A facile electrochemical chiral sensor for tryptophan enantiomers based on multiwalled carbon nanotube/hydroxypropyl-β-cyclodextrin functionalized carboxymethyl cellulose

Enhanced Enantioselective Discrimination Regulated by Achiral Ligands in Chiral Metal-Organic Frameworks

Enantioselective recognition is a fundamental property of chiral linkers in chiral metal-organic frameworks (CMOFs). However, clarifying the efficient enantioselective discrimination tailored by achiral linkers remains challenging to explain the chiral recognition mechanism and efficiency. Here, two CMOFs ([Zn2(l-Phe)2(bpa)2]n and [Zn2(l-Phe)2(bpe)2]n) with the completely different enantioselective recognition are synthesized from different nonchiral ligands and the same chiral ligands. The enantioselective recognition of CMOF is undoubtedly related to l-Phe, which differs in the hydrogen bonding to the Trp enantiomer. However, the electrochemical signals are weak and undifferentiated. [Zn2(l-Phe)2(bpe)2]n produces a flattened coplanar conformation with the -C═C- tether in the achiral ligand. The flattened achiral bpee ligand and its surrounding chiral phenylalanine molecules interact through multiple π-π stacking and hydrogen bonding, which together create a chiral sensor that facilitates the recognition of l-Trp. However, [Zn2(l-Phe)2(bpa)2]n produces a stepped conformation due to the -C-C- tether in the achiral ligand; despite the recognition effect of bpea, the recognition is unsatisfactory. Therefore, the chiral recognition of the two CMOFs stems from the synergistic effect between chiral and achiral ligands. This work shows that nonchiral ligands are also crucial in determining enantiomeric discrimination and opens up a new avenue for designing chiral materials.

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.

Chiral Materials: Progress, Applications, and Prospects

Chirality is a universal phenomenon in molecular and biological systems, denoting an asymmetric configurational property where an object cannot be superimposed onto its mirror image by any kind of translation or rotation, which is ubiquitous on the scale from neutrinos to spiral galaxies. Chirality plays a very important role in the life system. Many biological molecules in the life body show chirality, such as the "codebook" of the earth's biological diversity-DNA, nucleic acid, etc. Intriguingly, living organisms hierarchically consist of homochiral building blocks, for example, l-amino acids and d-sugars with unknown reason. When molecules with chirality interact with these chiral factors, only one conformation favors the positive development of life, that is, the chiral host environment can only selectively interact with chiral molecules of one of the conformations. The differences in chiral interactions are often manifested by chiral recognition, mutual matching, and interactions with chiral molecules, which means that the stereoselectivity of chiral molecules can produce changes in pharmacodynamics and pathology. Here, the latest investigations are summarized including the construction and applications of chiral materials based on natural small molecules as chiral source, natural biomacromolecules as chiral sources, and the material synthesized by design as a chiral source.

Enantioselective voltammetric sensor based on mesoporous graphitized carbon black Carbopack X and fulvene derivative

For medicine and pharmaceuticals, the problem of determining and recognizing the enantiomers of biologically active compounds is an actual issue because the enantiomers of the same substance can have different effects on living organisms. This paper describes the development of an enantioselective voltammetric sensor (EVS) based on a glassy carbon electrode (GCE) modified with mesoporous graphitized carbon black Carbopack X (CpX) and a fulvene derivative (1S,4R)-2-cyclopenta-2,4-dien-1-ylidene-1-isopropyl-4-methylcyclohexane (CpIPMC) for recognition and determination of tryptophan (Trp) enantiomers. Synthesized CpIPMC was characterized by 1 H and 13 C nuclear magnetic resonance (NMR), chromatography-mass spectrometry, and polarimetry. The proposed sensor platform was studied by Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS). Using the square-wave voltammetry (SWV), it was established that the developed sensor is an effective chiral platform for the quantitative determination of Trp enantiomers, including in a mixture and in biological fluids like urine and blood plasma, with adequate precision and recovery ranged from 96% to 101%.

Nanomaterial-based electrochemical sensors for detection of amino acids

Amino acids are the building blocks of proteins and muscle tissue. They also play a significant role in physiological processes related to energy, recovery, mood, muscle and brain function, fat burning and stimulating growth hormone or insulin secretion. Accurate determination of amino acids in biological fluids is necessary because any changes in their normal ranges in the body warn diseases like kidney disease, liver disease, type 2 diabetes and cancer. To date, many methods such as liquid chromatography, fluorescence mass spectrometry, etc. have been used for the determination of amino acids. Compared with the above techniques, electrochemical systems using modified electrodes offer a rapid, accurate, cheap, real-time analytical path through simple operations with high selectivity and sensitivity. Nanomaterials have found many interests to create smart electrochemical sensors in different application fields e.g. biomedical, environmental, and food analysis because of their exceptional properties. This review summarizes recent advances in the development of nanomaterial-based electrochemical sensors in 2017-2022 for the detection of amino acids in various matrices such as serum, urine, blood and pharmaceuticals.

Carboxymethyl cellulose-based materials as an alternative source for sustainable electrochemical devices: a review

In electrochemistry, bio-based materials are preferred over the traditional costly and synthetic polymers due to their abundance, versatility, sustainability and low cost. One of the bio-based polymers is carboxymethyl cellulose (CMC) which has become an overarching material in electrochemical devices pertaining to its amphiphilic nature with multi-carbon functional groups. Owing to its flexible framework with fascinating groups on its surface like hydroxide (-OH) and carboxylate (-COO-), CMC is able to be modified into conducting materials by blending it with other biopolymers, synthetic polymers, salts, acids and others. This blending has improved the profile of CMC by exploiting the ability of hydrogen bonding, swelling, adhesiveness and dispersion of charges and ions. These properties of CMC have made it possible to utilize this bio-sourced polymer in several applications as a conducting electrolyte, binder in electrodes, detector, sensor and active material in fuel cells, actuators and triboelectric nanogenerators (TENG). Thus, CMC based materials are cheap, environment friendly, hydrophilic, biodegradable, non-toxic and biocompatible which render it a desirable material in energy storage devices.

Preparation of fluorescein-modified polymer dots and their application in chiral discrimination of lysine enantiomers

Fluorescein-functionalized fluorescent polymer dots (F-PDs) were prepared by a facile one-pot method by magnetic stirring under mild conditions based on carboxymethylcellulose (CMC) and fluorescein as the precursors. The obtained F-PDs exhibited a nanoscale size of 3.2 ± 1.1 nm, excellent water solubility, and bright yellow fluorescence emission with a fluorescence quantum yield of 12.0%. The fluorescent probe displays rapid and sensitive chiral discrimination for lysine focused on different complexation abilities between lysine enantiomers and Cu²⁺. The concentration of L-lysine in the range 4 to 14 mM (R² = 0.997) was measured by the fluorescence intensity ratio (I₅₁₃/I₄₂₉); the exitation wavelength was set to λ_ex = 365 nm. The detection limit was 0.28 mM (3σ/slope). Importantly, this sensor accurately predicted the enantiomeric excess (ee) of lysine enantiomers at the designed concentration (lysine: 20 mM; Cu²⁺: 10 mM) ranges. The proposed sensor was successfully applied to determine L-lys (recovery: 95.8-101%; RSD: 0.465-3.34%) and ee values (recovery: 98.5-102%; RSD: 2.61-3.21%) in human urine samples using the standard addition method.

History of Cobaltabis(dicarbollide) in Potentiometry, No Need for Ionophores to Get an Excellent Selectivity

This work is a mini-review highlighting the relevance of the θ metallabis(dicarbollide) [3,3'-Co(1,2-C₂B₉H₁₁)₂]^- with its peculiar and differentiating characteristics, among them the capacity to generate hydrogen and dihydrogen bonds, to generate micelles and vesicles, to be able to be dissolved in water or benzene, to have a wide range of redox reversible couples and many more, and to use these properties, in this case, for producing potentiometric membrane sensors to monitor amine-containing drugs or other nitrogen-containing molecules. Sensors have been produced with this monoanionic cluster [3,3'-Co(1,2-C₂B₉H₁₁)₂]^-. Other monoanionic boron clusters are also discussed, but they are much fewer. It is noteworthy that most of the electrochemical sensor species incorporate an ammonium cation and that this cation is the species to be detected. Alternatively, the detection of the borate anion itself has also been studied, but with significantly fewer examples. The functions of the borate anion in the membrane are different, even as a doping agent for polypyrrole which was the conductive ground on which the PVC membrane was deposited. Apart from these cases related to closo borates, the bulk of the work has been devoted to sensors in which the θ metallabis (dicarbollide) [3,3'-Co(1,2-C₂B₉H₁₁)₂]^- is the key element. The metallabis (dicarbollide) anion, [3,3'-Co(1,2-C₂B₉H₁₁)₂]^-, has many applications; one of these is as new material used to prepare an ion-pair complex with bioactive protonable nitrogen containing compounds, [YH]_x[3,3'-Co(1,2-C₂B₉H₁₁)₂]_y as an active part of PVC membrane potentiometric sensors. The developed electrodes have Nernstian responses for target analytes, i.e., antibiotics, amino acids, neurotransmitters, analgesics, for some decades of concentrations, with a short response time, around 5 s, a good stability of membrane over 45 days, and an optimal selectivity, even for optical isomers, to be used also for real sample analysis and environmental, clinical, pharmaceutical and food analysis.

Highly Synergistic Sensor of Graphene Electrode Functionalized with Rutile TiO2 Microstructure to Detect L-Tryptophan Compound

Electrochemical processes are an effective method for detecting dangerous food ingredients. The synergetic between the reduction-oxidation (redox) processes inspired several papers and spurred research towards studying the new materials that can further adapt to optimize the rapid detection of chemical compounds. In this study, we report the eco-synthesis using graphene/TiO₂ rutile (G/TiO₂) electrode microstructures easily prepared through the physical method by mixing graphene and TiO₂ powder and its application for sensing L-tryptophan (Trp) compound. The material characterization results show that the graphene surface is smoother than the G/TiO₂ material. Graphene has been detected using X-ray diffraction (XRD) at a value of 2 thetas 26.39° and TiO₂ forms rutile crystals (110). The FTIR spectrum exhibits the functional groups from graphene of -OH, C-H, C=C, C-O, and TiO₂ identified with Ti-O bonds. The electrochemical test against G/TiO₂ electrode microstructures for Trp compound shows that 0.5 g TiO₂ rutile was the best composition functionalized with graphene material under 0.1M K₃[Fe(CN)₆] + 0.1M NaNO₃ electrolyte with a scan rate of 0.1 V/s. Determination of the detection limit was obtained at 0.005 mg/L with a HorRat value of 1.05%. The stability test was carried out for 25 days, and the addition of Pb(NO₃)₂ as an interference compound had a significant effect on the decrease in electrode performance.

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.