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

Molecular Hosts for the Sensing and Separation of 99TcO4. Chemistry 2024:e202401551. [PMID: 38779975 DOI: 10.1002/chem.202401551] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

In recent years, European Union member states have hastened energy policy deliberations to address supply and sustainability concerns, placing a significant emphasis on nuclear energy as a means to achieve decarbonization goals. However, despite its significant role in power generation, nuclear energy faces significant challenges linked to fuel reprocessing and waste disposal, that hinder its broader expansion. In this context, the separation of technetium represents a concerning issue. Indeed, technetium's catalytic activity can impede the extraction of uranium, neptunium, and plutonium, affecting waste reprocessing efficiency. Additionally, the stable form of technetium in aerobic conditions, pertechnetate (TcO4 -), poses risks of groundwater contamination due to its mobility and solubility. Hence, sensing and separation of TcO4 - is imperative for both nuclear fuel processing and minimising radioactive contamination in the environment. However, the binding of TcO4 - and its separation from contaminated solutions present challenges due to the acidic (or basic) waste components and the high ionic strength in real matrices. Supramolecular chemists have addressed these issues by designing receptors inspired by molecular recognition principles. This article explores recent advancements and future directions in TcO4 - sensing and separation (using extraction and sorption) with a focus on molecular hosts. Metal-organic receptors will also be discussed.

A Semiflexible Tetrahydrazone Macrocycle for Binding of Pyrophosphate and Smaller Anions

Macrocyclization has proven to be a useful design strategy in the development of efficient anion receptors. In addition to the ring size, the overall preorganization due to structural rigidity is key. To explore this in the context of developing an efficient pyrophosphate receptor, three macrocycles featuring a 26-membered interior ring size and similar H-bonding motifs have been synthesized, and their anion binding ability has been investigated. Computational studies and nuclear magnetic resonance (NMR) data showed different degrees of preorganization as a result of differences in flexibility. The interaction of the three macrocycles with chloride, dihydrogen phosphate, and dihydrogen pyrophosphate was investigated in solution by NMR and ultraviolet-visible spectroscopy and in the solid state by X-ray crystallography. The tetrahydrazone-based macrocycle featuring intermediate flexibility exhibited the best affinity for all three anions investigated. Our results suggest that in addition to the proper preorganization of binding groups in a macrocycle a certain degree of flexibility is also required for an optimal affinity with the target guest.

H-bond engineering as a general strategy for inhibiting twisted intramolecular charge transfer in donor-acceptor fluorescent probes: Reshaping the pre-twisting method

Twisted intramolecular charge transfer (TICT) is a fluorescence quenching mechanism that occurs in donor-acceptor (D‒A) molecules. Chemical engineering research into TICT regulation over the past 50 years has primarily focused on manipulating steric factors by introducing alkyl groups at the D-A junction (pre-twisting). Herein, we report a significant advance in TICT-based probes through the introducing of H-bond as an efficient strategy for suppressing TICT. Accordingly, ortho-Cl installation in the N-phenylpyrazine-2-carboxamide (PPC) platform can achieve complete reversal from the quenching mode to the light-up mode. This specific H-bonding (N-H⋯Cl) effectively blocks N-C(Ar) bond rotation, leading to fluorescence-ON. This suggested that TICT inhibition may be involved. Therefore, in a sharp contrast to the general nature of the pre-twisting method in rotor molecules, which involves incorporating steric hindrance at either the donor or acceptor moiety to enhance intramolecular rotation (promotion TICT), the ortho-H bonding strategy completely freezes D‒A bond twisting (suppression TICT), resulting in improved fluorescent intensity. Furthermore, the fluorophores were evaluated for Hg2+ detection and in vivo bio-imaging. Notably, Hg-complexation induced another fluorescence inversion (OFF-ON) by imposing spatial constraints on twisting freedom in 3,4-Cl-PPC. Taken together, this work provides a valid and generalizable tactic for the development of high-performance sensing fluorophores through inhibition of TICT.

Lab-on-a-molecule and multi-analyte sensing

The concept of a lab-on-a-molecule, which was proposed just short of two decades ago, has captured the imagination of scientists. From originally being proposed as an AND logic gate driven by three chemical inputs as a direct way of detecting congregations of chemical species, the definition of what constitutes a lab-on-a-molecule has broadened over the years. In this review, molecules that can detect multiple analytes by fluorescence, among other techniques, are reviewed and discussed, in the context of molecular logic and multi-analyte sensing. The review highlights challenges and suggestions for moving the frontiers of research in this field to the next dimension.

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.

Anion Sensing through Redox-Modulated Fluorescent Halogen Bonding and Hydrogen Bonding Hosts

Anion sensing via either optical or electrochemical readouts has separately received enormous attention, however, a judicious combination of the advantages of both modalities remains unexplored. Toward this goal, we herein disclose a series of novel, redox-active, fluorescent, halogen bonding (XB) and hydrogen bonding (HB) BODIPY-based anion sensors, wherein the introduction of a ferrocene motif induces remarkable changes in the fluorescence response. Extensive fluorescence anion titration, lifetime and electrochemical studies reveal anion binding-induced emission modulation through intramolecular photoinduced electron transfer (PET), the magnitude of which is dependent on the nature of both the XB/HB donor and anion. Impressively, the XB sensor outperformed its HB congener in terms of anion binding strength and fluorescence switching magnitude, displaying significant fluorescence turn-OFF upon anion binding. In contrast, redox-inactive control receptors display a turn-ON response, highlighting the pronounced impact of the introduction of the redox-active ferrocene on the optical sensing performance. Additionally, the redox-active ferrocene motif also serves as an electrochemical reporter group, enabling voltammetric anion sensing in competitive solvents. The combined advantages of both sensing modalities were further exploited in a novel, proof-of-principle, fluorescence spectroelectrochemical anion sensing approach, enabling simultaneous and sensitive read out of optical and electrochemical responses in multiple oxidation states and at very low receptor concentration.

Thiourea-Based Receptors for Anion Recognition and Signaling

This work reports on two thiourea-based receptors with pyridine and amine units including 1-naphthyl (MT1N) and 4-nytrophenyl (MT4N) as signaling units. For both compounds, their affinity and signaling ability toward various anions of different geometry and basicity in DMSO were studied using UV-vis, fluorescence, and 1H NMR techniques. Anion recognition studies revealed that both MT1N and MT4N have, in general, high affinities toward basic anions. In this regard, a higher acidity of the MT4N receptor was demonstrated. Furthermore, MT4N has a higher affinity for fluoride (log K1 = 5.98) than for the other anions and can effectively detect it through colorimetric changes that can be monitored by the UV-vis technique. The interaction between receptors and anions mainly involves the hydrogens of the amino and thiourea groups of the former. Complementary single-crystal X-ray diffraction studies and molecular modeling at the DFT level were also performed.

Recent applications of organic cages in sensing and separation processes in solution

Organic cages are three-dimensional polycyclic compounds of great interest in the scientific community due to their unique features, which generally include simple synthesis based on the dynamic covalent chemistry strategies, structural tunability and high selectivity. In this feature article, we present the advances over the last ten years in the application of organic cages as chemosensors or components in chemosensing devices for the determination of analytes (pollutants, analytes of biological interest) in complex aqueous media including wine, fruit juice, urine. Details on the recent applications of organic cages as selective (back-)extractants or masking agents for potential applications in relevant separation processes, such as the plutonium and uranium recovery by extraction, are also provided. Over the last ten years, organic cages with permanent porosity in the liquid and solid states have been highly appreciated as porous materials able to discriminate molecules of different sizes. These features, combined with good solvent processability and film-forming tendency, have proved useful in the fabrication of membranes for gas separation, solvent nanofiltration and water remediation processes. An overview of the recent applications of organic cages in membrane separation technologies is given.

Fluorescent styryl pyridine-N-oxide probes for imaging lipid droplets

Lipid droplets (LDs) have emerged as major regulators of cellular metabolism, encompassing lipid storage, membrane synthesis, viral replication, and protein degradation. Exclusive studies have suggested a direct link between LDs and cancer, as a notable abundance of LDs is found in cancerous cells. Therefore, monitoring the location, distribution, and movements of LDs is of paramount importance for understanding their involvement in biological processes. To target LDs, we designed and synthesized fluorophores with a styryl scaffold bearing electron-donating amino groups and pyridine-N-oxide, a zwitterionic acceptor moiety. We explored their photophysical properties in various solvents and conducted systematic DFT calculations on the synthesized fluorescent molecules, comparing them with neutral pyridine and cationic pyridinium styryl dyes. The results demonstrate that diphenylaminostyryl pyridine-N-oxide (TNO) shows excellent imaging of LDs, in contrast to the behavior of cationic styrylpyridinium (TNC), which primarily localizes within the mitochondria. Notably, pyridine N-oxide offers several benefits: an increased dipole moment facilitating charge separation between donors and acceptors, substantial HOMO and LUMO stabilization, improved water solubility, favorable redox properties, and bathochromic-shifted absorption/emission spectra, showing promise as a fluorescent tool for probing the cellular-biological realm.

Rational Design of the β-Bulge Gate in a Green Fluorescent Protein Accelerates the Kinetics of Sulfate Sensing

Detection of anions in complex aqueous media is a fundamental challenge with practical utility that can be addressed by supramolecular chemistry. Biomolecular hosts such as proteins can be used and adapted as an alternative to synthetic hosts. Here, we report how the mutagenesis of the β-bulge residues (D137 and W138) in mNeonGreen, a bright, monomeric fluorescent protein, unlocks and tunes the anion preference at physiological pH for sulfate, resulting in the turn-off sensor SulfOFF-1. This unprecedented sensing arises from an enhancement in the kinetics of binding, largely driven by position 138. In line with these data, molecular dynamics (MD) simulations capture how the coordinated entry and gating of sulfate into the β-barrel is eliminated upon mutagenesis to facilitate binding and fluorescence quenching.

Cinchona alkaloids - acid, anion-driven fluorescent INHIBIT logic gates with a receptor1-fluorophore-spacer-receptor2 format and PET and ICT mechanisms

The fluorescent natural products, quinine, quinidine, cinchonine and cinchonidine are demonstrated as H⁺-enabled, halide-disabled (Cl^-, Br^- or I^-) INHIBIT and INHIBIT-OR combinatorial logic gates in water. More fluorescent natural products with intrinsic logic properties await to be discovered.

Quinolinium-Based Fluorescent Probes for Dynamic pH Monitoring in Aqueous Media at High pH Using Fluorescence Lifetime Imaging

Spatiotemporal pH imaging using fluorescence lifetime imaging microscopy (FLIM) is an excellent technique for investigating dynamic (electro)chemical processes. However, probes that are responsive at high pH values are not available. Here, we describe the development and application of dedicated pH probes based on the 1-methyl-7-amino-quinolinium fluorophore. The high fluorescence lifetime and quantum yield, the high (photo)stability, and the inherent water solubility make the quinolinium fluorophore well suited for the development of FLIM probes. Due to the flexible fluorophore-spacer-receptor architecture, probe lifetimes are tunable in the pH range between 5.5 and 11. An additional fluorescence lifetime response, at tunable pH values between 11 and 13, is achieved by deprotonation of the aromatic amine at the quinolinium core. Probe lifetimes are hardly affected by temperature and the presence of most inorganic ions, thus making FLIM imaging highly reliable and convenient. At 0.1 mM probe concentrations, imaging at rates of 3 images per second, at a resolution of 4 μm, while measuring pH values up to 12 is achieved. This enables the pH imaging of dynamic electrochemical processes involving chemical reactions and mass transport.