Spherical GNPs of 13 nm and 27 nm were employed to construct glycan-GNPs. As most virus surface trimeric glycoprotein spikes are ∼13 nm in size, for example, the HIV surface trimeric gp160 spike, the 13 nm GNP (abbreviated as G13) was used to mimic virus spikes. Furthermore, a 27 nm GNP (G27s) was also employed to investigate how GNP size may impact its pH-dependent binding with DC-SIGN/R. G13 was synthesised by using the standard citrate reduction method as described previously. G27 was synthesised by citrate reduction with the addition of a small amount of NaOH following a literature protocol. Their detailed synthesis procedures were given in the Experimental Section. The average GNP core diameters were determined as ca. 13 nm for G13, and 27 nm for G27, respectively (see Supporting Information (SI), Figs. and). Both G13 and G27 gave a single plasmonic absorption peak at 520 nm for G13 and at 522 nm for G27, respectively, which are consistent to isolated individual GNPs. They both displayed a single hydrodynamic size distribution with a mean Dh of ∼16 nm with a polydispersity index, PdI, of 0.22 for G13, and ∼29 nm with a PdI of 0.21 for G27, confirming that they were uniform and aggregation-free GNPs. The concentrations of G13 and G27 were obtained by the Beer-Lambert law from their SPR peak absorbance at 520 or 522 nm using a molar extinction coefficient of 2.32 x 10 8 or 2.39 x 10 9 M -1 cm -1 for G13 or G27, respectively. Their detailed UV-vis spectra, TEM images and hydrodynamic size histograms were given in SI, Figs. and.