Engineering plasmonic nanomaterials for SERS applications

Abstract

Gold and silver nanoparticles show exceptional chemical, physical, and electrical properties. Most notably, they are capable of manipulating the light-matter interaction at the nanoscale. Their unique optical properties mainly arise from the generation of collective oscillations of the free conduction electrons, better known as localized surface plasmon resonances. Metal nanoparticles ability to absorb and concentrate the incident electromagnetic field at the nanostructure has been used in a variety of applications, including those based on the so-called plasmon-enhanced spectroscopies such as surface-enhanced Raman scattering (SERS) spectroscopy SERS combines the inherent structural specificity and high experimental flexibility of Raman spectroscopy with the extremely high sensitivity provided by the plasmonic-mediated enhancement. As a result, SERS spectroscopy emerged as an ultrasensitive analytical tool that has been continuously expanding its range of applications in several fields, including sensing and biosensing, biology, medicine, environmental monitoring, food safety, and catalysis, among others. In particular, further improvements in the engineering of low-cost, robust, sensitive substrates are of utmost importance to further boost the translation of SERS-based analytical tools into competitive, commercially viable applications. In this regard, it is also important to expand the molecular library of surface ligands that can equip the SERS platform with the required properties for a given application (e.g., selectivity, sensitivity, biocompatibility, stability, etc.). This dissertation aimed at strengthening the fundamental knowledge about the design and fabrication of advanced plasmonic materials with a special focus on their application as SERS platforms. Specifically, it was first carried out a systematic study on the cold-welding process of silver nanoparticles onto gold substrates that provides important insights into the role of different experimental parameters in determining the generation of well-defined bimetallic structures that retain the original gold substrate morphology. Indeed, the fabrication of bimetallic Au/Ag nanostructures via spontaneous nanowelding at room temperature remains to these days a relatively unexplored route to generating novel plasmonic materials with unique features that otherwise cannot be obtained via conventional chemical reductions of metal salts in solution. Secondly, a novel azobenzene derivative (AzoProbe) was successfully designed to yield highly SERS efficient and colloidally stable silver nanoparticle clusters for the sensitive detection and quantification of clinically relevant low molecular weight thiols. This offers a novel approach for engineering a highly efficient, low-cost platform for the SERS sensing of this class of biothiols which are highly active substances extensively involved in human physiology and their abnormal levels have been associated with various diseases. Finally, it was reported the production of a hybrid graded-index (GRIN) lens metallic metamaterial through the electrostatic assembly of gold nanoparticles onto a micrometric silica core. Metamaterials constitute a new frontier for sciences such as physic, material science, engineering, and chemistry thanks to the unique properties of these materials. Herein, a new experimental protocol was devised for fabricating a highly hierarchized metamaterial comprising a silica core covered by a total of 32 layers of densely packed gold nanoparticles of different sizes. Such a complex material experimentally reproduces the optical behaviour of a theoretically calculated graded-index (GRIN) lens. Most notably, the SERS intensity displays a particular pattern that is consistent with the concentration of the light at the center of the microsphere. In summary, this doctoral thesis provides contributions to the advancement of bimetallic plasmonic design and fabrication. Moreover, it expands the capability of SERS-based sensing at detecting and quantifying clinically relevant biomolecules, such as low molecular weight thiols, in complex fluids. Finally, it provides a novel experimental protocol to synthesize a graded-index (GRIN) lens metallic metamaterial using a micrometric silica core and ensembles of gold nanoparticles of different sizes.