Synthesis and Characterization of Janus Gold Nanoparticles

When gold nanoparticles are coated with binary mixtures of dislike ligand molecules, separation in the ligand shell occurs; if the particles are smaller than a threshold size the separation is solely enthalpy driven leading to the spontaneous formation of Janus particles

Structural and magnetic studies of core-shell and hollow nanoparticles

Nanoparticles can self-assemble or be directed to assemble into ordered structures, as determined from balance of forces acting upon them. In Chapter I, the concept of self-assembly and directed assembly is introduced followed by a discussion on surface and interparticle forces. Externally applied fields are particularly useful in directed assembly, and can be magnetic, electric, or optical forces in origin. In this chapter, we investigate methods leading to the directed assembly of cobalt nanoparticle rings around gold nanowires (nano-rotaxanes), and show that both dielectrophoretic and electrophoretic forces can be used to create such nanoscale heterostructures. Surfaces and material interfaces often play a significant role in governing the magnetic properties of nanoparticles. In Chapter II, we study the magnetic behaviors of core-shell Fe@Fe3O4 and hollow Fe 3O4 nanoparticles and reveal an unusual exchange-bias effect related to interfacial frozen spins. Hysteresis measurements of core–shell particles at 5 K after field cooling exhibit a large loop shift associated with unidirectional anisotropy, whereas Fe3O4 hollow nanoparticles support much smaller shifts. Both core–shell and hollow particles exhibit sharp demagnetization jumps at low fields associated with a sudden switching of shell moments. Temperature-dependent magnetizations of core–shell particles at high field show a deviation between field cooling and zero field cooling curves below 30 K, suggesting the presence of frozen spins at the interface. These frozen interfacial spins play an important role in mediating the exchange coupling between the ferromagnetic core and ferrimagnetic shell. We have also explored several experimental conditions that affect the relative intensity of the interfacial frozen spins such as temperature, repeated measuring field cycling, and age-dependent oxidation. With respect to nanoparticle synthesis, transitional-metal nanoparticles are often prepared by solvothermal routes or by the reduction of ionic salts. In chapter III, we show that TOPO has the potential to mediate the solvothermal synthesis of Co nanoparticles via a redox coupling process. The viability of the redox coupling is established by using TOP to convert Co-oleate into Co nanoparticles. Co nanoparticles synthesized from this route can then be coated with a thin shell of iron oxide, and further transformed into hollow cobalt ferrite nanoparticles upon heating or irradiation by a TEM electron beam. The transformation is accompanied by an enlargement of particle size, and is affected by environmental factors. Magnetic studies revealed that hollow cobalt ferrite nanoparticles possess higher blocking temperatures than their parent core–shell Co@FexOy nanoparticles. In Chapter IV, ultrathin Au nanowires (d\u3c2 nm) with extremely high aspect ratio were synthesized by reduction of Au(III) in oleylamine (OAm), and carefully analyzed by a combination of HRTEM and STM studies. While the nanowires are highly crystalline with apparent growth along the \u3c111\u3e direction, some segments are marked with amorphous defects, which we postulate to be an ionic Au(I)-OAm complex. The growth mechanism is proposed as template nucleation and anisotropic growth, in conjunction with oriented attachment

Monovalent ion-mediated charge-charge interactions drive aggregation of surface-functionalized gold nanoparticles

Monolayer-protected metal nanoparticles (NPs) are not only promising materials with a wide range of potential industrial and biological applications, but they are also a powerful tool to investigate the behaviour of matter at nanoscopic scales, including the stability of dispersions and colloidal systems. This stability is dependent on a delicate balance between attractive and repulsive interactions that occur in the solution, and it is described in quantitative terms by the classic Derjaguin-Landau-Vewey-Overbeek (DLVO) theory, that posits that aggregation between NPs is driven by van der Waals interactions and opposed by electrostatic interactions. To investigate the limits of this theory at the nanoscale, where the continuum assumptions required by the DLVO theory break down, here we investigate NP dimerization by computing the Potential of Mean Force (PMF) of this process using fully atomistic MD simulations. Serendipitously, we find that electrostatic interactions can lead to the formation of metastable NP dimers at physiological ion concentrations. These dimers are stabilized by complexes formed by negatively charged ligands belonging to distinct NPs that are bridged by positively charged monovalent ions present in solution. We validate our findings by collecting tomographic EM images of NPs in solution and by quantifying their radial distribution function, that shows a marked peak at interparticle distance comparable with that of MD simulations. Taken together, our results suggest that not only van der Waals interactions, but also electrostatic interactions mediated by monovalent ions at physiological concentrations, contribute to attraction between nano-sized charged objects at very short length scales

Ion-mediated charge-charge interactions drive aggregation of surface-functionalized gold nanoparticles

Monolayer-protected metal nanoparticles (NPs) are not only promising materials with a wide range of potential industrial and biological applications, but they are also a powerful tool to investigate the behavior of matter at nanoscopic scales, including the stability of dispersions and colloidal systems. This stability is dependent on a delicate balance between electrostatic and steric interactions that occur in the solution, and it is described in quantitative terms by the classic Derjaguin-Landau-Vewey-Overbeek (DLVO) theory, that posits that aggregation between NPs is driven by hydrophobic interactions and opposed by electrostatic interactions. To investigate the limits of this theory at the nanoscale, where the continuum assumptions required by the DLVO theory break down, here we investigate NP dimerization by computing the Potential of Mean Force (PMF) of this process using fully atomistic MD simulations. Serendipitously, we find that electrostatic interactions can lead to the formation of metastable NP dimers. These dimers are stabilized by complexes formed by negatively charged ligands belonging to distinct NPs that are bridged by positively charged ions present in solution. We validate our findings by collecting tomographic EM images of NPs in solution and by quantifying their radial distribution function, that shows a marked peak at interparticle distance comparable with that of MD simulations. Taken together, our results suggest that not only hydrophobic interactions, but also electrostatic interactions, contribute to attraction between nano-sized charged objects at very short length scales

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

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

Synthesis of magnetic cobalt ferrite nanoparticles with controlled morphology, monodispersity and composition: the influence of solvent, surfactant, reductant and synthetic conditions

In our present work, magnetic cobalt ferrite (CoFe2O4) nanoparticles have been successfully synthesised by thermal decomposition of Fe(III) and Co(II) acetylacetonate compounds in organic solvents in the presence of oleic acid (OA)/oleylamine (OLA) as surfactants and 1,2-hexadecanediol (HDD) or octadecanol (OCD-ol) as an accelerating agent. As a result, CoFe2O4 nanoparticles of different shapes were tightly controlled in size (range of 4-30 nm) and monodispersity (standard deviation only at ca. 5%). Experimental parameters, such as reaction time, temperature, surfactant concentration, solvent, precursor ratio, and accelerating agent, in particular, the role of HDD, OCD-ol, and OA/OLA have been intensively investigated in detail to discover the best conditions for the synthesis of the above magnetic nanoparticles. The obtained nanoparticles have been successfully applied for producing oriented carbon nanotubes (CNTs), and they have potential to be used in biomedical applications

Thioether-Functionalized Quinone-Based Resorcin[4]arene Cavitands: Electroswitchable Molecular Actuators

The utility of molecular actuators in nanoelectronics requires activation of mechanical motion by electric charge at the interface with conductive surfaces. We functionalized redox-active resorcin[4]arene-quinone cavitands with thioethers as surface-anchoring groups at the lower rim and investigated their propensity to act as electroswitchable actuators that can adopt two different conformations in response to changes in applied potential. Molecular design was assessed by DFT calculations and X-ray analysis. Electronic properties were experimentally studied in solution and thin films electrochemically, as well as by X-ray photoelectron spectroscopy on gold substrates. The redox interconversion between the oxidized (quinone, Q) and the reduced (semiquinone, SQ) state was monitored by UV-Vis-NIR spectroelectrochemistry and EPR spectroscopy. Reduction to the SQ state induces a conformational change, providing the basis for potential voltage-controlled molecular actuating devices

Determination and evaluation of the nonadditivity in wetting of molecularly heterogeneous surfaces

The interface between water and folded proteins is very complex. Proteins have “patchy” solvent-accessible areas composed of domains of varying hydrophobicity. The textbook understanding is that these domains contribute additively to interfacial properties (Cassie’s equation, CE). An ever-growing number of modeling papers question the validity of CE at molecular length scales, but there is no conclusive experiment to support this and no proposed new theoretical framework. Here, we study the wetting of model compounds with patchy surfaces differing solely in patchiness but not in composition. Were CE to be correct, these materials would have had the same solid–liquid work of adhesion (WSL) and time-averaged structure of interfacial water. We find considerable differences in WSL, and sum-frequency generation measurements of the interfacial water structure show distinctively different spectral features. Molecular-dynamics simulations of water on patchy surfaces capture the observed behaviors and point toward significant nonadditivity in water density and average orientation. They show that a description of the molecular arrangement on the surface is needed to predict its wetting properties. We propose a predictive model that considers, for every molecule, the contributions of its first-nearest neighbors as a descriptor to determine the wetting properties of the surface. The model is validated by measurements of WSL in multiple solvents, where large differences are observed for solvents whose effective diameter is smaller than ∼6 Å. The experiments and theoretical model proposed here provide a starting point to develop a comprehensive understanding of complex biological interfaces as well as for the engineering of synthetic ones

Quantitative 3D determination of self-assembled structures on nanoparticles using small angle neutron scattering

The ligand shell of a nanoparticle remains difficult to resolve, as the available characterization methods provide only qualitative information. Here, the authors introduce an approach based on small-angle neutron scattering that can quantitatively reveal the organization of ligands in mixed-monolayer nanoparticles