Acoustic-driven magnetic skyrmion motion.

Magnetic skyrmions have great potential for developing novel spintronic devices. The electrical manipulation of skyrmions has mainly relied on current-induced spin-orbit torques. Recently, it was suggested that the skyrmions could be more efficiently manipulated by surface acoustic waves (SAWs), an elastic wave that can couple with magnetic moment via the magnetoelastic effect. Here, by designing on-chip piezoelectric transducers that produce propagating SAW pulses, we experimentally demonstrate the directional motion of Néel-type skyrmions in Ta/CoFeB/MgO/Ta multilayers. We find that the shear horizontal wave effectively drives the motion of skyrmions, whereas the elastic wave with longitudinal and shear vertical displacements (Rayleigh wave) cannot produce the motion of skyrmions. A longitudinal motion along the SAW propagation direction and a transverse motion due to topological charge are simultaneously observed and further confirmed by our micromagnetic simulations. This work demonstrates that acoustic waves could be another promising approach for manipulating skyrmions, which could offer new opportunities for ultra-low power skyrmionics.

High-Performance T1-T2 Dual-Modal MRI Contrast Agents through Interface Engineering.

Iron oxide nanoparticles (IONPs) have been developed as contrast agents for T1- or T2-weighted magnetic resonance imaging (MRI) on account of their excellent physicochemical and biological properties. However, general strategies to improve longitudinal relaxivity (r1) often decrease transverse relaxivity (r2), thus synchronously strengthening the T1 and T2 enhancement effect of IONPs remains a challenge. Here, we report interface regulation and size tailoring of a group of FePt@Fe3O4 core-shell nanoparticles (NPs), which possess high r1 and r2 relaxivities. The increase of r1 and r2 is due to the enhancement of the saturation magnetization (Ms), which is a result of the strengthened exchange coupling across the core-shell interface. In vivo subcutaneous tumor study and brain glioma imaging revealed that FePt@Fe3O4 NPs can serve as a favorable T1-T2 dual-modal contrast agent. We envision that the core-shell NPs, through interface engineering, have great potential in preclinical and clinical MRI applications.

A puzzling insensitivity of magnon spin diffusion to the presence of 180-degree domain walls.

We present room-temperature measurements of magnon spin diffusion in epitaxial ferrimagnetic insulator MgAl0.5Fe1.5O4 (MAFO) thin films near zero applied magnetic field where the sample forms a multi-domain state. Due to a weak uniaxial magnetic anisotropy, the domains are separated primarily by 180° domain walls. We find, surprisingly, that the presence of the domain walls has very little effect on the spin diffusion - nonlocal spin transport signals in the multi-domain state retain at least 95% of the maximum signal strength measured for the spatially-uniform magnetic state, over distances at least five times the typical domain size. This result is in conflict with simple models of interactions between magnons and static domain walls, which predict that the spin polarization carried by the magnons reverses upon passage through a 180° domain wall.

Modulation of Magnetic Exchange Coupling via Constructing Bi- or Multimagnetic Heterointerfaces.

How to resolve contradictions between the nanoscale size and high saturation magnetization (Ms) remains one of the scientific challenges in nanoscale magnetism as the theoretical optimal Ms of nanocrystals is compromised by the surface spin disorder. Here, we proposed a novel nanotechnology solution, heterointerface constructions of exchange-coupling core-shell nanocrystals, to rearrange the surface spin for the enhancement of Ms of nanomagnetic materials. As a demonstration of this principle, single-interface coupling FePt@Fe3-δO4 core/shell nanocrystals and multi-interface coupling FePt@Fe3-δO4@MFe2O4 (M = Mn or Co) core/shell/shell nanocrystals were synthesized. The simulated and experimental results demonstrated that constructing coupling heterointerfaces orientates the overall magnetic moment, ultimately enhancing the Ms of nanomagnetic materials. Moreover, this work first demonstrated that the origin of coupling heterointerfaces arose from mismatched lattices rather than chemical composition mismatch at the core-shell interfaces, thus providing both a solution to unite different mechanisms and an explanation to explain the exchange coupling at heterointerfaces.

Anisotropic Magnon Spin Transport in Ultrathin Spinel Ferrite Thin Films─Evidence for Anisotropy in Exchange Stiffness.

Magnon-mediated spin flow in magnetically ordered insulators enables long-distance spin-based information transport with low dissipation. In the materials studied to date, no anisotropy has been observed in the magnon propagation length as a function of propagation direction. Here, we report measurements of magnon spin transport in a spinel ferrite, magnesium aluminum ferrite MgAl0.5Fe1.5O4 (MAFO), which has a substantial in-plane 4-fold magnetic anisotropy. We observe spin diffusion lengths > 0.8 µm at room temperature in 6 nm films, with spin diffusion lengths 30% longer along the easy axes compared to the hard axes. The sign of this difference is opposite to the effects just of anisotropy in the magnetic energy for a uniform magnetic state. We suggest instead that accounting for anisotropy in exchange stiffness is necessary to explain these results. These findings provide an approach for controlling magnon transport via strain, which opens new opportunities for designing magnonic devices.

Nanoparticle-Regulated Semiartificial Magnetotactic Bacteria with Tunable Magnetic Moment and Magnetic Sensitivity.

Micro-/nanomotors are widely used in micro-/nanoprocessing, cargo transportation, and other microscale tasks because of their ability to move independently. Many biological hybrid motors based on bacteria have been developed. Magnetotactic bacteria (MTB) have been employed as motors in biological systems because of their good biocompatibility and magnetotactic motion in magnetic fields. However, the magnetotaxis of MTB is difficult to control due to the lack of effective methods. Herein, a strategy that enables control over the motion of MTB is presented. By depositing synthetic Fe3 O4 magnetic nanoparticles on the surface of MTB, semiartificial magnetotactic bacteria (SAMTB) are produced. The overall magnetic properties of SAMTB, including saturation magnetization, residual magnetization, and blocking temperature, are regulated in a multivariate and multilevel fashion, thus regulating the magnetic sensitivity of SAMTB. This strategy provides a feasible method to manoeuvre MTB for applications in complex fluid environments, such as magnetic drug release systems and real-time tracking systems. Furthermore, this concept and methodology provide a paradigm for controlling the mobility of micro-/nanomotors based on natural small organisms.

One-Pot Synthesis of High-Quality Bimagnetic Core/Shell Nanocrystals with Diverse Exchange Coupling.

Exchange coupled bimagnetic core/shell nanoparticles are promising for emerging multiferroic and spintronic technologies compared with traditional, single-phase materials, as they deliver numerous appealing effects, such as large exchange bias, tailored coercivities, and tunable blocking temperatures. However, it remains a challenge to manipulate their magnetic properties via exchange coupling due to the lack of a straightforward method that enables the general preparation of desired composites. Here we report a robust and general one-pot approach for the synthesis of different kinds of bimagnetic core/shell nanostructures (BMCS NSs). The formation of highly crystalline and monodisperse BMCS NSs adopted a self-adaptive sequential growth, circumventing the employment of complex temperature control and elaborate seeded growth techniques. As a result of large lattice misfit, the presence of interfacial imperfections as an extra source of anisotropy induced diverse exchange coupling interactions in ferro-ferrimagnetic and ferro-antiferromagnetic systems, which had great effects on the improvement of the magnetic properties of BMCS NSs. We envision that this new strategy will open up exciting opportunities toward large-scalable production of such high-quality BMCS NSs, thereby greatly potentiating the prospective applications of nanomagnetic materials.

Nanoprobe-based force spectroscopy as a versatile platform for probing the mechanical adhesion of bacteria.

The first stage of biofilm-associated infections is commonly caused by initial adhesion of bacteria to intravascular tubes, catheters and other medical devices. The overuse of antibiotics to treat these infections has led to the spread of antibiotic resistance, which has made infections difficult to eradicate. It is crucial to develop advanced strategies to inhibit biofilm formation, avoiding the emergence of antibiotic resistance. Previously, it has been reported that substrate stiffness plays an important role in the initial attachment of bacteria. However, the mechanism of how the stiffness modulates the initial adhesion of bacteria remains unclear. Here, we developed magnetic nanoprobe-based force-induced remnant magnetization spectroscopy (FIRMS) as a new platform to measure the adhesion force of bacteria. Through examining the initial adhesion force and the adhesive protein, fibronectin-binding protein (FnBP), of Staphylococcus aureus (S. aureus), we found that the increase of the substrate stiffness promoted the expression of FnBP, thus enhancing the initial adhesion force of bacteria. Following the formation of initial adhesion, the substrates with soft stiffness delayed the biofilm formation, whereas those with moderate stiffness showed preferential promotion of the biofilm formation. We expect this versatile platform to be beneficial to the study of adhesion behaviors of bacteria that sheds light on the design of new medical materials to treat microbial infections.

Direct Measurement of Through-Bond Effects in Molecular Multivalent Interactions.

Multivalent interactions occur throughout biology, and have a number of characteristics that monovalent interactions do not. However, it remains challenging to directly measure the binding force of molecular multivalent interactions and identify the mechanism of interactions. In this study, the specific interaction between bivalent aptamer and thrombin has been measured directly and quantitatively by force-induced remnant magnetization spectroscopy to investigate the binding force and through-bond effects of the multivalent interactions. The measured differential binding forces enable through-bond effects in thrombin-aptamer complexes to be identified, where aptamer binding at exosite II produces visible effects on their binding at exosite I and vice versa. This method might be suitable for practical applications in the design of high-performance ligands.

Insulin-like growth factor type I selectively binds to G-quadruplex structures.

BACKGROUND: G-quadruplex has been viewed as a promising therapeutic target in oncology due to its potentially important roles in physiological and pathological processes. Emerging evidence suggests that the biological functions of G-quadruplexes are closely related to the binding of some proteins. Insulin-like growth factor type I (IGF-1), as a significant modulator of cell growth and development, may serve as a quadruplex-binding protein. METHODS: The binding affinity and selectivity of IGF-1 to different DNA motifs in solution were measured by using fluorescence spectroscopy, Surface Plasmon Resonance (SPR), and force-induced remnant magnetization (FIRM). The effects of IGF-1 on the formation and stability of G-quadruplex structures were evaluated by circular dichroism (CD) and melting fluorescence resonance energy transfer (FRET) spectroscopy. The influence of quadruplex-specific ligands on the binding of G-quadruplexes with IGF-1 was determined by FIRM. RESULTS: IGF-1 shows a binding specificity for G-quadruplex structures, especially the G-quadruplex structure with a parallel topology. The quadruplex-specific ligands TMPyP4 and PDS (Pyridostatin) can inhibit the interaction between G-quadruplexes and proteins. CONCLUSIONS: IGF-1 is demonstrated to selectively bind with G-quadruplex structures. The use of quadruplex-interactive ligands could modulate the binding of IGF-1 to G-quadruplexes. GENERAL SIGNIFICANCE: This study provides us with a new perspective to understand the possible physiological relationship between IGF-1 and G-quadruplexes and also conveys a strategy to regulate the interaction between G-quadruplex DNA and proteins.

Regulatory-sequence mechanical biosensor: A versatile platform for investigation of G-quadruplex/label-free protein interactions and tunable protein detection.

Mechanical biosensors can be used to quantitatively explore DNA-protein binding mechanisms by detecting targets at low concentrations or measuring force in single-molecule force spectroscopy. However, restrictions in single-molecule manipulation and labelling protocols have hindered the application for bulk analysis of label-free protein detection. Here, we present the integration of molecular force measurement and finely tunable detection of label-free proteins into one mechanical sensor. Regulatory-sequence force spectroscopy was obtained to investigate the binding force of DNA G-quadruplexes (GQ) and label-free protein. The dual control of regulatory sequences and mechanical forces induces the structure switching from DNA duplex to GQ/protein complex. It exhibits a synergistic effect, enabling the rational fine-tuning of the dynamic range for biosensing protein concentrations over eight orders of magnitude. Furthermore, this method was exploited to estimate the stability of the human telomeric DNA GQ by Ku protein and ligand methylpyridostatin. The results revealed that human telomeric GQ has two different binding sites for Ku protein and ligand. Force spectroscopy integrating label-free force measurement and tunable target detection holds great promise for use in biosensing, drug screening, targeted therapies, DNA nanotechnology, and fields in which GQ are of rapidly increasing importance.

Dual-Phase Separation in a Semiconfined System: Monodispersed Heterogeneous Block-Copolymer Membranes for Cell Encoding and Patterning.

Block copolymers (BCPs) have the capacity to self-assemble into a myriad of well-defined aggregate structures, offering great promise for the construction of drug delivery, photolithographic templates, and complex nanoscale assemblies. A uniqueness of these materials is their propensity to become kinetically frozen in non-equilibrium states, implying that the process of self-assembly can be utilized to remodel the resulting structures. Here, a new semiconfined system for processing the BCP self-assembly is constructed, in which an unusual dual-phase separation occurs, including nonsolvent-induced microphase separation and osmotically driven macrophase separation, ultimately yielding heterogeneous BCP membranes. These membranes with cellular dimensions show unique anisotropy that can be used for cell encoding and patterning, which are highly relevant to biology and medicine. This processing method not only provides new levels of tailorability to the structures and encapsulated contents of BCP assemblies, but can also be generalized to other block polymers, particularly those with attractive electronic and/or optical properties.