5 research outputs found
Interaction of Fe3+ meso-tetrakis (2,6-dichloro-3-sulfonatophenyl) porphyrin with cationic bilayers: magnetic switching of the porphyrin and magnetic induction at the interface
An organized multilayer was constructed by the layer-by-layer technique in which alternating layers of metalloporphyrin and dioctadecyldimethylammonium bromide bilayers were deposited onto an indium tin oxide surface electrode. The porphyrin molecules that are organized in the different layers showed a strong electroactivity with a well-defined electrochemical process. In LbL, electroactivity could be explained only by the occurrence of electron hoping. Thus, total KohnâSham density functional theory (KS-DFT) was performed to better understand the conditions responsible for the electroactivity of the metalloporphyrin layers intercalated by an insulating material. Total KS-DFT theory involves local density approximation energy calculations based on spin-polarized variant of KS-DFT theory. The results revealed a magnetization switching of the metalloporphyrin induced by the interaction with the surfactant bilayer accompanied by spin polarization of the porphyrin-interacting surfactant molecule. Although discrete, the surfactant magnetization had significant repercussions on the electron conductivity. Calculations also demonstrated loss of porphyrin symmetry promoted by a parent surfactant with a shorter hydrocarbon chain, ditetradecyldimethylammonium bromide. The calculation results were corroborated by experimental results obtained by the electron paramagnetic resonance and magnetic circular dichroism techniques.FAPESPCNPqCAPE
Origin of FM Ordering in Pristine Micro- and Nanostructured ZnO
An unexpected presence of ferromagnetic (FM) ordering in nanostructured nonmagnetic metal oxides has been reported previously. Though this property was attributed to the presence of defects, systematic exptl. and theor. studies to pinpoint its origin and mechanism are lacking. While it is widely believed that oxygen vacancies are responsible for FM ordering, surprisingly we find that annealing as-prepd. samples at low temp. (high temp.) in flowing oxygen actually enhances (diminishes) the FM ordering. For these reasons, we have prepd., annealed in different environments, and measured the ensuing magnetization in micrometer and nanoscale ZnO with varying crystallinity. We further find from our magnetization measurements and ab initio calcns. that a range of magnetic properties in ZnO can result, depending on the sample prepn. and annealing conditions. For example, within the same ZnO sample we have obsd. ferro- to para- and diamagnetic responses depending on the annealing conditions. We also explored the effects of surface states on the magnetic behavior of nanoscale ZnO through detailed calcns
pH-Dependent Synthesis of Anisotropic Gold Nanostructures by Bioinspired Cysteine-Containing Peptides
In
the present study, alkaline peptides AAAXCX (X = lysine or arginine
residues) were designed based on the conserved motif of the enzyme
thioredoxin and used for the synthesis of gold nanoparticles (GNPs)
in the pH range of 2â11. These peptides were compared with
free cysteine, the counterpart acidic peptides AAAECE and Îł-ECG
(glutathione), and the neutral peptide AAAACA. The objective was to
investigate the effect of the amino acids neighboring a cysteine residue
on the pH-dependent synthesis of gold nanocrystals. KohnâSham
density functional theory (KS-DFT) calculations indicated an increase
in the reducing capacity of AAAKCK favored by the successive deprotonation
of their ionizable groups at increasing pH values. Experimentally,
it was observed that gold speciation and the peptide structure also
have a strong influence on the synthesis and stabilization of GNPs.
AAAKCK produced GNPs at room temperature, in the whole investigated
pH range. By contrast, alkaline pH was the best condition for the
synthesis of GNP assisted by the AAARCR peptide. The acidic peptides
produced GNPs only in the presence of polyethylene glycol, and the
synthesis using AAAECE and Îł-ECG also required heating. The
ionization state of AAAKCK had a strong influence on the preferential
growth of the GNPs. Therefore, pH had a remarkable effect on the synthesis,
kinetics, size, shape, and polydispersity of GNPs produced using AAAKCK.
The AAAKCK peptide produced anisotropic decahedral and platelike nanocrystals
at acidic pH values and spherical GNPs at alkaline pH values. Both
alkaline peptides were also efficient capping agents for GNPs, but
they produced a significant difference in the zeta potential, probably
because of different orientations on the gold surface