Figure 4.

(A) (I) Exosomes were anchored on sulfate/latex beads by aldimine condensation followed by binding to a CD63-specific aptamer conjugated to biotin. Incubation with streptavidin-conjugated HRP converted colorless dopamine (DA) into brown-black colored polydopamine (PDA). (II) The color change is proportional to exosome concentrations. (III) The absorbance signals of the reaction can also be measured at 400 nm. (B) Schematics of the deposition of PDA onto the exosome membrane. (C) The color change of PDA deposition at the target site of HRP at different H₂O₂ concentrations. (D) The change in the color intensity is a function of time and H₂O₂ concentration. (E) Images of exosome samples after color development in the ExoAptaSensor. (F) Absorbance peak at 400 nm correlates with exosome concentrations. Reprinted from Development of a simple, sensitive, and selective colorimetric aptasensor for the detection of cancer-derived exosomes, Volume 169, Xu et al., 2020a, Page No. 112576, Copyright 2020, with permission from Elsevier.