Quantitative Rigidity-Emission Correlation Enabled by Dynamic Nanocluster-Protamine Condensates

Abstract

Restricting molecular motions within metal nanocluster (NC) assemblies provides the structural rigidity necessary to suppress non-radiative decay and trigger robust aggregation-induced emission (AIE). Despite this long-standing paradigm, the relationship between rigidity and AIE remains largely qualitative. Here, we report the protamine-directed assembly of AIE-active AuNCs into dynamic, biomolecular condensate-like structures and establish a quantitative correlation between structural rigidity and AIE enhancement. By modulating AuNC-protamine interactions through solution pH, we systematically tune the internal rigidity of those liquid-like condensates. Confocal laser scanning microscopy and fluorescence recovery after photobleaching experiments reveal highly dynamic assemblies with distinct molecular mobilities. Condensates formed at pH 10 exhibit the highest immobile fraction of similar to 54%, indicative of a gel-like structure with maximum rigidity, whereas those formed at pH 7 show the liquid-like dynamicity with the lowest rigidity (i.e., immobile fraction 38%). Notably, AIE enhancement follows the same trend, reaching a maximum at pH 10 and a minimum at pH 7. Correlating the mobility with AIE enhancement identifies the immobile fraction as a quantitative descriptor of rigidity in AIE-active cluster assemblies. These findings establish dynamic nanocluster condensates as an ideal platform for unveiling rigidity-guided photoluminescence enhancement.