Metal Ion Defects as Regulators of Uric Acid Surface Reactions in Manganese-Doped Zinc-Mediated Gold Nanocluster Assemblies

Defects play a crucial role in determining the properties of nanomaterials, yet their contribution to chemical reactivity - particularly in the context of assembled atomic clusters - remains poorly understood. In this work, we reveal how the introduction of defect states fundamentally alters the chemical reactivity landscape of atomically precise nanocluster assemblies. We show that uric acid (UA) engages in distinct chemical interactions with Mn²⁺-doped Zn-Au nanocluster assemblies (Mn-Zn Au NCs), compared to Zn-Au NCs assemblies lacking Mn²⁺ defects. While Zn-Au NCs responds to UA with enhanced fluorescence, Mn-Zn Au NCs exhibit selective quenching of the Zn-Au NC emission while retaining the Mn²⁺-related emission, thereby producing a concentration-dependent ratiometric signal detectable down to 0.566 ± 0.001 μM. The dual-emission behavior also drives a continuous shift in chromaticity coordinates, offering optical readout. Mechanistic analysis reveals that UA coordinates surface Zn²⁺ ions through specific functional groups, with binding governed by chelating-site arrangement and p<i>K</i>_a-dependent ionization states at near-physiological pH. This coordination enables energy transfers to surface Mn²⁺, leading to nonradiative dissipation and selective modulation of emission pathways. These findings establish Mn-Zn Au NCs as a unique nanocluster-based platform in which defect states dictate not only photophysical outcomes but also chemical reactivity, highlighting new opportunities for defect-engineered, ratiometric sensing at the nanoscale.