Naturally Occurring Proteins Direct Chiral Nanorod Aggregation

Serum albumin can template gold nanorods into chiral assemblies, but the aggregation mechanism is not entirely understood. We used circular dichroism spectroscopy and scanning electron microscopy to investigate the role of protein identity/shape, protein/nanorod ratio, and surfactants on chiral protein–nanorod aggregation. Three globular proteinsserum albumin, immunoglobulin, and transferrinproduced similarly sized chiral protein–nanorod aggregates. In solution these aggregates exhibited circular dichroism at the plasmon resonance that switched direction at specific protein/nanorod concentration ratios. Our explanation is that the extent of protein crowding influences protein conformation and therefore protein–protein interactions, which in turn direct nanorod aggregation into preferentially left- or right-handed structures. The fibrous proteins fibrinogen and fibrillar serum albumin also produced chiral nanorod aggregates but did not exhibit a ratio-dependent switch in the circular dichroism direction. In addition, cetyltrimethyl­ammonium bromide micelles prevented all aggregation, providing compelling evidence that protein–protein interactions are crucial for chiral protein–nanorod aggregate formation. The protein-dependent variations in circular dichroism and aggregation reported here present opportunities for future chiral nanostructure engineering and biosensing applications.