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Reversible Transference of Au Nanoparticles across the Water and Toluene Interface: A Langmuir Type Adsorption Mechanism Sihai Chen, Hiroshi Yao, and Keisaku Kimura* Department of Material Science, Faculty of Science, Himeji Institute of Technology, Harima Science Garden City, Kamigori, Hyogo 678-1297, Japan Received May 3, 2000. In Final Form: November 7, 2000 A novel method to reversibly modify the surface of gold nanoparticles is developed through electrostatic interaction between cationic tetraoctylammonium surfactant and anionic carboxylate groups. The particles can be extracted from the water phase into the toluene phase without aggregation after the adsorption of the surfactant on their surface. UV-visible spectroscopy reveals that a Langmuir type adsorption isotherm is observed during the transference process. The maximum adsorption amount of surfactants is equal to the total amount of carboxylate groups on the particle surface, indicating that the saturation adsorption behavior consistent with the Langmuir isotherm is solely caused by the 1:1 electrostatic charge interaction. Furthermore, through adjusting the dissociation states of carboxylate groups on the particle surface using hydrochloric acid, it is possible to redisperse the particles from toluene into the water layer because of the desorption of TOAB molecules from the particle surface. Studies using other methods such as infrared spectroscopy, energy-dispersive X-ray analysis, and transmission electron spectroscopy also support the above findings.
Introduction Promising usage in materials, chemical, and biological sciences has been predicted for thiolate-stabilized gold nanoparticles,1 which display several advantages over other conventional colloids2 such as the ability to be isolated and redissolved in solvents without irreversible aggregation, stability to air, and so forth. Since the first successful synthesis of long chain alkanethiolate stabilized gold nanoparticles,3 much effort has been devoted to the functionalization of these particles. Several methodologies have been developed: (i) direct one-step synthesis using thiolate molecules with predesigned functional groups, such as hydroxyl,4,5 carboxylate,6-8 and silane;9 (ii) placeexchange (replacement in the sulfur end of the molecules) synthesis starting from the other non-thiolate10 or thiolate11-13 stabilized gold nanoparticles; (iii) chemical interaction (reaction at the non-sulfur end) to further * To whom correspondence should be addressed. Fax: 81-79158-0161. Tel: 81-7915-8-0159. E-mail:
[email protected]. (1) Templeton, A. C.; Wuelfing, W. P.; Murray, R. W. Acc. Chem. Res. 2000, 33, 27-36. (2) Colloidal Gold: Principles, Methods and Applications; Hayat, M. A., Ed.; Academic Press: San Diego, CA, 1989; Vols. 1 and 2. (3) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801-802. (4) Brust, M.; Fink, J.; Bethell, D.; Schiffrin, D. J.; Kiely, C. J. J. Chem. Soc., Chem. Commun. 1995, 1655-1656. (5) Wuelfing, W. P.; Gross, S. M.; Miles, D. T.; Murray, R. W. J. Am. Chem. Soc. 1998, 120, 12696-12697. (6) Chen, S.; Kimura, K. Langmuir 1999, 15, 1075-1082. (7) Templeton, A. C.; Chen, S.; Gross, S. M.; Murray, R. W. Langmuir 1999, 15, 66-76. (8) Schaaff, T. G.; Knight, G.; Shafigullin, M. N.; Borkman, R. F.; Whetten, R. L. J. Phys. Chem. B 1998, 102, 10643-10646. (9) Buining, P. A.; Humbel, B. M.; Philipse, A. P.; Verkleij, A. J. Langmuir 1997, 13, 3921-3926. (10) Brown, L. O.; Hutchison, J. E. J. Am. Chem. Soc. 1997, 119, 12384-12385. (11) Hostetler, M. J.; Green, S. J.; Stokes, J. J.; Murray, R. W. J. Am. Chem. Soc. 1996, 118, 4212-4213. (12) Ingram, R. S.; Hostetler, M. J.; Murray, R. W. J. Am. Chem. Soc. 1997, 119, 9175-9178. (13) Hostetler, M. J.; Templeton, A. C.; Murray, R. W. Langmuir 1999, 15, 3782-3789.
functionalize the thiolate-stabilized particles, including SN2,14 coupling,15 and polymerization16 reactions. The functionalization is normally based on covalence-bonding interactions, yielding particles with fixed irreversible functionality. Different from all the above methods, in this report we present a novel reversible functionalization method to modify the gold nanoparticle surface, which is based on the electrostatic interaction between the negatively charged carboxylate groups on the gold particle surface and the positively charged tetraoctylammonium ions. The gold particles can be transferred from water into toluene with the help of tetraoctylammonium bromide (TOAB), and a Langmuir type isotherm is found during the transference of gold particles from water into toluene. A remarkable result is obtained after transference such that the particles can be redispersed into water by removing the surfactant through adjusting the acidity of the reaction mixture under ambient condition. Solvent exchange between polar and nonpolar solvents has been regarded as a main challenge for the preparation of metal nanoparticles because of the difficulties in preventing the particles from aggregation.17,18 Fendler et al. have used oleic acid as a phase transfer agent.17,19 When it is added as an anion into water, it coats the surface of silver nanoparticles. Upon acidification, the particles precipitate out of water and will then spontaneously transfer into organic solvents. They can be re-extracted into alkaline water by shaking. Underwood et al. trans(14) Templeton, A. C.; Hostetler, M. J.; Kraft, C. T.; Murray, R. W. J. Am. Chem. Soc. 1998, 120, 1906-1911. (15) Templeton, A. C.; Hostetler, M. J.; Warmoth, E. K.; Chen, S.; Hartshorn, C. M.; Krishnamurthy, V. M.; Forbes, M. D. E.; Murray, R. W. J. Am. Chem. Soc. 1998, 120, 4845-4849. (16) Watson, K. J.; Zhu, J.; Nguyen, S. T.; Mirkin, C. A. J. Am. Chem. Soc. 1999, 121, 462-463. (17) Fendler, J. H.; Meldrum, F. C. Adv. Mater. 1995, 7, 607-632. (18) Liz-Marzan, L. M.; Lado-Tourino, I. Langmuir 1996, 12, 35853589. (19) Meldrum, F. C.; Kotov, N. A.; Fendler, J. H. Langmuir 1994, 10, 2035-2040.
10.1021/la000644k CCC: $20.00 © 2001 American Chemical Society Published on Web 01/03/2001
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ferred gold particles from water into a range of organic solvents using a comb stabilizer.20 Liz-Marzan et al. applied nonionic ethoxylated surfactants to transfer silver nanoparticles between ethanol and cyclohexane.18 Thiolates with different chain lengths have been utilized as phase transfer agents between water and organic solvents for Ag,21 Au,22 and Ag2S23 nanoparticles. Compared with these methods, we use the thiolate-modified nanoparticles as the starting materials. Because these particles are very stable even in the dry state and have a well-defined surface structure,6 we can easily overcome the particle aggregation problem that is often met for the other anion-modified metal nanoparticles and study the transference processes quantitatively. Experimental Section Chemicals. All the reagents were used as received: from Wako, hydrogen tetrachloroaurate tetrahydrate (99%), sodium borohydride (>90%), Glutathione (reduced form) (GTR, 97%), mercaptosuccinic acid (MSA, 97%), hydrochloric acid (36%), sodium hydroxide (96%), toluene (>99.5%), methanol (99.8%), ethanol (99.5%), and other organic solvents which are all of reagent grade; from Sigma, N-(2-mercaptopropionyl)-glycine (MPG); from Aldrich, tetraoctylammonium bromide (TOAB, 99.5%); from Nacalai Tesque, tetra-n-propylammonium bromide; from Tokyo Kasei, all the other surfactants. Millipore water of high resistivity (>18.0 MΩ cm) was used. Particle Synthesis. Gold particles were prepared according to a similar procedure as reported before.6 Briefly, aqueous sodium borohydride solution (25 mL, 0.2 M) was added at 2.5 mL per min into 100 mL of methanol solution containing 5 × 10-3 M HAuCl4 and different amount of thiolates (GTR, MPG, or MSA) under vigorous stirring. The molar ratio between thiolate and HAuCl4 (S/Au) was adjusted to control the particle size. The particles used in this study were prepared with S/Au ) 1 (for 2.67 nm GTR-Au, 2.31 nm MPG-Au, and 3.35 nm MSA-Au particles) and S/Au ) 0.5 (for 3.45 nm GTR-Au particles). After the reaction, the precipitate was repeatedly washed with 20% (v/v) water-methanol, methanol and ethanol solutions through an ultrasonic dispersion-centrifugation process to remove inorganic or organic impurities. Finally, the particles were dried under vacuum. Transference Experiment. A preliminary work, which describes one-way transference from water to oil using TOAB as a surfactant was already reported.24 Typically, to 5 mL of a water solution containing 0.5 mg of Au nanoparticles was added 5 mL of toluene. To this upper toluene layer, different amounts of surfactant were then added. After being vigorously stirred for at least 30 min, the solution was left quiet until two clearly separated layers appeared. The separated solutions were then subjected to UV-vis measurement. All the experiments were conducted at 25 °C. When solid samples are required, such as in the experiments using Fourier transform infrared (FT-IR) spectroscopy and energy-dispersive analysis of X-rays (EDAX), 5 mg of gold particles was at first dissolved in 5 mL of water, followed by the addition of 5 mL of toluene containing TOAB; the concentration of TOAB was adjusted to get concentrated particle solutions in both layers. The solvents of the separated solution layers were then removed under vacuum. Choice of Surfactants. Using the 3.45 nm GTR-stabilized Au particles as an example, a series of cationic tetraalkylammonium surfactants with different alkyl groups were tested in order to find out the effect of the nature and concentration of the surfactants on the transference efficiency from the aqueous layer to the toluene layer. These surfactants include phenyltrimethylammonium chloride (C6H5N(CH3)3Cl), trimethylbenzylammo(20) Underwood, S.; Mulvaney, P. Langmuir 1994, 10, 3427-3430. (21) Meldrum, F. C.; Kotov, N. A.; Fendler, J. H. J. Chem. Soc., Faraday Trans. 1994, 90, 673-680. (22) Giersig, M.; Mulvaney, P. Langmuir 1993, 9, 3408-3413. (23) Motte, L.; Pileni, M. P. J. Phys. Chem. B 1998, 102, 4104-4109. (24) Yao, H.; Momozawa, O.; Hamatani, T.; Kimura, K. Bull. Chem. Soc. Jpn. 2000, 73, 2675-2678. A precise surface ion-pair model for MSA-TOAB-modified particles was presented.
Chen et al. nium chloride (C6H5CH2N(CH3)3Cl), benzoylcholine chloride (C6H5C(O)OCH2CH2N(CH3)3Cl), n-dodecyltrimethylammonium chloride (C12H25N(CH3)3Cl), distearyldimethylammonium chloride ((C18H37)2N(CH3)2Cl), tetra-n-propylammonium bromide ((C3H7)4NBr), tetrabutylammonium chloride ((C4H9)4NCl), and tetraoctylammonium bromide ((C8H17)4NBr). Transference was not observed for phenyl-containing and short alkyl chain (C3 or C4) surfactants even when their concentrations increased to 5 × 10-3 M. When the single-long-chain surfactant n-dodecyltrimethylammonium chloride was used, gold particles were found to form aggregates at the toluene-water interface, but no particles were found in the toluene layer as checked by a UV-vis spectrophotometer. In the case of the double-long-chain surfactant, distearyldimethylammonium chloride, the toluene layer displayed a transparent red color indicative of the transference of gold particles. However, large amounts of white flocculates were observed in the water layer, making it difficult to observe the change in water. The best result was met in the case of TOAB; transparent toluene and water layers were obtained, and the color change can be easily checked by a UV-vis spectrophotometer. This series of experiments showed that the transference efficiency is related to the hydrophobicity of the surfactants. In this paper, TOAB was thus used as the transference agent for further study. Analysis Techniques. A Hitachi-8100 transmission electron microscope (TEM), operated at 200 kV with 2.1 Å point-to-point resolution was used to size and analyze the particles. Fourier transform infrared spectra were measured with a Horiba FT210 infrared spectrophotometer. UV-vis absorption spectra were recorded on a Hitachi U-3210 spectrophotometer in the range of 300-900 nm with 2 nm resolutions. Quartz cuvettes with a 1 cm optical length were used. An EDAX DX-4 system attached to the Philips XL-20 scanning electron microscope (SEM) operated under an acceleration voltage of 6 kV was used to check the elements in the toluene or water layer after the transference experiments.
Results and Discussion Proposed Model of Au Nanoparticle Surface. The surface structure of MSA-modified Au nanoparticle powders has been clearly defined in a previous report:6 MSA molecules form a monolayer on the Au particle surface, they exist in the form of sodium carboxylate, and one MSA unit combines with one H2O molecule (Chart 1 in ref 6). We assume that this structure is also applicable to GTRor MPG-modified gold nanoparticles. This assumption is supported by the results from elemental analysis, FT-IR, and thermogravimetric measurement. For example, for the 2.67 nm GTR-modified Au nanoparticles, elemental analysis gave C, 11.21 (9.79); H, 1.54 (1.27); O, 11.36 (10.15); N, 3.71 (3.81); S, 2.60 (2.90). The calculated values in the brackets according to the MSA-modified gold nanoparticle model show consistency with the measured ones. The above model structure makes it easy to calculate the total number of carboxylate groups on the particle surface if the values of weight, diameter of nanoparticles, and packing density of the thiolate on the gold particle surface are known. It is also observed that GTR-modified gold nanoparticles display a superb stability, presumably because of their sterically bulky ligands. In the following, experiments are thus mainly focused on this kind of particle. Transference Isotherm. At first, the GTR-modified Au nanoparticles with a mean diameter of 2.67 nm were tested. In water solution, these particles displayed a typical plasmon absorption band peaked at around 510 nm. After transference into toluene solution, this peak red-shifted to about 514 nm (Figure 1); the 4 nm red-shift is caused by the increase of the solvent refractive index from 1.333 (water) to 1.494 (toluene) according to the Mie theory.20,25 (25) Mulvaney, P. Langmuir 1996, 12, 788-800.
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b is the saturation adsorption, and x is the number of adsorbed molecules per unit area. Because the absorbance Atol of Au particles in toluene obeys a simple Lambert-Beer law, Atol should be proportional to the total occupied number of COO- groups in toluene, if we assume that the spherical particles are monodispersed and the transferred particles are fully covered by TOAB at all COO- sites. Thus,
n1 ) k′Atol
(5)
The k′ value can be obtained by supposing that the maximum absorbance of gold particles in toluene Am,tol corresponds to the total COO- sites, ntotal, of the particles:
ntotal ) Figure 1. The absorption spectra of the GTR-Au nanoparticles in the toluene layer at different TOAB concentrations: (a) 4 × 10-6 M, (b) 1 × 10-5 M, (c) 2.2 × 10-5 M, (d) 3.2 × 10-5 M, (e) 6.4 × 10-5 M, and (f) 1.2 × 10-4 M. Five milliliters of 0.1 mg/mL 2.67 nm GTR-Au nanoparticles was first mixed with 5 mL of toluene containing TOAB under 30 min of vigorous stirring; the solution was then left quiet until two clear layers were observed. This process often takes less than 1/2 hour. The inset shows the change of the absorbance at 514 nm in toluene (O) and water (b) layers.
The Figure 1 inset shows the change of the 514 nm absorbance in the toluene and water layers with the concentration of TOAB. The absorbance in toluene increased at the expense of that in water upon increasing the TOAB concentration. This increase was found to obey a simple rule that is similar to the Langmuir adsorption isotherm used for the adsorption of gases on solid surfaces.26 Therefore, in the following we will give a detailed analysis of the transference process and propose a model to account for the transference mechanism. The total active sites on the particle surface for the adsorption of TOAB are assumed to be nmax, of which n1 are occupied and n0 ) nmax - n1 are free. The rate of desorption is taken to be proportional to n1 or equal to k1n1, and the rate of adsorption is proportional to the unoccupied sites n0 and to the concentration, c, of TOAB or equal to k2cn0. At equilibrium,
k1n1 ) k2cn0 ) k2c(nmax - n1)
(1)
n1 ac ) nmax 1 + ac
(2)
Then,
where
a)
k2 k1
(3)
For convenience in testing data, eq 2 may be put in the linear form with respect to the concentration.
1 c c ) + n1 anmax nmax
(4)
This equation can be read as P/x ) 1/ab + P/b, a familiar Langmuir plot, where P is the pressure, a is a constant, (26) Adamson, A. W. Physical Chemistry of Surfaces, 4th ed.; John Wiley & Sons: New York, 1982; p 521.
νπD2 6W ) k′Am,tol πD3F + 6πD2Ms/N0 S0
(6)
in which D is the diameter of the particle, W is the total weight of the particles used, F is the density of gold, S0 is the packing density of thiolate on the particle surface, ν is the carboxylate group number in one thiolate molecule, Ms is the assumed molecular weight according to the model, and N0 is Avogadro’s constant. Finally, we get
S0DF + 6Ms/N0 Am,tol 1 c c) + 6vW Atol anmax nmax
(7)
Upon calculating according to the absorbance of gold particles in water, we get a similar equation as follows:
(
)
Am,water S0DF + 6Ms/N0 c) 6vW Am,water - Awater 1 c + (8) anmax nmax
where Awater and Am,water represent the absorbance and maximum absorbance in water, respectively. Finally, we can get the maximum number nmax of TOAB molecules adsorbed on the particle surface and the adsorption/ desorption constant ratio a through eqs 4, 7, or 8 if a linear relation can be obtained from the experimental data. Straight lines were indeed found for the 2.67 nm GTRmodified particles using the 514 nm absorbance values either in toluene (Figure 2A) or in water (Figure 2B). Hence, this relationship reconfirms our model of monolayer formation. The calculated results are listed in Table 1. The maximum adsorption amount of TOAB molecules according to absorbance in toluene, nmax,tol, and that in water, nmax,water, are 6.76 × 1017 and 7.39 × 1017 (corresponding to concentrations of cmax,tol ) 2.24 × 10-4 M and cmax,water ) 2.45 × 10-4 M), respectively. They are both near to the total amount of COO- groups of ntotal ) 5.21 × 1017, indicating a 1:1 electrostatic interaction. The concentration of TOAB at maximum coverage matched well with the total transference of gold particles as marked with an arrow in the inset of Figure 1, showing a point of monolayer full adsorption. A different phenomenon was observed for the 3.45 nm GTR-modified gold nanoparticles. Presumably because of the increased size, some particles gathered at the toluene-water interface to form aggregates when the concentration of TOAB was smaller than 3.2 × 10-5 M. This was reflected in the UV-vis spectra, as shown in Figure 3c; the summation of the 514 nm absorbance in the toluene and water layers was found to be smaller than the expected 1.0-1.2. The calculated c/n1-c curves showed
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Table 1. Data Used for the Calculation According to Equations 4, 7, or 8 for Different Gold Particles and the Resultsa modifier
D (nm)
Ms
Am,tol
Am,water
ntotal (× 1017)
nmax,tol (× 1017)
cmax,tol (× 10-6M)
nmax,water (× 1017)
cmax,water (× 10-6M)
atol (× 104 M-1)
awater (× 104 M-1)
GTR GTR MPG MSA
2.67 3.45 2.31 3.35
368.3 368.3 175.2 211.2
0.72 1.18 1.10 0.45
0.62 1.12 0.80 0.60
5.21 4.35 3.52 5.02
6.76 4.82b 3.50b 5.21b
224 160 116 173
7.39 4.36 3.49 5.24
245 145 116 174
1.86 5.39 15.7 1.31
1.24 19.9 21.0 1.15
a The packing density of thiolate on the particle surface is assumed to be 15.2 Å2 (ref 6). b The linear relationship for the c/n -c curve 1 was obtained at higher TOAB concentrations.
Figure 3. Change of the absorbance at 514 nm in (a) toluene and (b) water layers as well as their summation (c) with the concentration of TOAB for the 3.45 nm GTR-Au nanoparticles. Five milliliters of 0.1 mg/mL particles was first mixed with 5 mL of toluene containing TOAB under 30 min of vigorous stirring, and then the spectra were taken after two clear layers were observed within 1/2 hour.
Figure 2. c/n1-c relationship for the 2.67 nm GTR-Au nanoparticles according to the 514 nm absorbance change in (A) toluene and (B) water layers. See text for further explanation. Other conditions are shown in Figure 1.
that according to the absorbance in toluene (Figure 4A), data points obtained at TOAB concentrations smaller than 3.2 × 10-5 M deviated from the straight line obtained at a higher concentration range. This deviation is caused by the decreased absorbance of gold particles in toluene because of the formation of aggregates at the interface. When calculating the c/n1-c curve according to the absorbance in water (that is, all the particles are supposed to be in the toluene layer except that in water), we got a perfect linear fitting of the experimental data (Figure 4B). This result clearly confirmed that the aggregates gathering at the toluene-water interface are covered by the TOAB but have not yet moved into the toluene layer. TEM observation showed that these aggregates contain stillseparated original particles. The calculated maximum TOAB adsorption amount (4.82 × 1017 and 4.36 × 1017, according to the absorbance in toluene at a higher TOAB concentration and that in the water layer, respectively) is again consistent with the total amount of COO- groups
of 4.35 × 1017. Similar results were also obtained for the MPG- and MSA-modified particles (see Table 1). Further Characterization. To further know the change of the surface states of Au particles before and after transference into the toluene layer, FT-IR spectra of the particles in the water and toluene layers were recorded as shown in Figure 5. The key peaks and their assignments are summarized in Table 2. First, we analyze the surface state of the GTR-modified Au particle powders used for the transference experiments by comparing it with that of GTR molecules (Figure 5 curves b and a). It was noted that a small peak at 2524 cm-1, which is attributed to the S-H stretching vibration mode, disappears when the GTR molecules adsorb on the gold particle surface, giving clear evidence that GTR anchors on the gold surface through the sulfur atom in the mercapto group. A sharp peak at 1713 cm-1 (in the range of 16801720 cm-1) in curve a, which belongs to the stretching vibration of the carbonyl groups of the carboxylic acid dimer,27 disappeared in curve b, showing that the dimer form of carboxylic acid does not exist on the particle surface. In fact, the double peaks of the carboxylate vibration at 1601 cm-1 (in the range of 1540-1650 cm-1, asymmetric) and 1400 cm-1 (in the range of 1360-1450 cm-1, symmetric) in curve b indicate that the organics exist as carboxyl salts on the particle surface.27a-c The existence of sodium which combines with the carboxylate group was detected by SEM-EDAX analysis. The atomic ratios between oxygen and sodium for the 2.67 and 3.45 nm particles were found to be 3.8((7):1 and 3.5((7):1, respectively. Compared to the standard 3:1 ratio for the (27) Colthup, N. B.; Daly, L. H.; Wiberley, S. E. Introduction to Infrared and Raman Spectroscopy, 3rd ed.; Academic Press: San Diego, CA, 1990; (a) p 291, (b) p 318, (c) p 315.
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Figure 5. FT-IR spectra of (a) GTR molecules; GTR-modified gold nanoparticles (b) as prepared, (c) in the water layer, and (d) in the toluene layer; (e) TOAB molecules. All spectra were acquired in the form of KBr plates, either by mixing the solid with KBr powders (a, b, c, and e) or by dropping the toluene solution on a KBr window and drying under vacuum (d). Figure 4. c/n1-c relationship for the 3.45 nm GTR-Au nanoparticles according to the 514 nm absorbance change in (A) toluene and (B) water layers. See text for further explanation. Other conditions are the same as in Figure 3.
pure sodium carboxylate salt of GTR, the amount of oxygen is excessive. This indicates that water exists on the particle surface, which is characterized by a broad O-H stretching vibration band in the range of 2700-3700 cm-1, peaked at 3462 cm-1; it even masked the vibration mode of the methylene group near 3000 cm-1 (Figure 5 curve b). Furthermore, thermogravimetric experiments show that the water content amounts to 1.68% (calculated 1.56%) and 2.63% (calculated 2.12%) for the above two sizes of particles, respectively. These results support that the particles used have the model structure as we proposed before. The surface structure of the particles in the water layer is almost the same as that of the original gold particles, as seen when comparing curve c to curve b in Figure 5. In contrast, TOAB molecules are found to cover the surface of the particles in toluene (comparing curves d and e in Figure 5). The characteristic CH2 vibration peaks at 723, 1460, 2852, and 2922 cm-1 as well as that of CH3 at 2954 cm-1 of TOAB (Figure 5 curve e) were clearly imposed on curve d. The peak values of the symmetric and asymmetric stretching vibrations of CH2 have been used as a sensitive indicator of the ordering of the alkyl chains.28,29 These two peaks at 2852 and 2924 cm-1 for the TOAB-modified gold particles are almost the same as those of solid TOAB (Table 2), implying that in the solid state highly ordered, all-trans alkyl chains may surround the particle surface. (28) Nuzzo, R. G.; Fusco, F. A.; Allara, D. L. J. Am. Chem. Soc. 1987, 109, 2358-2368. (29) Nuzzo, R. G.; Dubois, L. H.; Allara, D. L. J. Am. Chem. Soc. 1990, 112, 558-569.
SEM-EDAX analysis of the solid samples detected a very high weight ratio between carbon and gold of 1.34 for the particles in the toluene layer, whereas this ratio is only 0.08 for the particles in the water layer. This is consistent with the result that TOAB is covered on the particle surface in toluene. Sodium element was not found in the toluene layer, but about 3% of Br was detected from the particles in the water layer. These results clearly support the following reaction:
Aun(GTRNa2)m + 2m(C8H17)4NBr f (water)
(toluene) Aun{GTR[N(C8H17)4]2}m + 2mNaBr (9) (water) (toluene)
which is the main reaction for the transference of gold nanoparticles. Because the surface interaction between TOAB and the carboxylate groups on the gold particle surface does not affect the inner Helmholtz layer of the gold particle, the particles are very stable and no size change was observed after the transference into the toluene layer, as will be shown with TEM later. Effect of pH. Because the transference of gold nanoparticles is caused by the electrostatic adsorption of TOAB on the particle surface, it is thus expected that a change of the pH will affect the transference efficiency. In the water solution, gold nanoparticles were found to precipitate when pH < 4. Knowing that the pKa1 and pKa2 values of a simple dicarboxylic acid such as succinic acid are 4.19 and 5.64, respectively,30 and supposing that similar values are applicable to GTR, it is reasonable to relate the precipitation phenomenon to the change of the carboxylate (30) Wade, L. G., Jr. Organic Chemistry; Prentice Hall: New York, 1987; p 978.
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Table 2. Selected IR Band Positions and Assignments for the Samples Shown in Figure 5 GTR
GTR-Au (powder)
GTR-Au (in water)
GTR-Au (in toluene) 723
1396 1452 1537 1601 1660 1713 2524 2836 2909
1400 1450 1541 1600 1641
3248 3346
3278
1396 1527 1596 1640
1391 1464 1539 1603 1645 2852 2924 2954
2927
3305 3462
3413
TOAB
assignment
723 1375
CH2 in-phase rock CH3 symmetric deformation CdO in carboxylic acid salt (symmetric) CH2 scissor deformation C-N and N-H torsional motions CdO in carboxylic acid salt (asymmetric) amide CdO stretch CdO in carboxylic acid dimer S-H stretch CH2 symmetric stretch CH2 asymmetric stretch CH3 stretch N-H stretch N-H stretch OH‚‚‚O
1460
2852 2922 2954
Figure 6. (a) The absorption spectra of the 3.45 nm GTR-Au nanoparticles in water (0.1 mg/mL). (b) The absorption spectra of the 3.45 nm GTR-Au nanoparticles in the toluene layer after the following treatment: 5 mL of the above water solution was mixed with 5 mL of toluene containing 1.72 × 10-4 M TOAB under 30 min of vigorous stirring; after stirring, the solution was left quiet until two clear layers were observed. (c) The absorption spectra of the 3.45 nm GTR-Au nanoparticles in the water layer after the following treatment: the water layer was adjusted to pH ∼ 5, and then the above mixture was stirred for 10 min and left quiet for 1 day.
salt to the carboxylic acid form. The degree of dissociation using the pKa values cited above is calculated to be less than 20% for pH < 4. Under these conditions, gold particles cannot be transferred into the toluene layer regardless of the concentration of TOAB. At a higher pH value from 4 to 12, the transference efficiency of the 3.45 nm GTRmodified gold nanoparticles was found to be similar (6978%) under the same 1 × 10-5 M TOAB concentration, presumably because of the similar dissociated state of the carboxylate group in the higher pH solutions. Reverse Transference Process. Because the gold nanoparticles in toluene are covered by TOAB through electrostatic interaction, it should be possible to redisperse the particles into water, if we can remove the TOAB from the particle surface. One method is to change the carboxylate ion on the particle surface to carboxylic acid. However, in this case, as we have discussed in the last section, if the pH is too low gold particles themselves cannot be dispersed in water. In the experiment of Figure 6, the 3.45 nm GTR-modified gold particles were tested for the reverse transference process. After all the particles were transferred into toluene from water (see curves a and b), we then adjust
Figure 7. TEM photos of the 3.45 nm GTR-modified gold nanoparticles; (a), (b), and (c) correspond to the conditions shown for curves a, b, and c in Figure 6, respectively.
the pH of the clear water layer to ∼5 with hydrochloric acid and remix it with the toluene solution under stirring. After 10 min, the reaction mixture was left quiet at ambient condition for 1 day in order to reach a new equilibrium. It is clearly seen that some of the particles
Reversible Transference of Au Nanoparticles
come into the water layer again (curve c). We also noticed that the peak position of the plasmon band shifted from 510 nm (in water) to 518 nm (in toluene) and blue-shifted again to 510 nm (in water) during these processes (curves a, b, and c, respectively). No apparent aggregation and size change of the particles have been observed, as shown by the TEM photos in Figure 7. Conclusion We have demonstrated a novel reversible method to modify the gold nanoparticle surface based on the electrostatic interaction, making it possible to control the dispersibility of gold nanoparticles not only in a polar solvent but also in a nonpolar solvent. No size change of the same particle is observed before and after the transference. A Langmuir type adsorption isotherm is found during the transference of particles from water into toluene, and the total amount of carboxylate groups on the Au particle surface is calculated to be equal to the maximum adsorption amount of the surfactant. This clearly shows that the transference of gold nanoparticles is caused by the 1:1 electrostatic interaction between
Langmuir, Vol. 17, No. 3, 2001 739
negatively charged carboxylate groups on the particle surface and positively charged tetraoctylammonium cations. Acknowledgment. This work was supported in part by a fund from Mitsubishi Research Institute, Inc. and by Grants-in-aid for Scientific Research on Basic Research (A: 09304068) from the Ministry of Education, Science, Sports and Culture, Japan. The authors are indebted to the reviewer of this article for invaluable suggestions concerning the details of phase transfer experiments. Note Added During the revision of the present paper, we noticed that a similar strategy has been used for the modification of semiconductor CdTe clusters. See: Kurth, D. K.; Lehmann, P.; Lesser, C. J. Chem. Soc., Chem. Commun. 2000, 949-950. They used thioglycolic acid modified CdTe clusters as the starting materials and dimethyldioctadecylammonium bromide as the modifier to transfer the clusters from water into chloroform. LA000644K
