3,　3′,　5,　5′-tetramethylbenzidine　(TMB)　has　been　widely　used　as　a　chromogenic　substrate　for　colorimetric　immunoassays.　Normally,　the　colorless　TMB　is　oxidized　into　yellow　TMB2+　(in　acidic　solution)　to　indicate　the　presence　of　the　target　molecules.　However,　this　kind　of　monochromic　intensity　changes　seriously　confine　the　accuracy　of　visual　inspection.　Herein,　we　demonstrate　for　the　first　time　that　TMB2+　can　quantitatively　and　efficiently　etch　gold　nanorods　(AuNRs).　The　addition　of　AuNRs　into　a　solution　containing　different　amount　of　TMB2+　generates　vivid　color　responses　as　colorful　as　a　rainbow,　and　the　etching　process　can　be　finished　within　90 s.　As　a　result,　the　exact　concentration　of　TMB2+　can　be　easily　distinguished　with　the　naked　eyes　by　the　corresponding　solution　color.　Based　on　this　finding,　we　incorporate　AuNRs　into　the　well-developed,　commercially　available　horseradish　peroxidase　(HRP)-TMB　immunoassay　system,　so　that　it　can　be　utilized　for　semiquantitative　detection　of　a　broad　range　of　disease　biomarkers　with　the　naked　eyes　(termed　‘NEQ-IA’).　Carcinoembryonic　antigen　(CEA)　and　Prostate　specific　antigen　(PSA)　had　been　chosen　as　example　targets　to　test　the　feasibility　of　the　proposed　biosensor.　The　results　showed　good　accordance　with　the　conventional　methods.　Because　no　sophisticated　apparatus　but　human　eyes　are　used　as　the　readout,　the　proposed　NEQ-IA　could　be　a　good　supplementary　to　current　state-of-the-art　immunoassay　methods　for　those　applications　that　require　the　use　of　portable　and　affordable　devices,　for　example,　for　the　detection　of　disease　biomarkers　at　home　and　in　the　field.　©　2016　Elsevier　B.V.