Micron scale gold flowers as SERS substrates for the detection of rhodamine 6G

Abstract

Low symmetry gold nanomaterials gain interest with their diverse resonance modes stemming from their complex geometry. Their extinction sometimes extends to the near infrared (NIR) region, and this enables their diverse use as theranostic agents in nanomedicine. In this study, micron scale Au flowers with sharp tips were prepared with (4-carboxybutyl)triphenylphosphonium bromide as the additive. These Au microflowers were grown at 70 °C, which are not normal synthetic conditions for low symmetry gold nanomaterials because of their tendency to be organized into high symmetric materials at elevated temperatures. FTIR suggests that there is a possible coverage of triphenylphosphonium bromide on Au surfaces, and these microflowers show NIR-resonant absorption spectra. With these Au microflowers as the substrates, SERS signals of rhodamine 6G were observed with the enhancement factors over 10⁶. An intense peak at 608 cm⁻¹ was also found, which is not usually observed with 785 nm laser light and which is contributed by vibronic coupling or charge transfer effect. Overall enhancement, though, is thought to be due to the scattering from Au microflowers by the electromagnetic field enhancement around the tips. There are various metal oxides with sharp tips in the nanoscale or micron scale with the terms of urchins, stars, or flowers, but metals with sharp tips in the micron scale such as Au microflowers in this study are rarely found.