An　exonuclease-assisted　multicolor　aptasensor　was　developed　for　the　visual　detection　of　ochratoxin　A　(OTA).　It　is　based　on　the　etching　of　gold　nanorods　(AuNRs)　mediated　by　a　G-quadruplex-hemin　DNAzyme.　A　DNA　sequence　(AG4-OTA)　was　designed　that　comprises　a　hemin　aptamer　and　an　OTA　aptamer.　OTA　binds　to　AG4-OTA　to　form　an　antiparallel　G-quadruplex,　which　halts　its　digestion　by　exonuclease　I　(Exo　I)　from　the　3′-end　of　AG4-OTA.　Thus,　the　retained　hemin　aptamer　can　bind　to　hemin　to　form　a　G-quadruplex-hemin　DNAzyme.　This　DNAzyme　has　peroxidase-like　activity　that　catalyzes　the　oxidation　of　3,3′,5,5′-tetramethylbenzidine　(TMB)　by　H2O2　to　produce　its　diimine　derivative　(TMB2+)　in　acidic　solution.　TMB2+　can　etch　the　AuNRs　by　oxidizing　Au(0)　into　Au(I).　This　results　in　the　generation　of　rainbow-like　colors　and　provides　a　multicolor　platform　for　the　visual　detection　of　OTA.　The　assay　is　based　on　the　use　of　a　single　isolated　aptamer　and　possesses　obvious　advantages　such　as　multi-color　visual　inspection,　relatively　high　sensitivity　and　accuracy.　It　can　be　used　to　detect　as　little　as　30 nM　concentrations　of　OTA　by　visual　observation　and　even　10 nM　concentrations　by　spectrophotometry.　The　method　was　successfully　applied　to　the　determination　of　OTA　in　spiked　beer　where　it　gave　recoveries　of　101–108%,　with　a　relative　standard　deviation　(RSD,　n　=　5)　of <5%.　©　2018,　Springer-Verlag　GmbH　Austria,　part　of　Springer　Nature.