(384f) Small Molecule Diffusion Studies in High Aspect Ratio, Thin Film, Single Crystalline Metal Organic Frameworks

Metal-organic frameworks (MOFs) are a promising class of materials, composed of metal nodes and organic linkers coordinated to form a cage-like, porous crystalline structure. MOFs have gained a lot of recent attention due to their high internal surface area ( > 1000 m²/g), tunable porosity (2 – 40 Ã…), and variety of chemical functionalities, and can be utilized in numerous applications (i.e. separations, storage, sensing). While many studies have analyzed diffusion of gases through MOF pores, few have effectively studied the solution based diffusion within these materials. Most MOFs are created as low aspect ratio crystals, and it is impossible to visualize the position and rate of the diffusion front through the MOF. Further, in a MOF powder, grain boundaries and other defects have an exaggerated effect on diffusion coefficients.

We have developed a micro- and nano-fluidic device that confines the crystallization of MOFs into channels to synthesize thin film, high aspect ratio MOF crystals. For this work, we synthesized HKUST-1, a prototypical MOF frequently utilized in the field. By changing the channel dimensions, we show a variety of MOF morphologies, sizes, and aspect ratios. Furthermore, we performed loading studies with fluorescent small molecules (i.e. methylene blue, anthracene, riboflavin, rhodamine B, acridine orange) until a thermodynamic equilibrium was reached. We demonstrate size-based inclusion/exclusion within the crystalline framework. Molecules larger than the characteristic pore size for HKUST-1 did not enter the HKUST domains, while molecules smaller than the characteristic pore size did, as measured using confocal fluorescent microscopy (CFM). Further, we utilized image analysis methods to quantify the fluorescent intensity for specific guest molecules within the MOF as it relates to the bulk solution.

Finally, we designed studies to probe the timescale for diffusion of fluorescent molecules in HKUST-1. The first utilized top-down diffusion of methylene blue in HKUST-1, and we observed the increase in fluorescent intensity using CFM as a function of time. Knowing the depth of the channel, the diffusion coefficient was calculated to be 1.1x10^-16 m²/s, comparable to previous studies for solution based diffusion in MOFs. Next, we utilized flow through control of fluid transport in MOFs. With fluorescent microscopy, we are able to visualize the lateral diffusion front through a HKUST-1 single crystal and develop a diffusion coefficient for this MOF/guest molecule combination.