Electrochemical chiral recognition of tryptophan using a glassy carbon electrode modified with β-cyclodextrin and graphene

Highly sensitive enantioselective spectrofluorimetric determination of R-/S-mandelic acid using l-tryptophan-modified amino-functional silica-coated N-doped carbon dots as novel high-throughput chiral nanoprobes

Amino-functional silica-coated N-doped carbon dots (NH2-SiO2-CDs) were covalently modified by l-tryptophan (chiral selector) by producing an amide bond between carboxyl groups of L-try and amino groups of NH2-SiO2-CDs to develop a novel high throughput chiral nanoprobes (L-try-CONH-SiO2-CDs) for highly sensitive and enantioselective quantification of S-/R-mandelic acid (S-/R-Man). The method showed a great difference between S- and R-isomers (enantioselectivity coefficient = 4.17) due to the ultra-stability of the Meisenheimer complex that was formed between S-isomer and nanoprobe (KS-Man/KR-man = 2122.7, where K is the binding-constant). At optimal experimental conditions, two linear ranges of 0.5-25.0 (LOD of 0.05 μM) and 0.5-22.0 μM (LOD of 0.27 μM) for S- and R-Man, respectively, along with an enhanced sensitivity toward S-isomer (about 5.7-fold higher than R-isomer) were attained. High selectivity for the determination of mandelic acid was achieved compared to metal ions, amino acids, and sugars that commonly coexist with it. Intra-day as well as inter-day assays, respectively, showed RSD values of about 3.2 and 3.9%. The mechanistic studies were performed for proving the enantioselective behavior of the developed nanoprobe. The method was then used for S-/R-mandelic acid determination in bio-samples. The figures of merit for the method were found to be better than those already reported for enantioselective detection of R-/S-Man.

Chiral MOFs encapsulated by polymers with poly-metallic coordination as chiral biosensors

Chiral materials have drawn the widespread attention for their its chiral recognition ability. The design and synthesis of chiral material are of importance owing to the unpredictability in controlling chirality during the synthesis process. To circumvent problems, a chiral MOF (D-His-ZIF-8) was synthesized by ligand exchange of 2-methylimidazole (Hmim) on ZIF-8 by D-histidine (D-His), which can be treated as chiral host to distinguish amino acid enantiomers. The obtained D-His-ZIF-8 can provide chiral nanochannels for amino acid guests. Meanwhile, polynary transition-metal ion (Co²⁺ and Fe³⁺) coordinating with polydopamine (PDA) wrapped on the surface of D-His-ZIF-8 can increase the active sites. The electrochemical chiral recognition behavior showed that D-His-ZIF-8@CoFe-PDA exhibited good recognition of the tryptophan enantiomer (L/D-Trp) (working potential of -0.2 V vs. Hg/HgCl₂). The LOD and LOQ of L-Trp were 0.066 mM and 0.22 mM, respectively, while the LOD and LOQ of D-Trp were 0.15 mM and 0.50 mM, respectively. Finally, the usefulness of D-His-ZIF-8@CoFe-PDA/GCE was evaluated with a recovery of 94.4-103%. The analysis of real  samples shows that D-His-ZIF-8@CoFe-PDA/GCE is a feasible sensing platform for the detection of L-Trp and D-Trp.

Highly Efficient versus Null Electrochemical Enantioselective Recognition Controlled by Achiral Colinkers in Homochiral Metal-Organic Frameworks

Chiral materials capable of electrochemical enantiomeric recognition are highly desirable for many applications, but it is still very challenging to achieve high recognition efficiency for lack of the knowledge of structure-property relationships. Here, we report the completely distinct enantiomeric recognition related to slightly different achiral colinkers in isomorphic homochiral metal-organic frameworks with the same chiral linker. Cu-TBPBe, for which the achiral colinker has two pyridyl rings connected by ─CH═CH─, shows excellent enantioselectivity and sensitivity for electrochemical recognition of l-tryptophan (Trp) with a detection limit of 3.16 nM. The l-to-d ratio of differential pulse voltammetric (DPV) currents reaches 53, which is much higher than the values (2-14) reported for previous electrochemical sensors. By contrast, Cu-TBPBa, in which the achiral colinker has -CH₂-CH₂- between pyridyl rings, is incapable of discrimination between l-Trp and d-Trp. Structural and spectral analyses suggest that the achiral conjugated colinker and the chiral moieties around it cooperate to produce a chiral pocket in favor of enantioselective adsorption through multiple hydrogen-bonding and π-π stacking interactions. The work demonstrated that Cu-TBPBe can be used to fabricate reliable electrochemical sensors for ultrasensitive quantification of Trp enantiomers in racemic mixtures and in complex biological systems such as urine. The work also highlights that an achiral coligand can be of vital importance in determining enantiomeric discrimination, opening up a new avenue for the design of chiral sensing materials.

Appraising the electrocatalytic performance of beta-cyclodextrin embellished supramolecular recognition system for pernicious food colorants

The current research presents the evaluation of supramolecular proficiency of the designed platform for electrocatalytic determination of pernicious food colorants, amaranth and fast green. The approach involving surface modification of glassy carbon electrode with beta cyclodextrin decorated strontium ferrite reduced graphene oxide nanocomposite (SFrGO-βCD) to ensure fast and reversible electro-oxidation of hydroxyl groups of the colorant molecules. The synergy between SF and rGO facilitated the sensor with enhanced surface area and conductivity through faradic redox reaction. Tremendous decrease in the obtained values of peak separation potential and impedance as manifested in CV and EIS analysis, enabled by electrostatic interactions between surface functionalities of rGO and βCD has resulted in the significant augmentation of sensitivity. The value of charge transfer coefficient, number of electrons involved, nature of electron transport process at electrode electrolyte interface during the analysis of electrochemical detection were explored through CV experiments. Food samples analysis (without spiking) utilizing screen printed electrode manifested the sensor as portable device for real time monitoring. Outstanding detection limit (0.022 nM for amaranth and 0.051 nM for fast green), excellent regenerability (Relative standard deviation less than 3%) and apparent recovery rate (above 90%) of the modified electrode presented a colossal potential for the development of sustainable and commercially competitive electrochemical sensor in food sector.

Ultrasensitive Electrochemical Impedance Chiral Discrimination and Sensing of Tryptophan Isomers Based on Core-Shell-Structured Au-Ag Nanoparticles

Au-Ag nanoparticles (Au-Ag NPs) with a core-shell structure are prepared and used for ultrasensitive electrochemical impedance (EI) discrimination of the isomers of tryptophan (Trp). As revealed by circular dichroism, rotary polarization caused by the Au-Ag NPs is consistent with D-Trp but opposite to L-Trp, and thus, the Au-Ag NPs can selectively combine with D-Trp through preferential interactions. Compared with Au-Ag NPs, the composites of D-Trp and Au-Ag NPs (Au-Ag NPs/D-Trp) display significantly increased charge transfer resistance (R_ct); differently, the R_ct of Au-Ag NPs/L-Trp remains almost unchanged because the Au-Ag NPs exhibit poor affinity toward L-Trp. Therefore, ultrasensitive EI enantiodiscrimination of the isomers of Trp is realized even at an extremely low concentration of the Trp isomers (0.1 nM). In addition, it is successfully applied in the ultrasensitive determination of D-Trp at a low concentration level (0.1 nM∼10 μM).

Supramolecular assembly induced chiral interface for electrochemical recognition of tryptophan enantiomers

The β-CD@PEI-Fc chiral interface was prepared based on the supramolecular host-guest interaction between ferrocene (Fc) grafted polyethyleneimine (PEI-Fc) and chiral β-cyclodextrin (β-CD). SEM results show that β-CD@PEI-Fc interface has a regular spatial structure, which can effectively distinguish tryptophan (Trp) enantiomers. Under the optimal conditions, differential pulse voltammetry shows that the peak current ratio (Id/Il) of Trp enantiomers can reach 2.84 at 15 °C. More interestingly, the β-CD@PEI-Fc/GCE exhibited chiral recognition of d-Trp and l-Trp via water contact angle measurements. There was a good linear relationship between the peak current and the concentration of Trp enantiomers in the range from 0.005 mM to 0.10 mM. Finally, the chiral interface can be applied for quick detection of the proportion of isomers in Trp racemic solution, which is very important for chiral recognition in racemic mixture of chiral compounds. Meanwhile, the β-CD@PEI-Fc/GCE showed good stability and reproducibility.

An electrochemical chiral sensor based on the synergy of chiral ionic liquid and 3D-NGMWCNT for tryptophan enantioselective recognition

A facile chiral composite (3D-NGMWCNT@(S,S)-CIL) was prepared by integrating three-dimensional N-doped graphene oxide multi-walled carbon nanotubes (3D-NGMWCNT) and chiral ionic liquid ((S,S)-CIL) via electrodeposition. SEM, XRD, XPS, and electrochemical methods were used to characterize this composite and it revealed that the integrated 3D-NGMWCNT@(S,S)-CIL composite showed excellent electrochemical performance. Therefore, a 3D-NGMWCNT@(S,S)-CIL/GCE electrochemical sensor was constructed for enantioselective recognition of Trp enantiomers. The coefficient (I_L/I_D) of the 3D-NGMWCNT@(S,S)-CIL/GCE chiral sensor was 2.26 by differential pulse voltammograms (DPV), revealing that the synthesized 3D-NGMWCNT@(S,S)-CIL had a higher affinity for L-Trp than D-Trp. Moreover, UV-V is spectroscopy and a water contact angle test also proved this result. The 3D-NGMWCNT@(S,S)-CIL/GCE sensor had a detection limit of 0.024 μM and 0.055 μM, and sensitivity of 62.35 μA·mM^-1·cm^-2 and 30.40 μA·mM^-1·cm^-2 for L-Trp and D-Trp, respectively, with a linear response range of 0.01 to 5 mM. In addition, the 3D-NGMWCNT@(S,S)-CIL/GCE chiral sensor showed excellent stability, and good reproducibility and was applied to detect L-Trp or D-Trp in real samples. The novel 3D-NGMWCNT@(S,S)-CIL/GCE chiral sensor provides an efficient and convenient strategy for chiral enantioselective recognition. Schematic construction of the 3D-NGMWCNT@(S,S)-CIL/GCE chiral electrochemical sensors.

Chiral voltammetric sensor for tryptophan enantiomers by using a self-assembled multiwalled carbon nanotubes/polyaniline/sodium alginate composite

Due to the crucial role of amino acids in life sciences and pharmaceutics, identification of optical amino acid molecules is of great significance. In this study, the two materials (CNT and PANI) were combined together to obtain the magnification of electrochemical signal by substrate material (CNT/PANI). Then a self-assembled multiwalled carbon nanotubes/polyaniline/sodium alginate (CNT/PANI/SA) nanocomposite with chiral sites and conductive material was synthesized as the electrochemical sensing interface. Next, a novel electrochemical sensing interface was fabricated via modifying the as-prepared chiral material on a polished glassy carbon electrode (CNT/PANI/SA/GCE) for precisely, efficiently, and rapidly differentiation of tryptophan (Trp) enantiomers. It was observed that CNT/PANI/SA/GCE showed desirable stereoselective recognition effect in the variety of signal strength to peak current (Ip) to the different optical activity of Trp enantiomers. In the case of optimal conditions, the peak current ratio in the solution of l-Trp and d-Trp (I_D /I_L ) was observed to be 2.1 at CNT/PANI/SA/GCE by differential pulse voltammogram (DPV). UV-visible spectroscopy further showed that CNT/PANI/SA had a greater binding energy to l-Trp. Also different factors affecting the enantioselectivity of CNT/PANI/SA/GCE, such as the incubation time, pH, and dropcoating volume of CNT/PANI/SA were optimized. Moreover, the proposed CNT/PANI/SA/GCE showed excellent specific stereoselectivity and anti-interference ability. Besides, the proposed chiral sensing platform can be effectively applied in real samples to detect Trp enantiomers sensitively. This work inspires us a new path for the preparation of substrate material with excellent electrical conductivity, as well as extend its application potential in chiral recognition.

β-Cyclodextrin-Immobilized Ni/Graphene Electrode for Electrochemical Enantiorecognition of Phenylalanine

In this work, a Ni/graphene (Ni/G) electrode was designed and fabricated by plasma-enhanced chemical vapor deposition (PECVD) for the ultrasensitive recognition of d- and l-phenylalanine. Through a single-step PECVD process, the Ni/G electrode can achieve better hydrophilicity and larger catalytic surface area, which is beneficial for the electrochemical recognition of bio-objects. After surface modification with β-cyclodextrin, the Ni/G electrode can distinguish d-phenylalanine from l-phenylalanine according to a 0.09 V peak shift in differential pulse voltammetry tests. Moreover, this Ni/G electrode achieved a detection limit as low as 1 nM and a wide linear range from 1 nM to 10 mM toward l-phenylalanine, with great storage stability and working stability.

Electrochemical sensing of hydrogen peroxide using a glassy carbon electrode modified with multiwalled carbon nanotubes and zein nanoparticle composites: application to HepG2 cancer cell detection

A nanobiocomposite was prepared from multiwalled carbon nanotubes and zein nanoparticles. It was dispersed in water/ethanol and drop cast onto a glassy carbon electrode. The modified electrode can be used for electroreduction of H₂O₂ (typically at a working potential of -0.71 V vs. Ag/AgCl). The electrochemical properties of the electrode were investigated by cyclic voltammetry, linear sweep voltammetry, chronoamperometry and electrochemical impedance spectroscopy. Response to H₂O₂ is linear in the 0.049 to 22 μM concentration range, and the detection limit is 35 nM at pH 7.0. The sensor was successfully utilized for the measurement of H₂O₂ in a synthetic urine sample, and for monitoring the release of H₂O₂ from human dermal fibroblasts and human hepatocellular carcinoma cells. Graphical abstractSchematic representation of a novel metal- and enzyme-free electrochemical nanosensor. A glassy carbon electrode was modified with a nanocomposite prepared from multiwalled carbon nanotubes and zein nanoparticles. It was applied to the identification of liver cancer cells via sensing of H2O2 and has a very low detection limit.

D-amino acid electrochemical biosensor based on D-amino acid oxidase: Mechanism and high performance against enantiomer interference

For D-amino acid (DAA) electrochemical biosensors, it is necessary to achieve chiral recognition in racemic solutions or mixtures. However, common chiral recognition is only performed in a single isomer solution. Here, D-amino acid oxidase (DAAO) was used as a chiral selector, and carbon nanotubes (CNTs) as a signal amplifier to construct a non-mediator-style DAA biosensor. The biosensor showed high performance against enantiomer interference: in alanine (Ala) enantiomer mixtures, accurate quantification of D-Ala was achieved when the concentration ratio of L-Ala to D-Ala was 100. In Ala racemic solutions, the linear equation slope was almost consistent with that of standard D-Ala. This high performance was due to the combination of stereoselectivity (enzyme protein) and a catalytic reaction (redox center). The mechanism for the electrical signal change of the biosensor was explored and verified by cyclic voltammetry (CV). The results showed that (i) flavin adenine dinucleotide (FAD, redox center of DAAO) was a direct electroactive substance that produced a reduction peak current; in the presence of O₂, the amount of FAD increased leading to an increase of the reduction peak current. (ii) In the presence of DAA, the chemical reaction FAD+DAA → imino acids+ FADH₂ occurred and consumed FAD, which resulted in its decrease; thus, the reduction peak current also decreased. Under the same oxygen concentration, the linear decrease of the reduction peak current in the presence of DAA was due to FAD consumption. The biosensor was used for practical analyses in milk and urine samples with satisfactory results.

Surface-modified three-dimensional graphene nanosheets as a stationary phase for chromatographic separation of chiral drugs

Carbon-based stationary phases for chromatographic separation have been commercially available since the 1980s. Porous graphitic carbon liquid chromatography columns are known to be highly resistant to aggressive mobile phases and extreme pH values of solvents and eluents, an important advantage compared to conventional silica-based alternatives. In our work, we demonstrate a new variant of carbon-based stationary phases for liquid chromatography, specifically developed for chiral separation. Mesoporous three-dimensional graphene nanosheets (3D GNS), functionalized with tetracyanoethylene oxide (TCNEO) and (S)-(+)-2-pyrrolidinemethanol, demonstrate pharmaceutical-grade chiral separation of model ibuprofen and thalidomide racemic mixtures when used as Chiral Stationary Phases (CSPs), with performance parameters comparable to currently commercially available CSPs. Simple covalent attachment of functionalization groups to the surface of mesoporous three-dimensional graphene nanosheets makes these carbon-based CSPs chemically stable and up to an order of magnitude less expensive than standard silica-based analogues.

β-Cyclodextrin Functionalized Nanoporous Graphene Oxides for Efficient Resolution of Asparagine Enantiomers

Efficient resolution of racemic mixture has long been an attractive but challenging subject since Pasteur separated tartrate enantiomers in 19^th  century. Graphene oxide (GO) could be flexibly functionalized by using a variety of chiral host molecules and therefore, was expected to show excellent enantioselective resolution performance. However, this combination with efficient enantioselective resolution capability has been scarcely demonstrated. Here, nanoporous graphene oxides were produced and then covalently functionalized by using a chiral host material-β-cyclodextrin (β-CD). This chiral GO displayed enantioselective affinity toward the l-enantiomers of amino acids. In particular, >99 % of l-asparagine (Asn) was captured in a racemic solution of Asn while the adsorption of d-enantiomer was not observed. This remarkable resolution performance was subsequently modelled by using an attach-pull-release dynamic method. We expect this preliminary concept could be expanded to other chiral host molecules and be employed to current membrane separation technologies and finally show practical use for many other racemates.

Voltammetric chiral discrimination of tryptophan using a multilayer nanocomposite with implemented amino-modified β-cyclodextrin as recognition element

An electrochemical chiral multilayer nanocomposite was prepared by modifying a glassy carbon electrode (GCE) via opposite-charge adsorption of amino-modified β-cyclodextrin (NH₂-β-CD), gold-platinum core-shell microspheres (Au@Pts), polyethyleneimine (PEI), and multi-walled carbon nanotubes (MWCNTs). The modified GCE was applied to the enantioselective voltammetric determination of tryprophan (Trp). The Au@Pts enable an effective immobilization of the chiral selector (NH₂-β-CD) and enhance the electrochemical performance. Scanning electron microscopy, transmission electron microscopy, UV-vis spectroscopy, FTIR and electrochemical methods were used to characterize the nanocomposite. Trp enantiomers were then determined by differential pulse voltammetry (DPV) (with a peak potential of +0.7 V vs. Ag/AgCl). The recognition efficiency was expressed by an increase in peak height by about 32% for DPV determinations of L-Trp compared to D-Trp in case of a 5 mM Trp solution of pH 7.0. Response was linear in the 10 μM to 5.0 mM concentration range, and the limits of detection were 4.3 μM and 5.6 μM with electrochemical sensitivity of 43.5 μA·μM^-1·cm^-2 and 34.6 μA·μM^-1·cm^-2 for L-Trp and D-Trp, respectively (at S/N = 3). Graphical Abstract Schematic of an electrochemical chiral multilayer nanocomposite composed of multi-walled carbon nanotubes (MWCNTs), polyethyleneimine (PEI), gold-platinum core-shell microspheres (Au@Pt) and amino-modified β-cyclodextrin (NH₂-β-CD). It was prepared by modifying a glassy carbon electrode (GCE) for enantioselective voltammetric determination of tryptophan (Trp) enantiomers.

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.

Chiral recognition of naproxen enantiomers based on fluorescence quenching of bovine serum albumin-stabilized gold nanoclusters

A simple, fast and green method for chiral recognition of S- and R-naproxen has been introduced. The method was based on quenching of the fluorescence intensity of bovine serum albumin-stabilized gold nanoclusters in the presence of naproxen enantiomers. The quenching intensity in the presence of S-naproxen was higher than R-naproxen when phosphate buffer solution at pH7.0 was used. The chiral recognition occurred due to steric effect between bovine serum albumin conformation and naproxen enantiomers. Two linear determination range were established as 7.4×10^-7-9.1×10^-6 and 9.1×10^-6-3.1×10^-5molL^-1 for both enantiomers and detection limits of 7.4×10^-8molL^-1 and 9.5×10^-8molL^-1 were obtained for S- and R-naproxen, respectively. The developed method showed good repeatability and reproducibility for the analysis of a synthetic sample. To make the procedure applicable to biological samples, the removal of heavy metals from the sample is suggested before any analytical attempt.

The construction of electrochemical chiral interfaces using hydroxypropyl chitosan

A simple chiral electrochemical sensor based on hydroxypropyl chitosan (HPCS) covalently bound to multi-walled carbon nanotubes (MWCNTs) was developed for the recognition of mandelic acid (MA) enantiomers.

Preparation of β-cyclodextrin functionalized reduced graphene oxide: application for electrochemical determination of paracetamol

A sensitive electrochemical sensor of paracetamol was developed based on a β-CD/reduced graphene oxide modified electrode.

Chiral electrochemical recognition of tryptophan enantiomers at a multi-walled carbon nanotube–chitosan composite modified glassy carbon electrode

A multi-walled carbon nanotube–chitosan composite modified glassy carbon electrode (MWCNT–CS/GCE) was developed for the chiral recognition of tryptophan (Trp) enantiomers.