Shuangbing Han

How and Why Nanoparticle’s Curvature Regulates the Apparent pKa of the Coating Ligands

Dissociation of ionizable ligands immobilized on nanopaticles (NPs) depends on and can be regulated by the curvature of these particles as well as the size and the concentration of counterions. The apparent acid dissociation constant (pK(a)) of the NP-immobilized ligands lies between that of free ligands and ligands self-assembled on a flat surface. This phenomenon is explicitly rationalized by a theoretical model that accounts fully for the molecular details (size, shape, conformation, and charge distribution) of both the NPs and the counterions.

Nanoparticle Core/Shell Architectures within MOF Crystals Synthesized by Reaction Diffusion

Metal–organic frameworks (MOFs) have attracted considerable attention on account of their applications in molecular separations, gas storage, catalysis, and chemical sensing. Recently, there has been a growing interest in using these highly ordered microporous architectures as host matrices or templates with incorporated metal or metal oxide nanoclusters or nanoparticles (NPs). Such hybrid NPMOF structures are promising materials for gas storage and catalysis. The strategies available for the incorporation of metal or metal oxide NPs into MOFs include solvent-free gasphase loading, solution impregnation, incipient wetness impregnation, solid grinding, and microwave irradiation. Most of these techniques, however, involve relatively cumbersome processes such as pretreating the MOF (e.g., by solvent exchange or MOF activation) or particle loading (by introduction of reducing agents, heating, irradiation, etc.), and none allows any spatial control over the NP loading within the MOF crystals. Here, we describe a procedure in which reaction-diffusion processes inside the MOF crystals mediate the deposition of NPs either in a uniform or in a location-specific fashion, with the latter leading to the formation of core/shell architectures (which are of interest in the context of multistep catalysis). In our method, cyclodextrin-based MOF (CD-MOF) crystals are immersed in a metal (here, Ag or Au) salt solution, and the OH counterions—which are homogenously distributed in the CD-MOF at concentrations of about 1.33m—reduce this salt to the respective metal NPs. By coupling the diffusion of salt precursors with their reduction inside the CD-MOFs it is possible to deposit the NPs only at the core of the MOF crystal and such that the thickness of the NP-free shell depends on the concentration of the HAuCl4 used. Subsequent deposition of another type of NPs gives rise to core/ shell architectures. NPs of all types can be readily liberated by dissolving the CD-MOFs in water—for the core/shell NP/CDMOFs, the release of the two different types of NPs is then sequential. Two types of millimeter-sized CD-MOFs were used. The first type was synthesized from g-cyclodextrin (g-CD) and RbOH following the reported procedures (see Experimental Section for details). As illustrated in Figure 1a–c, these Rb-CD-MOF single crystals (for powder X-ray diffraction (PXRD) spectra, see Section 1 in the Supporting Information) were rectangular prisms up to about 2 2 1 mm in size with nanosized cavities (ca. 1.7 nm across) and 1D channels connecting them (channel cross-section: ca. 8 8 ). The second type of MOFwas also made from g-CD, but CsOH was used as the alkali metal source. These Cs-CDMOF crystals also comprised cavities of approximately 1.7 nm in diameter connected by channels with cross-sections of about 8 8 2 (Figure 1a,b). Following the procedure described in the Experimental Section, Cs-CD-MOF single crystals were grown that had an overall truncated-octahedron shape and the diameters of these crystals were up to 5 mm (Figure 1d, see also the PXRD spectra in Section 2 of the Supporting Information). In the context of the present work, the key feature of the CD-MOFs is that they contain hydroxide counterions (one per metal center) which can work either alone or cooperatively with the cyclodextin units to reduce metal salt Figure 1. a) A unit cell of Rbor Cs-CD-MOF crystals synthesized from g-CD and RbOH or CsOH, respectively (red: oxygen; gray: carbon; purple: Rb or Cs). b) The 1D channels in Rbor Cs-CD-MOF crystals. c) and d) The optical images of the millimeter-sized Rb-CD-MOF and Cs-CD-MOF crystals, respectively.

Imprinting Chemical and Responsive Micropatterns into Metal–Organic Frameworks

Wet stamping allows metal–organic framework (MOF) crystals to be imprinted with micropatterns of various organic chemicals. Printing the MOFs with photochromic molecules and pH indicators generates stimuli-responsive micropatterns which change their appearance upon contact with specific chemicals (see picture), thus reporting the environmental “status” of the crystal.

Charged nanoparticles as supramolecular surfactants for controlling the growth and stability of microcrystals

Microcrystals of desired sizes are important in a range of processes and materials, including controlled drug release, production of pharmaceutics and food, bio- and photocatalysis, thin-film solar cells and antibacterial fabrics. The growth of microcrystals can be controlled by a variety of agents, such as multivalent ions, charged small molecules, mixed cationic-anionic surfactants, polyelectrolytes and other polymers, micropatterned self-assembled monolayers, proteins and also biological organisms during biomineralization. However, the chief limitation of current approaches is that the growth-modifying agents are typically specific to the crystalizing material. Here, we show that oppositely charged nanoparticles can function as universal surfactants that control the growth and stability of microcrystals of monovalent or multivalent inorganic salts, and of charged organic molecules. We also show that the solubility of the microcrystals can be further tuned by varying the thickness of the nanoparticle surfactant layers and by reinforcing these layers with dithiol crosslinks.

A metal-organic framework stabilizes an occluded photocatalyst.

Occlusion and confinement of a [Ru(bpy)3 ]Cl2 photocatalyst in the cavities of a γ-cyclodextrin (CD) metal-organic framework (MOF) does not affect the catalysts activity but prevents its photodegradation. Additionally, the OH(-) ions and/or ROH groups present inside the CD-MOF act as electron donors and complete the catalytic cycle. The occlusion approach is a technically straightforward alternative to the covalent modification of MOF scaffolds with catalytic units.

Tunneling Electrical Connection to the Interior of Metal-Organic Frameworks

Metal-organic frameworks (MOFs) are typically poor electrical conductors, which limits their uses in sensors, fuel cells, batteries, and other applications that require electrically conductive, high surface area materials. Although metal nanoclusters (NCs) are often added to MOFs, the electrical properties of these hybrid materials have not yet been explored. Here, we show that adding NCs to a MOF not only imparts moderate electrical conductivity to an otherwise insulating material but also renders it photoconductive, with conductivity increasing by up to 4 orders of magnitude upon light irradiation. Because charge transport occurs via tunneling between spatially separated NCs that occupy a small percent of the MOFs volume, the pores remain largely open and accessible. While these phenomena are more pronounced in single-MOF crystals (here, Rb-CD-MOFs), they are also observed in films of smaller MOF crystallites (MIL-53). Additionally, we show that in the photoconductive MOFs, the effective diffusion coefficients of electrons can match the typical values of small molecules diffusing through MOFs; this property can open new vistas for the development of MOF electrodes and, in a wider context, of electroactive and light-harvesting MOFs.

Enhanced photocatalytic activity of hybrid Fe2O3–Pd nanoparticulate catalysts

“Hybrid” nanoparticles (NPs) comprising physically fused Fe2O3 and Pd domains can act as efficient photocatalysts, driving photoreduction of metal salts to metal nanoparticles. The overall photocatalytic redox cycle of the composite entails reduction on the Pd domain and oxidation on the Fe2O3 part. The photocatalytic activity of Fe2O3–Pd hybrids is three times higher than that of statistical mixtures of Fe2O3 NPs and Pd NPs.

Transport into Metal-Organic Frameworks from Solution Is Not Purely Diffusive**

Chemistry in motion: a combination of confocal microscopy and reaction-diffusion modeling provided a powerful toolkit with which solution transport into metal-organic framework crystals was studied. Commonly used pure diffusion models are insufficient to describe this process and, instead, it is necessary to account for the interactions of the guest molecules and the MOF scaffold.