Microporous thin films and ordered nanoparticle assemblies and coatings compose a rich subset of current nanotechnology initiatives with exciting applications spanning high-resolution molecular separations and chemical sensing, fuel cell membranes, nanoscale materials templating, colloidal lithography, protective coatings, and even biological applications. Realization of these materials demands rational, hierarchical device design in which structure-properties relations are developed to bridge molecular details with those at the nanoscopic, microscopic, and device levels. As such, recent nanotechnology roadmaps have espoused a synergistic theme, recognizing the need for intimate combination of theory, modeling, synthesis, and characterization for developing structure-properties relations directed at device design and structure and defect control.

Through my doctoral (Dept. of Chemical Engineering, University of Delaware with Professor Dion Vlachos) and postdoctoral (Dept. of Chemical Engineering and Materials Science, University of Minnesota with Professor Michael Tsapatsis) work, we have focused on several key nanomaterials issues including: 1) benign, facile synthesis of oxide nanoparticles as building units for nanomaterial devices, 2) fabrication of mono- and multilayer ordered nanoparticle coatings, 3) assembly of ordered nanoparticle-crystals, 4) characterization of microporous thin film polycrystallinity and defects, and 5) development of multscale computational frameworks to derive structure-properties relations for predicting device performance.

I. Multiscale approaches for elucidating structure-properties relations of diffusion through polycrystalline, microporous thin films (Ph.D. work with Prof. Dion Vlachos)

The ordered crystalline microstructure, nanometer-sized pores, cation tunability, and established techniques for thin film fabrication make microporous materials (e.g., zeolites) attractive for applications in high-resolution separations, membrane reactors, and chemical sensors.¹ The strongly coupled scales associated with molecular transport through polycrystalline membranes and lack of comprehensive structure-properties relations challenge the current design paradigm. To this end, we have developed multiscale modeling approaches (i.e., both continuum² and stochastic mesoscopic theories^3,4) for bridging disparate scales associated with molecular diffusion through zeolite membranes. Models of permeation through both prototype and realistic membranes have provided insight into the role of crystal terminations, strong adsorbate-adsorbate interactions, and underlying diffusion mechanisms in permeation.^5,6

Motivated by device sensitivity to crystal terminations, we have developed new protocols for advancing laser scanning confocal microscopy as a quantitative, non-destructive technique for imaging polycrystalline features of zeolite membranes, with interesting implications for even more general materials characterization. Efforts to push the quantitative limits of this technique have included correlative studies,⁷ comprehensive statistical image analysis,⁸ spatially-resolved pseudo-porosimetry studies using different sized dye molecules, and fabrication and application of calibration standards with nanometer scale features.⁹ This combination of modeling and non-destructive characterization has enabled development of structure properties relations linking polycrystallinity with membrane performance specifically for NaX zeolite films.⁸

II. Benign nanoparticle synthesis: Tuning particle properties, controlling ordered assembly, realizing multi- and monolayer nanoparticle films, and encapsulating living cells (Postdoctoral work with Prof. Michael Tsapatsis)

While my doctoral work revolved around films consisting of intergrown single crystals of a microporous material (zeolite), my postdoctoral work has explored the synthesis, characterization, and assembly of monodisperse oxide (i.e., silica) nanoparticles into porous continuous films. Realization of nanoparticle-crystals and controlled assembly of large-area nanoparticle films has implications spanning technologies of coatings and colloidal lithography to chemical sensing and biology. While silica nanoparticles are attractive for such applications, challenges to device fabrication in each of these areas derive, at least in part, from the lack until recently of a simple means for synthesizing stable and monodisperse nanoparticles, and the limited capabilities for rationally tuning particle size, porosity, order, and monolayer continuity. Complicating efforts especially in the biological arena are the harsh conditions (e.g., high pH) associated with the current paradigm for silica nanoparticle formation (i.e., hydrolysis of tetraethylorthosilicate in highly basic solutions in the presence of tetrapropylammonium cations). We have addressed a number of these key issues involving benign synthesis of oxide nanoparticles (i.e., silica¹⁰) and the ability to tailor their size and ordered assembly.¹¹ In particular, we have explored biomimetic approaches as a facile route for synthesizing stable silica nanoparticles,¹⁰ and have identified numerous handles (e.g., pH, silica content, hydrolysis and hydrothermal ageing temperature) for tuning particle diameter from less than 5 nm to more than 20 nm. In addition to particle size, we have identified conditions for controlling long-range (10 µm) ordered assembly of these nanoparticles, and for realizing continuous mono- and multilayer coatings over square centimeter areas.¹¹

Finally, we have harnessed the benign nature of this novel silica nanoparticle synthesis for applications in the biological arena. Through further neutralization of the lysine-silica sols by addition of compatible peptide oligomers (e.g., di-lysine), we are able to synthesize clear silica gels with tunable porosity and morphology, and have employed them for facile living cell encapsulation. The sustained viability of the encapsulated cells serves as proof-of-concept for their use as implantable “cellular reactors” capable of producing a metabolite of interest while being protected from immunological attack.

References

1. Snyder, M. A. ; Tsapatsis, M., Angewandte Chemie 2007, In press.

2. Snyder, M. A. ; Vlachos, D. G.; Katsoulakis, M. A., Chemical Engineering Science 2003, 58, 895.

3. Snyder, M. A. ; Vlachos, D. G., Molecular Simulation 2004, 30, 561.

4. Snyder, M. A. ; Chatterjee, A.; Vlachos, D. G., Computers & Chemical Engineering 2005, 29, 701.

5. Snyder, M. A. ; Vlachos, D. G., Journal of Chemical Physics 2005, 123.

6. Snyder, M. A. ; Vlachos, D. G., Journal of Chemical Physics 2005, 123.

7. Snyder, M. A. ; Lai, Z.; Tsapatsis, M.; Vlachos, D. G., Microporous and Mesoporous Materials 2004, 76, 29.

8. Snyder, M. A. ; Vlachos, D. G.; Nikolakis, V., Journal of Membrane Science 2007, 290, 1.

9. Snyder, M. A. ; Vlachos, D. G., Microporous and Mesoporous Materials 2007, 102, 101.

10. Davis, T. M.; Snyder, M. A. ; Krohn, J. E.; Tsapatsis, M., Chemistry of Materials 2006, 18, 5814.

11. Snyder, M. A.; Lee, J. A.; Davis, T. M.; Scriven, L. E.; Tsapatsis, M., 2007, Submitted.

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