THIOL-FUNCTIONALIZED GOLD AND SILVER NANOPARTICLES USING MIXED LIGANDS: A CLOSE LOOK AT THE ATOMIC STRUCTURE AND CHEMICO-PHYSICAL PROPERTIES BY SR-XPS AND SERS

Abstract

In recent years the field of Nanotechnology has shown increasing prominence in the scientific, regulatory, and public spheres. The development of nanosized systems (with dimensions in the range of 10-9 ÷ 10-7 m) has been improved more and more, exploring all the possible applications of nanostructured materials, from medicine to electronics to cultural heritage. Nanometric size and chemical versatility are the fundamental parameters that justify the huge nanomaterials diffusion. Among the different nano-sized systems, Noble Metal Nanoparticles (MNPs) are a very useful tool in the field of biotechnology, thanks to their peculiar properties. In recent years, MNPs have been used for diagnostic and therapeutic purposes, as well as for microscopy, bioimaging and immunology. Nanometric dimensions promote the nanoparticles use as carriers for delivering therapeutics to disease sites, opening up new therapeutic strategies in cancer therapy or antimicrobial treatments. Besides Drug Delivery, nanomaterials applications are emerging in anti-tumoral therapies, as for example Plasmonic Photo-Thermal Therapy (PPTT), Photodynamic Therapy (PDT) and Radiotherapy (RT). This PhD project focuses on the study of the structural, electronic and morphological properties of MNPs for innovative applications in nanomedicine and nano-biotechnology. A key point for the technological development of nano-structured materials and practical biomedical devices based on MNPs is the achievement of a fine control of the stability and toxicity of the system. The chemical functionalization of metal nanoparticles can be done by grafting appropriate organic ligands to metallic clusters surfaces. The molecule-capping method provides a reliable control of particle composition, shape and size distribution making the functionalized MNPs suitable for applications in the field of catalysis, nanoelectronics, sensing and bioanalysis. To answer to the constant demand for more stable and functional nanoparticles, this PhD project provides a deep study concerning the structural characterization, by means of Synchrotron Radiation-induced X-ray Photoelectron Spectroscopy (SR-XPS) and Surface-Enhanced Raman Spectroscopy (SERS), of innovative noble metal (Au, Ag) nanoparticles, which can selectively enter tumors, act as suitable candidate for drug delivery, for diagnostic or radiosensing applications. The here presented Au and Ag nanoparticles functionalized with mixed organic ligands, the 3-mercapto-1-propansulfonate (3MPS) and the 2-diethylaminoethanethiol hydrochloride (DEA), were prepared with different metal/thiol stoichiometric molar ratios with the aim to modulate the properties of the functionalized nanoparticles. Au and Ag nanoparticles, composed of a metallic core and a ligand shell show a peculiar optical behaviour and provide a very powerful tool for biotechnological applications. The molecular overlayer has been selected on purpose for the biomedical applications, as well as to stabilize the nanoparticles in aqueous medium. SR-XPS data allow to achieve a deep understanding of the influence of the thiols stoichiometric ratio on the electronic properties and stability of functionalized noble metal nanoparticles. SR-XPS provides information on the local bonding environment of a given species, and it has been demonstrated to be an unique tool for investigating the nature of the interaction at the capping agent/metal nanoparticle interface, as well as the chemical structure of MNPs surface. The study of the interaction between light and metallic nanostructures is a rapidly emerging research area known as plasmonics. SERS is a highly specific molecular technique that uses plasmonics to obtain detailed chemical information about molecules or molecular assemblies adsorbed or attached to nanostructured metallic surfaces. This powerful technique is based on the enhancement of the Raman signal of molecules situated in the near vicinity of metallic nanostructures to obtain detailed information regarding the identity of those molecules, with sensitivities down to single-molecule level. SERS has many advantages over ordinary spectroscopic analytical techniques such as extremely high sensitivity, molecular selectivity, intense signals and great precision. Thus, this technique is well suitable for bioanalytical investigation applied to small molecules, proteins, DNA, and biologically relevant nanoparticles. Complementary information about the electronic, chemical and molecular structure can be obtained combining SERS and SR-XPS data, allowing to evidence structural similarities and differences related to the noble metal choice. These results coherently suggest the effective 3MPS and DEA chemical adsorption to the nanoparticle surface, which allows to obtain a stable and biocompatible system, suitable for ad hoc further functionalization as required for several applications in biotechnology. It was observed that the NPs molecular overlayer is made up by covalent bonds between thiols and metallic surface atoms, without degradation of the molecules. SERS spectroscopy was employed for the investigation of the NP molecular conjugation. SERS spectra acquired on the capped AgNPs and AuNPs are in very good agreement and suggest that the same organic layer is covering the NPs. The conjugation with 3MPS and DEA is confirmed by comparing SERS data with the conventional Raman signature of the ligands. The reproducibility of SERS measurements points out the high purity and homogeneity of the system and the good integrity of the organic capping layer. In terms of penetration depth and size-sensitivity, SERS and SR-XPS are complementary techniques; in fact, while SERS is specifically sensitive to the metal/thiol interface, SR-XPS, by selecting an appropriate photonenergy, can be used to probe the most external layers of a surface. Such complementarity was specifically used in this research work to investigate the molecular organization of the mixed thiols around Au and Ag nanoparticles. The SR-XPS and SERS measurements carried out on the here presented mixed-thiol functionalized MNPs pointed out interesting information concerning changes arising in the molecular overlayer configuration due to different 3MPS:DEA stoichiometric molar ratios. A careful data analysis leaded to a new interpretation of the interaction dynamics between the two thiols and the metallic atoms on the NP surface. Thanks to the here reported investigation, we can conclude that AuNPs/3MPS/DEA and AgNPs/3MPS/DEA can be prepared on demand, obtaining nanostructured materials that are chemically stable and soluble in aqueous solution; the reported proof of concept is a mandatory prerequisite for AuNPs/3MPS/DEA and AgNPs/3MPS/DEA applicability in the fields of biophotonics and nanomedicine, through ad hoc further functionalization. The information acquired by SR-XPS and SERS were successfully used to stress the structural similarities and differences in the proposed systems, that play a key role in their chemico-physical behaviour, with foreseeable implications in noble metal NPs applicability in different fields of nanomedicine. In this work, the different sensitivity to the molecular overlayer of SR-XPS and SERS spectroscopies were used to explore the potential synergy of these techniques in order to give new insights in the field of nanomaterials.