Anhydride Post-Synthetic Modification in a Hierarchical Metal-Organic Framework.

Metal-organic frameworks (MOFs) are important porous materials. Post-synthetic modification (PSM) of MOFs via the pendant groups or secondary functional groups of organic linkers has been widely used to introduce new or enhance existing properties of MOFs for various practical applications. In this work, we have constructed, for the first time, a novel platform for PSM of MOFs by introducing an anhydride functional group into a hierarchically porous MOF (MIL-121) as an effective anchor. We have demonstrated that the combination of the high reactivity of anhydride and hierarchical porosity makes this protocol particularly novel and important, as it led to excellent opportunities of incorporating not only a wide variety of organic molecules with different sizes and chemical nature but also the noble metal complexes in MOFs. Specifically, we show that the anhydride group decorated in the MOF exhibits a high reactivity toward covalently binding 10 different guest molecules including alcohols, amines, thiols, and noble metal (Pt(II)/Pt(IV)) complexes, whereas the hierarchical pores created in the MOF allow the incorporation of guest species varying in size from methanol to larger molecules such as polyaromatic amines. This novel approach provides the community with a new avenue to prepare MOF-based materials for targeted applications. To illustrate this point, we furnish an example of using this new platform to prepare a Pt-based electrocatalyst which shows excellent catalytic activity toward the oxygen reduction reaction (ORR), a pivotal half-reaction in hydrogen-oxygen fuel cells and other energy storage and conversion devices.

Investigation of Au SAMs Photoclick Derivatization by PM-IRRAS.

In this work, we present a clean one-step process for modifying headgroups of self-assembled monolayers (SAMs) on gold using photo-enabled click chemistry. A thiolated, cyclopropenone-caged strained alkyne precursor was first functionalized onto a flat gold substrate through self-assembly. Exposure of the cyclopropenone SAM to UVA light initiated the efficient photochemical decarbonylation of the cyclopropenone moiety, revealing the strained alkyne capable of undergoing the interfacial strain-promoted alkyne-azide cycloaddition (SPAAC). Irradiated SAMs were derivatized with a series of model azides with varied hydrophobicity to demonstrate the generality of this chemical system for the modification and fine-tuning of the surface chemistry on gold substrates. SAMs were characterized at each step with polarization-modulation infrared reflection-absorption spectroscopy (PM-IRRAS) to confirm the successful functionalization and reactivity. Furthermore, to showcase the compatibility of this approach with biochemical applications, cyclopropenone SAMs were irradiated and modified with azide-bearing cell adhesion peptides to promote human fibroblast cell adhesion, and then imaged by live-cell fluorescence microscopy. Thus, the "photoclick" methodology reported here represents an improved, versatile, catalyst-free protocol that allows for a high degree of control over the modification of material surfaces, with applicability in materials science as well as biochemistry.

Nitrone-Modified Gold Nanoparticles: Synthesis, Characterization, and Their Potential as 18F-Labeled Positron Emission Tomography Probes via I-SPANC.

A novel bioorthogonal gold nanoparticle (AuNP) template displaying interfacial nitrone functional groups for bioorthogonal interfacial strain-promoted alkyne-nitrone cycloaddition reactions has been synthesized. These nitrone-AuNPs were characterized in detail using 1H nuclear magnetic resonance spectroscopy, transmission electron microscopy, thermogravimetric analysis, and X-ray photoelectron spectroscopy, and a nanoparticle raw formula was calculated. The ability to control the conjugation of molecules of interest at the molecular level onto the nitrone-AuNP template allowed us to create a novel methodology for the synthesis of AuNP-based radiolabeled probes.

Highly Electron-Deficient Pyridinium-Nitrones for Rapid and Tunable Inverse-Electron-Demand Strain-Promoted Alkyne-Nitrone Cycloaddition.

Highly accelerated inverse-electron-demand strain-promoted alkyne-nitrone cycloaddition (IED SPANC) between a stable cyclooctyne (bicyclo[6.1.0]nonyne (BCN)) and nitrones delocalized into a Cα-pyridinium functionality is reported, with the most electron-deficient "pyridinium-nitrone" displaying among the most rapid cycloadditions to BCN that is currently reported. Density functional theory (DFT) and X-ray crystallography are explored to rationalize the effects of N- and Cα-substituent modifications at the nitrone on IED SPANC reaction kinetics and the overall rapid reactivity of pyridinium-delocalized nitrones.

Dual-Bioorthogonal Molecular Tool: "Click-to-Release" and "Double-Click" Reactivity on Small Molecules and Material Surfaces.

The development of reactive moieties that enable molecular control of bond-forming and bond-breaking reactions within complex media is highly important in materials and biomaterials research as it provides opportunities to carefully manipulate small molecules and material surfaces in a reliable manner. Despite recent advances in the realization of new ligation strategies and "click-and-release" systems, there has been little development of multifunctional moieties that feature a broad range of chemical capabilities. To address this challenge, we designed a molecular tool that can utilize four well-defined bioorthogonal chemistries interchangeably for the attachment, replacement, and release of molecules within a system: the Staudinger-Bertozzi ligation (SBL), perfluoroaryl azide Staudinger reaction (PFAA-SR), strain-promoted alkyne-azide cycloaddition (SPAAC), and strain-promoted alkyne-nitrone cycloaddition (SPANC). We demonstrate "click-to-release" and "double-click" reactivity on small molecules and gold nanoparticles (AuNPs) as a model material substrate. As a proof of concept for material derivatization, we employed 5 nm AuNPs-functionalized with a Rhodamine B derivative and biotin through the double-click strategy-and showed their potential as a pretargeted delivery nanocarrier. This multifunctional molecular tool enables the design and production of molecular and material systems with unique, modular, and tunable dynamic properties that can be altered under mild and bioorthogonal conditions.

Harnessing complex fluid interfaces to control colloidal assembly and deposition.

HYPOTHESIS: Capillary interactions play an important role in directing colloidal assembly on fluid interfaces. Interface curvature is expected to influence not only individual particle migration on interfaces but also capillary forces between nearby particles. In drying droplets, we hypothesize that the assembly and deposition of particles bound to droplet surface are controlled by the interplay between capillary effects and evaporation-driven flow. EXPERIMENTS: Using lattice Boltzmann-Brownian dynamics (LB-BD) simulations, we modeled large-scale assembly of nanoparticles on fluid interfaces that have complex geometries and investigate the subsequent deposition upon complete evaporation. A systematic study was performed for geometrically-controlled sessile droplets whose surfaces exhibit varying curvature fields. FINDINGS: The simulations show that the particle dynamics on nonuniformly curved interfaces are anisotropic and governed by particle-pair capillary interactions and curvature-induced capillary migration. A theoretical model was developed to predict the capillarity-induced assembly. Using the curved surface as a template, drying droplets with surface-bound particles deposit distinct patterns as a result of the competition between the capillary effects and evaporation-induced convection. These findings could provide new opportunities in the directed assembly and deposition of colloidal particles with potential applications in fabricating functional materials from nanoscale building blocks.

Loading across the Periodic Table: Introducing 14 Different Metal Ions To Enhance Metal-Organic Framework Performance.

Loading metal guests within metal-organic frameworks (MOFs) via secondary functional groups is a promising route for introducing or enhancing MOF performance in various applications. In this work, 14 metal ions (Li+, Na+, K+, Mg2+, Ca2+, Ba2+, Zn2+, Co2+, Mn2+, Ag+, Cd2+, La3+, In3+, and Pb2+) have been successfully introduced within the MIL-121 MOF using a cost-efficient route involving free carboxylic groups on the linker. The local and long-range structure of the metal-loaded MOFs is characterized using multinuclear solid-state NMR and X-ray diffraction methods. Li/Mg/Ca-loaded MIL-121 and Ag nanoparticle-loaded MIL-121 exhibit enhanced H2 and CO2 adsorption; Ag nanoparticle-loaded MIL-121 also demonstrates remarkable catalytic activity in the reduction of 4-nitrophenol.

Fluorogenic Gold Nanoparticle (AuNP) Substrate: A Model for the Controlled Release of Molecules from AuNP Nanocarriers via Interfacial Staudinger-Bertozzi Ligation.

The ability to regulate small-molecule release from metallic nanoparticle substrates offers unprecedented opportunities for nanocarrier-based imaging, sensing, and drug-delivery applications. Herein we report a novel and highly specific release methodology off gold nanoparticle (AuNP) surfaces based on the bioorthogonal Staudinger-Bertozzi ligation. A thiol ligand bearing the molecular cargo, a Rhodamine B dye derivative, was synthesized and used to modify small water-soluble 5 nm AuNPs. Upon incorporation into the AuNP monolayer, we observed efficient quenching of the dye emission, resulting in a very low level of fluorescence emission that provided the baseline from which cargo release was monitored. We examined the ability of these AuNPs to react with azide molecules via Staudinger-Bertozzi ligation on the nanoparticle surface by monitoring the fluorescence emission after the introduction of an organic azide. We observed an immediate increase in emission intensity upon azide addition, which corresponded to the release of the dye into the bulk solution. The 31P NMR spectrum of the AuNP product also agrees with the formation of the ligation product. Thus this system represents a novel and highly specific release methodology off AuNP surfaces that can have potential applications in drug delivery, sensing, and materials science.

"Shine & Click" Photo-Induced Interfacial Unmasking of Strained Alkynes on Small Water-Soluble Gold Nanoparticles.

In this study, we report the design, synthesis, and characterization of small 3 nm water soluble gold nanoparticles (AuNPs) that feature cyclopropenone-masked strained alkyne moieties capable of undergoing interfacial strain-promoted cycloaddition (i-SPAAC) with azides after exposure to UV-A light. A strained alkyne precursor was incorporated onto AuNPs by direct ligand exchange of a thiol-modified cyclopropenone-masked dibenzocyclooctyne (photoDIBO) ligand. These photoDIBO-AuNPs were characterized by 1 H NMR, IR, and UV/Vis spectroscopy, as well as transmission electron microscopy (TEM) and thermogravimetric analysis (TGA), and the extent of modification was quantified. Upon irradiation with UV-A light, photoDIBO-AuNPs underwent efficient and quantitative regeneration of the parent strained alkyne by photochemical decarbonylation to afford DIBO-derivatized AuNPs. DIBO-AuNPs were found to react cleanly and rapidly (k=5.3×10-2 m-1 s-1 ) by an interfacial strain-promoted alkyne-azide cycloadditon (i-SPAAC) with benzyl azide, which served as a simple model system. Furthermore, DIBO-AuNPs were reacted with various azides and a nitrone (interfacial strain-promoted alkyne-nitrone cycloaddition, i-SPANC) to showcase the generality of this approach for the facile modification of AuNP surfaces and their properties. The cyclopropenone-based photo-triggered click chemistry at the interface of water-soluble AuNPs offers exciting opportunities for the atom-by-atom control and assembly of functional materials for applications in materials and biomaterials science as well as in chemical biology.

Small gold nanoparticles for interfacial Staudinger-Bertozzi ligation.

Small gold nanoparticles (AuNPs) that possess interfacial methyl-2-(diphenylphosphino)benzoate moieties have been successfully synthesized (Staudinger-AuNPs) and characterized by multi-nuclear MR spectroscopy, transmission electron microscopy (TEM), UV-Vis spectroscopy, thermogravimetric analysis, and X-ray photoelectron spectroscopy (XPS). In particular, XPS was remarkably sensitive for characterization of the novel nanomaterial, and in furnishing proof of its interfacial reactivity. These Staudinger-AuNPs were found to be stable to the oxidation of the phosphine center. The reaction with benzyl azide in a Staudinger-Bertozzi ligation, as a model system, was investigated using (31)P NMR spectroscopy. This demonstrated that the interfacial reaction was clean and quantitative. To showcase the potential utility of these Staudinger-AuNPs in bioorganic chemistry, a AuNP bioconjugate was prepared by reacting the Staudinger-AuNPs with a novel azide-labeled CRGDK peptide. The CRGDK peptide could be covalently attached to the AuNP efficiently, chemoselectively, and with a high loading.