In situ growth of positively-charged gold nanoparticles on single-walled carbon nanotubes as a highly active peroxidase mimetic and its application in biosensing.

We prepared a new positively-charged gold nanoparticle ((+)AuNPs)-single-walled carbon nanotubes (SWNTs) nanohybrids. The results showed that small (+)AuNPs dispersed uniformly on the surface of SWNTs. Importantly the resulting nanohybrids exhibited fascinating peroxidase-like activity, which can catalyze oxidation of the peroxidase substrate 3,3,5,5-tetramethylbenzidine (TMB) by H2O2 to develop a blue color in aqueous solution. Furthermore, single-stranded DNA (ss-DNA) can resist salt-induced (+)AuNPs-SWNTs nanohybrids aggregation, whereas double-stranded DNA (ds-DNA) can not inhibit salt-induced nanohybrids aggregation. Based on these unique properties of the (+)AuNPs-SWNTs nanohybrids, we developed a label-free colorimetric method for DNA hybridization detection. The response to target DNA concentration was linear in the range of 0.025-0.5μM with a detection limit of 2nM (3σ). Based on the specific recognition of aptamer, the method can be extended to detect non-nucleic acid targets such as cocaine. The present limit of detection for cocaine is 2nM. The method offers the advantages of simple, cheap, rapid and sensitive.