An optomechanical sensing technique that uses fluorescent tetrapod quantum dots to precisely measure the tensile strength of polymer fibers with minimal impact on their mechanical property could pave the way to stronger nanocomposite designs. Paul Alivisatos and colleagues at the University of California, Berkeley, and Lawrence Berkeley National Laboratory (Berkeley Lab) have incorporated a population of tetrapod quantum dots (tQDs) consisting of a cadmium-selenide (CdSe) core and four cadmium sulfide (CdS) arms into polymer fibers via electrospinning, a technique in which a large electric field is applied to droplets of polymer solution to create micro- and nanosized fibers. This is the first known application of electrospinning to tQDs. “The electrospinning process allowed us to put an enormous amount of tQDs, up to 20 percent by weight, into the fibers with minimal effects on the polymer's bulk mechanical properties,” said Alivisatos, Berkeley Lab director and the Larry and Diane Bock Professor of Nanotechnology at UC Berkeley. “The tQDs are capable of fluorescently monitoring not only simple uniaxial stress, but stress relaxation and behavior under cyclic varying loads. Furthermore, the tQDs are elastic and recoverable, and undergo no permanent change in sensing ability even upon many cycles of loading to failure.” Polymer nanocomposites contain fillers of nanoparticles dispersed throughout the polymer matrix. Exhibiting a range of enhanced mechanical properties, these materials have great potential for a broad range of biomedical and material applications. However, rational design has been hindered by a lack of detailed understanding of how they respond to stress at the micro- and nanoscale. Fluorescent tetrapod quantum dots, or tQDs (brown), serve as stress probes that allow precise measurement of polymer fiber tensile strength with minimal impact on the fiber’s mechanical properties. Inserts show relaxed tQDs (upper) and stressed tQDs (lower). Courtesy of Alivisatos group. “Understanding the interface between the polymer and the nanofiller and how stresses are transferred across that barrier are critical in reproducibly synthesizing composites,” Alivisatos said. “All of the established techniques for providing this information have drawbacks, including altering the molecular-level composition and structure of the polymer and potentially weakening mechanical properties such as toughness. It has therefore been of considerable interest to develop optical luminescent stress-sensing nanoparticles and find a way to embed them inside polymer fibers with minimal impact on the mechanical properties that are being sensed.” This challenge was met by electrospinning semiconductor tQDs of CdSe/Cds, which were developed in an earlier study by Alivisatos and his research group (See: Unique Luminescence Found in Nanocrystals). The CdSe/CdS tQDs are well-suited as nanoscale stress sensors because an applied stress will bend the arms of the tetrapods, causing a shift in the color of their fluorescence. The large electric field used in electrospinning results in a uniform dispersal of tQD aggregates throughout the polymer matrix, minimizing the formation of stress concentrations that would act to degrade the mechanical properties of the polymer. Electrospinning also provided a much stronger bond between the polymer fibers and the tQDs than a previous diffusion-based technique for using tQDs as stress probes that was reported two years ago by Alivisatos and his group. Much higher concentrations of tQDs could also be achieved with electrospinning rather than diffusion. When stress was applied to the polymer nanocomposites, elastic and plastic regions of deformation were easily observed as a shift in the fluorescence of the tQDs even at low particle concentrations. As particle concentrations were increased, a greater fluorescence shift per unit strain was observed. The tQDs acted as nonperturbing probes that tests proved were not adversely affecting the mechanical properties of the polymer fibers in any significant way. “We performed mechanical tests using a traditional tensile testing machine with all of our types of polymer fibers,” said Shilpa Raja, a member of Alivisatos' research group and co-lead author of the paper, which appeared in Nano Letters (doi: 10.1021/nl401999t). “While the tQDs undoubtedly change the composition of the fiber — it is no longer pure polylactic acid but instead a composite — we found that the mechanical properties of the composite and crystallinity of the polymer phase show minimal change.” The investigators believe their tQD probes should prove valuable for a variety of biological, imaging and materials engineering applications. From left, Andrew Olson, Shilpa Raja and Andrew Luong used electrospinning to incorporate tetrapod quantum dot stress probes into polymer fibers. They are members of the research group led by Paul Alivisatos. Courtesy of Roy Kaltschmidt, Berkeley Lab. “A big advantage in the development of new polymer nanocomposites would be to use tQDs to monitor stress buildups prior to material failure to see how the material was failing before it actually broke apart," said co-lead author Andrew Olson. “The tQDs could also help in the development of new smart materials by providing insight into why a composite either never exhibited a desired nanoparticle property, or stopped exhibiting it during deformation from normal usage.” For biological applications, the tQD is responsive to forces on the nanoNewton scale, which is the amount of force exerted by living cells as they move around within the body. A prime example of this is metastasizing cancer cells that move through the surrounding extracellular matrix. Other cells that exert force include the fibroblasts that help repair wounds, and cardiomyocytes, the muscle cells in the heart that beat. “All of these types of cells are known to exert nanoNewton forces, but it is very difficult to measure them,” Raja said. “We've done preliminary studies in which we have shown that cardiomyocytes on top of a layer of tQDs can be induced to beat, and the tQD layer will show fluorescent shifts in places where the cells are beating. This could be extended to a more biologically relevant environment in order to study the effects of chemicals and drugs on the metastasis of cancer cells.” The tQDs also could be used to make smart polymer nanocomposites that can sense when they have cracks or are about to fracture and can strengthen themselves in response. “With our technique, we are combining two fields that are usually separate and have never been combined on the nanoscale: optical sensing and polymer nanocomposite mechanical tunability,” Raja said. “As the tetrapods are incredibly strong — orders of magnitude stronger than typical polymers — ultimately they can make for stronger interfaces that can self-report impending fracture.” For more information, visit: www.lbl.gov