Stable versus Temporally Sensitive Optical Security Tags from Metal Nanoparticles.

Counterfeit goods are ubiquitous, causing economic fallout, device reliability issues, and health and safety concerns. To address this challenge, design principles for stable as well as temporally sensitive optical security tags from metal nanoparticles are established by comparing tags created from Ag-based nanorods, polymer-coated Ag-based nanorods, and Au nanorods. Both physically unclonable functions and unique images were created with these nanoparticles, and their colorimetric dark-field scattering responses were tracked as a function of time. The tags created from Ag-based nanorods showed the largest change in color response and were the most responsive to an increased temperature, followed by the polymer-coated Ag nanorods. The tags created from Au nanorods were the most temporally and thermally stable. The different temporal responses arise from variable sensitivity to surface oxidation. These findings outline how the composition and surface modification of nanoparticles can be designed and serve as indicators of time, structural heath, and/or tampering.