Highly Sensitive Electrochemical Sensor for the Detection of Anions in Water Based on a Redox-Active Monolayer Incorporating an Anion Receptor

Voltammetric Sensing of Chloride Based on a Redox-Active Complex: A Terpyridine-Co(II)-Dipyrromethene Functionalized Anion Receptor Deposited on a Gold Electrode

A redox-active complex containing Co(II) connected to a terpyridine (TPY) and dipyrromethene functionalized anion receptor (DPM-AR) was created on a gold electrode surface. This host-guest supramolecular system based on a redox-active layer was used for voltammetric detection of chloride anions in aqueous solutions. The sensing mechanism was based on the changes in the redox activity of the complex observed upon binding of the anion to the receptor. The electron transfer coefficient (α) and electron transfer rate constant (k0) for the modified gold electrodes were calculated based on Cyclic Voltammetry (CV) experiments results. On the other hand, the sensing abilities were examined using Square Wave Voltammetry (SWV). More importantly, the anion receptor was selective to chloride, resulting in the highest change in Co(II) current intensity and allowing to distinguish chloride, sulfate and bromide. The proposed system displayed the highest sensitivity to Cl- with a limit of detection of 0.50 fM. The order of selectivity was: Cl- > SO42- > Br-, which was confirmed by the binding constants (K) and reaction coupling efficiencies (RCE).

Selective Perchlorate Sensing Using Electrochemical Impedance Spectroscopy with Self-Assembled Monolayers of semiaza-Bambusurils

In the last two decades, perchlorate salts have been identified as environmental pollutants and recognized as potential substances affecting human health. We describe self-assembled monolayers (SAMs) of novel semiaza-bambus[6]urils (semiaza-BUs) equipped with thioethers or disulfide (dithiolane) functionalities as surface-anchoring groups on gold electrodes. Cyclic voltammetry (CV) with Fe(CN)6 3-/4- as a redox probe, together with X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and ellipsometry, were employed to characterize the interactions at the interface between the anchoring groups and the metal substrate. Data showed that the anion receptors' packing on the gold strongly depends on the anchoring group. As a result, SAMs of BUs with lipoic amide side chains show a concentration-dependent layer thickness. The BU SAMs are extremely stable on repeated electrochemical potential scans and can selectively recognize perchlorate anions. Our electrochemical impedance spectroscopy (EIS) studies indicated that semiaza-BU equipped with the lipoic amide side chains binds perchlorate (2-100 mM) preferentially over other anions such as F- , Cl- , I- , AcO- , H2 PO4 - , HPO4 2- , SO4 2- , NO2 - , NO3 - , or CO3 2- . The resistance performance is 10 to 100 times more efficient than SAMs containing all other tested anions.

Real-time Voltammetric Anion Sensing Under Flow*

The development of real‐life applicable ion sensors, in particular those capable of repeat use and long‐term monitoring, remains a formidable challenge. Herein, we demonstrate, in a proof‐of‐concept, the real‐time voltammetric sensing of anions under continuous flow in a 3D‐printed microfluidic system. Electro‐active anion receptive halogen bonding (XB) and hydrogen bonding (HB) ferrocene‐isophthalamide‐(iodo)triazole films were employed as exemplary sensory interfaces. Upon exposure to anions, the cathodic perturbations of the ferrocene redox‐transducer are monitored by repeat square‐wave voltammetry (SWV) cycling and peak fitting of the voltammograms by a custom‐written MATLAB script. This enables the facile and automated data processing of thousands of SW scans and is associated with an over one order‐of‐magnitude improvement in limits of detection. In addition, this improved analysis enables tuning of the measurement parameters such that high temporal resolution can be achieved. More generally, this new flow methodology is extendable to a variety of other analytes, including cations, and presents an important step towards translation of voltammetric ion sensors from laboratory to real‐world applications.

Beyond the Laviron Method: A New Mathematical Treatment for Analyzing the Faradaic Current in Reversible, Quasi-Reversible, and Irreversible Cyclic Voltammetry of Adsorbed Redox Species

A new algorithm that describes the faradaic current for elementary redox reactions in the cyclic voltammetric responses of persistently adsorbed species on metal electrodes at any scan rate is presented. This work does not assume electrochemical reversibility and instead demonstrates a set of equations that encapsulate how the forward and back charge-transfer rate constants influence the data as a function of the experimental time scale. The method presented here is compared against other approaches that rely on either finite-difference calculations or that require numerical approximation of improper integrals (i.e., ±infinity as a bound). The method here demonstrates that the current-potential data can be described by incomplete gamma functions, whose two arguments capture the relevant kinetic variables. Following the notation for the Butler-Volmer model of charge transfer, exact solutions are presented for the cases of the charge-transfer coefficient, α, equal to 1 or 0. A related algorithm based on these results affords calculation of current-potential data for 0 < α < 1, allowing comprehensive analysis (i.e., point by point) of voltammetric data throughout the reversible, quasi-reversible, and irreversible regimes. Accordingly, this work represents an alternative to the method of Laviron, i.e., analyzing just the peak splitting values, for experimentalists to understand and interpret their voltammetric data in totality.

Solvent Effects in Halogen and Hydrogen Bonding Mediated Electrochemical Anion Sensing in Aqueous Solution and at Interfaces

Sensing anionic species in competitive aqueous media is a well-recognised challenge to long-term applications across a multitude of fields. Herein, we report a comprehensive investigation of the electrochemical anion sensing performance of novel halogen bonding (XB) and hydrogen bonding (HB) bis-ferrocene-(iodo)triazole receptors in solution and at self-assembled monolayers (SAMs), in a range of increasingly competitive aqueous organic solvent media (ACN/H2 O). In solution, the XB sensor notably outperforms the HB sensor, with substantial anion recognition induced cathodic voltammetric responses of the ferrocene/ferrocenium redox couple persisting even in highly competitive aqueous solvent media of 20 % water content. The response to halides, in particular, shows a markedly lower sensitivity to increasing water content associated with a unique halide selectivity at unprecedented levels of solvent polarity. The HB sensor, in contrast, generally displayed a preference towards oxoanions. A significant surface-enhancement effect was observed for both XB/HB receptive films in all solvent systems, whereby the HB sensor generally displayed larger responses towards oxoanions than its halogen bonding analogue.

Supersensitive Detection of Anions in Pure Organic and Aqueous Media by Amino Acid Conjugated Ellman's Reagent

The last few decades witnessed a remarkable advancement in the field of molecular anion receptors. A variety of anion binding motifs have been discovered, and large number of designer molecular anion receptors with high selectivity are being reported. However, anion detection in an aqueous medium is still a formidable challenge as evident from only a miniscule of synthetic systems available in the literature. We, herein, report 5,5'-dithio-bis(2-nitrobenzoic acid) (Ellman's reagent) appended with amino acids as supersensitive anion sensors that can detect F^- and H₂PO₄^- ions in both aqueous as well as organic media. Interestingly, the sensors showed a dual response to anions, viz., chromogenic response in organic medium and electrochemical response in aqueous solutions. Various spectroscopic techniques such as UV-vis and ¹H NMR are used to investigate the binding studies in acetonitrile, whereas electrochemical methods such as cyclic voltammetry (CV) and differential pulse voltammetry (DPV) are employed to explore the anion binding in water. The host-guest complex stoichiometry and binding constants are calculated using the BindFit software. The geometry of host-guest complex has been optimized by the density functional theory (DFT) method. These molecules are versatile sensors since these function in both water and acetonitrile with extremely low limit of detection (LOD) up to 0.07 fM and limit of quantification (LOQ) up to 0.23 fM. To our knowledge, the present system is the first example of a sensor that can detect the lowest concentration of anions in water quantitatively. The minimalistic design strategy presented here opens up the innumerable possibilities for designing dual anion sensors in a one fell swoop.

Enhanced voltammetric anion sensing at halogen and hydrogen bonding ferrocenyl SAMs

Halogen bonding mediated electrochemical anion sensing has very recently been established as a potent platform for the selective and sensitive detection of anions, although the principles that govern binding and subsequent signal transduction remain poorly understood. Herein we address this challenge by providing a comprehensive study of novel redox-active halogen bonding (XB) and hydrogen bonding (HB) ferrocene-isophthalamide-(iodo)triazole receptors in solution and at self-assembled monolayers (SAMs). Under diffusive conditions the sensory performance of the XB sensor was significantly superior. In molecular films the XB and HB binding motifs both display a notably enhanced, but similar, response to specific anions. Importantly, the enhanced response of these films is rationalised by a consideration of the (interfacial) dielectric microenvironment. These effects, and the resolved relationship between anion binding and signal transduction, underpin an improved fundamental understanding of anion sensing at redox-active interfaces which will benefit not just the development of more potent, real-life relevant, sensors but also new tools to study host–guest interactions at interfaces.

Surface enhancement effects in the sensing of anions at redox-active molecular films are investigated in detail and rationalised based on a consideration of the dielectric binding microenvironment.

Electrochemical Anion Sensing: Supramolecular Approaches

Anions play a vital role in a broad range of environmental, technological, and physiological processes, making their detection/quantification valuable. Electroanalytical sensors offer much to the selective, sensitive, cheap, portable, and real-time analysis of anion presence where suitable combinations of selective (noncovalent) recognition and transduction can be integrated. Spurred on by significant developments in anion supramolecular chemistry, electrochemical anion sensing has received considerable attention in the past two decades. In this review, we provide a detailed overview of all electroanalytical techniques that have been used for this purpose, including voltammetric, impedimetric, capacititive, and potentiometric methods. We will confine our discussion to sensors that are based on synthetic anion receptors with a specific focus on reversible, noncovalent interactions, in particular, hydrogen- and halogen-bonding. Apart from their sensory properties, we will also discuss how electrochemical techniques can be used to study anion recognition processes (e.g., binding constant determination) and will furthermore provide a detailed outlook over future efforts and promising new avenues in this field.

The nanoscopic principles of capacitive ion sensing interfaces

Herein we discuss the operational principles of molecular interfaces that specifically recruit ions from an electrolyte solution and report this in a reagentless capacitive manner.

A robust electrochemical sensing of molecularly imprinted polymer prepared by using bifunctional monomer and its application in detection of cypermethrin

This work describes a hybrid electrochemical sensor for highly sensitive detection of pesticide cypermethrin (CYP). Firstly, Ag and N co-doped zinc oxide (Ag-N@ZnO) was produced by sol-gel method, and then Ag-N@ZnO was ultrasonically supported on activated carbon prepared from coconut husk (Ag-N@ZnO/CHAC). Finally, a layer of molecularly imprinted polymer (MIP) was in situ fabricated on glassy carbon electrode by electro-polymerization, with dopamine and resorcinol as dual functional monomers (DM), CYP acting as template (DM-MIP-Ag-N@ZnO/CHAC). Morphological features, composition information and electrochemical properties of DM-MIP-Ag-N@ZnO/CHAC were investigated in detail. It is worth to mention that for the first time response surface method was used to investigate the effect of double monomers and to optimize the ratio between template and monomers. Compared with typical one-monomer involving MIP, the MIP prepared with dual functional monomers (DMMIP) of monomers showed higher response and better selectivity. Under the optimal conditions, a calibration curve of current shift versus concentration of CYP was obtained in the range of 2 × 10^-13~8 × 10^-9 M, and the developed sensor gave a remarkably low detection limit (LOD) of 6.7 × 10^-14 M (S/N = 3). Determination of CYP in real samples was conducted quickly and accurately with our sensor. The DMMIP-Ag-N@ZnO/CHAC electrochemical sensor proposed in this paper has great potential in food safety, drug residue determination and environmental monitoring.