SCIENCE IN CHINA (Series B)
Vol. 45 No. 4
August 2002
Preparation, stability and two-dimensional ordered arrangement of gold nanoparticles capped by surfactants with different chain lengths ZHOU Xuehua (周学华)1,2, LI Jinru (李津如)1, LIU Chunyan (刘春艳)2 & JIANG long (江 龙)1 1. Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, China; 2. Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100039, China Correspondence should be addressed to Jiang Long (email:
[email protected]) Received March 7, 2002
Abstract Gold nanoparticles modified with C10NH2, C12NH2, C16NH2 and C18NH2 respectively have been prepared by the reverse micelle method. Nanoparticles stability and their two-dimensional (2D) ordered arrangement were studied by UV-Vis absorption spectra and LB technique. The factors, such as the chain length and the size distribution of particles, which affect the 2D ordered arrangement formation, are discussed. Experimental results show that the longer the chain length of surfactants capping the gold nanoparticles, the more stable the nanoparticles, and the more ordered 2D arrangement of gold nanoparticles. Keywords: surfactants, gold nanoparticles, 2D ordered arrangement.
The research on nanoparticles is very popular because of the particular properties of nanoparticles in electronics[1
—4]
, optics[1,5], magnetics[6] and other areas. In earlier 1915, Ostwald[7] put
forward the word of “Die Welt Der Vernachlässigten Dimensionen” to attract the attention of all scientists. Up to 1970s, the study of nanometer-sized particles attracted more people to enter this field. Today, the preparation of nanometer particles, as the base of nanostructure materials, has become sophisticated. However, there are few reports on the preparation of hydrophobic nanoparticles which are of great significance in nano-electronics. In this paper, the gold nanoparticles modified with fatty amines of different chain lengths were prepared and the sols of gold nanoparticles with different stabilities have been obtained. On the basis of the preparation of nanoparticles, more and more researchers begin to be interested in the 2D ordered aggregates of nanoparticles because the aggregates exhibit good applying perspective in the storage of information[8], microelectronic, and nonlinear optics[5] areas. Heath[5] led his group to prepare silver particles of 2.7 nm in diameter modified with hexanethiol and decanethiol respectively in 1997 and got 2D ordered arrangement of these silver particles. Their results showed that when the capped surfactants are decanethiol and hexanethiol, the distance between the metal surfaces is decreased from 1.2 nm to about 0.5 nm, and the films consisting of such ordered silver particles occur an insulator-to-metal transition. But Heath[9] thought that when the chain length of surfactants capping the particles was longer than C12, it would not be
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easy to form the compact 2D ordered arrangement. Schmid and his coworkers[10] took the polyethyleneimine as template to assemble the Au55 clusters with —SO3H shells into well-ordered cubic and hexagonal superlattices. In 2000, Mirkin group[11] and Alivisatos group[12] reported a new route to assemble the gold nanoparticles at the same time, providing a new thought for the nanoparticle organized assemblies. In their researches, the DNA were acted as the linker and cover. All the former work can be divided into two parts, the hydrophilic and hydrophobic nanoparticles. For the nanoelectronics, it is no doubt that the ordered arrangement of hydrophobic nanoparticles has more meaning. However, many further studies are needed in this field. On the basis of our previous work[13], our group has assembled gold nanoparticles capped by fatty amines with different chain lengths by LB technique to get different 2D ordered films, and stabilities of the films have been studied. 1
Experimental
1.1
Reagent C10NH2, C12NH2 and C18NH2 are analytical grade, from Beijing Reagent Co. C16NH2 (92%) used for organic synthesis from Merck Co. was purified by anhydrous ethanol. The other inorganic reagents are analytical grade.
1.2
Preparation of gold nanoparticles Gold nanoparticles capped by C10NH2, C12NH2, C16NH2, and C18NH2 respectively were prepared by reverse micelle method[13]. Take C18NH2/Au as an example. All manipulations were carried out at room temperature (22℃). First, dissolve certain amount octadecylamine into 100 mL of chloroform to form 0.02 mol/L solution; then a 0.01 mol/L HAuCl4· 4H2O in 20 mL of chloroform was added into the above solution, and the mixture was diluted to 200 mL. Next, the mixture was sonicated for at least 2 h in the dark at about 35℃. Following this, a 0.1 mol/L NaBH4 solution in 20 mL of ethanol was added dropwise while sonicating, causing an instant color change in the mixture from light yellow to dark purple. The mixture was continually sonicated for 2 h before the reaction stopped. Then the mixture in volume was decreased to around 5 mL by rotary evaporation. The purple precipitate was got by adding 400 mL anhydrous ethanol into the mixture and cooling to −20℃ for 20 h. The precipitate was filtered with 0.45 μm PTFE filter film, washed with a large amount of anhydrous ethanol and dried at room temperature to get purple dark solid. 1.3 2D arrangement of gold nanoparticles on the interface of air/water and preparation of sample The gold nanoparticles were dissolved into 20 mL of chloroform, and the size-selective precipitation was done as reported in literature[14]. The same amount methanol in batches was added into 20 mL of chloroform to increase the polarity of solvent, and the particles were separated out of the mixture according to the size of particles from big to small. Finally, the narrow-size distribution particles were obtained. In the experiment, the mean diameter of gold nanoparticles is about 5 nm (5.2, 4.0, 5.1 nm respectively). In order to compare the effect caused
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by the chain length of surfactants, we tried our best to select the same size particles in the experiment. It is very difficult to reach the goal in the separation process, however, we thought that in the above-mentioned error range of particles size, the size should have no effect on the conclusion about the effect of the chain length on the arrangement. The gold nanoparticles which were selected by size-selective precipitation were redispersed into chloroform to a concentration of about 0.5 mg/mL. At 12℃, 400 μL of material was dispersed across the water’s surface in the standard LB trough with a 100 μL glass syringe. At 25℃, the multistep creeping procedure was adopted. Then at 12℃, the compressed particle monolayer was transferred onto 230 mesh copper grids covered by Formvar at the surface pressure of 12 mN/m2 by parallel method, then the samples were finished and placed into the desiccator[13]. 1.4
Characteristics The UV-Vis absorption spectra of the colloids consisting of gold nanoparticles modified by fatty amines of different chain lengths were recorded on a Hitachi 2001 spectrophotometer in 24 h and 34 d respectively. TEM analysis was performed on a JEM100CX instrument operated at 100 kV. 2
Results and discussion
2.1
Stability of colloids consisting of gold nanoparticles Purple dark nanoparticles capped by C10NH2, C12NH2, C16NH2, and C18NH2 were redispersed into the same amount chloroform respectively and the UV-Vis spectra of colloids have been measured as shown in fig. 1. Fig. 1(a) shows that the maximum absorption value of colloids decreases in the order of surfactants from C18NH2, C16NH2, C12NH2 to C10NH2, and indicates that the last one is much smaller than the other three. During the preparations of nanoparticles, the concentrations of four kinds of fatty amines are the same, so we can think that under the same condition, the stabilities of forming reverse micelles of long chain fatty amines are C18NH2 > C16NH2 > C12NH2 > C10NH2. Affected by the stabilities of reverse micelles, the stabilities of the colloids consisting of gold nanoparticles are C18NH2/Au > C16NH2/Au > C12NH2/Au > C10NH2/Au. In addition, the absorption peak of colloid consisting of gold nanoparticles capped by C10NH2 is wider than the other three, and shifts red obviously. The order of the peak width of the other three is C18NH2 < C16NH2 < C12NH2, and the peaks shift red in the same way. Because the other conditions are the same in this system, the results of the UV-Vis spectra of colloids prove that the longer chains the fatty amines have, the smaller the gold nanoparticles will be and the narrower size distributions of nanoparticles will be obtained. Fig. 1(b) shows the absorption spectra of the same colloids placed for 34 d at room temperature (about 22℃). The maximum absorption value of colloid consisting of gold nanoparticles capped by C10NH2 after such a storage period becomes much smaller than that in fig. 1(a). Nevertheless, in spite of the maximum absorption values of colloids capped by C12NH2, C16NH2,
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C18NH2 also decreases in a certain degree compared with fig. 1(a), but it is not as much as that in the case of C10NH2. This fact shows that the stabilities of colloids are different. Under the same condition, the order of the stabilities of four colloids is C18NH2/Au > C16NH2/Au > C12NH2/Au > C10NH2/Au.
Fig. 1. UV-Vis absorption spectra of colloids of gold nanoparticles. (a) In 24 h, (b) 34 d. 1, 2, 3, and 4 stand for the colloids of gold nanoparticles modified with C10NH2, C12NH2, C16NH2, and C18NH2 respectively.
2.2
2D arrangement of gold nanoparticles Fig. 2 shows the 2D arrangement of gold nanoparticles modified by C12NH2, C16NH2, and
Fig. 2. 2D arrangement of gold nanoparticles capped by different long chain fatty amines. (a) C12NH2/Au, (b) C16NH2/Au, (c) C18NH2/Au.
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C18NH2. From fig. 2, it can be seen that the longer the surfactants capping the particles are, the more ordered the particles arrangements will be. From fig. 2(a), the particles show short range quasi one-dimensional structure. The gold particle number for each line is about 4, and the tropism of each line is uncertain. The distance between two particles in one line is about 5 nm. While in fig. 2(b), the number of the particles in a line increases apparently, and the distance between two particles is about 3 nm, shorter than that in fig. 2(a). The particles in fig. 2(c) show well-ordered hexagonal superlattices, the number of particles in a line reaches 20, and the distance decreases to 2 nm. According to the formula L(nm) = 0.25 + 0.127n (n, the number of carbon atom in the alkyl chain)[15], the lengths of C12NH2, C16NH2, and C18NH2 should be 1.774, 2.28, and 2.54 nm respectively, and the distances between two gold particles should be 3.55, 4.56, and 5.08 nm. The results from the TEM images are in contradiction to the calculated values. The longer the chains of the surfactants capping the particles, the shorter the distances between the two particles. This fact shows that the neighbouring fatty amines on the particle surface interpenetrate each other[16,17]. The longer the chains are, the deeper the penetration will be, and the closer the particles arrangements are. In addition, fig. 2 shows that with the increase of the chain length, the number of particles in one line rises. The nanoparticles modified with C18NH2 have formed a long-range ordered hexagonal lattice. It is obvious that the chain length of surfactants plays a very important role in forming the 2D ordered arrangement. Despite Heath’s viewpoint that it is difficult to assemble a 2D ordered aggregates when particles are capped by long chain molecules, there are still some reports to expect that the longer the molecules capping nanoparticles are, the more ordered the organized layers will be[18,19]. We think that the hydrophobic interaction of long chain fatty amines capping gold nanoparticles is the critical factor for assembling the 2D ordered closely packed nanoparticle arrangement. Usually, the hydrophobic interaction between long chain molecular capped on the particle surface prevents the nanoparticles from approaching closely, so it is difficult to form the 2D ordered packed arrangement. But with the method introduced in this paper, the long-range 2D ordered arrangement could be obtained. That is, put the monolayer on the air/water interface in an LB trough, “soften” the hydrophobic chain at a higher temperature, push and pull the floating barrier on the trough, and anneal the monolayer at a lower temperature at last. During the procedure, at a higher temperature, the motion of the surfactants becomes faster. The melting points of C12NH2, C16NH2, and C18NH2 are 28.3, 46.8, and 52.9℃, so at the temperature of our experiments, higher temperature has distinct effect on the motion of molecules. Floating barrier multistep creeping helps to overcome part of the hydrophobic interaction between alkyl chains and lightly adjusts the position of particles to reach the ordered structure. In summary, the chain length, the temperature change, and the floating barrier multistep creeping are all the important factors for assembling nanoparticles into a 2D ordered arrangement, and with the increase of the chain length, these factors ultimately lead to a more ordered arrangement.
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2.3
Stabilities of 2D ordered aggregates of gold nanoparticles For the long-range 2D ordered aggregates of gold nanoparticles modified with C18NH2, the aggregates are stable within a week. With time going on, the order of arrangement can be destroyed. It could be attributed to the entropy effect. During the formation of 2D arrangement by outside force, the entropy of the system decreases; when the finished samples are placed at room temperature, heat motion causes the entropy of the system to increase and the arrangement of particles is from ordered to non-ordered. If the samples are placed at a lower temperature, the 2D ordered arrangement should be more stable.
3
Conclusion
This paper reports the research on the preparation of gold nanoparticles modified with fatty amines with different chain lengths, the stabilities of colloids consisting of gold nanoparticles, the 2D arrangement of gold nanoparticles and the change of 2D ordered aggregates with time going on. The results show that the longer the chains, the smaller the gold nanoparticles prepared, the narrower the size distributions, and the more stable the colloids. By the LB technique used in the experiment, with the increase of the chain length, the 2D ordered range of gold nanoparticles has been extended. For the nanoparticles capped by C18NH2, the range reaches 1 μm 2, which overcomes the difficulty of assembling the nanoparticles capped by long chain molecules. These results suggest that the narrower size distribution of the nanoparticles and the longer chain surfactants will result in more ordered assemblies and larger area of the ordered arrangement. Acknowledgements 29733110).
This work was supported by the National Natural Science Foundation of China (Grant No.
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