(A) The schematics showing the reflectance-based multi-well plate colorimetric aptasensor using AuNPs (Reproduced with permission from [50] © 2015 Royal Society of Chemistry). (B) Schematic explanation of TET detection based on colorimetric triple-helix molecular switch (THMS). In the absence of Tetracycline (TET), THMS (Aptamer + STP) remains stable, resulting in AuNPs aggregation by salt, color changes from red to blue. In the presence of TET, aptamer binds to its target, the signal transduction probe (STP) leaves the THMS and adsorbs on the surface of AuNPs thus stabilizing them, so no color change (Reproduced with permission from [87] © 2015 Elsevier). (C). Illustration of the proposed gold nanoparticle-linked competition-based aptamer assay (Reproduced with permission from [89] © 2020 MDPI).