Inherently chiral electrodes: the tool for chiral voltammetry

Structural and Electronic Chirality in Inorganic Crystals: from Construction to Application

Chirality represents a fundamental characteristic inherent in nature, playing a pivotal role in the emergence of homochirality and the origin of life. While the principles of chirality in organic chemistry are well-documented, the exploration of chirality within inorganic crystal structures continues to evolve. This ongoing development is primarily due to the diverse nature of crystal/amorphous structures in inorganic materials, along with the intricate symmetrical and asymmetrical relationships in the geometry of their constituent atoms. In this review, we commence with a summary of the foundational concept of chirality in molecules and solid states matters. This is followed by an introduction of structural chirality and electronic chirality in three-dimensional and two-dimensional inorganic materials. The construction of chirality in inorganic materials is classified into physical photolithography, wet-chemistry method, self-assembly, and chiral imprinting. Highlighting the significance of this field, we also summarize the research progress of chiral inorganic materials for applications in optical activity, enantiomeric recognition and chiral sensing, selective adsorption and enantioselective separation, asymmetric synthesis and catalysis, and chirality-induced spin polarization. This review aims to provide a reference for ongoing research in chiral inorganic materials and potentially stimulate innovative strategies and novel applications in the realm of chirality.

Magnetic field-enhanced redox chemistry on-the-fly for enantioselective synthesis

Chemistry on-the-fly is an interesting concept, extensively studied in recent years due to its potential use for recognition, quantification and conversion of chemical species in solution. In this context, chemistry on-the-fly for asymmetric synthesis is a promising field of investigation, since it can help to overcome mass transport limitations, present for example in conventional organic electrosynthesis. Herein, the synergy between a magnetic field-enhanced self-electrophoretic propulsion mechanism and enantioselective redox chemistry on-the-fly is proposed as an efficient method to boost stereoselective conversion. We employ Janus swimmers as redox-active elements, exhibiting a well-controlled clockwise or anticlockwise motion with a speed that can be increased by one order of magnitude in the presence of an external magnetic field. While moving, these bifunctional objects convert spontaneously on-the-fly a prochiral molecule into a specific enantiomer with high enantiomeric excess. The magnetic field-enhanced self-mixing of the swimmers, based on the formation of local magnetohydrodynamic vortices, leads to a significant improvement of the reaction yield and the conversion rate.

Wireless electrochemical actuation of soft materials towards chiral stimuli

Different areas of modern chemistry, require wireless systems able to transfer chirality from the molecular to the macroscopic event. The ability to recognize the enantiomers of a chiral analyte is highly desired, since in the majority of cases such molecules present different physico-chemical properties that could lead, eventually, to dangerous or harmful interactions with the environment or the human body. From an electrochemical point of view, enantiomers have the same electrochemical behavior except when they interact in a chiral environment. In this Feature Article, different approaches for the electrochemical recognition of chiral information based on the actuation of conducting polymers are described. Such a dynamic behavior of π-conjugated materials is based on an electrochemically induced shrinking/swelling transition of the polymeric matrix. Since all the systems, described so far in the literature, are achiral and require a direct connection to a power supply, new strategies will be presented in the manuscript, concerning the implementation of chirality in electrochemical actuators and their use in a wireless manner through bipolar electrochemistry. Herein, the synergy between the wireless unconventional actuation and the outstanding enantiorecognition of inherent chiral oligomers is presented as an easy and straightforward read out of chiral information in solution. This approach presents different advantages in comparison to classic electrochemical systems such as its wireless nature and the possible real-time data acquisition.

Bipolar electrochemical rotors for the direct transduction of molecular chiral information

Efficient monitoring of chiral information of bioactive compounds has gained considerable attention, due to their involvement in different biochemical processes. In this work, we propose a novel dynamic system for the easy and straightforward recognition of chiral redox active molecules and its possible use for the efficient measurement of enantiomeric excess in solution. The approach is based on the synergy between the localized enantioselective oxidation of only one of the two antipodes of a chiral molecule and the produced charge-compensating asymmetric proton flux along a bipolar electrode. The resulting clockwise or anticlockwise rotation is triggered only when the probe with the right chirality is present in solution. The angle of rotation shows a linear correlation with the analyte concentration, enabling the quantification of enantiomeric ratios in mixtures where the two antipodes are present in solution. This device was successfully used to simultaneously measure different ratios of the enantiomers of 3,4-dihydroxyphenylalanine and tryptophan. The versatility of the proposed approach opens up the possibility to use such a dynamic system as a straightforward (bio)analytical tool for the qualitative and quantitative discrimination of different redox active chiral probes.

Electrocatalytic detection of noxious antioxidant diphenylamine in fruit samples with support of Cu@nanoporous carbon modified sensor

Herein, the facile synthesis of copper(II) and benzene-1,3,5-tricarboxylate (Cu-BTC) and copper nanoporous carbon (Cu@NPC) for the electrochemical detection of diphenylamine (DPA) was systematically investigated. The Cu-BTC and Cu@NPC materials structural, morphological, and thermal stability were evaluated and confirmed using FE-SEM, HR-TEM, XRD, FT-IR, and TGA. The electrocatalytic behavior of sensor materials was examined by cyclic voltammetry (CV) and differential pulse voltammetry (DPV). It is presumed that the structural stability and synergic effect exhibited in Cu@NPC are favorable for enhanced sensitivity and selectivity towards the detection of DPA. The Cu@NPC exhibited a wide linear range (0.09-396.82 μM) and the lowest limit of detection (5 nM). Furthermore, the real sample analysis of the sensor for the detection of DPA in apples and pears confirms its potential capability in practical application.

Metal-Organic Framework-Based Composites for the Detection and Monitoring of Pharmaceutical Compounds in Biological and Environmental Matrices

The production of synthetic drugs is considered a huge milestone in the healthcare sector, transforming the overall health, aging, and lifestyle of the general population. Due to the surge in production and consumption, pharmaceutical drugs have emerged as potential environmental pollutants that are toxic with low biodegradability. Traditional chromatographic techniques in practice are time-consuming and expensive, despite good precision. Alternatively, electroanalytical techniques are recently identified to be selective, rapid, sensitive, and easier for drug detection. Metal-organic frameworks (MOFs) are known for their intrinsic porous nature, high surface area, and diversity in structural design that provides credible drug-sensing capacities. Long-term reusability and maintaining chemo-structural integrity are major challenges that are countered by ligand-metal combinations, optimization of synthetic conditions, functionalization, and direct MOFs growth over the electrode surface. Moreover, chemical instability and lower conductivities limited the mass commercialization of MOF-based materials in the fields of biosensing, imaging, drug release, therapeutics, and clinical diagnostics. This review is dedicated to analyzing the various combinations of MOFs used for electrochemical detection of pharmaceutical drugs, comprising antibiotics, analgesics, anticancer, antituberculosis, and veterinary drugs. Furthermore, the relationship between the composition, morphology and structural properties of MOFs with their detection capabilities for each drug species is elucidated.

Doped Polythiophene Chiral Electrodes as Electrochemical Biosensors

π-conducting materials such as chiral polythiophenes exhibit excellent electrochemical stability in doped and undoped states on electrode surfaces (chiral electrodes), which help tune their physical and electronic properties for a wide range of uses. To overcome the limitations of traditional surface immobilization methods, an alternative pathway for the detection of organic and bioorganic targets using chiral electrodes has been developed. Moreover, chiral electrodes have the ability to carry functionalities, which helps the immobilization and recognition of bioorganic molecules. In this review, we describe the use of polythiophenes for the design of chiral electrodes and their applications as electrochemical biosensors.

Bipolar Electrochemical Measurement of Enantiomeric Excess with Inherently Chiral Polymer Actuators

Straightforward enantioselective analytical methods are very important for drug safety, considering that in certain cases one of the two enantiomers of a chiral molecule might be harmful for humans. In this work, we propose a simple system for the direct and easy read-out of the enantiomeric excess of 3,4-dihydroxyphenylalanine (DOPA) as a model analyte. A conducting oligomer, i.e. oligo-(3,3'-dibenzothiophene), bearing inherently chiral features, is electrogenerated on a polypyrrole film. The resulting freestanding hybrid material is used as a wireless enantioselective actuator in a bipolar electrochemical cell. Combining in a single setup two individual actuators with opposite chiral features allows a direct visual read-out of enantiomeric excess, as the bending amplitude of each of the two actuators is directly correlated with the concentration of the corresponding stereoisomer of the analyte. Optimization of the experimental parameters results in efficient bending, giving access to the percentage values of the enantiomeric excess in mixtures containing different ratios of the antipodes, thus opening the way to potential applications for chiral in situ analysis.

Hybrid light-emitting devices for the straightforward readout of chiral information

Bipolar electrochemistry has gained increasing attention in recent years as an attractive transduction concept in analytical chemistry in general and, more specifically, in the frame of chiral recognition. Herein, we use this concept of wireless electrochemistry, based on the combination of the enantioselective oxidation of a chiral probe with the emission of light from a light-emitting diode (LED), as an alternative for an easy and straightforward readout of the presence of chiral molecules in solution. A hybrid polymer-microelectronic device was designed, using an inherently chiral oligomer, that is, oligo-(3,3'-dibenzothiophene) and a polypyrrole strip as the anode and cathode of a miniaturized LED. The wireless induced redox reactions trigger light emission when the probe with the right chirality is present in solution, whereas no light emission is observed for the opposite enantiomer. The average light intensity shows a linear correlation with the analyte concentration, and the concept opens the possibility to quantify the enantiomeric excess in mixtures of the molecular antipodes.

Modulating the Enantiodiscrimination Features of Inherently Chiral Selectors by Molecular Design: A HPLC and Voltammetry Study Case with Atropisomeric 2,2'-Biindole-Based Monomers and Oligomer Films

A family of inherently chiral electroactive selectors based on the 2,2'-biindole atropisomeric scaffold, of easy synthesis and modulable functional properties, is studied in cascade in two enantioselection contexts. They are at first investigated as probes in enantioselective HPLC, studying molecular structure and temperature effects, and achieving very efficient semipreparative enantioseparation. The enantiomers thus obtained, of remarkable chiroptical features (optical rotation as well as circular dichroism), are successfully applied as selectors in chiral voltammetry in different media for discrimination of the enantiomers of chiral electroactive probes, either by conversion into enantiopure electroactive electrode surfaces by electrooligomerization on glassy carbon substrate (the two monomers with shorter alkyl chains), or as chiral additive in achiral ionic liquid (the monomer with longest alkyl chains). Discrimination is conveniently and reproducibly achieved in terms of significant potential differences for the two enantiomers, specularly inverting either probe or selector configuration. In one case successful discrimination is also observed with the two probe enantiomers concurrently present, either as racemate or with enantiomeric excesses, neatly accounted for by the peak current ratios.

Organic Electrochemistry: Molecular Syntheses with Potential

Efficient and selective molecular syntheses are paramount to inter alia biomolecular chemistry and material sciences as well as for practitioners in chemical, agrochemical, and pharmaceutical industries. Organic electrosynthesis has undergone a considerable renaissance and has thus in recent years emerged as an increasingly viable platform for the sustainable molecular assembly. In stark contrast to early strategies by innate reactivity, electrochemistry was recently merged with modern concepts of organic synthesis, such as transition-metal-catalyzed transformations for inter alia C-H functionalization and asymmetric catalysis. Herein, we highlight the unique potential of organic electrosynthesis for sustainable synthesis and catalysis, showcasing key aspects of exceptional selectivities, the synergism with photocatalysis, or dual electrocatalysis, and novel mechanisms in metallaelectrocatalysis until February of 2021.

Multimilligram-scale production implementation of atropisomers of 2,2'-bis(2,2'-bithiophene-5-yl)-3,3'-bithianaphthene

2,2'-Bis[2-(5,2'-bithienyl)]-3,3'-bithianaphthene (1) is the progenitor of a class of C₂ symmetric thiophene-based electroactive monomers that, when electrooxidized in the enantiomerically pure form, produce inherently chiral films endowed with outstanding electrochemical enantiorecognition properties. The enantioselective high-performance liquid chromatography (HPLC) is the only approach used so far to resolve the racemic form of 1 into enantiomers. In this work, an improved HPLC method for multimilligram enantiomer production is presented. Key factors controlling the enantioseparation, such as mobile phase composition and column temperature, were identified using a 100 × 4.6 mm i.d. Chiralpak IB-3 column and subsequently scaled up to a 250 × 10.0 mm i.d. Chiralpak IB column. In the optimized semipreparative conditions, about 34 mg of pure (P) and (M) enantiomers per hour could be produced.

Enantiospecific Molecular Fingerprinting Using Potential-Modulated Surface-Enhanced Raman Scattering to Achieve Label-Free Chiral Differentiation

Chiral differentiation is critical in diverse fields ranging from pharmaceutics to chiral synthesis. While surface-enhanced Raman scattering (SERS) offers molecule-specific vibrational information with high detection sensitivity, current strategies rely on indirect detection using additional selectors and cannot exploit SERS' key advantages for univocal and generic chiral differentiation. Here, we achieve direct, label-free SERS sensing of biologically important enantiomers by synergizing asymmetric nanoporous gold (NPG) nanoparticles with electrochemical-SERS to generate enantiospecific molecular fingerprints. Experimental and in silico studies reveal that chiral recognition is two pronged. First, the numerous surface atomic defects in NPG provide the necessary localized asymmetric environment to induce enantiospecific molecular adsorptions and interaction affinities. Concurrently, the applied potential drives and orients the enantiomers close to the NPG surface for maximal analyte-surface interactions. Notably, our strategy is versatile and can be readily extended to detect various enantiomers. Furthermore, we can achieve multiplex quantification of enantiomeric ratios with excellent predictive performance. Our combinatorial approach thus offers an important paradigm shift from current approaches to achieve label-free chiral SERS sensing of various enantiomers.

Characterization of Inherently Chiral Electrosynthesized Oligomeric Films by Voltammetry and Scanning Electrochemical Microscopy (SECM)

A voltammetric and scanning electrochemical microscopy (SECM) investigation was performed on an inherently chiral oligomer-coated gold electrode to establish its general properties (i.e., conductivity and topography), as well as its ability to discriminate chiral electroactive probe molecules. The electroactive monomer (S)-2,2′-bis(2,2′-bithiophene-5-yl)-3,3′-bibenzothiophene ((S)-BT₂T₄) was employed as reagent to electrodeposit, by cyclic voltammetry, the inherently chiral oligomer film of (S)-BT₂T₄ (oligo-(S)-BT₂T₄) onto the Au electrode surface (resulting in oligo-(S)-BT₂T₄-Au). SECM measurements, performed in either feedback or competition mode, using the redox mediators [Fe(CN)₆]⁴⁻ and [Fe(CN)₆]³⁻ in aqueous solutions, and ferrocene (Fc), (S)-FcEA, (R)-FcEA and rac-FcEA (FcEA is N,N-dimethyl-1-ferrocenylethylamine) in CH₃CN solutions, indicated that the oligomer film, as produced, was uncharged. The use of [Fe(CN)₆]³⁻ allowed establishing that the oligomer film behaved as a porous insulating membrane, presenting a rather rough surface. This was inferred from both the approach curves and linear and bidimensional SECM scans, which displayed negative feedback effects. The oligomer film acquired semiconducting or fully conducting properties when the Au electrode was biased at potential more positive than 0.6 V vs. Ag|AgCl|KCl. Under the latter conditions, the approach curves displayed positive feedback effects. SECM measurements, performed in competition mode, allowed verifying the discriminating ability of the oligo-(S)-BT₂T₄ film towards the (S)-FcEA and (R)-FcEA redox mediators, which confirmed the results obtained by cyclic voltammetry. SECM linear scans indicated that the enantiomeric discriminating ability of the oligo-(S)-BT₂T₄ was even across its entire surface.

Asymmetric reactions induced by electron spin polarization

Essential aspects of the chiral induced spin selectivity (CISS) effect and their implications for spin-controlled chemistry and asymmetric electrochemical reactions are described. The generation of oxygen through electrolysis is discussed as an example in which chirality-based spin-filtering and spin selection rules can be used to improve the reaction's efficiency and selectivity. Next the discussion shifts to illustrate how the spin selectivity of chiral molecules (CISS properties) allows one to use the electron spin as a chiral bias for inducing asymmetric reactions and promoting enantiospecific processes. Two enantioselective electrochemical reactions that have used polarized electron spins as a chiral reagent are described; enantioselective electroreduction to resolve an enantiomer from a racemic mixture and an oxidative electropolymerization to generate a chiral polymer from achiral monomers. A complementary approach that has used spin-polarized, but otherwise achiral, molecular films to enantiospecifically associate with one enantiomer from a racemic mixture is also discussed. Each of these reaction types use magnetized films to generate the spin polarized electrons and the enantiospecificity can be selected by choice of the magnetization direction, North pole versus South pole. Possible paths for future research in this area and its compatibility with existing methods based on chiral electrodes are discussed.

Electrochemical asymmetric synthesis of biologically active substances

Electrically driven oxidation and reduction reactions are well-established methods for synthesis even in the chemical industry, but asymmetric versions are still few. The mild conditions used, atom efficiency and low cost make these reactions a very attractive alternative to other methods of synthesis. Very fine tuning can be achieved based on minute changes in potentials, allowing only one functional group in a molecule to react in the presence of several others, which is ideal for applications in total synthesis. In this review, the literature in the field of asymmetric synthesis of biologically active substances over the last 10 years is surveyed.

Highly enantioselective "inherently chiral" electroactive materials based on a 2,2'-biindole atropisomeric scaffold

Chiral oligothiophene monomers with C 2 symmetry, based on 3,3'-bithiophene atropisomeric cores with high racemization barriers, have recently been shown to provide excellent chiral starting materials with high electroactivity for the easy preparation of enantiopure electroactive films endowed with powerful chirality manifestations. We now introduce an inherently chiral monomer based on a 2,2'-biindole core, as the prototype of a new inherently chiral monomer family, whose properties could be modulable through functionalization of the pyrrolic N atoms. By fast, regular electrooligomerization the new monomer yields inherently chiral films with high, reversible electroactivity and, above all, impressive enantioselectivity towards very different chiral probes, some of pharmaceutical interest, as general-scope electrode surfaces. Such results, while opening the way to a new, attractive inherently chiral selector class, nicely confirm the general validity of the inherent chirality strategy for chiral electrochemistry. Furthermore, the enantioselectivity of the new selectors not only holds with electroactive chiral probes, but also with circularly polarized light components as well as electron spins, resulting in good chiroptical and spin filter performances, which suggests fascinating correlations between the three contexts.

Highlighting spin selectivity properties of chiral electrode surfaces from redox potential modulation of an achiral probe under an applied magnetic field

Impressive spin-related effects are observed in cyclic voltammetry (CV) experiments performed under an applied magnetic field on a non-ferromagnetic electrode modified with a thin electroactive oligothiophene film, either "inherently chiral" or featuring chiral pendants with stereogenic centres. When flipping the magnet's north/south orientation, the CV peaks of two achiral, chemically reversible Fe(iii)/Fe(ii) redox couples in aqueous or organic solution undergo impressive potential shifts (up to nearly 0.5 V depending on protocol conditions), specularly by changing the film's (R)- or (S)-configuration. The magnitude of the potential shift decreases upon increasing both the polymer film thickness and the distance between the permanent magnet and the electrode surface. Such unprecedented spin-related redox potential modulation, obtained in the absence of a magnetic electrode acting as a spin injector, provides striking evidence (as well as an attractive evaluation criterion) of the spin selectivity properties of chiral thin films.

Hydroxypropyl β-cyclodextrin cross-linked multiwalled carbon nanotube-based chiral nanocomposite electrochemical sensors for the discrimination of multichiral drug atorvastatin isomers

Drugs having multiple chiral centres pose a greater risk to the human health as their pharmacological effects on human organs, cells and systems due to more number of enantiomers as compared to that of a single enantiomeric drug.

Efficient enantiorecognition of amino acids under a stimuli-responsive system: synthesis, characterization and application of electroactive rotaxane

An electroactive chiral rotaxane, consisting of a polymeric chiral ionic liquid as the flexible axle and 18-crown-6 as the wheel, is synthesized for efficient enantiorecognition of amino acids.

Thiahelicene-based inherently chiral films for enantioselective electroanalysis

Enantiopure inherently chiral films with helicoidal stereogenicity of outstanding enantiodiscrimination potentialities for chiral redox probes and as spin filters.

Chiral electroanalysis could be regarded as the highest recognition degree in electrochemical sensing, implying the ability to discriminate between specular images of an electroactive molecule, particularly in terms of significant peak potential difference. A groundbreaking strategy was recently proposed, based on the use of “inherently chiral” molecular selectors, with chirality and key functional properties originating from the same structural element. Large differences in peak potentials have been observed for the enantiomers of different chiral molecules, also of applicative interest, using different selectors, all of them based on atropisomeric biheteroaromatic scaffolds of axial stereogenicity. However, helicene systems also provide inherently chiral building blocks with attractive features. In this paper the enantiodiscrimination performances of enantiopure inherently chiral films obtained by electrooxidation of a thiahelicene monomer with helicoidal stereogenicity are presented for the first time. The outstanding potentialities of this novel approach are evaluated towards chiral probes with different chemical nature and bulkiness, in comparison with a representative case of the so far exploited class of inherently chiral selectors with axial stereogenicity. It is also verified that the high enantiodiscrimination ability holds as well for electron spins, as for atropisomeric selectors.

Spin-Dependent Processes Measured without a Permanent Magnet

A novel Hall circuit design that can be incorporated into a working electrode, which is used to probe spin-selective charge transfer and charge displacement processes, is reviewed herein. The general design of a Hall circuit based on a semiconductor heterostructure, which forms a shallow 2D electron gas and is used as an electrode, is described. Three different types of spin-selective processes have been studied with this device in the past: i) photoinduced charge exchange between quantum dots and the working electrode through chiral molecules is associated with spin polarization that creates a local magnetization and generates a Hall voltage; ii) charge polarization of chiral molecules by an applied voltage is accompanied by a spin polarization that generates a Hall voltage; and iii) cyclic voltammetry (current-voltage) measurements of electrochemical redox reactions that can be spin-analyzed by the Hall circuit to provide a third dimension (spin) in addition to the well-known current and voltage dimensions. The three studies reviewed open new doors into understanding both the spin current and the charge current in electronic materials and electrochemical processes.

Temporal multi-sensor system for voltammetric recognition of l- and d-tryptophan enantiomers based on generalized principal component analysis

The results of a quantitative reading of the cyclic voltammetry behavior of the tryptophan (Trp) enantiomers deposited on an electrochemically activated glassy carbon electrode (GCE) are presented.

Pulsed electroconversion for highly selective enantiomer synthesis

Asymmetric synthesis of molecules is of crucial importance to obtain pure chiral compounds, which are of primary interest in many areas including medicine, biotechnology, and chemistry. Various methods have been used very successfully to increase the enantiomeric yield of reaction pathways, but there is still room for the development of alternative highly enantioselective reaction concepts, either as a scientific challenge of tremendous fundamental significance, or owing to the increasing demand for enantiopure products, e.g., in the pharmaceutical industry. In this context, we report here a strategy for the synthesis of chiral compounds, based on pulsed electrochemical conversion. We illustrate the approach with the stereospecific electroreduction of a prochiral model molecule at chiral mesoporous metal structures, resulting in an enantiomeric excess of over 90%. This change of paradigm opens up promising reaction schemes for the straightforward synthesis of high-added-value molecules.

"Inherently Chiral" Ionic-Liquid Media: Effective Chiral Electroanalysis on Achiral Electrodes

To achieve enantioselective electroanalysis either chiral electrodes or chiral media are needed. High enantiodiscrimination properties can be granted by the "inherent chirality" strategy of developing molecular materials in which the stereogenic element responsible for chirality coincides with the molecular portion responsible for their specific properties, an approach recently yielding outstanding performances as electrode surfaces. Inherently chiral ionic liquids (ICILs) have now been prepared starting from atropisomeric 3,3'-bicollidine, synthesized from inexpensive reagents, resolved into antipodes without need of chiral HPLC and converted into long-chain dialkyl salts with melting points below room temperature. Both the new ICILs and shorter family terms, solid at room temperature, employed as low-concentration additives in achiral ILs, afford impressive enantioselection for the enantiomers of different probes on achiral electrodes, regularly increasing with additive concentration.