Interaction of κ-Carrageenan with Nickel, Cobalt, and Iron Hydroxides

Franca Jones, Helmut Co¨lfen,* and Markus Antonietti ... stabilizer to prevent the precipitation of iron oxides and hydroxides up to very high pH val...
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Biomacromolecules 2000, 1, 556-563

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Interaction of K-Carrageenan with Nickel, Cobalt, and Iron Hydroxides Franca Jones, Helmut Co¨lfen,* and Markus Antonietti Max Planck Institute of Colloids and Interfaces, Research Campus Potsdam/Golm, D-14424 Potsdam, Germany Received January 21, 2000; Revised Manuscript Received May 17, 2000

In a previous paper, it was shown that the functional polysaccharide κ-carrageenan acts as an efficient stabilizer to prevent the precipitation of iron oxides and hydroxides up to very high pH values (Jones, et al. Colloid Polym. Sci. 2000, 278, 491-501). Here, this process was investigated for its application to synthesize stable cobalt and nickel oxide particles. Nickel hydroxide nanoparticles were well stabilized for long periods, while the hydroxide of cobalt(II), although initially stable at pH 13, progressively showed some complex structure rearrangement, invoked by alkaline oxidation of the primary particles. The final product of this oxidation/self-assembly process are spherical, fluffy superstructures of cobalt(III) oxide platelets with an overall diameter of 12 µm, in coexistence with 65-75% of free κ-carrageenan. Introduction Organic-mineral hybrid systems have been the interest of much literature due to their novel properties that exceed the normal properties of the minerals themselves.2,3 In nature, formation of such minerals occurs at ambient temperature and pressure under relatively mild, aqueous conditions. Despite this, biopolymers are able to control the morphology, determine the phase present4,6 and/or “self-assemble” into superstructures,6 and the whole process is called “biomineralization” or “biomimetic mineralization”. An understanding of the mechanisms of the control of the hybrid structure setup by the functional biopolymers would deliver the synthetic chemist useful techniques for the production of tailored nanoparticles or larger three-dimensional hybrid systems. Previous literature has shown the formation of amorphous iron oxyhydroxide nanoparticles in protein assemblies, in polysaccharides, sulfonated dextrans and other natural polymers.1,7-15 In this paper, we want to probe how κ-carrageenan, which turned out to be very favorable for the control of the mineralization of iron oxide,1,7 influences precipitation of other elements from the iron group of the periodic table, cobalt and nickel. The structure of κ-carrageenan is presented in Figure 1. The reason for its good stabilization properties is that both the sulfate group and the three hydroxy groups are expected to interact with the mineral surface. In addition, its tendency to gel after binding creates a stable protection layer which adds a number of other advantageous characteristics. κ-Carrageenan is derived from seaweed and is already in commercial use in food or pharmaceutical products due to its low toxicity.16 Cobalt(II) and nickel(II) hydroxide were prepared in the presence of κ-carrageenan and the resulting hybrid colloids analyzed. These data are compared to the previous results * Corresponding author. Telephone: 49 331 567 9513. Fax: 49 331 567 9502. E-mail: [email protected].

Figure 1. Schematic diagram of the idealized structure of the repeat unit for κ-carrageenan.

of iron(III) oxyhydroxide/κ-carrageenan hybrid colloids. It will turn out that nickel hydroxide forms stable nanoparticles whereas in the case of cobalt oxide/hydroxide, a complicated oxidation/recrystallization process is detected which results in a new unconventional hybrid superstructure Materials and Methods Materials. κ-Carrageenan (GENUGEL carrageenan type X-8944, Copenhagen Pectin Factory), was obtained from Hercules. The used polymer fraction was the one characterized in ref 1, its molecular weight is Mw ) 380 000 g/mol (determined by analytical ultracentrifugation), the sulfate content is 2.7 mmol g-1 (25.9 wt %) dried solid, and the moisture content is 10.2 wt %, as received. Sodium hydroxide and hydrochloric acid (all AR reagents obtained from Fluka Chemika) were used to control the pH for the hydroxide preparation. Iron(III) chloride hexahydrate, cobalt(II) chloride hexahydrate, and nickel(II) chloride hexahydrate (all AR grade from Aldrich) were used for their hydroxide preparation. Nanoparticle Formation. The κ-carrageenan solution was prepared by boiling 2 g of carrageenan in 500 mL of water and dilution after cooling to 2 g L-1. Mineralization of the polymer-metal solution is described in refs 1 and 9. Although both nickel and cobalt(II) chloride are soluble at pH 7, acidification to pH 2 was performed prior to mineralization for consistency. Furthermore, this is a precaution to remove any carbonate ions which would precipitate as

10.1021/bm0055089 CCC: $19.00 © 2000 American Chemical Society Published on Web 08/30/2000

Mineralization of κ-Carrageenan

insoluble metal carbonates at more alkaline pH’s as well. Although carrageenan is sensitive to the acid hydrolysis of the R(1f3) linkages at a pH lower than 3-4 resulting in reduced gelling ability,16 the polymer gel is not hydrolyzed on a time scale of 24 h, retaining its gel strength. Alkaline pH does not cause hydrolysis.16 Briefly, the method of nanoparticle formation involves the following: Acidification of 350 mL polymer solution to pH 2, (using a Metrohm 716 DMS Titrino), dropwise addition of the metal chloride (0.2 M) to the stirred carrageenan solution at a molar ratio of 1:1 (metal cation:SO4), and then base addition to pH 13 (using a Metrohm 716 DMS Titrino). Methods. The product formed by mineralization was characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), atomic emission spectroscopy (AES), transmission electron microscopy (TEM), scanning electron microscopy (SEM), dynamic light scattering (DLS), analytical ultracentrifugation (AUC), gravitational field flow fractionation (Gr-FFF), and ultraviolet-visible absorption spectroscopy (UV-Vis). Powder XRD was performed on a PDS 120 (Nonius GmbH, Solingen, Germany) using Cu KR radiation (λ ) 1.54 Å). The samples were prepared at pH 13, which would have resulted in a large salt precipitate; therefore, they were ultrafiltered under nitrogen using a 10 nm cutoff membrane from Millipore until the conductivity was less than 3 µS. TEM photographs are from a Zeiss EM912 OMEGA by diluting the solutions in water and then drying the sample onto carbon coated copper grids. Scanning electron microscopy (SEM) was conducted on a DSM 940A (Carl Zeiss, Jena, Germany), by placing a drop of the solution on a carbon-coated stub, using filter paper to adsorb the excess solution and sputtering with platinum after drying. UV-vis spectra were obtained on a standard UVICON 931 (from Kontron Instruments) spectrophotometer using standard 1 cm quartz cuvettes with distilled water as the reference. Atomic emission spectroscopy (AES) was performed after dissolving the solids in concentrated sulfuric acid. The dynamic light scattering (DLS) experiments used a laboratory-built goniometer with temperature control. The details of the equipment and method can be found in ref 1. Several time-correlation functions were accumulated over a period of an hour at 90° and were subsequently inverse Laplace transformed. The resulting diffusion coefficient distributions were converted into the corresponding distributions of hydrodynamic radii by the Stokes-Einstein relation. All samples were investigated at 2.0 g L-1 κ-carrageenan without centrifugation to avoid loss of the larger microgel particles. Analytical ultracentrifugation (AUC) was performed on a Beckman Optima XL-I with integrated UV-Vis absorption and on-line interference optics at 25 °C in a self-made titanium or commercial Epon double sector centerpiece. The details of the experimental set up are discussed in ref 1. However, it is worth noting that sedimentation velocity experiments have been carried out and the metal containing components could be selectively detected (by UV-Vis) whereas the simultaneous application of the Rayleigh interference optics yields the sedimentation profile of all

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species as already demonstrated earlier.17-19 All sedimentation coefficients given in this paper refer to 25 °C and are not extrapolated to infinite dilution or corrected to the standard value in water due to the unknown hybrid particle density. Gravitational field flow fractionation (Gr-FFF) was built in-house. The clear, rigid polymethacrylate body of the cell was 30 mm thick, 80 mm wide and 600 mm long. Polyester spacer sheets of 0.10 mm (Schulze Siebdruck, Berlin, Germany) were used to form a standard parallelepiped channel of 471 mm end-to-end length, 21 mm mean breadth, with the last 21 mm of each end tapering to a point. Two spacers (0.20 mm) were used to generate the channel height. The blocks had a 4 mm O-ring installed to prevent leaks. Connection of tubing to the Gr-FFF cell was made with longthread nuts from Knauer (Berlin, Germany) and carried through 0.5 mm internal diameter peek tubing (Upchurch, WA). A micropiston chromatography pump with a 20 µL head volume (Postnova Analytics, Munich, Germany) followed by a 40 psi backpressure regulator controlled the flow rates. Channel flow was diverted through an eight port Valco valve (Houston, TX). A 5 µL injection was used throughout (Rheodyne 7125, CA). Spectrophotometric detection was at 350 nm (Knauer, Berlin, Germany) with 10 mm optical path length. The Gr-FFF carrier solution consisted of deionized water, 1 g L-1 Tween 20 (Sigma Chemicals), 0.2 g L-1 (3.0 mM) NaN3 (Fluka), and 2.0 g L-1 (35 mM) NaCl (Fluka). Gr-FFF was used since DLS cannot measure large particles which sediment out of the laser beam. Gr-FFF has the advantage of being able to fractionate and measure hydrodynamic sizes over the entire micrometer range.20 Results and Discussion Optical Examinations and Spectroscopy. The addition of Fe3+, Ni2+, and Co2+ to κ-carrageenan and the formation of their oxides/hydroxides by increasing the pH results in transparent solutions of polysaccharide-stabilized metal hydroxides at pH 13. The nickel produces the expected greentinged solution while the iron(III) solution is golden brown. The cobalt system is pink up to pH 7-8 whereupon the solution becomes blue up to pH 13. However, both the cobalt control and the mineralized carrageenan solution undergo further change and darken on the time scale of hours. This is clearly due to the oxidation of cobalt(II) to cobalt(III). Whereas the iron and nickel colloids stay stable on a time scale of months, the cobalt system develops a redispersable sediment at the bottom of the container. The presence of transparent, colloidally stable solutions of the hybrids enables the performance of UV-Vis spectra for the hydroxides mineralized in carrageenan, which are shown in Figure 2. The iron oxyhydroxide-carrageenan spectrum is similar to that expected for Fe3+ solutions at pH > 3.21 The color change of the cobalt system from pink (pH 7-8) to blue (pH 13) is the typical pH dependence of cobalt salts where an octahedral aqua ion converts to a tetrahedral complex, e.g., Co(OH)42-. The peaks at approximately 250 and 400 nm in wavelength do not correspond to the tetrahedral cobalt (which has peaks from 600 to 700 nm22). Instead we attribute

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Table 1. Phase and Crystallite Size (Radius in Nanometers) for the Respective Metal Hydroxide Controls and after Ultrafiltration of the pH 13 Mineralized κ-Carrageenan Fe(OH)3 {210}a

Ni(OH)2 {100}

Co(OH) {111}

phase

iron hydroxide with goethite

nickel hydroxide

cobalt hydroxide

phase crystallite size (radius, nm)

b 0.95

nickel hydroxide 9.1

cobalt hydroxide 21.9

reflection plane Control Mineralized Carrageenan

a

As determined from comparison to XRD spectra

b

The phase is as described in refs 9-13.

Figure 2. UV-vis spectra of ultrafiltered, mineralized solutions at pH 13 for the various metal hydroxides at a stoichiometric ratio of sulfate to metal ) 1. The cobalt sample was 2 months old.

them to the Co moieties located inside the nanoparticles which cannot undergo solvent induced surface transitions. The Ni(OH)2 system showed a small but significant absorbance in the 600-700 nm region typical of Ni2+ in the octahedrally coordinated configuration and a small absorption at 400 nm that can also be attributed to the nickel aqua ion.18 Due to its different electronic structure, the nickel system is not sensitive against oxidation. All systems show only a very slight decrease in absorption amplitude on extensive ultrafiltration, which underlines the very strong binding of the metal salts to the carrageenan microgels so that only a marginal amount can be released even under the conditions of ultrafiltration. In addition, the equilibrium concentration of free low molecular weight metal complexes seems to be very low. On longer terms, both the control and the mineralized carrageenan Co solution underwent further change whereby the blue color then darkened to a brown, which is the wellknown alkalic oxidation to cobalt(III) hydroxides. As this oxidation will turn out to be the driving mechanism for a complicated structure rearrangement, this oxidation was also quantified by UV spectroscopy (Figure 3). As seen in the UV-Vis spectrum where the 550-700 nm region is indicative of the tetrahedral Co(II) species,22 the cobalt oxidizes within 3 h. Even after oxidation was complete, the sample continues to change: the spectrum keeps its shape, but shifts slightly to higher wavelengths. This is the usual trend for ripening nanoparticles and suggests that the cobalt hydroxide units recrystallize or continue to grow.23 The FTIR of the polysaccharides containing Ni and Co at pH 13 (see supportive data) compared to the initial κ-carra-

Figure 3. UV-vis spectra of Co2+ in carrageenan at pH 13 vs time showing oxidation to Co3+.

geenan (spectrum in agreement with ref 24) revealed that the 979, 1135, and 1168 cm-1 peaks slightly shift to smaller frequencies by a maximum of 7 cm-1. These shifts were not found in the case of Fe(III) and may be indicative of the sulfate groups interacting somewhat more strongly with the hydroxides of cobalt and nickel after mineralization as compared to iron. As Co-hydroxide is amphoteric and can bind OH- in strongly alkaline solution at its surface via Co(OH)42- centers,25 the mode of interaction is interesting as the negative mineral surface should be repelled from the negative sulfate groups of the carrageenan. Obviously the sulfate ions replace some of the surface OH groups, which also explains the stronger, ligand-type binding. From the fact that all other FTIR peaks of the κ-carrageenan stay unchanged, it is deduced that, beside binding via the sulfate groups, the carrageenan structure essentially stays unchanged. X-ray Diffraction (XRD). Comparison of the spectra of the hybrids with those of the hydroxides precipitated without polymer showed in all cases mineral bands which are visible but very broadened suggesting amorphous or nanocrystalline solids.26 In the case of cobalt hydroxide, crystallinity could be proven by electron diffraction (see supportive information) whereas for the iron or nickel hydroxides, no diffraction rings or spots could be seen. The results of the X-ray diffraction experiments are summarized in Table 1. In all three cases (Fe, Co, Ni), we have compared the results of the mineralization reactions without (control sample) and with carrageenan. The iron oxyhydroxide nanocolloids diffractograms are similar to previously described iron(III) oxide-polysaccharide complexes1,9-13 but with fewer and broader bands; i.e., a XRD pattern consistent with an amorphous ferrihydrite or cell-contracted akaganite structure is observed. For the nickel

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hydroxide, both the control sample and the mineralized polysaccharide produce similar diffractograms, although the peaks with carrageenan are weaker and broader. This is attributed to smaller Ni(OH)2 synthesized in the presence of the biopolymer. The crystallite sizes, based on the reflection plane shown in brackets, were calculated from the line width of the strongest peak using the Scherrer equation27 and are listed in Table 1. Although in the case of the cobalt hydroxide only one scattering peak was observed, the line width analysis shows a well-crystallized solid as compared to the iron or the nickel hydroxides. The crystallinitiy was also checked by electron diffraction (see supportive data). The observed behavior is typical for needles or platelets where just one crystallographic direction is developed. A search through literature values for lattice parameters of cobalt(II) and cobalt(III) hydroxides suggests that the phase can only be as for the cobalt(III) hydroxide control sample. TEM and SEM. TEM micrographs of the mineralization reactions are shown in Figure 4, parts a-f, where the left side always shows the nanoparticles obtained without polymer, as compared to the right side where the results of the mineralization with polymer are depicted. In the case of cobalt at pH 13 (Figure 4, parts a and b), the κ-carrageenan seems to affect both primary particle size and superstructure: the tablet-like crystals found without polymer become slightly smaller (diameters of 20-30 nm), and the randomlike packing is turned into hollow, narrowly distributed shells or initially hollow polygons of 80 nm diameter which fill and sediment over time. It is underlined that this is the primary structure prior to the superstructure formation. Since the overall size goes well with the carrageenan-microgel size described in ref 1, it is assumed that a similar microgelation process takes place while the mineral is preferentially generated at the surface (speculatively attributed to its amphoteric character). The iron(III) oxyhydroxide formed without κ-carrageenan is very badly defined (Figure 4c), whereas in the presence of the charged polysaccharide, the predominant morphology is the one of particles in a gel “cage”.1 Sometimes, an interesting secondary morphology is found, (Figure 4d), showing strings of nanoparticles. Here, the nanoparticles bound to the carrageenan chain are contrasting the polymer helices which become observable that way. It is also interesting that in the carrageenan system the iron oxyhydroxide does not slowly transform to goethite even after 10 months, but the reference sample does, yielding elongated particles (see the rodlike particle in Figure 4c). The general shape of the nickel hydroxide appears the same in both the control and the mineralized κ-carrageenan sample (Figure 4, parts e and f), and from electron microscopy, there is no difference detectable apart from an indication of a smaller particle size in the presence of the polysaccharide. It must be remembered that the nickel hydroxide is colloidally stable in the presence of κ-carrageenan, where it is not without the polysaccharide. Thus, it appears that in this case the effect of carrageenan has simply been to hinder further agglomeration of the particles: it covers the crystal

Table 2. Sedimentation Coefficient Measured by AUC for the Various Samples at 2 g/L κ-Carrageenan and 25 °C sedimentation coeff at pH 13 after ultrafiltration (S) κ-carrageenan Fe3+ + carrageenan Ni2+ carrageenan Co3+ + carrageenana a

1.65 23.0 21.0 123.0

Co2+ oxidized but the sample is not aged.

surfaces, but does not influence the crystallization of the mineral. The well-defined cobalt/κ-carrageenan sediment formed after oxidation was characterized by SEM. It is seen that the formation of an interesting superstructure had occurred (Figure 5a). The superstructures were found to be roughly 10 µm in diameter, spherical and being composed of platelets with a size of 100-200 nm (Figures 5b and 6), which are “glued” together. The platelet morphology and aggregate structure of these spheres were confirmed by TEM on microtomed samples (Figure 6). Analytical Ultracentrifugation (AUC). Characterization of the samples with the analytical ultracentrifuge (AUC) showed that the (oxy)hydroxide nanoparticles are forming with the carrageenan polymer a common species with a higher sedimentation coefficient than the carrageenan alone (Table 2). The sedimentation coefficients are proportional to the size and density of the formed hybrid particles, and as Fe3+ and Ni2+, respectively, show similar sedimentation coefficients but the Co3+ hybrid sedimentation coefficient is much bigger, the size of these particles must be much bigger than of the other two systems in coincidence with TEM and DLS results. At pH 2, all ion-loaded carrageenan samples were observed to sediment as one species, and there is no free carrageenan present. Thus, at this pH the salts are homogeneously distributed throughout the solution. AUC after mineralization of Co2+ showed that the supernatant solution contained free carrageenan, and from the fringe shift and the known refractive index increment for carrageenan one can calculate that about 75% of the initial carrageenan was not attached to the mineral after the oxidation process (as the two sediment separately, see supportive information). The solids found in the cobalt oxyhydroxide-κ-carrageenan system were centrifuged and analyzed for their cobalt and sulfur content by AES. It was found that these solids contain ∼3 wt % S (which just can stem from the carrageenan), which corresponds to 9 wt % sulfate. Compared to the 25.9 wt % sulfate content of the initial κ-carrageenan, this means that the inorganic nanoparticles are still coupled to about 35% of the total carrageenan. This agrees reasonably well with the finding from AUC stating that about 75% of the carrageenan are released within 3 h after mineralization. Similar experiments were performed with the iron and nickel species, and here, the release of stabilizing polysaccharide was less pronounced in the case of iron (40% free carrageenan)1 but also about 75% for nickel. Dynamic Light Scattering (DLS) and Growth of Micrometer-Sized Particles. As already found for the iron hydroxide system,1 DLS on nickel hydroxide mineralized

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Figure 4. Different particle morphologies after purification with ultrafiltration: (a) cobalt hydroxide control; (b) cobalt hydroxide with carrageenan; (c) iron oxyhydroxide control; (d) iron oxyhydroxide with carrageenan at pH 13; (e) nickel hydroxide control; (f) nickel hydroxide with carrageenan.

in the presence of κ-carrageenan showed two species at approximately 4 and 160 nm in diameter. The smaller species could correspond to the nickel hydroxide with d ≈ 9 nm crystallite size from XRD whereas the bigger one is likely to be the polymer hybrid colloid (see also Figure 4f). For

the cobalt hydroxide system, the situation turned out to be more complicated as a slow particle growth could be observed. Time-dependent AUC experiments showed a clear increase of the sedimentation coefficient from about 110 to 220 S

Mineralization of κ-Carrageenan

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Figure 5. Cobalt hydroxide formed in the presence of carrageenan: (a) spherical “particles”; (b) close up of the “particles” showing they are aggregates of platelets.

Figure 7. Hydrodynamic radius (nm) determined from DLS results of cobalt(III) hydroxide + carrageenan at pH 13.

Figure 6. TEM of the superstructure produced by cobalt mineralized in carrageenan which was fixed in an epoxy resin and then microtomed. The aggregate is very loosely packed but is more dense than a flocculated sample without carrageenan. The scale bar represents 1000 nm.

after 12 days reflecting the significant particle growth (see supportive information). However, as the density of the particles is not known, the increase in the sedimentation coefficients cannot directly be correlated with an increase in the particle size so that additional information has to be sought to work out the time dependent increase in the particle size. For a better understanding of the growth process of the cobalt species following oxidation, samples were taken after different reaction times and analyzed with dynamic light scattering, Gr-FFF, and SEM. Figure 7 depicts the results of dynamic light scattering in the very first stages of the reaction (within the first hour). Because of the small sampling intervals, the data are quite noisy. It is seen that the 100 nm big primary mineralized structures (shown in Figure 4b) restructure themselves to approximately 600 nm big objects during the measurement. Comparison with the UV experiments suggests that this change is induced by oxidation from Co2+ to Co3+.

Already from the beginning, there is a second, larger species in the micrometer range which becomes more pronounced with time and starts to sediment after 24 h. These micrometer-sized particles are within the size range observed with the SEM experiments. To follow the behavior on larger size scales and longer times, Gr-FFF was employed (for experimental data, see Supporting Information). Here, after 48 h, a species having a hydrodynamic diameter of 14 µm, very close to the diameter observed in the SEM of approximately 10-12 µm can be observed although the majority of particles exhibit sizes of 30-50 µm. After aging the sample for 5 days the smaller particles have grown further and only a broad band indicative of diameters ranging from 19 to 60 µm is observed. The 2 months old sample shows two peaks representing diameters of approximately 54 and 60 µm. This suggests that the snowball superstructures, although they keep their structural identity (SEM), are not colloidally stable and aggregate further to multipletts. Mechanism of Biopolymer/Nanoparticle Hybrid Formation. The mechanism of interaction with carrageenan appears to be the same for all three cations, that is, physical cross-linking of the carrageenan at pH 2 through electrostatic interaction of the cations with the ionized sulfate groups (as proved by titration) is the initial stage in each case (see Figure 8 for the proposed mechanism). All the metals have very low hydroxide solubility products at 25 °C18 (Fe(OH)3 ) 4.7 × 10-17, Ni(OH)2 ) 5.54 ×10-16, Co(OH)2 ) 1.9 × 10-15) so the precipitation of the cation to the hydroxide

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Figure 8. Proposed mechanism for the formation of the selfassembled cobalt hydroxide hybrid.

should be essentially irreversible. The driving force for the nanoparticle formation stemming from the high supersaturation of the solutions can be considered as comparable as the maximum relative difference between the solubility products referred to the average magnitude of 10-16 is just about 12% on a logarithmic scale. The formation of the nanoparticle and the residual reversible electrostatic interaction of the sulfate groups on the carrageenan backbone with the nanoparticle surface results in a colloidally stable solution. There is a slight variation for cobalt. The cobalt(II) hydroxide has the highest solubility product of the three compounds whereas, on oxidation, the solubility of cobalt(III) hydroxide is much less (10-43 as compared to 10-15 mol/L).28 Thus, the nanoparticles of cobalt(II) hydroxide originally formed redissolve, and the cations further “migrate” to centers of precipitating cobalt(III) hydroxide forming larger particles and leaving behind some carrageenan which is no longer cross-linked. From UV-Vis data and dynamic particle size characterization, it is shown that the oxidation and the particle reconstitution is essentially complete after 3 h. The formation of a superstructure for cobalt in the presence of carrageenan is due to slow aggregation after the initial oxidation of Co(II) to Co(III). The larger Co(OH)3 particles interact less with the carrageenan. The formation of snowballlike spherical superstructures must be a product of energy minimization, as the gain in the lowering of the surface energy is balanced by the entropy loss upon aggregation. Some agglomeration of the particles (based on Gr-FFF and UV-Vis) also appears to be occurring. Conclusions and Outlook It has been shown that the formation of stable nanoparticles of metal hydroxides in carrageenan is applicable for a number of metals with application interest. For both 2+ and 3+ cations, the mineralization starts with carrageenan microgel formation, where the multivalent cations act as effective cross-links, as shown by AUC measurements. The precipitation of these bridging ions with the base results in stable hybrids where the nanoparticles are embedded in a stabilizing biopolymer shell. However, the stability in this system relies on the survival of the microgel structure. Iron oxyhydroxide and nickel hydroxide stability was observed because the reaction forming the metal hydroxide releases at most 50-75% of the carrageenan from the microgel structure after mineralization. AUC and AES measurements showed that on oxidation,

Jones et al.

cobalt mineralization resulted in 65-75% of the carrageenan being released. The disruption of the microgel and the weaker interaction of carrageenan with the cobalt(III) hydroxide allows aggregation to occur. Iron(III) oxyhydroxide nanoparticles are seen to be rather small and spherical while no distinct differences to the control are observed for nickel hydroxide. The cobalt(III) oxyhydroxide sediment, however, showed how the carrageenan acting as an in-situ flocculant can lead to a selfassembling system. The cobalt(III) hydroxide crystals (approx. 20 nm in radius) rearrange to polygons (100-200 nm in radius) which in turn further aggregate to form micrometersized (∼10 µm in diameter) snowball-like superstructures. As in all systems, no specific control of the polymer regarding the crystallite morphology or modification could be observed, the role of κ-carrageenan seems to be limited to cation binding, superstructure formation and stabilization. However, the cobalt hydroxide/carrageenan hybrid particles are a nice example for a complex superstructure formation over several length scales, and similar processes may well be observed with other mineral systems. Acknowledgment. Many thanks go to A. Vo¨lkel for the AUC work, I. Zenke for WAXS measurements, A. Peytcheva for the DLS measurements, and Dr. R. Hecker for the Gr-FFF work. We are also grateful to the Copenhagen Pectin Factory for the carrageenan sample. The Max Planck Society is acknowledged for financial support. H.C. furthermore thanks the Dr. Hermann Schnell foundation for financial support. Supporting Information Available. Plots of experimental data for the following techniques: AUC, DLS, Gr-FFF, electron diffraction, WAXS, and FTIR. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Jones, F.; Co¨lfen, H.; Antonietti, M. Colloid Polym. Sci. 2000, 278, 491-501. (2) Pileni, M. P. Cryst. Res. Technol. 1998, 33, 1155-1186. (3) Mann, S.; Archibald, D. D.; Didymus, J. M.; Douglas, T.; Heywood, B. R.; Meldrum, F. C.; Reeves, N. J. Science 1993, 261, 12861292. (4) Addadi, L.; Weiner, S. Angew. Chem., Int. Ed. 1992, 31, 153-169. (5) Weiner, S.; Addadi, L. J. Mater. Chem. 1997, 7, 689-702. (6) Mann, S.; Webb, J.; Williams, R. J. P. Biomineralization. Chemical and Biochemical PerspectiVes; VCH Verlagsgesellschaft mbH: Weinheim, Germany, 1989, and references therein. (7) Jones, F.; Co¨lfen, H.; Antonietti, M. 14th international Symposium on industrial crystallization; IChemE Ed.; Institution of Chemical Engineers: Rugby, England, 1999; Chapter 206. (8) Wong, K. K. W.; Douglas, T.; Gider, S.; Awschalom, D. D.; Mann, S. Chem. Mater. 1998, 10, 279-285. (9) Sipos, P.; St. Pierre, T. G.; Tombacz, E.; Webb, J. J. Inorg. Biochem. 1995, 58, 129-138. (10) Chan, P.; Chua-anusorn, W.; Nesterova, M.; Sipos, P.; St. Pierre, T. G.; Ward, J.; Webb, J. Aust. J. Chem. 1995, 48, 783-792. (11) Coe, E. M.; Bowen, L. H.; Speer, J. A.; Wang, Z.; Sayers, D. E.; Bereman, R. D. J. Inorg. Biochem. 1995, 58, 269-278. (12) Coe, E. M.; Bowen, L. H.; Bereman, R. D.; Speer, J. A.; Monte, W. T.; Scaggs, L. J. Inorg. Biochem. 1995, 57, 63-71. (13) Coe, E. M.; Bereman, R. D.; Monte, W. T. J. Inorg. Biochem. 1995, 60, 149-153. (14) Coe, E. M.; Bowen, L. H.; Speer, J. A.; Bereman, R. D. J. Inorg. Biochem. 1995, 57, 287-292. (15) Coe, E. M.; Bowen, L. H.; Bereman, R. D.; Speer, J. A.; Monte, W. T.; Scaggs, L. J. Inorg. Biochem. 1995, 57, 63-71.

Mineralization of κ-Carrageenan (16) Theerkeelsen, H. G. Carrageenan Chapter. In Industrial Gums; Whistler, R. L., BeMiller, J. N., Eds.; Academic Press Inc.: London, 1993. (17) Bronstein, L. M.; Sidorov, S. N.; Valetsky P. M.; Hartmann, J.; Co¨lfen, H.; Antonietti, M. Langmuir 1999, 15, 6256-6262. (18) Sidorov, S. N.; Bronstein, L. M.; Valetsky, P. M.; Hartman, J.; Co¨lfen, H.; Schnablegger, H.; Antonietti, M. J. Colloid Interface Sci. 1999, 212, 197-211. (19) Bronstein, L. M.; Sidorov, S. N.; Gourkova, A. Y.; Valetsky, P. M.; Hartmann, J.; Breulmann, M.; Co¨lfen, H.; Antonietti, M. Inorg. Chim. Acta 1998, 280, 348-354 (20) Co¨lfen, H.; Antonietti, M. AdV. Polym. Sci. 2000, 150, 67-187. (21) Mulay, L. N.; Selwood, P. W. J. Am. Chem. Soc. 1955, 77, 26932701. (22) Cotton, F. A.; Wilkinson, G. AdVanced Inorganic Chemistry, 4th ed.; John Wiley and Sons: New York, 1980.

Biomacromolecules, Vol. 1, No. 4, 2000 563 (23) Weller, H. Angew. Chem., Int. Ed. Engl. 1993, 32, 41-53. (24) Norton, I. T.; Goodall, D. M.; Morris, E. R.; Rees, D. A. J. Chem. Soc., Faraday Trans. 1 1983, 79, 2475-2488. (25) Hollemann, A. F., Wiberg, E., Eds. Lehrbuch der anorganischen Chemie, 91-100 Auflage; Walter de Gruyter: Berlin, New York, 1985; p 1149. (26) Klug H. P.; Alexander L. E. X-ray diffraction procedures for polycrystalline and amorphous materials, 2nd ed.; John Wiley: New York, 1974. (27) Scherrer, P. Go¨ tt. Nachr. 1918, 2, 98. (28) Lide, D. R., Ed. CRC Handbook of Chemistry and Physics, 73rd ed.; CRC Press: London, 1992-1993.

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