Raman, SERS, and DFT of Mauve Dye: Adsorption on Ag

Jun 4, 2015 - Department of Chemistry and Center of Analysis of Structures and Interfaces (CASI), The City College of New York, Convent Avenue at 138t...
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Raman, SERS, and DFT of Mauve Dye: Adsorption on Ag Nanoparticles M. V. Cañamares*,† and J. R. Lombardi‡ †

Instituto de Estructura de la Materia, IEM-CSIC, Serrano 121, 28006 Madrid, Spain Department of Chemistry and Center of Analysis of Structures and Interfaces (CASI), The City College of New York, Convent Avenue at 138th Street, New York, New York 10031, United States

‡

S Supporting Information *

ABSTRACT: Mauve was the first synthetic organic dyestuff to be manufactured industrially. The main components of the dye are mauveine A, B, B2, and C. These molecules show four nitrogen atoms in their structure. Therefore, they can interact with the SERS substrate by four different sites. In this study the Raman and SERS spectra of the mauve and its components were obtained in order to determine the adsorption mechanism on Ag nanoparticles. The comparison of the Raman and SERS spectra of mauve dye revealed several differences in the intensity and position of the Raman bands. Thus, a chemical interaction between the dye and the Ag nanoparticles was concluded. DFT calculations of the four mauveine molecules were carried out to aid in the assignments of the vibrational normal modes.

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INTRODUCTION Mauve was the first synthetic organic dyestuff to be manufactured industrially. The synthesis of this dye by William Henry Perkin and the establishment of a factory in 1862 to produce it commercially marked the beginnings of the modern dye industry.1 Until Perkin’s discovery, most dyes were natural compounds extracted from plants and animals, which show acidic properties. However, the basic properties of the new synthetic dye, associated with the N atom from the aniline, were relevant to the attachment of the dye to textile fiber.1 The main components of mauve are mauveine A and B; other components such as mauveine B2 and C were also discovered in 2007.2 The approximate relative amounts of each component in mauve dye are the following: mauveine A, 50%; B, 30%; B2, 10%; and C, 5%.3 The only differences among these molecules are the number and positions of the methyl groups attached to the aromatic rings (Figure 1). All of them show four nitrogen atoms, two of them in the pyrazine ring (N5 and N6) and the other two forming secondary and tertiary anime groups (N17 and N19, respectively). Raman spectroscopy is a potentially useful technique for the noninvasive analysis of textile dyes. However, most dyes examined exhibit intense fluorescence, and the use of surfaceenhanced Raman scattering (SERS)4 is needed. The potential of this technique in the analysis of natural and synthetic dyes has been amply demonstrated.5−11 Briefly, the adsorption of an organic dye on the surface of a metal nanoparticle results in an extremely large enhancement of the Raman scattering, accompanied by a significant quenching of the fluorescent © 2015 American Chemical Society

Figure 1. Structure and numbering of the main components of mauve: mauveine A (a), B (b), B2 (c), and C (d).

emission.4 This leads to ultrahigh sensitivity and the possibility to successfully analyze trace samples. In this study we endeavored to analyze the Raman and SERS spectra of the dye and its components in order to determine Received: March 18, 2015 Revised: June 3, 2015 Published: June 4, 2015 14297

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The Journal of Physical Chemistry C their interaction with Ag nanoparticles, in particular the adsorption mechanism and orientation of the molecules on the Ag surface. In a previous work, the best conditions to obtain the Raman and SERS spectra of mauve dye and the four mauveine molecules were determined.12 In this work the previously obtained Raman and SERS spectra were deeply analyzed in order to investigate the way the mauveine molecules are adsorbed on the Ag nanoparticles by applying the surface selection rules. These rules are best applied to planar surfaces,13 and the Ag nanoparticles used have curvature and roughness. However, the size of the mauveine molecules is much smaller than that of the Ag nanoparticles. Thus, despite the local roughness, the molecules experience, on the average, a planar surface. Density functional theory (DFT) calculations were carried out to aid in the assignments of the vibrational normal modes. Thus, the identification of the adsorption site of the mauveine molecules is enabled.

The components of mauve were analyzed by SERS directly on the TLC plate. The samples were prepared by placing 0.5 μL of the Ag nanoparticles and 0.25 μL of 0.5 M NaCl directly on top of each spot. The four purple spots were identified as mauveine A, B2, B, and C.12 Instrumentation. Ordinary Raman and SERS spectra were measured using a Bruker Senterra dispersive Raman microscope, with 633, dispersion through a 1200 line pairs/mm grating and detection with an Andor CCD camera. A modified Olympus BX51 microscope with long working distance 50× and 100× objectives was used. The size of the measured area was 4−5 and 2 μm, respectively. The output power was 0.2 mW. The ordinary Raman spectra were the result of three scans with 30 s of integration time. The parameters for the SERS spectra were set at 3 scans with an integration time of 5 s. The SERS analysis conducted directly on the TLC plates was the result of one scan of 5 s. DFT Calculations. Calculations were performed with Gaussian 0917 at the B3LYP level of theory and employing the 6-31+G* basis set. This basis set was chosen to be consistent with that used in our previous work.18,19 No imaginary wavenumbers were observed in the calculated spectrum. The assignments of the vibrational normal modes were based on the best fit comparison of the wavenumber of the calculated and experimental Raman bands. Scaling factors20 are commonly used in the calculated spectra in order to correct the correlation effects that are incompletely accounted for in DFT. In this work, the 0.965 and 0.95 were used in the regions 1800−1100 cm−1 and above 1800 cm−1, respectively.

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EXPERIMENTAL SECTION Materials. All the reagents employed were of analytical grade and purchased from Sigma-Aldrich except for p-toluidine, which was purchased from Fluka. All aqueous solutions were prepared using Millipore Milli-Q water. Mauve dye was synthesized in the laboratory following a modified Perkins’ original recipe.12 Thus, an aqueous solution containing p-toluidine, sulfuric acid, aniline, and o-toluidine was prepared. A potassium dichromate solution was added over the course of 1 h. The reaction was allowed to proceed for 2 h. The solution was filtered to afford a dark, purple−brown solid, which was washed and dried for 1 h at 110 °C. The solid was extracted for ∼48 h. The extracted solid was dried for 30 min at 102 °C. The dye was extracted from the crude with 25% methanol. The 25% methanol solution was evaporated to afford the purified dye. Thus, a dark purple product was obtained. Preparation of Ag Nanoparticles. Silver nanoparticles were prepared following the method of Lee and Meisel14 by means of reduction of silver nitrate with sodium citrate. A total of 1 mL of a 1% w/v trisodium citrate aqueous solution was added to 50 mL of a boiling 10−3 M silver nitrate aqueous solution. Boiling was continued for 1 h. The suspension of Ag nanoparticles showed grayish color. The characterization of these Ag nanoparticles by UV−vis spectroscopy and SEM was carried out by Cañamares et al.15 The average size of the Ag nanoparticles is 50−60 μm. SEM micrographs show that these nanoparticles have both spherical and rod-like shapes. The average enhanced factor of these AgNPs is 104.16 The activation of the obtained nanoparticles was carried out using a 0.5 M aqueous solution of NaCl. Preparation of Samples for Surface-Enhanced Raman Spectroscopy. Analysis of Mauve. A diluted solution of mauve was prepared in methanol 25%. Samples for the SERS analysis were prepared adding 1 μL of the dye solution to 2 μL of Ag nanoparticles activated with 0.25 or 1 μL of an aggregating solution. Analysis of Mauveine A, B, B2, and C. The components of mauve were previously separated by thin-layer chromatography (TLC) using a silica gel plate as the stationary phase and a 6:1:3 isobutanol:acetic acid:ethyl acetate solution as eluent. A small amount (∼2 μL) of the synthesized dye was deposited onto the TLC plate. Five spots were separated on the TLC plate, four purple and one red. These spots were visualized under a UV lamp at 365 and 375 nm.

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RESULTS AND DISCUSSION Mauveine A, B, B2, and C: DFT Calculations and Normal Modes Assignments. Figure 2 shows the DFToptimized structures of the main components of mauve. The main plane is formed by the anthracene derivative (rings I−III), while the benzene ring IV lies perpendicular to it. The other

Figure 2. Images of the DFT-optimized geometries of mauveine A (a), B (b), B2 (c), and C (d). 14298

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deformations of the methyl group bonded to C7, such as 459 and 441 cm−1, not observed in Tables S1 and S3 (Supporting Information). Raman and SERS Spectra of Mauve Dye: Adsorption Mechanism. The main Raman bands observed in the spectrum of mauve appear at 1637, 1617, 1604, 1566, 1465, 1391, 1385, 1347, 1245, 1183, 1074, 1019, 980, 824, 803, 764, 742, 687, 665, 615, 611, 561, 550, 501, 441, and 415 cm−1 (Figure 4a). On the other hand, the bands shown in the SERS

benzene ring, V, is also perpendicular to the antracene but a bit tilted with respect to ring IV. The calculated Raman spectra of the molecules appear in Figure 3. Tables S1−S4 (Supporting Information) show the

Figure 3. Scaled DFT-calculated Raman spectra of mauveine A (a), B (b), B2 (c), and C (d).

main DFT Raman bands of the four mauveine components together with the assignments derived from the calculations. The most intense bands in the spectra are located in three different regions: above 3000, in the 1800−1100 region, and around 450 cm−1. The first region shows bands assigned to ν(NH) (3536−3408 cm−1), ν(CH) in aromatic rings (just above 3000 cm−1), and ν(CH3) (just below 3000 cm−1). The different mauveine molecules have the same number of NH groups in the same positions; therefore, the bands assigned to those vibrations show the same intensity and wavenumber. It was noted before that the differences in the structure of these molecules are the number and positions of the methyl groups. Thus, the number and position of CH aromatic bonds are also different among the mauve components. For these reasons, the main variations among the Raman spectra of those molecules are shown around 3000 cm−1. The second region includes bands due to the main functional groups: ν(CC), ν(CN), and δ(CH). Here, the Raman calculated spectra of mauveine A and B2 are similar. Some differences can be found, such as the band at 1569 cm−1 assigned to the asymmetric bending of the CH3 attached to C14, only present in mauveine B2 (Tables S1 and S3, Supporting Information). The same happens for the mauveine B and C calculated spectra. The high differences between mauveine A/B and B2/C are due to the strong overlapping of normal bands assigned to the CH3 attached to C7, only present in mauveine B and C, such as the bands at 1418, 1237, and 1214 cm−1 assigned to δ(C27H3) (Tables S2−S4, Supporting Information). The third region corresponds to skeletal vibrations. As it happened in the previous region, bands due to mauveine A and B2 and B and C are similar among them. The assignment of the low-frequency region of the spectra is more complicated due to the relatively large number of normal modes that contribute to the observed vibrations. Tables S2 and S4 (Supporting Information) show some bands assigned to vibrations due to

Figure 4. Raman (a) and SERS (b) spectra of mauve dye. Excitation at 633 nm. The spectrum (a) was baseline corrected.

spectrum of the dye are located at 1641, 1613, 1601, 1560, 1540, 1510, 1447, 1404, 1390, 1347, 1278, 1248, 1210, 1185, 1134, 1017, 916, 822, 798, 763, 731, 710, 690, 670, 624, 616, 562, 549, 505, 470, 437, and 404 cm−1 (Figure 4b) . The observation of the differences in the wavenumbers and intensities of the bands observed in both spectra of mauve allows the deduction of the adsorption mechanism and the orientation of the dye on the Ag nanoparticles. An enhancement of the bands at 1560, 1540, 1390, 1347, 1134, 916, and 624 cm−1 is shown in the SERS spectra. A decrease of the intensity of the band at 1637 cm−1 is also observed. The normal mode assignment of those bands is shown in Table 2. Thus, the first band is associated with the deformation of the NH group (N19) and the stretching of ring V. The second one is assigned to δ(NH), together with ν(ring IV). The third band is due to different vibrations depending on the mauveine component. The band at 1347 cm−1 corresponds to stretching of rings I, II, and III, while the one at 1134 cm−1 is due to the deformation of CH in ring III. Finally, the last band can be assigned to in-plane deformations of the three main rings (I, II, III). The band showing a lower intensity in the SERS spectrum related to the Raman corresponds to the scissoring vibration on the NH2 group (N17). At first sight, the enhancement of the band at 624 cm−1 suggests a perpendicular orientation of the molecule on the Ag, according to SERS selection rules.21 Besides, N5 in ring II should be the most likely to interact with the Ag nanoparticles. However, the most enhanced bands are related to N19. For this reason, an adsorption of the mauveine molecules on the metallic surface by means of that nitrogen (N19) is proposed. Thus, the secondary amine group (N19) would be the closest to the Ag nanoparticles, while the primary amine (N17) would 14299

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Figure 5. Adsorption model on Ag nanoparticles of mauveine A and B2 (a) and B and C (b). The different substituents in C7 originating differences in the SERS spectra are marked by green elipses.

be the furthest (Figure 5). This interaction would explain the high intensity increase of the bands at 1566 and 1540 cm−1 and the weakening of the band at 1637 cm−1. Safranin O is a cationic dye whose structure (Figure S1, Supporting Information) is similar to that of mauveine molecules. It is therefore of value to study and compare the adsorption mechanism of safranin O with that of mauve. The Raman spectrum of safranin O (Figure S2a, Supporting Information) shows bands at 1638, 1553, 1526, 1517, 1480, 1379, 1247, 1128, 772, 753, 695, 616, and 594 cm−1. The bands observed in the SERS spectrum (Figure S2b, Supporting Information) are located at 1644, 1553, 1523, 1514, 1489, 1447, 1378, 1278, 1247, 1129, 813, 768, 754, 733, 659, 661, and 615 cm−1. It is clear that no differences are observed in the wavenumbers of both Raman and SERS spectra of safranin O. On the other hand, little variations in the relative intensities of the bands are also observed. For this reason, the identification of the most enhanced bands in the SERS spectrum becomes a difficult task. However, Lofrumento et al.22 proposed a perpendicular orientation of safranin O on Ag by means of the pyrazine nitrogen N5, based on the Herzberg−Teller selection rules approach developed by Lombardi and Birke.21 It has been shown that these selection rules are more accurate than the surface selection rules in predicting relative intensities of the SERS spectra, but they can only be applied for molecules with sufficient symmetry. In some cases, both types of selection rules give the same result. Safranin dye has a sufficiently high symmetry (C2ν), while the mauveine molecules have very low symmetry (C1). For this reason, the Herzberg−Teller selection rules can be applied to the study of the adsorption mechanism of the former molecule but not to the latter. Therefore, the effect was examined on more localized vibrations in the molecules in order to make the assignments and, thus, determine the orientation of the molecules on the surface. The greater differences between the Raman and SERS spectra of mauve dye suggest a different adsorption mechanism from saffranin O. The main structural variation between both dyes lies on the aromatic ring (V), bonded to N19. Therefore, the NH-ring V moiety is the most likely site for the adsorption of mauve dye on the Ag surface. At this point, it is of interest to study the SERS spectra of the four main components of mauve dye in order to evaluate the proposed adsorption mechanism. SERS Spectra of Mauveine A, B, B2, and C. SERS spectra of mauveine A, B, B2, and C obtained on a TLC plate12 are shown in Figure 6. The main SERS wavenumbers of each mauveine molecule are listed in Table 1. Regarding similarities and differences among these spectra, two regions can be observed. The SERS bands in the region

Figure 6. SERS spectra of mauveine A (a), B (b), B2 (c), and C (d).

between 1800 and 1000 cm−1 are quite similar in both positions and intensities. However, in the low-frequency region, below 850 cm−1, the highest differences are observed. SERS spectra of mauveine A and B2 are almost identical in the first region. The main bands in both spectra are centered at 1642, 1560, 1509, 1348, 1251, 1183, and 1133 cm−1. The same is observed with the spectra of mauveine B and C. In this case, the most intense bands are located at 1641, 1602, 1560, 1510, 1500, 1433, 1405, 1344, 1303, 1242, 1182, 1144, and 1018 cm−1. The assignments of these bands are shown in Table 2. The main differences between the spectra of mauveine A/B2 and mauveine B/C are the enhancement of the bands around 1602, 1433, 1405, 1303, 1242, and 1018 cm−1. Excepting the bands at 1602 and 1242 cm−1, assigned to ring and amino group vibrations, all the bands are related to δ(C27H3) (Tables S2 and S4, Supporting Information). The observation of those bands in the SERS spectra of mauveine B and C suggests the proximity of the CH3 group bonded to ring III to the Ag surface, as observed in Figure 5. Thus, the differences in the SERS spectra of mauveines A/B2 and B/C are explained. This fact is in accordance with the adsorption of the mauveine molecules by the aromatic nitrogen N5 or the amino nitrogen N17. The low wavenumber region of the SERS spectra shows a large number of different bands, depending on the structure of each mauveine molecule. These bands are assigned to in-plane deformations of the aromatic rings, out-of-plane deformations of rings and CH and NH groups, and twist vibrations of NH/ NH2. The most intense bands in that region of the SERS 14300

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Table 1. Main Experimental Raman and SERS Wavenumbers (cm−1) of Mauve Dye Together with the SERS Wavenumbers of Mauveine A, B2, B and Ca Raman mauveb 1637 1617 1604 1566

SERS mauveb

s m m s

1641 1613 1601 1560 1540 1510 1501 1463 1447 1404 1390 1347 1278 1248 1210 1185 1134

1465 w 1391 m-w 1385 m-w 1347 s 1245 m 1213 w 1183 m-w 1074w 1019 w 980 w 824 w 803 w 764 w 742 w 687 w 665 w

615 611 561 550

m sh m m-w

501 vw 441 vw 415 m-w

m sh w s w vw vw vw w vw m-w s vw w w m-w w

1642 1611 1599 1560 1541 1509 1502 1465 1445 1406 1386 1348 1276 1251 1209 1183 1133

SERS mauveine B2b

w sh vw m sh w vw sh vw vw vw s vw vw sh w w

1642 1613 1599 1560 1542 1512 1501 1643 1445 1406 1386 1349 1278 1249 1209 1182 1134

m sh w s sh w sh vw vw w w s w sh vw m m

1017 w

1015 vw

1016 w

822 798 767 731 710 690 670 649 636 624 616

m-w w w w vw vw w vw vw m sh

821 801 768 738

824 792 763 730 714 689 670 648 630

m w vw vw vw vw s vw vw

562 549 520 504 481 437

m m-w vw vw vw w

611 561 544 519 503 474 441

vw s vw vw vw w w

404 m-w a

SERS mauveine Ab

w w vw vw

689 w 647 vw 634 vw

SERS mauveine Bb

SERS mauveine Cb

1641 1614 1602 1560 1530 1510 1500

m sh w m sh w w

1641 1614 1602 1560 1539 1510 1500

m sh m s sh m m

1446 1405 1380 1344 1275 1242 1214 1184

sh w w s vw m sh w

1445 1404 1386 1344 1276 1242 1210 1182

sh m m s w m vw w

1018 w 984 vw 820 w 793 w 769 w 734 vw 689 661 648 638 626

vw vw vw vw sh

608 559 550 522 508 480 436 417 402

s sh m vw vw w m sh m

1017 w 985 vw 823 m 767 735 712 689

m vw vw vw

647 sh 634 m

615 s 565 546 517 503 473 441

w vw vw vw vw vw

407 w

406 m

609 564 549 523 508 485 433

vw s s vw vw w m

403 m

b

Excitation at 633 nm. vw, very weak; w, weak; m, medium; s, strong; sh, shoulder.

spectra of mauveine A (615 cm−1), B (608 cm−1), B2 (670 and 561 cm−1), and C (564 and 549 cm−1) are assigned to in-plane deformations of rings I, II, III, and IV and also to out-of-plane deformations of rings IV and V, in the case of the band at 549 cm−1. It is not clear that these bands are enhanced in the SERS spectra of mauve dye (Figure 4). Therefore, no information about the orientation of the molecules on the Ag nanoparticles can be obtained from the analysis of this region of the spectra, according to the SERS selection rules.21

and ring V (bonded to that N atom) and a decrease of the band assigned to the secondary amine (N19). Thus, the adsorption site was deduced to be N17. On the other hand, the analysis of the SERS spectra of the components of mauve, mauveine A, mauveine B, mauveine B2, and mauveine C shows differences in the spectra of mauveine A/B2 and B/C related to vibrations of CH3 bonded to ring III (C7). This observation supports the adsorption by N17, but also by the pyrazine N5. The adsorption of safranin O, a molecule with a structure similar to mauveines, takes part by the latter N. The SERS spectrum of that dye shows little differences in relation to the SERS of mauve. For this reason, the adsorption of the mauveine molecules on Ag nanoparticles was proposed to be through N17 and not by N5.

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CONCLUSIONS The deep study of the Raman and SERS spectra of mauve dye, together with the assignments of the bands obtained from the DFT calculations, lead to the deduction of the adsorption mechanism of the mauveine molecules on Ag nanoparticles. Comparison of the Raman and SERS spectra of the dye showed the enhancement of bands related to the tertiary amine (N17) 14301

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Table 2. Experimental Raman Wavenumbers of Mauve Dye Obtained at 633 nm Together with the Assignments of the Vibrational Normal Modes Derived from DFT Calculations (B3LYP/6-31+G**) of the Theoretical Raman Spectra of Mauveine A, B2, B, and Ca,b Raman

common modes assignmentsc

other mauveine A assignmentsc

other mauveine B2 assignmentsc

other mauveine B assignmentsc

other mauveine C assignmentsc

1637 1617 1604 1566 (1540) (1510) 1465 (1445) 1390 1385 1347 1244 1213 (1134) 1019 980 824 803 764 (749) (710) 687 665 (648) (630) 615 611 561 550 (520) 501 (470) 441 415 (404)

δsc(NH2) δsc(NH2)/νI,III(ring) νI,III,V(ring)/δ(NH) δ(NH)/νV(ring) ---ν(CN5,6,19) δ(C26H3) ------δ(C26H3) νI−III(ring) δIII(ring) δV(ring) δIII(CH) ρ(C26H3) ---γIII(CH) ---γII,III(ring) δI(ring) γV(ring) ---δI(ring) ------δII,IV(ring) δI,III(ring) δII(ring) ------γ(NH) γ(NH) γI,III(ring) γV(ring)/γIV(ring) δV(ring)

------δsc(NH2) ---δ(NH) νI(ring) δ(C18H3) δ(NH) νV(ring) δ(C18H3) ---------------N/A γV(CH)/δ(CN5,6C) γIV,V(CH)/δI,III(ring) γIV(CH)/γI(ring) N/A N/A γV(CH)/δI−IV(ring) N/A γIII(ring) δI,III(ring) ---N/A γIV(ring)/δI,III(ring) γV(ring)/δI−III(ring) γIV,V(ring)/δI−III(ring) δtw(NH2) ---δtw(NH2)/γ(NH) N/A ----

---νV(ring) νIV(ring)/δsc(NH2) ---νIV(ring) νIII(ring) ---δ(C18,26H3) νI−III(ring)/δ(C18H3) δ(C18H3) ---------------N/A ---δII−IV(ring) γI(ring) δII(ring) ---γI(ring) δII, III(ring) δIV(ring) γIII(ring) N/A ---δIV(ring) δI−III(ring)/γIV,V(ring) δtw(NH2) δtw(NH2) ---γII(ring) N/A ----

---νV(ring) ---------δ(NH)/νI,III(ring) N/A δ(C18,27H3) δ(CH3)/νV(ring) N/A ---δI(ring) νIII,IV(ring)/ν(CN19,6,5) N/A ρ(C27H3) γV(CH) γV(CH)/δII(ring) δV(ring)/ν(C−C26H3) γI(ring) γIV(CH) N/A δI−IV(ring) δV(ring)/γIII(ring) γIII(ring)/δV(ring) δI−III(ring) N/A ---δI,III(ring) γI−III(ring) γV(ring)/δIII(ring) ------γII(ring) -------

---νV(ring) ------δ(NH) δ(NH)/νI,III(ring) N/A δ(C18,27H3) δ(C27H3)/δ(NH)/νIII(ring) δ(C27H3)/νIII,V(ring)/δ(NH) ---δI(ring) δIII,IV(ring)/ν(CN5,6,19) N/A ρ(C27H3) γIV(CH) γV(CH) N/A ---δIII(ring) ---γI(ring) N/A γIII(ring)/δIV,V(ring) δI−III(ring) N/A ---δI,IV(ring) δI−III(ring)/γIV,V(ring) γI,V(ring)/δIII(ring) ------γII(ring) N/A ----

N/A: band not shown in the spectrum. b----: no additional modes. cν, stretching; δ, in-plane bending; δtw, twist deformation; δsc, scissoring deformation; γ, out-of-plane bending; ρ, rocking; s, symmetric; as, antisymmetric.

a

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ASSOCIATED CONTENT

ACKNOWLEDGMENTS We acknowledge Marco Leona and David A. Reagan for performing the DFT calculation of mauveine A and for carrying out the experimental part of the work, respectively. We are indebted to the National Institute of Justice (Department of Justice Award #2006-DN-BX-K034) and the City University Collaborative Incentive program (#80209). This work was further supported by the National Science Foundation grant number CHE-1402750 and by the City University of New York PSC-BHE Faculty Research Award Program. This work was also supported by Ministerio de Ciencia e Innovación de España (project FIS2010-15405). The table of contents/ abstract graphic adapted with permission from Perkin’s original mauve dye and shawl, 1856−1862. Copyright © Science Museum/Science & Society Picture Library.

S Supporting Information *

Full citation of reference 17, Tables S1−S4: Main experimental (SERS) and calculated Raman wavenumbers (cm−1) of mauveine A, B, B2, and C and assignments derived from the DFT calculations (B3LYP/6-31+G**). Figure S1: Structure of Safranin O. Figure S2: Raman and SERS spectra of safranin O. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b02619.

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AUTHOR INFORMATION

Corresponding Author

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*Fax: +34 91 564 5557. Phone: + 34 91 561 6800. E-mail: [email protected]. Notes

REFERENCES

(1) Ball, P. Chemistry: Perkin, the Mauve Maker. Nature 2006, 440, 429−429.

The authors declare no competing financial interest. 14302

DOI: 10.1021/acs.jpcc.5b02619 J. Phys. Chem. C 2015, 119, 14297−14303

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DOI: 10.1021/acs.jpcc.5b02619 J. Phys. Chem. C 2015, 119, 14297−14303