Chem. Mater. 2008, 20, 1221–1223
1221
Bolaamphiphile Surfactants as Nanoparticle Stabilizers: Application to Reversible Aggregation of Gold Nanoparticles Stéphanie Sistach, Kamil Rahme, Nelly Pérignon, Jean-Daniel Marty,* Nancy Lauth-de Viguerie, Fabienne Gauffre, and Christophe Mingotaud Laboratoire des IMRCP, UniVersité Paul Sabatier, CNRS UMR 5623, 31062 Toulouse Cedex 9, France ReceiVed October 29, 2007
Development of processes to render inorganic nanoparticles (NPs) dispersible and stable in water has received considerable attention in the past decade, the final nanomaterial having many applications in catalysis, material science, biology, and so forth. Surface chemical modification of the NPs is a popular example of such processes. For metal NPs, bifunctional molecules bearing a polar group and an anchoring function (e.g., thiol) on the NPs are highly used to get water-soluble NPs.1–3 But this chemical modification often modifies the properties or even totally passivates the final material.4,5 Adsorption of polymers on the NPs surface is another interesting way to stabilize NPs. However, the slow dynamics of polymers can strongly increase the time of its organization at the surface of the particle and makes it sometimes irreversible.6 The thick final polymer layer around the NPs could also be a strong limitation for many applications.7 Adsorption of surfactants on NPs is a third simple strategy which avoids the previous difficulties and is often used in organic solvent to get well-defined but hydrophobic NPs.8–10 In water, a few examples of stabilization have been also described. In that case, the surfactant adopts interdigitated or vesicle-like structures on the NP surface.11–13 This last architecture is maintained by alkyl tail interactions and is therefore relatively weak and quite difficult to obtain. To * To whom correspondence should be addressed. E-mail: marty@chimie. ups-tlse.fr.
(1) Reetz, M. T.; Quaiser, S. A. Angew. Chem., Int. Ed. Engl. 1995, 34, 2240–2241. (2) Shon, Y.-S.; Wuelfing, W. P.; Murray, R. W. Langmuir 2001, 17, 1255–1261. (3) Woehrle, G. H.; Brown, L. O.; Hutchison, J. E. J. Am. Chem. Soc. 2005, 127, 2172–2183. (4) Yee, C.; Kataby, G.; Ulman, A.; Prozorov, T.; White, H.; King, A.; Rafailovich, M.; Sokolov, J.; Gedanken, A. Langmuir 1999, 15, 7111– 7115. (5) Chauhan, B. P. S.; Rathore, J. S.; Glloxhani, N. Appl. Organomet. Chem. 2005, 19, 542–550. (6) Rahme, K.; Gauffre, F.; Marty, J.-D.; Payre, B.; Mingotaud, C. J. Phys. Chem. C 2007, 111, 7273–7279. (7) Mei, Y.; Lu, Y.; Polzer, F.; Ballauff, M.; Drechsler, M. Chem. Mater. 2007, 19, 1062–1069. (8) Peng, X. G.; Manna, L.; Yang, W. D.; Wickham, J.; Scher, E.; Kadavanich, A.; Alivisatos, A. P. Nature 2000, 404, 59–61. (9) Wang, C.; Daimon, H.; Lee, Y.; Kim, J.; Sun, S. J. Am. Chem. Soc. 2007, 129, 6974–6975. (10) Kahn, M. L.; Monge, M.; Snoeck, E.; Maisonnat, A.; Chaudret, B. Small 2005, 1, 221–224. (11) Patil, V.; Mayya, K. S.; Pradhan, S. D.; Sastry, M. J. Am. Chem. Soc. 1997, 119, 9281–9282. (12) Dubertret, B.; Skourides, P.; Norris, D. J.; Noireaux, V.; Brivanlou, A. H.; Libchaber, A. Science 2002, 298, 1759–1762. (13) Swami, A.; Kumar, A.; Sastry, M. Langmuir 2003, 19, 1168–1172.
Figure 1. Chemical structure of the bolaamphiphiles used in this work (with n ) 10, 12, 14, 16, and 20).
go beyond this strong limitation, we propose to choose bolaamphiphiles14 (i.e., surfactants bearing two polar heads connected by a hydrophobic chain) instead of the usual surfactants. Adsorption of such compounds on the NP surface should be a straightforward and general method to stabilize NPs in water. In this paper, we report experiments applying this concept on gold NPs. Because amino acids are wellknown to interact with the surface of gold NPs,15–19 bolaamphiphiles having amino acids as the polar head (more precisely, L-alanine) have been selected for this study (see structure in Figure 1). Following the literature,20 three-step syntheses (see Supporting Information) were performed to get a series of bolaamphiphiles (noted Bola-Cn) with various chain lengths (from 10 to 20 CH2 groups). All the studies reported here have been made at surfactant concentrations lower than their critical aggregation concentrations,20 to avoid any micelle or vesicle formation in the solution. The gold NPs were synthesized by reduction of HAuCl4 by NaBH4. Experimental conditions were set to get these negatively charged NPs with an average radius of approximately 5 ( 2 nm (see Supporting Information). The gold NPs stabilized by negative charges are sensitive to any increase of the ionic strength or change in pH which induces a charge neutralization and then aggregation. Thus, decreasing the pH of the gold NPs dispersion around 2 (i.e., below the pKa of the carboxylic functions of the bolaamphiphiles) modified the color of the solution from red to purple. This corresponded to a large change of the UV–visible spectrum of the solution: the surface plasmon band (SPB) of the NPs initially at 520 nm (associated to well-dispersed gold NPs) decreased strongly in absorbance (dashed line in Figure 2) and was accompanied by an increase in absorbance at higher wavelength (suggesting aggregated gold NPs21). When bolaamphiphile Bola-C20 was added in a low ratio [BolaC20]/[HAuCl4], a smaller red shift of the SPB was recorded when the pH was decreased to 2 (mixed line in Figure 2). For ratios higher than approximately 0.08, the SPB was maintained at 520 nm (solid line in Figure 2) and the gold NPs solution had a better short-term stability. For neutral (14) Fuhrhop, A. H.; Wang, T. Y. Chem. ReV. 2004, 104, 2901–2937. (15) Selvakannan, P. R.; Mandal, S.; Phadtare, S.; Pasricha, R.; Sastry, M. Langmuir 2003, 19, 3545–3549. (16) Joshi, H.; Shirude, P. S.; Bansal, V.; Ganesh, K. N.; Sastry, M. J. Phys. Chem. B 2004, 108, 11535–11540. (17) Bhargava, S. K.; Booth, J. M.; Agrawal, S.; Coloe, P.; Kar, G. Langmuir 2005, 21, 5949–5956. (18) Nayak, N. C.; Shin, K. J. Nanosci. Nanotechnol. 2006, 6, 3512–3516. (19) Aryal, S.; Bahadur, K. C. R.; Bhattarai, N.; Kim, C. K.; Kim, H. Y. J. Colloid Interface Sci. 2006, 299, 189–195. (20) Franceschi, S.; de Viguerie, N.; Rivière, M.; Lattes, A. New J. Chem. 1999, 23, 447–452. (21) Ung, T.; Liz-Marzan, L. M.; Mulvaney, P. J. Phys. Chem. B 2001, 105, 3441–3452.
10.1021/cm703091y CCC: $40.75 2008 American Chemical Society Published on Web 01/19/2008
1222
Chem. Mater., Vol. 20, No. 4, 2008
Communications
Figure 4. Solutions of gold NPs ([Bola-Cn] ) 80 µmol · L-1 and [Au(0)] ) 0.18 mmol · L-1): initial aspect, one day after addition of HCl (pH ca. 2) and then another day after addition of NaOH (pH ca. 8).
Figure 2. UV–visible spectra of gold NPs solutions, 55 min after a decrease of pH from 7 to 2 and for various concentrations of Bola-C20 (optical path length of 1 cm). Dashed line: 0 µmol · L-1. Mixed line: 3.7 µmol · L-1. Solid line: 48 µmol · L-1. [Au(0)] ) 0.21 mmol · L-1.
Figure 3. Evolution of the equivalent concentration of bolaamphiphile bound to gold NPs with the total concentration of bolaamphiphile for various lengths of the alkyl chain: open circle, n ) 10; open square, n ) 14; and full circle, n ) 20. [Au(0)] ) 0.18 mmol · L-1; size of the NPs, 5 ( 2 nm. Table 1. Characterization of the Adsorption of Bola-Cn of Various Alkyl Chain Lengths on Gold NPs and Qualitative Stability of the Final Solution Bola-Cn
n ) 10
n ) 12
stability after 1 h at pH ) 2 of the gold NPs K ((1500, in L · mol-1)
-
-
n ) 14 (
n ) 16 +
n ) 20 +
∼0
∼0
3800
9500
8300
pH, the observed stability can be as long as several months in the presence of the surfactant (few days without). These results clearly demonstrate that the selected bolaamphiphiles are indeed stabilizers for gold NPs in water. However, for short lengths of the hydrophobic chain (typically n ) 10–12), the stabilizing effect of the bolaamphiphile was lower and aggregation of gold NPs at low pH occurred much faster (see Supporting Information). To better understand this effect, the adsorption of the surfactant on the NPs has been quantified, using liquid 1H NMR spectroscopy (see Supporting Information). When an internal standard (dimethyl sulfoxide, DMSO) was used, titration of the free surfactant within the solution was possible and estimation of the quantity of amphiphiles adsorbed on the gold NPs was made for various concentrations of BolaCn. As shown in Figure 3, for short chains (n ) 10 or 12), the adsorbed quantity was too low to be accurately estimated. On the contrary, for lengths equal to 14 or higher, the quantity of surfactant adsorbed on the NPs
surface increased with the total concentration of bolaamphiphile. This increase was linear for low concentrations of surfactant before occurrence of some kind of saturation. Such a behavior clearly resembles an adsorption process following Langmuir’s model.22 No indication of hemimicelle or admicelle adsorption as in the case of usual surfactants (i.e., not bolaamphiphile) was found.23 Fit of the isotherms of Figure 3 by Langmuir’s model led to an estimation of an adsorption constant, K, and an area per adsorption site, S0. This area was estimated around 0.5 ( 0.3 nm2 for all Bola-Cn, which is quite compatible with their chemical structure. The absorption constant (see Table 1) was found to increase strongly for numbers of carbons above 14 and then remained more or less constant. The equivalent sorption energy (defined here by -RT ln K) was estimated around 22 kJ · mol-1 for n higher than 16. A direct qualitative correlation can be seen between the K value and the stability of the gold NPs (Table 1). All these results prove that the hydrophobic part of the bolaamphiphile plays a key role in the adsorption and stability of the adsorbed layer on the gold NPs. This phenomenon may be related to the poor solvatation of alkyl tails in water and/or to stabilization of the adsorbed layer around the NPs through interactions between the hydrophobic groups. The influence of this central part of the Bola-Cn was striking in the following experiments. After acidification below pH 2, total precipitation of the NPs occurred quickly for n ) 10–12 or after 1 day for n higher than 16. As shown in Figure 4, in the first case, the precipitate was relatively dense and had a dark, even black, color (suggesting an aggregation of the gold NPs). In the second case (i.e., long alkyl chain), it was more fluffy and had a red color. In a second step, when the solution pH was increased to about 8, the precipitate of Bola-C12 maintained this dark aspect and was clearly not dispersible. On the contrary, the flocculated NPs/Bola-C20 disappeared instantaneously upon shaking of the mixture, the final solution then exhibiting a red color similar to the original one. UV–visible spectra confirmed that the system returned to well-dispersed gold NPs. This precipitation-redispersion cycle was performed approximately 10 times until the ionic strength of the solution (increased by the neutralization reaction) became too high. The stabilizer Bola-C20 maintained the integrity of the NPs even during the flocculation of the metal NPs, presumably because of the high value of K. Therefore, we demonstrated that bolaamphiphile surfactants can be very efficient stabilizers for NPs in water. (22) Atkins, P. W. Physical chemistry; Oxford University Press: Oxford, 1986.
Communications
This new strategy using noncovalent bonding of bolaamphiphile species should be readily extended to NPs of any material. In addition, the previous surfactants offer the possibility to recover and redisperse easily NPs without any change in their aggregation state and properties: experiments are currently performed using this reversible aggregation ability in the case of catalytic NPs. (23) Parida, S. K.; Dash, S.; Patel, S.; Mishra, B. K. AdV. Colloid Interface Sci. 2006, 121, 77–110.
Chem. Mater., Vol. 20, No. 4, 2008 1223
Acknowledgment. The authors wish to thank B. Payré and I. Fourquaux for TEM measurements and the staff of the NMR service. This work was partially supported by an “ACI Jeunes Chercheurs”. Supporting Information Available: Bolaamphiphile and gold NPs synthesis and characterization, stability of NPs with bolaamphiphiles, NMR titration, and pKa determination of the bolaamphiphile carboxylic functions (PDF). This material is available free of charge via the Internet at http://pubs.acs.org. CM703091Y
