16642545

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www.elsevier.com/locate/jcis

Vibrational study of the metal–adsorbate interaction of phenylacetic acid

and

α

-phenylglycine on silver surfaces

J.L. Castro,

∗

M.R. López Ramírez, I. López Tocón, and J.C. Otero

Department of Physical Chemistry, Faculty of Sciences, University of Málaga, E-29071 Málaga, Spain

Received 9 July 2002; accepted 5 March 2003

Abstract

Raman and SERS spectra of phenylacetic acid andα-phenylglycine on silver sols have been recorded at several concentrations and pH values. Theα-phenylglycine has been also studied in D₂O. The respective vibrational assignments have been proposed and the analysis of the SERS spectra has made it possible to conclude that phenylacetic acid links to the metal through its carboxylate group only, while α-phenylglycine links also through its amino group. In both cases the aromatic ring seems to be almost perpendicular to the metal surface. On the other hand, the contribution of the charge transfer (CT) mechanism to the enhancement of the SERS spectra has been calculated as well and it is found to be very important in both molecules. The band most enhanced by this mechanism is that of vibration 8a, mainly in α-phenylglycine.

Keywords: Surface-enhanced Raman spectroscopy; Phenylacetic acid; Phenylglycine; Raman spectra

1. Introduction

SERS spectroscopy have been widely used to study the adsorption of many organic compounds, including car-boxylic acids and aminoacids, on several metal surfaces, namely Ag, Au, and Cu, because it makes it possible to de-cide which molecular species is adsorbed, which functional group links to the surface, and in many cases, the conforma-tion or surface orientaconforma-tion of the adsorbate.

Most published works dealing with carboxylic acids have to do with aromatic acids [1–12], while the number of publi-cations dealing with aliphatic acids is small. Among the very first SERS studies on aliphatic acids, those by Moskovits and Suh on maleic and fumaric acids [2], saturated mono-carboxylic acids from valeric to decanoic and dimono-carboxylic acids from oxalic to suberic [13], have to be mentioned. Our group has continued that work and extended the study to other molecules, namely formic, acetic, propionic, butyric, and acrylic acids, as well as some of their derivatives [14,15]. On the other hand, SERS has been widely used to ana-lyze proteins, peptides, and amino acids adsorbed on silver substrata [1,3,16–22]. The vibrational study of amino acids

* _{Corresponding author.}

E-mail address: [email protected] (J.C. Otero).

adsorbed on metal surfaces is quite complex, given that the species giving rise to the SERS records may be the anion, the cation, or even the zwitterion. Moreover, the interaction with the metal may take place through only one or both func-tional groups or even through an addifunc-tional funcfunc-tional group present in the side chain.

It is interesting to point out that some chemical transfor-mations can occurs in some SERS experiences, for instance, polymerizations [4,23], isomerizations [24], photodecompo-sitions [25], and hydrolysis reactions [26]. When any trans-formation occurs, the observed SERS record is the result of the competitive adsorption of all the species present in the solution. Some short-chain monocarboxylic acids or amino acids undergo a chemical transformation at low concentra-tions to give rise to the same anomalous SERS spectrum, ir-respective of the identity of the particular adsorbate [14,22]. In the aforementioned studies, the selective enhancement of some SERS bands is explained essentially on the ba-sis of the electromagnetic mechanism (EM) in connection with the surface orientation of the adsorbate. Nevertheless, the different enhancements shown by modes of the same symmetry cannot be easily explained on the basis of the EM mechanism, and could be attributed to the contribution of the CT mechanism. The SERS-CT enhancement mech-anism is a process analogous to a Raman Resonance one,

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but in SERS experiments the incoming photon produces the resonant transfer of an electron from the metal (M) to the adsorbate (A) and vice versa. The so formed transient radi-cal anion A− will be more stable if the molecule contains aromatic rings. In several works carried out by ourselves on methyl derivatives of pyridine and pyrazine it has been found that an essential feature of their SERS-CT spectra is the strong enhancement of mode 8a [27–29].

In this work we have recorded and analyzed the SERS spectra on silver colloids ofα-phenyl derivatives of acetic acid and glycine, which have been previously studied by our group [30].

2. Materials and methods

2.1. Preparation of the Ag colloid and samples for Raman and SERS

Colloidal silver solutions have been prepared in deionized and triply distilled water according to the method described by Creighton et al. [31], which basically consists of reducing a 10−3M AgNO3solution with an excess of NaBH4. In this

process, one volume of 10−3M AgNO3is added, dropwise

and with vigorous stirring, to three volumes of 2×10−3M NaBH4that has been previously cooled to a temperature of

between 0 and 5◦C. After stirring, the mixture is allowed to rest at room temperature for approximately 90 min. At a cer-tain instant, the appearance of a dark color in the heart of the solution is observed, and vigorous stirring at this moment is necessary to stabilize the colloidal solution. This is a trans-parent yellow solution, with a maximum in its absorption spectrum at 390 nm. Adsorbate is then added to the colloid as an aqueous solution in order to obtain the desired concen-tration. A change in the color of the system, from the initial yellowish to a final blue-greenish, is observed when the ad-sorbate is added to the colloid.

The colloidal solutions in deuterated water have been ob-tained by a similar method, using D2O as solvent.N

-deu-terated amino acids have been prepared by dissolving the nondeuterated reagents in D2O and recrystallizing them

thereafter. Further dissolution of deuterated amino acids in NaOD medium makes it possible to obtain the respective N-deuterated anions.

2.2. Instrumentation

Raman spectra were recorded with a Jobin–Yvon U-1000 double monochromator spectrometer fitted with a cooled Hamamatsu R943-02 photomultiplier, using the 514.5-nm exciting line from a Spectra Physics 2020 Ar+ gas laser. A constant slit width was used that allowed a spectral res-olution of 4 cm−1; the laser power reaching the sample was always 60 mW. In the case of liquid samples a quartz cell with a 1-cm path length was used, while a glass capillary was used for the microcrystalline solids. The measurement

of the band frequencies was done with the help of the laser plasma lines as frequency standards, whereby a precision of ±2 cm−1 was obtained under the operating conditions em-ployed.

3. Results and discussion

3.1. Phenylacetic acid

Figure 1 shows the Raman spectra of (a) solid pheny-lacetic acid and (b) 1 M sodium phenylacetate aqueous so-lution, as well as (c) the SERS spectrum on silver colloid of a 5×10−4M solution of the acid at pH 6. It has been found that changes in the bulk concentration of the adsorbate and changes of pH in the range from 6 to 11 do not produce vari-ations in the relative intensities of SERS bands.

The SERS spectrum in Fig. 1c is dominated by a very strong band at 1005 cm−1and three medium or strong bands at 1602, 1383, and 233 cm−1, respectively. On the basis of this spectrum, it is quite straightforward to decide that the species adsorbed on the metal is the anion, given that no ob-served band can be assigned to the stretching of the C=O group of the acid, while the symmetricνs(OCO) stretching is recorded at 1383 cm−1instead. Moreover, the pKa=4.28 of the acid points out that the anion is the majority species in the solution under the experimental conditions.

Table 1 collects the Raman and SERS wavenumbers of phenylacetic acid as well as the empirical assignments of the most significant bands. The assignments have been proposed

Fig. 1. Raman spectra of (a) solid phenylacetic acid, (b) 1 M aqueous solution of sodium phenylacetate at pH 14, and (c) SERS spectrum of 5×10−4M solution of phenylacetic acid on silver colloid at pH 7.

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Table 1

Wavenumbers (cm−1)of the main Raman bands of phenylacetic acid and proposed assignments

Acid (s) Salt (1 M) SERS Assignment

3069 3072 3066 2;ν(CH)

2930 2928 2926 νs(CH2)

1660 – – ν(C=O)

1608 1606 1602 8a;νring

1590 1588 1585 8b;νring

1410 1405 1420 δ(CH2)

– 1392 1383 νs(OCO)

1291 1289 3;δ(CH)/w(CH2)

1233 – – ν(CO)+δ(OH)

1206 1206 1223 tw(CH2)

1195 1200 1205 13;ν(CX)

1180 1188 1182 9a;δ(CH)

1005 1005 1005 12;δring

931 941 941 ν(C–COO)

843 847 843 10a;γ (CH)

752 772 766 1;νring

605 650 646 δor w(OCO)

622 622 620 6b;δring

562 558 6a;δring?

– – 233 ν(Ag-adsorbate)

Symbols:ν, stretching;δ, in-plane bending;γ, out-of-plane bending; tw, twisting; w, wagging; r, rocking; t, torsion.

on the basis of the works of Varsányi [32], Bellamy [33], and Roeges [34], as well as vibrational studies on molecules such as toluene [35,36], benzyl chloride [37], and 1-chloro-2 phenylethane [38].

According to the width and intensity enhancement of the band assigned toνs(OCO) at 1383 cm−1as well as its shift toward lower frequencies (9 cm−1), the coordination of the adsorbate to the metal seems to occur through the carboxy-late group. The rather strong intensities of the bands at 2926, 1289, 1223, 1205, 941, and 646 cm−1, which are assigned to vibrations of the carboxylate or groups in its neighborhood, indicate also that the adsorption essentially involves the an-ion. Provided that theνas(OCO) vibration is not observed, it

has been not possible to determine which kind of coordina-tion with the metal occurs.

No metal–aromatic ring interaction seems to take place, given that no significant red shifts of the bands assigned to ring modes are observed, especially 1;νringand 12;δring, and

moreover, no changes in their widths are observed. On the other hand, according to propensity rules of the EM mecha-nism, the plane of the aromatic ring should be more or less perpendicular to the metal surface, given that the strongest SERS bands correspond to totally symmetric A1modes and

vibration 2;ν(CH) is observed with noticeable intensity at 3066 cm−1[39].

In previous works [27–29], we have proposed a simple method to detect and to estimate the enhancement produced by the CT mechanism (SERS-CT) in a particular spectrum thanks to the particular features shown by the bands assigned to the 8a;νring and ν(CH) vibrations, which are

represen-tative of active and inactive modes in a SERS-CT process, respectively. Generally speaking, one accepts that the most

enhanced band in resonant Raman (RR) relates to the differ-ences between the equilibrium structures of the involved res-onant electronic states. This is the well-known Tsuboi rule, which accounts for the enhancement of the totally symmetric modes in RR (A-term) [40]. However, the transient excited state in SERS-CT corresponds to a charge transfer (CT) state of the metal–adsorbate complex (M–A) originated from pho-toinduced electron transfer from the metal surface to mole-cules similar to the ones here studied, yielding the formation of the corresponding radical anion M+–A−[27–29]. On this basis, the relative intensities observed in the SERS-CT of several adsorbates are strictly compatible with the differ-ences between the geometries of the neutral molecules and their respective radical anions, which are closely related to the shape of LUMO of the adsorbate where the transfered electron located. When the electron is tranferred to the adsor-bate their nuclei are displaced from their equilibrium posi-tions. Although the LUMO of the neutral benzene is doubly degenerated (Fig. 2a), the C2–C3 and C5–C6 bondlengths

should be significantly modified in both cases due to the re-spective bonding and antibonding interactions between the π-orbitals of the respective pairs of atoms [29]. These mole-cular deformations are very similar to mode 8a (Fig. 2b) and explain the main role of this vibration in the SERS of benezene, pyridine, pyridazine, and derivatives [27–29].

In contrast, ν(CH) vibrations are not part of the chro-mophore in the resonant CT-process, given that the electron is mainly located in the aromatic ring and not in the CH bonds. On this basis, the CT enhancement of each band “i” in a particular SERS record can be estimated by removing all the other contributions included in the EM term according to the expression [28]

ICT,i=ISERS,i−IEM,i=ISERS,i−Isol,i,

(a)

(b)

Fig. 2. Pictorial representation of (a) the doubly degenerated LUMO orbitals of benzene and (b) the vibration 8a of benzene.

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Table 2

Relative Raman and SERS intensities of the A1fundamentals of phenylac-etate as normalized to mode 2;ν(CH) (SERS-CT intensities)

Mode νSERS Isol ISERS ICT %CT

2;ν(CH) 3066 100 100 0 0

8a;νring 1602 27 522 495 95

19a;νring 1500 25 ∼25 ∼100

13;ν(CX) 1205 20 133 113 85

9a;δ(CH) 1182 8 22 14 64

18a;δ(CH) 1031 88 155 67 43

12;δring 1005 4000 1110 710 64

1;νring 766 20 78 58 74

6a;δring 558? 33 ∼33 ∼100

whereIEM,iis the SERS intensity of theith band, due exclu-sively to the non-CT mechanism. Given that a SERS spec-trum without any CT contribution is not available, we have supposed thatIEM,i is similar to the respective intensity in the Raman spectrum of the phenylacetate aqueous solution Isol,i. Additionally, all the intensities must be normalized with respect to mode 2;ν(CH), provided that it is assumed that the SERS intensity of this vibration is due to the EM mechanism exclusively. Accordingly, the relative contribu-tion of the CT mechanism to the intensities of each SERS band can be estimated to be

%CTIi=100(ICT,i/ISERS,i).

Table 2 collects the Raman and SERS intensities of the A1

fundamentals of the phenylacetate ion as well as the cal-culated CT intensities. One can see that the intensity of vibration 12, which is by far the strongest one in the Ra-man spectrum of the solution, is significantly decreased in SERS, while that of vibration 8a, very weak in Raman, is enormously enhanced in SERS. Our results indicate that the CT mechanism is responsible for 95% of the SERS intensity of mode 8a, in accordance with previous results obtained by ourselves, which confirms once again that the enhancement of mode 8a is characteristic of the presence of the CT mech-anism in the SERS of this kind of molecules [27–29].

3.2. α-Phenylglycine

Three ionic species can be present in the aqueous solu-tions ofα-phenylglycine, namely: (I) Ph–CH(NH+₃)–COOH, (II) Ph–CH(NH+₃)–COO−, (III) Ph–CH(NH2)–COO−.

Additionally, the neutral species Ph–CH(NH2)–COOH

(IV) could be adsorbed on the metal surface.

No data concerning the acidity of the ionic species of α-phenylglycine have been found in the literature, but data involvingα-alanine andα-phenylalanine are as follows: pK1

and pK2ofα-alanine are 2.33 and 9.71, respectively, while

those ofα-phenylalanine are in turn 2.18 and 9.09, respec-tively. Given that the respective values of glycine are 2.34 and 9.58, it is sensible to suppose that substitution of a methylenic hydrogen by a phenyl group will not affect so

Fig. 3. Raman spectra of (a) solidα-phenylglycine, (b) 1 M aqueous so-lution of sodiumα-phenylglycinate at pH 14, and (c) SERS spectrum of 5×10−4M solution ofα-phenylglycine on silver colloid at pH 7.

much that values, and consequently the isoelectric point of α-phenylglycine should be ca. pI≈6.

It comes out therefore, that the majority species in 1 M aqueous solutions of α-phenylglycine at pH 2 will be the cationic species I, while at pH 10, the majority species will be the anion III and in turn, the zwitterion II will be predom-inant at pH close to the isoelectric point. Figure 3 shows the Raman spectra of (a) solidα-phenylglycine, (b) 1 M aqueous solution of sodium phenylglycinate at pH≈14, and (c) the SERS spectrum of 5×10−4M α-phenylglycine on silver colloid at pH ≈7. Figure 4 shows in turn (a) the Raman spectrum of 1 M solution of sodium phenylglycinate in D2O

at pD≈14 and (b) the SERS spectrum of 5×10−4M α -phenylglycine in D2O silver colloid at pD≈7.

It is seen that SERS spectra do not undergo noticeable changes when the concentration of either the adsorbate or the pH is changed, which in principle allows us to deduce that the adsorbed species is the phenylglycinate anion III. Table 3 summarizes the wavenumbers of Raman and SERS spectra in H2O as well as the empirical assignments of the most

im-portant bands. The proposed assignments are done on the ba-sis of the vibrational spectra of phenylacetic acid and some amino acids, namely glycine [3,30,41,42],α-alanine [3,30, 42,43] andα-phenylalanine [44].

The analysis of the SERS spectra points out that the car-boxylic group is ionized and interacts directly with the metal surface. The bands recorded at 1408 cm−1 in H2O and at

1380 cm−1in D2O, assigned to modeνs(OCO), appear to be stronger and broader than in the respective Raman spec-tra of the solutions. In the SERS recorded in D2O, they show

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Fig. 4. (a) Raman spectrum of a 1 M aqueous solution ofα-phenylglycine in D2O at pD 14 and (b) SERS spectrum of a 5×10−4 M solution of α-phenylglycine on silver colloid in D2O at pD 7.

Table 3

Wavenumbers (cm−1)of the main Raman bands ofα-phenylglycine and proposed assignments

Zwitterion (s) Anion (1 M) SERS Assignment

– 3308 3272 νs(NH2)

3068 3070 3066 2;ν(CH)

2942 2928 ν(CH)alif

– 1640 νas(OCO)

1614 δ(NH2)

1608 1606 1600 8a;νring

1588 1588 1580 8b;νring

1460 1460 1454 19b;νring

1402 1404 1408 νs(OCO)

1274 1260 1253 3;δ(CH)

1201 1197 1209 13;ν(CX)

1188 1186 1184 9a;δ(CH)

1162 1160 1162 9b;δ(CH)

1003 1003 1001 12;δring

920 927 929 ν(C–COO)

853 857 855 10a;γ (CH)

772 784 784 1;νring

630 640 δor w(OCO)

618 618 618 6b;δring

482 486 486 6a;δring?

– – 220 ν(Ag-adsorbate)

a red shift of some 22 cm−1, while in H2O, a small but

sig-nificant blue shift amounting to 4 cm−1, quite unusual in this type of adsorbate, is seem. Perhapsνs(OCO) couples with other vibrations, probablyδ(NH2)/δ(ND2) when linked

to the metal.

The bands recorded at 3272 and 2398 cm−1in H2O and

D2O, respectively, are assigned to –NH2 and –ND2

sym-metric stretchings, respectively, given that νs(NH+₃) and νs(ND+₃) are usually observed in the ranges of 3080± 50 and 2225 ±75 cm−1, respectively, in many amino acids [33]. Additionally, the frequency of 2;ν(CH) recorded at 2928 cm−1is in turn characteristic of the presence of the amino group [33]. As a consequence, the spectroscopic results indicate thatα-phenylglycine adsorbs as phenylgly-cinate ion (species III) even at pH values where the majority species is the zwitterion. Moreover, the fact that the SERS wavenumbers ofνs(NH2) andνs(ND2) exhibit an important

downshift makes evident that the amino group interacts with the metal too.

Two bands are recorded at 1614 and 1640 cm−1 in the SERS on H2O. We proposed the assignment of the

aforementioned bands toδ(NH2) andνas(OCO) vibrations,

respectively, given that only one band at 1626 cm−1 is recorded with medium intensity in D2O, due probably to

vi-brationνas(OCO). The δ(NH2) mode appears in the range

1610±15 cm−1 in the spectra of the sodium salts of sim-ple amino acids [34] or close to 1620 cm−1in complexes of α-phenylalanine with transition metals [44]. The wavenum-ber differenceν(OCO)=νas−νs amounts to 232 cm−1in

the SERS recorded from samples in H2O, while it amounts

only some 140 cm−1 in the spectra of alkaline carboxylate salts. That higher values suggest that coordination of the car-boxylate group to the metal is unidentate, which removes the equivalence of both oxygen atoms to get a pseudoester struc-ture R–C(=O)–O–M [45].

On the basis of the selection rules of the EM mechanism the benzene ring should be almost perpendicular to the metal surface. This could explain why the most enhanced SERS bands, with the exception of some modes of the substituent group, are totally symmetric A1ring modes, and why mode

2;ν(CH) is observed with noticeable intensity at 3066 cm−1. All the mentioned considerations seem to point out that the phenylglycinate anion adsorbs on the metal surface in an ori-entation looking like that shown in Fig. 5.

Fig. 5. Proposed orientation of theα-phenylglycinate anion adsorbed on the metal surface.

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Table 4

Relative Raman and SERS intensities of the A1fundamentals of phenylgly-cinate as normalized to the mode 2;ν(CH) (SERS-CT intensities)

Mode νSERS Isol ISERS ICT %CT

2;ν(CH) 3066 100 100 0 0

8a;νring 1600 44 2150 2106 98

19a;νring 1498 20 ∼20 ∼100

13;ν(CX) 1209 53 750 697 93

9a;δ(CH) 1184 26 110 84 76

18a;δ(CH) 1029 79 150 71 47

12;δring 1001 294 1000 796 80

1;νring 784 15 50 35 70

6a;δring 486? 7 80 73 91

The contribution of the CT mechanism to the SERS en-hancement has been calculated according to the already ex-plained methodology. Table 4 collects the SERS-CT intensi-ties of the A1ring fundamental vibrations. It is to be stressed

that vibration 8a is even more enhanced than in phenylacetic acid and shows intensity double that of mode 12, being therefore the strongest band in the spectrum. Our results in-dicate that its SERS enhancement is due almost exclusively to the charge transfer mechanism. Comparison of results allows to establish that the contribution of the CT mech-anism is larger in the aminoacid, probably because of the involvement of the amino group in the coordination to the metal, as the analysis of the SERS spectra shows. When the amino group is involved, the adsorbate–metal coordination gets stronger than in phenylacetic acid and the orbitals of both moieties get more mixed, which favors the photoin-duced charge transfer process.

The relevant role vibration 8a plays in the SERS-CT mechanism in aromatic molecules is due to this particu-lar vibration connecting the equilibrium geometries of both electronic states involved in the resonant process, namely the ground states of the neutral molecule and of the radi-cal anion. Moreover, vibration 8a exhibits a very stable fre-quency in the whole series of benzene-like molecules and their derivatives, appearing always near to 1600 cm−1. On this basis, we have proposed a “propensity rule” to detect the presence of the CT mechanism in any SERS spectrum, which consists of the significant enhancement of this normal mode [27–29,46].

Finally, in the SERS here studied, no odd bands that could be originated by molecular species other than the adsorbates themselves have been recorded. As mentioned before, some anomalous bands have been recorded in the SERS of sev-eral aliphatic organic acids and simple amino acids, which have been explained on the basis of a Hoffmann-like chemi-cal reaction catalyzed by the metal, yielding the elimination of a hydrogen molecule fromα- and β-positions contigu-ous to the carboxylate. The double bond formed between α- andβ-carbon atoms originates a characteristic band in the respective Raman and SERS spectra. In the case of pheny-lacetic acid orα-phenylalanine, that hydrogen elimination

cannot take place because theβ-C-atom has no hydrogen to be eliminated. This behavior confirms our hypothesis con-cerning the presence of anomalous bands in some SERS of these kind of organic molecules.

4. Summary

Molecules of phenylacetic acid andα-phenylglycine ad-sorbed on silver colloids link to the metal through their func-tional groups, while the aromatic rings are orientated almost perpendicular to the metal surface.

The CT mechanism contributes to the enhancement of the observed SERS spectra in a very important way, which has been detected in the basis of the strong enhancement of vi-bration 8a in both cases, especially inα-phenylglycine.

The observed spectra show that no chemical transforma-tion of the adsorbates take place, probably because of the lack of H-atoms in the respectiveβ-C-atoms.

Acknowledgments

The authors express their gratitude to the Spanish MCYT for financial support through Project BQU2000/1353. M.R. López Ramírez acknowledges the Spanish MCYT for Grant FP20000-5950.

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