Even if nanomaterials have been produced for centuries, nowadays nanotechnologies offer us a wide range of new materials (such as superconductors, high-resistance materials, restoration products, and new analytical sensors). Gold and silver nanoparticles (NPs) have recently been used to increase the detection power of Raman spectroscopy—a spectroscopic technique that uses lasers to analyze molecules. In many cases, the low concentration of the molecules-of-interest makes it difficult to identify the characteristic signals. It is especially challenging to distinguish the characteristic signals from other types of emissions, mainly fluorescence, that can come from the sample. Metal NPs enhance those signals, allowing for their identification over the “noise” around them. This technique has been called Surface Enhanced Raman Spectroscopy (SERS).
SERS is a powerful tool for the identification of different materials present in cultural heritage, such as dyes. Dyes are organic, soluble colorants; meanwhile pigments are organic or inorganic solids, which usually are insoluble. In the paper published by Amato et al., the term “pigment” is used, even though both samples analyzed are actually dyes. The development of SERS for the identification of dyes in cultural heritage has been increasing. It has been successful for studying lakes, which are dyes absorbed into an inert salt that makes them insoluble (basically, the dye becomes a pigment!), so they can be used for painting. Also, these methods allow for the identification of dyes in manuscripts and inks.
As we know, non-invasive techniques have gained many followers in the last few years, since avoiding sampling represents a huge improvement that helps scientists preserve the integrity of objects and allows in-situ analysis. Scientists have used SERS as a tool for the challenging non-invasive study of dyes and lakes.
The SERS effect (the enhancement of the dye signal) is produced when the molecules are adsorbed on or close enough to the metal (Au or Ag) NPs. These particles create plasmon resonance, an electric interaction with the light of the laser, giving rise to a resonance with the signal of the molecule. The places where this happens are called “hot-spots.” If you are interested in learning more about this method, the book by Eric C. Le Ru and Pablo G. Etchegoin is a good start! If the interaction between the molecule and the NPs is fundamental, how can it be done in a non-invasive way? An interesting solution is presented by Amato et al.
Many of us are familiar with gels for cleaning your hands or setting your hairstyle. Their physical properties have been used in conservation mainly for cleaning paintings and documents. Typically, gels are used to reduce or control the interaction between the solvent inside the gel and the paint layers. The same principle has been exploited by these authors to develop a non-invasive SERS method using Ag NPs, both commercial and “homemade” (synthesized in the lab). The gel, in this case agarose gel, controls the interaction between the molecule of interest and the NPs, which are suspended in water inside the gel. Entrapping the NPs in a gel network makes it easier to remove the NPs from the object’s surface. Other methods apply the NPs directly onto the surface, which can produce stains. In this case, a partial dissolution and absorption of the dye into the gel occurs and the adsorbed molecules, after drying, interact with the NPs and give rise to the SERS effect (Figure 1). This methodology has been used before by Lofrumento, et al. and represents an improvement to previous research by Doerthy et al., which used cellulose film instead of agargel.
Amato and his collaborators studied two dyes, alizarin (a component of Madder dye) and Crystal Violet (more precisely named Basic Violet 3). Crystal violet is a synthetic dye commonly used in textiles and inks, which is still employed in commercial felt-tip pens today. They have tested their method on paper “dyed” with these two dyes. The results are promising, showing good capacity for the identification of the colorants without damaging the surface.
What is the innovation of this system? Studying dyes in cultural heritage is a challenging task, because they are commonly present in a very low concentration. Thus their identification is almost impossible using traditional Raman spectroscopy due to the strong fluorescence that obscures all the characteristic signals. Even if SERS methods are well known these days, a sample is often required or the application of NPs on the surface produces stains. The use of gel systems allows for the identification of the colorant, avoids surface damages, and keeps the advantages of the SERS technique.
Even though more tests should be done to set-up experimental parameters for complex samples, there is no doubt it is a promising improvement and a clever adaptation of the gel systems combined with the SERS technique. It is clear that this methodology can have a widespread application.
All figures adapted with permission from Simonpietro Agnello from the University of Palermo.