Nirmesh Jain

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.

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.

Polyelectrolyte–surfactant complexes at interfaces and in bulk

We have investigated the interactions between anionic polyelectrolytes and a cationic surfactant at the air/water interface and in bulk, for increasing surfactant concentrations. Mixed aggregates are formed at the air/water surface at extremely low surfactant concentrations. Above a critical aggregation concentration, a viscosity drop indicates that polymer chains undergo a rapid collapse. At higher surfactant concentrations, light scattering shows the existence of larger structures, which are surprisingly monodisperse. Their size increases with surfactant concentration. During this bulk evolution, surface tension remains constant, suggesting that the surface aggregates remain unchanged.

Anomalous clouding behavior of an ethylene oxide–propylene oxide block copolymer in aqueous solution

Abstract Partial phase diagram of a polyethylene oxide–polypropylene oxide–polyethylene oxide polymer (EO 6 –PO 30 –EO 6 ), pluronic L62, in water, shows two cloud points (CP) in the concentration range (5–25 g dl −1 ). The phase behavior of L62 solutions markedly changes in the presence of neutral salts. While micellization of L62 is not favored by the presence of sodium chloride exerting ‘salting out’ action, sodium thiocynate which increase CP (salting in) favors micellization of the copolymer. The results of double CP and salt effect are discussed in terms of micellization of the copolymer. The critical micelle concentration (CMC) as determined by surface tension, dye spectral change and hydrodynamic micelle size from dynamic light scattering are reported and compared with similar systems.

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.

Species measurements in the beam of an ionic liquid ferrofluid capillary electrospray source under magnetic stress

Three solutions of an ionic liquid ferrofluid (ILFF) using 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIM-NTf2) as the carrier liquid were emitted from a capillary electrospray source and the resulting beam was measured using a time-of-flight mass spectrometer (TOF-MS). The solutions had 3.04, 5.98, and 8.80 wt% iron oxide nanoparticles making them susceptible to magnetic fields. A Helmholtz coil was used to impose a gradientfree magnetic stress onto the electrospray source. Mass spectra were collected in the spray from each of the solutions, with and without the imposed magnetic field. The magnetic stress caused an increase in the peak intensity of distinct ion species (n = 0 or n = 1) at lower energy defects suggesting that the stress causes ions to be born at higher energy. The ratio of the ion peak intensity with magnetic field to ion peak intensity at zero magnetic field was proportional to the concentration of nanoparticles. The magnetic stress did not significantly affect the large mass distributions until the nanoparticle concentration reached 8.80 wt% in the fluid.

Application of small-angle neutron scattering to the study of forces between magnetically chained monodisperse ferrofluid emulsion droplets

The optical magnetic chaining technique (MCT) developed by Leal-Calderon, Stora, Mondain-Monval, Poulin & Bibette [Phys. Rev. Lett. (1994), 72, 2959–2962] allows precise measurements of force profiles between droplets in monodisperse ferrofluid emulsions. However, the method lacks an in situ determination of droplet size and, therefore, requires a combination of separately acquired measurements of droplet chain periodicity versus an applied magnetic field from optical Bragg scattering and droplet diameter inferred from dynamic light scattering (DLS) to recover surface force–distance profiles between the colloidal particles. Compound refractive lens (CRL) focused small-angle neutron scattering (SANS) MCT should result in more consistent measurements of droplet size (form factor measurements in the absence of field) and droplet chaining period (from structure factor peaks when the magnetic field is applied), and, with access to shorter length scales, extend force measurements to closer approaches than possible by optical measurements. This article reports on CRL-SANS measurements of monodisperse ferrofluid emulsion droplets aligned in straight chains by an applied field perpendicular to the incident beam direction. Analysis of the scattering from the closely spaced droplets required algorithms that carefully treated resolution and its effect on mean scattering vector magnitudes in order to determine droplet size and chain periods to sufficient accuracy. At lower applied fields, scattering patterns indicate structural correlations transverse to the magnetic field direction owing to the formation of intermediate structures in early chain growth.

Species measurements in the beam of an ionic liquid ferrofluid electrospray source

An ionic liquid ferrofluid (ILFF) using 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIM-NTf2) as the carrier liquid was emitted from the peak formed via the Rosensweig instability and the electrospray beam was measured using quadrupole and TOF mass spectrometry. The Rosensweig instability peak source (RIPS) was found to operate in three stable emission current modes: transient-emission, lowcurrent, and high-current. Both quadrupole and linear TOF mass spectra for the final two modes were collected, and revealed that the charged-particle species within the electrospray beam varied between the two emission current modes. No correlation between the magnetic field strength and the collected mass spectra was measured. Mass flow measurements using a quartz crystal microbalance revealed that the RIPS only operated at a high mass flow rate of 3-5 ng/s during the startup transient-emission mode and otherwise ran at mass flow rates of 0.03-0.3 ng/s indicating the absence of droplet species during the majority of emission. The RIPS quadrupole mass spectra were compared to a pure ionic liquid needle source quadrupole spectra and it was discovered that multiple species exist in the ILFF electrospray that are attributed to fragments respective ion species, with some species partially comprised of fragments from the polymer used for steric stabilization of the nanoparticles; specifically the block that comprises the stabilization group, poly(N,N-dimethylacrylamide), and the end functionalizing group, CH3CHCOOH.

The effects of magnetic surface stress on electrospray of an ionic liquid ferrofluid

Four solutions of an ionic liquid ferrofluid (ILFF) using 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIM-NTf2) as the carrier liquid were emitted from a capillary electrospray source and its beam was measured using a time-of-flight mass spectrometer (TOF-MS) and a downstream stack of Faraday plates. The solutions had 3.04, 5.98, 8.80, and 14.15 wt% iron-oxide nanoparticles making them susceptible to magnetic fields. A Helmholtz coil was used to impose a magnetic stress onto the electrospray source. The addition of nanoparticles to neat IL increased viscosity, and decreased conductivity and