Molecular Structure of 3-Amino-5-mercapto-1,2,4-triazole Self

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J. Phys. Chem. C 2007, 111, 17397-17403

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Molecular Structure of 3-Amino-5-mercapto-1,2,4-triazole Self-Assembled Monolayers on Ag and Au Surfaces Beata Wrzosek and Jolanta Bukowska* Department of Chemistry, UniVersity of Warsaw, Pasteur Street 1, 02-093 Warsaw, Poland ReceiVed: July 12, 2007; In Final Form: August 30, 2007

Molecular structure of adlayers formed from 3-amino-5-mercapto-1,2,4-triazole (AMT) solutions on silver and gold surfaces was investigated with surface-enhanced Raman scattering (SERS) and X-ray photoelectron (XPS) spectroscopies. Two types of structures were identified, dependent on the concentration of the AMT in the solutions, pH values, and the potential applied to the electrode. In the presence of sufficient amount of the adsorbate molecules in the vicinity of the metal surface, AMT adsorbs in the thiolate form through the metal-thiolate bond, in more or less perpendicular orientation to the metal surface. This structure is stabilized by stacking interactions between triazole rings and by hydrogen bonding between amine groups of adjacent AMT molecules and/or between adsorbed AMT and the molecules in the solution. However, in the case of insufficient concentration of the adsorbate at the surface, the AMT molecules also adsorb through the ring nitrogens, in the form deprotonated in the triazole ring, with the ring lying flat on the metal surface. Quick, reversible switching between those two monolayer structures is possible by applying an appropriate electrode potential.

Introduction Self-assembled monolayers (SAMs) of organic compounds having thiol groups are widely used to obtain functional molecular surfaces.1 Such surfaces are designed for a great variety of purposes, e.g., in fabricating chemical or biological sensors, biofuel cell elements, and optoelectronic or bioelectronic devices. The great advantage of the monolayers formed from sulfur-containing molecules (thiols, sulfides, or disulfides) is their strong interaction with a metal (Ag, Au, and Cu) surface, resulting in a stable metal-sulfur chemical bond.1-3 Among this type of coating, alkanethiols and their ω-substituted analogues are very extensively exploited, because of their ability to create an interface with well-defined composition and structure and with easily controlled chemical and physical properties.1 Aromatic thiols, including heteroaromatic thiols, were less extensively investigated, despite their promising properties in electron-transfer processes on modified electrodes.4-8 In recent years, various electrodes modified with self-assembled monolayers of six-membered heterocyclic compounds such as 4-mercaptopyridine or bis(4-pyridyl) disulfide were successfully applied for promoting the electrochemistry of electron-transfer proteins (e.g., cytochrome c).4-6 The redox response of cytochrome c on gold electrodes modified with nucleic acid base derivatives such as 2-amino-6-purinethiol, 6-amino-8-purinethiol, and 2-thiouracil were also reported.7,8 It has been recognized that the electrochemical response of cytochrome c on these monolayers strongly depends on structural properties of promoter adlayers and that the formation of hydrogen bonds with complementary nucleic acid base may in some cases inhibit the electron transfer between protein and gold electrode. The electron-transfer kinetics of various redox probes on the monolayers of 2-mercaptobenzothiazole and 2-mercaptobenzimidazole was shown to depend on protonation/deprotonation * Corresponding author. E-mail: [email protected]. Fax: +48 22 822 59 96.

processes on the heterocyclic nitrogen atom, that may result in the structural reorganization of the monolayers.9 It was also demonstrated that SAMs of heteroaromatic thiols may be successfully used in voltammetric sensors for determination of uric acid (2,5-dimercapto-1,3,4-thiodiazole, mercaptobenzimidazole SAMs)10,11 and dopamine (SAM of 3-amino-5-mercapto1,2,4-triazole)12 in the presence of ascorbic acid. It was suggested that the thickness of the monolayer,11 suitable orientation of the thiol molecule with respect to the electrode surface, and a favorable interaction through hydrogen bonds between analyte molecules and SAM10 play a decisive role in the detection of uric and ascorbic acids. 3-Amino-5-mercapto1,2,4-triazole was also examined (among other thiol compounds) for its potential as a coating for liquid-phase quartz-crystal microbalance applications.13 In the present paper we report results of surface-enhanced Raman scattering (SERS) and X-ray photoelectron spectroscopy (XPS) studies of 3-amino-5-mercapto-1,2,4-triazole (AMT) (formula in Figure 1) adlayers on silver and gold supports. It will be shown how various factors, such as pH of the solution, the possibility of interaction of the surface-immobilized AMT molecules with AMT in the solution, and/or an electrode potential determine the molecular structure of the monolayer. The possible consequences for application of AMT-modified Au and Ag electrodes in molecular recognition and biosensing will be discussed. Experimental Section Chemicals. All chemicals were purchased by commercial suppliers (Aldrich, POCH, and ChemPur) and used without further purification. Phosphate buffer solutions (typical ionic strength 50 mM) were prepared from appropriate conjugated acid-base pairs of Na3PO4, Na2HPO4, NaH2PO4, H3PO4. Roughening Procedures for Ag and Au Electrodes. Before the chemical modification, the polycrystalline silver electrodes

10.1021/jp075442c CCC: $37.00 © 2007 American Chemical Society Published on Web 10/25/2007

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Figure 1. Tautomeric (thiol -thione) and deprotonated forms of AMT.

were electrochemically roughened by three successive positivenegative cycles in 0.1 M KCl aqueous solution from -0.3 to +0.3 to -0.3 V at a sweep rate of 5 mV s-1 in a separate cell. The cycling was finished at -0.3 V, and then the silver electrode was kept for 1 min at -0.4 V. For gold, 20 cycles with the potential changing from -0.5 to +1.2 to -0.5 V at a sweep rate of 50 mV s-1 were applied. Platinum and Ag/AgCl/1 M KClaq electrodes were used as counter and reference electrodes, respectively. Raman Measurements. Raman spectra were recorded with a Jobin Yvon-Spex T64000 Raman spectrometer equipped with a Kaiser holographic notch filter, 600 grooves/mm holographic grating and a 1024 × 256 pixels nitrogen-cooled charge-coupled device detector. The Raman spectrometer was equipped with an Olympus BX40 microscope with a 50× long distance objective. A Laser-Tech model LJ-800 mixed Ar/Kr laser provided excitation radiation of 647.1 and 514.5 nm with a power less than 10 mW at the sample in the case of SERS spectrum and about 100 mW at the head of the laser in the case of normal Raman (NR) spectra. The integration time ranged from 0.5 to 20 s for SERS spectra and from 50 to 200s for the normal Raman spectra. XPS Measurements. A high-resolution scanning Auger spectrometer Microlab 350 (Thermo Electron - Thermo VG Scientific) equipped with a FEG-tip (Field-Emission Electron Gun) and a twin anode source (AlKR and MgKR) was used for the SEM and XPS analysis. XPS experiments was executed by Al KR (hV ) 1456.6 eV) anode X-ray source operated at 15 kV and emission current intensity of 20 mA. The surveyed spectra and high-resolution spectra from the total surface area of 0.2 cm2 were recorded using 150 and 50 eV pass energy. A linear or Shirley background subtraction was made to obtain XPS signal intensity. The peaks were fitted using an asymmetric Gaussian/Lorentzian mixed function. The position of the carbon C1s peak was assumed to be at 285 eV and used as an internal standard to determine the binding energy of other photoelectron peaks. An Avantage-based data system software (version 3.44) was used for data acquisition and processing. Results and Discussion Normal Raman Spectra of 3-Amino-5-mercapto-1,2,4triazole. 3-Amino-5-mercapto-1,2,4-triazole (AMT), like other azoles substituted with an SH group, may exist in thiol or thione tautomeric forms (Figure 1). In principle the actual form of this compound may be determined with the help of infrared and Raman spectra. Figure 2 shows normal Raman spectra of AMT in a solid state (spectrum a) and in aqueous solutions of varying pH values (spectra b-d). Randomness of intermolecular forces in solution cause the respective vibrational bands in the spectra to considerably broaden in comparison to those in the solidstate spectrum. In the high-frequency region (2600 - 3350 cm-1) of the spectrum of the crystalline AMT there are several bands that can be ascribed to the stretching vibrations of the NH (in the triazole ring) and NH2 groups.14,15 Lack of the SH stretching band in the spectrum of the solid sample, which is expected within the 2500-2600 cm-1 frequency range, may

Figure 2. Normal Raman spectra of AMT (a) solid sample; (b) 50 mM solution (natural pH ) 4.7); (c, d) alkaline solutions (pH 12) in H2O (c) and D2O (d). λexc ) 514.5 nm.

indicate that in the solid-state AMT exists in the thione form. According to the literature data for mercaptotriazoles,14-16 the band corresponding to the CdS stretching vibration is located within the range of 430-530 cm-1. Therefore, the 461 or 494 cm-1 band seen in the Raman spectrum of the solid sample may be most probably ascribed to the ν(CdS) vibration of the thione form of AMT. The solid-state Raman spectrum does not significantly differ from the AMT spectrum in 50 mM aqueous solution, demonstrating that the thione form is also predominant in solution at natural pH (about 4.7). The spectral pattern for dissolved AMT significantly changes in alkaline medium (pH > 9). Under such conditions, AMT exists in the form of a thiolate anion (pKa for deprotonation of the Sexo proton of AMT is 8.2317). The nitrogen atom in the triazole ring of the AMT molecule in solution remains protonated even at pH 12, since the positions of all the bands due to ring and NH deformation vibrations (1000-1600 cm-1) change upon hydrogen-deuterium exchange (compare spectra c and d in Figure 2). The vibrational spectra of five-membered N-heterocyclic derivatives have been extensively investigated, and vibrational assignments of 1,2,4-triazole and its derivatives structurally related to AMT, like 3-mercapto-1,2,4-triazole and 3,5-diamino-1,2,4-triazole, are available in the literature.18-22 Unfortunately, no assignment can be found for AMT. However, by comparing the SERS spectra of AMT with the normal Raman spectra of the AMT in the solid state (Figure 2a) and in solutions of H2O and D2O at varying pH values (Figure 2b-d), we may easily identify species present at the metal surface.

3-Amino-5-mercapto-1,2,4-triazole SAMs on Ag and Au

Figure 3. SERS spectra of AMT on the Ag surface (a) in presence of 10 mM AMT in buffer solution (pH 5.5); (b) after partial desorption in buffer solution (pH 5.5). λexc ) 514.5 nm.

SERS Experiments. In general we observed two types of SERS spectra in our experiments, as illustrated in Figure 3. This figure shows the SERS spectra of AMT adsorbed on a silver electrode in the presence of adsorbate molecules in solution (spectrum a) and after prolonged storage (at least 3 h) of the AMT-modified Ag electrode in pure buffer solution (pH 5.5) (spectrum b). As seen in Figure 3, the SERS spectra in both cases are quite different in comparison to the normal Raman spectrum of crystalline AMT. The SERS spectrum of the AMTmodified electrode recorded in the absence of adsorbate in solution (spectrum b) is dominated by a very strong band at 1360 cm-1, some bands that were visible in the spectrum “a” disappear or become considerably shifted (1020, 1155, 1490, 1545, 1640 cm-1), and several new features appear (530, 930, 1360 cm-1). However, in the case of the SERS spectrum recorded in the presence of AMT in solution (spectrum a) there is a close resemblance to the normal Raman spectrum of the alkaline solution of AMT, shown in Figure 2. Selective surface enhancement may cause the relative intensities in the SERS spectra to be changed. The respective bands are shifted and some new features appear within 1000-1100 cm-1 frequency range as a result of bonding to the metal surface. We also observed high increase in the relative intensity of the band at 480 cm-1. According to the literature data, this band may be due to vibration involving C-S stretching.20 This assignment was confirmed by hydrogen/deuterium exchange experiments, which will be further discussed. Strong enhancement of the ν(C-S) band intensity, observed in the surface spectra, may point to the upright orientation of the AMT molecule with respect to the surface. It is worth mentioning that the spectral changes illustrated in Figure 3 are totally reversible, i.e., after transferring the Ag/AMT sample, that exhibits spectrum b, to the buffer solution containing AMT, spectrum a is restored. Similar differences in the AMT spectra to those visible in Figure 3 are found in the series of SERS spectra of monolayers grown from solutions of varying AMT concentration, collected in the presence of the adsorbate (Figure 4). This experiment, which shows evolution from the one type spectrum, characteristic for low concentrations of AMT (Figure 4b-d) or its absence (Figure 3b), to the second type spectrum characteristic for higher concentrations (Figure 4f), may suggest that in the presence of a sufficient amount of AMT in the solution (concentrations higher than 10-4 M), adsorbed AMT molecules are able to interact through hydrogen bonds with the adjacent molecules at the surface and/or with the molecules in the bulk. Such

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Figure 4. SERS spectra of AMT on Ag surface for varying concentrations of AMT solutions: (a) 10-7; (b) 10-6; (c) 10-5; (d) 10-4; (e) 3 × 10-4; (f) 10-3 M. λexc ) 647.1 nm.

Figure 5. Influence of hydrogen/deuterium exchange on the SERS spectra of AMT recorded in presence of 10 mM (a, a′) and absence (b, b′) of AMT in the solution. Spectra a and b were recorded in H2O and spectra a′ and b′ in D2O. λexc ) 514.5 nm.

interaction can induce more or less perpendicular orientation of the triazole ring with respect to the metal surface, in which both the nitrogen atom in the ring and the NH2 groups may be engaged in H-bonding with neighboring molecules. This geometry may be additionally stabilized by stacking interactions between triazole rings oriented parallel to each other (structure a, exhibiting spectrum a in Figure 3). When the surface coverage of AMT molecules diminishes due to insufficient concentration of the AMT in the solution (submonolayer coverage) or after partial desorption in pure buffer solution, AMT molecules may interact with the metal surface not only through the sulfur atom but also through the nitrogen atoms of the triazole ring, oriented more or less parallel to the surface (structure b, exhibiting spectrum b in Figure 3). It is highly probable that in this configuration the ring nitrogen atoms are deprotonated. This hypothesis has been confirmed in the following experiments. In Figure 5 the effect of hydrogen/deuterium (H/D) isotope exchange in the SERS spectra of both types are presented. All the bands in the spectrum of the AMT monolayer recorded in the presence of adsorbate molecules in solution (spectra a, a′ in Figure 5) change their positions upon isotopic exchange. The

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Figure 6. SERS spectra of AMT (10 mM solution) adsorbed on Ag at varying pH values: (a) 2.2; (b) 5.5; (c) 10.6; (d) 12.3. λexc ) 647.1 nm.

cm-1

only exception is the band at 480 most probably ascribed to the vibration involving a C-S bond. It indicates that adsorbed molecules preserve protons in the triazole ring, since most of the bands observed in the spectra are assigned to the triazole ring vibrations.18-22 The SERS spectrum recorded after thorough rinsing of the AMT monolayer with buffer solution, followed by transfer to pure water (H2O or D2O) (spectra b,b′ in Figure 5), is on the other hand completely insensitive to the H/D exchange, thus giving evidence that vibrations corresponding to the bands in this spectrum originate from the molecules deprotonated in the triazole ring. This situation is quite different from that found for AMT in solution, in which the AMT molecules remain protonated even at alkaline pH (about 12) (see discussion of the spectra in Figure 2). Additional evidence that confirms proposed assignment of spectrum b in Figure 3 to deprotonated AMT molecules can be found in the SERS spectra, recorded at varying pH values (Figure 6). The strong band at 1360 cm-1 (assigned to the C-N stretching vibrations of the triazole ring16), characteristic for the spectrum b in Figure 3, appears at pH > 10 (despite the presence of AMT in the solution), and its relative intensity increases for more alkaline solutions with a simultaneous intensity decrease of the bands corresponding to the spectrum observed in acidic solutions. It is worth mentioning that the pKa value for 1,2,4-triazole (the respective value for AMT has not been found), corresponding to deprotonation of the N atom, amounts to 10.26,23 being in fine agreement with the changes observed in the SERS spectra shown in Figure 6, for adsorbed AMT. Solution pH also influences the kinetics of transformation of the AMT monolayer of one type (structure a) into the monolayer of the second type (structure b), during desorption in the buffer solutions of varying pH values (data not shown). In more alkaline solutions the spectral pattern changes considerably more rapidly, suggesting that this process is associated with the deprotonation of the triazole ring of AMT molecules. The spectrum corresponding to the parallel-oriented, deprotonated AMT molecules (structure b) is observed at the early stages of adsorption (Figure 7). In the case of dilute AMT solution (1 mM) this spectrum (characterized by the 1360 cm-1 feature) survives after 1 h of monolayer growth, but for relatively concentrated solutions (10 mM) this spectrum was hardly detected just after 20 s. Transition from one type of monolayer to the second type can also be induced by the potential applied to the electrode. As visible in Figure 8, with increasing electrode potential, the 1360 cm-1 band intensity increases and finally, at sufficiently positive potential, the second type spectrum (spectrum b) is exclusively observed, pointing to deprotonation of the heterocyclic nitrogen.

Wrzosek and Bukowska

Figure 7. Time evolution of the AMT spectrum during adlayer growth on the Ag surface from a 1 mM solution: (a) 1 min; (b) 3 min; (c) 10 min; (d) 60 min; (e) 60 min, the spectrum recorded with the laser beam focused at another place of the surface. λexc ) 514.5 nm.

Figure 8. Potential dependence of the SERS spectrum of AMT adsorbed on the Ag electrode from the solution (10 mM; pH 5.5): (a) -0.4; (b) -0.15; (c) 0; (d) +0.2 V vs Ag/AgCl. λexc ) 647.1 nm

TABLE 1: Values of the Potential Applied to Silver and Gold Electrodes, Covered with AMT Monolayers, Corresponding to Equal Intensities of the 1360 cm-1 Band, Characteristic for Spectrum “b”, and of the 1325 cm-1 Band, Characteristic for Spectrum “a” in Figure 3 E (V) vs Ag/AgCl conditions

Ag

Au

AMT (10 mM) + buffer pH 2.2 pure buffer pH 2.2 pure buffer pH 5.5 pure buffer pH 12.3

+0.15 +0.15 -0.15 -0.6

+0.4 +0.4 +0.2 -0.5

This result seems quite reasonable because the pH value in the vicinity of the electrode may increase with increasing positive charge at the electrode surface, due to charge compensation by adsorbed OH- anions. The potential value corresponding to appearance of this band in the AMT spectrum depends on the pH of the solution (see Table 1) and shifts to more negative values on increasing pH. It indicates that at more negative potentials the surface pKa, defined as a solution pH at which the molar fraction of protonated molecules at the surface is equal to 0.5, shifts to higher values, which is in fine agreement with the results for other molecular bases such as 4-mercaptopyridine and 4-aminothiophenol.24 It is worth mentioning that the potential value corresponding to the beginning of the structural transformation of adsorbed AMT is not influenced by the AMT molecules present in the solution, indicating that the surface charge plays a decisive role in this process.

3-Amino-5-mercapto-1,2,4-triazole SAMs on Ag and Au

Figure 9. SERS spectra of the AMT monolayer grown on the Au electrode in the presence of 10 mM (a, b) and absence (a′, b′) of AMT in solution at pH 5.5 (a, a′) and 12.3 (b, b′). λexc ) 647.1 nm.

Similar SERS experiments were carried out for AMT adsorbed at the Au electrode. In Figure 9 the SERS spectra of the AMT monolayer on Au are presented at pH 5.5 and 12.3 in the presence and the absence of the adsorbate in the solution. As seen from comparison of the respective spectra at pH 12.3 (compare Figure 9b with Figure 6d) the relative intensity of the 1360 cm-1 feature in the presence of AMT in the solution is considerably smaller for AMT adsorbed on Au support, as compared to the Ag electrode. However, after transfer of the AMT/Au electrode to the pure buffer solution (pH 12.3), the

J. Phys. Chem. C, Vol. 111, No. 46, 2007 17401 SERS spectrum on the Au electrode (Figure 9b′) was exactly the same as that for the AMT/Ag system (Figure 3b). Interestingly, the relative intensities as well as the band frequencies in the SERS spectra of AMT do not depend on the kind of the metal support. The only difference consists of the relative amount of two molecular forms of AMT under the same experimental conditions. Similar to the AMT/Ag system, the structural changes in the AMT adlayer on the Au support can also be induced by the electrode potential. As shown in Table 1, the potential value corresponding to the beginning of the molecular transformation is for Au considerably more positive than that for the Ag support in the same AMT solution, in agreement with the relation between the values of the potential of zero charge for Ag and Au electrodes.25,26 Our experiments are reminiscent of the results reported for benzotriazole (BTA) adsorbed on a Cu(100) electrode.27 In this case, two structures were identified at the surface with in situ STM (scanning tunneling microscopy), the ordered phase of flat-lying molecules, and the disordered phase of perpendicularly oriented BTA, forming long chains, in which molecules were oriented parallel to each other. Most recently, similar dynamic changes of the adsorption mode ascribed to the changes of orientation were reported for 2-amino-5-mercapto-1,3,4-thiadiazole at a silver electrode.16 XPS Experiments. XPS experiments were performed for the Ag and Au electrodes modified with the AMT monolayers. The

Figure 10. XPS N(1s) and S (2p) peaks of the AMT monolayer on Ag (a, a′) and Au (b) support. (a, b) “Mixed” structure of the adlayer; (a′) structure of the “b” type.

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TABLE 2: Binding Energies of N 1s and S 2p3/2 for AMT Monolayers on Ag and Au Surfaces EB (eV) sample Ag Au

dN- -NH2 -NH- -S-Me ···SdC

“mixed” “a” and “b” 399.4 400.6 structure “b” structure 399.2 400.5 “mixed” structure 398.9 400.2

401.6

162.4

401.3

162.5 162.3

163.9

samples for XPS measurements were prepared with the same procedure as for the SERS experiments. The monolayers were self-assembled on the Ag and Au electrodes by immersing in 10 mM AMT solution for a few hours. The AMT-modified electrodes were then thoroughly rinsed with pure water and dried in air. The “b” structure of the monolayer was obtained by prolonged desorption in the buffer solution (pH 12.3). Both flat and electrochemically roughened surfaces were examined, and no differences between those two substrates were found in the XPS spectra of adsorbed molecules. To control the actual structure of the AMT monolayer, the SERS spectra of the samples before and after XPS experiments have been recorded. A series of XPS spectra for S(2p) and N(1s) are shown in Figure 10. All the binding energies for various types of the N and S atoms in AMT molecules adsorbed on Ag and Au supports are collected in Table 2. The S(2p) peak was deconvoluted, yielding two pairs of peaks due to spin-orbit coupling. The S(2p3/2) peak for a monolayer exhibiting a “mixed “type spectrum on the Ag support has two components corresponding to binding energies close to 162 eV (main peak) and about 164 eV. In the case of the monolayer of the “b” structure on Ag and the monolayer on the Au substrate (“mixed” structure), the higher energy component was not observed. The binding energy of 162 eV is characteristic of a thiolate group bound to the metal surface,28,29 giving strong evidence that AMT adsorbs on the Ag and Au surfaces in a thiolate form. A very weak component at 164 eV may be ascribed to the presence of a very small amount of the thione tautomer of AMT30 or physisorbed AMT in a thiol (SH) form.28 The N(1s) peaks were best fitted with two (Figure 10a′) components in the case of a monolayer identified as the “b” structure or three (Figure 10a and 10b) components in the case of the “mixed” structure. The peak at 400.2-400.6 eV is characteristic of amine groups (NH2),31 while that at lower energies (near 399 eV) can be attributed to imine nitrogen (d N-)32 in the ring and/or to the nitrogen atoms of the deprotonated triazole ring in an anionic form. The third component, that occurs at 401.3 eV (for Au) and 401.6 eV (for Ag) in the case of monolayers exhibiting the “mixed” type SERS spectrum, with features characteristic for both types of monolayers (spectra a and b in Figure 3), can be attributed to protonated (NH) nitrogens in the triazole ring,32,33 in agreement with the wellknown observation that attachment of the proton to the N atom results in a core-level shift to higher binding energy. However, the highest energy peak has not been found in the XPS spectra of the monolayers characterized by the SERS spectrum shown in Figure 3b (structure b), confirming the conclusion about deprotonation of the triazole ring in this structure, drawn on the basis of the SERS experiments. As shown in Figure 1, deprotonated AMT molecules may exist in anionic or neutral, imine form. The XPS results for the monolayer of the “b” type point to the presence of the NH2 groups (component at 400.5 eV). However, this component may be also ascribed to the protonated imine nitrogen (substituent dNH group). The imine structure of the AMT molecules in the monolayer of the “b” type remains in good agreement with the SERS results. We

observed a dramatic intensity increase of the SERS spectrum for the “b” structure, that made recording the SERS spectrum possible on the macroscopically flat Ag support. Very high relative intensity of the SERS spectrum of the “b” type structure suggests that in this case the resonance Raman effect may contribute to the total enhancement of the Raman scattering. This effect seems to be quite probable in the imine structure, which is composed of the system of three (two -CdN- and one -NdN-) coupled double bonds (see Figure 1). This problem will be described in a separate paper. Conclusions The SERS and XPS experiments confirmed that AMT adsorbs on both Ag and Au surfaces by forming a thiolate bond with the metal. The negligible admixture of the thione form was found only on the Ag supports. Two different structures of the AMT monolayers were identified. At the early stages of selfassembly, the AMT molecules interact with the surface also through the nitrogen atoms of the deprotonated triazole rings, oriented flat to the metal surface. Deprotonated AMT molecules may exist in the anionic form with an NH2 substituent or as an imine structure involving a dNH substituent. In the subsequent steps of monolayer formation, the AMT molecules change their orientation with respect to the surface, with simultaneous protonation of the nitrogen in the ring. This structure is most probably stabilized by a network of hydrogen bonds and stacking interactions of the aromatic rings oriented parallel to each other. The relative amount of the respective structure critically depends on the concentration of the adsorbate in the solution, its pH value, and the potential applied to the electrode. The monolayer structure can be reversibly tuned between the two molecular states described, by changing one or more of those parameters. The structure of AMT molecules constituting the monolayer may play a decisive role in the ability of this monolayer to act as a linkage layer for binding the biologically active molecules to the surface, for at least two reasons. First, in many cases free NH2 groups are necessary for effective protein binding. Second, the electron transfer to/from the metal may be strongly influenced by the electronic structure and orientation of the molecules constituting the monolayer. Acknowledgment. This work was financially supported by the Ministry of Science and Higher Education Project No. N N204 0366 33 for years 2007-2008. M. Pisarek, Ph.D. (Institute of Physical Chemistry, Polish Academy of Science), is acknowledged for recording and resolving the XPS spectra. References and Notes (1) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R.; Whitesides, G. M. Chem. ReV. 2005, 105, 1103. (2) Ulman, A. Chem. ReV. 1996, 96, 1533. (3) Whitesides, G. M.; Laibnis, P. E. Langmuir 1990, 6, 87. (4) Taniguchi, I.; Iseki, M.; Yamaguchi, H.; Yasukouchi, K. J. Chem. Soc., Chem. Commun. 1982, 1032. (5) Taniguchi, I.; Toyosawa, K.; Yamaguchi, H.; Yasukouchi, K. J. Electroanal. Chem. 1982, 140, 187. (6) Taniguchi, I.; Yoshimoto, S.; Yoshida, M.; Kobayashi, S.; Miyawaki, T.; Aono, Y.; Sunatsuki, Y.; Taira, H. Electrochim. Acta 2000, 45, 2843. (7) Sato, Y.; Mizutani, F. J. Electroanal. Chem. 1999, 473, 99. (8) Sato, Y.; Mizutani, F. Electrochim. Acta 2000, 45, 2869. (9) Jun, Y. Y.; Beng, K. S. Electrochem. Commun. 2004, 6, 87. (10) Raj, C. R.; Ohsaka, T. J. Electroanal. Chem. 2003, 540, 69. (11) Chance, J. J.; Purdy, W. C. Thin Solid Films 1998, 335, 237. (12) Kalimuthu, P.; John, S. A. Electrochem. Commun. 2005, 7, 1271. (13) Yixin, S.; Wang, S. F. Microchim. Acta 2006, 154, 115. (14) Elhajji, A.; Ouijja, N.; Saidi Idrissi, M.; Garrigou-Lagrange, C. Spectrochim. Acta, Part A 1997, 53, 699.

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