Using a home-made, anvil pressure-cell mounted to a probe suitable for use with a commercial magnetometer, the photo and thermal responses of the magnetism of CoFe@CrCr-PBA (PBA = Prussian blue analogue) core@shell nanoparticles were studied down to 5 K and up to 0.5 GPa in 100 G. The effect of pressure on the magnetic ordering temperatures of the CoFe-PBA core (≈25 K) and the CrCr-PBA shell (≈200 K) along with a shift of the relaxation temperature of the photo-CTIST (charge-transfer-induced spin-transition) of the CoFe-PBA core (≈125 K), were similar to the behaviors reported for the single-phase materials. Specifically, although the magnetic ordering temperature of the CrCr-PBA shell shifted to higher temperatures, the relaxation temperature of the photo-CTIST of the CoFe-PBA core moved to lower temperatures when the pressure was increased, thereby lowering the temperature range under which the CrCr-PBA component can be photoswitched.Externally applied pressure applied to CoFe@CrCr Prussian blue analogue core@shell nanoparticles shifts the magnetic ordering temperatures of the CoFe-PBA core, the CrCr-PBA shell, and the photo-CTIST of the CoFe-PBA core. Specifically, the magnetic ordering temperature of the CrCr-PBA shell shifted to higher temperatures, while the relaxation temperature of the photo-CTIST of the CoFe-PBA core moved to lower temperatures.

A library of relative absorptivities of the C-N stretching modes for a series of Prussian blue analogues (PBAs), AxMy[M′(CN)6]z·nH2O (A = alkali cation; M, M′ = transition metals), has been assembled based on analysis of Fourier transform infrared (FTIR) spectra. Chemical variables including transition metal identity, cation identity, stoichiometry, and oxidation state have been considered. The absorptivities for individual PBAs were then applied to determine the composition of core–shell particles with varying shell thickness and the results correlate well with analyses derived from energy dispersive X-ray spectroscopy (EDS). Knowledge of relative absorptivities allows generally accessible FTIR methods to contribute to quantitative characterization of multiple phase Prussian blue analogue samples and PBA heterostructures.A library of relative absorptivities of the C-N stretching modes for a series of Prussian blue analogues (PBAs) has been assembled based on analysis of Fourier transform infrared (FTIR) spectra. Individual absorptivities were applied to determine compositions of core–shell particles. The results correlate well with energy dispersive X-ray spectroscopy (EDS).Download high-res image (82KB)Download full-size image

The thermodynamically favored phases in a family of two-dimensional (2D) hybrid perovskites, (C6H13NH3)2MCl4 (M = Cu2+, Mn2+, Cd2+), can be reversibly switched by changing the medium in which particles are immersed. Differential scanning calorimetry and X-ray diffraction show that the temperature of a known solid–solid phase transition in the series of layered perovskites shifts to lower temperature when exposing the particles to organic solvents. For the Cu2+ and Cd2+ members of the series, the shift in transition temperature changes the thermodynamically favored phase at room temperature. Interactions between the solvent and the particle surface alter the surface stress, thereby reducing the effective pressure, leading to a change in transition temperature. The mechanism likely explains some anomalous behavior in related 2D perovskites and could factor into potential applications of hybrid perovskite-based materials.

A series of photomagnetic coordination polymer core–shell heterostructures, based on the light-switchable Prussian blue analogue RbaCob[Fe(CN)6]c·mH2O (RbCoFe-PBA) as the core and the ferromagnetic KjNik[Cr(CN)6]l·nH2O (KNiCr-PBA) as the shell, was studied using powder X-ray diffraction, down to 100 K, and magnetometry, down to 2 K, to investigate the influence of the shell thickness on light-induced magnetization changes and gain insight into the mechanism. The core material is known to undergo a charge-transfer-induced spin transition (CTIST), and synchrotron powder diffraction was used to monitor structural changes in both the core and the shell associated with the thermally and optically induced CTIST of the core. Significant lattice contraction in the RbCoFe-PBA core upon cooling through the high-spin to the low-spin state transition near ∼260 K induces strain on the KNiCr-PBA shells. This lattice strain in the shell can be relieved either by thermal cycling back to high temperature or by using light to access the metastable high-spin state of the core at low temperature. The different extents of strain in the KNiCr-PBA shell are reflected in low-temperature, low-field magnetization versus temperature data in the light and dark states. A broader magnetic transition at Tc ≈ 70 K in the dark state relative to the light state reflects the greater dispersion of nearest-neighbor contacts and exchange energies induced by the structural distortions of the strained state. Analyses for different shell thicknesses, coupled with high-field magnetization data, support a mechanism whereby the light-induced magnetization changes in the KNiCr-PBA shell are due to realignment of the local magnetic anisotropy as a result of the structural changes in the shell associated with the optical CTIST of the core. Through magnetization and structural analyses, the depth to which the properties of the shell are influenced by the core–shell architecture was estimated to be between 40 and 50 nm.

Different routes for preparing zirconium phosphonate-modified surfaces for immobilizing biomolecular probes are compared. Two chemical-modification approaches were explored to form self-assembled monolayers on commercially available primary amine-functionalized slides, and the resulting surfaces were compared to well-characterized zirconium phosphonate monolayer-modified supports prepared using Langmuir–Blodgett methods. When using POCl3 as the amine phosphorylating agent followed by treatment with zirconyl chloride, the result was not a zirconium-phosphonate monolayer, as commonly assumed in the literature, but rather the process gives adsorbed zirconium oxide/hydroxide species and to a lower extent adsorbed zirconium phosphate and/or phosphonate. Reactions giving rise to these products were modeled in homogeneous-phase studies. Nevertheless, each of the three modified surfaces effectively immobilized phosphopeptides and phosphopeptide tags fused to an affinity protein. Unexpectedly, the zirconium oxide/hydroxide modified surface, formed by treating the amine-coated slides with POCl3/Zr4+, afforded better immobilization of the peptides and proteins and efficient capture of their targets.

The magnetic properties of a series of core–shell particles based on the Prussian blue analogues K0.1Cu[Fe(CN)6]0.7·3.5H2O and K0.1Ni[Fe(CN)6]0.7·4.4H2O (CuFe-PBA@NiFe-PBA) are investigated as a function of electrochemical titration and lithium ion insertion. The particles, with average size 305 ± 50 nm, are reduced using the galvanostatic intermittent titration technique to prepare 10 samples of Lix(CuFe-PBA@NiFe-PBA) with 0 ≤ x ≤ 1.0. Magnetization as a function of temperature for each member of the series reveals the ferromagnetic ordering of the individual components, NiFe-PBA (Tc ∼ 24 K) and CuFe-PBA (Tc ∼ 18 K). The magnetic ordering of each component is suppressed upon reduction and Li+ incorporation, but in a stepwise fashion with the CuFe-PBA core reduced before the NiFe-PBA shell. The separate reductions of the core and shell are also seen in magnetization vs field measurements at low temperature. By introducing a lattice-gas model, the enthalpy changes (ΔHi) associated with each redox couple after Li-ion insertion were calculated and applied to the mean field approximation to reproduce the magnetic transition temperatures. The results are significant as the CuFe-PBA@NiFe-PBA particles had previously been shown to exhibit superior performance over the individual components as cathode materials for lithium ion batteries, although the stepwise reduction had not previously been discerned. Furthermore, the report is the first showing the systematic control of magnetism in core–shell coordination polymer heterostructures by electrochemical guest ion insertion.

The heterogeneous growth of a series of Prussian blue analogues on nickel hexacyanocobaltate seeds to produce coordination polymer heterostructues was investigated to determine the influence of lattice misfit over the growth mode. For small lattice misfits, the observation of a core–shell morphology indicates the growth of a pseudomorphic layer while larger lattice misfits result in the growth of islands. A structural study suggests an efficient mechanical coupling of the substrate and overlayer materials. The difference in lattice constants between the heterostructure components induces anisotropic strain that is relieved by switching to a different growth mode.

A one-step synthesis of Prussian blue nanoparticles possessing a concentration gradient of Gd3+ counterions, g-Gd-PB, has been developed, and the potential for the particles to perform as both MRI positive contrast agents and photothermal therapy agents is demonstrated. The synthesis of potassium/gadolinium ironhexacyanoferrate is performed under increasing concentration of Gd3+ ions forming particles with a higher concentration of gadolinium toward the outer layers. The proton relaxivity (r1) measured for the particles is 12.3 mM−1 s−1, and T1 weighted images of phantoms containing the particles show their potential as MRI contrast agents. In addition, the Prussian blue host can rapidly and efficiently convert energy from near-IR light into thermal energy, allowing g-Gd-PB to be used as a photothermal therapy agent. The photothermal properties are demonstrated by measuring temperature changes of particle suspensions under irradiation and by photothermal ablation of CCRF-CEM cancer cells.

Coordination polymer thin film heterostructures of the Prussian blue analogue NiIIb[CrIII(CN)6]0.7·nH2O (NiCr-PBA) and the 3D Hofmann-like spin crossover compound Fe(azpy)[Pt(CN)4]·xH2O {azpy = 4,4′-azopyridine} have been developed, and spin transition properties have been characterized via SQUID magnetometry and Raman spectroscopy. The magnetic response of the ferromagnetic NiCr-PBA layer (Tc ≈ 70 K) can be altered by inducing the LIESST effect (light-induced excited spin state trapping) in the coupled paramagnetic Fe(II) spin crossover material. Whereas an increase in magnetization is measured for the single-phase Fe(azpy)[Pt(CN)4]·xH2O, a decrease in magnetization is observed for the heterostructure. These results indicate the LIESST effect alone cannot account for the sign and magnitude of the magnetization change in the heterostructure, but the temperature profile of the magnetization shows that significant changes in the NiCr-PBA network are correlated to the spin state of the Hofmann-like SCO network.

Particles of formula Rb0.24Co[Fe(CN)6]0.74@K0.10Co[Cr(CN)6]0.70·nH2O with a light-responsive rubidium cobalt hexacyanoferrate (RbCoFe) core and a magnetic potassium cobalt hexacyanochromate (KCoCr) shell have been prepared and exhibit light-induced changes in the magnetization of the normally light-insensitive KCoCr shell, a new property resulting from the synergy between the core and shell of a coordination polymer heterostructure. A single batch of 135 ± 12 nm RbCoFe particles are used as seeds to generate three different core@shell samples, with KCoCr shell thicknesses of approximately 11, 23 and 37 nm, to probe the influence of the shell thickness over the particles’ morphology and structural and magnetic properties. Synchrotron powder X-ray diffraction reveals that structural changes in the shell accompany the charge transfer induced spin transition (CTIST) of the core, giving direct evidence that the photomagnetic response of the shell is magnetomechanical in origin. The depth to which the KCoCr shell contributes to changes in magnetization is estimated to be approximately 24 nm when using a model that assumes a constant magnetic response of the core within the series of particles. In turn, the presence of the shell changes the nature of the CTIST of the core. As opposed to the usually observed first order transition exhibiting hysteresis, the CTIST becomes continuous in the core@shell particles.

We demonstrate that core–shell nanoparticles consisting of two different Prussian blue analogues, one high capacity and the other robust, can provide enhanced rate capability as cathode materials in sodium-ion batteries.

The influence of particle size on the electrochemical properties of guest-ion storage materials has attracted much attention because of the extensive need for long cycle-life, high energy density, and high power batteries. The present work describes a systematic study of the effect of particle size on the guest-ion storage capabilities of a cyanide-bridged coordination polymer. A series of nickel hexacyanoferrate particles ranging from approximately 40 to 400 nm were synthesized by a co-precipitation method and were used as the cathode material for both Li-ion and Na-ion insertion/extraction experiments using organic electrolyte. A large polarization was observed for the largest particles during Li-ion cycling, indicating a heterogeneous ion concentration within the lattice. As a consequence, the available capacity of Li-ion intercalation at high rates is significantly improved by reducing the particle size. On the other hand, Na-ion intercalation shows excellent rate capability regardless of the particle size.

Three different sizes of Eu0.2Gd0.8PO4·H2O nanoparticles have been prepared to investigate the particle size influence on water proton relaxivity. Longitudinal relaxivity (r1) values increase for smaller particles, reaching as high as r1 = 6.13 mM–1 s–1 for a sample of 40 ± 4 nm particles, which, with a ratio of transverse/longitudinal relaxivity, r2/r1 = 1.27, are shown to be effective positive contrast agents. The correlation between relaxivity and the surface-to-volume ratio implies that access to surface Gd3+ sites is the principal factor affecting relaxivity. On the other hand, although ionic molar relaxivity decreases for larger particles, the relaxivity per particle can be significantly greater. Gadolinium-based nanoparticles doped with fluorescent lanthanide elements have attracted attention for their dual-imaging abilities, combining magnetic resonance imaging (MRI) and fluorescence imaging agents. In both in vitro experiments with HeLa cells and in vivo experiments with C. elegans, strong red fluorescence is observed from Eu0.2Gd0.8PO4·H2O with high resolution, demonstrating the parallel use of the particles as fluorescence imaging agents.

The attachment of affinity proteins onto zirconium phosphonate coated glass slides was investigated by fusing a short phosphorylated peptide sequence at one extremity to enable selective bonding to the active surface via the formation of zirconium phosphate coordinate covalent bonds. In a model study, the binding of short peptides containing zero to four phosphorylated serine units and a biotin end-group was assessed by surface plasmon resonance-enhanced ellipsometry (SPREE) as well as in a microarray format using fluorescence detection of AlexaFluor 647-labeled streptavidin. Significant binding to the zirconated surface was only observed in the case of the phosphopeptides, with the best performance, as judged by streptavidin capture, observed for peptides with three or four phosphorylation sites and when spotted at pH 3. When fusing similar phosphopeptide tags to the affinity protein, the presence of four phosphate groups in the tag allows efficient immobilization of the proteins and efficient capture of their target.

Prussian blue analogues (PBAs) have recently been proposed as electrode materials for low-cost, long-cycle-life, and high-power batteries. However, high-capacity bimetallic examples show poor cycle stability due to surface instabilities of the reduced states. The present work demonstrates that, relative to single-component materials, higher capacity and longer cycle stability are achieved when using Prussian blue analogue core@shell particle heterostructures as the cathode material for Li-ion storage. Particle heterostructures with a size dispersion centered at 210 nm composed of a high-capacity K0.1Cu[Fe(CN)6]0.7·3.8H2O (CuFe-PBA) core and lower capacity but highly stable shell of K0.1Ni[Fe(CN)6]0.7·4.1H2O have been prepared and characterized. The heterostructures lead to the coexistence of both high capacity and long cycle stability because the shell protects the otherwise reactive surface of the highly reduced state of the CuFe-PBA core. Furthermore, interfacial coupling to the shell suppresses a known structural phase transition in the CuFe-PBA core, providing further evidence of synergy between the core and shell. The structure and chemical state of the heterostructure during electrochemical cycling have been monitored with ex situ X-ray diffraction and X-ray absorption experiments and compared to the behavior of the individual components.

A facile surfactant-free route to synthesize uniform Rb0.4M4[Fe(CN)6]2.8·7.2H2O (M = Co, Ni) hollow nanoparticles is described. Rb1.6Mn4[Fe(CN)6]3.2·4.8H2O serves as a sacrificial/removable core in the synthesis of core@shell heterostructures. After dissolution of the cores under very mild conditions, the crystalline hollow nanocubes feature well-defined micro-, meso-, and macropores. The surfactant-free approach preserves the reactivity of the Prussian blue analogue surface as evidenced by the subsequent synthesis of hollow shell@shell heterostructures.Keywords: coordination polymer heterostructures; hierarchical pore system; hollow nanocubes; Prussian blue analogue nanoparticles;

The controlled synthesis of monodisperse nanoparticles of the cubic Prussian blue analogue iron(II) hexacyanochromate(III) is reported along with a kinetic study, using cyanide stretching frequencies, showing the variations of the activation energy (Ea) of the linkage isomerism as a function of the particle size. Highly reproducible, cubic-shaped iron(II) hexacyanochromate(III) nanocrystals, with sizes ranging from 2 to 50 nm, are synthesized using a microemulsion technique, whereas a bulk synthesis yields nonuniform less monodisperse particles with sizes greater than 100 nm. Monitoring the cyanide stretching frequency with FTIR spectroscopy shows that the rate of isomerization is faster for smaller particles. Moreover, a kinetic analysis at different temperatures (255 K ≤ T ≤ 321 K) gives insight into the evolution of Ea with the particle size. Finally, time-dependent powder X-ray diffraction and net magnetization confirm the FTIR observations. The data are interpreted within the concept of a simple two-component model with different activation energies for structures near the surface of the solid and within the bulk.

The Prussian blue analogue K0.4Co1.3[Fe(CN)6]·nH2O has previously been shown to exhibit three distinct phases at low temperature. In addition to the well-studied high-temperature ‘high spin’ and low-temperature ‘low spin’ phases, with unit cells of ∼10.3 and ∼9.95 Å, respectively, an intermediate phase with unit cell a = 10.09 Å was recently reported, although it existed as a minority constituent in a mixed-phase material. Herein we report the synthesis of 130 nm K0.4Co1.3[Fe(CN)6]·4.4 H2O particles that form solely the intermediate phase at temperatures below the normal charge transfer induced spin transition. Compared with those formed from a bulk synthesis, the nanosized particles are shown to have the same chemical composition at high temperature, but they undergo different structural transitions with thermal cycling.Nanosized K0.4Co1.3[Fe(CN)6]·4.4 H2O Prussian blue analogue particles are shown to undergo different structural transitions with thermal cycling relative to bulk sized particles with the same chemical and charge-state composition at high temperature. Particle size reduction can be applied to isolate an intermediate phase in nanosized particles. This intermediate phase only occurs as a minor component in a mixture of phases in the bulk material.

The crystal structures and magnetic properties of four μ-(1,5) singly bridging dicyanamide complexes, specifically Mn{N(CN)2}2(DMSO)2 (1), Fe{N(CN)2}2(DMSO)2 (2), Co{N(CN)2}2(DMSO)2 (3), and Ni{N(CN)2}2(DMSO)2 (4), (DMSO = dimethyl sulfoxide) are presented. The compounds are isostructural, in which the MII ions adopt a distorted octahedral geometry, coordinating to four nitrile N atoms of the dicyanamide ligand and to the two DMSO molecules, giving rise to (4,4) pseudo-2D square grid sheets. Magnetic interactions in these materials are via the five atom dicyanamide bridge [NCNCN], which is known to propagate weak magnetic exchange, and the complexes do not undergo long-range ordering down to 2 K. Single crystal X-ray diffraction, FTIR spectroscopy, CHN analyses, and SQUID magnetometry were used to characterize these materials.The crystal structures and magnetic properties of four layered dicyanamide based coordination polymers are presented. All four compounds are isostructural, forming two dimensional (4,4) pseudo square grid sheets. The compounds are paramagnetic, down to 2 K, with weak antiferromagnetic interactions.

Films of photomagnetic Prussian blue analogue KjCok[Fe(CN)6] (CoFe-PBA) were deposited on thin manganite substrates (La0.4Pr0.6)0.67Ca0.33MnO3 (LPCMO) to investigate whether the photoinduced changes of the CoFe-PBA can give rise to changes in the transport properties of the LPCMO. When the CoFe-PBA was directly deposited on the LPCMO substrate, the photomagnetic properties of the CoFe-PBA were not present because of oxidation of the CoII to CoIII by the Mn in the substrate. The CoII oxidation can be suppressed by using a buffer layer of the non-photomagnetic KjNik[Cr(CN)]6 (NiCr-PBA) between the CoFe-PBA and the LPCMO substrate. Using the buffer layer, the photomagnetic properties of the CoFe-PBA are recovered. Additionally, the transport properties of the LPCMO substrate are not affected by the light-induced changes in the CoFe-PBA.Graphical abstractFilms of photomagnetic Prussian blue analogue KjCok[Fe(CN)6] (CoFe-PBA) were deposited on thin manganite substrates (La0.4Pr0.6)0.67Ca0.33MnO3 (LPCMO). When the CoFe-PBA was directly deposited on the LPCMO substrate, the photomagnetic properties of the CoFe-PBA were not present because of oxidation of the CoII to CoIII, which can be suppressed by using a buffer layer of the non-photomagnetic KjNik[Cr(CN)]6 (NiCr-PBA). The transport properties of the LPCMO are not affected by the light-induced changes in the CoFe-PBA.

Magnetic studies on the Prussian blue analogues (PBAs) LixCuy[Fe(CN)6]z·mH2O (LiCuFe–PBA) and LijNik[Cr(CN)6]l·nH2O (LiNiCr–PBA), as well as LiCuFe@LiNiCr–PBA core–shell heterostructures, have been conducted under pressures ranging from ambient to ≈1.4 GPa and at temperatures of 2–90 K. The results for the single component CuFe–PBA indicate robust magnetic properties under the range of pressures studied where a Tc = 20 K was observed at all pressures. Our pressure studies of single component NiCr–PBA are consistent with previously published results by other workers below 1.0 GPa. However, at pressures above 1.0 GPa, the decrease in magnetization is accompanied by a decrease in the Tc, an indication of changes in the superexchange value. The results obtained with the single component samples can be mapped onto the observations of the heterostructures.The effect of pressure on the magnetization of LiCuFe@LiNiCr Prussian blue analogue core@shell particles (inset, scale: 100 nm) is measured up to ≈1.4 GPa. A decrease in magnetization is observed with increasing pressure together with two transition temperatures, Tclow=20KandTchigh=70K corresponding to the ordering of the LiCuFe–PBA core and LiNiCr–PBA shell, respectively. The results indicate a robust Tclow at all pressures. On the other hand, a decrease in Tchigh is measured at pressures above 1.0 GPa.

Oligonucleotide modified gadolinium phosphate nanoparticles have been prepared and their magnetic resonance relaxivity properties measured. Nanoparticles of GdPO4·H2O were synthesized in a water/oil microemulsion using IGEPAL CO-520 as surfactant, resulting in 50 to 100 nm particles that are highly dispersible and stable in water. Using surface modification chemistry previously established for zirconium phosphonate surfaces, the particles are directly modified with 5′-phosphate terminated oligonucleotides, and the specific interaction of the divalent phosphate with Gd3+ sites at the surface is demonstrated. The ability of the modified nanoparticles to act as MRI contrast agents was determined by performing MR relaxivity measurements at 14.1 T. Solutions of nanopure water, Feridex, and Omniscan (FDA approved contrast agents) in 0.25% agarose were used for comparison and control purposes. MRI data confirm that GdPO4·H2O nanoparticles have relaxivities (r1, r2) comparable to those of commercially available contrast agents. In addition, the data suggest that biofunctionalization of the surface of the nanoparticles does not prevent their function as MRI contrast agents.

Skeletonized zirconium phosphonate surfaces are used to support planar lipid bilayers and are shown to be viable substrates for studying transmembrane proteins. The skeletonized surfaces provide space between the bilayer and the solid support to enable protein insertion and avoid denaturation. The skeletonized zirconium octadecylphosphonate surfaces were prepared using Langmuir–Blodgett techniques by mixing octadecanol with octadecylphosphonic acid. After zirconation of the transferred monolayer, rinsing the coating with organic solvent removes the octadecanol, leaving holes in the film ranging from ∼50 to ∼500 nm in diameter, depending on the octadecanol content. Upon subsequent deposition of a lipid bilayer, either by vesicle fusion or by Langmuir–Blodgett/Langmuir–Schaefer techniques, the lipid assemblies span the holes providing reservoirs beneath the bilayer. The viability of the supported bilayers as model membranes for transmembrane proteins was demonstrated by examining two approaches for incorporating the proteins. The BK channel protein inserts directly into a preformed bilayer on the skeletonized surface, in contrast to a bilayer on a nonskeletonized film, for which the protein associates only weakly. As a second approach, the integrin α5β1 was reconstituted in lipid vesicles, and its inclusion in supported bilayers on the skeletonized surface was achieved by vesicle fusion. The integrin retains its ability to recognize the extracellular matrix protein fibronectin when supported on the skeletonized film, again in contrast to the response if the bilayer is supported on a nonskeletonized film.

Many applications of molecule based magnets, whether they are in information storage, displays, or as components in electronic or spintronic devices, will require putting the active materials on a surface or interfacing them with other components. Although there are many examples of molecule-based magnets, the families of tetracyanoethylene (TCNE) based magnets and Prussian-blue analogs possess materials properties that are close to those required for practical applications, and are the most advanced with respect to studies as thin films. This critical review focuses on fabrication and characterization of thin films of TCNE and Prussian-blue analog coordination polymer magnets. Emphasis is on current developments in thin film heterostructures and potential spintronics applications (135 references).

Heterostructured thin films consisting of distinct layers of the Prussian blue analogues RbaCob[Fe(CN)6]c·mH2O (CoFe PBA) and RbjMk[Cr(CN)6]l·nH2O (MCr PBA, where M = Ni or Co) have been fabricated, and their photomagnetic properties have been investigated. The CoFe PBA is known to be photoactive, with light induced changes in the unit cell size and the spin states below ∼150 K and magnetic order below ∼20 K. The NiCr and CoCr PBAs do not have native photoeffects, but are known to have higher magnetic ordering temperatures (TCNiCr ∼ 70 K, TCCoCr ∼ 30 K), and a pressure dependence of the magnetization. The layered heterostructures are synthesized using aqueous chemistry and sequential adsorption techniques that allow for fine control of layer thickness. Some of the heterostructured films show photoinduced magnetization changes up to the ordering temperatures of the MCr PBA component, behavior that is not seen when the individual materials are measured separately. A variety of different layer arrangements and thicknesses has been investigated with the goal of identifying structures that optimize the photocontrol of the magnetic response in the MCr PBA lattices, which are in intimate contact with the photoactive CoFe PBA lattices. The new behavior is optimized when the constituent layers have thicknesses on the order of hundreds of nanometers. When layers are too thin, it is shown that mixing of ions at the interface between PBA components leads to mixed-metal phases. The concurrence of the maximum temperature of the large photomagnetic effect with the native ordering temperature of the MCr PBA lattice, as well as its magnetic field dependence, supports the interpretation that the photocontrol is the result of photoinduced structural changes in the CoFe PBA lattice coupling to the MCr PBA component of the heterostructure, inducing random magnetic anisotropy.Keywords: film; heterostructure; magnetism; photoinduced magnetism; Prussian blue analogue;

Core/shell and core/shell/shell particles comprised of the Prussian blue analogues KjNik[Cr(CN)6]l·nH2O (A) and RbaCob[Fe(CN)6]c·mH2O (B) have been prepared for the purpose of studying persistent photoinduced magnetization in the heterostructures. Synthetic procedures have been refined to allow controlled growth of relatively thick (50–100 nm) consecutive layers of the Prussian blue analogues while minimizing the mixing of materials at the interfaces. Through changes in the order in which the two components are added, particles with AB, ABA, BA, and BAB sequences have been prepared. The two Prussian blue analogues were chosen because B is photoswitchable, and A is ferromagnetic with a relatively high magnetic ordering temperature, ∼70 K, although it is not known to exhibit photoinduced changes in its magnetic properties. Magnetization measurements on the heterostructured particles performed prior to irradiation show behavior characteristic of the individual components. On the other hand, after irradiation with visible light, the heterostructures undergo persistent photoinduced changes in magnetization associated with both the B and A analogues. The results suggest that structural changes in the photoactive B component distort the normally photoinactive A component, leading to a change in its magnetization.

Heterostructured ABA thin films consisting of two different Prussian blue analogues, where A is a ferromagnet and B is a photoinducible ferrimagnet, have been fabricated for the first time. This novel arrangement allows the magnetization to be decreased by irradiation with white light and significantly increases the ordering temperature of the photoinduced magnetism from 18 to 75 K.

Urinary stones are commonly composed of an inorganic component, calcium oxalate, or calcium phosphate and an organic matrix of lipids, carbohydrates, and proteinaceous matter. Of interest is the role that the organic matrix elements may play as catalysts for the heterogeneous nucleation of the calcium salts, and a number of studies have examined the role of lipids in calcium oxalate monohydrate (COM) formation. In this study, products of lipid hydrolysis from phospholipase A2 (PLA2) are examined for their effect on COM formation using Langmuir monolayers as model lipid membrane assemblies. The enzyme PLA2 hydrolyzes DPPC monolayers in the presence of a supersaturated calcium oxalate subphase, inducing the rapid and plentiful nucleation of calcium oxalate at the lipid interface. To investigate the cause of increased crystal formation in the presence of the enzyme, Langmuir monolayers modeling the hydrolysis products were investigated. Calcium oxalate crystal growth at a ternary monolayer of dipalmitoylphosphatidylcholine (DPPC), palmitic acid (PA), and a 22-carbon chain lysophospholipid (22:0 Lyso PC) dramatically increases relative to monolayers of just DPPC. Binary monolayers of DPPC with either PA or the 22:0 Lyso PC and single-component monolayers of PA were also studied. It is demonstrated that the fatty acid generated during lipid hydrolysis causes a significant increase in the extent of heterogeneous nucleation of calcium oxalate from supersaturated solutions. The results imply a possible link between increased phospholipase activity, which is associated with hyperoxaluria, and calcium oxalate precipitation.

Tuning the composition of the ternary transition-metal Prussian blue analogue NaαNi1−xCox[Fe(CN)6]β·nH2O allows the sign of the photoinduced change in magnetization to be controlled. The parent cobalt hexacyanoferrate material is well-known to display photoinduced and thermal charge-transfer-induced spin transitions (CTISTs). Upon partial replacement of Co ion sites with NiII, irradiation with halogen light can cause either an increase or a decrease in magnetization, depending upon the extent of NiII substitution, the applied field, and the temperature. For all compositions with x > 0, photoexcitation generates new moments according to the same mechanism observed for the parent x = 1 compound. However, the presence of NiII introduces a superexchange of opposite sign, providing a mechanism for controlling the sign of the change in magnetization with applied light. Additionally, dilution of the spin-crossover material reduces the magnitude and hysteresis of the thermal CTIST effect. These effects can be qualitatively explained by simple mean-field models.

Supported lipid bilayers that can fully represent biological cell membranes are attractive biomimetic models for biophysical and biomedical applications. In this study, we develop a new approach to engineering stable supported lipid membranes and demonstrate their utility for the study of protein−membrane interactions. This system uses a zirconium phosphonate monolayer to modify a substrate and generate a reactive surface that tethers the lipid membrane via a highly covalent bond between surface zirconium ions and divalent phosphate groups in the lipid assembly, for example, from phosphatidic acid. An advantage of the approach is that the zirconium phosphonate modifier can be applied to nearly any surface, allowing the same methods to be used on glass, gold, silicon, or plastic supports. The lipid bilayers are formed by vesicle fusion, either directly on the zirconated surface to form symmetric bilayers or following deposition of a Langmuir−Blodgett lipid layer to generate asymmetric bilayers. The membrane formation was studied by surface plasmon resonance enhanced ellipsometry (SPREE) as the phosphatidic acid composition was varied. We found that 10% of phosphatidic acid generates supported lipid bilayers stable to dehydration. The two-dimensional fluidity of these systems was characterized by fluorescence recovery after photobleaching (FRAP) measurements. Uniform, mobile supported lipid bilayers with lipid diffusion coefficients of ∼4 μm2/s were obtained. SPREE was also used to measure kinetic parameters of the binding of melittin, a bee venom peptide, to asymmetric lipid bilayers with different electrostatic properties. The results are comparable to those obtained by other research groups, confirming that the model membranes behave as expected. Overall, the results of this study prove that supported lipid bilayers on zirconium phosphonate inorganic surfaces make up an attractive biomimetic system that is highly stable, can be used with multiple substrates, and does not require any biomolecule synthetic modifications.

The photoinduced magnetism in thin films of the Prussian blue analogue AjCok[Fe(CN)6]l·nH2O deposited on Melinex solid supports is anisotropic, exhibiting a photoinduced increase when oriented parallel to the external magnetic field and a decrease when oriented perpendicular. The anisotropic behavior is observed for films less than ∼200 nm thick below nominally 10 K, which is less than the ferrimagnetic ordering temperature (TC) of 17 K, and in applied fields of less than ∼1.5 kG. The thin films with formulas Rb0.7Co4[Fe(CN)6]3.0, Rb2.3Co4[Fe(CN)6]3.1, and K0.5Co4[Fe(CN)6]3.2 were generated using sequential adsorption methods, alternately immersing the Melinex solid support in solutions of the constituent Co2+ and [Fe(CN)6]3− ions. Film thickness is controlled by the number of deposition cycles and by variations in the deposition protocols. Measurements on films with different alkali ions and with different stoichiometry indicate that the microscopic mechanism for the photoinduced magnetization is the same in the films as in the bulk material. The unique anisotropy is qualitatively associated with the interface between the magnetic film and the solid support. Detailed studies of the influence of film thickness, applied field strength, and temperature support this description.

The surface coverage of phosphorylated oligonucleotides immobilized on a zirconium-phosphonate surface was analyzed using X-ray photoelectron spectroscopy (XPS). By quantifying the intensity of the N 1s signal originating from the oligonucleotide and the Zr 3d peak from the metal-phosphonate surface, the surface coverage of the oligonucleotide could be calculated with a modified substrate-overlayer model. We found relatively low surface coverages indicating that once covalently bound via the terminal phosphate the polymer chain further physisorbs to the surface limiting the adsorption of additional molecules.

Recent theories have revealed topological differences between one-dimensional (1D) Heisenberg antiferromagnets (HAFs) with odd and even integer spins (S). With this renewed interest, we have investigated the low-field magnetic susceptibility, specific heat, and high-field magnetization of single-crystal samples of MnCl3(bpy), bpy=2,2t-bipyridine, which is regarded as an S = 2 quasi-1D HAF. With the magnetic field oriented along the a*, b and c directions, the magnetic susceptibilities possess broad maxima around 100K. When the magnetic field is parallel to the b-axis, a sharp peak at 11K is observed in the magnetic susceptibility, while a smaller feature is detected in the specific heat, indicating a transition to long-range antiferromagnetic order. At 1.7K, the high-field magnetization curve along the c-axis shows a spin-flop transition, while the responses in the other directions show monotonic increases. These experimental findings confirm that MnCl3(bpy) is not an ideal model system to study the S=2 Haldane phase.

Cubic heterostructured core@shell particles of Prussian blue analogues, composed of a shell (∼80 nm thick) of ferromagnetic K0.3Ni[Cr(CN)6]0.8·1.3H2O (A), Tc∼70 K, surrounding a bulk (∼350 nm) core of photoactive ferrimagnetic Rb0.4Co[Fe(CN)6]0.8·1.2H2O (B), Tc∼20 K, have been studied. Below nominally 70 K, these CoFe@NiCr samples exhibit a persistent photoinduced decrease in low-field magnetization, and these results resemble data from other BA core@shell particles and analogous ABA heterostructured films. This net decrease suggests that the photoinduced lattice expansion in the B layer generates a strain-induced decrease in the magnetization of the NiCr layer, similar to a pressure-induced decrease observed by others in a similar, pure NiCr material and by us in our CoFe@NiCr cubes. Upon further examination, the data also reveal a significant portion of the NiCr shell whose magnetic superexchange, J, is perturbed by the photoinduced strain from the CoFe constituent.Graphical abstractBelow 70 K, Prussian blue analogue core@shell particles of K0.3Ni[Cr(CN)6]0.8·1.3H2O surrounding Rb0.4Co[Fe(CN)6]0.8·1.2H2O exhibit a photoinduced decrease in magnetization, which suggests a strain-induced decrease in the magnetization of the NiCr–PBA, as well as an accompanying perturbation of its superexchange, resulting from this photoinduced strain.Download full-size image

The local structure within the Co–Fe atomic array of the photoswitchable coordination polymer magnet, K0.3Co[Fe(CN)6]0.77·nH2O, is directly observed during charge transfer induced spin transition (CTIST), a solid–solid phase change, using high-resolution transmission electron microscopy (HRTEM). Along with the low-spin (LS) or thermally quenched high-spin (HS) states normally observed in CTIST solids at low temperature, slow cooling of K0.3Co[Fe(CN)6]0.77·nH2O results in an intermediate phase containing both HS and LS domains with short coherence length. By mapping individual metal–metal distances, the nanometer-scale HS domains are directly visualized within the LS array. Temperature-dependent analyses allow monitoring of HS domain coarsening along the warming branch of the CTIST, providing direct visualization of the elastic process and insight into the mechanism of phase propagation. Normally sensitive to electron beam damage, the low-temperature TEM measurements of the porous coordination polymer are enabled by using appropriate ionic liquids instead of usual conductive thin-film coatings, an approach that should find general utility in related classes of materials.

New nanometer scale heterostructure particles of the two-dimensional Hofmann-like Fe(II) spin-crossover network, Fe(phpy)2[Ni(CN)4]·0.5H2O {phpy = 4-phenylpyridine}, and the Prussian blue analogue K0.4Ni1.0[Cr(CN)6]0.8·nH2O (NiCr-PBA) have been developed, exhibiting synergistic photomagnetic effects, whereby the LIESST (light-induced electron spin-state trapping) effect in the Hofmann-like material induces a magnetization change in the NiCr-PBA. A variety of microscopic and spectroscopic techniques demonstrate the heterogeneous growth of the NiCr-PBA on the Hofmann seed particles and show the Hofmann compound retains its thermal and photoinduced spin transition properties in the heterostructure. The photoinduced magnetization change in the NiCr-PBA network arises from coupling of the two lattices despite dissimilar structure types. Isothermal magnetization minor hysteresis loop studies at 5 K show light absorption leads to changes in the local anisotropy of NiCr-PBA magnetic domains, providing direct evidence for a general magnetomechanical mechanism of light-switchable magnetism in coordination polymer heterostructures combining a photoactive material with a magnet.

Many applications of molecule based magnets, whether they are in information storage, displays, or as components in electronic or spintronic devices, will require putting the active materials on a surface or interfacing them with other components. Although there are many examples of molecule-based magnets, the families of tetracyanoethylene (TCNE) based magnets and Prussian-blue analogs possess materials properties that are close to those required for practical applications, and are the most advanced with respect to studies as thin films. This critical review focuses on fabrication and characterization of thin films of TCNE and Prussian-blue analog coordination polymer magnets. Emphasis is on current developments in thin film heterostructures and potential spintronics applications (135 references).

We demonstrate that core–shell nanoparticles consisting of two different Prussian blue analogues, one high capacity and the other robust, can provide enhanced rate capability as cathode materials in sodium-ion batteries.