Insertion of Gold Nanoparticles in Fluid Mesophases: Size Filtering

ARTICLE pubs.acs.org/JPCC

Insertion of Gold Nanoparticles in Fluid Mesophases: Size Filtering and Control of Interactions B. Pansu,* A. Lecchi, D. Constantin, M. Imp
eror-Clerc, M. Veber, and I. Dozov Laboratoire de Physique des Solides UMR-CNRS 8502, Universit
e Paris-Sud, 91405 ORSAY Cedex, France

bS Supporting Information ABSTRACT: Hydrophobically coated gold nanoparticles have been inserted in swollen surfactant lamellar phases. Insertion of the gold nanoparticles is observed only above a critical hydrophobic thickness that depends on the particle size. Such a system can thus be used as a size filter for hydrophobically coated gold nanoparticles. Above this critical thickness, large amounts of nanoparticles can be inserted, forming a 2D fluid in each lamella. The interaction between the nanoparticles has been investigated using small-angle X-ray scattering experiments on well-oriented doped lamellar samples. A strong repulsive interaction between the particles when confined in the lamellar phase is observed. This interaction, which depends on the oil swelling rate, is highly competitive with the attractive van der Waals interaction and can thus be used to tailor the interaction between gold nanoparticles.

’ INTRODUCTION Revolutionary developments in the synthesis of nanosized particles have created huge expectations in the past few years for the use of such materials in various areas such as medical applications or high-technology devices. Individual properties of nanosized particles are drastically altered compared with the bulk material, as, for example, the electronic properties of nanosized metallic, magnetic, or semiconducting particles. However the most challenging part for new applications comes from their collective behavior and, in the past decade, significant progress has been made to obtain multidimensional assemblies of nanoparticles with controlled morphology.1
3 In this vein, self-assembly is certainly a very promising technique and a general route for preparing large and organized structures of all kinds of nanoparticles at low cost and high yield. Controlling the interactions between nanocolloids, especially when dispersed in a fluid medium, is of primary importance to drive self-assembly. Because of their electronic structure, the van der Waals attraction between metallic colloids like gold colloids is quite high; therefore, a strong enough repulsion is needed to stabilize them and prevent irreversible aggregation. Because of this strong van der Waals interaction, the nanoparticles can selforganize in supracrystals even if they are small4,5 but in an irreversible way. Electrostatic forces in water or steric interaction induced by polymer coating in water or in organic solvent are commonly used for stabilization. Recent efforts geared toward combining nanoparticles and other materials to promote and control their self-assembly. Among many candidates, liquid crystals have great advantages because they combine both anisotropic order and mobility.6 Lyotropic lamellar phases in which amphiphilic molecules self-assemble in water to form fluid layers that stack with a quasi-long-range periodicity are a r 2011 American Chemical Society

promising system to achieve this goal. Indeed, they can incorporate either hydrophilic particles in the water regions or hydrophobic ones inside the lipophilic regions. Moreover, some lamellar phases can be easily swollen with oil or with water, remaining stable up to large solvent-to-surfactant ratio, leading to a perfect control of the confining layers thickness. In both cases, particles are expected to be confined in a 2D space. Since the pioneering work of Fabre et al.7 on ferrofluid-doped lamellar phases, many mixed systems combining particles and lamellar phases have been explored but mainly in terms of stability, and only a few stable systems have yet been observed.8
11 Previous experiments have shown that a better stability of doped lamellar phases is observed when interactions between the layers are dominated by thermically induced steric repulsions rather than by electrostatic ones.12 Much effort has also been dedicated to understand how membrane proteins interact within lipid membranes, and many theoretical and numerical works have been devoted to this problem,13 predicting either additional attraction or repulsion. Recently, inorganic clusters (1 nm in diameter) have been used to mimic protein behavior, and we have shown that these clusters when inserted in lipid layers interact with an additional repulsion described by a potential of ∼5 kT in amplitude and 1.5 nm in lateral range.14 The purpose of this Article is to show that a significant amount of gold, hydrophobically coated nanoparticles can be inserted in self-assembled surfactant systems. The nanoparticles that have been used are nanometric gold particles (diameter 2 nm) stabilized by alcane-thiols that can be dispersed in an organic Received: May 18, 2011 Revised: July 11, 2011 Published: July 18, 2011 17682

dx.doi.org/10.1021/jp2046189 | J. Phys. Chem. C 2011, 115, 17682–17687

The Journal of Physical Chemistry C

ARTICLE

solvent such as dodecane. The lyotropic lamellar phase in which they have been inserted is a quaternary system (sodium dodecyl sulfate, pentanol, water, dodecane)15,16 and is known to be sterically stabilized and able to incorporate large amounts of dodecane. The nanoparticles are thus expected to be confined in the nanometric hydrophobic layers whose thickness depends on the swelling rate in dodecane (Figure 1). This insertion is possible only if the hydrophobic thickness is large enough, depending on the particles size. Above this critical thickness, large amounts of gold nanoparticles can be inserted in the lamellar phase, but their confinement inside the hydrophobic layers induces an additional repulsive interaction that dominates the van der Waals attraction.

’ EXPERIMENTAL SECTION Materials. Gold nanoparticles have been synthesized using the Brust
Schiffrin method,17,18 followed by digestive ripening19 to reduce polydispersity. The gold nanoparticles thus prepared can be redispersed in dodecane without irreversible aggregation. Their surface properties can be tailored by changing the chain length of the grafted ligands, and two different ligands have been used: hexanethiol (Au-C6) and dodecanethiol (Au-C12). The diameter of the gold nanoparticles has been adjusted to 2 nm, that is, the value of the typical thickness of the SDS surfactant bilayers. Because of the large amounts of particles required for the different experiments, different batches had to be used, but the particle core size has been measured for all batches. Gold nanoparticles have been redispersed in a mixture of 91 wt % dodecane (Aldrich 99%) and 9 wt % pentanol (Sigma Aldrich, 99%). Indeed, because of pentanol dissolution in dodecane, a better stability of the lamellar phase is observed when swelled with this mixture compared with pure dodecane. Gold dispersions with various weight fractions of nanoparticles have been prepared: 1, 5, 10, 15, and 20% w/w. A stock lamellar phase solution without dodecane has been prepared with 30.6% w/w SDS (Fluka 99%), 22% w/w pentanol (Sigma Aldrich 99%), and 47.4% w/w ultrapure water (Millipore). Small amounts (100 mg) of this primary lamellar phase have been swollen with various gold dispersions. In the following, the swelling will be defined by the weight ratio of the added dodecane/pentanol mixture to the stock lamellar solution. Below some critical swelling, the particles even at low concentration do not mix with the lamellar phase. Above this critical swelling, a large amount of gold nanoparticles can easily be incorporated in the lamellar phase without any phase separation. Different swellings (from 10 to 30% w/w by steps of 2.5% w/w) have been tested to determine the critical swelling allowing insertion. Three different swellings, above the critical swelling for both particles, have been used, and they will be referred to next as A, B, and C. The three ratios for A, B, and C are, respectively, equal to 30, 50, and 70% w/w. The final composition of doped samples is equivalent to the addition of different amounts of gold nanoparticles to the same swollen lamellar phases (A, B, or C). Following the phase diagram of Safinya et al.,15,16 these three different swellings are expected to give an additional hydrophobic layer thickness e equal to 1.4 (A), 2.7 (B), and 3.9 nm (C), whereas the lamellar period of the stock lamellar phase is 3.9 nm. For the X-ray experiments, the samples were introduced either in standard capillaries (diameter 0.7 mm) using smooth centrifugation or in flat glass capillaries (VitroCom),

Figure 1. Schematic drawing of the doped lamellar phase: d is the lamellar thickness, δ is the total hydrophobic thickness, and e is the additional hydrophobic thickness due to swelling. For δ < δc, the gold nanoparticles cannot be inserted.

0.1 mm thick and 2 mm wide by gently sucking in the lamellar phase using a syringe. The capillaries were then sealed to avoid evaporation. Method. The gold nanoparticle core size (diameter mean value and polydispersity) has been measured by SAXS (smallangle X-ray scattering) on a dilute dispersion (1.5 larger than the mean distance in bulk dispersion; therefore, the steric contribution that can be computed using a 2D hard disk model24 has been neglected. The interaction potential Uadd(qr) can thus be estimated using

ð3Þ

The global lateral interaction is clearly repulsive and has been modeled by a Gaussian function, with amplitude U0 and range ξ

a

dlam is the measured lamellar period (in nanometers), e is the excess of hydrophobic thickness (in nanometers) due to swelling, cs is the mean surface concentration of nanoparticles in each layer (in nm
2), ϕ is the apparent disk surface fraction occupied by the particles in each layer, and L is the mean lateral distance between particles in nanometers inside each layer, as deduced from composition and dlam.

1
1 Sðqr Þ

UðrÞ ¼ U0 expð
ðr=ξÞ2 =2Þ

ð4Þ

~ r Þ ¼ 2πU0 ξ2 exp½
q2r ξ2 =2 Uðq

ð5Þ

A comparison of the values of the amplitude U0 and range ξ deduced from Gaussian fits for different samples is given in Table 1.

’ CONCLUSIONS Hydrophobically gold nanoparticles can be inserted in swollen lamellar systems, mainly sterically stabilized. This insertion is easy when the hydrophobic thickness is large enough. No insertion is observed below a critical thickness. Lamellar phases can thus be used as a size filter for hydrophobically coated gold nanoparticles. The critical hydrophobic thickness allowing insertion of the gold nanoparticles has been measured to be 0.85 times the apparent particle diameter (core + ligand shell). The particles can be inserted even if they strongly disturb the lamellae. Above this critical thickness, quite large amounts of gold nanoparticles can be inserted in the lamellar phase, but their confinement inside the hydrophobic layers induces an additional repulsive interaction that dominates the van der Waals attraction. This additional repulsion is quite large in magnitude and in range and totally dominates the van der Waals attraction. It cannot be attributed to electrostatic interaction between the inserted objects, as already seen for transmembrane proteins.25 The magnitude of the confinement-induced repulsive potential increases with decreasing swelling rate. It also depends on the global size of the particles and increases with the capping agent length. The lateral range of this interaction clearly depends on the swelling rate and is roughly proportional to the lamellar period. Complementary experiments are, however, needed to determine precisely the influence of the particle size on the lateral range. For these experiments, homeotropic alignment for which the membranes are parallel to the flat part of the capillaries would help to improve the experimental resolution of S(qr). 17686

dx.doi.org/10.1021/jp2046189 |J. Phys. Chem. C 2011, 115, 17682–17687

The Journal of Physical Chemistry C The origin of this repulsive interaction is still under debate. The global particle diameter (3.2 nm for Au-C6 and 4.0 nm for Au-C12) is of the same order of magnitude as the additional thickness e induced by swelling (from 1.4 to 3.9 nm). In the case of the smallest swelling (A), it is clear that the layers are strongly disturbed by the particles and that this can induce additional, repulsion, as already observed in liquid crystals or in lipid membranes. Particles are also expected to favor defects like bridges between the layers, but additional repulsion is still observed when the particle size is comparable to the additional thickness e induced by swelling. For all swellings, effective membrane fluctuations are certainly pinned by the particles. In fluctuating lamellar systems, the in-plane correlation length ξ0 is known to be proportional to the mean separation between membranes, and the in-plane lateral range ξ of the additional repulsive interaction could be influenced by ξ0. Membrane fluctuations would thus generate repulsive interaction between inserted objects and not only attractive ones, as usually theoretically predicted. The role of the membrane flexibility thus needs to be explored to understand the physical origin of this repulsion. This will be done in forthcoming experiments by varying the water layer thickness.26 Swollen lyotropic lamellar phases have thus proved to be able to incorporate a large amount of small nanoparticles and have proved that confinement can induce an original and strong repulsive interaction between particles. This interaction dominates the van der Waals attraction and this system thus reveals to be very promising for nanoparticle self-assembly. Further experiments on the phase stability of these swollen phases to nanoparticle concentration increases are in progress. It is also worthwhile to test the effect of using other metallic cores.

’ ASSOCIATED CONTENT

bS

Supporting Information. Form factor of polydisperse spheres, capping layer characterization, structure factor of hard spheres: Percus-Yevick approach, and attractive interaction: van der Waals or Yukawa potential. This material is available free of charge via the Internet at http://pubs.acs.org.

ARTICLE

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’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT The ESRF is gratefully acknowledged for the allocated beamtime (02-01800), and we thank C. Rochas for competent and kind support. Giulia Fornasieri (ICCMO, Paris Sud-11) is acknowledged for introducing B.P. to TGA characterization. ’ REFERENCES (1) Dutta, J.; Hofmann, H. Encycl. Nanosci. Nanotechnol. 2003, X, 1. (2) Kinge, S.; Crego-Calama, M.; Reinhoudt, D. N. Chem. Phys. Chem. 2008, 9, 20. (3) Grzelczak, M.; Vermant, J.; Furts, E. M.; Liz-Marzan, L. M. ACS Nano 2010, 4, 3591. (4) Pileni, M. P. Surf. Sci. 2009, 603, 1498. (5) Abecassis, B.; Testard, F.; Spalla, O. Phys. Rev. Lett. 2008, 100, 115504. (6) Hegmann, T.; Qi, H.; Marx, V. M. J. Inorg. Organomet. Polym. Mater. 2007, 17, 483. 17687

dx.doi.org/10.1021/jp2046189 |J. Phys. Chem. C 2011, 115, 17682–17687