Adsorption of (S)–(+)–O-acetylmandelic acid on gold nanoparticle surfaces investigated by surface-enhanced Raman scattering

Chemical Physics Letters 467 (2008) 136–139

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Adsorption of (S)–(+)–O-acetylmandelic acid on gold nanoparticle surfaces investigated by surface-enhanced Raman scattering Jang Suk Kwon a, Heung Bae Jeon b, Sung Ik Yang c, Jung Jin Oh d,*, Sang-Woo Joo a,* a

Department of Chemistry, Soongsil University, Sangdo 5 dong, Seoul 156-743, Republic of Korea Department of Chemistry, Kwangwoon University, Seoul 139-701, Republic of Korea c College of Environment and Applied Chemistry, Kyung Hee University, Yongin 446-701, Republic of Korea d Department of Chemistry, Sookmyung Women’s University, Seoul 140-742, Republic of Korea b

a r t i c l e

i n f o

Article history: Received 6 July 2008 In final form 3 November 2008 Available online 7 November 2008

a b s t r a c t Adsorption of (S)–(+)–O-acetylmandelic acid on gold nanoparticle surfaces was examined by means of surface-enhanced Raman scattering. The adsorbate was presumed to bind to the surface via the carboxylate functional group due to the presence of symmetric stretching bands at 1350 cm 1. Nearly identical spectral features to those of phenylacetic acid in conjunction with the disappearance of the C@O stretching band at 1700 cm 1 were exhibited in the SERS spectra. These observations suggest scission of the carboxyl group into a benzyl radical intermediate on gold nanoparticle surfaces. Ó 2008 Published by Elsevier B.V.

1. Introduction Since its discovery [1], surface-enhanced Raman scattering (SERS) has been widely used as an ultra-sensitive spectroscopic tool for interface studies to monitor chemical reactions occurring in colloidal surfaces [2–4]. SERS can provide chemically specific information on the basis of the unique vibrational modes of target adsorbates [5]. In a previous SERS study, aromatic sulfides adsorbed on colloidal Ag surfaces were found to undergo surface reactions involving facile cleavage of C–S bonds [6,7]. Benzyl phenyl sulfide (BPS, C6H5CH2–S–C6H5) was exclusively decomposed into benzenethiolate (C6H5–S) on the silver surface while dibenzyl sulfide (DBS, C6H5CH2–S–CH2C6H5) was converted into benzyl thiolate (C6H5CH2–S). Benzyl methyl sulfide (BMS, C6H5CH2–S–CH3) decomposed to result in the formation of benzyl thiolate (C6H5CH2–S) via S–CH3 bond scission on silver. Silver colloidal nanoparticles prepared by citrate-reduction were found to be a useful substrate for studying the BPS photoreaction via visible irradiation [8–10]. However, this photoreaction does not occur on gold surfaces [11]. Visible-light-absorbing gold nanoparticles (AuNPs) were recently investigated by Falvey et al. [12]. For the compound N-methyl-picoliniumphenylacetate, the N-methylpicolinium and carboxylate functional groups were produced via an electron transfer process mediated by AuNPs. A good electron donor, dithiothreitol was used to initiate the electron transfer with a 300 W broad-band tungsten-filament lamp. In this research we used SERS

* Corresponding authors. Fax: +1 82 2 820 0434 (S.-W. Joo); +1 82 2 710 9977 (J.J. Oh). E-mail addresses: [email protected] (J.J. Oh), [email protected] (S.-W. Joo). 0009-2614/$ - see front matter Ó 2008 Published by Elsevier B.V. doi:10.1016/j.cplett.2008.11.003

to study the adsorption characteristics of (S)–(+)–O-acetylmandelic acid (AMA) and examine its eventual photoreactions in aqueous Au sols prepared by citrate-reduction without using dithiothreitol by means of SERS. 2. Methodology (S)–(+)–O-Acetylmandelic acid (AMA) (99%), phenylacetic acid (PA) (99%), and acetic acid (AA) (99.99+%) were purchased from Sigma Aldrich. The methods of preparation of the aqueous Au colloidal nanoparticles were reported previously [13]. A 133.5 mg portion of KAuCl4 purchased from Aldrich was dissolved in 250 mL of distilled water and the solution was brought to boiling. A solution of 1% sodium citrate (25 mL) was added to the vigorously stirred KAuCl4 solution. Boiling was continued for approximately 20 min. The resulting Au solution was stable for several weeks. Fig. 1 illustrates the experimental scheme for the adsorption of AMA on gold nanoparticles via laser irradiations at 633 nm or 785 nm [14]. 3. Results and discussion As shown in Fig. 2a, citrate-reduced Au colloidal particles exhibited a UV–Vis absorption maximum at 520 nm ascribed from surface plasmon bands. Upon the addition of the AMA compounds, the position of the absorption maximum is expected to gradually red-shift and the spectrum becomes smooth over 520 nm, with a long tail extending up to 700 nm due to the aggregation of Au nanoparticles [15]. Fig. 2b shows the UV–Vis absorption spectral change after injection of AMA for gold particles in aqueous solution. For the gold particles derivatized with AMA, the kmax value

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J.S. Kwon et al. / Chemical Physics Letters 467 (2008) 136–139

O

H3C

O

H

Au

O

CH3

O O

O

O OH

Au Nanoparticle

Fig. 1. Schematic diagrams of the photorelease of AMA on gold nanoparticles.

523

0.8

(a) 0.6

785

(b)

633

0.4

525

Absorbance (Arbitrary Unit)

1.0

0.2

300

400

500 600 Wavelength (nm)

700

800

Fig. 2. Visible absorbance spectra (a) before and (b) after the addition of 10 AMA to citrate reduced-Au colloidal nanoparticles.

3

M

was found at 525 and 700 nm. This indicated an unsubstantial decrease in inter-particle distance between gold particles due to AMA [15]. Fig. 2 is presented to compare the shifted position of surface plasmon band of Au nanoparticles and the excitation wavelengths. After the addition of AMA, the surface plasmon band at 525 nm appeared to be red-shifted to the longer wavelength region at 700 nm as exhibited in Fig. 2. Although the excitation wavelengths of 633 and 785 nm are far away from the plasmon band maximum at 525 nm, they could be in resonance with the broad region at 700 nm for the aggregated particles. Although the broad band above 700 nm is assumed to occur due to the aggregation effects of nanoparticles, it seems also possible that there may be an additional contribution of charge transfer [14] between the metal and adsorbates in the UV–Vis spectrum. Fig. 3 shows the ordinary Raman (OR) spectrum of AMA in neat solid state, Au SERS spectra of AMA and PA, respectively, using irra-

diation at 633 nm. To obtain information on the surface structure, it is necessary to analyze spectral changes according to the adsorption process. A correct vibrational assignment is prerequisite in this respect. Consulting the earlier vibrational assignments [16,17], we analyzed Raman spectra in Fig. 3. Spectral data and appropriate vibrational assignment of AMA are summarized in Table 1. The concentrations of AMA in the Au nanocolloids were estimated to be 10 3 M. According to the dynamic light scattering measurement, the average diameters of gold particles were 30– 50 nm. This implies that the SERS spectra as shown in Fig. 3c and e should be higher than that needs to cover the total surface area of gold particles [3]. For the SERS spectra, the absence of the m(C@O) stretching band at 1700 cm 1 appeared in the ordinary Raman spectrum indicates that AMA is chemisorbed on the surfaces by rupture of the carboxylate group. For a better comparison, the SERS spectrum of PA is also illustrated as shown in Fig. 3e. The SERS spectral feature of AMA upon adsorption on Au seems quite analogous to that of PA. Although not shown here, it is noteworthy that the benzene ring CH stretching band is identified at 3060 cm 1. It has been well documented in the literature that the presence of the ring CH stretching band in an SERS spectrum is indicative of a vertical (or at least tilted) orientation of the benzene ring moiety on a metal substrate [18]. The position of the CC stretching m8a band of the benzene ring in AMA in gold nanocolloids appeared at 1597 cm 1, while it was observed at 1602 cm 1 in the aqueous solution state. The slight decrease in frequency of the ring breathing m12 mode at 1000 cm 1 may imply that a direct ring p orbital interaction with gold substrates would be low. These results suggest a vertical or titled orientation of the phenyl ring in AMA on gold. The spectral pattern on Au nanoparticle surfaces at 785 nm appeared to be almost analogous with those at 633 nm, as shown in Fig. 4. Most ring modes were found to belong to those of in-plane modes [16,17]. The weakness of the out-of-plane bands supported that the adsorbate should have a rather vertical structure [5]. It is

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J.S. Kwon et al. / Chemical Physics Letters 467 (2008) 136–139 -

ν(C=O)

i

Table 1 Spectral data and vibrational assignment of AMAa.

ν(Au-O)

ν(COO )

Intensity (Arbitrary Unit)

(g) (f) (e) (d) (c) (b) (a) 1500

1000 -1 Raman Shift (cm )

ii Intensity (Arbitrary Unit)

γ(CH2)

500

9a

(c) 9a

ν(C-O-C)

OR Solid

Solution

In-plane 3064 1602 1587 1459 1198 1181 1028 1000 765 615

3053 1602 1587 1459 1192 1183 1027 1000 768 615

Out-of-plane 731 691 521 Substituents 2937 1743 1684 1459 1255

(a)

mCH (2) mCC (8a) mCC (8b) mCC + dCH (19b) mCH (13) bCH (9a) bCH (18a) bCCC (12) bCCC (1) bCCC (6b)

cCH (11) cCCC (4) cCCC (16b)

705 691 521 2940 1758 1697 1653

1383

ν(C-O-C)

3056 1597

1182 1025 1000 761

1258

1160 861

Assignmentc

1455

(b) γ(CH2)

Au SERSb

1160 865

2932

m(CH2) m(C@O) weak H-bondd m(C@O) weak H-bondd m(C@O) strong H-bondd

1450 1250 1531 1354 1330

bs(CH2) (scissor, i.p.)e c(CH2) (wag or twist, o.p.)e manti-symmetric (COO )f msymmetric (COO )f msymmetric (COO )f manti-symmetric (COC)g bas(CH2) (rock, i.p.)e m(Au–O)h

865 253

Unit in cm 1. Spectral positions taken by 632.8 nm. c Based on Refs. [16,17]. PA is not listed here due to the previous report. Wilson notation in the parenthesis and atomic symbols with m; stretching, b; in-plane bending c; out-of-plane bending modes. The symmetry may correspond to C1 point group. d Based on Ref. [20]. e i.p.: In-plane; o.p.: out-of-plane. f Based on Refs. [17,22]. g Based on Ref. [21]. h Based on Ref. [23]. a

Fig. 3. (i) OR spectra of AMA in its (a) solid and (b) aqueous state at 1800–150 cm 1. SERS spectra of 10 3 M (c) AMA. (d) OR and (e) 10 3 M SERS spectra of PA. (f) OR and (g) 10 3 M SERS spectra of AA. OR spectra could only be obtained for a highly concentrated aqueous solution under basic condition using NaOH. Please note that the C@O stretching bands were observed in the OR spectra presumably due to their supersaturated condition. All spectra were obtained using 633 nm irradiation. (ii) An expanded view of the m(C–O–C) stretching region between 1300 and 1050 cm 1 for its (a) solid and (b) aqueous state. (c) SERS spectrum of 10 3 M AMA.

also possible that the charge transfer resonance mechanism [19] should affect the SERS intensities. This may be evidenced by different enhancements at 633 nm from those at 785 nm. For example, the SERS peaks at 1530 and 870 cm 1 appeared to be more enhanced via irradiation at 633 nm than that at 785 nm. The different enhancements of the vibrational bands at 1530 and 870 cm 1 may also be due to the different laser excitation and enhancement in coincidence with the surface plasmon bands of Au nanoparticles. We have to mention that the SERS spectral features appeared to change depending on the solvent and mixing conditions. From the visible spectra in Fig. 2, it is not absolutely certain whether there should be any charge transfer resonance for AMA on gold. Referring to our data that the SERS spectra looked different depending on the excitation wavelengths as shown in Figs. 3 and 4, it is expected however that there would be a charge transfer resonance between the adsorbed species and metal surfaces. We have to mention that there are several C@O bands found in the ordinary Raman spectra, which could be ascribed to the intermolecular hydrogen bonding as previously reported [20]. After the scission of the carboxyl group of AMA, the adsorbate should be the same as the case when PA is adsorbed on Au as depicted in Fig. 1. Nearly identical spectral features to those of phenylacetic acid in conjunction with the disappearance of the C@O stretching band at 1700 cm 1 were observed in the SERS spectra. This result may provide evidence that the possibility of chemisorptions on

1345

1200 1100 -1 Raman Shift (cm )

1358 1334

1300

Intensity (Arbitrary Unit)

b

(c) -

ν(COO )

(b) ν(C=O)

(a) 1500

1000 -1 Raman Shift (cm )

500

Fig. 4. OR spectra of AMA in its solid state at 1800–350 cm 1. SERS spectra of 10 3 M (b) AMA and (c) PA on citrate-reduced Au nanoparticles. All spectra were obtained using 785 nm irradiation. We could not scan the low frequency region below 350 cm 1 with using 785 nm Raman spectrometer set-up, differently from the case of 633 nm.

the carboxylate moiety of AMA without scission should be low. The rupture of the O–H bond and the breakdown of the C@O double bond to a single bond C–O bond would result in the vanishing of the C@O vibrational modes. The C@O vibrational mode detectable in solution was not found in the SERS spectra as in Fig. 3. It

Intensity (Arbitrary Unit)

J.S. Kwon et al. / Chemical Physics Letters 467 (2008) 136–139

1800

(g) (f) (e)

139

tions, the C@O stretching bands at 1700 cm 1 all disappeared. This result may suggest that the dissociation could occur in an efficient way without laser illumination. Our work should be potentially useful to understand the surface-induced scission of a certain functional moiety from the self-assembled monolayers on metal surfaces.

(d)

1600 -1 Raman Shift (cm )

(c)

4. Conclusion

(b) (a)

The results of these SERS experiments on Au nanoparticle surfaces indicated that the carboxyl group of AMA could be detached upon adsorption. Unlike previous research, no electron donor, such as dithiothreitol, was used in this research with Raman experimental conditions of 633 nm or 785 nm. Even without the addition of any electron donors, the carboxylate group was excised from AMA on the surfaces on gold nanoparticle surfaces.

1400

Fig. 5. Time-dependent SERS spectra of AMA on Au nanoparticle surfaces at 1800– 1400 cm 1. With an irradiation time of (a) 5, (b) 10, (c) 15, (d) 20, (e) 30, (f) 35, and (g) 40 s. The C@O stretching band could not be observed even at the beginning of the irradiation using 633 nm. We could not obtain a better signal to noise ratio with 785 nm irradiation. The gradual increase of the band at 1530 cm 1 is assumed to be due to the adsorption of the carboxyl group.

is also noteworthy that we could not find the C–O–C stretching mode at 1160 cm 1 [21] in the SERS spectrum but found in the OR spectrum in the solution. As magnified in Fig. 3ii, we could not clearly observe the m(C–O–C) band at 1160 cm 1, in the SERS spectrum of AMA on Au nanoparticles, whereas it was apparently found in the OR spectra for the solid state and the aqueous solution. Although the spectral region at 1160 cm 1 appeared to locate closely at the wing of the broadened band of the ring 9a mode, it is quite likely that the m(C–O–C) band should be either absent or considerably weakened upon adsorption on the Au nanoparticles. At the carboxylate stretching band region in the SERS spectra of AMA using 633 nm excitation, the split bands were found at 1354 and 1330 cm 1, whereas a band was observed at 1360 cm 1 for PA and the broad peaks appeared between 1395 cm 1 for AA as previously reported [17,22]. Under the irradiation at 785 nm, the two split bands appeared at 1358 and 1334 cm 1 for the symmetric carboxylate stretching band as similar to the case of the 633 nm excitation. The two distinct peaks in the symmetric m(COO ) stretching region present for AMA, differently from the case of PA may be explained by the CH3COO moiety adsorbed on Au after the scission of AMA. We checked the SERS spectrum of acetic acid on Au and found the broad peak between 1380 and 1330 cm 1 in the symmetric m(COO ) stretching region. The broad band at 250 cm 1 that can be ascribed to the Au–O stretching vibration [23], only found in the SERS spectra of AMA, PA, and AA also indicated that the molecules should adsorb chemically on Au with the carboxylate group. These findings supported the view that adsorbates should undergo a scission of the carboxyl group into a benzyl radical intermediate on gold nanoparticle surfaces as depicted in Fig. 1. To examine any eventual photoreaction behaviors we obtained the SERS spectra as early as possible spectra with only 5 s illumination as shown in Fig. 5. Under our experimental condi-

Acknowledgments S.W.J would like to thank Prof. Mino Yang and Prof. Kwan Kim for kind considerations. This work was supported by the Korea Science and Engineering Foundation (KOSEF R01-2006-000-10017-0), Nano R&D program through the Korea Science and Engineering Foundation funded by the Ministry of Education, Science and Technology (2008-04308), the Seoul R&BD Program, the Soongsil University Research Fund, and the Sookmyung Women’s University Research Grants 2007. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23]

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