GNPs are universal outstanding quenchers for a wide range of fluorophores, due to high molar extinction coefficients and broad absorption spectrum. Moreover, GNP-mediated fluorescence quenching follows a nano surface energy transfer (NSET) mechanism, where the QE shows an inverse 4 th power dependence of distance, d, i.e. QE = 1/[1 + (d/d0) ]. As a result, GNP-based fluorescence quenching can occur over a much longer distance than organic quenchers relying on the Förster resonance energy transfer (FRET) mechanism, where QE shows an inverse 6 th power dependence on distance, R; i.e. QE = 1/[1 + (R/R0) 6 ]. Therefore, GNP quenchers can be orders of magnitude more efficient than organic quenchers (with quenching efficiency, QE, as high as 99.97% being reported) , making them well-suited for probing MLGIs between dye labelled lectins and glycan-GNPs. To determine the pH-dependency of MLGI between DC-SIGN/R and Gx-DiMan, fluorescence spectra were recorded at 0.4 pH unit intervals over a range from 4.6 to 7.4 for Atto-643-labelled DC-SIGN/R in the absence and presence of Gx-DiMan. A fixed concentration of 5 nM for G13-DiMan and 0.5 nM for G27, respectively, was employed to compensate for the much higher extinction coefficient of G27 over G13 (∼10 fold), ensuring that they have similar UV-vis absorbance to minimise any possible difference caused by GNP's inner filter effect (SI, Fig. ). A fixed protein: Gx-DiMan molar ratio of 10:1 for G13-DiMan or 60:1 for G27-DiMan was used for fluorescence quenching measurement. These ratios were lower than those required to form a closely packed monolayer of DC-SIGN molecules on the Gx-DiMan surfaces (estimated as ∼43 and ∼92 for G13-DiMan and G27-DiMan, respectively, based on Gx-DiMan surface areas calculated using their Dhs, and a footprint are of ∼35 nm 2 for each bound DC-SIGN molecule). Thus, surface saturation should not be a limiting factor for Gx-DiMan-DC-SIGN binding (hence fluorescence quenching). All fluorescence measurements were carried out in an MES (2-(N-morpholino)ethane sulfonic acid) buffer, which is well-suited for buffering within this pH range. The fluorescence of Atto-643 dye alone was independent of pH over this range (see SI, Figs.