In recent years, the use of functionalized gold nanoparticles has enabled sensitive detection of nucleic acids, small molecules, and cells. These tests are based on the “cross-linking aggregation” method, which uses the interaction between the modified ligand (RNA or DNA) on the surface of the functionalized gold nanoparticles and the target analyte to overcome the repulsive force between the particles to aggregate. Because the color of gold nanoparticles is more sensitive to aggregation, the target analyte can be detected by observing the color change.