To ensure all CRDs conjugated on the GNP surface are oriented and available for binding, the N-terminal amine in DC-SIGN ECD or CRD was selected for linker labeling. The pKa of N-terminal αamine is > 2 pH units lower than protein surface ε-amines of lysine residues (e.g. ~6.0-8.0 vs. ~10.5). Thus, labeling was conducted at pH 6.2, ensuring that only the α-amine, but not ε-amines, was non-protonated and available to react with TFP ester to form a stable amide linkage. Incubating the protein with LA-EG11-Tz-TFP at 1: 1.5 molar ratio for ~20 min was sufficient to produce single linker labeled proteins in ~18% and ~22% yields for ECD (denoted as LA-EG11-Tz-ECD) and CRD (denoted as LA-EG11-Tz-CRD), respectively. Extending the incubation time led to the formation of dual-labeled proteins. The same condition was used to label linker LA-EG11-TFP, giving single-linker labeled CRD (denoted as LA-EG11-CRD) in ~19% yield and a very small amount (~2%) of duallabeled CRD (denoted as (LA-EG11)2-CRD) (SI, Section 4).