Gold wrinkles boost SERS

Polydimethylsiloxane can be used to produce wrinkles on a glass surface to pattern lines of gold which are twice as effective as conventional SERS substrates, according to German and Spanish researchers.

Nicolás Pazos-Pérez, Alexandra Schweikart, and Andreas Fery of the Physical Chemistry Department, at the University of Bayreuth, in Germany and Weihai Ni, Ramón Alvarez-Puebla, and Luis Liz-Marzán of the Departamento of Química Física, and Unidad Asociada CSIC-Universidade de Vigo, in Spain, have developed a versatile method for making improved SERS substrates. The team produced a wrinkled strip of the silicon-based organic polymer, polydimethylsiloxane, and pressed it into a gold nanoparticle solution on a glass slide. Once they had evaporated off the solvent, the team removed the wrinkled "stamp" leaving a pattern of parallel lines of gold nanoparticles.

Surface-enhanced Raman spectroscopy (SERS) relies on a roughened metal surface to allow spectroscopists to boost the Raman signal sufficiently to produce useful vibrational fingerprints of a sample. Unfortunately, as anyone who has endeavoured to produce a uniform roughened surface will know it is common that preparative methods are commonly irreproducible, which means that quantitative applications of the ultra-sensitive technique are limited. Liz-Marzán says the team's new technique could pave the way for the large-scale development of highly sensitive quantitative SERS platforms.

The team explains that a wide variety of substrates has been engineered to optimize metallic nanostructures, such as gold and silver island films produced by physical vapour deposition, layer-by-layer assembly, electron beam lithography, nanoimprint lithography, and nanoparticle self-assembly. These have been used to enhance fields and even to undertake single molecule detection. But, hot spots formed in the SERS substrates made using the majority of these approaches means that signal intensity is not reproducible over a wide area, hence the limited application of the technique for quantitative experiments.

"Other interesting alternatives to control SERS intensity include the use of plasmon standing waves or Wood anomalies in gratings," the team says, "however, both methods sacrifice intensity in order to achieve reproducibility. Thus, to date, the closest solution to this problem is probably the use of nanosphere lithography coupled to film over nanospheres (NSL-FON), mainly developed by Van Duyne's Group." With all of this in mind, the team turned to the possibility of the organization of pre-synthesized nanoparticle colloids using modern lithographic techniques. This, they explain, "appears as a viable alternative, if both the size and shape of the particles, as well as their organization on the substrate can be properly controlled."

The team previously demonstrated that a wrinkled surface can act as an effective template for the controlled assembly of nanoparticles from a solution using either the dip coating or spin coating technique. The team thus created wrinkled surfaces (verified by atomic force microscopy) with wrinkles of different wavelengths (340 and 500 nm) and depths (23 and 77 nm, respectively) in the organosilicon polymer, using plasma oxidation of stretched polymer samples and their subsequent relaxation into a wrinkled state.

The team demonstrated proof of principle laser excitation at 633 nm on a substrate exposed to benzenethiol in the gas phase. A control experiment used a conventional physically evaporated gold island film. The team emphasises that they carried out both dark field optical characterization and also SERS mapping on the same specific areas of each substrate. "The SERS spectrum of benzenethiol, characterized by the CH bending and ring breathings, could be readily acquired from all points in the image," the team says. "Further, in both cases the intensity of the SERS signal was highly uniform, in strong contrast to that obtained for the gold island film."