Synthesis, optical characterization and sensing application of gold nanostars

Resumo

Raman scattering is a powerful analytical technique for molecular analysis that allows molecular identification, detection, and quantification of analytes even in complex matrices. Such specific and sensitive molecular analysis is of key importance in many basic and practical fields of modern science and technology. Examples of such fields are medicine, pharmacy, biology, environmental monitoring, forensic science, archaeology, homeland security. In fact, in all the areas of research where molecular investigations are required, Raman scattering has proven extremely useful. Unfortunately, Raman scattering is characterized by extremely low cross-sections (efficiency). In fact, only one into every ten thousand photons is inelastically scattered. This low cross-section may be overcome, to some extent, by excitation the target molecule at a wavelength close to the isolated absorption lines of the molecule. This resonance effect provides an enhancement of the Raman cross section of few orders of magnitude. However, every molecular system is characterized by a different electronic absorption spectrum, ranging from the UV to the NIR and resulting, in practice, in the need of having a wide window of excitation lines. Further, by using Raman in resonance, the sensitivity of the scattering process is not high enough for the effective detection of trace analytes in the real life. Surface enhanced Raman scattering (SERS) is known to provide highly sensitive detection of molecular species, even down to the single molecule regime. The implementation of SERS in biosciences is becoming very popular as it provides significant information about the studied system even directly within complex environments such as biological fluids, living tissues and cells. SERS can be achieved and maximized by carefully controlling both electromagnetic and chemical effects, mainly through careful design of the optical enhancing substrates. Therefore, the preparation of optical platforms with optimized properties is a very dynamic field of research, and, as there is no universal best SERS platform, careful consideration of the analytical problem is required before choosing/designing a SERS sensor. The activity of the Colloid Chemistry Group, at the University of Vigo, is strongly focused on the size and shape control of gold and silver nanostructures, which are the principal metals used in SERS. Controlling the morphology provides a method to tune the optical and spectroscopic response of metallic nanostructures. Moreover, the importance of tailoring the morphology of the nanoparticles is related to the recently demonstrated observation that, in anisotropic particles, localized surface plasmon resonances (LSPRs) are not homogeneously distributed throughout the whole particle surface but give rise to a concentration of electromagnetic fields in several specific regions within the nanoparticle. Because of these characteristics, gold nanoparticles with anisotropic morphology are excellent candidates for SERS enhancement. In this context, our group has reported a simple strategy for the controlled synthesis of spiked gold nanoparticles with well-defined optical response. These anisotropic particles, known as nanostars, are extremely efficient as the core act as perfect nanoantennas while the spikes concentrate the electromagnetic field in a very small region at their apexes. Our main objective in this thesis has been to demonstrate the ability of gold nanostars to function as reproducible, stable and simple SERS-active platforms for sensing applications. In order to achieve this goal, we carried out a complete study of the optical and enhancing properties of gold nanostars"; subsequently, we tested their applicability in combination with SERS as an analytical method;" and finally, we examined their performance in environmental and biomedical assays. To illustrate the obtained results, this thesis has been structured in five chapters. Thus, a general overview of the Raman effect and SERS mechanism is provided in Chapter 1. In this chapter, we also explain the importance of electromagnetic field concentration in metallic nanostructures for the design of SERS substrates. In fact, this chapter provides the basic knowledge to understand the work presented within for subsequent chapters. Chapter 2 is dedicated to describe the synthesis and optical characterization of gold nanostars. We start with a brief introduction of the synthetic method used for the formation of simple gold nanostars, as well as their crystallographic study using high resolution transmission electron microscopy (HRTEM). On the basis of these previous results, we characterize the LSPR modes of single nanostars both experimentally and theoretically. Within this chapter, we also present a way to finely tune the LSPR response through the reshaping mechanism that these particles undergo in the presence of CTAB. In Chapter 3, we demonstrate the extremely strong concentration of electromagnetic field at the tip of gold nanostars. Because of this concentration, gold nanostars constitute an excellent and relevant alternative for their use in the so-called surface enhanced spectroscopies (SES). Thus, we discuss the development of new and simple ways to create large SERS enhancements of the order of 1010 by taking advantage of the high quantum confinement produced when the probed molecules are in between the tips of a gold nanostar and a planar metallic surface (sandwich configuration). This procedure yields enhancement factors higher than those previously reported using conventional hot spots. Our results pave the road towards high-yield, controllable, SERS-based ultradetection. On the basis of the results presented in Chapter 3, we studied the analytical applicability of gold nanostars. Thus, we describe in Chapter 4 their performance in common SES experiments, including SERS, surface enhanced resonance Raman scattering (SERRS), and surface enhanced fluorescence (SEF). Subsequently, we demonstrate the potential of the sandwich configuration for ultradetection of non-functionalized analytes. Finally, we describe a method for the remote detection of photosensitive molecules, using the sandwich configuration. This method is based on the generation of propagating plasmons on the gold smooth surfaces due to the presence of gold nanostars. Finally, Chapter 5 focuses on exploring the applicability of the outstanding optical properties of gold nanostars for two different biomedical assays. First, we demonstrate their use for the design of an enzyme-linked immunoassay that detects metabolites down to the attomolar (aM) regime, directly in complex fluids. Second, we take advantage of the large field enhancement generated at the tips of gold nanostars, for the fabrication of SERS-encoded particles for in vivo bio-imaging. The results of this thesis, and the impact on the field are then presented in a detailed conclusions chapter (Chapter 6). The impact on applications along with some future directions are also discussed. In the last part of this thesis few selected appendices are presented. A general synopsis, in Spanish, of this dissertation is also included at the end of the thesis as required by the regulation of Universidade de Vigo.