In order to understand the discrepancy between the experiments and simula�ons, we performed extended simula�ons in Supplementary Informa�on S4.4. The phase plots revealed how the concentra�on of the reac�ve layer (i.e., the concentra�on of available amine groups on the surface) could influence the dependencies of the system as a func�on of flow and pH. Fig. highlights that the delay �me is highly dependent on the effec�ve concentra�on of amine groups, and that the single-layer devices do not show the same dependency. We therefore used layer-by-layer deposi�on to develop a polyelectrolyte mul�layer (PEM)-coated surface (Supplementary Informa�on, S5.3). To our sa�sfac�on, we observed that the 𝑡𝑡 𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑 in a PEM-coated device (with three layers), indeed, followed the same trend as predicted in the model. Notably, the delay �me at pH 8 became much slower, exceeding the �me of our experiments (Supplementary Informa�on, Fig. ). Nevertheless, we observed a significant increase in the delay �me when we compared the responses across the examined pH range in the three-layer devices with the single-layer ones.