The immunosensor showed a dynamic range from 0.1 pg mL−1 to 10 ng mL−1 and a LOD of (0.037 ± 0.002) pg mL−1 of HER2 protein in 25-fold diluted human serum and was able to differentiate between HER2-positive (SK-BR-3) and HER2-negative breast cancer cells (MCF-7 and Hela) and to detect 26 SK-BR-3 cells mL−1 in human serum samples. Here, ERBB2 is linked to breast cancer.