NANO LETTERS
From “Nano-bags” to “Micro-pouches”. Understanding and Tweaking Flocculation-based Processes for the Preparation of New Nanoparticle-Composites
2008 Vol. 8, No. 11 3598-3604
Gre´gory F. Schneider†,‡ and Gero Decher*,† Centre National de la Recherche Scientifique (CNRS UPR022), Institut Charles Sadron, 23 rue du Loess BP 84047, 67034 Strasbourg Cedex 2, France, and UniVersite´ Louis Pasteur (ULP), 1 rue Blaise Pascal, F-67008 Strasbourg-Cedex, France Received May 27, 2008; Revised Manuscript Received September 1, 2008
ABSTRACT The control of simple parameters involved in the process of classic bridging flocculation allow the preparation and fine-tuning a new class of hybrid nanomaterials with respect to size, composition, and morphology. The resulting nanoparticle-filled “nano-bags” are obtained in aqueous suspension by mixing three basic components, a polyelectrolyte, a multivalent ion, and nanoparticles in different ratios. The size range in which nano- and micropouches can be prepared seems to start at about 25 nm; these are oligo-nanoparticle aggregates whose size are clearly related to the size of the nanoparticles themselves and seem to extend up to about 5 µm. By controlling the stoichiometric balance between the global number of positive and negative charges on the polycation and on the multivalent anion and by controlling the absolute concentrations and the ratios, namely of the polyelectrolyte and the nanoparticles, one has access to a wide range of different nanopouch morphologies and compositions. Interestingly, the process of nanopouch formation seems not to be restricted to a single type of nanoparticles since, at least, citrate-stabilized gold and iron oxide nanoparticles showed indistinguishable results in transmission electron microscopy. The outer surface of the nanoparticle-filled nanopouches is easily functionalized further through layer-by-layer assembly. As a first example, we have enhanced the stability of nanopouch suspensions at increased ionic strength by electrostatic adsorption of a polyanion on the nanobag exterior.
Soft matter composites are an interesting class of multimaterials in which the properties of different components are combined by interfacing them in a controlled way, for example, by using self-assembly or growth processes. While it is in general easy to grow or to assemble larger structures it is much more difficult and therefore much less explored to control an assembly or growth process in such a way that the composite material has a certain dimension and/or composition. The best known example for size-limited and shape-controlled self-assembly is probably the aggregation of surfactants in micelles.1 Another well-known example for the size-limited assembly of a single component is the formation of nanosized “mushrooms” reported by Stupp et * To whom correspondence should be addressed. E-mail: decher@ ics.u-strasbg.fr. † Centre National de la Recherche Scientifique (CNRS UPR022), Institut Charles Sadron, and Universite´ Louis Pasteur (ULP). ‡ Present address: Department of Chemistry and Chemical Biology, Harvard University, 12 Oxford Street, Cambridge, Massachusetts 02138. 10.1021/nl801511w CCC: $40.75 Published on Web 10/14/2008
2008 American Chemical Society
al.2 which makes use of the self-organization potential of block-copolymers.3-5 A powerful method for interfacing many different components in a very defined way is the so-called layer-by-layer deposition of multimaterial films6,7 that we have developed since the early 1990s and that has become very popular in the meantime.8-16 However, this technique is based on a multistep assembly approach and therefore is restricted to the fabrication of materials for which multistep assembly is an acceptable option. A very interesting combination of components is the combination of nanoparticles with polymers if one can achieve a high degree of structural control.17 While the interaction between interacting species such as polymers and solids in the form of particles is of great technical importance (flocculation) and has been studied over many decades, it has been extremely difficult to exercise structural control in flocculation processes. People were predominantly interested in the process and much less in the formation of the resulting
composite materials. However, when composition, morphology, and internal structures of nanoparticle/polymer composites are controlled well, systems with very useful properties can be engineered as illustrated, for example, by the work of Mirkin et al. on DNA-nanoparticle systems.18,19 On the other hand, DNA is not a polymer of choice for the large part of materials science and only recently a more general approach was introduced by Rotello et al., the socalled “polymer-mediated nanoparticle assembly”.20-23 This concept is based on the idea of controlling the interaction of nanoparticles and polymers by making use of the selective interactions between certain functional groups. However, for the case of simple electrostatic interactions between the components in two- and three-component systems, the structural control was somewhat limited to the induction of certain morphology of the resulting flocculated materials. Significantly better control over aggregate morphology was exercised by Stucky and Wong et al., who investigated the flocculation of gold nanoparticles with poly-L-lysine, a process which yields hollow microcapsules.24 They established for the first time that a simple two component system, a dispersion of charged nanoparticles in presence of an oppositively charged polypeptide, can give rise to aggregates with astonishing morphologies such as micron-sized spherical objects with small polydispersity.24 We recently investigated a similar system for completely different reasons (e.g., to optimize the functionalization of single gold nanoparticles by wrapping around an oppositively charged polyelectrolyte, therefore to avoid flocculation25,26) and found that mixtures of the polycation poly(allyl amine) with gold nanoparticles did not yield very interesting morphologies or spherical aggregates in the parameter space explored by us.27 In twocomponent systems (e.g., nanoparticles and an oppositively charged polyelectrolyte without a multivalent ion), we found mostly necklacelike aggregates27 of particles similar to what was observed by Rotello before.21 While monovalent salts at concentrations below the “salting out” limit mostly have an influence on the solution conformation of the respective polyions,29 the addition of bivalent or multivalent ions may lead to flocculation through bridging or multicenter interactions.29,30 We extend here this work on controlled flocculation-based assembly toward the fabrication of differently sized and shaped objects with controlled composition: nanoparticle-filled “nano-bags” and “micro-pouches”. If the interaction between different molecular, macromolecular or colloidal species in solution or suspension exceeds a certain threshold (flocculation threshold), one observes a phase separation into a solid and a liquid phase often termed adsorption/aggregation, complexation, coagulation or flocculation. Flocculation and related phenomena are very important in science and technology and have broad applications both in avoiding a precipitation (keeping emulsions, dispersions, and suspensions stable) or in controlling the precipitate being formed (solid-liquid separation, water treatment, etc). While theoretical approaches on particle/ polymer and polymer/polymer interaction exist in the literature,31,32 it is rather difficult to carry out experiments and to Nano Lett., Vol. 8, No. 11, 2008
structurally characterize the nature of the formed aggregates due to the highly dynamic nature of such systems, especially at increased concentrations or in multicomponent systems. In general, many factors that are difficult to control are involved in the process of flocculation, and one must consider the chemical nature and the concentrations of the reacting partners, their stoichiometry, the way of addition, the agitation, the size of particles (for particles), the degree of polymerization (for polymers), the viscosity of the solution, the temperature, to name a few. For electrostatic interactions (such as in polyelectrolyte complexes33,34), one could add ionic strength, pH, and type of ions to the parameter list. In this manuscript we present a model system for flocculation in which the nanoparticles used provide in situ information on interparticle distances and/or aggregate sizes by color changes associated with these two parameters. Interestingly, the enhanced control that we attain for this model flocculation process allows fabricating new flocculation-based nanomaterials of a controlled nanoparticle content (e.g., nanoparticle loading) over a well-defined and controlled size range going from several nanometers to several micrometers. We describe how the composition of the three component system controls the formation and the morphology of nanoaggregates in the early stages of the flocculation regime. Results and Discussion. The flocculating system reported here is based on three components: a small trivalent ion (trisodium citrate), a polycation (poly(allyl amine hydrochloride); PAH), and negatively charged, citrate-stabilized, nanoparticles composed of either gold or iron oxide. We report in detail on how the most important parameters can be used to drive flocculation in a desired direction (that is control over nanoparticle loading and flocs size). The initial motivation for studying a three component systems (that is by including nanoparticles) was to obtain a maximum amount of information about the flocculation products using the plasmon resonance of gold nanoparticles, a color signal easily picked up even by the naked eye which reports on the distance between gold nanoparticles embedded in a floc and thus their degree of aggregation (i.e., the nanoparticle loading). We then generalized the results obtained with gold to iron oxide nanoparticles. A typical batch yielding to floc formation consists in the injection of a solution of PAH (e.g., n positive charges, n+) in a solution containing citrate (e.g., m negative charges, m-) in presence or not of nanoparticles (e.g., a number p of nanoparticles, that was kept constant and equivalent to a concentration of 1.2 nM in the whole study). We observed that no flocculation occurs for n > m, that is, when the number of positive charges from PAH is in excess compared to negative charges from citrate. This is somewhat similar to the case of polyelectrolyte complexes in which the two polyions have very different degrees of polymerization.32 In order to refer to the relative distribution of n versus m, we therefore define a stoichiometric balance between negative charges from citrate and positive charges from PAH using the usual definition (n+/m-). A given stoichiometric balance (n+/m-) can be indistinguishably defined for different 3599
Figure 1. Transmission electron micrographs of individual flocs formed through the flocculation of a three component system: poly(allyl amine hydrochloride) (PAH, blue, a total of n positive charges; e.g., n+), trisodium citrate (red, a total of m negative charges; e.g., m-) and either gold (Au) or iron oxide (Fe2O3) citrate-stabilized nanoparticles (p ) const; 1.2 nM in nanoparticles). Mixtures are characterized by the stoichiometric balance between positive and negative charges of the flocculating system (e.g., (n+/m-)) and by a proportionality coefficient x (e.g., [x]), the latter representing the total concentration of flocculating partners. The negative charges resulting from the citrate molecules adsorbed on the gold nanoparticles is considered to be negligible compared to concentration of other active species in the system. TEM micrographs surrounded by colored boxes were further investigated in Figure 2 (same color code). Unlabeled scale bars represents 50 nm (a,b). Samples containing gold nanoparticles were stained using uranyl acetate. Fe2O3 samples were not stained because the contrast of iron oxide particles was not sufficient to discern the nanoparticles within an aggregate in presence of a uranyl acetate staining procedure.
overall concentrations of reactants (e.g., x·n+ and x·m-), where x is a proportionality coefficient that accounts for a proportional deviation in concentration of flocculating partners from a standard experimental condition (e.g., x ) 1 represents the standard and accounts for a constant citrate concentration of 23.5 mg/L). This proportionality coefficient x appears in the notation of the stoichiometric balance as a bracket, such that, in a sample defined by (n+/m-)[x], the total concentration of flocculating reactants (e.g., citrate and PAH) is x-times higher than in the standard (see Materials and Methods for more details). The influence of the stoichiometric balance (n+/m-)[x] on flocs morphologies was studied in two sets of experiments (Figure 1). In a first set of experiments (e.g., the standard, with x ) 1), we kept the citrate concentration constant (23.5 mg/L), and we varied the concentration of PAH from an equal concentration of positive and negative charges (e.g., (1/1)[1]) up to a 7-fold decrease in positive charge content (e.g., (1/7)[1]) either in absence (Figure 1a, p ) 0) or in presence of gold nanoparticles (Figure 1b, p ) 1, e.g., [AuNPs] ) 1.2 nM). In a second set of experiments, we varied the total concentration of flocculating partners (Figure 1c,d, with 0.2 < x < 8) while maintaining a constant stoichiometric balance (1/2) and a constant nanoparticle concentration of 1.2 nM (Figures 1c,d). Transmission electron micrographs (TEM) revealed that the presence of nanoparticles is not required to observe the formation of the flocs (Figure 1a), suggesting that only the interaction of PAH with citrate drives their formation. Interestingly, in the presence of gold nanoparticles, decreasing values of n+ yield an increase in the degree of nanoparticle loading while the flocs (e.g., “nano-bags”) exhibit a rather constant size ((1/2)[1] is 264 ( 141 nm, (1/4)[1] is 207 ( 52 nm, and (1/7)[1] is 230 ( 65 nm; statistics were carried on more than 500 flocs by TEM). The degree of nanoparticle loading ranges from less than ten nanoparticles per aggregate in (1/1)[1] to aggregate saturated 3600
Figure 2. (a) UV/visible spectra of floc dispersions for different stoichiometric balances (n+/m-)[1]. (b) Statistical size distribution of flocs (1/2)[x] with x ranging from x ∼ 0.4 (e.g., [Citrate] ) 0.01 mg/mL) to x ∼ 20 (e.g., [Citrate]) ) 0.5 mg/mL. TEM micrographs of some flocs shown in Figure 2b are shown in Figure 1b (respect the same color code).
with larges numbers of nanoparticles in (1/7)[1] (this large number of nanoparticles prevents the determination of exact numbers of nanoparticles per bags). Nano Lett., Vol. 8, No. 11, 2008
Figure 3. TEM micrographs of flocs formed for two stoichiometric balances (1/2 and 1/6; respectively panels a and b) for two proportionality coefficients (x ) 1 and x ) 4; respectively left and right images). Each TEM micrograph within each box represents an overview of the sample (top images) and an image at a higher magnification on a region of interest (bottom images).
Rather than using TEM, the control over the nanoparticle loading can also be monitored by less time-consuming techniques such as UV/vis spectroscopy (Figure 2a) since a given nanoparticle loading and morphology corresponds to a specific UV/vis signature, thus reducing the characterization of such aggregates to a simple color-matching procedure. While the stoichiometric balance controls the degree of loading (Figure 1a-b), a control of the size of the flocs is exercised by varying the total concentration of the flocculating materials (e.g., for constant (n+/m-) and different x; see Figure 1c-d). A variation of ∼40-fold in the total concentration of PAH/Citrate (e.g., from (1/2)[0.2] to (1/2)[8]) yield aggregates with an average size ranging from tens of nanometers to a couple of microns (Figure 2b) with a rather constant particle loading (i.e., an approximately constant number of nanoparticles per area, as seen by the qualitative comparison of TEM micrographs in Figure 1c). Also, as shown in Figure 1d, such a control over the floc morphology is not only restricted to the use of gold nanoparticles, since iron oxide nanoparticles showed indistinguishable results (that is, nano- to micron-size iron oxide-embedded flocs; Figure 1d). Control over floc size is valid for any degree of loading and vice versa. This is illustrated by Figure 3, depicting all possible combinations for two stoichiometric balances (1/2 and 1/6) and two proportionality coefficients (x ) 1 and x ) 4). As mentioned above, lower n+ yield flocs with higher Nano Lett., Vol. 8, No. 11, 2008
Figure 4. TEM micrographs illustrating the effect of increased ionic strengths ([NaCl] ) 9 g/L) on the stability of (1/2)[1] type flocs (top, middle) and illustration of their further stabilization using poly(styrene sulfonate) (PSS) (bottom). In the absence of a PSS protecting layer, the nanoparticle payload is released from the nanobags (middle micrograph).
loading densities (e.g., 1/2 flocs are denser than 1/6 flocs), independently of their sizes (e.g., x ) 1 versus x ) 4). These hybrid nano/micropouches, with a reasonably small polydispersity and filled with a controlled amount of nanoparticles, may potentially be interesting for therapeutic applications. We therefore investigated whether such aggregates remain stable (that is, of indistinguishable morphology) in the presence of NaCl at increased ionic strengths ([NaCl] ) 9 g/L; e.g., 154 mM) as illustrated for (1/2)[1] flocs (Figure 4a). After the concentration of NaCl reached an isotonic level, TEM micrographs revealed that all aggregate structures disappeared and that their nanoparticle payload is completely released (Figure 4b) through, probably, the electrostatic screening by sodium and chloride ions (that is, a dissolution of the nanobags in presence of NaCl).35 However, if poly(styrene sulfonate) (PSS) is adsorbed in a layer-by-layer deposition process presumably on the outer surface of the aggregates, their stability is considerably improved and their suspension remains stable even toward centrifugation which is required to remove unadsorbed PSS (Figure 4c). At the present time we do not know to which mechanism this stabilization with a single “layer” of PSS can be attributed. Possibilities are that a single PSS layer acts as a barrier to NaCl (which is rather unlikely in view of ref 36), or that PSS mechanically stabilizes the soft aggregates like an additional skin, or that PSS exchanges to some extent with the citrate complexed inside the aggregate while preserving its morphology. We are planning to address this question in future work. 3601
Conclusions.This study shows that nanoparticle-filled nanobags and micropouches can be obtained from a flocculation process involving a three component system: a polyelectrolyte, a multivalent ion, and nanoparticles. The synthesis of such aggregates is performed by simply mixing the three components and allows for a precise control over their nanoparticle loading capacity and their size and somewhat also their morphology. The major control parameters of this process are the stoichiometric balance between the global number of positive and negative charges which influences the nanoparticle loading and the absolute concentration of the polycation and of the multivalent ion which influences mostly the size of aggregates. Mechanistically, when compared to monovalent ions, the interaction between a trivalent ion and an oppositiely charged polyelectrolyte will likely occur through a multidentate (or multivalent) binding process, resulting in complexes with different Debye lengths, the latter depending on the valency of the ion used. One possible result of this multidentate binding process is the bridging of the polyelectrolyte chain by the multivalent ion, yielding unusual (e.g., cross-linked) conformations of poly(allyl amine) chains that (we infer) further self-assemble intermolecularly and generate the spherical structures observed in this paper. Most importantly, the flocculation process seems to be tolerant toward the chemical nature of the nanoparticles used. At least in the case of citrate-stabilized gold and iron oxide nanoparticles, the composition, the sizes and morphologies of the aggregates were highly reproducible. It is certainly also interesting that the properties of such aggregates can further be modified by postsynthetic steps; while unmodified aggregates dissolve at ionic strengths of about 0.15 mol/L NaCl thereby releasing their nanoparticle payload, they can easily be stabilized by adsorbing a single additional PSS layer using layer-by-layer deposition to their outer surface. Future experiments will show how this PSS layer forms (with or without an exchange with citrate) and how this further modification can be used to control nanobag and micropouch stability and the release of the nanoparticle payload. Materials and Methods. Synthesis of Citrate-Stabilized Gold Nanoparticles. The water used in all experiments was prepared in a three-stage Millipore Milli-Q purification system (resistivity higher than 18.2 MΩ·cm-1). Gold colloids (13.5 nm sized) were prepared as described previously25 using the standard reduction of tetrachloroauric(III) acid (HAuCl4·3H2O) (Sigma-Aldrich) with trisodium citrate dihydrate (Na3C6H5O7·2H2O) (Sigma-Aldrich) without filtration. All experiments described here were performed with the same stock solution prepared by mixing of 70 mL of 38.8 mM sodium citrate solution and 700 mL of 1 mM HAuCl4·3H2O solution previously brought to a rolling boil with vigorous stirring. After their synthesis, gold colloids (1.25 × 10-8 mol·L-1 gold colloids; concentration determined by elements conservation and by considering a gold colloid density equal to gold in bulk) were kept in the dark at a temperature of 5 °C. This solution was stable over more than three months. For controlling sodium citrate concentration, several aliquots of the colloid stock solution were centrifuged with 3602
Heraeus Pico centrifuge in 1.5 mL polypropylene Eppendorf Safe Lock Tubes for 3 h at 7000 rpm. The supernatant (1480 µL) was removed from each tube and replaced by the same volume of ultrapure water (Millipore). Gold nanoparticles were then diluted ten times (one thousand times for iron oxide nanoparticles; cf. next section) with citrate-buffered solution to reach specific citrate bulk concentration (23.5 mg/L for the standard, that is for x ) 1). The final volume after dilution is of 10 mL and citrate bulk concentrations are ranged from 1 to 500 mg/L. Synthesis of Citrate-Stabilized Iron Oxide Nanoparticles. The synthetic procedure was inspired from the previous work of Cabuil with slight modifications.37 Ultrapure water was deoxygenated by stirring under argon bubbling during six hours. One g (1.043 g) of FeCl2 (Fluka) and 2.6 g (2.653 g) of FeCl3 (Fluka) (stoichiometry 1/2) were dissolved in 13 mL of ultrapure water containing 1 mL of hydrochloric acid (37 wt %, Prolabo). The 13 mL mixture was added dropwise under vigorous stirring in 125 mL of a 2 M NaOH solution previously brought to a rolling boil under an argon atmosphere. The reflux was continued for one hour. Under reflux, 16 mL of a HNO3 solution (68 wt %) was added dropwise into the vortex to neutralize hydroxides and reach an acidic pH. Ultrapure water containing 1.8 g of ferric nitrate (5 mL) was then injected under reflux. Reflux was continued for one hour, and the reaction setup regularly was purged with argon to remove as-formed red and toxic nitrous gas (extreme caution is therefore required during heating). In a last step, 6 g of sodium citrate solid (Aldrich) was added under reflux and vigorous stirring. Reflux was continued for half an hour. The heating mantel was then removed and the solution stirred slowly for an additional 20 min. When the solution reached room temperature, the 200 mL maghemite dispersion was precipitated with 300 mL of analytical acetone (Riedel) and magnetically decanted by using a permanent magnet and the supernatant was discarded. Colloids were then redispersed in ultra pure water and the magnetic decantation process repeated three times. After 2-3 cycles, it became impossible to precipitate particles with acetone, which is a gage of a good stability of the iron oxide nanoparticles dispersion. At this stage, excess of acetone was removed by rotating evaporation (25 °C, 400 mbar) and the evaporation continued to evaporate a small quantity of water (45 °C, 30 mbar). Particles were redispersed in a final volume of 500 mL of water and the weight fraction of maghemite was estimated by weighting 4 mL of the dispersion after complete solvent evaporation (5 days at 300 °C). We found 2.5 mg in 4 mL, that is, ∼4 mM in Fe2O3 and ∼740 nM in nanoparticles (8 nm is the average diameter of the nanoparticles as revealed by TEM and 5.24 g/cm3 the density of bulk Fe2O3). This solution was diluted one thousand times for floc formation (e.g., a concentration in nanoparticles of ∼0.7 nM after dilution). Formation of Nanobags/Micropouches. Polyelectrolytes used are poly(allyl amine hydrochloride) (PAH) Mw ) 15 000 g/mol and poly(styrene sulfonate) (PSS) Mw ) 70 000 g/mol (Sigma-Aldrich). They were used without further purification. All glassware was submitted to a cleaning step using a Nano Lett., Vol. 8, No. 11, 2008
solution of ammonium persulfate in pure sulfuric acid and further rinsed with ultra pure water. For all experiments, 15 µL of a solution containing different concentrations of poly(allyl amine hydrochloride) (from 0.01 to 160 mg/mL) were added to 10 mL of a citrate solution containing different concentrations of trisodium citrate (from 0.1 to 500 mg/L) in presence or not of nanoparticles (1.2 nM for gold nanoparticles and 0.74 nM for iron oxide nanoparticles) followed by magnetic stirring for one minute after mixing. A stoichiometric balance of (1/1)[1] means that the system is composed of a stoichiometric number of positive and negative charges (e.g., n+ ) m-). All stoichiometries (n+; m-)[1] were formed at a bulk concentration of citrate of 23.5 mg/L. A stoichiometric balance of (1/2)10 means that two times less the cationic charges are present compared to negative charges and that a bulk concentration of citrate of 235 mg/L was used (the concentration of PAH was also multiplied by a factor 10 compared to x ) 1 to conserve the stoichiometry 1/2). As an example, the condition (1/2)[1] ((1/4)[1]) corresponds to a number of NH3+ from PAH two times lower (four times lower) than the number of COO- from citrate. This stoichiometry was obtained by injecting 15 µL of PAH at 7.47 mg/mL (15 µL of PAH at 7.47/4 mg/mL; e.g., 1.87 mg/mL) into 10 mL of a citrate solution of a concentration of 23.5 mg/L (23.5 mg/L; the citrate concentration is kept the same for experimental conditions with undistinguishable x) under vigorous stirring (750 rpm); in presence or not of nanoparticles. Immediately after the addition of PAH, the agitation was reduced to 200-300 rpm and stirring continued for an additional minute. Because of high adsorption of flocs and poly(allyl amine) on negatively charged glass vessels, products of the reaction were immediately transferred into PMMA glasswares. The samples were then stored at room temperature without stirring during two hours prior being investigated by UV-vis spectroscopy and transmission electron microscopy (TEM). Native gold nanoparticles are denoted by (0/1)). As another example, the stoichiometry (1/2)4 was obtained with a 4-fold increase in the absolute concentration of flocculating materials (as opposed to (1/2)[1]), except for gold nanoparticles, which concentration was kept unchanged, that is, 1.2 nM (same as the one in (1/2)[1]). To prepare this sample, we injected 15 µL of PAH at 7.47 × 4 mg/mL (e.g., 29.88 mg/mL) into 10 mL of a citrate solution at a concentration in citrate of 23.5 × 4 mg/L (e.g., 94.0 mg/L). Stirring and storage procedures were kept the same. Stabilization of Nanobags/Micropouches with PSS. The stabilization of the flocs with PSS was performed in a PMMA UV-vis cuvette by injecting into the formed (1/2)[1] flocs (1.5 mL), 10 µL of a concentrated solution of PSS (70 000 g/mol) to reach a final concentration in PSS of 2.5 mg/mL. After a hand-shaking homogenization, the sample was kept at room temperature for 30 min and the excess of PSS (e.g., unadsorbed PSS) was removed by centrifugation (e.g., the supernatant was discarded). The sample was then redispersed in ultrapure water (two centrifugation steps were performed Nano Lett., Vol. 8, No. 11, 2008
at 13 000 rpm for 15 min). After the last redispersion step (final volume of 1.5 mL), 10 µL of a concentrated NaCl solution was added in order to reach a concentration of 9 g/L in NaCl. Techniques. UV-vis spectroscopy was carried out on a Cary 500 UV-vis spectrophotometer in the range from 185 to 800 nm in Hellma Quartz Suprasil cuvettes with 10 mm length of the light path. Each of the samples reported here were also observed with a TEM operating at 120 kV (Philips CM12) equipped with a Megaview III Soft Imaging System camera. TEM grids were prepared from solutions containing flocs, where flocs were adsorbed from a 7.5 µL drop during 3 min on 400 mesh carbon-coated copper grids (SPI 2040c or 3040c). After 3 min, the drops were removed with a Whatmann filter n°4 (Wf4) and immediately replaced by a 7.5 µL of a solution containing 2 wt % of uranyl acetate previously filtered through a 0.2 µm cellulose acetate filter (this procedure was omitted for iron-oxide containing flocs). The staining solution was allowed to stay in contact with the sample for 2 min and was then removed by touching with a Wf4 filter paper. Acknowledgment. The following support is gratefully acknowledged: Ministe`re de l’Education Nationale, de la Recherche et de la Technologie, the C.N.R.S, the Region Alsace, and the Institut Universitaire de France. We thank A. Thierry and M. Schmutz for their TEM advice and their invaluable technical support. References (1) Wennerstro¨m, H.; Lindman, B. Phys. Rep. 1979, 52, 1–86. (2) Stupp, S. I.; LeBonheur, V.; Walker, K.; Li, L. S.; Huggins, K. E.; Keser, M.; Amstutz, A. Science 1997, 276, 384–389. (3) Zhang, L. F.; Yu, K.; Eisenberg, A. Science 1996, 272, 1777–1779. (4) Goldacker, T.; Abetz, V.; Stadler, R.; Erukhimovich, I.; Leibler, L. Nature 1999, 398, 137–139. (5) Ruzette, A. V.; Leibler, L. Nat. Mater. 2005, 4, 19–31. (6) Decher, G. Science 1997, 277, 1232–1237. (7) Decher, G.; Hong, J.-D.; Schmitt, J. Thin Solid Films 1992, 210/211, 831–835. (8) Multilayer thin films: Sequential assembly of nanocomposite materials; Decher, G., Schlenoff, J. B., Eds.; Wiley-VCH: Weinheim, 2003. (9) Decher, G. In ComprehensiVe Supramolecular Chemistry; Sauvage, J.-P., Hosseini, M. W., Eds.; Pergamon Press: Oxford, 1996; Vol. 9, pp 507-528. (10) Decher, G.; Eckle, M.; Schmitt, J.; Struth, B. Curr. Opin. Colloid Interface Sci. 1998, 3, 32–39. (11) Bertrand, P.; Jonas, A.; Laschewsky, A.; Legras, R. Macromol. Rapid Commun. 2000, 21, 319–348. (12) Hammond, P. T. Curr. Opin. Colloid Interface Sci. 2000, 4, 430– 442. (13) Tang, Z.; Wang, Y.; Podsiadlo, P.; Kotov, N. A. AdV. Mater. 2006, 18, 3203–3224. (14) Johnston, A. P. R.; Cortez, C.; Angelatos, A. S.; Caruso, F. Curr. Opin. Colloid Interface Sci. 2006, 11, 203–209. (15) Ariga, K.; Hill, J. P.; Ji, Q. Phys. Chem. Chem. Phys. 2007, 9, 2319– 2340. (16) Jessel, N.; Lavalle, P.; Ball, V.; Ogier, J.; Senger, B.; Picart, C.; Schaaf, P.; Voegel, J.-C.; Decher, G. In Elements of Macromolecular Structural Control; Gnanou, Y., Leibler, L., Matyiaszewski, K., Eds.; WileyVCH: Weinheim, 2007; Vol. 2, pp 1249-1306. (17) Balazs, A. C.; Emrick, T.; Russell, T. P. Science 2006, 314, 1107– 1110. (18) Elghanian, R.; Storhoff, J. J.; Mucic, R. C.; Letsinger, R. L.; Mirkin, C. A. Science 1997, 277, 1078–1081. (19) Storhoff, J. J.; Mirkin, C. A. Chem. ReV. 1999, 99, 1849–1862. (20) Boal, A. K.; Ilhan, F.; DeRouchey, J. E.; Thurn-Albrecht, T.; Russell, T. P.; Rotello, V. M. Nature 2000, 404, 746–748. 3603
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