We use the bead based system for nucleic acid and protein assay quantification. Primary antibodies (a) are immobilized on nanofibers (b) of agarose beads (c) and loaded onto a silicon chip with anisotopically etched wells (d). Fluid is delivered in a microfluidic structure, flowing over and around the bead array, exiting underneath each microsphere through square fluid outlets. Using this setup, known or unknown concentrations of analytes of interest are delivered to the bead array where they bind to the primary antibodies and are captured in a second step using fluorophore pre-labeled secondary antibodies. The fluorescent signal (c) is measured using an epifluorescent microscope.
The bead-based system is incorporated in a partially integrated (e) and a fully integrated (e) programmable bio-nano-chip (PBNC) microfluidic flow cell. The partially integrated PBNC flow cell is assembled by sandwiching a microfabricated silicon chip between two poly(methyl methacrylate) (PMMA) inserts which serve as an inlet and outlet for fluid delivery using syringe or peristaltic pumps. A stainless steel environment is used to seal the flow cell components into place, providing an optical access window through the upper PMMA insert (e). In contrast, the fully integrated PBNC platform is a modular labcard built with blister packs to hold reagents and a waste reservoir (f) as well as a cost-effective stainless steel chip for agarose bead housing. The labcard can conveniently be placed into a portable and self contained analyzer for biomarker analysis. These arrangements in a bead-based system allow simultaneous optical and solution access.
Using this bead-based system we have reported detection limits as low as femtomolar, but more commonly in the nano to picomolar range for a variety of biomarkers relevant to cancer and cardiovascular disease. More recently we have adapted the bead-based system for trace detection of drugs.