Highly sensitive electrochemical immunosensor based on SiO₂ nanospheres for detection of EGFR as colorectal cancer biomarker

Colorectal cancer is one of the most common cancers, and its incidence rate keeps increasing worldwide. Epidermal growth factor receptor (EGFR) is highly expressed in colorectal cancer cells, and its activation leads to the activation of downstream kinase pathways to promote cell growth, resulting in tumour growth and progression. EGFR is expressed in only small amounts in normal cells, however, it is overexpressed in tumour cells and is a potential tumour marker, thus the detection of which is of great importance. This work proposes a novel and highly sensitive electrochemical immunosensor based on nucleic acid aptamer recognition and silver deposition for EGFR detection. The key novelty of this work is the use of SiO2 nanospheres to enhance the sensitivity. First, SiO2 nanospheres were synthesized and modified on the surface of a gold electrode using a selfassembly process. This provided a large surface area to allow high loading of capture antibodies. The antibody was then immobilized on the SiO2 nanosphere modified electrode surface to form an immunosandwich complex with the antigen and biotin-labelled nucleic acid aptamer. Afterwards, the affinity-labelled alkaline phosphatase was immobilized onto the electrode with the specific reaction of biotin and affinity. The obtained product was subjected to silver deposition in the substrate solution. The large surface area provided by the SiO2 nanospheres allowed significantly improved sensitivity. Experiments were performed to optimize the concentration of immobilized antibodies and biotin-labelled nucleic acid aptamer. The experimental results show a good linearity of EGFR concentration in the range of 1 to 1000 ng/mL, with the lowest detection limit up to 0.06 ng/mL. This method has high sensitivity and stability, which allows it to have a broad application prospects in the clinical field.