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Surface Science 601 (2007) 4586–4597

Influence of aliphatic spacer group on adsorption mechanismsof phosphonate derivatives of L-phenylalanine: Surface-enhanced

Raman, Raman, and infrared studies

E. Podstawka a,*, A. Kudelski b, P. Kafarski c, L.M. Proniewicz d

a Laser Raman Laboratory, Regional Laboratory of Physicochemical Analysis and Structural Research, Jagiellonian University,

ul. Ingardena 3, 30-060 Krakow, Polandb Faculty of Chemistry, University of Warsaw, ul. L. Pasteura 1, 02-093 Warsaw, Poland

c Institute of Organic Chemistry, Biochemistry and Biotechnology, Wroclaw Technical University, ul. Wybrze_ze Wyspiańskiego 27, 50-370 Wroclaw, Polandd Chemical Physics Division, Faculty of Chemistry, Jagiellonian University, ul. Ingardena 3, 30-060 Krakow, Poland

Received 23 March 2007; accepted for publication 9 July 2007Available online 21 July 2007

Abstract

The nature of phosphonopeptides containing N-terminal L-phenylalanine (L-Phe), namely L-Phe-DL-NH–CH(CH(CH3)2)–PO3H2 (A),L-Phe-L-NH–CH(CH3)–PO3H2 (B), and L-Phe-DL-NH–CH(CH2CH2COOH)–PO3H2 (C) (Fig. 1 presents molecular structure of thesemolecules), adsorbed on electrochemically roughened and colloidal silver surfaces has been explored by surface-enhanced Raman spec-troscopy (SERS). To reveal adsorption mechanism of these species on the basis of their SERS spectra at first Fourier-transform Raman(FT-RS) and absorption infrared (FT-IR) spectra of non-adsorbed molecules were measured. Examination of enhancement, frequencyshifts, and changes in relative intensities of SERS bands due to adsorption and surface roughens variation reveals that the tilted com-pounds adsorb on the electrochemically roughened silver substrate in similar way, while they behave differently on the colloidal silver sur-face. A stronger enhancement of in-plane ring vibrations of the L-Phe ring, i.e., m3 and m18b (B2), over these of the A2 symmetry in all SERSspectra on the electrochemically roughened silver substrate suggests that the ring interacts with this surface adopting slightly deflect ori-entation from the perpendicular one. Also, enhancement of P@O and –CH2–/–CH3 fragments vibrations points out that they are involvedin adsorption process on this substrate. This conclusion was drawn on the basis of the enhancement of 1274–1279 and 1138–1152(m(P@O)), 1393–1400 (d(CH) + qb(CNH2) + m(C–C@O) + d(CH3)), �1455 (d(CCH3/CCH2) + qb(CH3/CH2), and 1505–1512 cm�1(d(CH2) + Phe(m19a)) bands. Although a relative intensity ratio of these bands in the presented SERS spectra is different. On the otherhand, on the colloidal silver nanoparticles, the aromatic ring of all molecules is lying flat or takes almost parallel orientation to this surface.Besides, A interacts also via P-terminal group (568, 765, 827, 1040, and 1150 cm�1), whereas B mainly through NH2–C–(C@O)–CNH–(712 and 1255 cm�1). In the case of C, it adsorbs on the silver colloidal surface mainly through the aromatic ring of L-Phe, while otherfragments of the molecule are in close proximity to this surface as comes off the weak enhancement of bands due to the aliphatic vibrations.� 2007 Elsevier B.V. All rights reserved.

Keywords: Surface enhanced Raman scattering, SERS; Fourier-transform Raman scattering, FT-RS; Fourier-transform infrared spectroscopy, FT-IR;Silver colloid; Electrochemically roughened silver substrate; L-Phenylalanine phosphonate derivatives

1. Introduction

The establishment of vibrational techniques, i.e., infra-red (IR) and Raman (RS), in chemical analysis offers the

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* Corresponding author. Tel.: +48 12 6632255; fax: +48 12 6340515.E-mail address: [email protected] (E. Podstawka).

possibility to identify behaviour of various samples onthe basis of vibrational bands being characteristic for cer-tain functional groups [1,2]. Due to the different selectionrules that govern IR and Raman spectroscopies, these tech-niques are complementary providing insight into the vari-ous polar and non-polar moieties of a given molecule notobserved solely with one of vibrational technique. The

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E. Podstawka et al. / Surface Science 601 (2007) 4586–4597 4587

nature of these techniques along with their chemical speci-ficity makes them ideally suited for elucidation of molecu-lar structure, surface processes, and interface reactions[3,4]. Thus, they have been successfully employed to studymany biological systems, including peptide interactions,protein folding, and diseased tissues [5–10]. Additionally,the Raman scattering is being developed as a tool forin vivo diagnostics [11]. However, the normal Ramanscattering suffers of extremely poor efficiency. This gener-ally limits usefulness of this technique to samples that arerelatively concentrated. A technique that overcomes thislimitation is surface enhanced Raman scattering (SERS).In SERS, the molecule of interest is attached to a nano-scopically structured metallic surface (metal nano-resona-tors), which provides an increase in the observed Ramanscattering by many orders of magnitude (up to 1014–1015)allowing for detection limit that can approach the singlemolecule level [12,13]. It thus can be employed to investi-gate ligand-receptor binding of significance for drug-pro-tein complexes and differences in individual molecules,such as catalysis rates of a single enzyme [14]. Theenormous Raman scattering enhancement results fromthe strong increase of local electromagnetic field in closevicinity to metal (i.e., silver or gold) surfaces that accom-panies excitation of surface plasmon resonances in themetals [15]. The most popular and universal substrate usedfor SERS is silver that can either be used in the form of ananoscale roughened surface onto which the sample isadsorbed, or as a colloidal suspension. One disadvantageof SERS is difficult spectra interpretation. The signalenhancement is so dramatic that very weak Raman bandsbarely observed in ordinary Raman spectra can appear inSERS. Moreover, because of chemical interactions withmetal surfaces, certain bands, which are strong in conven-tional Raman, might not be present in SERS at all.Although the interpretation, as discussed above, can bedefficult, SERS has a surface selection rule that states thatnormal mode with polarizability derivative componentsperpendicular to the metal surface will be the most en-hanced [16,17]. Therefore, specific information regardingthe packing and orientation (adsorbed structure) of bio-physical molecules on the metal surface is availablethrough the utilization of SERS.

Phosphonopeptides investigated in this work, i.e.,L-Phe-DL-NH–CH(CH(CH3)2)–PO3H2 (A), L-Phe-L-NH–CH(CH3)–PO3H2 (B), and L-Phe-DL-NH–CH(CH2CH2-COOH)–PO3H2 (C), are a class of peptidomimetics inwhich C-terminal carboxylic group is replaced by phos-phonic moiety. Despite the differences of both groups con-sidering acidity (phosphonic acid is more acidic with thedifference of at least three pKa units), shape (phosphonicgroup is tetrahedral versus flat carboxylate), and size(phosphonic group is considerably larger) they competewith their carboxylic counterparts for the sites of enzymesand other cell receptors [18]. Thus, their physiologic activ-ity ranges from herbicidal, antibacterial to anticancer[18,19]. On the other hand, phosphonopeptides might be

also considered as chemoreceptors for some amino acidand peptides [20].

The biological importance and supramolecular proper-ties of these molecules has motivated us to a number ofspectroscopic studies on these species. In view of gettinginformation about their adsorbed structures, changes inthe adsorption mechanism upon side-group or surfaceroughens variation, and manner of ligand-receptorapproaching we measured the SERS spectra of these mol-ecules adsorbed on the electrochemically roughened andcolloidal silver substrates. To propose the proper geometryof these species on different silver surfaces it was importantto define their non adsorbed molecular structure based ontheir Raman and IR spectra.

2. Experimental

2.1. Peptide synthesis

Phosphonopeptides were synthesized according to previ-ously described procedure [21]. Their purity and chemicalstructures were proved by means of the 1H, 31P, and 13CNMR spectra (Bruker Avance DRX 300 MHz spectrome-ter) and electrospray mass spectrometry (Finnigan MatTSQ 700).

2.2. FT-Raman spectroscopy

FT-Raman measurements were performed for phospho-nopeptides placed in a glass capillary tube. FT-RS spectrawere recorded on a Bio-Rad step-scan spectrometer (modelFTS 6000) combined with a Bio-Rad Raman Accessory(model FTS 40) and liquid-nitrogen-cooled germaniumdetector. Typically, 400 scans were collected with the reso-lution of 4 cm�1. Excitation at 1064 nm was used from aSpectra-Physics continuum-wave Nd3+:YAG laser (modelTopaz T10-106c).

2.3. FT-IR spectroscopy

Thin palettes containing 1 mg of each phosphonopep-tide dispersed in 200 mg of KBr were used for the FT-IRmeasurement. The spectra were recorded at room temper-ature as an average of 30 scans using a Brucker infraredspectrometer (model EQUINOX 55) equipped with aNernst rod as the excitation source and a DT–GS detectorin the 400–4000 cm�1 range with the spectral resolution of4 cm�1.

2.4. SERS spectroscopy

2.4.1. Silver colloids

AgNO3 and NaBH4 were purchased from Sigma–Al-drich Co (Poznan, Poland) and used without further puri-fication. A solution of the colloidal silver was prepared

4588 E. Podstawka et al. / Surface Science 601 (2007) 4586–4597

three times according to the standard procedure [22].Briefly, 8.5 mg of AgNO3 dissolved in 50 mL deionizedwater at 4 �C was added drop-wise to 150 mL of 1 mMsolution of NaBH4 immersed in an ice-bath and stirred vig-orously. After the addition of AgNO3 was completed, theresulting pale-yellow solution was stirred and maintainedat 4 �C for approximately 1 h. The excitation spectra ofthree batches of the Ag sol prepared in this manner showedan absorbance maximum at 396 nm.

Aqueous sample solutions were prepared by dissolvingthe samples in deionized water. The concentration of thesamples before mixing with the colloid was set at 10�4 M.The freshly prepared aqueous sample solution was addedto the silver sol (the final sample concentration in the silvercolloid was �10�5 M) and brought to pH = 8.3.

The SERS spectra of compounds investigated in thiswork were collected twice for each batch of the three silvercolloids using a triple grating spectrometer (Jobin Yvon, T64000). A liquid-nitrogen-cooled CCD detector (JobinYvon, model CCD3000) was used in these measurements.The spectral resolution of 4 cm�1 was set. The 514.1 nmline of an Ar-ion laser (Spectra-Physics, model 2025) wasused as an excitation source. Laser power at the samplewas set at 20 mW (�0.5 W/cm2).

The SERS spectra were recorded in the same time periodafter sample addition. The obtained spectra were almostidentical except for small differences (up to �5%) in someband intensities. No spectral changes that could be associ-ated with sample decomposition or desorption processwere observed during these measurements.

2.4.2. Macroscopic silver substrates

Before adsorption of phosphonopeptide, silver sub-strates were roughened electrochemically by three succes-sive oxidation–reduction cycles in a 0.1 M KCl aqueoussolution from �0.3 to 0.3 to �0.3 V at a sweep rate of5 mV s�1. The cycling was finished at �0.3 V, then the ap-plied potential was changed to �0.4 V and the silver elec-trode was kept for 5 min at this potential; after that theworking electrode was removed at an open circuit potentialand very carefully rinsed with water. The roughening wascarried out in a conventional three-electrode cell with alarge platinum sheet as the counter-electrode and an0.1 M KCl AgCl|Ag electrode as the reference (all poten-tials are referred to the potential of this electrode).

O

NH2

NHCH3

CH3

PO3H2

O

NH2

NH

CH3

Fig. 1. Molecular structures of L-Phe-DL-NH–CH–(CH(CH3)2)–PO3H2 (A),COOH)–PO3H2 (C).

SERS spectra at the macroscopic silver substrates wererecorded with an ISA T64000 (Jobin Yvon) Raman spec-trometer equipped with Kaiser SuperNotch-Plus holo-graphic filters, 600 grooves/mm holographic grating, anOlympus BX40 microscope with a 50 · long distance objec-tive, and 1024 · 256 pixels nitrogen-cooled CCD detector.A Laser-Tech model LJ-800 mixed argon/krypton laserprovided excitation radiation of 514.5 nm. Power at samplewas set at �4 mW.

3. Results and discussion

3.1. Fourier-transform Raman and infrared spectroscopies

The peptides investigated in this work are L-Phe–NH–CH(–R)–PO3H2, where: –R denotes isopropyl (analogueof valine), methyl (analogue of alanine), and propionic acid(analogue of glutamic acid) for A, B, and C, respectively(see Fig. 1). Thus, their Raman and infrared spectra showsimilar patterns, except the frequency regions characteristicfor stretching and deformation vibrations of these sidegroups. In the next paragraphs, we briefly discuss onlythe Raman and IR bands useful for the interpretation ofthe SERS spectra that allow identification of the adsorbedstructures of these analogues. Such typical strategy allowsone to suggest a mechanism of interaction of investigatedmolecules with the colloidal and electrochemically rough-ened silver substrates.

L-Phe has a number of aromatic ring vibrations thathave been intensively studied by vibrational spectroscopy[23–27]. These characteristic vibrations appears as bandsthat show up in the FT-RS spectra of A, B, and C ana-logues (Fig. 2) around 621, 1003, 1030–1026, 1203–1207,1585, and 1605 cm�1. They are due to the in-plane ringdeformation (m6b), symmetric ring breathing (m12), in-planeCH bending (m18a), phenyl-C stretching (m7a), and two in-plane ring stretching vibrations (m8b and m8a), respectively.Other less characteristic spectral features, of medium tolow intensity, associated with the L-Phe residue are ob-served around 941–958 (m5), 1065–1088 (m18b), 1342–1354(m3), 1439–1450 (m19b), 1497–1504 (m19a), and 3059–3065(m2) cm

�1 [25]. Table 1 summarizes detailed frequencies ofthe L-Phe bands observed for these species.

The FT-IR spectra presented in Fig. 3, in the spectralrange of 400–3700 cm�1 show several of the above men-

PO3H2

O

NH2

NH COOH

PO3H2

L-Phe-L-NH–CH–(CH3)–PO3H2 (B), and L-Phe-DL-NH–CH–(CH2CH2-

Fig. 2. FT-RS spectra of solid L-Phe-DL-NH–CH–(CH(CH3)2)–PO3H2 (A), L-Phe-L-NH–CH–(CH3)–PO3H2 (B), and L-Phe-DL-NH–CH–(CH2CH2-COOH)–PO3H2 (C) in the spectral range of 3200–150 cm

�1.


tioned modes. For example, the band due to m8a is clearlyobserved in the FT-IR spectra of A (1610 cm�1) and B(1604 cm�1) only, whereas in the case of C it is maskedby a very strong absorption at 1670 cm�1. Among the in-plane ring-bending modes, m19a, m8b, m11, m3, m9a/15, and m7aemerge in the FT-IR spectra. For all analogues investigatedhere the four former modes are distinctly observed near1498, 1076, 747, and 1367 cm�1, respectively. The m9a/15mode appears as a shoulder for C (at 1175 cm�1) only,while is hidden under the �1147 cm�1 band for A and B.The last above mentioned band is observed at 1214 and1218 cm�1 for A and B, respectively.

The presence of the phosphonate group is manifested inthe discussed vibrational spectra by several bands. Some ofthese bands appear around 1261–1273, 1147–1157, 820–829, and 745–756 cm�1 (see Figs. 2 and 3). The formertwo above mentioned spectral features are mainly due tothe P@O stretching vibrations (m(P@O)). Whereas, the nexttwo are assigned to the m(P–O) + qb(POH) + m(C–P) andms(O–P–O) modes, respectively [26–31]. Besides these, the–PO3H2 group may also contribute the �667, 595–541,and 500–412 cm�1 spectral futures.

The FT-RS and FT-IR spectra (Figs. 2 and 3) of phos-phonopeptides exhibit also several bands that have contri-bution from the –CNH2 group vibrations. One of themappears at the high-frequency range and is due to the N–H stretching vibrations (m(NH)). Five other bands of theamine group are seen in the frequency range of 1270–450 cm�1. These bands are due to the qb(CNH2),qb(CNH2), qb(CNH2 + doop(CNH2), m(C–N), doop(CNH2),and qop.t(NH2) vibrations and appear at around 1396–1412(A and C), 1304–1312, �1133 (A and C), 1065 (A), 911–933, 806 (B), and 455–489 cm�1, respectively. All of theabove mentioned bands are relatively broad and overlapwith other bands expected in this range (see Table 1 fordetailed band assignment to the normal coordinates).

The FT-IR spectra (Fig. 3) depict one additional bandbetween 1659–1678 cm�1 associated with the das(NH2)mode.

In the Raman spectrum of C a very weak feature near1288 cm�1 is enhanced. It could be due to the amide IIImode that overlaps with other bands present in this region.The amide III band arises mainly from a combination ofthe in-phase and out-of-phase NH bending and CNstretching and is clearly observed only in the Raman spec-tra [32]. A Raman band at 1663–1678 cm�1 and a broad IRband at around 1659–1670 cm�1 are assigned to the amideI vibrations. The broadness of this band in the all FT-IRspectra presented here arises, as was suggested by Asherand co-workers [33], from its splitting into two subbandsthat are probably derived from the coupling of the amideI motion with the bending modes of water molecules thatare hydrogen bonded to the amide group. It mainly in-volves the C@O stretching, Ca–C–N deformation, andCN stretching [34]. On the other hand, the IR-active amideII results from the N–H bending and C–N stretching vibra-tions. In the spectra presented in this work, it is observed inthe range of 1521–1561 cm�1 as a broad of medium inten-sity IR band.

The absence of the methyl group in C is manifested inthe spectral regions characteristic for different stretchingand bending vibrations of this moiety. Thus, the 3001–2974, 2932–2957, 2893, and 2866–2874 cm�1 spectral fea-tures (see Table 1 for detailed band positions) for A andB are assigned to the aliphatic m(CH), mas(CH3/CH2),ms(CH3), and ms(CH2) modes, respectively. Whereas, inthe spectra of C (Figs. 2C and 3C) the bands at 2932–2959 and 2856–2870 cm�1 are due to mas(CH2) and ms(CH2),respectively. In addition, in the lower-frequency range sev-eral deformation modes of the –CH3/–CH2–/–CH< frag-ments are observed. For example, the –CH3 groups of Acontributes to the 1462, 1065, and 933–914 cm�1 Raman

Table 1Wavenumbers and proposed band assignments for FT-RS and FT-IR spectra of L-Phe-DL-NH–CH–(CH(CH3)2)–PO3H2 (A), L-Phe-L-NH–CH–(CH3)–PO3H2 (B), and L-Phe-DL-NH–CH–(CH2CH2COOH)–PO3H2 (C)

Frequency (cm�1) Assignment

A B C

FT-RS FT-IR FT-RS FT-IR FT-RS FT-IR

3339 3300 3264 ms(NH)3233

3059 3069 3059 3062 3065 3076 Phe(m2)3034 3031 3031 m(C–H)aromatic

3001 2974 2976 m(C–H)aliphatic2955 2957 2932 2933 2959 2939 mas(CH3) and/or mas(CH2)

2933 29322893 ms(CH3)2866 2870 2874 2870 2870 2856 ms(CH2)

1739 1734 m(COO�)1663 1664 1663 1659 1678 1670 Amide I, das(NH2)1605 1610 1605 1604 1605 Phe(m8a)1585 1585 1585 Phe(m8b)

1556 1550 1561 Amide II1521 1525

1497 1498 1504 1498 1497 1499 Phe(m19a)1462 1469 d(CH3), qb(CCH3), d(CCH3)1447 1457 1450 1455 1439 1455 Phe(m19b), d(CH2)

14411404 1405 1396 1412 1411 d(CH), qb(CNH2), m(C–C@O), d(CH3)

1385 1378 qb(CCH3), m(CCCH3)1342 1367 1346 1354 1367 qw(CH2), Phe(m3)1312 1304 1306 1312 Phe(m14), qw(CH2), qb(CNH2), Amide III

1288 d(PC(–H)C), qb(PC(–H)C), qb(CNA(–H)CA)1269 1270 1268 1261 1273 m(P@O), qb(PC(–H)C), d(PC(–H)C), m(C–P)1254 1254 1237 Phe(m13), qt(CH2), m(C–N)1203 1218 1207 1214 1207 Phe(m7a)1180 1180 1184 1175 Phe(m9a)/Phe(m15)1157 1147 1157 1148 1157 m(P@O), d(CH2), d(CC(–H)N)1111 1133 1132 d(CH2), qb(CNH2), d(C@OC(–H,N)C),

qb(C@OC(–H)N) m(C–N), m(C–C), doop(CNH2)1065 1082 1076 1073 1088 1072 Phe(m18b) m(C–CCH3 ), m(N–C), qb(CC–CCH3 )

10651030 1030 1034 1026 1030 Phe(m18a)1003 998 1003 989 1003 Phe(m12)958 941 Phe(m5)/Phe(m17a)933 911 918 919 926 Phe, doop(CNH2), Amide, m(C–CC–CCH3 ),

d(CCH3), qb(CCH3)914887 887 887 898 Phe, doop(CNH2), m(C–C)821 821 829 820 m(P–O), qb(POH), m(C–P)748 747 752 745 756 750 Phe(m11), s(O–P–O)

698 705 699 701 Phe(m4), doop(CNA(–H)CA)668 648 667 qw(CPO), das(PO3H2)

621 621 621 Phe(m6b)596 595 579 qt(P–(OH)2)

567 567 551 556 561 das(PO3H2), qb(CPO)541

496 500 qs(PO3H2)489 464 489 486

455 469 d(CPO), qop.t(NH2)412 439 432 416 ds(PO3H2)

370 385 366 d(C–C–N), d(C–C–C)293 316 312243 177 189185


bands (Fig. 2A) and the 1469, 1385, 1065, and 911 cm�1 IRbands (Fig. 3A). While the –CH2– group (A, B, and C)

gives input to the �1447, �1342, 1312, 1254, and1157 cm�1 vibrational features that are due to the different

Fig. 3. FT-IR spectra of solid L-Phe-DL-NH–CH–(CH(CH3)2)–PO3H2 (A), L-Phe-L-NH–CH–(CH3)–PO3H2 (B), and L-Phe-DL-NH–CH–(CH2CH2-COOH)–PO3H2 (C) in the spectral range of 3700–400 cm

�1.


deformation modes (see Table 1 for detailed bandsassignment).

3.2. Surface-enhanced Raman scattering

3.2.1. Electrochemically roughened silver substrates

Fig. 4 compares surface enhanced Raman (SERS) spec-tra, in the 1750–600 cm�1 spectral range, of investigatedphosphonopeptides immobilized on the electrochemically

Fig. 4. SERS spectra of L-Phe-DL-NH–CH–(CH(CH3)2)–PO3H2 (A), L-Phe-LPO3H2 (C) adsorbed on electrochemically roughened silver substrate in thexcitation wavelength: 514.5 nm; power at sample �4 mW.

roughened silver surface. Proposed SERS bands assign-ment to the normal coordinates, based on the DFT calcu-lations for similar L-Phe phosphonate dipeptides [31],published data on a-phenylglycine [35] and L-phenylalanine-alanine and -valine phosphonodipeptides adsorbed on thecolloidal and macroscopic silver surface [26,27], togetherwith the observed frequencies is given in Table 2. As is seenin Fig. 4, the number of the observed SERS vibrations isreduced in comparison to the vibrations observed in the

-NH–CH–(CH3)–PO3H2 (B), and L-Phe-DL-NH–CH–(CH2CH2COOH)–e spectral range of 1750–600 cm�1. Measurement conditions: �10�4 M;


ordinary Raman spectra (Fig. 2). This is due to furtherconstrains introduced by molecular orientation at the sur-face interface and the variation of the tangential and nor-mal components of the electric field at the surface thatdetermine the SERS selection rules [16,17]. Additionally,changes in relative band intensities of L-Phe show deviationof the ring orientation from the perpendicular orientationto the surface. According to surface selection rules withassuming a predominant normal component, it is expectedthat the normal modes with polarizability derivative com-ponents perpendicular to the surface should dominate theSERS spectra [16,36,37]. For the aromatic ring of L-Pheof dipeptdes investigated here that is of the C2v symmetry,with the assumption that the molecular axes are such that zcontains the C2 axis of symmetry and yz is the molecular

Table 2Frequency and proposed band assignments for SERS spectra of L-Phe-DL-NHL-Phe-DL-NH–CH–(CH2CH2COOH)–PO3H2 (C)

Frequency (cm�1) in SERS spectra

A B

Colloid Macroscopic substrate Colloid Macroscopic substrate

3074 30712964 29832935 29382873 287828541604 1599 1602 16021583 1568 1585

1508 15051455 1457 1455 1455

14321392 1398 1393

13531317 1317

1245 1278 1255 12741244 1227

1209 12091150 1138 1162 1149

1081 10831040

10331003 1010 1004 1006958 954911 909

859827 827765

720696 699 712

683 679665

617 620568 559494 497210 208

plane, it is possible to determine orientation of this ringwith respect to the enhancing substrate. Therefore, thestrongest bands in the SERS spectra should belong to nor-mal modes transforming as zz. The next intense bandsshould belong to modes transforming as xz and yz. Finally,modes transforming as xx, xy, and yy should have veryweak intensity and may not be observed at all. Applyingthis treatment to the presented SERS spectra, the B2 modes(transforming as yz) would be expected to be large onlywhen the ring in-plane vibrations have a large componentperpendicular to the surface, that is, when the ring is‘‘standing up’’. In this orientation, the A1 and B1 modes(transforming as xx, yy, and zz, and xz, respectively) arealso expected to exhibit good enhancement. Finally, vibra-tional modes of the A2 symmetry (transforming as xy)

–CH–(CH(CH3)2)–PO3H2 (A), L-Phe-L-NH–CH–(CH3)–PO3H2 (B), and

Assignment

C

Colloid Macroscopic substrate

3071 Phe(m2)m(C–H)

2932 mas(CH3) or mas(CH2)ms(CH3)

2862 ms(CH3) or ms(CH2)1600 1599 Phe(m8a)1580 1568 Phe(m8b)1561 Amide II15261493 1512 Phe(m19a), d(CH2), Amide II

1458 d(CCH3/CCH2), qb(CH3/CH2)Phe(m19b), d(CH2)

1389 1400 d(CH), qb(CNH2), m(C–C@O),(CH3)

1346 qw(CH2), Phe(m3)1331 Phe(m14), qw(CH2), qb(CNH2),

Amide III1269 1279 m(P@O)

1248 Phe(m13), qt(CH2), m(C–N)1209 Phe(m7a)1161 1152 m(P@O), d(CH2)

11431114 m(C–N), m(C–C), doop(CNH2)1083 Phe(m18b)

m(Ca–N), m(C–P)1033 Phe(m18a)1002 1012 Phe(m12)956 Phe(m5)/Phe(m17a)906 doop(CNH2), d(CCH3), qb(CCH3)

Phe(m10a), m(P–O), qb(POH)836 mas(OPO)

752 Phe(m11), ms(O–P–O)Amide V, qwðPO2�3 Þ

705 705 d(NH2–C–(O@)C–NH)684 Phe(m4), d(CH)

615 Phe(m6b)566 dasðPO2�3 Þ, qb(CPO)

qt(NH2), d(C–C–N)214 Ag-molecule


should not be apparent since they have no z polarizabilityderivative component. On the other hand, for the L-Phering laying flat on the surface, only the A2, A1, and B1modes should scatter effectively [38–46].

In the SERS spectrum of A several of the L-Phe ringmodes of the A1 and B2 symmetry are observed what pointsout that A interacts with the electrochemically roughenedsilver substrate. These modes are enhanced at 1599 (A1,m8a), 1568 (B2, m8b), 1508 (A1, m19a), �1432 (B2, m19b),�1351 (B2, m3), 1244 (A1, m13), and 1010 (A1, m12) cm�1.The SERS bands of A1 symmetry decrease in their relativeintensity (Fig. 4A) in comparison to the relative intensityshown in the ordinary Raman spectrum (Fig. 2A), whilethat of the B2 symmetry practically do not change theirintensity. This phenomenon may suggest that the L-Phering adopts slightly tilted from the vertical orientation tothe macroscopic substrate. Also, considering that boththe bandwidths of these modes increase (by 5–8 cm�1)and the maxima of intensity of these modes shift(D = �7�17 cm�1) upon surface adsorption, the possibilityof direct interaction between L-Phe ring of A and silver sur-face seems to be rather high. In the discussed SERS spec-trum (Fig. 4A) bands due to vibrations of the otherfragments of the molecule are also enhanced. So, theP@O moiety contributes to the 1278 and 1138 cm�1 bands(m(P@O)). Although deformation vibrations of the –CH2–fragment (d(CH2)) may be also enhanced around 1138cm�1. This statement is supported by the enhancement ofthe 1508, 1457, 1398, and 1244 cm�1 spectral features ofthe d(CH2), d(CCH3/CCH2) + qb(CH3/CH2), d(CH3), andqt(CH2) + m(C–N) modes, respectively. The 1398 cm

�1

band dominates the A spectrum (Fig. 4A). Thus, it couldbe also due to the combination of the –CNH2 bending(qb(CNH2)) with the C–C@O stretching (m(C–C@O)) vibra-tions. This may explain its strong enhancement. In turn,this suggests that the –CNH2 moiety strongly adsorb onthe silver surface. The relatively strong enhancement ofthe 1244 cm�1 band due also to m(C–N) confirms abovestatement.

The last discussed band in the SERS spectrum of A onthe macroscopic silver substrate appears at 1508 cm�1. Itcould be due not only to the deformations of the –CH2–group but also to the m19a and amide II vibrations. Fre-quency of this spectral feature is lower than its expectedvalue for the amide II band. This arises from the changesin the bond order of the CA–NA and CA@OA bonds ofthe amide moiety that for similar L-Phe phosphonatedipeptides were calculated to be �1.5 [31]. This phenome-non could explain the absence of the C@O stretching band(amide I) in the spectra. However, the medium-low inten-sity of the 1508 cm�1 band in comparison to its strongintensity in the SERS spectra of other two molecules mayimply that the amide bond lies flat on the silver surface[26] or does not interact directly with the macroscopic sil-ver surface. Neither significant band broadening nor largefrequency shift is observed for all above mentioned bands.Since the band shift and band broadening of these modes

are not as dramatic as those of the ring modes, the adsorp-tion of A on the electrochemically roughened silver surfaceis supposed to occur mainly via the aromatic ring. Whileits P@O, –CNH2, and –CH3/–CH2– fragments interactnot so strongly with this surface. However, consideringenormous enhancement of the 1398 cm�1 band, it seemsthat the –CNH2 moiety rather adsorb on the silver surface,whereas the methyl/methane and P@O fragments assist inthe A binding to this surface than are positioned very closeto it.

Similar analysis may be lead for other two analoguesinvestigated in this work, i.e., B and C, that pattern ofthe SERS spectra (Fig. 4B and C) resembles shape of theA SERS spectrum. The frequency and width of the mostof the enhanced bands are only slightly different betweenthe SERS spectra presented in Fig. 4. However, relativeintensities for some of bands are remarkably different be-tween these spectra. Anyway, the 1505 cm�1 band due tom19a + d(CH2) + Amide II dominates the SERS spectrumof B immobilized on the electrochemically roughened silversurface. This implies that the amide bond of B stronglyinteracts with the silver substrate and the –CH2–/–CH3fragments assist in this binding. This phenomenon maybe confirmed by both the observed band broadening forthis spectral feature and the enhancement of the otherbands due to the methyl/methylene groups vibrations, i.e.,1455 (d(CCH3/CCH2) + qb(CH3/CH2)), 1432 (d(CH2)),1353 (qw(CH2)), 1227 (qt(CH2)), and 1148 (d(CH2)) cm

�1.In the case of C (Fig. 4C), the 1512 cm�1 band decreasesin intensity in comparison to the intensity exhibited bythe corresponding band for B (Fig. 4B) and increases inintensity in relation to that shown for A (Fig. 4A). Thisintensity variation suggests that the amide bond of C inter-acts with the silver substrate, whereas strongly binds to thesilver for B but in the case of A assists in the adsorption onthe electrochemically roughened silver substrate.

Medium-weak enhancement of the 1455 cm�1 band(d(CCH3/CCH2) + qb(CH3/CH2)) and low intensity ofthe 1393 cm�1 spectral feature (qb(CNH2) + d(CH3)) inthe B SERS spectrum together with the weak and strongenhancement of the corresponding modes for A and C sug-gests that the –CNH2 group for B is rather remote from themacroscopic silver surface. Whereas, for A and C itstrongly interacts with this surface. The comparison ofthe relative intensity of the 1227–1248 cm�1 band betweenthe spectra confirms above conclusions for B and A, only.The relative intensity of this spectral feature changes indirection: A > B > C, what points out that vibrations ofL-Phe and –CH2– mainly contribute to this band. So, thestrong enhancement of the 1400 cm�1 band could be ex-plained by strong interactions of the carboxyl group of Bwith the silver.

Other most prominent bands of the B and C SERS spec-tra are due to the m(P@O) and ring modes. In fact, theSERS bands at 1274 and 1149 cm�1 for B and 1279 and1143 cm�1 for C that are partially ascribed to the P@Ostretching are relatively stronger than the ring ones and


stronger than these for A. So, the relative intensity of thesetwo bands changes in direction: B > C > A. This tendencysuggests that the P@O moiety of B and C stronger interactswith the electrochemically roughened silver surface than inthe case of A. Whereas the strength of the interactions ofthe L-Phe aromatic ring with the silver substrate is weakerfor B and C than for A. In addition, in the SERS spectra ofB and C all the ring modes of the B2 symmetry were foundto be more enhanced than A1 ones. That is the reverse sit-uation that for the ordinary Raman spectra. As predictedfrom the surface selection rules the more distinctive spec-tral features of the B2 symmetry in the SERS spectramay point out the perpendicular orientation of the aro-matic ring on the Ag surface. Thus, it can be concludedthat the L-Phe ring almost ‘‘stands up’’ on the macroscopicsilvers surface.

3.2.2. Adsorption mechanism on colloidal silver

Fig. 5 presents the SERS spectra, in the frequency rangeof 3200–150 cm�1, measured for phosphonopeptides con-taining L-phenylalanine when adsorbed on the colloidal sil-ver surface. The detailed frequencies of the enhanced bandsin these spectra together with their assignment to the nor-mal coordinates are listed in Table 2. This table addition-ally compares the values with the frequencies shown inthe SERS spectra of these molecules immobilized on themacroscopic silver substrate.

As is expected, in the SERS spectra of A, B, and C ad-sorbed on the silver colloidal surface the characteristic setof the L-Phe ring modes, i.e., m2, m8a, m8b, m7a, m18a, m12,and m6b, is observed. The bands of these modes are rela-tively weakly enhanced, especially m12. However, the A1symmetry modes (m2, m8a, m7a, m18a, and m12) are stronger en-hanced in these SERS spectra than the modes of the B2 andA2 symmetry. According to the surface selection rules suchbehaviour of the L-Phe ring modes suggests tilted close to

Fig. 5. SERS spectra of L-Phe-DL-NH–CH–(CH(CH3)2)–PO3H2 (A), L-Phe-PO3H2 (C) adsorbed at silver colloidal surface in the spectral range of 3200–110�5 M; excitation wavelength, 514.5 nm; power at sample, 20 mW.

flat orientation of the ring on the colloidal silver surface.Mentioned above significant intensity decrease of the1003 cm�1 band indicates no direct interaction betweenthe L-Phe ring of these analogues with the silver nanoparti-cles what is probably caused by distance effect. This state-ment is supported by the lack of a significant frequencyshift of the ring modes and their only slightly higher band-widths [47]. Further careful comparison of the intensity ofthe 1003 cm�1 band between the spectra shows that itsintensity decreases in direction: A > B > C. This indicatesthat the ring of A is located closer to the silver colloidal sur-face than that of B that in turn should lying closer thanthat of the C analogue.

The enhancement effect of intense Raman bands of ad-sorbed molecule depends strongly on the distance fromthe metal surfaces. In the molecular mechanism, theenhancement of Raman scattering involves chemisorptionand/or charge transfer between adsorbed molecules andthe metal surface and thus is short range in origin [48–51]. Moreover, the electromagnetic fields of surface plas-mons decay exponentially with the increase of the distancefrom the metal surface [48–53].

Castro et al. in their SERS study on a-phenylglycinedeposited on the silver surface showed that the most en-hanced bands are the totally symmetric A1 ring modesand this is why the C–H stretching vibration of the phenyl-alanine ring (m2) is observed with noticeable intensity. Thus,based on the surface selection rules of EM (electromagneticmechanism) the Authors stated that the L-Phe ring lies per-pendicularly to that surface [35]. The above considerationstogether with the appearance of the m2 mode (low-fre-quency shoulder at �3071 cm�1) of weak intensity in theSERS spectra of A, B, and C (Fig. 5) support our conclu-sions that the L-Phe ring for these dipeptides adopts orien-tation slightly deviated from the horizontal position to thecolloidal silver surface (Fig. 6).

L-NH–CH–(CH3)–PO3H2 (B), and L-Phe-DL-NH–CH–(CH2CH2COOH)–50 cm�1. Measurement conditions: sample concentrations in silver colloid,


The bands of medium-strong intensity observed in the Aspectrum at 1150, 827, and 568 cm�1 are typical SERSvibrations of the phosphonate group. The assignment ofthese bands is as fallows: m(P@O) + d(CH2), mas(OPO),and daðPO2�3 Þ þ qbðCPOÞ. In addition, two other bandsdue to the m(P@O) and mas(OPO) modes are enhanced inthe SERS spectrum of A on the colloidal silver. The formerone is observed as a higher frequency shoulder at the1245 cm�1 band (m13), while the latter one is relatively theweakest band of the P-terminal group and is seen at765 cm�1. The high enhancement of the P@O bond vibra-tions may imply that this moiety, in comparison to otherparts of A, binds to the silver nanoparticles. In the SERSspectra of the other two analogues, B and C, on the colloi-dal silver surface all of the above mentioned bands, exceptmas(OPO), are enhanced (see Table 2 for detailed band posi-tions). However, decrease in their intensities in comparisonto the intensities exhibited in the A SERS spectrum, except1255 cm�1 band of B and 1161 cm�1 band for C, is ob-served. The high intensity and/or broadening of thesetwo spectral features are probably joined with overlappingof different modes that are expected to be enhanced aroundthese frequencies. Thus, it can be concluded that B and Cinteract with the silver nanoparticles in such a way thatthe P@O bond interacts with the surface, while two otheroxygen’s of the phosphonate group are in close range tothis surface.

Special attention should be devoted also to a broadband with the maximum around 696–712 cm�1 due to the

Fig. 6. Possible manner of binding of L-Phe-DL-NH–CH–(CH(CH3)2)–PO(CH2CH2COOH)–PO3H2 (C) adsorbed at colloidal and electrochemically rou

deformation vibrations of the NH2–C–(O@)C–NH– frag-ment. This spectral feature is relatively strongly enhancedin the B SERS spectrum, while in the SERS spectra ofthe other two analogues is only slightly marked. Therebyindicating, the amide bond of B takes place in the adsorp-tion mechanism on the silver nanoparticles. This statementmay be supported by the enhancement of the amide bands,i.e., I, II, and III, that are usually enhanced around 1610–1640, 1520–1550, and 1240–1290 cm�1 [6]. Unfortunately,in the higher frequency region in the SERS spectra of allmolecules investigated in this work the broad envelope ofbands is observed. However, in the case of C several max-imas in this frequency range may be pointed, i.e., 1561,1526, and 1493 cm�1. Thus, it is reasonable to state thatthe NH2–C–(O@)C–NH– fragment of B strongly interactswith the silver colloidal surface with the tilted geometry ofthe amide bond with respect to the surface. In differentmanner is oriented the amide bond of A and C. It seemsthat for these two analogues the amide bond adopts closeto horizontal orientation on the silver surface.

Five different vibrations of the amine group may contrib-ute to the spectral features in the frequency range of 1330–490 cm�1. The SERS bands of A (Fig. 5A) at 1392 and 1317,1245, �911, and 494 cm�1 are due to the –CNH2 bend-ing (qb(CNH2)), C–N stretching (m(C–N)), out of plane–CNH2 deformation (doop(CNH2)), and –NH2 twistingcoupled with C–C–N deformation (qt(NH2) + d(C–C–N))vibrations, respectively. These bands are slightly enhanced.Similar behaviour of these bands is also observed for C. In

3H2 (A), L-Phe-L-NH–CH–(CH3)–PO3H2 (B), and L-Phe-DL-NH–CH–ghened silver surfaces.


the SERS spectrum of B (Fig. 5B), all these bands are barelyobserved, except that at 1255 cm�1, and appear at similarfrequencies as for A (see Table 2 for detailed band posi-tions). This observation points out that the –CNH2 frag-ments of A and C does not take part directly in theinteraction with the silver nanoparticles. While, in the caseof B it assists in the dipeptide binding to the substrate.

In the higher frequency region of the SERS spectra ofthree peptides investigated here bands due to the pure C–H bond vibrations are observed, i.e., the –CH3/–CH2– anti-symmetric (mas(CH3/CH2)) and –CH3/–CH2– symmetricstretching (ms(CH3/CH2)) vibrations. Their exact frequen-cies are listed in Table 2. These bands are very weakly en-hanced in comparison to they enhancement in the ordinaryRaman spectra (Fig. 2). This indicates that methyl and/ormethylene groups are rather far away from the colloidal sil-ver surface. As well, in the lower frequency range we didnot observe clearly the characteristic SERS bands originat-ing from these two groups.

4. Conclusions

In the present work, the nature of phosphonatederivatives of L-Phe-DL-NH–CH(CH(CH3)2)–PO3H2 (A),L-Phe-L-NH–CH (CH3)–PO3H2 (B), and L-Phe-DL-NH–CH(CH2CH2COOH)–PO3H2 (C), adsorbed on the electro-chemically roughened and colloidal silver surfaces has beeninvestigated. Despite different so-called spacer group inthese molecules (isopropyl, methyl, and propionic acidfor A, B, and C, respectively) their Raman and infraredspectral patterns are very similar. These spectra differ onlyin the frequency region characteristic for the methane,methylene, and carbonyl group vibrations. On the otherhand, the pattern of the SERS spectra of adsorbed specieson the electrochemically roughened silver substrate differmarkedly from that of species adsorbed on the colloidalsilver surface. This clearly points out that variation of thesilver surface affects adsorption mechanism of thesemolecules. In addition, the SERS spectra of all analogueson the macroscopic silver surface (or on the colloidal silvernanoparticles) show similar shape. Although they differ inthe relative intensity ratios of the enhanced bands. Thisphenomenon depicts influence of the spacer group on thestrength of the interactions of the proper fragments ofthe molecule with the silver substrate.

We have shown that the ring of all L-Phe phosphono-peptides deposited on the electrochemically roughened sil-ver surface adopts tilted to close to vertical orientation,while on the colloidal silver nanoparticles it is lying at anangle close to flat orientation. The strength of this interac-tion is high in the case of the macroscopic substrate and de-creases in direction: A > C P B, whereas it is lower forcolloidal silver substrate suggesting that the ring does not‘‘stacked’’ to this substrate. We also have shown that theP@O moiety of all molecules on both surfaces is involvedin the adsorption process. However, the strength of thisinteractions is different in each case (B > C > A on the mac-

roscopic silver substrate and A > B � C on the colloidal sil-ver). In addition, in the case of the colloidal silver substratethe O–P–O fragment of A assists in adsorption process,whereas for B and C is in close range to this surface. Wealso proved that –CH2–/–CH3 groups assist in the dipep-tides interaction on the electrochemically roughened silversubstrate, while for peptides adsorbed on the colloidal sil-ver they are rather away from the metal surface. Besides,only for A the NH2–C–(O@)C–NH link is lying flat or doesnot interact with the macroscopic silver. While for peptideson the silver nanoparticles it interacts with the substrate insuch a way that the amide bond for B adopts tilted geom-etry but in the case of A and C close to horizontalorientation.

Acknowledgement

This work was supported by the Adam Krzy _zanowskiFound, 2006/2007 (to E.P.).

References

[1] I.R. Lewis, H.G.M. Edwards (Eds.), Handbook of Raman Spectros-copy. Practical Spectroscopy, Marcel Dekker Inc., New York, 2001.

[2] M.J. Pelletier (Ed.), Analytical Applications of Raman Spectroscopy,Blackwell Science Ltd., Oxford, 1999.

[3] J.R. Baena, B. Lendl, Curr. Opin. Chem. Biol. 8 (2004) 534.[4] C. Krafft et al., Vibr. Spectrosc. 32 (2003) 75.[5] R.S. Conroy, C. Danilowicz, Contemp. Phys. 45 (2004) 277.[6] A.E. Głowacka, E. Podstawka, M.H. Szczęsna-Antczak, H. Kalin-

owska, T. Antczak, Comp. Biochem. Phys. 140 (2005) 321.[7] Z. Huang, A. McWilliams, H. Lui, D.I. McLean, S. Lam, H. Zeng,

Int. J. Cancer 107 (2003) 1047.[8] E. Podstawka, Y. Ozaki, L.M. Proniewicz, Appl. Spectrosc. 58 (2004)

1147.[9] E. Podstawka, Biopolymers, submitted for publication.

[10] E. Podstawka, E. Sikorska, L.M. Proniewicz, B. Lammek, Biopoly-mers 83 (2006) 193.

[11] P.J. Caspers, G.W. Lucassen, G.J. Puppels, Biophys. J. 85 (2003) 572.[12] P. Etchegoin, R.C. Maher, L.F. Cohen, H. Hartigan, R.J.C. Brown,

M.J.T. Milton, J.C. Gallop, Chem. Phys. Lett. 375 (2003) 84.[13] Ch. Zander, J. Enderlein, R.A. Keller (Eds.), Single-Molecule

Detection in Solution Methods and Applications, Wiley, Berlin, 2002.[14] R.C. Maher, M. Dalley, E.C. Le Ru, L.F. Cohen, P.G. Etchegoin, H.

Hartigan, R.J.C. Brown, M.J.T. Milton, J. Chem. Phys. 121 (2004)8901.

[15] K. Kneipp et al., J. Phys.: Condens. Mat. 14 (2002) R597.[16] M. Moskovits, J. Chem. Phys. 77 (1982) 4408.[17] M. Moskovits, J.S. Suh, J. Phys. Chem. 88 (1984) 5526.[18] P. Kafarski, B. Lejczak, Aminophosphonic and Aminophosphinic

Acids, in: V.P. Kukhar, H.R. Hudson (Eds.), Chemistry andBiological Activity, John Wiley & Sons, 2000, p. 173.

[19] P. Kafarski, B. Lejczak, Phosphorous Sulfur Silicon 63 (1991) 193.[20] P. Młynarz, E. Rudzińska, Ł. Berlicki and P. Kafarski, Curr. Org.

Chem. 10, in press.[21] B. Lejczak, P. Kafarski, H. Sztajer, P. Mastalerz, J. Med. Chem. 29

(1986) 221.[22] E. Podstawka, Y. Ozaki, L.M. Proniewicz, Appl. Spectrosc. 58 (2004)

570.[23] R.P. Rava, T.G. Spiro, J. Phys. Chem. 89 (1985) 1856.[24] S.A. Asher, M. Ludwig, C.R. Johnson, J. Am. Chem. Soc. 108 (1986)

3186.[25] B.J.M. Rajkumur, V. Ramakrishnam, Spectrochim. Acta Part A 58

(2002) 1923.


[26] E. Podstawka, R. Borszowska, M. Grabowska, M. Drąg, P. Kafarski,L.M. Proniewicz, Surf. Sci. 599 (2005) 207.

[27] E. Podstawka, H. Kozłowski, L.M. Proniewicz, J. Raman Spectrosc.37 (2006) 574.

[28] N.B. Colthup, L.H. Daley, S.E. Wilberey (Eds.), Introduction toInfrared and Raman Spectroscopy, Academic Press, New York, 1975.

[29] M.T.S. Rosado, M.L.R.S. Duarte, R. Fausto, J. Mol. Struct. 410/411(1997) 343.

[30] G. Niaura, A.K. Gaigalas, V.L. Vilker, J. Phys. Chem. B 101 (1997)9250.

[31] E. Podstawka, A. Kudelski, and L.M. Proniewicz, Surf. Sci. submit-ted for publication.

[32] R. Schweitzer-Stenner, J. Raman Spectrosc. 32 (2002) 711.[33] X.G. Chen, R. Schweitzer-Stenner, S. Krimm, N.G. Mirikin, S.A.

Asher, J. Am. Chem. Soc. 116 (1994) 11141.[34] J. Grdadolnik, Y. Maréchal, Biopolymers (Biospectroscopy). 62

(2001) 54.[35] J.L. Castro, M.R. Lopez-Ramirez, I. Lopez Tocon, J.C. Otero, J.

Colloid Interf. Sci. 263 (2003) 357.[36] V.M. Hallmark, A. Campion, J. Chem. Phys. 84 (1986) 2933.[37] J.A. Creighton, R.J.H. Clark, R.E. Hester (Eds.), Spectroscopy of

Surfaces, John Wiley & Sons, New York, 1988.[38] M. Moskovits, D.P. Dilella, J. Chem. Phys. 73 (1980) 6068.

[39] P. Gao, M.J. Weaver, J. Phys. Chem. 89 (1985) 5040.[40] R.A. Wolkow, M. Moskovits, J. Chem. Phys. 84 (1986) 5196.[41] M. Moskovits, D.P. Dilella, K.J. Maynard, Langmuir 4 (1988) 67.[42] X.P. Gao, J.P. Davies, M.J. Weaver, J. Phys. Chem. 94 (1990)

6858.[43] P.A. Lund, R.R. Smardzewski, D.E. Tevault, Chem. Phys. Lett. 89

(1982) 508.[44] M. Litorja, C.L. Haynes, A.J. Haes, T.R. Jensen, R.P. Van Duyne, J.

Phys. Chem. 105 (2001) 6907.[45] M.W. Howard, R.P. Cooney, Chem. Phys. Lett. 87 (1982) 299.[46] X.-X. Li, Q.-J. Huang, W.I. Petrov, Y.-T. Xie, Q. Luo, X. Yu, Y.-J.

Yan, J. Raman Spectrosc. 36 (2005) 555.[47] S.W. Joo, S.W. Han, K. Kim, J. Phys. Chem. B. 103 (1999) 10831.[48] A. Ott, I. Mrotzek, H. Grabhorn, W. Akemann, J. Phys. Condens.

Mat. 4 (1992) 1143.[49] E. Koglin, J.-M. Sequaris, Top. Curr. Chem. 134 (1986) 1.[50] C. Murray, D. Allara, J. Chem. Phys. 76 (1982) 1290.[51] R. Venkatachalam, F. Boerio, P. Roth, W. Tsai, J. Polym. Sci. Polym.

Phys. 26 (1988) 2447.[52] M. Moskovits, Rev. Mod. Phys. 57 (1985) 783.[53] R.L. Garrel, Anal. Chem. 61 (1989) 401A.

Influence of aliphatic spacer group on adsorption mechanisms of phosphonate derivatives of l-phenylalanine: Surface-enhanced Raman, Raman, and infrared studiesIntroductionExperimentalPeptide synthesisFT-Raman spectroscopyFT-IR spectroscopySERS spectroscopySilver colloidsMacroscopic silver substrates

Results and discussionFourier-transform Raman and infrared spectroscopiesSurface-enhanced Raman scatteringElectrochemically roughened silver substratesAdsorption mechanism on colloidal silver

ConclusionsAcknowledgementReferences

inﬂuence of aliphatic spacer group on adsorption ...proniewi/pdf/publ154.pdfinﬂuence of...

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