nanorod-coated pnipam microgels: thermoresponsive optical properties

Microgels

DOI: 10.1002/smll.200700078

Nanorod-Coated PNIPAM Microgels: ThermoresponsiveOptical PropertiesMatthias Karg, Isabel Pastoriza-Santos, Jorge P�rez-Juste, Thomas Hellweg,*and Luis M. Liz-Marz n*

Nanoparticles and in particular gold nanorods have interesting opticalproperties arising from two well-differentiated plasmon modes. Thefrequency of such modes can be altered by their chemical environmentand coupling with neighboring rods. This study investigates newcomposite materials made of gold nanorods adsorbed on thermorespon-sive poly(N-isopropylacrylamide) (PNIPAM) microgels. It is shown thatthe thermally induced collapse of the polymer network inside theparticles leads to a red shift of the longitudinal plasmon band of the goldrods, which is found to be fully reversible.

Keywords:· gold· microgels· nanoparticles· nanorods· plasmon resonance

1. Introduction

The intense research dedicated to the study of poly(N-isopropylACHTUNGTRENNUNGacrylamide) (PNIPAM) microgels is motivatedby their thermoresponsive character, which has grantedthem the classification into the group of smart materials.[1–5]

The thermal response of these spherical polymer colloidsmanifests itself in large size variations around a characteris-tic transition temperature, which can be readily estimatedby means of dynamic light scattering (DLS)[4,6] and cryogen-

ic transmission electron microscopy (TEM).[7] The associat-ed morphological changes of the polymer network havebeen extensively studied using small-angle neutron scatter-ing.[4, 8, 9] This characteristic swelling behavior can be alteredthrough copolymerization with organic co-monomers, suchas acrylic acid,[10] leading not only to tailored shifts of thetransition temperature but also affecting other parameterssuch as the swelling ratio and sensitivity toward solution pHor ionic strength, and providing PNIPAM microgels with alarge flexibility for many applications. Because of theirstrong light scattering, dispersions of PNIPAM microgels inwater are turbid, even at low concentrations, and scatteringincreases during the collapse of the particles because of asignificant variation of the refractive index when the poly-mer gets denser. Interestingly, these particles combine prop-erties of typical colloids (e.g., crystallization) with propertiesof responsive polymer systems. Microgels respond muchfaster upon changes of external stimuli, such as temperature,pH, and ionic strength, than their macroscopic counter-parts[11–13] and hence they are interesting for a number of ap-plications including sensors,[14] drug delivery,[15] and separa-tion media.[16] However, many of these applications can begreatly improved if additional functionalities are incorporat-ed into the microgel particles.

Recently, several approaches to the preparation ofhybrid structures made of inorganic nanoparticles and mi-crogels have been reported, mainly comprising well-defined

[*] M. Karg, Prof. T. HellwegStranski-Laboratorium f�r Physikalische undTheoretische ChemieTU Berlin, Strasse des 17. Juni 12410623 Berlin (Germany)Fax: (+49)303-142-6602E-mail: [email protected]

Dr. I. Pastoriza-Santos, Dr. J. P:rez-Juste, Prof. L M. Liz-Marz;nDepartamento de Qu>mica F>sica and Unidad Asociada CSICUniversidade de Vigo36310 Vigo (Spain)Fax: (+34)986-812-556E-mail: [email protected]

Prof. T. HellwegUniversitDt Bayreuth, Physikalische Chemie IUniversitDtsstraße 3095440 Bayreuth (Germany)

Supporting information for this article is available on the WWWunder http://www.small-journal.com or from the author.

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full papers T. Hellweg, L. M. Liz-Marz;n, et al.

core–shell structures[17–20] or microgels with a distribution ofnanoparticles in the polymer network.[7,17] We are interestedin the preparation of hybrids combining the exciting me-chanical properties of PNIPAM microgels with materialscomprising optical properties that can either sense or affectthe volume transition of the microgels. An obvious choicefor such inorganic materials are noble-metal nanoparticles,which display optical properties arising from localized sur-face plasmon resonances in the visible and near-IR regions,so that the resonance frequency is strongly influenced byparameters such as particle size and shape, dielectric proper-ties of the environment, and interparticle interactions.[21]

Gold nanoparticles in particular have been extensively stud-ied, including the synthesis of particles with different shapessuch as spheres,[22,23] rods,[24] platelets,[25,26] polyhedrons,[27] oreven multipods.[28] Each of these geometries displays differ-ent optical behavior, and responds differently to changes insize and shape, since different plasmon modes can be ac-commodated within the particles.[29] One of the most inter-esting systems from the optical point of view are gold nano-rods, since these nanoparticles exhibit strong anisotropy,which is responsible for characteristic UV/Vis spectra withtwo well-separated plasmon resonance bands, arising fromelectron oscillation along (longitudinal surface plasmon)and across (transverse surface plasmon) the long axis.[30]

While the transverse plasmon band is typically locatedaround 510–520 nm and varies little with the nanorod di-mensions, the longitudinal plasmon band is located athigher wavelengths and its position mainly depends on theaspect ratio of the rod, but can also be strongly affected bythe dielectric environment and the proximity of other nano-rods,[31] which can be conveniently used as an indication forsensing changes in their neighborhood.

In the present paper we report on the preparation andoptical characterization of hybrid particles made ofPNIPAM microgels with high surface charge, coated withgold nanorods (see Scheme 1). While Kumacheva and co-

workers[32,33] have used a similar system to study photother-mally induced particle deswelling, our work is focused onoptical changes arising from the influence that microgelswelling and collapse have on the average distance betweengold nanorods. Since this should result in a plasmon shift(color change), it will provide a high sensitivity, which canbe obtained by simple optical monitoring. This will un-doubtedly improve the capabilities of these smart systemsfor sensing applications as explained above. The depositionof nanorods on the negatively charged microgel wasACHTUNGTRENNUNGachieved upon surface modification of the gold particles bywrapping them with two polyelectrolyte layers, poly(styrenesulfonate) (PSS) and poly(allylamine hydrochloride) (PAH),to obtain a uniform, positive surface charge, as describedearlier.[24] We present a systematic study of the effect ofPNIPAM volume changes on the composite optical re-sponse, by using samples with different nanorod surface cov-erage. The morphology of the particles was characterized byTEM, while UV/Vis spectroscopy was used to monitor theoptical changes. As a reference, the swelling behavior of thepure PNIPAM microgels was studied by dynamic light scat-tering (DLS).

2. Results and Discussion

The preparation of the microgel–nanorod composite col-loids was based on electrostatic interactions, which requiredprior surface modification of the rods. The PNIPAM micro-gel particles typically have a negative surface charge, themagnitude of which can be varied by changing the concen-tration of the inorganic initiator and/or by copolymerizationwith charged co-monomers such as acrylic acid.[10] In thiswork, the particles were synthesized with a crosslinker den-sity of 5% (see Experimental Section), and a zeta potentialof �17 mV was determined by measuring a dilute solutionof the microgel at pH 5.5. This value of the zeta potential istypical for PNIPAM particles prepared with charged initia-tors, as was recently shown by Zha and Bao, and can bevaried by changing the amount of initiator.[34, 35] On theother hand, since freshly made gold nanorods are stabilizedwith the cationic surfactant cetyltrimethyl ammonium bro-mide (CTAB), they have a positive surface chargeACHTUNGTRENNUNG(+20 mV).[36] However, the cleaning process to removeexcess CTAB strongly hinders the colloidal stability of thegold nanorods and hence their sensitivity with respect toionic-strength-induced precipitation is increased. Therefore,prior to the assembly, it was necessary to coat the nanorodswith a polyelectrolyte bilayer (PSS+PAH) so as to enhancecolloidal stability and achieve a positively charged surfaceon the nanorods. After this procedure, adsorption of nano-rods on the microgel spheres was found to be fast and quan-titative (no nanorods remained free in solution). Thus, bysimply mixing a concentrated microgel dispersion with dif-ferent amounts of polyelectrolyte-coated gold nanorods(Au-rods@PSS@PAH) in water, it was straightforward toprepare a series of nanorod-coated microgel spheres withdifferent surface coverage. The amounts used and other pa-rameters for each sample in the series are given in Table 1,

Scheme 1. Sketch of a collapsed PNIPAM microgel partly coveredwith polyelectrolyte-coated gold nanorods (Au-rods@PSS@PAH).

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Polymer Microgels with Nanorod Coatings

where the nanorod coverageis seen to increase from sam-ple a to sample e.

The characterization ofthe resulting composite col-loids was carried out byTEM, scanning electron mi-croscopy (SEM) and UV/Visspectroscopy. The spectrum(Supporting Information,Figure S1) of the pure Au-rods@PSS@PAH reveals nosigns of aggregation duringpolyelectrolyte wrapping,since the longitudinal andtransverse plasmon bands arevery well defined. They dis-play maxima centered at860 nm and 512 nm, respec-tively, which agrees very wellwith the expected values forthe average dimensions de-termined from TEM images(average length=57 nm;average diameter=15 nm;average aspect ratio=3.8).The distribution of nanorodson the microgel spheres wasinvestigated by TEM andSEM (see Figure 1). Theimages show that most of thenanorods are located on thesurface of the dried PNIPAMparticles and that the distri-bution of nanoparticles israther homogeneous. Only afew rods seem not to be at-tached to the microgelspheres, but this is likely dueto the drying process on theTEM grids, during which themicrogel collapses. It can beclearly seen that the numberof gold particles per microgelsphere increases from sam-ple a to e (see N values inTable 1). However, even forsample e with the highestnanorod loading no completecoverage is observed. This isalso reflected in the UV/Visspectra of the gold-coatedmicrogel colloids, which areonly slightly red-shifted (seeTable 1) as compared to thespectrum of the free goldnanorods in aqueous disper-sion. For sample a the shift isprobably due to the in-

Table 1. Description of the samples (samples a–e)[a] investigated in this work.

Sample VAu [mL] N[b] lmax at 15 8C [nm][c] lmax at 50 8C [nm][c] Dl [nm][d] Ttrans [8C][e]

a 0.075 3.5 865 874 9 34b 0.270 12.8 869 880 11 34c 0.540 25.1 868 892 24 34d 0.810 38.3 875 900 25 35e 1.000 51.9 876 904 28 36

[a] Samples prepared from same mass of microgel (0.60 mg) and same volume (1 mL) of gold nanorodsolutions, which were prepared from different volumes of a stock solution ([Au-rods]=2.77 mm in termsof gold) resulting in different nanorod coverage increasing from sample a to sample e. [b] N is the abso-lute number of polyelectrolyte-coated gold nanorods per microgel particle, as determined from TEMimages by counting the overall number of rods and dividing by the number of microgel particles.[c] Wavelength of the maximum of the longitudinal band at 15 8C and 50 8C. [d] Magnitude of the bandshift. [e] Transition temperature.

Figure 1. a1) SEM image of nanorod-loaded PNIPAM microgel (sample a), tilted by 458. a2–e) TEM imagesof PNIPAM microgels (samples a–e) covered with different amounts of gold nanorods (Au-rods@PSS@-PAH).

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creased refractive index of the microgel (�1.39 versus 1.34estimated on the basis of ellipsometry measurements), whilefor samples b–e plasmon coupling can play an additionalrole in the shift.

The influence of swelling and deswelling of thePNIPAM core particle as a function of temperature, on theoptical properties of the nanorod-covered microgel particleswas studied by UV/Vis spectroscopy. Spectra were recordedfor a temperature range between 15 8C (completely swollenmicrogel) and 50 8C (completely collapsed microgel), andthe results suggest the existence of two different gold-nano-rod-loading regimes, with low (sample a) and high (sam-ples b–e) surface coverage. Figure 2 (upper panel) shows theUV/Vis spectra of sample a. Two different effects can be ob-served in the spectral evolution arising from the tempera-ture increase: 1) an increase of absorbance at low wave-lengths (up to 650 nm), and 2) a red shift and increase inthe intensity of the longitudinal plasmon band. A qualita-

tively similar shift induced by the microgel collapse was al-ready observed for spherical silver particles embeddedinside the internal network of a PNIPAM microgel by Bal-lauff and co-workers.[7] In the present case both effects canbe attributed to an increase in the refractive index of themicrogel particle during collapse (and compacting) of themicrogel, which results in an increase of the Rayleigh scat-tering (this effect, mainly observed at lower wavelengths inthe UV/Vis spectra, is also observed for pure microgel parti-cles),[3,37] as well as in an increase of the local refractiveindex near the gold nanorods (producing a red shift and en-hancement of the longitudinal plasmon band).[32,33] In thissample, the possibility of electronic interactions amongnanorods in the collapsed state of the microgel can be con-sidered negligible because of the low coverage of the micro-gel surface (see Figure 2, top).

The behavior is different for higher coverages, as exem-plified in the lower panel of Figure 2, which displays the

Figure 2. UV/Vis spectra for two PNIPAM microgel/gold nanorod samples (top: sample a; bottom: sample e) at different temperatures (15, 20,25, 28, 30, 31, 32, 33, 34, 35, 37, 40, 45, and 50 8C). The spectra on the right-hand side are expansions of those on the left.



UV/Vis spectral evolution for sample e at temperatures be-tween 15 8C and 50 8C. We can again distinguish two differ-ent effects during the deswelling and collapse of the micro-gel. At low wavelengths the absorbance rises due to an in-crease in the scattering power of the microgel cores, but thechanges of the longitudinal plasmon band are quite differ-ent, with a red shift, damping, and broadening of the band.These effects are readily interpreted in terms of a decreasein the distances between nanorods upon reduction of theparticle volume when the microgel shrinks. When the goldnanorod coverage is high enough, the interparticle distancesare short enough for plasmon coupling to occur because ofelectronic interactions. These interactions have been classi-fied into tip-to-tip, tip-to-side, and side-to-side interac-tions,[31] the first two being more probable and also strongerthan the latter, and resulting in both cases in a red shift anda decrease in the intensity of the longitudinal plasmon band.For instance, the tip-to-tip self-assembly of gold nanorods,and the accompanying optical effects have been studied byseveral authors,[38–41] invariably showing, not only a red shift,but also a broadening of the band. However, as mentionedabove, the microgel collapse also leads to an increase in thelocal refractive index around the nanorods, which shouldadd to the red-shifting of the longitudinal plasmon reso-nance, but increases its intensity. Therefore, the deswellingprocess will always result in a red shift of the plasmon bandas the sum of the effects of refractive index (independentlyof the loading) and electronic interactions (only for highloading), while the increase or decrease of the band intensi-ty will depend on the dominant effect, and in turn on theloading. We discuss these effects in more detail below.

In Figure 3 the position of the longitudinal band is plot-ted as a function of temperature for samples with differentsurface coverage. As a reference, also the deswelling curve

of the corresponding pure microgel particles is depicted interms of the swelling ratio a[42]

a ¼ Vcollapsed=Vswollen ¼ ðRh=R0Þ3 ð1Þ

where R0 is the hydrodynamic radius in the swollen stateand Rh corresponds to the particle radius in the collapsedstate.

A small red shift of �10 nm is observed for sample a.Due to the low surface coverage in this case, the shift canbe related exclusively to the refractive index effect. Howev-er, for samples with a higher surface coverage (b–e), themagnitude of the experimentally determined red shift in-creases when the amount of gold nanorods is increased (seealso Table 1), clearly revealing that, apart from the refrac-tive index effect, the plasmon shift stems from electronic in-teractions between rods attached to the same particle.

Regardless of the magnitude of the shift observed ineach case, the band position as a function of temperaturequalitatively reproduces the swelling curve of the uncoatedmicrogel (see Figure 3), though for higher surface coveragethe transition temperature is slightly shifted towards highervalues in comparison to the pure microgel particles. Thistemperature shift might be due to the increased ionicstrength in the presence of the polyelectrolyte-coated nano-rods.

Apart from the maximum position, also the variation ofthe intensity of the longitudinal plasmon resonance with in-creasing temperature, for both surface coverage regimes,should be carefully analyzed. For a better understanding ofthis effect, Figure 4 shows the changes in the normalized in-tensity as a function of temperature. By plotting the normal-ized intensity of the band, which is determined according toEquation (2) one can clearly observe the change of the in-

Figure 3. Maximum of the longitudinal plasmon band of nanorod-loaded PNIPAM microgels as a function of temperature for differentloadings (solid symbols). Sample descriptions are listed in Table 1.Deswelling curve of the corresponding pure microgel particles interms of the swelling ratio a (open circles).

Figure 4. Change in the normalized intensities of the longitudinalband of loaded PNIPAM microgels as a function of temperature.Sample descriptions are listed in Table 1.



tensity at the maximum as a function of temperature (seeFigure 4).

Inormalized ¼ IMaxð15 �CÞ=IMaxðTÞ ð2Þ

Here, T is the temperature associated with the individualmeasurements. The intensity obtained from the spectrum atT=15 8C is taken as a reference since at this temperaturethe microgels are usually completely swollen.[37] Two differ-ent types of behavior can be clearly distinguished. For lowsurface coverage (sample a), the normalized intensity in-creases with increasing temperature, which we have attribut-ed to the increase of the local refractive index upon micro-gel collapse. However, in the high-surface-coverage regime(samples b–e), the decrease in normalized intensity ob-served in Figure 4 stems from the overall contribution oftwo opposing effects arising when the temperature is in-creased: an increase in the local refractive index and an in-crease in the magnitude of the electronic interactions be-tween neighboring nanorods. The latter effect becomesmore dominant as the amount of rods on each microgel par-ticle is increased (from sample a to e), and thus results in ared shift and decrease of the intensity of the longitudinalplasmon band, thus leading to a decrease in the normalizedintensity at 50 8C, as shown in Figure 4. For sample b, thenormalized intensity decreases by 3% for a temperature in-crease from 15 to 50 8C, while for sample e the decrease isaround 13 %.

Since the volume-phase transition of a pure PNIPAMmicrogel is fully reversible,[43] the thermoresponsive opticalbehavior of the composite particles is expected to be rever-sible, too, unless aggregation of rods takes place in the col-lapsed state. The confirmation of this fully reversible opticalresponse is shown in Figure 5, where the maximum wave-

length of the longitudinal plasmon band has been plotted asa function of the number of swelling and collapse events,which clearly indicates that the original rod distribution isrecovered after each cycle.

3. Conclusions

PNIPAM microgels can be homogeneously covered withsurface-modified gold nanorods. Apart from other interest-ing effects,[32,33] we demonstrate here that nanorod opticalproperties can be used to monitor the thermoresponsive be-havior of PNIPAM microgel colloids. The surface coverage(number of rods per microgel sphere) can be controlledsimply by varying the microgel to nanorod ratio whenmixing solutions of both (oppositely charged) particle types,as well as through the charge density on the microgel andthe nanorod surface. This aspect will be further investigatedin the future. The optical study of the hybrid materials pre-sented here demonstrates that the collapse of the microgelcan induce a red shift of the longitudinal plasmon band ofthe gold nanorods by as much as 28 nm. In addition,changes in the absorbance and the band width also occur,and all these effects are fully reversible when the tempera-ture of the system is lowered again. The reason behind theobserved spectral changes lies mainly in the increase of thenanorod density during the shrinking process of the micro-gel, but the associated changes in the refractive index of themicrogel during collapse also have an influence. In thefuture we wish to extend this work towards microgels withhigher surface charge and want to explore whether chargecan affect the red shift. Moreover, highly charged microgelcores should lead to elevated surface coverages and henceto stronger plasmon-frequency shifts.

4. Experimental Section

Materials: N-isopropylacrylamide (NIPAM; Aldrich) was re-crystallized from hexane. N,N-methylenebisacrylamide (BIS;Fluka) and potassium peroxodisulfate (KPS; Fluka) were used asreceived. Tetrachloroauric acid, silver nitrate, sodium borohy-dride, sodium chloride, cetyltrimethyl ammonium bromide(CTAB), and ascorbic acid (all Aldrich) were used without furtherpurification. Poly(styrene sulfonate) (PSS, Mw=14900; Poly-sciences) and poly(allylamine hydrochloride) (PAH, Mw=15000;Aldrich) were used as received. Water was purified using a Milli-Q system (Millipore).

Synthesis of the PNIPAM microgel: Conventional precipitationpolymerization was employed for the microgel synthesis, as de-scribed elsewhere.[44] To prepare microgels with a crosslinkerdensity of 5% and a higher surface charge, a solution of N-iso-propylacrylamide (NIPAM, 226 mg) and the crosslinker N,N-meth-ylenebisacrylamide (BIS, 16 mg) dissolved in Milli-Q water(20 mL) was prepared in a three-necked flask equipped with areflux condenser and a magnetic stirrer. The mixture was heatedto 70 8C and maintained under a nitrogen atmosphere to remove

Figure 5. Position of the maximum of the longitudinal plasmon bandof gold nanorods assembled on PNIPAM microgels, as a function of anumber of swelling and collapse events (alternating between 20 8Cand 40 8C).



oxygen. After an equilibration time of 30 min the polymerizationwas started by the rapid addition of a solution of potassium per-oxodisulfate (KPS, 20 mg) dissolved in MilliQ water (1 mL). Afterone minute the colorless solution became turbid and the reac-tion was allowed to proceed for 4 h at 70 8C. Afterwards, thewhite mixture was allowed to cool down to room temperatureovernight under continuous stirring. The microgel dispersion wasthen filtered with a membrane filter (Sartorius, RC) of 0.45 mmpore size to remove aggregates and large impurities if present.To remove small oligomers and unreacted monomers the disper-sion was centrifuged (30 min at 4000 rpm and 40 8C) and redis-persed in water three times.

Synthesis of gold nanorods. Gold nanorods with an averagelength of 57 nm and an average diameter of 15 nm were pre-pared and coated with PSS and PAH as reported by Pastoriza-Santos et al.[24] Briefly, a gold-seed solution was prepared by re-duction of tetrachloroauric acid with sodium borohydride (5 mLof 0.25 mm HAuCl4 in aqueous 0.1m CTAB solution with 0.3 mLof 0.02m NaBH4) and the growth of the rods using 24 mL of theseed solution in a growth solution containing 0.1m CTAB,0.5 mm HAuCl4, 0.6 mm ascorbic acid, and 0.12 mm silver ni-trate. The polyelectrolyte coating was carried out in two steps.First, the desired volume (typically 10 mL) of the nanorod solu-tion was centrifuged (20 min, 8000 rpm), the supernatant dis-carded and the precipitate redispersed in 5 mL Milli-Q water.Subsequently, it was added dropwise to 5 mL of a PSS (2 gL,�1,6 mm NaCl) aqueous solution (previously sonicated for 30 min)under vigorous stirring. Stirring was continued for 3 h, the solu-tion was centrifuged twice at 4500 rpm to remove excess poly-electrolyte, and redispersed in 5 mL of Milli-Q water. Thereafter,it was added dropwise to 5 mL of PAH (2 gL,�1, 6 mm NaCl)aqueous solution (previously sonicated for 30 min) under vigo-rous stirring. Stirring was continued for 3 h, the solution wascentrifuged at 4500 rpm, to eliminate excess PAH, and redis-persed in the desired amount of Milli-Q water.

Assembly of gold nanorods on PNIPAM microgels: To coverthe microgel with the inorganic particles, different amounts ofan aqueous stock solution of the polyelectrolyte-coated goldnanorods ([Au]=2.77 mm)�1 were redispersed in 1 mL water andadded to aqueous microgel dispersions (0.60 mg in 0.06 mL)under sonication (see Table 1 for further information).

Characterization: Images of the nanorod–microgel compositeparticles were obtained using transmission electron microscopy(TEM, JEOL JEM 1010 microscope, acceleration voltage 100 kV)and scanning electron microscopy (SEM, JEOL JSM 6700F, accel-eration voltage 5 kV). For both techniques specimens were pre-pared on copper grids (coated with a carbon membrane). Onedrop (7 mL) of each highly diluted aqueous dispersion of thenanorod-loaded PNIPAM microgels was dried on the grids. Thethermoresponsive behavior of the pure PNIPAM microgel was in-vestigated by means of dynamic light scattering (DLS). Correla-tion functions were recorded at a constant scattering angle of908 using an ALV goniometer setup with a frequency-doubledNd:YAG laser as light source (532 nm), with an output power of150 mW. The swelling curve was obtained by measuring at differ-ent temperatures between 15 8C and 50 8C controlled using aHaake thermostat working with an accuracy of �0.1 K and a tol-uene matching bath. The correlation functions were generatedusing an ALV-5000/E multiple t digital correlator and subse-

quently analyzed by inverse Laplace transformation (CONTIN[45]).UV-vis spectra were recorded employing an Agilent 8453 spectro-photometer with a temperature-controlled sample holder with anaccuracy of �0.1 8C.

Acknowledgements

This work has been supported by the Spanish Ministerio deEducaci!n y Ciencia, through Grants Nos. MAT2004-02991,NAN2004-09133-C03-03, and by the DFG within the frame-work of the priority program SPP 1259. The authors thank Ja-cinto P0rez-Borrajo for his assistance with the SEM measure-ments.

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Received: January 31, 2007Published online on May 8, 2007



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