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Intermittent jumping force is an operational atomic-force microscopy mode that produces simultaneous topography and tip-sample maximum-adhesion images based on force spectroscopy. In this work, the operation conditions have been implemented scanning in a repulsive regime and applying very low forces, thus avoiding unspecific tip-sample forces. Remarkably, adhesion images give only specific rupture events, becoming qualitative and quantitative molecular recognition maps obtained at reasonably fast rates, which is a great advantage compared to the force–volume modes. This procedure has been used to go further in discriminating between two similar protein molecules, avidin and streptavidin, in hybrid samples. The adhesion maps generated scanning with biotinylated probes showed features identified as avidin molecules, in the range of 40–80 pN; meanwhile, streptavidin molecules rendered 120–170 pN at the selected working conditions. The gathered results evidence that repulsive jumping force mode applying very small forces allows the identification of biomolecules through the specific rupture forces of the complexes and could serve to identify receptors on membranes or samples or be applied to design ultrasensitive detection technologies.

Magnetoferritin molecules with an average inorganic core diameter of 5.7±1.6 nm and polycrystalline internal structure were investigated by a combination of transmission electron microscopy, magnetic susceptibility, magnetization, and electron magnetic resonance (EMR) experiments. The temperature and frequency dependence of the magnetic susceptibility allowed for the determination of the magnetic anisotropy on an experimental time scale which spans from seconds to nanoseconds. In addition, angle-dependent EMR experiments were carried out for the determination of the nanoparticle symmetry and internal magnetic field. Due to the large surface to volume ratio, the nanoparticles show larger and uniaxial rather than cubic magnetic anisotropies compared to bulk maghemite and magnetite. © 2014 Elsevier B.V.

In this work we report the development and calibration of an ultrasensitive SQUID susceptometer capable of operating over an extremely wide frequency range (0.001 Hz-1 MHz) at extremely low temperatures (13 mK). Starting with 2-stage SQUID sensors fabricated at PTB-Berlin, an integrated susceptometer with experimental spin sensitivity of 104 Bohr magneton/Hz1/2 is obtained by rerouting some SQUID input circuit connections. Modification of the chips is carried out using Focused Ion Beam Induced Deposition (FIBID) of amorphous W, using W(CO)6 as precursor gas. We have demonstrated that superconducting connections between W FIBID lines and Nb films can be fabricated and that they operate at 4.2 K, providing a powerful technique for fixing errors in device designs, repairing damaged circuits, or modifying existing ones. The microsusceptometer offers new possibilities in the study of thin films or even monolayers of nanosized magnets, and has potential applications in diverse fields such as quantum computing, high-density information storage or on-chip magnetic refrigeration.

We report a detailed experimental study of maghemite nanoparticles, with sizes ranging from 1.6 to 6 nm, synthesized inside a biological mould of apoferritin. The structural characterization of the inorganic cores, using TEM and x-ray diffraction, reveals a low degree of crystalline order, possibly arising from the nucleation and growth of multiple domains inside each molecule. We have also investigated the molecular structure by means of atomic force microscopy in liquid. We find that the synthesis of nanoparticles inside apoferritin leads to a small, but measurable, decrease in the external diameter of the protein, probably associated with conformational changes. The magnetic response of the maghemite cores has been studied by a combination of techniques, including ac susceptibility, dc magnetization and Mössbauer spectroscopy. From the equilibrium magnetic response, we have determined the distribution of magnetic moments per molecule. The results show highly reduced magnetic moments. This effect cannot be ascribed solely to the canting of spins located at the particle surface but, instead, it suggests that magnetoferritin cores have a highly disordered magnetic structure in which the contributions of different domains compensate each other. Finally, we have also determined, for each sample, the distribution of the activation energies required for the magnetization reversal and, from this, the size-dependent magnetic anisotropy constant K. We find that K is enormously enhanced with respect to the maghemite bulk value and that it increases with decreasing size. The Mössbauer spectra suggest that low-symmetry atomic sites, probably located at the particle surface and at the interfaces between different crystalline domains, are the likely source of the enhanced magnetic anisotropy.

In this manuscript we demonstrate the spatially controlled immobilization of ferritin proteins by directly writing them on a wide range of substrates of technological interest. Optical and fluorescence microscopy, AFM and TOF-SIMS studies confirm the successful deposition of the protein on those surfaces. Control on nanostructure shape and size, by miniaturizing the dot-like features down to a 100 nm, demonstrates the particular capabilities of the DPN approach. Ultimately, this study gives the opportunity to design nanoparticle-based arrays regarding the growing interest in the use of nanoparticles as structural and functional elements for fabricating nanodevices. Herein, we demonstrate how the protein shell of ferritins can be removed by a simple heat-treatment process while maintaining the encapsulated inorganic nanoparticle intact on the same location of the nanoarray. As a result, this study establishes how direct-write DPN approach could give the opportunity to design not only protein-based nanoarrays but also nanoparticle-based nanoarrays with high-resolution and control.

Desired numbers of proteins can be positioned on surfaces by using dip-pen nanolithography (DPN). Ferritin nanoarrays are fabricated by direct-writing nanodroplets of a ferritin solution on TEM surfaces. After the contact-angle value of these droplets is determined, the concentration of the inking protein solution and the diameter of the dot-like feature can be modulated to position a single ferritin particle on a surface (see figure).