Flow-based microimmunoassay.

A small-volume heterogeneous immunoassay system is demonstrated in microchannels exploiting magnetic manipulations of small paramagnetic particles (1-2-microm diameter). The small-diameter particles help to create a high surface-area-to-volume ratio that generates the sensitivity for the small detection volumes. Flow characteristics of the magnetically formed packed bed within the channel allow the assay to be carried out quickly (minutes) while passing appropriate volumes of both samples and reagents (microliters to hundreds of nanoliters) through the system. The assay is demonstrated with a direct interaction of fluorescein isothiocyanate (FITC) with an immobilized anti-FITC conjugate in which a small-volume sample (< 1 microL) reaches 90% of maximum signal in 3 min. Heterogeneous sandwich assays are demonstrated with parathyroid hormone (PTH) and interleukin-5 (IL-5). Both the PTH assay and IL-5 assays were carried out on microliter volumes and demonstrated physiologically relevant sensitivity (approximately microg/L).