Circulatingproteins are also valuable biomarkers for cancer detection,as they can diffuse into the bloodstream from the tumor microenvironment.Using this principle, a sensor for detecting cytokeratin 19 (CK19),a lung cancer marker, was developed with graphene oxide modified bycarboxyl groups attached to gold chips via cystamine. The sensor functionalized with anti-CK19 antibodies achievedan exceptional detection limit of 0.05 pg/mL. Here, KRT19 is linked to lung cancer.