Solvent Effect on Formation of Cysteamine Self ... - IOPscience

Jpn. J. Appl. Phys. Vol. 42 (2003) pp. 236–241 Part 1, No. 1, January 2003 #2003 The Japan Society of Applied Physics

Solvent Effect on Formation of Cysteamine Self-Assembled Monolayers on Au(111) Sang Yun LEE, Jaegeun N OH1 , Eisuke I TO1 , Haiwon LEE and Masahiko HARA1 y Department of Chemistry, Hanyang University, Seoul 133-791, Korea 1 Local Spatio-Temporal Functions Laboratory, Frontier Research System, RIKEN (The Institute of Physical and Chemical Research), 2-1 Hirosawa, Wako, Saitama 351-0198, Japan (Received May 20, 2002; accepted for publication September 5, 2002)

Cysteamine (CA) self-assembled monolayers (SAMs) formed in various solutions on Au(111) were examined by scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS) to understand the solvent effect on the SAM structure. The STM study revealed that the surface structure of CA SAMs prepared in polar protic solutions was strongly influenced by immersion time, while there were no significant structural changes in the SAMs prepared in nonpolar and polar aprotic solutions. This result implies that the proticity of the solvent and the immersion time play important roles in determining the surface structures of the amino-terminated CA SAMs due to the coadsorptoin of solvent molecules onto the clean monolayers, which are stabilized by hydrogen bonding between polar protic solvents and the clean monolayers. In addition, our STM and XPS results for CA SAMs on Au(111) suggest the existence of two different structural conformations, i.e., trans and gauche conformers. [DOI: 10.1143/JJAP.42.236] KEYWORDS: cysteamine, self-assembled monolayers (SAMs), solvent effect, scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS)

1.

To build well-organized SAMs, on the other hand, it is essential to understand their surface structures on a nanometer scale. However, in general, the molecular structures for CA SAMs on gold could not be resolved by STM because of the weak lateral interaction between alkyl chains, the hydrophilic surface of the terminal amino groups which can interact with additional molecules such as solvents, and reorganization of the monolayers due to the formation of hydrogen bonds.11,15) Despite such numerous difficulties in obtaining molecular-scale STM images for CA SAMs, recently, Kawasaki et al. reported the first molecularly p resolved STM image showing the well-ordered 7 3 structure, adopting multiple adsorption sites of the sulfur headgroups on the Au(111) lattice.12) The solvent is regarded as one of the important factors for controlling surface structures of SAMs.9,16–18) For example, it is revealed that the size of well-ordered domains and the number and size of depressions in alkanethiol SAMs on gold are strongly influenced by the solvent.16) The domain formation of 4-mercaptopyridine SAMs on gold from the corresponding aqueous and ethanolic solutions was investigated by STM and it was found that the surface morphologies of the SAMs were significantly changed by the properties of the solvents used.18) In the present study, to understand solvent effect on the surface structures of CA SAMs on gold, we have investigated by STM and XPS the SAM samples formed from various CA solutions prepared by using water, ethanol, N,N,-dimethylformamide (DMF), and toluene as solvents. We believe that our results on a nanometer scale will be useful to obtain uniform CA SAMs on gold, which are very important to enhance the efficiency of immobilization for target molecules onto the cysteamine-modified surface.

Introduction

Organic self-assembled monolayers (SAMs) prepared by organosulfur compounds on metal surfaces have been extensively studied because of their wide varieties of potential applications such as nanolithography, chemical sensors, corrosion inhibition, wetting control, and molecular electronic devices.1) In particular, alkanethiol SAMs on gold have been thoroughly studied due to their highly reproducible and well-ordered structures, high stability, and ease of preparation.2–6) The surface properties of the SAMs can be also precisely tuned by varying the terminal functionalities of the monolayers. On the other hand, cysteamine (CA) SAMs on metal surfaces have been widely used as a linking adlayer because of the bifunctional molecular properties of CA where the sulfur headgroups bind to metal surfaces while the amino groups can be used for the immobilization of various target molecules such as proteins, biomolecules, and nanoparticles.7–11) In order to understand the structures and adsorption behaviors of CA molecules on metal surfaces, the SAMs on metal surface were characterized by various surface sensitive techniques such as ellipsometry, surfaceenhanced Raman scattering (SERS), X-ray photoelectron spectroscopy (XPS), cyclic voltametry (CV), and scanning tunneling microscopy (STM).9–14) As a result, it has been revealed that CA SAMs form spontaneously by a chemical reaction between sulfur headgroups and metal surfaces as the case of alkanethiol SAMs. However, contrary to the case of alkanethiol SAMs, the structures of CA SAMs are quite complicated due to the presence of amino groups in CA molecules that interact strongly with metal surfaces. It was found that the conformation of CA molecules adsorbed on silver strongly depends on the difference in the method of sample preparation such as concentration of solution, solvent, pH, and immersion time by SERS measurement.9,10) Therefore, it is pointed out that special care is required to obtain uniform SAMs terminated by amino groups, which are desired for various applications.

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Experimental

Cysteamine (CA, HS(CH2 )2 NH2 ) was purchased from Tokyo Chemical Industry (Japan) and was purified by recrystallization with a mixed solvent of diethyl ether and ethanol. All organic solvents used in this study were purchased from Wako Pure Chemical Industry (Japan) and were of analytical reagent grade. The water was of Milli-Q

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Results and Discussion

In order to understand the surface structure of aminoterminated CA SAMs on gold, it is worth comparing the surface structure of CA SAMs with that of PT SAMs. Although the PT has a nearly identical molecular length to that of the CA, each of these molecules has quite a different functional group attached to one end, i.e., the methyl group in PT and the amino group in CA. Due to such different molecular structures, PT and CA SAMs on gold have hydrophobic and hydrophilic surface properties, respectively. As mentioned above, the structures of alkanethiol SAMs on gold are fairly well understood, whereas those of CA SAMs are still very unclear as a result of a complex interplay due to both interactions between the amino groups and gold as well as between the sulfur headgroups and gold. STM is one of the most powerful tools for observing surface structures of organic monolayers on metal surfaces with a nanometer-scale precision.4,6,12,15–23) However, very few STM studies for CA SAMs on gold have been reported to date.11,12) From STM observations in this study, we will provide the first structural details for CA SAMs on Au(111) formed from various solutions as well as from an aqueous solution as a function of immersion time. Figure 1(a) shows the surface structure of PT SAMs on Au(111). As with alkanethiol SAMs having different alkyl chains,2,4,6,16,17,23) PT molecules form uniform SAMs with

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(a)

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quality (Millipore, Bedford, MA), and was bubbled using nitrogen gas for 10 min prior to use. The Au(111) substrates on mica were prepared by vacuum deposition as described in the literature.4) CA SAMs were prepared by dipping the gold substrates into 1 mM CA solutions prepared by the appropriate solvents for 2 h. Note that this immersion time is sufficient to form fully covered SAMs since it was confirmed by XPS measurement that the nearly fully covered SAMs on gold were formed within 5 min.12) In addition, to elucidate solvent effect on the structures of the SAMs as a function of immersion time, the SAMs prepared in an aqueous solution after immersion for 5 min, 1 h, and 2 h were examined by STM and XPS. Propanethiol (PT) SAMs were also prepared by dipping the gold substrates into a 1 mM ethanolic solution to compare the surface structure of PT SAMs with that of CA SAMs. After the SAM samples were removed from the solutions, the samples were immediately and thoroughly rinsed with the corresponding solvents. STM measurements were performed using NanoScope IIIa (Digital Instruments, Santa Barbara, CA) with a commercial Pt/Ir (80/20) tip. All STM images were obtained using a constant current mode under an ambient condition at room temperature. The typical imaging conditions of bias voltage and tunneling current are 500 mV and 300 pA, respectively. High-resolution XPS spectra were obtained using a VG ESCALAB 250 system with a monochromatic Al K X-ray source (1486.6 eV). The binding energies were calibrated by the Au 4f7=2 peak (83.9 eV) and the X-ray power was 200 W. The pass energy of the analyzer and the takeoff angle of photoelectrons were set at 20 eV and 90 , respectively. We measured survey, Au 4f, C 1s, O 1s, and S 2p spectra within 2 h.

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(b)

Distance (nm) Fig. 1. (a) STM image of PT SAMs on Au(111) formed from a 1 mM ethanolic solution after immersion for 2 h. Scan size was 100 nm 100 nm. (b) The height profile was obtained along with the black lines indicated in STM image of (a).

numerous depressions on the gold surface. The depth of which is nearly depressions is measured to be 2.5 A, consistent with the diameter of a gold atom [Fig. 1(b)]. It is generally believed that the depressions originate from the lateral displacement of gold atoms on the surface rather than the traditional chemical etching process during SAM formation.24) CA SAMs on Au(111) were also prepared from 1 mM toluenic, DMF, ethanolic, and aqueous solutions, and their surface structures for each CA SAM sample are shown in Figs. 2(a)–2(d). The structures of CA SAMs are significantly different from those observed for PT SAMs. In particular, STM images of CA SAMs on Au(111) obtained from toluenic and DMF solutions [Figs. 2(a) and 2(b)] clearly show three different reproducible height profiles, as indicated by A, B, and C in the images. Here, the darkest area (A) corresponds to the depression formed as a result of chemisorption of CA molecules. Unlike PT SAMs on gold, CA SAMs have a rough surface with many different features in the size, shape, and distribution of depressions. These marked differences in the structures of two SAM systems are ascribed to the presence of the amino functional groups in CA molecules. Actually, the smaller size and larger number of depressions for CA SAMs were observed compared to those in the case of PT SAMs. This result strongly implies that the migration rate of adsorbate-gold complex for CA SAMs is much slower than in the case of alkanethiol SAMs, since the presence of the amino groups in CA SAMs can provide additional strong interactions between the amino groups and the gold surface as well as can contribute to the formation of a hydrogen bonding network between CA

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(b) A C C

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Fig. 2. STM images of CA SAMs on Au(111) formed from 1 mM (a) toluenic, (b) DMF, (c) ethanolic, and (d) aqueous solutions of CA after immersion for 2 h. Scan sizes were 100 nm 100 nm.

molecules, resulting in the formation of a rigid CA monolayer. On the other hand, in comparison with alkanethiol SAMs having much more favorable all-trans conformation on metal surfaces, it was suggested by SERS study that CA SAMs on silver have two distinct structural conformations: one is trans conformer with a free amino group facing away from the surface and the other is gauche conformer with the amino group closed to the silver surface.9,10) Thus, it is reasonable to assume that similar structural conformations would be expected for CA SAMs on the gold surface, resulting in an inhomogeneous monolayer surface. Hence, it is considered that the trans conformers in STM imaging can be observed as brighter molecular features than those observed from the gauche conformers, because the adsorbed molecules in the trans conformers are oriented perpendicular to the surface, but the adsorbed molecules in the gauche conformers are oriented nearly parallel to the surface, resulting in such a height difference in STM imaging. Therefore, we suggest here that dark (B) and bright (C) regions result from the existence of two conformers, gauche and trans, as expected for CA SAMs on silver by SERS measurement. Furthermore, surface morphologies of CA SAMs prepared from ethanolic and aqueous [Figs. 2(c) and 2(d)] were

noticeably different from those of CA SAMs prepared from toluenic and DMF solutions [Figs. 2(a) and 2(b)]. When ethanol and water were used as solvents, more defective SAM structures with dark and large regions were observed, as shown in Figs. 2(c) and 2(d). This result means that the surface structure of CA SAMs was strongly influenced by the solvents used for SAM preparation. We note, here, that such defects are not the depressions formed by coalescence of depressions from a small size to a large one. The origin of such observed SAM structures will be discussed below in more detail. It is well known that ethanol and water are polar protic solvents, and toluene and DMF are nonpolar and polar aprotic solvents, respectively. In considering the relationship between the observed STM results and the properties of solvents, it is clear that the proticity of solvents such as ethanol and water is responsible for such structural changes, which may be related to the coadsorption of solvent molecules onto the hydrophilic SAM surface via the formation of hydrogen bonds between the terminal amino groups of monolayer and solvents. Meanwhile, it is also found that water solvent affects the monolayer more seriously compared to ethanol solvent. To reveal the origin of the unusual surface structures of CA SAMs on Au(111) prepared from polar protic solutions,


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(b) C

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D

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Fig. 3. STM images of CA SAMs on Au(111) formed from a 1 mM aqueous solution as a function of immersion time. Immersion times were (a) 1 min, (b) 5 min, (c) 1 h, and (d) 2 h. The height profiles of (a)0 , (b)0 , (c)0 , and (d)0 were obtained along with the black lines indicated in STM images of (a), (b), (c), and (d), respectively. Scan sizes were 100 nm 100 nm.

we investigated the surface structures of CA SAMs formed from an aqueous solution as a function of immersion time. STM images in Fig. 3 show structural changes of CA SAMs as the immersion time increases. STM images in Figs. 3(a) and 3(b) were taken after immersion for 1 min and 5 min, respectively. From these SAM samples, interestingly, we observed very similar monolayer structures to those obtained from nonpolar and polar aprotic solutions after immersion

for 2 h [see Figs. 2(a) and 2(b)]. However, after immersion for 1 h, the surface structure was markedly changed, as shown in Fig. 3(c). Although a depression (A) and monolayer (B and C) still remained, some domains with a bright feature, as indicated by D, newly appeared. After immersion for 2 h, finally, the depression and a clean surface of the monolayer were not clearly observed [Fig. 3(d)]. This can be attributed to the development of new domains on the clean


SAM surface increasing with the immersion time. To understand the formation of new domains more clearly, we checked the height profiles along with the black lines in each STM image as shown in Figs. 3(a)0 –3(d)0 . Before the formation of new domains, the height differences between depressions (A) and domains (C) for CA SAMs on to Au(111) were measured to be in the range from 1.5 A as shown in Figs. 3(a)0 and 3(b)0 . It is noteworthy to 2.5 A, mention that such variations in the height difference are probably due to the difference in various orientations of adsorbed molecules and due to the degree of crystallization of CA SAMs, resulting in different tunneling mechanisms. On the other hand, after the formation of new domains, the height differences between dark and bright regions suddenly increased with increasing the immersion time [Figs. 3(c)0 and 3(d)0 ]. The height values were measured to be in the to 4.2 A. It is evident, on a nanometerrange from 2.5 A scale, that the formation of new domains occurred on the clean monolayer. It was expected based on molecular dynamics simulations and contact angle measurements that the hydrophilic surface of SAMs consisting of OH and NH2 terminal groups can interact strongly with solvents such as water and ethanol, leading to the formation of a bilayer structure, which is stabilized by hydrogen bonding between solvents and the clean monolayer.15) Hence, it can be considered that the formation of new domains revealed by our STM study is mainly due to the coadsorption of solvent molecules such as water and ethanol on the clean hydrophilic surface. As discussed above, the formation of new domains from CA solutions prepared by nonpolar and polar aprotic solvents was not observed. From our STM results, it is reasonable to conclude that the hydrogen bonding between the amino terminated SAMs and polar protic solvents as well as the hydrogen bonding in the coadsorbate overlayer itself plays a major role in the formation of new domains, i.e., coadsorbates. In addition, we would like to point out that the coadsorption of polar protic solvent molecules onto the clean monolayer is required for an appropriate immersion time, reflecting slow kinetics of this process. This means that the solvents do not affect the SAM structure at the initial SAM growth stage. On the other hand, another possibility for the formation of new domains will be coadsorption of CA molecules, not via solvents. To examine this possibility, we examined checked XPS spectra in the S 2p region of CA SAMs on Au(111) formed from an aqueous solution as a function of immersion time (Fig. 4). In the S 2p XPS spectrum for CA SAMs on Au(111), we observed four different sulfur species (S1–S4). Each sulfur component is composed of two 2p3=2 and 2p1=2 peaks having the intensity ratio of 2 : 1 by the spin-spin splitting effect.25–27) The main S1 component for S 2p3=2 and 2p1=2 peaks was observed at 162.05 and 163.25 eV, respectively, which can be attributed to the bound sulfur on gold as observed from closed-packed and well-ordered alkanethiol SAMs on gold.25–27) This result implies the formation of CA SAMs chemisorbed on Au(111). On the other hand, the S2 component for S 2p3=2 and 2p1=2 peaks was found at 163.35 and 164.55 eV, respectively. Together with the S1 component, this component is often observed from alkanethiol SAMs, which is mainly derived from the unbound sulfur or physisorbed multilayers as revealed in the

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Binding energy (eV) Fig. 4. XPS spectra in the S 2p region of CA SAMs on Au(111) formed from a 1 mM aqueous solution as a function of immersion time. Immersion times were (a) 5 min, (b) 1 h, and (c) 2 h. Here, S1 and S3 peaks are assigned to the bound sulfurs, and S2 and S4 peaks are assigned to the unbound sulfur and the oxidized sulfur, respectively. Note that the relative peak intensity of the unbound sulfurs (S2) against the bound sulfurs (S1 and S3) for CA SAM samples regardless of immersion time was estimated to be 0:16 0:02.

literature.25) The S3 component for S 2p3=2 and 2p1=2 peaks was observed at 161.05 and 162.25 eV, respectively. Although this component is assigned to the bound sulfur, the origin of this component is still unclear.27) Compared to the S1 sulfur component, to decrease the binding energy, the S3 component is required for stronger interaction between the sulfur headgroups and gold. Thus, it has been suggested that the most likely reason for the the presence of the S3 component is hybridization change in adsorbed molecules from sp3 to sp hybridized sulfur on metal surfaces.27,28) This component for CA SAMs was observed as one of the main peaks, whereas it was only observed in alkanethiol SAMs on gold at the initial stage of SAM growth or after annealing.27) As discussed above, it has been revealed that CA molecules adsorbed on silver have two structural arrangements, i.e., trans and gauche conformers.9,10) As indicated by regions B and C in the STM images of CA SAMs on Au(111), we observed two different phases, which can be ascribed to the existence of two conformers in CA SAMs on Au(111). Therefore, it is reasonable to consider that the S3 component is mainly due to the presence of the gauche conformer of adsorbed CA molecules because the molecules in this conformer need strong interactions with gold surface, resulting in sp hybridized sulfur. Finally, the S4 component


with high binding energy for S 2p3=2 and 2p1=2 peaks was found at 167.59 and 168.79 eV, respectively. This component is often observed in various SAM systems, which is mainly due to the existence of a small amount of oxidized sulfur such as sulfornates.29,30) Despite a drastic change of surface structure of CA SAMs on Au(111) after 1 h immersion (see Fig. 2), no significant change in XPS spectra of S 2p region was observed. If physisorbed CA molecules as coadsorbates on CA SAMs remain, an unbound sulfur species (S2 component) with a strong intensity should be observed from CA SAM samples prepared after 1 h or 2 h immersion. However, the XPS result obtained here does not show any significant intensity changes as a function of immersion time. For example, the relative peak intensity of the unbound sulfurs (S2) against the bound sulfurs (S1 and S3) for CA SAM samples regardless of immersion time was measured to be 0:16 0:02. This result strongly suggests that the formation of new domains, i.e., coadsorbates, onto the clean CA SAMs is mainly due to coadsorption of polar protic solvents rather than coadsorption of CA molecules. 4.

Conclusion

From a nanoscopic viewpoint, the solvent effect on the formation of CA SAMs in various solutions on Au(111) was investigated by STM. In contrast to the nonpolar and polar aprotic solvents, it is found that the polar protic solvents such as water and ethanol significantly affect the structure of CA SAMs on Au(111) via coadsorption of solvent molecules onto the clean monolayer, which is stabilized by hydrogen bonding between polar protic solvents and the aminoterminated CA monolayers. We observed that this coadsorption process occurs slowly as a function of immersion time. The XPS result clearly reveals that the coadsorption process onto the clean monolayers is mainly originated from the coadsorption of polar protic solvents, not the CA molecules. In addition, our STM and XPS results for CA SAMs on Au(111) suggest the existence of two different structural conformations, i.e., trans and gauche conformers. By choosing nonpolar and polar aprotic solvents or by using the shorter immersion time from a polar protic solution in the preparation of CA SAMs on gold, we could obtain highly qualified CA SAMs, which can improve the efficiency of immobilization for target molecules onto the amino-terminated CA SAMs. Acknowledgement This work was partially supported by the National

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