Crystallization of high aspect ratio HKUST-1 thin films in nanoconfined channels for selective small molecule uptake

Metal organic frameworks (MOFs) are a leading material researched for use in multiple applications, including separations, storage, drug delivery, and catalysis. As the library of pore architectures and synthesis techniques expands, it becomes important to achieve application specific morphologies (e.g. high aspect ratio rods, plates and nanosheets). In this work, we present unique morphologies of a prototypical MOF, HKUST-1, achieved using crystallization within a set of nano-confined fluidic channels fabricated on cyclic olefin copolymer by the high-fidelity hot embossing imprinting method. The picoliter volume synthesis environment is hypothesized to bias the balance between nucleation and growth to obtain high aspect ratio large-crystalline domains, which are grown in defined morphologies due to the patterned nanochannel shapes. This work achieves HKUST-1 crystalline domains with aspect ratios greater than 2500, an improvement upon results obtained using chemical modulation and other confinement methods. High aspect ratio crystals form as a direct result of the degree of confinement present in the nanochannels. HKUST-1 crystals are characterized using optical microscopy and scanning electron microscopy with energy dispersive spectroscopy. Porosity of the MOF and selective molecular uptake is demonstrated with the inclusion of anthracene and methylene blue within the HKUST-1 framework, and with exclusion of rhodamine B and riboflavin, characterized using a confocal fluorescence microscope. We attribute this selectivity to the analyte size and electrostatic characteristics.