Using the static analyzer, we evaluated sensor performance on artificial urine samples spiked with creatinine, whilst varying the CE:WE area ratio (Figs. 5a, 5b and S9). Interestingly, although increasing the relative WE area amplifies the signal and lowers the detection limit (Fig. ), response linearity and analytical sensitivity are gradually lost due to the insufficient CE size (Fig. ). We found that a CE:WE of 2:1 showed optimal performance, with a detection limit of 0.56±0.07 mM and an analytical sensitivity of 4.83±0.25 μA mM -1 . Note that in addition to magnitude, the standard deviation of the sensitivity values indicates the response linearity of the system. When compared to the static analyzers, paper-based flow-injection analyzers offer a number of key advantages (Figs. 5e, 5f and S10). First, they allow the injection of multiple samples into a single device (at rates in excess of 1 sample per minute), substantially reducing reagent consumption (carrier flow rate of 7-12 μL min -1 , Fig. ) and sample volumes (down to 1 μL, Fig. ). Second, the device-to-device variation is eliminated since all measurements can be performed on a single device, leaving only variations resulting from the injection process, which is approximately 6 % in our FIA devices (Figs. and). Additionally, the signals (unit: Coulomb) are quantified by integrating currents over time as the sample flows through the electrode, thus averaging the noise from the measurement readout. Surprisingly, and despite the weaker signals generated by the smaller WEs, we observed detection limits below 0.32 mM regardless of the relative electrode area (Fig. ). This is due to ability of the FIA device to detect small deviations from the steady baseline established by the carrier flow. On the other hand, the sensitivity shares a similar trend with that of static analyzers, showing a reduced sensitivity with decreasing CE area (Fig. ).