Methods of assaying a biological target are disclosed. The method comprises: (a) providing a sample containing the biological target; (b) providing biotin-labeled first aptamers conjugated to a gold nanoparticle (GNP), and second aptamers conjugated to a magnetic bead, wherein the first and the second aptamers exhibit specific binding affinities to the target; (c) incubating the sample with the first and the second aptamers to obtain target-bound aptamers; (d) separating the target-bound aptamers from unbound aptamers; (e) eluting the first aptamers from the GNP; (f) incubating the eluted biotin-labeled first aptamers with streptavidin-magnetic beads and reporter gold nanoparticles (GNPs) to obtain a complex comprising: the bead, the first aptamers, attached to the bead; and the reporter GNPs captured by the bead through the first aptamers; (g) eluting the reporter GNPs captured; and (h) detecting the target by measuring and analyzing a light-scattering signal of the eluted reporter GNPs.