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Exploring the Effect of Intermolecular H‑Bonding: A Study on Charge-Transfer Contribution to Surface-Enhanced Raman Scattering of p‑Mercaptobenzoic Acid Yue Wang,† Wei Ji,‡ Huimin Sui,† Yasutaka Kitahama,‡ Weidong Ruan,† Yukihiro Ozaki,‡ and Bing Zhao†,* †

State Key Laboratory of Supramolecular Structure and Materials, Jilin University, Changchun 130012, People’s Republic of China Department of Chemistry, School of Science and Technology, KwanseiGakuin University, Sanda, Hyogo 669-1337, Japan

‡

S Supporting Information *

ABSTRACT: We report the significant effect of intermolecular hydrogen bonding (H-bonding) on surface-enhanced Raman scattering (SERS) spectra in which the vibrational frequencies and intensities of some characteristic peaks of p-mercaptobenzoic acid (MBA) change with varying concentrations of aniline. These changes can be attributed to modifications in the electronic structure of the MBA molecule and the conjugation of the system under the influence of H-bonding. Of remarkable note is that the nontotally symmetric (b2) mode of MBA is dramatically enhanced, which can be considered as a manifestation of the charge-transfer (CT) transition process in the system. By comparing SERS spectra obtained under normal and basic conditions, the effect caused by H-bonding can be further understood. These results manifest that the CT resonance between MBA and Ag NPs through Herzberg−Teller contributions can be promoted by H-bonding. The current work may, therefore, be instructive for studying the influence of H-bonding on the electronic structure of molecules in a system via SERS technique.

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bonding studies,14−16 possible modifications to the polarizability of a molecule caused by H-bonding are still ambiguous. Therefore, a better understanding of H-bonding via SERS spectroscopy is necessary for a deeper comprehension of the modification to the electronic structure of a molecule resulting from H-bonding interaction. Moreover, the results of such a study can also be instructive for developing SERS analytical techniques based on the characteristic of a SERS probe molecule affected by H-bonding. It is widely recognized that there are two contributions to the Raman enhancement, namely, the electromagnetic (EM) and chemical (CE) enhancement mechanisms.2,3,17−21 Unlike the EM enhancement, which mainly stems from surface plasmon resonance on a metal substrate, the CE enhancement is a resonance-like process in which charge-transfer (CT) occurs between the adsorbed molecules and the metal surface.20,21 The transition energy of this CT process depends on the energy difference between the Fermi level of the metal and the molecular orbital of the adsorbate.22 As a result, the CT process can contribute to the enhancement up to several orders of magnitude in a SERS spectrum, in addition to greatly affecting

INTRODUCTION Surface-enhanced Raman scattering (SERS) is a powerful technique for exploring the structure, orientation, adsorption, and quantitative and qualitative analysis of a molecule due to the molecular specificity directly associated with vibrational modes.1−3 Although SERS has aroused great interest and has been intensively investigated,2−7 there still remain some fundamental questions that are poorly understood or even neglected. These questions arise from the sensitivity of SERS to not only slight details of molecular structure, but also to local environment.4−6 Factors such as potential and pH have been studied to understand the impact of the local environment on the SERS profiles of many adsorbates.4,5 Actually, influences on the SERS spectra are not merely in these aspects. Many intermolecular interactions present in the system, so far as it is known,8,9 can significantly affect the SERS signal of adsorbed molecules by modifying the electronic molecular structure of molecules.10−12 However, the effect of intermolecular interaction on its SERS profiles has been simply neglected in many cases, which may restrict some of the practical applications and also lead to incomplete conclusions. This is the case with hydrogen-bonding (H-bonding),12 a commonly occurring intermolecular interaction in many systems. H-bonding has been known to strongly affect some intrinsic properties and performance of molecules.12,13 Despite great advances in H© 2014 American Chemical Society

Received: March 13, 2014 Revised: April 26, 2014 Published: April 28, 2014 10191

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Scheme 1. Illustration of the Formation of the Ag NPs/MBA/Aniline Intermolecular System

The substrates were prepared via a self-assembled method. The positively charged polyelectrostatic (poly(diallyldimethylammonium chloride)) adhered on a hydroxylated glass slide surface for the adsorption of negatively charged Ag NPs. A layer of Ag NPs was assembled on the glass surface by electrostatic interaction and the substrates were complete. MBA molecules were then adsorbed on the assembled Ag NPs through the formation of Au−S bonds. After exhaustive rinsing by ethanol and drying by nitrogen, the substrates with the adsorbed MBA were soaked in ethanolic solutions of aniline with different concentrations for 12 h and then used directly for in-solution SERS measurements. Scheme 1 illustrates the process for fabricating the Ag NPs/MBA/aniline intermolecular system. MBA and aniline solutions used were prepared with ethanol. The concentration of MBA was 10−3 M, while the aniline solutions were in the range from 10−8 to 10−2 M. To eliminate the pH interference, a series of ethanolic solutions of aniline containing NaOH at a concentration of 10−3 M was prepared. The UV−vis spectura was recorded on a Shimadzu UV-3600 spectrophotometer. All Raman and SERS spectra were recorded in a Horiba-Jobin-Yvon LabRAM ARAMIS spectrometer equipped with a 633 nm He−Ne exciting laser with an effective power of 3 mW reaching the sample. The laser was focused on the surface of the sample through a 50× longdistance objective lens with a 1 μm spot size. SERS spectra were obtained with an acquisition time of 10 s using a holographic grating of 1200 grooves/mm. The Raman band of the silicon wafer at 520.7 cm−1 was used to calibrate the spectrometer.

the relative intensities of different vibrational modes of an adsorbate. Over the past few years, many organic sulfur compounds have been used as SERS probe molecules to demonstrate the importance of the contribution of CT to a SERS spectrum, which is owing to their strong SERS signals and characteristic features.23−25 Our group has been focusing on investigating the CT contributions by studying the changes in the SERS spectra of some organic sulfur molecules.26−28 p-Mercaptobenzoic acid (MBA), a simple aromatic molecule containing both carboxyl and thiol functional groups, has, in particular, attracted much attention because of its potential applications in pH sensors and ion detection.29−32 Many studies have been carried out using MBA in various systems; nevertheless, there are still some important factors, including the influence of intermolecular Hbonding, that remain to be understood. It can be expected that H-bonding effects the contribution of CT to the SERS spectra, since intermolecular H-bonding could modify the electronic structure of the metal−adsorbate system.33 In this work, we systematically study the CT-induced SERS spectra of MBA under the influence of different numbers of intermolecular H-bonds, which leads to a variation in the relative intensity of the SERS signal and frequency shifts of some bands in the SERS spectra. To simplify complications associated with a solution environment and minimize the diversity of the orientations of MBA in Ag nanocolloids, a selfassembled monolayer of MBA at the surface of self-assembled Ag nanoparticles (Ag NPs) was used as a system (Ag-MBA). By comparing SERS spectra obtained under normal and basic conditions, the effect caused by H-bonding can be further understood.

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RESULTS AND DISCUSSION Figure 1A shows the SERS spectra of MBA as a function of aniline concentration ranging from 0 to 10−8 M (0, 10−8, 10−7, 10−6, 10−5, 10−4, 10−3, 10−2 M). The band assignments of MBA are summarized in Table 1, as acquired from previous reports.35,36 The spectrum, which shows the intense SERS features characteristic of MBA, is dominated by the bands at 1075 cm−1, assigned to the in-plane ring breathing mode coupled with ν (C−S), and at 1584 cm−1 ascribed to the aromatic ν(CC) mode.35 Upon exposure of the Ag-MBA complex to ethanolic solutions of aniline with different concentrations, we observed that both the spectral shapes and the intensities of the SERS spectra differed significantly. Figure 1B plots the SERS intensity at 1075 cm−1 versus the concentration of aniline. The relative intensities of many characteristic peaks changed with the aniline concentration. The most prominent changes are observed for the bands at 417 cm−1, assigned to the ν (C−S) mode, at 691 and 713 cm−1,

EXPERIMENTAL SECTION The p-mercaptobenzoic acid and aniline molecules were purchased from Sigma-Aldrich Chemical Co., and other chemicals (analytic grade) were obtained from Beijing Chemical Reagent Factory. All the chemicals were used without any further purification. The distilled and deionized water from a Milli-Q-plus system with the resistivity greater than 18.0 MΩ· cm was used in preparing the Ag nanocolloids and the Ag NPs substrates. Ag nanocolloids were synthesized by the process of Lee and Meisel.34 Briefly, for a 100 mL solution, 18 mg silver nitrate was added and brought to boiling. A 1% sodium citrate solution with the volume of 2 mL was used as reductant. The solution was kept on 85 °C for 45 min. The greenish yellow sol shows a single extinction maximum at 406 nm (see Supporting Information, SI, Figure S1). The size of the Ag NPs in the nanocolloids is with an average diameter of ∼70 nm. 10192

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Notably, the intensity of the bands at 998 and 1022 cm−1 increase with the concentration of aniline. These two bands have frequently appeared in previous reports of the SERS spectra of MBA; despite this, the assignments and the cause behind the peak have always remained ambiguous owing to large disparities in the intensities obtained with different systems.29−32 To illustrate that these bands were SERS features belonging to MBA and not to the introduced aniline, a control experiment was carried out. By comparing the SERS spectrum of aniline and MBA at a concentration of 10−3 M under the same test conditions (SI Figure S2(b)), it can be noted that the SERS response of aniline is much weaker than that of MBA, and an obvious signal corresponding to aniline cannot even be obtained under the experimental conditions. Accordingly, the intensification of these two bands indicates the changes in the polarizability of the bonds for the corresponding vibrational modes of MBA caused by the formation of the intermolecular H-bond between MBA and aniline, increasing the degree of conjugation and further modifying the electronic structure of the phenyl ring in the MBA-Ag complex.37,38 It is, therefore, not surprising that the intensities of these bands are associated with a change in the polarizability of the phenyl ring of MBA. Considering that the in-plane ring breathing coupled ν (C− S) band is the strongest and remains more insusceptible before and after the addition of aniline, the SERS spectra of MBA in varying concentrations of aniline are normalized to the band at 1075 cm−1, as illustrated in Figure 2A. Distinct relative intensity variations in the SERS spectra of MBA with increasing concentrations of aniline were observed. The bands corresponding to MBA at 417, 691, 998, and 1022 cm−1, respectively assigned to the C−S stretching, the C−H out-of-plane deformation, and the vibrations of the aromatic ring breathing modes, exhibit a dramatic increase with the increase in the concentration of aniline. An obvious increasing trend for the relative intensities of the above-mentioned bands versus the concentration of aniline is illustrated in Figure 2B. The increase in the intensity of these in-plane and out-of-plane modes further demonstrate that the overall spectral changes are mainly determined by the modification in the electronic structure of the MBA molecule as a result of forming H-bonds.35−37 The fact that these remarkable increases in intensity arise due to intermolecular H-bonding instead of bonding to aniline was further verified by measuring the SERS spectra of the MBA-Ag complex under repeated cycling conditions by immersing the complex in 10−3 M aniline and ethanol alternately, as shown in Figure 3. For the SERS spectra recorded under aniline−ethanol cycling conditions, we initially measured the SERS spectra of the MBA-Ag complex in 10−3 M aniline, as shown in Figure 3(A-a). After exhaustive rinsing using ethanol and drying under nitrogen, the MBA-Ag complex was immersed in ethanol to measure SERS spectra. Then, the MBA-Ag complex was taken out from ethanol and dried under nitrogen. The abovementioned procedure constitutes one cycle. We then repeated the same procedure for another three cycles to obtain the SERS spectra of MBA shown in Figure 3(A). In each cycle, dramatically different SERS spectra were observed for MBA in aniline and ethanol. However, when we immersed the MBAAg complex in aniline again, the distinct enhancement in the intensities of the MBA bands at 417, 691, 998, and 1022 cm−1 could be observed again. The intensities of these bands could be cycled repeatedly. By monitoring the change in the intensities of these bands, we illustrate that the SERS spectra for MBA in aniline solution and that dipped in ethanol are

Figure 1. (A) The SERS spectrum of MBA at various aniline concentrations of 0, 10−8, 10−7, 10−6, 10−5, 10−4, 10−3, and 10−2 M (ah, respectively). (B) The variation in the intensity of the band at 1075 cm−1in the SERS spectrum of MBA in (A) as a function of the log concentration of aniline.

Table 1. Wavenumbers and Assignments of Bands in a SERS Spectrum of the MBA-Modified SERS Substratea wavenumber (cm−1)

band assignments

417 691 713 998 1022 1075 1365 1572 1584

ν (C−S) C−H out-of-plane deformation γ (CCC) out of plane in-plane ring breathing in-plane ring breathing in-plane ring breathing + ν (C−S) COO− stretching nontotally symmetric ν (CC), (b2) totally symmetric ν (CC), (a1)

a ν, stretching; γ, bending. For ring vibrations, the corresponding vibrational modes of benzene and the symmetry species under C2v symmetry are indicated.

respectively attributed to the C−H out-of-plane deformation mode and the γ (CCC) out-of-plane bending mode, at 998 and 1022 cm−1, ascribed to the in-plane ring breathing modes, and at 1572 and 1584 cm−1, assigned to the nontotally symmetric ν (CC) and totally symmetric ν (CC) mode, respectively. 10193

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Figure 3. (A) SERS spectra of MBA dipped alternately in 10−3 M aniline and ethanol (from a to h). (B) Repetitive cycling of the SERS spectra for the MBA-Ag complex measured by monitoring the change in the intensities of the bands at 998 and 1022 cm−1. The SERS spectra are normalized by the band at 1075 cm−1.

Figure 2. (A) Normalized SERS spectrum of MBA for the band at 1075 cm−1 upon exposure to varying of aniline; the concentrations of aniline are 0, 10−8, 10−7, 10−6, 10−5, 10−4, 10−3, and 10−2 M (a−h, respectively). (B) Intensities of the bands at 417, 691, 998, and 1022 cm−1with respect to that of the band at 1075 cm−1in the SERS spectra of (A) versus the log concentration of aniline.

more intuitive and obvious reflection of H-bonding formed between the carboxyl group of MBA and the amine group of aniline. Consequently, this could certainly affect the MBA bands in the SERS spectra. The most striking changes in the presence of aniline can be seen for the bands at 1572 and 1584 cm−1, attributed to the nontotally symmetric CC vibration (b2 mode) and totally symmetric vibration mode (a1 mode), respectively, as shown in Figure 5A.35 It is obvious that the intensification in the b2 mode of MBA occurs with the increase in aniline concentration. The selectively enhanced b2 mode can be considered as an evidence for the participation of a CT resonance transition between the adsorbed MBA molecules and the Ag NPs. As mentioned above, the intermolecular H-bonds formed by MBA and aniline, which could alter the degree of conjugation of the system, leading to the redistribution of charge and modifications to the electronic molecular structure.36,37 It turns out that the energy of the excitation laser is suitable for the resonant SERS-CT states, and the CT process can occur through Herzberg−Teller coupling when the excitation energy matches the CT states.21,25 Therefore, H-bonding could facilitate CT transition from the Fermi level of the Ag NPs to the lowest unoccupied molecular orbitals (LUMO) of the MBA molecules. With increasing concentration of the aniline solution, the number of H-bonds increase, and the charge of the MBA-Ag complex can be redistributed further, resulting in a more suitable CT state involved in the SERS-CT process. For a favorable SERS-CT

dramatically different. This phenomenon further suggests that the enhancement in the intensities of these bands do not result from bonding with aniline, but arise from the intermolecular Hbonding. Since the change in the vibrational modes of MBA associated with the bands are known to reflect the interfacial contribution to the modification in the polarizability of the phenyl ring,36−38 it is reasonable that intermolecular H-bonding plays a critical role in the redistribution of charge in the MBAAg complex. Another intriguing observation from the spectra in Figure 4A,B is the progressive red shift for both the bands at 1075 and 1365 cm−1, assigned to the in-plane ring breathing coupled with ν (C−S) and COO− stretching modes, respectively. The inplane ring breathing mode coupled with ν (C−S) (Figure 4A) is usually a spectral marker for monitoring modifications in the electronic structure of the MBA-Ag complex caused by external factors such as the interactions involving a substituent group of the phenyl ring.35,37 This red shift can be considered a reflection of the effect of H-bonding on the redistribution of charge in the adsorbed MBA molecule. Another possible interpretation for the downshift of this band is the stress caused by intermolecular interaction on introducing an aniline molecule into the MBA-Ag system.39 In the case of the COO− stretching mode (Figure 4B), the SERS band exhibits a distinct red shift with increasing aniline concentrations along with a simultaneous decrease in the intensity of the band. It is a 10194

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Figure 5. (A) Details of the 1540−1615 cm−1spectral region of the SERS spectra illustrated in Figure 2 (A). (B) Intensity ratio (I1572 (b2)/ I1584 (a1)) and the degree of CT for MBA as a function of the log concentration of aniline. Figure 4. Details of the 1050−1100 cm−1 (A) and 1330−1410 cm−1 (B) spectral regions of the SERS spectra illustrated in Figure 2(A), respectively.

symmetric with the SERS signal only contributions from SPR, whose intensity is denoted I0(SPR), and for this line Ik(SPR) = I0(SPR). While the other line is nontotally symmetric (intensity denoted Ik(CT)). It is the measured intensity in the region of the spectrum where CT resonance makes an additional contribution to the SERS intensity excluding the contribution of SPR. In this case, Ik(SPR) is normally be small or zero. We selected two lines, the totally symmetric line located at 1584 cm−1 (a1) and the nontotally symmetric line at 1572 cm−1 (b2), for the investigation of the CT mechanism. The band was selected as Ik(CT) because it is fairly intense and relatively isolated from interference by nearby bands. Then eq 1 can be approximately expressed as follows:

process, the excitation energy could better match the energy difference between the Fermi level of the Ag NPs and the LUMO of MBA. In this case, the energy difference would gradually suit for the excitation energy by forming more Hbonds, that is, the formation of H-bond shifts the LUMO level of MBA correspondingly downward, thus reduces the energy difference. Accordingly, the intensity of the b2 mode of MBA located at 1572 cm−1 is further enhanced. For quantitatively estimating the influence of intermolecular H-bonding on the contribution of CT resonance to the SERS intensity, we used the concept of the “degree of CT (pCT)”, proposed by Lombardi et al.21,40 The pCT(k) of a k-bond can be determined using the following equation: k

ρCT(k) =

ρCT =

1+

k

I (CT) − I (SPR) I k(CT) + I 0(SPR)

b2 a1 b2 a1

(2)

The pCT values for MBA in the intermolecular H-bonding system along with the intensity ratio of the bands at 1572 and 1584 cm−1 is plotted as a function of the log concentration of aniline in Figure 5B. It is evident that pCT values show an increasing trend with aniline concentration, indicating that the

(1)

Here, k is an index used to identify individual molecular lines in the Raman spectrum. We choose the intensities of two lines in a spectral region for better understanding. One line is totally 10195

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Ag NPs to the LUMO of the adsorbed MBA molecules promoted by H-bonding. By comparing SERS spectra obtained under normal and basic conditions, the influence of H-bonding on the SERS spectrum of the adsorbate can be further understood. We believe that this study will pave the way to realizing the significant influence of intermolecular H-bonding on SERS spectroscopy.

b2 mode of MBA was selectively enhanced via the CT resonance transition. With further redistribution of the charge in the surface complex, a more energetically favorable CT state, which could better match the energy of the excitation laser, was involved in the SERS-CT process. Thus, the observed intensification of the b2 mode is associated with the number of H-bonds, which, in turn, is related to the concentration of aniline. In the experiments described above, the pH of the system was not controlled. Since the structures of both MBA and aniline are sensitive to pH, we conducted another experiment under basic conditions to eliminate any possible interference from changes in the pH of the system. We prepared a series of ethanolic solutions of aniline containing NaOH at a concentration of 10−3 M. Under these basic conditions, it can be assumed that both MBA and aniline molecules were present in their basic form, as suggested by their pKa values.41,42 However, similar results were obtained under basic conditions, whereby the intensities of the b2 mode and other characteristic peaks exhibited a concentration dependence on the aniline solution (Figure 6). The only difference was observed for the

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ASSOCIATED CONTENT

S Supporting Information *

The UV−vis spectrum and the morphology of the Ag NPs. The SERS spectra of Ag NPs, aniline, and MBA, and the correlative elucidation. This material is available free of charge via the Internet at http://pubs.acs.org

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AUTHOR INFORMATION

Corresponding Author

*Tel: +86 431 8516 8473; fax: +86 431 8519 3421; e-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the Natural Science Foundation of China (Grant Nos. 21103062, 21273091, 21221063, and 201327803) and the Specialized Research Fund for the Doctoral Program of Higher Education of China (Grant No. 20110061110017).

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REFERENCES

(1) Le Ru, E. C.; Etchegoin, P. G. Principles of Surface-Enhanced Raman Spectroscopy and Related Plasmonic Effects; Elsevier: Amsterdam, The Netherlands, 2009. (2) Moskovits, M. Surface-Enhanced Spectroscopy. Rev. Mod. Phys. 1985, 57, 783−826. (3) Campion, A.; Kambhampati, P. Surface-Enhanced Raman Scatering. Chem. Soc. Rev. 1998, 27, 241−250. (4) Lombardi, J. R.; Birke, R. L.; Sanchez, L. A.; Bernard, I.; Sun, S. C. The Effect of Molecular Structure on Voltage Induced Shifts of Charge Transfer Excitation in Surface Enhanced Raman Scattering. Chem. Phys. Lett. 1984, 104, 240−247. (5) Hu, J. W.; Zhao, B.; Xu, W. Q.; Li, B. F.; Fan, Y. G. SurfaceEnhanced Raman Spectroscopy Study on the Structure Changes of 4Mercaptopyridine Adsorbed on Silver Substrates and Silver Colloids. Spectrochim. Acta A 2002, 58, 2827−2834. (6) Kim, K.; Kim, K. L.; Choi, J. Y.; Shin, D.; Shin, K. S. Effect of Volatile Organic Chemicals on Surface-enhanced Raman Scattering of 4-Aminobenzenethiol on Ag: Comparison with the Potential Dependence. Phys. Chem. Chem. Phys. 2011, 13, 15603−15609. (7) Ren, B.; Lin, X. F.; Yang, Z. L.; Liu, G. K.; Aroca, R. F.; Mao, B. W.; Tian, Z. Q. Surface-Enhanced Raman Scattering in the Ultraviolet Spectral Region: UV-SERS on Rhodium and Ruthenium Electrodes. J. Am. Chem. Soc. 2003, 125, 9598−9599. (8) Grajcar, L.; Baron, M. H. A SERS Probe of Adenyl Residues Available for Intermolecular Interactions. Part IAdenyl “Fingerprint”. J. Raman Spectrosc. 2001, 32, 912−918. (9) Umadevi, M.; Ramasubbu, A.; Vanelle, P.; Ramakrishnan, V. Spectral Investigations on 2-Methyl-1,4-naphthoquinone: Solvent Effects, Host−Guest Interactions and SERS. J. Raman Spectrosc. 2003, 34, 112−120. (10) Yang, G. C.; Si, Y. L.; Geng, Y.; Yu, F.; Wu, Q. X.; Su, Z. M. Charge Transport and Electronic Properties of N-heteroquinones: Quadruple Weak Hydrogen Bonds and Strong π−π Stacking Interactions. Theor. Chem. Acc. 2011, 128, 257−264.

Figure 6. SERS spectra of MBA with various aniline concentrations of 0, 10−8, 10−7, 10−6, 10−5, 10−4, 10−3, and 10−2 M (a−h, respectively), in basic conditions containing 10−3 M NaOH.

band at 1365 cm−1, which is assigned to the COO− stretching mode. Unlike under normal conditions, where the intensity of the COO− band reduced with increasing aniline concentration, under basic conditions, an obvious peak can still be observed even at high concentrations of aniline. This is consistent with previously reported SERS spectra of MBA, in which the vibrational stretching mode of COO− would result in an increase in mode intensity.31,32 Therefore, intermolecular Hbonding interactions between aniline and MBA, not the change in pH on introducing aniline molecules in the system, is the reason for the significant differences in the SERS spectral pattern of MBA.

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CONCLUSIONS In summary, we have investigated the important influence of the intermolecular H-bonding of a system on the electronic structure of molecules in a system via SERS technique. We found that some frequency shifts and changes in the intensities of some characteristic peaks of MBA can occur on H-bonding. Moreover, the enhanced b2 mode of MBA can be considered as a manifestation of the CT transition from the Fermi level of the 10196

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dx.doi.org/10.1021/jp5025284 | J. Phys. Chem. C 2014, 118, 10191−10197