Lanthanide metal chelate security feature

A method of authenticating an article, comprising:

• providing an article including a security feature;
• and
• irradiating the article with a radiation source to excite the security feature,
• wherein the security feature comprises:
• a carrier;
• and
• an additive disposed in or on the carrier,
• wherein the additive is a chelate comprising:
• a lanthanide metal;
• and
• a ligand of formula (1), formula (2), or formula (3),
• wherein each of R1-R7 in formula (1) is independently selected from the group consisting of H, OH, NH2, Cl, F, OMe, OAr, OCF3, CF3, alkyl, aryl, phenyl, OPh, and heteroaromatic,
• wherein each of R1-R5 in formula (2) is independently selected from the group consisting of H, OH, NH2, Cl, F, OMe, OAr, OCF3, CF3, NMe2, CN, alkyl, aryl, phenyl, OPh, and heteroaromatic, and wherein R6 in formula (2) is selected from the group consisting of H, NH2, Cl, F, OMe, OAr, OCF3, CF3, NMe2, CN, alkyl, aryl, phenyl, OPh, and heteroaromatic, and
• wherein each of R1-R5 in formula (3) is independently selected from the group consisting of H, OH, NH2, Cl, F, OMe, OAr, OCF3, CF3, alkyl, aryl, phenyl, OPh, and heteroaromatic.