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Ferromagnetic nanowires are finding use as untethered sensors and actuators for probing micro- and nanoscale biophysical phenomena, such as for localized sensing and application of forces and torques on biological samples, for tissue heating through magnetic hyperthermia, and for microrheology. Quantifying the magnetic properties of individual isolated nanowires is crucial for such applications. Dynamic cantilever magnetometry is used to measure the magnetic properties of individual sub-500 nm diameter polycrystalline nanowires of Ni and Ni80 Co20 fabricated by template-assisted electrochemical deposition. The values are compared with bulk, ensemble measurements when the nanowires are still embedded within their growth matrix. It is found that single-particle and ensemble measurements of nanowires yield significantly different results that reflect inter-nanowire interactions and chemical modifications of the sample during the release process from the growth matrix. The results highlight the importance of performing single-particle characterization for objects that will be used as individual magnetic nanoactuators or nanosensors in biomedical applications.

Inspired by the movement of bacteria and other microorganisms, researchers have developed artificial helical micro‐ and nanorobots that can perform corkscrew locomotion or helical path swimming under external energy actuation. In this paper, for the first time the locomotion of nonhelical multifunctional nanorobots that can swim in helical klinotactic trajectories, similarly to rod‐shaped bacteria, under rotating magnetic fields is investigated. These nanorobots consist of a rigid ferromagnetic nickel head connected to a rhodium tail by a flexible hydrogel‐based hollow hinge composed of chemically responsive chitosan and alginate multilayers. This design allows nanoswimmers switching between different dynamic behaviors—from in‐plane tumbling to helical klinotactic swimming—by varying the rotating magnetic field frequency and strength. It also adds a rich spectrum of swimming capabilities that can be adjusted by varying the type of applied magnetic fields and/or frequencies. A theoretical model is developed to analyze the propulsion mechanisms and predict the swimming behavior at distinct rotating magnetic frequencies. The model shows good agreement with the experimental results. Additionally, the biomedical capabilities of the nanoswimmers as drug delivery platforms are demonstrated. Unlike previous designs constitute metallic segments, the proposed nanoswimmers can encapsulate drugs into their hollow hinge and successfully release them to cells., Advanced Science, 8 (8), ISSN:2198-3844

We report for the first time on a novel Atomic Force Microscopy (AFM) technique with dual actuation capabilities using both piezo and magnetic bead actuation for advanced single-molecule force spectroscopy experiments. The experiments are performed by manipulating functionalized magnetic microbeads using an electromagnet against a stationary AFM cantilever. Magnetic actuation has been demonstrated for AFM before to actuate cantilevers, but here we report for the first time a method where we keep the cantilever stationary and accomplish actuation via free-manipulated microstructures. This method leads to a significant reduction of mechanical drift in the system since the experiments are performed without a need for a hard surface and the measured force between the cantilever and the bead is inherently differential. In addition, shrinking the size of the actuator can minimize hydrodynamic forces affecting the AFM cantilever. We conducted single-molecule force spectroscopy and force-clamp experiments with biotin/streptavidin as a model system using the new method. The new method reported herein allows applying constant force on the beads to perform force-clamp experiments without any active feedback, which might be crucial for a deeper understanding of interaction between biomolecules.

We report on the simplest magnetic nanowire-based surface walker that is able to change its propulsion mechanism near a surface boundary as a function of the applied rotating magnetic field frequency. The nanowires are made of CoPt alloy with semihard magnetic properties synthesized by means of template-assisted galvanostatic electrodeposition. The semihard magnetic behavior of the nanowires allows for programming their alignment with an applied magnetic field as they can retain their magnetization direction after premagnetizing them. By engineering the macroscopic magnetization, the nanowires' speed and locomotion mechanism are set to tumbling, precession, or rolling depending on the frequency of an applied rotating magnetic field. Also, we present a mathematical analysis that predicts the translational speed of the nanowire near the surface, showing a very good agreement with experimental results. Interestingly, the maximal speed is obtained at an optimal frequency (∼10 Hz), which is far below the theoretical step-out frequency (∼345 Hz). Finally, vortices are found by tracking polystyrene microbeads, trapped around the CoPt nanowire, when they are propelled by precession and rolling motion.

Pollen tubes are tip-growing plant cells that deliver the sperm cells to the ovules for double fertilization of the egg cell and the endosperm. Various directional cues can trigger the reorientation of pollen tube growth direction on their passage through the female tissues. Among the external stimuli, protons serve an important, regulatory role in the control of pollen tube growth. The generation of local guidance cues has been challenging when investigating the mechanisms of perception and processing of such directional triggers in pollen tubes. Here, we developed and characterized a microelectrode device to generate a local proton gradient and proton flux through water electrolysis. We confirmed that the cytoplasmic pH of pollen tubes varied with environmental pH change. Depending on the position of the pollen tube tip relative to the proton gradient, we observed alterations in the growth behavior, such as bursting at the tip, change in growth direction, or complete growth arrest. Bursting and growth arrest support the hypothesis that changes in the extracellular H+ concentration may interfere with cell wall integrity and actin polymerization at the growing tip. A change in growth direction for some pollen tubes implies that they can perceive the local proton gradient and respond to it. We also showed that the growth rate is directly correlated with the extracellular pH in the tip region. Our microelectrode approach provides a simple method to generate protons and investigate their effect on plant cell growth.

Polymeric magnetic spherical microparticles are employed as sensors/actuators in lab-on-a-chip applications, small-scale robotics and biomedical/biophysical assays. Achieving controlled stable motion of the microparticles in a fluid environment using low intensity magnetic fields is necessary to achieve much of their technological potential; this requires that the microparticle is magnetically anisotropic, which is difficult to achieve in spheres. Here we have developed a simple method to synthesise anisotropic ellipsoidal microparticles (average eccentricity 0.60 ± 0.14) by applying a magnetic field during synthesis, using a nanocomposite of polycaprolactone (PCL) with Fe3O4 nanowires. The “microellipsoids” are thoroughly characterised using optical microscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDX). Their suitability for magnetically controlled motion is demonstrated by analysing their rotation in low magnetic fields (0.1, 1, 5, 10 and 20 mT) at varying rotational frequencies (1 Hz and 5 Hz). The microellipsoids are able to follow smoothly and continuously the magnetic field, while commercial spherical particles fail to continuously follow the magnetic field, and oscillate backwards and forwards resulting in much lower average angular speeds. Furthermore, only 23 % of commercial particles analysed rotated at 1 Hz and 26 % at 5 Hz, whereas 77 % of our ellipsoidal particles rotated at 1 Hz, and 74 % did at 5 Hz.

We exploit electrolyte confinement effects in porous membranes to electrosynthesize a two-engine micromachine from a single electrolyte solution. Taking advantage of the often undesired overplating observed in template-assisted deposition, we can produce micromachines consisting of hollow microstructures with a compositionally graded mushroom-like shape that consist of a manganese oxide-based stipe and an iron oxide-based cap. While the stipe acts as the catalytic motor of the machine, the cap serves as a magnetic steering wheel.

AAO membranes become essential for fabricating nano-building blocks. However, the integration of these nano-building blocks in complex machinery is still challenging, mainly due to the fragility of these membranes. In this work, we overcome this drawback by developing a new integrative process which enables the support of a highly-ordered nanoporous membrane onto a mechanically robust substrate such as silicon. The fabrication of supported AAO (SAAO) membranes is achieved by transferring an AAO layer onto a Si substrate via a Au/Au compressive bonding process. Two types of AAOs were prepared for this bonding process to demonstrate the universality of our technology: mild-anodized AAO (MA-AAO) and pulse-anodized AAO (PA-AAO). We also demonstrate that the newly developed SAAO membranes are suitable for electrodeposition of nanostructures. Problems such as membrane handling or electrolyte leakage occurring in conventional AAO membranes are avoided, so that Ni nanostructures with well-controlled dimensions and uniform lengths are obtained. The high-aspect ratio Ni nanostructures have the potential to be used in various applications, such as biosensing and energy storage.

Nanoletters, 15 (7), ISSN:2079-4991

Conventional photocatalytic micromotors are limited to the use of specific wavelengths of light due to their narrow light absorption spectrum, which limits their effectiveness for applications in biomedicine and environmental remediation. We present a multiwavelength light-responsive Janus micromotor consisting of a black TiO

ACS Nano, 11 (6), ISSN:1936-0851, ISSN:1936-086X

Biocompatibility and high responsiveness to magnetic fields are fundamental requisites to translate magnetic small-scale robots into clinical applications. The magnetic element iron exhibits the highest saturation magnetization and magnetic susceptibility while exhibiting excellent biocompatibility characteristics. Here, a process to reliably fabricate iron microrobots by means of template-assisted electrodeposition in 3D-printed micromolds is presented. The 3D molds are fabricated using a modified two-photon absorption configuration, which overcomes previous limitations such as the use of transparent substrates, low writing speeds, and limited depth of field. By optimizing the geometrical parameters of the 3D molds, metallic structures with complex features can be fabricated. Fe microrollers and microswimmers are realized that demonstrate motion at ≈20 body lengths per second, perform 3D motion in viscous environments, and overcome higher flow velocities than those of "conventional 3D printed helical microswimmers." The cytotoxicity of these microrobots is assessed by culturing them with human colorectal cancer (HCT116) cells for four days, demonstrating their good biocompatibility characteristics. Finally, preliminary results regarding the degradation of iron structures in simulated gastric acid liquid are provided.

Nano Letters, 16 (8), ISSN:1530-6984, ISSN:1530-6992

ACS Nano, 10 (11), ISSN:1936-0851, ISSN:1936-086X

Recent advances in nanorobotic manipulation of ferromagnetic nanowires bring new avenues for applications in the biomedical area, such as targeted drug delivery, diagnostics or localized surgery. However, probing a single nanowire and monitoring its dynamics remains a challenge since it demands high precision sensing, high-resolution imaging, and stable operations in fluidic environments. Here, we report on a novel method of imaging and sensing magnetic fields from a single ferromagnetic nanowire with an atomic-scale sensor in diamond, i.e. diamond nitrogen-vacancy (NV) defect center. The distribution of static magnetic fields around a single Co nanowire is mapped out by spatially distributed NV centers and the obtained image is further compared with numerical simulation for quantitative analysis. DC field measurements such as continuous-wave ODMR and Ramsey sequence are used in the paper and sub Gauss level of field sensing is demonstrated. By imaging magnetic fields at a single nanowire level, this work represents an important step toward tracking and probing of ferromagnetic nanowires in biomedical applications.

Micro- and nanorobots have shown great potential for applications in various fields, including minimally invasive surgery, targeted therapy, cell manipulation, environmental monitoring, and water remediation. Recent progress in the design, fabrication, and operation of these miniaturized devices has greatly enhanced their versatility. In this report, the most recent progress on the manipulation of small-scale robots based on power sources, such as magnetic fields, light, acoustic waves, electric fields, thermal energy, or combinations of these, is surveyed. The design and propulsion mechanism of micro- and nanorobots are the focus of this article. Their fabrication and applications are also briefly discussed.

Correction for ‘High precision, localized proton gradients and fluxes generated by a microelectrode device induce differential growth behaviors of pollen tubes’ by Chengzhi Hu et al., Lab Chip, 2017, 17, 671–680.

Electrodeposition is a versatile method, which enables the fabrication of a variety of wire-like nanoarchitectures such as nanowires, nanorods, and nanotubes. By means of template-assisted electrodeposition, segmented Au/Co/Au nanowires are grown in anodic aluminum oxide templates from two different electrolytes. To tailor the properties of the cobalt segments, several electrochemical conditions are studied as a function of current density, pulse deposition, and pH. The morphology, crystal structure, and magnetic properties are accordingly investigated. Changes in the deposition conditions affect the cobalt electrocrystallization process directly. Cobalt tends to crystallize mainly in the hexagonal close-packed structure, which is the reason cobalt might not accommodate satisfactorily on the face-centered cubic Au surface or vice versa. We demonstrate that by modifying the electrolyte and the applied current densities, changes in the texture and the crystalline structure of cobalt lead to a good quality connection between dissimilar segments. In particular, lowering the bath pH, or using pulse plating at a high overpotential, produces polycrystalline fcc Co and thus well-connected Co/Au bimetallic junctions with smooth interface. These are crucial factors to be carefully considered taking into account that nanowires are potential building blocks in micro- and nanoelectromechanical systems.

A procedure based on freezing and thawing was developed to induce the rupture of adsorbed lipid vesicles on solid surfaces into supported lipid bilayers (SLBs). The SLB assembly exploits the phase transition of both lipids and water during freezing. It enables SLB formation independent of the type of substrates and lipids as long as the vesicles spontaneously adsorb onto the surface. The created SLB is a single bilayer, and has a diffusion coefficient of (0.6–4) × 10−8 cm2 s−1 on TiO2, which is in the same range as the SLBs formed by conventional techniques. The presented approach has the advantages of both the Langmuir–Blodgett method (the versatility in the selection of lipids and substrates) and vesicle fusion (self-assembly) at the same time.