Microchip-Based Capillary Electrochromatography of Similar Molecules: Molecular Imprint Polymers as Stationary Phases for Lab-on-a-Chip Devices

Alyssa C. Henry

National Institute of Standards and Technology, Analytical Chemistry Division, 100 Bureau Drive, Stop 8394, Gaithersburg, MD USA

The miniaturization of several workhorses of analytical chemistry has been the primary focus of a plethora of research groups in the chemical, physical, and engineering communities. Microanalytical separation devices, microarrays, and microreaction platforms are some of the projects of researchers interested in decreasing the size of many analytical tools. Microdevices fabricated from glass or other silicon-based materials have, thus far, been the norm due to their ease of fabrication as well as the well-documented chemical surface modification chemistries of these materials. However, the diversity of microsystem manufacturing protocols employing plastic substrates, the inexpensive nature of plastics, and the documentation of plastic surface chemical modification protocols have allowed for the entrance of polymeric materials into the microdevice fabrication arena.

Microanalytical separation devices, or "lab-on-a-chip" devices, while still in their infancy, have been used for various separations. These separations, performed on both glass and plastic microchips, have included proteins, DNA, and other molecules of biological interest. As the microanalytical separation device technology grows, it will be necessary to separate chiral molecules as well as other molecules whose retention times are similar or identical. In order to afford such a separation, molecularly imprinted polymers (MIPs) can be used as stationary phases. In this technology, functional groups on a template molecule are allowed to interact with functional groups on one or more monomers. The monomers are then polymerized in the presence of a crosslinker, thus solidifying the interactions between the functional groups on the template molecule and monomers and producing a highly rigid polymer. The template molecule is then extracted from the polymer matrix, yielding a polymer with "imprints" specific to the template molecule.

Research in our laboratory has focused on the covalent tethering of MIPs to plastic surfaces and microchannels for open-tubular capillary electrochromatography devices. This covalent tethering is possible due to the chemical modification of the plastic surface such that a reactive functional group, such as a primary amine, exists on the surface of the plastic. The amine can be coupled to the carboxylic acid of methacrylic acid and thus terminate the surface of the plastic in alkene groups. The alkenes then participate in the polymerization of the MIP. Surface analytical techniques such as reflection-absorption infrared spectroscopy, water contact angle studies, and fluorescence imaging techniques have been used as characterization tools in this research.

Selectivity studies of the MIPs were performed by imprinting a fluorescent dye, extracting the dye from the polymer matrix, and assessing the fluorescence of the extract solutions. In addition, electrophoretic mobility studies were performed on dyes that were exposed to microchannels employing an MIP stationary phase. The results from these studies will be presented.