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Pedro Tartaj , * Maria P. Morales , * Teresita Gonzalez-Carreño , * Sabino Veintemillas-Verdaguer , * and Carlos J. Serna *

The Iron Oxides Strike Back: From Biomedical Applications to Energy Storage Devices and Photoelectrochemical Water Splitting

Abundance combined with facile synthesis, easy accessibility to different oxidation states and polymorphs, variety of electronic and magnetic proper-ties, low biotoxicity and natural elimination make of iron oxides a prototype of the ideal functional material. In this research news, we briefl y describe some of the fundaments and perspectives of the use of iron oxides in biomedicine, energy storage devices (anodes for lithium ion batteries), photoelectrochem-ical water splitting and other forms of catalysis.

1. Introduction

Iron components occur largely as ferromagnesium minerals in the earth. During weathering these minerals (primary minerals) dissolve and the released iron precipitates as ferric oxides and hydroxides. Thus, iron oxides (including hydroxides) are ubiq-uitous in nature ( Figure 1 ). [ 1 ] In most compounds iron is in the trivalent state, but magnetite contains Fe 2 + . The coordination number in ionically bonded structures (Fe-O included) is gov-erned by the relative size of oppositely charged ions. Transition metals such as iron ions normally have, owing to their relatively small ionic radii, a preference for tetrahedral and octahedral coordination (only to mention that iron ions in SrFeO 2 adopts an exotic square planar oxygen coordination). [ 2 ] It is the Fe 3d electrons that determine the electronic, magnetic and some spectroscopy properties for the iron oxides. The set of the fi ve d orbitals is split by the electrostatic fi eld of the surrounding ligands (negatively charged O 2 − /OH − ions). As a result the Fe d orbitals do not have the same energy. This fact infl uences various thermodynamic and other properties of the Fe com-pounds. For example Fe 2 + occupies octahedral sites while Fe 3 + has no preference for tetrahedral or octahedral coordination.

Iron oxides in natural settings are represented by a variety of minerals that range from well crystalline (hematite, goethite, lepidocrocite and maghemite/magnetite) to poorly crystalline

© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, WeinheAdv. Mater. 2011, 23, 5243–5249

Dr. P. Tartaj , Dr. M. P. Morales , Dr. T. Gonzalez-Carreño , Dr. S. Veintemillas-Verdaguer , Prof. C. J. Serna Instituto de Ciencia de Materiales de Madrid (CSIC) Campus de Cantoblanco, 28049 Madrid, Spain E-mail: ptartaj@icmm.csic.es; puerto@icmm.csic.es; teresita@icmm.csic.es; sabino@icmm.csic.es; cjserna@icmm.csic.es

DOI: 10.1002/adma.201101368

(2-line and 6-line ferrihydrite, schwert-mannite, feroxyhyte, and “green rust”). [ 3 ] Among the well crystalline iron oxide phases, hematite ( α -Fe 2 O 3 ) and goethite ( α -FeOOH) defi ne the energetic and ther-modynamic minimum of the system Fe 2 O 3 -H 2 O with magnetite being the most favorable phase in Fe 2 + rich envi-ronments. [ 1 , 3 ] The structure of hematite (corundum structure) can be understood in terms of an arrangement of O 2 − anions in a hexagonal close-packed lattice with

the Fe 3 + cations occupying two-thirds of the octahedral inter-stitials. The structure of goethite by far the most common iron oxide in soils can be understood in terms of an arrange-ment of O 2 − /OH − anions in a hexagonal close-packed lattice along the [100] direction with the Fe 3 + cations occupying half of the octahedral interstitials (unit cell is orthorhombic). [ 1 ] Lepi-docrocite ( γ -FeOOH, unit cell also orthorhombic) is comprised of a cubic close-packed array of O 2 − /OH − ions with the Fe 3 + cat-ions ordering to form zigzag sheets of Fe-octahedra, each layer being held together by hydrogen bonds. [ 1 ] Finally, in magnetite/maghemite (Fe 3 O 4 , γ -Fe 2 O 3 ) the iron and oxygen atoms arrange in a cubic inverse spinel structure, with O 2 − anions forming a cubic close-packed array and the Fe cations occupying intersti-tial tetrahedral and octahedral sites. [ 1 ] Maghemite only differs from magnetite in that all or most of the Fe is in the trivalent state. Cation vacancies compensate for the oxidation of Fe (II) cations.

Magnetically, the permanent magnetism in iron oxides arises from the magnetic exchange coupling between different sub-lattices. This coupling generates a variety of magnetic phases from antiferromagnetic to ferrimagnetic going through a series of relatively exotic magnetic phases associated for example with canting of two sublattices or canting at surface. [ 1 , 4 ] The magnetic structure of maghemite (the one used in biomedical applications) consists of antiferromagnetic coupled Fe 3 + cations of different coordination located in two different sublattices. The antiferromagnetic coupling occurs through the O 2 − anions (superexchange interaction). The ferrimagnetism arises from the different number of spins in the two sublattices (80–90 emu g − 1 for bulk maghemite at room temperature). The variety in mag-netic properties also depends on crystal order and particle size (surface and fi nite size effects). In ionic compounds the orienta-tion of each moment at surface can be altered as a result of com-peting exchange interactions in an incomplete coordination shell

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Figure 1 . Iron oxides are ubiquitous in nature. Upper left pictures portrait an iron oxide dust storm that swept over Sydney in September 2009 and iron oxides precipitating from the Rio Tinto water in SW Spain. The rest of the pictures portrait soils containing different iron oxides. We thank Profs. Vidal Barron and Jose Torrent for kindly providing these pictures, some of which were shown by Prof. Torrent in the opening ceremony speech for the 2009-2010 academic year at the University of Cordoba, Spain.

for surface ions. [ 5 ] This can lead to a disordered spin confi gura-tion near the surface and a reduced average net moment relative to the bulk material. [ 4 ] Ferrihydrite (accepted chemical formulas, Fe 5 HO 8 · 4H 2 O or 5Fe 2 O 3 · 9H 2 O) constitutes the crystal core of ferritin, the storage protein essential to cellular iron metabo-lism. Its principal function is to store a bioavailable reserve of intracellular iron in a nontoxic form via oxidation of Fe 2 + to Fe 3 + . The degree of crystallinity of ferrihydrite is variable and ranges from quasi-amorphous solids over poorly crystalline two-line ferrihydrite to a more ordered six-line ferrihydrite. [ 6 ] It has been recently shown that the magnetic moment of ferrihydrite can reach values close to nanosized magnetite (45 emu/g). [ 7 ]

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Abundance combined with facile synthesis in the laboratory, easy accessibility to different oxidation states and polymorphs, and fi nally the variety of electronic and magnetic properties make of iron oxides a prototype of an “ideal functional material” ( Figure 2 ). It is rather logical to understand why developing valuable materials containing iron oxide is of real interest. Furthermore, iron oxide especially in the trivalent oxidation state has so far shown rela-tively low biotoxicity. [ 8 ] This feature has signifi cantly expanded the applicability of iron oxides to the emerging fi eld of Nano medicine. In fact, our opinion is that biomedical applications alongwith some important contributions coming from photocatalysis (strictly photoelectrochemical water splitting), energy storage

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Figure 2 . Abundance (see Figure 1 ) combined with facile synthesis in the laboratory, easy accessibility to different oxidation states and polymorphs, and fi nally the variety of electronic and magnetic properties make of iron oxides a prototype of an “ideal functional material”. (A) Humans from the Upper Paleolithic used iron oxide as pigments (Altamira, Northern Spain). (B) Iron oxide particles of defi ned shape and size can be easily prepared in the Laboratory and (C) can be manipulated by an external magnetic fi eld gradient. Iron oxide nanoparticles can for example display superparamagnetic behavior. (E) Energy levels for hematite are adequate for photocatalytic oxidation of water under visible light. (F) Iron oxides can show low citotoxicity effects.

devices (mainly anodes for lithium ion batteries) and other forms of catalysis are the responsible for “the iron oxides to strike back”. We must notice that the interest in photoelectrochemical water splitting and energy storage devices has grown exponentially during the last 2–3 years for obvious reasons. From the chemist approach, reasons for the renew interest in “traditional” applica-tions of iron oxides and more modern applications are advances in synthetic and processing techniques that have made possible to reach an unequalled control over size, geometry and mesostructu-ration. From the physicist approach the interest comes from the unique characteristics of nanosized units themselves and the way they interact each other and with the surrounding media. [ 9 ]

In this research news, we briefl y describe some of the fun-daments and perspectives of the use of iron oxides in bio-medicine, energy storage devices (anodes for lithium ion bat-teries), photoelectrochemical water splitting and other forms of catalysis. Driven both by our own degree of expertise and the interest in the relatively new fi eld of Nanomedicine (new appli-cations appear regularly) we put special emphasis on this topic. Of course, we are aware of the numerous applications of iron oxides. Magnetite (Fe 3 O 4 ), for example, fi nds applications in the emerging fi eld of Spintronics. Magnetite is a half-metal fer-rimagnet with a very high Curie temperature (860 K) and has a spin gap located in the majority density of states, and localized states in the minority band. [ 10 ] Because of their high surface area and reactivity, the poorly crystalline iron oxide minerals are active in many processes, such as adsorption and transport of metals from acid mine drainage waters. [ 1 , 3 ] Iron oxides are therefore studied as effective sorbents to remove toxic materials from polluted water and nuclear waste streams due to their high surface areas and affi nities for metal ions. [ 11 ] For obvious reasons goethite and hematite are also used as pigments in the color industry while maghemite was extensively used in mag-netic recording during the 1970–1980s.

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2. Fundamentals and Perspectives

2.1. Biomedical Applications

Reasons for the interest of iron oxide magnetic nanoparticles in biomedical applications come at fi rst by the fact that some of the iron oxide phases can be manipulated by an external magnetic fi eld gradient. This property opens the possibility to use these systems in magnetic separation of for example cells, mechanical manipulation of cells or drug delivery. [ 8 , 12 ] Among these three applications drug delivery is arguably the one that have generated more interest. The use of magnetic carriers for drug delivery aims to target drug to specifi c sites through the selective application of a magnetic fi eld, and to achieve controlled release of high, localized concentrations of drug by retention of the carriers in the region of interest. [ 13 ] Controlled release, for example, can be achieved using a combination of a thermosensitive polymer and iron oxide magnetic nanoparti-cles that as we better describe below might act as localized heat sources when exposed to an alternating magnetic fi eld. [ 14 ] Com-bining drug delivery and gene therapy in a single particle has the potential to enhance the transfection effi ciency or to achieve a synergic effect of drug and gene therapy. [ 15 ]

Nuclear magnetic resonance imaging (MRI) has been regarded as a powerful imaging tool as a result of its noninva-sive nature, high spatial resolution and tomographic capabili-ties, but its low signal sensitivity has been a major limitation. The development of MRI as a clinical diagnostic modality has prompted the need for contrast agents. [ 8 ] Historically, the most commonly used MRI contrast agents are small paramagnetic metal chelates, which act by shortening T 1 relaxation times (the time constant describing the return movement of a group of nuclei, protons usually, to the fi eld direction). Superpara-magnetic iron oxide nanoparticles act by mainly shortening T 2

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Figure 3 . Rat brain nuclear magnetic resonance contrast images pre and post injection (5, 30 and 60 min) with DMSA functionalized iron oxide 4 nm particles (seen as a black contrast in the area inside the circle). The time-course of DMSA nanoparticle residence in the brain was 1 h and no residual material was observed thereafter. Nanoparticles accumulate transiently in lateral, third and fourth ventricles and most likely in some blood vessels of the brain.

relaxation times (the time constant describing the relaxation constant of usually protons interfering with each other). [ 8 ] Basi-cally, the presence of superparamagnetic iron oxide signifi cantly alter the magnetic local fi elds around protons ( Figure 3 ). The signal enhancement caused by conventional iron oxide nanopar-ticles (the only FDA approved product), however, is still unsatis-factory compared to that obtained with other imaging modalities such as fl uorescence and PET. Ferrites with high magnetization (Zn 0.4 Mn 0.6 Fe 2 O 4 ) and/or aggregation of iron oxide ferrites are strategies that are currently used to enhance T 2 capabilities. [ 16 ] Following the high magnetization criteria one could argue that the research should be shift to Fe or FeCo alloys but these mate-rials must be protected against oxidation and still there are some issues about toxicity that have not been fully solved.

Iron oxide magnetic particles are suitable for use in biosensors because most biological species are not magnetic, which means that there is inherently low background noise. [ 17 ] New methods have been presented in order to quantify the amount of biomole-cules attached to iron oxides in a liquid using magnetic sensors based on different technologies (magnetorelaxometry, magnetore-sistance, Hall effect or SQUID sensors) or using their inherent capability to enhance electrochemical signals or optical properties of the noble metal forming a multifunctional component with the nanomagnets and the biomolecule (magnetoplasmonic). [ 17 , 18 ] In this latter case the magnetic functionality of the nanocomposite is used for separation purposes and the optical properties of the noble metal for its detection. High sensitivity, small size, quick response, resistance to aggressive medium, and low price (not so restrictive in nano medicine) are the fi gures of merit that the next generation of biosensors based on iron oxide must improve.

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A multicomponent approach seems a good pathway to follow in order to increase sensing capabilities.

Iron oxide magnetic nanoparticles exposed to an alternating magnetic fi eld might act as localized heat sources at certain target regions inside the human body. The heating of magnetic oxide particles with low electrical conductivity in an external alternating magnetic fi eld is mainly due to either loss processes during the reversal of coupled spins within the particles or due to frictional losses if the particles rotate in an environment of appropriate viscosity. Inductive heating of magnetic oxide parti-cles (i.e. via eddy currents) is negligible due to the low electrical conductivity. [ 8 ] One of the last trends in hyperthermia treatment is the use of iron oxide ferrite particles with sizes around the monodomain–multidomain transition. Gonzalez-Fernandez and co-workers have presented a study on the magnetic properties of bare and silica-coated ferrite nanoparticles with sizes between 5 and 110 nm. [ 19 ] Their results show a strong dependence of the power absorption with the particle size, with a maximum around 30 nm, as expected for a Neel relaxation mechanism in single-domain particles. Another trend, which is as aforemen-tioned a general one in nanomedicine, is to combine different therapy approaches with targeting and monitoring. For example, results on the chemistry of photosensitizers and the preferential uptake of porphyrin derivatives in tumors are in the basis of the interest in obtaining iron oxide magnetic nanomaterials doped with porphyrin derivatives. [ 20 ] Phototherapy which is a form of photocatalysis combined with magnetoheating, magnetodiagnos-tics and magnetotargeting constitute a typical example of the so-called theranostic agents.

2.2. Photoelectrochemical Water Splitting (PWS)

Production of hydrogen is mainly generated from processing of fossil fuels which produces CO 2 as a by-product. [ 21 ] Global warming demands the search for other more environmental friendly technologies. Generation of hydrogen from water splitting reaches this demand and so there is a need in devel-oping strategies for water splitting. Different approaches are currently investigated such as water electrolysis, thermo and biophotolysis. In fact, as an example of the multifunctionality of iron oxides there is a two-step thermochemical water split-ting cycle which makes use of solar energy and the Fe 3 O 4 /FeO redox pair. [ 22 ] PWS has been envisioned as a promising strategy for collecting the energy of sunlight and storing it in the form of chemical bonds. [ 21 ] While in photoelectrochemical solar cells the net gain in free energy is zero, in PWS there is a gain in free energy associated with the production of hydrogen.

H 2 generation by PWS was ignited after the discovering in 1972 by Fujishima and Honda of the phenomenon of photo-catalytic splitting of water on a TiO 2 electrode under ultravi-olet (UV) light. [ 23 ] Although TiO 2 is the most widely studied material for PWS its wide band gap (3.0–3.2 eV) restricts absorption to the UV region. Characteristics such as high nat-ural abundance, oxidative robustness, environmental friend-liness, band gap (2.1 eV absorbs visible light) and valence band edge potential ( > 1 V to that required for water oxida-tion) at fi rst sight provide good capabilities for commercial

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use of photoanodes based on hematite ( α -Fe 2 O 3 ). [ 24 ] Although hematite conduction band edge position is too far positive to produce hydrogen via the reduction of protons, it could be utilized as the photoanode for water oxidation in a tandem photoelectrochemical device. However, there are limitations associated with a large overpotential for water oxidation (0.8–1.0 V RHE (reversible hydrogen potential) ), a short hole-diffusion length ( ∼ 2-4 nm, an indicator of rapid electron–hole recom-bination), low electron mobility (10 − 1 cm 2 V − 1 s − 1 ) and rela-tively low absorption coeffi cient. [24a] These limitations clearly indicate that any development in effi ciency must lie in the preparation of porous hematite nanostructures. [ 25 ] In any case there is a need for high overpotentials to achieve large current densities and strategies to increase this fi gure of merit are mainly based on doping. [ 26 ] Particularly, it has been recently shown that strategies based on biology that aims to separate the tasks of photon absorption and catalysis can produce hematite photoanodes with better effi ciencies. [ 27 ] Finally, just as another example of the multifunctionality of iron oxide based compounds, photocatodes based on p-CaFe 2 O 4 com-bined with n-TiO 2 anodes have shown enhanced effi ciency for water splitting. [ 28 ] CaFe 2 O 4 is a p-type semiconductor with a band gap of 1.9 eV and conduction and valence band edges of −0.6 and + 1.3 V RHE , respectively, which are suitable for reducing water.

2.3. Energy Storage Devices (Anodes for Lithium Ion Batteries

The development of next-generation lithium ion batteries is a key to the success of electric and hybrid electric vehicles, next generation electronic devices and implantable medical devices. [ 29 , 30 ] Ideal batteries should be inexpensive, have high energy density, and be made from environmentally friendly materials. In particular iron oxides as anode materials can react with lithium to give metal nanoparticles through conver-sion reactions. [ 31 ] Metallic lithium is an excellent anode mate-rial but its use in rechargeable Li-ion batteries leads to serious safety problems. [ 30 , 31 ] Carbonaceous materials are possible alternatives for the anode and dominate current commercial batteries. Reports on the electrochemical reduction of hema-tite ( α -Fe 2 O 3 ) with metallic Li date back to the earlier 1980s. [ 32 ] However, it was not up to the pioneering work of Tarascon and co-workers in 2000 on the reversible full reduction of 3d-metal oxides that the research on hematite electrodes was reignited. [ 33 ] The theoretical capacity for full reduction of hematite to give metallic iron is 1007 mAh g − 1 . However, a fundamental drawback of hematite anodes is the rapid loss of capacity during the fi rst cycles when operating at voltages able to fully reduce them to metallic iron. Lately signifi cant progress in cyclability has been reached when working with magnetite/carbon composites. For example, electrodes fabri-cated with magnetite nanorods and single-walled carbon nano-tubes show a stable capacity of 600 mAh g − 1 at a relatively high rate (10 C). [ 34 ] Still the performance of iron oxides is lower when compared to cobalt oxides (though iron oxides are sig-nifi cantly less expensive and toxic) and specially with respect to silicon nanowires. In any case, it seems that any signifi cant improvement in the electrochemical capabilities of iron oxides

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must be complemented with an adequate electrode processing (especially in nanosized particles where aggregation and interface effects are more relevant). [ 35 ] Interest in hematite anodes is not only based on its full reduction to give metallic iron but also in results reported by Tarascon and co-workers on nanoscale effects. [ 36 ] These authors clearly showed that hematite nanocrystals give phases that although only inserted 0.6 Li/hematite could theoretically insert up to 1 Li/hematite (170 mAh g − 1 ) at a relatively high operating voltage (1.6V vs. Li/Li + ). This relatively high operating voltage though it reduces the specifi c energy of the device could make these anodes intrinsically safer compared to graphite, which has an oper-ating voltage close to Li electroplating potential and thus raises concerns over its safety. Some recent studies carried out on porous hematite nanorods have suggested that these anodes could both operate at a voltage and retain a capacity similar to that of nanostructured lithium titanates anodes if actions are taken to prevent signifi cant electrochemical grinding. [ 37 ]

2.4. Catalytic Applications

Catalysis plays a vital role in the chemical industry. The basis for the widespread use of iron oxides in catalysis is abundance com-bined with the capabilities of iron oxides to undergo redox proc-esses and have good reactivity. Arguably, most of the interest nowadays of iron oxides is in the catalytic decomposition of H 2 O 2 (Fenton reaction) that is used to oxidize contaminants. [ 38 ] Basically, the process consist on the generation of highly oxi-dant hydroxyl radicals from H 2 O 2 and iron oxides through redox processes. Another example of the interest of iron oxides in catalysis is the dehydrogenation of ethylbenzene to produce styrene. Styrene is commercially produced by the dehydrogena-tion of ethylbenzene in the presence of a large quantity of steam at temperatures from 600 to 700 ° C. Replacement of steam by CO 2 combined with iron oxide based catalysts is believed to be an energy-saving and environmentally friendly alternative. [ 39 ] Another example of the interest of iron oxides in catalysis is the Fischer − Tropsch reaction, which is regaining considerable atten-tion. The production of liquid transportation fuels from bio-mass involves fi rst gasifi cation of biomass. This synthesis gas is converted into long chain hydrocarbons in a Fischer–Tropsch process. [ 40 ] Briefl y, in the Fischer–Tropsch synthesis a mixture of carbon monoxide and hydrogen is converted into liquid hydrocarbons. This conversion requires temperature, pressure and the use of low-cost catalysts such as iron and cobalt-based catalysts (cobalt-based are more effi cient though iron-based are signifi cantly more abundant). In the particular case of iron oxides though the mechanism of this reaction is still not clear, it seems that the high activity is associated with the formation of iron carbides. [ 41 ] Current trends in the use of iron oxides in the Fischer–Tropsch synthesis is the use of nanosized particles that can increase the effi ciency an order of magnitude. [ 42 ] Not strictly related with the capabilities of iron oxides as catalysts, just mention that magnetically-driven separation makes the recovery of catalysts in a liquid-phase reaction much easier than by cross-fl ow fi ltration and centrifugation, especially when the catalysts are in the sub-micrometer size range. Such small and magnetically separable catalysts could combine the advantages

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of high dispersion and reactivity with easy separation. [ 43 ] Finally, only to mention that iron oxides are not the panacea for all the catalytic applications. For example, a fundamental problem that iron oxides cannot evade is their inherent high solubility at acid pH (especially important for catalysis).

3. Summary and Conclusions

Having briefl y described some of the fundaments and perspec-tives of the use of iron oxides in biomedicine, energy storage devices (anodes for lithium ion batteries), photoelectrochemical water splitting and other forms of catalysis, we here outline some of the strategies to follow to enhance capabilities for these applications. In order to enhance T 2 capabilities it seems clear that strategies based on the use of ferrite aggregates with high magnetization should be followed. Controlled release using any physically sensitive material (say temperature or pH for example) in combination with magnetic nanoparticles should be the best strategy for drug delivery. High sensitivity, small size, quick response, resistance to aggressive medium, and low price (not so restrictive in nanomedicine) are the fi gures of merit that the next generation of biosensors based on iron oxide must comply. A multicomponent approach seems a good pathway to follow in order to increase sensing capabilities. For hyperthermia treatment we are of the opinion that the use of iron oxide ferrite particles with sizes around the monodomain–multidomain transition could be a good strategy to enhance heat absorption. In order to increase the effi ciency of photo-anodes based on iron oxide, it seems clear that strategies based on doping combined with advances in processing should be fol-lowed. In batteries there is a need for increasing the stability (better cyclability) of anodes based on iron oxide. Magnetite/carbon composites are promising candidates to reach this goal. Strategies that aim to produce adequate nanostructures of these materials seem the ones to follow. In catalysis, for example current trends in the use of iron oxides in the Fischer-Tropsch synthesis are based on the use of nanosized particles that can increase the effi ciency an order of magnitude. Finally, it is important to keep in mind that the idea behind using iron oxide in different applications is not only based on its perform-ance but also in its abundance, facile preparation, and tested low biotoxicity and natural elimination (i.e., the performance can be lower when compared to other materials).

Acknowledgements All the authors have contributed equally to the work. Financial support of this work from the Spanish Ministerio of Ciencia e Innovacion under projects MAT2008-01489 and MAT2008-03224.

Published online: July 8, 2011

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