Physisorption and Chemisorption of a Self-Assembled Monolayer by

Jul 22, 2014 - The conventional adsorption model of. Langmuir is suitable only for the fast initial adsorption step (i.e., physisorption) in the proce...
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Physisorption and Chemisorption of a Self-Assembled Monolayer by the Quartz Crystal Microbalance Ji Li and Yong J. Yuan* Laboratory of Biosensing and MicroMechatronics, School of Materials Science and Engineering, Southwest Jiaotong University, Chengdu, Sichuan 610031, China ABSTRACT: A new adsorption model was developed to investigate the adsorption process of SAMs in the gaseous scenario, and the results were verified by using a prototype quartz crystal microbalance (QCM) sensor. In the experimental study, the observed properties in the gaseous scenario did not conform to the conventional theories well but matched the proposed adsorption model better. Hence, an optimal methodology for the theoretical study of adsorption process of SAMs was developed. The conventional adsorption model of Langmuir is suitable only for the fast initial adsorption step (i.e., physisorption) in the process of forming SAMs in the liquid scenario. Here, the rates of adsorption and desorption (ka, kd) at different temperatures were investigated. The activation barrier Ea = 59.738 kJ/ mol was obtained by the Arrhenius equation. The result agreed well with that obtained experimentally. More importantly, this study has established a new avenue of QCM chip applications.

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Karpovich et al.17 studied the adsorption of two alkanethiols, i.e., octadecanethiol and octanethiol in stirred hexane and cyclohexane solutions at room temperature over a broad range of composition (10−3 to 10−1 mM). They reported quite different observations from those of Shimazu et al.: the adsorption of both thiols from either solvent was very rapid, apparently complete within seconds, and there was no subsequent long-time-scale process. They verified that a monolayer of alkanethiol was formed using infrared reflection and showed that the kinetics was adequately described by the simple reversible Langmuir adsorption model. The literature reports only the kinetics of adsorption at room temperature, and all determinations occurred in the liquid phase. In this paper, the kinetic data with a temperature dependence were detected in the gas phase by using a prototype QCM device. The next section gives key experimental details, including the measurement of the shift resonance frequency and the kinetics of simulation.

INTRODUCTION In the past two decades, the immobilization of self-assembled monolayers (SAMs) has drawn considerable attention in optical,1,2 electronic,3 chemical,4,5 medicine,6 microfabrication,7,8 and biological fields.9,10 SAMs with molecules containing thiol and short hydrocarbon chains are most commonly immobilized onto Au(111) substrates, and the immobilization process normally takes place in ethanol solution.11 The reason for choosing gold as the substrate for adsorption is that gold is relatively easy to clean and the monolayers are relatively easy to produce via a Au−S bond. The short hydrocarbon chain SAMs are simple molecules that exhibit all the properties and degrees of freedom necessary for complete monolayer formation. While there has been considerable work carried out on the structure and properties of equilibrated alkanethiol assemblies on Au, there have been relatively few studies of the kinetics of the formation process in relation to temperature. Kelvin et al.12 have studied the different carbon length of alkanethiols onto gold surfaces by surface plasmon resonance (SPR) and determined that the kinetics of adsorption could be described by the Langmuir equation. Kim et al.13 have studied the selfassembly process of octadecanethiol. Their results showed that there are two stages in the process: (1) rapid and disordered adsorption and (2) accumulation and rearrangement under the control of thermodynamics. In addition, the lower solubility is more conducive to forming the monolayer. QCM has been used to study the layer-formation process.14,15 Shimazu et al.16 studied layers formed from stirred ferrocenylundecanethiol in hexane at room temperature, and the total frequency shift suggested monolayer formation. © 2014 American Chemical Society

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EXPERIMENTAL SECTION

QCM and Sample Preparation. All solutions were of analytical grade; ultrapure water was generated by OMNI (Research Scientific Instruments Company) and was used when necessary. With regard to the effect of chain length on SAM stability,18 chemical reagent 11mercapto-1-undecanol (MUO) was used as obtained from Sigma. Ethanol is the most commonly used solvent for thiol molecules. It solvates a variety of thiol molecules, and it is available in high purity. Received: May 24, 2014 Revised: July 9, 2014 Published: July 22, 2014 9637

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Several studies suggest that SAM formation occurs faster in certain nonpolar solvents, e.g., heptane or hexane, than in ethanol.19,20 However, it seems that these SAMs are less organized than those formed in ethanol.21 Bain et al.20 found that from strongly diluted solutions in the micromolar range only imperfect monolayers are formed. Therefore, typically solutions in the millimolar range are used. In this study, a 5 mM thiol solution was prepared. QCM chips were from International Crystal Manufacturing, Inc with a nominal resonance frequency of 10 MHz. The chips consisted of a circular piezoelectric crystal (about 5 mm in diameter) sandwiched between gold electrodes. According to the Sauerbrey relationship,22 an increase in the mass on the gold surface would cause the oscillation frequency to decrease. Hence, the frequency change caused by the self-assembled monolayer was utilized to determine the kinetics of adsorption processes. In this study, a 1 Hz measured frequency (in the gas phase) corresponds to a 1.36 ng mass increase. When the SAMs were prepared under ambient conditions, the cleanliness of the gold substrate has a strong influence on the growth behavior.23 All QCM chips were cleaned by immersion in hot piranha solution (3:1 concentrated H2SO4/30%H2O2) for 30 min and then rinsed in ultrapure water followed by ethanol. Finally, chips were dried with high-purity nitrogen. To prepare a well-assembled thiol monolayer, the cleaned QCM chips were immersed in the thiol solution for 2, 4, 6, and 8 h with the temperature set at 20, 40, and 60 °C, respectively. Before resonance frequency measurements, the MUO-modified QCM chips were washed with ethanol followed by ultrapure water and then dried in a stream of high-purity nitrogen. Resonance Frequency Measurement. A prototype bondrupture device24 was used to carry out frequency measurement experiments. The resonance frequency of QCM chips was detected in three stages. The following three resonance frequencies were measured: (1) resonance frequency f 0 of the cleaned bare chips, (2) resonance frequency f1 of chip-immobilized thiol molecules, and (3) shift resonance frequencies Δf of QCM chips obtained by the difference between f 0 and f1. The surface coverage of self-assembled thiol molecules was characterized by the immobilization temperature and period. A prototype QCM device was used to detect the resonance frequency of the QCM chip in three stages: the bare chip, the chip immobilized with thiol molecules, and the chipe after the removal of thiol molecules. The experimental data also show that when thiol molecules were removed from the gold surface, the resonance frequency of the QCM chip was found to have the same value as that of the bare gold chip. The shift in resonance frequency caused by the immobilization of thiol molecules can therefore be utilized to quantitatively determine the coverage ratio of thiol molecules self-assembled on the Au(111) surface. The immobilization times of 2, 4, 6, and 8 h were used with temperature set at 20, 40, and 60 °C, respectively. Compared to the S−Au bond, the S−H bond is less stable due to oxygen oxidation. Hence, the chips were put into a holder immersed in MUO solution after purging with nitrogen.

Figure 1. Shift resonance frequency at different temperatures and immobilization times.

resonating crystal increases its inertia and lowers the resonance frequency by Δf Δf = −

2f0 2 A c66 ̅ ρq

Δm (1)

where f 0 is the resonance frequency (Hz), Δf is the frequency change (Hz), Δm is the mass change (g), c6̅ 6 is the shear modulus of quartz for AT-cut crystal (c6̅ 6 = 2.947 × 1011 g· cm−1·s−2), ρq is the density of quartz (ρq = 2.648 g/cm3), and A is the piezoelectric area (cm2). By formula derivation, eq 1 is expressed as Δf = −αβA′

(2)

where α = 2f 0, β = f 0/(c6̅ 6ρq) , and A′ = m/A. Assuming the adsorbed alkanethiol monolayer have a 31/2 ×31/2 R30° structure, the areal density A′ for 11-mercapto-1-undecanol is estimated to be 1.849 × 10−7 g/cm2, and the corresponding frequency shift for a monolayer is the same to the significant figures permitted by the instrument at 41 Hz. In the Langmuir adsorption kinetics,25 the rate of formation of the monolayers can be expressed as 1/2

dθ = ka(1 − θ )C − kdθ dt

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(3)

where θ is a unitless quantity which expresses the fraction of available sites that have reacted (or equivalently, the fraction of a monolayer formed), C is the thiol concentration in the solution, and ka and kd are the rate constants for the adsorption and desorption processes, respectively. Integrating eq 3 leads to

RESULTS AND DISCUSSION Experimental Results. Figure 1 show the effect of temperature and time on the shift resonance frequency which is directly related to thiol molecule immobilization onto one side of the gold surface. At the end of 2 h, only 9 Hz was detected at 20 °C whereas shift resonance frequencies of 17 ± 5, 24 ± 6, and 28 ± 5 Hz were detected after 4, 6, and 8 h, respectively. At 40 °C, the shift resonance frequency was increased to 18 ± 5 Hz by immobilized thiol molecules after 2 h and to 25 ± 5, 33 ± 6, and 35 ± 5 Hz after 4, 6, and 8 h, respectively. As compared to 20 and 40 °C, the thiol molecules were more easily adsorbed onto the gold surface at 60 °C. The value of Δf was up to 28 ± 5 Hz after 2 h of immobilization and 40 ± 5 Hz after 4, 6, and 8 h. According to the Sauerbrey equation,16 the deposition of a thin rigid overlayer with areal density A′ (g/cm2) onto a

C

θ (t ) =

C+

kd ka

(1 − e−(kaC + kd)t ) (4)

We set kobs = kaC + kd

(5)

where kobs is an apparent adsorption rate and K′ =

C C+

9638

kd ka

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Manipulating the above equations results in eq 7: θ(t ) = K ′[1 − exp(−kobst )]

(7)

According to the experimental results as shown in Figure 1, it can be seen that a coverage ratio of 1 corresponds to the shift resonance frequency at approximately 40 Hz. Hence the fractional coverage θ is equal to Δf/40. Figure 2 shows the fitting curves at different temperatures. This figure also shows the representative data for the surface

Figure 3. Comparable results between experiment and simulation.

and the environment of detection. First, the kinetic analyses were considered only for physisorption compared to experiments. Two stages of an adsorption process (i.e., physisorption and chemisorption) were therefore proposed as illustrated in Figure 4a,b. The randomly physisorbed MUO molecules would

Figure 2. Fitting curve of the relationship between time and the coverage ratio.

coverage ratio during the formation of the thiol layer on Au from a 5 mM HS (CH2)11OH/EtOH solution. The data fitted the Langmuir adsorption equation perfectly, with kobs = 1.01122 × 10−5, 4.43216 × 10−5, and 1.91041 × 10−4 s−1 at temperatures of 20, 40, and 60 °C, respectively. In addition, parameters ka, kd, and K′ were calculated using eqs 3 and 4, and the results are displayed in Table 1.

Figure 4. Schematic of two different adsorbed situations of molecules: (a) physisorption and (b) chemisorption.

Table 1. Kinetics Parameters at Different Temperatures −3

C = 5 × 10

20 °C 40 °C 60 °C

mol/L

−1

kobs (s ) 1.01122 × 10−5 4.43216 × 10−5 1.91041 × 10−4

−1

ka (mol/L)

0.58124 × 10−2 1.12367 × 10−2 3.78651 × 10−2

occupy more space since the physisorption is of a lower order of magnitude than chemisorption. The number of molecules in the physisroption stage is smaller than for chemisorption. Furthermore, the detection process was performed in the gaseous phase. However, in the simulation, the detection was assumed to be in the liquid environment. This is to mimic the work of Kelvin et al.15 In their study, the data for adsorption were determined in the liquid phase. Their kinetics of adsorption were calculated using Langmuir equation. In liquids, both adsorption and desorption processes (physisorption) occur simultaneously. However, in the experimental process, the thiol molecules were first adsorbed in the liquid onto the gold surface (chemisorption). After the chip was removed from the liquid, the coverage ratio was then obtained in the gaseous phase where the simultaneous adsorption and desorption processes would not occur. In this study, which is close to the most sensing applications’ scenario of SAMs, we propose a new kinetic model applicable to the detection of thiol molecules in the gaseous phase. The proposed kinetic model would overcome the discrepancies in the results experienced between experimental and simulation processes. Adsorption Mechanism. Kinetics studies of alkanethiol adsorption onto the Au(111) surface have shown two distinct adsorption kinetics: a very fast step (physisorption), which

−1

kd (s ) 1.89501 × 10−5 1.18622 × 10−5 0.17155 × 10−5

The results in Figure 2 indicate that the adsorption rate of layer formation increases with the rise in temperature. The results are logical as elevated temperatures increase the rate of dissociation of MUO molecules either physisorbed or chemisorbed onto the gold surface. A higher temperature therefore enhances the possibility of the system crossing activation barriers for processes such as chain reorganization and lateral rearrangements of the adsorbents as compared to those at room temperature. Simulation Results. The experimental values of the apparent adsorption rate with different temperatures were utilized to carry out the kinetics simulation by CARLOS.26 Figure 3 shows that the experimental results at three temperature settings do not data fit those obtained using the simulation process based on CARLOS theory. The experimental values appear to be higher. There are two plausible explanations that lie in the difference in the stage of adsorption 9639

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Figure 5. Kinetics model of alkanethiol adsorption: (a) adsorption process and (b) the proposed model.

atom, and blue ball is the entity which represents adsorbed hydrosulfuryl. Here, the lattice constant of the adsorbed interface is set to approximately 5 Å so that the adsorbed atoms are packed as 31/2 × 31/2 R30° on the surface.29 The area of simulation was set as 250 Å × 250 Å, which was a periodic boundary. The total time of adsorption was set at 30 000 s with a sampling interval of 1 s. Figure 6a,b presents the situation of chemisorption and the fitting curves of simulation data calculated using the proposed new model with the detection of thiol molecules in the gaseous phase and also the experimental results by QCM. The data show an almost perfect match between the results obtained using the simulation and experimental methods. Hydrosulfuryl was more easily dissociated and bonded to gold atoms due to the temperature increase which causes molecules to adsorb more energy over the barrier. This indicates that the adsorption process of MUO molecules on the Au(111) surface is in accordance with the Arrhenius equation and the adsorption rate of the model is related to the activation energy, the pre-exponential factor, and the temperature. Finally, the areal densities A′ for a monolayer at different temperatures were calculated based on eq 1, which were 1.237, 1.5468, and 1.894 g/cm2 at 20, 40, and 60 °C, respectively.

takes a few minutes, and a slow step (chemisorption), which lasts several hours. The thickness and contact angles reach their final values 27 at the end of both physisorption and chemisorption. In this study, chemisorption took place immediately after physisorption when hydrosulfuryl adsorbed onto Au(111) as shown in Figure 5a. However, according to the kinetic theory of adsorption, the velocity of physisorption was ignored because the physisorption step was too fast while only the chemisorption process was considered. In the chemisorption process, the activation barrier (Ea) is an important parameter which dominates the ease or complexity of reaction. Activation barriers (Ea) during the assembly process were calculated using the Arrhenius equation ln(k) =

−Ea + ln(A) RT

(8)

where k is the rate of reaction, T is the temperature (in Kelvin), A here is the pre-exponential factor (s−1), Ea is the activation energy (kJ/mol), and R is the universal gas constant. Alternatively, eq 7 can be expressed as ⎛k ⎞ E ⎛1 1⎞ ln⎜ 1 ⎟ = − a ⎜ − ⎟ R ⎝ T2 T1 ⎠ ⎝ k2 ⎠

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(9)

CONCLUSIONS In this work, the effect of different temperatures on immobilizing thiol molecules onto a gold surface was studied. The results obtained show that the rate of adsorption increased with a rise in temperature. In the process of monolayer formation, the rates of adsorption and desorption were both calculated using the Langmuir equation, with adsorption rates of 1.01122 × 10−5, 4.43216 × 10−5, and 1.91041 × 10−4 s−1 at 20, 40, and 60 °C, respectively. However, comparing the rates obtained using the simulation method based on the Langmuir equation with the detection of thiol molecule adsorption in the liquid phase did not match the experimental adsorption detection data obtained in the gaseous phase. A new kinetics model proposed using a simulation process with the detection of thiol molecule adsorption in the gaseous phase using the Arrhenius equation in the calculation results in a perfect match with the experimental results. The average value of the calculated activation energy is 59.738 kJ/mol, and the areal densities of adsorption were 1.237, 1.5468, and 1.894 g/cm2 at

Here it was considered that the rate of reaction is approximately equal to the apparent adsorption rate because physisorption was not considered. Hence, the activation energy at different temperatures as indicated in Table 1 can be calculated using eq 8, with the average value of the activation energy being Ea = 59.738 kJ/mol. This result is comparable to that obtained experimentally by Baradwlia et al.28 The related parameters are shown in Table 2. On the basis of this theory, a relevant adsorption interface was built as shown in Figure 5b. The yellow ball is the gold Table 2. Kinetics Parameters at Different Temperatures C = 5 × 10−3 mol/L 20 °C 40 °C 60 °C

kobs (s−1)

A (s−1) −5

1.01122 × 10 4.43216 × 10−5 1.91041 × 10−4

Ea (kJ/mol) −6

5.90793 × 10 4.58507 × 10−6 4.30836 × 10−6

Ea1 = 56.357 Ea2 = 63.265 Ea3 = 59.592 Ea = 59.738 kJ/mol 9640

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Figure 6. (a) Number of molecules after chemisorption for 8 h. (b) Comparable results between experiment and the new model.

20, 40, and 60 °C, respectively. Hence, the adsorption rate was related to the activation energy, pre-exponential factor, and temperature. In conclusion, the results of this paper will provide a theoretical platform for studying the effect of temperature on SAM stability and establishing a new avenue for QCM chip applications.

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(8) Frisble, C. D.; Martin, J. R.; Duff, R. R.; Wrighton, M. S. Use of High Lateral Resolution Secondary Ion Mass Spectrometry to Characterize Self-Assembled Monolayers on Microfabricated Structures. J. Am. Chem. Soc. 1992, 114, 7142−7145. (9) You, C. C.; De, M.; Rotello, V. M. Monolayer-Protected Nanoparticle−Protein Interactions. Curr. Opin. Chem. Biol. 2005, 9, 639−646. (10) You, C. C.; De, M.; Han, G.; Rotello, V. M. Tunable Inhibition and Denaturation of α-Chymotrypsin with Amino Acid-Functionalized Gold Nanoparticles. J. Am. Chem. Soc. 2005, 127, 12837−12881. (11) Schreiber, F. Structure and Growth of Self-Assembling Monolayers. Prog. Surf. Sci. 2000, 65, 151−256. (12) Kelvin, A. P.; Georgiadis, R. In situ kinetics of Self-Assembly by Surface Plasma Resonance Spectroscopy. Langmuir 1996, 12, 4731− 4740. (13) Kim, H. J.; Kwak, S.; Kim, Y. S. Adsorption kinetics of alkanethiols studied by quartz crystal microbalance. Thin Solid Films. 1998, 327-329, 191−194. (14) Luan, Y. F.; Li, D.; Wang, Y. W.; Liu, X. L.; Brash, J. L.; Chen, H. 125I-Radiolabeling, Surface Plasmon Resonance, and Quartz Crystal Microbalance with Dissipation: Three Tools to Compare Protein Adsorption on Surfaces of Different Wettability. Langmuir 2014, 30, 1029−1035. (15) Hromadová, M.; Pospíšil, L.; Sokolová, R.; Bulíčková, J.; Hof, M.; Fischer-Durand, N.; Salmain, M. Atrazine-Based Self-Assembled Monolayers and Their Interaction with Anti-Atrazine Antibody: Building of an Immunosensor. Langmuir 2013, 29, 16084−16092. (16) Shimazu, K.; Yag, I.; Sato, Y.; Uosaki, K. In situ and dynamic monitoring of the self-assembling and redox processes of a ferrocenylundecanethiol monolayer by electrochemical quartz crystal microbalance. Langmuir 1992, 8, 1385−1387. (17) Karpovich, D. S.; Blanchard, G. J. Direct Measurement of the Adsorption Kinetics of Alkanethiolate Self-Assembled Monolayers on a Microcrystalline Gold Surface. Langmuir 1994, 10, 3315−3322. (18) Israelachvili, J. N. Intermolecular and Surface Forces; Academic Press: San Diego, CA, 1994. (19) Peterlinz, K. A.; Georgiadis, R. In Situ Kinetics of Self-Assembly by Surface Plasmon Resonance Spectroscopy. Langmuir 1996, 12, 4731−4740. (20) Bain, C. D.; Troughton, E. B.; Tao, Y. T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. Formation of monolayer films by the spontaneous assembly of organic thiols from solution onto gold. J. Am. Chem. Soc. 1989, 111, 321−335. (21) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Self-Assembled Monolayers of Thiolates on Metals as a Form of Nanotechnology. Chem. Rev. 2005, 105, 1103−1169.

AUTHOR INFORMATION

Corresponding Author

*Tel: 28 8760-0980. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge support from the National Natural Science Foundation of China under General Program Funds 30870664 and 31170954 to Y.J.Y.

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