Ultrasensitive Electrochemical Immunosensor for Oral Cancer Biomarker IL-6 Using Carbon Nanotube Forest Electrodes and Multilabel Amplification

Squamous cell carcinomas of head and neck (HNSCC) are associated with immune, inflammatory, and angiogenic responses involving interleukin-6 (IL-6). This article reports an ultrasensitive electrochemical immunosensor for human IL-6 and proof-of-concept studies of IL-6 detection in HNSCC cells. Single wall carbon nanotube (SWNT) forests with attached capture antibodies (Ab 1 ) for IL-6 were used in an electrochemical sandwich immunoassay protocol using enzyme label horseradish peroxidase (HRP) to measure very low (≤30 pg mL −1 ) and elevated levels of IL-6. Two levels of multienzyme labeling were used to measure a broad concentration range of IL-6 in a representative panel of HNSCC cells. Secondary antibodies (Ab 2 ) attached to carboxylated multiwall carbon nanotubes with 106 HRP labels per 100 nm gave the highest sensitivity of 19.3 nA mL (pg IL-6) −1 cm −2 and the best detection limit (DL) of 0.5 pg mL −1 (25 fM) for IL-6 in 10 μL of calf serum. For more concentrated samples, biotinylated Ab 2 bound to streptavidin-HRP to provide 14−16 labels per antigen was used. These immunosensors accurately measured secreted IL-6 in a wide range of HNSCC cells demonstrated by excellent correlations with standard enzyme-linked immunosorbent assays (ELISA), suggesting that SWNT immunosensors combined with multilabel detection have excellent promise for detecting IL-6 in research and clinical applications.