Surfactant-Free\nMethod to Prevent Gold Nanoparticle\nAggregation and Its Surface-Enhanced Raman Scattering Applications

The introduction of surfactants to stabilize colloidal\ncitrate-reduced\ngold nanoparticles (prevent aggregation) is usually used in surface-enhanced\nRaman scattering (SERS) applications. However, the surfactants have\nmany drawbacks for SERS applications, such as increasing the SERS\nbackground and blocking surface active sites. Here, we develop a surfactant-free\nmethod to stabilize colloidal cit-AuNPs based on alkali regulation,\nand this method can prevent gold nanoparticle aggregation under different\nharsh treatments, including ligand modification, centrifugation-based\nwashing/enrichment, and salt addition. The SERS spectra, density functional\ntheory simulation, and ζ potentials of cit-AuNPs indicate that\nthe stability of enhanced cit-AuNPs under alkaline conditions is attributed\nto both the increased negative charge density (by ∼6 times\nfrom pH 7 to 12) and the molecular configuration on the metal surface.\nCompared with surfactant-based methods, this method can well maintain\nthe inherent optical and interface properties of nanoparticles, avoid\nthe SERS background, and avoid blocking of the surface active site\ndue to the presence of surfactants. This method will enable AuNPs\nto have a wide range of applications in areas such as highly sensitive\nSERS sensors.