The FRET ratio-C relationship for DC-SIGN binding with QR-DiMan was fitted very nicely (R 2 > 0.99) by the Hill equation and revealed a 𝐾 d of 0.5 ± 0.1 nM at a PQR of 1:1 (Figure ; Table ). This represents a massive 1.8 millionfold enhancement of affinity (𝛽) over the corresponding DiMan-CRD monovalent binding (𝐾 d = 0.9 mM), and a perglycan normalized affinity enhancement (𝛽/𝑁) of ~4,900 (Table ). Interestingly, this affinity is ~4-fold as strong as that of QD-DiMan•DC-SIGN binding (𝐾 d = 2.1 ± 0.5 nM). The sub-nanomolar 𝐾 d , here, demonstrates the high suitability of QR-DiMan for potent DC-SIGN targeting. The difference in affinity between QR-DiMan and QD-DiMan in DC-SIGN binding could be due to subtle changes in the interglycan distances and/or glycan display curvatures, allowing the former to have better spatial and/or orientation match with DC-SIGN's four binding sites to form stronger binding than the latter. The FRET ratio for DC-SIGNR binding is considerably lower than that of DC-SIGN at PQR = 1 under equivalent conditions, implying a weaker binding compared to the former. This result is fully consistent with that of DC-SIGN/R binding with QD-DiMan reported previously. The overall FRET ratio-C relationship for DC-SIGNR binding with QR-DiMan could be fitted by the Hill equation (R 2 = 0.990), giving an apparent Kd of 3.3 ± 0.9 nM (grey broken line), although several data points were found to deviate considerably from the fitting curve (Figure ). Thus, the resulting FRET ratio-C relationship appeared to display biphasic binding behavior, where the C-dependency of the FRET ratio exhibits a secondary increase at higher concentrations. By fitting only the first few data points at the low concentration range (C ≤ 10 nM), a good fit (R 2 > 0.996) with an apparent 𝐾 d of 0.3 ± 0.1 nM was obtained (Figure ). This 𝐾 d value is comparable to that of DC-SIGN, suggesting that a similar interaction is taking place, likely involving the same degree of binding multivalency (e.g. binding of all 4 CRDs to the same QR-DiMan). The appearance of the broader (slower increasing signal) secondary binding phase suggests that further binding can occur at higher Cs, which may indicate the formation of an additional crosslinking interaction, similar to that observed previously with QDs. This is presumably because at higher Cs, the relatively weak inter-QR binding becomes accessible. In order to obtain a more accurate overall binding affinity between DC-SIGNR and QR-DiMan, the binding assay was performed using a PQR of 10:1 which significantly improved the FRET signals (Figure ). Fitting the resulting FRET ratio-C relationship using eq 1 gave an apparent binding 𝐾 d of 48 ± 9 nM (Figure ), which is ~13fold stronger than that of the QD-DiMan-DC-SIGNR binding under equivalent conditions (𝐾 d = ~633 nM, PQR = 10). The binding affinity enhancement for DC-SIGNR is more substantial than that for DC-SIGN, which is expected due to the presence of the additional high affinity 1:1 binding component as observed with DC-SIGNR under at PQR of 1.