A label-free and sandwich aptamer-based electrochemical biosensor for simple and sensitive determination of cocaine was developed based on double-strand nucleotide aptamer. Capture ss-DNA was immobilized on gold surfaces by means of self-assembly technique to fabricate cocaine aptamer-based biosensor. In the presence of target cocaine and detected aptamer probe, two fragments folded with cocaine and formed a sandwich complex on the electrode surface. Based on the difference binding ability of [Ru(NH₃)₆]^3＋ to ss-DNA and sandwich complex, a simple and highly sensitive electrochemical method was developed for the determination of cocaine by detecting the charge of [Ru(NH₃)₆]³⁺ adsorbed on the electrode surface by chronocoulometry. Under the optimized conditions, the charge response was linear with the concentration of cocaine from 1 mmol/L to 50 mmol/L. The detection limit was 0.1 mmol/L. The developed method allows the detection of cocaine in a serum sample with recovery in 96.4%～104%. This design strategy is easy to generalize for any aptamer without prior knowledge of its secondary or tertiary structure, and would be used as a simple, sensitive and general tool for biomolecule detection.

Key words: label-free, electrochemistry, sensor, aptamer, cocaine