Engineering Gap‐Enhanced Raman Tags for High‐Performance Security Inks: A Focus on Raman Tag Density and Stability

Abstract Gap‐enhanced Raman tags (GERTs) represent a promising class of nanomaterials for security ink applications, offering superior anti‐counterfeiting capabilities through their rich and distinctive surface‐enhanced Raman scattering (SERS) characteristics. A key parameter influencing their performance is the Raman tag loading density, which directly impacts the colloidal stability of the GERT ink, the critical factor for ensuring uniform ink, long‐term reliability, and retention of SERS performance under various environmental conditions. Therefore, the loading density of Raman tags is optimized, and the effects of 4‐(2‐hydroxyethyl)‐1‐piperazineethanesulfonic acid (HEPES) buffer and chloroauric acid (HAuCl ₄ ) concentrations during shell formation are investigated to enhance SERS activity without compromising colloidal integrity. The resulting GERT inks exhibit good colloidal stability and generate strong and reproducible SERS activity when applied via handwriting on filter paper, along with invisibility, high photostability, and the capability of multiplexed steganographic encoding. Furthermore, the long‐term stability of the synthesized GERT ink is confirmed through Raman imaging after one year of ambient storage, with encoded patterns remaining detectable, thereby demonstrating the robustness of the optimized ink condition. These findings highlight the potential of GERT‐based security inks as a versatile and scalable platform for practical applications in secure labeling, document authentication, and anti‐counterfeiting technologies.