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.
This Perspective gives an overview of the modulated self-assembly of MOFs - incorporating additives and alternative precursors into syntheses - focusing on its varying influences on crystallization mechanisms, physical properties, and applications.
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.
The chemical and mechanical stabilities of zirconium metal‐organic frameworks (MOFs) make them ideal platforms for postsynthetic modification. This microreview provides an overview of the various techniques for modification and the functionalities that can be incorporated into zirconium MOFs to facilitate different applications.
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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.
Support vector machine (SVM) was applied for land-cover characterization using MODIS time-series data. Classification performance was examined with respect to training sample size, sample ...variability, and landscape homogeneity (purity). The results were compared to two conventional nonparametric image classification algorithms: multilayer perceptron neural networks (NN) and classification and regression trees (CART). For 2001 MODIS time-series data, SVM generated overall accuracies ranging from 77% to 80% for training sample sizes from 20 to 800 pixels per class, compared to 67–76% and 62–73% for NN and CART, respectively. These results indicated that SVM’s had superior generalization capability, particularly with respect to small training sample sizes. There was also less variability of SVM performance when classification trials were repeated using different training sets. Additionally, classification accuracies were directly related to sample homogeneity/heterogeneity. The overall accuracies for the SVM algorithm were 91% (Kappa=0.77) and 64% (Kappa=0.34) for homogeneous and heterogeneous pixels, respectively. The inclusion of heterogeneous pixels in the training sample did not increase overall accuracies. Also, the SVM performance was examined for the classification of multiple year MODIS time-series data at annual intervals. Finally, using only the SVM output values, a method was developed to directly classify pixel purity. Approximately 65% of pixels within the Albemarle–Pamlico Basin study area were labeled as “functionally homogeneous” with an overall classification accuracy of 91% (Kappa=0.79). The results indicated a high potential for regional scale operational land-cover characterization applications.
The properties of metal–organic frameworks (MOFs) can be tuned by postsynthetic modification (PSM) to introduce specific functionalities after their synthesis. Typically, PSM is carried out on ...pendant functional groups or through metal/ligand exchange, preserving the structure of the MOF. We report herein the bromination of integral alkyne units in a pair of Zr4+ and Hf4+ MOFs, which proceeds stereoselectively in a single-crystal to single-crystal manner. The chemical and mechanical changes in the MOFs are extensively characterized, including the crystal structures of the postsynthetically brominated materials, which show a mechanical contraction of up to 3.7% in volume. The combination of stability and chemical reactivity in these MOFs leads to the possibility of tuning mechanical properties by chemical transformation while also opening up new routes to internal pore functionalization.
Metal–organic frameworks (MOFs) are emerging as leading candidates for nanoscale drug delivery, as a consequence of their high drug capacities, ease of functionality, and the ability to carefully ...engineer key physical properties. Despite many anticancer treatment regimens consisting of a cocktail of different drugs, examples of delivery of multiple drugs from one MOF are rare, potentially hampered by difficulties in postsynthetic loading of more than one cargo molecule. Herein, we report a new strategy, multivariate modulation, which allows incorporation of up to three drugs in the Zr MOF UiO‐66 by defect‐loading. The drugs are added to one‐pot solvothermal synthesis and are distributed throughout the MOF at defect sites by coordination to the metal clusters. This tight binding comes with retention of crystallinity and porosity, allowing a fourth drug to be postsynthetically loaded into the MOFs to yield nanoparticles loaded with cocktails of drugs that show enhancements in selective anticancer cytotoxicity against MCF‐7 breast cancer cells in vitro. We believe that multivariate modulation is a significant advance in the application of MOFs in biomedicine, and anticipate the protocol will also be adopted in other areas of MOF chemistry, to easily produce defective MOFs with arrays of highly functionalised pores for potential application in gas separations and catalysis.
Multivariate modulation: Inclusion of multiple modulators into one‐pot self‐assembly of MOFs yields UiO‐66 nanoparticles with three drugs loaded at defect sites throughout the structure, yet still sufficient porosity to postsynthetically store a fourth, selectivity‐enhancing, anticancer drug.
3D printing techniques allow the laboratory‐scale design and production of reactionware tailored to specific experimental requirements. To increase the range and versatility of reactionware devices, ...sealed, monolithic reactors suitable for use in hydrothermal synthesis have been digitally designed and realized. The fabrication process allows the introduction of reaction mixtures directly into the reactors during the production, and also enables the manufacture of devices of varying scales and geometries unavailable in traditional equipment. The utility of these devices is shown by the use of 3D printed, high‐throughput array reactors to discover two new coordination polymers, optimize the synthesis of one of these, and scale‐up its synthesis using larger reactors produced on the same 3D printer. Reactors were also used to produce phase‐pure samples of coordination polymers MIL‐96 and HKUST‐1, in yields comparable to synthesis in traditional apparatus.
Form and function: Sealed, monolithic reactors suitable for use in hydrothermal synthesis are digitally designed and realized using 3D printing techniques. The utility of these reactors is shown by the fabrication of high‐throughput‐array reactors which were used to discover two new coordination polymers, and to optimize and scale up the synthesis of one of these.
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