Controlled High-Yield Assembly of Gold Nanoparticles via Amide Bond Formation

Abstract

Assembly of gold nanoparticles (AuNPs) enhances their plasmonic properties, including visible coloration, local electric field generation, hot-carrier production, and photothermal heating. While assembling AuNPs through chemical reactions that create solid, well-defined covalent linkages is highly desirable, achieving such assemblies with high efficiency remains challenging. During surface functionalisation and interparticle reactions, AuNPs are prone to aggregation, which compromises colloidal stability and yield. Such uncontrolled agglomeration can easily be mistaken for successful covalent assembly, because instability-driven clustering—and even nonspecific electrostatic association between oppositely charged particles—can produce structures that resemble the intended covalently bonded nanoassemblies. Here, we present a general strategy for assembling AuNPs through covalent amide linkages, providing detailed experimental guidelines and critical precautions to avoid these pitfalls and to achieve reproducible, high-yield assembly. To prevent aggregation during ligand exchange, AuNPs are immobilised on glass substrates and functionalised with amine groups (NH₂–AuNPs). Alkylamines such as 6-amino-1-hexanethiol outperform arylamines because of their higher nucleophilicity towards activated carboxyl groups. We prepare carboxyl-functionalised AuNPs (COOH–AuNPs) by ligand exchange with mercaptoalkanoic acids and find that removing unbound ligands is essential for high-yield assembly. Hydrophilic discrete PEG spacers stabilise COOH–AuNPs during repeated centrifugation for purification. In the presence of EDC, NH₂–AuNPs and COOH–AuNPs form covalently linked assemblies with yields of 95 ± 5%. The resulting nanoassemblies exhibit well-defined 1:1 or 1:2 core–satellite stoichiometries, reflecting the limited availability of activated carboxyl groups. Raman spectroscopy confirms the formation of interparticle amide bonds. Finally, we demonstrate that this method is broadly applicable to the high-yield assembly of AuNPs across diverse shapes (nanospheres, nanocubes, nanorods) and sizes (14–101 nm). This strategy provides a versatile platform for constructing plasmonic nanoassemblies with chemical reactions.