Rice University researchers discovered that putting nanotube pillars between sheets of graphene could create hybrid structures with a unique balance of strength, toughness and ductility throughout all three dimensions. Carbon nanomaterials are common now as flat sheets, nanotubes and spheres, and they’re being eyed for use as building blocks in hybrid structures with unique properties for electronics, heat transport and strength. The Rice team is laying a theoretical foundation for such structures by analyzing how the blocks’ junctions influence the properties of the desired materials. Rice materials scientist Rouzbeh Shahsavari and alumnus Navid Sakhavand calculated how various links, particularly between carbon nanotubes and graphene, would affect the final hybrid’s properties in all directions. They found that introducing junctions would add extra flexibility while maintaining almost the same strength when compared with materials made of layered graphene. Their results appear this week in the journal ("Junction configuration-induced mechanisms govern elastic and inelastic deformations in hybrid carbon nanomaterials"). 
Carbon nanotube pillars between sheets of graphene may create hybrid structures with a unique balance of strength, toughness and ductility throughout all three dimensions, according to Rice University scientists. Five, seven or eight-atom rings at the junctions can force the graphene to wrinkle. (Illustration: Shuo Zhao and Lei Tao/Rice University) Carbon nanotubes are rolled-up arrays of perfect hexagons of atoms; graphene is a rolled-out sheet of the same. Both are super-strong and excel at transmitting electrons and heat. But when the two are joined, the way the atoms are arranged can influence all those properties. “Some labs are actively trying to make these materials or measure properties like the strength of single nanotubes and graphene sheets,” Shahsavari said. “But we want to see what happens and quantitatively predict the properties of hybrid versions of graphene and nanotubes. These hybrid structures impart new properties and functionality that are absent in their parent structures — graphene and nanotubes.” To that end, the lab assembled three-dimensional computer models of “pillared graphene nanostructures,” akin to the boron-nitride structures modeled in a previous study to analyze heat transfer between layers. “This time we were interested in a comprehensive understanding of the elastic and inelastic properties of 3-D carbon materials to test their mechanical strength and deformation mechanisms,” Shahsavari said. “We compared our 3-D hybrid structures with the properties of 2-D stacked graphene sheets and 1-D carbon nanotubes.” Layered sheets of graphene keep their properties in-plane, but exhibit little stiffness or thermal conductance from sheet to sheet, he said. But pillared graphene models showed far better strength and stiffness and a 42 percent improvement in out-of-plane ductility, the ability to deform under stress without breaking. The latter allows pillared graphene to exhibit remarkable toughness along out-of-plane directions, a feature that is not possible in 2-D stacked graphene sheets or 1-D carbon nanotubes, Shahsavari said. The researchers calculated how the atoms’ inherent energies force hexagons to take on or lose atoms to neighboring rings, depending on how they join with their neighbors. By forcing five, seven or even eight-atom rings, they found they could gain a measure of control over the hybrid’s mechanical properties. Turning the nanotubes in a way that forced wrinkles in the graphene sheets added further flexibility and shear compliance, Shahsavari said. When the material did fracture, the researchers found it far more likely for this to happen at the eight-member rings, where much of the strain gathers when stressed. That leads to the notion the hybrids can be tuned to fail under particular circumstances. “This is the first time anyone has created such a comprehensive atomistic ‘lens’ to look at the junction-mediated properties of 3-D carbon nanomaterials,” Shahsavari said. “We believe the principles can be applied to other low-dimensional materials such as boron nitride and molybdenum/tungsten or the combinations thereof.”
read more "Pillared graphene gains strength"
Carbon nanotube pillars between sheets of graphene may create hybrid structures with a unique balance of strength, toughness and ductility throughout all three dimensions, according to Rice University scientists. Five, seven or eight-atom rings at the junctions can force the graphene to wrinkle. (Illustration: Shuo Zhao and Lei Tao/Rice University) Carbon nanotubes are rolled-up arrays of perfect hexagons of atoms; graphene is a rolled-out sheet of the same. Both are super-strong and excel at transmitting electrons and heat. But when the two are joined, the way the atoms are arranged can influence all those properties. “Some labs are actively trying to make these materials or measure properties like the strength of single nanotubes and graphene sheets,” Shahsavari said. “But we want to see what happens and quantitatively predict the properties of hybrid versions of graphene and nanotubes. These hybrid structures impart new properties and functionality that are absent in their parent structures — graphene and nanotubes.” To that end, the lab assembled three-dimensional computer models of “pillared graphene nanostructures,” akin to the boron-nitride structures modeled in a previous study to analyze heat transfer between layers. “This time we were interested in a comprehensive understanding of the elastic and inelastic properties of 3-D carbon materials to test their mechanical strength and deformation mechanisms,” Shahsavari said. “We compared our 3-D hybrid structures with the properties of 2-D stacked graphene sheets and 1-D carbon nanotubes.” Layered sheets of graphene keep their properties in-plane, but exhibit little stiffness or thermal conductance from sheet to sheet, he said. But pillared graphene models showed far better strength and stiffness and a 42 percent improvement in out-of-plane ductility, the ability to deform under stress without breaking. The latter allows pillared graphene to exhibit remarkable toughness along out-of-plane directions, a feature that is not possible in 2-D stacked graphene sheets or 1-D carbon nanotubes, Shahsavari said. The researchers calculated how the atoms’ inherent energies force hexagons to take on or lose atoms to neighboring rings, depending on how they join with their neighbors. By forcing five, seven or even eight-atom rings, they found they could gain a measure of control over the hybrid’s mechanical properties. Turning the nanotubes in a way that forced wrinkles in the graphene sheets added further flexibility and shear compliance, Shahsavari said. When the material did fracture, the researchers found it far more likely for this to happen at the eight-member rings, where much of the strain gathers when stressed. That leads to the notion the hybrids can be tuned to fail under particular circumstances. “This is the first time anyone has created such a comprehensive atomistic ‘lens’ to look at the junction-mediated properties of 3-D carbon nanomaterials,” Shahsavari said. “We believe the principles can be applied to other low-dimensional materials such as boron nitride and molybdenum/tungsten or the combinations thereof.”
Researchers from the National University of Singapore, comprising (from left to right) Professor T. Venky Venkatesan, Mr Michal Marcin Dykas, Assistant Professor Chester Lee Drum, Assistant Professor James Kah and Mr Abhijeet Patra, have developed a technique to observe, in real time, how individual blood components interact and modify advanced nanoparticle therapeutics. Challenges of using nanoparticles in diagnostic and drug delivery systems With their small size and multiple functionalities, nanoparticles have attracted intense attention as both diagnostic and drug delivery systems. However, within minutes of being delivered into the bloodstream, nanoparticles are covered with a shell of serum proteins, also known as a protein ‘corona’. “The binding of serum proteins can profoundly change the behaviour of nanoparticles, at times leading to rapid clearance by the body and a diminished clinical outcome,” said Asst Prof Kah. Existing methods such as mass spectroscopy and diffusional radius estimation, although useful for studying important nanoparticle parameters, are unable to provide detailed, real-time binding kinetics. Novel method to understand nano-bio interactions The NUS team, together with external collaborator Professor Bo Liedberg from the Nanyang Technological University, showed highly reproducible kinetics for the binding between gold nanoparticles and the four most common serum proteins: human serum albumin, fibrinogen, apolipoprotein A-1, and polyclonal IgG. “What was remarkable about this project was the initiative taken by Abhijeet Patra, my graduate student from NUS Graduate School for Integrative Sciences and Engineering, in conceptualising the problem, and bringing together the various teams in NUS and beyond to make this a successful programme,” said Prof Venkatesan. “The key development is the use of a new technique using surface plasmon resonance (SPR) technology to measure the protein corona formed when common proteins in the bloodstream bind to nanoparticles,” he added. The researchers first immobilised the gold nanoparticles to the surface of a SPR sensor chip with a linker molecule. The chip was specially modified with an alginate polymer layer which both provided a negative charge and active sites for ligand immobilisation, and prevented non-specific binding. Using a 6 x 6 microfluidic channel array, they studied up to 36 nanoparticle-protein interactions in a single experiment, running test samples alongside experimental controls. “Reproducibility and reliability have been a bottleneck in the studies of protein coronas,” said Mr Abhijeet Patra. “The quality and reliability of the data depends most importantly upon the design of good control experiments. Our multiplexed SPR setup was therefore key to ensuring the reliability of our data.” Testing different concentrations of each of the four proteins, the team found that apolipoprotein A-1 had the highest binding affinity for the gold nanoparticle surface, with an association constant almost 100 times that of the lowest affinity protein, polyclonal IgG. “Our results show that the rate of association, rather than dissociation, is the main determinant of binding with the tested blood components,” said Asst Prof Drum. The multiplex SPR system was also used to study the effect of modification with polyethylene (PEG), a synthetic polymer commonly used in nanoparticle formulations to prevent protein accumulation. The researchers found that shorter PEG chains (2-10 kilodaltons) are about three to four times more effective than longer PEG chains (20-30 kilodaltons) at preventing corona formation. “The modular nature of our protocol allows us to study any nanoparticle which can be chemically tethered to the sensing surface,” explained Asst Prof Drum. “Using our technique, we can quickly evaluate a series of nanoparticle-based drug formulations before conducting in vivo studies, thereby resulting in savings in time and money and a reduction of in vivo testing,” he added. The researchers plan to use the technology to quantitatively study protein corona formation for a variety of nanoparticle formulations, and rationally design nanomedicines for applications in cardiovascular diseases and cancer.
