Metal-organic framework/Nb4C3Tx MXene composites for ultrasensitive detection of dopamine

An easy, in situ growth approach led to the formation of several composites of metal-organic framewoks and Nb4C3Tx MXenes mixed intimately at the submicron scale. The high affinity of MXene surface for dopamine, enhanced by a nanostructuration induced by MOFs, resulted in superior sensing performances. The system exhibited good linearity over the 1-100 nM range, with an excellent limit of detection of 0.2 nM.

A novel style of 2D Hofmann-type coordination polymer incorporated trigonal prismatic coordination geometry with bidentate co-ligands

A novel 2D Hofmann-type framework was prepared with a bidentate co-ligand, 5,5'-dimethyl-2,2'-bipyridyl (dmbpy), which forces the curvature of the layer. X-ray diffraction analysis demonstrated that the coordination polymers, MnII(dmbpy)[MVN(CN)4] (MV = Mn (1) and Cr (2)), formed a considerably corrugated 2D cyanide-bridged network with a quasi C4v symmetric building unit, [CrVN(CN)4]2-, and trigonal prismatic coordination geometry around MnII. Compound 2 demonstrated a metamagnetic-like ordering at 14.4 K, caused by the intra- and inter-layer antiferromagnetic interactions between CrV (S = 1/2) and MnII (S = 5/2), and a weak ferromagnetic behaviour at 2 K reflecting the single-ion anisotropy of CrV and structural anisotropy.

Strongly Enhanced Polarization in a Ferroelectric Crystal by Conduction-Proton Flow

Ion conductors comprising noncentrosymmetric frameworks have emerged as new functional materials. However, strongly correlated polarity functionality and ion transport have not been achieved. Herein, we report a ferroelectric proton conductor, K2MnN(CN)4·H2O (1·H2O), exhibiting the strong correlation between its polar skeleton and conductive ions that generate anomalous ferroelectricity via the proton-bias phenomenon. The application of an electric field of ±1 kV/cm (0.1 Hz) on 1·H2O at 298 K produced the ferroelectricity (polarization = 1.5 × 104 μC/cm2), which was enhanced by the ferroelectric-skeleton-trapped conductive protons. Furthermore, the strong polarity-proton transport coupling of 1·H2O induced a proton-rectification-like directional ion-conductive behavior that could be adjusted by the magnitude and direction of DC electric fields. Moreover, 1·H2O exhibited reversible polarity switching between the polar 1·H2O and its dehydrated form, 1, with a centrosymmetric structure comprising an order-disorder-type transition of the nitrido-bridged chains.

Janus-Type Mixed-Valent Copper-Cyanido Honeycomb Layers

The synthesis of Janus-type layers, which possess front and back sides that consist of different structures, remains a major challenge in the field of two-dimensional materials. In this study, two Janus-type layered coordination polymers, namely, CuII(NEtH2)(NMe2H·H2O)CuI(CN)3 (1) and CuII(NMe2H)(NMe2H·H2O)CuI(CN)3 (2), were synthesized via a simple one-pot procedure using copper(II) nitrate and sodium cyanido in mixed solutions of dimethylamine and ethylamine. Uniquely, 1 and 2 were composed of cyanido-bridged neutral layers and exhibited a CuICuII mixed-valent state. Meanwhile, using a solution of pure dimethylamine for the synthesis yielded the monovalent three-dimensional framework (NMe2H2)[CuI2(CN)3] (3). Results indicated that the simultaneous use of two mixed amines gave rise to the controlled reduction of CuII ions during the reaction. In addition, each face of the layers was coordinated by different amines on the axial positions of the CuII sites, resulting in anisotropic Janus layers. Furthermore, the thermal expansion behavior of 2 was investigated, demonstrating that the neutral [CuICuII(CN)3] layer was relatively rigid compared with the analogous anionic [CuI2(CN)3]- layer.

Efficient water-based purification of metal-organic polyhedra by centrifugal ultrafiltration

An efficient water-based purification strategy for metal-organic polyhedra (MOPs) using commercially available centrifugal ultrafiltration membranes was developed. Having a diameter above 3 nm, MOPs were almost fully retained by the filters, while free ligands and other impurities were washed away. MOP retention also enabled efficient counter-ion exchange. This method paves the way for the application of MOPs with biological systems.

Structural-transformation-induced Drastic Luminescence Changes in an Organic-Inorganic Hybrid [ReN(CN)4]2- Salt Triggered by Chemical Stimuli

We synthesized a (1-propylpyridinium)2[ReN(CN)4]-type organic-inorganic hybrid exhibiting water-vapor-induced drastic structural changes of the [ReN(CN)4]2- assemblies. Specifically, upon exposure to water vapor, dehydrated nitrido-bridged chains were converted to hydrated cyanido-bridged tetranuclear clusters via rearrangements of large molecular building units in the crystals. These switchable assembly forms display substantially different photo-physical properties, although in both cases the emission is caused by a metal-centered d-d transition. The nitrido-bridged chain exhibited a near-infrared (749 nm) emission, which blue-shifted as the temperature increased, while a visible (561 nm) emission and its red shift was demonstrated by the cyanido-bridged cluster.

An approach to MOFaxanes by threading ultralong polymers through metal-organic framework microcrystals

Mechanically interlocked architecture has inspired the fabrication of numerous molecular systems, such as rotaxanes, catenanes, molecular knots, and their polymeric analogues. However, to date, the studies in this field have only focused on the molecular-scale integrity and topology of its unique penetrating structure. Thus, the topological material design of such architectures has not been fully explored from the nano- to the macroscopic scale. Here, we propose a supramolecular interlocked system, MOFaxane, comprised of long chain molecules penetrating a microcrystal of metal-organic framework (MOF). In this study, we describe the synthesis of polypseudoMOFaxane that is one of the MOFaxane family. This has a polythreaded structure in which multiple polymer chains thread a single MOF microcrystal, forming a topological network in the bulk state. The topological crosslinking architecture is obtained by simply mixing polymers and MOFs, and displays characteristics distinct from those of conventional polyrotaxane materials, including suppression of unthreading reactions.

Water-Soluble Ionic Metal-Organic Polyhedra as a Versatile Platform for Enzyme Bio-immobilization

Metal-organic polyhedra (MOPs) can act as elementary structural units for the design of modular porous materials; however, their association with biological systems remains greatly restricted by their typically low stabilities and solubilities in water. Herein, we describe the preparation of novel MOPs bearing either anionic or cationic groups and exhibiting a high affinity for proteins. Simple mixing of the protein bovine serum albumin (BSA) and ionic MOP aqueous solutions resulted in the spontaneous formation of MOP-protein assemblies, in a colloidal state or as solid precipitates depending on the initial mixing ratio. The versatility of the method was further illustrated using two enzymes, catalase and cytochrome c, with different sizes and isoelectric points (pI's) below and above 7. This mode of assembly led to the high retention of catalytic activity and enabled recyclability. Furthermore, the co-immobilization of cytochrome c with highly charged MOPs resulted in a substantial 44-fold increase of its catalytic activity.

Negative Thermal Expansion of Undulating Coordination Layers through Interlayer Interaction

The negative thermal expansion (NTE) of solid-state materials is of significance in various fields, but a very rare phenomenon. In this study, we carried out a meta-analysis for the anisotropic thermal expansion behavior of fifteen two-dimensional coordination polymers [M(salen)]₂[M'(CN)₄(solvent)] (M = Mn, Fe; M' = MnN, ReN, Pt, Pt(I₂)_x; x = 0.18, 0.45, 0.85, 1.0; solvent = H₂O, MeOH, MeCN) with a newly synthesized [Fe(salen)]₂[MnN(CN)₄(MeCN)]. Consequently, we successfully demonstrate the unusual NTE of the undulating coordination layers by an expansion deformation of the layers via strong interlayer interaction within the layer stacking. Notably, the layer volume of [Mn(salen)]₂[ReN(CN)₄] with its powder form decreases with a large NTE coefficient, α_layer-volume = -27 × 10^-6 K^-1 (100-500 K). This is a significantly large value despite the increase in layer thickness along the layer contraction based on the anisotropic transformation of undulating layers. Conversely, the analysis demonstrates that the chemical modification of the layers to enhance intralayer interaction rather than interlayer interaction switches a direction of the layer anisotropy, yielding positive thermal expansion materials with the coefficient of the layer volume reaching +92 × 10^-6 K^-1.

Vapor-Induced Conversion of a Centrosymmetric Organic-Inorganic Hybrid Crystal into a Proton-Conducting Second-Harmonic-Generation-Active Material

Chemical responsivity in materials is essential to build systems with switchable functionalities. However, polarity-switchable materials are still rare because inducing a symmetry breaking of the crystal structure by adsorbing chemical species is difficult. In this study, we demonstrate that a molecular organic-inorganic hybrid crystal of (NEt₄)₂[MnN(CN)₄] (1) undergoes polarity switching induced by water vapor and transforms into a rare example of proton-conducting second-harmonic-generation-active material. Centrosymmetric 1 transforms into noncentrosymmetric polar 1·3H₂O and 1·MeOH by accommodating water and methanol molecules, respectively. However, only water vapor causes a spontaneous single-crystal-to-single-crystal transition. Moreover, 1·3H₂O shows proton conduction with 2.3 × 10^-6 S/cm at 298 K and a relative humidity of 80%.

Zero area thermal expansion of honeycomb layers via double distortion relaxation in (PPh4)[Cu2(CN)3]

The zero area TE of cyanide-bridged honeycomb layers occurs by complementary structural changes in the cation and anion counterparts.

Flexibility Control of Two-Dimensional Coordination Polymers by Crystal Morphology: Water Adsorption and Thermal Expansion

Layer flexibility in two-dimensional coordination polymers (2D-CPs) contributes to several functional materials as it results in anisotropic structural response to external stimuli. Chemical modification is a common technique for modifying layer structures. This study demonstrates that crystal morphology of a cyanide-bridged 2D-CP of type [Mn(salen)]₂ [ReN(CN)₄ ] (1) consisting of flexible undulating layers significantly impacts the layer configuration and assembly. Nanoplates of 1 showed an in-plane contraction of layers with a longer interlayer distance compared to the micrometer-sized rod-type particles. These effects by crystal morphology on the structure of the 2D-CP impacted the structural flexibility, resulting in dual-functional changes: the enhancement of the sensitivity of structural transformation to water adsorption and modification of anisotropic thermal expansion of 1. Moreover, the nanoplates incorporated new adsorption sites within the layers, resulting in the uptake of an additional water molecule compared to the micrometer-sized rods.

Crystalline assembly of metal-organic polyhedra driven by ionic interactions with polyoxometalates

Charge-driven self-assembly of cationic zirconium-based metal-organic polyhedra (MOPs) with polyoxometalates (POMs) leads to a series of porous crystalline salts, prepared by simple mixing of soluble precursors. The reactivity of immobilized POMs was greatly increased, as demonstrated by their fast reduction by hydrazine vapors, without loss of structural integrity.

Guest-selective and reversible magnetic phase switching in a pseudo-pillared-layer porous magnet

A novel porous magnet consisting of cationic two-dimensional (2-D) layers extended by Fe^III-CN-Ni^II linkages and pseudo-pillar dianions was synthesized. The size-selective guest adsorption behaviour of water and methanol molecules originates from the narrow bottle-neck-type pores in the flexible pseudo-pillared-layer structure, which results in the switching of the magnetic phases from antiferromagnetic to ferromagnetic, involving significant changes in the interlayer distance.

Guest-Tunable Excited States in a Cyanide-Bridged Luminescent Coordination Polymer

The excited-state energy was tuned successfully by guest molecules in a cyanide-bridged luminescent coordination polymer (CP). Methanol or ethanol vapor reversibly and significantly changed the luminescent color of the CP between green and yellow (Δλ_em = 32 nm). These vapors did not significantly affect the environment around the luminophore in the ground state of the CP, whereas they modulated the excited states for the resulting bathochromic shift. The time-resolved photoluminescent spectra of the CP systems showed that solvent adsorption enhanced the energetic relaxation in the excited states. Furthermore, time-resolved infrared spectroscopy indicated that cyanide bridging in the CP became more flexible in the excited states than that in the ground state, highlighting the sensitivity of the excited states to external stimuli, such as the guest vapor. Overall, guest-tunable excited states will allow the more straightforward design of sensing materials by characterizing the transient excited states.

Coordination Geometry Changes in Amorphous Cyanide-Bridged Metal-Organic Frameworks upon Water Adsorption

Amorphous coordination polymers and metal-organic frameworks (MOFs) have attracted much attention owing to their various functionalities. Here, we demonstrate the tunable water adsorption behavior of a series of amorphous cyanide-bridged MOFs with different metals (M[Ni(CN)₄]: MNi; M = Mn, Fe, and Co). All three compounds adsorb up to six water molecules at a certain vapor pressure (P_ads) and undergo conversion to crystalline Hofmann-type MOFs, M(H₂O)₂[Ni(CN)₄]·4H₂O (MNi-H₂O; M = Mn, Fe, and Co). The P_ads of MnNi, FeNi, and CoNi for water adsorption is P/P₀ = 0.4, 0.6, and 0.9, respectively. Although the amorphous nature of these materials prevented structural elucidation using X-ray crystallography techniques, the local-scale structure around the N-coordinated M²⁺ centers was analyzed using L_2,3-, K-edge X-ray absorption fine structure, and magnetic measurements. Upon hydration, the coordination geometry of these metal centers changed from tetrahedral to octahedral, resulting in significant reorganization of the MOF local structure. On the other hand, Ni[Ni(CN)₄] (NiNi) containing square-planar Ni²⁺ centers did not undergo significant structural transformation and therefore abruptly adsorbed H₂O in the low-pressure region. We could thus define how changes in the bond lengths and coordination geometry are related to the adsorption properties of amorphous MOF systems.

Controlling the Packing of Metal-Organic Layers by Inclusion of Polymer Guests

The preparation of metal-organic structures with a controlled degree of disorder is currently one of the most promising fields of materials science. Here, we describe the effect of guest polymer chains on the transformation of a metal-organic framework (MOF). Heating a pillared MOF at a controlled temperature resulted in the exclusive removal of the pillar ligands, while the connectivity of the metal-organic square-grid layers was maintained. In the absence of a polymer, 2D-layers rearranged to form a new crystalline phase. In contrast, the presence of a polymer in the MOF inhibited totally the recrystallization, leading to a turbostratic phase with layers threaded and maintained apart by the polymer chains. This work demonstrates a new synthetic approach toward the preparation of anisotropic metal-organic materials with controlled disorder. It also reveals how guests can dramatically modify the conversion of host MOFs, even though no chemical reaction occurs between them.

Selective sorting of polymers with different terminal groups using metal-organic frameworks

Separation of high-molecular-weight polymers differing just by one monomeric unit remains a challenging task. Here, we describe a protocol using metal-organic frameworks (MOFs) for the efficient separation and purification of mixtures of polymers that differ only by their terminal groups. In this process, polymer chains are inserted by threading one of their extremities through a series of MOF nanowindows. Selected termini can be adjusted by tuning the MOF structure, and the insertion methodology. Accordingly, MOFs with permanently opened pores allow for the complete separation of poly(ethylene glycol) (PEG) based on steric hindrance of the terminal groups. Excellent separation is achieved, even for high molecular weights (20 kDa). Furthermore, the dynamic character of a flexible MOF is used to separate PEG mixtures with very similar terminal moieties, such as OH, OMe, and OEt, as the slight difference of polarity in these groups significantly changes the pore opening kinetics.

The separation of high molecular weight polymers composed of the same number of monomeric units remains highly challenging. Here, the authors show that efficient separation and purification of mixtures of polymers that differ only by their terminal groups can be achieved through polymer threading in metal-organic framework channels.

Core-Shell Silver Nanoparticles in Endodontic Disinfection Solutions Enable Long-Term Antimicrobial Effect on Oral Biofilms

To achieve effective long-term disinfection of the root canals, we synthesized core-shell silver nanoparticles (AgNPs@SiO₂) and used them to develop two irrigation solutions containing sodium phytate (SP) and ethylene glycol-bis(β-aminoethyl ether)N,N,N',N'-tetraacetic acid (EGTA), respectively. Ex vivo studies with instrumented root canals revealed that the developed irrigation solutions can effectively remove the smear layer from the dentinal surfaces. Further in vitro experiments with single- and multispecies biofilms demonstrated for the first time that AgNPs@SiO₂-based irrigation solutions possess excellent antimicrobial activities for at least 7 days, whereas the bare AgNPs lose the activity almost immediately and do not show any antibacterial activity after 2 days. The long-term antimicrobial activity exhibited by AgNPs@SiO₂ solutions can be attributed to the sustainable availability of soluble silver, even after 7 days. Both solutions showed lower cytotoxicity toward human gingival fibroblasts compared to the conventionally used solution (3% NaOCl and 17% EDTA). Irrigation solutions containing AgNP@SiO₂ may therefore be highly promising for applications needing a long-term antimicrobial effect.

Opening of an Accessible Microporosity in an Otherwise Nonporous Metal-Organic Framework by Polymeric Guests

The development of highly porous metal-organic frameworks (MOFs) is greatly sought after, due to their wide range of applications. As an alternative to the development of new structures, we propose to obtain new stable configurations for flexible MOFs by insertion of polymeric guests. The guests prevent the otherwise spontaneous closing of the host frameworks and result in stable opened forms. Introduced at a fraction of the maximal capacity, polymer chains cause an opening of the occupied nanochannels, and because of the MOF reticular stiffness, this opening is propagated to the neighboring nanochannels that become accessible for adsorption. Composites were obtained by in situ polymerization of vinyl monomers in the nanochannels of an otherwise nonporous MOF, resulting in homogeneously loaded materials with a significant increase of porosity (S_BET = 920 m²/g). In addition, by limiting the accessible configurations for the framework and forbidding the formation of a reactive intermediate, the polymeric guest prevented the thermal degradation of the host MOF even at very low loading (as low as 3 wt %) and increased its stability domain by more than 200 °C.

Host-guest chemistry with water-soluble gold nanoparticle supraspheres

The uptake of molecular guests, a hallmark of the supramolecular chemistry of cages and containers, has yet to be documented for soluble assemblies of metal nanoparticles. Here we demonstrate that gold nanoparticle-based supraspheres serve as a host for the hydrophobic uptake, transport and subsequent release of over two million organic guests, exceeding by five orders of magnitude the capacities of individual supramolecular cages or containers and rivalling those of zeolites and metal-organic frameworks on a mass-per-volume basis. The supraspheres are prepared in water by adding hexanethiol to polyoxometalate-protected 4 nm gold nanoparticles. Each 200 nm assembly contains hydrophobic cavities between the estimated 27,400 gold building blocks that are connected to one another by nanometre-sized pores. This gives a percolated network that effectively absorbs large numbers of molecules from water, including 600,000, 2,100,000 and 2,600,000 molecules (35, 190 and 234 g l^-1) of para-dichorobenzene, bisphenol A and trinitrotoluene, respectively.

Preparation of polythiophene microrods with ordered chain alignment using nanoporous coordination template

The properties of polymeric materials can be amplified by macromolecular alignment.