Figure 1.

(a) Schematics of GCS. A porous substrate is coated with a sensing probe that can react with an analyte specifically and change color. The top of the porous substrate is covered with a layer of acrylic to prevent direct exposure of the sensing probe to the analyte from the top, such that the analyte can only diffuse from the edge of the substrate to react with the sensing probe. The local reaction creates a color gradient, and GCS tracks the color gradient shift along the substrate over time. (b) Snapshots of the intensity profiles show shifting of the color gradient over time.