Nanodots　are　attractive　stimuli-responsive　luminescence　materials　for　anti-counterfeiting　and　information　encryption.　However,　their　applications　are　limited　by　low　water　solubility　and　single-mode　information　identification　by　naked　eyes　under　UV　light　illumination.　Herein,　we　report　one　type　of　new　nanodots,　main-chain　imidazolium-based　ionic　polymer　dots　(IPDs).　There　is　no　edge　effect　in　IPDs,　and　the　ionic　groups　are　homogenously　distributed　in　the　entire　dot.　IPDs　exhibit　high　water　solubility,　good　stability,　narrow　size　distribution,　low　toxicity,　and　exceptional　optical　performance　without　additional　modification.　Written　information　using　aqueous　IPD　solution　is　invisible　in　natural　light,　but　can　be　recognized　by　a　portable　UV　lamp.　Moreover,　they　can　be　further　encrypted　and　decrypted　using　easily　available　and　nontoxic　sodium　carbonate　and　acetic　acid,　respectively.　The　encrypted　information　is　invisible　in　natural　light　and/or　UV　light.　This　study　provides　a　new　prospect　for　high-level　data　recording　and　security　protection　by　using　water-soluble　IPDs　as　invisible　security　ink.　©　2018　American　Chemical　Society.