Magnetite nanoparticles (MNPs) coated with poly(acrylic acid-co-maleic acid) polyelectrolyte (PAM) have been prepared with the aim of improving colloidal stability of core-shell nanoparticles for biomedical applications and enhancing the durability of the coating shells. FTIR-ATR measurements reveal two types of interaction of PAM with MNPs: hydrogen bonding and innersphere metal-carboxylate complex formation. The mechanism of the latter is ligand exchange between uncharged -OH groups of the surface and -COO -anionic moieties of the polyelectrolyte as revealed by adsorption and electrokinetic experiments. The aqueous dispersion of PAM@MNP particles (magnetic fluids -MFs) tolerate physiological salt concentration at composition corresponding to the plateau of the high-affinity adsorption isotherm. The plateau is reached at small amount of added PAM and at low concentration of non-adsorbed PAM, making PAM highly efficient for coating MNPs. The adsorbed PAM layer is not desorbed during dilution. The performance of the PAM shell is superior to that of polyacrylic acid (PAA), often used in biocompatible MFs. This is explained by the different adsorption mechanisms, namely metalcarboxylate cannot form in the case of PAA. Molecular-level understanding of the protective shell formation on MNPs presented here improves fundamentally the colloidal techniques used in coreshell nanoparticle production for nanotechnology applications. This is a PDF file of an unedited manuscript that has been accepted for publication. Final edited form is published in Langmuir 2012, 28, 16638−16646. http://dx
Graphical abstractH-type isotherms HA, PAM, GA, and CA Highlights Organic acids either stabilize or destabilize oxide nanoparticles in natural waters. The stabilizing/destabilizing effect depends on pH, salinity and organic concentration. Specific configuration of carboxylic groups is necessary to surface complexation. Surface complexation leads to high affinity adsorption isotherms. Higher molecular weight organic acids provide better stability than smaller ones. AbstractThe adsorption of different organic acids and their influence on the pH-dependent charging, salt tolerance and so the colloidal stability of magnetite nanoparticles are compared. Adsorption isotherms of citric acid -CA, gallic acid -GA, poly(acrylic acid) -PAA, poly(acrylic-co-maleic acid) -PAM and humic acid -HA were measured. The pH-dependent charge state of MNPs was characterized by electrophoretic mobility and their aggregation by dynamic light scattering. The salt tolerance was tested in coagulation kinetic experiments. Although the adsorption capacities, the type of bonding (either H-bonds or metal ioncarboxylate complexes) and so the bond strengths are significantly different, the following general trends have been found. Small amount of organic acids at pH < ~8 (the pH of PZC of magnetite) -relevant condition in natural waters -only neutralizes the positive charges, and so promotes the aggregation and sedimentation of nanoparticles. Greater amounts of organic acid, above the charge neutralization, cause the sign reversal of particle charge, and at high
Corresponding authorsM. Szekeres (szekeres@chem.u-szeged.hu) and E. Tombácz (tombacz@chem.u-szeged.hu) Graphical abstract AbstractMagnetite nanoparticles (MNPs) with biocompatible coatings are good candidates for MRI (magnetic resonance imaging) contrasting, magnetic hyperthermia treatments and drug delivery systems. The spontaneous surface induced polymerization of dissolved organic matter on environmental mineral particles inspired us to prepare carboxylated core-shell MNPs by using a ubiquitous polyphenolic precursor. Through the adsorption and in-situ surface polymerization of gallic acid (GA), a polygallate (PGA) coating is formed on the nanoparticles (PGA@MNP) with possible antioxidant capacity. The Polygallate on magnetite nanoparticles Humic matter on clays 2This is a PDF file of an unedited manuscript that has been accepted for publication. Final edited form is published in Langmuir, 2014, 30:15451−15461. http://dx.doi.org/10.1021/la5038102 present work explores the mechanism of polymerization with the help of potentiometric acid-base titration, dynamic light scattering (for particle size and zeta potential determination), UV-Vis (UVvisible light spectroscopy), FTIR-ATR (Fourier-transformed infrared spectroscopy by attenuated total reflection) and XPS (X-ray photoelectron spectroscopy) techniques. We observed the formation of ester and ether linkages between gallate monomers both in solution and in the adsorbed state. Higher polymers were formed in the course of several weeks both on the surface of nanoparticles and in the dispersion medium. The ratio of the absorbances of PGA supernatants at 400 and 600 nm (i.e., the E4/E6 ratio commonly used to characterize the degree of polymerization of humic materials) was determined to be 4.3, similar to that of humic acids. Combined XPS, dynamic light scattering and FTIR-ATR results revealed that prior to polymerization, the GA monomers became oxidized to polycarboxylic acids due to ring opening while Fe 3+ ions reduced to Fe 2+ . Our published results on the colloidal and chemical stability of PGA@MNPs are referenced thoroughly in the present work. Detailed studies on biocompatibility, antioxidant property and biomedical applicability of the particles will be published.
Magnetite nanoparticles (MNP)
Nanoparticles do not exist in thermodynamical equilibrium because of high surface free energy, thus they have only kinetic stability. Spontaneous changes can be delayed by designed surface coating. In biomedical applications, superparamagnetic iron oxide nanoparticles (SPIONs) require an optimized coating in order to fulfil the expectation of medicine regulatory agencies and ultimately that of biocompatibility. In this work, we show the high surface reactivity of naked SPIONs due to ≡Fe-OH sites, which can react with H/OH to form pH- and ionic strength-dependent charges. We explain the post-coating of naked SPIONs with organic polyacids via multi-site complex bonds formed spontaneously. The excess polyacids can be removed from the medium. The free COOH groups in coating are prone to react with active biomolecules like proteins. Charging and pH- and salt-dependent behaviour of carboxylated SPIONs were characterized quantitatively. The interrelation between the coating quality and colloidal stability measured under biorelevant conditions is discussed. Our coagulation kinetics results allow us to predict colloidal stability both on storage and in use; however, a simpler method would be required to test SPION preparations. Haemocompatibility tests (smears) support our qualification for good and bad SPION manufacturing; the latter 'promises' fatal outcome .
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