Postsynthetic modification of zirconium metal-organic frameworks

Metal-organic frameworks (MOFs) have been in the spotlight for a number of years due to their chemical and topological versatility. As MOF research has progressed, highly functionalised materials have become desirable for specific applications, and in many cases the limitations of direct synthesis have been realised. This has resulted in the search for alternative synthetic routes, with postsynthetic modification (PSM), a term used to collectively describe the functionalisation of pre-synthesised MOFs whilst maintaining their desired characteristics, becoming a topic of interest. Advances in the scope of reactions performed are reported regularly; however reactions requiring harsh conditions can result in degradation of the framework. Zirconium-based MOFs present high chemical, thermal and mechanical stabilities, offering wider opportunities for the scope of reaction conditions that can be tolerated, which has seen a number of successful examples reported. This microreview discusses pertinent examples of PSM resulting in enhanced properties for specific applications, alongside fundamental transformations, which are categorised broadly into covalent modifications, surface transformations, metalations, linker and metal exchange, and cluster modifications

Enhancing anticancer cytotoxicity through bimodal drug delivery from ultrasmall Zr MOF nanoparticles

Dual delivery of dichloroacetate and 5-fluorouracil from Zr MOFs into cancer cells is found to enhance in vitro cytotoxicity. Tuning particle size and, more significantly, surface chemistry, further improves cytotoxicity by promoting caveolae-mediated endocytosis and cytosolic cargo delivery

The surface chemistry of metal-organic frameworks and their applications

Modifying the outer surfaces of metal-organic frameworks (MOFs) has received considerably less attention than functionalization of the bulk, despite the range of physical and chemical properties that can be tuned by controlling MOF surface chemistry. In this Frontier article, we summarise developments over the last five years in both functionalizing and visualizing the outer surfaces of MOFs, with particular focus on their application as surface-modified nanoparticles for drug delivery and in the enhanced self-assembly of hybrid materials

Psychophysiological insomnia and idiopathic insomnia: the role of self-regulatory behaviour systems

Further research into identifying the mechanisms that underlie the development and maintenance of insomnia and its different subtypes is required. Neurobiological motivational systems are thought to mediate our experiences of negative and positive affect and are implicated in the etiology of psychiatric disorders, but their role in insomnia is unknown. The present study aimed to compare self-reported sensitivity to these systems across Psychophysiological Insomnia (PI) and Idiopathic Insomnia (IdI). Sixty one adults with PI (n = 20) and IdI (n = 20), and Good Sleepers (n = 21), completed measures of sleep characteristics, Behavioural Inhibition Sensitivity (BIS), Behavioural Activation Sensitivity (BAS), Sleep Effort, Depression and Anxiety. As predicted the PI group reported significantly greater BIS sensitivity compared with the IdI and GS groups. However, no significant differences were found between groups on BAS sensitivity. Post-hoc analysis revealed significant differences between the insomnia groups on sleep effort when age was included as a covariate. Depression and anxiety did not moderate the relationships between the other outcome variables. The findings support the notion that PI is associated with a specific tendency toward threat sensitivity, a tendency absent in IdI. This is consistent with contemporary thinking on PI that this group exhibits greater vulnerability to stress-related sleep disturbance, whereas IdI is a more stable insomnia subtype that may be less reactive to circumstances. Accordingly, this suggests that different psychological treatment approaches are indicated for these subtypes with PI requiring re-conditioning forms of CBT and IdI requiring a more acceptance based approach

Modulated self-assembly of metal-organic frameworks

Exercising fine control over the synthesis of metal-organic frameworks (MOFs) is key to ensuring reproducibility of physical properties such as crystallinity, particle size, morphology, porosity, defectivity, and surface chemistry. The principle of modulated self-assembly – incorporation of modulator molecules into synthetic mixtures – has emerged as the primary means to this end. This perspective article will detail the development of modulated synthesis, focusing primarily on coordination modulation, from a technique initially intended to cap the growth of MOF crystals to one that is now used regularly to enhance crystallinity, control particle size, induce defectivity and select specific phases. The various mechanistic driving forces will be discussed, as well as the influence of modulation on physical properties and how this can facilitate potential applications. Modulation is also increasingly being used to exert kinetic control over self-assembly; examples of phase selection and the development of new protocols to induce this will be provided. Finally, the application of modulated self-assembly to alternative materials will be discussed, and future perspectives on the area given

Crystallographic investigation into the self-assembly, guest binding, and flexibility of urea functionalised metal-organic frameworks

Introduction of hydrogen bond functionality into metal-organic frameworks can enhance guest binding and activation, but a combination of linker flexibility and interligand hydrogen bonding often results in the generation of unwanted structures where the functionality is masked. Herein, we describe the self-assembly of three materials, where Cd2+, Ca2+, and Zn2+ are linked by N,Nʹ-bis(4-carboxyphenyl)urea, and examine the effect of the urea units on structure formation, the generation of unusual secondary building units, structural flexibility, and guest binding. The flexibility of the Zn MOF is probed through single-crystal to single-crystal transformations upon exchange of DMF guests for CS2, showing that the lability of the [Zn4O(RCO2)6] cluster towards solvation enables the urea linkers to adopt distorted conformations as the MOF breathes, even facilitating rotation from the trans/trans to the trans/cis conformation without compromising the overall topology. The results have significant implications in the mechanistic understanding of the hydrolytic stability of MOFs, and in preparing heterogeneous organocatalysts

Square vortex lattice at anomalously low magnetic fields in electron-doped Nd1.85_{1.85}Ce0.15_{0.15}CuO4_{4}

We report here on the first direct observations of the vortex lattice in the bulk of electron-doped Nd$_{1.85}$Ce$_{0.15}$CuO$_{4}$ single crystals. Using small angle neutron scattering, we have observed a square vortex lattice with the nearest-neighbors oriented at 45$^{\circ}$ from the Cu-O bond direction, which is consistent with theories based on the d-wave superconducting gap. However, the square symmetry persists down to unusually low magnetic fields. Moreover, the diffracted intensity from the vortex lattice is found to decrease rapidly with increasing magnetic field.Comment: 4 pages, 4 Figures, accepted for publication in Phys. Rev. Let

Synthetic considerations in the self-assembly of coordination polymers of pyridine-functionalised hybrid Mn-Anderson polyoxometalates

The incorporation of polyoxometalates (POMs) as structural units into ordered porous constructs such as metal-organic frameworks (MOFs) is desirable for a range of applications where intrinsic properties inherited from both the MOF and POM are utilised, including catalysis and magnetic data storage. The controlled self-assembly of targeted MOF topologies containing POM units is hampered by the wide range of oxo and hydroxo units on the peripheries of POMs that can act as coordinating groups towards linking metal cations leading to a diverse range of structures, but incorporation of organic donor units into hybrid POMs offers an alternative methodology to programmably synthesise POM/MOF conjugates. Herein, we report six coordination polymers obtained serendipitously wherein Zn2+ and Cu2+ link pyridine-appended Mn-Anderson clusters into two- and three-dimensional network solids with complex connectivities and topologies. Careful inspection of their solid-state structures has allowed us to identify common structure-directing features across these coordination polymers, including a square motif where two Zn2+ cations bridge two POMs. By correlating certain structural motifs with synthetic conditions we have formulated a series of design considerations for the self-assembly of coordination polymers of hybrid POMs, encompassing the selection of reaction conditions, co-ligands and linking metal cations. We anticipate that these synthetic guidelines will inform the future assembly of hybrid POMs into functional MOF materials

Triple trouble for XZ Tau : deep imaging with the Jansky Very Large Array

DF gratefully acknowledges support from STFC grant ST/J001422/1. RJI acknowledges support in the form of ERC Advanced Investigator programme, cosmicism. EI acknowledges funding from CONICYT/FONDECYT postdoctoral project no.: 3130504.We present new observations of the XZ Tau system made at high angular resolution (55 mas) with the Karl G. Jansky Very Large Array (VLA) at a wavelength of 7 mm. Observations of XZ Tau made with the VLA in 2004 appeared to show a triple-star system, with XZ Tau A resolved into two sources, XZ Tau A and XZ Tau C. The angular separation of XZ Tau A and C (0.09 arcsec) suggested a projected orbital separation of around 13 au with a possible orbital period of around 40 yr. Our follow-up observations were obtained approximately 8 yr later, a fifth of this putative orbital period, and should therefore allow us to constrain the orbital parameters of XZ Tau C, and evaluate the possibility that a recent periastron passage of C coincided with the launch of extended optical outflows from XZ Tau A. Despite improved sensitivity and resolution, as compared with the 2004 observations, we find no evidence of XZ Tau C in our data. Components A and B are detected with a signal-to-noise ratio greater than 10; their orbital motions are consistent with previous studies of the system, although the emission from XZ Tau A appears to be weaker. Three possible interpretations are offered: either XZ Tau C is transiting XZ Tau A, which is broadly consistent with the periastron passage hypothesis, or the emission seen in 2004 was that of a transient, or XZ Tau C does not exist. A fourth interpretation, that XZ Tau C was ejected from the system, is dismissed due to the lack of angular momentum redistribution in the orbits of XZ Tau A and XZ Tau B that would result from such an event. Transients are rare but cannot be ruled out in a T Tauri system known to exhibit variable behaviour. Our observations are insufficient to distinguish between the remaining possibilities, at least not until we obtain further VLA observations at a sufficiently later time. A further non-detection would allow us to reject the transit hypothesis, and the periastron passage of XZ Tau C as agent of XZ Tau A's outflows.Publisher PDFPeer reviewe

Application of zirconium MOFs in drug delivery and biomedicine

Nanoparticulate metal-organic frameworks (MOFs) have the requisite high storage capacities, tailorable structures, ease of functionalisation, and excellent biocompatibilities for application as nanoscale drug delivery devices (DDSs). Zirconium MOFs in particular combine optimal stability towards hydrolysis and postsynthetic modification with low toxicity, and so are particularly suited for biological applications. This review covers the use of Zr MOFs as DDSs with focus on the different physical properties that makes them attractive for use. The various methods for modifying the surfaces of Zr MOFs are described with pertinent examples of the resulting enhancements in aqueous stability, colloidal dispersion, and stimuli-responsive drug release. The in vitro and in vivo application of Zr MOFs for photodynamic therapy and drug delivery are discussed with respect to the structural features of the MOFs and their surface functionality, and perspectives on their future applications and analogous hafnium MOFs are given