A Novel Approach to Synthesise a Dual-Mode Luminescent Composite Pigment for Uncloneable High-Security Codes to Combat Counterfeiting.

A strategy is demonstrated to protect valuable items, such as currency, pharmaceuticals, important documents, etc. against counterfeiting, by marking them with luminescent security codes. These luminescent security codes were printed by employing luminescent ink formulated from a cost effective dual-mode luminescent composite pigment of Gd Yb Er O and Zn Mn S phosphors using commercially available PVC Gold medium. In the composite, Gd Yb Er O and Zn Mn S account for upconversion and downconversion processes, respectively. The synthesis procedure of the composite involves the admixing of Gd Yb Er O nanorods and Zn Mn S phosphor, synthesised by hydrothermal and facile solid-state reaction methods, respectively. The structural, morphological, microstructural, and photoluminescent features of Gd Yb Er O nanorods, Zn Mn S phosphor and composite were characterised by using XRD, SEM, TEM, and photoluminescence (PL) techniques, respectively. The distribution of PL intensity of the printed pattern was examined by using confocal PL mapping microscopy. The obtained results reveal that security codes printed using ink formulated from this bi-luminescent composite pigment provide dual-stage security against counterfeiting.