As covalent organic frameworks (COFs) are coming of age, the lack of effective approaches to achieve crystalline and centimeter-scale-homogeneous COF films remains a significant bottleneck toward advancing the application of COFs in optoelectronic devices. Here, we present the synthesis of colloidal COF nanoplates, with lateral sizes of ∼200 nm and average heights of 35 nm, and their utilization as photocathodes for solar hydrogen evolution. The resulting COF nanoplate colloid exhibits a unimodal particle-size distribution and an exceptional colloidal stability without showing agglomeration after storage for 10 months and enables smooth, homogeneous, and thickness-tunable COF nanofilms via spin coating. Photoelectrodes comprising COF nanofilms were fabricated for photoelectrochemical (PEC) solar-to-hydrogen conversion. By rationally designing multicomponent photoelectrode architectures including a polymer donor/COF heterojunction and a hole-transport layer, charge recombination in COFs is mitigated, resulting in a significantly increased photocurrent density and an extremely positive onset potential for PEC hydrogen evolution (over +1 V against the reversible hydrogen electrode), among the best of classical semiconductor-based photocathodes. This work thus paves the way toward fabricating solution-processed large-scale COF nanofilms and heterojunction architectures and their use in solar-energy-conversion devices.
While micromachines with tailored functionalities enable therapeutic applications in biological environments, their controlled motion and targeted drug delivery in biological media require sophisticated designs for practical applications. Covalent organic frameworks (COFs), a new generation of crystalline and nanoporous polymers, offer new perspectives for light-driven microswimmers in heterogeneous biological environments including intraocular fluids, thus setting the stage for biomedical applications such as retinal drug delivery. Two different types of COFs, uniformly spherical TABP-PDA-COF sub-micrometer particles and texturally nanoporous, micrometer-sized TpAzo-COF particles are described and compared as lightdriven microrobots. They can be used as highly efficient visible-light-driven drug carriers in aqueous ionic and cellular media. Their absorption ranging down to red light enables phototaxis even in deeper and viscous biological media, while the organic nature of COFs ensures their biocompatibility. Their inherently porous structures with ≈2.6 and ≈3.4 nm pores, and large surface areas allow for targeted and efficient drug loading even for insoluble drugs, which can be released on demand. Additionally, indocyanine green (ICG) dye loading in the pores enables photoacoustic imaging, optical coherence tomography, and hyperthermia in operando conditions. This real-time visualization of the drug-loaded COF microswimmers enables unique insights into the action of photoactive porous drug carriers for therapeutic applications.
Mesoporous catalyst supports that mimic the spatially confined active sites of enzymes can aid in the development of highly selective molecular heterogeneous catalysts. Nontemplated mesoporous SiO 2 (NT-mSiO 2 ) materials with open porosity, tunable pore sizes, and high diffusivity are promising candidates in this regard. However, the operationalization of such materials strongly depends on the controlled passivation of their external pore surfaces. This enables catalyst molecules to be selectively immobilized on the internal pore surface where the desired spatial confinement effects can be observed. In this work, confocal laser scanning microscopy (CLSM) is presented as a viable analytical tool to visualize the passivation efficiency and permeability of NT-mSiO 2 platelets consisting of interconnected mesopores (d pore = 9.4 nm) with positive pore wall curvatures. CLSM investigations with representative fluorescent probe molecules show that after pore-filling with Pluronic P123, the passivating film is constrained to the external platelet surface. The permeability of different passivating films based on mono and bifunctional silanes is compared. A pyrene-based organosilane is used as a tracer molecule to determine the covalent functionalization susceptibility of passivated NT-mSiO 2 platelets. Additionally, SiO 2 nanospheres with modular particle sizes are synthesized using an L-lysine-mediated sol−gel process and assembled into NT-mSiO 2 with tunable pore sizes. Hexamethyldisilazane-passivated NT-mSiO 2 (d pore = 4.3 nm) is used as a catalyst support for the immobilization of cationic molybdenum imido alkylidene N-heterocyclic carbene complexes to study the effect of confinement on monomacrocyclization selectivity in ring-closing olefin metathesis reactions. A 31% enhancement in monomacrocyclization selectivity is observed when compared to the homogeneous catalyst.
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