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Functional Core/Shell Nanoparticles via Layer-by-Layer Assembly. Investigation of the Experimental Parameters for Controlling Particle Aggregation and for Enhancing Dispersion Stability Gre´gory Schneider† and Gero Decher* Institut Charles Sadron, CNRS UPR022, 6 rue Boussingault, F-67083 Strasbourg Cedex, France, and UniVersite´ Louis Pasteur (ULP), 1 rue Blaise Pascal, F-67008 Strasbourg Cedex, France ReceiVed July 19, 2007 Gold nanoparticles (AuNPs) with a size of 13.5 nm were synthesized using well-established methods as described earlier by Turkevich (Turkevich, J.; Stevenson, P. C.; Hillier, J. Discuss. Faraday Soc. 1961, 11, 55-75) and Frens (Frens, G. Nature (London), Phys. Sci. 1973, 241, 20-22) using citrate as the reducing agent. It has already been reported that such AuNPs can easily be coated with polymeric shells using electrostatic layer-by-layer assembly of certain polyelectrolytes. Here, we show which parameters, namely, the polyelectrolyte concentration, the contour length of the polyelectrolyte chain, and the ionic strength, are preventing bridging flocculation during polyelectrolyte adsorption and enhancing the stability of the colloidal dispersion. For the preparation of individually coated particles with high yield, we identified optimal conditions such as the degree of polymerization of the polyelectrolytes used, the polyelectrolyte concentration, the nanoparticle concentration, and the concentration of added NaCl during multilayer buildup. Surprisingly, such functional nanoparticles are obtained with highest yield at a moderate excess of polyions. In contrast to expectations, a larger excess of polyions leads again to slight destabilization of the dispersion. The present findings raise our confidence to establish layer-by-layer deposition as a general method for functionalizing even different nanoparticles using a single method.
Introduction Layer-by-layer (LBL) deposition is an established method for the fabrication of multicomposite ultrathin films on solid surfaces.1-11 Some years ago, this approach was very successfully extended to the fabrication of micrometric core/shell colloids and hollow capsules by Mo¨hwald and colleagues.12-18 The approach is highly interesting, as colloids or capsules can then be easily functionalized and their core can be manipulated by exchange processes across the membrane. First attempts to prepare * To whom correspondence should be addressed. E-mail: decher@ ics.u-strasbg.fr. Fax: 00 33 (0)388 414 099. † Present address: Department of Chemistry and Chemical Biology, Harvard University, 12 Oxford Street, Cambridge, Massachusetts 02138. (1) Decher, G.; Schlenoff, J. B. Multilayer thin films: Sequential assembly of nanocomposite materials; Wiley-VCH: Weinheim, 2003; p 524. (2) Decher, G. In Layered nanoarchitectures Via directed assembly of anionic and cationic molecules; Sauvage, J.-P., Hosseini, M. W., Eds.; Pergamon Press: Oxford, 1996; Vol. 9, pp 507-528. (3) Decher, G. Science 1997, 277, 1232-1237. (4) Decher, G.; Eckle, M.; Schmitt, J.; Struth, B. Curr. Opin. Colloid Interface Sci. 1998, 3, 32-39. (5) Decher, G.; Hong, J.-D. Makromol. Chem. 1991, 46, 321-327. (6) Decher, G.; Hong, J.-D. Ber. Bunsen-Ges. Phys. Chem. 1991, 95, 14301434. (7) Decher, G.; Hong, J.-D.; Schmitt, J. Thin Solid Films 1992, 210/211, 831835. (8) Bertrand, P.; Jonas, A.; Laschewsky, A.; Legras, R. Macromol. Rapid Commun. 2000, 21, 319-348. (9) Hammond, P. T. Curr. Opin. Colloid Interface Sci. 2000, 4, 430-442. (10) Freemantle, M. Chem. Eng. News 2002, 80, 44-48. (11) Gorman, J. Sci. News 2003, 164, No. 6. (12) Caruso, F.; Caruso, R. A.; Mo¨hwald, H. Science 1998, 282, 1111-1114. (13) Donath, E.; Sukhorukov, G.; Caruso, F.; Davis, D. P.; Mo¨hwald, H. Angew. Chem., Int. Ed. 1998, 37, 2201-2205. (14) Shenoy, D. B.; Antipov, A. A.; Sukhorukov, G. B.; Mo¨hwald, H. Biomacromolecules 2003, 4, 265-272. (15) Sukhorukov, G.; Fery, A.; Mo¨hwald, H. Prog. Polym. Sci. 2005, 30, 885-897. (16) Caruso, F. Chem.sEur. J. 2000, 6, 413-419. (17) Dai, Z. F.; Dahne, L.; Mo¨hwald, H.; Tiersch, B. Angew. Chem., Int. Ed. 2002, 41, 4019-4022. (18) Shchukin, D. G.; Sukhorukov, G. B.; Mo¨hwald, H. Angew. Chem., Int. Ed. 42, 4472-4475.
nanometric core/shell particles and capsules were reported by Caruso and colleagues.19-21 At nanometric particle sizes, one is confronted with a surface curvature which makes the wrapping of polyelectrolytes around such small objects difficult.22-35 The problem of wrapping polyelectrolytes around small nanoparticles is of considerable importance in biology, especially in the process of DNA replication. In living cells, DNA must be strongly condensed to fit into the cell nucleus, but this condensation must be reversible to allow for duplication of genetic material. This reversible condensation is achieved in nature by complexation with histone proteins, the size of which is very similar to the size of the gold nanoparticles used in the present study. The previous work of Caruso and colleagues on gold nanoparticles coated with LBL shells describes systems of up to a maximum of eight layers with an unknown yield of individual and aggregated particles.19-21 This earlier work by Caruso has outlined the possibility to coat gold particles of a diameter in the nanometer range using layer-by-layer deposition; however, they (19) Mayya, K. S.; Schoeler, B.; Caruso, F. AdV. Funct. Mater. 2003, 13, 183-188. (20) Gittins, D. I.; Caruso, F. AdV. Mater. 2000, 12, 1947-1949. (21) Gittins, D. I.; Caruso, F. J. Phys. Chem. B 2001, 105, 6846-6852. (22) Kunze, K. K.; Netz, R. R. Phys. ReV. Lett. 2000, 85, 4389-4392. (23) Netz, R. R.; Joanny, J. F. Macromolecules 1999, 32, 9026-9040. (24) Messina, R.; Holm, C.; Kremer, K. Langmuir 2003, 19, 44734482. (25) Netz, R. R.; Joanny, J. F. Macromolecules 1999, 32, 9026-9040. (26) Castelnovo, M.; Joanny, J.-F. Langmuir 2000, 16, 7524-7532. (27) Kunze, K. K.; Netz, R. R. Phys. ReV. Lett. 2000, 85, 4389-4392. (28) Messina, R.; Holm, C.; Kremer, K. Phys. ReV. E 2002, 65, 041805. (29) Messina, R.; Holm, C.; Kremer, K. J. Chem. Phys. 2002, 117, 2947. (30) Messina, R.; Holm, C.; Kremer, K. J. Polym. Sci., Part B: Polym. Phys. 2004, 42, 3557-3569. (31) Netz, R. R.; Joanny, J.-F. Macromolecules 1999, 32, 9013-9025. (32) Akinchina, A.; Linse, P. J. Phys. Chem. B 2003, 107, 8011-8021. (33) Penchagnula, V.; Jeon, J.; Rusling, J. F.; Dobrynin, A. V. Langmuir 2005, 21, 1118-1125. (34) Boroudjerci, H.; Netz, R. R. Eur. Phys. Lett. 2003, 64, 413-419. (35) Stukan, M. R.; Lobaskin, V.; Holm, C.; Vinogradova, O. I. Phys. ReV. E 2006, 73.
10.1021/la7021837 CCC: $40.75 © 2008 American Chemical Society Published on Web 01/29/2008
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Scheme 1. Oversimplified Representations of Particles Being Aggregated by Bridging Flocculation (a) and Being Individually Wrapped by Polymer Chains (b); Lower Part Shows Electron Micrographs of Aggregated Particles (c) and of Particles Predominantly Ensheathed Individually (d)
report the absolute necessity to first increase the surface charge of the particles by thiol chemistry.36-39 Very recently, we40,41 and others42,43 described the assembly of up to 20 and more layers on nanoparticle templates with a diameter of 13.5 nm or smaller, which requires robust coating conditions and a high stability of the nanoparticle dispersion. A few months ago, Berret et al. described the control of clustering of superparamagnetic nanoparticles using ionic/neutral block copolymers; however, aggregation numbers of nanoparticles smaller than about five were not reported.44 For the case of the functionalization of individual nanoparticles with multifunctional polymeric shells (i.e., a large number of layers), the deposition of each individual layer must result in very high yields of individual particles to be recovered after each adsorption step. Our first manuscripts40,41 and the work of Channa et al.42 and Hong et al.43 represent more than a proof of concept that even very small nanoparticles can easily be functionalized using layerby-layer deposition; however, no one has yet investigated the experimental parameters in detail that are involved in this process. Due to the high interest in functional nanoparticles in general, for example, for medical diagnostics and as new nanotherapeutic agents, but also with respect to the interest in the wrapping/ unwrapping of polyelectrolytes around oppositely charged nanospheres or with respect to the interest to stabilize dispersions (36) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Chem. ReV. 2005, 105, 1103-1169. (37) Bain, C. D.; Troughton, E. B.; Tao, Y. T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321-335. (38) Nuzzo, R. G.; Fusco, F. A.; Allara, D. L. J. Am. Chem. Soc. 1987, 109, 2358-2368. (39) Weisbecker, C. S.; Merritt, M. V.; Whitesides, G. M. Langmuir 1996, 12, 3763-3772. (40) Schneider, G.; Decher, D. Nano Lett. 2004, 4, 1833-1839. (41) Schneider, G.; Decher, G.; Nerambourg, N.; Praho, R.; Werts, M. H. V.; Blanchard-Desce, M. Nano Lett. 2006, 6, 530-536. (42) Chanana, M.; Gliozzi, A.; Diaspro, A.; Chodnevskaja, I.; Huewel, S.; Moskalenko, V.; Ulrichs, K.; Galla, H. J.; Krol, S. Nano Lett. 2005, 5, 26052612. (43) Hong, X.; Li, J.; Wang, M. J.; Xu, J. J.; Guo, W.; Li, J. H.; Bai, Y. B.; Li, T. J. Chem. Mater. 2004, 16, 4022-4027. (44) Berret, J. F.; Schonbeck, N.; Gazeau, F.; El Kharrat, D.; Sandre, O.; Vacher, A.; Airiau, M. J. Am. Chem. Soc. 2006, 128, 1755-1761.
or emulsions by reducing bridging flocculation, it seemed timely to investigate the conditions for nanoparticle wrapping in more details. The final goal of the present investigation is to derive a set of experimental conditions which lead to (i) a minimum excess of polyion chains per nanoparticle; (ii) a maximum amount of colloids per volume; (iii) a maximum recovery of nanoparticles per adsorption cycle; (iv) a minimum number of aggregated particles; (v) the deposition of a minimum number of layers for achieving a certain degree of functionalization; (vi) a speed-up of the of the characterization work during optimization cycles by using rapid in situ methods (e.g., color changes) after calibration with more direct methods such as transmission electron microscopy (TEM); and (vii) a minimum of process time per adsorbed layer (i.e., reduction of washing steps).
Results and Discussion To obtain well-defined gold-core/LBL-shell nanoparticles in high yields, it is necessary to avoid bridging flocculation45-48 (Scheme 1). This can be achieved by moving away from charge stoichiometry between particles and oppositely charged polyelectrolytes and by limiting the polyelectrolyte excess for maintaining conditions that remain practical for synthesis and handling. There are three important parameters that are expected to play a major role: (1) the length of the polyelectrolyte which is a function of the degree of polymerization, (2) the average distance between the nanoparticles which is adjusted via the nanoparticle concentration, and (3) the stoichiometric excess of either nanoparticles or polyelectrolyte chains. Scheme 1 shows simplified drawings combined with their transmission electron micrograph analogues of gold nanoparticles (AuNPs) in the flocculated state and in the isolated state. Whether it is carried out on micro- or on nanoscaled templates, the assembly of layers on oppositely charged particles requires (45) Hiemenz, P. C. Principles of Colloids, Surface Chemistry, 2nd ed.; Marcel Dekker, Inc.: New York, 1989. (46) Pugh, T. L.; Heller, W. J. Polym. Sci. 1960, 47, 219-227. (47) Vincent, B. AdV. Colloid Interface Sci. 1974, 4, 193-277. (48) Heller, W.; Pugh, T. L. J. Polym. Sci. 1960, 47, 203-217.
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to work more or less close to flocculation conditions. Depending on the charge stoichiometry between particles and polyelectrolytes, one can identify three more or less distinct regimes: (i) Regime I has colloids in excess of polyelectrolytes; this results in incomplete surface coverage and only very few free polyion chains left in solution. This regime would be adequate for adsorbing the next polyelectrolyte layer (due to the absence of the previous polyion in solution), but the suspension tends to be weakly stabilized and there is only little surface functionality from the previous layer to which the next layer could bind. Although highly desirable, it would be difficult to fabricate multilayer coatings on particles using regime I conditions. (ii) Regime II has colloids and polyelectrolytes in approximately equal charge proportion; it is also known as the flocculation regime and must completely be avoided. In fact, one should even avoid getting close to stoichiometric conditions unless at a very high dilution. (iii) Regime III has polyelectrolytes in excess of colloids; this results in excellent surface coverage and a very good stability of the suspension, permitting one even to avoid high dilution of the particle suspension during layer adsorption. The drawback is that each adsorption step leads to a huge excess of polyelectrolyte chains remaining in solution that must be removed prior to the adsorption of each next layer. It seems to be obvious that the experimental conditions must be adapted to regime III, although it would be much more desirable to be able to work in regime I (parameter 1: stoichiometric excess of the oppositely charged polyelectrolyte). In general, one should expect that in regime III adsorption is favored over bridging flocculation if individual nanoparticles are sufficiently separated from each other (parameter 2: nanoparticle concentration) and if the length of the oppositely charged polyelectrolytes is sufficiently small (parameter 3: degree of polymerization). As salt screens polyelectrolyte charges, it would also be desirable to work at high ionic strength to increase the thickness of the adsorbed layer, but this would also lead to a dramatic reduction of the electrostatic stabilization of the particles, which suggests that this parameter must be adjusted with care (parameter 4: concentration of added salt). The last important parameter is a parameter specific to gold nanoparticles prepared by citrate reduction and is only relevant for the deposition of the first layer. If there is an excess of citrate in the suspension of the native particles, the citrate acts as a flocculation agent for the polycations typically deposited as the first layer. It will be shown in a forthcoming manuscript that the flocculation between polyelectrolytes and oppositely charged multivalent ions such as sodium citrate can be used to build, in a straightforward way, new composite nanomaterials. In the present study, we therefore only report on systems with a minimum amount of citrate that is required to maintain dispersion stability (i.e., ∼0.4 mM citrate).40 It is immediately clear that a multidimensional parameter space cannot possibly be explored in full detail with respect to every single parameter. So, for very practical reasons, we sometimes only explore two values of a parameter, for example, only two molar masses of a single polyion, to show a general trend. The effects of the variation of the parameters mentioned above on the dispersion behavior are investigated by several methods: (1) sample color (a very powerful qualitative observation), (2) spectral characteristics of the suspensions (namely the changes in the position and intensity of the plasmon band of the nanoparticles), (3) dynamic light scattering (DLS), and (4) qualitative and quantitative transmission electron microscopy (TEM). Here, gold nanoparticles serve as a model system that can easily be centrifuged and whose plasmon band provides
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very precise information on the aggregation state of the gold colloids.20,21,40,46,49-57 The plasmon shift originates from essentially two different phenomena: the aggregation of single particles into aggregates and/or the adsorption of polyelectrolyte layers onto the particle surface. Typically, if the plasmon band is displaced by changes of the dielectric environment caused, for example, by the adsorption of polyelectrolytes, the frequency shift is about 10 nm or less after the deposition of 20 consecutive layers.40,55 Whereas when gold nanoparticles aggregate, notably by bridging flocculation, the shift is much higher; depending on the number of particles in the aggregate, the shift can easily exceed 150 nm as revealed from studies on planar films.55 Using the methods above, we show how the control of these parameters allows the reaction to disfavor aggregation/flocculation and to favor polyelectrolyte wrapping and suspension stability. As a consequence, in the conclusions of this manuscript, we present improved experimental conditions for the synthesis of Au-core/LBL-shell nanoparticles. Adsorption of the First Polycation Layer. In general, the undesirable aggregation of AuNPs is followed in situ by observing the color change of the suspensions (intensity and the position of the plasmon absorption band), whereas the morphologies of the aggregates (i.e., the number of AuNPs per aggregate) are determined ex situ by TEM, including a statistical evaluation of the aggregate composition. Figure 1 details the aggregation behavior of native AuNPs in the presence of poly(allylamine hydrochloride) (PAH) in regime I as a function of the number of PAH chains per nanoparticle (i.e., the regime where the AuNPs are in excess). It turns out that all four methods agree very well, which provides the advantage to simplify and speed up the evaluation of the aggregation state of AuNP/PAH dispersions by only monitoring color changes of the dispersion. When PAH is added to a dispersion of AuNPs at amounts going from 0.1 to 10 polyelectrolyte chains per AuNP (0.1 to 10 PCs/NP), the intensity of the plasmon band of AuNPs at 520 nm decreases in favor of a contribution around 600-650 nm (Figure 1b). It is easy to follow the color change from bordeaux red (0 PCs/NP) to purple (2-4 PCs/NP) and to blue (5 PCs/NP) with naked eyes (Figure 1a). A grayish-black color indicates strong particle aggregation (10 PCs/NP) that often results after a few hours in complete precipitation. Figure 1d shows the statistics obtained by TEM of the number of Au-cores per aggregate obtained by TEM, sampling 2000 Au-cores for each experimental condition, showing an increasing amount of AuNPs per aggregate when the PAH excess increases from 0 to 10 PCs/NP. The transition from regime II (10 PCs/NP) to regime III (excess of PAH) is described in Figure 2. UV/vis spectra in red (4000-200 PCs/NP) have a plasmon band whose maximum is centered at 519-519.5 nm which corresponds to a red-shift of about 1.5-2.0 nm compared to the band for native AuNPs (dotted line). The intensity of the band centered at 600-650 nm increases when the excess of PAH decreases (i.e., when the PC/NP ratio approaches regime II). For PC/NP ratios of about 100-20 PCs/NP (Figure 2b), aggregation occurs fairly rapidly and it (49) Daniel, M. C.; Astruc, D. Chem. ReV. 2004, 104, 293-346. (50) Elghanian, R.; Storhoff, J. J.; Mucic, R. C.; Letsinger, R. L.; Mirkin, C. A. Science 1997, 277, 1078-1081. (51) Faraday, M. Philos. Trans. R. Soc. London 1857, 147, 145-181. (52) Frens, G. Nature (London), Phys. Sci. 1973, 241, 20-22. (53) Henglein, A. Chem. ReV. 1989, 89, 1861-1873. (54) Mulvaney, P. Langmuir 1996, 12, 788-800. (55) Schmitt, J.; Decher, G.; Dressick, W. J.; Brandow, S. L.; Geer, R. E.; Shashidhar, R.; Calvert, J. M. AdV. Mater. 1997, 9, 61-65. (56) Schmitt, J.; Machtle, P.; Eck, D.; Mo¨hwald, H.; Helm, C. A. Langmuir 1999, 15, 3256-3266. (57) Underwood, S.; Mulvaney, P. Langmuir 1994, 10, 3427-3430.
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Figure 1. (a) Photographic images showing the color of dispersions of AuNPs at a concentration of 1.2 nM in the presence of different amounts of PAH per nanoparticle (excess ratios of 0, 2, 4, 5, and 10 PCs/NP). (b) UV/vis spectra as obtained from the cuvettes in (a) (0 PCs/NP, dashed trace; 0.1 PCs/NP, red trace; 2 PCs/NP, purple trace; 4 PCs/NP, blue trace; 5 PCs/NP, orange trace; 10 PCs/NP, green trace). (c) TEM micrographs of samples corresponding to the dispersions described in (a) and (b). (d) Quantitative statistical analysis on the observed fraction of species bearing n Au-cores per aggregate.
Figure 2. (a) Photographic images showing the color of dispersions of Au/NPs at a concentration of 1.2 nmol/L in the presence of different excess ratios of PAH per nanoparticle (10-2000 PCs/NP). (b) UV/ vis spectra as obtained from the cuvettes in (a) (0 PCs/NP, dashed trace; 0.1 PCs/NP, red trace; 2 PCs/NP, purple trace; 4 PCs/NP, blue trace; 5 PCs/NP, orange trace; 10 PCs/NP, green trace). (c) TEM micrographs of samples corresponding to the dispersions shown in (a) and (b). (d) Quantitative statistical analysis on the observed fraction of species bearing n Au-cores per aggregate.
is easy to follow the color change by the naked eye (Figure 2a), going again from purple-red (100 PCs/NP) to purple (40 PCs/NP) and bluish-purple (30 PCs/NP). The spectra in Figure
2b show the decrease of the plasmon band of isolated particles (at 520-525 nm) accompanied by an increase of the plasmon band corresponding to aggregates at 600-650 nm. Statistical
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Schneider and Decher Table 1. Percentage of Particles Embedded in Aggregates of Hydrodynamic Radii Higher than 150 nm as a Function of the Number of PAH Chains Per Particle as Revealed by Dynamic Light Scattering
Figure 3. Relative optical density given by the integration of spectra in the range of 440-800 nm of the AuNP plasmon band as a function of the polymer excess ratio PCs/NP. The three different regimes (I, II, and III) are identified in terms of charge stoichiometry and in terms of the ratio of polycation chains per nanoparticle. The dip at 1:1 stoichiometry is due to a rapid sedimentation of aggregated particles at the point of maximum flocculation.
analysis (Figure 2d) of 2000 Au-cores by TEM (Figure 2c) corroborates and quantifies that this increasing contribution of the plasmon band at 600-650 nm is due to a continuous decrease of the fraction of single particles in favor of aggregates composed of multiple Au-cores. Figure 2b shows that there is a noticeable plasmon shift (7 nm) between an excess ratio of polyion chains of 200 PCs/NP (Figure 2b, red traces) and 100 PCs/NP (Figure 2b, blue traces). Statistical evaluation of the TEM micrographs reveals that this shift is due to the formation of aggregates with more than 10 Au-cores. Within a range of excess ratios of 4000-200 PCs/NP, the aggregation remains negligible as compared to AuNPs with lower PC/NP numbers (9 < PCs/NP < 100). TEM investigations substantiate that plasmon shifts of less than 2 nm can mainly be attributed to the wrapping of polyelectrolyte chains around the nanoparticles, with very little indication for particle bridging. Determination of the Surface Charge of AuNPs and the Minimum Amount of Citrate Required for Their Stabilization. To develop a more quantitative understanding of these three regimes and to correlate these ratios with the charge balance between polyelectrolytes and nanoparticles, the relative integrated optical densities of the plasmon band were plotted as a function of the PC/NP ratio (Figure 3). For this purpose, spectra were integrated from 440 to 800 nm (to pick up all aggregation induced color changes) and normalized with respect to AuNP concentrations. This analysis yields a maximum of aggregation within regime II for an excess ratio of about 10 PCs/NP. The dip in the trace of the optical density obtained for the ratio of 10 PCs/NP is due to the rapid sedimentation of flocculated nanoparticles prior to completion of the spectral scan. Consequently, the lowest value of the optical density in regime II can be considered to represent the 1:1 charge stoichiometry. Assuming that 10 chains of PAH (i.e., the PC/NP ratio found at the 1:1 stoichiometry) carry 1600 positive charges based on an average degree of polymerization of PAH of 160 (assuming complete protonation at pH 5.5), one can imply that each AuNP carries about 1600 negative charges. Since the pKa values of citrate adsorbed on the surface of gold nanoparticles are not known, one cannot estimate the exact amount of citrate molecules per nanoparticle. However, as each citrate can have between one and three negative charges, the number of citrate molecules per nanoparticle is expected to be between 530 and 1600. At a diameter of 13.5 nm, a nanoparticle has a surface area of about 575 nm2, and thus, the area per charge is 0.36 nm2, a value which is in good agreement with the size
polyelectrolyte chains per nanoparticle (PAH 15K)
percentage of nanoparticles with an apparent hydrodynamic radius > 150 nm
60 000 100 000 150 000 200 000
18% 25% 35% 40%
of a monomer repeat unit or with the size of a citrate molecule. We therefore conclude that the stabilizing layer of citrate molecules around a gold nanoparticle corresponds to a monolayer comprising about 1600 (or slightly less) molecules at a particle diameter of 13.5 nm. Determination of the Optimum Excess of Polyelectrolyte Chains during the Adsorption of the First Layer. Dynamic light scattering (DLS) was performed on the different AuNP/ PAH mixtures discussed above; the results are summarized in Table 1. Considering the optical density of the gold particles, the refractive index of the polyelectrolyte chains, and the refractive index of the surrounding medium, these results should be interpreted with care; however, DLS indicates the presence of isolated particles with an estimated hydrodynamic radius of around 20 nm but with an increasing proportion of aggregates with a size of 150-200 nm for a number of polyelectrolyte chains per particle exceeding 60 000 PCs/NP (Table 1). The quality of the electrosteric stabilization affects mainly the particle recovery after centrifugation, which should be as high as possible. Figure 4 details the stability of particles toward centrifugation analyzed by monitoring the hydrodynamic radius by DLS and the position of the plasmon band by UV/vis spectroscopy. Yields for the recovery of redispersable AuNPs versus the number of centrifugation cycles (centrifugation is used to remove the polyelectrolyte chains in excess) were determined by monitoring changes within the absorbance of the AuNP dispersions (Figure 4a). Yields after the second centrifugation cycle are less than 60% at an excess of chains smaller than 10 000 PCs/NP, and reach a plateau at 60% at an excess of chains between 10 000 and 200 000 PCs/NP. The highest yield, 80%, is observed for 60 000 PCs/NP. For a number of PCs/NP lower than 60 000, the second centrifugation step leads to a loss of 20% of nanoparticles. This 20% loss of material appears as an undispersible pellet at the bottom of the centrifugation vial. More interestingly, for a number of PCs/NP higher than 60 000, yields of recovery become independent of the number of centrifugation cycles, which indicates that the stabilization of AuNPs by PAH has reached an optimum. At this excess of PAH, AuNP/PAH mixtures have a minimal hydrodynamic radius after two centrifugation steps (Figure 4b, 60 000 PCs/NP). Quantitative statistical analysis of TEM images with respect to the number of Au-cores per aggregate for 10 000 < PCs/NP < 60 000 revealed an increasing amount of twin, triple, and higher numbers of Au-cores per aggregate when the amount of PAH per particle is decreased, also explaining the maximized hydrodynamic radius for 20 000 PCs/NP (Figure 4b). After two centrifugations, at an excess of PAH of 60 000 PCs/NP, the yield of recovered particles is ∼80%, with 97% being single particles, 2% being twin aggregates and 1% corresponding to aggregates bearing more than three nanoparticles. For 30 000 PCs/NP (recovery yield of 60%), a larger amount of multiple Au-core aggregates is observed: 87.5% as single particles, 9.5% as twin aggregates, 2% as triple aggregates, and 1% as being composed of more than four nanoparticles. The positions of the plasmon band as a function
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Figure 5. Influence of the length of the polyelectrolyte chains and effect of the nanoparticle concentration on the aggregation state during multilayer buildup. The positions of the plasmon bands are used to show the corresponding trends in a qualitative way. (a) Position of the plasmon peaks versus the number of adsorbed layers for functionalization involving low molecular weight PAH (15 000 g/mol) and PSS (13 500 g/mol) (short chains) and high molecular weight PAH (70 000 g/mol) and PSS (70 000 g/mol) (long chains). For comparison of the influence of the degree of polymerization, the absolute quantities of the polyelectrolytes used were kept constant (same number of polyelectrolyte repeat units in both cases). (b) Position of the plasmon peaks versus the number of adsorbed layers for the case of low molecular weight polyelectrolytes for two different concentrations of AuNPs (6 and 44 nM). Figure 4. Effect of the number of PAH chains per gold nanoparticle. (a) Recovery yields after one (empty squares) and two (full squares) centrifugation/redispersion cycles versus the number of PAH chains per AuNP. The dashed lines have no physical meaning; they only serve as guides to the eyes. (b) Particle diameters as determined by dynamic light scattering (DLS) as a function of the number of PAH chains per AuNP. (c) Position of the plasmon peak as a function of the number of PAH chains per AuNP.
of PAH excess and number of centrifugations are represented in Figure 4c. Low PC/NP ratios (below 10 000 PCs/NP) lead to large plasmon shifts and poor particle recovery. For a value of PCs/NP higher than 20 000, the maximum of the plasmon band reaches a plateau around 519 ( 0.5 nm. All techniques agreed on the requirement to work at a PAH stoichiometric excess of 60 000 PCs/NP or higher. For a PAH excess above 60 000 PCs/NP, one observes a slight tendency toward aggregation that may arise from increasing ionic strength due to the PAH counterions. Effect of the Degree of Polymerization and of the Nanoparticle Concentration. When comparing long (PAH, PSS: MW ) 70 000 g/mol) and short polyelectrolytes (PAH: MW ) 15 000 g/mol; PSS: MW ) 13 500 g/mol) by optical spectros-
copy (Figure 5a), one clearly observes that the shorter chains cause less shifting of the plasmon band and thus less aggregation. As expected, longer polymer chains will bridge and aggregate particles whereas smaller ones will limit bridging flocculation. Transmission electron micrographs corresponding to samples represented in Figure 5a are those reported in Scheme 1 for gold nanoparticles functionalized with seven polyelectrolyte layers using polyelectrolytes of high molar mass (PAH, PSS: MW = 70 000 g/mol, Scheme 1c) and low molar mass (PAH: MW = 15 000 g/mol; PSS: MW = 13 500 g/mol, Scheme 1d). Similarly, a higher particle concentration is expected to cause aggregation and bridging flocculation as well. Figure 5b shows the plasmon band position versus the nanoparticle concentration. A higher nanoparticle concentration (44 nM) causes a larger shift of the plasmon band, whereas a lower particle concentration (6 nM) leads to smaller spectral changes and thus to less bridging flocculation. TEM investigations were carried out using short polymer chains (estimated contour lengths of 40 nm for PAH with a molar mass of 15 000 g/mol and 17 nm for PSS with a molar mass of 13 500 g/mol using a length of the repeat unit of vinyl monomers of 0.25
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nm) at an excess of 30 000 PAH chains per nanoparticle and 66 000 chains of PSS during the layer adsorption steps. A quantitative statistical analysis of nanoparticle/aggregate morphologies performed with more than 2000 nanoparticles coated with nine layers reveals that approximately 80% of the population consists of individual particles at a particle concentration of 6 nM (average interparticle distance of 650 nm), and less than 60% of the population consists of individual particles when the concentration of nanoparticles is 7.3 times higher (44 nM, average interparticle distance of 350 nm). This clearly shows that interparticle distances 10-20 times larger than the contour length of the polymer are not yet sufficient to completely suppress aggregation at this stoichiometric excess of more than 30 000 PCs/NP. While this seems surprising at first sight, one should realize that the interparticle distances are dramatically reduced during centrifugation when particles are forced together in very small volumes (i.e., concentrated up to 75-100 times by centrifugation). Consequences of Polymer Stoichiometric Excess and Contour Length of the Adsorbing Polymer during Multilayer Buildup. For the functionalization of gold nanoparticles with one PAH layer, it has been shown that nanoparticles become considerably stabilized with increasing polyelectrolyte excess. In this section, the multilayer growth was studied as a function of polymer excess while the aggregation state of the AuNPs and the characteristics of the multilayer shell were studied as a function of the excess for values of 15 000, 30 000, and 60 000 PCs/NP for PAH with a molar mass of 15 000 g/mol, for values of 16 500, 33 000, and 66 000 PCs/NP for PSS with a molar mass of 13 500 g/mol, and for values of 35 000, 70 000, and 140 000 PCs/NP for PSS with a molar mass of 6500 g/mol (Figure 6). For simplifying the discussion, we only refer to the number of PCs/NP for PAH, as those for PSS could be obtained by multiplying these values by a factor 15 000/13 500 for PSS with a molar mass of 13 500 g/mol and 15 000/6500 for PSS with a molar mass of 6500 g/mol. The position of the plasmon peak was plotted as a function of the number of adsorbed layers for both the low and high molecular weight of PSS (Figure 6a and b, respectively). For a better understanding of the parameters causing the plasmon shift during multilayer construction, both the adsorbed polyelectrolyte shell thickness and the degree of aggregation of particles have been investigated by TEM. Average yields of nanoparticle recovery for each condition have also been studied. All characterizations were performed after each adsorbed layer. It is shown in Figure 6a and b that a higher polymer excess per particle induces only a small plasmon shift independently of the PSS molecular weight. TEM statistical investigations on the multilayer membrane thickness after nine layers do not show significant variations between excess ratios of 15 000 (3.4 ( 0.8 nm), 30 000 (3.6 ( 0.7 nm), and 60 000 PCs/NP (3.4 ( 0.7 nm). The statistical evaluation of the number of Au-cores per aggregate (Figure 6c) reveals an increasing degree of aggregation when the number of PCs/NP decreases, independent of the molecular weight of PSS, and explains that the lower is the polyelectrolyte excess, the higher is the aggregation state of particles and also the higher is the corresponding plasmon shift. Yields of particle recovery after each deposited layer decrease when the ratio of PCs/NP decreases (94% per adsorption cycle for 60 000 PCs/NP, 90% for 30 000 PCs/NP, and 85% for 15 000 PCs/NP). One should also notice that the particle recovery yields always increase for increasing number of deposited layers. As expected, we observe less bridging of particles for shorter polymer chains. However, using smaller polyelectrolyte chains also results in smaller
Schneider and Decher
Figure 6. Spectral data and statistics depicting the effect of the number of PCs/NP on multilayer growth. (a and b) Position of plasmon peaks as a function of the number of adsorbed layers for an increasing PCs/NP excess ratio for two molecular weights of PSS (6500 g/mol and 13 500 g/mol). (c) Statistics revealed by TEM micrograph analysis on the ratio of species bearing n Au-cores per nanoaggregate as a function of the molecular weight of PSS and as a function of the PCs/NP excess ratio.
nanoparticle recovery yields, which is apparently due to the reduced electrosteric stabilization when the particles are decorated with shorter polymer chains. Figure 7a and b presents the variation of the position of the plasmon peak as a function of the layer number for low (MW ) 6500 g/mol) and high (MW ) 13 500 g/mol) molecular weights of PSS and as a function of the PAH excess ratio (30 000 and 60 000 PCs/NP). Figure 7c represents the evolution of the integrated intensity of the peak at 225 nm (absorbance of the PSS chromophore) as a function of the PSS layer number and corresponds to the amount of adsorbed PSS. In Figure 7a and b, it is shown that plasmon shifts are smaller when the molecular
Core/Shell NPs Via Layer-by-Layer Assembly
Langmuir, Vol. 24, No. 5, 2008 1785 Scheme 2. General Trends Resulting from the Studies of the Effect of the Polymer Stoichiometric Excess (Parameter 1), the Concentration of AuNPs (Parameter 2), and the PSS Degree of Polymerization (or Contour Length) (Parameter 3)a
a The stoichiometric excess (PCs/NP) refers to the number of PAH chains per nanoparticle, and the values for PSS are obtained by multiplication with a factor of 15 000/13 500 for PSS 13.5K and by a factor of 15 000/6500 for PSS 6.5K.
Figure 7. Spectral data as a function of the number of deposited layer pairs and of the degree of polymerization of PSS. The number of layer pairs is identical to the number of PSS layers in the multilayered film. In these plots, the degree of polymerization of the polyelectrolyte is varied but the total number of monomer repeat units is kept constant by adjusting the polymer concentration accordingly. The concentration of gold nanoparticles is 3 nmol/L. (a) Position of the plasmon peak versus the number of adsorbed layers for two distinct molecular weights of PSS (MW ) 13 500 g/mol, DP ) 65 and MW ) 6500 g/mol, DP ) 32). Experiments were performed at a constant stoichiometric excess of 60 000 PCs/NP for PAH, 70 000 PCs/NP for PSS 13.5K, and 140 000 PCs/NP for PSS 6.5K. (b) As (a) but with a polyelectrolyte stoichiometric excess two times lower: 30 000 PCs/NP for PAH, 35 000 PCs/NP for PSS 13.5K, and 70 000 PCs/NP for PSS 6.5K. (c) Integrated intensity of PSS absorbance at 225 nm versus the number of adsorbed layers for long (13.5K) and short PSS (6.5K).
weight of PSS decreases. Similarly, integrated intensities of PSS absorbance plotted as a function of the number of deposited layers show that a shorter polymer chain leads to a smaller amount of adsorbed PSS (difference of a factor of 1.3 as compared to PSS with a molar mass of 13 500 g/mol). This has been
corroborated by TEM evaluation of the shell thickness which, after nine layers, shows a 1.2 times thicker shell for PSS with a molar mass of 13 500 g/mol as compared to PSS with the shorter chain length (3.0 ( 0.5 nm for the shorter PSS chains and 3.6 ( 0.5 nm for the longer PSS chains). In addition, the statistical evaluation of the number of Au-cores per aggregate (Figure 6c) reveals a higher ratio of isolated nanoparticles when the shorter PSS is used. This further corroborates that short polymer chains reduce bridging flocculation during the multilayer buildup but with shorter polyelectrolyte chains they also reduce the stabilization of coated nanoparticles. With the previously described results on the effects of the polymer stoichiometric excess (parameter 1), of the AuNP concentration (parameter 2), and of the degree of polymerization of the polyelectrolyte chains (parameter 3), we have been able to better quantify a general trend for the layer-by-layer ensheathing of nanoparticles which is in perfect agreement with the classic rules of colloid science (Scheme 2): the longer the polymer chain, the lower the polymer concentration, and the higher the nanoparticle concentration are, the higher is the aggregation as induced by bridging flocculation. However, we have shown in Figure 5 that, at a stoichiometric excess exceeding 60 000 PCs/ NP, the tendency of the particles to aggregate increases slightly, which is not predicted by simple classic arguments and which might originate from the effect of a higher ionic strength (due to polyelectrolyte counterions) on the conformation of the polyelectrolytes during adsorption. Optimized coating conditions (a molar mass of PAH of 15 000 g/mol at an excess ratio of 60 000 PCs/NP, a molar mass of PSS of 13 500 g/mol at an excess ratio of 66 000 PCs/NP, and a nanoparticle concentration of 3 nmol/L) gave an 85% yield of single coated nanoparticles after the deposition of 10 layers. Figure 8 reports the characterization of AuNPs coated according to these conditions. Figure 8a shows normalized UV/vis spectra (A(440 nm) ) 0.5) as a function of the layer thickness; the shift and the intensity of the plasmon band are reported in Figure 8b as
1786 Langmuir, Vol. 24, No. 5, 2008
Figure 8. (a) UV/vis spectroscopic data of Au-core/polyelectrolyteshell nanoparticles as a function of the number of deposited layers (normalized intensities). (b) Integrated intensity and position of plasmon peaks as a function of the number of adsorbed layers. (c) Raw data of the zeta potential versus the adsorption cycle.
a function of the number of deposited layers. Those observations are in perfect agreement with theoretical considerations by Helm and Eck.58 The yield of recovered particles after 10 layers is 54.1%, which corresponds to an average recovery yield of 94% per adsorbed layer. In addition, and to verify the charge inversion of nanoparticles during functionalization, zeta potential measurements were carried out at a constant AuNP concentration of 6 nmol/L (A440 nm )1.0) after each adsorbed layer, and they are reported in Figure 8c. Tunable Layer Thicknesses with Added Salt. The effect of added salt during layer deposition was investigated using the experimental conditions reported in the previous section. Salt (NaCl) was only added to the polyelectrolyte solutions during layer deposition, and the washing steps were carried out in pure water. NaCl concentrations ranged from 0.001 to 0.4 M. The positions of the plasmon band, the integrated intensities at 225 (58) Eck, D.; Helm, C. Langmuir 2001, 17, 257-260.
Schneider and Decher
Figure 9. Effect of ionic strength on multilayer construction. (a) Position of the plasmon peak versus the number of adsorbed layers for different ionic strengths ranging from 0 to 0.4 M NaCl. (b) Left: Integrated intensity of the peak at 225 nm corresponding to PSS chromophore groups versus the number of adsorbed layers for different ionic strengths. Right: TEM micrographs of individual Au-core/polyelectrolyte nanoparticles as a function of ionic strength. (c) Statistics revealed by TEM on the ratio of species bearing n Au-cores per nanoaggregate.
nm, the statistical distribution of the number of Au-cores in aggregates, and TEM pictures of coated nanoparticles are shown in Figure 9. As in the case of films on flat surfaces,59,60 an increased ionic strength increases the shell thickness (Figure 9b), but it also is accompanied by a larger shift of the plasmon band (Figure 9a). The recovery yields per adsorption cycle versus the ionic strength are shown in Figure 9b. As expected from charge screening arguments, recovery yields of individual particles increase with decreasing ionic strength: 44% at a NaCl concentration of 0.4 M, 73% at 0.1 M, 81% at 0.08 M, 92% at 0.01 M, 89% at 0.001 M, and 94% in the absence of salt. The statistics on the number of Au-cores per aggregate are presented (59) Dubas, S. T.; Schlenoff, J. B. Langmuir 2001, 17, 7725-7727. (60) Ladam, G.; Schaad, P.; Voegel, J. C.; Schaaf, P.; Decher, G.; Cuisinier, F. Langmuir 2000, 16, 1249-1255.
Core/Shell NPs Via Layer-by-Layer Assembly
Figure 10. Effect of ionic strength on the stability of AuNPs coated with 10 layers bearing PSS as the terminal layer. (a) Raw UV/vis spectral data for AuNPs coated with 10 pairs of PAH/PSS layers dispersed in different concentrations of NaCl. (b) TEM micrographs corresponding to these conditions (scale bar: 200 nm).
in Figure 9c. At an ionic strength of 0.4 M, more than 90% of the observed species are composed of more than five Au-cores; at 0.1 M, the majority of the species consist of oligo-aggregates bearing two Au-cores on average. This makes the ionic strength an interesting experimental parameter for controlling the composition of small colloidal aggregates. Influence of the Ionic Strength on the Dispersion Stability. AuNPs coated with 10 layers were successively dispersed in media containing an increasing amount of NaCl ranging from 0 to 0.1 M, and the dispersions were analyzed by UV/vis spectroscopy (Figure 10a) and by TEM (Figure 10b). The dotted line in Figure 10a corresponds to Au/(PAH/PSS)5 dispersed in pure water. The plasmon band is located at 526.9 nm. When the ionic strength is stepwise increased, the plasmon maximum is shifted slightly to higher wavelengths: 527.8 nm (NaCl 0.02 M), 528.4 nm (NaCl 0.05 M), and 532.4 nm (0.10 M). This shift is accompanied by an overall absorbance decrease due to the onset of aggregation induced by the increased electrostatic screening. TEM micrographs (Figure 10b) show that single nanoparticles move closer to each other when the ionic strength increases. For particles coated with 10 layers, the plasmon band is shifted to a maximum value of 533 nm. The plasmon shift is higher for a smaller number of deposited layers (e.g., 545 nm for AuNP/(PAH/PSS)2/PAH). This is easily explained by the fact that the minimum distance between the gold cores, which determines the position of the plasmon absorption band, is given by shell thickness. Consequently, a higher membrane thickness leads to a smaller displacement of the plasmon band.
Conclusion At present, we have defined practical concentrations of nanoparticle suspensions at about 50-200 µg of Au/mL which, in molar terms, corresponds to 3-12 nmol/L in gold nanoparticles of a diameter of 13.5 nm. At a total surface area of about 10-40
Langmuir, Vol. 24, No. 5, 2008 1787
cm2/mL and a surface coverage of approximately 1 mg of polyelectrolyte per layer for 1 m2 of surface area, this corresponds to about 10-40 µg of polymer per layer per milliliter of suspension, which is in the range required for some typical applications such as drug delivery and (bio)detection (these values can be slightly increased by concentrating the particles by centrifugation). At the same time, the excess of polymer chains per particle must be kept as small as possible, since a larger excess also increases the amount of wasted polymer per adsorption step. This is important for the case of functional polymers. We have analyzed and discussed several experimental parameters that control the flocculation of the gold nanoparticle/ polyelectrolyte system and the functionalization of particles with a maximum of individual particles in the dispersion and high yields of recovery. The length of the polyions is a major parameter for avoiding flocculation while obtaining good rates of polyion deposition at the particle surface at the same time. If the chains are too long, bridging flocculation is favored; if the chains are too short, the yields of particle recovery are reduced. For a scale-up of the system, one would like to work with a maximum concentration of particles in suspension. However, small distances between particles also favor bridging flocculation and must be avoided. We found that a particle concentration of 3 nmol/L during polymer deposition is a good compromise for the system described here. We also reported here optimized conditions for coating AuNPs with LbL assembled shells (i.e., a polymer mass of about 15 000 g/mol and a polymer excess ratio of about 60 000 chains per particle at a particle concentration of 3 nmol/L) which lead to a recovery of individual particles higher than 90% per adsorption cycle accompanied only by very small amounts of oligomeric aggregates. As for planar surfaces, increased ionic strength during layer deposition leads to a higher adsorbed thickness per layer but also to a reduced electrostatic stabilization which is detrimental to the stability of the suspension. Multilayer growth is faster with salt (e.g., less adsorption cycles are required to reach a given thickness), but the reduced yield outweighs this benefit. Thus, it is best to functionalize the AuNPs in the absence of added salt to achieve high yields of particle recovery. The comparably poor stability of these systems in buffers at physiological ionic strength or in serum will be addressed in a forthcoming manuscript. Of course, different types of particles and different polyelectrolytes will slightly influence the conditions reported here, but we have identified some of the parameters that should allow one to guide and accelerate such an optimization on different systems. Materials and Methods The polyelectrolytes used for multilayer deposition were poly(allylamine hydrochloride) (PAH) MW ) 15 000 and 70 000 g/mol and were purchased from Sigma-Aldrich. Poly(sodium-4-styrenesulfonate) (PSS) MW ) 13 500 and 6500 g/mol was purchased from Polymer Standard Service, PSS of MW ) 70 000 g/mol was purchased from Sigma-Aldrich. All polyelectrolytes were used without further purification. Small molecular weights were chosen such that the polymer coils would not exceed too much the colloid size. The water used in all experiments was prepared in a three-stage Millipore Milli-Q purification system (resistivity higher than 18.2 MΩ/cm) and was air-equilibrated before use. All glassware was cleaned in pure sulfuric acid and rinsed with ultrapure water. Synthesis of Gold Nanoparticles (AuNPs). Gold colloids (13.5 nm size) were prepared as described previously61 using the standard (61) Grabar, K. C.; Freeman, R. G.; Hommer, M. B.; Natan, M. J. Anal. Chem. 1995, 67, 735-743.
1788 Langmuir, Vol. 24, No. 5, 2008 reduction of tetrachloroauric(III) acid (HAuCl4‚3H2O) (SigmaAldrich) with trisodium citrate dihydrate52,62 (Na3C6H5O7‚2H2O) (Sigma-Aldrich) without filtration. All experiments described here were performed with the same stock solution prepared by mixing 70 mL of 38.8 mM sodium citrate solution with 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 gold colloids, assuming a density for gold colloids identical to that of gold in bulk) were kept in the dark at a temperature of 5 °C. This solution was stable over more than 3 months. Preparation of Suitable AuNPs for Direct Coating with PAH. For controlling the sodium citrate concentration, several aliquots of the colloid stock solution were centrifuged in a 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 Milli-Q water. At this stage, the quantity of free sodium citrate is reduced by a factor 75. This centrifugation/redispersion cycle was carried out just one time. A quantitative determination of the remaining concentration of citrate was not attempted, but it was assumed to be less than 1.3% of the initial concentration. This “citrate free” gold suspension solution is of a concentration of about 12 nmol/L (solution A) according to a calculation by considering the conservation of gold during synthesis and by using a density of the gold nanoparticles similar to that of bulky gold. The pristine NP stock solution and “citrate free” NPs (solution A) exhibit undistinguishable UV/vis spectra (i.e., no particles are lost during this washing step). Experiments Concerning the Variation of the Polymer Excess Ratio (PC/NP) during Adsorption of the First Layer (Parameter 1). (i) Regimes I, II, and III. A gold suspension (solution A) was diluted ten times with ultrapure water to reach a final volume of 10 mL. The concentration of gold nanoparticles is then expected to be 1.2 nmol/L. Under vigorous stirring (750 rpm), several 15 µL aliquots of PAH solution (MW ) 15 000 g/mol) with different concentrations were added. The mixture was agitated, and then stirring was reduced to 200-300 rpm for an additional 1 min. The products of the reaction were transferred in poly(methyl methacrylate) (PMMA) glasswares and were allowed to stand for 2 h before performing physicochemical analysis. Photographs were performed in quartz cuvettes for technical reasons. Concentrations of injected PAH solutions ranged from 0.01 to 320 mg/mL to obtain desired amounts of PCs/NP. For example, 8.33 PCs/NP were obtained by injecting 15 µL of a solution of PAH at a concentration of 0.1 mg/mL in 10 mL of a AuNP dispersion at a concentration of 1.2 nmol/L in particles (also a 10 times dilution of the AuNP stock solution that was previously centrifuged once before dilution (e.g., solution A diluted 10 times)). Aliquots were measured with 0.1 µL precision. A TEM grid was prepared for each condition. For that, particles were adsorbed from a 7.5 µL drop during 3 min on 400 mesh copper grids (SPI 2040c or 3040c) coated by floatation in distilled water of an evaporated carbon film deposited on a freshly cleaved mica surface. The drops were then removed by absorption with a Whatmann filter no. 4 (Wf4). The aggregation state of AuNPs was obtained by integrating the plasmon band reported in Figures 1b and 2b between 440 and 800 nm and by renormalizing the spectra with respect to the lowest value obtained. This renormalized smaller integration value corresponds to native particles and has been considered to be the zero point, and it corresponds to the baseline of Figure 3 which shows the flocculating diagram of the AuNP dispersion as a function of the PC/NP ratio. (ii) Regime III: Determination of the Optimum Excess of Polymer Chains. An amount of 1500 µL of “citrate free” gold dispersion at a concentration of 6 nmol/L (dilution of solution A by a factor 2) was added under vigorous stirring to a total volume of 1500 µL of ultrapure water containing several aliquots of a stock solution of 30 mg/mL PAH (MW ) 15 000 g/mol) to reach a number of polyelectrolyte chains ranging from 100 to 200 000 PCs/NP. For example, the case of 60 000 PCs/NP was obtained by mixing 270 (62) Turkevich, J.; Stevenson, P. C.; Hillier, J. Discuss. Faraday Soc. 1951, 11, 55-75.
Schneider and Decher µL of the PAH stock solution with 1230 µL of ultrapure water and by injecting under stirring 1500 µL of a AuNP solution composed of solution A diluted twice. After mixing, the dispersion was stirred during 12 h and then centrifuged for 1 h at 15 000 rpm in 1.5 mL Eppendorf Safe Lock tubes; the surpernatant was then carefully removed and replaced by ultrapure water, and the centrifugation procedure was repeated one more time. Twelve hours after mixing, after the first centrifugation/redispersion cycle and after the second centrifugation/redispersion cycle, dynamic light scattering (DLS) and UV/vis spectroscopy measurements of the redispersed samples (3 nmol/L) were performed. Hydrodynamic radii reported in Figure 4 correspond to samples that were centrifuged twice. Yields were calculated according to the absorbance values at 440 nm, where the plasmon contribution is minimal. Effects of AuNP Concentration and Polyelectrolyte Molecular Weights (Figure 5). We used the same conditions as the ones we described before40 (30 000 PCs/NP). An amount of 50 mg of polyelectrolyte was dissolved in a volume of 9 mL of ultrapure water for an injection of 9 mL of particles at 12 nmol/L (e.g., 6 nM AuNPs after injection), and 370 mg of polyelectrolyte was dissolved in 9 mL of ultrapure water for an injection of 9 mL of particles at 88 nmol/L (reported as 44 nM after injection, also 7.33 times more concentrated). Functionalization of Gold Nanoparticles with Five Pairs of PAH/PSS Layers. (i) Without Salt. The procedure was described in detail previously.40 For layer deposition, several aliquots of the colloid stock solution were centrifuged in a Heraeus Pico centrifuge in 1.5 mL polypropylene Eppendorf Safe Lock tubes for 3 h at 7000 rpm. The supernatant was removed from each tube and replaced by a certain volume of Milli-Q water. If particles are dispersed in the same volume as the volume extracted, the concentration of AuNPs is 12 nM and the remaining free sodium citrate is assumed to be lower than 1.2% of the initial concentration. In a typical batch, 9 mL of this solution (of a desired concentration) was added dropwise, under vigorous stirring, to 9 mL of Milli-Q water containing a certain mass of PAH previously sonicated in an ultrasonic bath during a few seconds and stirred for 2 h. The concentration of AuNPs and of PAH was adapted to reach a desired AuNPs concentration and a certain number of PCs/NP. For example, 60 000 PCs/NP at a AuNP concentration of 3 nM corresponds to the injection of 9 mL of AuNPs at a concentration of 6 nM (solution A diluted twice) in 9 mL of ultrapure water containing 50 mg of PAH. The reaction vessel for layer deposition is composed of standard glass. After mixing AuNPs and PAH, agitation was reduced and the mixture was kept under stirring at room temperature in the dark during 12 h. This mixture (18 mL) was conditioned in some 1.5 mL Eppendorf Safe Lock tubes and centrifuged during 100 min at 13 000 rpm. The supernatant was removed, and the colloids were redispersed in ultrapure water. This centrifugation process was repeated one more time for the concentrations of colloids and polymers specified here. The total redispersion volume after this second centrifugation was adapted to obtain a final volume of 9 mL. Gold nanoparticles covered with PAH were then added dropwise, under vigorous stirring, to 9 mL of a solution of Milli-Q water containing the same weight of PSS (MW ) 6500, 13 500, or 70 000 g/mol) previously sonicated in an ultrasonic bath during a few seconds and stirred for 2 h. Agitation was reduced, and the mixture was kept under stirring at room temperature in the dark during about 12 h and then recentrifuged. This procedure was followed in cycles to obtain 10 consecutive polyelectrolyte layers. An exception remains for the high concentrated batch that involves a concentration of particles of 44 nM. It was prepared as follows: After centrifugation to remove the citrate, 44 liquid pellets of 44 Eppendorf tubes containing initially 1.5 mL of AuNPs at 12 nM were dispersed in a total volume of 9 mL of ultrapure water. The solution reached a final concentration of 88 nM and was injected like previously in 9 mL of ultrapure water containing 365 mg of PAH (15 000 g/mol). Centrifugation/redispersion cycles were repeated here three times to eliminate all unadsorbed polymers (due to the higher concentration of PAH and PSS under this condition), and the pellets were redispersed in at least a total volume of 9 mL of water. This solution containing particles coated with one layer
Core/Shell NPs Via Layer-by-Layer Assembly of PAH was then injected in 9 mL of a solution containing 365 mg of PSS (13 500 g/mol) under vigorous stirring. After mixing, agitation was reduced and the dispersion was allowed to stir during 12 h and was then centrifuged three times like previously. The procedure was repeated to adsorb up to 10 layers. The multilayer growth described for excess ratios of 15 000, 30 000, and 60 000 PCs/NP was carried out as follows: For 60 000 PCs/NP, 9 mL of solution A diluted twice was added to 9 mL of a solution of ultrapure water containing 50 mg of PAH (MW) 15 000 g/mol. For 30 000 PCs/NP, 9 mL of solution A (not diluted) was added to 9 mL of a solution of ultrapure water containing 50 mg of PAH (MW) 15 000 g/mol. For 15 000 PCs/NP, 9 mL of solution A (concentrated twice) was added to 9 mL of a solution of ultrapure water containing 50 mg of PAH (MW) 15 000 g/mol. After 12 h of moderate agitation, the dispersions were centrifuged twice as reported previously,40 redispersed after two centrifugation steps to reach a final volume of 9 mL, and then dropwise injected in 9 mL of solutions containing 50 mg each of PSS of molecular weights of 6500 and 13 500 g/mol. The procedure was followed in cycle to coat the particles with at least 10 polyelectrolyte layers. (ii) In the Presence of NaCl. Functionalization of AuNPs in the presence of NaCl was carried out by adding NaCl in the polyelectrolyte mixture before injection of the nanoparticles. For example,
Langmuir, Vol. 24, No. 5, 2008 1789 9 mL of AuNPs (6 nmol/L, solution A diluted twice) was added in 9 mL of a solution containing 50 mg of PAH and 105.9 mg of NaCl if the concentration of NaCl is required to be 0.1 M in the final volume (e.g., 18 mL). Particle Characterization. Electron micrographs were obtained with a transmission electron microscope (Philips CM12) operating at 120 kV and equipped with a Megaview III Soft Imaging System camera. Observations were carried out in the bright field imaging mode. UV/vis spectroscopy was carried out on a Cary 500 UV/vis spectrophotometer in the spectral range of 185-800 nm in Hellma Quartz Suprasil cuvettes with 10 mm length of the light path, and in the spectral range of 300-800 nm in PMMA cuvettes with 10 mm length of the light path. Dynamic light scattering was performed on a Nano-ZS (Malvern, U.K.) zetasizer at a scattering angle of 173°, and data were analyzed using the DTS(Nano) program. Zeta potential measurements were performed on the same apparatus, and the results are reported here as raw data. Statistics. Statistics on the number of Au-cores per aggregate were performed by counting more than 2000 Au-cores on TEM micrographs for each condition. LA7021837
