An Efficient Method for Covalent Surface Functionalization of Ultrasmall Metallic Nanoparticles by Surface Azidation Followed by Copper‐Catalyzed Azide‐Alkyne Cycloaddition (Click Chemistry).

The azidation of glutathione (GSH)‐functionalized ultrasmall gold nanoparticles (2 nm) by the azide transfer reagent imidazole‐1‐sulfonyl azide hydrogen sulfate leads to azide‐terminated nanoparticles with high yield. A subsequent copper‐catalyzed azide‐alkyne cycloaddition (CuAAC), i. e. a click reaction, leads to covalently functionalized nanoparticles. This was demonstrated with two alkyne‐functionalized dyes, i. e. FAM‐alkyne and AlexaFluor‐647‐alkyne, that were covalently coupled to the nanoparticles. The integrity of the glutathione ligand and the successful surface azidation were demonstrated by one‐dimensional and two‐dimensional NMR spectroscopy. The surface composition of the nanoparticles was determined by quantitative NMR spectroscopy and UV/vis spectroscopy. Each nanoparticle carries 125 glutathione molecules of which 118 were substituted by an azide group. After dye conjugation, either 6 FAM molecules or 11 AlexaFluor‐647 molecules were present on each nanoparticle, respectively. The dye‐clicked nanoparticles were highly fluorescent due to the absence of surface plasmon resonance. The post‐functionalization of GSH avoids a chemical reaction of a functional ligand during the reduction reaction, gives a high yield (up to 50 mg nanoparticles per batch), is based on water as solvent, and is applicable for metallic nanoparticles in general. [ABSTRACT FROM AUTHOR]