This study presents a single-component colorimetric nanosensor based on gold nanoparticles functionalized with hairpin DNA (H-SNA) for detecting the SARS-CoV-2 RdRp gene after PCR amplification. In this system, the hairpin DNA is conjugated to the surface of gold nanoparticles and consists of a stem-loop structure. The loop region contains a sequence that is complementary to the target gene, while the stem includes two palindromic arms. Upon the addition of an activating probe, which is complementary to the loop sequence, the hairpin structure unfolds. This unfolding allows the palindromic arms to interact, leading to the aggregation of nanoparticles and resulting in a visible color change from red to blue. In positive samples containing the target gene, the activating probe hybridizes with the target amplicon during the polymerase chain reaction (PCR) and is degraded by the 5'-exonuclease activity of the DNA polymerase enzyme. As a result, when the H-SNA solution is added to the final PCR product, the hairpin structure remains closed, and the solution retains its red color. Conversely, in negative samples that lack the target gene, the activating probe remains intact during PCR. After adding the H-SNA solution to the final PCR product, the probe binds to the hairpin structure, causing it to unfold. This interaction leads to nanoparticle aggregation and results in a color change of the solution to blue. By incorporating two palindromic arms on either side of any genomic sequence, this method can potentially be adapted for the detection of other viral genomes.