Determination of Solvation Layer Thickness by a Magnetophotonic Approach

Derjaguin–Landau–Verwey–Overbeek (DLVO) theory fails in explaining the superior stability of colloid particles in aqueous suspensions under conditions of high ionic strengths where electrostatic forces are effectively screened. Accumulating evidence shows that the formation of a thin rigid layer of solvent molecules in the vicinity of a colloidal particle surface provides an additional repulsive interaction when the interparticle distance is reduced to several nanometers. The effective determination of the thickness of the solvation layer however remains a challenge. Here, we demonstrate a simple yet powerful magnetophotonic technique that can be used to study the thickness of the solvation layers formed on the colloidal silica surface in various polar solvents. A relationship between the hydrogen-bonding ability of the solvents and the thickness of solvation layer on colloidal silica surfaces has been identified; this observation is found to be consistent with the previously proposed hydrogen-bonding origin of the solvation force

Magnetically Actuated Liquid Crystals

Ferrimagnetic inorganic nanorods have been used as building blocks to construct liquid crystals with optical properties that can be instantly and reversibly controlled by manipulating the nanorod orientation using considerably weak external magnetic fields (1 mT). Under an alternating magnetic field, they exhibit an optical switching frequency above 100 Hz, which is comparable to the performance of commercial liquid crystals based on electrical switching. By combining magnetic alignment and lithography processes, it is also possible to create patterns of different polarizations in a thin composite film and control over the transmittance of light in particular areas. Developing such magnetically responsive liquid crystals opens the door toward various applications, which may benefit from the instantaneous and contactless nature of magnetic manipulation

Nanocrystalline TiO₂‑Catalyzed Photoreversible Color Switching

We report a novel photoreversible color switching system based on the photocatalytic activity of TiO₂ nanocrystals and the redox-driven color switching property of methylene blue (MB). This system rapidly changes from blue to colorless under UV irradiation and recovers its original blue color under visible light irradiation. We have identified four major competing reactions that contribute to the photoreversible switching, among which two are dominant: the decoloration process is mainly driven by the reduction of MB to leuco MB by photogenerated electrons from TiO₂ nanocrystals under UV irradiation, and the recoloration process operates by the TiO₂-induced self-catalyzed oxidation of LMB under visible irradiation. Compared with the conventional color switching systems based on photoisomerization of chromophores, our system has not only low toxicity but also significantly improved switching rate and cycling performance. It is envisioned that this photoreversible system may promise unique opportunities for many light-driven actuating or color switching applications

Magnetic Assembly and Patterning of General Nanoscale Materials through Nonmagnetic Templates

Applied magnetic field represents an effective tool to rapidly assemble micro- and nanoscale magnetic objects into defined structures. Ordered assembly is typically achieved by using magnetic micropatterns, for which the downside is that they require advanced microfabrication techniques to produce. In addition, most conventional magnetic assembly strategies are restricted to target objects that possess magnetic properties. Herein we present a general strategy that allows convenient magnetically driven assembly of nonmagnetic objects in defined locations with high spatial resolution. The process involves immersing a polymer relief pattern in a uniformly magnetized ferrofluid, which modulates the local magnetic fields around the pattern. Nonmagnetic target objects dispersed in the same ferrofluid can then be magnetically assembled at positions defined by the polymer pattern. As the nonmagnetic polymer patterns can be conveniently fabricated at low cost through photolithography and soft-lithography processes, our method provides a general yet very effective means to assemble a wide range of nonmagnetic objects with controlled spatial distribution, paving the way toward patterning functional microstructures

Magnetic Assembly and Patterning of General Nanoscale Materials through Nonmagnetic Templates

Applied magnetic field represents an effective tool to rapidly assemble micro- and nanoscale magnetic objects into defined structures. Ordered assembly is typically achieved by using magnetic micropatterns, for which the downside is that they require advanced microfabrication techniques to produce. In addition, most conventional magnetic assembly strategies are restricted to target objects that possess magnetic properties. Herein we present a general strategy that allows convenient magnetically driven assembly of nonmagnetic objects in defined locations with high spatial resolution. The process involves immersing a polymer relief pattern in a uniformly magnetized ferrofluid, which modulates the local magnetic fields around the pattern. Nonmagnetic target objects dispersed in the same ferrofluid can then be magnetically assembled at positions defined by the polymer pattern. As the nonmagnetic polymer patterns can be conveniently fabricated at low cost through photolithography and soft-lithography processes, our method provides a general yet very effective means to assemble a wide range of nonmagnetic objects with controlled spatial distribution, paving the way toward patterning functional microstructures

Hemodynamic responses to six reversal frequencies examined in an event-related experiment.

<p>The symbols and error bars in each panel (mean ± SEM) show the average of original time course obtained from eight subjects. The solid curve in each panel shows the estimated hemodynamic response averaged from eight subjects (for clarity, error bars of estimated hemodynamic response are not shown). To quantify hemodynamic responses to different reversal frequencies, four parameters were extracted and are given in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0099547#pone-0099547-t001" target="_blank">Table 1</a>.</p

Assembly and Photonic Properties of Superparamagnetic Colloids in Complex Magnetic Fields

Interparticle magnetic dipole force has been found to drive the formation of dynamic superparamagnetic colloidal particle chains that can lead to the creation of photonic nanostructures with rapidly and reversibly tunable structural colors in the visible and near-infrared spectrum. Although most studies on magnetic assembly utilize simple permanent magnets or electromagnets, magnetic fields, in principle, can be more complex, allowing the localized modulation of assembly and subsequent creation of complex superstructures. To explore the potential applications of a magnetically tunable photonic system, we study the assembly of magnetic colloidal particles in the complex magnetic field produced by a nonideal linear Halbach array. We demonstrate that a horizontal magnetic field sandwiched between two vertical fields would allow one to change the orientation of the particle chains, producing a high contrast in color patterns. A phase transition of Fe₃O₄@SiO₂ particles from linear particle chains to three-dimensional crystals is found to be determined by the interplay of the magnetic dipole force and packing force, as well as the strong electrostatic force. While a color pattern with tunable structures and diffractions can be instantly created when the particles are assembled in the form of linear chains in the regions with vertical fields, the large field gradient in the horizontal orientation may destabilize the chain structures and produces a pattern of 3D crystals that compliments that of initial chain assemblies. Our study not only demonstrates the great potential of magnetically responsive photonic structures in the visual graphic applications such as signage and security documents but also points out the potential challenge in pattern stability when the particle assemblies are subjected to complex magnetic fields that often involve large field gradients

Breath-Taking Patterns: Discontinuous Hydrophilic Regions for Photonic Crystal Beads Assembly and Patterns Revisualization

Surfaces patterned with hydrophilic and hydrophobic regions provide robust and versatile means for investigating the wetting behaviors of liquids, surface properties analysis, and producing patterned arrays. However, the fabrication of integral and uniform arrays onto these open systems remains a challenge, thus restricting them from being used in practical applications. Here, we present a simple yet powerful approach for the fabrication of water droplet arrays and the assembly of photonic crystal bead arrays based on hydrophilic–hydrophobic patterned substrates. Various integral arrays are simply prepared in a high-quality output with a low cost, large scale, and uniform size control. By simply taking a breath, which brings moisture to the substrate surface, complex hydrophilic–hydrophobic outlined images can be revisualized in the discontinuous hydrophilic regions. Integration of hydrogel photonic crystal bead arrays into the “breath-taking” process results in breath-responsive photonic crystal beads, which can change their colors upon a mild exhalation. This state-of-the-art technology not only provides an effective methodology for the preparation of patterned arrays but also demonstrates intriguing applications in information storage and biochemical sensors

Photonic Labyrinths: Two-Dimensional Dynamic Magnetic Assembly and <i>in Situ</i> Solidification

Creating novel structures by self-assembly processes and fixing the resultant assemblies are both critical to the design and fabrication of functional materials through bottom-up approaches. We demonstrate magnetically induced self-assembly of 2D photonic labyrinth structures and their solidification through a sol–gel method. The photonic labyrinth structures can be patterned into more regular arrangements using nonmagnetic substrates. This work may provide a platform for fabricating novel materials and devices with complex morphologies and spatial configurations

Magnetic Tuning of Plasmonic Excitation of Gold Nanorods

By using gold nanorods as an example, we report the dynamic and reversible tuning of the plasmonic property of anisotropically shaped colloidal metal nanostructures by controlling their orientation using external magnetic fields. The magnetic orientational control enables instant and selective excitation of the plasmon modes of AuNRs through the manipulation of the field direction relative to the directions of incidence and polarization of light