2930 Bull.KoreanChem. Soc. 2009, Vol. 30, No. 12 DOI 10.5012/bkcs.2009.30.12.2930
Mak Soon Kimet al.
Surface-enhanced Raman Spectroscopy of Benzimidazolic Fungicides:
Benzimidazole and Thiabendazole
Mak Soon Kim, Min Kyung Kim, Chul JaeLee： YoungMee Jung,* and Mu Sang Lee*
Department of Chemistry Education,KyungpookNational University, Daegu702-701, Korea. *E-mail:mslee@knu.ac.kr
‘Division of Chemical Industry, YeungnamCollege of Science & Technology, Daegu 705-703, Korea
* Department of Chemistry,Kangwon National University, Chunchon 200-701,Korea Received September2, 2009, Accepted October 7, 2009
Surface-enhanced Raman Scattering (SERS) spectroscopy is applied to the study of the adsorption of benzoimidazolic fungicides benzimidazole (BIZ) and thiabendazole (TBZ) on silver mirrors. The influence of pH on the adsorption mechanism was investigated. In case of BIZ, two different adsorption mechanisms are deduced depending on the experimental conditions: via the n electrons of the ring in neutral conditions and through an ionic pairing of protonated nitrogen atom with the chloride adsorbed on the metal surface. The SERS spectra of TBZ revealed that most molecules were adsorbed on silver surface by the n electrons in neutral and acidic conditions but in acid conditions, some molecules were adsorbed via the sulfur and nitrogen atoms tilted slightly to the surface.
KeyWords: Surface-enhanced Ramanspectroscopy,Fungicide, Benzimidazole, Thiabendazole
Introduction
Thiabendazole (TBZ, 4-(1H-1,3-benzodiazol-2-yl)-1,3-thi- azole) which belongs to the benzimidazole derivative, is a fungicide and parasiticide. It is known by the brandnames Min- tezol,Tresaderm,andAcrobetec. It isused primarily tocontrol mold,blight, and otherfungally caused diseases infruits(e.g.
oranges) and vegetables. TBZ is alsoused as a foodadditive, a preservative. For example, it isappliedto bananas toensure freshness, andis acommon ingredientin the waxesapplied to the skin of citrus fruits. Medicinally, thiabendazole is also a chelating agent, whichmeans that it is used medicinally to bind metalsincase ofmetalpoisoning or antimonypoisoning.As anantiparasitic,it is able tocontrol roundworms,hookworms, andother helminth species which attack wild animals, live
stockand humans.1-3 Benzimidazole (BIZ, 1H-benzimidazole) also is known for commercialand biological importance as phar maceuticals and fungicides. Despiteitsextensive using in the fields indicated above, itissurprisingthe fact thatthere is non completevibrationalassignment for themoleculeinthe scien
tificliteratures. The onlyvibrationalspectroscopicstudies are limited to a briefreport of theinfraredspectrum.4-8
Surface-enhanced Raman spectroscopy (SERS) is a very use
ful techniquein theanalysis of theadsorption of molecules on metal surfaces. The enhancement of the Raman signalinduced by metal nanoparticles, which is the basis of SERS spectro scopy,canbe understoodas a contributionfromtwodifferent mechanisms: electromagnetic andcharge-transfer mechani- sms.9-12In theelectromagneticmechanism,the intensity of the surface plasmonresonance of themetal dependsonmany fac
tors includingthe wavelength of the incident light and the mor phology of themetalsurface.10 The roleof these nanoparticles is correlated with themechanisminvolvedin the SERS enhan cement, beingessentialthe formation of surfacenanoparticles toinduceahuge intensification of Raman emission through localizedplasmonresonance in orderto obtain highquality
SERS spectra.13-16Recently, Saito etal.17haveshownthatthe usingof silver mirror reaction produced a thinsilverislands film on siliconwafer,which shows an order of magnitudesuperior surfaceenhancement propertywhen compared to a conventional SERS substratemade by vacuum evaporation. Ourgroup show ed that the silver mirrorsubstrateshave an excellent surface enhancement on theRamansignal.18,19
Therefore,in this paper, westudiedthe adsorptiongeometry ofBIZand TBZ onsilver mirrors by SERS technique. The SERS study was done by testingthe influence of pH. In orderto assist in the interpretation of theRaman spectra, we haveobtained pre liminaryassignments bymeans of theprogram HyperChem®
(release7.0). PM3Hamiltonian wasused to calculatethe Raman frequencies of BIZ and TBZinthis program.
ExperimentalSection
Analytical-reagentgrade BIZ, TBZ, silvernitrate, chloric acid, sodium hydroxide,d-glucose and ammonia water were purchased fromAldrichCo. and used without further purifi cation. Allsolutionswere prepared with triplydistilled water and methanol. Silver mirror substrate was prepared by the Tollen's method,which is widely used for theidentification of aldehyde.20 A glass plateof25 mm x 10mm x 1 mm was put in aculturedish. A 10mL of 5%-silver ammonia solution and 5 mL of 10%-sodiumhydroxide solutionweremixed in the culturedish. Then, 10mL of 5%-D-glucosewasadded tothe mixing solution. Afew minutes later,thecolor of thesolution turnedto yellow, then darkbrown. In the meanwhile, silver ionswerereduced anddeposited onto theglassplate toform a fine silverfilmcalled a silver mirror. Afterwithdrawing the silver mirror from a culturedish, itwas washed with distilled waterand dried in room temperature. To make use of the preparedmirror, a 1gL of 1.0 x 10-3MBIZorTBZ solution wasdropped onto it. After the evaporation of thesolvents,the substratewas ready for SERS analysis. The pH of thesolutions
wasadjustedby injecting the necessary amounts of reagent grade HCl and NaOH solutions.
Raman and SERS spectra were recordedon a NicoletAlmega XR spectrophotometer in combination with a Thermo Scientific DXR Ramanmicroscopeby using radiation at 780 nm from diode laser for excitation. The laser power and confocalpinhole size were 100 mW and 25 pm, respectively. The images of the Scanning electron microscope (SEM) were obtained with a HitachiS-4200.
Results and Discussion
As shown in Figure 1, themolecules of BIZ and TBZare planar structures. Generally, the lone-pair electron and aromatic n systemwere easilybound to a metal surface. Hence, weassu med that BIZ and TBZcouldbeadsorbedon metalsurface by thesulfur,thenitrogen atom or the n electrons of the ring. The SEM image of silver mirror substrate prepared by Tollen's methodis showninFigure2.
Figure 3 shows the surface-enhancedRaman spectrum of BIZ adsorbedonsilvermirror(a)andthe ordinary Raman spec trum of BIZ insolid state(b)and 0.1 M solution(c). Itis obvious that there are great-enhancedeffects intheSER spectrum of BIZ on the roughened silver surface. The strongerRaman bands at 553,782 and 1012 cm-1 in SERSare assignedto 8 (N-H) oop, 8(C-H)oop and 8 (C-C-C)oop, respectively. The weaker band at 1279 cm-1 isassignedVnng and 8(C-H)ip. Theband frequencies and their assignmentsbased onliterature and cal culationaregiven in Table 1.21.22 To performthe PM3 calcula tions, we focused on thevibrationalmodes of BIZ. Figure 4 presents a diagram of thenormal mode frequencyof553,782,
1012 and 1363 cm-1. Asis wellknown thatif the adsorption occursviadonation of n-system electrons, the plane of the aro
matic ring would lie parallel to thesurface and thetotallysym metricin-plane stretchingmodeswouldbeexpected to show little orno surface enhancement, andcertain out-of-plane vibra tions would then show enhancement as per allowanceof the symmetry. This line of approach hasbeen successfullyapplied by many investigators.23,24Besides,theC-Hstretchingband at 3072 cm-1in ordinaryRaman spectrum of BIZ becameweaker in SERS remarkably. Therefore, it is thought that BIZ is adsorb ed on tothe silver surface with its molecular planelying flat, asisshowninFigure 6a.
In Figure 5, theSER spectraof BIZatpH 2.0, 7.0and 12.0 areshown in the wavenumber range200 ~ 1800 cm-1 and nearby 3000 cm-1. The spectralpattern of theacidicsolutionspectrum differs from that of the neutral and basic solutions. The signi ficantchangein acidic condition is theintensity increase of the 628,942, and1174 cm-1modes. Also,the 1353, 1513 and
1624 cm-1modes areenhanced. The discussion aboutthe orien tation of the BIZ moleculesto thesilversurface has been made by applyingthe results of "surface selectionrules" in thein plane andout-of-planevibrations.Therefore,accordingto these observationsmentionedabove in Figure 5, itis thoughtthat BIZ is adsorbed on tothesilversurfacevia thenitrogenatom withits molecular plane standing perpendicularly.These ad sorptionsare possibleby the formation ofa strong ionic pair a nitrogen atom with theanion(in this casechloride) previously
Figure 1. Chemical structure of the fungicides: (a) benzimidazole and (b) thiabendazole.
Figure 2. The SEM image of silver mirror.
500 1000 1500 2000 2500 3000
Wavenumber (cm-1)
Figure 3. (a) SERS of BIZ (1.0 乂 10-3 M) and ordinary Raman spectra of (b) solid state and (c) 0.1 M BIZ solution in methanol.
adsorbed onto themetal, asalsoobserved for other cationic species.25-27 The interactionof theprotonated N atom of BIZ with Cl- seemstobe so strong thata CT occurs between the chloride and BIZ in acidic conditions.28 When the H2SO4 or HNO3 solutionwasusedinstead of HCl,the SERS spectrum was notenhanced. But the SERS spectrum of the BIZ addedto the H2SO4 and KCl solutionwas similar to Figure 5(a). These experimentalresults supporttheformation of a strong ionic pair. Theenhancement of the bandat 3072 cm-1(Figure 5.inset
2932 Bull.KoreanChem. Soc. 2009, Vol. 30, No. 12 Mak Soon Kimet al.
Table 1. Wavenumbers (cm-1) and assignments corresponding to the calculated and experimental spectra of BIZ.
Calcul
ation
aSolid Raman
aSERS (pH = 7)
aSERS
(pH = 2) "Assignment
425 424(w) 427(vw) - 8(C-C-C)ip
556 549(m) 553(m) 550(m) 8(N-H)oop, 8(N-C-N)oop 611 628(m) 632(m) 628(m) 8(C-C-C)ip
771 782(vs) 782(vs) 780(vs) 8(C-H)oop
951 - - 942(m) C-C ring breathing
956 962(w) 958(vw) - 8(C-H)oop
1000 1008(s) 1012(s) 1012(s) 8(C-C-C)oop 1151 1142(m) 1138(w) - 8(C-H)ip
1151 - - 1174(m) 8(C-H)ip
1291 1276(s) 1279(m) 1283(s) Vring + 8(C-H)ip 1291 1309(sh) 1309(sh) - 8(C-H)ip 1408 1368(m) 1363(m) 1353(s) v(C=C) 1425 1422(m) 1422(m) - v(C=C) 1495 1484(sh) - 1451(w) 8(N-H)ip 1495 1501(w) 1501(w) 1513(w) v(C=C) 1607 1593(w) 1597(w) - v(C=C) 1639 1622(vw) 1627(vw) 1624(w) v(C=N) 3080 3072(w) 3072(vw) - v(C-H)
3084 3118(vw) - - v(N-H)
as= strong;m =medium; w = weak; vw = very; vs = very strong;sh= shoulder; bv = stretching; 8 = bending; ip=in plane; oop = out of plane.
(n
쯔•
A_su
오 드
500 1000 1500 2000 2500 3000
Wavenumber (cm-1)
Figure 5. SERS spectra of 1.0 x 10-3 M benzimidazole on silver mirror at (a) pH = 2.0, (b) pH = 7.0, and (c) pH = 12.0.
Figune 6. The plausible adsorption orientation of BIZ onto the silver mirror surface in (a) neutral and (b) acidic conditions.
(c) ? (d)
(n
쯔•
A_su
오 -
500 1000 1500 2000 2500 3000
Figure 4. Vibrational modes of (a) 553, (b) 782, (c) 1012 and (d) 1363 cm-1 for BIZ. The arrows represent the relative displacement of the nuclei for that specific vibrational mode.
Wavenumber (cm-1)
Figure 7. (a) SERS of TBZ (1.0 乂 10-3 M) and ordinary Raman spectra of (b) solid state and (c) 0.1 M TBZ solution in methanol.
spectra), which is assigned to the stretching mode of C-H, further supports the adsorption orientation as shown in Figure6b. The SERS spectrum ofBIZrevealed little enhancementat pH =12.0 as shown inFigure 5c, since the adsorption of moleculesis supposedto be hinderedby hydroxide ions previouslyadsorbed on to metal.It would rathersimilartothe ordinary Ramanspec trum of 0.1MBIZsolutions. In Figure 6, theplausibleadsorp
tion orientationsof BIZ ontothesilvermirror surfacein neutral and acidicconditionswerepresented.
Figure7 showsthesurface-enhancedRaman spectrumof TBZ adsorbed onsilvermirror(a) and the ordinary Raman spec trum of TBZ in solid state (b) and0.1 M solution (c). Theordi nary Ramanspectrum of TBZsolution alsocannotbeobtained as thesame as BIZ.Themost significantchange in SER spec-
(a) Table 2. Wavenumbers (cm-1) and assignments corresponding to the calculated and experimental spectra of TBZ.
Calcul
ation
aSolid Raman
aSERS (pH=7)
aSERS
(pH=3) bAssignment
298 298(w) - - 8(C-S-C)ip
578 - 575(w) 573(w) 8(C-C-C)ip
635 641(w) 632(w) 641(w) 8(C-C-C)ip, 8(S-C-N)ip, 771 786(m) 786(s) 781(s) 8(C-H)oop
839 - 891(m) 889(m) 8(C-C-S)oop, 8(C-H)oop 988 993(sh) 992(sh) 987(m) v(C-S)
1010 1016(m) 1018(s) 1015(s) 8(C-H)oop
1194 - 1203(w) 1203(w) 8(C-H)ip
1297 1282(s) 1282(m) 1282(w) "ring
1315 1313(w) v(C=C)
1362 1360(w) v(C=C)
1434 1415(vw) 1432(w) "ring
1451 1462(m) 1458(vw) 1470(w) v(C=N) 1584 1585(vs) 1585(s) 1601(m) "ring + "(C=N) 1639 1629(sh) 1631(m) "(C=N)
3077 3086(vw) "(C-H)
as= strong;m =medium; w = weak; vw = very; vs = very strong;sh= shoulder; bv = stretching; 8 = bending; ip=in plane; oop = out of plane.
(c)
trumisthe strongintensity decrease of theband 1585, 1462 and 1282cm-1. These are attributedto totally ring stretching vibrations as alsoconfirmedbycomparison with assignments made for similarstructuralpesticides.22,29 While, the strong bands of intensityappearingat 786,1018 and 891cm-1 in the SERS spectrum may beassigned to C-H out-of planebending modes and C-C-Sout-of-plane bending mode, respectively. The band frequenciesand their assignments based onliteratureand calculation are giveninTable 2.21,30 Asshown in Figure 1, TBZ has delocalized n electrons in themolecule of planarstructure, sincethe benzimidazole and thiazolerings are not allowed to rotate. Therefore, according to the SERS selectionrule, the strong intensities ofthese out-of-planevibration modes in the SERS suggest the parallel orientation of TBZ molecules to the silver surface as shown in Figure 10a. Figure8presentsdia grams of the normal mode vibration of786, 891, 1018 and 1585 cm-1 which areremarkablepeaks in the SERS.
In other tocomparethe adsorptiongeometryaccordingto the pH,we obtainedtheSER spectra ofTBZ at pH2.0, 7.0 and 12.0 asshown inFigure 9. The spectralpatterns of TBZ in acidic andneutral solutions weresimilar.Only the bands of 987 and
1631cm-1 in theacidicSERSareenhanced a little which may be assignedtov(C-S) and v(C=N), respectively. Also, the C=C and C=N stretchingmodes aresplitted slightly in 1200〜1500 cm-1 region. Therefore,most moleculeswerealsoadsorbedon surface viathe delocalized n electrons in planarrings and some moleculeswereadsorbed via a sulfur andnitrogenatomstilted slightly to the silver surface, in acidic solutions. The enhance
mentof the bandat3086cm-1(Figure9. inset spectra), which is assigned to thestretching mode of C-H,further supportsthis adsorptionorientation The plausibleadsorptionorientations of BTZ inneutral and acidicsolutionswere presented inFigure
Figure 8. Vibrational modes of (a) 786, (b) 891, (c) 1018 and (d) 1585 cm-1 for TBZ. The arrows represent the relative displacement of the nuclei for that specific vibrational mode.
(d)
500 1000 1500 2000 2500 3000
Wavenumber (cm-1)
(a)
Figure 9. SERS spectra of 1.0 x 10-3 M TBZ on silver mirror at (a) pH = 2.0, (b) pH = 7.0, and (c) pH = 12.0.
Figure 10. The plausible adsorption orientation of TBZ onto the silver mirror surface in neutral and acidic conditions.
10. TheSERSspectrum of the basic solution of TBZcouldnot beenhancedasthe same reasonas BIZ.
Conclusion
SERSspectroscopy allowsfor the achievement of vibrational spectra of 1.0 x 10-3 MBIZ and TBZfungicidesolutions. In SERS spectroscopy, there isno necessary preprocessing of
2934 Bull.KoreanChem. Soc. 2009, Vol. 30, No. 12 Mak Soon Kimet al.
samplesolutions,thereforethis techniquecouldbeemployed for the detection of such fungicides in theenvironments.We havemeasured the SER spectraof BIZ and TBZadsorbed on silver mirror substrates. In case of BIZ, completelydifferent adsorption mechanisms are possible depending on the pH conditions.In neutral conditions,the adsorption of BIZoccurs via delocalized n electrons parallel to thesurface andin acidic conditions,the interaction of the protonated N atom with chlo ride ion lead to perpendicular adsorption tothesurface. The adsorptions of TBZ havesimilar mechanisms. Most adsorptions occurparallel tothesurfacevia thedelocalized n electronsin planar rings but some adsorptions are slightly tilted viathe sulfur and nitrogenatomsin acidicconditions. In basic condi
tions,the SERspectra of both of BIZ and TBZ couldnot be obtainedsincetheadsorption of moleculesis supposed to be hinderedbyhydroxide ions.
Acknowledgments. This workwassupported by Korea Re searchFoundation GrantNo.KRF-2006-005-j02401 and by KyungpookNational University Research Fund,2007(KRF- 2004-005-c-00006).
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