The invention relates to methods and kits for DNA The method comprises a) contacting the sample with a plurality of metal nanoparticles functionalised with one or more RNA probes; b) forming a heteroduplex between the target DNA and RNA probe on an RNA-functionalised metal nanoparticle; c) contacting the heteroduplex of target DNA and RNA probe on the RNA-functionalised metal nanoparticle with an enzyme that cleaves RNA in a DNA-RNA heteroduplex thereby releasing the target DNA; d) repeating steps (b) and (c) until all, or substantially all, of the RNA probes on the plurality of RNA-functionalised metal nanoparticles have been cleaved from the metal nanoparticles; and e) aggregating the metal nanoparticles from which all, or substantially all, of the RNA probes have been cleaved, thereby indicating the presence of the target DNA.