Dynamic Properties of Self-Assembled Monolayers of Mercapto Oligo

Jul 1, 2014 - Department of Information Science and Biomedical Engineering, Graduate School of Science and Engineering, Kagoshima University, 1-21-40 ...
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Dynamic Properties of Self-Assembled Monolayers of Mercapto Oligo(ethylene oxide) Methyl Ether on an Oscillating Solid−Liquid Interface Minoru Yoshimoto,*,† Keita Honda,† Shigeru Kurosawa,‡ and Mutsuo Tanaka*,§ †

Department of Information Science and Biomedical Engineering, Graduate School of Science and Engineering, Kagoshima University, 1-21-40 Korimoto, Kagoshima 890-0065, Japan ‡ Research Institute for Environmental Management Technology, National Institute of Advanced Industrial Science and Technology (AIST), AIST Tsukuba West, 16-1 Onogawa, Tsukuba, Ibaraki 305-8569, Japan § Biomedical Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Central 6, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8566, Japan S Supporting Information *

ABSTRACT: The dynamic properties of chemisorbed soft matter on a solid−liquid interface oscillating at megahertz were investigated using a quartz crystal microbalance (QCM). As a chemisorbed soft matter, we employed the self-assembled monolayers of mercapto oligo(ethylene oxide) methyl ethers, HS(CH2CH2O)nCH3 (n = 5, 11, 12, 19, 27, 35, and 43), where those molecular weights had unity. The systematic analyses on the basis of the Voight model revealed that the molecular thickness moving with the solid−liquid interface oscillating at megahertz frequencies, the resonant length, is 8.8 nm, where the frequency dependence of the resonant length is not considered. On the other hand, the analyses based on the Debye process revealed that the logarithm of the resonant length linearly decreases with the logarithm of 2πF, where F is the frequency of the QCM, and varies from 17.3 (9 MHz) to 12.4 nm (81 MHz). Those values in the Debye process were within twice that of the Voight model and were approximately consistent with that in the Voight model. On the basis of the experimental data, we proposed the equation with the resonant length of HS(CH2CH2O)nCH3 as a function of frequency. Moreover, we discussed the difference between the chemisorbed and the physisorbed molecules on the solid−liquid interface oscillating at megahertz frequencies.

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polymers, and synthetic polyelectrolytes from solutions.20−23 Moreover, the simultaneous measurement of the resonantfrequency shift and energy dissipation shift has enabled the QCM to supply its utilization as a high-frequency interfacial rheometer.24 This finding has adrastic impact on the field of biosensors with soft elements. However, the theory of the QCM rheometer is based on the Voight model, consisting of a spring and a dashpot in parallel, without considering the exact physical model. Therefore, in applying the QCM rheometer as a biosensor, it is essential to clear the physical properties of soft matter molecules on an oscillating solid−liquid interface. In a Newtonian liquid, the thickness moving with an oscillating plate, resonant length, is called viscous penetration depth. In the case of pure water, it is reported to be 177 nm with 9 MHz at 25 °C.25 In the present day, this concept is applied to soft matter dissolved in liquid and adsorbed on a solid−liquid interface.22,23 However, soft matter has a resonant length corresponding to its inherent relaxation time. Recently, we have employed poly(ethylene oxide) (PEO) as a model of

INTRODUCTION Adsorption of soft matter (proteins, DNA, lipid, polymers, and so on) onto a solid−liquid interface has been the focus of a considerable research effort from the point of view of theory and experiment for several decades.1−19 This interest is due to the importance of a thin coating of soft matter in many applications, including the field of biosensors.20−23 A biosensor is an analytical device composed of a biological sensing element integrated within a signal transducer. Its main feature is the high selectivity and sensitivity on the basis of molecular recognition between a biological sensing element and an analyte. The molecular recognition alters the properties of the sensor surface and enables suitable transducers to convert the biochemical interaction into the electronic information translated into a quantitative signal. As one of those transducers, a quartz crystal microbalance (QCM) is useful.22,23 The QCM techniques rely on the quartz crystal operating in thickness-shearing mode at megahertz. A small amount of mass deposited on the surface of the QCM leads to its resonantfrequency shift. Thus, the mass changes on the subnanogram order can be detected by the measurement of a resonantfrequency shift. Therefore, the QCM has traditionally been used as a mass sensor for adsorption of, for example, proteins, © 2014 American Chemical Society

Received: November 14, 2013 Revised: June 23, 2014 Published: July 1, 2014 16067

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was mounted level with the water surface and the immersion depth of the QCM was set at 0.5 cm. The temperature of cell was kept at 25 ± 0.1 °C. Under this condition, we carried out the impedance measurement of the QCM. An impedance analyzer (Agilent Technologies 4395A) was used for the measurement of impedance properties of the QCM (Figure S1a, Supporting Information). The impedance data and phase data associated with 801 frequency data points centered at the frequency of minimum impedance were recorded on a personal computer. The values of the resonant-frequency shift, ΔF, and the energy dissipation shift, ΔD, at the first, third, fifth, seventh, and ninth overtones of a fundamental resonant frequency were calculated using admittance analysis.20,22 Cyclic Voltammetry Measurement. The dry mass of the SAM on a gold electrode was determined using cyclic voltammetry (CV) for calculation of ρeff and heff, where ρeff and heff are the effective density and effective thickness of a thin layer, respectively. The treatment of the QCM prior to measurements in the CV was the same as that of the impedance measurements. The electrochemical reductive desorption method on a gold surface was adopted to determine the molecular concentration of a thin layer.35 The CV was carried out in 1 M potassium hydroxide solution at a room temperature, using a PAR 263A (EG&G Princeton Applied Research) potentiostat equipped with an external potential scanner (model 175, EG&G PARC). The sweep rate was 20 mV/s and the electrolyte solution was deaerated by flowing nitrogen. The saturated calomel electrode (SCE) and Pt wire were used as reference and counter electrodes for the electrochemical measurements, respectively.

soft matter and have reported the resonant length of PEO molecules on an oscillating solid−liquid interface.25 When a 9 MHz QCM was immersed into PEO solutions, the viscous penetration depth and the physisorbed thin layer were generated on the QCM. In this case, it was found that the resonant length of PEO with the oscillating plate of 9 MHz is 5.4 nm at 25 ± 0.1 °C in both regions of the viscous penetration depth and physisorbed thin layer. This value is the inherent resonant length of PEO corresponding to the relaxation time at 9 MHz. In general, it is recognized that the resonant length of soft matter is roughly 10 nm on the order of megahertz.26 This value is much smaller than that for pure water, 177 nm. On the other hand, the information about the resonant length of chemisorbed molecules on the solid−liquid interface oscillating at MHz frequencies is still not obtained. In many cases of biosensors, self-assembled monolayers (SAMs) composed of oligo(ethylene oxide) have been employed to prevent nonspecific adsorption of soft matter onto a solid−liquid interface.12,27−34 Alkanethiols are known to form SAMs through chemisorption on a gold surface with bond formation. Thus, we considered that the SAMs composed of oligo(ethylene oxide) could be useful as a model of chemisorbed soft matter on an oscillating solid−liquid interface. Therefore, in the present paper, we examined in detail dynamic properties for the chemisorbed molecules of oligo(ethylene oxide) on the solid−liquid interface oscillating at megahertz. In concrete terms, with the help of the QCM, we focused on determination for the resonant length of SAMs moving with an oscillating solid−liquid interface, where a hydrophilic gold surface was employed. We investigated the resonant length corresponding to the intrinsic relaxation time of an oligomer molecule based on the Voight model and the Debye process. Moreover, these analyses enable us to discuss the difference between the chemisorbed and physisorbed molecules on an oscillating solid−liquid interface. In this paper, we used the SAMs constructed from mercapto oligo(ethylene oxide) methyl ethers, HS(CH2CH2O)nCH3 (n = 5, 11, 12, 19, 27, 35, and 43). The compounds, HS(CH2CH2O)nCH3, where n = 5, 11, 12, 19, 27, 35, and 43, hereafter are referred to as (EO)5, (EO)11, (EO)12, (EO)19, (EO)27, (EO)35, and (EO)43, respectively.

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RESULTS AND DICUSSION

In all the experiments, we used the SAMs of (EO)n with closedpacked mass on the gold electrode of the QCM. The closedpacked condition was verified by the QCM or CV. These results of experiments indicated that the immersion time of more than 24 h in compound solutions is good enough to construct the SAMs with closed-packed condition on the gold electrode (Figures S2 and S3, Supporting Information). Therefore, the QCM was left in compound solutions for 24 h prior to the impedance or CV measurements. In order to examine the dynamic properties of SAMs, the two types of analyses of the Voight model and the Debye process were employed. First, we investigated the SAMs based on the Voight model and next the Debye process. Resonant Length on the Basis of the Voight Model. We measured the values of ΔF and ΔD of the seven types of compounds at the first, third, fifth, seventh, and ninth overtones of a fundamental resonant frequency (Figure S4, Supporting Information). In all the compounds, the ΔF values increase linearly with the resonant frequency, and the ΔD values decrease linearly. In order to obtain insight into the resonant length of SAM, using the data of Figure S4 (Supporting Information), we calculated the sensed mass, mQCM, the effective hydrated density, ρeff, and the effective hydrated thickness, heff, of SAM according to the equations based on the Voight model. The expression for the signal changes due to SAM in a Newtonian liquid can be derived in the form24,36

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EXPERIMENTAL SECTION Synthesis of Mercapto Oligo(Ethylene Oxide) Methyl Ethers. The compounds (EO)5, (EO)11, (EO)12, (EO)19, (EO)27, (EO)35, and (EO)43 were synthesized by the methods described in the Supporting Information, where those molecular weights had unity. In other words, the variance values of molecular weights were 0. Sample Preparation. A 9 MHz AT-cut QCM with a pair of gold electrodes was used for all the experiments. The QCM was purchased from Nihon Dempa Kogyo (Tokyo, Japan). One side of the QCM was sealed with a blank quartz crystal casing, maintaining it in an air environment (Figure S1b of Supporting Information). Aqueous 1 mM solutions of (EO)n were employed in all the experiments. The SAMs were prepared by immersing the QCMs in (EO)n solutions at 25 ± 0.1 °C. The QCMs with SAMs were rinsed with pure water after construction of the SAMs and were used for the impedance and CV measurements. QCM Measurement. The cell was 8 mL and had a water jacket to maintain constant temperature. The QCM with SAM was immersed into the cell with pure water, where the QCM 16068

dx.doi.org/10.1021/jp411186n | J. Phys. Chem. C 2014, 118, 16067−16073

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ΔD =

⎧ ⎫ ηsωq 2 hsρω ⎪ s q ⎪ ⎬ ⎨1 − ηbρb 2 2 2 ⎪ ⎪ 2πρq hq ⎩ ρs (μs + ωq ηs ) ⎭

molecular lengths of (EO)35 and (EO)43 are longer than the resonant length. Therefore, next, we investigated the heff values of the SAMs. Using eqs 4 and 5, we can calculate heff for the five types of compounds up to (EO)27. However, we cannot estimate the exact values of heff for (EO)35 and (EO)43, because the values of mQCM are smaller than those of mCV. In other words, we cannot evaluate the ρeff values. On that account, we need to estimate those by the other method. Here, we attempted to evaluate the heff values for the SAMs of (EO)35 and (EO)43 from the tendency of the relative water content in SAM, w%, in relation to the molecular weight of (EO)n, Mn. The w% value was calculated by37,38 mQCM − mCV w% = 100 × mQCM (6)

(1)

μω s q

2hsηbρb ρq hq

Article

2

μs + ωq 2ηs 2

(2)

where ωq is the angular frequency of the QCM, hq and ρq are the quartz crystal thickness and density, ηb and ρb are the bulk liquid viscosity and density, and hs, ρs, μs, and ηs are the thickness, density, elastic shear modulus, and shear viscosity of the SAM, respectively. Furthermore, the mass per unit area, mQCM, is obtained as mQCM = hsρs

(3)

The mQCM value means the mass of the SAM, including the mechanically trapped water. The CV was used as a complementary technique. The CV can measure the number of molecules bound to gold, i.e., the dry mass of SAM. The CV charts for electrochemical reductive desorption of (EO)n are shown in Figure S3 of the Supporting Information. The values of ρeff and heff of the SAM can be yielded by combining the CV and impedance data as mQCM − mCV m ρeff = ρW + ρs CV mQCM mQCM (4)

heff =

On the basis of eq 6, we calculated the w% values. Figure 2 indicates that the w% values increase with Mn, and have the

mQCM ρeff

(5)

where mCV is the mass per unit area measured by the CV, ρW is the water density of 997 kg/m3, and ρs is the density of SAM estimated as 1200 kg/m.27,28,37,38 Using the data of harmonic overtones of a fundamental resonant frequency, the parameter values of eqs 1 and 2 were evaluated using the genetic algorithm in MATLAB. Figure 1

Figure 2. Relative water content of the SAM as a function of the molecular weight of (EO)n. The error bar represents a standard deviation. Measurements were repeated six times. The values of relative water content of (EO)35 and (EO)43 were estimated from the tendency of the relative water content against molecular weight and were supposed as 20%. Those are denoted as ○.

constant value of 20% above (EO)19. This result means that the w% values for (EO)35 and (EO)43 are able to be evaluated as 20%. In addition, we assumed that the water included in the SAM is uniformly distributed, because eqs 1 and 2 are employed under the uniform thin layer (Figure 3). Under the conditions of a uniform thin layer and 20% of w% in the SAMs of (EO)35 and (EO)43, we evaluated the heff values using eqs 1−5. Figure 4 indicates the thickness of SAM as a function of Mn. The theoretical values of thickness of the 7/2 helical and the all-trans conformation against Mn are also illustrated in Figure 4. The heff values increase with Mn up to (EO)27, and have the values between the 7/2 helical and the alltrans conformation. In contrast, those show the constant value of 8.8 nm above (EO)27. Since heff have to exist between thicknesses of the 7/2 helical and the all-trans conformation, this value of 8.8 nm is considered as the resonant length in the chemisorbed SAMs of (EO)n. However, this value does not consider the frequency dependence of the resonant length. Therefore, we attempted to investigate it in the next section. Resonant Length on the Basis of the Debye Process. In this section, we discuss the frequency dependence of the

Figure 1. Mass as a function of the molecular weight of (EO)n. ● and ■ denote mQCM and mCV, respectively. The error bar represents a standard deviation. Measurements were repeated six times.

shows the results for mQCM and mCV as a function of the molecular weight of (EO)n. The mCV values increase linearly with the molecular weight of (EO)n. On the other hand, the mQCM values become constant above (EO)27. In general, the mQCM values are larger the mCV values. However, the values of mQCM of (EO)35 and (EO)43 are smaller than those of mCV. This means that the QCM cannot accurately measure the mass values of those SAMs. That is, it is considered that the 16069

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Figure 5. Sensitivity at 9 MHz as a function of the molecular weight of (EO)n. The error bar represents a standard deviation. Measurements were repeated six times. The solid line shows the value fitted to the experimental data using the real part of eq 9 Figure 3. Illustration of the SAM of (EO)n on gold. The water molecules are uniformly distributed in the SAM.

J *(ω) =

ΔJ 1 + iωτ

(8)

where ΔJ is the relaxation strength of a molecule, τ is the relaxation time of a molecule, and ω is the angular frequency of an oscillating plate.25,39 Separation into the real and the imaginary part yields J *(ω) = J ′ − iJ ″ =

resonant length from the viewpoint of the sensitivity in the QCM. The sensitivity, Sn, is able to be calculated using the following equation:

ΔF mCV

(9)

This equation shows that the imaginary part has the maximum value at ω = 1/τ. That is, the molecule has the maximum energy loss at this point and resonates with an oscillating plate. Furthermore, at this point, the value of the real part is ΔJ/2. This means that ΔJ/2 is related to the resonant length. Therefore, we adapted this concept to the present experimental data. In our experiments, Sn corresponds to J′ of eq 9 and Mn is comparable to ωτ. Therefore, we fitted the real part of eq 9 to the experimental data of Sn (Figure 5). In this case, the data points do not cover the whole range. However, this fitting is sufficient to evaluate the value of ΔJ/2, because the real part of eq 9 has the point symmetry at ΔJ/2. As a result, we found that the value of Sn corresponding to ΔJ/2 is 9.9 × 107 Hz/(g cm−2). Next, we estimated the relationship between ΔJ/2 and the resonant length of (EO)n. From Figure 5, the Mn value for 9.9 × 107 Hz/(g cm−2) is 2.4 × 103 g/mol. In this molecular weight, the theoretical molecular lengths of the 7/2 helical and the all-trans conformation are 15.2 and 19.4 nm, respectively.27 This means that the resonant length at 9 MHz exists between 15.2 and 19.4 nm. In the same way as 9 MHz, we evaluated the values of Mn for other harmonics of the QCM (Figure S5, Supporting Information). Generally, in polymer, the relationship between τ and Mn is given by

Figure 4. Thickness of the SAM as a function of the molecular weight of (EO)n. ● denotes the values calculated from the experimental data. The error bar represents a standard deviation. Measurements were repeated six times. ■ and ▲ denote the thickness values of the 7/2 helical and the all-trans conformation calculated on the basis of ref 27, respectively.

Sn =

ΔJ ΔJωτ −i 2 2 1+ωτ 1 + ω 2τ 2

(7)

In all the SAMs constructed from the seven types of (EO)n, we evaluated the Sn values at 9 MHz on the assumption that the structures of SAMs are uniform and w% has a constant value of 0%. In this case, the error of the SAM density is estimated as ca. 3% at w% = 20%. Figure 5 shows the result in this approximation. The values of Sn against Mn vary in a singletime relaxation process. This result means that we can evaluate the resonant length using the Debye process. In the Debye process, the dynamical compliance, J*(ω), is given by

τ ∝ Mnν

(10)

where ν is a constant.39,40 According to eq 10, log Mn was plotted against log ω, where ω = 1/τ. Figure 6a shows that log Mn decreases linearly with log ω. This means that the relationship between ω and Mn follows eq 10. Next, we calculated the length values of the 7/2 helical and the all-trans 16070

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Figure 6. Frequency dependence of (EO)n. (a) Frequency dependence of Mn. (b) Frequency dependence of the resonant length. ● and ■ denote the theoretical values of thickness of the 7/2 helical and the all-trans conformation, respectively. The solid line shows the value calculated from eq 11

conformation corresponding to Mn. Those logarithm values also decrease linearly with log ω (Figure 6b). Here, we introduce the average value for lengths of the 7/2 helical and the all-trans conformation as the resonant length, RL (m). This attempt leads to the following relationship: log RL = − 6.591 − 0.151 log ω

difference of the resonant length. However, we reported that the resonant length of PEO in the region of the viscous penetration depth is the same as that of the physisorbed PEO.25 This fact may indicate that the force of the physical adsorption between gold and PEO on the solid−liquid interface is weak and then that the slip phenomenon occurs on its interface, where the slip phenomenon means that the oscillating QCM electrode and the adjacent PEO do not move at the same velocity.20,41 As a result, both the structure and the slip phenomenon may cause differences between the physical and the chemical adsorption. That is, we suggest the following two points: (1) The resonant length of (OE)n in the chemical adsorption is some 3 times larger than that of the physical adsorption, and (2) in the physical adsorption of (OE)n, the slip phenomenon occurs on solid−liquid interface.

(11)

The value calculated with eq 11 is shown in Figure 6b. This estimation of the resonant length is useful to obtain the frequency dependence of resonant length on (EO)n. For example, we are able to estimate 24.1 nm from eq 11 with 1 MHz. The oscillation amplitude of (EO)n molecule decreases exponentially with distance from the oscillating solid−liquid interface (Figure S6, Supporting Information). The resonant length means the thickness corresponding to 1/e of the oscillation amplitude, A, of the oscillating solid−liquid interface of the QCM. Comparison between Analyses of the Voight Model and the Debye Process. The resonant length in the Voight model is 8.8 nm. On the other hand, the resonant length estimated from eq 11 is dependent on the frequency and varies from 17.3 (9 MHz) to 12.4 nm (81 MHz). The value of 8.8 nm in the Voight model is roughly near the one of 12.4 (81 MHz) in the Debye process. The value of 8.8 nm may be affected by the data of high frequencies, because we used those in the analysis due to the Voight model. At any rate, those values of the Debye process are within twice the value of the Voight model and are approximately consistent with that of the Voight model. Comparison between the Physical and the Chemical Adsorption. We discuss the difference between the physical and the chemical adsorption. In a previous paper, based on the Debye process, we estimated that the resonant length of PEO in the physical adsorption as 5.4 nm at 9 MHz.25 On the other hand, that of (OE)n in the chemical adsorption is estimated as 17.3 nm. This value is some 3 times larger than that of the physical adsorption. The PEO molecules in the physical adsorption are bound to the solid−liquid interface in the random coil configuration. On the other hand, the compounds of (OE)n in the chemical adsorption are bound to it in the amorphous structure on the basis of the 7/2 helical and the all-trans conformation. The structure of the physisorbed thin layer is different from that of the chemisorbed one. It is considered that the viscoelasticity, dependent on the structure of thin layers, also affects the

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CONCLUSIONS We studied the dynamic properties of the chemical adsorption on an oscillating plate using SAMs of (OE)n. This study revealed the following novel points: (1) in the case of the Voight model, the resonant length of (EO)n in the chemical adsorption is estimated as 8.8 nm, and (2) in the case of the Debye process, the resonant length of (EO)n in the chemical adsorption is dependent on the QCM frequency and varies linearly according to eq 11. That is, in the chemical adsorption, we defined the resonant length of (EO)n and found a novel relationship between resonant length and frequency. The above results indicate the physical properties of the SAMs constructed from (OE)n. However, the soft matter, such as proteins, DNA, lipid, and polymers, may also show the same behavior. In the mass measurement of the QCM, the resonant length of (OE)n in this work may provide the index of the measurable limit of soft matter.

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ASSOCIATED CONTENT

* Supporting Information S

Synthesis of compounds, schematic illustration of an experimental apparatus (Figure S1), time series of (EO)n (Figure S2), CV charts of electrochemical reductive desorption of (EO)n (Figure S3), ΔF and ΔD against the frequency of the QCM (Figure S4), sensitivity as a function of molecular weight of (EO)n (Figure S5), and schematic diagram on the oscillating 16071

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solid−liquid interface of the QCM (Figure S6). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*M.Y. e-mail: [email protected]. *M.T. e-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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