Two Metal−Molecule Binding Modes for Peptide Molecular Junctions

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J. Phys. Chem. C 2007, 111, 12860-12865

Two Metal-Molecule Binding Modes for Peptide Molecular Junctions Slawomir Sek† Department of Chemistry, UniVersity of Warsaw, Pasteura 1, 02093 Warsaw, Poland ReceiVed: May 22, 2007; In Final Form: June 25, 2007

The electrical behavior of gold-molecule-gold junctions incorporating 1,12-dodecanedithiol and 11-amino1-undecanethiol was investigated using scanning tunneling spectroscopy. The replacement of one terminal thiol group by the amine resulted in a change of the electronic conductance of the junction reflecting the difference in the nature of metal-molecule binding. The results were compared with the conductance of the molecular junctions incorporating short peptide consisting of a cystamine linker, L-alanine, and L-cysteine with an alternatively deprotected thiol or amine group. Using this approach, it is possible to form goldmolecule contacts using thiol or amine functional groups. The electron transmission through the peptide junctions formed using two different binding modes was relatively efficient. However, the junctions with gold-sulfur contacts showed a higher conductance as compared to those with a gold-amine contact.

Introduction The electronic conduction properties of single organic molecules became an important issue in the studies of electrontransfer processes.1 Understanding the electric behavior of the single molecules fixed between two metal electrodes is crucial for the future development of molecular electronics and nanoscale devices.2-4 In a simple metal-molecule-metal junction, there are several important factors that determine the electrical behavior of the system. The first one is the chemical structure of the molecular bridge linking two metal electrodes. Using different bridging units, one can control the efficiency of electron transmission through the junction and symmetry of the currentvoltage response.5-10 Recently, it was shown that the conductance of the molecular junctions could be strongly influenced by the geometry of the metal-molecule binding site11,12 and the order within the molecular assembly trapped between two metallic electrodes.12 In the latter case, it was demonstrated that for alkanedithiols, the structural transition of the alkyl chains from a gauche to an all-trans conformation leads to significant differences in the measured conductance. The variety of transitional conformers contributes also to the broad distribution of the measured currents. Thus, the determination of the single molecule conductance becomes difficult in such cases, and more complex analysis of the data is required.12 Another important factor controlling the junction behavior is the nature of the contacts, which depends on the choice of the metal electrodes and the terminal functional groups, which connect the molecules to the electrodes. In this way, it is possible to control the strength of the interactions between the molecule and the electrode. The gold-sulfur contact is most commonly used for connecting the molecules to metal surfaces,5-16 but other combinations are also known. A number of experimental studies has demonstrated that some functional groups such as the pyridyl, amine, selenide, and isocyanide groups can be considered as an alternative for thiol in Au-S binding.14,17-21 Moreover, other metals, such as Pt, Pd, or Ag, could replace gold electrodes.22-25 By choosing different metal-linker combinations, tuning the junction properties becomes possible. It was shown that the nature of the †

Corresponding author. E-mail: [email protected].

metal-molecule contact influences the conductance and shape of the current-voltage characteristics observed for molecular junctions. For example, Kushmerick and co-workers observed that the extent of coupling between chemical linker and metal electrode determines the conductance of the junction and the extent of current rectification.23,26 In this paper, the conductance measurements of the molecular junctions with symmetric and asymmetric gold-molecule contacts are reported. The experimental approach was based on the molecular junction method, which involves the entrapment of R,ω-functionalized molecules between a gold substrate and a gold tip of a scanning tunneling microscope. Several groups have demonstrated the use of this approach for the conductance measurements of alkanedithiols,14,15 dithiolated DNA,27-29 peptides,7,8 viologens,15,30,31 diamines,17,18 and diisonitryles.17 In this study, the junctions were formed using the self-assembled monolayers of 1,12-dodecanedithiol (DDT), 11amino-1-undecanethiol (AUT), and a short peptide consisting of a cystamine linker, L-alanine, and L-cysteine with protected amine and thiol moieties (CSA-Ala-(Boc)Cys(Acm)). In the latter case, the molecules were alternatively deprotected using a simple peptide chemistry to obtain free amine or thiol groups available for the adsorption on a gold scanning tunneling microscope tip. In this way, it was possible to introduce the peptide molecules into the junctions using two different binding modes: Au-S or Au-N. It should be noted that the terminal functional groups were chosen so as to provide a well-defined contact between the metal and the molecule. The amino group was chosen as an alternative for the thiol moiety because it can be considered to be a convenient functionality for coupling the molecules to metal leads in molecular junctions incorporating peptides. The effect of the replacement of the thiol group by an amine at one of the two gold-molecule contacts on the efficiency of electron transmission through the junction was considered. Experimental Procedures The starting materials for synthesis were purchased from Sigma, Aldrich, Fluka, and POCh (Gliwice, Poland). AUT was

10.1021/jp073960h CCC: $37.00 © 2007 American Chemical Society Published on Web 08/03/2007

Binding Modes for Peptide Molecular Junctions purchased from Prochimia (Sopot, Poland), and it was used without additional purification. Synthesis of DDT. A mixture of 7 mmol of 1,12-dibromododecane, 14 mmol of thiourea, and 80 mL of 99.9% ethanol was refluxed under Ar atmosphere for 18 h. Then, the solvent was removed under vacuum, and the solution of 20 mmol of sodium hydroxide in 80 mL of water was added. The resulting mixture was further refluxed under Ar atmosphere for 2 h, and then it was cooled to room temperature. The solution was extracted with three 50 mL portions of diethyl ether, and combined organic extracts were dried over anhydrous MgSO4. The solvent was removed under the vacuum, and the crude product was recrystallized twice from 99.9% ethanol to yield a white solid. 1H NMR (500 MHz) CDCl3: 2.52 ppm (4H, m); 1.59 (4H, m); 1.34 ppm (2H, t); 1.26-1.30 (16H, broad). EI MS: m/z ) 234 (M+). Synthesis of CSA-Ala-(Boc)Cys(Acm). A mixture of 5.5 mmol of HOBt, 5.5 mmol of EDC, and 5 mmol of Boc-L-Ala in 50 mL of dichloromethane was stirred for 30 min at 0 °C. Then, the solution of 2.5 mmol of cystamine hydrochloride and 5 mmol of triethylamine in 50 mL of dichloromethane was added. The resulting solution was stirred for further 12 h. The reaction mixture was extracted subsequently with 50 mL portions of a saturated aqueous solution of NaHCO3, 10% citric acid, saturated NaHCO3, and water. The organic phase was dried over anhydrous MgSO4 and filtered, and the solvent was removed using a rotary evaporator. The resulting solid was treated with 10 mL of trifluoroacetic acid. After 30 min, the solvent was removed under vacuum, and the mixture of 15 mL of triethylamine and 50 mL of dichloromethane was added to the resulting solid. The solution was left for 30 min. In a similar way, the mixture of 5.5 mmol of HOBt, 5.5 mmol of EDC, and 5 mmol of Boc-l-Cys(Acm) in 50 mL of dichloromethane was prepared and stirred for 30 min at 0 °C. Then, the mixtures were combined together, and the resulting solution was stirred for a further 12 h. After that, the reaction mixture was extracted subsequently with 50 mL portions of saturated aqueous solution of NaHCO3, 10% citric acid, saturated NaHCO3, and water. The organic phase was dried over anhydrous MgSO4 and filtered, and the solvent was removed under vacuum. The crude product was purified using flash column chromatography on silica gel. ESI-MS: m/z ) 865.5 [M + Na]+. Gold on glass samples were purchased from Arrandee (Werther, Germany). The substrates were 200-300 nm thick gold films evaporated on borosilicate glass pre-coated with a 1-4 nm chromium layer. Each substrate was flame annealed before use. This treatment leads to the formation of atomically flat Au (111) terraces.32 The adsorption of DDT, AUT, and CSA-Ala-(Boc)Cys(Acm) was carried out by self-assembly from 1 mM solutions in ethanol. The substrates were soaked for 18 h. Following the monolayer deposition, the samples were washed with ethanol and water and then dried in an Ar stream. It should be noted that in the case of CSA-Ala-(Boc)Cys(Acm), the presence of the Acm protecting group prevents the adsorption of peptide through Cys. Thus, the molecules adsorb on gold through sulfur atoms present in the cystamine linker. Using this approach, it was possible to obtain a monolayer with a uniform orientation of molecules. Prior to the junction formation, the substrates modified with peptide molecules were subjected to the deprotection procedures. Free thiol or amine groups in the external plane of the monolayer were obtained by removing the Acm or Boc protecting groups, respectively. To remove the Acm protecting group, the peptidemodified substrates were immersed in deoxygenated water, and

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Figure 1. Schematic view of self-assembled monolayers of DDT and AUT adsorbed on the gold surface.

then the pH was adjusted to 4.0 with small amounts of acetic acid and ammonia. The solution over the substrates was mixed by an argon stream. Subsequently, 100 mg of mercury (II) acetate was added, and the solution was mixed for 60 min. In the next step, 0.2 mL of β-mercaptoethanol was added, and the solution was bubbled with argon for another 15 min. Finally, the substrates were removed from the solution and rinsed with large amounts of water. Dry samples were ready for the formation of the junctions with the top Au-S contact. Deprotection of the amine group involved soaking the peptidemodified gold substrates in trifluoroacetic acid for 30 min. Then, the samples were rinsed with water and dried. After this treatment, the samples were suitable for the formation of junctions with the Au-N top contact. Electrochemical desorption experiments were carried out in a three-electrode cell using Ag wire as the reference electrode and platinum foil as the counter electrode. The supporting electrolyte was 0.1 M NaOH. The oxygen was removed from the solution using an Ar stream. The cyclic voltammetry curves were obtained using a CHI750B bipotentiostat. All experiments were carried out at room temperature. Scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) experiments were performed with a MultiMode SPM working with a Nanoscope IIIa controller equipped with a low-current converter (Digital Instruments, Santa Barbara, CA). The data were taken under ambient conditions in air. Gold tips were prepared by cutting a 0.25 mm gold wire (99.99%, Aldrich). The junctions were formed by a method similar to that described by Haiss.15 A gold scanning tunneling microscope tip was placed at a given location on the surface of the monolayer-modified sample, and the current setting determined the distance between the tip and the substrate. The initial value of the current was in the range of 0.1-6.0 nA. These settings were sufficient to bring the tip into contact with the molecules forming the assembly. When the proper distance was achieved, the molecules were expected to bridge the tip and the substrate. Further, the feedback was disabled, the tip was lifted at a rate of 10 nm/s while keeping constant the x-y position, and the current was recorded as a function of the tip-sample distance. The conductance of the junctions was measured at the bias voltage of -0.4 V. Results and Discussion A schematic view of DDT and AUT molecules adsorbed on a gold surface is shown in Figure 1. To verify the quality of the monolayers composed of 1,12- DDT and AUT, a series of

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Figure 2. Cyclic voltammetric curves recorded for gold electrodes modified with self-assembled monolayers of DDT (black) and AUT (gray). Scan rate: 0.05 V/s; supporting electrolyte: 0.1 M NaOH.

experiments was performed, which gave the information about the packing density of the molecules on gold substrates. The surface coverage for SAMs was determined from the electrochemical desorption experiments, which were performed in a 0.1 M NaOH aqueous solution. The examples of cyclic voltammetric curves obtained for SAMs of DDT and AUT are shown in Figure 2. When the potential of the thiol-modified gold electrode is swept to the negative values, a reduction peak appears, which corresponds to desorption of the thiol molecules from the metal surface. At the reverse scan, the oxidation peak is observed, which is attributed to subsequent adsorption of the molecules. Assuming that one electron is involved in the reduction process, the surface coverage of the electrode can be calculated using the area of the desorption peak.33 The values found from these experiments were (7.4 ( 1.0) × 10-10 mol/ cm2 for DDT and (7.8 ( 1.2) × 10-10 mol/cm2 for AUT. Using these numbers, it is possible to calculate mean molecular areas for DDT and AUT, which were 22 and 21 Å2, respectively. Both values are close to 20 Å2 reported for simple nalkanethiols.33,34 This may indicate that DDT and AUT form molecular assemblies with similar packing densities as compared to simple n-alkanethiols. Figure 3 shows representative current-distance curves recorded in junction experiments with bare gold (gray curves) and the samples modified with monolayers of DDT and AUT (black curves). Each curve was obtained from a single junction experiment. The current transients observed for bare gold are characteristic for tunneling through the empty gap between the tip and the substrate. The current decreases exponentially with an increasing distance between the scanning tunneling microscope tip and the gold surface. When the gold samples were modified with a monolayer of DDT or AUT, the current transients were substantially changed, which is reasonable because the molecules adsorbed on the metal surface were able to bridge the tip and the substrate, creating the tunneling pathway. Thus, when the scanning tunneling microscope tip is lifted, the distance between the tip and the substrate changes, but the length of the tunneling pathway remains constant as long as the molecule is bonded to the metallic contacts. As a result, the current-distance curves recorded in the presence of pre-adsorbed molecules on the gold surface display a welldefined plateau, which is attributed to the conductance through the molecule(s) incorporated into the junction.14,15,30 As soon as the contact is broken, the current drops rapidly to very low values, reflecting a poor conductivity of the gap. The insets

Figure 3. Examples of current-distance curves obtained for molecular junctions incorporating DDT (A) and AUT (B). Gray dashed curves were recorded for bare gold substrates. Insets show respective histograms constructed on the basis of current-distance curves. Bias voltage: -0.4 V.

shown in Figure 3 present histograms obtained on the basis of a large number of current-distance curves. The histograms were constructed using only those current-distance curves, which displayed well-defined current steps. In the case of DDT as well as for AUT, we observed pronounced peaks, indicating that some particular current values appear more frequently on current-distance curves than others. These consecutive maxima can be ascribed to the conductance of one or two molecules trapped between the scanning tunneling microscope tip and the substrate.14,15,30 The currents corresponding to the first maximum were used for further analysis. Comparing the current values obtained for DDT and AUT, it is evident that the DDT molecule has a slightly larger conductance than AUT. The conductance values for DDT and AUT are 1.5 × 10-6G0 and 1.2 × 10-6G0, respectively, where G0 is the conductance quantum defined by 2e2/h, and its value is 7.75 × 10-5 Ω-1. This result is surprising since the DDT molecule is longer by one methylene unit as compared to AUT, but on the other hand, the junctions incorporating the DDT and AUT molecules differ in the top contacts (i.e., scanning tunneling microscope tip-molecule contacts). In the case of DDT, we have an Au-S contact, while for AUT there is an Au-N contact. Thus, we have two factors that may contribute to the conductance differencesthe length of the bridge and the nature of the top contacts. The results obtained in the present work clearly show that the strength of the interactions between the gold scanning tunneling microscope tip and the terminal groups of DDT and AUT have significant contributions to the overall measured conductance. Moreover, this effect compensates the contribution

Binding Modes for Peptide Molecular Junctions from the difference in length of the DDT and AUT bridges. Following the paper by Heath and co-workers,35 the aminegold surface interaction can be described as a weak covalent bond. In the case of the thiol-gold interaction, the formation of the covalent bond is also expected, but differential scanning calorimetry measurements performed on Au nanocrystals showed that the Au-N interaction is much weaker than the Au-S interaction.35 Thus, the electronic conductances obtained for AUT and DDT seem to correlate with those observations, and a stronger interaction on the Au-S top contact results in a more efficient electron transmission, although the length of the electron-transfer pathway is longer for DDT. Such a conclusion is supported by the results obtained by Chen and co-workers.36 These authors investigated the conductance of alkanes terminated with a thiol, amine, or carboxylic group using a STMbased break junction approach. They found that the conductance depends on the nature of the gold-molecule binding and decreases in the following sequence: Au-SH > Au-NH2 > Au-COOH. The differences in the conductance reflected the binding strength between the gold electrodes and the anchoring groups. On the other hand, the results reported by Chu and coworkers indicate that the amine-gold contact is more efficient for charge transfer than the sulfur-gold contact.37 The discrepancy between their results and those presented by Chen et al. and in the present paper may result from several reasons. First of all, there are the differences in the experimental approaches and the structural details of the junctions. Chu and co-workers measured the conductance in nanoparticle-bridged gold nanogaps using the monolayers of monosubstituted alkane derivatives. In such a case, we can expect that the arrangement of the metal-molecule contacts as well as the molecular conformation may be different as compared to STM-based methods utilizing disubstituted alkanes. As was pointed out in Chen et al.’s paper, the differences in the tunneling probability could be expected since the molecules trapped within the junction are under compression in a nanoparticle-based approach, while in the STM-based method, the molecules are actually stretched.37 These findings have some important implications for conductance measurements of the junctions incorporating peptides. If we consider that the monolayer of the peptide with the cysteine (Cys) or penicillamine (Pen) residue at the N-terminus is exposed to the gold scanning tunneling microscope tip, there are two functional groups available for the adsorption on the scanning tunneling microscope probe. This is related to the fact that the terminal amino acid contains a thiol moiety as well as an amine group. At these conditions, we could observe two sets of conductance corresponding to two possible binding modess Au-S and Au-N. Distinction between these two modes can be made on the basis of conductance values. The higher values can be ascribed to Au-S binding, while the lower values should correspond to Au-N interactions. Of course, the probability of the particular binding event will depend on several factors. For example, the Au-N binding may be difficult due to the protonation of the amine groups or the unfavorable location of the --NH2 substituents. Another conclusion from the results presented above is that the presence of Cys or Pen at the N-terminus of the peptide is not necessary for the junction formation and that other amino acids can be used for the coupling of molecules to the scanning tunneling microscope tip since the Au-N binding may replace the Au-S contact. To verify these considerations experimentally, the junctions were formed using self-assembled monolayers of short peptide consisting of a cystamine linker, L-alanine, and L-cysteine with

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Figure 4. Structure of the peptide CSA-Ala-(Boc)Cys(Acm).

Figure 5. Cyclic voltammetric curve recorded for gold electrode modified with a self-assembled monolayer of CSA-Ala-(Boc)Cys(Acm). Scan rate: 0.05 V/s; supporting electrolyte: 0.1 M NaOH.

protected amine and thiol moietiessCSA-Ala-(Boc)Cys(Acm). The detailed structure of the molecule is given in Figure 4. It should be noted that CSA-Ala-(Boc)Cys(Acm) is a disulfide, but after adsorption on gold, it forms monolayers, which are indistinguishable from those prepared using thiols. As was stated previously, the surface coverage of SAMs formed by CSA-Ala(Boc)Cys(Acm) was determined from the electrochemical desorption experiments performed in a 0.1 M NaOH aqueous solution. An example of the cyclic voltammetric curve is shown in Figure 5. The surface coverage obtained for CSA-Ala-(Boc)Cys(Acm) was (5.5 ( 0.3) × 10-10 mol/cm2, and this corresponds to a molecular area (per single chain) of 30 Å2. This value is higher than that observed for DDT and AUT, which is reasonable since the adsorbed CSA-Ala-(Boc)Cys(Acm) contains a bulky t-butyl substituent. Prior to the junction formation, the substrates modified with CSA-Ala-(Boc)Cys(Acm) monolayers were subjected to deprotection procedures to obtain alternatively free amine or thiol groups in the external plane of the monolayer. Thus, upon removing the Boc protecting group, we obtain CSA-Ala-Cys(Acm), and the amine functionality is available for the adsorption on the scanning tunneling microscope gold tip, while after cleavage of the Acm group, we obtain CSA-Ala-(Boc)Cys, and the free thiol group is exposed. Figure 6 presents examples of current-distance curves recorded in junction experiments with the bare gold (gray curves) and samples modified with monolayers of CSA-Ala(Boc)Cys and CSA-Ala-Cys(Acm) (black curves). Each curve was recorded in a single junction experiment. The insets in Figure 6 show the distribution of the current values measured during the junction experiments. As was stated previously, the currents corresponding to the first maximum were used for further analysis. In this case, we observed a similar relation

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Sek bridges and peptides studied in this work. Therefore, the higher I(SH)/I(NH2) ratio obtained for the peptide may reflect the different polarizability of the peptide and n-alkyl bridges. As a result, the overall conductance of the junction can be strongly influenced by a more complex electronic structure of the entire bridge. Conclusion It has been demonstrated that the efficiency of electron transmission through STM-based molecular junctions is influenced by the nature of the tip-molecule binding. The electronic conductance of DDT and AUT molecules shows that the contribution from the gold-molecule contacts is different if the thiol group is replaced by an amine moiety. In the case of goldsulfur binding, the electronic conductance is higher as compared to the gold-amine contact. This difference seems to reflect the strength of the interactions between the scanning tunneling microscope tip and the terminal functional groups of the molecules incorporated into the junction. Nevertheless, goldamine contact seems to be relatively efficient for electron transmission. In this paper, it was shown that Au-N binding can be considered to be an alternative for gold-sulfur contact, especially in the case of the junctions incorporating short peptides. As a result, the presence of Cys or Pen residues at the N-terminus of the peptide is not essential for the junction formation. In this way, a broader range of amino acid sequences can be probed. Acknowledgment. This work was supported by the Ministry of Scientific Research and Information Technology (Projects PBZ 18-KBN-098/T09/2003 and 3 T09A 01627).

Figure 6. Examples of current-distance curves obtained for the molecular junctions incorporating CSA-Ala-(Boc)Cys (A) and CSAAla-Cys(Acm) (B). Gray dashed curves were recorded for bare gold substrates. Insets show the respective histograms constructed on the basis of current-distance curves. Bias voltage: -0.4 V.

between the conductances of the alternatively deprotected peptide as for DDT and AUT (i.e., the junctions with the Au-S top contact produced larger currents as compared to the junctions with Au-N binding). The conductance values for CSA-Ala(Boc)Cys and CSA-Ala-Cys(Acm) are 4.8 × 10-5G0 and 2.2 × 10-5G0, respectively. It should be noted that similar to for DDT and AUT, the junctions formed with an alternatively deprotected peptide differ not only in the top gold-molecule binding but also in the number of chemical groups separating the top and bottom contacts. As a result, the electron-transfer pathway for CSA-Ala-(Boc)Cys is longer by one methylene unit as compared to CSA-Ala-Cys(Acm). Thus, direct comparison of the currents measured in the presence of the Au-S and Au-N top contacts can be made. If we assume that I(SH) denotes the current measured for the junctions with the Au-S top contact and that I(NH2) denotes the current measured for the junctions with the Au-N top contact, we can compare the ratios I(SH)/I(NH2) for the DDT and AUT pair and the CSAAla-(Boc)Cys and CSA-Ala-Cys(Acm) pair. In the first case (i.e., the DDT and AUT pair), the ratio I(SH)/I(NH2) is 1.25, while for the second case (i.e., the CSA-Ala-(Boc)Cys and CSAAla-Cys(Acm) pair), the ratio I(SH)/I(NH2) equals 2.16. These results suggest that in the case of peptides, the contribution of the top contact to the overall conductance of the junction is stronger as compared to simple n-alkyl systems (i.e., DDT and AUT). Such an observation is rather surprising since the Au-S and Au-N binding modes are actually the same for n-alkyl

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