Functional Gradient Inverse Opal Carbon Monoliths with Directional and Multinary Porosity

Achieving Functionality and Multifunctionality through Bulk and Interfacial Structuring of Colloidal-Crystal-Templated Materials

Over the past 25 years, the field of colloidal crystal templating of inverse opal or three-dimensionally ordered macroporous (3DOM) structures has made tremendous progress. The degree of structural control over multiple length scales, understanding of mechanical properties, and complexity of systems in which 3DOM materials are a component have increased substantially. In addition, we are now seeing applications of 3DOM materials that make use of multiple features of their architecture at the same time. This Feature Article focuses on the different properties of 3DOM materials that provide functionality, including a relatively large surface area, the interconnectedness of the pores and the resulting good accessibility of the internal surface, the nanostructured features of the walls, the structural hierarchy and periodicity, well-defined surface roughness, and relative mechanical robustness at low density. It provides representative examples that illustrate the properties of interest related to applications including energy storage and conversion systems, sensors, catalysts, sorbents, photonics, actuators, and biomedical materials or devices.

Time-Temperature Integrating Optical Sensors Based on Gradient Colloidal Crystals

Manipulation-free and autonomous recording of temperature states for extended periods of time is of increasing importance for food spoilage and battery safety assessment. An optical readout is preferred for low-tech visual inspection. Here, a concept for time-temperature integrators based on colloidal crystals is introduced. Two unique features in this class of advanced materials are combined: 1) the film-formation kinetics can be controlled by orders of magnitude based on mixtures of particles with distinct glass transition temperatures. 2) A gradual variation of the particle mixture along a linear gradient of the colloidal crystal enables local readout. Tailor-made latex particles of identical size but different glass transition temperatures provide a homogenous photonic stopband. The disappearance of this opalescence is directly related to the local particle ratio and the exposure to a time and temperature combination. This sensing material can be adjusted to report extended intermediate and short excessive temperature events, which makes it specifically suitable for long-term tracing and threshold applications.

Ultracentrifugation Techniques for the Ordering of Nanoparticles

A centrifugal field can provide an external force for the ordering of nanoparticles. Especially with the knowledge from in-situ characterization by analytical (ultra)centrifugation, nanoparticle ordering can be rationally realized in preparative (ultra)centrifugation. This review summarizes the work back to the 1990s, where intuitive use of centrifugation was achieved for the fabrication of colloidal crystals to the very recent work where analytical (ultra)centrifugation is employed to tailor-make concentration gradients for advanced materials. This review is divided into three main parts. In the introduction part, the history of ordering microbeads in gravity is discussed and with the size of particles reduced to nanometers, a centrifugal field is necessary. In the next part, the research on the ordering of nanoparticles in analytical and preparative centrifugation in recent decades is described. In the last part, the applications of the functional materials, fabricated from centrifugation-induced nanoparticle superstructures are briefly discussed.

Adhesion Enhancement of Micropillar Array by Combining the Adhesive Design from Gecko and Tree Frog

It has long been demonstrated the gecko-inspired micropillar array with T-shape tips possesses the best adhesion performance of a given material. The further enhancement of the adhesion performances of T-shape micropillars can offer redundant adhesion to compensate for the inevitable improper contacts. Here, the array of T-shape polydimethylsiloxane (PDMS) micropillars is incorporated with gradient dispersed calcium carbonate nanoparticles in the micropillar stalk, termed as T-shape gradient micropillars (TG), possessing the modulus gradient with stiff tip and soft root. The gradient modulus in TG facilitates the contact formation and regulates the stress at the detaching interface, resulting in a 4.6 times adhesion and 2.4 times friction as compared with the pure PDMS T-shape micropillar arrays. The study here provides a new design strategy for the super-strong structured dry adhesives.

Putting a New Spin on It: Gradient Centrifugation for Analytical and Preparative Applications

Gradient centrifugation is an important technique in chemistry, biology, materials science and engineering. It has big potential beyond the well-known centrifugation for separation of molecules and particles. Various possibilities for special analysis and separation of particles, but also preparative applications like the production of gradient materials and controlled polymerizations exist. In all examples, a gradient of physical and/or chemical properties is generated by centrifugation and used for the further application. In this Concept article, selected examples of gradient centrifugation are presented, to show important developments in the field and discuss their applications, potential, and limitations. It concludes by analysing future trends of gradient centrifugation that are relevant for academic and industrial usage.

Continuous Gradient Nanoporous Film Enabled by Delayed Directional Diffusion of Solvent and Selective Swelling

Nature-inspired porous structures are highly desired in the fields of new materials, sustainable energy, biological and chemical science, and so forth. Here, a new strategy for the fabrication of continuous, gradient nanoporous polystyrene- block-poly(2-vinylpyridine) (PS- b-P2VP) film is established. The continuous nanopore gradient along the direction of film thickness (∼120 μm) is achieved via delayed directional diffusion of dynamic binary solvent of ethanol/water and selective swelling of P2VP domains. Ethanol in binary solvent diffuses into the film from one side to another, which is retarded by the water gate as water is concentrated at the film surface. The delayed diffusion matches the swelling rate of P2VP domains, forming the continuous nanopore gradient normal to the film surface.

Nanoparticle Gradient Materials by Centrifugation

Nanoparticle gradient materials are a unique class of functional materials. They combine the specific properties of nanoparticles with macroscopic materials. A continuous spatial gradient of the nanoparticle concentration leads to diverse physical property profiles. Therefore, these materials have a remarkable potential for applications in optics, electronics, and sensors. A novel approach for the defined and controlled synthesis of this material class is the fabrication in ultracentrifugal fields. The formation of a nanoparticle gradient by sedimentation in a gelatin solution is monitored online with optical systems inside an analytical ultracentrifuge. As soon as the desired nanoparticle concentration gradient is generated, the material is solidified by gelation and the desired gradient is fixed in the material. Application of the established theory of analytical ultracentrifugation allows simulations of the sedimentation process of the nanoparticles in advance. Thus, desired nanoparticle gradient materials can also be tailor-made and fabricated on a preparative scale. This is demonstrated for the example of spherical gold nanoparticles of different sizes, gold nanorods, mixtures thereof, and spherical superparamagnetic iron oxide nanoparticles.