Chiral recognition of tryptophan enantiomers using chitosan-capped silver nanoparticles: Scanometry and spectrophotometry approaches

Synthesis of chiral hollow polymer microspheres and their applications in the spectroscopic chiral discrimination of tryptophan isomers

Hollow polymer microspheres (HPMs) were synthesized, which were then hydrolyzed in aqueous ammonia to produce carboxyl (-COOH) groups on their surface. L-phenylalanine (L-Phe) was grafted to the hydrolyzed HPMs (H-HPMs) through amidation reactions, endowing the H-HPMs with chirality. The resultant chiral HPMs (C-HPMs) were used for the chiral discrimination of tryptophan (Trp) isomers. Due to the same rotatorydirection of L-Phe and L-Trp, the C-HPMs showed greatly higher selectivity toward L-Trp than its isomer. After being adsorbed by the C-HPMs, the absorbance of the residual L-Trp is significantly lower than that of the residual D-Trp, and thus spectroscopic chiral discrimination of the Trp isomers was successfully achieved. The Trp isomers were also discriminated by the chiral solid polymer microspheres (C-SPMs), while the difference in the absorbance of the residual L-Trp and D-Trp is remarkably smaller than that obtained by the C-HPMs. The outstanding discrimination capability of the C-HPMs might be ascribed to their high surface permeability resulted from their unique hollow structure.

Highly Sensitive and Selective Detection of L-Tryptophan by ECL Using Boron-Doped Diamond Electrodes

L-tryptophan is an amino acid that is essential to the metabolism of humans. Therefore, there is a high interest for its detection in biological fluids including blood, urine, and saliva for medical studies, but also in food products. Towards this goal, we report on a new electrochemiluminescence (ECL) method for L-tryptophan detection involving the in situ production of hydrogen peroxide at the surface of boron-doped diamond (BDD) electrodes. We demonstrate that the ECL response efficiency is directly related to H2O2 production at the electrode surface and propose a mechanism for the ECL emission of L-tryptophan. After optimizing the analytical conditions, we show that the ECL response to L-tryptophan is directly linear with concentration in the range of 0.005 to 1 µM. We achieved a limit of detection of 0.4 nM and limit of quantification of 1.4 nM in phosphate buffer saline (PBS, pH 7.4). Good selectivity against other indolic compounds (serotonin, 3-methylindole, tryptamine, indole) potentially found in biological fluids was observed, thus making this approach highly promising for quantifying L-tryptophan in a broad range of aqueous matrices of interest.

A signal amplification for Trp isomers electrochemical recognition based on PEDOT:PSS and CS/PAA multilayers

In this work, enhanced tryptophan (Trp) isomers recognition was successfully demonstrated on (CS/PAA)3.5@PEDOT:PSS/GCE, a multilayer chiral sensor with good stability and reproducibility. The (CS/PAA)n multilayers chiral interface was first fabricated via alternating self-assembly of chiral chitosan (CS) and achiral polyacrylic acid (PAA). Conductive PEDOT:PSS was then compounded with (CS/PAA)n multilayers to obtain the chiral sensor for the electrochemical recognition of Trp isomers. The structure of the sensor and its chirality properties for Trp isomers were characterized by fourier transform infrared spectroscopy (FT-IR)，scanning electron microscopy (SEM) and electrochemical methods. The SEM images showed uniform distribution of PEDOT:PSS in the multilayer films, which changed the internal structure of the (CS/PAA)3.5. Consequently, (CS/PAA)3.5@PEDOT:PSS multilayers rendered more chiral centers in addition to improved good conductivity, which significantly amplified the oxidation peak current ratio of D-Trp to L-Trp (ID/IL) up to 6.71 at 25 °C. In addition, a linear relationship was observed between the peak current and Trp enantiomer concentration in the range of 0.002-0.15 mM, and the detection limits of D-Trp and L-Trp were 0.33 and 0.67 μM, respectively. More importantly, the percentage of D-Trp in non-racemic Trp enantiomers mixture solutions were successfully determined on the chiral interface, showing its effectiveness and promising potential in practical applications.

Fluorescent/Colorimetric Dual-Mode Discriminating Gln and Val Enantiomers Based on Carbon Dots

Discrimination and quantification of amino acid (AA) enantiomers are particularly important for diagnosing and treating diseases. Recently, dual-mode probes have gained a lot of research interest because they can catch more detecting information compared with the single-mode probes. Thus, it is of great significance to develop a dual-mode sensor realizing AA enantiomer discrimination conveniently and efficiently. In this work, carbon dot L-TCDs were prepared by N-methyl-1,2-benzenediamine dihydrochloride (OTD) and l-tryptophan. With the assistance of H2O2, L-TCDs show an excellent discrimination performance for enantiomers of glutamine (Gln) and valine (Val) in both fluorescent and colorimetric modes. The fluorescence enantioselectivity of Gln (FD/FL) and Val (FL/FD) is 5.29 and 4.13, respectively, and the colorimetric enantioselectivity of Gln (ID/IL) and Val (IL/ID) is 13.26 and 3.42, individually. The chiral recognition mechanism of L-TCDs was systematically studied. L-TCDs can be etched by H2O2, and the participation of AA enantiomers results in different amounts of the released OTD, which provides fluorescent and colorimetric signals for identifying and quantifying the enantiomers of Gln and Val. This work provides a more convenient and flexible dual-mode sensing strategy for discriminating AA enantiomers, which is expected to be of great value in facile and high-throughput chiral recognition.

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.

Magnetic Molecularly Imprinted Chitosan Combined with a Paper-Based Analytical Device for the Smartphone Discrimination of Tryptophan Enantiomers

The separation of enantiomers plays a critical role in pharmaceutical development, ensuring therapeutic efficacy, safety, and patent protection. It enables the production of enantiopure drugs and enhances our understanding of the properties of chiral compounds. In this study, a straightforward and effective chiral detection strategy was developed for distinguishing between tryptophan (TRP) enantiomers. The approach involved the preparation of a magnetic molecularly imprinted chitosan (MMIC) for preparation of the sample, which was combined with a nitrocellulose membrane (a paper-based analytical device, PAD) integrated with D-TRP covalently grafted with polymethacrylic acid (PAD-PMA_D-TRP). Discriminating between the TRP enantiomers was achieved using AuNPs as a colorimetric probe. Indeed, the presence of D-TRP rapidly induced the aggregation of AuNPs due to its strong affinity to PAD-PMA_D-TRP, resulting in a noticeable change in the color of the AuNPs from red to purple. On the other hand, L-TRP did not induce any color changes. The chiral analysis could be easily performed with the naked eye and/or a smartphone. The developed method exhibited a detection limit of 3.3 µM, and it was successfully applied to detect TRP in serum samples, demonstrating good recovery rates. The proposed procedure is characterized by its simplicity, cost-effectiveness, rapidity, and ease of operation.

Surface modified nanoparticles and their applications for enantioselective detection, analysis, and separation of various chiral compounds

The development of efficient enantioselective detection, analysis, and separation relies significantly on molecular interaction. In the scale of molecular interaction, nanomaterials have a significant influence on the performance of enantioselective recognitions. The use of nanomaterials for enantioselective recognition involved synthesizing new materials and immobilization techniques to produce various surface-modified nanoparticles that are either encapsulated or attached to surfaces, as well as layers and coatings. The combination of surface-modified nanomaterials and chiral selectors can improve enantioselective recognition. This review aims to offer engagement insights into the production and application of surface-modified nanomaterials to achieve sensitive and selective detection, better chiral analysis, and separation of numerous chiral compounds.

Constructing electrochemical sensor using molecular-imprinted polysaccharide for rapid identification and determination of l-tryptophan in diet

An imbalance of l-tryptophan (l-Trp), a basic component of a healthy diet, is harmful to human health. Traditional methods for detecting l-Trp have many limitations. To correct a deficiency or excess of l-Trp in human diets, it is necessary to develop a novel method that is rapid, low-cost, and high-sensitivity. Herein, a molecularly imprinted polysaccharide electrochemical sensor termed MIP/CS/MWCNTs/GCE (molecularly imprinted polymer/chitosan/multiwalled carbon nanotubes/glassy carbon electrode) targeting l-Trp was first constructed on a glassy carbon electrode, which was modified with multiwalled carbon nanotubes and chitosan using bifunctional monomers. The MIP/CS/MWCNTs/GCE obtained a wide linear range (1-300 μM) for detecting l-Trp and accurately detected the proportion of l-Trp in mixtures of Trp enantiomers. In milk samples, the spiked recoveries of l-Trp were 86.50 to 99.65%. The MIP/CS/MWCNTs/GCE electrochemical sensor possessed good recognition and detection performance for l-Trp and has promising potential for practical application.

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.

Enantioseparation/Recognition based on nano techniques/materials

Enantiomers show different behaviors in interaction with the chiral environment. Due to their identical chemical structure and their wide application in various industries, such as agriculture, medicine, pesticide, food, and so forth, their separation is of great importance. Today, the term "nano" is frequently encountered in all fields. Technology and measuring devices are moving towards miniaturization, and the usage of nanomaterials in all sectors is expanding substantially. Given that scientists have recently attempted to apply miniaturized techniques known as nano-liquid chromatography/capillary-liquid chromatography, which were originally accomplished in 1988, as well as the widespread usage of nanomaterials for chiral resolution (back in 1989), this comprehensive study was developed. Searching the terms "nano" and "enantiomer separation" on scientific websites such as Scopus, Google Scholar, and Web of Science yields articles that either use miniaturized instruments or apply nanomaterials as chiral selectors with a variety of chemical and electrochemical detection techniques, which are discussed in this article.

A chiral sensing platform based on a multi-substituted ferrocene-cuprous ion complex for the discrimination of electroactive amino acid isomers

An electrochemical chiral sensing platform based on a multi-substituted ferrocene-cuprous ion (Cu⁺) complex is constructed for the discrimination of electroactive amino acid (AA) isomers. Due to the opposite configurations of the AA isomers, the developed multi-substituted ferrocene-Cu⁺ can preferably combine with a right-handed AA (D-AA) isomer to form the ternary complex of multi-substituted ferrocene-Cu⁺-D-AA through π-π interactions, resulting in higher peak currents of D-AA. Therefore, the isomers of electroactive AA can be successfully discriminated. Among the tested electroactive AA isomers, the chiral sensing platform exhibits higher discrimination capability toward the isomers of tryptophan (Trp) than that of tyrosine (Tyr) and cysteine (Cys), which might be ascribed to the stronger π-π interactions between the benzene ring of the multi-substituted ferrocene and the indole ring of the Trp isomers.

Design of surface nanostructures for chirality sensing based on quartz crystal microbalance

Quartz crystal microbalance (QCM) has been widely used for various sensing applications, including chirality detection due to the high sensitivity to nanogram or picogram mass changes, fast response, real-time detection, easy operation, suitability in different media, and low experimental cost. The sensing performance of QCM is dependent on the surface design of the recognition layers. Various strategies have been employed for studying the relationship between the structural features and the specific detection of chiral isomers. This review provides an overview of the construction of chiral sensing layers by various nanostructures and materials in the QCM system, which include organic molecules, supermolecular assemblies, inorganic nanostructures, and metal surfaces. The sensing mechanisms based on these surface nanostructures and the related potentials for chiral detection by the QCM system are also summarized.

A Critical Review of Analytical Applications of Chitosan as a Sustainable Chemical with Functions Galore

Biomass and biowastes stand as sustainable and cost-effective environmentally benign alternative feedstock. Chitosan is a biocompatible, bioactive, and biodegradable biopolymer derived from chitin to achieve eight aspects out of the 12 green chemistry principles. Chitosan got significant attention in several fields including chemical analysis, in addition to chemical functionally, which enabled its use as adsorbent and its structural crosslinking using various crosslinkers. The physicochemical, technological, and optical properties of chitosan have been extensively exploited in analysis. Mainly, deacetylation degree and molecular weight are controlling its properties and hence controlling its functions. This review presents a structure, properties, and functions relationships of chitosan. It also aims to provide an overview of the different functions that chitosan can serve in each analytical technique such as supporting matrix, catalyst…etc. The contribution of chitosan in improving the ecological performance is discussed in each technique.

Chiral recognition of tryptophan enantiomers with UV-Vis spectrophotometry approach by using L-cysteine modified ZnFe2O4 nanoparticles in the presence of Cu2

Amino acids play a very important role in the fields of pharmacy and biochemistry, and the identification of amino acid enantiomers has become a research hotspot. In this study, chiral nanomaterials ZnFe₂O₄-L-Cys (Cys = cysteine) were prepared by the mechanical stirring method and characterizad by different kinds of techniques. The effect of pH and Cu²⁺ on the recognition of tryptophan by chiral nanomaterials ZnFe₂O₄-L-Cys was further explored by ultraviolet-visible spectroscopy. The experimental results show that when the pH of the recognition environment is neutral, ZnFe₂O₄-L-Cys can be used as chiral selectors for tryptophan enantiomers in the presence of Cu²⁺ and the absorbance of L-Trp is always stronger than D-Trp within a certain concentration range, which provides a novel and convenient way for the chiral recognition of tryptophan enantiomers.

Enantioselective chitosan-based racemic ketoprofen imprinted polymer: Chiral recognition and resolution study

This work presented a novel racemic imprinting process employing the chiral properties of chitosan monomer. The preparation of racemic ketoprofen (RS-KTP) imprinted polymer (RS-MIP) was conducted using glutaraldehyde as a crosslinker. The nature of elution solvent affected the % desorption ratio suggesting a heterogenous nature of the formed binding sites. Good imprinting was indicated by an imprinting factor of 3.50 for S-KTP. The enantioselectivity of the RS-MIP was indicated by enantioselectivity coefficient of 2.31 and % enantiomeric excess (%ee) of 28.55%. A SPE cartridge packed with RS-MIP enabled resolution of RS-KTP using gradient elution solvent system. Scatchard plot revealed two binding sites types of different affinity towards S-KTP and density observed for the RS-MIP. The binding capacity of RS-MIP showed observed dependence on the % ee of S-KTP indicating its enantioselectivity. The success of using racemic template for the preparation of enantioselective MIP brings a new possibility to achieve enantioseparation of racemic mixtures having very expensive or unavailable pure enantiomers.

Recent Advances in Electrochemical and Optical Sensors for Detecting Tryptophan and Melatonin

Tryptophan and melatonin are pleiotropic molecules, each capable of influencing several cellular, biochemical, and physiological responses. Therefore, sensitive detection of tryptophan and melatonin in pharmaceutical and human samples is crucial for human well-being. Mass spectrometry, high-performance liquid chromatography, and capillary electrophoresis are common methods for both tryptophan and melatonin analysis; however, these methods require copious amounts of time, money, and manpower. Novel electrochemical and optical detection tools have been subjects of intensive research due to their ability to offer a better signal-to-noise ratio, high specificity, ultra-sensitivity, and wide dynamic range. Recently, researchers have designed sensitive and selective electrochemical and optical platforms by using new surface modifications, microfabrication techniques, and the decoration of diverse nanomaterials with unique properties for the detection of tryptophan and melatonin. However, there is a scarcity of review articles addressing the recent developments in the electrochemical and optical detection of tryptophan and melatonin. Here, we provide a critical and objective review of high-sensitivity tryptophan and melatonin sensors that have been developed over the past six years (2015 onwards). We review the principles, performance, and limitations of these sensors. We also address critical aspects of sensitivity and selectivity, limit and range of detection, fabrication process and time, durability, and biocompatibility. Finally, we discuss challenges related to tryptophan and melatonin detection and present future outlooks.

Simultaneous determination of the concentration and enantiomeric excess of amino acids with a coumarin-derived achiral probe

The chirality of amino acids plays an important role in biological and medical sciences. The development of achiral small-molecule probes that can simultaneously determine the absolute configuration, enantiomeric excess, and total concentration of amino acids is significant. We reported the currently available achiral coumarin aldehyde probe that could form Schiff bases with free amino acids at room temperature to induce CD signals and change UV-vis signals. The red-shifted UV-vis signals were independent of the substrate's chirality and could be used to determine the total concentration. Conversely, the enantioselective CD signals could be used to determine the absolute configuration and enantiomeric excess. The usefulness and practicability of this sensing method were demonstrated by analyzing 6 non-racemic phenylalanine samples with different enantiomeric compositions and concentrations.

Application of nanoparticles in chiral analysis and chiral separation

Chiral molecules in relation to particular biological roles are stereoselective. Enantiomers differ significantly in their biochemical responses in biological environment. Despite the current advancement in drug discovery and pharmaceutical biotechnology, the chiral separation of some racemic mixtures continues to be one of the greatest challenges, because the available techniques are too costly and time consuming for the assessment of therapeutic drugs in the early stages of development worldwide. Various nanoparticles became one of the most investigated and explored nanotechnology-derived nanostructures especially in chirality where several studies are reported to improve enantiomeric separation of different racemic mixtures. The production of surface-modified nanoparticles has contributed to these limitations in terms of sensitivity, accuracy, and enantioselectivity that can be optimized and therefore makes these surface-modified nanoparticles convenient for enantiomeric identification and separation.

Colorimetric recognition of aromatic amino acid enantiomers by gluconic acid-capped gold nanoparticles

In this work, we prepared gold nanoparticles (AuNPs) by employing gluconic acid (GlcA) as reducing-cum-stabilizing agent. The proposed GlcA-AuNPs successfully worked as a colorimetric sensor for visual chiral recognition of aromatic amino acid enantiomers, namely tyrosine (D/L-Tyr), phenylalanine (D/L-Phe), and tryptophan (D/L-Trp). After adding L-types to GlcA-AuNPs solution, the color of the mixture changed from red to purple (or gray), while no obvious color change occurred on the addition of D-types. The effect can be detected by naked eyes. The particles have been characterized by transmission electron microscopy, Fourier-transform infrared spectroscopy, zeta potential, the dynamic light scattering analysis as well as UV-Vis spectroscopy. This assay can be used to determine the enantiomeric excess of L-Trp in the range from 0 to + 100%. The method has advantages in simplicity, sensitivity, fast response, and low cost.

Rapid microwave-assisted biosynthesis of chitooligosaccharide coated silver nanoparticles: assessments of antimicrobial activity for paediatric pulp therapy

Silver nanoparticles (AgNPs) have gained much attention in the treatment of dental caries due to their excellent antimicrobial activities. In this study, we have biosynthesised chitooligosaccharide coated silver nanoparticles (COS-AgNPs) using a rapid microwave-assisted method. The physical and chemical characterisations of biosynthesised COS-AgNPs were investigated using UV–visible spectroscopy (UV–vis), dynamic light scattering (DLS), Fourier transform infrared spectroscopy (FTIR), X-Ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM) equipped with selected area electron diffraction (SAED). Further, the developed COS-AgNPs were assessed for antimicrobial activity on Streptococcus mutans. From the DLS analysis, the average particle size of the COS-AgNPs was found to be ∼32 nm. The morphologies of the COS-AgNPs were monodisperse and spherical in shape. XRD peaks are coherent with the standard peaks of AgNPs. Antimicrobial activity of COS-AgNPs showed a significant zone of inhibition due to the synergistic effect of COS-AgNPs. Furthermore, the antibacterial potential of developed COS-AgNPs-Ca(OH)2 paste was checked against Streptococcus mutans, Staphylococcus aureus, and Enterococcus faecalis. COS-AgNPs based composite paste showed better inhibition zone against the tested microbes. Hence, COS-AgNPs and COS-AgNPs-Ca(OH)2 based paste materials will be promising biomaterials for pulp therapy in paediatric dentistry.

Immobilization of 6-O-α-maltosyl-β-cyclodextrin on the surface of black phosphorus nanosheets for selective chiral recognition of tyrosine enantiomers

A novel chiral sensing platform, 6-O-α-maltosyl-β-cyclodextrin (Mal-βCD)-based film, is proposed for selective electrochemical recognition of tyrosine (Tyr) enantiomers. Black phosphorus nanosheets (BP NSs) and Mal-βCD modified glassy carbon electrode (Mal-βCD/BP NSs/GCE) were prepared by a layer-to-layer drop-casting method, and the platform was easy to fabricate and facile to operate. It is proposed that the amino and hydroxyl groups of the Tyr enantiomers and the chiral hydroxyl groups of Mal-βCD selectively form intermolecular hydrogen bonds to dominate effective chiral recognition. Two linear equations of Ip (μA) = 11.40 C_L-Tyr (mM) + 0.28 (R² = 0.99147) and Ip (μA) = 7.96 C_D-Tyr (mM) + 0.22 (R² = 0.99583) in the concentration range 0.01-1.00 mM have been obtained. The limits of detection (S/N=3) for L-Tyr and D-Tyr were 4.81 and 6.89 µM, respectively. An interesting phenomenon was that the value of I_L-Tyr/I_D-Tyr (1.51) in this work was slightly higher than the value of I_L-Trp/I_D-Trp (1.49) reported in our previous study, where tryptophan (Trp) enantiomers were electrochemically recognized by Nafion (NF)-stabilized BPNSs-G2-β-CD composite. The two similar sensors fabricated by different methods showed different recognition ability toward either Tyr or Trp enantiomers, and the underlying mechanism was discussed in detail. More importantly, the proposed chiral sensor enables prediction of the percentages of D-Tyr in racemic Tyr mixtures. The chiral sensor may provide a novel approach for the fabrication of novel chiral platforms in the practical detection of L- or D-enantiomer in racemic Tyr mixtures.Graphical abstract.

Chirality at the Nanoparticle Surface: Functionalization and Applications

Chiral molecules, such as amino acids and carbohydrates, are the building blocks of nature. As a consequence, most natural supramolecular structures, such as enzymes and receptors, are able to distinguish among different orientations in space of functional groups, and enantiomers of chiral drugs usually have different pharmacokinetic properties and physiological effects. In this regard, the ability to recognize a single enantiomer from a racemic mixture is of paramount importance. Alternatively, the capacity to synthetize preferentially one enantiomer over another through a catalytic process can eliminate (or at least simplify) the subsequent isolation of only one enantiomer. The advent of nanotechnology has led to noteworthy improvements in many fields, from material science to nanomedicine. Similarly, nanoparticles functionalized with chiral molecules have been exploited in several fields. In this review, we report the recent advances of the use of chiral nanoparticles grouped in four major areas, i.e., enantioselective recognition, asymmetric catalysis, biosensing, and biomedicine.

One-pot synthesis of an anionic cyclodextrin-stabilized bifunctional gold nanoparticles for visual chiral sensing and catalytic reduction

A facile one-pot synthetic method for preparing gold nanoparticles by employing sugammadex (SUG), a carboxylic acid functionalized γ-cyclodextrin derivative, as reducing-cum-stabilizing agent herein was reported for the first time. The SUG protected gold nanoparticles (SUG-AuNPs) can work as a colorimetric sensor for visual chiral recognition of α-amino acids enantiomers, especially for lysine (Lys) and asparagine (Asn) enantiomers. The chiral recognition assay was successfully applied to determining the enantiometric excess of L-Lys and L-Asn ranging from -100 % to 100 % respectively. Moreover, the prepared SUG-AuNPs was found to exhibit efficient catalytic activity towards the reduction of toxic 4-nitrophenol by NaBH₄ and the efficiency of the system was further demonstrated through the reduction of other typical nitroaromatics under mild condition. The as-synthesized SUG-AuNPs shows good performance for both chiral sensing and reduction activity and thus may facilitate the practical application in the area of both chiral discrimination and catalysis.

Optical nanoprobes for chiral discrimination

Chiral recognition can be achieved by exploiting chiral properties of nanoparticles within various colorimetric and luminescent sensing systems.

Development of a selective and sensitive colour reagent for gold and silver ions and its application to desktop scanner analysis

Desktop scanners can be favorable alternatives to sophisticated spectrophotometers for the assessment of analytes in complex real samples. Distinctively, our method has been thoroughly investigated, optimized, validated and successfully applied to the assessment of silver and gold in complex real samples, applying syringal rhodanine (SR) as a novel specifically tailored chromogenic reagent and using a desktop scanner as a versatile sensor. Maximum colour absorbance was obtained in the presence of cetylpyridinium chloride (CPC) and cetyltrimethylammonium chloride (CTAC) for silver and gold chelates, respectively. For each metal ion, two ternary complexes were formed depending on the SR concentration with stoichiometries of 1 : 1 : 1 and 1 : 2 : 3 (Ag-SR-CPC) and 1 : 2 : 3 and 1 : 3 : 4 (Au-SR-CTAC), respectively. The methods adhered to Beer's law for 0.15-2.5 and 0.15-2.25 μg mL-1 with detection limits of 0.0089 and 0.0163 μg mL-1 for silver and gold, respectively. The molar absorptivities were 3.63 × 104 and 6.15 × 104 L mol-1 cm-1 at 550 nm and 554 nm, with Sandell's sensitivity indexes of 0.0029 and 0.0032 μg cm-2, respectively. The method was successfully applied to the assessment of silver and gold in a wide range of complex environmental samples.

Perylene-functionalized graphene sheets modified with β-cyclodextrin for the voltammetric discrimination of phenylalanine enantiomers

A facile approach was reported to synthesize β-cyclodextrin functionalized graphene that is bridged by 3,4,9,10-perylene tetracarboxylic acid (rGO-PTCA-CD) via a chemical route that involves the functionalization of rGO with PTCA followed by covalently cross-linking NH₂-β-CD. The as-prepared rGO-PTCA-CD was characterized by Fourier transform infrared spectroscopy, scanning electron microscopy, X-ray photoelectron spectroscopy and electrochemical methods. The working electrodes were thoroughly studied for the cyclic voltammetry by using [Fe(CN)₆]^4-/3- as redox probe and using ferrocene as an internal standard. Furthermore, rGO-PTCA-CD was successfully applied to the recognition of phenylalanine enantiomers. The host-guest inclusion interaction between rGO-PTCA-CD and the phenylalanine enantiomers was investigated by differential pulse voltammetry with Fc used as a competitor. The recognition result showed that the rGO-PTCA-CD-modified glassy carbon electrode exhibited higher chiral recognition capability for L-Phe than for D-Phe with an enantioselectivity coefficient of 2.07. The proposed modified electrode had a limit of detection of 0.08 nM and 0.2 nM (S/N = 3) for L-Phe and D-Phe, respectively, with a linear response range of 0.01 mM to 5 mM, which was ascribed to the synergy of the rGO-PTCA (e.g., its excellent electrochemical performance) and β-CD (e.g., the hydrophobic inner cavity with good molecular recognition and enrichment abilities).

Enantiomeric Recognition and Separation by Chiral Nanoparticles

Chiral molecules are stereoselective with regard to specific biological functions. Enantiomers differ considerably in their physiological reactions with the human body. Safeguarding the quality and safety of drugs requires an efficient analytical platform by which to selectively probe chiral compounds to ensure the extraction of single enantiomers. Asymmetric synthesis is a mature approach to the production of single enantiomers; however, it is poorly suited to mass production and allows for only specific enantioselective reactions. Furthermore, it is too expensive and time-consuming for the evaluation of therapeutic drugs in the early stages of development. These limitations have prompted the development of surface-modified nanoparticles using amino acids, chiral organic ligands, or functional groups as chiral selectors applicable to a racemic mixture of chiral molecules. The fact that these combinations can be optimized in terms of sensitivity, specificity, and enantioselectivity makes them ideal for enantiomeric recognition and separation. In chiral resolution, molecules bond selectively to particle surfaces according to homochiral interactions, whereupon an enantiopure compound is extracted from the solution through a simple filtration process. In this review article, we discuss the fabrication of chiral nanoparticles and look at the ways their distinctive surface properties have been adopted in enantiomeric recognition and separation.

Yolk–shell structured Au@Ag@mSiO2 as a probe for sensing cysteine enantiomers and Cu2+ based on circular dichroism

Novel yolk–shell structured Au@Ag@mSiO₂ was fabricated and used as a probe for recognition and quantification of cysteine enantiomers and Cu²⁺.

Silver Nanomaterials in Contemporary Molecular Physiology Research

Silver nanoparticles have numerous potential applications in engineering, industry, biology and medicine. Because of their unique chemical properties, they have become the focus of many research teams all over the world. Silver nanoparticles may exhibit significant antimicrobial and anticancer effects, and they may be a valuable part of various bioassays and biosensors. However, the research on biological and medical uses of AgNPs is related with numerous potential problems and challenges that need to be overcome in the years ahead. Possible toxic effects of silver nanoparticles on living organisms represent a great concern, both in clinical medicine and public health. Nevertheless, in the future, it may be expected that all metallic nanomaterials, including the ones made from silver will greatly benefit almost all natural scientific fields. In this short review, we focus on the recent research on silver nanoparticles in experimental physiology, as well as other areas of fundamental and clinical medicine.