Stabilization of Gold Nanocrystals in Tryptic Soy Broth Bacterial Growth Medium

Gold nanocrystals (AuNCs) have been recently attracting interest as potential antimicrobial agents. One of the hurdles hindering quantitative studies in this area is the limited stability of AuNCs in common bacterial growth media (BGMs) with respect to flocculation. In this work, we investigated the stability of AuNCs with sizes from ∼3 to ∼40 nm in three common BGMs: Tryptic soy broth, Luria-Bertani broth, and Mueller Hinton broth. We find that AuNCs prepared by standard methods, with citrate or citrate/tannic acid passivation, irreversibly flocculate in all studied BGMs. However, when the freshly synthesized AuNCs are subjected to a surface treatment with N'-2-hydroxy-ethylpiperazine-N'-2 ethane-sulfonic acid, they become stable in Tryptic soy broth at concentrations up to 500 μg of Au per mL of medium, for all studied NC sizes. Based on the results of surface enhanced Raman spectroscopy and Fourier transfer infrared studies, we attribute this to the formation of a protective layer on the surface of AuNCs, which prevents the formation of flocculation-inducing protein corona. A structural model explaining the effect is proposed. The stabilization procedure described here paves the way for systematic studies of interactions of AuNCs with bacteria.