A Coat of Many Functions

DOI: 10.1126/science.1242895, 1458 (2013);341 Science Dmitry Shchukin and Helmuth Möhwald A Coat of Many FunctionsThis copy is for your personal, non-commercial use only.clicking here.colleagues, clients, or customers by , you can order high-quality copies for your If you wish to distribute this article to othershere.following the guidelines can be obtained by Permission to republish or repurpose articles or portions of articles): October 18, 2013 (this information is current as of The following resources related to this article are available online at/content/341/6153/1458.full.html version of this article at:including high-resolution figures, can be found in the online Updated information and services,/cgi/collection/mat_sci Materials Sciencesubject collections:This article appears in the following registered trademark of AAAS.is a Science 2013 by the American Association for the Advancement of Science; all rights reserved. The title Copyright American Association for the Advancement of Science, 1200 New York Avenue NW, Washington, DC 20005. (print ISSN 0036-8075; online ISSN 1095-9203) is published weekly, except the last week in December, by the Science o n O c t o b e r 18, 2013w w w .s c i e n c e m a g .o r g D o w n l o a d e d f r o m27 SEPTEMBER 2013 VOL 341 SCIENCE 1458PERSPECTIVEStute ~30% of atmospheric emissions each year, with the remainder attributed to terres-trial and oceanic emissions that originate from legacy anthropogenic and natural mercury sources ( 3). Terrestrial and oceanic mercury reservoirs will continue to grow and release more mercury under all but the most stringent emissions controls scenarios, meaning that aggressive emissions reductions are required to stabilize background concentrations in the atmosphere and global oceans ( 4). However, future emissions trajectories are highly uncer-tain and depend on changes both in the global economy and in technology penetration.Global biogeochemical cycling of mer-cury will also be affected by rapid climate change (see the fi gure). Weaker global circu-lation and elevated temperatures will affect atmospheric oxidation rates and patterns of deposition globally. Increased precipitation intensity and incidence of extreme storm events are likely to lead to increased mercury inputs to aquatic systems through direct depo-sition, runoff, and erosion. Terrestrial soils are the largest global mercury reservoirs ( 4) and atmospheric mercury sequestration will likely increase in areas where ecosystem primary productivity is stimulated by increases in pre-cipitation and elevated atmospheric CO 2 ( 12).Furthermore, increases in the frequency, scale, and intensity of wildfi res are likely to mobilize vast stores of mercury in terrestrial soils ( 4). Changes in ocean circulation, pro-ductivity, and growth of oxygen minimum zones will likely alter rates and patterns of methylmercury formation in seawater ( 6). In polar regions, higher temperatures and declin-ing sea ice could enhance oceanic mercury losses through elevated evasion of Hg 0. How-ever, changes in primary productivity and species composition in Arctic ice-free waters have also been associated with an increase in biological mercury concentrations due to dif-ferences in feeding preferences ( 13). Many studies have suggested that climate change will exacerbate methylmercury production and bioaccumulation in aquatic ecosystems, but improved understanding of impacts on global mercury biogeochemistry is necessary for anticipating future human and wildlife exposures and risks.Mercury cycling and bioaccumulation is also affected by other human-driven changes (see the fi gure). For example, increased ozone concentrations since preindustrial times are thought to have increased the residence time of mercury in the atmosphere by 66% through interactions with bromine ( 4). This change, coupled with greater emissions, has promoted an overall increase in the worldwide distri-bution and accumulation of anthropogenicmercury. Widespread nutrient enrichment of coastal ecosystems is likely to increase meth-ylmercury formation in many ecosystems ( 6). Increasing consumption of lower–mercury containing fish from aquaculture (particu-larly in China) and fi shing pressure that has decreased the size and trophic level of fi sh consumed in many regions, have partially off-set increases due to rising pollution levels.The links between environmental mer-cury cycling and major global change driv-ers (warming, hydrology, and emission con-trols) are reasonably understood. However, it remains challenging to forecast a future envi-ronment driven by multiple synergistic and antagonistic drivers operating simultaneously. New and rapidly developing scientifi c tools such as the application of high-resolution mass spectroscopy to fi ngerprint sources and key processes ( 14) and genetic markers for the capacity to methylate mercury ( 15) will help to improve understanding of the cycling and health impacts of environmental mercury.References1. O. Serrano, A. Martinez-Cortizas, M. A. Mateo, H. Biester,R. Bindler, Global Biogeochem. Cycles 27, 21 (2013). 2. D. G. Streets et al ., Environ. Sci. Technol. 45, 10485(2011).3. U.N. Environment Programme, Global Mercury Assessment2013: Sources, Emissions, Releases and Environmental Transport (UNEP Chemicals Branch, Geneva, Switzerland, 2013).4. H. M. Amos, D. J. Jacob, D. G. Streets, E. M. Sunderland,Global Biogeochem. Cycles 27, 410 (2013).5. W. F. Fitzgerald, D. R. Engstrom, R. P. Mason, E. A. Nater,Environ. Sci. 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Parks et al ., Science 339, 1332 (2013).10.1126/science.1242838A Coat of Many FunctionsMATERIALS SCIENCEDmitry Shchukin 1 and Helmuth Möhwald 2Smart coatings are designed to be sensitive to various external and internal stimuli, thereby enhancing the surface functionality of materials.The past decade has seen great interest in the development of smart materi-als with autonomic functionalities. Among them smart coatings have a special niche, filling the position at the interface between bulk solid substrate and liquid or gaseous external environment. This makes them uniquely well suited for such appli-cations as corrosion protection, detection and delivery of bioactive species, and anti-fouling. They can provide either autonomic response to fluctuations and variations of the coating integrity (disruption, melting) or stimulated response to changes in the exter-nal environment (magnetic or electromag-netic fi elds). The response action depends on the functionalities that the coatings attain during their preparation. The main chal-lenges are to introduce these improvements, maintain them through all manufacturingsteps and material life cycle, and use them effi ciently when demanded.The development of smart coatings pos-sessing rapid or sustained feedback activ-ity in response to external impacts will be an enabling technology for the fabrication of high-tech products with multifunctional surfaces. In general, the coatings combine passive properties inherited from classical coating design (barrier, color, adhesion) and engineered active parts, which are sensitive to instant or gradual impacts occurring either in the coating matrix (pH changes, cracks) or in the environment surrounding the coat-ing (light, temperature, humidity). After exposure to certain impact(s), the active part of the smart coatings responds in order to restore the coating functionality, thus reduc-ing the negative effect of the impact on the coating (self-healing concept) or launching additional properties of the coating interface (bioactivity, detection). The coatings should also have several passive and active function-alities (e.g., antirefl ection, antifungal, anti-corrosion) exhibiting synergistic effects, thus1Stephenson Institute for Renewable Energy, Department of Chemistry, University of Liverpool, Liverpool L69 7ZD, UK. 2Department of Interfaces, Max Planck Institute of Colloids and Interfaces, D-14476 Golm, Germany. E-mail: d.shchukin@Published by AAAS SCIENCE VOL 341 27 SEPTEMBER 20131459PERSPECTIVESrendering the surface of any coated substrate functional and responsive. The key problem is to introduce multiple functions that sur-vive the coating manufacture, do not inter-act adversely, and preserve their function throughout the coating life cycle.There are several ways toward a smart and active coating. Besides other methods (vas-cular, based on tubular networks, or intrinsic, based on mobility of the components of the coating matrix) for design of the active com-ponents of smart coatings ( 1), incorporation of encapsulated active material in the coating matrix is the most promising technology ( 2,3). The high versatility of encapsulation tech-nologies, active agents, and shell components allows selection of the appropriate library of nanocontainers with the desired characteris-tics (size, sensitivity of the shell, compatibil-ity to the matrix), thereby isolating the prob-lems associated with each individual function (see the fi gure).The most sophisticate d part of smart coatings is the development of nanocontain-ers with sensitive shells, high loading capac-ity, and good affi nity for the coating matrix. The containers have to be robust, sensitive to external stimuli, and well dispersed in the coating. Being uniformly distributed in the coating, the nanocontainers keep the active material in a trapped state, avoiding undesir-able interaction between the active compo-nent and the matrix and spontaneous leak-age. Each of these parameters is essential for the active property of the smart coatings. The nanocontainer shell becomes sensitive as a result of the weak forces between the shell components during shell formation: electrostatic, pH-bonding, supramolecular, or hydrophobic forces in a layer-by-layer de position of opposite ly inte racting spe -cies (polyelectrolytes, nanoparticles, bioma-terials) on the surface of the capsule scaf-fold (mesoporous oxide nanoparticle, oil core, polymer shell, etc.) ( 4). Strong forces such as covalent binding are not sensitive enough to react to weak external stimuli. The following triggers have been shown to be applicable for opening or closing of the container shell: local pH changes, temper-ature changes, electromagnetic irradiation, mechanical pressure (including ultrasonic treatment), humidity, electric (electrochemi-cal) potential, ionic strength, and dielectric permeability of the solvent.The simplest trigger for opening or clos-ing of the capsule shell is the pH shift in the local environment. Polyelectrolyte capsules, hydrogels, and emulsions with weak acidic or basic functional groups in the shell can be made sensitive, demonstrating reversibleand/or irreversible changes of the shell per-meability in a wide pH range ( 5). For open-ing by electromagnetic irradiation, the cap-sule should have light-sensitive components in the shell: nanoparticles ( 6) for infrared or ultraviolet light, or dyes for visible light ( 7). Mechanical impact requires a certain level of rigidity or brittleness of the shell because an elastic shell can undergo deformation under pressure but not rupture ( 8); containers with diameters of <100 nm can survive reasonable mechanical force because they tend to escape from the force direction.The smart coating concept has provided effective anticorrosion self-healing coatings and bioactive coatings. Mesoporous nano-containers (SiO 2, TiO 2, CeO 2) and halloysite nanotubes have been used in smart polymer and sol-gel coatings ( 9, 10). The outer surface of the inorganic containers was coated with pH-sensitive weak polyelectrolytes or other polymers to provide controlled release of theencapsulated inhibitor. The main advantageof the composite inorganic nanocontainers is their small size (typically <200 nm), mak-ing them compatible with thin coatings and not affecting mechanical and optical proper-ties of the coatings. The triggering mechanism of inhibitor release is local changes of the pH value caused by the corrosion process, which in turn cause the pH-sensitive polymer or poly-electrolyte shell to open the pores and release encapsulated inhibitor. The main drawback of such containers is the low loading capacity due to the inner mesoporous structure. Clay nanoparticles demonstrated release of encap-sulated inhibitors induced by ion-exchange mechanisms ( 11). Anionic inhibitor was elec-trostatically bound inside the clay nanoparti-cles and then exchanged to Cl – ions in sodium chloride media. There are also several demon-strations of encapsulation of healing agents orbiocides inside nanocontainers with oil inte-rior and polymer shell. Polyurethane micro-capsules containing hexamethylene diisocya-nate were manufactured via interfacial polym-erization of commercial methylene diphenyl diisocyanate prepolymer and 1,4-butanediol in an oil-in-water emulsion ( 12). Isocyanates are reactive with moisture and can be used as a catalyst-free healing agent. The crack was sealed and healed autonomously to retard the diffusion of salt ions and thus protect the sub-strate from the corrosion process.The demonstrated approaches for the fab-rication of smart coatings have been great de ve lopme nts for functional organic and composite nanocontainers able to encapsu-late active material and release it on demand. However, it remains a challenge to have all functions optimized that would allow the development of a universal approach that combines everything in the one process. Fur-ther transfer of these approaches to the tech-nology level will be dictated by the impact of industrial (technical, economic, and eco-logical) requirements on the coating. Much work still remains in gaining a better under-standing of the detailed mechanism of shell permeation and structure of the inner voids. The composition of all elements of the feed-back active system has to provide good phys-icochemical compatibility in terms of such prope rtie s as dispe rsibility and colloidal stability of initial fl uid coating formulation, adhesion between containers and host matrix, and adhesion between matrix and substrate in the cured state. Demonstrating the approach with one type of coating with high economic impact, in addition to preserving raw mate-rial, will open routes to many other function-alities and applications.References and Notes 1. B. J. Blaiszik et al ., Annu. Rev. Mater. Res. 40, 179 (2010).2. D. G. Shchukin, H. Möhwald, Small 3, 926 (2007).3. S. R. White et al ., Science 409, 794 (2001).4. G. Schneider, G. Decher, Nano Lett. 4, 1833 (2004).5. D. G. Shchukin, G. B. Sukhorukov, Adv. Mater. 16, 671(2004).6. A. Muñoz Javier et al ., Langmuir 24, 12517 (2008).7. X. Tao, J. Li, H. Möhwald, Chemistry 10, 3397 (2004).8. T. A. Kolesnikova et al ., Adv. Funct. Mater. 20, 1189 (2010).9. D. Borisova et al ., ACS Appl. Mater. Interfaces 5, 80 (2013).10.I. A. Kartsonakis, A. C. Balaskas, E. P . Koumoulos, C. A.Charitidis, G. Kordas, Corros. Sci. 65, 481 (2012).11. J. Tedim et al ., ACS Appl. Mater. Interfaces 2, 1528 (2010). 12. M. Huang, J. Yang, J. Mater. Chem. 21, 11123 (2011). Acknowledgments: Supported by the European Union’s 7th Framework Programme (project “MUST”—Multilevel protection of materials for vehicles by smart nanocontainers), and the German Ministry for Education and Research (BMBF) under theForMat and NanoFutur programmes.Light, heat,pressure∆pH, ∆DamageΦNanocontainer-based smart coatings. A fi ne dis-persion of nanocapsules lends active functionality to the coating matrix. Capsules can be loaded with var-ious active materials depending on the functional-ities required. The sensitivity of the capsule shell can be adjusted to different stimuli (e.g., pH changes or changes in the electrochemical potential, φ) for opening and release of the encapsulated active material by nanoengineering of the shell compo-nents and structure. [Adapted from ( 2)]10.1126/science.1242895C R ED I T : C . S M I T H /S C IE N C EPublished by AAAS。