Five serum microRNAs for detection and predicting of ovarian cancer

Article Cite This: J. Phys. Chem. C 2019, 123, 23026−23036

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Adsorption, Chemical Enhancement, and Low-Lying Excited States of p‑Methylbenzenethiol on Silver and Gold Nanoparticle Surfaces: A Surface Enhanced Raman Spectroscopy and Density Functional Theory Study Rui Wang,† Xiao-Ru Shen,† Meng Zhang,† Rajkumar Devasenathipathy,† Ran Pang,† De-Yin Wu,*,† Jingdong Zhang,‡ Jens Ulstrup,‡ and Zhong-Qun Tian† Downloaded via UNIV OF ALABAMA BIRMINGHAM on October 21, 2019 at 18:52:28 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

†

State Key Laboratory of Physical Chemistry of Solid Surfaces, Collaborative Innovation Center of Chemistry for Energy Materials, and Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen, 361005, China ‡ Department of Chemistry, Technical University of Denmark, DK-2800 Kongens Lyngby, Denmark S Supporting Information *

ABSTRACT: Adsorption and chemical enhancement of p-methylbenzenethiol (PMBT) on silver and gold nanoparticle surfaces have been studied using surface enhanced Raman spectroscopy (SERS) and density functional theory (DFT) calculations. In normal Raman spectra, the Raman intensity of the molecule is sensitive to methyl substitution at the para position. DFT calculations for the Raman spectrum of PMBT reproduces well the Raman spectrum in nonpolar solution relative to PMBT in powder. This accords with the order of the PMBT molecules in the solid. The SERS results of PMBT adsorbed on Au and Ag nanoparticles indicate that the Raman intensity in the low-wavenumber region increases with increasing excitation wavelength. The electronic structures of low-lying excited states have been explored for this increase in different PMBT-S-metal cluster complexes. DFT results indicate that low-energy excited states are in fact present and originate from two types of excitations, one localized at the sulfur−silver/gold bonding region and another one from a charge transfer state excited from PMBT to the silver and gold surfaces. Both interfacial excited states contribute significantly to the chemical enhancement mechanism and change relative Raman intensities of adsorbed PMBT. The chemical bonding interaction and the interfacial energy level alignment are therefore important to understand SERS processes of PMBT adsorbed on noble metal surfaces of nanostructures.

1. INTRODUCTION Surface enhanced Raman scattering spectroscopy (SERS) is one of the most sensitive surface spectroscopic detection techniques.1−4 The ultrahigh sensitivity is caused mainly by the superior surface plasmon resonance (SPR) effect,5 which elevates the enhancement factor to several orders of magnitude compared with the Raman signals from molecules in the bulk. It has been widely accepted that such enhancement effects arise mainly from two mechanisms: (i) electromagnetic field enhancement (EM) and (ii) chemical enhancement (CE). Further, metals such as gold, silver, and copper have been found to possess conspicuous free electron properties. Excitation in the visible region can induce collective oscillations of the conduction band electrons on these metal surfaces and acquire a huge SPR effect. This effect can effectively convert far-field light into near-field light, resulting in huge EM enhancement. On the basis of surface selection rules,6 the EM mechanism, however, results in approximately the same enhancement factor for the same symmetry modes. The CE mechanism is attributed to chemical binding © 2019 American Chemical Society

interaction and photodriven charge/intervalence transfer (PDCT) between adsorbates and metal.7−9 Chemical binding interaction leads to redistribution of the charges in the electronic ground states, thus directly affecting the geometric and electronic structures of the adsorbed molecules, and the charge transfer mechanism ultimately leads to a change of the Raman spectra in a resonance-like Raman scattering process of the molecule-metal system.10,11 The charge transfer direction depends on the interfacial energy level alignment between the metal and the target molecules, which is closely related to the electronic structures of the adsorbed molecules, the Fermi level of the metal, the applied potential, and the wavelength of the excitation light. Although the enhancement effect of the CE mechanism is small, it still affects significantly the relative Raman intensities of the vibrations with the same symmetry irreducible representation. In order to understand the Received: July 6, 2019 Revised: September 2, 2019 Published: September 3, 2019 23026

DOI: 10.1021/acs.jpcc.9b06431 J. Phys. Chem. C 2019, 123, 23026−23036

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The Journal of Physical Chemistry C

used to investigate the CE enhancement mechanism of PMBT on Au and Ag, the nature of the interface bonding, energy level arrangement and excited states are not completely clear. With a view of further clarifying the role of the CE mechanism for adsorption structures of PMBT as a representative of aromatic thiols, the present work focuses on two aspects: (i) to study the PMBT adsorption structures of PMBT on metal nanostructures by a combination of experimental work and theoretical calculations, specifically the PMBT adsorption configurations on Au and Ag nanoparticles (NPs); (ii) to investigate the CE mechanism and explore the effect of interfacial chemical bonding on the Raman spectra of PMBT, further to discuss the interfacial energy level alignment, and to explore the low-lying excited states resulting from interfacial adsorption. Particularly, the SERS enhancement of several specific PMBT Raman peaks in resonance with low-lying excited states was investigated. Our DFT calculations show that the low-lying excited states arise mainly from the contribution of interfacial intervalence states of the Au−S and Ag−S bonds and the charge transfer from adsorbed PMBT to the metals. Such a detailed clarification of the nature of lowlying excited Au−S and Ag−S states and their role in the CE enhancement mechanism of SERS of aromatic thiols has not been reported before.

relationship between the SERS enhancement effects and the molecular Raman signals, it is therefore essential to clarify the chemical enhancement magnified by the EM mechanism. Aromatic thiophenol compounds are extensively employed as probe molecules in SERS spectroscopy, based on (i) their strong adsorption on the surface of gold and silver nanostructures, (ii) formation of self-assembled monolayers (SAMs), and (iii) strong SERS response. p-Methylbenzenethiol (PMBT) is a para-methyl substituted aromatic thiophenol, which generates a stable and strong SERS signal when adsorbed on the surface of nanostructured metals.12−22 In recent years, PMBT has been used as SERS probe molecules23−28 to study the SPR effect of nanostructures, surface molecular adsorption conformation, and Raman signal enhancement mechanisms or charge transfer processes at the interfaces. To understand these interfacial mechanisms, the adsorption geometry of molecules on the metal surface, such as adsorption orientation and surface adsorption site, as well as surface chemical bonding,29−32 electronic properties of lowlying excited states arising from photoexcitation,33−35 and relaxation processes of hot carriers,9,36,37 must be understood. The surface adsorption configuration directly affects the SERS signal, and single-crystal surfaces are often used in studies of this PMBT aspect. For example, Endo and associates used Xray absorption fine-structure spectroscopy at the near carbon K-edge (C K-NEXAFS) to analyze PMBT SAMs and suggested that the PMBT molecules are adsorbed on Au(111) with a tilt angle around 60°.38 Theoretical approaches can provide important insight on these issues. Fan and associates proposed that the surface adsorption energy and adsorption tilt angle for PMBT adsorbed on Au(111) decreases and the Au−S bond length increases as the coverage increases.39 Recently, Ikeda and co-workers studied the desorption behavior of PMBT on Au(111) and Au(100) by electrochemical THz-SERS. They suggested that PMBT molecules are adsorbed at the bridge sites of Au(100) but at both the bridge and hollow sites on Au(111). Moreover, the bridge site adsorption was more stable than hollow site adsorption on gold nanoparticles.26 SERS spectra of PMBT on a copper foil roughened by HNO3 were first observed with a SERS enhancement factor reaching 105.20 Jia and co-workers studied the SERS spectra of PMBT molecules on colloidal silver and proposed that the SERS signal under 514.5 nm light excitation arose mainly from the EM enhancement with a small contribution from chemical enhancement.40 When the incident excitation light resonates with the excited states of PMBT-Ag, the maximum enhancement factor can reach 106. They also suggested that charge transfer enhancement plays an important role and that the charge transfer direction was from molecule to metal. Futamata and co-workers found that oxidation of PMBT to pmercaptobenzoic acid occurred at 532 nm light excitation in the gap between the Ag film and the Ag nanoparticles.41,42 Ikeda and associates studied the SERS enhancement effect of PMBT on different Au single-crystal faces based on the gap mode on the low-index crystal planes and proposed that charge transfer enhancement had a significant effect on the SERS intensity of the nontotally symmetric vibrations on the highindex crystal faces. They further proposed that the direction of charge transfer was from molecule to metal.28 On the basis of crystal structure analysis it was reported recently that aromatic thiophenols on small metal nanoparticles form surface complexes.43 However, although the EM amplification was

2. MATERIALS AND METHODS 2.1. Chemical Reagents. p-Methylbenzenethiol (PMBT, 98%) was from Sigma-Aldrich and tetrabutylammonium nitrate (TBA+NO3̅, 98%) from Alfar Aesar. Other chemicals were from Sinopharm Chemical Reagent Co. Ltd. All chemicals were used without further purification. Deionized ultrapure water (UPW, 18.2 MΩ·cm) from a Millipore Milli-Q system was used throughout. 2.2. SERS Substrate Preparation and SERS Measurements. Self-assembled nanostructured films were prepared using a simple method previously described.44,45 Specifically, 5 mL of presynthesized NPs (S1, Supporting Information) colloidal solution, 3 mL of dichloromethane, and 1.3 mL of 2.5 × 10−4 M TBA + NO 3̅ solution were mixed. The 2D nanoparticle arrays were obtained by vigorously shaking the mixed solution, followed by attachment to a silicon wafer to obtain a self-assembled NP substrate. The substrates were placed in PMBT ethanol solutions with different concentrations for 15 h to finally obtain various SAMs. One mM PMBT solution was used in all SERS measurements. Raman spectra were recorded using a confocal microscope Raman instrument Renishaw in Via (Renishaw plc, England). An Olympus objective (50×, N.A. 0.5) was used for Raman detection. 532, 561, 633, and 785 nm laser lines were used. Before the measurement of PMBT, the standard silicon was used to correct the Raman frequencies to 520.6 cm−1 at different excitation wavelengths and to make sure that the Raman intensities were the maximum under different measuring conditions. 2.3. Computational Details. A molecule-metallic cluster model was used to obtain molecular adsorbate structures and interfacial electronic structures. The sizes of metallic clusters include Ag3, Ag5, Ag7, Ag13, Ag19, and Au3, Au5, Au13, Au19. The density functional theory (DFT) approach in the B3LYP46−48 form was used for geometry optimizations and vibrational spectral calculations, performed using the Gaussian 09 program.49 The basis sets for carbon, sulfur, and hydrogen atoms were 6-311+ G(d, p).50,51 The valence electrons and 23027

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Figure 1. Raman and SERS spectra of PMBT. The Raman spectra of pure solid PMBT on different surface measuring sites (a) and (b) with a laser power = 1.705 mW; (c) SERS spectrum of PMBT on 53 ± 4 nm AuNPs, laser power = 0.018 mW; (d) SERS spectrum of PMBT on 81 ± 6 nm AgNPs, laser power = 0.188 mW. PMBT concentration 1 mM. 633 nm laser line.

3. RESULTS AND DISCUSSION 3.1. Raman and SERS Spectra of PMBT Molecules Adsorbed on Ag and Au Nanoparticles. Figure 1a,b shows the Raman spectra of PMBT on different surface measuring points of solid PMBT samples. The Raman spectrum in Figure 1a is consistent with the results of previous studies.15,61 The strongest Raman peak of solid PMBT at 1099 cm−1 is assigned to the ring breathing mode generated by the mercapto substitution, while the weak Raman peak at 1599 cm−1 is assigned to the υ8a mode of the benzene ring. Other Raman peaks at 639, 796, 1186, and 1212 cm−1 accord with previous reports.15,61 Notably, enhanced Raman peaks at 379 and 1599 cm−1 are observed in Figure 1b after changing the measured solid PMBT surface. This reflects the anisotropy of solid PMBT, which exhibits different properties in different directions. The intensities of Raman vibrational modes are thus significantly affected by the targeted surface. The anisotropy disappears as PMBT solid is dissolved in CCl4 solvent, and there is no difference between the recorded Raman spectrum of PMBT in liquid solution (Figure S2) and the simulated spectrum (Figure S3a). In addition, the variation of the υ8a mode in the Raman spectra of solid PMBT also shows that this Raman peak has a large depolarization ratio. According to our DFT calculations, the depolarization ratio of the υ8a vibrational mode was evaluated to be 0.726. The value of this ratio should vanish when the Raman peak is fully polarized.57 These results substantiate that the simulated Raman spectrum is reliable for PMBT and that anisotropy must be considered to understand the experimental solid state Raman spectra. Enhanced and some red-shifted Raman peaks relative to pure solid PMBT (Figure 1a,b) appeared in SERS of PMBT adsorbed on AuNPs (Figure 1c) and AgNPs (Figure 1d). Notably, the strongest peaks derive from ring stretching vibration and are red-shifted to 1077 and 1076 cm−1 after adsorption on AuNPs and AgNPs, respectively. This is in contrast to the 1099 cm−1 peak before adsorption of PMBT.

inner shells for all metal atoms were described using the LanL2DZ basis set and the corresponding relativistic effective core potentials, respectively.52,53 The scaled quantum-mechanical force field (SQMF) procedure was used to assign all the fundamental frequencies according to the potential energy distribution (PED).54 The Scale 2.0 program was used to obtain the magnitude of PED based on the force constant matrix of B3LYP calculations.55,82 We chose the scaling factors of 0.981 for the vibrational modes below 2000 cm−1, and 0.967 for other vibrational frequencies to compare theoretical and experimental Raman spectra.56 These scaling factors were used to correct the incompleteness of the theoretical approaches and basis sets, as well as the neglect of vibrational anharmonicity. Raman intensities were estimated by combining the optimized geometries with the differential Raman scattering cross-section (DRSC) as follows:57,58 (∼ ν0 − ∼ νi )4 (2π )4 h ij dσ yz jj zz = Si · 2 ∼· 45 8π cνi 1 − exp( −hc∼ νi /kBT ) k dΩ {i

(1)

ji dα zyz ji dγ zyz zz + 7jjj z Si = 45jjjj z j dQ zz k dQ i { k i{

(2)

2

2

where h, c, kB, and T are the Planck constant, the speed of light, the Boltzmann constant, and the Kelvin temperature, respectively. ν̃0 and ν̃i are the frequencies (in cm−1) of incident light and of the ith vibrational mode. Si is the Raman scattering factor (in Å4/amu) that can be directly calculated using the Gaussian 09 program.49 dα/dQi and dγ/dQi are derivatives of isotropic and anisotropic polarizabilities with respect to a given normal coordinate, respectively. The frequency-dependent polarizabilities were calculated using the coupled perturbation methods.59 The polarizability derivatives were obtained by numerically differentiating the analytic polarizability with respect to the nuclear coordinates.60 23028

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The Journal of Physical Chemistry C The second strongest peaks are 387 and 1593 cm−1 peaks on AgNPs and 388 and 1593 cm−1 on AuNPs. The SERS spectra are different from the spectra of the solid molecule but consistent with reported experimental results.25,62 In addition, the frequencies and relative intensities of the SERS spectra of PMBT on AgNPs and AuNPs are very similar. Assuming that the point group of the PMBT molecule is approximately C2V, the EM enhancement on the gold and silver surfaces should have little impact on the relative intensities of the totally symmetric a1 modes. The relative intensities of these modes are thus mainly controlled by the CE effect. At the same time, the Au−S and Ag−S bonds are particularly strong,63 so the differences in the SERS spectra of PMBT on AuNPs and AgNPs are quite small. Some PMBT Raman peaks have been assigned previously by comparison with DFT calculations,26,28 but a comprehensive analysis of all the peaks is not available. To correlate the molecular local structure with the SERS spectra clearly, we have undertaken a detailed analysis of the Raman peaks of the PMBT molecule. Table S1 summarizes the vibrational frequencies calculated by the SQMF method, the experimental Raman frequencies, and the assignments of the PMBT fundamental frequencies based on the potential energy distributions. The atomic numbers involved are shown in Figure S4. Combining with Figure 1 and the vibrational analysis of Table S1, we can clearly identify the experimentally observed Raman peaks with the PMBT vibrational modes from the theoretical analysis. We note further the following three points: (i) The a1 vibrational modes associated with the methyl group do not directly contribute to the observed Raman spectra, whereas they affect significantly the relative intensities of the Raman peaks associated with the benzene ring. As shown in the simulated Raman spectrum in Figure S3b, the 1000 cm−1 peak is strong for thiophenol, but the intensity of this peak decreases significantly or even disappears for PMBT. (ii) As shown in Table S1, the frequencies of the C−H stretching mode on the benzene ring, the C−H stretching mode on the methyl group, and the S−H stretching mode are overestimated and differ greatly from the experimentally observed frequencies. These fundamental vibrations are, however, well assigned. (iii) The deviations of scaled DFT frequencies from the experimental values for other peaks in the middle- and low-frequency regions are small and only about 5 cm−1. The differences between the two peaks associated with the CC stretching mode on the benzene ring are all about 10 cm−1. The scaled frequencies are 1088 and 1609 cm−1, and the experimental values 1099 and 1599 cm−1. 3.2. Simulated Raman Spectra of PMBT-Agn and PMBT-Aum. Figure 2 displays the simulated Raman spectra of PMBT-silver clusters. Using these cluster models, we calculated the Raman spectra when the sulfur atom in the thiyl radical binds to the top, bridge, and hollow sites of these silver clusters, respectively, and compared them with the experimental SERS spectra. In Figure 2, the 375 cm−1 peak is strong at the top site, and very weak at the hollow site but at the bridge site in accordance with our experimental results. At the same time, the other peaks in bridge adsorption accord with the experimental results. We therefore conclude that the simulated Raman spectra of the bridge adsorption configuration best reproduce the experimental results. The results also indicate that the relative intensities of the four main characteristic PMBT Raman peaks vary according to the

Figure 2. Simulated Raman spectra of PMBT at different adsorption sites of silver. (a) and (d) PMBT at top sites; (b) and (c) at bridge sites; (e) and (f) PMBT at hollow sites.

different adsorption sites, but the spectra of PMBT adsorbed at Ag silver bridge sites coincide with experimentally recorded peaks, supporting that the bridge sites dominate among the PMBT adsorption sites on Ag. Previous results show that the adsorption energies of aromatic thiophenol compounds at bridge sites are large and that the bridge adsorption is in fact the most stable.26,64 On comparing Figure 2 with the simulated normal Raman spectra Figure S3a, the 796 cm−1 peak intensity (υCC + υC1C2 + βbenzene; β denotes the in-plane bending) decreases significantly compared with Raman spectra in the solid, and the 913 cm−1 peak corresponding to the C−S−H bending mode disappears due to breaking of the S−H bond. This result is consistent with the experimental results and with comparison of the Raman spectra before and after adsorption, supporting the rationale of the theoretical method. It is noted that the theoretical calculations clearly accord with the four strong characteristic peaks at 387, 623, 1076, and 1593 cm−1 in the experimental SERS spectra. Vibrational analysis (shown in Table 1 and Table S2) shows that these four peaks correspond to the vibrational modes with a1 symmetry: C−S stretching (υ7a), ring deformation (υ6a), ring breathing (υ1), and ring stretching (υ8a) vibrations, respectively. Notably, the wavenumbers of 387, 623, and 1076 cm−1 are all related to C−S stretching. In addition, when PMBT adsorbs at silver surfaces, the 214 cm−1 peak was observed in the experimental SERS spectrum (Figure 1d). This peak can be assigned to the Ag−S bond stretching, further indicative of Ag−S bond formation on the surfaces. These results are consistent with previous reports.65,66 Figure 3 presents simulated SERS spectra of PMBT interacting with the different Au clusters. Similar to the simulation of PMBT-silver clusters, the intensities of the peaks at 796 cm−1 significantly decrease compared with the experimental SERS spectra of PMBT on AuNPs, the 913 cm−1 Raman peak assigned to C−S−H bending mode disappears, and the SERS peak assigned to the Au−S stretching mode appears at 238 cm−1. The SERS spectra of PMBT on the single-crystalline Au surfaces were theoretically calculated previously, where the symmetric b2 in-plane modes were observed.26,28 However, only the a1 modes were observed in 23029

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Table 1. Theoretical Scaled Frequencies (freq., cm−1), Experimental Raman Frequencies (expt., cm−1), the Assignments, and Potential Energy Distribution (PED, %) of the Chosen Modes of Ag5-PMBT and Au5-PMBT Ag5-PMBT

Au5-PMBT

freq

expt

assignment (PED)a

freq

expt

ref26

assignment (PED)a

1604 1489 1388 1207 1185 1114 1074 1010 792 702 613 489 375 303 287 198

1593 1485 1381 1212 1184

υCC(90) + βbenzene(10) βCH in benzene(53) + υCC(22) βCH3(100) υC1C2 (40) + βbenzene(12) βCH in benzene (75) + υCC(23) βCH in benzene(57) + υCC(33) υC = C(51) + υCS(19) βbenzene(62) + υC = C(10) υCC(46) + βbenzene(21) τbenzene(100) υCS(39) + βbenzene(35) + υCS(34) + υC1C2(19) τbenzene(43) + γSC(27) + γC1C2(20) βbenzene(36) + υCS(32) τbenzene(60) + γSC(33) βbenzene (31) + υAg−S(26) + βCS(22) υAg−S(49) + υAg−Ag(21)

1603 1490 1388 1206 1186 1117 1072 1010 793 722 611 495 380 322 302 232

1593 1482 1376 1210 1180

1590

1077 1013 795 696 623 488 388 324

1077 1011

238

235

υCC(86) + βbenzene(14) βCH in benzene (60) + υCC(36) βCH3(100) υC1C2 (41) + υCC(27) + βbenzene(12) βCH in benzene (78) + υCC(22) βCH in benzene(57) + υCC(23) υCC(65) + υCS(16) βbenzene(64) + υCC(25) υCC(58) + βbenzene(22) τbenzene(100) βbenzene(35) + υCS(34) + υC1C2(19) τbenzene(42) + γSC(25) + γC1C2(21) βbenzene(35) + υCS(32) γC1C2(31) + γSC(22) + τbenzene(18) + υAu−S(11) γC1C2(38) + υAu−S(20) + βSC(12) υAu−S(54) + τC1C2(11)

1076 1012 797 695 623 486 387 315 214

623 390 325

Approximate description of the modes (υ, stretching; β, in-plane bending; γ, out-of-plane bending; τ, torsion).

a

The intensities of the PMBT υ7a mode are similar at the bridge sites on gold and silver surfaces, indicating that the adsorption site has little effect on the SERS of the molecule on the gold clusters. 3.3. Concentration-Dependent SERS of PMBT. We consider next the impacts of surface coverage on the CE effect and discuss two aspects of the impact on the SERS spectra. (i) The coverage on the surfaces of the metal substrates can affect the absolute Raman intensities per unit area.68−71 (ii) The coverage affects the adsorption orientation and adsorption sites of molecules on the surfaces, which further determine the relative intensities of the molecular Raman signals.65,72 Figure 4 shows the PMBT molecular concentration-dependent SERS signal intensity at different bulk concentrations. The intensities of the four major PMBT SERS peaks as a function of concentration in the range from 0.10 to 500 μM are shown in Figure 4a. The metal nanoparticle substrates were dipped in PMBT ethanol solution for 15 h so as to reach adsorption equilibrium before the SERS measurements. When the molecular concentration is lower than 0.1 μM, the SERS signal is too small to be detected, indicating that the coverage of adsorbed molecules on the surface of the metal nanoparticles is too low. Figure 4b shows the logarithm of the molecular concentration-dependent peak areas of the four characteristic vibrational peaks. The intensity of each vibrational peak rises with the PMBT bulk phase concentration, and their integral intensities reach maxima when the concentrations reach 50 μM. Tripathi et al. studied the adsorption kinetics of PMBT on the Au surfaces using SERS. Assuming that the adsorption behavior of the molecule on the metal surface conforms with the Langmuir adsorption model,68,71 the following equation could be fitted to the variation of the SERS signal with concentration: ÅÄÅ ÑÉ Å KeqC ÑÑÑ maxÅ Å ÑÑ IC = IC ÅÅ ÅÅ 1 + KeqC ÑÑÑ (3) ÅÇ ÑÖ

Figure 3. Simulated Raman spectra of PMBT at different adsorption sites of gold clusters. (a) PMBT-Au3 at the top site; (b) PMBT-Au5 at the bridge site; (c) PMBT-Au13 and (d) PMBT-Au19 at the hollow site.

our SERS spectra. We assigned the 623 and 390 cm−1 bands in PMBT-Au5 to the mixed modes of the benzene ring deformation (υ6a) and the C−S stretching (υ7a). This is quite different from the previous assignments.26,28 Comparing the SERS spectra of different sites on Au, we found that PMBT adsorption on Au is similar to that on Ag and that bridge adsorption dominates. Besides, Bodappa et al.67 examined the structural differences of Au−S bond stretching vibrations of monolayer mercapto molecules on the Au144 cluster and on a polycrystalline gold surface, and concluded that the adsorption of 2-phenylethanethiol on polycrystalline Au planes is dominated by the bridge sites. This is consistent with the results of the present work. The theoretical simulations of PMBT on Au and Ag thus show that the adsorption of PMBT molecules is primarily at the bridge sites. On the basis of the comparison with simulated Raman spectra, the interactions of PMBT with gold clusters are stronger than with silver clusters. This is supported from the frequencies of Au−S slightly higher than those of Ag−S.

where C is the bulk concentration, IC the peak area of a spectral feature at the concentration C, Imax the maximum C observed peak area of that spectral feature, and Keq the 23030

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Figure 4. SERS of PMBT on AuNPs as a function of bulk molecular concentration. (a) Concentration-dependent SERS spectra after immersion in PMBT ethanol solution for 15 h. (b) Variation of SERS intensities of the 387, 624, 1080, and 1600 cm−1 peaks as a function of PMBT concentration. Laser line λ = 785 nm.

Figure 5. Excitation wavelength-dependent SERS spectra of PMBT adsorbed on silver and gold nanoparticle films. (a) SERS of PMBT on the AgNP film; (b) SERS of PMBT on the AuNP film. Here the intensities of other peaks are normalized with respect to the 1593 cm−1 peak.

molecular adsorption equilibrium constant. The solid lines in Figure 4b are the fitting results using the Langmuir adsorption isotherm (r2 ≥ 0.9). The equilibrium isotherms of PATP on spherical Au nanoparticles of 130 nm were reported in a previous study to get a fitting equilibrium constant.73 As shown in Figure 4b, our fitted Keq value is smaller than the reported values due to the different molecules, which also can be attributed to the size and morphology differences of the nanoparticles. In addition, the fitted Keq value of 387 cm−1 peak differs from three other fitted values, and this suggests that this vibrational mode intensity is more sensitive to the coverage and orientation change of the molecule. When the PMBT bulk concentration is higher than 50 μM, the molecular adsorption on the surface can be considered to have reached saturation, i.e., the full-monolayer adsorption, and the bulk phase concentration of molecules has no further effect on the SERS signal. Consequently, when the concentration of PMBT is large (≥50 μM), the adsorption configuration of PMBT is stable, and the adsorption orientation does not change further with the concentration of the solution. 3.4. Excitation Wavelength-Dependent SERS of PMBT on Ag and Au. The SERS signals of full molecular monolayers adsorbed on the surfaces of the metal nanoparticles at different excitation wavelengths were recorded to analyze the CE mechanism of PMBT (Figure 5). The relative intensities of other SERS peaks were normalized based on the intensities of the 1593 cm−1 peak. Besides, the effect of quantum efficiency (QE) of the CCD detector has been reduced according to the correction of the QE difference in different wavelength regions using the QE curve of the detector (Figure S5). It can be seen that the intensities of the three peaks associated with the C−S stretching below 1200 cm−1 increase with increasing excitation wavelength. Table S3

lists the relative intensity values evaluated based on Figure 5. The relative intensities increase several-fold with changing excitation wavelength from 532 to 785 nm or with the decrease of excitation energy (Figure S6). This result shows that the CE effect becomes stronger and that the selectivity of SERS intensities of the low-frequency modes of the PMBT-metal systems is enhanced with decreasing excitation energy. The absolute DRCS of the ith Raman active vibrational mode of the molecules can be represented by eqs 1 and 2. From these equations, the molecular Raman intensities were found to depend on the frequencies of incident light ν̃0, the Raman shifts ν̃i of vibrational modes, and temperature T, etc. There are two origins of the enhancement effect: (i) The term (ν̃0−ν̃i)4, i.e., as the excitation wavelength increases, the relative Raman intensities of the low-frequency Raman peaks are enhanced. Table S3 also shows the results when the term (ν̃0−ν̃i)4 appearing in eq 1 has been divided out, and notably, the relative intensities still increase as the excitation wavelength increases. (ii) The increment of the polarizability derivatives is another possibility. It is assumed that certain low-lying intervalence states exist in the PMBT-silver or -gold interface,74 leading to an increase in the polarizability derivatives when the photonic energy is reduced. The three modes are all closely associated with the C−S stretching. It is therefore appropriate to search for localized low-lying excited states formed when PMBT is adsorbed on the metal surfaces. When the excitation light resonates with the surface localized lowlying excited states, the vibrational modes on the metal surfaces significantly increase. 23031

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thus there are four occupied π orbitals. The relative energy level order of PMBT molecules after binding with Au and Ag clusters was first analyzed (Figure 6). The calculated LUMO

3.5. Chemical Enhancement Effect. To understand the SERS process of PMBT, we first evaluate the static chemical enhancement factors, which mainly reflect chemical enhancement effects on Raman signals caused by the binding interaction between molecules and the metal surfaces. Table 2 shows the static chemical enhancement factors calculated Table 2. Chemical Enhancement Factors (EF) Estimated on the Basis of Theoretical Calculations at the UB3LYP/6311+G(d,p) Level modes

υ7a, 388 cm−1

υ6a, 623 cm−1

υ1, 1077 cm−1

υ8a, 1593 cm−1

PMBT-Ag PMBT-Ag3(T) PMBT-Ag3(B) PMBT-Ag5(B) PMBT-Ag7(T) PMBT-Ag7(B) PMBT-Ag13(H) PMBT-Ag19(H) PMBT-Au(T) PMBT-Au3(T) PMBT-Au5(B) PMBT-Au13(H) PMBT-Au19(H)

13.1 21.3 5.5 5.2 2.4 2.8 2.3 2.4 9.8 54.0 5.6 8.4 5.9

17.6 483.1 13.0 12.5 6.8 9.7 12.7 13.6 8.9 68.8 11.3 28.5 19.6

19.3 17.3 9.9 9.3 4.2 8.0 13.8 17.1 13.8 93.2 8.5 24.6 19.4

14.3 9.8 2.4 6.0 2.7 4.7 11.5 13.7 12.5 81.4 5.6 17.0 15.4

Figure 6. Energy levels alignments of PMBT, PMBT-Agn (n = 1, 3, 5, 7, 13, 19), and PMBT-Aum (m = 1, 3, 5, 13, 19). The red, purple, yellow, gray, green, and blue zones denote the LUMOs of the benzene ring, antibonding orbitals of Ag−S, antibonding orbitals of Au−S, bonding orbitals of Ag−S and HOMOs of the benzene ring, and bonding orbitals of Au−S and HOMOs of the benzene ring and metallic orbitals, respectively.

and LUMO-1 orbital levels for PMBT using B3LYP are at −0.65 and −0.50 eV, and the occupied π orbitals at −6.02, −7.00, −8.06, and −10.28 eV. After binding to the metal clusters, the energies of the unoccupied orbitals of PMBT are not lower than −1.0 eV (referred to the vacuum level), while the level changes of last three occupied orbitals are small. Meanwhile, if the Fermi level of Ag is at −4.3 eV78 and of Au at −5.0 eV,79 the effects of the latter three levels (-8.06 and −10.28 eV) in PMBT on SERS are negligible. In the general SERS experiments, the energy levels related to molecules and metallic interfaces can therefore be considered as being of three types, i.e., the S-metal antibonding orbitals above the metal Fermi level, the S-metal bonding orbitals, and the HOMO of PMBT molecules both below the Fermi level. These orbitals may produce three kinds of low-lying excited states that affect the SERS intensity, i.e., the transition from the metal to the S−M antibonding orbitals, and the transitions from the HOMO of PMBT, and from the S−M bonding orbitals to the metal states above the Fermi level. The first one forms a broken S−M bond, while the latter two involve a localized intervalence S−M bonding interaction and a charge transfer state. In our calculations, the energy level differences between the S−M antibonding and bonding orbitals are larger than 3.0 eV. The effect of the excited state directly related to the electronic transition between these two orbitals on the Raman intensity can therefore be neglected. The Ag cluster-molecule models are here mainly used to calculate the Raman spectra, which clearly reflect the resonance-like Raman scattering process associated with the S−Ag bonding and the HOMO π orbitals. Table S6 gives the transition energies, configuration coefficients, and oscillator strengths of the low-lying excited states calculated using different molecule-silver clusters. Except for the S2 state of PMBT-Ag, i.e., with only a single Ag-atom, which has a large oscillator strength (0.25), other clusters for these low-lying excited states with the transition energy less than 2.6 eV have small oscillator strengths. Figure 7 presents the simulated preresonance Raman spectra considering the two lowest lying

based on the DRSC of PMBT molecules interacting with Ag and Au clusters. Table S4 provides the data including Raman shifts, Raman scattering factors (RSF), and DRSC of characteristic peaks of PMBT interacting with Ag and Au clusters. For the normal Raman spectra of PMBT, we used B3LYP to calculate the DRSC of the main Raman peaks at the excitation wavelength of 632.8 nm. For comparison, we also calculate the DRSC of thiophenol listed in Table S5. The experimental DRSC of thiophenol has been reported,69,75,76 and the calculated DRSC values for the peaks between 950− 1590 cm−1 accord with these experimental values. However, for PMBT the Raman intensity at 1090 cm−1 significantly increases due to the introduction of the para-methyl group, while the intensity of the Raman peak associated with the ringtriangular distortion is significantly weakened. On the basis of the theoretical DRSC values of PMBT, the chemical enhancement factors produced by chemisorbed bonding are between 1 and 490. PMBT-Ag3 and PMBT-Au3 models describe the largest enhancement factor for the atop adsorption. The adsorption at bridge and hollow sites only contributes small static chemical enhancement factors. This is mainly due to the additional participation of lone pair orbitals on the sulfur atom in the bonding at the bridge and hollow sites, which significantly reduces the polarizability derivatives of the vibrational modes associated with the sulfur atom, resulting in the decrease of static Raman intensities. When the mercapto groups are bound to Au, bonding and antibonding orbitals are created, which may result in new surface states at the interface of the metal and molecules affecting the SERS spectra of the adsorbed molecules. A recent review illustrates the chemical bonding mechanisms of aromatic thiophenols on the surfaces of AuNPs.74,77 The electronic structure relations between the SERS of PMBT and the surfaces of the Au and Ag nanostructures are rooted in these energy level changes. For PMBT, a strong p-π conjugate is formed between the sulfur atom and the benzene ring, and 23032

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adsorption coordination bonds and the HOMO localized in PMBT, which significantly affect the relative intensities of Raman spectra of adsorbed molecules. Finally, we emphasize that the chemical enhancement from the low-lying states contributes to the vibrational modes selectively related to the C−S and S−M bonds as well as the benzene ring in other aromatic thiol compounds. Our calculate results also showed that the chemical binding interaction has a similar influence on the SERS of PMBT strongly adsorbed on silver and gold nanoparticles.

4. CONCLUSION We have investigated the Raman spectra and the SERS of PMBT adsorbed on Ag and Au nanostructures using both experimental and theoretical approaches. Along with the experimental measurements, the vibrational fundamentals of the free PMBT and PMBT adsorbed on metallic clusters have been assigned. The DRSC values of PMBT could be reasonably predicted. The relative intensities of the Raman spectra accord with those in experimental Raman spectra of solid PMBT and PMBT in CCl4 solution. We further conducted experimental and theoretical analyses on the SERS spectra of PMBT on the surface of Ag and Au nanostructures. The relationships between SERS intensity, surface coverage, and excitation wavelength were analyzed experimentally under the assumption of the same EM enhancement. The increase in wavelength significantly raises the relative intensities of the low-wavenumber vibrational modes. According to the vibrational analysis, these are mainly associated with sulfur in the bound thiyl group. DFT calculations of the Raman spectra of PMBT adsorbed on small Ag and Au clusters suggest that the recorded Raman spectra accord best with PMBT adsorption at bridge sites on the metallic NPs. The chemical enhancement factors for PMBT on Ag and Au nanostructures were calculated. The static chemical enhancement factors were first estimated. Although mercapto sulfur can form strong chemical bonds with Ag and Au, in most cases, the enhancement factor is less than 2 orders of magnitude. We analyzed the energy level alignment of the interface between PMBT and the metals, which directly affects the contribution of the interfacial low-lying excited states to the SERS chemical enhancement. The DFT calculations demonstrate that the LUMO and LUMO-1 π orbital levels of the PMBT are not lower than −1.0 eV (referred to the vacuum level), while the high occupied energy levels directly related to the SERS process, mainly the S−M bonding orbital and the molecular HOMO orbital. Under the excitation by visible light, these orbitals form intervalence-like interfacial charge transfer states and low-lying excited states localized in the S−M bond region, thereby affecting the SERS spectra of PMBT. If the molecular symmetry is taken to be C2V, the wavelength of the excitation light-dependent relative Raman intensity is likely to result mainly from these low-lying excited states, which include the contribution of the interfacial charge transfer states. Finally, the chemical enhancement implied in the change of relative SERS signals can be used to extract the information of the interface energy level alignment and photochemical processes in nanoscale interfaces. Overall,our study based on broad experimental SERS mapping and comprehensive theoretical support has thus provided molecular scale insight into the electronic and vibrational behavior of aromatic thiols adsorbed

Figure 7. Simulated preresonance Raman spectra of Ag-PMBT and Ag7-PMBT complexes using different incident wavelengths: (a) 785 nm; (b) 680 nm; (c) 551 nm; (d) 803 nm; (e) 634 nm; and (f) 532 nm.

singlet excited states (S1 and S2) of PMBT-Ag and PMBT-Ag7. The two states of S1 and S2 can be assigned to the transitions from the bonding orbital and the HOMO to the unoccupied metal energy level. Figure 7a−c shows preresonant Raman spectra of PMBT-Ag closely associated with the S1 and S2 states. Since the oscillator strength of PMBT-Ag S2 state is large, the excitation energy used here is smaller than the excited state energy by about 0.30 eV in Figure 7c. From the DRSC used in Figure 7a−c, the enhancements of the Raman signals resulting from these low-lying excited states can reach 1−3 orders of magnitude. This is in agreement with the PDCT mechanism enhancement of the SERS signal within 3 orders of magnitude. The chemical enhancement effect is selective to the vibrational modes. For example, in Figure 7b, relative intensities of the 354, 616, and 1062 cm−1 peaks associated with the υ7a, υ6a, and υ1 modes are enhanced (Figure 7c). Nevertheless, the increase in relative intensities of 248 cm−1 associated with the S−Ag bonds is obviously pronounced. Figure 7d−f shows the calculated preresonant Raman spectra using the PMBT-Ag7 model. Since the oscillator strength in each excited state used here is small, excitation lights with energy lower than the energy of the corresponding excited states of S1 and S2 by only 0.1 eV are used in our calculations. The changes of relative Raman intensities are caused by the electronic energy levels of the S−M bonding and the HOMO in PMBT in PMBT-Ag7. The relative intensities of the SERS spectra considering the interface electron level may therefore differ significantly from the Raman spectra calculated based on the static polarizability. A previous theoretical study of the adsorption of thiophenol on silver clusters concluded that there was no charge transfer enhancement mechanism between 1.6 and 3.0 eV, and that the enhancement mainly came from the excited state of the metal cluster itself.80 On the basis of first-principle density functional calculation, Zayak et al. proposed that the charge transfer direction for the adsorption of thiophenol on gold clusters was from molecule to metal.81 Ikeda et al. proposed that for PMBT molecules there was a charge transfer from molecule to metal on the gold electrode.28 Our calculations indicate that the low-lying excited states of PMBT adsorbed on the surface of gold and silver derive mainly from the electronic transitions associated with surface 23033

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on nanostructured Au- and Ag-materials to a new degree of detail.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.9b06431.

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Synthetic details of nanoparticles, experimental Raman spectra of PMBT, simulated Raman spectra of thiophenol, calculated data for the geometric structures of chemisorbed states, complete vibrational analyses and assignment of the fundamentals of PMBT, PMBT-Ag5, and PMBT-Au5, Raman scattering factors and corresponding cross sections of selective vibrational modes, and electronic transition properties of low-lying excited states (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Ran Pang: 0000-0001-5246-389X De-Yin Wu: 0000-0001-5260-2861 Jingdong Zhang: 0000-0002-0889-7057 Jens Ulstrup: 0000-0002-2601-7906 Zhong-Qun Tian: 0000-0002-9775-8189 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge support from the National Natural Science Foundation of China (NSFC) (Nos. 21533006, 201703183, 201773197, 21621091), the Chinese Ministry of Science and Technology (Y2018YFC1602802 and 2015CB932303), and Funds of State Key Laboratory of Physical Chemistry of Solid Surface and Fujian Science and Technology Office. Financial support from Otto Mønsted foundation is greatly appreciated for providing D.Y.W with a visiting professorship in Denmark.

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REFERENCES

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DOI: 10.1021/acs.jpcc.9b06431 J. Phys. Chem. C 2019, 123, 23026−23036