Hydrophilic colloids meet inorganics: synthesis and functionalization of metal and metal oxide nanoparticles and nanohybrids for biotechnology and sensing studies

This Ph.D. thesis focused on the synthesis and characterization of functionalized metal nanoparticles (MNPs, M = Au, Ag, Pd), metal oxide nanoparticles (MOxNPs, i.e. TiO2NPs and Fe2O3NPs) and related nanohybrids (TiO2-Ag and Fe2O3-Ag). In particular, efforts were devoted to the development of multifunctional nanoplatforms useful in sensing and biotechnology studies. The control of the size, morphology and properties of MNPs, MOxNPs and related hybrid systems, was carried out by wet chemical methods, i.e., starting from the reduction of metal precursor or by coprecipitation method. The versatility of these materials, especially in modulating physicochemical properties and relative colloidal stability in aqueous or organic media, was achieved choosing hydrophilic and hydrophobic thiols, yielding isolated or interconnected nanoparticles. In detail, hydrophilic AgNPs, both covalently functionalized with a 3-mercapto-1-propanesulfonate (3MPS) thiol layer and non-covalently with a synthetic polyacrylic acid (PAA) polymer were evaluated. Long-term colloidal stability was assessed by spectroscopy investigations, achieving high stability in the case of AgNPs-3MPS over one month. These two systems were evaluated for two distinct applications: AgNPs-3MPS as potential tool to enhance the mycorrhization process in plant growth, whereas AgNPs-PAA as optical sensor for heavy metal detection in water. Experimental results showed that both systems exhibited excellent performance in their respective applications. Titania nanoparticles (TiO2NPs) were selected for the loading of an anticancer prodrug, 5-aminolevolinic acid (ALA), because of the importance of this nanocomposite in photodynamic cancer therapy. The conjugation was studied in three different pHs (3, 7, 9) and a drug loading of η% = (15 ± 1) % was achieved. SR-XPS results on the TiO2NPs-ALA conjugates revealed the stabilizing effect of TiO2NPs onto the chemical structure of attached-ALA against degradation, at different pHs conditions, i.e., pH 7 and 9. Subsequently, the synergy of metal oxide NPs, i.e., TiO2NPs or Fe2O3NPs, with AgNPs was achieved through a bifunctional silane linker, MPTMS, bearing –O and –SH moieties in its opposing sites, to mediate the covalent attachment of TiO2- or Fe2O3- to AgNPs and enhance their colloidal stability as well. ICP-OES analysis confirmed the tunable decoration of AgNPs obtained by varying the stoichiometric metal oxide/AgNO3 weight ratios during the synthesis process and tested their advanced applications. For instance, TiO2-Ag nanohybrid was selected for antibacterial studies on E. coli and S. aureus strains and EPR studies allowed to understand the action mechanism. The results revealed a synergistic and photoactive antibacterial behavior antibacterial effect of the TiO2-Ag nanohybrid, which exceeds the efficacy of the individual components, even at extremely low dilutions with high antibacterial stability over one month. Additionally, this system was designed as a resistive sensor for toxic gas detection, indicating that the TiO2-Ag nanohybrids exhibited high sensitivity toward H2S gas at room temperature. Finally, TiO2-Ag nanohybrid was tested as a new label for lateral flow assay studies at the Catalan Institute of Nanoscience and Nanotechnology (ICN2), under the supervision of Prof. Arben Merkoçi. In parallel, the Fe/Ag nanohybrid synthetized and studied within the framework of nanoprimes and germination tests, showed an increase in germination rate after the application of the Fe/Ag nanohybrids at really low concentrations (0.01 – 1) ppm compared to the controls. Finally, hydrophobic MNPs, in particular Pd-, Au- and AgNPs stabilized with bifunctional and organometallic binding thiols were also studied in this work. Palladium nanoparticles (PdNPs) networks stabilized with different bifunctional thiols, were deposited onto interdigitated electrode and tested as resistive sensor for the detection of volatile organic compounds (VOCs). The electrical measurements and sensing response towards BTX (benzene, toluene, and p-xylene) and common interfering gases were investigated. In parallel, AgNPs, AuNPs and PdNPs, were appropriately functionalized with rod-like organometallic dithiol containing square-planar Pt(II), i.e., Pt-DEBP, and were structurally characterized by means of SR-XPS and TEM experiments. These systems can be exploited for their use in optoelectronics applications.