Scientists program the lifetime of self-assembled nanostructures

Materials that self-assemble and self-destruct once their work is done are highly advantageous for a number of applications – as components in temporary data storage systems or for medical devices. For example, such materials could seal blood vessels during surgery and re-open them subsequently. Dr. Andreas Walther, research group leader at DWI – Leibniz Institute for Interactive Materials in Aachen, developed an aqueous system that uses a single starting point to induce self-assembly formation, whose stability is pre-programmed with a lifetime before disassembly occurs without any additional external signal – hence presenting an artificial self-regulation mechanism in closed conditions. Their results are published as this week’s cover article in ("Generic Concept to Program the Time Domain of Self-Assemblies with a Self-Regulation Mechanism"). Programmed with a self-destruction mechanism: Scientists at DWI – Leibniz Institute for Interactive Materials can program self-assembly, lifetime and degradation of nanostructures, consisting of single polymer strands. The process is initiated by adding a base. It then runs autonomously, regulating itself. (Image: Thomas Heuser /DWI) Biologically inspired principles for synthesis of complex materials are one of Andreas Walther’s key research interests. To allow the preparation of very small, elaborate objects, nanotechnology uses self-assembly. Usually, in man-made self-assemblies, molecular interactions guide tiny building blocks to aggregate into 3D architectures until equilibrium is reached. However, nature goes one step further and prevents certain processes from reaching equilibrium. Assembly competes with disassembly, and self-regulation occurs. For example, microtubules, components of the cytoskeleton, continuously grow, shrink and rearrange. Once they run out of their biological fuel, they will disassemble. This motivated Andreas Walther and his team to develop an aqueous, closed system, in which the precise balance between assembly reaction and programmed activation of the degradation reaction controls the lifetime of the materials. A single starting injection initiates the whole process, which distinguishes this new approach from current responsive systems that always require a second signal to trigger the disassembly. The approach uses pH changes to control the process. The scientists press the start button by adding a base and a dormant deactivator. This first rapidly increases the pH and the building blocks – block copolymers, nanoparticles or peptides – then assemble into a three-dimensional structure. At the same time, the change of pH stimulates the dormant deactivator. PhD student Thomas Heuser explains: “The dormant deactivator slowly becomes activated and triggers an off-switch. But it takes a while before the off-switch unfolds its full potential. Depending on the molecular structure of the deactivator, this can be minutes, hours or a whole day. Until then, the self-assembled nanostructures remain stable.” Currently a hydrolytic reaction is used to activate the dormant deactivator. However, Andreas Walther and his team are already working on more sophisticated versions, which include an enzymatic reaction to slowly start the self-destruction mechanism.

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Engineers devise optical method for producing high-res, 3-D images of nanoscale objects (w/video)

To design the next generation of optical devices, ranging from efficient solar panels to LEDs to optical transistors, engineers will need a 3-dimensional image depicting how light interacts with these objects on the nanoscale. Unfortunately, the physics of light has thrown up a roadblock in traditional imaging techniques: the smaller the object, the lower the image's resolution in 3-D. Now, engineers at Stanford and the FOM Institute AMOLF, a research laboratory in the Netherlands, have developed a technique that makes it possible to visualize the optical properties of objects that are several thousandths the size of a grain of sand, in 3-D and with nanometer-scale resolution. The research is detailed in the current issue of ("Nanoscale optical tomography with cathodoluminescence spectroscopy").

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The technique involves a unique combination of two technologies, cathodoluminescence and tomography, enabling the generation of 3-D maps of the optical landscape of objects, said study lead author Ashwin Atre, a graduate student in the lab group of Jennifer Dionne, an assistant professor of materials science and engineering. The target object in this proof-of-principle experiment was a gold-coated crescent 250 nanometers in diameter – several hundred times as thin as a human hair. To study the optical properties of the crescent, they first imaged it using a modified scanning electron microscope. As the focused electron beam passed through the object, it excited the crescent energetically, causing it to emit photons, a process known as cathodoluminescence. Both the intensity and the wavelength of the emitted photons depended on which part of the object the electron beam excited, Atre said. For instance, the gold shell at the base of the object emitted photons of shorter wavelengths than when the beam passed near the gap at the tips of the crescent. By scanning the beam back and forth over the object, the engineers created a 2-D image of these optical properties. Each pixel in this image also contained information about the wavelength of emitted photons across visible and near-infrared wavelengths. This 2-D cathodoluminescence spectral imaging technique, pioneered by the AMOLF team, revealed the characteristic ways in which light interacts with this nanometer-scale object. "Interpreting a 2-D image, however, can be quite limiting," Atre said. "It's like trying to recognize a person by their shadow. We really wanted to improve upon that with our work." To push the technique into the third dimension, the engineers tilted the nanocrescent and rescanned it, collecting 2-D emission data at a number of angles, each providing greater specificity to the location of the optical signal. By using tomography to combine this tilt-series of 2-D images, similar to how 2-D X-ray images of a human body are stitched together to produce a 3-D CT image, Atre and his colleagues created a 3-D map of the object's optical properties. This experimental map reveals sources of light emission in the structure with a spatial resolution on the order of 10 nanometers. For decades, techniques to image light-matter interactions with sub-diffraction-limited resolution have been limited to 2D. "This work could enable a new era of 3D optical imaging with nanometer-scale spatial and spectral resolution," said Dionne, who is an affiliate of the Stanford Institute for Materials and Energy Sciences at SLAC. The technique can be used to probe many systems in which light is emitted upon electron excitation. "It has applications for testing various types of engineered and natural materials," Atre said. "For instance, it could be used in manufacturing LEDs to optimize the way light is emitted, or in solar panels to improve the absorption of light by the active materials." The technique could even be modified for imaging biological systems without the need for fluorescent labels. In addition to Atre and Dionne, the research was co-authored by Aitzol Garcia-Etxarri, a postdoctoral fellow at Stanford now at DIPC in Spain, and by Benjamin Brenny, Toon Coenen and Albert Polman, all of the FOM Institute AMOLF in the Netherlands. The paper was the capstone of Atre's doctoral thesis, and he is the first graduate from Dionne's lab.

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