Remarkably selective and enantiodifferentiating sensing of histidine by a fluorescent homochiral Zn-MOF based on pyrene-tetralactic acid

Chiral Metal-Organic Frameworks with Spectroscopic Methods: Towards Chemical Sensor Devices

Chiral Metal-Organic Frameworks (CMOFs) are a rapidly growing field reflecting their potential as selective and sensitive chemical sensors for chiral analytes. The highly tuneable nature of CMOFs enables the size, shape, and non-covalent interactions to be optimised towards specific analytes to engender strong intermolecular interactions and sensing responses. While CMOFs as chiral chemical sensor devices have been explored with electrochemical methods including differential pulse voltammetry (DPV), bipolar and chemiresistive sensing techniques, the CMOFs as chiral chemical sensors using spectroscopic methods has received significantly less attention. This review examines the synthesis of CMOFs for chemical sensors with spectroscopic methods such as photoluminescence, circular dichroism, and solid-state nuclear magnetic resonance with a view towards their incorporation into chemical sensor devices. Future directions of the field are highlighted for the generation of functional devices.

RhB-Embedded Mn-MOF with Cyclotriphosphazene Skeleton as Dual-Emission Sensor for Putrescine as well as Smart Fluorescent Response of Aromatic Diamines and Nitrophenol

Luminescent metal-organic framework composites with multiple luminescence emissions have been efficient sensing platforms. Herein, a fluorescent sensor (RhB@1-0.4) with dual-emission fluorescence properties was prepared by introducing rhodamine B (RhB) into the framework of complex 1, [Mn2.5(HCPCP)(H2O)4]·(CH3CN)0.5 [HCPCP = hexa-(4-carboxyl-phenoxy)-cyclotriphosphazene and CH3CN = acetonitrile), which is a novel crystalline two-dimensional (2D) coordinated organic framework material. It is a highly desirable material, realizing a ratiometric fluorescence response to putrescine with a high signal-to-noise ratio, and the detection limit can be as low as 6.8 μM. In addition, RhB@1-0.4 exhibited a better fluorescent sensing performance for aromatic diamines and nitrophenols compared with that of complex 1. It is a potential functionalized MOF material for the application of multichannel fluorescence sensing.

A pH-Stable Tb-MOF as Luminescence Sensor for Highly Sensitive Detection of Amino Acids through Diverse Sensing Mechanism

A luminescent Tb-MOF with excellent stability and dual-emitting properties was constructed with an amide-functionalized tetracarboxylate ligand. Tb-MOFs were initially assembled on one-dimensional Tb3+ chains, then formed a two-dimensional double-decker layer through the synergistic linking of organic ligands and bridging formic acid anions, and further fabricated the final three-dimensional structure through the connection of the organic ligands. Powder X-ray diffraction experiments revealed that Tb-MOFs not only exhibited excellent stability in water but also maintained structural integrity in the pH range of 2-12. Importantly, this Tb-MOF provided the first example of a metal-organic framework (MOF)-based luminescence sensor that can simultaneously detect two acid amino acids (aspartic and glutamic acids) through a turn-off sensing mechanism and two basic amino acids (lysine and arginine acids) through unusual turn-on and turn-off-on sensing mechanisms. Moreover, high sensitivity, low detection limit, and excellent recyclability of this sensor endow Tb-MOFs with great potential as a highly efficient amino acid fluorescence sensor in chemical detection and biological environments.

A Chiral Metal-Organic Framework Prepared on Large-Scale for Sensitive and Enantioselective Fluorescence Recognition

MOF-based luminescent sensors have garnered considerable attention due to their potential in recognition and discrimination with high sensitivity, selectivity, and fast response in the last decades. Herein, this work describes the bulk preparation of a novel luminescent homochiral MOF, namely, [Cd(s-L)](NO₃)₂ (MOF-1), from an enantiopure pyridyl-functionalized ligand with rigid binaphthol skeleton under mild synthetic condition. Except for the features of porosity and crystallinity, the MOF-1 has also been characterized with water-stability, luminescence, and homochirality. Most important, the MOF-1 exhibits highly sensitive molecular recognition toward the4-nitrobenzoic acid (NBC) and moderate enantioselective detection of proline, arginine, and 1-phenylethanol.

One Molecular Probe with Opposite Enantioselective Fluorescence Enhancement at Two Distinct Emissions

It is discovered that one enantiomer of a chiral substrate can greatly enhance the fluorescence of one molecular probe at one emitting signal (λ₁ = 517 nm), while the opposite enantiomer of the substrate greatly enhances the fluorescence of the same probe at a distinctively different emission (λ₂ = 575 nm). This probe is made of a 1,1'-binaphthyl-based chiral dialdehyde that in combination with Zn²⁺ under slightly acidic conditions shows a chemoselective and enantioselective fluorescent response to histidine. The opposite enantioselective fluorescent responses of the probe at two emissions allow it to be used to determine both the concentration and the enantiomeric composition of the substrate using a single probe. The mechanistic study has revealed two very different reaction pathways when the two enantiomers of the substrate are treated with the probe. These reaction pathways generate two different products, one dimeric and another polymeric, with very different emissions.

Ratiometric fluorescence detection of artemisinin based on photoluminescent Zn-MOF combined with hemin as catalyst

Artemisinin (ART) is a type of frontline drug to treat drug-resistant falciparum malaria. Simple, accurate and selective determination of ART is significant to monitor its clinical pharmaceutical efficacy. Herein, a new ratiometric fluorescence method has been designed for the determination of ART with Zn-MOF as fluorescence reference and hemin as catalyst, respectively. Zn-MOF possesses intrinsic fluorescence at 443 nm owing to 2-aminoterephthalic acid ligand. When o-phenylenediamine (OPD) is mixed with hemin, a weak fluorescent signal at 570 nm ascribed to oxidized product of OPD (oxOPD) is observed. In the presence of ART, hemin can catalyze ART to break its peroxide bridge and release a large number of reactive oxygen species, which effectively oxidize OPD into luminescent oxOPD. Therefore, the fluorescence at 570 nm is enhanced significantly while the fluorescence of Zn-MOF remains basically unchanged. Thus, a ratiometric fluorescence sensing platform has been constructed for the detection of ART. This method exhibits wider linear range (0.15 μM-150 μM) with detection limit of 50 nM. This novel and selective method has been used to detect ART in compound naphthoquinone phosphate tablets.

Synthesis, characterization, and heparin-binding study of a self-assembled p-cymene-Ru(II) metallocycle based on a 4-amino-1,8-naphthalimide Tröger's base supramolecular scaffold

We report the very first example of a self-assembled p-cymene-Ru(II) metallocycle based on a green emitting 4-amino-1,8-naphthalimide Tröger's base (TBNap) supramolecular scaffold. A new cleft-shaped TBNap-derived di-4-picolyl donor was synthesized and reacted in a 2 : 2 stoichiometry ratio with a dinuclear Ru(II) acceptor (Ru-A) to generate a [2 + 2] self-assembled metallocycle (TBNap-Ru-MC) in good yield. Both TBNap and TBNap-Ru-MC showed positive solvatochromism in different solvents with varying polarities. In addition, the binding propensity of cationic TBNap-Ru-MC toward the heparin polyanion was determined using fluorescence titration studies. The initial fluorescence emission of TBNap-Ru-MC was quenched upon the gradual addition of the heparin polyanion, and the Stern-Volmer quenching constant (K_SV) was calculated to be 3.97 × 10⁵ M^-1.

Highly Efficient versus Null Electrochemical Enantioselective Recognition Controlled by Achiral Colinkers in Homochiral Metal-Organic Frameworks

Chiral materials capable of electrochemical enantiomeric recognition are highly desirable for many applications, but it is still very challenging to achieve high recognition efficiency for lack of the knowledge of structure-property relationships. Here, we report the completely distinct enantiomeric recognition related to slightly different achiral colinkers in isomorphic homochiral metal-organic frameworks with the same chiral linker. Cu-TBPBe, for which the achiral colinker has two pyridyl rings connected by ─CH═CH─, shows excellent enantioselectivity and sensitivity for electrochemical recognition of l-tryptophan (Trp) with a detection limit of 3.16 nM. The l-to-d ratio of differential pulse voltammetric (DPV) currents reaches 53, which is much higher than the values (2-14) reported for previous electrochemical sensors. By contrast, Cu-TBPBa, in which the achiral colinker has -CH₂-CH₂- between pyridyl rings, is incapable of discrimination between l-Trp and d-Trp. Structural and spectral analyses suggest that the achiral conjugated colinker and the chiral moieties around it cooperate to produce a chiral pocket in favor of enantioselective adsorption through multiple hydrogen-bonding and π-π stacking interactions. The work demonstrated that Cu-TBPBe can be used to fabricate reliable electrochemical sensors for ultrasensitive quantification of Trp enantiomers in racemic mixtures and in complex biological systems such as urine. The work also highlights that an achiral coligand can be of vital importance in determining enantiomeric discrimination, opening up a new avenue for the design of chiral sensing materials.

Brønsted Acid-Functionalized Ionic Co(II) Framework: A Tailored Vessel for Electrocatalytic Oxygen Evolution and Size-Exclusive Optical Speciation of Biothiols

Metal-organic frameworks (MOFs) not only combine globally demanded renewable energy generation and environmental remediation onto a single platform but also rationalize structure-performance synergies to devise smarter materials with remarkable performance. The robust and non-interpenetrated cationic MOF exemplifies a unique bifunctional scaffold for the efficient electrochemical oxygen evolution reaction (OER) and ultrasensitive monitoring of biohazards. The microporous framework containing Brønsted acid-functionalized [Co₂(μ₂-OH)(CO₂)₂] secondary building units (SBUs) exhibits remarkable OER performance in 1 M KOH, requiring 410 mV overpotential to obtain 10 mA cm^-2 anodic current density, and a low Tafel slope of 55 mV/dec with 93.1% Faradaic efficiency. Apart from the high turnover frequency and electrochemically assessable surface area, steady OER performance over 500 cycles under potentiodynamic and potentiostatic conditions result in long-term catalyst durability. The highly emissive attribute from nitrogen-rich fluorescent struts benefits the MOF in recyclable and selective fluoro-detection of three biothiols (l-cysteine, homocysteine, and glutathione) in water with a fast response time. In addition to colorimetric monitoring in the solid and solution phases, control experiments validate size-exclusive biothiol speciation through molecular-dimension-mediated pore diffusion. The role of SBUs in the OER mechanism is detailed from density functional theory-derived free energy analysis, which also validates the importance of accessible N-sites in sensing via portraying framework-analyte supramolecular interactions.

Recent advances in the tuning of the organic framework materials - The selections of ligands, reaction conditions, and post-synthesis approaches

Organic framework materials, particularly metal-organic frameworks (MOFs), graphene-organic frameworks (GOFs), and covalent organic frameworks (COFs), have led to the revolution across fields including catalysts, sensors, gas capture, and biology mainly owing to their ultra-high surface area-to-volume ratio, on-demand tunable crystal structures, and unique surface properties. While the wet chemistry routes have been the predominant synthesis approach, the crystal phase, morphological parameters, and physicochemical properties of organic framework materials are largely affected by various synthesis parameters and precursors. In this work, we specifically review the influences of synthesis parameters towards crystal structures and chemical compositions of organic framework materials, including selected ligand types and lengths, reaction temperature/solvent/reactant compositions, as well as post-synthesis modification approaches. More importantly, the subsequent impacts on the general electronic, mechanical, surface chemical, and thermal properties as well as the consequent variation in performances towards catalytic, desalination, gas sensing, and gas storage applications are critically discussed. Finally, the current challenges and prospects of organic framework materials are provided.

Homochiral Eu3+@MOF Composite for the Enantioselective Detection and Separation of (R/S)-Ornidazole

The development of homochiral materials for the enantioselective detection and separation of chiral drugs is in high demand for the pharmaceutical industry. Herein, an anionic homochiral metal-organic framework (HMOF) with in situ generated [Me₂NH₂]⁺ counterions, {[Me₂NH₂]₂[Zn₂(d-L)₂(HCO₂)(OH)]·5H₂O}_n (HMOF-1), was synthesized using a d-camphorate-derived enantiopure dicarboxylate ligand, 4,4'-[[(1R,3S)-1,2,2-trimethylcyclopentane-1,3-dicarbonyl]bis(azanediyl)]dibenzoic acid (d-H₂L) via a simple solvothermal method. Interestingly, HMOF-1 could be used as a parent framework to encapsulate Eu³⁺ cations via an ion-exchange process, yielding an Eu³⁺@HMOF-1 composite with dual-luminescent centers. The obtained Eu³⁺@HMOF-1 has high chemical stability and good luminescence stability in water. Importantly, Eu³⁺@HMOF-1 exhibits enhanced enantioselectivity and sensitivity in the detection of an important chiral nitroimidazole antibiotic, (R/S)-ornidazole (ONZ) in comparison to HMOF-1 under the same aqueous conditions. The enantiomeric excess (ee) value of the ONZ enantiomers can be accurately determined by the ratio of dual emission from the ligand and Eu³⁺. In addition, Eu³⁺@HMOF-1 shows the enantioselective separation of racemic ONZ enantiomers with an ee value of 86.6%. This work provides a simple strategy for the preparation of Ln^III-incorporated HMOF composite materials for the simultaneous enantioselective detection and separation of chiral drugs.

Turn-on and blue-shift fluorescence sensor toward L-histidine based on stable CdII metal-organic framework with tetranuclear cluster units

A novel Cd^II-based two-fold interpenetrated metal-organic framework (MOF), namely {[Cd₂(BTDB)₂(4,4-bpy)]·DMF}_n (JXUST-14), (H2BTDB = 4,4'-(benzo[c][1,2,5]thiadiazole-4,7-diyl)dibenzoic acid and 4,4-bpy = 4,4-bipyridine), has been prepared and characterized. Single-crystal structure determination reveals that JXUST-14 has a tetranuclear cluster based 6-connected pcu topological network with Schlafli symbol {4¹²·6³}. When soaked in common organic solvents and aqueous solutions with diverse pH values of 2-12 for 48 h, JXUST-14 remains stable. JXUST-14 is a highly selective and sensitive luminescent sensor for L-histidine (His) with a detection limit of 11.1 ppm. JXUST-14 is the first Cd^II-based MOF for the detection of His via turn-on and fluorescence blue-shift effects. Experimental study and theoretical calculation suggest that the sensing process can be mainly attributed to a charge transfer and energy transfer mechanism. More interestingly, LED lamps of JXUST-14 and JXUST-14@His were successfully developed, which endowed efficient sensitivity for His detection and thus provide great potential for future applications.

A chiral binaphthyl-based coordination polymer as an enantioselective fluorescence sensor

A fluorescent, chiral coordination polymer (CP) with a novel topology has been synthesised using a dipyridyl ligand derived from 1,1'-bi-2-naphthol (BINOL). Enantioselectivity ratios up to 2.61 were obtained in fluorescence sensing studies with chiral analytes.

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.

Zwitterionic Luminescent 2D Metal-Organic Framework Nanosheets (LMONs): Selective Turn-On Fluorescence Sensing of Dihydrogen Phosphate

While a plethora of organic linkers based on carboxylic acids have been utilized in the construction of MOFs, zwitterionic linkers that typify the attributes of naturally occurring amino acids have been exploited only scarcely to the best of our knowledge. Zwitterionic interior characteristics should be expected to impart unique properties to the resultant MOFs with a high potential to interact with guest species through electrostatic interactions. In our investigations with bis(p-carboxyphenyl)imidazolylarenes as a novel class of linkers for the development of functional MOFs, we have found that bisimidazole-tetracarboxylic acid H₄DMBI undergoes metal-assisted self-assembly with Zn(NO₃)₂ to yield a layered MOF (Zn-DMBI). In the latter, the linker serves as a two-connecting linker with imidazoles and carboxylic acids behaving as zwitterions. The layers are offset stacked in the crystal structure and are bound firmly by hydrogen bonds between imidazolium and carboxylate ions. Such a packing precludes fluorescence from being observed due to self-quenching. However, exfoliation into zwitterionic 2D metal-organic nanosheets (MONs) by sonication in methanol for 1 h liberates palpable fluorescence. Furthermore, the suspension of luminescent MONs (LMONs) in methanol permits selective sensing of anions; in particular, dihydrogen phosphate (H₂PO₄^-) that is complementary to the zwitterions in terms of hydrogen bond donor and acceptor sites is observed with fluorescence enhancement by 120%, leading to its detection at a sub-parts-per-million (0.13 ppm) level. Thus, access to zwitterionic 2D MONs and their application for selective anion sensing with "turn-on" fluorescence are demonstrated by a rational de novo bottom-up approach.

Stable bifunctional ZnII-based sensor toward acetylacetone and L-histidine by fluorescence red shift and turn-on effect

A new coordination polymer [Zn(bbip)(NH2-BDC)]n (JXUST-15, bbip = 2,6-bis(benzimidazol-1-yl)pyridine and NH2-H2BDC = 2-aminoterephthalic acid) has been synthesized by mixed ligand strategy. The structure analysis shows that JXUST-15 takes a two-dimensional...

Chemoselective and Enantioselective Fluorescent Identification of Specific Amino Acid Enantiomers

The enantiomers of chiral amino acids play versatile roles in biological systems including humans. They are also very useful in the asymmetric synthesis of diverse chiral organic compounds. Therefore, identification...

Homochiral porous coordination polymer of EuIII for metal ion sensing and enantioselective adsorption

A novel multifunctional homochiral porous metal–organic framework was obtained by combining the luminescent component Eu(iii) with an enantiopure triangular polycarboxylic ligand.

Dual-Functional Coordination Polymer with High Proton Conductivity and a Low-Detection-Limit Fluorescent Probe

A coordination polymer with dual functions of high proton conductivity and highly sensitive fluorescent sensors demonstrates a great application potential. In this work, a cadmium-based coordination polymer (denoted as CP 1) with hydrothermal stability was synthesized. The abundant coordination water, lattice water, and amino groups make an extended hydrogen-bonding pathway for efficient proton migration, which endows CP 1 with the highest proton conductivity of 2.41 × 10^-3 S·cm^-1 at 353 K and 98% RH. Especially, the proton conductivity of the chitosan (CS) hybrid membrane containing CP 1 reaches a maximum value of 2.62 × 10^-2 S·cm^-1 under 343 K and 98% RH, which increases almost 7 times higher than that of the pure CS membrane due to the host-guest collaboration. Furthermore, luminescence studies revealed that CP 1 is a high-sensitivity and good-selectivity fluorescent probe for the detection of trace amounts of l-histidine with a lowest detection limit of 1.0 × 10^-8 M.

Mechanoluminescence and aggregation-enhanced emission (AEE) of an In-MOF based on a 9,9'-diphenyl-9H-fluorene tetraacid linker

A water-stable In-MOF, constructed based on a conformationally-flexible tetraacid linker, i.e., 2,7-bis(3,5-dicarboxyphenyl)-9,9'-diphenyl-9H-fluorene, i.e., H4DPF, is shown to exhibit a significantly enhanced solid-state fluorescence quantum yield (φf) of 23% in comparison with that of the linker (φfca. 4%) as a consequence of rigidification of the latter by metalation. Application of external stimulus in the form of grinding of the In-MOF leads to a drastic enhancement by 29%, φf from 23 to 52%. Solid-state absorption and emission spectra show that the absorption in the region of 368-550 nm gets diminished with a concomitant change in the emission maximum with a blue shift upon grinding. Fluorescence enhancement with grinding is correlated with a gradual reduction in the size of the particles, as established by SEM analysis. MOF particle aggregation has been invoked to account for the observed fluorescence enhancement in addition to a subtle conformational change in the structure of the linker upon grinding. Intriguingly, the ground MOF particles exhibit aggregation behaviour in the DMF-water solvent system with the emission further increasing up to 75% for the increase in the water fraction (fw) from 0 to 60%; hydrophobic aggregation of particles evidently leads to a change in the conformation of the linker and particle aggregation-enhanced emission (AEE). De-aggregation of particles ensues for fw = 70-90%, as reflected by a gradual decrease in the emission intensity. It is shown that the suspension of ground In-MOF particles in water permits sensing of metal ions, in particular Al3+ ions, by fluorescence quenching with detection at a sub-ppb level. The observed results comprise first demonstration of both mechanoluminescence and AEE of MOF particles.

Turn-On Fluorescence Enantioselective Sensing of Hydroxyl Carboxylic Enantiomers by Metal-Organic Framework Nanosheets with a Homochiral Tetracarboxylate of Cyclohexane Diamide

Two-dimensional (2D) metal-organic frameworks (MOFs) have attracted growing interest due to excellent performance in gas separation, energy conversion and storage, catalysis, and sensing, but their homochirality and exfoliation as well as related enantioselective catalysis and sensing remain a stage of pending exploration owing to the scarcity of homochiral MOFs and intrinsic aggregation of nanosheets. Herein, a homochiral 2D MOF (HMOF-3) with polymeric chirality, good thermostability, and solvent stability is designed and constructed by a homochiral organic ligand 5,5'-((1R,2R)-cyclohexane dicarbonyl bis(azanediyl)) diisophthalic acid (R,R-CHCAIP), a ditopic coligand 4,4'-bipyridine, and Zn salts. Remarkably, HMOF-3 can be exfoliated via solvent-assisted sonication to achieve 2D HMOF-3 nanosheets (HMOF-3-NS), which exhibit a sensitive turn-on effect with the fluorescence enhancement up to 63.5 times in the presence of R/S-mandelic acid, d/l-tartaric acid, d/l-lactic acid, d/l-alanine, and d/l-tryptophan. More importantly, the high surface area, polymeric chirality environment, and highly accessible functional sites on the surface of HMOF-3 nanosheets enable close contact with probed enantiomers, leading to highly enantioselective and sensitive sensing. The turn-on mechanism of host-guest-assisted electronic transfer is confirmed by DFT calculation and the relative experiment. This work highlights the promise of homochiral 2D MOF nanosheets for enantioselective sensing applications.

Efficient chemosensors for toxic pollutants based on photoluminescent Zn(ii) and Cd(ii) metal-organic networks

Optical sensors with high sensitivity and selectivity, as important analytical tools for chemical and environmental research, can be realized by straightforward synthesis of luminescent one-, two- and three-dimensional Zn(ii) and Cd(ii) crystalline coordination arrays (CPs and MOFs). In these materials with emission centers typically based on charge transfer and intraligand emissions, the quantitative detection of specific analytes, as pesticides or anions, is probed by monitoring real-time changes in their photoluminescence and color emission properties. Pesticides/herbicides have extensive uses in agriculture and household applications. Also, a large amount of metal salts of cyanide is widely used in several industrial processes such as mining and plastic manufacturing. Acute or chronic exposure to these compounds can produce high levels of toxicity in humans, animals and plants. Due to environmental concerns associated with the accumulation of these noxious species in food products and water supplies, there is an urgent and growing need to develop direct, fast, accurate and low-cost sensing methodologies. In this critical frontier, we discuss the effective strategies, chemical stability, luminescence properties, sensitivity and selectivity of recently developed hybrid Zn(ii)/Cd(ii)-organic materials with analytical applications in the direct sensing of pesticides, herbicides and cyanide ions in the aqueous phase and organic solvents.

Homochiral metal-organic frameworks for enantioseparation

Obtaining homochiral compounds is of high importance to human health and environmental sustainability. Currently, enantioseparation is one of the most effective approaches to obtain homochiral compounds. Thanks to their controlled synthesis and high efficiency, homochiral metal-organic frameworks (HMOFs) are one of the most widely studied porous materials to enable enantioseparation. In this review, we discuss the chiral pocket model in depth as the key to unlock enantioselective separation mechanisms in HMOFs. In particular, we classify our discussion of these chiral pockets (also regarded as "molecular traps") into: (a) achiral/chiral linker based helical channels as a result of packing modality; and (b) chiral pores inherited from chiral ligands. Driven by a number of mechanisms of enantioseparation, conceptual advances have been recently made in the design of HMOFs for achieving high enantioseparation performances. Herein, these are systematically categorised and discussed. Further we elucidate various applications of HMOFs as regards enantioseparation, systematically classifying them into their use for purification and related analytical utility according to the reported examples. Last but not the least, we discuss the challenges and perspectives concerning the rational design of HMOFs and their corresponding enantioseparations.

Pyrene-based metal organic frameworks: from synthesis to applications

Pyrene is one of the most widely investigated aromatic hydrocarbons given to its unique optical and electronic properties. Hence, pyrene-based ligands have been attractive for the synthesis of metal-organic frameworks (MOFs) in the last few years. In this review, we will focus on the most important characteristics of pyrene, in addition to the development and synthesis of pyrene-based molecules as bridging ligands to be used in MOF structures. We will summarize the synthesis attempts, as well as the post-synthetic modifications of pyrene-based MOFs by the incorporation of metals or ligands in the structure. The discussion of promising results of such MOFs in several applications; including luminescence, photocatalysis, adsorption and separation, heterogeneous catalysis, electrochemical applications and bio-medical applications will be highlighted. Finally, some insights and future prospects will be given based on the studies discussed in the review. This review will pave the way for the researchers in the field for the design and development of novel pyrene-based structures and their utilization for different applications.

Multi-emission metal-organic framework composites for multicomponent ratiometric fluorescence sensing: recent developments and future challenges

Ratiometric fluorescence sensors that are achieved via the ratiometric fluorescence intensity changes of emission peaks based on multi-emission fluorescence probes show a huge advantage. However, the preparation of these multi-emission fluorescence probes is a key challenge, as it is related to having more fluorescence groups with the same excitation but different emission wavelengths, and their assembly is not a simple mixing process. More fluorescent groups or molecules can be assembled into the multi-emission fluorescence probe by covalent bonds and coordination interactions, or by loading in metal-organic frameworks (MOFs). MOFs are excellent candidates for constructing complexes with the capability of multicomponent ratiometric fluorescence sensing, but there are some problems that need to be considered. For example, not all fluorophores can be stably loaded in the MOFs' pores, usually due to the size, surface charge and intrinsic properties of the fluorophore. In turn, it is also related to the structure of the MOF, metal nodes, and properties of the organic ligands. This review first introduces the advantages of the MOF-based multi-component fluorescence sensors, and then discusses the synthesis, classification and application of fluorescent MOFs or MOF composites for multi-component ratiometric fluorescence detection. Particular emphasis is focused on the potential, types and characteristics for sensing and biological applications, and the main challenges and limitations are further explored. This review might be helpful for those researchers interested in the application of multi-component ratiometric fluorescence sensing based on fluorescent MOFs or MOF composites.

Determining the concentration and enantiomeric composition of histidine using one fluorescent probe

A chemoselective as well as enantioselective fluorescent probe has been developed to determine both the concentration and enantiomeric composition of the biologically important amino acid histidine by measuring the fluorescence responses when excited at two different wavelengths.

Metal–organic frameworks as photoluminescent biosensing platforms: mechanisms and applications

Recent progress of MOF-based photoluminescent platforms: a comprehensive overview of their applications in biosensing and underlying mechanisms.

Enantioselective Fluorescent Recognition of Free Amino Acids: Challenges and Opportunities

Fluorescent probes that can discriminate enantiomers of amino acids in organic media or aqueous solution are discussed. This Minireview focuses on recent progress in the studies of three classes of probes including those made of cyclodextrins, 1,1'-binaphthyl compounds, and nanomaterials, and uses them to illustrate the design strategies, applications, and limitations in this area. These probes are potentially useful for rapid analysis of asymmetric reactions for amino acid synthesis as well as the real-time imaging of amino acids in biological systems. The challenges in these applications are analyzed. Working in this field of enantioselective fluorescent recognition of amino acids offers great opportunities to make new scientific discoveries and to develop important practical applications.

Temperature-Dependent Emission and Turn-Off Fluorescence Sensing of Hazardous "Quat" Herbicides in Water by a Zn-MOF Based on a Semi-Rigid Dibenzochrysene Tetraacetic Acid Linker

A zinc metal-organic framework, i.e., Zn-MOF (Zn-DBC), with ca. 27% solvent-accessible void volume was synthesized from a rationally designed tetraacid based on sterically insulated dibenzo[g,p]chrysene core; the latter inherently features concave shapes. Due to rigidification of the fluorophore in the MOF, Zn-DBC exhibits a respectable fluorescence quantum yield of ca. 30% in the solid state. The fluorescent and water-stable Zn-DBC MOF was found to display intriguing temperature-dependent emission behavior with an activation barrier of 1.06 kcal/mol for radiationless deactivation from the singlet-excited state. It is shown that the Zn-MOF can be employed as an efficient sensory material for detection of hazardous "quat" dicationic herbicides in water by diffusion-limited "turn-off" fluorescence. Due to confinement of the cationic guest analytes within the pores of the MOF, the fluorescence quenching via excited-state charge transfer mechanism is shown to depend on the molecular size of the analyte in addition to the redox potentials. Remarkably, Zn-DBC permits sensing of DQ, a well-known toxic "quat" herbicide, with a detection limit as low as 2.8 ppm in water. The unique structural attributes of the Zn-MOF for highly efficient fluorescence sensing of toxic herbicides in water are thus exemplified for the first time.

Intriguing pH-modulated Luminescence Chameleon System based on Postsynthetic Modified Dual-emitting Eu3+@Mn-MOF and Its Application for Histidine Chemosensor

Due to the disruption by other nonanalyte factors, single-emission probes have been limited in complicated detecting systems. In this work, a pH-modulated luminescence chameleon system based on lanthanide-based MOF (Eu³⁺@Mn-MOF), with stable structure and miraculous dual-emitting fluorescent properties, was synthesized by a postsynthetic modification (PSM) strategy of a simple hydrothermal and agitation method. Amazingly, not only can the Eu³⁺@Mn-MOF emit a broad emission at 500 nm attributed to the ligand-based fluorescence emission but it can also exhibit the characteristic emission of Eu³⁺ ions responding to the antenna effect. Moreover, the Eu³⁺@Mn-MOF displays an interesting luminescence color transition between acidic and basic solutions. Inspired by this phenomenon, a pH-modulated luminescence chameleon system was first constructed and employed to detect histidine, a kind of basic amino acid for a variety of biological matters, causing a unique fluorescence signal of the ratio-dependent color to change from yellow to light pink which differs from the color change of other water-soluble amino acids. Therefore, Eu³⁺@Mn-MOF can be as a practical pH-modulated luminescence chameleon system chemsensor for sensing histidine with low detection limit, high sensitivity, and rapid sensing time. In conclusion, the postsynthetic modified Eu³⁺@Mn-MOF has outstanding applications in the fields of chemical detection and human health.

Uranyl Organic Framework as a Highly Selective and Sensitive Turn-on and Turn-off Luminescent Sensor for Dual Functional Detection Arginine and MnO4. Inorg Chem 2020;59:5004-5017. [PMID: 32207299 DOI: 10.1021/acs.inorgchem.0c00236]

Five new uranyl coordination polymers were prepared by the hydrothermal method based on 5-nitroisophthalic acid (H₂nip) as (UO₂)(nip)(2,2'-bpy) (1), (H₂4,4'-bpy)·[(UO₂)₃(nip)₄]·(4,4'-bpy) (2), (H₂bpe)·[(UO₂)_0.5(nip)] (3), (H₂ bpp)·[(UO₂)₂-(nip)₃]·H₂O (4), and (H₂tmp)·[(UO₂)(nip)₂](5) [2,2'-bpy = 2,2'-bipyridine, 4,4'-bpy = 4,4'-bipyridine, bpe = 4,4'-vinylenedipyridine, bpp = 4,4' -trimethylenedipyridine, tmp = tetramethylpyrazine]. All of these synthesized complexes have been characterized by single crystal and powder X-ray diffraction, IR spectra, thermogravimetric analysis, elemental analysis, and luminescent properties. In particular, it is found that compounds 1 and 4 can be used as a luminescent sensor to efficiently detect arginine in aqueous solution by means of "turn-on"; the detection limits were 1.06 × 10^-6 and 6.42 × 10^-6 mol/L, respectively. Moreover, 4 can also be used as a bifunctional sensor for selective sensing of MnO₄^- anion by "turn-off". The detection limit of MnO₄^- in water was 1.79 × 10^-6 mol/L; the K_sv was 1.88 × 10⁴. The sensing effect of arginine in simulated grape juice samples and MnO₄^- in simulated river water samples was also investigated by this sensing system with high recovery. In addition, the possible mechanism of sensing arginine and MnO₄^- in the aqueous solution was discussed.

Zn(ii)/Cd(ii) based mixed ligand coordination polymers as fluorosensors for aqueous phase detection of hazardous pollutants

The recent developments and prospects of fluorosensors with a handful of recent examples based on mixed ligand Zn(ii)/Cd(ii) coordination polymers for aqueous phase detection of organic as well as inorganic pollutants have been discussed.

A luminescent terbium coordination polymer as a multifunctional water-stable sensor for detection of Pb2+ ions, PO43− ions, Cr2O72− ions, and some amino acids

It is the first Ln-CP fluorescence probe for synchronous determination of Tyr and Trp in the presence of other amino acids.