Determination of cocaine on banknotes through an aptamer-based electrochemiluminescence biosensor

A novel electrochemiluminescence (ECL) “sandwich” biosensor has been developed to detect cocaine. The sandwich biosensor was fabricated on the basis of the fact that a single aptamer could be split into two fragments and the two dissociated parts could form a folded, associated complex in the presence of targets. One of these (capture probe), which had hexane–thiol at its 5′-terminus, was immobilized on a gold electrode via thiol–gold binding. The other one (detection probe) was labeled with the ECL reagent tris(2,2′-bipyridyl)ruthenium(II)-doped silica nanoparticles (RuSiNPs) at its 3′-terminus. Owing to the weak interaction between the two fragments, the sensor exhibited a low ECL signal in the absence of cocaine. After the target cocaine had been added to the solution, it induced association of the two fragments and stabilized the associated complexes, leading to immobilization of RuSiNPs on the electrode surface, and the ECL detected on the electrode surface was enhanced. The enhanced ECL intensity was directly proportional to the logarithm of the cocaine concentration in the range from 1.0 × 10⁻⁹ to1.0 × 10⁻¹¹ mol/L, with a detection limit of 3.7 × 10⁻¹² mol/L. The biosensor was applied to detect trace amounts of cocaine on banknotes with satisfactory results.