Adriana P. Herrera

Colloidal dispersions of monodisperse magnetite nanoparticles modified with poly(ethylene glycol).

Monodisperse magnetite nanoparticles modified with poly(ethylene glycol) (PEG) were synthesized using a silane functionalized PEG obtained by reacting 3-aminopropyl triethoxysilane with carboxylic acid-methoxy PEG (mPEG-COOH) using amide reactions. Transmission electron microscopy (TEM), dynamic light scattering (DLS), and zeta potential measurements show the particles are monodisperse (sigma(gv) approximately 0.2) and stable in water for pH of 3-9 and ionic strengths, up to 0.3 M NaCl. Thermogravimetric analysis coupled with TEM and DLS indicates formation of a dense graft layer on the particle surface. An analysis of the interparticle interaction energy indicates that the particles are stabilized by strong steric repulsions between PEG chains on their surface.

Synthesis and functionalization of magnetite nanoparticles with aminopropylsilane and carboxymethyldextran

Magnetite nanoparticles were functionalized with carboxymethyldextran (CMDx) covalently attached to the particles to prevent polymer desorption in biomedical applications. Carbodiimide chemistry was used to react carboxylic groups (–COOH) present in the CMDx molecule with amine end groups (NH2) previously grafted onto the nanoparticle surface by condensation of aminopropylsilane (APS) molecules. This method produces highly stable suspensions of magnetite nanoparticles with electrostatic and steric repulsion and no particle precipitation in the presence of electrolytes.

Multifunctional magnetite nanoparticles coated with fluorescent thermo-responsive polymeric shells

We have synthesized multifunctional magnetite nanoparticles coated with N-isopropylacrylamide (NIPAM) and a fluorescent modified acrylamide (FMA) monomer. The temperature of the medium surrounding a collection of nanoparticles may be determined by monitoring fluorescence changes in the FMA monomer resulting from the decrease in polarity of the NIPAM copolymer chain with increasing temperature. The possibility to induce localized heating while allowing non-invasive monitoring of temperature, makes these nanoparticles attractive for biomedical applications.

Effect of poly(ethylene oxide)-silane graft molecular weight on the colloidal properties of iron oxide nanoparticles for biomedical applications

The size, charge, and stability of colloidal suspensions of magnetic nanoparticles with narrow size distribution and grafted with poly(ethylene glycol)-silane of different molecular weights were studied in water, biological buffers, and cell culture media. X-ray photoelectron spectroscopy provided information on the chemical nature of the nanoparticle surface, indicating the particle surfaces consisted of a mixture of amine groups and grafted polymer. The results indicate that the exposure of the amine groups on the surface decreased as the molecular weight of the polymer increased. The hydrodynamic diameters correlated with PEG graft molecular weight and were in agreement with a distributed density model for the thickness of a polymer shell end-grafted to a particle core. This indicates that the particles obtained consist of single iron oxide cores coated with a polymer brush. Particle surface charge and hydrodynamic diameter were measured as a function of pH, ionic strength, and in biological buffers and cell culture media. DLVO theory was used to analyze the particle stability considering electrostatic, magnetic, steric, and van der Waals interactions. Experimental results and colloidal stability theory indicated that stability changes from electrostatically mediated for a graft molecular weight of 750 g/mol to sterically mediated at molecular weights of 1000 g/mol and above. These results indicate that a graft molecular weight above 1000 g/mol is needed to produce particles that are stable in a wide range of pH and ionic strength, and in cell culture media.

The effect of grafting method on the colloidal stability and in vitro cytotoxicity of carboxymethyl dextran coated magnetic nanoparticles

The colloidal stability and cytotoxicity of iron oxide nanoparticles coated with adsorbed or covalently-bound carboxymethyl dextran (CMDx) was studied in biologically relevant buffers and in MCF-7 cell culture. Comparisons were also made with aminopropylsilane coated nanopartciles which are an intermediate step in obtaining the particles with covalently bound CMDx. Particles with covalently-bound CMDx were colloidally stable in all the buffers and in cell culture. Conversely, particles with aminopropylsilane and adsorbed CMDx precipitated under the conditions tested. Aminopropylsilane coated nanoparticles were only stable in deionized water and pH < 5. Nanoparticles with adsorbed CMDx precipitate due to desorption of CMDx in the presence of phosphates. Nanoparticles with aminopropylsilane and adsorbed CMDx were significantly more cytotoxic in the absence of magnetic field than nanoparticles with covalently-bound CMDx, which correlates with their poor colloidal stability. Both CMDx coated nanoparticles were equally effective in decreasing MCF-7 cancer cell viability by magnetic fluid hyperthermia (MFH), to levels of around 4–6%, compared to untreated samples. This study illustrates the importance of grafting method on obtaining nanoparticles suitable for biomedical applications.

Monitoring colloidal stability of polymer-coated magnetic nanoparticles using AC susceptibility measurements

The application of the response of magnetic nanoparticles to oscillating magnetic fields to probe transitions in colloidal state and structure of polymer-coated nanoparticles is demonstrated. Cobalt ferrite nanoparticles with narrow size distribution were prepared and shown to respond to oscillating magnetic fields through a Brownian relaxation mechanism, which is dependent on the mechanical coupling between the particle dipoles and the surrounding matrix. These nanoparticles were coated with covalently-attached poly(N-isopropylacrylamide) (pNIPAM) or poly(N-isopropylmethacrylamide) (pNIPMAM) through free radical polymerization. The temperature induced transitions of colloidal suspensions of these nanoparticles were studied through a combination of differential scanning calorimetry (DSC), dynamic light scattering (DLS), and AC susceptibility measurements. In the pNIPAM coated nanoparticles excellent agreement was found for a transition temperature of approximately 30 degrees C by all three methods, although the AC susceptibility measurements indicated aggregation which was not evident from the DLS results. Small-angle neutron scattering (SANS) results obtained for pNIPAM coated nanoparticles confirmed that aggregation indeed occurs above the lower critical transition temperature of pNIPAM. For the pNIPMAM coated nanoparticles DLS and AC susceptibility measurements indicated aggregation at a temperature of approximately 33-35 degrees C, much lower than the transition temperature peak at 40 degrees C observed by DSC. However, the transition observed by DSC is very broad, hence it is possible that aggregation begins to occur at temperatures lower than the peak, as indicated by the AC susceptibility and DLS results. These experiments and observations demonstrate the possibility of using AC susceptibility measurements to probe transitions in colloidal suspensions induced by external stimuli. Because magnetic measurements do not require optical transparency, these methods could be applied even in concentrated or opaque systems, in which light scattering techniques encounter technical problems.

Tissue-specific direct microtransfer of nanomaterials into Drosophila embryos as a versatile in vivo test bed for nanomaterial toxicity assessment.

Nanomaterials are the subject of intense research, focused on their synthesis, modification, and biomedical applications. Increased nanomaterial production and their wide range of applications imply a higher risk of human and environmental exposure. Unfortunately, neither environmental effects nor toxicity of nanomaterials to organisms are fully understood. Cost-effective, rapid toxicity assays requiring minimal amounts of materials are needed to establish both their biomedical potential and environmental safety standards. Drosophila exemplifies an efficient and cost-effective model organism with a vast repertoire of in vivo tools and techniques, all with high-throughput scalability and screening feasibility throughout its life cycle. Here we report tissue specific nanomaterial assessment through direct microtransfer into target tissues. We tested several nanomaterials with potential biomedical applications such as single-wall carbon nanotubes, multiwall carbon nanotubes, silver, gold, titanium dioxide, and iron oxide nanoparticles. Assessment of nanomaterial toxicity was conducted by evaluating progression through developmental morphological milestones in Drosophila. This cost-effective assessment method is amenable to high-throughput screening.

Oriented Growth of α-MnO2 Nanorods Using Natural Extracts from Grape Stems and Apple Peels

We report on the synthesis of alpha manganese dioxide (α-MnO2) nanorods using natural extracts from Vitis vinifera grape stems and Malus domestica ‘Cortland’ apple peels. We used a two-step method to produce highly crystalline α-MnO2 nanorods: (1) reduction of KMnO4 in the presence of natural extracts to initiate the nucleation process; and (2) a thermal treatment to enable further solid-state growth of the nuclei. Transmission electron microscopy (TEM) and field emission scanning electron microscopy (FESEM) images provided direct evidence of the morphology of the nanorods and these images were used to propose nucleation and growth mechanisms. We found that the α-MnO2 nanorods synthesized using natural extracts exhibit structural and magnetic properties similar to those of nanoparticles synthesized via traditional chemical routes. Furthermore, Fourier transform infrared (FTIR) shows that the particle growth of the α-MnO2 nanorods appears to be controlled by the presence of natural capping agents during the thermal treatment. We also evaluated the catalytic activity of the nanorods in the degradation of aqueous solutions of indigo carmine dye, highlighting the potential use of these materials to clean dye-polluted water.

Development and validation of a 10 kHz–1 MHz magnetic susceptometer with constant excitation field

The design and validation of a mutual inductance AC susceptometer with constant excitation field of up to 4.25 Oe, operating at frequencies from 10 kHz to 1 MHz, is presented. Considerations such as parasitic capacitances between wire turns and sensing bridge electronics were taken into account in order to extend the operating frequency range. An 18AWG wire with considerable insulator thickness was used for coil construction to keep parasitic capacitive reactance negligible relative to coil inductive reactance, and to obtain controlled field operation. A high speed instrumentation amplifier (slew rate over 33 V/μs) was designed and constructed using voltage feedback LM7171 operational amplifiers. The system was calibrated with Dy2O3 to account for mismatches in signal amplitude and phase shifts due to the electronics, coil coupling and imperfections, and external disturbances. AC susceptometer operation in the 10 kHz–1 MHz frequency range was validated by measuring the complex susceptibility of cobalt fer...

Chromatographic analysis of phytochemicals components present in mangifera indica leaves for the synthesis of silver nanoparticles by AgNO3 reduction

It was studied the green synthesis of silver nanoparticles (AgNPs) from the reduction of a silver nitrate solution (1 and 10mM) in the presence of an extract of mangifera indica leaves. Phytochemicals components present in extracts of mango leaves were determined using a GC-MS chromatograph. The results showed the presence of the phenolic compound pyrogallol (26.9% wt/5mL of extract) and oleic acid (29.1% wt/5mL of extract), which are useful for the reduction of the metallic salt AgNO3 and the stabilization of silver nanoparticles. The synthesized nanoparticles were characterized by UV visible spectroscopy (UV-vis), evidencing absorbances at wavelengths of 417nm (AgNPs-1) and 414nm (AgNPs- 10), which are characteristic peaks of this metallic nanoparticles. Scanning Electron Microscopy (SEM) was used to determine the size of the synthesized nanoparticles. A particle size of about 28±7nm was observed for the AgNPs-1 sample and 26±5nm for the AgNPs-10. This suggests the advantages of green chemistry to obtain silver nanoparticles with a narrow size distribution.