Ln-containing　polyoxoniobates　(PONbs)　have　appealing　applications　in　luminescence,　information　encryption　and　magnetic　fields,　but　the　synthesis　of　PONbs　containing　high-nuclearity　Ln-O　clusters　is　challenging　due　to　the　easy　hydrolysis　of　Ln3　+　ions　in　alkaline　environments.　In　this　paper,　we　are　able　to　integrate　CO3　2-　and　high-nuclearity　Ln-O　clusters　into　PONb　to　construct　an　inorganic　giant　Eu19　-embedded　PONb　H49　K16　Na13　(H2　O)63　[Eu21　O2　(OH)7　(H2　O)5　(Nb7　O22　)10　(Nb2　O6　)2　(CO3　)18　]　91H2　O　(　1　),　which　contains　the　highest　nuclearity　Eu-O　clusters　and　the　largest　number　of　Eu3　+　ions　among　PONbs.　In　addition,　the　film　that　was　prepared　by　mixing　1　with　gelatin　and　glycerol,　exhibits　reversible　luminescence　switching　behavior　under　acid/alkali　stimulation　and　has　been　used　to　create　a　fluorescence-encoded　information　approach.　This　work　paves　a　feasible　strategy　for　the　construction　of　high-nuclearity　Ln-O　cluster-containing　PONbs　and　the　expansion　of　the　application　of　Ln-containing　PONbs　in　information　encryption.　(c)　2024　Published　by　Elsevier　B.V.　on　behalf　of　Chinese　Chemical　Society　and　Institute　of　Materia　Medica,　Chinese　Academy　of　Medical　Sciences.