Optimizing gold nanostars as a colloid-based surface-enhanced Raman scattering (SERS) substrate

Journal of Optics, 2015, 17 pp 114013 

Shuai He¹, Malvin Wei Cherng Kang¹, Fahima Jaleel Khan¹, Eddie Khay Ming Tan², Miguel Arnold Reyes³and James Chen Yong Kah¹

The one-pot seedless protocol provides a facile approach in the synthesis of gold nanostars (AuNS) that involves only three reagents, gold (III) chloride (HAuCl₄), silver nitrate (AgNO₃) and ascorbic acid (C₆H₈O₆). While studies correlating the synthesis parameters of the seed-mediated protocol to surface-enhance Raman scattering (SERS) enhancement is well reported, the same understanding of the one-pot seedless protocol is limited. Here, we aim to elucidate how the synthesis parameters of AuNS from the one-pot seedless protocol, the AuNS concentration, surface passivation and aggregation level affect the colloidal SERS enhancement. Using crystal violet (CV) as a Raman probe molecule, we found that the SERS enhancement increases with Au³⁺/C₆H₈O₆ molar ratio up to 0.60 and Au³⁺/Ag⁺ molar ratio up to 18. Although the surfactant, cetyltrimethylammonium bromide (CTAB) maintained colloidal stability, it reduced the SERS enhancement. Interestingly, the SERS enhancement did not increase monotonically with AuNS concentration, but decreased when AuNS concentration was beyond 15 pM. The SERS enhancement also increased with the increasing level of salt-induced aggregation of AuNS, but only within a few minutes. While the concept of SERS with colloidal nanostructures is not new, we have shown for the first time, a detailed systematic study of various parameters that affect the SERS enhancement of AuNS synthesized using a one-pot seedless protocol. This study enables us to optimize the SERS enhancement of AuNS at the synthesis level to make them effective colloid-based SERS substrates for potential use in intracellular biosensing.