New approach for time-resolved and dynamic investigations on nanoparticles agglomeration

Nanoparticle (NP) colloidal stability plays a crucial role in biomedical application not only for human and environmental safety but also for NP efficiency and functionality. NP agglomeration is considered as a possible process in monodispersed NP colloidal solutions, which drastically affects colloidal stability. This process is triggered by changes in the physicochemical properties of the surrounding media, such as ionic strength (IS), pH value, or presence of biomolecules. Despite different available characterization methods for nanoparticles (NPs), there is a lack of information about the underlying mechanisms at the early stage of dynamic behaviors, namely changing in NP size distribution and structure while placing them from a stable colloidal solution to a new media like biological fluids. In this study, an advancedin situapproach is presented that combines small angle X-ray scattering (SAXS) and microfluidics, allowing label-free, direct, time-resolved, and dynamic observations of the early stage of NP interaction/agglomeration initiated by environmental changes. It is shown for silica NPs that the presence of protein in the media enormously accelerates the NP agglomeration process compared to respective changes in IS and pH. High IS results in a staring agglomeration process after 40 min, though, in case of protein presence in media, this time decreased enormously to 48 s. These time scales show that this method is sensitive and precise in depicting the dynamics of fast and slow NP interactions in colloidal conditions and therefore supports understanding the colloidal stability of NPs in various media concluding in safe and efficient NP designing for various applications.

Unravelling the precipitation pathway of biorelevant minerals: from fundamental studies to biomedical applications

Agnese Carino

The formation pathway of biominerals archetypes, namely calcium carbonate (CaC) and calcium phosphates (CaPs), was investigated using a multidisciplinary approach. The time-resolved analytical data were collected using a controlled-composition method and interpreted using a computational tool. Some basic-science questions were identified arising from long-lasting debates on the details about the solid formation mechanism. Some of these questions are here addressed by the accurate experimental data collection, the application of new cutting-edge techniques, and the development of a rigorous thermodynamic-kinetic model for the entire solid formation process from solution. The time-resolved data were collected analysing the aqueous species in-situ. In parallel, the amount of chemicals added into the reactor, to maintain iso-pH conditions, was mathematically correlated to the bonded carbonate or phosphate ions, allowing the collection of two independent series of data and the assessment of both the cation and the anion during the solid formation process. This analytical approach went beyond the state-of-the-art. The complex mechanism was partitioned into its sub-processes, and a new strategy was identified to indirectly disclose the nature of the building units involved in the solid growth. Some assumptions embedded in the derivation of the nucleation theory did not compromise the theoretical validity of the framework, and the thermodynamic quantities expressed in the formulation have to be properly related to the nucleating phase and not extended as intensive properties of the solid. As main scientific results, for both CaC and CaPs systems, the experimental data are consistent with a classical nucleation and crystallisation mechanism, where primary, secondary, and diffusion limited growth are taken into account. The associated thermodynamic and kinetic quantities, such as the surface energy for primary and secondary nucleation as well as the nucleation and growth rate are reported and critically discussed in this dissertation. The models allowed detailed investigations in the pre-nucleation zone with the aim of evaluating the presence of clusters at controlled T, pH, ionic strength, and cation/anion ratio against saturation level. An experimental setup was developed to study clusters in-situ: a reactive horizontal micrometric pulsation-free liquid jet was investigated with a focused synchrotron X-ray beam, and the small angle scattering signals were collected for both CaC and CaPs systems. Preliminary results confirm the presence of clusters and attempts to identify their size distribution are ongoing. The agreement between experimental data, standard and cutting-edge analytical investigations, and modelling results allowed the definition of a plausible solid formation pathway, for both the investigated systems, based on the nucleation theory framework, in which the plausible influence of clusters was assessed. Beside the basic-science activities, some applications in the biomedical field were also evaluated. Among them, the controlled growth of CaPs phase on pre-treated Ti implants, e.g., dental implants, was achieved. This application opened exciting perspectives toward the production and the commercialisation of implants characterised by a faster healing time and improved stability. The scientific results were published in peer-reviewed papers and an international patent application; two additional papers are under finalisation.

EPFL2018

In-situ Investigations on Gold Nanoparticles Stabilization Mechanisms in Biological Environments Containing HSA

Antonia Neels, Quy Ong Khac, Francesco Stellacci

Nanoparticles (NPs) developments advance innovative biomedical applications. However, complex interactions and the low colloidal stability of NPs in biological media restrict their widespread utilization. The influence of NPs properties on the colloidal stability for gold NPs with 5 and 40 nm in diameter with two surface modifications, methoxy-polyethylene glycol-sulfhydryl (PEG) and citrate, in NaCl and human serum albumin (HSA) protein solution, is investigated. This study is based on small-angle X-ray scattering (SAXS) methods allowing the in-situ monitoring of interactions in physiological conditions. The PEG coating provides high colloidal stability for NPs of both sizes. For 5 nm NPs in NaCl solution, a stable 3D self-assembled body-centered cubic (BCC) arrangement is detected with an interparticle distance of 20.7 +/- 0.1 nm. In protein solution, this distance increases to 21.9 +/- 0.1 nm by protein penetration inside the ordered structure. For citrate-capped NPs, a different mechanism is observed. The protein particles attach to the NPs surfaces, and an appropriate concentration of proteins results in a stable suspension. Cryogenic transmission electron microscopy (Cryo-TEM), UV-visible spectroscopy, and dynamic light scattering (DLS) support the SAXS results. The findings will pave the way to design and synthesize NPs with controlled behaviors in biomedical applications.

WILEY-V C H VERLAG GMBH2021

Molecular insights into the electrical double layer of colloidal oxide particles obtained through second harmonic scattering

Marie Bischoff

Photocatalytic applications play an essential role in the search for alternative energy sources and environmental decontamination techniques. It is of fundamental interest to understand on a molecular level the aqueous solid/liquid interface, where photocatalytic reactions occur, to improve the reaction efficiency.Surface charge, ion diffusion and adsorption are all elements that can have a significant impact on reaction rates and, in the case of nanoparticles, colloidal stability. However, not many experimental methods are able to probe the electrical double layer (EDL) and its peculiar properties, particularly when it comes to colloidal nanoparticles directly dispersed in solution. In this thesis we apply polarimetric angle-resolved second harmonic scattering (AR-SHS) to study the EDL of colloidal oxide particles in aqueous environments. The insulating oxide silica (SiO2) and the semiconducting metal oxide titanium dioxide (TiO2) are investigated and further compared. We show that from AR-SHS measurements, we can extract the surface potential of the particles with respect to bulk liquid and the surface susceptibility, as a measure of interfacial molecular orientation. This can be achieved without assuming a specific charge distribution in the EDL, and hence without employing a model such as the Gouy-Chapman (GC) or the Gouy-Chapman-Stern (GCS). Through electrophoretic mobility measurements, we additionally obtain the zeta potential. Knowing the surface potential, the surface susceptibility and the zeta potential enables us to establish a molecular-level picture of the oxide particle/liquid interface as a function of ionic strength and pH. First, we provide evidence that a compact layer of hydrated ions is formed at the 300 nm amorphous SiO2/aqueous interface at high pH and high NaCl concentrations (> 1mM). Computation of surface charge density values based on our measured surface potential values and using the GC and GCS models are in good agreement with literature values.Next, we study the EDL of 100 nm amorphous TiO2 particles as a function of NaCl concentration and of basic pH. Three regions, reflecting three different phenomena occurring as a function of ionic strength, can be identified. First, inner-sphere adsorption at the lowest concentrations, then formation of a diffuse layer of counterions and finally, accumulation of hydrated counterions near the interface. Similar regions are observed for 100 nm SiO2 particles as a function of NaCl concentration. The TiO2 surface is found to have a stronger affinity for Na+ ions than SiO2.We further evaluate ion-specific effects at the SiO2 and amorphous TiO2 colloidal aqueous interface. The addition of NaCl, RbCl or CaCl2 to the solution leads to relativedifferences in surface potential and in the evolution of the interfacial H-bonding network as a function of ionic strength, which reveal surface and cation- specific preferences for inner- and outer-sphere adsorption.Finally, we demonstrate that AR-SHS measurements as a function of pH can be used to determine the pKas of 100 nm anatase TiO2 particles. Our data reveals a correlation between the change in orientation of interfacial water molecules and the pKas of surface TiO2 groups. This work shows that AR-SHS is a powerful tool to uncover the EDL structure of colloidal oxide particles, thus contributing to a better understanding of interfacial processes important to a variety of (photo-)catalytic applications.