Aggregation of Ag6-Derived Clusters in Dialdehyde Hydroxypropyl Methylcellulose for Fluorescent Security Ink

Abstract

Aggregation-induced emission (AIE) is a powerful route to amplify weakly emissive metal nanoclusters, yet simultaneously achieving AIE, structural precision, and ink compatibility remains challenging. Here, we demonstrate an aldehyde-functional cellulose ether as a soft, switchable template that regulates aggregation and emission of Ag₆-derived cluster species. Periodate oxidation converts hydroxypropyl methylcellulose (HPMC) into dialdehyde HPMC (DHPMC), which forms swollen, homogeneous domains in water but collapses into confining microenvironments in ethanol-rich media. In the presence of 2-mercaptonicotinic acid (H₂mna), DHPMC promotes the formation of Ag₆-derived cluster species; the corresponding Ag₆(H₂mna)₆ crystalline core is established in the crystalline state by single-crystal X-ray diffraction, while negative-mode ESI-MS supports the presence of Ag₆-derived species in solution. Notably, DHPMC converts inefficient aggregation into emissive aggregation. In 85 vol% ethanol, DHPMC/H₂mna@AgNCs shows a dominant emission at 578 nm (λ_ex = 385 nm), a quantum yield of 1.70%, and an average lifetime of 63.64 ns, reflecting suppressed non-radiative decay in a microenvironment-sensitive excited state. Structural and microscopic analyses further indicate that, in the presence of H₂mna, the DHPMC-containing formulations favor cluster-derived Ag-containing domains rather than plasmonic Ag nanoparticles. Coupled with printing-adapted rheology and antibacterial performance, this platform enables fluorescent patterns and latent fingerprint visualization, providing a practical polymer-cluster co-design strategy for multifunctional security inks.