Rapid colorimetric detection of HIV-1 RNase H activity has been achieved via a low-cost, convenient approach, based on oligonucleotide-unmodified gold nanoparticles.The colorimetric assays relies on different binding affinities of a single-stranded DNA (ssDNA) and a double-stranded (dsDNA) toward unmodified gold nanoparticles.Neg. charged ssDNA sequences can effectively stabilize nanoparticles against salt-induced aggregation, providing a convenient route for colorimetric assays without surface biomodification of the nanoparticles.A synthetic RNA-DNA duplex substrate is first incubated with HIV-1 reverse transcriptase (RT).Under predefined buffer conditions, the HIV-1 RT should effectively cleave the RNA into fragments, resulting in the dissociation of ssDNA and ssRNA probes at room temperatureUpon addition of gold nanoparticles to the solution, the dissociated probes can form a charged protecting layer on the surface of the nanoparticles and subsequently stabilize the nanoparticles at a precisely defined salt concentrationHowever, if there is no HIV-1 RT or if the enzyme is inactive, the RNA-DNA duplex should remain intact.Upon adding gold nanoparticles, salt-induced aggregation of the particles occurs with a concomitant change in color.