Rodica Turcu

Structure of Polydopamine: A Never-Ending Story?

Polydopamine (PDA) formed by the oxidation of dopamine is an important polymer, in particular, for coating various surfaces. It is composed of dihydroxyindole, indoledione, and dopamine units, which are assumed to be covalently linked. Although PDA has been applied in a manifold way, its structure is still under discussion. Similarities have been observed in melanins/eumelanins as naturally occurring, deeply colored polymer pigments derived from L-DOPA. Recently, an alternative structure was proposed for PDA wherein dihydroxyindoline, indolinedione, and eventually dopamine units are not covalently linked to each other but are held together by hydrogen bonding between oxygen atoms or π stacking. In this study, we show that this structural proposal is very unlikely to occur taking into account unambiguous results obtained by different analytical methods, among them (13)C CPPI MAS NMR (cross-polarization polarization-inversion magic angle spinning NMR), (1)H MAS NMR (magic angle spinning NMR), and ES-HRMS (electrospray ionization high-resolution mass spectrometry) for the first time in addition to XPS (X-ray photoelectron spectroscopy) and FTIR spectroscopy. The results give rise to a verified structural assignment of PDA wherein dihydroxyindole and indoledione units with different degrees of (un)saturation are covalently linked by C-C bonds between their benzene rings. Furthermore, proof of open-chain (dopamine) monomer units in PDA is provided. Advanced DFT calculations imply the arrangements of several PDA chains preferably by quinone-hydroquinone-type interactions in a parallel or antiparallel manner. From all of these results, a number of hypotheses published before could be experimentally supported or were found to be contradictory, thus leading to a better understanding of the PDA structure.

Magnetic iron oxide nanoparticles: Recent trends in design and synthesis of magnetoresponsive nanosystems.

Recent developments in nanotechnology and application of magnetic nanoparticles, in particular in magnetic iron oxide nanosystems, offer exciting possibilities for nanomedicine. Facile and precise synthesis procedures, high magnetic response, tunable morphologies and multiple bio-functionalities of single- and multi-core magnetic particles designed for nanomedicine applications are thoroughly appraised. This review focuses on the structural and magnetic characterization of the cores, the synthesis of single- and multicore iron oxide NPs, especially the design of the latter, as well as their protection, stabilization and functionalization by desired coating in order to protect against the corrosion of core, to prevent non-specific protein adsorption and particle aggregation in biological media, and to provide binding sites for targeting and therapeutic agents.

The effect of initial conductivity and doping anions on gas sensitivity of conducting polypyrrole films to NH3

Abstract The action of ammonia on free-standing polypyrrole films doped with ClO4− and TsO− having a high conductivity in the range 10–65 S cm−1 has been investigated. The gas sensitivity of conducting polypyrrole films to NH3, at room temperature, is dependent on the initial conductivity of the polypyrrole films and doping anions. A low conductivity causes an increase in sensitivity of polypyrrole films to NH3, The films containing ClO4− counterions display a high sensitivity, in the range 88 to 155% at 10 ppm NH3, whereas those including TsO− have a low sensitivity of around 20%. It is found that the sensitivity values of ClO4−- and TsO−-doped polypyrrole films remain constant for a period of 60 days in an ambient environment. The polypyrrole films formed by electrochemical oxidative polymerization are found to posses detectable sensitivity in the form of an increase in resistivity for NH3 gas.

Structure and in vitro biological testing of water-based ferrofluids stabilized by monocarboxylic acids.

Water-based ferrofluids (magnetic fluids) with double-layer steric stabilization by short monocarboxylic acids (lauric and myristic acids) are considered to be a potential source of magnetic nanoparticles in brain cancer (glioblastoma) treatment. Structure characterization in the absence of an external magnetic field is performed, including transmission electron microscopy, magnetization analysis, and small-angle neutron scattering with contrast variation. It is shown that despite the good stability of the systems a significant part of the magnetite nanoparticles are in aggregates, whose inner structure depends on the stabilizer used. In particular, an incomplete coating of magnetite particles is concluded in the case of myristic acid stabilization. The ferrofluids keep their structure unchanged when added to the cancer cell medium. The intracellular accumulations of magnetite from the ferrofluids added to cancer cell cultures as well as its cytotoxicity with respect to human brain cells are investigated.

Mechanism of in Situ Surface Polymerization of Gallic Acid in an Environmental-Inspired Preparation of Carboxylated Core–Shell Magnetite Nanoparticles

Magnetite 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 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 (UV-visible 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 poly(carboxylic acid)s 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.

Diazo transfer at polydopamine – a new way to functionalization

The possibility of introducing azido functions onto polydopamine by diazo transfer making use of existing aminoethyl moieties was verified at polydopamine-coated magnetite nanoparticles. The resulting azido-functionalized Fe3O4@polydopamine nanoparticles serve as a magnetic nano-platform for the introduction of interesting applicatory functions by click-chemistry (CuAAC) as exemplified by linking biotin, tetraacetylglucose, dansyl and proline.

Magnetically induced phase condensation in an aqueous dispersion of magnetic nanogels

The applicability of aqueous magnetic colloids depends on their colloidal stability under the influence of various factors like external magnetic field and temperature. The magnetically induced phase condensation of magnetic colloids leads to the formation of spindle-like condensed phase drops of highly packed magnetic colloidal particles. The condensed phase drops are aligned parallel to the external magnetic field and may grow up to several microns thickness and tens or even hundreds of microns length. Thus, the magnetically induced phase condensation could be an advantage in magnetic separation applications, whereas in magnetic drug targeting applications it could lead to blood vessel clogging as well as to a significant decrease of the specific surface. We present the results of an experimental study regarding the influence of the external magnetic field and temperature on the magnetically induced phase condensation in an aqueous dispersion of pNIPA magnetic nanogels. A theoretical model was developed for the analysis of the data from forward light scattering experiments. It was found that the volume weight of the condensed phase increases with passing time, with increasing field intensity and temperature decrease. Using the proposed model, the magnetic field intensity dependence of the initial supersaturation of the sample was calculated.

Melanin-like polydopa amides – synthesis and application in functionalization of magnetic nanoparticles

Polydopa amides are obtained by oxidative polymerization of amides of L-dopa. These products are analogues of naturally occurring melanins and of polydopamine. They can be used as shells for magnetic core shell nanoparticles. The materials are characterized by FTIR-spectroscopy, 13C ss-NMR spectroscopy, magnetic measurements, TEM and TGA. Comparison with the structures of melanins or polydopamine shows similarities, i.e. cyclized indole monomer units and non-cyclized dopa amide moieties. The majority of the amide groups is maintained during the oxidative polymerization. The functional groups linked to the amide N-atoms open a way of connecting functions to the organic shell that have interesting applicatory potential, e.g. in biomedicine.

Magnetite–polylactic acid nanoparticles by surface initiated organocatalysis ring opening polymerization

Organocatalysis by 4-N,N-dimethylaminopyridine was employed for ring opening polymerization of lactide initiated at magnetic nanoparticles covered by glycerol phosphate or ascorbic acid phosphate. The resulting magnetite–polylactic acid nanoparticles exhibit high colloidal stability in water and alcohol. Their morphology was investigated by transmission electron microscopy and the chemical structure was elucidated by Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy. The increase in mass after coating the nanoparticles was determined by thermogravimetric analysis, while dynamic light scattering revealed the increase in hydrodynamic size. Magnetic measurements revealed superparamagnetic behavior and high magnetization values. The magnetite–polylactic acid nanoparticles were further used for magnetic tagging of biotin.

Refinement of Magnetite Nanoparticles by Coating with Organic Stabilizers

Magnetite nanoparticles are of great importance in nanotechnology and nanomedicine and have found manifold applications. Here, the effect of coating of magnetite nanoparticles with organic stabilizers, such as O-phosphoryl ethanolamine, glycerol phosphate, phospho-l-ascorbic acid, phospho-d,l-serine, glycolic acid, lactic acid, d,l-malic acid, and d,l-mandelic acid was studied. Remarkably, this procedure led to an improvement of saturation magnetization in three cases rather than to an unfavorable decrease as usually observed. Detailed X-ray powder diffraction investigations revealed that changes in the average crystallite occurred in the coating process. Surprisingly, changes of the average crystallite sizes in either direction were further observed, when the exposure time to the stabilizer was increased. These results imply a new mechanism for the well-known coating of magnetite nanoparticles with stabilizers. Instead of the hitherto accepted simple anchoring of the stabilizers to the magnetite nanoparticle surfaces, a more complex recrystallization mechanism is likely, wherein partial re-dispersion of magnetite moieties from the nanoparticles and re-deposition are involved. The results can help producers and users of magnetite nanoparticles to obtain optimal results in the production of core shell magnetite nanoparticles.