Bioorthogonal conjugation of NIR luminescent gold nanoclusters with multifunctional polymers: insights into binding efficiency, kinetics, and optical properties

The bioorthogonal reactivity of gold nanoclusters (AuNCs) with macromolecules such as polymers is strongly influenced by their physicochemical parameters, which govern both reaction selectivity and kinetics. Using a quartz crystal microbalance with dissipation monitoring (QCM-D), we demonstrate that the strain-promoted azide-alkyne cycloaddition (SPAAC) reaction between azide-functionalized AuNCs and dibenzyl cyclooctyne (DBCO)-functionalized polymers in aqueous environments is highly specific yet proceeds at a relatively slow rate. Under diluted conditions (μM range), this reaction occurs predominantly at the level of individual nanoclusters, resulting in surface functionalization without extensive network formation. Optical characterization studies reveal a temperature- and concentration-dependent enhancement of AuNC photoluminescence upon their SPAAC conjugation with polymers. Furthermore, increased DBCO density in the polymer correlates with stronger photoluminescence enhancement, suggesting accelerated SPAAC kinetics and denser polymer coverage, consistent with the QCM-D observations. The insights gained from this work provide valuable guidance for an efficient bioorthogonal coupling between multifunctional AuNCs and polymers under biological conditions. Moreover, the developed experimental approach offers a versatile platform to investigate the formation and dynamics of various types of AuNC assemblies, including individual functional clusters and extended networks by delivering complementary information on kinetics, specificity, and luminescence properties.