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

Emerging frontiers in chiral metal-organic framework membranes: diverse synthesis techniques and applications

Chirality is a basic and universal property in nature, refering to the asymmetry of molecules, where they do not coincide with their mirror images. Chiral materials, in multiple forms, usually exhibit unique physical phenomena such as chiral luminescence and distinctive chemical properties. Metal-organic framework (MOF) membranes have high porosity and abundant active sites; thus, they are an excellent candidate for functionalization. With the involvement of chiral units, chiral MOF membranes demonstrate great potential in applications such as chiral sensing, separation and luminescence. In this review, we first introduce the up-to-date preparation methods for chiral MOF membranes, including direct and indirect methods, and then discuss their applications in enantiomer recognition, chiral separation, and circularly polarized luminescence. Finally, we summarize the challenges in developing chiral MOF membranes and provide a perspective on future developments.

Recent advance for enantiorecognition of chiral drugs sensing: Electrochemical, electrochemiluminescent and photoelectrochemical application

Chiral isomers show different behaviours and properties in physiological activities. It is of great significance to find productive approach to realize the recognition of enantiomers, which is a key issue in biochemical and pharmaceutical fields. Nowadays, chiral identification can be successfully achieved according to the discrepancies of special signals correlated with different enantiomers of multiple electrode structures. Electrochemical technologies have attracted wide interest in enantioselective analysis because of its unique merits, such as the economic and miniaturized instruments, simplified and environmental-friendly sample preparations. This review summarizes the development trends of electrochemical sensing in the enantiospecific analysis of chiral drugs, expounds the enantiospecific recognition mechanism between chiral selector and target enantiomers based on general electrochemical, electrochemiluminescent and photoelectrochemical sensors, respectively. In addition, this review attempts to predict the future application of electrochemical, electrochemiluminescent and photoelectrochemical-based technologies in the enantioselective recognition and detection.

Preparation of D-NCCDs and its application in fluorescent/colorimetric dual-mode discrimination of glutamine enantiomers

A new type of carbon dots (D-NCCDs) was synthesized by 3, 5-diaminobenzoic acid, N,N-dimethyl-o-phenylenediamine, and D-cysteine. The morphology and structure of D-NCCDs were investigated by transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and FT-IR spectra, and the chirality was characterized by circular dichroism. In the presence of hydrogen peroxide, the fluorescence of D-NCCDs at 487 nm (λex = 410 nm) showed great discrimination ability towards glutamine enantiomers. The ratio of fluorescence intensity (F/F0) to the concentration of D-Gln showed good linearity in the range 0.5-10 mM, with a detection limits of 0.11 mM. Meanwhile, the color of the solution gradually changed from light yellow to yellow-brown. The UV-Vis absorption ratio (A/A0) at 410 nm showed good linearity with the concentration of D-Gln in the range 0.5 to 20 mM; the detection limit is 7.7 μM. But the fluorescence and absorbance of D-NCCDs showed no significant change after the addition of L-glutamine. Thus, fluorescence and colorimetry dual-mode discrimination of glutamine enantiomers was achieved. The fluorescence enantioselectivity of Gln (FL-Gln/FD-Gln) is 1.62, and the colorimetric enantioselectivity of Gln (AD-Gln/AL-Gln) is 2.14. The chiral discrimination mechanism of D-NCCDs to Gln enantiomers was also investigated systematically. This work not only can discriminate glutamine enantiomers with high sensitivity and convenience, but also offers a new strategy for preparing new dual mode chiral nanoprobes.

Recent progress of chiral metal-organic frameworks in enantioselective separation and detection

Chiral compounds are abundantly distributed in both the natural world and biological systems. It is crucial to identify and detect chiral compounds in living systems or to separate and determine them in the natural environment. Many researchers have developed a range of chiral materials with different functionalizations to separate and detect chiral substances. Chiral metal-organic frameworks (CMOFs) have the potential to be used in enantioselective separation and detection due to their large surface areas, regulated framework topologies, particular substrate interactions, and accessible chiral sites. CMOFs contribute significantly to the development of enantiomer separation and detection in medicine, agriculture, food, environment, and other fields. This review focuses on four synthesis methods of CMOFs and their applications in chiral separation and chiral sensing in the past five years, mainly including chromatographic separation, membrane separation, optical sensing, electrochemical sensing, and other sensing methods. Finally, the challenges and potential growth direction of CMOFs in enantiomer separation and detection are discussed and prospected.

From individuals to families: design and application of self-similar chiral nanomaterials

Establishing an intimate relationship between similar individuals is the beginning of self-extension. Various self-similar chiral nanomaterials can be designed using an individual-to-family approach, accomplishing self-extension. This self-similarity facilitates chiral communication, transmission, and amplification of synthons. We focus on describing the marriage of discrete cages to develop self-similar extended frameworks. The advantages of utilizing cage-based frameworks for chiral recognition, enantioseparation, chiral catalysis and sensing are highlighted. To further promote self-extension, fractal chiral nanomaterials with self-similar and iterated architectures have attracted tremendous attention. The beauty of a fractal family tree lies in its ability to capture the complexity and interconnectedness of a family's lineage. As a type of fractal material, nanoflowers possess an overarching importance in chiral amplification due to their large surface-to-volume ratio. This review summarizes the design and application of state-of-the-art self-similar chiral nanomaterials including cage-based extended frameworks, fractal nanomaterials, and nanoflowers. We hope this formation process from individuals to families will inherit and broaden this great chirality.

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.

π-π+ Interaction Promoting the Absorption of Electroactive Chiral Selectors into the Cavity of Conductive Covalent Organic Framework for Enantioselective Sensing of Electrochemically Silent Molecules

To date, achieving enantioselective electroanalysis for electrochemically silent chiral molecules is still highly desired. Here, an ionic covalent organic framework (COF) consisting of the pyridinium cation was derived from the tripyridinium Zincke salt and 1,4-phenylenediamine in a one-pot reaction. The electrochemical measurements revealed that the ionic backbone contributed to the electron transfer with a low charge transfer resistance. Besides, the π-π+ interaction between the pyridinium cation and ferrocenyl unit can promote the absorption of electroactive chiral ferrocenyl reagents into the hole of COF, so as to afford the electrochemical signals by themselves, replacing the testing enantiomers. As a result, the electroactive complex used as an electrochemical platform was highly effective at enantiomerically recognizing amino alcohols (prolinol, valinol, leucinol, and alaninol) and amino acids (methionine, serine, and penicillamine), giving the ratios of current intensity between l- and d-enantiomers in the range of 1.46-1.72. Moreover, the density functional theory calculations determined the possible intermolecular interactions between the testing enantiomers and chiral selector: namely, hydrogen bonds and electrostatic attractions. Overall, the present work offers an effective strategy to enlarge the electrochemical scope for chiral recognition based on electroactive chiral COFs.

Potential Application Prospects of Biomolecule-Modified Two-Dimensional Chiral Nanomaterials in Biomedicine

Chirality, one of the most fundamental properties of natural molecules, plays a significant role in biochemical reactions. Nanomaterials with chiral characteristics have superior properties, such as catalytic properties, optoelectronic properties, and photothermal properties, which have significant potential for specific applications in nanomedicine. Biomolecular modifications such as nucleic acids, peptides, proteins, and polysaccharides are sources of chirality for nanomaterials with great potential for application in addition to intrinsic chirality, artificial macromolecules, and metals. Two-dimensional (2D) nanomaterials, as opposed to other dimensions, due to proper surface area, extensive modification sites, drug loading potential, and simplicity of preparation, are prepared and utilized in diagnostic applications, drug delivery research, and tumor therapy. Current advanced studies on 2D chiral nanomaterials for biomedicine are focused on novel chiral development, structural control, and materials sustainability applications. However, despite the advances in biomedical research, chiral 2D nanomaterials still confront challenges such as the difficulty of synthesis, quality control, batch preparation, chiral stability, and chiral recognition and selectivity. This review aims to provide a comprehensive overview of the origins, synthesis, applications, and challenges of 2D chiral nanomaterials with biomolecules as cargo and chiral modifications and highlight their potential roles in biomedicine.

A chiral metal-organic framework/cyclodextrin sensing interface for the chiral discrimination of tryptophan enantiomers

A chiral metal-organic framework (CMOF) was synthesized by introducing L-histidine (L-His) to zeolitic imidazolate framework-8 (ZIF-8) and then grafting with carboxymethyl-β-cyclodextrin (CM-β-CD). Compared with L-His-ZIF-8, the CM-β-CD-functionalized L-His-ZIF-8 (L-His-ZIF-8-CD) showed significantly enhanced discrimination ability for the tryptophan (Trp) enantiomers owing to the inherent chirality of CM-β-CD. The specificity of the chiral interface was also studied, and the results indicated that the discrimination ability for Trp enantiomers is significantly stronger than that for the enantiomers of cysteine (Cys) and tyrosine (Tyr), which might be due to the better matching between the indole ring of Trp and the chiral cavity of CM-β-CD.

Capsulation of EBTAC into ZIF-8 for the development of a signal-on fluorescent biosensor to detect alkaline phosphatase

Diseases such as liver cancer, extrahepatic biliary obstruction and osteocarcinoma are closely associated with the abnormal level of alkaline phosphatase (ALP). Hence, it is essential to develop a convenient assay to detect ALP activity. Herein, a novel signal-on fluorescent biosensor on account of the fluorescence signal of the aggregation-induced emission (AIE) fluorochrome 2,2',2'',2'''-((ethene-1,1,2,2-tetrayltetrakis(benzene-4,1-diyl))tetrakis(oxy))tetraacetic acid (EBTAC) encapsulated zeolitic imidazolate framework-8 (ZIF-8@EBTAC) was designed to monitor ALP. Due to the aggregation-induced emission of EBTAC, the synthetic ZIF-8@EBTAC shows robust fluorescence. Once pyrophosphate (ppi) was added, its complexation with Zn2+ in ZIF-8 triggered the collapse of the ZIF-8 framework, releasing encapsulated EBTAC molecules and restoring to free state, leading to the dramatical decrease in fluorescence. ALP could catalyze the hydrolysis of ppi to phosphate (pi), which is difficult to bind to Zn2+ and has little effect on the fluorescence of ZIF-8@EBTAC. Therefore, with the assistance of the substrate ppi, the ultimate fluorescence of ZIF-8@EBTAC was positively related with ALP activity. The constructed biosensor was able to monitor the ALP activity well from 0.01 to 100 U L-1, and a detection limit of 0.01 U L-1 was achieved. Based on the ability of EBTAC serving as a fluorescent probe with aggregation-induced luminescence properties, this proposed design can be applied to diverse targets and provide new ideas for the establishment of fluorescent biosensors.

Revelation of the Electrochemical Chiral Recognition of l-Proline-Tuned Zr-MOFs

Chirality plays an extremely important role in nature. In this work, a highly ordered and non-clustered crystalline material UiO-88-LP was synthesized by using l-proline (l-Pro)-tuning Zr-MOF and the solvothermal method, which was then modified on the glassy carbon electrode (GCE) to construct an electrochemical chiral interface for the recognition of tryptophan (Trp) configuration. UiO-88-LP composites were characterized by scanning electron microscopy, X-ray transmission diffraction, X-ray photoelectron spectroscopy, and Fourier transform infrared (FT-IR) spectroscopy. After optimization of the experimental conditions, redox peaks for l-Trp and d-Trp were clearly observed at the UiO-88-LP/GCE electrochemical sensing interface with a peak-to-current ratio (IL/ID) of 2.47. The peak current was positively correlated with the concentration of Trp. The electrochemical recognition behavior of l-Trp and d-Trp was investigated by differential pulse voltammetry. The electrochemical characterization showed that UiO-88-LP/GCE had an enantiomeric resolution of amino acids. The recognition mechanism showed that l-Pro entering the UiO-66 molecular cage provided a site for the system to be recognized, so the purpose of recognition was achieved. The relevant data provide theoretical support for the practical application of UiO-88-LP in electrochemical sensors.