Metal nanostructures decorated with silver nanostars: a novel highly efficient SERS substrate for dyes and pigments detection

The detection of artistic molecules such as dyes and/or pictorial binders employed in the field of Cultural Heritage requires more and more sensitive techniques. The surface enhanced Raman spectroscopy (SERS) represents a valuable option due to its ability to reveal molecules adsorbed or bound onto nanostructured interfaces. In order to maximize the enhancement of the Raman effect, different Ag and Au nanomaterials where bound together to produce hierarchical structures. In particular, silver nanostars (AgNSs) were chosen because of the strong enhancement caused by their anisotropic shape and they were prepared by one-pot chemical synthesis [1]. The AgNSs were used to decorate two different gold nanostructures, namely Au nanowires (AuNWs) and sphere segment void (SSV) structured surfaces. The AuNWs were obtained in the shape of ensembles of nanoelectrodes by templated elecrtroless deposition in nanoporous membranes [2,3] while the SSV substrates were prepared via gold electrodeposition by using a monolayer of polystyrene spheres as template [4]. The enhancement of the Raman effect was tested at first through the employment of benzenethiol, a well known Raman probe. Thereafter, cochineal lake-pigment, in particular, was chosen as target owing to its use throughout the history of art. Lakes are complex coloring pigments in which an organic dye (e.g. cochineal, madder or kermes) is adsorbed onto solid particles of inorganic salts. This composite structure, composed by an organic molecule chelated by metallic ions, makes the Raman detection of lakes extremely challenging. However, the described issue is overcome by employing highly sensitive SERS substrates as those prepared in the context of this thesis. We compared the magnitude of the enhancements obtained with different SERS substrates: standing alone AgNSs, AuNWs ensembles and SSV substrates and hierarchical nanostructures composed by AgNSs combined to AuNWs (AgNSs@AuNWs) and to SSV substrates (AgNSs@SSV). Particularly strong enhancements were recorded in the case of the AgNSs@SSV substrates, showing the high potential of the so called particle-in-cavity (PIC) architectures [5]. Concluding, we synthesized highly effective hierarchical nanostructures whose future employment lies in the production of extremely sensitive SERS biosensors to be applied in the detection of artistic dyes and binders. References [1] García-Leis A., García-Ramos J. V., Sánchez-Cortés S., J. Phys. Chem. C, 2013, 117, 7791. [2] De Leo M., Kuhn A., Ugo P., Electroanal., 2007, 19 , 227. [3] Ugo P., Moretto L. M., Template Deposition of Metals, Ed. C. Zosky, Handbook of Electrochemistry, chapter 16.2, Elsevier, Amsterdam, NL, 2007, 678. [4] Mahajan S., Cole R. M., Soares B. F., Pelfrey S. H., Russell A. E., Baumberg J. J., Bartlett P. N., Phys. Chem. C, 2009, 113 (21), 9284. [5] Speed J. D., Johnson R. P., Hugall J. T., Lal N. N., Bartlett P. N., Baumberg J. J., Russell A. E., Chem. Commun., 2011, 47, 6335.