Rapid Analysis in Continuous-Flow Electrochemical Paper-Based Analytical Devices

A simple and low-cost continuous-flow (CF) electrochemical paper-based analytical
device (ePAD) coupled with thermoplastic electrodes (TPEs) was developed. The fast, continuous
flow combined with flow injection analysis was made possible by adding two inlet reservoirs to the
same paper-based hollow channel flowing over detection electrodes, terminating in a fan-shaped
pumping reservoir. The upstream inlet reservoir was filled with buffer and provided constant flow
through the device. Sample injections were performed by adding 2 μL of the sample to the
downstream sample inlet. Differences in flow resistance resulted in sample plugs displacing buffer
as the solution flowed over the working electrodes. The electrodes were fabricated by mixing
carbon black and polycaprolactone (50% w/w). CF-TPE-ePADs were characterized with
chronoamperometry using ferrocenylmethyl trimethylammonium as the electrochemical probe.
Optimized flow rates and injection volumes gave analysis times roughly an order of magnitude faster than those of previously
reported flow injection analysis ePADs. To demonstrate applicability, the CF-TPE-ePADs were used to quantify caffeic acid in three
different tea samples. The proposed method had a linear range from 10 to 500 μmol L−1 and limits of detection and quantification of
2.5 and 8.3 μmol L−1
, respectively. Our approach is promising for fabricating simple, inexpensive, yet high-performance, flow
injection analysis devices using paper substrates and easy-to-make electrodes that do not require external mechanical pumping
systems or complicated valves.
KEYWORDS: flow injection analysis, paper microfluidics, thermoplastic electrodes, electrochemical sensor, caffeic acid, tea samples