Figure 1. HRP-accelerated dopamine polymerization and deposition.

(a) Schematic illustration of the EASE technology. Dopamine (colorless) slowly oxidizes in the presence of air (O₂ as oxidant) and produces brown-black PDA. This polymerization process can be sped up by approximately 300 times under HRP catalysis (H₂O₂ as oxidant). (b) Visual observation of dopamine polymerization under conventional and HRP catalyzed conditions at various time points. (c) Extinction measured at 700 nm for the samples showing in (b). EASE produces the same optical extinction (700 nm) in 48 s compared to 4 h under conventional conditions. (d) HRP-catalyzed PDA deposition on solid supports. When protein density on the solid support is low (for example only HRP is present), majority of the PDA molecules diffuse away. For solid supports (e.g., flat surface and membrane) with high protein density (e.g., in cells and surfaces blocked with protein molecules for reduced nonspecific binding), rapid and localized deposition of PDA occurs due to the reactivity of PDA to nearby amines (rich in proteins), leading to formation of a dark spot. (e) Images showing membranes immobilized with BSA alone, HRP alone, or HRP/BSA before and after exposing to dopamine. Scale bar, 5 mm.