Described SPAAC experiments were conducted by mixing solutions of AuNP-4 with fluorescein-DBCO (FITC-DBCO; ratio of N3/DBCO of 2:1) in H2O/EtOH. The process of the conjugation was monitored by UV-vis through the decrease of the DBCO specific absorbance band at 310 nm over a period of 3 h (Figure , E) and further confirmed by FT-IR (see ESI for spectrum, Figure ) and XPS. Disappearance of the band confirmed the full conversions of the free DBCO moiety in the corresponding triazole species attached to the PEG shell of AuNPs. Subsequent fluorescence emission spectra of such conjugated AuNP-6 show a clear emission at 524 nm compared to unconjugated AuNP-4, which does not show any fluorescence when exited at 460 nm (see ESI for spectra; Figure ). Comparing the signal intensity of conjugated FITC to FITC-DBCO at the initially used concentration a significant emission signal enhancement by Förster resonance energy transfer (FRET) is observed, which is supported by the structural features of AuNP-6 (AuNP dTEM, ~ 6 nm, ligand shell thickness 8-9 nm, see ESI for TEM, Figure ) and the good overlap of the emission of FITC with the SP band of employed AuNPs (λmax, 515 nm).