Temperature dependence of electron magnetic resonance spectra of iron oxide nanoparticles mineralized in Listeria innocua protein cages.

Electron magnetic resonance (EMR) spectroscopy was used to determine the magnetic properties of maghemite (γ-Fe(2)O(3)) nanoparticles formed within size-constraining Listeria innocua (LDps)-(DNA-binding protein from starved cells) protein cages that have an inner diameter of 5 nm. Variable-temperature X-band EMR spectra exhibited broad asymmetric resonances with a superimposed narrow peak at a gyromagnetic factor of g ≈ 2. The resonance structure, which depends on both superparamagnetic fluctuations and inhomogeneous broadening, changes dramatically as a function of temperature, and the overall linewidth becomes narrower with increasing temperature. Here, we compare two different models to simulate temperature-dependent lineshape trends. The temperature dependence for both models is derived from a Langevin behavior of the linewidth resulting from "anisotropy melting." The first uses either a truncated log-normal distribution of particle sizes or a bi-modal distribution and then a Landau-Liftshitz lineshape to describe the nanoparticle resonances. The essential feature of this model is that small particles have narrow linewidths and account for the g ≈ 2 feature with a constant resonance field, whereas larger particles have broad linewidths and undergo a shift in resonance field. The second model assumes uniform particles with a diameter around 4 nm and a random distribution of uniaxial anisotropy axes. This model uses a more precise calculation of the linewidth due to superparamagnetic fluctuations and a random distribution of anisotropies. Sharp features in the spectrum near g ≈ 2 are qualitatively predicted at high temperatures. Both models can account for many features of the observed spectra, although each has deficiencies. The first model leads to a nonphysical increase in magnetic moment as the temperature is increased if a log normal distribution of particles sizes is used. Introducing a bi-modal distribution of particle sizes resolves the unphysical increase in moment with temperature. The second model predicts low-temperature spectra that differ significantly from the observed spectra. The anisotropy energy density K(1), determined by fitting the temperature-dependent linewidths, was ∼50 kJ/m(3), which is considerably larger than that of bulk maghemite. The work presented here indicates that the magnetic properties of these size-constrained nanoparticles and more generally metal oxide nanoparticles with diameters d < 5 nm are complex and that currently existing models are not sufficient for determining their magnetic resonance signatures.

Size and crystallinity in protein-templated inorganic nanoparticles.

Protein cages such as ferritins and virus capsids have been used as containers to synthesize a wide variety of protein-templated inorganic nanoparticles. While identification of the inorganic crystal phase has been successful in some cases, very little is known about the detailed nanoscale structure of the inorganic component. We have used pair distribution function analysis of total X-ray scattering to measure the crystalline domain size in nanoparticles of ferrihydrite, γ-Fe2O3, Mn3O4, CoPt, and FePt grown inside 24-meric ferritin cages from H. sapiens and P. furiosus. The material properties of these protein-templated nanoparticles are influenced by processes at a variety of length scales: the chemistry of the material determines the precise arrangement of atoms at very short distances, while the interior volume of the protein cage constrains the maximum nanoparticle size attainable. At intermediate length scales, the size of coherent crystalline domains appears to be constrained by the arrangement of crystal nucleation sites on the interior of the cage. Based on these observations, some potential synthetic strategies for the control of crystalline domain size in protein-templated nanoparticles are suggested.

Photochemical mineralization of europium, titanium, and iron oxyhydroxide nanoparticles in the ferritin protein cage.

The Fe storage protein ferritin was used as a size-constrained reaction vessel for the photoreduction and reoxidation of complexed Eu, Fe, and Ti precursors for the formation of oxyhydroxide nanoparticles. The resultant materials were characterized by dynamic light scattering, gel electrophoresis, UV-vis spectroscopy, and transmission electron microscopy. The photoreduction and reoxidation process is inspired by biological sequestration mechanisms observed in some marine siderophore systems.

Synthetic control over magnetic moment and exchange bias in all-oxide materials encapsulated within a spherical protein cage.

This work focuses on the synthetic control of magnetic properties of mixed oxide magnetic nanoparticles of the general formula Fe(3-x)Co(x)O(4) (x < or = 0.33) in the protein cage ferritin. In this biomimetic approach, variations in the chemical synthesis result in the formation of single-phase Fe(3-x)Co(x)O(4) alloys or intimately mixed binary phase Fe/Co oxides, modifying the chemical structure and magnetic behavior of these particles, as characterized by static and dynamic magnetization measurements and X-ray absorption spectroscopy.

Assembly of multilayer films incorporating a viral protein cage architecture.

Protein cage architectures such as viral capsids, heat shock proteins, ferritins, and DNA-binding proteins are nanoscale modular subunits that can be used to expand the structural and functional range of composite materials. Here, layer-by-layer (LbL) assembly was used to incorporate cowpea chlorotic mottle virus (CCMV) into multilayer films. Three types of multilayer films were prepared. In the first type, ionic interactions were employed to assemble CCMV into triple layers. In the second type, complementary biological interactions (streptavidin/biotin) were used for this purpose. In a third variation of LbL assembly, complementary biological interactions were employed to produce nanotextured films that exhibit in-plane order over a micron scale without the need to adsorb onto a prepatterned template.

Influence of electrostatic interactions on the surface adsorption of a viral protein cage.

The Cowpea chlorotic mottle virus (CCMV) provides a useful protein-cage architecture that can be used for the size- and shape-constrained chemistry of nanomaterials. The control of surface assembly is necessary for the realization of many applications of these nanoscale reaction vessels. Electrostatic interactions provide a useful (and reversible) method for controlled surface assembly. CCMV absorption behavior was studied on Formvar, bare Si, Formvar-coated Si, and Si modified by aminopropyltriethoxysilane (APS). Transmission electron microscopy (TEM), atomic force microscopy (AFM), and attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) were used to characterize the CCMV surface adsorption. Combined AFM and ATR-FTIR data indicated that the viral coverage on the modified surfaces was approximately 84% of the jamming limit predicted by the random sequential adsorption (RSA) model. According to the ATR-FTIR results, surface coverage was not increased at higher ionic strengths nor at a pH near the isoelectric point (pI) of the virus. The Langmuir model was used to provide a description of the kinetic absorption behavior.

A5InPb8 (A = K, Rb): an apparent zintl phase with lead tetrahedra interbridged by mu6-In atoms.

Reaction of elemental In, Pb, and K, or Rb within welded Ta containers at 900 degrees C followed by subsequent annealing at 350 degrees C gives the new phases A5InPb8 (A = K, Rb). These crystallize in the trigonal space group R3m (No. 166, Z = 3) with cell dimensions of a = 6.8835(6) and 6.885(1) A and c = 37.591(5) and 37.64(2) A for K5InPb8 and Rb5InPb8, respectively. The structure contains clusters built of pairs of Pb4 tetrahedra that are interbridged by a mu6-In atom. The InPb8 units, which in the isolated case would behave as ideal 40-electron Wade's rule clusters, are weakly interlinked into sheets in the ab plane by long (3.5 A) intercluster Pb-Pb interactions. According to the EHTB calculations, these cause a broadening of the valence band and thus generate a number of new states at the Fermi level. Compound K5InPb8 is metallic (rho298 approximately 42 microohms cm, (delta rho/delta T)/rho approximately 1.4(2) x 10(-1) K(-1)) which is in agreement with the expectations according to calculations on the anion network.

Synthesis, structure, and bonding of the nonclassical Zintl phase K5As3Pb3.

The title phase was synthesized by direct fusion of a stoichiometric amount of the elements followed by annealing at 650 degrees C for 3 weeks. The compound crystallizes in the orthorhombic space group Pnma (No. 62), Z = 4, with a = 19.451(6) A, b = 12.164(3) A, c = 6.581(1) A. The compound is made up of As(3)Pb(3)(5-) crown clusters that can be likened geometrically and electronically to 6-atom hypho-clusters derived from a tricapped trigonal prismatic closo parent. These crowns are interconnected via intercluster Pb-Pb bonds to form infinite chains along the b axis, which means the compound contains an extra two cations and two electrons per formula unit. Extended Hückel calculations indicate that the two additional electrons per cluster are accommodated in pi states on the cluster and predict that the phase should be semiconducting. The latter is confirmed by microwave resistivity measurements, rho(298) = 1.0 x 10(2) microOmega.cm; (deltarho/deltaT)/rho = -0.14(3) K(-)(1).

2-D array formation of genetically engineered viral cages on au surfaces and imaging by atomic force microscopy.

The preparation and subsequent imaging of a two-dimensional array of a genetically and chemically modified cowpea chlorotic mottle virus (CCMV) is described. The genetic mutation provides symmetrically dispersed exposed thiol groups on the outer surface of the virus capsid. These functional groups can be used to covalently bind the capsid to smooth Au substrate. AFM imaging suggests that the genetic mutation by itself does not promote array formation but, rather, aggregation through disulfide linkages. However, breaking the symmetry of the capsid using a solid-phase approach and chemically passivating the exposed thiol groups with iodoacetic acid results in a capsid with exposed thiols only on one side of the particle. These symmetry-broken capsids were able to form self-assembled monolayers (SAM) on a Au surface.

Synthesis and structure of the metallic K6Tl17: a layered tetrahedral star structure related to that of Cr3Si.

The title compound, the Tl-richest in the K-Tl system, has been synthesized in Ta containers via direct reaction of the elements at 400 degrees C followed by quenching to room temperature and subsequent annealing at 150 degrees C for 4 weeks. It crystallizes in the orthorhombic space group Cccm (No. 66) with a = 16.625(1) A, b = 23.594(2) A, c = 15.369(2) A (22 degrees C), and Z = 8. Two different Tl(12) units consisting of augmented tetrahedral stars are condensed into layers of such tetrahedra, and further Tl(2) dumbbells and the potassium cations also interconnect the stars and layers into a three-dimensional network. The former anionic Tl(8) subunits clearly resemble those in the heteroatomic 3-D structure of cubic Cr(3)Si before their augmentation with bridging atoms. The compound is metallic (rho(270) = 22.6 micro omega x cm, alpha = 0.0023 K(-)(1)) and shows Pauli-like paramagnetic susceptibility (chi(296) = 1.1 x 10(-4) emu/mol). EHTB calculations illustrate the importance of Tl p-orbital bonding, the positive Tl-Tl overlap populations up to E(F), and greater strengths of the Tl-Tl bonding between and about the surface of the augmented Tl(12) units. Cations between the thallium layers play specific and important roles in the structure.

Polyatomic clusters of the triel elements. Palladium-centered clusters of thallium in A(8)Tl(11)Pd, A = Cs, Rb, K.

Reactions of the elements within welded Ta containers at approximately 600 degrees C followed by slow cooling give new A(8)Tl(11)Pd(x) products from an apparently continuous encapsulation of Pd atoms into the pentacapped trigonal prismatic anions in the isotypic rhombohedral (R3 macro c) A(8)Tl(11) phases. All systems also produce other phases at x < 1 as well, the simplest being the cesium system in which only trigonal Pd(13)Tl(9) is also formed. Cs(8)Tl(11)Pd(0.84(1)) was characterized by single-crystal means as close to the upper x limit in that system (R3 macro c, Z = 6, a = 10.610(1) A, c = 54.683(8) A). The Pd insertion causes an expansion of the D(3) host anion, particularly about the waist, to generate a trigonal bipyramidal PdTl(5) unit (d(Pd-Tl) approximately 2.6-2.8 A) centered within a somewhat larger Tl(6) trigonal prism, the remainder of the Tl(11) cluster. Strong Tl cage bonding is retained. Extended Hückel calculations show significant involvement of all Tl 6s, 6p and Pd 4d, 5s, 5p orbital sets in the central and cage bonding. The last valence electron is considered to be delocalized in a conduction band, as in A(8)Tr(11) examples, rather than occupying an antibonding e'' LUMO across a gap of approximately 2.4 eV.