Highly sensitive paper-based analytical devices with the introduction of a large-volume sample via continuous flow

The implementation of continuous flow in paper-based analytical devices (PADs) was challenging because of the largevolume introduction that was created; but this allowed for the development of novel types of PADs for preconcentration, separation, and sensitive detection. In this study, pump-free continuous flow was applied to a distance-based PAD for the determination of iron ions. Continuous flow enabled the introduction of a volume that exceeded what was necessary to fill the hydrophilic channel of a PAD. Thus, this continuous-flow method significantly improved both the limits of detection (LOD) and the limits of quantification (LOQ) for a distance-based PAD by increasing the sample volume that could be introduced into the PAD. The values for LOD and LOQ were 20 and 26 ppb, respectively, which were more than 150-times lower than that obtained using a small sample volume (50 μL), and were comparable to those of inductively coupled plasma-atomic emission spectrometry. The continuous-flow technique was applicable to the determination of iron ions at levels of several tens of ppb in natural water without preconcentration.