Surface-Initiated Reversible Addition-Fragmentation Chain Transfer Polymerization (SI-RAFT) to Produce Molecularly Imprinted Polymers on Graphene Oxide for Electrochemical Sensing of Methylparathion

A nonenzymatic redox-responsive sensor was put forward for the detection of methylparathion (MP) by designing globular nanostructures of molecularly imprinted polymers on graphene oxide (GO@MIPs) via surface-initiated reversible addition-fragmentation chain transfer polymerization (SI-RAFT). Fourier transform infrared (FTIR) spectroscopy, field emission scanning electron microscopy (FESEM), and small-angle X-ray scattering (SAXS) studies have confirmed the successful formation of receptor layers of MIPs on RAFT agent-functionalized GO sheets. The electrochemical signal with an amplified current response was attained because of the enhanced diffusion rate of ions at the interface provided by widening the pore size of the MIP film. The analytical response of GO@MIPs, validated by recording square-wave anodic stripping voltammetry (SWASV) at varying MP concentrations, followed the linear response between 0.2 and 200 ng/mL. Under optimized conditions, the sensor exhibited a limit of detection of 4.25 ng/mL with high selectivity over other interfering ions or molecules. The anti-interfering ability and good recovery (%) in food samples directed the use of the proposed sensor toward real-time monitoring and also toward future mimicking of surfaces.

Selective binding of oxalate by a tris-ureido calix[6]tube in a protic environment

Due to their significant role in industry and biological systems, the interest in selectively recognizing and detecting small dicarboxylates has grown in recent years. In this study, we report on the binding properties of a family of tubular-shaped heterotritopic receptors based on bis-calix[6]arenes, which contain three (thio)urea bridges (C3U and C3TU) or six urea bridges (C6U), toward dicarboxylates. While poor binding properties were observed by NMR for the newly synthesized C6U, receptors C3U and C3TU exhibited a unique ability to cooperatively complex a dicarboxylate anion sandwiched between two ammonium ions. The three ions are complexed in contact and aligned within the tubular shape of the receptor, forming cascade complexes that are stable even in a competitive environment. The different binding properties between the receptors were rationalized in terms of size, flexibility, H-bond donor ability, and intramolecular H-bonding within the anion binding pocket between the calixarene cavities. With C3U, a rare selectivity for oxalate over other small dicarboxylates and various bicharged anions was observed. Molecular modeling of the cascade complex indicated that the oxalate anion is stabilized by an array of hydrogen bonds with the urea bridges of the receptor and both propylammonium cations nested within the calixarene cavities.

Solution- and gas-phase study of binding of ammonium and bisammonium hydrocarbons to oxacalix[4]arene carboxylate

Oxacalixarenes represent a distinctive class of macrocyclic compounds, which are closely related to the parent calixarene family, offering binding motifs characteristic of calixarenes and crown ethers. Nevertheless, they still lack extensive characterization in terms of molecular recognition properties and the subsequent practical applicability. We present here the results of binding studies of an oxacalix[4]arene carboxylate macrocycle toward a variety of organic ammonium cationic species. Our results show that the substituents attached to the guest ammonium compound largely influence the binding strengths of the host. Furthermore, we show that the characteristic binding pattern changes upon transition from the gas phase to solution in terms of the governing intermolecular interactions. We identify the key factors affecting host-guest binding efficacy and suggest rules for the important molecular structural motifs of the interacting parts of ammonium guest species and the macrocycle to facilitate sensing of ammonium cations.

Anthraquinone-1,8-Derived (Pseudo-) Crown and Lariat Ethers: Design and Applications as Fluorescent and Chromogenic Ion (Pair) Sensors

Cyclic polyamine/ethers embedded with anthraquinone moieties and functional pendants, are structural analogues of crown ethers and (oxo-) cyclams, and could be utilized as sensitive and selective chemosensors towards metal cations. Those pseudo- (similar but geometrically distinct) crown and lariat ethers show various cation-binding patterns and stoichiometry, being modulated by donor type, cavity size and pendants' chelating ability. The luminescent and chromogenic properties also differ a lot along with the derivation of the parental macrocycle. Methodological designing including synthesis and post-functionalization through nucleophilic substitution, Mannich condensation etc., as well as the sensing performance of those pseudo- crown and lariat ethers are summarized in this review, basing on the spectroscopic, voltammetric and X-ray crystallographic determinations. Anion effect in sensing cations is evaluated according to the ion-pair recognition theory. Those results shed some light on exemplifying the anions' role in bioinorganic systems including metalloenzymes.

Triazolated calix[4]semitubes: assembling strategies towards long multicalixarene architectures

Cone and 1,3-alternate calix[4]arenes bearing pairs of 2-azidoethyl or propargyl groups, and 1,3-alternate calix[4]arenes having four 2-azidoethyl, four propargyl groups or pairs of 2-azidoethyl and silylated propargyl groups were explored...

Heteroditopic Calix[6]arene Based Intervowen and Interlocked Molecular Devices

Since the dawn of supramolecular chemistry, calixarenes have been employed as platforms onto which functional groups and binding sites can be loaded in a regio- and stereocontrolled manner for the recognition of charged and neutral species. Despite their wider annulus, potentially suitable to bind larger guests, the larger members of the calixarene series have been relatively less employed, mainly because of the synthetic difficulties to control their conformational flexibility and their regioselective functionalization. In this account, we will present the achievements gained during the last two decades on the use of the calix[6]arene as a platform to build-up structures in which the macrocycle acts as a wheel for the synthesis of oriented (pseudo)rotaxanes. We also account on how these calix[6]arene hosts affect the reactivity or spectroscopic properties of their bound guests.

Molecular Recognition of Zwitterions with Artificial Receptors

Many biomolecules exist as internal ion pairs or zwitterions within a biologically relevant pH range. Despite their importance, the molecular recognition of this type of systems is specially challenging due to their strong solvation in aqueous media, and their trend to form folded or self-assembled structures by pairing of charges of different sign. In this Minireview, we will discuss the molecular recognition of zwitterions using non-natural, synthetic receptors. This contribution does not intend to make a full in-depth revision of the existing research in the field, but a personal overview with selected representative examples from the recent literature.

Tuning the Fluorescence Through Reorientation of the Axle in Calix[6]arene-Based Pseudorotaxanes

This work describes a calix[6]arene-based wheel that binds, in non-polar media, a stilbazolium salt to yield a mixture of pseudorotaxane orientational isomers. The isomer's abundance ratio evolves with time and can be reversibly tuned by adjusting the temperature. The spectroscopic properties, and notably the emission spectrum, of the bound guest depend on its orientation inside the non-palindromic wheel, suggesting such a system as a switch with spectroscopic readout.

Ditopic Receptors Based on Dihomooxacalix[4]arenes Bearing Phenylurea Moieties With Electron-Withdrawing Groups for Anions and Organic Ion Pairs

Two bidentate dihomooxacalix[4]arene receptors bearing phenylurea moieties substituted with electron-withdrawing groups at the lower rim via a butyl spacer (CF₃-Phurea 5b and NO₂ Phurea 5c) were obtained in the cone conformation in solution, as shown by NMR. The X-ray crystal structure of 5b is reported. The binding affinity of these receptors toward several relevant anions was investigated by ¹H NMR, UV-Vis absorption in different solvents, and fluorescence titrations. Compounds 5b and 5c were also tested as ditopic receptors for organic ion pairs, namely monoamine neurotransmitters and trace amine hydrochlorides by ¹H NMR studies. The data showed that both receptors follow the same trend and, in comparison with the unsubstituted phenylurea 5a, they exhibit a significant enhancement on their host-guest properties, owing to the increased acidity of their urea NH protons. NO₂-Phurea 5c is the best anion receptor, displaying the strongest complexation for F⁻, closely followed by the oxoanions BzO⁻, AcO⁻, and HSO4-. Concerning ion pair recognition, both ditopic receptors presented an outstanding efficiency for the amine hydrochlorides, mainly 5c, with association constants higher than 10⁹ M⁻² in the case of phenylethylamine and tyramine.

Macrocycles as Ion Pair Receptors

Cation and anion recognition have both played central roles in the development of supramolecular chemistry. Much of the associated research has focused on the development of receptors for individual cations or anions, as well as their applications in different areas. Rarely is complexation of the counterions considered. In contrast, ion pair recognition chemistry, emerging from cation and anion coordination chemistry, is a specific research field where co-complexation of both anions and cations, so-called ion pairs, is the center of focus. Systems used for the purpose, known as ion pair receptors, are typically di- or polytopic hosts that contain recognition sites for both cations and anions and which permit the concurrent binding of multiple ions. The field of ion pair recognition has blossomed during the past decades. Several smaller reviews on the topic were published roughly 5 years ago. They provided a summary of synthetic progress and detailed the various limiting ion recognition modes displayed by both acyclic and macrocyclic ion pair receptors known at the time. The present review is designed to provide a comprehensive and up-to-date overview of the chemistry of macrocycle-based ion pair receptors. We specifically focus on the relationship between structure and ion pair recognition, as well as applications of ion pair receptors in sensor development, cation and anion extraction, ion transport, and logic gate construction.

Shape-Selective Recognition of Quaternary Ammonium Chloride Ion Pairs

Synthetic receptors that recognize ion pairs are potentially useful for many technical applications, but to date there has been little work on selective recognition of quaternary ammonium (Q⁺) ion pairs. This study measured the affinity of a tetralactam macrocycle for 11 different Q⁺·Cl^- salts in chloroform solution. In each case, NMR spectroscopy was used to determine the association constant ( K_a) and the structure of the associated complex. K_a was found to depend strongly on the molecular shape of Q⁺ and was enhanced when Q⁺ could penetrate the macrocycle cavity and engage in attractive noncovalent interactions with the macrocycle's NH residues and aromatic sidewalls. The highest measured K_a of 7.9 × 10³ M^-1 was obtained when Q⁺ was a p-CN-substituted benzylic trimethylammonium. This high-affinity Q⁺·Cl^- ion pair was used as a template to enhance the synthetic yield of macrocyclization reactions that produce the tetralactam receptor or structurally related derivatives. In addition, a permanently interlocked rotaxane was prepared by capping the end of a noncovalent complex composed of the tetralactam macrocycle threaded by a reactive benzylic cation. The synthetic method provides access to a new family of rotaxanated ion pairs that can likely act as anion sensors, molecular shuttles, or transport molecules.