METAL NANOPARTICLES FOR SENSING (II): The role of metal nanoparticle surface in the development of SERS or SEF applications.
As we mention in our previous post (GOLD NANOPARTICLE FOR SENSING (I)), the interaction between light and gold nanoparticle surface causes a collective oscillation of the electronic cloud of the metal leading also to the generation of strong electric fields on the nanoparticle surface. However, besides gold nanoparticles, there are other nanomaterials such as silver nanoparticles or alloyed gold silver nanoparticles which show typical LSPR bands in the visible range.
The generation of these strong electric fields on the metallic nanoparticles surface can be useful in the development of different sensing approaches, one example of that is Surface Enhance Raman Scattering (SERS). SERS1 is based in the increase of Raman dispersion intensity of a molecule in the presence of a nanometallic structure. When a Raman active molecule is absorbed over a metallic surface, an enhancement in the intensity of Raman lines by a factor of 5 or 6 orders of magnitude takes place. Generally speaking, the metal surface favours the promotion of electron from the fundamental level to the excitated level, therefore an increase in the Raman intensity signal is observed. In this sense, metallic nanoparticles are an excellent platform to immobilize Raman active molecules. The information extracted from Raman spectra is quite similar to IR spectra, although it should be taken into account that water molecules do not show signal in Raman Spectroscopy, which is advantage when we are working with an aqueous media. Moreover, the progress in the synthetic procedures of metallic nanoparticles has allowed the improvement of these types of methodologies. Usually silver nanoparticles exhibit stronger Raman enhancement, so they are more adequate to be employed in SERS applications.
Despite the fact that the synthesis of silver nanoparticles is a well-know and easy procedure, an appropriate control over the size of the nanoparticle to get uniform nanoparticle in size is still a challenging task, and sometimes difficult to perform in a reproducible and scale-up way, besides the nanoparticle surface coverage is a key parameter in SERS applications. Most of synthetic routes to obtain silver nanoparticles imply the use of significate quantities of surfactant or polymers to get good quality particles. However, this type of capping agents are frequently difficult to remove or displace from the nanoparticle surface. The presence of these capping agents arising from the synthesis procedure usually limits the applications of metallic nanoparticles for SERS biosensing, since these molecules avoid the adsorption of Raman active molecule or even interfere in the analysis. In this line Nanovex Biotechnologies metallic nanoparticles provides high quality metallic nanoparticles with high purity, surfactant or polymer free.
Apart from the use of metallic nanoparticles as basic platform to enhance the Raman signal, metallic nanoparticles could be also employed as fluorescence enhancers. This approach is known as Surface Enhanced Fluorescence2 (SEF) or Metal Enhanced Fluorescence (MEF). Moreover, taking again advantage of the interaction between the electron cloud in the nanoparticle surface and the presence of a specific fluorophore next to metal nanoparticle surface some limitations of intrinsic fluorescence could be overcome. Fluorescence is a sensitive technique, although this sensitivity is limited sometimes due to the lack of photostability of luminescence compounds. SEF is a valuable tool to enhance the optical properties of conventional fluorophores, although in addition to the increase in the sensitivity of the fluorescence detection it is useful to solve the lack of photostability of some fluorophores. Once more in this approach the state of the metal nanoparticle surface and distance between nanoparticle and surface are some of the key parameters to develop a successful application.
Considering the crucial role of metallic nanoparticle surface in the development of these sensing approaches among others, Nanovex Biotechnologies offers several types of metallic nanoparticles (gold nanoparticles, silver nanoparticles and alloyed gold silver nanoparticles) showing LSPR bands in a wide range of the visible spectra
- Ríos Castro A., Moreno Bondi M.C., Simonet Suau B. M., Spectroscopic techniques in analytical chemistry, Vol. I, Chapter 10 (Ed. Sintesis).
- Asselin J., Legros P., Gregorie A., Boudreau D., Correlating metal-enhanced fluorescence and structural properties in Ag@SiO2 core-shell nanoparticles. Plasmonics 11 (5) 1369-1376 2016.