Growing Symmetrical Gold Nanostars

Chemists can now make symmetrical and consistently shaped gold nanostars (J. Am. Chem. Soc. 2015, DOI: 10.1021/jacs.5b05321).

Symmetrical or not, nanosized gold nuggets with jutting arms are useful for selectively killing cancer cells and boosting surface-enhanced Raman spectroscopy (SERS) signals. Yet previous nanostar-building methods create particles with wildly asymmetrical shapes, leading to inconsistent performance in SERS or in cancer therapies.

So Xianmao Lu’s group at the National University of Singapore made symmetrical nanostars by starting with a known synthesis for symmetrical gold icosahedral seeds, each with 20 faces (J. Phys. Chem. C 2008, DOI: 10.1021/jp7109498). Then they place the gold kernels in a solution containing dimethylamine (DMA). DMA binds with the gold surface and helps control the growth of six-sided pyramids on each face, forming 20 symmetrical spikes. The method results in consistently shaped nanostars; the team estimates that 95% have the same geometry.

The unique symmetry of these nanostars allows five of its arms to land on a flat surface. The researchers then tested their performance in SERS. First the team scattered 4-mercaptobenzoic acid, a standard SERS reference compound, on a gold nanosheet and then added the stars. As the stars landed on the flat sheet, they trapped the analyte molecules inside their tips and greatly magnified the signal.

SERS signals enhanced by symmetric nanostars are nearly four times as strong as signals enhanced by asymmetric ones. The equal-sided stars offer more reproducible results. The SERS enhancement from symmetric nanostars varied within one order of magnitude, while results from their asymmetrical cousins varied within two orders of magnitude. Such predictability means they’re well-suited for helping researchers study how nanostars absorb and scatter light, Lu says.