Analysis of Adsorption and Binding Behaviors of Silver Nanoparticles

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Analysis of Adsorption and Binding Behaviors of Silver Nanoparticles onto a Pyridyl-Terminated Surface Using XPS and AFM Nobuko Fukuda,*,† Naoyuki Ishida,† Kenichi Nomura,† Tong Wang,† Kaoru Tamada,‡,§ and Hirobumi Ushijima† †

Flexible Electronics Research Center (FLEC), National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Higashi, Tsukuba 305-8565, Japan ‡ Research Institute of Electrical Communication (RIEC), Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan

bS Supporting Information ABSTRACT: In this study, we analyzed adsorption and binding behaviors of citrate-capped silver nanoparticles (AgNPs) on a pyridyl-terminated surface using X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). Adsorption of the AgNPs onto the pyridyl-terminated silicon wafer surface was completed through pH-controlled sol immersion. The adsorption occurred predominantly at a pH less than the pKb value of the pyridyl group and more than the pKa1 of citric acid, indicating that the driving force behind adsorption was electrostatic interaction. Adsorption of citrate onto the pyridyl group also occurred at pKa1 < pH < pKb without AgNPs. According to XPS in the N1s region, larger deprotonation from the pyridinium-formed pyridyl groups was demonstrated subsequent to adsorption of the AgNPs. The deprotonation from the pyridinium indicates the formation of the neutral pyridyl group as the counterpart of hydrogen bonding with the carboxyl group of citrate. The binding state between the pyridyl group and citrate surrounding AgNPs is expected to be kept stable through hydrogen bonding and van der Waals force derived from the AgNPs approach to the pyridyl surface.

1. INTRODUCTION Metal nanoparticles are popular for potential use in various applications due to their unique electronic, chemical, and optical properties. These interesting properties have been demonstrated not only theoretically but also experimentally using advanced synthesis, preparation, and assembly of metal nanoparticles.1
10 Capping materials surrounding the metal cores determine the manner in which the metal nanoparticles assemble and the properties of the resulting structures. For example, gold nanoparticles capped with DNA oligonucleotide-thiols are wellknown to form self-assembled superstructures through hybridization with the complementary DNA oligonucleotide in aqueous media.11 Metal nanoparticles capped with alkanethiols and fatty acids show good dispersion in organic solvents such as chloroform and toluene.12
14 The metal nanoparticles form closely packed two-dimensional arrays on the water surface, and the resulting two-dimensional (2-D) arrays can be transferred onto solid surfaces using the Langmuir
Blodgett (LB) technique.15,16 The resulting 2-D arrays show interesting plasmonic characteristics depending on the distance between metal nanoparticles. The simplest and most common capping material is citrate, which encourages the dispersion of metal nanoparticles in aqueous media and imparts negative charge to the metal nanoparticles. The citrate-capped metal nanoparticles can be assembled on positively charged substrate surfaces.17,18 The method for the assembly of the metal nanoparticles is just r 2011 American Chemical Society

immersion into the citrate-capped metal sol. Although it is difficult to gain the periodicity of nanoparticles, the arrays obtained from the assembly show plasmonic absorption. Adsorption behaviors based on electrostatic interaction between oppositely charged materials strongly depend on the pH of aqueous media.19
21 The pKa of the charged material is a key factor that determines the material states of dispersion, aggregation, and absorption. Also, detailed studies on the pH dependence of adsorption provide the result that the pKa of an adsorbed material on the surface differs from that in solution.22 The result assists in clarification of the binding state between materials following adsorption. Additionally, the detailed binding state is important in the determination of reaction conditions (e.g., pH, solvent, and temperature) for the next treatments in the architect multilayers. Negatively charged metal nanoparticles in aqueous media are often assembled onto the positively charged primary aminoterminated surfaces of the substrates. The primary amino groups are used because they ionize at the pH of a fresh sol containing the citrate-capped metal nanoparticles, and electrostatic interactions are responsible for the assembly. The pyridyl group is an amino group that is readily used as a ligand for forming Received: January 21, 2011 Revised: September 17, 2011 Published: September 28, 2011 12916

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Figure 1. Chemical structure of 2-(4-pyridylethyl)triethoxysilane (PySi).

Figure 3. The survey (a) and the high-resolution N1s (b) XP spectra of the PySi-modified surface. Immersion time for modification in the 1.0 mM PySi ethanol solution was 120 min.

Figure 2. Relationships between immersion times and static water contact angles (a) and concentration of nitrogen to silicon atoms (N/Si) determined from the PySi-modified surface XP spectra on a silicon wafer (b).

photo- and electrochemical-sensitive metal complexes.23 Adsorption of metal nanoparticles and formation of metal complex via a pyridyl-terminated surface should provide enhanced photo- and electrochemical-sensitive properties. The pKb value of the pyridyl group tends to be much lower than those of the other amino groups.24 Thus, a nanoparticle assembly whose behavior is different from that of the other amino groups can be expected. To utilize the interesting properties of the pyridyl group and metal nanoparticles, the pH dependence of adsorption and the binding state requires detailed studies. Reported here is the preparation of a pyridyl-terminated surface on a silicon wafer and pH-adjusted silver sol. The pH dependence of citrate-capped silver nanoparticles (AgNPs) adsorption onto the pyridyl surface was quantitatively evaluated using X-ray photoelectron spectroscopy (XPS) and morphologically analyzed using atomic force microscopy (AFM).

2. MATERIALS AND METHODS 2.1. Materials. 2-(4-Pyridylethyl)triethoxysilane (PySi, Figure 1) was purchased from Gelest, Inc. Silver nitrate (AgNO3) was purchased from Kanto Chemical Co., Inc. Sodium borohydride (NaBH4) was purchased from Aldrich. Trisodium citrate and all the solvents were purchased from Wako Pure Chemical Industries, Ltd. Duolite C225LFH cation-exchange resin (particle size 0.65 ( 0.05 mm) was purchased

from Sumika Chemtex Co., Ltd. All the chemicals were used without further purification. 2.2. Surface Modification of PySi. A silicon (100) wafer (p-type, 15
50 μΩ) was used as the substrate. The substrate was cleaned with 50% hydrochloric acid in methanol and rinsed twice with ethanol. Then, the substrate was immersed into concentrated sulfuric acid and rinsed with Milli-Q water.25 After drying, the static water contact angle was below 5°. The hydrophilic substrate was immersed into 1.0 mM PySi in ethanol at room temperature (rt). The substrates used for experiments in adsorption of AgNPs were immersed into the ethanol solution for 120 min. The PySi-treated surface was washed twice with ethanol, dried with a stream of nitrogen gas, and baked at 100 °C for 5 min. 2.3. Preparation and Deposition of AgNPs. An aqueous solution (100 mL) containing 0.1 mM AgNO3 and 0.1 mM trisodium citrate as a stabilizer was stirred at rt, and 10 mL of a 2.0 mM NaBH4 aqueous solution was continuously added over 4 h.26 The pH of the resulting fresh silver sol was 7.8 at rt. The pH of the silver sol was adjusted to the desired pH by adding the cation-exchange resin to the sol5 while measuring the pH using a Mettler Toledo MP120 pH meter. After attaining the desired pH, the resin was removed from the sol using filter paper. The adjusted lower limit pH was 3.3. The PySi-modified substrate was immersed into the pH-adjusted silver sol at rt for adsorption of AgNPs and then rinsed with Milli-Q water. Then, the substrate was dried under a stream of nitrogen gas. For the control experiment, the PySi-modified substrate was immersed into the aqueous solution without AgNPs and the pH of the 0.1 mM trisodium citrate aqueous solution was adjusted using the same method above. 2.4. Measurements. The static water contact angles were measured with a Kyowa DM500 contact angle meter. UV
vis absorption spectra were measured with a Hitachi U-4100 spectrophotometer. Transmission electron microscope (TEM) images of the AgNPs were taken with a Hitachi H9000 on carbon-coated grids. The XPS was measured with a PHI 5000 VersaProbe spectrometer (ULVACPHI, Inc.) using a monochromatic Al Kα source (1486.6 eV, 100 V) with a takeoff angle of 75°. The collected spectra were analyzed using a curve-fitting program, XPSPEAK4.1. All the spectra were corrected for 12917

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Langmuir background signals using the Shirley algorithm prior to curve-fitting. The high-resolution spectral curve fitting was completed using some constraints to reduce free parameters. All the peak shapes were limited to Gaussian ones. The constrained values of the peak region and the fwhm (full width half-maximum) were decided using specific criteria. First, curve-fitting for the spectrum of the PySi surface before immersion into the silver sol was measured several times using some peak regions and fwhms referenced in the NIST X-ray photoelectron spectroscopy database.27 Finally the peak region and fwhm in the best fitted curve were employed as the constrained values for the curve-fitting of other spectra. Atomic concentration was estimated from peak areas using appropriate reflective sensitive factors. The peak assignments were referenced to the C1s aliphatic carbon set at 284.5 eV. AFM images were captured with a Shimadzu nano search microscope SFT-3500. All the images were taken with silicon cantilevers in the tapping mode. The tapping frequency was between 283.8 and 360 kHz, and the spring constant was 42 N m
1. The surface charge of the AgNPs was measured as the ζ-potential using laser Doppler electrophoresis (Malvern Zetasizer Nano ZS). The pH-adjusted silver sol in a disposable capillary cell was used for the measurement.

3. RESULTS AND DISCUSSION 3.1. Surface Modification with PySi. The static water contact angles on the PySi-modified surface were measured at various

Figure 4. UV
vis absorption spectra of silver sols at pH 3.3, 5.0, and 7.8.

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immersion times into the 1.0 mM PySi ethanol solution, as shown in Figure 2a. The contact angle before immersion was below 5° and increased with immersion, eventually reaching saturation in 30 min. The concentration of the nitrogen to silicon atoms (N/Si) at the PySi-modified surface on the silicon wafer was estimated from XPS measurement with saturation shown in Figure 2b. These results indicate that modification with PySi onto the hydrophilic silicon surface was completed in a 30 min period. The Si2p, C1s, O1s, and N1s peaks originating from the PySi surface are shown in Figure 3a. In the high-resolution N1s spectrum (Figure 3b), the two peaks (401.8 and 399.4 eV, fwhm 2.3 eV) assigned to the two different conditions of nitrogen are distinctly visible; the higher and lower binding energy peaks correspond to protonated and deprotonated nitrogen, respectively.28,29 These two peaks were also detected at the surfaces immersed into the PySi solution for 10, 30, and 60 min. The modified pyridyl groups were partially protonated and formed pyridinium after rinsing with ethanol and drying. In the following experiments, the PySi surface was immersed for 120 min, as this provided enough time for complete modification. 3.2. Adsorption of AgNPs on the Pyridyl Surface. The pH of the silver sol before cation exchange was 7.8. The diameter of the AgNPs was estimated to be 6.8 ( 2.3 nm from the TEM image obtained using the carbon-coated grid dropped in the sol at pH 7.8 as shown in the Supporting Information (Figure S1). The pH of the silver sol was adjusted to 5.0 and 3.3 using cationexchange resin. UV
vis absorption spectra of the silver sols showed the typical absorption band due to localized plasmons of AgNPs, as shown in Figure 4. The peaks are slightly lowered and broadened by the treatment with the cation-exchange resin. The peak wavelength at 396 nm does not change by adjusting the pH. As a result, it seems that the size and dispersion condition of the nanoparticles were not largely influenced by the sol pH. Adsorption of the AgNPs onto the PySi-modified surface was determined using immersion into silver sols with adjusted pH values of 3.3, 5.0, and 7.8. Figure 5a shows the XP survey spectra of the surfaces after immersion for 900 min in the silver sols.

Figure 5. XP survey spectra (a) and high-resolution Ag3d XP spectra (b) of surfaces immersed into silver sols at pH 3.3, 5.0, and 7.8 for 900 min. 12918

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Figure 6. AFM images of surfaces immersed in silver sol at pH 3.3 for 20 min (a), 60 min (b), and 900 min (c) and at pH 7.8 for 900 min (d).

Figure 7. Relationship between silver sol immersion time and the silver to silicon atom (Ag/Si) concentration ratio obtained from XP spectra at pH3.3 (b), 5.0 (O), and 7.8 (2). The lines were added to guide the eye and do not represent a trend line.

Figure 8. Relationship between the silver sol pH and the AgNP ζ-potentials. The error bars indicate the standard deviation of the collected ζ-potentials.

The peaks originating from the AgNPs (Ag3d) were found on the surfaces immersed in the silver sols at pH 3.3 and 5.0, while

no adsorption was observed at pH 7.8. Figure 5b shows the highresolution Ag3d XP spectra of the surfaces immersed in the silver 12919

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sols at pH 3.3, 5.0, and 7.8. The peaks at 368 and 374 eV observed in the case of pH 3.3 and 5.0 are attributed to Ag3d5/2 and Ag3d3/2, respectively.28 The AFM images indicate that the amount of adsorbed AgNPs at pH 3.3 increases with the immersion time, as shown in Figure 6a
c, but no adsorption at pH 7.8 was observed, as shown in Figure 6d. The adsorbed AgNP dependence on the immersion time is shown in Figure 7. The amount of adsorption at pH 3.3 and 5.0 drastically increases in the first 120 min and then achieves equilibrium. The adsorption of AgNPs onto a PySi surface is fundamental. The pKa association constants for citric acid are reported as pKa1 = 2.9, pKa2 = 4.3, and pKa3 = 5.6.30 It is expected that the AgNPs have negative charge between a pH of 3.3 and 7.8. The ζ-potential of the AgNPs showed negative values within the pH range, as shown in Figure 8. By contrast, the pKb for protonation of 4-methylpyridine is reported as 6.02,24 and the pKb of the pyridyl group in PySi is presumed to be nearly the same. The pyridyl group in PySi must be neutral at pH > pKb. Figure 9 shows the relationship between the pH of the silver sol and the amount of the adsorbed AgNPs after 120 min immersion. Adsorption of AgNPs was observed at pH < 6, while no adsorption

of AgNPs was observed at pH > 6. From the results, the adsorption of the AgNPs was determined to be a consequence of electrostatic attraction between the positively charged PySi surface and the negatively charged AgNPs at pKa1 pKb was a result of poor electrostatic interaction on the PySi surface. The experiment at pH < pKa1 of citrate was not completed because the lower limit of the adjusted pH using the cation-exchange resin was 3.3. If the silver sol was adjusted to pH < pKa1 by addition of acid, it is expected that the AgNPs surface charge would become neutral and the electrostatic interaction decrease. Thus, it is unlikely that the AgNPs would electrostatically adsorb onto the PySi surface. The AgNPs aggregation increases and the dispersion decreases with decreasing pH values.31 The aggregated

Figure 9. Relationship between the silver sol pH and silver to silicon atom (Ag/Si) concentration ratio obtained using the XPS measurement. Immersion time of the PySi surface into the silver sol was 120 min.

Figure 11. Relationship between silver sol immersion time at pH 3.3 and the percentage of deprotonated nitrogen atoms estimated from the XP spectra shown in Figure 10.

Figure 10. High-resolution N1s XP spectra after silver sol immersion for 0 min (a), 20 min (b), 60 min (c), 120 min (d), 480 min (e), and 900 min (f) at pH 3.3. 12920

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Table 1. Percentages of Protonated and Deprotonated Nitrogen Atomsa with AgNPs at pH 3.3

without AgNPs at pH3.3

immersion time (min)

protonated N (%)

deprotonated N (%)

protonated N (%) (%)

deprotonated N (%)

0

62.9

37.1

62.9

37.1

20

37.5

62.5

39.3

60.7

60

21.8

78.2

50.0

50.0

900

8.8

91.2

31.2

68.8

a

Percentages are estimated from high-resolution N1s XP spectra of the PySi surface after immersion into the silver sol (Figure 10) and the citrate aqueous solution without AgNPs (Figure S3 in Supporting Information) at pH 3.3.

AgNPs might approach the PySi surface through sedimentation and immobilize on it by van der Waals force. 3.3. Binding State of the Pyridyl Group and AgNPs. Figure 10 shows the high-resolution N1s XP spectra of the PySi surfaces after immersion in the silver sol at pH 3.3. The spectra exhibit two peaks at 399.4 and 401.8 eV with each fwhm of 2.3 eV corresponding to deprotonated and protonated nitrogen, respectively. The deprotonated and protonated nitrogen signals strengthened and weakened, respectively, with increased immersion time. The percentage of deprotonated nitrogen was estimated using XP spectra curve-fitting shown in Figure 10. The percentage was plotted against the immersion time, as shown in Figure 11. Deprotonation rapidly proceeded in the initial 60
120 min and exceeded 90% by 900 min. At pH 3.3, the deprotonation was >80% for 120 min, as observed from the surfaces adsorbed with AgNPs at pH < pKb of PySi (Figure S2 in Supporting Information). The behavior displays some correlation with the binding of AgNPs to the PySi surface following electrostatic adsorption, as shown in Figure 7. Next, the influence of AgNPs on deprotonation was observed by immersing the PySi surface into the citrate aqueous solution adjusted to pH 3.3 without AgNPs and then rinsed with Milli-Q water. The percentages of protonated and deprotonated nitrogen atoms in this case were also estimated from the XP spectra curve-fitting (see Figure S3 in Supporting Information), as shown in Table 1. Deprotonation from pyridinium was also observed even in the case without AgNPs; however, the presence of AgNPs accelerates deprotonation. Alternatively, the ratio of carbon to nitrogen (C/N) and oxygen to nitrogen (O/N) atoms before and after immersion into the solution without AgNPs for 15 h was calculated from the XP spectra in the C1s, O1s, and N1s regions (see Figure S4 in Supporting Information). The ratios of C/N before and after immersion were estimated to be 14.4 and 22.0, respectively. The ratio before immersion did not correspond to the stoichiometric amount of PySi, most likely due to a carbon-containing contamination on the measured surface. The ratios of O/N before and after immersion were estimated to be 72.8 and 96.0, respectively. The increase of the ratios after immersion is probably due to adsorption of citrate through electrostatic interaction. From the results, some sort of binding force derived from the deprotonation is expected to exist between the pyridyl group and citrate with or without AgNPs as well as van der Waals force. Hao et al. assigned the binding state between the pyridyl groups surrounding gold nanoparticles and the polymer carboxyl groups as hydrogen bonding using Fourier transform infrared (FTIR) spectroscopy. The assignment was based on the formation of OH groups as an indication of hydrogen bonding.32 However, extinction of the protonated pyridyl groups by hydrogen bonding was not assigned from the FTIR spectra, due to absorption peak

overlap. Deprotonation from the pyridyl group of the PySi surface was observed and attributed to the formation of the neutral pyridyl group as the counterpart of hydrogen bonding with the carboxyl group of citrate. It is believed that the binding state is maintained through hydrogen bonding and van der Waals force, which were resistant to Milli-Q water (pH ≈ 5.7 > pKa3 of citric acid) rinsing. The higher percentage of deprotonated nitrogen atoms with AgNPs would be due to electrostatic adsorption of citrate concentrated by surrounding the AgNP core. Although acceleration of deprotonation and interaction between the AgNP core and the pyridyl group might have some correlations, it is unclear within our experiment. The citrate surrounding the AgNPs must be fully protonated after adsorption from the sol at pH 3.3 on the PySi surface, in spite of pH > pKa1. The trisodium citrate- and NaBH4-derived sodium ions (Na+) act as counterions of citrate and were estimated to be completely exchanged for protons. No XPS signal in the Na1s region was observed from the AgNP-adsorbed PySi surface obtained by immersion into the sol at pH 3.3. The adsorption of citrate surrounding AgNPs at pH 3.3 (H2cit
) on the positively charged PySi surface (PyH+) and the binding processes is represented as follows PyHþ þ H2 cit
f Py 3 3 3 H3 cit

ð1Þ

At pH > 3.3, Na+ must exist in the sol and the concentration is higher with increased pH. However, the XPS signal in the Na1s region was not detected from the AgNP-adsorbed PySi surface at pH 4.3, 5.0, and 5.7. The citrate surrounding AgNPs on the PySi surface seems to be in the fully protonated state, even at pH > pKa2, pKa3, as mentioned in eq 1. Interestingly, Mudunkotuwa et al. have reported that the pKa of adsorbed citrate on TiO2 nanoparticles differs from that in solution.22 The pH dependence on acid dissociation for citrate surrounding AgNPs adsorbed on the PySi surface may differ from that in the sol.

4. CONCLUSION The pH dependence on adsorption of the citrate-capped AgNPs onto the pyridyl surface by immersion into the pHadjusted silver sols was evaluated. At pH < pKa1 of citric acid < pKb of the pyridyl group, adsorption of AgNPs occurs due to the charge difference between the negatively charged AgNPs and the positively charged pyridyl surface. At pH > pKb, no adsorption of AgNPs occurs due to decreased charge difference between AgNPs and the neutral pyridyl surface. The pH at the border of adsorption and no adsorption is approximately 6. The adsorption of AgNPs achieves equilibrium as the immersion time advances because the positive surface of the pyridyl layer compensates by the adsorption of AgNPs and decreases electrostatic attraction. 12921

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Langmuir In addition, we demonstrated that deprotonation from the pyridyl group occurs after electrostatic adsorption of the AgNPs using the N1s XP spectra. The deprotonation in conjunction with the AgNPs electrostatic adsorption is considered to affect the binding state between citrate surrounding AgNPs and the pyridyl surface. It was interpreted that the deprotonation indicates formation of the neutral pyridyl group as a counterpart of hydrogen bonding with the carboxyl group of citrate. Generally, particle
substrate adsorption behavior in aqueous solution strongly depends on the solution pH. The pH affects the particle dispersion and the charged state of the surface. There are countless combinations of materials forming particles and surfaces. Each combination has an optimal pH condition for adsorption. For example, ascorbate, malate, and gallate have been widely used as passivation materials of metal nanoparticles as well as citrate. The pKa values of the acids corresponding to these salts differ from that of citrate. Thus, the pH range for electrostatic adsorption of nanoparticles prepared using these salts onto the pyridyl surface must be different from that of the citrate-capped nanoparticles. In contrast, primary amino derivatives are typically used as positively charged materials. The pKb of normal primary amines is 9
10, suggesting that they have a wide pH range for electrostatic adsorption of the negatively charged nanoparticles. In this work, we chose PySi as a material for pyridyl-terminated monolayer formation and the positively charged surface. The pyridyl group has also been reported as a ligand that forms metal complexes through coordination bond with metals and metal ions. For example, modification with metal nanoparticles and photosensitive metal complexes on the same PySi surface should provide emission enhancement from the metal complexes. Here, it was clear that the pH range for electrostatic adsorption of AgNPs on the PySi surface is pKa1 < pH < 6. Although it is narrower than that in the case of the primary amine, the stable binding state between AgNPs and the PySi surface was shown even after rinsing with Milli-Q water at pH > pKa3 of citric acid and drying similar to the primary amino group reports. The result implies that the modification with other materials on the AgNP-adsorbed PySi surface does not require special pH, solvent conditions. Future work on the interactions between pyridyl derivatives and other functional materials is necessary for designing and preparing promising materials and surfaces.

’ ASSOCIATED CONTENT

bS

Supporting Information. Additional information as noted in the text. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Present Addresses §

Institute for Materials Chemistry and Engineering (IMCE), Kyushu University, Japan.

’ ACKNOWLEDGMENT This work was partially supported by a Grant-in-Aid for Scientific Research (B) (Grant No. 21310067) from the Ministry

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of Education, Culture, Sports, Science and Technology of Japan. We thank Mr. Shinji Fujii, Waseda University, for his assistance with the preparation of the AgNPs and TEM observations. N.F. is deeply grateful for Dr. Naoto Koshizaki’s effort to restore the XPS equipment after the huge earthquake.

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