We present a short overview of the influence of inter-particle interactions on the properties of magnetic nanoparticles. Strong magnetic dipole interactions between ferromagnetic or ferrimagnetic particles, that would be superparamagnetic if isolated, can result in a collective state of nanoparticles. This collective state has many similarities to spin-glasses. In samples of aggregated magnetic nanoparticles, exchange interactions are often important and this can also lead to a strong suppression of superparamagnetic relaxation. The temperature dependence of the order parameter in samples of strongly interacting hematite nanoparticles or goethite grains is well described by a simple mean field model. Exchange interactions between nanoparticles with different orientations of the easy axes can also result in a rotation of the sub-lattice magnetization directions.
dipole interactions; exchange interactions; spin structure; superferromagnetism; superparamagnetic relaxation
We have replaced the periodic Prandtl–Tomlinson model with an atomic-scale friction model with a random roughness term describing the surface roughness of micro-electromechanical systems (MEMS) devices with sliding surfaces. This new model is shown to exhibit the same features as previously reported experimental MEMS friction loop data. The correlation function of the surface roughness is shown to play a critical role in the modelling. It is experimentally obtained by probing the sidewall surfaces of a MEMS device flipped upright in on-chip hinges with an AFM (atomic force microscope). The addition of a modulation term to the model allows us to also simulate the effect of vibration-induced friction reduction (normal-force modulation), as a function of both vibration amplitude and frequency. The results obtained agree very well with measurement data reported previously.
MEMS; microscale friction reduction; normal force modulation; stochastic Prandtl–Tomlinson model; surface roughness
The trajectories of differently shaped nanoparticles manipulated by atomic force microscopy are related to the scan path of the probing tip. The direction of motion of the nanoparticles is essentially fixed by the distance b between consecutive scan lines. Well-defined formulas are obtained in the case of rigid nanospheres and nanowires. Numeric results are provided for symmetric nanostars. As a result, orienting the fast scan direction perpendicular to the desired direction of motion and reducing b well below the linear size of the particles turns out to be an efficient way to control the nanomanipulation process.
atomic force microscopy; nanomanipulation; nanoparticles
Wavelet transform analysis is applied to a thermally excited cantilever to get insights into fundamental thermodynamical properties of its motion. The shortcomings of the widely used Fourier analysis are briefly discussed to put into perspective the wavelet transform analysis, used to describe the temporal evolution of the spectral content of the thermal oscillations of a cantilever with an interacting tip. This analysis allows to retrieve the force gradients, the forces and the Hamaker constant in a measurement time of less than 40 ms.
AFM; force; graphite; thermal excitation; wavelet transforms
Rapid and accurate measurements of protein biomarkers, pathogens and cells in biological samples could provide useful information for early disease diagnosis, treatment monitoring, and design of personalized medicine. In general, biological samples have only negligible magnetic susceptibility. Thus, using magnetic nanoparticles for biosensing not only enhances sensitivity but also effectively reduces sample preparation needs. This review focuses on the use of magnetic nanoparticles for in vitro detection of biomolecules and cells based on magnetic resonance effects. This detection platform, termed diagnostic magnetic resonance (DMR), exploits magnetic nanoparticles as proximity sensors, which modulate the spin–spin relaxation time of water molecules surrounding molecularly-targeted nanoparticles. By developing more effective magnetic nanoparticle biosensors, DMR detection limits for various target moieties have been considerably improved over the last few years. Already, a library of magnetic nanoparticles has been developed, in which a wide range of targets, including DNA/mRNA, proteins, small molecules/drugs, bacteria, and tumor cells, have been quantified. More recently, the capabilities of DMR technology have been further advanced with new developments such as miniaturized nuclear magnetic resonance detectors, better magnetic nanoparticles and novel conjugational methods. These developments have enabled parallel and sensitive measurements to be made from small volume samples. Thus, the DMR technology is a highly attractive platform for portable, low-cost, and efficient biomolecular detection within a biomedical setting.
biosensor; diagnostics; magnetic nanoparticle; microfluidics; nuclear magnetic resonance
A glassy carbon electrode (GC) was modified by multi-walled carbon nanotubes (MWCNTs). The modified electrode showed a pair of redox peaks that resulted from the oxygen-containing functional groups on the nanotube surface. A recombinant thermostable dye-linked L-proline dehydrogenase (L-proDH) from hyperthermophilic archaeon (Thermococcus profundus) was further immobilized by physical adsorption. The modified electrode (GC/MWCNTs/L-proDH) exhibited an electrocatalytic signal for L-proline compared to bare GC, GC/L-proDH and GC/MWCNTs electrodes, which suggested that the presence of MWCNTs efficiently enhances electron transfer between the active site of enzyme and electrode surface. The immobilized L-proDH showed a typical Michaelis–Menten catalytic response with lower apparent constant.
dye-linked L-proline dehydrogenase; electrocatalysis; electron transfer; multi-walled carbon nanotube
Single crystal ZnO nanotube arrays were synthesized at low temperature in an aqueous solution containing zinc nitrate and hexamethylenetetramine. It was found that the pH value of the reaction solution played an important role in mediating the growth of ZnO nanostructures. A change in the growth temperature might change the pH value of the solution and bring about the structure conversion of ZnO from nanorods to nanotubes. It was proposed that the ZnO nanorods were initially formed while the reaction solution was at a relatively high temperature (~90 °C) and therefore enriched with colloidal Zn(OH)2, which allowed a fast growth of ZnO nanocrystals along the  orientation to form nanorods. A decrease in the reaction temperature yielded a supersaturated solution, resulting in an increase in the concentration of OH− ions as well as the pH value of the solution. Colloidal Zn(OH)2 in the supersaturated solution trended to precipitate. However, because of a slow diffusion process in view of the low temperature and low concentration of the colloidal Zn(OH)2, the growth of the (001) plane of ZnO nanorods was limited and only occurred at the edge of the nanorods, eventually leading to the formation of a nanotube shape. In addition, it was demonstrated that the pH might impact the surface energy difference between the polar and non-polar faces of the ZnO crystal. Such a surface energy difference became small at high pH and hereby the prioritized growth of ZnO crystal along the  orientation was suppressed, facilitating the formation of nanotubes. This paper demonstrates a new strategy for the fabrication of ZnO nanotubes on a large scale and presents a more comprehensive understanding of the growth of tube-shaped ZnO in aqueous solution at low temperature.
growth from solutions; nanorods; nanotubes; supersaturation; ZnO
Polyvinyl alcohol-capped CdSe quantum dots, with a size within their quantum confinement limit, were prepared in aqueous solution at room temperature, by a simple and environmentally friendly chemical method. The size of the CdSe quantum dots was found to be dependent on the concentrations of the precursors of cadmium and selenium ions, as well as on the aging time and the reaction temperature; all of which could be used conveniently for tuning the size of the particles, as well as their optical properties. The synthesized quantum dots were characterized by optical absorption spectroscopy, fluorescence spectroscopy, X-ray diffraction, atomic force microscopy and transmission electron microscopy. The samples were fluorescent at room temperature; the green fluorescence was assigned to band edge emission, and the near-infrared fluorescence peaks at about 665 and 865 nm were assigned to shallow and deep trap states emissions, respectively. The quantum dots were fairly stable up to several days.
cadmium selenide; electron microscopy; energy dispersive X-ray analysis (EDAX); semiconductor
The results of the investigation of the structural and magnetic (static and dynamic) properties of an assembly of metallic Fe nanoparticles synthesized by an organometallic chemical method are described. These nanoparticles are embedded in a polymer, monodisperse, with a diameter below 2 nm, which corresponds to a number of around 200 atoms. The X-ray absorption near-edge structure and Mössbauer spectrum are characteristic of metallic Fe. The structural studies by wide angle X-ray scattering indicate an original polytetrahedral atomic arrangement similar to that of β-Mn, characterized by a short-range order. The average magnetic moment per Fe atom is raised to 2.59 µB (for comparison, bulk value of metallic Fe: 2.2 µB). Even if the spontaneous magnetization decreases rapidly as compared to bulk materials, it remains enhanced even up to room temperature. The gyromagnetic ratio measured by ferromagnetic resonance is of the same order as that of bulk Fe, which allows us to conclude that the orbital and spin contributions increase at the same rate. A large magnetic anisotropy for metallic Fe has been measured up to (3.7 ± 1.0)·105 J/m3. Precise analysis of the low temperature Mössbauer spectra, show a broad distribution of large hyperfine fields. The largest hyperfine fields display the largest isomer shifts. This indicates a progressive increase of the magnetic moment inside the particle from the core to the outer shell. The components corresponding to the large hyperfine fields with large isomer shifts are indeed characteristic of surface atoms.
iron nanoparticles; magnetic properties; organometallic synthesis; size effects; structure
Magnetic γ-Fe2O3 nanoparticles with a mean diameter of 20 nm and size distribution of 7% were chemically synthesized and spin-coated on top of a Si-substrate. As a result, the particles self-assembled into a monolayer with hexagonal close-packed order. Subsequently, the nanoparticle array was coated with a Co layer of 20 nm thickness. The magnetic properties of this composite nanoparticle/thin film system were investigated by magnetometry and related to high-resolution transmission electron microscopy studies. Herein three systems were compared: i.e. a reference sample with only the particle monolayer, a composite system where the particle array was ion-milled prior to the deposition of a thin Co film on top, and a similar composite system but without ion-milling. The nanoparticle array showed a collective super-spin behavior due to dipolar interparticle coupling. In the composite system, we observed a decoupling into two nanoparticle subsystems. In the ion-milled system, the nanoparticle layer served as a magnetic flux guide as observed by magnetic force microscopy. Moreover, an exchange bias effect was found, which is likely to be due to oxygen exchange between the iron oxide and the Co layer, and thus forming of an antiferromagnetic CoO layer at the γ-Fe2O3/Co interface.
exchange bias; iron oxide nanoparticles; nanoparticle self-assembly; nanoparticle-thinfilm composite; super-spin glass interaction
Highly luminescent nanocomposites were prepared by incorporating CdSe/CdS core/shell nanorods into different polymer matrices. The resulting nanocomposites show high transparency of up to 93%. A photoluminescence quantum efficiency of 70% was obtained, with an optimum combination of nanorod (0.05 wt %) and at a UV-initiator concentration of 0.1 wt % for poly(lauryl methacrylate). Nanorods tend to agglomerate in cellulose triacetate.
CdSe; luminescence lifetime; nanocomposites; nanorods; quantum yield
We present a short review of the magnetic excitations in nanoparticles below the superparamagnetic blocking temperature. In this temperature regime, the magnetic dynamics in nanoparticles is dominated by uniform excitations, and this leads to a linear temperature dependence of the magnetization and the magnetic hyperfine field, in contrast to the Bloch T
3/2 law in bulk materials. The temperature dependence of the average magnetization is conveniently studied by Mössbauer spectroscopy. The energy of the uniform excitations of magnetic nanoparticles can be studied by inelastic neutron scattering.
collective magnetic excitations; Mössbauer spectroscopy; neutron scattering; spin waves; superparamagnetic relaxation
Hydrothermally grown ZnO nanorods have inherent crystalline defects primarily due to oxygen vacancies that enhance optical absorption in the visible spectrum, opening up possibilities for visible light photocatalysis. Comparison of photocatalytic activity of ZnO nanorods and nanoparticle films on a test contaminant methylene blue with visible light irradiation at 72 kilolux (klx) showed that ZnO nanorods are 12–24% more active than ZnO nanoparticulate films. This can be directly attributed to the increased effective surface area for adsorption of target contaminant molecules. Defects, in the form of interstitials and vacancies, were intentionally created by faster growth of the nanorods by microwave activation. Visible light photocatalytic activity was observed to improve by ≈8% attributed to the availability of more electron deficient sites on the nanorod surfaces. Engineered defect creation in nanostructured photocatalysts could be an attractive solution for visible light photocatalysis.
defects; nanoparticle; nanorod; photocatalysis; pollutant; ZnO
Two dimensional island arrays and honeycomb patterns consisting of ZnO nanocrystal clusters were fabricated on predefined TiO2 seed patterns prepared by vacuum free, aerosol assisted wet-chemical synthesis. The TiO2 seed patterns were prepared by applying an aerosol of a water soluble titanium complex on hexagonally close-packed polystyrene bead arrays for different lengths of time. Scanning electron microscopy revealed that a dot array grows into a honeycomb shape as increasing amounts of the precursor were deposited. ZnO nucleation on substrates with a dot array and honeycomb patterns resulted in the formation of two discrete patterns with contrasting fill fractions of the materials.
aerosol; photonic crystal; polystyrene bead; TiO2; ZnO
The concept of reticular chemistry is investigated to explore the applicability of the formation of Covalent Organic Frameworks (COFs) from their defined individual building blocks. Thus, we have designed, optimized and investigated a set of reported and hypothetical 2D COFs using Density Functional Theory (DFT) and the related Density Functional based tight-binding (DFTB) method. Linear, trigonal and hexagonal building blocks have been selected for designing hexagonal COF layers. High-symmetry AA and AB stackings are considered, as well as low-symmetry serrated and inclined stackings of the layers. The latter ones are only slightly modified compared to the high-symmetry forms, but show higher energetic stability. Experimental XRD patterns found in literature also support stackings with highest formation energies. All stacking forms vary in their interlayer separations and band gaps; however, their electronic densities of states (DOS) are similar and not significantly different from that of a monolayer. The band gaps are found to be in the range of 1.7–4.0 eV. COFs built of building blocks with a greater number of aromatic rings have smaller band gaps.
covalent organic frameworks; DFTB; energetic and electronic properties; layer stacking; XRD
We propose an original route to prepare magnetic alloy nanoparticles with uniform size and shape by using nanosecond annealing under pulsed laser irradiation. As demonstrated here on CoPt nanoparticles, flash laser annealing gives an unprecedented opportunity to control the size and the shape of bimetallic nanoparticles without changing their composition. The mechanisms involved in the complete reshaping of the nanoparticle thin films are discussed and it is also shown that order-disorder phase transformations occur under laser irradiation. This technique is then very interesting for magnetic alloy nanoparticles studies and applications because it opens up a new way to fabricate size-controlled spherical nanoparticles with narrow size dispersion.
magnetic alloy nanoparticles; nanoparticle morphology; nanosecond pulsed laser annealing; order-disorder transformation
This paper highlights recent advances in synthesis, self-assembly and sensing applications of monodisperse magnetic Co and Co-alloyed nanoparticles. A brief introduction to solution phase synthesis techniques as well as the magnetic properties and aspects of the self-assembly process of nanoparticles will be given with the emphasis placed on selected applications, before recent developments of particles in sensor devices are outlined. Here, the paper focuses on the fabrication of granular magnetoresistive sensors by the employment of particles themselves as sensing layers. The role of interparticle interactions is discussed.
bottom-up particle synthesis; dipolar particle coupling; granular giant magnetoresistance sensor; magnetic nanoparticles; self-assembly
Polymers are often used to modify surface properties to control interfacial processes. Their sensitivity to solvent conditions and ability to undergo conformational transitions makes polymers attractive in tailoring surface properties with specific functionalities leading to applications in diverse areas ranging from tribology to colloidal stability and medicine. A key example is polyethylene glycol (PEG), which is widely used as a protein-resistant coating given its low toxicity and biocompatibility. We report here a microcantilever-based sensor for the in situ characterization of PEG monolayer formation on Au using the “grafting to” approach. Moreover, we demonstrate how microcantilevers can be used to monitor conformational changes in the grafted PEG layer in different solvent conditions. This is supported by atomic force microscope (AFM) images and force–distance curve measurements of the microcantilever chip surface, which show that the grafted PEG undergoes a reversible collapse when switching between good and poor solvent conditions, respectively.
AFM; cantilever sensor; polyethylene glycol; polymer brush; reversible collapse; static mode
Monatomic (Fe, Co) and bimetallic (FePt and CoPt) nanoparticles were prepared by exploiting the self-organization of precursor loaded reverse micelles. Achievements and limitations of the preparation approach are critically discussed. We show that self-assembled metallic nanoparticles can be prepared with diameters d = 2–12 nm and interparticle distances D = 20–140 nm on various substrates. Structural, electronic and magnetic properties of the particle arrays were characterized by several techniques to give a comprehensive view of the high quality of the method. For Co nanoparticles, it is demonstrated that magnetostatic interactions can be neglected for distances which are at least 6 times larger than the particle diameter. Focus is placed on FePt alloy nanoparticles which show a huge magnetic anisotropy in the L10 phase, however, this is still less by a factor of 3–4 when compared to the anisotropy of the bulk counterpart. A similar observation was also found for CoPt nanoparticles (NPs). These results are related to imperfect crystal structures as revealed by HRTEM as well as to compositional distributions of the prepared particles. Interestingly, the results demonstrate that the averaged effective magnetic anisotropy of FePt nanoparticles does not strongly depend on size. Consequently, magnetization stability should scale linearly with the volume of the NPs and give rise to a critical value for stability at ambient temperature. Indeed, for diameters above 6 nm such stability is observed for the current FePt and CoPt NPs. Finally, the long-term conservation of nanoparticles by Au photoseeding is presented.
Co; CoPt; core–shell particles; FePt; magnetic anisotropy; magnetic particles; plasma etching; reverse micelles; self-assembly