QCM based enantioselective discrimination of enantiomers by a pair of serine derived homochiral coordination polymers

A free carboxyl-decorated metal-organic framework with 3D helical chirality for highly enantioselective recognition

With the judicious selection of a designed polycarboxylate derived from L-phenylalanine, (S)-5-(((1-carboxy-2-phenylethyl)amino)methyl)isophthalic acid (H3L), a novel homochiral metal-organic framework decorated with a free carboxyl, {[Cu2(HL)2(bipy)]∙2H2O}n (Cu-MOF), has been designed and synthesized in a solvothermal process. The result of single crystal X-ray diffraction analysis showed that Cu-MOF had the character of a three-dimensional structure with helical chirality. As we expected, in Cu-MOF, one accessible free carboxylic acid group on H3L pointed toward the spiral channels, and the other two -COOH groups were utilized in bonding. The enantioseparation performance of Cu-MOF was thoroughly investigated and the results showed that Cu-MOF can specifically recognize S-1-(1-naphthyl) ethanol (S-NE) with enantiomeric excess (ee) value of 99.35 %, which was much higher than the other three racemates. The appropriate size together with suitable interaction sites played an important role in enantioseparations. Inspired by the excellent chiral recognition effects towards S-NE, the chiral recognition mechanism was experimentally clarified. A fully agreement observed in 13C CP MAS NMR analysis as well as the X-ray photoelectron spectroscopy (XPS) determination revealed that a strong hydrogen bonding interaction forces existed between the hydroxyl of the optical S-NE and the decorated -COOH in the chiral framework. The control experiment further identified the decisive role of the uncoordinated carboxyl group in Cu-MOF. In addition, the strong intermolecular off-set π-π interactions between the phenyl ring involved with the coordinated COO- groups in Cu-MOF and the naphthyl ring of S-NE, was the another important factor for the specifical enantioseparation of S-enantiomer. On the basis of strong intermolecular hydrogen bonding, NE racemates were enantioselective discriminated and enantiomeric purity can be determined by means of Raman scattering spectroscopy.

Biomimetic chiral hydrogen-bonded organic-inorganic frameworks

Assembly ubiquitously occurs in nature and gives birth to numerous functional biomaterials and sophisticated organisms. In this work, chiral hydrogen-bonded organic-inorganic frameworks (HOIFs) are synthesized via biomimicking the self-assembly process from amino acids to proteins. Enjoying the homohelical configurations analogous to α-helix, the HOIFs exhibit remarkable chiroptical activity including the chiral fluorescence (glum = 1.7 × 10-3) that is untouched among the previously reported hydrogen-bonded frameworks. Benefitting from the dynamic feature of hydrogen bonding, HOIFs enable enantio-discrimination of chiral aliphatic substrates with imperceivable steric discrepancy based on fluorescent change. Moreover, the disassembled HOIFs after recognition applications are capable of being facilely regenerated and self-purified via aprotic solvent-induced reassembly, leading to at least three consecutive cycles without losing the enantioselectivity. The underlying mechanism of chirality bias is decoded by the experimental isothermal titration calorimetry together with theoretic simulation.

Chiral MOF Derived Wearable Logic Sensor for Intuitive Discrimination of Physiologically Active Enantiomer

Chiral sensors have attracted growing interest due to their application in health monitoring. However, rational design of wearable logic chiral sensors remains a great challenge. In this work, a dual responsive chiral sensor RT@CDMOF is prepared through in situ self-assembly of chiral γ-cyclodextrin metal-organic framework (CDMOF), rhodamine 6G hydrazide (RGH), and tetracyanovinylindane (TCN). The embedded RGH and TCN inherit the chirality of host CDMOF, producing dual changes both in fluorescence and reflectance. RT@CDMOF is explored as a dual channel sensor for chiral discrimination of lactate enantiomers. Comprehensive mechanistic studies reveal the chiral binding process, and carboxylate dissociation is confirmed by impedance and solid-state 1 H nuclear magnetic resonance (NMR). A flexible membrane sensor is successfully fabricated based on RT@CDMOF for wearable health monitoring. Practical evaluation confirms the potential of fabricated membrane sensor in point-of-care health monitoring by indexing the exercise intensity. Based on above, a chiral IMPLICATION logic unit can be successfully achieved, demonstrating the promising potential of RT@CDMOF in design and assembly of novel smart devices. This work may open a new avenue to the rational design of logic chiral sensors for wearable health monitoring applications.

Chiral Metal-Organic Frameworks

In the past two decades, metal-organic frameworks (MOFs) or porous coordination polymers (PCPs) assembled from metal ions or clusters and organic linkers via metal-ligand coordination bonds have captivated significant scientific interest on account of their high crystallinity, exceptional porosity, and tunable pore size, high modularity, and diverse functionality. The opportunity to achieve functional porous materials by design with promising properties, unattainable for solid-state materials in general, distinguishes MOFs from other classes of materials, in particular, traditional porous materials such as activated carbon, silica, and zeolites, thereby leading to complementary properties. Scientists have conducted intense research in the production of chiral MOF (CMOF) materials for specific applications including but not limited to chiral recognition, separation, and catalysis since the discovery of the first functional CMOF (i.e., d- or l-POST-1). At present, CMOFs have become interdisciplinary between chirality chemistry, coordination chemistry, and material chemistry, which involve in many subjects including chemistry, physics, optics, medicine, pharmacology, biology, crystal engineering, environmental science, etc. In this review, we will systematically summarize the recent progress of CMOFs regarding design strategies, synthetic approaches, and cutting-edge applications. In particular, we will highlight the successful implementation of CMOFs in asymmetric catalysis, enantioselective separation, enantioselective recognition, and sensing. We envision that this review will provide readers a good understanding of CMOF chemistry and, more importantly, facilitate research endeavors for the rational design of multifunctional CMOFs and their industrial implementation.

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.

Chiral Recognition on Bare Gold Surfaces by Quartz Crystal Microbalance

Quartz crystal microbalance (QCM) is one of the powerful tools for the studies of molecular recognition and chiral discrimination. Its efficiency mainly relies on the design of the functional sensitive layer on the electrode surface. However, the organic sensitive layer may easily cause dissipation of oscillation or detachment and weaken the signal transfer during the molecular recognition processes. In this work, we reveal for the first time that the bare metal surface without the organic selector layer has the capability for chiral recognition in the QCM system. During the adsorption of various chiral amino acids, relatively higher selectivity of D-enantiomers on gold (Au) surface was shown by the QCM detection. Based on analyses of the surface crystalline structure and density functional theory calculations, we demonstrate that the chiral nature of Au surface plays an important role in the selective binding of specific D-amino acids. These results may open new insights on chiral detection by QCM system. It will also promote the construction of novel chiral sensing systems with both efficient detection and separation capability.

Engineering Homochiral MOFs in TiO2 Nanotubes as Enantioselective Photoelectrochemical Electrode for Chiral Recognition

Enantioselective sensing of chiral molecules is an important issue for both biomedical research and the pharmaceutical industry. Here, an enantioselective photoelectrochemical (PEC) sensor was constructed by integrating TiO₂ nanotubes (NTs) with metal-organic frameworks (MOFs) for the identification of enantiomers. TiO₂ NTs prepared by electrochemical anodization can not only be used as the PEC platform but also as the metal-ion precursor to react with terephthalic acid (BDC) to form MIL-125(Ti) in situ. A postsynthetic exchange (PSE) method was used for exchanging the ligand of MIL-125 by 2-aminoterephthalic acid (BDC-NH₂) for further functionalization. Homochirality was then successfully introduced into achiral MIL-125-NH₂ by postsynthetic modification (PSM) with l-histidine (l-His). The resulting homochiral metal-organic frameworks (MOF)-in-NT architecture exhibits excellent discrimination ability for the chiral recognition of 3,4-dihydroxyphenylalanine (l/d-DOPA) enantiomers. Moreover, by adjusting the charge-carrier separation-induced photocurrent variation mechanism, the as-proposed homochiral PEC electrode exhibits a broad application potential for the discrimination of enantiomers. Because of the construction of binder-free monochiral MOF-in-NT structure directly on a Ti-metal substrate, the valuable feature is that the PEC sensing platform can be used directly, thereby providing a stable, simplified, and low-cost sensing device for the recognition of chiral enantiomers.

Enantioselective Recognition and Separation of C2 Symmetric Substances via Chiral Metal-Organic Frameworks

A promising route toward the enantioselective recognition and separation of racemic molecules is the design of chiral metal-organic frameworks (CMOFs) with high enantioselectivity and stability. Herein, we report porous CMOFs Δ- and Λ-RuEu-MOFs constructed from the D₃-symmetry helical chiral Ru(phen)₃-derived tricarboxylate ligand and Eu₂ units, which can be utilized as adsorbents for the enantioselective recognition and separation of 1,1'-bi-2-naphthol (BINOL) derivatives. Investigation of the circular dichroism enantiodifferentiation between the host and guest suggested that Δ- and Λ-RuEu-MOFs can be employed as chiral sensors to discriminate axial enantiomers due to their diastereomeric host-guest relationship. Density functional theory calculations reveal that chiral recognition is attributed to the distinguishing binding affinities stemming from N···H-O hydrogen bonds and π-π stacking between the host and guest. Moreover, the reticulate structure of Δ- and Λ-RuEu-MOFs can be readily recycled and reused for the successive enantioselective separation of BINOL up to 80% ee.

Chiral UiO-MOFs based QCM sensors for enantioselective discrimination of hazardous biomolecule

Developments of enantioselective devices for discriminating bio-enantiomers is of significant importance. Due to the vital role of Cysteine (Cys) in biological processes and the hazardous effect of its D-enantiomer, discriminating Cys enantiomers without auxiliary enzyme is highly wanted. In this work, a pair of UiO-MOF enantiomers (UiO-tart) have been fabricated through post-modification, which could be further fabricated into enantiomeric sensing devices (UiO-tart@Au). By employing the Quartz Crystal Microbalance (QCM) technology, gravimetric discrimination of Cys enantiomers could be achieved. UiO-tart@Au is highly enantioselective, and the afforded enantioselective factor (5.97 ± 0.54) represents the best performance reported ever. In the fabricated device, MOF layer acts as the chiral selector for specific Cys enantiomer, and the reaction between the captured Cys enantiomer and Au results in the mass growth of the system. Solid-phase extraction (SPE) gives an e.e. value of 71.6 ± 3.8%, substantially confirming the chiral-selector role of UiO-tart. DFT calculations indicate that enantiomeric H-bonding effect and greater reaction enthalpy should be the reason. To the best of our knowledge, this work represents the first example of chiral tartaric acid derived MOF sensors for enantioselective discrimination of Cys, suggesting a promising potential of developing chiral MOFs based devices for enhanced enantioselective application.