Recent advances in metal-organic frameworks for stimuli-responsive drug delivery

After entering the human body, drugs for treating diseases, which are prone to delivery and release in an uncontrolled manner, are affected by various factors. Based on this, many researchers utilize various microenvironmental changes encountered during drug delivery to trigger drug release and have proposed stimuli-responsive drug delivery systems. In recent years, metal-organic frameworks (MOFs) have become promising stimuli-responsive agents to release the loaded therapeutic agents at the target site to achieve more precise drug delivery due to their high drug loading, excellent biocompatibility, and high stimuli-responsiveness. The MOF-based stimuli-responsive systems can respond to various stimuli under pathological conditions at the site of the lesion, releasing the loaded therapeutic agent in a controlled manner, and improving the accuracy and safety of drug delivery. Due to the changes in different physical and chemical factors in the pathological process of diseases, the construction of stimuli-responsive systems based on MOFs has become a new direction in drug delivery and controlled release. Based on the background of the rapidly increasing attention to MOFs applied in drug delivery, we aim to review various MOF-based stimuli-responsive drug delivery systems and their response mechanisms to various stimuli. In addition, the current challenges and future perspectives of MOF-based stimuli-responsive drug delivery systems are also discussed in this review.

Noncovalent chemistry of xenon opens the door for anesthetic xenon recovery using Bio-MOFs

Designing an inexpensive and highly efficient recovery process for xenon (Xe) is gaining importance in the development of sustainable applications. Using metal organic frameworks (MOFs) for separating Xe from anesthetic gas mixtures has been a recent topic studied rarely and superficially in the literature. We theoretically investigated Xe recovery performances of 43 biological MOFs (Bio-MOFs) formed by biocompatible metal cations and biological endogenous linkers. Xe uptakes and Xe permeabilities in its binary mixtures with CO2, O2, and N2 were investigated by applying Grand Canonical Monte Carlo and Molecular Dynamics simulations. Materials with metalloporphyrin, hexacarboxylate, triazine, or pyrazole ligands, dimetallic paddlewheel units, relatively large pore sizes (PLD > 5 Å and LCD > 10 Å), large void fractions (≈0.8), and large surface areas (>2900 m2 g-1) have been determined as top performing Bio-MOFs for Xe recovery. By applying Density Functional Theory simulations and generating electron density difference maps, we determined that Xe-host interactions in the top performing Bio-MOFs are maximized mainly due to noncovalent interactions of Xe, such as charge-induced dipole and aerogen-π interactions. Polarized Xe atoms in the vicinity of cations/anions as well as π systems are fingerprints of enhanced guest-host interactions. Our results show examples of rarely studied aerogen interactions that play a critical role in selective adsorption of Xe in nanoporous materials.

An europium(III) metal-organic framework as a multi-responsive luminescent sensor for highly sensitive and selective detection of 4-nitrophenol and I- and Fe3+ ions in water

A luminescence sensor based on an europium(III)-based lanthanide-organic framework, [Eu(BCB)(DMF)]·(DMF)_1.5(H₂O)₂ (1), was synthesized via a solvothermal method using 4,4',4''-benzenetricarbonyltribenzoic acid (H₃BCB) as a bridging ligand. Single-crystal X-ray diffraction indicates that Eu centers are eight-coordinated with a trigonal dodecahedron and a square antiprismatic configuration, and adjacent Eu atoms are bridged by BCB organic linkers to form a 3D rod-packing structure. Photoluminescence studies show that compound 1 emits bright red luminescence and behaves as a multi-responsive luminescent sensor toward 4-nitrophenol (4-NP) and I^- and Fe³⁺ ions in water with high sensitivity, selectivity and low detection limits. Furthermore, the possible luminescence sensing mechanisms were also investigated by PXRD analysis, UV-vis spectroscopy and X-ray photoelectron spectroscopy (XPS). The recognition mechanism for 4-NP and I^- ions can be attributed to the competition absorption and that for Fe³⁺ ions is considered to be a multi-quenching mechanism dominated by competition absorption. This study demonstrates that the lanthanide-based MOF might be a promising candidate for the detection of 4-NP and I^- and Fe³⁺ ions in aqueous medium.

An Eu(III)-functionalized Sr-based metal-organic framework for fluorometric determination of Cr(III) and Cr(VI) ions

An Eu(III)-functionalized strontium-organic framework (Eu³⁺@Sr-MOF) was synthesized by encapsulating Eu³⁺ ions into the pores of Sr-MOF via postsynthetic modification (PSM) and fully characterized by XRD, SEM, EDS, FTIR, and TGA. The luminescent properties of Eu³⁺@Sr-MOF were measured and investigated in detail, which demonstrated that Eu³⁺@Sr-MOF not only possessed excellent water and chemical stabilities in a wide pH range (5-10) but also displayed red luminescence with excitation/emission maxima at 332/617 nm. The Eu³⁺@Sr-MOF can be considered as a highly selective and sensitive recyclable luminescence probe for sensing Cr(III) and Cr(VI) based on strong luminescence quenching effects; it can be recycled at least five times. The luminescence probe for Cr³⁺, CrO₄^2-, and Cr₂O₇^2- shows low detection limits (0.15 μM, 0.17 μM, and 0.13 μM, respectively), relatively broad linear ranges (0.6-150 μM, 0.1-200 μM and 0.4-180 μM, respectively), and high quenching constants K_SV. The signals can be observed by the naked eye under UV light of 365 nm. The method was applied to the determination of total Cr in environmental water samples. Graphical abstract.

A Luminescent Mg-Metal-Organic Framework for Sustained Release of 5-Fluorouracil: Appropriate Host-Guest Interaction and Satisfied Acid-Base Resistance

It is important to achieve a moderate sustained release rate for drug delivery, so it is critical to regulate the host-guest interactions for the rational design of a carrier. In this work, a nano-sized biocompatible metal-organic framework (MOF), Mg(H₂TBAPy)(H₂O)₃·C₄H₈O₂ (TDL-Mg), was constructed by employing π-conjugated 1,3,6,8-tetrakis(p-benzoic acid)pyrene (H₄TBAPy) as a ligand and used for 5-fluorouracil (5-FU) loading (28.2 wt %) and sustained slow release. TDL-Mg exhibits a 3D supramolecular architecture featuring a 1D rectangle channel with a size of 6.2 × 8.1 Ų and a Brunauer-Emmett-Teller surface area of 627 m²·g^-1. Channel microenvironment analysis shows that the rigid H₂TBAPy^2- ligand adopts special torsion to stabilize the channels and offer rich π-binding sites; the partially deprotonated carboxyls not only participate in the formation of strong hydrogen bonds but also create a mild pH buffer environment for biological applications. Suitable host-guest interactions are generated by the synergistic effect of polydirectional hydrogen bonds, multiple π-interactions, and confined channels, which allow 5-FU@TDL-Mg to release 76% of load in 72 h, a medically reasonable rate. Microcalorimetry was used to directly quantify these host-guest interactions with a moderate enthalpy of 22.3 kJ·mol^-1, which provides a distinctive thermodynamic interpretation for understanding the relationship between the MOF design and the drug release rate. Additionally, the nano-sized 5-FU@TDL-Mg can be taken up by mouse breast cancer cells (4T1 cells) for imaging based on the dramatic fluorescence change during the release of 5-FU, exhibiting potential applications in biological systems.

Two coordination polymers constructed by 5-[(4-carboxyphenoxy)methyl]benzene-1,3-dicarboxylic acid and 2,2'-bipyridine: syntheses, structures and luminescence properties

Coordination polymers (CPs) have attracted increasing interest in recent years. In this work, two new CPs, namely poly[[aquabis(2,2'-bipyridine-κ²N,N'){μ₃-5-[(4-carboxylatophenoxy)methyl]benzene-1,3-dicarboxylato-κ⁴O¹,O^1':O³:O⁵}(μ-formato-κ³O:O,O')dicadmium(II)] monohydrate], {[Cd₂(C₁₆H₉O₇)(HCO₂)(C₁₀H₈N₂)₂(H₂O)]·H₂O}_n (1), and poly[[(2,2'-bipyridine-κ²N,N'){μ₃-5-[(4-carboxylphenoxy)methyl]benzene-1,3-dicarboxylato-κ⁴O¹,O^1':O³:O⁵}manganese(II)] sesquihydrate], {[Mn(C₁₆H₁₀O₇)(C₁₀H₈N₂)]·1.5H₂O}_n (2), have been prepared using the tricarboxylic acid 5-[(4-carboxyphenoxy)methyl]benzene-1,3-dicarboxylic acid and 2,2'-bipyridine under hydrothermal conditions. CP 1 displays a two-dimensional layer structure which is further extended into a three-dimensional (3D) supramolecular structure via intermolecular π-π interactions, while CP 2 shows a different 3D supramolecular structure extended from one-dimensional ladder chains by intermolecular π-π interactions. In addition, the solid-state luminescence spectra of 1 and 2 were studied at room temperature.

Fluorescent "Turn-on" Sensing Based on Metal-Organic Frameworks (MOFs)

Metal-organic frameworks (MOFs) have evolved as an exciting class of materials in the domain of porous materials. The unique features of these materials arise from the combined properties of metal ions/clusters and organic struts which form the building blocks of these fascinating architectures. Among other multifarious applications, MOFs have shown tremendous applications as sensory materials for a wide variety of species. The signal transduction induced mechanism in these confined nanospaces generate optical output in response to a particular analyte which can be detected by wide variety of detection techniques. Fluorometric methods of sensing is one of widely studied method over past few decades. MOF-based fluorometric detection is a key research theme developed over the past few years. In this review, we give a brief overview of the recent developments of MOFs as "turn-on" sensors for a wide range of analytes (viz. cations, anions, volatile organic compounds (VOCs), etc.).

Zinc-based metal-organic frameworks as nontoxic and biodegradable platforms for biomedical applications: review study

Development of biomedical systems for controllable drug delivery systems and construction of biosensors is imperative to reduce side effects of common treatment techniques and enhance the therapeutic efficacy. To address this issue, metal-organic frameworks (MOFs) as hybrid porous polymeric structures have attracted worldwide attention due to their unprecedented opportunities in vast range of applications in diverse fields including chemistry, biological, and medicinal science as gas storage/separation, sensing, and drug delivery systems. Recently, biomedical application has become an interesting and promising issue for development and usage of multi-functional MOFs. Flexible chemical composition and versatile porous structure of MOFs enable the engineering and enhancement of their medical formulation and functionality as practical carriers for whether therapeutic or imaging agents. One important point in this domain is the efficient delivery of drugs in the body using nontoxic and biodegradable carriers. This review brings together the literatures that addressing the biomedical applications of Zinc-based MOFs (i.e. as drug delivery systems or nontoxic agent in matter of therapeutic applications) to present recent achievements in this interesting field.

Screening metal-organic frameworks for capturing radioactive gas Rn in indoor air

The radioactive gas Rn in indoor air shows severe harm to the human body and even induces the occurence of lung cancer. Finding an excellent candidate that can efficiently and selectively adsorb Rn, is still a great challenge. In this work, we use the grand canonical ensemble Monte Carlo simulation to systematically investigate the selectivities of 23 different kinds of representative metal organic framework (MOFs) for Rn separation from the Rn/N₂ and Rn/O₂ mixtures at 298 K. Results indicate that ZIF-12, HKUST-1, IRMOF-62 and ZIF-11 are four kinds of excellent candidates for capturing Rn. In particular, the selectivities of ZIF-12 for Rn/N₂ and Rn/O₂ mixtures with Rn molar fraction X_Rn = 0.001 reach ∼2500 and 1200 at 1 atm and 298 K, respectively. Moreover, when the Rn concentration becomes smaller, the selectivity of ZIF-12 for Rn would increase significantly. In short, we found that ZIF-12 is indeed a very promising candidate for trapping radioactive Rn in indoor air.

A stable luminescent zinc–organic framework as a dual-sensor toward Cu2+ and Cr2O72−, and excellent platform-encapsulated Ln3+ for systematic color tuning and white-light emission

A stable three-fold interpenetrated framework (Zn-MOF), showing chemical sensing for Cu²⁺ and Cr₂O₇²⁻, tunable luminescence and white-light emission after encapsulating lanthanide cations, was synthesized and characterized.

Syntheses and crystal structures of new dinuclear lanthanide complexes based on 3-(4-hydroxyphenyl)propanoic acid: Hirshfeld surface analyses and photoluminescence sensing

A series of ten new lanthanide complexes, [Ln₂(HPA)₆(bipy)₂·4H₂O·2bipy], were synthesized and their photoluminescence sensing and Hirshfeld surface analyses performed.

Dual-Stimulus-Triggered Programmable Drug Release and Luminescent Ratiometric pH Sensing from Chemically Stable Biocompatible Zinc Metal-Organic Framework

Metal-organic frameworks (MOFs), as drug delivery carriers, with high loading capacity and controllable release behavior can provide a more efficacious therapy in cancer treatments. In our work, a novel biocompatible zinc MOF Zn-cpon-1 with the (3,6)-connected rtl 3D topological network was designed and synthesized via employing ClO₄^- anion as template. The optically and chemically stable Zn-cpon-1 could be verified as a pH-responsive dual-emission platform and excellent drug delivery carrier with higher 5-fluorouracil (5-FU) (44.75 wt %) loading behavior than 6-mercaptopurine (6-MP) (4.79 wt %) ascribed to the influence of size and shape matching. The multiple interactions between Zn-cpon-1 and 5-FU drug molecules have been discussed and evidenced, which could be quantitatively estimated via the rate constant related to the topological structure. Specially, the gust release behavior of 5-FU@Zn-cpon-1 microcrystal was described and programmed via the Weibull distribution model and could be dual-triggered by the temperature and pH stimulus. This study illustrates that the Zn-cpon-1 without any postmodification performs a favorable potential of being used as biomedical drug delivery alternative carriers in effective drug payload, flexible release administration, and superior dual-stimuli responsiveness.

Resorcin[4]arene-Based Microporous Metal-Organic Framework as an Efficient Catalyst for CO2 Cycloaddition with Epoxides and Highly Selective Luminescent Sensing of Cr2O72

A stable microporous anionic metal-organic framework (MOF), [(CH₃)₂NH₂]₆[Cd₃L(H₂O)₂]·12H₂O (1), has been synthesized via solvothermal assembly of a new resorcin[4]arene-functionalized dodecacarboxylic acid (H₁₂L) and Cd(II) cations. The constructed MOF (1) was characterized by single-crystal X-ray diffraction and other physicochemical analyses. 1 exhibits a fascinating 3D microporous framework structure, in which the free water molecules and the [(CH₃)₂NH₂]⁺ cations were located. Remarkably, the exposed Lewis acid Cd(II) sites of activated 1 make it an efficient heterogeneous catalyst for the cycloaddition of CO₂ with epoxides at 1 and 20 atm. Importantly, the activated samples of 1 can be reused at least five circles with excellent catalytic performance. Moreover, the fluorescence detection of Cr₂O₇^2- and Fe³⁺ was studied by using 1 as a potential luminescent sensor.

Cycloaddition Reactions of Cobalt-Complexed Macrocyclic Alkynes: The Transannular Pauson-Khand Reaction

The Pauson-Khand reaction is a powerful tool for the synthesis of cyclopentenones through the efficient [2 + 2 + 1] cycloaddition of dicobalt alkyne complexes with alkenes. While intermolecular and intramolecular variants are widely known, transannular versions of this reaction are unknown and the basis of this study. Macrocyclic enyne and dienyne complexes were readily synthesized by palladium(II)-catalyzed oxidative macrocyclizations of bis(vinyl boronate esters) or ring-closing metathesis reactions followed by complexation with dicobalt octacarbonyl. Several reaction modalities of these macrocyclic complexes were uncovered. In addition to the first successful transannular Pauson-Khand reactions, other intermolecular and transannular cycloaddition reactions included intermolecular Pauson-Khand reactions, transannular [4 + 2] cycloaddition reactions, intermolecular [2 + 2 + 2] cycloaddition reactions, and intermolecular [2 + 2 + 1 + 1] cycloaddition reactions. The structural and reaction requirements for each process are presented.

Multifunctional Metal-Organic Framework Nanoprobe for Cathepsin B-Activated Cancer Cell Imaging and Chemo-Photodynamic Therapy

Integration of a photodynamic therapy platform with a drug-delivery system in a porous structure is an urgent challenge for enhanced anticancer therapy. Here, an amino-functionalized metal-organic framework (MOF), which is useful as efficient delivery vehicle for drugs and provides the -NH₂ group for postsynthetic modification, is chosen and well-designed for cell imaging and chemo-photodynamic therapy. The multifunctional MOF nanoprobe was first assembled with camptothecine drug via noncovalent encapsulation and then bound with folic acid as the targeted element and chlorine e6 (Ce6)-labeled CaB substrate peptide as the recognition moiety and signal switch. The designed MOF probe can realize cathepsin B-activated cancer cell imaging and chemo-photodynamic dual-therapy combining Ce6 as the photosensitizer and the camptothecine drug. Compared with the individual treatment, the dual-functional nanoprobe presents an enhanced treatment efficiency in terms of the time of chemotherapy, laser power, and irradiation time of the photodynamic therapy, which has been confirmed in cancer cells and in vivo assays. This work presents a significant example of the MOF nanoprobe as an intracellular switch and shows great potential in cancer cell targeted imaging and multiple therapies.

Luminescent Functionalised Supramolecular Coordination Polymers Based on an Aromatic Carboxylic Acid Ligand for Sensing Hg2+ Ions

Two luminescent functionalised supramolecular coordination polymers, namely, [Zn(TPDC-2CH3)(H2O)2]·H2O (1) and [Cd(TPDC-2CH3)(H2O)4]·H2O (2), were successfully synthesised by the reaction of 2′,5′-dimethyl-[1,1′:4′,1″-terphenyl]-4,4″-dicarboxylic acid (H2TPDC-2CH3) with Zn2+ and Cd2+ ions, respectively. X-Ray crystallographic analysis reveals that both compounds 1 and 2 exhibit fascinating 3D supramolecular networks, in which metal ions are linked by ligands to form a 1D chain which further extends to a 3D structure through the interaction of hydrogen bonding. The use of 1 and 2 as luminescent sensors for the optical detection of metal ions: Na+, K+, Hg2+, Ag+, Ca2+, Co2+, Ni2+, Mn2+, Cu2+, Zn2+, Cd2+, Pb2+, Mg2+, Al3+, Fe3+, Fe2+, In3+, Bi3+, and Cr3+ was carried out in aqueous solution, and the results indicated that compound 1 could effectively detect Hg2+ ions among various cations at room temperature, with a detection limit of 3.6 × 10−15 M.

Visible detection of copper ions using a fluorescent probe based on red carbon dots and zirconium metal–organic frameworks

Specific spectral energy transfer leads to a gradually increasing quenching effect on the fluorescence of UiO-66-(COOH)₂with a higher concentration of Cu²⁺, causing a visible color change from blue to red.