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    Locating Reactive Groups on Nanomaterials with Gold Nanoclusters: Toward a Surface Reactive Site Map
    Nanoparticles (NPs) are often functionalized with reactive groups like amines or thiols for the subsequent conjugation of further molecules, e.g., stabilizing polymers, drugs and proteins or targeting cells or specific diseases, etc. In addition to the quantitative estimation of the reactive conjugation sites, their nanoscale and molecular positioning and local arrangement on single nanoparticles becomes more and more important for tailored engineering and design of functional nanomaterials. Here, we use maleimide or sulfo-succinimidyl ester modified 1.4 nm gold nanoclusters (AuNCs) to specifically label reactive thiol and amine groups with sub –2 nm precision on metal oxide and polymeric nanostructures. We confirm the binding of AuNCs by measuring and modelling sedimentation properties using analytical centrifugation, image their surface distribution and surface distances by transmission electron microscopy (TEM), and compare the results to ensemble measurements of numbers of reactive surface groups obtained by common photometric assays. We map thiol and amine groups introduced on silica NPs (SiNPs), titania stars (Ti), silica inverse opals (SiOps), and polystyrene NPs (PS NPs). We show that the method is suitable to map local, clustered inhomogeneities of the reactive sites on single SiNPs introduced by masking certain areas during surface functionalization. Mapping precise positions of reactive surface groups is essential for the design and the tailored ligation of multifunctional nanomaterials.
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