Engineering the morphology and organization of gold nanostructures for sers detection

Abstract

Since its discovery, Surface-enhanced Raman Scattering (SERS) has become one of the most powerful and intensively studied spectroscopic analytical techniques. The electric near-field enhancement created by illumination of metallic nanostructures provides SERS with the ability to overcome the main drawback of standard Raman scattering spectroscopy, namely its low sensitivity. Many efforts are therefore currently devoted toward the fabrication of high-performance, homogeneous and reproducible SERS substrates by means of the most advanced methods, both top-down and bottom-up. Metallic nanoparticles represent an attractive route to the design of SERS supports with suitable properties. Among all the available metals and related alloys, gold and silver are the principal materials of choice because of their special interaction with light. Applications of SERS spectroscopy are foreseen in a wide variety of fields like medicine, biology, forensic science, archaeology, pharmacy and others. The activity of the Bionanoplasmonics Laboratory at CIC biomaGUNE is focused on the chemical synthesis of Au and Ag nanoparticles with different sizes and shapes. The research in the group is also dedicated to the functionalization of the above-mentioned nanoparticles toward driving interparticle and particle-support interactions, to achieve control over self-assembly processes. Being able to tune morphology and organization allows the design of efficient SERS substrates since size, shape and interparticle distance are crucial parameters to create highly localized areas where the electric field is largely enhanced (hot-spots), leading to a suitable response of the substrate. The scope of this thesis lies within the search for innovative and efficient SERS substrates that may become useful in the identification and quantification of specific chemical species. With the objective of expanding the potential field of application, different strategies have been followed in this process. After a brief general introduction, the selected strategies are presented in the subsequent chapters. Each of these chapters describes the ideas behind the work, as well as the experimental set-up, results and conclusions. Key bibliographic references are also provided, which can guide interested readers toward acquiring a more complete knowledge of the subject. General considerations about the Raman effect and SERS spectroscopy are provided in Chapter 1, which are important for understanding the basic theoretical concepts behind these spectroscopy techniques. The chapter is also dedicated to discuss the optical properties of Au and Ag nanoparticles, as these are the most frequently used materials in SERS. A final section is aimed to give an overview on some common procedures to fabricate SERS substrates and a series of representative examples is discussed. Chapter 2 deals with a chemical method leading to the vertical growth of Au seeds from a glass substrate. The highly anisotropic growth mechanism results in closely packed Au nanowires, which present interesting plasmonic properties. The possibility to tune the plasmon band by changing the length and diameter of each nanowire allows the choice of the optimal morphology according to the targeted SERS application. A particular application is demonstrated for this system in the detection of analytes in the gas phase. Apart from efficiency, aspects like low cost and easy fabrication process are also important and should be taken into consideration during the development of a SERS substrate. In Chapter 3 standard industrial paper is used as a solid support for the adsorption of plasmonic nanoparticles. With no need for additional treatment of the paper support, the nanoparticles can be deposited until complete coverage of the paper fibers and directly used as a SERS support. In order to avoid problems related to the drying step, nanoparticles were deposited in the form of an ink, using a simple fountain pen. It is thus possible to obtain flexible SERS substrates where the nanoparticles are uniformly deposited into the desired pattern with no need of any expensive instrumentation or technique. One of the problems when dealing with colloidal nanoparticles is that their SERS efficiency strongly depends on the stabilizing agents used in the synthesis process. Surfactants with high affinity for the metal prevent the analyte from getting close to the nanoparticle surface, where the electric field enhancement responsible for SERS detection is present. In chapter 4 we introduce a process for replacing the stabilizing agent covering the nanoparticle surface to facilitate adsorption of analyte molecules. This method allowed us to compare several morphologies with identical surface chemistry as well as reaching a limit of detection in the nanomolar range. Apart from solid supports, colloidal solutions themselves can also be used as liquid substrates for SERS spectroscopy. This approach presents several advantages like reduced sample degradation or improved uniformity in the distribution of the analyte, and therefore represents a valid alternative to solid supports. We introduce in Chapter 5 a hybrid SERS substrate that combines the optical properties of Au nanoparticles and the magnetic response of magnetite nanocrystals. We were thus able to create a system whose SERS sensitivity can be increased by several orders of magnitude upon application of an external magnetic field. As a whole, this thesis is expected to contribute toward the development of novel plasmonic nanoplatforms as sensible and reliable SERS substrates for sensing applications in various field.