Described are compositions and methods effective to improve the security obtainable for red phosphorescent inks printed, such as by ink jet printing. Specially designed polymeric chelating agents are water soluble and have a high chelating efficiency for lanthanide cations and are themselves strongly adsorbed on polymeric substrates, especially paper. The polymeric chelating agents comprise a polymer chain of either general formulae (1) or (2) containing pyridine-N-oxide, quinoline-N-oxide or isoquinoline-N oxide pendent ligand groups A, where m and n are independent integers in the range of from 1 to 10, and the number average molecular weight of the polymers lies in the range from 1×103 to 5×105: In one aspect a polymeric chelate as described is produced in situ in an aqueous ink formulation and is subsequently printed onto a substrate and dried. In another, a substrate is pre-treated with a chelate as described and dried, and then subsequently printed with an aqueous solution of a fluorescent rare earth salt. In both cases, the result is a wet-fast, intensely luminescent print.