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

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.

A naphthalimide functionalized chitosan-based fluorescent probe for specific detection and efficient adsorption of Cu2

As one of the most abundant metal ions, Cu²⁺ has turned into a great threat to human health and the natural environment due to its widely utilized in various industries. In this paper, a chitosan-based fluorescent probe CTS-NA-HY for detection and adsorption of Cu²⁺ was rationally prepared. CTS-NA-HY exhibited a specific "turn off" fluorescence response to Cu²⁺ and the fluorescence color changed from bright yellow to colorless. It possessed satisfactory detection performance to Cu²⁺ including good selectivity and anti-interference, low detection limit (29 nM) and wide pH range (4-9). The detection mechanism was confirmed by Job's plot, X-ray photoelectron spectroscopy, FT-IR and ¹H NMR analysis. Additionally, the probe CTS-NA-HY was capacity of determining Cu²⁺ in environmental water and soil samples. Besides, CTS-NA-HY-based hydrogel could also remove Cu²⁺ in aqueous solution effectively, which the ability of adsorption was greatly improved compared with original chitosan hydrogel.

Probing the Role of the Bridging Nitrogen in the Signaling Mechanism of an Anthracene-Boronic Acid Sugar Sensor and a Different Version of the PET-Based Mechanism

The N-quaternized derivative 5 of the James-Shinkai anthracene-boronic acid fluorescence sugar sensor 1 was prepared to probe the role of the bridging nitrogen in the signaling mechanism of 1. Both 5 and 1 contain positively charged bridging groups NMe⁺ or NH⁺, respectively, but 5 lacks the ability to form the intramolecular ammonium-boronate doubly ionic hydrogen bond present in 1. Receptors 1 and 5 display opposite fluorescence vs pH profiles with a small turn-on effect of the sugar binding to the zwitterion of 5 in contrast to a large effect observed with 1. It is concluded that the ammonium-boronate hydrogen bond is essential for the signaling mechanism of 1. Its possible function is enabling the PET quenching effect by shifting the NH⁺ proton toward boronate anion inside the hydrogen bond, the degree of which is modulated by the ester formation with diols affecting the basicity of boronate anion. This mechanism agrees with observed signaling selectivity of 1 toward a series of di- and polyols of variable structures as well as with the behavior of 1 in buffered D₂O and methanol solvents at controlled pH and provides an addition to the established "loose bolt" mechanism signaling mode essential for receptors with nonpolar fluorophores.

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.

Sharing the salt bowl: Counterion identity drives N-alkyl resorcinarene affinity for pyrophosphate in water

N-Alkyl ammonium resorcinarene chloride receptors, NARX4, have been shown to act as high-sensitivity detectors of pyrophosphate (PPi), a biomarker of disease, in aqueous media through the chloride-to-PPi exchange [NAR(Cl)4 to...

Fluorescent Chemosensors Based on Polyamine Ligands: A Review

Polyamine ligands are water-soluble receptors that are able to coordinate, depending on their protonation degree, either metal ions, anionic, or neutral species. Furthermore, the presence of fluorescent signaling units allows an immediate visual response/signal. For these reasons, they can find applications in a wide variety of fields, mainly those where aqueous media is necessary, such as biological studies, wastewater analysis, soil contamination, etc. This review provides an overview of the recent developments in the research of chemosensors based on polyamine ligands functionalized with fluorescent signaling units. The discussion focuses on the design, synthesis, and physicochemical properties of this type of fluorescent chemosensors in order to analyze the applications associated to the sensing of metal ions, anions, and neutral molecules of environmental and/or biological interest. To facilitate a quick access and overview of all the chemosensors covered in this review, a summary table of the chemosensor structures and analytes, with all the corresponding references, is also presented.

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.

Bis-N-Heterocyclic Carbene Complexes of Coinage Metals Containing Four Naphthalimide Units: A Structure-Emission Properties Relationship Study

Naphthalimide derivatives provide highly versatile self-assembled systems and aggregated forms with fascinating emission properties that make them potential candidates for many applications such as bioimaging and sensing. Although various aggregated species of naphthalimide derivatives have been well documented, little is known about the correlation between their structure and photophysical properties. Here the preparation of a series of tetrameric naphthalimide molecules in which naphthalimide units are linked by bis-N-heterocyclic carbene complexes of coinage metals is described. An in-depth structural investigation into these tetramers has been carried out in solution and the solid state using spectroscopic methods, X-ray crystallography, and computational methods. The experimental and calculated data indicate that the magnitude of the intramolecular interchromophoric π-interactions increases either by an increase in the metal ionic radius or on going from the solid to the solution state. These tetrameric naphthalimide compounds show intramolecular excimeric emissions in the solid and solution phases. However, the quantum yield efficiencies of these excimeric emissions show a trend similar to that for the intramolecular π-interactions either by going from the solution to the solid state or with an increase in the metal ionic radius. Surprisingly, the amine derivative analogues of the silver(I) compound showed an unusual increase in the emission quantum yield efficiency to 92% in solution due to intramolecular hydrogen bonds between amine substituents on adjacent naphthalimde units.

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.

Synthesis, Photophysical Characterization, and Sensor Activity of New 1,8-Naphthalimide Derivatives

Three new 1,8-naphthalimide derivatives M1-M3 with different substituents at the C-4 position have been synthesized and characterized. Their photophysical properties have been investigated in organic solvents of different polarity, and their fluorescence intensity was found to depend strongly on both the polarity of the solvents and the type of substituent at C-4. For compounds M1 and M2 having a tertiary amino group linked via an ethylene bridge to the chromophore system, high quantum yield was observed only in non-polar media, whereas for compound M3, the quantum efficiency did not depend on the medium polarity. The effect of different metal ions (Ag⁺, Ba²⁺, Cu²⁺, Co²⁺, Mg²⁺, Pb²⁺, Sr²⁺, Fe³⁺, and Sn²⁺) on the fluorescence emission of compounds M1 and M2 was investigated. A significant enhancement has been observed in the presence of Ag⁺, Pb²⁺, Sn²⁺, Co²⁺, Fe³⁺, as this effect is expressed more preferably in the case of M2. Both compounds have shown significant pH dependence, as the fluorescence intensity was low in alkaline medium and has been enhanced more than 20-fold in acidic medium. The metal ions and pH do not affect the fluorescence intensity of M3. Density-functional theory (DFT) and Time-dependent density-functional theory (TDDFT) quantum chemical calculations are employed in deciphering the intimate mechanism of sensor mechanism. The functional properties of M1 and M2 were compared with polyamidoamine (PAMAM) dendrimers of different generations modified with 1,8-naphthalimide.

Fluorescence detection of phosphonates in water by a naphthalimide-based receptor and its derived cryopolymers

The concentration of compound M in cryopolymers has a dramatic influence on the fluorescence response in the presence of phosphonates.

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.

A dual-responsive luminescent metal–organic framework as a recyclable luminescent probe for the highly effective detection of pyrophosphate and nitrofurantoin

Luminescent ZTMOF-1 can discriminately detect PPi and NFT with high selectivity, sensitivity and stability.

Versatile fluorophores for bioimaging applications: π-expanded naphthalimide derivatives with skeletal and appendage diversity

Four novel fluorescent cores bearing a transformable functional group based on a π-expanded naphthalimide including a fused pyranone or furan ring have been prepared.