We proposed a FRET immunosensing for detection of CA15-3 tumor marker by highly biospecific interactions between CA 15-3 antigen and the corresponding antibody and aptamer. In this sandwich type immunoassay, CA15-3 antibody-functionalized carbon dots and AuNPs labeled PAMAM-Dendrimer/aptamer were used as donor/acceptor, respectively. When CA 15-3 Ag was added to homogenous immunoassay, the strong complex interaction between CA15-3 Ab-CA15-3 Ag- aptamer caused in more coming closer carbon dot and AuNPs and more decreasing fluorescence signal. The decreased fluorescence intensity was linear at three ranges including in concentration range 1.1 μUmL−1 to 16 μU mL−1 with regression of R2=0.9879, at the concentration range 16 μU mL−1 to 0.163 mU mL−1 with regression of R2=0.9944 and at the concentration range 0.163 mU mL−1 to 5.0 mU mL−1 with regression of R2=0.9805. The detection limit of the FRET immunoassay was 0.9 μU mL−1. This assay revealed good sensitivity and specificity with MDA-MB-231 breast cancer cells concentrations from 1000 to 40000 cells/mL with correlation coefficient of 0.9955 and detection limit of 300 cells/mL (3 cells in 10 μL of injected sample). In addition, this FRET immunosensing is applicable in diluted human serum. The recovery values were in the range of 95.86–96.97% for CA 15-3 Ag in spiked serum sample with RSD lower than 7.3%. The proposed immunoassay could be a valid model for establishing other immunoassays for detection of different cancer tumor markers with relevant antigens and antibodies.
