Redox-active Receptors Research Articles

A novel near-infrared optical and redox-active receptor for the multi-model detection of Hg2+ in water and living cells

A key issue for constructing optical and redox-active receptors is how to conjugate a specific sensing kernel with a multi-signal-responsive system to carry out multi-feature analysis. Mercury is considered to be highly toxic to human health and ecological security. In this work, we present a novel near-infrared optical and redox-active receptor that can sense Hg2+ at ppb level in aqueous media via multi-model monitors with a low detection limit of 8.4 × 10−9 M (1.68 ppb). This receptor features a visible detection, ‘off-on’ fluorescence response, and efficient electrochemistry assessment, as well as pH-insensitivity to Hg2+ with high sensitivity. In view of its marked near-infrared emission and fluorescence enhancement, we successfully applied this receptor to visualize Hg2+ in live cells. Furthermore, a possible sensing model was established and rationalized with theoretical studies.

New Pyridine-Bridged Ferrocene-Rhodamine Receptor for the Multifeature Detection of Hg2+ in Water and Living Cells.

A challenge in the design of optical and redox-active receptors is how to combine a specific recognition center with an efficient responsive system to facilely achieve multifeature detection in biological and environmental analyses. Herein, a novel ferrocene–rhodamine receptor conjugated with a pyridine bridge was designed and synthesized. This receptor can sensitively sense Hg2+ in aqueous media via chromogenic, fluorogenic, and electrochemical multisignal outputs with a low detection limit and fast response time. Moreover, it can be qualified as a fluorescent probe for effectively monitoring Hg2+ in living cells. A plausible recognition mode was proposed and rationalized with theoretical calculations.

A reaction-type receptor for the multi-feature detection of Hg2+ in water and living cells

The first optical and redox-active receptor of reaction-type has been developed for efficiently multi-model survey of Hg2+.

Two novel ferrocenyl dipeptide-like compounds generated via the Ugi four-component reaction.

The Ugi four-component reaction, a powerful method for the synthesis of diverse dipeptide-like derivatives in combinatorial chemistry, was used to synthesize (S)-1'-{N-[1-(anthracen-9-yl)-2-(tert-butylamino)-2-oxoethyl]-N-(4-methoxyphenyl)carbamoyl}ferrocene-1-carboxylic acid dichloromethane disolvate, [Fe(C6H5O2)(C33H31N2O3)]·2CH2Cl2, (I), and (S)-2-(anthracen-9-yl)-N-tert-butyl-2-[N-(4-methylphenyl)ferrocenylformamido]acetamide, [Fe(C5H5)(C33H31N2O2)], (II). They adopt broadly similar molecular conformations, with near-eclipsed cyclopentadienyl rings and near-perpendicular amide planes in their dipeptide-like chains, one of which is almost coplanar with its attached cyclopentadienyl ring but perpendicular to the aromatic ring bound to the N atom. In the supramolecular structure of (I), a two-dimensional network is constructed based on molecular dimers and a combination of intermolecular O-H···O, N-H···O and C-H···O hydrogen bonds, forming R2(2)(11), R2(2)(16), R2(2)(22) and C(9) motifs. These two-dimensional networks are connected by C-H···O and C-H···Cl contacts to create a three-dimensional framework, where one dichloromethane solvent molecule acts as a bridge between two neighbouring networks. In the packing of (II), classical hydrogen bonds are absent and an infinite one-dimensional chain is generated via a combination of C-H···O hydrogen bonds and C-H···π interactions, producing a C(7) motif. This work describes a simple synthesis and the supramolecuar structures of ferrocenyl dipeptide-like compounds and is significant in the development of redox-active receptors.

“Wurster-Type” Ureas as Redox-Active Receptors for Anions

Four redox-active receptors, 1-4, based on the incorporation of p-phenylenediamine(s) within a urea framework, were synthesized, and the affinities of two for a series of anions were quantified through UV-vis and NMR spectroscopic studies. The structure of 1 was confirmed by X-ray crystallography. For the oxoanions studied, complex stabilities approached 10(6) M(-1) in acetonitrile and decreased with the decreasing basicity of the anion (CH(3)COO(-) > C(6)H(5)COO(-) > H(2)PO(4)(-) > NO(2)(-) > NO(3)(-)). The presence of the urea functionality caused an increase in the oxidation potential of the p-phenylenediamine subunit compared to that of free p-phenylenediamine. Electrochemical studies of the anion complexes revealed two-wave behavior with the appearance of a second oxidation wave cathodic to that in the free receptors and characteristic of the bound anion. Ab initio DFT studies of a representative acetate complex revealed the consequences of host oxidation state on complex structure.

A novel ferrocene-based thiacalix[4]arene ditopic receptor for electrochemical sensing of europium(III) and dihydrogen phosphate ions

We have synthesized a ferrocene-based 1,3-alternate thiacalix[4]arene ditopic receptor 3 that contains four identical polyether arms terminated with the ferrocene amide moieties. Our studies have revealed that this redox-active receptor can be used as an electrochemical sensor to recognize both europium (Eu 3+) and dihydrogen phosphate ( H 2 PO 4 - ) ions with a high selectivity.

Ferrocene-substituted calix[4]pyrrole modified carbon paste electrodes for anion detection in water

A neutral redox-active receptor (ferrocene functionalized calix[4]pyrrole) was used as an active component in carbon paste electrodes and ion-selective electrodes ISEs, for the detection of anions in aqueous solution. Measurements with the carbon paste electrodes were conducted using Osteryoung square-wave voltammetry. At the surface of the modified electrode the redox potential of ferrocene-substituted calix[4]pyrrole was found to be at ca. 400 mV. Amongst the anions studied, dihydrogen phosphate and fluoride caused the strongest decrease of peak current (approximately 25%), followed by bromide and chloride. This sensitivity sequence was compared with that obtained with ISEs containing the ferrocene-substituted calix[4]pyrrole. The influence of the interface between host molecule environment and aqueous solution containing target anion on the molecular recognition process and electrochemical signal generation has been investigated.

Electrochemical sensing of anions by redox-active receptors built on the ferrocenyl cyclam framework

A range of ferrocenyl derivatives, L 1 – L 3 and L 1 Cu, has been prepared and their electrochemical recognition properties in organic solvents have been studied. These compounds are built on the same cyclam framework (ferrocenyl trans di-substitued cyclam), but they differ by the nature of their anions binding sites. While L 1 and its corresponding N-methylated L 2 derivative can interact with anionic species via H-bonding, strong electrostatic and hard acid–hard base interactions with anionic species are mainly involved in the ammonium derivative L 3 and the copper(II) complex L 1 Cu, respectively. Significant CV perturbations in the characteristic potential of the Fc/Fc + subunit in the presence of anions were found with L 1 and L 3 . Selectivity of the electrochemical process is likely to be based on the formation of multiple H-bonds between the secondary amines of L 1 and the phosphate anions (both H-donor and acceptor) when the topology of the ammonium derivatives, L 3 , governs mainly the anion complexation. Clear cyclic voltammetric features, such as a well-defined two-wave behavior allowing an accurate amperometric titration, require that the redox receptor contain an appropriate binding site and the ferrocene group in close proximity. Lewis acid–base interactions were shown to influence more deeply the electroactivity of Lewis acid center, namely the copper center in L 1 Cu, than that of the ferrocenyl redox probe that is not directly connected to the associated anion. However, the precipitation of the host structure (e.g., L 3 or L 1 Cu) in the presence of oxo-anions shows that a compromise has to be found between the establishment of strong interactions and the formation of soluble ion-pairs based complexes.

Bis(calix[4]diquinone) receptors: cesium- and rubidium-selective redox-active ionophores.

A new class of redox-active ionophore comprised of two calix[4]diquinone moieties connected through either alkylene or pyridylene linkages has been developed. Spectroscopic and electrochemical investigations, X-ray crystal structure analyses, and molecular modeling studies show butylene- and propylene-linked members of this family of redox-active receptors exhibit remarkable selectivity preferences and substantial electrochemical recognition effects toward cesium and rubidium cations.

Asymmetric synthesis of chiral alpha-ferrocenylalkylamines and their use in the preparation of chiral redox-active receptors.

A new strategy for the asymmetric synthesis of chiral primary alpha-ferrocenylalkylamines has been utilized to generate homochiral redox-active receptors that bind chiral carboxylate anions with moderate enantioselectivity and undergo a redox response to complexation. [structure: see text]

Electrochemical Recognition of Ions with Self-assembled Monlayers of Calixarenes

Quinone-functionalized calix[4]arenes with carboxylic acid groups or disulfide group were prepared and their spontaneous adsorption respectively on silver and gold surface was studied. Since the cavity-like structure of calixarenes was immobilized, they exhibited selective affinity towards specific metal ions in aqueous media. Voltammetric as well as EQCM and spectroscopic studies showed the well-ordered deposition of organic receptors and entrapment of metal ions. And it also was found that the repeated capture and removal of metal ions reversibly with chelating agents such as ethylenediaminetetraacetic acid (EDTA) was possible. This is the first example, in our knowledge, of voltammetric detection of metal ions in aqueous media using a chemically modified electrode with redoxactive macrocyclic receptors.

Voltammetric investigation of new boronic ester-substituted triphenylamines-based redox receptors in solution and attached to an electrode surface. Effects of F − as an anionic guest

Triphenylamines mono-, di- and trisubstituted by boronic ester unit(s) were designed as powerful redox-active receptors for Lewis hard bases like fluoride anion. Their voltammetric behaviour was found to be dramatically changed upon the addition of this halide. Depending on the degree of substitution of triphenylamines, the binding of F − to the boron atom led to the appearance of one to three new redox system(s). The binding constants determined for their neutral form ranged from 1.0×10 2 to 4.0×10 2 and were dramatically enhanced upon their oxidation into radical cation (3.0×10 5–1.6×10 7). The fixation of such electroactive compounds to the electrode surface has been achieved from the anodic oxidation of a vinyl-substituted bipodal receptor. The polymer films showed a reversible and stable response in a dried organic medium. Unfortunately, the voltammetric changes indicative of a complexation phenomenon were not observed in the presence of F − and only a degradation of the film electroactivity was noticed.

Cesium- and Rubidium-Selective Redox-Active Bis(calix[4]diquinone) Ionophores.

The length of the bridging alkylene chain between two calix[4]diquinone moieties determines the selectivity of the receptors for Group 1 metal ions. These redox-active receptors exhibit remarkable selectivity preferences and substantial electrochemical recognition effects towards Cs+ and Rb+ ions. The solid-state crystal structure of the Cs+ complex of a bis(calix[4]diquinone) derivative is shown (green: Cs+ , red: oxygen).

Boronic ester-substituted terpyridine metal complex as a novel fluoride-sensitive redox receptor

The boronic ester-disubstituted terpyridine 1 Fe(II) complex [Fe(II) 1 2] 2+ has been used as a redox-active receptor for fluoride. Upon binding of F − to the boronic ester moieties, the Fe(II)/Fe(III) oxidation peak of this metal complex is shifted toward less positive potentials. The magnitude of the shift (around 160 mV) is much larger than that measured with Cl − (20 mV) whereas other anionic species (i.e. Br −, NO 3 − and SO 4 2−) do not interact. Such a variation is consistent with a more than 500-fold increase of the F −-binding constant upon the oxidation of Fe(II) to Fe(III). Further UV–Vis spectroscopy data would indicate the formation of two complexes of stoichiometries 1:1 and 1:2 (receptor:fluoride), successively.

Electrochemical sensing of fluoride and sugars with a boronic acid-substituted bipyridine Fe(II) complex in solution and attached onto an electrode surface

The electrochemical behaviour of a boronic acid-substituted 2,2′-bipyridine 1 Fe(II) complex ([Fe(II) 1 3] 2+) has been investigated in aqueous buffered solutions, in the absence and in the presence of F − or various sugars as guest species. The binding of the halide to the boron atom led to a shift of the Fe(II)/Fe(III) oxidation peak towards less positive potentials whereas the sugars, and particularly d-fructose, had an opposite effect. With the aim of developing electrochemical sensors, the immobilization of this redox-active receptor onto an electrode surface has been successfully achieved from an electropolymerizable organometallic assembly consisting of the ligand 1 and a N-substituted pyrrole derivative. Its anodic oxidation yielded polymer films which exhibited stable electroactive properties in CH 3CN. Owing to the total loss of their electroactivity in aqueous media, only the effect of F − on the response of the immobilized Fe(II)/Fe(III) system has been examined in CH 3CN. An unusual complexation-induced catalytic phenomenon was evidenced in the presence of this guest anion.

Electrochemical recognition of metal cations by redox-active receptors in homogeneous solution and in polymer films: some relevant examples

Examples of several molecular redox-active receptors containing a ferrocene redox centre and crown ether or bipyridyl moieties as binding site are presented. Their electrochemical recognition properties towards groups I and II (crown ether-based receptors) and Cu + (2,2′-bipyridine-based receptors) metal cations have been investigated by cyclic voltammetry in homogeneous solution. Among the four ferrocene-crown ether studied, Fcc 3 allowed the effective amperometric recognition of Ba 2+ and Ca 2+ cations owing to the growth of a new redox peak system at the expense of the original Fc/Fc + wave (two-wave behavior). In addition, comparison of the electrochemical features in the presence of Cu + cations of ferrocene monosubstituted (Fcb 1) and disubstituted (Fcb 2) by bipyridyl groups emphasized the beneficial “sandwich” effect on the recognition behavior of ferrocene-based receptors. Polymer films coated onto electrodes surfaces by electropolymerization of pyrrole-substituted Fcc 3 and Fcb 2 retained the complexation and the amperometric properties, studied by cyclic voltammetry, of the molecular receptors. Furthermore, capacity measurements using electrochemical impedance spectroscopy (EIS) appeared as a good method for detection and titration of metal cations with these polymer-modified electrodes.

Mechanisms of electrochemical recognition of cations, anions and neutral guest species by redox-active receptor molecules

This short review highlights the mechanisms involved in electrochemically sensing cationic, anionic and neutral guest species by redox-active receptors and is an update to a previously published review article (P.D. Beer, P.A. Gale, Z. Chen, Adv. Phys. Org. Chem. 31 (1998) 1). Mechanisms of redox–complexation coupling are discussed together with recent examples of redox-responsive molecular receptors from the literature that illustrate them.

Selective electrochemical recognition of mercury in water by a redox-functionalised aza-oxa crown derivative

The ability of the new cyclic aza-oxa redox-active receptor N-ferrocenylmethyl-1,4,7-trioxa-10-azacyclododecane to selectively and electrochemically recognize Hg2+ in water over other commonly water-present metal ions is reported.

Quantitative determination of metal ions and anions in aqueous solution by using pH-responsive redox-active receptors

The redox-active polyazacyloalkane 1,4,8,11-ferrocenylmethyl-1,4,8,11-tetraazacyclotetradecane L 1 is a versatile receptor for the quantitative electrochemical determination of metal ions (Cu 2+ , Zn 2+ , Cd 2+ ) and ATP in aqueous solution.

Highly branched ferrocene-functionalised polyazacycloalkanes as electroactive receptors for transition-metal ions

Reaction of N,N′,N″,N‴-tetrakis (2-aminoethyl)-1,4,8,11-tetraazacyclotetradecane (taec) with ferrocenecarbaldehyde and further reduction with LiAlH4 resulted in the synthesis of the redox-active receptor N,N′,N″,N‴-tetrakis(4-ferrocenyl-3-azabutyl)-1,4,8, 11-tetraazacyclotetradecane (tfabc). The protonation behaviour of tfabc and its complex formation with CuII, ZnII and CdII have been studied in tetrahydrofuran–water (70 : 30 v/v)(0.1 mol dm–3 NBut4ClO4, 25 °C). The potentiometric and electrochemical data indicate the formation of 1 : 1 and 1 : 2 (tfabc: M2+) stoichiometries. The E½ values for tfabc shift as a function of the pH from 389 (pH 10) to 482 mV (pH 2.5) and E½vs. pH curves for the tfabc–H+–M2+ systems (M = Cu, Zn or Cd) show that tfabc electrochemically recognises the presence of copper(II) selectively at pH < 5. The shift of E½ appears to be caused by electrostatic forces and the E½vs. pH curves can be predicted in terms of the distribution diagrams of the tfabc–H+–M2+ systems.