At present, surface modified gold and silver nanoparticles are widely applied as colorimetric DNA detectors. Binding affinity of ssDNA and dsDNA onto the metal nanoparticles are different. Using Leptospira interogans as a model, a novel DNA detection was designed based on the binding affinity of DNA in corporation with aggregation and oxidation properties of gold and silver nanoparticles. The ssDNA of Leptospira interogans prevented aggregation of silver and gold nanoparticles conducted by saline solution. On the other hand, after hybridisation with the probe DNA, aggregation of gold and silver nanoparticles can be observed as the change of colour of the metal nanoparticles. In oxidation process, dsDNA of probe and target Leptospira interogans DNA prevent oxidation of silver nanoparticles. In contrast, the bleaching of the yellow colour of silver nanoparticles by H2O2 oxidation can be observed in ssDNA/silver nanoparticles mixture. The colour changing of solutions can be calculated from light absorption spectra of nanoparticles
