Surfactant-free coating of thiols on gold nanoparticles using sonochemistry: A study of competing processes

• A sonochemical method for coating AuNPs with thiols avoids the need for surfactants and purification steps.
• Competition between particle fusion and “digestive ripening” determines the morphology of the coated particles.
• Short sonication times lead to particle fusion, while longer times (>15 min) lead to the formation of larger aggregates.
• The effect of thiols is concentration-dependent; excess thiols lead to pronounced digestive ripening and changes in size.
• Long-chain thiols enhance digestive ripening compared to shorter-chain thiols, resulting in “capsule-like” morphologies.

A method for the surfactant-free coating of gold nanoparticles with thiols using sonochemistry is presented. The gold nanoparticles were prepared by a modified Zsigmondy method, affording good control over the particle-size distribution, and the thiol coating was performed by the sonication of a biphasic system consisting of a nanoparticle suspension in water and thiols in toluene. The effects of two important reaction parameters on the particle morphology, viz. sonication time and thiol concentration, were investigated in detail using transmission electron microscopy. The effect of the thiol chain length was also studied. We show that the morphology of the coated particles is determined through a competition between two opposing effects: particle fusion, due to the sonication conditions, and digestive ripening, due to the action of the thiols. Additionally, we illustrate the utility of our technique for various applications, including surface-enhanced Raman scattering from bound molecules, and further functionalization using a thiol-exchange reaction. Our technique paves the way for an efficient synthesis of thiol-coated AuNPs of different shapes and sizes, suitable for a range of diverse applications.