There is a growing need for cancerous exosome detection towards potential non-invasive cancer diagnosis. Researchers from King’s College London amied to develop a reliable colorimetric aptasensor for sensitive and specific detection of circulating cancer-derived exosomes. In this design, the researchers first captured target exosomes by latex beads via aldimine condensation, followed by bio-recognition using a specific CD63 aptamer, which was conjugated to horseradish peroxidase (HRP) through biotin-streptavidin binding. Colorimetric detection was achieved in 10 min via enzymatic catalysis to produce dark coloured polydopamine (PDA) from colourless substrate dopamine (DA) in especially prepared H2O2 reaction solution. The sensitivity was enhanced by in situ deposition of PDA around exosome particles to strengthen the developed colorimetric signal, which could be directly observed by naked eye. Signal quantification was carried out by absorbance measurement. The colour intensity correlates to the CD63 amount and the limit of detection can be as low as 7.7 × 103 particle/mL, improved by 3-5 orders of magnitude from conventional Dot-blot methods. The aptasensor showed specificity to HER2 and integrin αvβ6 positive, cell culture-derived, breast and pancreatic cancer-derived exosomes, respectively, when the correct aptamer sequence was used. Overall, a sensitive and selective colorimetric aptasensor was successfully developed for detecting cancer-derived exosomes facilitated by HRP-accelerated DA polymerization and in situ PDA deposition. This versatile aptasensor holds great potential for future development of point-of-care detection kits for cancer diagnosis in a clinical setting.