Polymeric Membrane Electrodes Using Calix[4]pyrrole Bis/Tetra-Phosphonate Cavitands as Ionophores for Potentiometric Acetylcholine Sensing with High Selectivity

Flexible Acetylcholine Neural Probe with a Hydrophobic Laser-Induced Graphene Electrode and a Fluorous-Phase Sensing Membrane

This work develops the first laser-induced graphene (LIG)-based electrochemical sensor with a superhydrophobic fluorous membrane for a flexible acetylcholine (ACh) sensor. ACh regulates several physiological functions, including synaptic transmission and glandular secretion. The ACh sensing membrane is doped with a fluorophilic cation-exchanger that can selectively measure ACh based on the inherent selectivity of the fluorous phase for hydrophobic ions, such as ACh. The fluorous-phase sensor improves the selectivity for ACh over Na+ and K+ by 2 orders of magnitude (compared to traditional lipophilic membranes), thus lowering the detection limit in artificial cerebrospinal fluid (aCSF) from 331 to 0.38 μ M, thereby allowing measurement in physiologically relevant ranges of ACh. Engraving LIG under argon creates a hydrophobic surface with a 133.7° contact angle, which minimizes the formation of a water layer. The flexible solid-contact LIG fluorous sensor exhibited a slope of 59.3 mV/decade in aCSF and retained function after 20 bending cycles, thereby paving the way for studying ACh's role in memory and neurodegenerative diseases.

FAST (Flexible Acetylcholine Sensing Thread): Real-Time Detection of Acetylcholine with a Flexible Solid-Contact Potentiometric Sensor

Acetylcholine (ACh) is involved in memory and learning and has implications in neurodegenerative diseases; it is therefore important to study the dynamics of ACh in the brain. This work creates a flexible solid-contact potentiometric sensor for in vitro and in vivo recording of ACh in the brain and tissue homogenate. We fabricate this sensor using a 250 μm diameter cotton yarn coated with a flexible conductive ink and an ACh sensing membrane that contains a calix[4]arene ionophore. The exposed ion-to-electron transducer was sealed with a 2.5 μm thick Parylene C coating to maintain the flexibility of the sensor. The resulting diameter of the flexible ACh sensing thread (FAST) was 400 μm. The FAST showed a linear response range from 1.0 μM to 10.0 mM in deionized water, with a near-Nernstian slope of 56.11 mV/decade and a limit of detection of 2.6 μM. In artificial cerebrospinal fluid, the limit of detection increased to 20 μM due to the background signal of ionic content of the cerebrospinal fluid. The FAST showed a signal stability of 226 μV/h over 24 h. We show that FAST can measure ACh dynamics in sheep brain tissue and sheep brain homogenate after ACh spiking. FAST is the first flexible electrochemical sensor for monitoring ACh dynamics in the brain.

Aryl- and Superaryl-Extended Calix[4]pyrroles: From Syntheses to Potential Applications

The incorporation of aryl substituents at the meso-positions of calix[4]pyrrole (C4P) scaffolds produces aryl-extended (AE) and super-aryl-extended (SAE) calix[4]pyrroles. The cone conformation of the all-α isomers of "multi-wall" AE-C4Ps and SAE-C4Ps displays deep aromatic clefts or cavities. In particular, "four-wall" receptors feature an aromatic polar cavity closed at one end with four convergent pyrrole rings and fully open at the opposite end. This makes AE- and SAE-C4P scaffolds effective receptors for the molecular recognition of negatively charged ions and neutral guest molecules with donor-acceptor and hydrogen bonding motifs. In addition, adequately functionalized all-α isomers of multi wall AE- and SAE-C4P scaffolds self-assemble into uni-molecular and supra-molecular aggregates displaying capsular and cage-like structures. The self-assembly process requires the presence of template ions or molecules that lock the C4P cone conformation and complementing the inner polar functions and volumes of their cavities. We envisioned performing an in-depth revision of AE- and SAE-C4P scaffolds owing to their importance in different domains such as supramolecular chemistry, biology, material sciences and pharmaceutical chemistry. Herewith, besides the synthetic details on the elaboration of their structures, we also draw attention to their diverse applications. The organization of this review is mainly based on the number of "walls" present in the AE-C4P derivatives and their structural modifications. The sections are further divided based on the C4P functions and applications. The authors are convinced that this review will be of interest to researchers working in the general area of supramolecular chemistry as well as those involved in the study of the binding properties and applications of C4P derivatives.

Tunable Construction of Sandwich-Type Double-[1 + 1] and Half-Folded [2 + 2] Schiff-Base Complexes Controlled by the Combination of Primary and Secondary Template Effects

The first-row transition-metal ions Mn²⁺-Cu²⁺ could serve as effective templates to construct three types of double-[1 + 1], [2 + 2], and [1 + 1] Schiff-base dinuclear macrocyclic complexes in the presence of dialdehydes with different pendant arms and a common 1,8-diamine. The extremely flexible nature of macrocyclic ligands allows for the multiple template-directed syntheses, but the final products could be finely tuned by the subtle variations of Mn²⁺-Cu²⁺ ions in a 3d-electronic configuration, radius, and coordination number/geometry as well as the auxiliary (pendant-armed and anionic) template effect at the same time. Two borderlines are observed at the Co²⁺ ion for forming double-[1 + 1] and [2 + 2] metallacycles involving the H₂pdd precursor and the [1 + 1] Cu²⁺ complex for double-[1 + 1] and [2 + 2] macrocycles containing the H₂hpdd unit, respectively. The structural diversity is originated from the non-perfect match between [1 + 1]/[2 + 2] Schiff-base macrocycles and dinuclear metal centers; hence, a compromise between the metal coordination modes and alterations of the ligand conformation takes place.

Anchoring H-Bond Donating/Accepting Pyrrolic Derivatives on Preorganized Scaffolds: Conformationally Switchable Bipedal/Tripodal and Locked Molecular Cage Ionophores for Potentiometric Sensing of Phosphate and Fluoride

The ionophore properties of a myriad of conformationally switchable bipedal/tripodal receptors and locked molecular cages were evaluated here for the first time to fabricate potentiometric sensors for the determination of environmentally important phosphate and fluoride. Owing to the competent ionophore properties such as high binding selectivity and affinity, the developed ion-selective electrodes displayed response preference for phosphate and fluoride with a selectivity pattern that differs distinctly from traditional Hofmeister series. Binding constants of the ionophore-anion complexes are determined to underscore how modifications in the preorganization and H-bond donating/accepting ability of a given series of ionophores can be exploited to improve the performance for potentiometric sensing. While conformationally switchable bipedal/tripodal ionophores prefer tetrahedral oxyanions, locked molecular cages shift their preference to spherical halides gradually. Nernstian potential responses with good reversibility to target anions can be observed when shifting the optimized membrane electrodes in aqueous solutions within the concentration range of 10^-6.5-10^-2.0 M. Moreover, potentiometric determination of phosphate and fluoride in mineral water, soil, and tap water samples was achieved in a low μM concentration range with high accuracy, confirming their promising utility in real world applications.

Embedding of Functionalized Coordination Cages and a Molecular Knot in a Polymeric Membrane for Potentiometric Sensing of Environmentally Important Oxyanions and Halides

Three kinds of coordination cages and a molecular knot with inductively activated ⁺P-H, N-H, or C-H hydrogen bond donors anchoring in the functionalized cavities were inspected as ionophores to develop polymeric membrane ISEs for potentiometric sensing of environmentally important oxyanions and halides. The proposed ISEs displayed significant preference for perrhenate, phosphate, or chloride with a selectivity pattern distinctively different from the sequence depending on the Gibbs free energy of hydration owing to the high degree of shape, charge, and size selectivity originating from the rigidity and complementarity of the binding cavities. To gain further insight into the response characters of the proposed ISEs, the binding constants of ionophore-anion complexes in the membrane phase were investigated, and the binding affinity, together with the Hofmeister series, correlates well with the determined selectivity pattern of the proposed ISEs. Optimizing the composition of the membrane such as lipophilic additives and plasticizers produced ISEs displaying Nernstian/near-Nernstian potentiometric responses to primary anions with a wide linear range, improved detection limits, good reversibility, and satisfying lifetime. Potentiometric determination of perrhenate, phosphate, and chloride in river water, mineral water, and artificial serum samples was achieved with good recovery and accuracy using the proposed ISEs, demonstrating their potential for real-life applications. These results will shed new light on how novel ionophores could be designed for potentiometric sensing and broaden the scope of host-guest chemistry of coordination cages and molecular knots.

Nanosensing of Acetylcholine Molecules: Influence of the Association Mechanism

A bullet-shaped nanopore surface modified by two polyelectrolyte (PE) layers, an inner polyethyleneimine (PEI) layer and an outer p-sulfonatocalix[4]-arene (SCX4) layer, is applied to sense trace levels of acetylcholine (Ach) molecules. We show that the higher the order of the association reaction of Ach with SCX4, the smaller the difference between the ionic current when Ach is present and that when it is absent, and so is the difference in the space charge density. In addition, the larger the binding constant K of that reaction, the lower the detection limit but narrower the detection range. Choosing pH 7 is most appropriate because if the pH is low, the concentration polarization of H⁺ is significant, and as it gets high, both PE layers become uncharged. At pH 7 and K = 2 × 10⁷ L/mol, the detection limit of the nanopore ranges from 1 to 10 nM, which is orders of magnitude lower than that of the other approaches.

Recent developments in calix[4]pyrrole (C4P)-based supramolecular functional systems

Recent advances with calix[4]pyrrole-based supramolecular functional entities in the fields of molecular recognition (receptors, sensors, and metal ion caged systems), self-assembly (polymers), photo/pH-responsive molecular switches and catalysis are reviewed.