Nanocrystalline Ag microflowers as a versatile SERS platform

Abstract

In this paper, the synthesis of Ag microflowers for use as manipulable and reusable substrates in surface enhanced Raman spectroscopy (SERS) is demonstrated, working with ultra-low volumes of the analyte. Flower-like AgBr crystallites with a growth direction of 〈110〉 were first obtained by thermolysing a complex obtained by the stabilization of (AgCl₂)⁻ anions with tetraoctylammonium bromide. NaBH₄ reduction leads to the formation of porous Ag microflowers (50–100 μm) with interconnected nanoparticles. The coupling of the nanoparticles in the microflower results in broadband extinction from visible to IR wavelengths, facilitating SERS using both red and green wavelengths. Using thiophenol as test analyte, uniform SERS enhancement factors in the range of 10⁶–10⁸ have been achieved from different parts of the microflower. The microflowers have been used for labeled and non-labeled detection of both single- and double-stranded DNA and using simple manipulation techniques, SERS data have been collected from ultra-low volumes of the analyte solution (∼0.34 nL). The reusability of the substrate for SERS over multiple cycles involving a rapid and efficient wet chemical cleaning procedure is also demonstrated. Finally, by placing the microflower in a microfluidic device, chemical reactions have been examined in situ.