In a trend of the µTAS and Lab-on-a-Chip researches, biosensors have been incorporated in various microfluidic systems. In particular, amperometric sensors have been used for clinical diagnosis such as cancer screening and blood testing. In general, a difficulty in conducting detection in miniaturized systems is to lower the detection limit. With a planar electrode used in conventional microsystems, a large proportion of analyte molecules pass over the electrode without any reactions. In other words, most of the analytes is wasted without being used for the analysis. A solution to this problem is to improve the collection efficiency of the electrode using a three-dimensional microscopic structure. In this study, we used electrodes with micropillars and demonstrated their effectiveness to increase the detection sensitivity.

　Micropillars (50 µm high) were formed on the working electrode with a thick-film photoresist SU-8. The active area of the working electrode was 500 µm × 500 µm. The flow channel was 500 µm in width and 55 µm in height. To examine the collection efficiency, micropillar structures of 50 µm in height and 30, 20, or 10 µm in diameter were fabricated (Fig. 1). The current increased as the surface area of the micropillar electrodes increased (Fig. 2a). The current on the electrode of micropillars of 10 µm in diameter was 12 times larger than that on the flat electrode, although the areal ratio of the two electrodes was only 4.4. In order to improve the detection sensitivity further, the micropillar electrodes were additionally modified with porous gold black to increase the active surface area (Fig. 1). By forming gold black, a 22-fold increase in the output current was observed compared with the flat electrode (Fig. 2b).

Fig. 1 SEM images of the electrodes. Diameter of pillars: (a)30 µm; (b)20 µm; (c) 10 µm. Scale bar:10 µm. (d) Flat electrode surface with gold black (d). Scale bar:2.5 µm

Fig. 2 Oxidation current of L-ascorbic acid.