Our work focuses on the use of porous photopolymerized sol-gel (PSG) materials to create chromatographic media for separations as well as on-line chemical reactors. We developed a synthetic method based on sol-gel chemistry (hydrolysis and condensation reactions) and photochemistry for the preparation of a photopolymerized sol-gel (PSG) monolith. The methacrylate group of a trialkoxysilane reagent is photoactivated to produce PSG in a 5-min reaction. The light source can be UV or visible. Our sol-gel technique includes template-based processing where the silicate matrix is assembled around a suitable template or porogen to form cavities of a specific size and shape within the cross-linked host. With the presence of free silanol groups on the PSG monolith a variety of different functional groups can be covalently grafted to the monolith surface, allowing us to tailor the selectivity of the monolith.
We have had considerable success in recent years with the use of PSG materials for separation and preconcentration of dilute mixtures of analytes. The on-line preconcentration feature of our macroporous PSG monolith offers an alternative to existing sample enrichment schemes. While the typical injected sample volume ranges from 1 nL to 30 nL, the PSG monolith allows for injection volumes up to 10 μL because of high mass transfer and high convective flow in the monolith structure, allowing up to 1000-fold preconcentration of dilute test samples. We are currently working on creating a protein and peptide concentrator on-line with capillary electrophoresis.
The PSG monolith can be thought of as a building block for the preparation of different on-column chemical devices, including enzyme microreactors, immunoaffinity materials through the bonding of an antibody, and catalytic reactors for organic transformations. These devices are formed in a capillary column downstream from separation techniques such as capillary electrophoresis and capillary electrochromatography. We have demonstrated the use of trypsin-immobilized PSG materials for on-line enzymatic digestions with 2000 times enhancement in the digestion rate of an artificial substrate as compared to the bulk solution rate. Our success with trypsin-PSG materials for capillary columns is the basis for our current work that involves the creation of enzyme microreactors in plastic pipette tips.
More generally, we want to expand the use of the PSG material for on-line organic reactions. The PSG monolith can be used to entrap catalytic materials or the monolith can be used as a support matrix for chemicals that can effect a chemical reaction. For example, a PSG material with covalently bound amine groups has the potential to be used in Knoevenagel condensation reactions at room temperature with downstream chromatographic or electrophoretic separation of the products. A PSG chemical reactor will allow us to combine synthesis, separation, and detection of products in one step in our capillary system. The advantages of such a device include small volumes of reagents and starting materials and an increase in the number of different reactions that can be run. In addition, there is some evidence to suggest that immobilization of a catalyst can lead to improvements in the efficiency and activity of the catalyst.