Hydrosulfide (HS- ) Recognition and Sensing in Water by Halogen Bonding Hosts

Phosphate selective binding and sensing by halogen bonding tripodal copper(II) metallo-receptors in aqueous media

Combining the potency of non-covalent halogen bonding (XB) with metal ion coordination, the synthesis and characterisation of a series of hydrophilic XB tripodal Cu(II) metallo-receptors, strategically designed for tetrahedral anion guest binding and sensing in aqueous media is described. The reported metallo-hosts contain a tripodal C3-symmetric tris-iodotriazole XB donor anion recognition motif terminally functionalised with tri(ethylene glycol) and permethylated β-cyclodextrin functionalities to impart aqueous solubility. Optical UV-vis anion binding studies in combination with unprecedented quantitative EPR anion titration investigations reveal the XB Cu(II) metallo-receptors exhibit strong and selective phosphate recognition over a range of other monocharged anionic species in competitive aqueous solution containing 40% water, notably outperforming a hydrogen bonding (HB) Cu(II) metallo-receptor counterpart. Electrochemical studies demonstrate further the capability of the metallo-receptors to sense anions via significant cathodic perturbations of the respective Cu(II)/Cu(I) redox couple.

An overview of analytical methods for detecting endogenous hydrogen sulfide (H2S) in plants

The significance of hydrogen sulfide (H2S) as a crucial gasotransmitter has been shown extensively in plants, and endogenous H2S is often modulated to activate H2S signaling when plants respond to numerous developmental and environmental cues. Consequently, elucidating the H2S physiological concentrations and the H2S generation intensity of plants is key to understanding the activation mechanism of H2S signaling, which has attracted increasing attention. Currently, a variety of reaction-based methods have been reported for monitoring H2S concentration in vivo and in vitro. In this review, we summarize and describe in detail several methods for quantifying and bioimaging endogenous H2S in plants systems, mainly the spectrophotometer-dependent methylene blue (MB) method and fluorescence probes, including the reaction mechanisms, design strategies, response principles, and application details. Moreover, we also summarize the advantages and disadvantages of these methods as well as the research scenarios in which they are applicable. We expect that this review will provide some guidelines on the selection of methods for H2S sensing and the comprehensive investigations into H2S signaling in plants.

Activity-Based Fluorescent Probes for Hydrogen Sulfide and Related Reactive Sulfur Species

Hydrogen sulfide (H2S) is not only a well-established toxic gas but also an important small molecule bioregulator in all kingdoms of life. In contemporary biology, H2S is often classified as a "gasotransmitter," meaning that it is an endogenously produced membrane permeable gas that carries out essential cellular processes. Fluorescent probes for H2S and related reactive sulfur species (RSS) detection provide an important cornerstone for investigating the multifaceted roles of these important small molecules in complex biological systems. A now common approach to develop such tools is to develop "activity-based probes" that couple a specific H2S-mediated chemical reaction to a fluorescent output. This Review covers the different types of such probes and also highlights the chemical mechanisms by which each probe type is activated by specific RSS. Common examples include reduction of oxidized nitrogen motifs, disulfide exchange, electrophilic reactions, metal precipitation, and metal coordination. In addition, we also outline complementary activity-based probes for imaging reductant-labile and sulfane sulfur species, including persulfides and polysulfides. For probes highlighted in this Review, we focus on small molecule systems with demonstrated compatibility in cellular systems or related applications. Building from breadth of reported activity-based strategies and application, we also highlight key unmet challenges and future opportunities for advancing activity-based probes for H2S and related RSS.

Reversible Hydrosulfide (HS-) Binding Using Exclusively C-H Hydrogen-Bonding Interactions in Imidazolium Hosts

H2S is a physiologically important signaling molecule with complex roles in biology and exists primarily as HS- at physiological pH. Despite this anionic character, few investigations have focused on the molecular recognition and reversible binding of this important biological anion. Using a series of imidazole and imidazolium host molecules, we investigate the role of preorganization and charge on HS- binding. Using a macrocyclic bis-imidazolium receptor, we demonstrate the unexpected 2:1 host-guest binding of HS-, which was characterized both in solution and by X-ray crystallography. To the best of our knowledge, this is the first example of this binding stoichiometry for HS- binding. Moreover, the short C-H···S distances of 2.53, 2.54, 2.76, and 2.79 Å are well within the sum of the van der Waals radii of the interacting atoms, which is consistent with strong C-H···S interactions.

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.

Construction of Halogen-Bonded Organic Frameworks (XOFs) as Novel Efficient Iodinating Agents

The structural diversity and the various applications of organic frameworks have attracted much attention in recent years. Recently, halogen-bonded organic frameworks (XOFs) became a novel member of these materials, thereby facilitating the exploration of the interesting structures as well as functions. Here we present two types of [N···I⁺···N] connected XOFs (XOF-TPy and XOF-TPEB) with two tridentate ligands as building blocks. XOF-TPy and XOF-TPEB were characterized by ¹H NMR, UV-vis, X-ray photoelectron spectroscopy (XPS), IR, SEM, and HR-TEM. Two-dimensional (2D) structural models were established based on powder X-ray diffraction (PXRD) data and theoretical simulations. Further experiment showed that these XOFs were excellent iodinating agents for the substituted arylboronic acids with either the electron-donating or electron-withdrawing groups upon heating without any catalyst. This research not only brings further understanding to the XOFs but also extends the applications of XOFs.

A Fluorescent Cage for Supramolecular Sensing of 3‐Nitrotyrosine in Human Blood Serum

3‐Nitrotyrosine (NT) is generated by the action of peroxynitrite and other reactive nitrogen species (RNS), and as a consequence it is accumulated in inflammation‐associated conditions. This is particularly relevant in kidney disease, where NT concentration in blood is considerably high. Therefore, NT is a crucial biomarker of renal damage, although it has been underestimated in clinical diagnosis due to the lack of an appropriate sensing method. Herein we report the first fluorescent supramolecular sensor for such a relevant compound: Fluorescence by rotational restriction of tetraphenylethenes (TPE) in a covalent cage is selectively quenched in human blood serum by 3‐nitrotyrosine (NT) that binds to the cage with high affinity, allowing a limit of detection within the reported physiological concentrations of NT in chronic kidney disease.

Halogen bonding and chalcogen bonding mediated sensing

Sigma-hole interactions, in particular halogen bonding (XB) and chalcogen bonding (ChB), have become indispensable tools in supramolecular chemistry, with wide-ranging applications in crystal engineering, catalysis and materials chemistry as well as anion recognition, transport and sensing. The latter has very rapidly developed in recent years and is becoming a mature research area in its own right. This can be attributed to the numerous advantages sigma-hole interactions imbue in sensor design, in particular high degrees of selectivity, sensitivity and the capability for sensing in aqueous media. Herein, we provide the first detailed overview of all developments in the field of XB and ChB mediated sensing, in particular the detection of anions but also neutral (gaseous) Lewis bases. This includes a wide range of optical colorimetric and luminescent sensors as well as an array of electrochemical sensors, most notably redox-active host systems. In addition, we discuss a range of other sensor designs, including capacitive sensors and chemiresistors, and provide a detailed overview and outlook for future fundamental developments in the field. Importantly the sensing concepts and methodologies described herein for the XB and ChB mediated sensing of anions, are generically applicable for the development of supramolecular receptors and sensors in general, including those for cations and neutral molecules employing a wide array of non-covalent interactions. As such we believe this review to be a useful guide to both the supramolecular and general chemistry community with interests in the fields of host-guest recognition and small molecule sensing. Moreover, we also highlight the need for a broader integration of supramolecular chemistry, analytical chemistry, synthetic chemistry and materials science in the development of the next generation of potent sensors.

Hydrogen sulphide-triggered theranostic prodrugs based on the dynamic chemistry of tetrazines

Dynamic nucleophilic aromatic substitution of tetrazines (S_NTz) has been employed to build theranostic prodrugs that are activated by hydrogen sulfide. H₂S is typically found in high concentrations in some kinds of cancer cells and it is able to trigger the disassembly of tetrazine prodrugs. In such a way, a dual release of drugs and/or fluorescent compounds can be selectively triggered.