Plasmonic gold assembliessynthesis, characterization and sensing applications

Resumo

Surface-enhanced Raman scattering (SERS) is a vibrational spectroscopic technique known to provide ultrasensitive detection and quantitative analysis of molecules located on or nearby a plasmonic surface, even down to the single-molecule regime. This analytical technique offers unambiguous identification by the characteristic Raman vibrational fingerprint, overcoming the inherently low efficiency of Raman spectroscopy. These features convert SERS into a promising tool for its implementation in diverse areas as biomedicine, environmental chemistry, analytical chemistry, and, overall, sensing applications. However, some key issues need improvement to maximise their application both in research and market, including careful control on the design and fabrication of plasmonic substrates. The SERS enhancement is strongly dependent on the electromagnetic field resulting from the excitation of the localised surface plasmon resonance (LSPR) present in plasmonic nanoparticles. Furthermore, this electromagnetic field is enhanced at the gap of nanoparticles assemblies (hot spot), especially if they form highly ordered nanostructures (supercrystal) due to collective antenna effects. For these reasons, the development of strategies to control the plasmonic nanoparticles assembly in random or orderly structures allows to design and fabricate plasmonic substrates with extremely high SERS efficiency for their employment in sensing. Bearing this in mind, the scope of this dissertation is to investigate the assembly of Au nanoparticles with different sizes and morphologies, on the one hand in colloidal solution and on the other hand in predesigned templates forming highly ordered nanostructures employing micro/millifluidic. Moreover, their use as sensing platforms based on SERS is also studied. This PhD thesis is framed in the context of the project entitled “Microfluidic-induced Hierarchical 3D assemblies of Metal nanoparticles for advanced catalytic and sensing applications” funded by the Spanish Ministry of Science, Innovation to the Functional NanoBiomaterials Group (former Colloid Chemistry Group). The FunNanoBio group has a long experience in the synthesis, characterisation, surface modification and assembly of plasmonic nanoparticles with well-defined and tunable optical properties, as well as their later application in catalysis or sensing (SERS and LSPR). The thesis is divided into four chapters. Chapter 1 is a general introduction about plasmonic nanoparticles and their optical properties, strategies to fabricate plasmonic assemblies and the resultant optical properties, a basic introduction of the different characterisation techniques employed in the thesis, and the basic principles of Raman scattering and SERS. This chapter is aimed to provide the basic knowledge that the reader will need to understand the work presented within the subsequent chapters. The remaining chapters describe the different studies carried out during the thesis and their major conclusions, organised in an introduction, results and discussion, conclusion, and experimental section. In addition, each of the experimental chapters includes an appendix with extra figures and discussions that can help the reader to follow the conclusions. Chapter 2 investigates the colloidal assembly of citrate-stabilised Au nanospheres induced by a bolaform surfactant with two polar heads. The characterisation of the Au nanoparticles assembly together with molecular dynamic simulations allows us to analyse the role of the bolaform surfactant in the process. The reproducibility and optical properties of the assemblies motivated us to develop a label-free millifluidic sensor based on SERS using colloidal Au sphere assemblies. Time-resolved UV-Vis and SERS spectroscopies are performed to optimise the sensor work conditions. The sensor efficiency, attributed to the fact that the target molecule is trapped inside the generated hot spots, is tested for detection of different drugs in terms of sensitivity, reliability, and quantification capabilities. Mathematical methods such as principal components analysis (PCA) or machine learning tools (in collaboration with Prof. Patón from UPM) are applied to improve the SERS analysis and demonstrate the possibility of multiplex detection. Finally, we investigate the integration of the system into a millifluidic chip to automate the whole assembly process. Chapter 3 is dedicated to the fabrication and characterisation of single-domain plasmonic supercrystal inside a microfluidic chip. Gold nanoparticles with different shapes and sizes nanoparticles are synthesised using wet chemical methods, and their assembly within the microfluidic platform is studied by scanning electron microscopy (SEM), small-angle X-ray scattering (SAXS), and SAXS-synchrotron. SAXS and SAXS-synchrotron characterisation were carried out in collaboration with Matière et rayonnement (Matrix) group at the Laboratorie Physique des Solides (LPS) in Orsay (France). Chapter 4 is devoted to evaluating the best Au supercrystal microfluidic platform obtained in Chapter 3 as a potential SERS sensor. One critical limitation of plasmonic supercrystals for their application is the low molecule infiltration ability. Therefore, three-dimensional SERS characterisation of the microfluidic sensor is performed to demonstrate the infiltration of analytes. Besides, SERS simulations using M3 frequency domain full-wave solver and large-scale supercrystal models are performed to investigate how the interparticle separation and disorder within the supercrystal affect the SERS efficiency. Different analytes with and without gold affinity, and even in a biological complex media, are employed in the study. Finally, the incorporation of a silica-based chromatographic unit in the plasmonic microfluidic chip is performed and the charge selective capability of the sensing platform is investigated. Finally, general conclusions are summarised as a final section at the end of the thesis. Also, a brief summary in Galician is included to fulfil the requirements established by the regulation of the University of Vigo. Altogether, this thesis is expected to improve the conventional SERS sensing systems based on gold nanoparticles assembly.