is a summary of efforts devoted to the creation of de novo pyrrole-based receptors, as well as the anion recognition chemistry of naturally occurring pyrrolic systems as prodigiosins and their synthetic analogues.