Aggregation of Ag ₆ ‐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 Ag 6 ‐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 (H 2 mna), DHPMC promotes the formation of Ag 6 ‐derived cluster species; the corresponding Ag 6 (H 2 mna) 6 crystalline core is established in the crystalline state by single‐crystal X‐ray diffraction, while negative‐mode ESI‐MS supports the presence of Ag 6 ‐derived species in solution. Notably, DHPMC converts inefficient aggregation into emissive aggregation. In 85 vol% ethanol, DHPMC/H 2 mna@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 H 2 mna, 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.