Spectrochimica Acta Part A 60 (2004) 1845–1852

Surface-enhanced Raman scattering of a series ofn-hydroxybenzoicacids (n = P, M and O) on the silver nano-particles

Di Wu, Yan Fang∗

Beijing Key Lab of Nano-Photonics and Nano-Structure (NPNS), Department of Physics, Capital Normal University, Beijing 100037, PR China

Received 20 July 2003; accepted 2 October 2003

Abstract

Surface-enhanced Raman scattering (SERS) spectra of a series ofn-hydroxybenzoic acids (n-HBA, n = P, M and O) adsorbed on the silvernano-particles were studied, respectively, in the silver colloidal solution and on the dried silver-coated filter paper. On the same substrate, thedifferent molecules’ SERS spectra were different, while on the different substrates the same molecules’ SERS spectra were also different.Significant changes were found in the SERS spectra of PHBA molecules adsorbed on the two substrates, and the changes found in MHBA’sspectra on two substrates were next to PHBA’s, while almost no changes were found in the spectra of OHBA molecules. Moreover, it wasfound, on the filter paper, that the SERS spectra of the same molecules would change with the coverage density of the silver nano-particles. Theanalyses showed that the origins of these changes were the different adsorption behavior of molecules adsorbed on the silver nano-particles.Because in these three molecules the relative positions of the carboxyls and hydroxyls on the benzenes are different, the adsorption behaviorsof these three molecules adsorbed on the silver surfaces are also different. The experimental results suggest that the surface characteristic ofthe substrate and the surface configuration of the adsorbate could exert a great influence on the adsorption behavior of the adsorbates on thesubstrates.© 2004 Elsevier B.V. All rights reserved.

Keywords:Adsorption behavior; SERS;n-Hydroxybenzoic acid; Silver nano-particles; Filter paper

1. Introduction

Since the discovery of surface-enhanced Raman scatter-ing (SERS) by Fleischmann et al.[1] in 1974, this tech-nology has shown great advantage in high sensitivity andhigh quenching of the fluorescence. Now, SERS technologyhas been well established for obtaining detailed informa-tion of molecules adsorbed on the surfaces of silver, gold orother noble metals, such as the adsorption configuration ofmolecules and the interaction mechanism of the moleculeswith the surfaces of substrates[2–7], in which many re-searchers are interested.

The vibrational enhancement of molecules adsorbed onthe metal surfaces lies on the adsorption behavior of themolecules. While one of the most important factors thatcould affect the molecules’ adsorption behavior is the sur-faces characteristic of the SERS-active substrates. It is

∗ Corresponding author. Tel.:+86-10-68902965;fax: +86-10-68902965.

E-mail address:talick@tsinghua.org.cn (Y. Fang).

because of more or less differences of the adsorption config-urations that the SERS spectra would have more or less dif-ferences for the same molecules adsorbed on the differencesubstrate. Currently, there are many kinds of SERS-activesubstrates in the wild SERS applications, such as the nega-tively or positively charged silver colloidal solution[8–12],acid-etching silver foil[13], ANOTOP filter filtrated withsilver colloidal solution[14,15], metal island film formedthrough thermal evaporation[16–18], laser ablated silverplate [19,20], and so on. Different surface information ofthe adsorbate can be obtained by using different substrates.The silver-coated filter paper is also a highly SERS-activesubstrate[21,22], which can greatly simplify the experi-mental procedures and is worth studying and developingfurther.

Another important factor that could affect the molecules’adsorption behavior is the surface configuration of themolecules themselves. For allotropes, which are composedof the same elements but whose configurations are notsame, the adsorption behaviors on the same substrate arenot same either.

1386-1425/$ – see front matter © 2004 Elsevier B.V. All rights reserved.doi:10.1016/j.saa.2003.10.001

1846 D. Wu, Y. Fang / Spectrochimica Acta Part A 60 (2004) 1845–1852

In our work, the SERS spectra of a series ofn-hydroxy-benzoic acids (n-HBA, n = P, M and O) adsorbed on thesilver nano-particles were studied, respectively, in the sil-ver colloidal solution and on the dried silver-coated fil-ter paper. On the same substrate, the different molecules’SERS spectra were different. On different substrates, thesame molecules’ SERS spectra were also more or less dif-ferent. Significant changes were found in the SERS spectraof PHBA molecules adsorbed on the two substrates, and thechanges found in MHBA’s spectra on two substrates werenext to PHBA’s, while almost no changes were found in thespectra of OHBA molecules. Moreover, it was found, on thefilter paper, that the SERS spectra of the same moleculeswould change with the silver nano-particles’ coverage den-sity. The analyses showed that these changes arose from thedifferent adsorption behavior of molecules adsorbed on thesilver nano-particles, indicating that the surface character-istic of the substrate and the surface configuration of theadsorbate can exert a great influence on the adsorption be-havior of the adsorbate.

2. Experimental

2.1. Preparation of silver colloid

Silver colloid was prepared according to Lee and Meisel’smethod[23]. Silver nitrate (90 mg) was dissolved in 500 mldeionized water and was rapidly heated to boiling before10 ml of a 1% trisodium citrate aqueous solution was drop-wise added accompanied by vigorously stirring. The mixedsolution was kept boiling for a further 10 min. Then, agreen–gray silver colloidal solution was obtained, whichcould be stable for several days or weeks.

2.2. Preparation of samples for SERS measurement

One milliliter silver colloidal solution was uniformlyadded dropwise onto two layers of quantitative andslow-speed filter papers (∅7 cm), then the papers weredried for about 10 min. Repeating these procedures, a se-ries of filter papers with different coverage density ofsilver nano-particles were obtained. Continuing adding1 ml n-HBA (n = P, M, or O) aqueous solution ontothe silver-coated filter papers and drying them, a seriesof dried samples for SERS measurement were obtained,whose proportions betweenn-HBA molecules and silvernano-particles are different.

2.3. Instrumentation

The Raman spectra were obtained by the RFS 100 s−1

Bruker NIR-FT Raman spectrophotometer. The operatedwavelength is 1064 nm. The resolution was 2 cm−1 and 180◦geometry was employed. The output laser power, whichcould not induce the change of the adsorbate–substrate

system, was 150 mW in the case of filter paper, 200 mWin the case of colloidal solution, and 50 mW in the case ofsolid powder.

3. Results and discussion

The solid FT-Raman spectrum (a), FT-SERS spectra insilver colloidal solution (b), and on the silver-coated filterpaper (c) for PHBA (Fig. 1), MHBA (Fig. 2) and OHBA(Fig. 3) are, respectively, represented inFigs. 1–3. In thesefigures, (c) are the original spectra, while (b) are 5–10 timesof the original spectra. It is obvious that the SERS activity ofthe silver-coated filter paper is superior to that of the silver

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1583

1494

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1167

1165

(c)

(b)

(a)

Ram

an In

tens

ity

Raman Shift / cm -1

Fig. 1. The Raman spectrum of solid PHBA (a), and the SERS spectra ofPHBA in the silver colloidal solution (b) and on the silver-coated filterpaper (c).

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1694

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1039 11

63

(c)

(a)

(b)

Ram

an In

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ity

Raman Shift cm -1

Fig. 2. The Raman spectrum of solid MHBA (a), and the SERS spectraof MHBA in the silver colloidal solution (b) and on the silver-coatedfilter paper (c).

D. Wu, Y. Fang / Spectrochimica Acta Part A 60 (2004) 1845–1852 1847

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1008

(a)

(b)

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Ram

an In

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ity

Raman Shift cm -1

Fig. 3. The Raman spectrum of solid OHBA (a), and the SERS spectra ofOHBA in the silver colloidal solution (b) and on the silver-coated filterpaper (c).

colloidal solution, and the information reflected by (c) is alsomore than that by (b), indicating that the silver-coated filterpaper is a highly SERS-active substrate. Further observingthese spectra, significant changes were found in the SERSspectra of PHBA molecules adsorbed on the two substrates,and the changes found in the spectra of MHBA moleculesadsorbed on two substrates were next to PHBA’s, while al-most no changes were found in the spectra of the OHBAmolecules.Figs. 4–6represent, respectively, the SERS spec-tra of PHBA, MHBA and OHBA molecules adsorbed onthe filter paper with different silver nano-particles coveragedensity. We can find that on the filter paper the SERS spec-tra of the same molecules would change more or less withthe coverage density. All of these changes show that the ad-sorption behavior of molecules changes more or less with

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Fig. 4. The SERS spectra of 1 ml PHBA solution filtrated on the filterpaper coated with 1 ml (a), 2 ml (b), 3 ml (c) and 4 ml (d) silver colloidalsolution, respectively.

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63 (d)

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Fig. 5. The SERS spectra of 1 ml MHBA solution filtrated on the filterpaper coated with 1 ml (a), 2 ml (b), 3 ml (c) and 4 ml (d) silver colloidalsolution, respectively.

the surface characteristic of the substrate and the surfaceconfiguration of the molecules themselves.Table 1summa-rizes the positions, intensities and vibrational assignmentsof the SERS bands forn-HBA (n = P, M and O) in silvercolloidal solution, respectively. The corresponding informa-tion of these three compounds on the silver-coated filter pa-per is summarized inTable 2. The vibrational assignmentsare acquired by comparing these spectra to each other andreferring to[24–34].

3.1. SERS of n-HBA (n= P, M and O) in the silvercolloidal solution

The SERS spectrum of PHBA molecules in the silvercolloidal solution is represented inFig. 1b. At 1612 and

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Fig. 6. The SERS spectra of 1 ml OHBA filtrated on the filter paper coatedwith 1 ml (a), 2 ml (b), 3 ml (c) and 4 ml (d) silver colloidal solution,respectively.

1848 D. Wu, Y. Fang / Spectrochimica Acta Part A 60 (2004) 1845–1852

Table 1The wavenumbers, intensities and band assignments of the SERS bands forn-HBA (n = P, M and O) adsorbed on the silver nano-particles in the silvercolloidal solution

PHBA (cm−1) MHBA (cm−1) OHBA (cm−1) Assignment Modea

3074(w) 3083(mw) 3073(mw) ν(C–H) 20a1612(s) 1608(s) 1623(m) ν(CC) 8a1600(ms) 1576(w) ν(CC) 8b1518(w) 1519(w) ν(CC) 19a

1481(w) ν(CC) 19b1381(m) 1390(m) 1384(m) νs(COO−)

1309(m) ν(CC) 141278(w) 1283(w) 1250(ms) ν(C–O)

1238(mw) β(O–H)1144(ms) 1136(mw) 1145(m) β(C–H) 18b

1096(w) β(C–H) 151035(ms) β(C–H) 18a

948(vw) 1001(s) 1005(w) Ring breathing 12860(vs) 841(mw) 865(w) ν(C–COO−)

799(mw) 810(vs) γ(C–H) 11632(w) Ring breathing 6b414(w) 567(w) Ring breathing 6a

382(w) 409(w) Ring out-of-plane skeletal vibration 16b

Note: s, strong; w, weak; m, medium; v, very;ν, stretching;νs, symmetric stretching;β, in-plane bending;γ, out-of-plane bending.a Wilson vibration number.

1600 cm−1, twoν(CC) bands are enhanced greatly. But thereare no shifts of these two bands compared with the cor-responding bands in the Raman spectrum of solid PHBA(Fig. 1a), indicating that the interaction between the benzenering of PHBA molecules and the silver nano-particles is stilla physical interaction. The relative weakness of theν(C–O)band at 1278 cm−1 indicates that the distances between thehydroxyls and the surfaces of silver nano-particles are rel-atively far, while the appearance of theνs (COO−) band at

Table 2The wavenumbers, intensities and band assignments of the SERS bands forn-HBA (n = P, M and O) molecules adsorbed on the silver nano-particlescoated on the filter paper

PHBA (cm−1) MHBA (cm−1) OHBA (cm−1) Assignment Modea

3066(vw) 3064(mw) 3063(m) ν(C–H) 20a1606(s) 1617(m) ν(CC) 8a

1583(s) 1580(w) ν(CC) 8b1494(ms) 1511(w) ν(CC) 19a1466(mw) 1479(w) ν(CC) 19b1353(m) 1388(m) 1379(m) νs(COO−)

1309(m) ν(CC) 141278(w) 1249(ms) ν(C–O)

1239(s) 1239(mw) β(O–H)1165(m) β(C–H) 9a

1163(w) β(C–H) 9b1140(m) 1135(w) 1144(ms) β(C–H) 18b

1094(w) 151039(w) 1033(ms) β(C–H) 18a

1005(w) 1002(vs) 1008(w) Ring breathing 12857(w) 854(w) 863(vw) ν(C–COO−)

794(m) 808(s) γ(C–H) 11807(m) γ(C–H) 10a632(m) Ring breathing 6b421(ms) 554(w) 569(w) Ring breathing 6a

384(w) 405(w) Ring out-of-plane skeletal vibration 16b

Note: s, strong; w, weak; m, medium; v, very;ν, stretching;νs, symmetric stretching;β, in-plane bending;γ, out-of-plane bending.a Wilson vibration number.

1381 cm−1 and the great enhancement of theν(C–COO−)band at 860 cm−1 indicate that PHBA molecules adsorbon the surfaces of the silver nano-particles through theircarboxyls. According to the SERS selection rule[35,36],vibrations deriving their intensities from a large value ofαzz (z being the local surface normal), would be the mostintense in the SERS spectrum. In particular, the adsorp-tion configuration of compounds which have a planar struc-ture can be determined from the relative magnitude of the

D. Wu, Y. Fang / Spectrochimica Acta Part A 60 (2004) 1845–1852 1849

O O-

OH

O

O-OH

(a) (b)

Fig. 7. Two most possible adsorption configurations of PHBA on the silver nano-particles.

O O-

OH

O

-O OH

(a) (b)

Fig. 8. Two most possible adsorption configurations of MHBA on the silver nano-particles.

intensity of theν(C–H) bands in their SERS spectra. Theappearance ofν(C–H) band at 3074 cm−1 and the great en-hancement ofβ(C–H) band at 1144 cm−1 and two bandsassigned to carboxyl vibrations at 1381 and 860 cm−1 men-tioned above indicate that in the silver colloidal solutionPHBA molecules stand perpendicularly on the surfaces ofthe silver nano-particles through their carboxyls, as is shownin Fig. 7a.

The SERS spectrum of MHBA molecules in the silvercolloidal solution is shown inFig. 2b. The appearance ofνs(COO−) band at 1390 cm−1 indicates that the interactionbetween the carboxyls of MHBA molecules and the sur-faces of silver nano-particles are chemical interaction. Butthe weakness of theν(C–COO−) band at 841 cm−1 sug-gests that this bond is not perpendicular to the surface of sil-ver. The simultaneous appearance ofν(C–O) band at 1283and theβ(O–H) band at 1238 cm−1 suggest that the interac-tions between the hydroxyls and the silver surfaces are alsochemical interaction. At 1608 and 1001 cm−1, two strongpeaks appear, which are respectively assigned toν(CC) ofbenzene ring and ring breathing vibration. According to theSERS selection rule mentioned above, the surfaces of ben-zene rings are most probably perpendicular or near perpen-dicular to the silver surfaces. So, the interaction betweenMHBA molecules and the silver nano-particles is most prob-ably through both the carboxyls and the hydroxyls, and thesurfaces of the benzenes are perpendicular or near perpen-dicular to the surfaces of silver nano-particles, as is shownin Fig. 8b.

The SERS spectrum of OHBA molecules in the silvercolloidal solution is shown inFig. 3b. The appearance ofνs(COO−) band at 1384 cm−1 indicate that the interactionbetween the carboxyls of OHBA molecules and the sur-faces of the silver nano-particles are still chemical inter-action. The weak or middle strong peaks in the range of1623–1481 cm−1 are all assigned toν(CC) of benzene. Theband at 1250 cm−1 is assigned toν(C–O). The three bandsat 1145, 1096 and 1034 cm−1 are all assigned toβ(C–H).The shoulder peak at 3073 cm−1 is assigned toν(C–H). Thestrong peak at 810 cm−1 is assigned toγ(C–H). The simulta-neous appearances of these bands assigned to both in-planeand out-of-plane vibrations of C–H band suggest that OHBAmolecules are probably tilted on the surfaces of the silvernano-particles through their carboxyls and the orientations ofOHBA molecules are probably various, as is shown inFig. 9.

O

O-

OH

Fig. 9. The most possible adsorption configuration of OHBA on the silvernano-particles.

1850 D. Wu, Y. Fang / Spectrochimica Acta Part A 60 (2004) 1845–1852

3.2. SERS of n-HBA (n= P, M and O) on the silver-coatedfilter paper

3.2.1. SERS of PHBA on the silver-coated filter paperThe SERS spectrum of PHBA molecules adsorbed on the

surfaces of the silver nano-particles coated on the filter pa-per is shown inFig. 1c. It can be seen that this spectrumis very different from that of PHBA molecules in the sil-ver colloidal solution (Fig. 1b). In Fig. 1c, there is only onestrong band at 1583 cm−1 which is assigned to theν(CC)of benzenes. Moreover, there is significant red shift of thisband compared with the 1600 cm−1 band inFig. 1a, indicat-ing that the average energy of benzene ring is relatively lowin the case of filter paper. So, the benzene rings of PHBAmolecules most probably chemisorb on the surfaces of thesilver nano-particles. Theνs(COO−) band at 1353 cm−1 isalso moderately enhanced, and significant red shift is alsofound by comparing it with the 1381 cm−1 band inFig. 1b.While the ν(C–COO−) band is found to be very weak at857 cm−1 in Fig. 1c, it is strongly enhanced at 860 cm−1 inFig. 1b, indicating that on the filter paper the interactionsbetween the carboxyls of PHBA molecules and the surfacesof silver nano-particles are still chemical interactions, butthe orientations of PHBA molecules on the surfaces of sil-ver nano-particles change greatly and the C–COO− bondis probably not perpendicular to the surfaces of the silvernano-particles. Moreover, some bands assigned to benzenevibrations are all strongly enhanced, such asβ(C–H) bandat 1165 and 1140 cm−1, γ(C–H) band at 807 cm−1, benzenering breathing bands at 632 and 421 cm−1, while in Fig. 1bthese bands are all very weak or are not found, indicatingthat the distances between the benzenes of PHBA moleculesand the silver surfaces are shortened.ν(C–H) band near3000 cm−1 is not found, whileβ(O–H) band at 1239 cm−1

are strongly enhanced, indicating that PHBA molecules mostprobably lie flat on the surfaces of the silver nano-particlesand the surfaces of the benzenes are parallel or near par-allel to the silver surfaces, as is shown inFig. 7b. This isin accordance with the significant red shifts of 1583 and1353 cm−1 bands, which mean that the average energy ofsystem is lowered.

The SERS spectra of 1 ml PHBA solution filtrated on thefilter papers coated with 1–4 ml silver colloidal solution are,respectively, represented inFig. 4a–d. From these spectra,it can be seen that the more the silver nano-particles arecoated onto the filter paper, the stronger the intensities ofmost bands are. It is interesting that there are two excep-tions at 857 and 806 cm−1. When the silver colloidal solu-tion is 1 ml (Fig. 4a), 857 cm−1 band appears firstly; whenthe solution increases to 2 ml (Fig. 4b), 806 cm−1 band ap-pears but is still weaker than 857 cm−1 band; when the so-lution is up to 3 ml (Fig. 4c), 806 cm−1 band is strongerthan 857 cm−1 band; while, when the solution is up to 4 ml(Fig. 4d), 806 cm−1 band has greatly exceeded 857 cm−1

band. It is noteworthy thatνs(COO−) band at 1352 cm−1 ap-pears with the appearance ofν(C–COO−) band at 857 cm−1,

and is enhanced with the enhancement of 857 cm−1 band.While ν(C–O) band at 1241 cm−1 changes with the corre-sponding changes ofγ(C–H) band at 806 cm−1.

The reasons for these changes are probably that when thesilver nano-particles are relatively few, the lack of the ad-sorptive sites results in the adsorptive competition betweenthe hydroxyls and the carboxyls of PHBA molecules. As aconsequence of competition, PHBA molecules stand perpen-dicularly on the surfaces of the silver nano-particles throughtheir carboxyls (Fig. 7a) because of its relatively stabiliza-tion and easiness. In this case, 1352 and 857 cm−1 bandsassigned to the carboxylic vibrations are enhanced greatly,while the bands assigned to hydroxy vibrations and C–Hout-of-plane bending are relatively weak. However, when thesilver nano-particles increase, there are enough adsorptivesites on the surfaces of substrate. So, the PHBA moleculescan easily lie flat on the surfaces of the silver nano-particles,and the benzene rings are parallel or near parallel to the sil-ver surfaces (Fig. 7b). In this case, the vibrations of the car-boxyls, the hydroxyls and the benzene rings are all enhancedgreatly. From these spectra, it can be seen that the latter ad-sorption configuration is the main configuration of PHBAmolecules adsorbed on the silver nano-particles coated onthe filter paper.

3.2.2. SERS of MHBA on the silver-coated filter paperThe SERS spectrum of MHBA molecules adsorbed on

the silver nano-particles coated on the filter paper is repre-sented inFig. 2c. It can be seen that this spectrum is verysimilar to the SERS spectrum of MHBA molecules in thesilver colloidal solution (Fig. 2b). From the analyses above,we can deduce that on the silver-coated filter paper MHBAmolecules also adsorb on the surfaces of silver nano-particlesthrough both the carboxyls and the hydroxyls, and the planesof the benzene rings are perpendicular or near perpendicularto the surfaces of the silver nano-particles.

The SERS spectra of 1 ml MHBA solution filtrated on thefilter paper coated with 1–4 ml silver colloidal solution are,respectively, represented inFig. 5a–d. It is obvious that onthe silver-coated filter paper the changes in the SERS spectraof MHBA molecules with the increase in the coverage den-sity of the silver nano-particles are more significant than thatof PHBA. Of these changes, the most obvious change is thatof ν(CC) band of benzene at 1606 cm−1, which is greatlyenhanced with the increase in the silver coverage density, in-dicating that the distances between benzenes and the silversurfaces are shortened. The next most obvious changes arethe changes ofνs(COO−) band at 1388 cm−1 and benzenering breathing band at 1002 cm−1. With the increase in thesilver coverage density, 1388 cm−1 band changes from thestrongest band, relatively, to the middle strong band, while1001 cm−1 band changes from the middle strong band tothe strongest band. These suggest that the angle formed bythe carboxyls and the silver surfaces changes with the sil-ver coverage density, and the distances between the benzenerings and the silver surfaces are shortened.

D. Wu, Y. Fang / Spectrochimica Acta Part A 60 (2004) 1845–1852 1851

Additionally, there are two group of weak peaks whichchange with the silver coverage density. One group isν(C–O) band at 1278 cm−1 andβ(O–H) band at 1239 cm−1.With the increase in the silver coverage density, the formerband is weakened but the later band is strengthened, indi-cating that the average angle formed by the hydroxyls andthe silver surfaces changes. Another group isν(C–COO−)band at 854 cm−1, which is weakened by the increase inthe silver density, andγ(C–H) band at 794 cm−1, whichis strengthened by the increasing density, indicating thatthe average angle formed by the carboxyls and the silversurfaces also changes and the average distance between thebenzene rings and the silver surfaces is shortened.

So, the adsorptive configuration of MHBA molecules ad-sorbed on the surfaces of silver nano-particles changes withthe silver coverage density. When the silver nano-particlesare relatively few, the SERS-active sites are not adequate,indicating that MHBA molecules stand perpendicularly onthe surfaces of the silver nano-particles through the car-boxyls. When the amount of silver nano-particles increases,the SERS-active sites also increase. For lowering the systemenergy effectively, the adsorption configuration of MHBAmolecules gradually transits through only the carboxyls tothrough both the carboxyls and the hydroxyls, while the ben-zene ring planes are always perpendicular or near perpen-dicular to the surfaces of the silver nano-particles. From thechanges ofFig. 5, it can be seen that on the silver-coatedfilter paper these two adsorption configurations of MHBAmolecules adsorbed on the surfaces of silver nano-particlesare both main configurations.

3.2.3. SERS of OHBA on the silver-coated filter paperThe SERS spectrum of OHBA molecules adsorbed on the

silver nano-particles coated on the filter paper is representedin Fig. 3c. It is obvious that this spectrum is also very simi-lar to the SERS spectrum of OHBA molecules in the silvercolloidal solution (Fig. 3b), indicating that the adsorptionconfigurations of MHBA molecules on two substrates aresame or similar. From the analyses before, we can judgethat on the silver-coated filter paper OHBA molecules areprobably tilted on the surfaces of the silver nano-particlesthrough their carboxyls and also there are several orienta-tions of OHBA molecules.

The SERS spectra of 1 ml OHBA solution filtrated on thefilter paper coated with 1–4 ml silver colloidal solution arerespectively represented inFig. 6a–d. From these spectra,it can be seen that the relative positions and intensities ofOHBA molecules’ Raman bands do not change with theincrease in the silver coverage density on the filter paper. Theintensities of all the bands simply increase with the increasein the silver coverage density, indicating that there is almostno change in adsorption configurations of OHBA moleculeswith the change in the silver coverage density. It is only thatthe increase in the SERS-active sites with the increase in thesilver nano-particles can result in more adsorption of OHBAmolecules on the surfaces of the silver nano-particles.

4. Conclusions

The adsorption behavior of the adsorbate on the substratewould be greatly affected by the surface characteristic of thesubstrate and the surface configuration of the adsorbate.

On different substrates, the adsorption behaviors of thesame adsorbates are different. For example, in the silver col-loidal solution PHBA molecules stand perpendicularly onthe surfaces of the silver nano-particles through their car-boxyls, while on the silver-coated filter paper they lie flat onthe silver surfaces. When the amount of silver nano-particleschanges from relatively few to relatively rich, the orien-tations of PHBA molecules would gradually transit fromstanding perpendicularly (Fig. 7a) to lying flat (Fig. 7b) onthe silver surfaces. For MHBA molecules, though the lastadsorption behavior on the silver-coated filter paper is sameto that in the silver colloidal solution, the orientations ofMHBA molecules on the silver still transit with the increas-ing in the silver coverage density on the filter paper throughonly the carboxyls (Fig. 8a) to through both the carboxylsand the hydroxyls (Fig. 8b). Studying the changes of theSERS spectra for the same adsorbate on different substratesand further finding the differences of all adsorption behav-iors can reflect the different characteristics of the surfaceconfiguration of the adsorbate from different angles, whichprovide a powerful technology for further studying the sur-face configuration of molecules.

For the same substrate, though the composition of differ-ent adsorbate is same, if the surface configurations of ad-sorbates are different the adsorption behaviors are generallydifferent. PHBA, MHBA and OHBA are three allotropesof C7H6O3, which is composed of the same elements butwhose surface configurations are different. In the silver col-loidal solution, PHBA molecules stand perpendicularly onthe surfaces of the silver nano-particles through their car-boxyls, MHBA molecules stand by side on the silver sur-faces through their carboxyls and hydroxyls, while OHBAmolecules are tilted on the silver surfaces through their car-boxyls. Considering the surface configuration of these threeallotropes, these differences of the adsorption configurationsare reasonable. For PHBA molecule, the carboxyl and thehydroxyl are opposite on the benzene ring, and the distancesbetween them are relatively far. In the silver solution, ifPHBA molecules lie flat on the silver surfaces, the energyof the adsorbate–substrate system cannot overcome the dis-turbance of the water molecules’ striking. Since standing onthe silver through the ionized carboxyls is more stable thanstanding through hydroxyls, so most PHBA molecules standperpendicularly on the silver surfaces only through the car-boxyls. For MHBA molecules, the carboxyl and hydroxylare separated by one position of the benzene ring. Stand-ing on the silver surfaces through carboxyls and hydroxyls,MHBA molecules not only can overcome the disturbanceof water but also can lower the system energy effectively.So, MHBA molecules stand by side on the silver surfacesthrough their carboxyls and hydroxyls. For OHBA molecule,

1852 D. Wu, Y. Fang / Spectrochimica Acta Part A 60 (2004) 1845–1852

the carboxyl and the hydroxyl are adjacent on the benzenering, and the distances between them are relatively near, in-dicating that OHBA molecules cannot stand through them.So, OHBA molecules can only tilt on silver surfaces throughtheir ionized carboxyls. On the dried filter paper coated withsilver nano-particles, the disturbance energy of the circum-stance is lowered, so PHBA molecules can easily lie flat onthe silver surfaces. The MHBA and OHBA molecules, be-cause of the surface configurations themselves, still adsorbon silver with the same adsorption configurations as that inthe silver colloidal solution. By studying the changes of theSERS spectra of allotropes, the adsorption behavior of theadsorbate on the substrate can be further affirmed.

Acknowledgements

The authors are grateful for the support to this researchby the National Natural Science Foundation of China andthe Natural Science Foundation of Beijing.

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