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

ABSTRACT

We present the ability to create unique morphologies of a prototypical metal organic framework (MOF), HKUST-1, by carrying out its crystallization within a set of nano-confined fluidic channels. These channels are fabricated on cyclic olefin copolymer by the high-fidelity hot embossing imprinting method. The picoliter volume synthesis in the nanochannels is hypothesized to bias the balance between nucleation and growth rates to obtain high aspect ratio large-crystalline domains of HKUST-1, which are grown in defined morphologies due to the patterned nanochannels. Confined crystal growth is achieved in nanofluidic channels as shallow as 50 nm. HKUST-1 crystalline domains with aspect ratios greater than 2500, and lengths up to 144 µm are obtained using the nanochannels, exceeding values obtained using chemical modulation and other confinement methods. 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 through 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. Nanoconfined crystallization of MOFs can thus yield control over crystalline morphology to create ideal MOF crystals for enabling selective molecular enrinchment and sensing.