Self-Assembly of Iron Oxide-Poly(ethylene glycol) Core–Shell Nanoparticles at Liquid–Liquid Interfaces

Nanoparticles (NPs) play an increasingly important role in the fabrication of functional advanced materials. Two major steps need to be carried out in order to achieve control of the material properties. First of all, the properties of the single NPs have to be under control, especially in relation to colloidal stability; aggregation and corrosion negate all the benefits associated to the nanoscopic dimensions. Secondly, the assembly process has to be controlled to achieve a material with the desired properties. We propose here to use stabilized ceramic NPs consisting of a magnetite core, coated by a poly(ethylene glycol) (PEG) shell and study their assembly at polar/non-polar liquid interfaces, en route to fabricating functional NP membranes. These NPs show extraordinary stability in aqueous solutions achieved by anchoring linear PEG chains through an end-terminating nitroDOPA group to their surface. Furthermore, the core and shell sizes of these NPs can be independently varied with ease. We first describe the details of the NP synthesis and stabilization in bulk solutions, discussing the PEG molecular weight needed to achieve bulk stability. Subsequently, we demonstrate self-assembly of these particles at liquid–liquid interfaces (SALI) into monolayers of stable properties. SALI has been chosen as path for the assembly given its suitability for fabricating two-dimensional materials. We report here results from pendant drop tensiometry which illustrate the kinetics of NP adsorption at the liquid–liquid interface and highlight the role played by the molecular weight of the PEG shell in the interfacial assembly. In particular we show that the requisites to ensure particle stability at a liquid interface are more stringent compared to the bulk case

Membrane interaction of pegylated superparamagnetic nanoparticles

Iron oxide core-shell nanoparticles are gaining ever increasing interest for separation and imaging in biotechnology and biomedicine1,2, due to supposed low cytotoxicity and their superparamagnetic properties. Hydrophilic polymer-coated nanoparticles are believed to have low nonspecific interactions in biological systems, but much additional work in-vitro and in-vivo is needed to understand their detailed interactions with proteins, membranes and cells. We investigated monodisperse (SD\u3c5%), single-crystalline and superparamagnetic magnetite nanoparticles of different core size and densely grafted with poly(ethylene glycol) (Mw=5kDa), with particular emphasis on their interaction with biological membranes. Membrane interactions will determine nonspecific recognition and uptake by cells. These nanoparticles demonstrated no cytotoxicity and low cell uptake in in-vitro culture of HeLa and HEK cell lines. However, using Quartz Crystal Microbalance (QCM) a strong DLVO-type interaction could be demonstrated with anionic membranes that simulate eukaryote membranes. This interaction was only present in nonphysiological buffer with low ionic strength. Only low, weak and transient binding was observed to zwiterionic phosphocholine membranes. Core size seems to have an effect, with the smallest core size (3.3nm) yielding the strongest interactions while 8nm cores displayed almost no interaction. These results imply that dense polymer grafting and nanoparticle curvature are crucial parameters to control interactions between biomedical core-shell nanoparticles and their biomolecular environment, in particular cell membranes. The interaction between nanoparticle and membrane was furthermore shown to not perturb membrane structure by Differential Scanning Calorimetry (DSC). Please click Additional Files below to see the full abstract

Stabilization and functionalization of iron oxide nanoparticles for biomedical applications

Superparamagnetic iron oxide nanoparticles (NPs) are used in a rapidly expanding number of research and practical applications in the biomedical field, including magnetic cell labeling separation and tracking, for therapeutic purposes in hyperthermia and drug delivery, and for diagnostic purposes, e.g., as contrast agents for magnetic resonance imaging. These applications require good NP stability at physiological conditions, close control over NP size and controlled surface presentation of functionalities. This review is focused on different aspects of the stability of superparamagnetic iron oxide NPs, from its practical definition to its implementation by molecular design of the dispersant shell around the iron oxide core and further on to its influence on the magnetic properties of the superparamagnetic iron oxide NPs. Special attention is given to the selection of molecular anchors for the dispersant shell, because of their importance to ensure colloidal and functional stability of sterically stabilized superparamagnetic iron oxide NPs. We further detail how dispersants have been optimized to gain close control over iron oxide NP stability, size and functionalities by independently considering the influences of anchors and the attached sterically repulsive polymer brushes. A critical evaluation of different strategies to stabilize and functionalize core-shell superparamagnetic iron oxide NPs as well as a brief introduction to characterization methods to compare those strategies is given. © 2011 The Royal Society of Chemistry

Rupture Pathway of Phosphatidylcholine Liposomes on Silicon Dioxide

We have investigated the pathway by which unilamellar POPC liposomes upon adsorption undergo rupture and form a supported lipid bilayer (SLB) on a SiO2 surface. Biotinylated lipids were selectively incorporated in the outer monolayer of POPC liposomes to create liposomes with asymmetric lipid compositions in the outer and inner leaflets. The specific binding of neutravidin and anti-biotin to SLBs formed by liposome fusion, prior to and after equilibrated flip-flop between the upper and lower monolayers in the SLB, were then investigated. It was concluded that the lipids in the outer monolayer of the vesicle predominantly end up on the SLB side facing the SiO2 substrate, as demonstrated by having maximum 30–40% of lipids in the liposome outer monolayer orienting towards the bulk after forming the SLB

Synthesis of short-range ordered aluminosilicates at ambient conditions

We report here on structure-related aggregation effects of short-range ordered aluminosilicates (SROAS) that have to be considered in the development of synthesis protocols and may be relevant for the properties of SROAS in the environment. We synthesized SROAS of variable composition by neutralizing aqueous aluminium chloride with sodium orthosilicate at ambient temperature and pressure. We determined elemental composition, visualized morphology by microscopic techniques, and resolved mineral structure by solid-state ²⁹Si and ²⁷Al nuclear magnetic resonance and Fourier-transform infrared spectroscopy. Nitrogen sorption revealed substantial surface loss of Al-rich SROAS that resembled proto-imogolite formed in soils and sediments due to aggregation upon freezing. The effect was less pronounced in Si-rich SROAS, indicating a structure-dependent effect on spatial arrangement of mass at the submicron scale. Cryomilling efficiently fractured aggregates but did not change the magnitude of specific surface area. Since accessibility of surface functional groups is a prerequisite for sequestration of substances, elucidating physical and chemical processes of aggregation as a function of composition and crystallinity may improve our understanding of the reactivity of SROAS in the environment

Switching Transport through Nanopores with pH-Responsive Polymer Brushes for Controlled Ion Permeability

Several nanoporous platforms were functionalized with pH-responsive poly(methacrylic acid) (PMAA) brushes using surface-initiated atom transfer radical polymerization (SI-ATRP). The growth of the PMAA brush and its pH-responsive behavior from the nanoporous platforms were confirmed by scanning electron microscopy (SEM), Fourier transform infrared (FTIR) spectroscopy, and atomic force microscopy (AFM). The swelling behavior of the pH-responsive PMAA brushes grafted only from the nanopore walls was investigated by AFM in aqueous liquid environment with pH values of 4 and 8. AFM images displayed open nanopores at pH 4 and closed ones at pH 8, which rationalizes their use as gating platforms. Ion conductivity across the nanopores was investigated with current–voltage measurements at various pH values. Enhanced higher resistance across the nanopores was observed in a neutral polymer brush state (lower pH values) and lower resistance when the brush was charged (higher pH values). By adding a fluorescent dye in an environment of pH 4 or pH 8 at one side of the PMAA-brush functionalized nanopore array chips, diffusion across the nanopores was followed. These experiments displayed faster diffusion rates of the fluorescent molecules at pH 4 (PMAA neutral state, open pores) and slower diffusion at pH 8 (PMAA charged state, closed pores) showing the potential of this technology toward nanoscale valve applications

On the Formation of Supported Phospholipid Bilayers

Lipid bilayers are a central component of both the structure and function of biological membranes, like the one surrounding cells of living organisms. Since the lipid bilayer provides a host matrix for membrane proteins, which perform many of the specialized tasks in cells like energy, signal and material transduction, researchers have tried to develop artificial model membranes to study a variety of membrane protein functions. One of the most promising model systems during the last two decades have been the supported lipid bilayer, which is compatible with plenty of surface analytical tools and has many potential applications, including drug screening, medical diagnostics and non-fouling surfaces. Supported phospholipid bilayers (SPBs) are increasingly prepared through lipid vesicle fusion with surfaces. The process by which a planar membrane is formed from a spherical vesicle is still not fully understood. It provides an interesting case of molecular self-assembly, from a physics point of view, in addition to being important for preparing high quality membrane mimics for applications. In this work vesicle adsorption to several hydrophilic surfaces has been studied. However, most of the work is focused on phosphatidylcholine lipid vesicle adsorption and subsequent SPB formation on SiO2. In the course of determining the detailed kinetics of the vesicle-to-SPB transformation, new instrumentation and methods for data interpretations were developed, which are also presented in this thesis. A combination of quartz crystal microbalance with dissipation monitoring (QCM-D) and surface plasmon resonance (SPR) measurements was used to monitor the time-evolution of the vesicle and SPB abundance on the surface. These results were compared to atomic force microscope (AFM) measurements to get the microscopic distribution. It was possible to calculate the critical coverage of vesicles on the surface, required to trigger rupture. A bulk vesicle size and concentration dependent desorption of lipids to complete the SPB was also quantified. Single-technique, QCM-D, measurements of the influence of surface chemistry, osmotic pressure, lipid concentration, vesicle size and temperature were also performed. It was possible to find factors facilitating vesicle rupture and high-quality SPB formation (e.g. increased temperature and hypotonic vesicles), as well as factors inhibiting vesicle rupture and causing defects in the final SPB (e.g. low temperature and high concentrations). Combined with the QCM-D/SPR results, these measurements also made it possible to refine our understanding of the rupture mechanism. The presented results strongly indicate that SPB-formation is dominated by SPB-edge induced rupture of neighboring vesicles, where the interaction of vesicles in the solution with the edges seems to play a crucial role. Monte Carlo computer simulations were performed to find a mechanistic picture that could explain the observed temperature dependence of the SPB formation. A good correspondence between theoretical and experimental results was found, both qualitatively and quantitatively. Furthermore, in an experiment, where the outer monolayer of the vesicles was selectively tagged, it was shown that the opening and adsorption of the vesicle membrane follows a pathway leaving the outer monolayer primarily facing the substrate after SPB formation

On the Formation of Supported Phospholipid Bilayers

Lipid bilayers are a central component of both the structure and function of biological membranes, like the one surrounding cells of living organisms. Since the lipid bilayer provides a host matrix for membrane proteins, which perform many of the specialized tasks in cells like energy, signal and material transduction, researchers have tried to develop artificial model membranes to study a variety of membrane protein functions. One of the most promising model systems during the last two decades have been the supported lipid bilayer, which is compatible with plenty of surface analytical tools and has many potential applications, including drug screening, medical diagnostics and non-fouling surfaces. <p />Supported phospholipid bilayers (SPBs) are increasingly prepared through lipid vesicle fusion with surfaces. The process by which a planar membrane is formed from a spherical vesicle is still not fully understood. It provides an interesting case of molecular self-assembly, from a physics point of view, in addition to being important for preparing high quality membrane mimics for applications. <p />In this work vesicle adsorption to several hydrophilic surfaces has been studied. However, most of the work is focused on phosphatidylcholine lipid vesicle adsorption and subsequent SPB formation on SiO<font size="-1">₂</font>. In the course of determining the detailed kinetics of the vesicle-to-SPB transformation, new instrumentation and methods for data interpretations were developed, which are also presented in this thesis. A combination of quartz crystal microbalance with dissipation monitoring (QCM-D) and surface plasmon resonance (SPR) measurements was used to monitor the time-evolution of the vesicle and SPB abundance on the surface. These results were compared to atomic force microscope (AFM) measurements to get the microscopic distribution. It was possible to calculate the critical coverage of vesicles on the surface, required to trigger rupture. A bulk vesicle size and concentration dependent desorption of lipids to complete the SPB was also quantified. <p />Single-technique, QCM-D, measurements of the influence of surface chemistry, osmotic pressure, lipid concentration, vesicle size and temperature were also performed. It was possible to find factors facilitating vesicle rupture and high-quality SPB formation (e.g. increased temperature and hypotonic vesicles), as well as factors inhibiting vesicle rupture and causing defects in the final SPB (e.g. low temperature and high concentrations). Combined with the QCM-D/SPR results, these measurements also made it possible to refine our understanding of the rupture mechanism. The presented results strongly indicate that SPB-formation is dominated by SPB-edge induced rupture of neighboring vesicles, where the interaction of vesicles in the solution with the edges seems to play a crucial role. Monte Carlo computer simulations were performed to find a mechanistic picture that could explain the observed temperature dependence of the SPB formation. A good correspondence between theoretical and experimental results was found, both qualitatively and quantitatively. Furthermore, in an experiment, where the outer monolayer of the vesicles was selectively tagged, it was shown that the opening and adsorption of the vesicle membrane follows a pathway leaving the outer monolayer primarily facing the substrate after SPB formation