In conventional microfluidic bioassays, short optical path length and extremely small quantities of analytes sometimes can significantly reduce detectability. Various sample preconcentration techniques have been reported for improving the detectability of bioassays. In the present study, we developed a novel preconcentration technique, “double sweeping”, utilizing cationic and anionic micelles simultaneously. Microscopic observations demonstrated that double sweeping enabled a sample solution, which initially filled a capillary, to be focused into an extremely narrow band. Initially, the sample molecules are swept from cathode to anode, or anode to cathode, based on conventional sweeping with an anionic or cationic micellar solution, respectively. Then, the fronts of the swept bands collide with each other in the capillary and halt at the interface between the bands. The sample molecules in the micellar solutions continue to move toward the interface because of the electrophoretic migration of the micelles, which results in further focusing and suppression of the band broadening due to molecular diffusion. We demonstrated that higher preconcentration efficiency was achieved using double sweeping than using conventional sweeping. In addition, double sweeping was successfully incorporated into a multiple enzyme activity assay using an arrayed reagent-release capillary, resulting in simple, rapid, simultaneous, and highly sensitive assays of caspase-3, alkaline phosphatase, and trypsin.

A single-step, easy-to-use enzyme immunoassay capillary sensor, composed of functional multilayer coatings, was developed in this study. The coatings were composed of substrate-immobilized hydrophobic coating, hydrogel coating, and soluble coating containing an enzyme-labeled antibody. The response mechanism involved a spontaneous immunoreaction triggered by capillary action-mediated introduction of a sample antigen solution and subsequent separation of unreacted enzyme-labeled antibodies and antigen-enzyme-labeled antibody complexes by the molecular sieving effect of the hydrogel. An enzyme reaction at the substrate-immobilized hydrophobic coating/hydrogel coating interface resulted in a protein-selective fluorescence response. An antigen concentration-dependent response was obtained for diagnostic marker protein samples (hemoglobin A1c (HbA1c), 7.14–16.7 mg mL−1; alpha-fetoprotein (AFP), 1.4–140 ng mL−1; C-reactive protein (CRP), 0.5–10 μg mL−1) that cover a clinically important concentration range. The successful measurement of CRP in diluted serum samples demonstrated the application of this capillary sensor.