Micron Scale Gold Flowers as SERS Substrates for the Detection of Rhodamine 6G

Abstract

Low symmetry gold nanomaterials gain interests with their diverse resonance modes stemming from their complex geometry. Their extinction sometimes extends to 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 is not a normal synthetic condition for low symmetry gold nanomaterials because of their tendency to be organized into high symmetric materials at the elevated temperature. 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. 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 nanoscale or micron scale with the terms of urchins, stars, or flowers, but metals with sharp tips in micron scale such as Au microflowers in this study are rarely found.