Electrochemical chiral sensing of tryptophan enantiomers by using 3D nitrogen-doped reduced graphene oxide and self-assembled polysaccharides

Synthesis of chiral graphene structures and their comprehensive applications: a critical review

From a molecular viewpoint, chirality is a crucial factor in biological processes. Enantiomers of a molecule have identical chemical and physical properties, but chiral molecules found in species exist in one enantiomer form throughout life, growth, and evolution. Chiral graphene materials have considerable potential for application in various domains because of their unique structural framework, properties, and controlled synthesis, including chiral creation, segregation, and transmission. This review article provides an in-depth analysis of the synthesis of chiral graphene materials reported over the past decade, including chiral nanoribbons, chiral tunneling, chiral dichroism, chiral recognition, and chiral transfer. The second segment focuses on the diverse applications of chiral graphene in biological engineering, electrochemical sensors, and photodetectors. Finally, we discuss research challenges and potential future uses, along with probable outcomes.

Functionalized Chiral Materials for Use in Chiral Sensors

Chirality represents a fundamental attribute within living systems and is a pervasive phenomenon in the natural world. The identification and analysis of chiral materials within natural environments and biological systems hold paramount importance in clinical, chemical, and biological sciences. Within chiral analysis, there is a burgeoning focus on developing chiral sensors exhibiting exceptional selectivity, sensitivity, and stability, marking it as a forefront area of research. In the past decade (2013-2023), approximately 1990 papers concerning the application of various chiral materials in chiral sensors have been published. Biological materials and nanomaterials have important applications in the development of chiral sensors, which accounting for 26.67% and 45.24% of the material-related applications in these sensors, respectively; moreover, the development of chiral nanomaterials is closely related to the development of portable and stable chiral sensors. Natural chiral materials, utilized as selective recognition units, are combined with carriers characterized by good physical and chemical properties through functionalization to form various functional chiral materials, which improve the recognition efficiency of chiral sensors. In this article, from the perspective of biological materials, polymer materials, nanomaterials, and other functional chiral materials, the applications of chiral sensors are summarized and the research prospects of chiral sensors are discussed.

Stereoselective Voltammetric Biosensor for Myo-Inositol and D-Chiro-Inositol Recognition

This paper describes the development of a simple voltammetric biosensor for the stereoselective discrimination of myo-inositol (myo-Ins) and D-chiro-inositol (D-chiro-Ins) by means of bovine serum albumin (BSA) adsorption onto a multi-walled carbon nanotube (MWCNT) graphite screen-printed electrode (MWCNT-GSPE), previously functionalized by the electropolymerization of methylene blue (MB). After a morphological characterization, the enantioselective biosensor platform was electrochemically characterized after each modification step by differential pulse voltammetry (DPV) and electrochemical impedance spectroscopy (EIS). The results show that the binding affinity between myo-Ins and BSA was higher than that between D-chiro-Ins and BSA, confirming the different interactions exhibited by the novel BSA/MB/MWCNT/GSPE platform towards the two diastereoisomers. The biosensor showed a linear response towards both stereoisomers in the range of 2-100 μM, with LODs of 0.5 and 1 μM for myo-Ins and D-chiro-Ins, respectively. Moreover, a stereoselectivity coefficient α of 1.6 was found, with association constants of 0.90 and 0.79, for the two stereoisomers, respectively. Lastly, the proposed biosensor allowed for the determination of the stereoisomeric composition of myo-/D-chiro-Ins mixtures in commercial pharmaceutical preparations, and thus, it is expected to be successfully applied in the chiral analysis of pharmaceuticals and illicit drugs of forensic interest.

A sensitive electrochemical sensor for chiral detection of tryptophan enantiomers by using carbon black and β‑cyclodextrin

A chiral sensor for the electrochemical identification of tryptophan (Trp) isomers is described. The electrochemical sensor was prepared based on the combination of (a) carbon black (CB-COOH) as conductive material, (b) Cu2+-modified β-cyclodextrin (Cu-β-CD), and (c) β-CD-based metal-organic frameworks (β-CD-MOF) as chiral selectors. The Cu-β-CD can be self-assembled into the CB-COOH and β-CD-MOF through electrostatic interactions, which was characterized by zeta potential analysis. UV-vis spectroscopy proved that Cu-β-CD displays a higher combination for D-Trp than L-Trp, and the β-CD-MOF at the surface of the GCE has a higher affinity for L-Trp than D-Trp, which endow an easier permeation of L-Trp to the surface of the electrode, thus leading to a larger electrochemical signal of differential pulse voltammetry (DPV). The enantioselectivity for L-Trp over D-Trp (IL/ID) is 2.13, with a low detection limit for D-Trp (11.18 μM) and L-Trp (5.48 μM). In addition, the proposed chiral sensor can be chosen to determine  the percentage of D-Trp in enantiomer mixture solutions and real sample detection with a recovery from 98.2 to 102.8% for L-Trp and 97.9 to 101.1% for D-Trp.

Facilitating two-electron oxygen reduction with pyrrolic nitrogen sites for electrochemical hydrogen peroxide production

Electrocatalytic hydrogen peroxide (H₂O₂) production via the two-electron oxygen reduction reaction is a promising alternative to the energy-intensive and high-pollution anthraquinone oxidation process. However, developing advanced electrocatalysts with high H₂O₂ yield, selectivity, and durability is still challenging, because of the limited quantity and easy passivation of active sites on typical metal-containing catalysts, especially for the state-of-the-art single-atom ones. To address this, we report a graphene/mesoporous carbon composite for high-rate and high-efficiency 2e^- oxygen reduction catalysis. The coordination of pyrrolic-N sites -modulates the adsorption configuration of the *OOH species to provide a kinetically favorable pathway for H₂O₂ production. Consequently, the H₂O₂ yield approaches 30 mol g^-1 h^-1 with a Faradaic efficiency of 80% and excellent durability, yielding a high H₂O₂ concentration of 7.2 g L^-1. This strategy of manipulating the adsorption configuration of reactants with multiple non-metal active sites provides a strategy to design efficient and durable metal-free electrocatalyst for 2e^- oxygen reduction.

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.

Design and Electrochemical Chiral Sensing of the Robust Sandwich Chiral Composite d-His-ZIF-8@Au@ZIF-8

In the pursuit of chiral materials with significant chiral recognition effects and stability, various strategies have been explored, among which the integration of metal nanoparticles and chiral metal-organic frameworks (CMOFs) is highly promising. However, metal nanoparticles (MNPs)/CMOFs show high chiral properties but inferior stabilities due to the MNPs being easily detached from the outside layer under certain conditions. Sandwich MOFs@MNPs@CMOF chiral materials can overcome this dilemma because the sandwich structure can maximize the regulation of the chiral interface activity, while the controlled outer layer can stop the MNPs from falling off in the procedure of chiral recognition. Here, a novel sandwich chiral material (d-His-ZIF-8@Au@ZIF-8) was synthesized by a ligand-assisted strategy with a well-defined sandwich morphology and chiral recognition capabilities. The electrochemical chiral recognition showed that d-His-ZIF-8@Au@ZIF-8 was the most efficient for the enantiomer of phenylalanine (Phe). This experiment presents a novel perspective for the fabrication of a chiral electrochemical sensing platform based on a solid sandwich chiral nanocomposite.

Genetically engineered bacterium-modified magnetic particles assisted chiral recognition and colorimetric determination of D/L-tryptophan in millets

Chiral recognition of enantiomers has always been a thorny issue since they exhibit the same properties under an achiral environment. Herein, polydopamine-functionalized magnetic particles (MP@PDA) were synthesized to immobilize the genetically engineered bacterium Escherichia coli DH5α (MP@PDA-E. coli). L-tryptophan (Trp) instead of D-Trp can be stereo-specifically degraded by tryptophanase in E. coli. The degradation product indole reacts with 4-dimethylaminobenzaldehyde to generate a rose-red adduct. Thus, MP@PDA-E. coli was employed to fabricate a chiral colorimetric method for chiral recognition and determination of L-Trp. The method averts the purification of tryptophanase. More importantly, tryptophanase demonstrates excellent enantioselective ability for L-Trp. The method can not only quantitatively detect L-Trp but also realize the measurement of the enantiomer percentage in the enantiomeric mixture. The feasibility was verified by detecting L-Trp in millet samples from different origins. Furthermore, a portable device was fabricated to make the method more convenient.

The fabrication of a highly electroactive chiral-interface self-assembled Cu(ii)-coordinated binary-polysaccharide composite for the differential pulse voltammetry (DPV) detection of tryptophan isomers

It is of significance to fabricate excellently performing chiral carbon nanocomposites for chiral electrochemical detection applications.

Electrochemical Amino Acid Sensing: A Review on Challenges and Achievements

The rapid growth of research in electrochemistry in the last decade has resulted in a significant advancement in exploiting electrochemical strategies for assessing biological substances. Among these, amino acids are of utmost interest due to their key role in human health. Indeed, an unbalanced amino acid level is the origin of several metabolic and genetic diseases, which has led to a great need for effective and reliable evaluation methods. This review is an effort to summarize and present both challenges and achievements in electrochemical amino acid sensing from the last decade (from 2010 onwards) to show where limitations and advantages stem from. In this review, we place special emphasis on five well-known electroactive amino acids, namely cysteine, tyrosine, tryptophan, methionine and histidine. The recent research and achievements in this area and significant performance metrics of the proposed electrochemical sensors, including the limit of detection, sensitivity, stability, linear dynamic range(s) and applicability in real sample analysis, are summarized and presented in separate sections. More than 400 recent scientific studies were included in this review to portray a rich set of ideas and exemplify the capabilities of the electrochemical strategies to detect these essential biomolecules at trace and even ultra-trace levels. Finally, we discuss, in the last section, the remaining issues and the opportunities to push the boundaries of our knowledge in amino acid electrochemistry even further.

Chiral Nematic Coatings Based on Cellulose Nanocrystals as a Multiplexing Platform for Humidity Sensing and Dual Anticounterfeiting

The need for a precise regulation of the properties of chiral nematic structures in response to external stimuli is addressed. Self-assembled iridescent coatings are produced under the effect of electrostatic interactions between cellulose nanocrystals and poly(acrylic acid), endowing a high anisotropic dissymmetry (>0.3) and sensitivity to environmental humidity (13.1 nm/1% at 68-75% relative humidity, RH). The phenomena associated with shifts in selective light reflection (green to orange) and polarization, facilitate tunable transmitted colors (blue to orange) at given rotation angles (RA). Such properties are conveniently integrated into a "RH-RA-color" ternary code that is introduced as an anticounterfeiting technology, taking advantage of multicolor patterns that conveniently track with changes in RH and RA. The proposed charge-driven assembly opens new opportunities for chiral nematic materials that enable precise optical sensing and information encryption.

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 (ID /IL ) 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.

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 Graphene Hybrid Materials: Structures, Properties, and Chiral Applications

Chirality has become an important research subject. The research areas associated with chirality are under substantial development. Meanwhile, graphene is a rapidly growing star material and has hard-wired into diverse disciplines. Rational combination of graphene and chirality undoubtedly creates unprecedented functional materials and may also lead to great findings. This hypothesis has been clearly justified by the sizable number of studies. Unfortunately, there has not been any previous review paper summarizing the scattered studies and advancements on this topic so far. This overview paper attempts to review the progress made in chiral materials developed from graphene and their derivatives, with the hope of providing a systemic knowledge about the construction of chiral graphenes and chiral applications thereof. Recently emerging directions, existing challenges, and future perspectives are also presented. It is hoped this paper will arouse more interest and promote further faster progress in these significant research areas.

A voltammetric sensor for simultaneous determination of hydroquinone and catechol by using a heterojunction prepared from gold nanoparticle and graphitic carbon nitride

An electrochemical sensor is described for the simultaneous determination of hydroquinone (HQ) and catechol (CC) based on a nanocomposite consisting of gold nanoparticles and graphitic carbon nitride (g-C₃N₄). The nanocomposite was synthesized via one-step thermal polymerization route and characterized by X-ray diffraction, transmission electron microscopy, and Fourier transform infrared techniques. The results confirmed the close contact between gold nanoparticles and g-C₃N₄. The nanocomposites exhibited the enhanced electrocatalytic redox towards HQ and CC. A glassy carbon electrode was modified with the nanocomposite to obtain a sensor that exhibited favorable analytical properties in the simultaneous detection of HQ and CC, with voltammetric peaks typically near -0.14 and - 0.02 V (vs. saturated calomel electrode). Linear responses are found between 1.0 and 320 μM for HQ (with a 0.3 μM detection limit; at S/N = 3), and between 0.1 and 320 μM for CC (with a 0.04 μM detection limit; at S/N = 3). The sensor was applied for the simultaneous determination of HQ and CC in spiked water samples, and acceptable recoveries were achieved. The superior sensing properties of the electrode are attributed to the synergy between the microstructure (heterojunction and porosity) and the π interactions between phenolic isomers and g-C₃N₄. Graphical abstractA novel electrochemical sensor is demonstrated for the simultaneous determination of hydroquinone and catechol based on a nanocomposite consisting of gold nanoparticles (AuNPs) and graphitic carbon nitride (g-C₃N₄).