Understanding the Equilibria of Thio Compounds Adsorbed on Gold

Article pubs.acs.org/JPCC

Understanding the Equilibria of Thio Compounds Adsorbed on Gold by Surface-Enhanced Raman Scattering and Density Functional Theory Calculations Tércio de F. Paulo,† Rômulo A. Ando,† Izaura C. N. Diógenes,*,‡ and Marcia L. A. Temperini*,† †

Instituto de Química, Universidade de São Paulo, Cx. Postal 26077, São Paulo-SP, Brasil 05508-000 Departamento de Química Orgânica e Inorgânica, Universidade Federal do Ceará, Cx. Postal 6021, Fortaleza, Ceará, Brasil 60455-970

‡

S Supporting Information *

ABSTRACT: The self-assembled monolayers (SAMs) derived from thionicotinamide (TNA), thioisonicotinamide (iTNA), and 5-(4-pyridyl)-1,3,4-oxadiazole-2-thiol (Hpyt) on gold have been characterized via surface-enhanced Raman scattering (SERS) as a function of pH and applied potential. Density functional theory calculations performed on the molecule/metal interaction model reinforced the vibrational assignments of the SERS spectra. Despite the structural similarity, these compounds presented different behaviors depending on the pH and applied potential with the iTNA SAM being the most affected. Upon adsorption and at pH 6, Hpyt SAM is not protonated while TNA and iTNA SAMs are partially and completely protonated, respectively. The results presented herein, besides being helpful for the understanding of the formation of the SAMs, can shed light on the understanding of the different responses observed for the cytochrome c metalloprotein by using the SAMs derived from these molecules.

■

at the first cycle to irreversible shape17 in the subsequent cycles, indicating that this SAM is no longer able to assess the ET reaction of Cyt c. Surface-enhanced Raman scattering (SERS) has been demonstrated to be a powerful tool to study the conformation and the properties of SAMs adsorbed on nanostructured (i.e., roughened) metallic surfaces.18−23 In this work, we have investigated, by means of Raman and SERS techniques, the structure of TNA, iTNA, and Hpyt species and their SAMs addressing the influence of pH and applied potential on them. Moreover, density functional theory (DFT) calculations, performed on a molecule/metal interaction model, were used to support the vibrational assignments.

INTRODUCTION Self-assembled monolayers (SAMs) have attracted much attention due to the ease of formation of highly ordered structures1−3 The attachment of functional groups to SAMs has been widely used to deliberately transfer specific properties to metallic surfaces.4,5 Depending on the physical−chemistry properties, such modified surfaces, usually gold, have been used in a range of applications including chemical and biological sensors.3−9 Such manipulations provide or enhance the selectivity and sensitivity toward, for instance, the assessment of electron transfer (ET) reactions of metalloproteins, such as cytochrome c (Cyt c) and myoglobin,1,9−12 which play key roles in the respiratory chain and oxygen storage in muscle tissue,13 respectively. In addition, the use of SAMs as a biological sensor has the advantage of providing a favorable environment for the analysis of target compounds, making it less susceptible to interferences. In all cases, however, the understanding of the structure and formation process of a given SAM is a prerequisite for successful use. Thionicotinamide (TNA), thioisonicotinamide (iTNA), and 5-(4-pyridyl)-1,3,4-oxadiazole-2-thiol (Hpyt) species onto gold have been used10,14 to assess the ET reaction of Cyt c. The redox behavior of Cyt c has been shown to be dependent on the nature and orientation of the pyridine tail group of these SAMs. For Hpyt SAM, the half-wave potential, E1/2, is observed at 0.008 V vs Ag/AgCl being consistent with the value reported for the native form of this protein.15 For TNA SAM a positive shift is observed (E1/2 = 0.16 V vs Ag/AgCl) and was assigned16 to the positive charge density of the modified surface. For iTNA SAM, the voltammograms change from quasi-reversible © 2013 American Chemical Society

■

EXPERIMENTAL SECTION Thioisonicotinamide, thionicotinamide, 5-(4-pyridyl)-1,3,4-oxadiazole-2-thiol, KOH, HCl, and KCl were purchased from Aldrich and used as received. Aqueous solutions were prepared using Millipore water of at least 18 MΩ cm−1 resistance. KCl (0.1 mol L−1) was used as supporting electrolyte. The pH value was adjusted by the addition of HCl and KOH. The polycrystalline gold substrates used for SERS acquisition were polished with 600 mesh and subsequently with 1200 and 2000 mesh sandpaper and thoroughly rinsed with deionized water. The details of the oxidation−reduction cycles procedure can be found elsewhere.24 Briefly, the working electrode was Received: January 8, 2013 Revised: February 28, 2013 Published: March 5, 2013 6275

dx.doi.org/10.1021/jp400235y | J. Phys. Chem. C 2013, 117, 6275−6283

The Journal of Physical Chemistry C

Article

submitted to 25 cycles of oxidation and reduction in 0.1 mol L−1 KCl in the range of potentials between −0.3 and +1.3 V vs Ag/AgCl at a scan rate of 0.1 V s−1. The activation of the gold surfaces and the application of the potential for the acquisition of SERS with applied potential (EC-SERS) were obtained by using an Autolab PGSTAT 101 (Metrohm Autolab) potentiostat. The surfaces were modified by immersing the SERS active gold electrodes in 2.0 × 10−3 mol L−1 aqueous solution (pH ∼ 6) of the modifier compounds for 30 min, in accordance with previous kinetics studies on the adsorption processes of iTNA, TNA, and Hpyt.10,14,25 After modification, the surfaces were rinsed with ethanol and water and then dried under nitrogen flux. The ex situ SERS spectra were obtained in air contact while the in situ SERS spectra were recorded in aqueous KCl solution without the presence of the modifier species in solution. Normal Raman and SERS spectra were acquired on a Renishaw Raman InVia equipped with a CCD detector and coupled to an Olympus microscope (BTH2). The excitation radiation was the 632.8 nm line (He−Ne laser, Renishaw RL633) with 8 μW at the sample. The laser beam was focused on the sample by a ×50 long distance objective (NA = 55) or by ×63 water immersion objective (NA = 0.99). Computational Details. The quantum chemical calculations were performed with the aid of the GAUSSIAN 03 (Gaussian Inc., Wallingford, CT) software26 for the free thionicotinamide, thioisonicotinamide, and 5-(4-pyridyl)-1,3,4oxadiazole-2-thiol molecules and for those bound to an Au atom to simulate the metallic surface.27,28 The ground state geometries were fully optimized by DFT with the B3LYP hybrid functional (Becke’s gradient corrected exchange correlation in conjunction with the Lee−Yang−Parr correlation functional with three parameters).29−31 The calculations of the molecules attached to Au were performed using the gen keyword, where the atoms (C, H, N, O, and S) were calculated with the 6-311++G(d,p) basis set and the Au atom with the LANL2DZ basis set considering a pseudo potential. The vibrational frequency analyses were carried out and no imaginary frequencies were found, indicating that the optimized geometries were in a minimum of the potential energy surface. The simulated Raman spectra were plotted using the Maple 9.5 software from an analysis code written by D. J. Ross, using a bandwidth of 5 cm−1 and a scaling factor of 0.9679 for the calculated harmonic vibrational wavenumbers considering the functional (B3LYP) and the 6-311++G(d,p) basis set.32

Scheme 1. Possible Structures for Nonadsorbed and Adsorbed Hpyt

1 shows the experimental and calculated normal Raman and the ex situ SERS spectra of Hpyt. The most relevant bands and the respective assignments are displayed in Table 1. Other simulated spectra for Hpyt are shown in Figure S1 of the Supporting Information. The spectra shown in Figure 1a,b present the vibrational profiles of the Hpyt molecule in the solid and aqueous states, respectively. The vibrational assignment for Hpyt in the solid state is not straightforward due to the tautomeric equilibrium. However, the absence of the band assigned33 to S−H stretching at ca. 2550 cm−1 and the agreement with the calculated Raman spectrum for the thione isomer (Figure 1d) hint that the thione tautomer is predominant in solid state. This conclusion is reinforced by the band at 1078 cm−1, which is assigned to the stretching modes of the N(3)-protonated oxadiazole ring (Table 1). On the other hand, the spectrum obtained in solution shows a broad band at 1022 cm−1, which is assigned to the υ(NN) vibrational mode of thiol oxadiazole ring (N(3)depronated), indicating that the thiol form is favored in aqueous solution. The assignment of the thione and thiol tautomers in solid and solution phases, respectively, is reinforced by the calculated spectra, which are shown in Figure 1d,e. The ex situ SERS spectrum (Figure 1c) of Hpyt adsorbed on gold surface is relatively similar to the normal Raman spectrum in aqueous phase (Figure 1b). By accounting that the thiol form is favored in solution and the adsorption process takes place by immersing the substrate in aqueous solution, it seems reasonable to consider that the chemisorption occurs as an oxidative addition of the RS-H bound to the Au(0) surface to give Au(I)-SR followed by a reductive elimination of the hydrogen (1/2H2).35−37 In fact, the best match is obtained with the calculated spectrum for the Hpyt thiolate (Figure 1f). Aiming to investigate the protonation equilibrium of Hpyt SAM on gold, in situ SERS spectra were acquired at different pH values. Figure 2 shows the spectra obtained at pH 6 and 1 (Figure S2 of the Supporting Information presents the spectra obtained at pH values ranging from 7 to 1). At pH 6, the in situ SERS spectrum is very similar to the ex situ one (Figure 1c), suggesting the predominance of the thiolate species in both air and aqueous media.

■

RESULTS AND DISCUSSION The vibrational characterization of the nonadsorbed and adsorbed thio compound species on gold was performed by Raman spectroscopy. DFT calculations were carried out to support the vibrational assignments based on the optimized geometries of the studied molecules. 5-(4-Pyridinyl)-1,3,4-oxadiazole-2-thiol. The thio compound 5-(4-pyridinyl)-1,3,4-oxadiazole-2-thiol, which contains an oxadiazole and a pyridine ring in its structure, is very sensitive to pH of the medium with tautomeric and protonation equilibria being simultaneously operative, as illustrated in Scheme 1. The characterization of Hpyt in solid and aqueous phases was performed by means of Raman spectroscopy and DFT calculations to obtain the optimized geometries and the calculated spectra for the thiol/thione Hpyt tautomers. Figure 6276



Article

Figure 1. Normal Raman spectra recorded for solid (a) and aqueous (b) samples of Hpyt; (c) ex situ SERS spectrum obtained after the modification of Au surface with Hpyt. Optimized geometries and calculated spectra considering (d) thione and (e) thiol tautomers of nonadsorbed Hpyt, and (f) adsorbed Hpyt on Au atom.

Table 1. Experimental and Calculated Wavenumber Values Obtained from the Raman and SERS Spectra of Hpyt and Their Corresponding Vibrational Assignments wavenumber/cm−1 normal Raman solid

DFT

959 w 1013 m

974 1035

1078 m 1215 w

1065 1202

1538 m

1619 vs a

aqueous

in situ SERS DFT

ex situ SERS

DFT

pH 6

DFT

pH 1

DFT

1022 m

1010

1025 m

1010

1028 m

1010

1034 s

1061

1215

1192 1448

1541 s

1541 s

1202 1420 1526

1215

1541 m

1202 1420 1526

1215

1535

1205 1466 1525

1567 vs

1577

1585

1578 m 1620 s

1557 1585

1575 s 1609 vs

1557 1585

1575 s 1609 vs

1557 1585

1609 m 1640 s

1595 1621

vibrational assignments ϕ12 υ(COC)a+ υ(NN)a υ(NN)a ϕ18a + υ(NN)a β(CH) υ(CN)a ϕ8b + υ(CN)a ϕ9b + υ(CN)a ϕ8b + υ(CN)a υ(CN)a + ϕ8a ϕ8a

Oxadiazole ring; Varsanyi notation34 for six-membered rings was employed to assign the pyridine ring modes.

of the N atom of the pyridine ring, and it is in accordance with a previous report on the pKa of Hpyt SAM on gold.14 The best match between the experimental spectrum at pH 1 and the simulated data was achieved with the calculated spectrum for the Hpyt protonated in both pyridine N and oxadiazole N(3) atoms, as illustrated in Figure 2b,d. Therefore, the pKa at 1.3 corresponds to the protonation of oxadiazole N(3) atom. Other simulated spectra are shown in Figure S1 of the Supporting Information. Thioisonicotinamide and Thionicotinamide. Although TNA and iTNA isomers present structures less complex than that of Hpyt, they are also affected by both tautomeric (thioamide/imidothiol) and protonation equilibria38 as illustrated in Scheme 2. Therefore, the observed pKa values should be considered as composed of several individual ionization and the tautomeric equilibria constants.39 These compounds can

As can be ascertained from the experimental spectra displayed in Figure 2, there is a meaningful change in the spectral profiles with the change in the pH of the electrolyte medium with the bands between 1541 to 1640 cm−1 being the most affected. By accounting for the fact that the most probable protonation sites are the N3 atom and the N atom of the pyridine ring (see Scheme 1), the equilibrium was monitored by the ratio between the intensity of these bands in relation to the band at 1215 cm−1, assigned to the C−H bending, which is not affected by the pH. The plot of the relative intensities as a function of pH is presented in Figure 3. The curves displayed above indicate two equilibria of protonation with pKa values at 4.5 and 1.3. Although the DFT vibrational analysis indicates a large mixture of vibrational modes of pyridine and oxadiazole rings for such bands (see Table 1), the pKa value of 4.5 is consistent with the protonation 6277



Article

Figure 2. In situ SERS spectra of Hpyt SAM on gold electrode in 0.1 mol L−1 KCl at pH (a) 6 and (b) 1. Optimized geometries and calculated spectra considering different protonation states of the Hpyt molecule bound to a gold atom: (c) N-deprotonated and (d) N-protonated in both pyridine and oxadiazole N(3) atoms (see Scheme 1 for the numbering).

assume different structures depending on the medium and kind of surface where they are adsorbed. Using normal Raman and SERS, we have tried to characterize the predominant species at different conditions. Figures 4a and 5a present the normal Raman spectra of TNA and iTNA in solid state, respectively, where the tautomers can coexist. The absence, in both spectra, of the band assigned33 to S−H stretching at ca. 2550 cm−1 suggests that the thioamide is the predominant tautomer in this phase for both compounds. This assignment is supported by the calculated spectra that are illustrated in Figures 4c and 5c. The most relevant modes of TNA and iTNA along with the assignments are presented in Table 2. Aqueous or ethanolic solutions of TNA and iTNA have been used by our group in preparing SAMs on gold.10,25 In solution, such molecules can undergo several equilibria and assume different structures depending on the medium conditions such as pH and solvent polarity. At pH ∼ 6, only the tautomeric equilibria is operative and the structures 3 or 4 (TNA) and 9 or

Figure 3. Relative Raman intensities as a function of pH: (○) I1541/ I1215, (■) I1575/I1215, (Δ) I1609/I1215, and (▼) I1640/I1215 . Lines are the sigmoidal fits of the data.

Scheme 2. Possible Tautomeric and Protonation Equilibria for TNA and iTNA Thio Compounds

6278



Article

Figure 4. (a) Normal Raman spectrum recorded for TNA solid sample; (b) ex situ SERS spectrum of TNA SAM; calculated spectra and optimized geometries considering (c) thioamide tautomer of TNA and TNA bound to a gold atom with (d) RCN+H2 and (e) RC−N+H3 structures.

Figure 5. (a) Normal Raman spectrum recorded for iTNA solid sample; (b) ex situ SERS spectrum of iTNA SAM; optimized geometries and calculated spectra considering (c) thioamide tautomer of iTNA and (d) iTNA bound to a gold atom with RCN+H2 structures.

totally ruled out. In order to get insights on the understanding of such spectral changes, simulated spectra of possible structures taking into account the tautomeric and protonation equilibria were obtained and are shown in Schemes 2 and 3. Other simulated spectra are illustrated in Figures S3−S6 of the Supporting Information. The forms 3 and 4 for TNA and 9 and 10 for iTNA are the most likely species in aqueous solutions. Upon adsorption, the forms 3′ and/or 4′, and 10′ are the most likely forms of TNA and iTNA, respectively, due to the similarity with the calculated spectra (Figures 4 and 5). It can be seen in the ex situ SERS spectra that the bands assigned to the thioamide (R(NH2)CS) disappear while the new bands associated with the R (= N+H2)C-SR fragment (Figures 4 and 5 and Table 2) appear. This result indicates the decrease and increase of the bond orders of CS and CNH2 fragments, respectively, and a positive charge density on the NH2 fragment.

10 (iTNA) are expected since the observed pKa values reported40−42 for pyridine and NH2 groups are ca. 3 and 10, respectively. Unfortunately, due to the low solubility of TNA and iTNA in water, we were not able to detect the respective vibrational modes via Raman spectroscopy in aqueous solution. On the other hand, the enhancement of the exciting and scattering radiation near the roughened gold electrode and the interaction of the adsorbed modifiers with the gold atoms of the surface (SERS effect) allow the SAMs derived from TNA and iTNA to be analyzed on surface. The ex situ SERS spectra acquired for these SAMs are very different in relation to the normal Raman spectra as can be seen in Figures 4 and 5. The ρ(NH2) + υ(CS) vibrational modes of thioamide group at 918 and 1307 cm−1 for TNA (Figure 4a and Table 2) and 939 and 1299 cm−1 for iTNA (Figure 5a and Table 2) disappear after adsorption (Figures 4b and 5b). This result indicates that the differences might be due to structural changes after adsorption, even though the surface selection rules cannot be 6279



Article

Table 2. Experimental and Calculated Wavenumber Values Obtained from the Raman and SERS Spectra of TNA and iTNA and Their Corresponding Vibrational Assignments Thionicotinamide (TNA) wavenumber/cm−1 normal Raman

DFT

918 m 1035 sh 1040 vs

885 1004 1024

1307 s

1281

1576 m 1587 vs

1550 1569

ex situ SERS

DFT 3′

1024 m 1045 m

970 1022

1411 w 1488 m

1403 1468

1583 vs

1563

DFT 4′

SERS pH 6

991 1022

1563

SERS pH 1

DFT 6′

1024 s 1045 m

1024 m 1045 s 1241 m

1006 1022 1256

1411 1488

1513 s

1451

1630 m

1609

1586 vs

vibrational assignments ρ(NH2) + υ(CS) ϕ12 ϕ1 ρ(NH2) ρ(NH2) + υ(CS) + β(CH) υ(CNH2) + β(CH) β(CH) + δ(NH2) ϕ8b + δ(NH2) ϕ8a ϕ8a + δ(NH2)

Thioisonicotinamide (iTNA) wavenumber/cm−1 normal Raman 939 999 1156 1299

m vs m s

1600 s

DFT 892 997 1113 1280

1574

ex situ SERS

DFT 10′

SERS pH 6

SERS pH 1

DFT 12′

1004 m 1197 s

975 1116

1004 m 1197 s

1004 s 1197 vs

988 1116

1479 sh 1497 vs 1593 vs

1429 1466 1556

1479 1497 1600 1630

1479 1497 1600 1630

1437 1480

vs sh s s

vs sh sh s

1615

vibrational assignments ρ(NH2) + υ(CS) ϕ12 ϕ18a + ρ(NH2) ρ(NH2) + υ(CS)+ β(CH) υ(CNH2) + δ(NH2) ϕ19a ϕ8a + δ(NH2) ϕ8a + δ(NH2)

Scheme 3. Possible Equilibria for TNA and iTNA Compounds Adsorbed on Gold

For iTNA SAM, the protonation of the N atom of the pyridine ring is observed at both pH values as indicated by the band at 1630 cm−1 in Figure 6b. At pH 6, however, both protonated and deprotonated species are observed on surface as indicated by the observation of the band at 1600 cm−1 which disappears at pH 1 when the adsorbed molecule is completely protonated. Previous data reported by our group have indicated that the SAMs formed with the thio compounds Hpyt, TNA, and iTNA on gold present different capabilities to assess the ET reaction of the Cyt c metalloprotein.10,14 Although all these SAMs present the pyridine as tail group, the differences observed in

In order to gain further insight on the protonation of the N atom of the pyridine ring of the modifiers, SERS spectra of TNA and iTNA SAMs (Figure 6) were acquired in aqueous medium at pH 6 and 1. For TNA SAM, the in situ SERS spectrum obtained at pH 6 (Figure 6a) is quite similar to that obtained in air (Figure 4a). At pH 1, the protonation of the pyridine group is evidenced by the band at 1630 cm−1, which is assigned to the characteristic ϕ8a mode of protonated pyridine. The intensity of the band at 1513 cm−1, assigned to the ν(CNH2) mode, also increases indicating that the form 6′ is the most likely at pH 1 (see Scheme 3). 6280



Article

within the potential range normally used in the Cyt c electrochemical studies, i.e., from 0.3 to −0.3 V vs Ag/AgCl. Electrochemical SERS (EC-SERS) spectra were obtained for Hpyt, TNA, and iTNA SAMs at pH 6 and are illustrated in Figure 7. The SERS spectra obtained for Hpyt (Figure 7a) and TNA (Figure 7b) showed virtually no dependence on the applied potential, indicating no structural change of these species from 0.3 to −0.3 V vs Ag/AgCl. On the other hand, for iTNA SAM, the EC-SERS spectra presented in Figure 7c show different profiles. It is well-known that the applied potential may affect the charge density on the adsorbed molecules and the values of pH near the surface.46 As a consequence, the protonation equilibrium as well as the orientation of the adsorbed molecules should be affected. In accordance with the results presented in this work, only the iTNA SAM was shown to be affected by the applied potential within the studied potential range (from −0.3 to +0.3 V vs Ag/AgCl). The pH-sensitive counterpart bands appear at 1600/1630 cm−1 and 1479/1500 cm−1. As the potential becomes more positive than −0.1 V vs Ag/AgCl, some of the adsorbed iTNA molecules are converted to the Nprotonated form while at negative potential the molecules on the surface are N-deprotonated. Our hypothesis is that at negative potentials, the NH2 fragment, which is positively charged, is closer to the surface than at positive potentials, as suggested in Scheme 4. Accordingly, the formation of hydrogen bonding between the N atom of the pyridine ring and the NH2 group is favored for iTNA in comparison to TNA due to the para position of the former, which decreases the distance between the N and NH2 fragments of the adjacent molecules. In fact, STM images have indicated that the average distance between the adsorbed TNA molecules ranges from 0.55 to 0.85 nm10. Assuming that iTNA molecules assemble at similar distances, it is reasonable to expect hydrogen bonding formation at potentials more positive than −0.1 V. The suggestion illustrated in Scheme 4 is reinforced by the observation in the EC-SERS of iTNA (Figure 7) at potentials more positive than −0.1 V of the band at 1630 cm−1, which is assigned to the stretching mode ϕ8a ν(CC) of N-protonated pyridine ring. In addition, the typical broadening assigned to the formation of hydrogen bonding47 is observed for the band at 1479 cm−1, which is assigned to the υ(CNH2) + δ(NH2) modes.

Figure 6. SERS spectra of SAMs derived from (a) TNA and (b) iTNA on gold in an 0.1 mol L−1 KCl aqueous solution at different pH values.

the ET reaction of Cyt c have been tentatively assigned to the charge density of the adsorbed molecules, which is directly affected by the protonation equilibrium. Other factors such as the position of the terminal end group in relation to the headgroup have also been considered. Among these molecules, the best results (current and potential parameters) for the ET reaction of Cyt c were observed for the Hpyt SAM,14 which presents a nonprotonated configuration at physiological pH. According to the SERS spectra presented in this work, both the TNA and iTNA SAMs are partially or completely protonated at pH 6, indicating an increase in the positive charge density on surface. This conclusion explains the positive shift in the halfwave potential (E1/2) value of Cyt c when TNA SAM is used.10 Such result is in accordance with the hypothesis of Whitesides et al.16 in which SAMs containing positively charged groups cause the redox potential of Cyt c to shift to positive values. For iTNA SAM, the results indicate a complete protonation at physiological pH and, therefore, a higher positive charge density that, in turn, results in a higher repulsion in relation to the Cyt c molecules in solution, which are positively charged at this pH.13 The SERS data commented above reinforce the wellknown statement that pyridine group plays an important role in the ET reaction of Cyt c, although the mechanism is not yet fully understood.43−45 Another relevant point that must be addressed is the effect of the applied potential on the nature of the SAMS, particularly

Figure 7. EC-SERS spectra of SAMs derived from (a) Hpyt, (b) TNA, and (c) iTNA on gold recorded at different potentials in 0.1 mol L−1 KCl (pH 6). 6281



Article

Scheme 4. Schematic Model to Represent the Influence of the Applied Potential on the Orientation of iTNA and iTNA SAMs

Notes

Concerning the Cyt c electrochemistry, the EC-SERS results showing the influence of the applied potential on the nature of iTNA SAM explain the noncapability of this SAM to assess the ET reaction of Cyt c17. In fact, the voltammograms of the gold electrode modified with iTNA in solution containing Cyt c change from quasi-reversible at the first cycle to irreversible in the subsequent cycles.17

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS T.F.P. is thankful for the grant from FAPESP (2011/12479-3). M.L.A.T., R.A.A., and I.C.N.D. are thankful to CNPq for the grants and FAPESP, FUNCAP (PRONEM PRN-004000065.01.00/10 SPU No. 10582696-0), and CAPES for the financial support.

■

CONCLUSION By combining the analysis of the Raman and SERS spectra of thionicotinamide, thioisonicotinamide, and 5-(4-pyridil)-1,3,4oxadiazole-2-thiol with the results of the DFT calculations, we were able to establish which species are adsorbed on the electrode surface and to demonstrate the effect of pH and applied potential on the nature of the SAMs. The normal Raman spectra indicated that the thione of Hpyt and the thioamide tautomer of TNA and iTNA are predominant in solid state while the thiol and imidothiol tautomers, respectively, are favored in aqueous solution. Despite the structural similarity, these compounds presented different behaviors depending on the pH and applied potential. In addition, iTNA SAM showed to be strongly affected by the applied potential (from −0.3 to +0.3 V vs Ag/AgCl) with the experimental spectroscopic data being well supported by the DFT calculated spectra. Upon adsorption and at pH 6, Hpyt SAM is not protonated while TNA and iTNA SAMs are partially and completely protonated, respectively. The in situ and ex situ spectra of iTNA SAM explain the electrochemical data observed for Cyt c. We guess that the results presented herein, besides being helpful for the understanding of the formation of the SAMs, can shed light on the understanding of the different responses observed for the Cyt c metalloprotein by using the SAMs derived from these molecules.

■

■

(1) Yue, H.; Waldeck, D. H.; Schrock, K.; Kirby, D.; Knorr, K.; S., S.; Rosmus, J.; Clark, R. A. Multiple Sites for Electron Tunneling between Cytochrome c and Mixed Self-Assembled Monolayers. J. Phys. Chem. C 2008, 112, 2514−2521. (2) Jin, Q.; Rodriguez, J. A.; Li, C. Z.; Darici, Y.; Tao, N. J. SelfAssembly of Aromatic Thiols on Au(111). Surf. Sci. 1999, 425, 101− 111. (3) Xue, Y.; Zimmt, M. B. Patterned Monolayer Self-Assembly Programmed by Side Chain Shape: Four-Component Gratings. J. Am. Chem. Soc. 2012, 134, 4513−4516. (4) Pons-Siepermann, I. C.; Glotzer, S. C. Design of Patchy Particles Using Quaternary Self-Assembled Monolayers. ACS Nano 2012, 6, 3919−3924. (5) Elemans, J. A. A. W.; Lei, S.; De Feyter, S. Molecular and Supramolecular Networks on Surfaces: From Two-Dimensional Crystal Engineering to Reactivity. Angew. Chem., Int. Ed. 2009, 48, 7298−7332. (6) Chen, D.; Li, J. Interfacial Design and Functionlization on Metal Electrodes through Self-Assembled Monolayer. Surf. Sci. Rep. 2006, 61, 445−463. (7) Freire, R. S.; Pessoa, C. A.; Kubota, L. T. Self-assembled monolayers applications for the development of electrochemical sensors. Quim. Nova 2003, 26, 381−389. (8) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Self-Assembled Monolayers of Thiolates on Metals as a Form of Nanotechnology. Chem. Rev. 2005, 105, 1103−1170. (9) Paulo, T. F.; Diógenes, I. C. N.; Abruña, H. D. Direct Electrochemistry and Electrocatalysis of Myoglobin Immobilized on LCysteine Self-Assembled Gold Electrode. Langmuir 2011, 27, 2052− 2057. (10) Paulo, T. F.; Pinheiro, S. D.; Da Silva, M. A. S.; Lopes, L. G. D.; Pinheiro, L. S.; Aquino, G. F. A.; Temperini, M. L. A.; Neto, P. D.; Diógenes, I. C. N. Thionicotinamide SAM on Gold: Adsorption Studies and Electroactivity. Electroanalysis 2009, 21, 1081−1089. (11) Davis, K. L.; Drews, B. J.; Yue, H.; Waldeck, D. H.; Knorr, K.; Clark, R. A. Electron-Transfer Kinetics of Covalently Attached

ASSOCIATED CONTENT

S Supporting Information *

Figures S1−S6 as mentioned in the text. This material is available free of charge via the Internet at http://pubs.acs.org.

■

REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (M.L.A.T); [email protected] (I.C.N.D). 6282



Article

Cytochrome c/SAM/Au Electrode Assemblies. J. Phys. Chem. C 2008, 112, 6571−6576. (12) Diógenes, I. C. N.; Nart, F. C.; Barreto, M.; Temperini, M. L. A.; Moreira, I. D. SERRS study of [Ru(CN)5(pyS)]4‑ SAM and Cytochrome c: A Suggestion toward the Heterogeneous Molecular Recognition. J. Solid State Electrochem. 2007, 11, 1585−1590. (13) Van Gelder, B. F.; Slater, E. C. The Extinction Coefficient of Cytochrome c. Biochim. Biophy. Acta 1962, 58, 593−595. (14) Paulo, T. F.; da Silva, M. A. S.; Pinheiro, S. D.; Meyer, E.; Pinheiro, L. S.; Freire, J. A.; Tanaka, A. A.; de Lima, P.; Moreira, I. D.; Diógenes, I. C. N. 5-(4-Pyridinyl)-1,3,4-oxadiazole-2-thiol on Gold: SAM Formation and Electroactivity. J. Braz. Chem. Soc. 2008, 19, 711−719. (15) Eddowes, M. J.; Hill, H. A. O. Electrochemistry of Horse Heart Cytochrome c. J. Am. Chem. Soc. 1979, 101, 4461−4464. (16) Chen, X.; Ferrigno, R.; Yang, J.; Whitesides, G. M. Redox Properties of Cytochrome c Adsorbed on Self-Assembled Monolayers: A Probe for Protein Conformation and Orientation. Langmuir 2002, 18, 7009−7015. (17) Paulo, T. F.; Sousa, T. P.; S., A. D. d.; Bernhardt, P. V.; Diógenes, I. C. N., 2013, submitted for publication. (18) Pinheiro, L. S.; Temperini, M. L. A. SERS Effect of PyridineNormal-Aldehyde Thiosemicarbazone on a Silver Electrode. J. Electroanal. Chem. 1990, 295, 169−181. (19) Diógenes, I. C. N.; Nart, F. C.; Temperini, M. L. A.; Moreira, I. D. The [Ru(CN)5(pyS)4‑] Complex, an Efficient Self-Assembled Monolayer for the Cytochrome c Heterogeneous Electron Transfer Studies. Inorg. Chem. 2001, 40, 4884−4889. (20) Lorén, A.; Engelbrektsson, J.; Eliasson, C.; Josefson, M.; Abrahamsson, J.; Abrahamsson, K. Self-Assembled Monolayer Coating for Normalization of Surface Enhanced Raman Spectra. Nano Lett. 2004, 4, 309−312. (21) Kim, K.; Yoon, J. K.; Lee, H. B.; Shin, D.; Shin, K. S. SurfaceEnhanced Raman Scattering of 4-Aminobenzenethiol in Ag Sol: Relative Intensity of a1- and b2-Type Bands Invariant against Aggregation of Ag Nanoparticles. Langmuir 2011, 27, 4526−4531. (22) Kim, K.; Kim, K. L.; Shin, D.; Choi, J.-Y.; Shin, K. S. SurfaceEnhanced Raman Scattering of 4-Aminobenzenethiol on Ag and Au: pH Dependence of b2-Type Bands. J. Phys. Chem. C 2012, 116, 4774− 4779. (23) Caldwell, W. B.; Campbell, D. J.; Chen, K.; Herr, B. R.; Mirkin, C. A.; Malik, A.; Durbin, M. K.; Dutta, P.; Huang, K. G. A Highly Ordered Self-Assembled Monolayer Film of an Azobenzenealkanethiol on Au(111): Electrochemical Properties and Structural Characterization by Synchrotron in-Plane X-ray Diffraction, Atomic Force Microscopy, and Surface-Enhanced Raman Spectroscopy. J. Am. Chem. Soc. 1995, 117, 6071−6082. (24) Gao, P.; Gosztola, D.; Leung, L.-W. H.; Weaver, M. J. SurfaceEnhanced Raman Scattering at Gold Electrodes: Dependence on Electrochemical Pretreatment Conditions and Comparisons with Silver. Electroanal. Chem. Interf. Electrochem. 1987, 233, 211−222. (25) Paulo, T. F.; Abruna, H. D.; Diógenes, I. C. Thermodynamic, Kinetic, Surface pK(a), and Structural Aspects of Self-Assembled Monolayers of Thio Compounds on Gold. Langmuir 2012, 28, 17825−17831. (26) Frisch, M. J.; ; Trucks, G. W.; ; Schlegel, H. B.; ; Scuseria, G. E.; ; Robb, M. A.; ; Cheeseman, J. R.; ; Montgomery, J. A.; ; Vreven, T.; ; Kudin, K. N.; ; Burant, J. C.; et al. Gaussian 03 (Revision D.1); Gaussian, Inc.: Wallingford, CT, 2004. (27) Santos Costa, J. C.; Ando, R. A.; Sant’Ana, A. C.; Rossi, L. M.; Santos, P. S.; Temperini, M. L. A.; Corio, P. High Performance Gold Nanorods and Silver Nanocubes in Surface-Enhanced Raman Spectroscopy of Pesticides. Phys. Chem. Chem. Phys. 2009, 11, 7491−7498. (28) Costa, J. C. S.; Ando, R. M. A.; Camargo, P. H. C.; Corio, P. Understanding the Effect of Adsorption Geometry over Substrate Selectivity in the Surface-Enhanced Raman Scattering Spectra of Simazine and Atrazine. J. Phys. Chem. C 2011, 115, 4184−4190.

(29) Stephens, P. J.; Devlin, F. J.; Chabalowski, C. F.; Frisch, M. J. Ab Initio Calculation of Vibrational Absorption and Circular Dichroism Spectra Using Density Functional Force Fields. J. Phys. Chem. 1994, 98, 11623−11627. (30) Lee, C.; Yang, W.; Parr, R. G. Development of the Colle-Salvetti Correlation-Energy Formula into a Functional of the Electron Density. Phys. Rev. B 1988, 37, 785−789. (31) Becke, A. D. Density-Functional Thermochemistry. III. The Role of Exact Exchange. J. Chem. Phys. 1993, 98, 5648−5652. (32) Andersson, M. P.; Uvdal, P. New Scale Factors for Harmonic Vibrational Frequencies Using the B3LYP Density Functional Method with the Triple-ζ Basis Set 6-311+G(d,p). J. Phys. Chem. A 2005, 109, 2937−2941. (33) Joo, H. T.; Kim, M. S.; Kim, K. Surface-Enhanced Raman Scattering of Benzenethiol in Silver Sol. J. Raman Spectrosc. 1987, 18, 57−60. (34) Varsanyi, G. Assignments for Vibrational Spectra of 700 Benzene Derivatives; Wiley: New York, 1974. (35) Sellers, H.; Ulman, A.; Shnidman, Y.; Eilers, J. E. Structure and Binding of Alkanethiolates on Gold and Silver Surfaces: Implications for Self-Assembled Monolayers. J. Am. Chem. Soc. 1993, 115, 9389− 9401. (36) Ulman, A. Formation and Structure of Self-Assembled Monolayers. Chem. Rev. 1996, 96, 1533−1554. (37) Yang, C. S. C.; Richter, L. J.; Stephenson, J. C.; Briggman, K. A. In Situ, Vibrationally Resonant Sum Frequency Spectroscopy Study of the Self-Assembly of Dioctadecyl Disulfide on Gold. Langmuir 2002, 18, 7549−7556. (38) Matysiak, J.; Niewiadomy, A.; Ż abińska, A.; Rózẏ ło, J. K. Structure and Retention of 2,4-Dihydroxythiobenzanilides in a Reversed-Phase System. J. Chromatogr. A 1999, 830, 491−496. (39) Jang, Y. H.; Goddard, W. A.; Noyes, K. T.; Sowers, L. C.; Hwang, S.; Chung, D. S. First Principles Calculations of the Tautomers and pKa Values of 8-Oxoguanine: Implications for Mutagenicity and Repair. Chem. Res. Toxicol. 2002, 15, 1023−1035. (40) Casasnovas, R.; Frau, J.; Ortega-Castro, J.; Salvà, A.; Donoso, J.; Muñoz, F. Absolute and Relative pKa Calculations of Mono and Diprotic Pyridines by Quantum Methods. J. Mol. Struct.: THEOCHEM 2009, 912, 5−12. (41) Dey, G. R.; Naik, D. B.; Kishore, K. Pulse Radiolysis of Thionicotinamide in Aqueous Solutions: Formation of Resonance Stabilized Species on One Electron Oxidation. Res. Chem. Intermed. 2003, 29, 147−156. (42) Jackson, M. D.; Schmidt, M. T.; Oppenheimer, N. J.; Denu, J. M. Mechanism of Nicotinamide Inhibition and Transglycosidation by Sir2 Histone/Protein Deacetylases. J. Biol. Chem. 2003, 278, 50985− 50998. (43) Volkov, A. N.; Nicholls, P.; Worrall, J. A. R. The Complex of Cytochrome c and Cytochrome c Peroxidase: The End of the Road? Biochim. Biophys. Acta 2011, 1807, 1482−1503. (44) Fedurco, M. Redox Reactions of Heme-Containing Metalloproteins: Dynamic Effects of Self-Assembled Monolayers on Thermodynamics and Kinetics of Cytochrome c Electron-Transfer Reactions. Coord. Chem. Rev. 2000, 209, 263−331. (45) Zhao, H. Z.; Du, Q.; Li, Z. S.; Yang, Q. Z. Mechanisms for the Direct Electron Transfer of Cytochrome c Induced by Multi-Walled Carbon Nanotubes. Sensors 2012, 12, 10450−10462. (46) Dorain, P. B.; Von Raben, K. U.; Richard, K. C. Voltage Dependence of the pH at the Interface of a Ag Electrode: A SERS Measurement of a Phosphate-Phosphoric Acid System. Surf. Sci. 1984, 148, 439−452. (47) Coulson, C. A.; Robertson, G. N. A Theory of the Broadening of the Infrared Absorption Spectra of Hydrogen-Bonded Species. Proc. R. Soc. London, Ser. A. 1974, 337, 167−197.

6283


Understanding the Equilibria of Thio Compounds Adsorbed on Gold

Recommend Documents