Surface-Enhanced Hyper Raman Spectra of Aromatic Thiols on Gold and Silver Nanoparticles.

We report the two-photon excited nonresonant surface-enhanced hyper Raman scattering (SEHRS) spectra of six aromatic thiol molecules during their interaction with gold and silver nanostructures. SEHRS spectra were obtained from thiophenol, benzyl mercaptan, and phenylethyl mercaptan and from the three isomers 2-aminothiophenol (2-ATP), 3-aminothiophenol (3-ATP), and 4-aminothiophenol (4-ATP). All SEHRS spectra were excited off-resonance at a wavelength of 1064 nm and compared to surface-enhanced Raman scattering (SERS) spectra excited at 785 nm or at 633 nm. The SEHRS spectra show a different interaction of thiophenol, benzyl mercaptan, and phenylethyl mercaptan with silver and gold nanostructures. Density functional theory calculations were used to support band assignments, in particular, for the unknown SERS spectrum of 3-ATP, and identify a band of phenylethyl mercaptan as a vibrational mode unique to the SEHRS spectrum and very weak in the Raman and infrared spectra. 2-ATP, 3-ATP, and 4-ATP show a different interaction with gold nanostructures that was found to depend on pH. Bands in the SEHRS spectrum of 2-ATP could be assigned to 2,2'-dimercaptoazobenzene, suggested to be obtained in a plasmon-assisted reaction that occurred during the SEHRS experiment. The results provide the basis for a better characterization of organic thiols at surfaces in a variety of fields, including surface functionalization and plasmonic catalysis.

Introduction

The structure of organic molecules on metal surfaces is a topic of increased interest in life and materials sciences, as it can significantly modify the physical and chemical properties of the interface.[1] Information on the organization of the adsorbed molecules can help us to understand and control the interfacial properties. The strong affinity of sulfur to gold and silver has resulted in organic thiolates becoming one of the most important and extensively investigated classes of compounds that have been used to improve the thermodynamic stability, electrical conductivity, and chemical reactivity of noble metal nanoscopic and macroscopic materials.[2−6]

Surface-enhanced Raman scattering (SERS)[7−9] as a sensitive tool for probing of molecule–metal interactions with high selectivity has been used to characterize the interaction of organic molecules,[10−13] including thiolates,[14,15] with the surface of nanostructured metals. Because of several advantages that the two-photon excited process of hyper Raman scattering (HRS) offers, the nonlinear analogue of SERS, surface-enhanced hyper Raman scattering (SEHRS),[16] has gained increasing importance for studying organic structures and materials.[17−21] In particular, HRS follows different selection rules than those acting in RS, and therefore, SEHRS can provide complementary vibrational information, specifically from IR active and silent modes.[18,22,23] Moreover, SEHRS is more sensitive than SERS with respect to adsorbate orientation and surface environmental changes.[17,24−27] The high local field enhancements observed recently in resonant SEHRS experiments with gold nanostructures[28,29] are very promising for exploiting SEHRS as an approach to probe molecules on gold surfaces excited off-resonance as well.

In this work, we examine the structure and interactions of aromatic thiols on gold and silver nanoparticles from the viewpoint of nonlinear spectroscopy using SEHRS. We report the nonresonant SEHRS spectra of thiophenol, benzyl mercaptan, and phenylethyl mercaptan, as well as of 2-aminothiophenol (2-ATP), 3-aminothiophenol (3-ATP), and 4-aminothiophenol (4-ATP) obtained at an excitation wavelength of 1064 nm. The differences between the Raman and hyper Raman selection rules become evident by comparing SEHRS data with one-photon excited SERS spectra from identical samples. As will be shown here, the two-photon spectra reveal different interactions of the thiol molecules with the metal surface under varied experimental conditions and complement the information obtained by SERS.

Materials and Methods

Gold(III) chloride trihydrate (HAuCl4·3H2O, 99.9% trace metals basis), silver nitrate (99.9999% trace metals basis), 2-ATP, 3-ATP, 4-ATP, thiophenol, benzyl mercaptan, and phenylethyl mercaptan were purchased from Sigma-Aldrich. Trisodium citrate dihydrate (99%) was obtained from Th. Geyer, and sodium chloride was purchased from J.T.Baker. All solutions were prepared using Milli-Q water (USF Elga PURELAB Plus purification system).

Citrate-reduced gold nanoparticles of size 50 ± 10 nm were prepared according to the protocol by Frens.[30] HAuCl4 solution (50 mL, 0.3 mM) was heated to boiling, then 450 μL trisodium citrate solution (1% by weight) was added, and the reaction mixture was kept boiling for 30 min. Citrate-reduced silver nanoparticles of size 115 ± 30 nm were produced by the Lee and Meisel method.[31] AgNO3 (46 mg) was dissolved in 245 mL of water and heated to boiling with extensive stirring. Next, sodium citrate solution (5 mL, 0.04 M) was added dropwise, and the reaction mixture was kept boiling for 1 h.

UV–vis spectra were recorded on a UV–vis–NIR double-beam spectrophotometer (V-670, JASCO) in the wavelength range between 300 and 1200 nm in quartz cuvettes of 10 mm path length. Transmission electron micrographs were taken using a Tecnai G2 20 TWIN instrument operating at 200 kV.

UV–vis absorbance spectra and transmission electron micrographs of the colloids are displayed in Figure . To obtain high local field enhancement that is necessary for the SEHRS experiments, the nanoparticles were aggregated with sodium chloride.

Figure 1

UV–vis absorbance spectra of different batches of citrate-stabilized silver nanoparticles (green) and gold nanoparticles (red) used in SEHRS experiments with aromatic thiols. Dashed spectra were measured in the presence of 0.1 M sodium chloride. Insets show transmission electron micrographs of the nanoparticles. The average size determined by TEM of the gold particles is 50 ± 10 nm and that of the silver particles is 115 ± 30 nm.

In the SEHRS and SERS experiments with thiophenol, benzyl mercaptan, and phenylethyl mercaptan, the gold and silver colloids (120 μL) were mixed with 15 μL of 1 M NaCl solution, and finally, 15 μL of 10–4 M of the respective thiol solution was added. In SEHRS and SERS experiments with aminothiophenols, 100 μL of gold nanoparticles were mixed with 10 μL of 1 M HCl, NaCl, and NaOH solutions, and finally, 10 μL of 10–4 M 2-ATP, 3-ATP, and 4-ATP solutions were added. pH in the samples with HCl was 2, with NaCl 3.5, and with NaOH 13. Each sample was measured directly after preparation.

SEHRS spectra were excited with a 1064 nm mode-locked laser producing 7 ps pulses at 76 MHz repetition rate. SERS spectra were excited using continuous-wave (cw) excitation, a diode laser operating at 785 nm (Toptica), and a HeNe laser operating at 633 nm (Thorlabs). The liquid samples were placed in microcontainers, the excitation light was focused onto the samples through a microscope objective (NA 0.3), and the RS and HRS light was detected using a liquid nitrogen-cooled CCD. Spectral resolution was 3–6 cm–1, considering the full spectral range. Excitation intensities and accumulation times used for each sample are stated in the figure captions. All spectra were frequency-calibrated using a spectrum of toluene. The SEHRS spectra were averaged and background-corrected using an automatic background correction algorithm.[32]

Vibrational frequencies and normal Raman spectra of the molecules were calculated according to density functional theory (DFT) with Gaussian 09[33] using the B3LYP functional[34] and the 6-311G** basis set.[35]

Results and Discussion

SEHRS of Thiophenol, Benzyl Mercaptan, and Phenylethyl Mercaptan

To measure the SEHRS and SERS spectra, an aqueous solution of the respective thiol was mixed with citrate-stabilized gold or silver nanoparticles. SEHRS spectra of thiophenol, benzyl mercaptan, and phenylethyl mercaptan were acquired using ps laser excitation at 1064 nm (Figure A,C,E), and one-photon SERS spectra of identical samples were measured with a cw laser operating at 785 nm (Figure B,D,F). In the following discussion, band assignments (Table ) are based on previous Raman, SERS, and DFT work[36−40] and also on vibrational frequencies and Raman spectra of the respective molecule in the gas phase that were calculated here using DFT. The SEHRS spectrum of thiophenol on silver nanoparticles (Figure A, red line) is in agreement with a previously reported SEHRS spectrum using 1550 nm excitation.[41] The SEHRS spectra of benzyl mercaptan on gold and silver nanoparticles (Figure C) and the SEHRS spectra of phenylethyl mercaptan (Figure E) as well as its SERS spectra (Figure F) have not been reported so far. The SERS spectra of thiophenol (Figure B) and benzyl mercaptan (Figure C) are consistent with SERS spectra measured on gold electrodes reported in the literature.[38]

Figure 2

(A,C,E) SEHRS spectra and (B,D,F) SERS spectra of thiophenol (A,B), benzyl mercaptan (C,D), and phenylethyl mercaptan (E,F) with gold (blue lines) and silver (red lines) nanoparticles. The bands marked in green are associated with modes involving C–S vibrations. All spectra are normalized relative to the intensity of the ring C–C stretching at around 1600 cm–1 of each molecule and are averages of 30 spectra. The inset in C shows the magnified not-corrected spectrum in the range between 1400 and 400 cm–1. Excitation: 1064 nm (A,C,E) and 785 nm (B,D,F); laser intensity in (A,C,E): 4.5 × 109 W cm–2 (peak intensity) and in (B,D,F): 4 × 105 W cm–2 (cw); acquisition time: 60 s (A,C,E, blue lines), 10 s (A,C,E, red lines), and 1 s (B,D,F); concentration: 10–5 M.

Table 1

Raman Shift Values in the Spectra of Thiophenol (PhSH), Benzyl Mercaptan (PhMeSH), and Phenylethyl Mercaptan (PhEtSH) and the Proposed Band Assignment Based on Refs (36−40) and Calculated Vibrational Frequencies and Raman Spectra of the Respective Molecule in the Gas Phase with DFT (See Table S1, Figure S2A–C)

Raman shift [cm–1]
PhSH	PhMeSH	PhEtSH	assignmenta
1573	1600	1602	symm i.p. C–C ring str
	1563	1584	asymm i.p. C–C ring str + CH2 twist
		1557	i.p. C–C–H ring bend
		1495	CH2 bend + i.p. C–C–H ring bend
	1494		i.p. C–C–H ring bend
1472			i.p. C–C str + C–H bend + C–S str
	1451		CH2 bend
1436			i.p. C–C str + C–H bend + C–S bend
		1415	CH2 bend + i.p. C–C–H ring bend
	1420	1373	i.p. C–C–H ring bend and/or
	1375	1335	i.p. C–C ring str and/or
	1320	1313	CH2 bend
1367			i.p. C–H ring bend
1325			i.p. C–C ring str
1271			i.p. C–H ring bend
		1260	(Ph)C–C(Et) bend
	1222		CH2 wag + C–C(Ph) str
		1203	(Ph)C–C(Et) str
	1202		(Ph)C–C(Me) str + CH2 wag
1180	1182	1178	i.p. C–H ring bend
1156	1156		i.p. C–H ring bend
	1140	1156	i.p. C–H ring bend + CH2 twist
1108			i.p. C–C–H bend + C–C–S bend
1073			i.p. C–C–C bend + C–S str
1022	1030	1030	i.p. C–H ring bend
		1015	i.p. ring bend + C–C (alkyl) str + S–C bend
999	1003	1002	i.p. C–C–C ring bend (ring breathing)
953		950	o.o.p. C–C–H bend
	913		o.o.p. ring C–H bend + S–C bend
		908	o.o.p. ring C–H bend or asymm C–C–S str
897			o.o.p. C–C–H bend
	850	870	o.o.p. ring C–H bend or asymm C–C–S str
834		824	o.o.p. C–H ring bend
	805		o.o.p. ring C–H bend or asymm C–C–S str
737	760	758	o.o.p. C–H ring bend
		705	i.p. C–C–H ring bend
	700		o.o.p. C–H + C–C ring bend
691			i.p. C–C–C ring bend + C–S str
	650	683	S–C str
		621	o.o.p. C–C–H ring bend
	620		i.p. C–C ring bend
615			i.p. C–C–C ring bend + C–S str
	558	560	o.o.p. C–C–H ring bend
473	471		o.o.p. C–C–C bend
		493	o.o.p. C–C–C bend + CH2 bend
419			i.p. C–S str
	407		o.o.p. C–C–C bend
		403	o.o.p. C–C–C bend + CH2 bend

i.p., in-plane; o.o.p., out-of-plane; str, stretching; bend, bending; symm, symmetric; asymm, asymmetric; twist, twisting; wag, wagging; and ring, phenyl ring.

Comparing the overall signals, stronger SEHRS signals were obtained from thiophenol and benzyl mercaptan, compared to phenylethyl mercaptan (compare the scale bars in Figure A,C with those in Figure E). The silver nanostructures (Figure A,C,E, red scale bars) yielded stronger enhancement than the gold nanostructures (Figure A,C,E, blue scale bars). We will now first discuss the general differences between the SEHRS and SERS spectra of all three molecules (compare Figure A with 2B, Figure C with 2D, and Figure E with 2F) and then compare the spectra obtained with the silver and the gold nanostructures (compare red and blue traces in every panel of Figure ).

The SEHRS spectra are dominated by bands assigned to C–C stretching vibrations of the ring at 1573 cm–1 for thiophenol (Figure A), 1600 cm–1 for benzyl mercaptan (Figure C), and 1602 cm–1 for phenylethyl mercaptan (Figure E), respectively. In contrast, in the SERS spectra of all molecules (Figure B,D,F), the in-plane bending modes of the phenyl ring around 1000–1100 cm–1 are the most prominent bands. In agreement with our previous report on the SEHRS spectra of aromatic amino acids[25] and nucleobases,[23] also here in the SEHRS spectra of the aromatic thiols, the signals assigned to the ring-breathing (C–C–C bending) modes ∼1000 cm–1 are greatly diminished compared to those in the one-photon spectra. Both SEHRS and SERS spectra of all three molecules show strong contributions from modes that include the C–S stretching vibrations (all bands marked in green in Figure (36,37)), especially the spectra obtained with silver nanoparticles (Figure , all panels, red lines). The SEHRS spectra of benzyl mercaptan (Figure C) and phenylethyl mercaptan (Figure E) also exhibit bands that could be assigned to (phenyl)C–C(alkyl chain) and CH2 vibrations in the 1200–1230 cm–1 spectral range (assignments in Table ). For all three molecules, most of the weaker out-of-plane ring deformation modes below 1000 cm–1 (cf. Table ), for example, ∼950 cm–1 (Figure B), ∼800, ∼760 cm–1 (Figure C,D), or ∼820 cm–1 (Figure E,F) are more pronounced in the SERS spectra.

Interestingly, in the SEHRS spectrum of phenylethyl mercaptan on silver nanostructures (Figure E, red line and Figure S1A), a strong contribution at 1015 cm–1 is observed, and on gold nanostructures (Figure E, blue line), this band appears as a shoulder of the band at 1002 cm–1 assigned to the ring-breathing mode. Our DFT calculation indicates the existence of a C–C stretching mode of the ethyl group combined with C–S bending vibration that is located between the strongly pronounced in-plane C–H and in-plane C–C–C bending (ring breathing) modes that are very characteristic for monosubstituted benzenes (see Supporting Information Table S1) and that are present in the SERS spectra of phenylethyl mercaptan at 1030 and 1002 cm–1, respectively (Figures F and S1B). The DFT calculation shows that this mode has very low IR and Raman activity (Table S1, mode 24), in spite of its strong contribution to the SEHRS spectra.

Comparing the spectra obtained with gold and silver nanostructures, both one- and two-photon excited spectra show more or less the same bands but they differ in relative intensities. To highlight the differences between the spectra obtained with gold and silver nanoparticles, the respective SEHRS and SERS spectra in each panel of Figure were normalized to the band of the symmetric C–C stretching vibration of the phenyl ring at around 1600 cm–1 of the respective molecule. The most pronounced differences when using the two kinds of plasmonic substrates are found in the SEHRS spectra of thiophenol (Figure A). There, the intensity of all bands in its SEHRS spectrum with silver nanoparticles (Figure A, red line) decreases relative to the band at 1573 cm–1, when the spectrum is acquired with gold nanoparticles (Figure A, blue line). With gold nanoparticles, in particular, the bands at 1108, 1073, 691, and 419 cm–1 that are associated with C–S vibrations and in-plane C–C–C bending modes of the phenyl ring (cf. Table ) have much lower relative intensity (Figure A, blue trace). This is less prominent for the other bands, for example, the pure C–H bending at 1022 cm–1 and the ring breathing at 999 cm–1. These differences between the SEHRS spectra obtained with silver and gold nanostructures are much stronger than the differences between the SERS spectra of thiophenol with gold and silver nanoparticles (compare red and blue lines in Figure A with 2B). Interestingly, the bands at 419 and 1073 cm–1 involving C–S stretching modes in the SERS spectrum with gold particles are slightly more intense than with silver particles (compare red and blue lines in Figure B), although in SEHRS, the opposite was observed (Figure A). These variations in band intensities in the spectra obtained with gold and silver nanostructures point to a different interaction of the thiophenol molecule with the nanoparticle surface, in agreement with previous work suggesting based on SERS data that the azimuthal angle of rotation about the C2 axis of the phenyl ring differs for gold and silver.[42] Here, the spectral differences in SEHRS are much more pronounced, which supports previous observations[17,24,25] that SEHRS is more sensitive than SERS with respect to the orientation of the molecule on the metal surface.

For both benzyl mercaptan (Figure C,D) and phenylethyl mercaptan (Figure E,F), the SEHRS (Figure C,E) and the SERS spectra (Figure D,F) differ, depending on whether they were obtained with silver (red lines) or gold nanoparticles (blue lines). In all SEHRS and SERS spectra (Figure C–F), the bands associated with C–S stretching vibrations are much more intense in the spectra with silver nanostructures (at 650 and at 683 cm–1 in Figure C,D and 2E,F, respectively). The SEHRS spectrum of phenylethyl mercaptan obtained with silver nanoparticles (Figure E, red line) has contributions from the out-of-plane ring-bending modes at 824, 758, 560, and 493 cm–1, while in its SEHRS spectrum with gold nanoparticles (Figure E, blue line), these modes are not observed. In contrast, in the SERS spectra, they are present with both gold and silver nanostructures, and the modes at 560 and 758 cm–1 are even more pronounced with the gold nanoparticles than with silver nanoparticles (compare blue and red lines in Figure F). Specifically, the SERS spectra of benzyl mercaptan (Figure D) and phenylethyl mercaptan (Figure F) differ in the out-of-plane ring modes in the 950–800 cm–1 spectral region, for example, at 850 and 805 cm–1 (Figure D, compare red and blue lines, and Table ), and at 950 and 870 cm–1 (Figure F, compare red and blue lines, and Table ), respectively.

The differences between the respective SEHRS and SERS spectra of benzyl mercaptan and phenylethyl mercaptan indicate a different orientation and/or interaction of the molecules on the gold and silver surfaces. Specifically, the decreased intensity of bands associated with C–S vibrations mentioned above (Figure C–F, green labels) in both SEHRS and SERS with gold nanoparticles as well as different intensity ratios for some out-of-plane ring modes (Figure D,F) suggest that the orientation of the phenyl ring with respect to the metal surface is different for gold and silver, in line with the observations for thiophenol discussed above. More precisely, the data point toward a more direct interaction of the phenyl ring with the gold surface. This could be due to the different nature of the interaction between the thiols with gold and with silver, but it might also be influenced by the amount and coverage of the different nanoparticles with the capping citrate molecules.

In all three molecules, the combination of SEHRS with SERS provides complementary vibrational information suitable to identify the different interaction of the molecules with the two kinds of plasmonic substrates. The results obtained with benzyl mercaptan and phenylethyl mercaptan clearly demonstrate that the orientation of these molecules with respect to the surfaces of both gold and silver nanostructures is more versatile than in thiophenol, where the different interaction is mainly detected in the SEHRS rather than in the SERS spectra.

SEHRS of Aminothiophenol Isomers and the Influence of pH

SEHRS spectra of the three isomers 4-ATP, 2-ATP, and 3-ATP at 1064 nm were obtained with citrate-stabilized gold nanostructures at three different pH values (Figure A,C,E). SERS spectra from the identical samples were excited at a wavelength of 633 nm (Figure B,D,F) for comparison. To facilitate the comparison of the spectra of one sample at different pHs, the spectra in each panel were normalized to the signal of the symmetric C–C stretching/symmetric NH2 deformation mode in the range 1560–1590 cm–1 of the respective molecule. 4-ATP yielded stronger signals compared to 3-ATP and 2-ATP in both SEHRS and SERS experiments (compare the scale bars in Figure A with those in Figure C,E). While the SERS spectra of 4-ATP (Figure B) and 2-ATP (Figure D) are in agreement with previously reported spectra from the literature,[43−46] the SERS spectrum of 3-ATP has not been reported so far. Assignments for some of the bands observed in the spectra (Table ) were therefore also made based on Raman spectra of the three molecules calculated with DFT (Figure S2D,E,F). The SEHRS spectra of the different isomers vary greatly at the same pH (compare spectra of the same colors in Figure A,D,F).

Figure 3

(A,C,E) SEHRS spectra and (B,D,F) SERS spectra of 4-ATP (A,B), 2-ATP (C,D), and 3-ATP (E,F) with gold nanoparticles at pH 2 (black lines), 3.5 (red lines), and pH 13 (blue lines). All spectra are normalized relative to the intensity of the band at around 1570 cm–1 of each molecule and are averages of 30 spectra. The inset in (E) shows the magnified not-corrected spectrum in the range between 1500 and 700 cm–1. The bands marked in green are associated with vibrations of DMAB. Excitation: 1064 nm (A,C,E) and 633 nm (B,D,F); laser intensity in (A,C,E): 6 × 109 W cm–2 (peak intensity) and in (B,D,F): 1 × 105 W cm–2 (cw); acquisition time: 60 s (A,C,E, blue lines) and 1 s (B,D,F); concentration: 8.3 × 10–6 M.

Table 2

Raman Shift Values in the Spectra of 2-ATP, 3-ATP, and 4-ATP and the Proposed Band Assignment Based on refs (43, 45, 46, 49, 54) and Calculated Vibrational Frequencies and Raman Spectra of the Respective Molecule in the Gas Phase with DFT (Figure S2D–F)

Raman shift [cm–1]
2-ATP	3-ATP	4-ATP	assignmenta
1581	1600	1595	symm NH2 bend + symm C–C str
1564	1575	1588	symm C–C str + symm NH2 bend
		1484	i.p. C–C–H bend + C–S and C–N str
1469	1436		i.p. C–C str + i.p. C–H bend
1313	1300		C–N str + C–C bend
1290	1257	1284	C–N str + C–C bend
1229	1203	1178	i.p. C–H bend
1158	1166		i.p. C–H bend
1092			asymm NH2 bend + C–S str
		1076	C–S str + i.p. C–C–C bend
	1073		C–S str + i.p. C–H bend
1053			i.p. C–H bend
	1033		i.p. C–H bend + asymm NH2 bend
1028			i.p. C–C–C bend (ring breathing)
		1002	i.p. C–C–C bend + C–S str
	999		i.p. C–C–C bend (ring breathing)
	884		i.p. C–C–C bend + C–N str
	864		i.p. C–C–C bend + C–N str
		810	i.p. C–C–C bend + C–S str + C–N str (ring breathing)
	683		C–C bend + C–S str
560		633	i.p. C–C–C bend + C–S str
	532		i.p. C–C bend + o.o.p. NH2 bend
387	415		i.p. C–S bend
		388	i.p. C–C(S)–C bend + C–S str + C–C(N)–C bend
1433		1447	N=N str of dimerization product
1375		1385	N=N str of dimerization product
1121		1143	C–N str of dimerization product

i.p., in plane; o.o.p., out of plane; str, stretching; bend, bending; symm, symmetric; and asymm, asymmetric.

The SEHRS spectra of 4-ATP under acidic conditions (Figure A, black and red lines) show strong contributions from the symmetric C–C stretching of the phenyl ring combined with the symmetric NH2 bending at 1588 cm–1 and the C–S stretching combined with the C–C–C bending mode of the ring at 1076 cm–1 (Table ).[46] Both lines are also very strong in the SERS spectra (Figure B, black and red lines), although in the SERS spectra, the relative intensity of the 1588 cm–1 phenyl ring stretch is weaker. Furthermore, bands associated with in-plane ring-bending modes that include C–S stretching vibrations at 1484, 633, and 388 cm–1 are observed in both SEHRS and SERS spectra of 4-ATP (Figure A,B). In SEHRS, the intensities of the bands at 1484, 1178, 1076, and 633 cm–1 decrease significantly with increasing pH (compare black, red, and blue lines in Figure A). This can be associated with the high degree of protonation of the amino groups of 4-ATP for pH values below 4,[47] which is lost at high pH.

In the SERS spectrum of 4-ATP at pH 13 (Figure B, blue line), further pronounced differences compared to the spectra obtained at acidic pH (Figure B, black and red lines) are found. Particularly, several new bands are observed in the spectral range between 1100 and 1500 cm–1, including signals at 1447, 1385, and 1143 cm–1 (Figure B, green labels). In previously discussed SERS spectra of 4-ATP in the basic environment on gold nanoparticles using 633 nm excitation,[48] they were proven to arise from the product of the plasmon-assisted catalytic dimerization of 4-ATP to 4,4′-dimercaptoazobenzene (4,4′-DMAB).[48−50] In the SEHRS spectrum at pH 13 (Figure A, blue line), none of the bands associated with 4,4′-DMAB vibrations are present.

Similar to the SEHRS spectra of 4-ATP, the SEHRS spectra of 2-ATP (Figure C) and 3-ATP (Figure E) are also clearly dominated by the band because of the symmetric C–C stretching and symmetric NH2 bending modes at 1564 and at 1575 cm–1, respectively (cf. Table ). The contributions of the second symmetric NH2 bending combined with the symmetric C–C stretching mode become more prominent under acidic pH in both SEHRS and SERS spectra, visible as a shoulder in the black and red 2-ATP SEHRS spectrum (Figure C) and as a distinct band in the 2-ATP SERS spectrum (Figure D) at 1581 cm–1. In 3-ATP, this band is observed as a shoulder at 1600 cm–1 in the SEHRS and SERS spectra measured at pH 2 (black traces in Figure E,F, respectively). At pH 2, also bands assigned to modes that include asymmetric NH2 bending vibrations (see Table ) are more pronounced in the SEHRS spectra of 2-ATP (Figure C, black line, band at 1092 cm–1) and 3-ATP (Figure E, black line, band at 1033 cm–1). In the one-photon SERS spectra of 2-ATP (Figure D) and 3-ATP (Figure F), the changes with varying pH are more distinct. As an example, the intensity ratio of the bands at 815 and 838 cm–1 in the SERS spectrum of 2-ATP (Figure D, compare black and blue line) and that at 862 and 884 cm–1 in the spectrum of 3-ATP (Figure F, compare black and blue line) are reversed when pH is changed between pH 2 and pH 13.

As observed in the spectrum of 4-ATP, the SERS spectrum of 2-ATP at pH 13 shows contributions from new modes at 1433, 1375, and 1121 cm–1 (Figure D, blue line, green labels) that are absent in the spectra obtained under acidic conditions (Figure D, black and red lines). This indicates that also 2-ATP undergoes a dimerization reaction at the given excitation conditions. The band at 1433 cm–1 associated with N=N stretching of the dimerization product[49] is much more intense in the SERS spectrum of 2-ATP than in the SERS spectrum of 4-ATP (compare blue lines in Figure B,D). Interestingly, the N=N stretching mode is observed also in the SEHRS spectrum of 2-ATP at pH 13 (Figure C, blue line). The latter indicates that the dimerization of 2-ATP must have also occurred when using 1064 nm laser excitation. We have shown that, in order for SEHRS on gold nanoaggregates to be efficient, plasmon resonances at the NIR excitation wavelength play a major role.[28] These plasmon resonances in the NIR are suggested to assist the plasmon-catalyzed oxidation reaction of 2-ATP here. We have also previously observed DMAB formation at an excitation wavelength of 785 nm,[51] where we also provided evidence that surface metal oxide species, stabilized at the high pH, are important in the plasmon-catalyzed dimerization of 4-ATP. The production of such surface metal oxides on gold nanoparticles is proposed to occur by activation of 3O2 in a surface plasmon-assisted process that can employ both electron transfer from the gold nanoparticles and a local temperature increase.[48,51] Especially, the high peak intensities coming from the ps laser pulses used for excitation of the SEHRS could facilitate this activation. Although the absence of the bands typical of DMAB in the SEHRS spectrum of 4-ATP could indicate that the dimerization does not take place under excitation with 1064 nm despite their presence in the 633 nm-excited SERS spectrum, it is possible that the (few) characteristic modes of the reaction product are not enhanced in the SEHRS spectrum of 4-ATP. The much stronger contributions from the DMAB in the SERS spectra of 2-ATP discussed above and the presence of the N=N stretching band of DMAB in the SEHRS spectrum of 2-ATP point to a more efficient generation of the dimer, possibly due to the different interactions of 2-ATP with the surface that could result in a different efficiency of the plasmon-assisted reaction steps and/or an orientation of the 2-ATP monomers that favor dimer formation.

In contrast, the SERS and SEHRS spectra of 3-ATP at pH 13 do not exhibit any new peaks compared to the spectra at pH 2 and 3.5 (Figure E,F). From the results reported for 4-ATP,[52,53] it is expected that dimerization of 3-ATP does not occur at pH 2 and that the spectra of 3-ATP therefore do not show contributions from DMAB. The similarity of the three 3-ATP SEHRS and SERS spectra and the absence of bands typical of DMAB suggest that probably all spectra in Figure E,F were acquired under conditions where only the 3-ATP monomers are present. The spectral differences, in particular, the band at 1203 cm–1 associated with an in-plane C–H bending mode that disappears at pH 13 (e.g., compare red and blue lines in Figure F and in the inset in Figure E), must be due to a different protonation state and/or different orientation with respect to the gold nanoparticle surface.

Conclusions

Nonresonant SEHRS spectra of the three aromatic thiols, thiophenol, benzyl mercaptan, and phenylethyl mercaptan using aggregates of gold and silver nanoparticles were obtained with 1064 nm laser excitation. Comparing the SEHRS data with 785 nm-excited SERS spectra from the identical samples, the complementarity of both types of vibrational spectral information becomes evident. In agreement with nonresonant SEHRS spectra of other aromatic compounds, the ring-breathing modes (in-plane C–C–C bending vibrations) in the SEHRS spectra are much weaker than in the SERS spectra or even absent. A typical band was observed in the spectrum of phenylethyl mercaptan and assigned based on DFT calculation, illustrating that the SEHRS spectra can reveal modes that are very weak in IR or Raman spectra.

The great dissimilarity of the SEHRS spectra of thiophenol when measured on gold and silver nanostructures, in spite of the great similarity of the respective SERS spectra, highlights the potential of SEHRS for the characterization of thiol-functionalized surfaces. Bands assigned to the functional groups that are expected to interact with the metal surface, such as the thiol group, are very prominent in the SEHRS spectra. Also, in-plane symmetric C–C stretching vibrations of the phenyl ring show very strong contributions in SEHRS. The SEHRS spectra obtained with the two types of metal nanoparticles indicate a different interaction with the two kinds of plasmonic substrates, with a more direct interaction of gold nanoparticles with the phenyl ring. The results strongly suggest SEHRS as a tool to probe interactions of organic molecules with nanoparticles under varied environmental conditions.

The nonresonant SEHRS spectra of the aminothiophenol isomers 2-ATP, 3-ATP, and 4-ATP, obtained on gold nanostructures, together with their SERS spectra excited at 633 nm, indicate a different interaction of the different isomers with the gold. When pH in the local environment is varied, structural changes, that include both the direct de-/protonation of the amino group and the emergence of new molecular species, can be monitored.

The SEHRS spectrum of 2-ATP at high pH contains a band that was assigned to the dimer DMAB based on comparison with the SERS spectra of 2-ATP and 4-ATP excited at 633 nm. We conclude that it is a product of the plasmon-assisted oxidation reaction of 2-ATP taking place here at an excitation wavelength of 1064 nm. Although the dependence of product formation on laser intensity, presence of other metal species, and the influence of other ligand molecules, as well as a comparison with the mechanisms discussed for this reaction at visible wavelengths will have to be the subject of future studies, the observation of the plasmon-catalyzed reaction by SEHRS suggests that the combination of SEHRS and SERS could be very beneficial for the further elucidation of plasmon-assisted reaction mechanisms, especially in the NIR frequency range.