Chemical Education Today
Research Advances: Paper Batteries, Phototriggered Microcapsules, and Oil-Free Plastic Production by Angela G. King Department of Chemistry, Wake Forest University, Winston-Salem, North Carolina 27109
[email protected] Chemists continue to work at the forefront of materials science research. Recent advances include application of bioengineering to produce plastics from renewable biomass instead of petroleum, generation of paper-based batteries, and development of phototriggerable microcapsules for chemical delivery. Paper Battery May Power Electronics in Clothing and Packaging Material Imagine that you receive a gift wrapped in paper you really do treasure and want to carefully fold and save. That is because the wrapping paper lights up with words like “Happy Birthday” or “Happy Holidays” thanks to a built-in battery, an amazing battery made out of paper. That is one potential application of a new battery made of cellulose. Albert Mihranyan and colleagues report that scientists are trying to develop light, eco-friendly, and inexpensive batteries consisting entirely of nonmetal parts (1). The most promising materials include so-called conductive polymers or “plastic electronics”. One conductive polymer, polypyrrole (PPy), shows promise, but was often regarded as too inefficient for commercial batteries. The scientists realized, however, that by coating PPy on a large surface area substrate and carefully tailoring the thickness of the PPy coating, both the charging capacity and the chargedischarge rates can be drastically improved (2). The secret behind the performance of this battery is the presence of the homogeneous, uninterrupted, and nano-thin coating of PPy on individual cellulose fibers, which in turn can be molded into paper sheets of exceptionally high internal porosity. The scientists employed cellulose extracted from a certain species of green algae, Cladophora sp., and displayed 100 times the surface area of cellulose found in paper. That surface area was key to allowing the new device to hold and discharge electricity very efficiently (Figure 1). The innovative design of the battery cell is simple yet elegant. Both electrodes consist of identical pieces of the composite paper, and they are separated by ordinary filter paper soaked with sodium chloride serving as the electrolyte. The potential difference results from differences between oxidized and reduced pieces of the functional PPy layer. According to the scientists, the battery recharged faster than conventional rechargeable batteries, displayed only a 6% loss in capacity over 100 subsequent charge-discharge cycles, and appears well suited for applications involving flexible electronics, such as clothing and packaging (Figure 2). Additionally, low-cost and very large 464
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Figure 1. Cladophora cellulose-PPy conductive paper composite: (A) SEM micrograph taken with a magnification factor of 10 000; (B) TEM image of the cellulose composite fiber; (C) schematic image; and (D) photograph of the composite paper battery cell before and after sealing it into a polymer-coated aluminum pouch. Reprinted with permission from ref 2. Copyright 2009 American Chemical Society.
energy-storage devices with electrodes several square yards in size could potentially be made in the future. Educators interested in discussing this area of research with their students may also want to consider the hands-on assembly of either a conducting polymer thin film or a light-emitting device; they might also consider demonstrating a polymer battery to stimulate student interest (3). Next-Generation Microcapsules Deliver “Chemicals on Demand” Scientists in California are reporting development of a new generation of the microcapsules used in carbon-free copy paper. Conventional microcapsules burst and release ink when subjected to pressure from a pen (4, 5). The new microcapsules (MCs) burst when exposed to light, releasing their contents in ways that could have wide-ranging commercial uses from home and personal care to medicine.
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Vol. 87 No. 5 May 2010 pubs.acs.org/jchemeduc r 2010 American Chemical Society and Division of Chemical Education, Inc. 10.1021/ed100154x Published on Web 03/23/2010
Chemical Education Today
Figure 2. Charge-discharge curves (A) and charge capacities (B) obtained with the conductive paper composite cell for currents with ranges of 10-320 mA. The capacities were normalized with respect to the total amount of cellulose/PPy composite used in the cell. Reprinted with permission from ref 2. Copyright 2009 American Chemical Society.
Figure 4. Schematic showing how CNT-containing microcapsules could be triggered by light to release a reactant (A), yielding desired product C. Image provided by J. Frechet and used with permission.
Figure 3. Toluene filled polyamide microcapsules containing 1 wt % CNTs: (a) optical image of MCs in a scintillation vial; (b) optical image of MCs in oil; (c) scanning electron micrograph of MCs; (d) scanning electron micrograph of crushed MCs with CNTs (white bundles) visible in the interior, exterior, and incorporated into the wall. Reprinted with permission from ref 6. Copyright 2009 American Chemical Society.
Jean Frechet, Alex Zettl, and colleagues note that liquid-filled MCs have many uses other than for carbon-free copy paper, including applications in self-healing plastics (6). Such plastics contain one group of MCs filled with a monomer and another group filled with a catalyst. When scratches rip open the MCs, the contents flow, mix, and form a seal. MCs that burst when exposed to light would have great advantages, the scientists say. Light could be focused to a pinpoint to kill cancer cells, for instance, or shone over a large area to print a pattern by way of these light-triggerable, liquid-filled MCs. Stefan Pastine and David Okawa, members of Frechet's research team, spearheaded the development of new MCs, which consist of nylon spheres about the size of a grain of sand. These spheres enclose a liquid chemical sprinkled with carbon nanotubes (CNTs) as an optothermal triggering element (Figure 3). The MCs have a solid polyamide shell synthesized through an interfacial polymerization technique. The protective shell is substantial, generating an audible popping sound when physically crushed. But the polyamide MCs are breachable by irradiation. The CNTs inside the MC convert laser light to heat that bursts the nylon capsule, releasing the chemical (6). Using such a system, doctors (for example) might
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inject microcapsules containing anticancer drugs and a strong lightabsorbing material to specific cells and make the capsules burst upon exposure to laser light, delivering their contents precisely where and when they are needed in the body (Figure 4). As a demonstration, the scientists polymerized dicyclopentadiene (DCPD) via a remotely initiated ring-opening metathesis reaction (7). Without laser bursting of the Grubb's catalyst containing MCs dispersed in neat DCPD, there was no noticeable increase in viscosity after weeks. However, when laser irradiation burst the MCs and released the catalyst, the scientists detected gelling within minutes and hardening within an hour. Engineered E. coli Produces Plastics from Biomass A team of pioneering South Korean scientists has succeeded in producing the polymers used in everyday plastics through bioengineering instead of through the use of fossil-fuel-based chemicals. Led by Professor Sang Yup Lee, the scientists from KAIST and the Korean chemical company LG Chem focused their research on polylactic acid (PLA), a biobased polymer that holds the key to producing plastics through natural and renewable resources (8, 9). “The polyesters and other polymers we use everyday are mostly derived from fossil oils made through the refinery or chemical process”, said Lee. “The idea of producing polymers from renewable biomass has attracted much attention due to the increasing concerns of environmental problems and the limited nature of fossil resources. PLA is considered a good alternative to petroleum based plastics as it is both biodegradable and has a low toxicity to humans.” Until now, PLA has been produced in a two-step fermentation and chemical process of polymerization that is both complex and expensive. Now, through the use of a metabolically engineered strain
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of E. coli, the team has developed a one-stage process that produces polylactic acid and its copolymers through direct fermentation. This makes the renewable production of PLA and lactate-containing copolymers cheaper and more commercially viable (10). “By developing a strategy that combines metabolic engineering and enzyme engineering, we've developed an efficient biobased one-step production process for PLA and its copolymers”, said Lee. “This means that a developed E. coli strain is now capable of efficiently producing unnatural polymers, through a one-step fermentation process.” This combined approach of systems-level metabolic engineering and enzyme engineering now allows for the production of polymer- and polyester-based products through direct microbial fermentation of renewable resources. “Global warming and other environmental problems are urging us to develop sustainable processes based on renewable resources”, concluded Lee. “This new strategy should be generally useful for developing other engineered organisms capable of producing various unnatural polymers by direct fermentation from renewable resources.” Chemical educators striving to introduce green chemistry approaches could combine a discussion of this research with use of a published classroom activity on plastic identification, including PLA (11). Literature Cited 1. Nanotechnology and Functional Materials Home Page. http:// personal.teknik.uu.se/Teknikvetenskaper/nfm/index.html (accessed Mar 2010).
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2. Strømme Nystrom, G.; Razaq, A.; Strømme, M.; Nyholm, L.; Mihranyan, A. Ultrafast All-Polymer Paper-Based Batteries. Nano Lett. 2009, 9, 3635–3639. 3. See, for example J. Chem. Educ. 2010, 87, 208-211; 2004, 81, 1620-1623; and 2008, 85, 1067-1070. 4. More information on Frechet's research can be found online at http://frechet.cchem.berkeley.edu/ (accessed Mar 2010). 5. The principles behind carbonless copy paper have previously been explained to chemical educators in this Journal. See J. Chem. Educ. 1998, 75, 1119-1120 and 2009, 86, 464A-464B. 6. Pastine, S. J.; Okawa, D.; Zettl, A.; Frechet, J. M. J. Chemicals on Demand with Phototriggerable Microcapsules. J. Am. Chem. Soc. 2009, 131, 13586–13587. 7. Research Advances has previously reported separately on microcapsules research and ring-opening metathesis polymerizations. See, respectively, J. Chem. Educ. 2006, 83, 522-526 and 2009, 86, 414-416. 8. Sang Yup Lee Curriculum Vitae. http://mbel.kaist.ac.kr/lab/family/professor.html (accessed Mar 2010). 9. Jung, Y. K.; Kim, T. Y.; Park, S. I.; Lee, S. Y. Metabolic Engineering of Escherichia coli for the Production of Polylactic Acid and Its Copolymers. Biotechnol. Bioeng. 2010, 105, 161–171. 10. Yang, T. H.; Kim, T. W.; Kang, H. O.; Lee, S.; Lee, E. J.; Lim, S.; Oh, S. O.; Song, A.; Park, S. J.; Lee, S. Y. Biosynthesis of Polylactic Acid and its Copolymers Using Evolved Propionate CoA Transferase and PHA Synthase. Biotechnol. Bioeng. 2010, 105, 150–160. 11. Harris, M. E.; Walker, B. A Novel, Simplified Scheme for Plastics Identification. J. Chem. Educ. 2010, 87, 147–149.
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