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.

Automated summary

In this study, glutathione-functionalized gold nanoparticles were azidated and covalently functionalized through a click reaction successfully coupling alkyne-functionalized dyes. The surface composition and successful azidation were confirmed by NMR and UV/vis spectroscopy. The dye-clicked nanoparticles exhibited high fluorescence, high yield, and applicability for metallic nanoparticles.