Article Cite This: Langmuir XXXX, XXX, XXX−XXX
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Investigation of Local Hydration Structures of Alkanethiol SelfAssembled Monolayers with Different Molecular Structures by FMAFM Akito Fujita, Kei Kobayashi, and Hirofumi Yamada* Department of Electronic Science and Engineering, Kyoto University, Kyoto 615-8510, Japan
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S Supporting Information *
ABSTRACT: Hydration structures play crucial roles in a wide variety of chemical and biological phenomena. However, the key factors that determine a hydration structure remain an open question. Most recent studies have focused on the electrostatic interactions between the surface charges and dipoles of water molecules, which are determined by the atomic/ionic species of the outermost solid surface, as the dominating factor. The number of studies on the correlation between the hydration structure and the atomic-scale surface corrugation has been limited. In this study, we investigated the hydration structures of alkanethiol self-assembled monolayers terminated with a hydroxyl group using frequency-modulated atomic force microscopy. We observed two molecular structures, namely, the (√3 × √3)R30° structure and the c(4 × 2) superlattice structure, and found that their hydration structures are different mainly because of the slight differences in their molecular arrangements. This result suggests that a slight difference in the molecular/atomic arrangements as well as the atomic/ionic species in the outermost solid surface strongly influences the local hydration structures.
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well as surface atoms and molecules.8−10 We recently developed a technique to visualize two-dimensional (2D) and three-dimensional local hydration structures with a subnanometer scale resolution based on the force mapping technique by FM-AFM.11,12 By this method, one can measure the in-plane hydration structure in addition to the out-of-plane hydration structures on any surface areas of concern, which is in contrast to the fact that the conventional analytical techniques only provide information about the out-of-plane hydration structures averaged over a certain area. As FM-AFM allows us to image surface structures and visualize the local hydration structures in real space at the same time, it is an ideal tool to investigate the relationship between the atomic-scale arrangement of surface atoms and hydration structures. We applied this technique to study the local hydration structures on various alkali halides surfaces and found that the in-plane hydration structures depend on the atomic periodicity of the surface with respect to the size of a single water molecule.13 More recently, we applied this technique to study the local hydration structures on various silicate surfaces, such as albite and apophyllite, and reported that the local hydration structures depend on the arrangements of the silicon−oxygen tetrahedral at the surface.14 In this study, we investigated the relationship between the local hydration structures and the molecular arrangements of self-assembled monolayers (SAMs)
INTRODUCTION The local structure of water molecules facing a solid surface, called a hydration structure, is known to significantly vary from the bulk structure and affect the phenomena occurring at the interface, such as crystal growth,1 biofunctions,2 and chemical reactions.3 Hydration structures of various substrates have been intensively studied by X-ray/neutron reflectivity4,5 and sum-frequency generation,6 but the relationship between the surface and the hydration structures is quite complicated and still not fully understood. The hydration structures result from a competition of two different interactions; that is, the interaction between the water molecules and the surface and the interaction between the water molecules themselves. As the former interaction is governed by the electrostatic interactions between the surface charges and dipoles of water molecules, the arrangement of water molecules at the interface is determined by the atomic/ionic species of the outermost solid surface. However, as water molecules have a finite volume, the hydration structures are significantly influenced by the atomic-scale arrangement of surface atoms and ions with respect to the size and shape of the water molecules.7 Even for an atomically flat surface, the corrugation of surface atoms may range from 10 to several tens of picometers.4 Although these numbers are very low, they are comparable to a fraction of the size of a single water molecule and large enough to reconstruct the entire hydration structure. In the last decade, frequency-modulated atomic force microscopy (FM-AFM) has proven to be capable of atomic-/molecular-scale imaging of local hydration layers as © XXXX American Chemical Society
Received: September 7, 2018 Revised: October 30, 2018 Published: November 15, 2018 A
DOI: 10.1021/acs.langmuir.8b03052 Langmuir XXXX, XXX, XXX−XXX
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in a 0.1 M KCl solution to suppress any long-range electrostatic forces.11 Experiments were conducted with a lab-developed FM-AFM based on a commercial apparatus (SPM-9600, Shimadzu). The frequency shift was detected by a homemade phase-locked loop detector, and the controller was programmed based on LabVIEW (National Instruments).12 Silicon cantilevers with backside gold coating were used. UV ozone cleaning of the cantilevers was conducted for 15 min prior to their use. This not only stabilized the experiments by removing any contamination from the tip, but also hydrated the tip apex which was crucial to obtain reproducible hydration measurements. The spring constant and resonance frequency were deduced by conducting a fast Fourier transformation on the Brownian spectrum.25 The typical spring constant and resonance frequency in a liquid were 20 N/m and 140 kHz, respectively. The topographic images were obtained in the XY scan mode with an amplitude of 1 nm peak-topeak such that the tip does not follow the hydration layers but the molecular surface, whereas the hydration structures were measured with an amplitude of 0.3 nm peak-to-peak. The hydration structures were measured by a previously reported 2D force mapping technique.12 Frequency shift versus distance curves were consecutively collected along the surface in the ZX scan mode and used to construct a 2D frequency shift map. The frequency shift versus distance curves were converted to force versus distance curves using the Sader method.26 The 2D force maps were obtained after the topographic images were acquired. Data processing was conducted using WSxM27 or lab-made Python programs.
in order to gain more insight into the local hydration structures at the solid−liquid interfaces and their effects on the surface phenomena. Alkanethiol SAMs on gold have been widely used for surface modification and applied in the fields of electronics15 and nanopatterning.16 The molecules are known to form densely packed SAMs on the Au(111) surface, which are characterized as the (√3 × √3)R30° structure and the c(4 × 2) superlattice structure. In both structures, the molecules are hexagonally packed on the gold surface with a distance of 0.5 nm between each other. Although the molecules are tilted by about 30° from the surface normal in both structures, the arrangement of the molecules are uniform and slightly different in the (√3 × √3)R30° structure and the c(4 × 2) superlattice structure, respectively.17 It has been reported that the height of the end groups from the Au(111) surface is slightly different in the c(4 × 2) superlattice structure, presumably because of the difference in the twist angle of the molecule.18,19 These two molecular structures are ideal samples for investigating the relationship between the local hydration structures and the molecular arrangements as they have the same surface composition and surface charges and only differ in their surface molecular arrangements. As these molecular structures are stabilized by a van der Waals force between the alkyl chains, the structures depend on the length of the alkyl chain. When the chain length is long, such as around 16 or more, the (√3 × √3)R30° structure is frequently observed, whereas for short chain lengths of around 8 or lower, the c(4 × 2) superlattice structure is dominant. For intermediate chain lengths of around 10, the two structures coexist,20 and the molecular structures can be controlled by post-annealing.21 In this study, we investigated the hydration structures of hydroxyl(OH)-terminated alkanethiol SAMs with the (√3 × √3)R30° structure and the c(4 × 2) superlattice structure using FM-AFM. We chose the OH-terminated alkanethiol SAMs because they are hydrophilic and their hydration structures have often been studied.22,23 We fabricated the OH-terminated alkanethiol SAMs with the (√3 × √3)R30° structure and the c(4 × 2) superlattice structure and investigated the local hydration structure on these molecular structures. We compared the hydration force versus distance curves measured on the two molecular structures and discussed the relationship between the local hydration structures and the molecular arrangements.
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RESULTS AND DISCUSSION We first prepared C11OH SAMs at room temperature and found that the two structures coexisted. However, the domains were small, and the density of defects was high, which made it difficult to reproducibly measure the hydration structure in a single domain. The influence of immersion temperature on SAM quality is presented in the Supporting Information. It has been reported that post-annealing of alkanethiol SAMs fabricated in the gas phase enhances the coverage ratio of the c(4 × 2) superlattice structure21 and that the preparation of alkanethiol SAMs in a heated solution enlarges the domain size.28,29 Referring to these publications, we heated the sample during immersion and obtained the SAMs with the c(4 × 2) superlattice structure and a large domain size. Therefore, we studied the (√3 × √3)R30° structure domain by the SAMs prepared at room temperature and the c(4 × 2) superlattice structure domain prepared at 78 °C. Figure 1a shows a large-scan topographic image of the C11OH SAM prepared at 78 °C. We can see etch pits and domain boundaries that are typically observed for the alkanethiol SAMs.30 Figure 1b,c shows small-scan topographic images taken on the C11OH SAM prepared at room temperature and 78 °C, respectively. All images in Figure 1 were obtained in 0.1 M KCl solutions. The hexagonal structure observed in Figure 1b is the (√3 × √3)R30° structure. On the other hand, the molecules observed in Figure 1c showed a “zigzag”-like pattern with a rectangular unit cell, that is the c(4 × 2) superlattice.31 We found that the molecules in the unit cell have four different height states with the maximum height difference of around 50 pm, which well agrees with previous scanning tunneling microscopy (STM) investigations of CH3and OH-terminated alkanethiol SAMs.32,33 It should be noted that the c(4 × 2) superlattice structure has also been reported for OH-terminated alkanethiol SAMs under aqueous conditions by electrochemical STM34,35 and FM-AFM.23 We followed the nomenclature commonly used for CH 3terminated alkanethiol SAMs. We found the molecular packing
METHODS
11-Mercapto-1-undecanol (C11OH, purity 97%) was purchased from Sigma-Aldrich, and ethanol (purity 99.5%) was purchased from Kishida Chemical. Mica substrates purchased from Furuuchi Chemical were mounted on a heating stage and heated at 430 °C for more than 24 h in a vacuum chamber.24 Gold was then deposited on the mica substrate in vacuum at a thickness rate of 0.1 nm/s to obtain an atomically flat Au(111) film with a thickness of about 150 nm. We continued heating the gold film for at least 90 min after the deposition to let the gold atoms migrate and form large terraces. The gold films were then immersed in an ethanol solution containing alkanethiol molecules at a concentration of 10 μM. The samples were then kept at 78 °C in an oven or at room temperature for 2 h to several days. Heating the sample during SAM fabrication reduced the number of defects and formed wide domains. The samples were rinsed with pure ethanol and deionized water and dried in flowing nitrogen gas before observation. The measurements were performed B
DOI: 10.1021/acs.langmuir.8b03052 Langmuir XXXX, XXX, XXX−XXX
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Figure 1. Topographic images of C11OH SAM obtained in 0.1 M KCl solutions. (a) Large-scan topographic image of a C11OH SAM. (b) Molecular-scale topographic image and schematic of the (√3 × √3)R30° structure. (c) Molecular-scale topographic image and cartoon of the c(4 × 2) superlattice structure (ζ-phase).
Figure 2. (a,b) 2D frequency shift maps taken on the (√3 × √3) R30° structure and the c(4 × 2) superlattice structure of C11OH SAMs, respectively. Red and blue arrows indicate the positions where the frequency shift versus distance curves in Figure 3a,b were extracted, respectively. Orange curve represents the height of the tip at which the frequency shift reached the threshold, which corresponds to the cross-sectional profiles of molecular surfaces, which are schematically drawn in (c,d). White and black arrows in Figure 3a,b and 3c,d, respectively, show the directions where the frequency shift vs distance curves were consecutively collected.
in the domain observed in Figure 1c to be very close to that of the ζ-phase among the various phases of the c(4 × 2) superlattice structure (β-, γ-, δ-, ε-, and ζ-phases),17,36 whereas the (√3 × √3)R30° structure as shown in Figure 1b is referred to as the α-phase. Although the origin of the c(4 × 2) superlattice structure is out of the scope of this paper, we will briefly review the previous studies. The c(4 × 2) superlattice structure has been reported not only by STM but by various methods such as helium diffraction,18 low-energy electron diffraction,37 grazing incidence X-ray diffraction,38,39 and FM-AFM.31 Early studies suggested that the two states were due to different tilt or twist angles of the alkanethiol molecule,18,40 but recent papers propose that the structure is derived from the complex gold/ sulfur interface.41 Recently, theoretical work has suggested that the height modulation is caused by steric effects of the neighboring alkanethiol molecules which are derived from the complex gold/sulfur interface.42 Figure 2a,b shows the 2D frequency shift maps of the (√3 × √3)R30° structure and the c(4 × 2) superlattice structure, respectively. As Figure 2a was recorded in the ZX scan mode 30 min after the last topographic image was obtained in the XY scan mode, the exact location where Figure 2a was recorded could not be identified (see the Supporting Information for details). Figure 2b was recorded on the domain shown in Figure 1c 8 min after Figure 1c was recorded. Figure 2a was recorded along the [101̅] direction, which is the direction of the nearest-neighbor molecule, and Figure 2b was recorded along the [12̅1] direction indicated in Figure 1c. These directions are defined against the Au(111) plane. In the force mapping technique, when the frequency shift exceeds the threshold, the tip retracts; thus, there are no data points in the bottom part of the picture (white area) and the boundary indicated by the orange curve reflects the topography of the molecular surface. Protrusions and valleys can be found in the image, which are indicated by red and blue arrows, respectively. The protrusions and valleys found in Figure 2 reflect the morphology of C11OH SAM. Figure 2c,d shows the schematics of the cross sections of molecular surface along the [101̅] direction of the (√3 × √3)R30° structure and along the [12̅1] direction of the c(4 × 2) superlattice structure, respectively. In the frequency shift maps (Figure 2a,b), the
bright and dark contrasts are recognized above the SAM surface (orange curve). These contrasts correspond to the positive and negative frequency shifts of the cantilever resonance frequency, respectively. In Figure 2a, fine dot patterns were observed in the center part of the 2D frequency shift map. The reason why it is only observed in the center part is possibly due to the mismatch of the alignment between the scan direction (X) of the 2D frequency shift map and the molecular row of the (√3 × √3)R30° structure with a mismatch angle of about 6°. A remarkable difference was recognized between the hydration structures of the (√3 × √3)R30° structure and the c(4 × 2) superlattice structure. The biggest difference was the lateral distance between the dark spots found over the protrusions (depicted by blue arrows) of 0.5 nm in Figure 2a and 1.0 nm in Figure 2b. The distance of 1.0 nm does not match with the distance of the nearest-neighbor molecules, but it corresponds to the distance between the molecules with the same height when we scan in the [12̅1] direction of the c(4 × 2) superlattice structure, namely, the unit cell length along this direction. On the basis of these results, we can describe a consistent explanation of how the topography of the surface affects the hydration structure. In the (√3 × √3)R30° structure, all of the molecules are on the same plane and the distance between the molecules is 0.5 nm. This is longer than the diameter of a single water molecule, which is 0.28 nm, and the water molecules have enough space to position themselves between the thiol molecules. In the c(4 × 2) superlattice structure, the distance between the nearest neighbors is still 0.5 nm, but hydroxyl groups are not on the same plane and have a height difference of around 50 pm as already mentioned. In this case, the spaces over the lower thiol molecules are better suited for the water molecules and are positioned over the lower hydroxyl groups in the c(4 × 2) superlattice structure. Therefore, the periodicity and the amplitude of the corrugation alter the hydration structure and is strong evidence that topography is a significant factor affecting the hydration C
DOI: 10.1021/acs.langmuir.8b03052 Langmuir XXXX, XXX, XXX−XXX
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Langmuir structure. This is quite surprising because the height difference in the c(4 × 2) superlattice structure measured in this study was around 50 pm, which is only about 15% of the size of a single water molecule. Some papers have already reported the 2D frequency shift images of OH-terminated alkanethiol SAMs. Hiasa et al. investigated the hydration structure of C11OH and 6mercaptohexanol (C6OH) SAMs using FM-AFM. 22,23 C11OH SAM showed a (√3 × √3)R30° structure, and the frequency shift oscillation was found over the valleys (indicated by blue arrows) of the frequency shift map, which is consistent with this study. However, C6OH SAM showed a c(4 × 2) superlattice structure and the oscillation appeared on the protrusion, which disagrees with our result. It is hard to say what caused these differences, but maybe the difference in chain length or phase of the two SAMs are the reasons. The molecular structure of C11OH SAM in this study closely resembles the ζ-phase, whereas the molecular structure of C6OH SAM in the previous investigation suggested an εphase-like structure. A detailed picture of the molecular structure of each phase is displayed in the Supporting Information. The effect of molecular arrangements on hydration structures as already mentioned also explains why the hydration structures of the ζ-phase c(4 × 2) superlattice structure were different from those of the previously reported ε-phase c(4 × 2) superlattice structures (see the Supporting Information). It should be noted here that we also measured the hydration structure of C6OH SAMs. Although we could not determine the phase, we obtained the 2D frequency shift map with the frequency shift oscillation on the protrusions, which agrees with the previous study.23 Figure 3a,b shows the frequency shift versus vertical distance curves extracted from Figure 2a,b, respectively. We chose the limit point (where the frequency shift exceeds the threshold) of the curve over the valley as the origin in both graphs. The frequency shift versus distance curve over the protrusions was shifted away from the origin, and the shifted distance corresponded to the height difference between the valley and protrusions in the 2D frequency shift map (Figure 2a,b). The frequency shift versus distance curves over the protrusions and valleys was completely different for both structures; a characteristic peak was observed at the distance of about 0.3 nm above the surface on the valleys, whereas it was further away on the protrusions. The period of oscillation was around 0.3 nm, which matches the size of a single water molecule. Figure 3c,d shows the force versus distance curves converted from the frequency shift versus distance curves using Sader’s method.26 Although it is not easy to determine whether the tip retracts at the “true” surface in the force mapping technique, we consider that the tip reached the sample and did not stop on the hydration layer above it. In fact, the threshold of the frequency shift was set higher than previous FM-AFM studies of the alkanethiol SAMs.22,23 Hence, we calculated the force curves only for the range where the frequency shift does not exceed 1 kHz to discuss the hydration structures. Although the frequency shift and force curves are useful in gaining some information about the local hydration structure, it is not easy to interpret these curves regarding the local water molecule distribution. One approach is to assume that the tip is hydrated and a water molecule is positioned in front of the tip apex, which is called the solvation tip approximation (STA).43,44 Despite this simple assumption, the STA matches extremely well with the free-energy calculations including the tip, which
Figure 3. (a,b) Frequency shift vs distance curves extracted from Figure 2a,b, respectively. Each curve was obtained by averaging the curves recorded on three consecutive pixels whose center was indicated by the arrows in Figure 2c,d. (c,d) Force vs distance curves converted from (a,b), respectively. (e,f) Local water molecule densities obtained using eq 2. using eq 2.
is usually expensive to calculate. In the STA, the force versus distance curves and the local water molecule distribution are related by the following equation:43,44 f (z ) =
kBT dρ(z) ρ (z ) d z
(1)
where z is the distance between the tip and the sample, f(z) is the force acting on the tip, ρ(z) is the local water molecule distribution, kB is the Boltzmann constant, and T is the temperature of the system. By integrating and taking the exponential function of both sides, ρ(z) can be obtained by the following equation: ij ∫ f (z)dz yz j zz ρ(z) = expjjj z jj kBT zzz k {
(2)
Empirically, the force versus distance curves obtained by FM-AFM are known to monotonically increase when approaching the surface other than oscillation forces,45,46 which act as the background. Kilpatrick suggested that the monotonic component is due to the entropic energy required to exclude ions that are diffusing at the solid−liquid interface.45 They fitted the force versus distance curves using the following formula:45 D
DOI: 10.1021/acs.langmuir.8b03052 Langmuir XXXX, XXX, XXX−XXX
ÄÅ É l ÅÅ 2π (z + φ) ÑÑÑ ji z zy o o Å Å ÑÑÑexpjjj− zzz + A m f (z) = 2πR m oAocosÅÅÅ ÑÑ j λ z o σ Å ÑÖ k o { o Ç n | ij z yzo o expjjj− zzz} j λ zo k m {o ~
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periodicity of the surface. The results strongly suggest that the surface topography has a huge impact on the entire hydration structure.
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ASSOCIATED CONTENT
S Supporting Information *
(3)
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.8b03052.
where R is the tip radius, Ao and Am are the magnitudes of the oscillation and monotonic repulsion forces, respectively, φ is the phase shift for adjusting the start point of the circular function, σo is the distance between the oscillations, and λo and λm are the decay lengths of the oscillation and monotonic repulsion forces, respectively. We adopted Kilpatrick’s conclusions and defined the monotonic increase as the shortrange background and subtracted it from the force converted from Sader’s method.46 Referring to eq 2, we then integrated the force curve by applying the trapezoidal rule and used the exponential function to obtain the local water molecule distribution. The details of the converting process can be found in the Supporting Information. Figure 3e,f shows the local water molecule distribution following the previously mentioned procedures. As been expected from the frequency shift versus distance curves, we found that the local water molecule distribution on the protrusions and valleys differed for both structures. In the density profiles predicted by the STA, a peak in the molecular density appears at the distance of the highest force gradient that is located closer to the surface than a force peak, namely, the left shoulder of a force peak (see eq 1). Despite the twostage conversions of the frequency shift curves to the force curves and then to the molecular density curves, we found that the peak positions in the molecular density curves happened to be close to the dip (frequency minimum) positions in the frequency shift curves for both structures, especially for those obtained on the valleys in the molecular corrugations. Roughly speaking, the density of the water molecules is higher in the dark spots above the molecular surface found in the 2D frequency shift images shown in Figure 2a,b, which correspond to the region above the gaps in the (√3 × √3)R30° structure and the top of the lower molecule in the c(4 × 2) superlattice structure, respectively. As mentioned before, STA has a huge impact on the interpretation of the force maps obtained by FM-AFM because, otherwise, the full free-energy calculation of the atomistic models including the model tip is required to theoretically calculate the force versus distance curves.43,47 STA reinforces FM-AFM as a more powerful tool to investigate hydration structures on complicated biomolecules.
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Discussion about the effect of the immersion temperature on SAM formation, further results about the hydration structure of the (√3 × √3)R30° phase, figures of different phases found in the c(4 × 2) superlattice structure, discussion about the hydration structure of C6OH SAMs and explanation of the water molecule distribution calculation procedure (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Akito Fujita: 0000-0001-8463-338X Author Contributions
A.F. performed the FM-AFM experiments. K.K. developed the AFM instruments and electronics. H.Y. supervised the whole study. All authors have discussed the results and been involved in writing the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank Dr. Toru Utsunomiya for fruitful discussions. This work was supported by JSPS Grants-in-Aid for Scientific Research grand numbers 17H06122 and 16H03870.
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REFERENCES
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CONCLUSIONS Hydration structures of C11OH SAMs were measured using the force mapping technique of FM-AFM in a 0.1 M KCl solution. The 2D frequency shift maps on two different molecular structures, that is, the (√3 × √3)R30° structure and the c(4 × 2) superlattice structure, were obtained in this study. The 2D frequency shift images showed unambiguous differences in the hydration structures, where the lateral periodicity of the dark dot patterns was different in the two structures. The frequency shift versus distance curves were first converted to force versus distance curves and then converted to local water molecule distributions using STA. The results show that the water molecule distribution depends on the atomic-scale corrugation of the surface, and the lateral periodicity of the water molecules corresponds to the lateral E
DOI: 10.1021/acs.langmuir.8b03052 Langmuir XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.langmuir.8b03052 Langmuir XXXX, XXX, XXX−XXX
