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Microfluidics Research 

Two-dimensional stilbene MOF
Two-dimensional stilbene MOF

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Research in Microfluidic Transport at Sandia National Laboratories

Nanoporous coordination polymers

Extended solids comprised of inorganic and organic components are attracting considerable attention because of their exceptional properties, chemical tailorability, and broad range of potential applications. Recently, a new class of nanoporous coordination polymers known as metal organic frameworks (MOFs) was created that have immense potential for understanding and exploiting molecular interactions in pores. MOFs are crystalline materials with tunable, monolithic pore sizes and cavity properties. Their properties exceed those of virtually all other porous materials, including the lowest density and highest surface area for a crystalline material, tunable photoluminescence, and high capacity for molecular adsorption. These exciting properties are achieved by coupling inorganic clusters with tunable organic ligands that serve as struts, allowing facile manipulation of pore size and surface area through ligand selection. The so-called isoreticular MOFs (for example, see IRMOF-1, Figs. 1, 2) are particularly attractive because of their cubic structure and interchangeable organic linkers.

IRMOF-1 figure 1
Fig. 1. The prototypical MOF structure (IRMOF-1). ZnO4 tetrahedra (blue) are joined by organic linkers (O, red, C, black), giving an extended 3D cubic framework with inter-connected pores of 11.2 Â aperture width and 18.5Â pore (yellow sphere) diameter [1].
Figure 2, IRMOF-1
Fig. 2 Space-filling model showing the cubic nature of IRMOF pores [2].

The development of MOFs for real-world applications is a nascent, but growing area. We are currently working in several areas to realize the exciting potential of these materials:

Fig. 3. Microcantilever design with built-in piezoresistive stress sensor
Fig. 3. Microcantilever design with built-in piezoresistive stress sensor designed and fabricated at Georgia Tech by the research group of Prof. Peter Hesketh.
Integration with MEMS devices

The growth of MOF layers on substrates is essential to gaining full use of their properties. At present we are growing MOFs on self assembled monolayers (SAMs) and on oxide materials deposited by atomic layer deposition. We are now able to grow the zinc-carboxylate MOF IRMOF-1 on microcantilever devices (Fig. 3), as seen in Fig. 4, and are testing these for use as chemical sensors.

Fluorescent MOFs
We are exploring the potential for MOFs for use as chemical sensors. The open pores and ability to tailor the chemical nature of the pore suggests molecular recognition possibilities. One area in which we have made considerable progress is the development of fluorescent MOFs. Our concept is to design materials with organic linkers such as stilbene that display significant solvochromatic shifts so that adsorption of guest molecules in the pore will lead to observable changes in the optical emission. Stilbene MOFs we created exhibit reversible solvent-dependent fluorescence spectra. An example is the 3-D MOF shown in Fig. 5, which exhibits reversible solvochromatic shifts in its fluorescence when guest molecules are exchanged in the pores.

Fig 4
Figure 4. Microcantilever coated by Sandia with crystals of IRMOF-1, using a self assembled monolayer to attach crystals to the surface. Note region of device in upper left hand corner where no MOF deposited , demonstrating ability to pattern the material.
IRMOF- in the presence of water
Fig. 5. Three-dimensional nanoporous strucure of a new MOF composed of Zn4O clusters linked by stilbene dicarboxylic acid linkers. Inset shows single-crystal fluorescence generated by 325-nm excitation.

MOF-adsorbate interactions
Understanding the interactions of solvent molecules with the MOF framework is essential to developing a rational design approach. We are using molecular dynamics to simulate these interactions, employing a non-bonded forcefield we developed for IRMOFs. These calculations predict the limits of stability of IRMOF-1 in the presence of water (Fig. 6), which disrupts the tetrahedral coordination of the Zn(II) ions (Fig. 7).


Fig 5
Figure 6. Simulated lattice parameter as a function of water content. The dashed line indicates the trend toward a much smaller lattice parameter (≈20 Â) at higher water
fig 6
Fig. 7. Molecular dynamics prediction of the disruption of the IRMOF-1 structure at 2.3 % water. The color scheme is Zn (purple), O (red), C (gray), and H (white), with ZnO4 tetrahedra represented as polygons. The picture shows the formation of hydrogen-bonded chains of water molecules, which disrupt the normal tetrahedral coordination around the Zn(II) ions in the framework.

Publications

References

  1. Eddaoudi, M.; Kim, J.; Rosi, N.; Vodak, D.; Wachter, J.; O'Keefe, M.; Yaghi, O. M."Systematic design of pore size and functionality in isoreticular MOFs and their application in methane storage," Science 2002, 295, 469.
  2. Eddaoudi, M.; Moler, D. B.; Li, H.; Chen, B.; Reineke, T. M.; O'Keefe, M.; Yaghi, O. M."Modular chemistry: secondary building units as a basis for the design of highly porous and robust metal-organic carboxylate frameworks," Acc. Chem. Res. 2001, 34, 319.
  3. Greathouse, J. A.; Allendorf, M. D."The interaction of water with MOF-5 simulated by molecular dynamics," J. Am. Chem. Soc. 2006, 106, 10678.