Research Highlights


Enclosed Pillar Arrays Integrated on a Fluidic Platform for On-Chip Separations and Analysis
Nickolay V. Lavrik1, Lisa C. Taylor2 and Michael J. Sepaniak2

1 Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, TN 37830, 2 Department of Chemistry, University of Tennessee, Knoxville, TN 37996

Achievement

A new approach to robust sealed networks of highly ordered nanofluidic channels (Fig. 1) has been developed and used to demonstrate liquid phase separation approaching the theoretical limit imposed by diffusion. By contrast to synthesis of conventional nanoporous materials characterized by a high degree of disorder and heterogeneity, our approach is based on deterministic patterning and involves a sequence of photolithography, anisotropic reactive ion etching (RIE) of silicon, and plasma enhanced chemical vapor deposition of (PECVD) of silicon oxides. In our experiments we studied dispersion of picoliter sample plugs injected into the system (Fig. 2) and found that plate heights as low as 0.76 ┬Ám for a 10 mm long model bed can be achieved. Our theoretical calculations confirm that ordered pillar arrays with submicron pore sizes combine high permeability with extremely small theoretical plate heights.

Significance

As the sizes of the separation media shrink, the random nature and polydispersity of porosity becomes a significant factor that contributes to dispersion of chromatographic zones. An important fundamental question arises whether perfectly ordered networks of nanoscale pores may offer substantial advantages over traditional disordered porous media used in packed and monolithic chromatographic columns. By refining previously established fabrication processes and implementing novel technological sequences, we demonstrated a model system based on highly ordered Si pillar arrays that are integrated into a system of on-chip fluidic channels scalable well into the submicron range. Our experiments showed that the implemented structures satisfy fundamental criteria of high performance separation while also providing high fabrication throughput. We anticipate that the demonstrated system will facilitate further studies of ordered nanoporous structures as a platform for emerging separation techniques.

Publication

This work is published in Lab-on-a-Chip, 2010, DOI: 10.1039/B920275G

Acknowledgements

This research at Oak Ridge National Laboratory's Center for Nanophase Materials Sciences was sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy. LT and MS also acknowledge support by the US Environmental Protection Agency STAR Program under grant EPA-83274001 with the University of Tennessee.

Fig. 1. Top: Cross-sectional SEM images illustrating different stages of capping of high aspect ratio Si pillars with PECVD silicon oxide: (a) before capping, (b) partially capped pillars and (c) completely sealed pillar array. Bottom: fragments of the chip viewed in the SEM at a 30 degrees tilt: (d) before and (e) after capping with PECVD silicon oxide.
 
Fig. 2 Intensity profiles and corresponding fluorescence images of a dye plug (10-4 M Rhodamine B solution in MeOH) migrating down the model pillar bed at a velocity of 1.7 mm s-1. The experimental intensity profile (open circles in the plot) extracted from the image of the plug centered at x=5.7 mm is shown together with the Gaussian fit (solid lines) of intensity profiles for each image.