Steric Stabilization of γ-Fe2O3 Superparamagnetic Nanoparticles in a Hydrophobic Ionic Liquid and the Magnetorheological Behavior of the Ferrofluid.

Hydrophobic ionic liquid ferrofluids (ILFFs) are studied for use in electrospray thrusters for microsatellite propulsion under nonatmospheric and in high-temperature environments. We synthesized a hydrophobic ILFF by dispersing sterically stabilized γ-Fe2O3 nanoparticles (NPs) in the ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide. A diblock copolymer, C4-RAFT-AA10-DEAm60, was synthesized to facilitate multipoint bidentate anchoring to the NP through the acrylic acid block. The DEAm60 layer was incorporated to generate steric repulsion between particles to protect against the aggregation of magnetized particles arising from dipole-dipole attraction. The effect of shearing and variation in the magnetic field strength on the steric repulsion was examined using the DLVO theory. The effect of varying the magnetic field strength and particle concentration on the viscoelastic properties of the ferrofluid was evaluated using rheometry. The viscosity of the ferrofluid increased with the magnetic field strength, indicating that the magnetized particles assembled into a structure. The level of straining required to break down the structure formed by the magnetized particles increased with the magnetic field strength and particle concentration. The absence of particle interlocking during shearing was indicated by the smooth viscosity versus shear rate traces. The DLVO analysis showed that increasing the magnetic attraction between the particles causes the DEAm60 brush layers on the particles to overlap more, resulting in an increase in the steric repulsion. As overlapping increases, osmotic repulsion is caused before progressing to a strong elastic repulsion. The effect of the polymer solubility and particle interaction due to hydrodynamic forces on the steric repulsion was also analyzed.

The interaction of sterically stabilized magnetic nanoparticles with fresh human red blood cells.

Sterically stabilized superparamagnetic iron oxide nanoparticles (SPIONs) were incubated with fresh human erythrocytes (red blood cells [RBCs]) to explore their potential application as magnetic resonance imaging contrast agents. The chemical shift and linewidth of (133)Cs(+) resonances from inside and outside the RBCs in (133)Cs nuclear magnetic resonance spectra were monitored as a function of time. Thus, we investigated whether SPIONs of two different core sizes and with three different types of polymeric stabilizers entered metabolically active RBCs, consuming glucose at 37°C. The SPIONs broadened the extracellular (133)Cs(+) nuclear magnetic resonance, and brought about a small change in its chemical shift to a higher frequency; while the intracellular resonance remained unchanged in both amplitude and chemical shift. This situation pertained over incubation times of up to 90 minutes. If the SPIONs had entered the RBCs, the intracellular resonance would have become broader and possibly even shifted. Therefore, we concluded that our SPIONs did not enter the RBCs. In addition, the T 2 relaxivity of the small and large particles was 368 and 953 mM(-1) s(-1), respectively (three and nine times that of the most effective commercially available samples). This suggests that these new SPIONs will provide a superior performance to any others reported thus far as magnetic resonance imaging contrast agents.

Self-assembling array of magnetoelectrostatic jets from the surface of a superparamagnetic ionic liquid.

Electrospray is a versatile technology used, for example, to ionize biomolecules for mass spectrometry, create nanofibers and nanowires, and propel spacecraft in orbit. Traditionally, electrospray is achieved via microfabricated capillary needle electrodes that are used to create the fluid jets. Here we report on multiple parallel jetting instabilities realized through the application of simultaneous electric and magnetic fields to the surface of a superparamagnetic electrically conducting ionic liquid with no needle electrodes. The ionic liquid ferrofluid is synthesized by suspending magnetic nanoparticles in a room-temperature molten salt carrier liquid. Two ILFFs are reported: one based on ethylammonium nitrate (EAN) and the other based on EMIM-NTf2. The ILFFs display an electrical conductivity of 0.63 S/m and a relative magnetic permeability as high as 10. When coincident electric and magnetic fields are applied to these liquids, the result is a self-assembling array of emitters that are composed entirely of the colloidal fluid. An analysis of the magnetic surface stress induced on the ILFF shows that the electric field required for transition to spray can be reduced by as much as 4.5 × 10(7) V/m compared to purely electrostatic spray. Ferrofluid mode studies in nonuniform magnetic fields show that it is feasible to realize arrays with up to 16 emitters/mm(2).

The composition and end-group functionality of sterically stabilized nanoparticles enhances the effectiveness of co-administered cytotoxins.

Diffusion of active cytotoxic agents throughout an entire solid tumour is a particular challenge to successful drug delivery. Here we show the simple and robust generation of non-toxic, 10-15 nm superparamagnetic iron oxide nanoparticles (SPIONs) that have been sterically stabilized by either 100% anionic or 100% cationic or 100% neutral end-functionalized steric stabilizers or by novel combinations of cationic and neutral end-functionalized polymer. When these nanoparticles were co-administered with various anti-cancer drugs, a significant increase in the diffusion and effectiveness of the cytotoxin in a 3-dimensional model of a solid tumour was shown for specific combinations of surface functionality and cytotoxin. The critical determinant of enhanced cytotoxin diffusion and effectiveness was the end functionality of the steric stabilizers and not the core composition (either iron oxide, silica or gold). We provide evidence that SPIONs stabilized with heterogeneous steric stabilizers enhance nuclear uptake of doxorubicin across multiple cell layers.

Stable and water-tolerant ionic liquid ferrofluids.

Ionic liquid ferrofluids have been prepared containing both bare and sterically stabilized 8-12 nm diameter superparamagnetic iron oxide nanoparticles, which remain stable for several months in both protic ethylammonium and aprotic imidazolium room-temperature ionic liquids. These ferrofluids exhibit spiking in static magnetic fields similar to conventional aqueous and nonaqueous ferrofluids. Ferrofluid stability was verified by following the flocculation and settling behavior of dilute nanoparticle dispersions. Although bare nanoparticles showed excellent stability in some ILs, they were unstable in others, and exhibited limited water tolerance. Stability was achieved by incorporating a thin polymeric steric stabilization layer designed to be compatible with the IL. This confers the added benefit of imbuing the ILF with a high tolerance to water.

Optimized steric stabilization of aqueous ferrofluids and magnetic nanoparticles.

The preparation and properties of an aqueous ferrofluid consisting of a concentrated (>65 wt %) dispersion of sterically stabilized superparamagnetic, iron oxide (maghemite) nanoparticles stable for several months at high ionic strength and over a broad pH range is described. The 6-8 nm diameter nanoparticles are individually coated with a short poly(acrylic acid)-b-poly(acrylamide) copolymer, designed to form the thinnest possible steric stabilizing layer while remaining strongly attached to the iron oxide surface over a wide range of nanoparticle concentrations. Thermogravimetric analysis yields an iron oxide content of 76 wt % in the dried particles, consistent with a dry polymer coating of approximately 1 nm in thickness, while the poly(acrylamide) chain length indicated by electrospray mass spectrometry is consistent with the 4-5 nm increase in the hydrodynamic radius observed by light scattering when the poly(acrylamide) stabilizing chains are solvated. Saturation magnetization experiments indicate nonmagnetic surface layers resulting from the strong chemical attachment of the poly(acrylic acid) block to the particle surface, also observed by Fourier transform infrared spectroscopy.

Interaction of cationic surfactants with carboxymethylcellulose in aqueous media.

We have examined the polymer-surfactant interaction in mixed solutions of the cationic surfactants, i.e., dodecyltrimethylammonium chloride, dodecyltrimethylammonium bromide, tetradecyltrimethylammonium bromide, hexadecyltrimethylammonium bromide, tetradecyltriphenylphosphonium bromide, and tetradecylpyridinium bromide and a semiflexible anionic polyelectrolyte carboxymethylcellulose in water and aqueous salt solutions by various techniques: tensiometry, viscosimetry or ion-selective electrode method, and dynamic light scattering. We have investigated the effect of varying surfactant chain length, head group size, counterion, and ionic strength on the critical aggregation concentration (CAC) of mixed polymer surfactant systems and the collapse of the polymer molecule under different solution conditions. The CAC decreases with increasing alkyl chain length. Above a certain surfactant concentration, mixed aggregates start growing until their macroscopic phase separation. The growth is more rapid with greater surfactant tail length and with increasing head group size. This is attributed in both cases to the increasing hydrophobic interaction between polymer and surfactant. Among surfactants with monovalent halide counterions, iodide induces the strongest binding, reflected by the onset of growth of the mixed aggregates at low surfactant concentration. This is perhaps related to the decreasing hydration of the counterion from chloride to iodide. The surfactant concentration at which the viscosity of the solution starts to decrease sharply is smaller than the CAC, and probably reflects polymer chain shrinkage due to noncooperative binding.

Critical aggregation concentration in mixed solutions of anionic polyelectrolytes and cationic surfactants.

We have examined the polymer/surfactant interaction in mixed aqueous solutions of cationic surfactants and anionic polyelectrolytes combining various techniques: tensiometry, potentiometry with surfactant-selective electrodes, and viscosimetry. We have investigated the role of varying polymer charge density, polymer concentration, surfactant chain length, polymer backbone rigidity, and molecular weight on the critical aggregation concentration (Cac) of mixed polymer/surfactant systems. The Cac of these systems, estimated from tensiometry and potentiometry, is found to be in close agreement. Different Cac variations with polymer charge density and surfactant chain length were observed with polymers having persistence lengths either smaller or larger than surfactant micelle size, which might reflect a different type of molecular organization in the polymer/surfactant complexes. The surfactant concentration at which the viscosity starts to decrease sharply is different from the Cac and probably reflects the polymer chain shrinkage due to surfactant binding.

Time-dependent complexation of glucose-reduced gold nanoparticles with octadecylamine Langmuir monolayers.

We report on the reduction of aqueous chloroaurate ions by glucose to form gold nanoparticles of uniform size. We further demonstrate the complexation of these particles with octadecylamine (ODA) monolayers at the air-water interface. Pressure-area (pi-A) isotherms as a function of time of complexation revealed a significant expansion of the monolayer. Surface pressure variation with time for constant areas after spreading of the monolayer was carried out to observe the kinetics of complexation of the colloidal particles at the interface. The kinetics of complexation of the particles at the interface was also monitored by Brewster angle microscopy (BAM) measurements. Langmuir-Blodgett films of the particles complexed with ODA were formed at a subphase pH of 9 onto different substrates. Quartz crystal microgravimetry (QCM) was used to quantify the amount of particles deposited per immersion cycle of the quartz crystal. The LB films were further characterized by UV-vis and transmission electron microscopy (TEM) measurements. TEM measurements indicate a close packed and equidistant arrangement of colloidal particles in the LB film, probably due to hydrogen-bonding interactions.