A newly developed electrochemical biosensor for the determination of superoxide anions (O(2)˙(-)) released from cancer cells using potassium-doped multi-walled carbon nanotubes (KMWNTs)-1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM]PF(6)) ionic liquid composite gels is demonstrated. The KMWNTs-[BMIM]PF(6) can electrocatalyze oxygen reduction to generate a strong current signal in neutral solution. Compared with KMWNTs without [BMIM]PF(6) or MWNTs-[BMIM]PF(6) composites, the KMWNTs-[BMIM]PF(6) can enhance the oxygen reduction peak current by 6.2-fold and 2.8-fold, which greatly increases the detection sensitivity of oxygen. Then, O(2)˙(-) biosensors are fabricated by mixing superoxide dismutase (SOD) in the KMWNTs-[BMIM]PF(6) gels via monitoring oxygen produced by an enzymic reaction between SOD/O(2)˙(-) without the help of electron mediators. The resulting biosensors show a linear range from 0.04 to 38 μM with a high sensitivity of 98.2 μA mM(-1), and a lower detection limit of 0.024 μM. The common interferents such as hydrogen peroxide (H(2)O(2)), ascorbic acid (AA), uric acid (UA), and metabolites of neurotransmitters, do not interfere with the detection of O(2)˙(-). The proposed biosensor is tested to determine O(2)˙(-) in vitro and from liver cancer and leukemia cells and shows good application potential in biological electrochemistry.
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Li et al. (2011) studied this question.