Adsorption of Carboxylate-Modified Gold Nanoparticles on an

Nov 30, 2006 - Gold nanoparticles (Au NPs) were prepared and surface-modified by mercaptosuccinic acid (MSA) to render a surface with carboxylic acid ...
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Langmuir 2007, 23, 3123-3127

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Adsorption of Carboxylate-Modified Gold Nanoparticles on an Octadecylamine Monolayer at the Air/Liquid Interface Lian-Hua Chen, Anna Dudek, Yuh-Lang Lee,* and Chien-Hsiang Chang Department of Chemical Engineering, National Cheng Kung UniVersity, Tainan 70101, Taiwan ReceiVed September 22, 2006. In Final Form: NoVember 30, 2006 Gold nanoparticles (Au NPs) were prepared and surface-modified by mercaptosuccinic acid (MSA) to render a surface with carboxylic acid groups (MSA-Au). Octadecylamine (ODA) was used as a template monolayer to adsorb the Au NPs dispersed in the subphase. The effect of MSA concentration on the incorporation of Au NPs on the ODA monolayer and the relevant behavior of the mixed monolayer were studied using the pressure-area (π-A) isotherm and transmission electron microscopy (TEM) observations. The experimental results showed that the adsorbed density of Au NPs is low without the surface modification by MSA. When MSA was added into the Au NP-containing subphase, the incorporation amount of Au NPs increased with increasing MSA concentration up to ∼1 × 10-5 M for the particle density of 1.3 × 1011 particles/mL. With a further increase in the MSA concentration, the adsorbed particle density decreases due to competitive adsorption between the free MSA molecules and the MSA-Au NPs. It is inferred that free MSA molecules adsorb more easily than the MSA-Au NPs on the ODA monolayer. Therefore, an excess amount of MSA present in the subphase is detrimental to the incorporation of gold particles. The study on the monolayer behavior also shows that the π-A isotherm of the ODA monolayer shifts right when small amounts of Au NPs or free MSA molecules are incorporated. However, when larger amounts of particles are adsorbed at the air/liquid interface, a left shift of the π-A isotherm appears, probably due to the adsorption of ODA molecules onto the particle surface and the transferring of the particles from beneath the ODA monolayer to the air/water interface. According to the present method, it is possible to prepare uniform particulate films of controlled densities by controlling the particle concentration in the subphase, the MSA concentration, and the surface pressure of a mixed monolayer.

Introduction Nanoparticles are at the center of research interests because of their unique physicochemical and optoelectronic properties,1,2 which can be useful in fundamental and application aspects of studies. Order arrays of nanoparticles have been applied in optical, electronic, and biosensing devices,3-8 and their utilization is strongly dependent on the available techniques employed to immobilize the nanoparticles onto solid substrates. Usually, the immobilizations are performed using a covalent or an electrostatic interaction between the nanoparticles and solid substrates.9-14 Self-assembly techniques and Langmuir-Blodgett (LB) deposition are common methods used to fabricate particulate films. Although the self-assembly technique is fast and versatile, LB deposition allows precise control over the organization of * To whom correspondence should be addressed. Telephone: 886-62757575 ext 62693. Fax: 886-6-2344496. E-mail: [email protected]. (1) Grunwaldt, J. D.; Kiener, C.; Wogerbauer, C.; Baiker, A. J. Catal. 1999, 181, 223. (2) Henglein, A.; Meisel, D. Langmuir 1998, 14, 7392. (3) Famul, M.; Mayer, G. Nature 2006, 439, 666. (4) Martensson, T.; Svensson, C. P. T.; Wacaser, B. A.; Larsson, M. W.; Seifert, W.; Deppert, K.; Gustafsson, A.; Wallenberg, L. R.; Samuelson, L. Nano Lett. 2004, 4, 1987. (5) Wang, H.; Li, H.; Xue, B.; Wang, Z.; Meng, Q.; Chen, L. J. Am. Chem. Soc. 2005, 127 (17), 6394. (6) Han, G.; Guo, B.; Zhang, L.; Yang, B. AdV. Mater. 2006, 18, 1709. (7) Chen, J.; Wiley, B.; Li, Z. Y.; Campbell, D.; Saeki, F.; Cang, H.; Au, L.; Lee, J.; Li, X.; Xia, Y. AdV. Mater. 2005, 17, 2255. (8) Concertino, A.; Capobianchi, A.; Valentini, A.; Cirillo, E. N. M. AdV. Mater. 2003, 15 (13), 1103. (9) Sastry, M.; Patil, V.; Mayya, K. S.; Paranjape, D. V.; Singh, P.; Sainkar, S. R. Thin Solid Films 1998, 324, 239. (10) Sastry, M.; Gole, A.; Patil, V. Thin Solid Films 2001, 384, 125. (11) Mayya, K. M.; Jain, N.; Gole, A.; Langevin, D.; Sastry, M. J. Colloid Interface Sci. 2004, 270 (1), 133. (12) Mayya, K. S.; Patil, V.; Sastry, M. Langmuir 1997, 13 (9), 2575. (13) Bandyopadhyay, K.; Patil, V.; Vijayamohanan, K.; Sastry, M. Langmuir 1997, 13, 5244. (14) Sastry, M.; Patil, V.; Sainkar, S. R. J. Phys. Chem. B 1998, 102, 1404.

the system.15 Several assembly strategies through the use of the LB technique have been elaborated: that is, binding of nanoparticles to a Langmuir monolayer (a) by using the electrostatic attraction between particles in the subphase and a surfactant monolayer,12,16-19 (b) by the direct spreading of surfactant capped particles at the air/liquid interface,20-22 and (c) by postdepostion treatment of LB films containing precursors of nanoparticles.23,24 The strategy based on electrostatic interaction allows control of the size and structure of the particles. Nanoparticles such as Au, Ag, and CdS were synthesized via a colloidal route and were surface-modified with suitable functional groups25-28 to render a particle with a charged surface. Different compounds such as 4-carboxythiophenol (4-CTP),12,16 laurylamine,17 octadecylamine,25 and cysteine29 have been used to cap the nanoparticle surface to enhance particle adsorption on a template monolayer. (15) Roberts, G., Ed. Langmuir-Blodgett Films; Plenum: New York, 1990. (16) Mayya, K. S.; Sastry, M. Langmuir 1998, 14, 74. (17) Swami, A.; Kumar, A.; Selvakannan, P. R.; Mandal, S.; Sastry, M. J. Colloid Interface Sci. 2003, 260 (2), 367. (18) Patil, V.; Mayya, K. S.; Pradham, S. D.; Sastry, M. J. Am. Chem. Soc. 1997, 119, 9281. (19) Sastry, M.; Mayya, K. S.; Patil, V. Langmuir 1998, 14, 5921. (20) Meldrum, F. C.; Kotov, N. A.; Fendler, J. H. Langmuir 1994, 10, 2035. (21) Chi, L. F.; Krakers, S.; Hartig, M.; Fuchs, H.; Schmid, G. Thin Solid Films 1998, 327-329, 520. (22) Abe, K.; Hanada, T.; Yoshida, Y.; Tanigaki, N.; Takiguchi, H.; Nagasawa, H.; Nakamoto, M.; Yamaguchi, T.; Yase, K. Thin Solid Films 1998, 327-329, 524. (23) Elliot, D. J.; Furlong, D. N.; Grieser, F. Colloids Surf., A 1999, 155, 101. (24) Kumar, N. P.; Narang, S. N.; Major, S.; Witta, S.; Talwar, S. S.; Dubczek, P.; Amenitsch, H.; Berntorff, S. Colloids Surf., A. 2002, 198-200, 59. (25) Sastry, M.; Rao, M.; Ganesh, K. N. Acc. Chem. Res. 2002, 35 (10), 847. (26) Mayya, K. S.; Patil, V.; Kumar, P. M.; Sastry, M. Thin Solid Films 1998, 312, 300. (27) Schmitt, J.; Decher, G.; Dressick, W. J.; Brandow, S. L.; Geer, R. E.; Shashidhar, R.; Calvert, J. M. AdV. Mater. 1997, 9, 61. (28) Mayya, K. S.; Sastry, M. Langmuir 1998, 14, 6344. (29) Mayya, K. M.; Gole, A.; Jain, N.; Phadtare, S.; Langevin, D.; Sastry, M. Langmuir 2003, 19, 9147.

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The carboxylic acid groups on 4-CTP or the cysteine-capped nanoparticles enable the adsorption of the particles on a positively charged monolayer such as octadecylamine (ODA). The complexation of the surface-modified nanoparticle with the monolayer was reported to be a function of charge on the particle and monolayer, which may be controlled by varying the pH of the subphase.30 Although previous reports show that the adsorption of nanoparticles on an ODA monolayer was enhanced by the modification of the nanoparticles, the adsorbed particle density is still limited within a monolayer. The reason is probably that the higher pH (∼8-10) required to ionize the carboxylic acids on the particles is detrimental to the protonization of amines. The contrary effect of the pH value on the ionization of the carboxylic acid and amine groups limits the interaction between the particles and an ODA monolayer. To solve this problem, mercaptosuccinic acid (MSA) was used as a capping agent to modify the gold nanoparticles in this work. Because two carboxylic acid groups are present in a MSA molecule, MSA has a lower pKa (pKa1 ) 3.30). The high degree of MSA ionization in an aqueous solution has been shown to enhance the adsorption of a cationic surfactant on the MSA-modified CdS nanoparticle.31 It is also expected that MSA-modified gold nanoparticles (MSAAu) should have a stronger interaction with an ODA monolayer. Various concentrations of MSA were used to modify the gold nanoparticles (Au NPs). The effect of MSA concentration (the degree of the surface modification) as well as the competitiveadsorption effect of free MSA molecules on the incorporation of MSA-Au on an ODA monolayer were studied using the characteristics of the surface pressure-area (π-A) isotherm and transmission electron microscopy (TEM) observations. Experimental Section Materials. Octadecylamine (ODA, >99%, Aldrich), chloroform (>99%, J.T. Baker), trisodium citrate dihydrate (>99%, J.T. Baker), hydrogen tetrachloroaurate (HAuCl4, >99%, Alfa Aesar), and 2-mercaptosuccinic acid (HOOCCH2CH(SH)COOH, MSA, >97%, Aldrich) were used as received. Doubly distilled water purified with a Milli-Q apparatus supplied by Millipore (resistivity g18.2 MΩ cm) was used for all the experiments of this study. Au Colloid Solution Preparation. An aqueous solution (200 mL) containing 57.6 mg of chloroauric acid (HAuCl4) was boiled, and then another 200 mL aqueous solution of 14.7 mM sodium citrate was added. By continuously boiling this mixture, a gold hydrosol formed after ∼15 min. After cooling the hydrosol, the capping of the gold clusters was performed by adding MSA into the hydrosol solution without further purification. Various amounts of MSA were used to study the concentration impact. The hydrosol pH was adjusted to ∼7 using ammonia. Mixed ODA/Au Monolayer and LB Film Preparation. The monolayer experiments were performed in a computer-controlled film-balance apparatus (KSV minitrough) constructed by KSV Instruments Ltd., Finland. A Teflon trough with a working area of 32 × 7.5 cm2 was placed on a vibration isolation table and closed in an environmental chamber. The film surface pressure at the air/ water interface was measured by using the Wilhelmy plate arrangement attached to a microbalance. ODA was dissolved in chloroform to prepare a stock solution with a concentration of 1 mg/mL. The stock solution was spread on the subphase of the gold hydrosol using a microsyringe (Hamilton Co., U.S.A.). After allowing a waiting period for solvent evaporation and for the adsorption of the Au nanoparticles, the monolayer was compressed at a rate of 5 mm/min. A surface pressure-area (π-A) isotherm was obtained by continuous compression of the monolayer using two moving barriers. (30) Sastry, M.; Mayya, K. S.; Patil, V.; Paranjape, D. V.; Hegde, S. G. J. Phys. Chem. B 1997, 101, 4954. (31) Shen, Y. J.; Lee, Y. L.; Yang, Y. M. J. Phys. Chem. B 2006, 110, 9556.

Chen et al.

Figure 1. Pressure-area (π-A) isotherms of ODA monolayers on pure water (a) or on subphases containing unmodified Au NPs (be). The particle concentration for curves b-d is 1.3 × 1011 particles/ mL (LD), and the allowable adsorption time before compression is 0.5 h (b), 2.0 h (c), or 3.0 h (d). For curve e, the particle concentration is higher (6.5 × 1011 particles/mL, HD), and the adsorption time is 2 h. The pH values of all the subphases are controlled at 7. To examine the adsorption situation of Au NPs in the monolayer, the monolayers were transferred onto a carbon-coated copper grid by vertical deposition (LB deposition) at a dipping speed of 1 mm/ min. The LB films were observed using a high-resolution transmission electron microscope (Hitachi H-7500).

Results and Discussion Adsorption Behavior of Unmodified Au NPs. Figure 1 shows the π-A isotherms of the ODA monolayer on the aqueous subphases with or without gold nanoparticles. In the absence of Au NPs, the ODA monolayer exhibits highly condensed and low compressible characteristics, resembling the results reported in the literature.12,25,32 The limiting area estimated by extrapolating the constant-slope region of the isotherm to zero surface pressure is ∼21 Å2/molecule, which corresponds to the cross-sectional area of a hydrocarbon chain.33 In the presence of Au NPs, the position of the isotherm is dependent on the interval of the adsorption period allowed before compression. For a waiting time of 0.5 h, the isotherm exhibits a right shift and a more expanded state in comparison with that of the pure ODA monolayer, ascribable to the adsorption of nanoparticles. When the adsorption time of Au NPs was prolonged to 2.0 or 3.0 h, the π-A isotherm was found to shift left when compared to the pure monolayer. This phenomenon is unusual since an increase in the adsorption time is supposed to increase the amount of Au NPs adsorbed and thus result in a more expanded monolayer.12 A left shift of the isotherm implies that fewer ODA molecules are present at the air/liquid interface. One possible reason for the loss of ODA molecules is the high solubility of ODA in the water subphase.34 This effect is excluded here because no significant shift of the isotherm was found when the pure ODA monolayer was compressed after a 3.0 h waiting period (results not shown). This result also implies that the solubility of ODA molecules in the aqueous solution of pH ) 7 does not significantly affect the π-A isotherms. To examine the adsorption behavior of Au NPs on the monolayer, the monolayers were transferred onto carbon-coated copper TEM grids at a molecular area of 20 Å2/ODA molecule. The transfer under a constant molecular area is for comparing the adsorbed amount of Au NPs at the same molecular density. (32) Lee, Y. L. Langmuir 1999, 15, 1796. (33) Petty, M. C., Ed. Langmuir-Blodgett Films. An Introduction; Cambridge University Press: New York, 1996. (34) Ganguly, P.; Paranjape, D. V.; Rondelez, F. Langmuir 1997, 13, 5433.

Adsorption of Au Nanoparticles on ODA Monolayers

Figure 2. TEM images for unmodified Au NPs adsorbed on ODA monolayers. The monolayers were transferred at a molecular area of 20 Å2/ODA molecule. The particle concentration in the subphase is 1.3 × 1011 particles/mL (a and b) or 6.5 × 1011 particles/mL (c). The monolayers were compressed after 0.5 h (a) or 2.0 h (b and c) of adsorption time.

Figure 3. Proposed model illustrating the arrangement of Au NPs and ODA molecules at the air/liquid interface. (a) For a short adsorption period, the adsorbed particles are few and stay mainly beneath the ODA monolayer. (b) When the adsorption time is long enough, more particles are adsorbed and some particles are able to transfer to the air/liquid interface.

The TEM images shown in Figure 2 demonstrate a very low density of Au NPs for an adsorption time of 0.5 h. The density increased slightly when the adsorption time was prolonged to 2.0 or 3.0 h (both have similar results as shown in Figure 2b). The similar densities obtained for the 2.0 and 3.0 h adsorption times indicate that a 2.0 h waiting period is enough to reach the equilibrium state of particle adsorption. Therefore, the adsorption time in the following experiments is controlled at 2.0 h. In the case of increasing the concentration of Au NPs 5 times from the value of 1.3 × 1011 to 6.5 × 1011 particles/mL, the π-A isotherm shifts further left (Figure 1, curve e) and the adsorbed density of the Au NPs increases (Figure 2c). Apparently, both prolonging the adsorption period and increasing the concentration of Au NPs lead to a higher adsorbed amount of Au NP as well as to the left shift of the π-A isotherm. A model is proposed here to explain the above phenomena as illustrated in Figure 3. When a short adsorption time is adopted, only few Au NPs adsorb, and they are adsorbed mainly to the amine groups of ODA beneath the air/liquid interface (Figure 3a). The ODA monolayer becomes more extended and compressible due to the incorporation of Au NPs in this case. As the adsorption time is prolonged, more Au NPs can be adsorbed and, moreover, the adsorbed NPs have the chance to make contact with other ODA molecules nearby. Due to electrostatic interaction, an ODA molecule will be incorporated on the particle surface once it meets a Au NP. When a sufficient amount of ODA molecules is adsorbed on a Au NP, the nanoparticle becomes more hydrophobic, and it is more stable for the particle to exist at the air/liquid interface rather than beneath the interface. As shown in Figure 3b, many of the ODA molecules are lost from

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the air/liquid interface to the particle surface. This transfer also causes the left shift of the π-A isotherm. Although the incorporation amount of Au NPs in the ODA monolayer can be enhanced by controlling the adsorption time or by adjusting the concentration of Au NPs in the subphase, the adsorbed density is still low and the adsorbed particles tend to aggregate, which can also be observed in Figure 2. Surface modification of the gold surface is thus proposed not only for giving a higher interaction between ODA and Au NPs but also for preventing aggregation between particles. Adsorption Behavior of MSA-Modified Au NPs. In this work, surface modification was performed by adding MSA to the aqueous solution containing the Au NPs. No further separation process was utilized to remove the residual MSA, if any, in the gold hydrosol. Therefore, both MSA-modified Au particles (MSA-Au) and free MSA molecules can interact with the ODA monolayer. Besides, when MSA molecules adsorb on the Au NP surface, the citrate ions are released into the bulk solution. The effect of the citrate ions on the π-A isotherm of the ODA monolayer was first examined by using a subphase containing 7.5 mM sodium citrate (with the pH value adjusted to ∼7.0). No significant shift of the ODA isotherm was observed when compared with the isotherm obtained on pure water. Therefore, it can be concluded that the citrate ions released during the surface modification of Au NPs do not cause the shift of an ODA isotherm and that the variation of the isotherms in the following studies is attributed to the adsorption of MSA and/or MSA-Au. The effect of MSA present in the subphase on the behavior of an ODA monolayer was studied first. Figure 4a shows the π-A isotherms of ODA monolayers on aqueous subphases containing different concentrations of MSA. The isotherms were found to gradually shift right and become more expanded with increasing MSA concentration. The collapse pressure of the isotherm does not change much when the MSA concentration is less than 2.8 × 10-8 M, but it decreases significantly to ∼40 mN/m when the MSA concentration is elevated to above 2.3 × 10-6 M. The present result indicates that the isotherm of an ODA monolayer is highly influenced by the presence of MSA. Due to electrostatic interaction between the carboxylic acid and amine groups, the head group of an ODA molecule becomes bulky when a MSA molecule is adsorbed, which not only leads to a higher liftoff area of the isotherm but also prevents the close contact of alkyl chains between ODA molecules. When Au NPs of constant density (1.3 × 1011/mL) were present simultaneously with MSA in the subphase, the results shown in Figure 4b also demonstrate that the π-A isotherms gradually shift right with increasing MSA concentration. However, the characteristics of these isotherms are different from those without Au NPs. For a MSA concentration as low as 1.4 × 10-10 M (curve b in Figure 4a), the presence of MSA alone does not induce a significant change to the isotherm of ODA. When Au NPs were added (curve b in Figure 4b), the liftoff area of the isotherm increases, the collapse pressure decreases, and the isotherm becomes more compressible. The adsorption behavior of Au NPs without and with the addition of MSA is also different as seen by comparing curve c in Figure 1 with curve b in Figure 4b. The left shift of the isotherm found for the longer adsorption time of the unmodified Au NPs (curve c in Figure 1) becomes moderate due to the presence of MSA (curve b in Figure 4b). This result seems to suggest that the Au NPs modified by MSA reduce the loss of ODA molecules from the air/liquid interface. When the MSA concentration is increased to 2.8 × 10-8 M, the relative positions of the isotherms do not change significantly, although the isotherms slightly shift right in the presence of

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Figure 5. TEM images of MSA-modified Au NPs adsorbed on ODA monolayers. The particle concentration in the subphase is 1.3 × 1011 particles/mL, and the monolayers were transferred at a molecular area of 20 Å2/ODA molecule. The concentrations of MSA are 1.4 × 10-10 M (a), 2.8 × 10-8 M (b), 2.3 × 10-6 M (c), 1 × 10-5 M (d), 1 × 10-4 M (e), and 2.1 × 10 -3 M (f). All monolayers were compressed after 2.0 h of adsorption time.

Figure 4. Pressure-area (π-A) isotherms of ODA monolayers on aqueous subphases containing various concentrations of MSA in the absence (a) or presence (b) of Au NPs. The concentration of Au NPs is 1.3 × 1011 particles/mL. The monolayers were compressed after 2.0 h of adsorption time.

MSA. For the two MSA concentrations discussed above, the residual MSA in the subphase containing the Au NPs should be insignificant and the variation of the π-A isotherm is mainly controlled by the adsorbed Au NPs. When the MSA concentration is increased to 2.3 × 10-6 M, both isotherms with (curve d in Figure 4b) and without (curve d in Figure 4a) Au NPs exhibit compressible characteristics. The isotherm without Au NPs slightly locates right with respect to the isotherm with Au NPs, indicating that the free MSA molecules have a more significant effect than the NPs in expanding the ODA monolayer. The adsorption of Au NPs in the presence of MSA was examined by transferring the monolayer at 20 Å2/ODA molecule. To confirm that the NPs deposited on the substrate are transferred from the monolayer rather than adsorbed from the subphase, the LB transfer was first performed in a subphase containing MSA and Au PNs but in the absence of an ODA monolayer. Under TEM observation, no significant amount of Au NPs was observed, indicating that the adsorption of Au NPs from the bulk is insignificant in this system. Therefore, the NPs observed on a LB film prepared in the presence of an ODA monolayer are transferred from the monolayer. Figure 5 shows the TEM images of LB films prepared in the presence of various concentrations of MSA. Compared with the results from films prepared in the absence of MSA, the adsorbed density of Au NPs increases even at a MSA concentration as low as 1.4 × 10-10 M (Figure 5a). Moreover, the density increases with increasing MSA concentration and the maximum adsorption

amount appears at ∼2.3 × 10-6-1 × 10-5 M MSA (Figure 5c and d). However, the adsorbed density decreases when the MSA concentration is further increased (10-4 M, Figure 5e) and only few particles were observed at a MSA concentration of 2.1 × 10-3 M (Figure 5f). Such results clearly indicate that the residual MSA molecules in the subphase decrease the adsorption of MSAAu NPs. It is inferred that the free MSA molecules adsorb more easily than the gold particles on the ODA monolayer. Once the positively charged moiety of an ODA molecule is associated with the MSA molecules, it cannot incorporate further with the Au NPs; therefore, the adsorbed density of the Au NPs is decreased. The optimal concentration of MSA required for the maximum adsorption of Au NPs was estimated from the TEM images. A concentration range between 2.3 × 10-6 and 1 × 10-5 M is estimated for a colloidal density of 1.3 × 1011 Au NPs/mL. If the occupied area of a MSA molecule on the gold surface is taken to be 21.4 Å2,35 the theoretical MSA concentration required to completely cover the surface of all Au NPs (18 nm in mean diameter) is estimated to be 5.9 × 10-5 M. The optimal concentration obtained here is lower than the theoretical value. Such results may suggest that the MSA molecules pack more loosely on the particle surface than theoretically predicted. For the purpose of increasing the adsorbed density of the Au NPs, both the Au NP density and MSA concentration in the subphase were increased 5 times to 6.5 × 1011 particles/mL and 5 × 10-5 M, respectively. Figure 6 shows the π-A isotherms before and after such variation. The high particle density isotherm (HD, curve d) shifts left with respect to the low density one (LD, curve c). The TEM image obtained by transferring the monolayer at 20 Å2/ODA molecule (Figure 7a) reveals an increase of the adsorbed density due to the elevation of the particle density in the subphase. Such results also indicate that the adsorption state of particles on a template monolayer cannot be evaluated simply from the shift of the isotherm. The left shift of the isotherm for a monolayer with a higher incorporation of Au NPs is different from that previously reported by Mayya et al.18 The phenomenon found here can also be attributed to the loss of ODA molecules by adsorption onto the particle surface. Besides, when a sufficient amount of Au NPs is adsorbed at the air/liquid interface, the (35) Daniel, M. C.; Astruc, D. Chem. ReV. 2004, 104, 293.

Adsorption of Au Nanoparticles on ODA Monolayers

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Another method to increase the particle density is by compressing the monolayer and then depositing the LB film at higher surface pressures. Figure 7 also shows the TEM images of LB films transferred at 15.5 Å2/ODA molecule (π ) 20 mN/ m, Figure 7b) and 7.6 Å2/ODA molecule (π ) 57 mN/m, Figure 7c) for the monolayer on a subphase of high colloidal density (curve d in Figure 6). Apparently, the particle density increases with decreasing surface area. For the film transferred at 7.6 Å2/ ODA molecule, the Au NPs almost completely cover the entire surface, which confirms the applicability of this method to prepare uniform particulate films of close-packed structures.

Conclusion Figure 6. Pressure-area (π-A) isotherms of ODA monolayers. A comparison between monolayers on gold hydrosol of high (HD, 6.5 × 1011 particles/mL) and low (LD, 1.3 × 1011 particles/mL) densities. The ratio of MSA concentration to particle density was kept constant. All monolayers were compressed after 2.0 h of adsorption time.

Figure 7. TEM images of MSA-modified Au NPs adsorbed on ODA monolayers. The particle concentration in the subphase is 6.5 × 1011 particles/mL, and the monolayers were transferred at various molecular areas of 20 (a), 15.5 (b), and 7.6 Å2/ODA molecule (c). All monolayers were compressed after 2.0 h of adsorption time.

monolayer behavior is dominated by the particles rather than by the template monolayer as is the case shown in Figure 6.

Au NPs were surface-modified by mercaptosuccinic acid (MSA). The effects of MSA concentration on the incorporation of Au NPs to the template ODA monolayer and on the mixed monolayer (ODA/Au NPs) behavior were studied. The results show that the adsorbed density of MSA-Au NPs is higher than that of Au NPs without surface modification and increases with increasing concentration of MSA up to ∼1 × 10-5 M for the particle density of 1.3 × 1011 particles/mL. However, an excess amount of MSA present in the subphase is unfavorable for gold particle incorporation. When a high amount of particles is adsorbed at the air/liquid interface, a left shift of the π-A isotherm is observed, probably due to the adsorption of ODA molecules on the surface of particles and their transfer to the air/liquid interface. Moreover, the particle density increases as the surface pressure of the mixed monolayer increases. This study shows that, through the control of particle and MSA concentration, as well as the compression of the monolayer, a uniform film with a close-packed structure can be obtained. Acknowledgment. The support of this research by the National Science Council of Taiwan through Grant No. NSC 95-2214-E-006-024 is gratefully acknowledged. LA062779H