Surface-enhanced Raman Scattering of Af l atoxin B1on Silver by DFT Method

Si-min Gao,Hong-yan Wang,Yue-xia Lin

School of Physical Science and Technology,Southwest Jiao tong University,Chengdu 610031,China

(Dated:Received on January 23,2013;Accepted on September 23,2013)

Surface-enhanced Raman Scattering of Af l atoxin B1on Silver by DFT Method

Si-min Gao,Hong-yan Wang?,Yue-xia Lin

School of Physical Science and Technology,Southwest Jiao tong University,Chengdu 610031,China

(Dated:Received on January 23,2013;Accepted on September 23,2013)

The structure,electrostatic properties,and Raman spectra of af l atoxin B1(AFB1) andAFB1-AgcomplexarestudiedbydensityfunctionaltheorywithB3LYP/6-311G(d,p)/Lanl2dz basis set.The results show that the surface-enhanced Raman scattering (SERS)and pre-resonance Raman spectra of AFB1-Ag complex strongly depend on the adsorption site and the excitation wavelength of the incident light.The SERS factors are found to enhance 102-103order compared to normal Raman spectrum of AFB1molecule due to the larger static polarizabilities of the AFB1-Ag complex,which directly results in the stronger chemical enhancement in SERS spectra.The pre-resonance Raman spectra of AFB1-Ag complex are explored at 266,482,785,and 1064 nm incident light wavelength,in which the enhancement factors are about 102-104,mainly caused by the charge-transfer excitation resonance.The vibrational modes are analyzed to explain the relationship between the vibrational direction and the enhanced Raman intensities.

Af l atoxin B1,Surface-enhanced Raman scattering spectrum,Pre-resonance Raman spectra,Density functional theory

I.INTRODUCTION

A fl atoxins(AFs)are a group of hepatoxic,carcinogenic,mutagenic,and teratogenic mycotoxins,which are mainly produced by Aspergillus fl avus,A.parastiticus,and A.anomies[1].As the most common mycotoxins detected in human food and animal feed,AFs have drawn an increasing attention because of their frequent occurrence in cereal,cotton and groundnuts[2].Among more than 20 AFs derivatives,a fl atoxin B1(AFB1)has the most powerful toxicity and carcinogenicity to animals and human,which can cause malignant tumors in various animals and primary hepatocellular carcinoma in human[3].The toxicity and carcinogenicity of AFB1are associated with its DNA binding properties,as well as its teratogenic properties to cause malformations in many organs in embryos[4].

In order to remove or destroy the toxin,many chemical,biological,and physical methods are used to detect AFs in contaminated crops[5].Recently some novel biosensors based on surface plasmon resonance[6],for example surface enhanced Raman scattering(SERS) [7-9],have been developed to implement the rapid detection of chemical and biological samples.SERS, fi rstly observed by Fleischmann et al.in 1974,has been employed successfully for qualitative micro analyses because of the high sensitivity and selectivity[10-12]. However,it is not easy to apply SERS to quantitative microanalyses because of the intensity and a band shift of the enhancement of a Raman spectra depending on the vibrational mode[13].Dif f erent vibrational modes correspond to dif f erent intensity and band shift.The SERS is af f ected by the molecular structural changes and the conditions of a metal colloid or a substrate. Based on metallic nanostructure substrates[14],the enhancement factor induced by nanostructure can reach as much as 14 to 15 orders of magnitude,which allows the SERS technique to be sensitive enough to detect small amount of molecules,even single molecule[15, 16].AFB1is a Raman-active compound which can be detected or identif i ed by SERS microscopy after capturing.Some experiments on the SERS technique have been performed to detect and track af l atoxins toxin by our cooperation group[17].

In order to explore the SERS mechanism of AFB1adsorbed on Ag nanoparticles,density functional theory (DFT)method is used to analyze the AFB1-Ag complex structures and properties.The SERS spectra and preresonance Raman spectra of AFB1molecule have been calculated to explore the SERS enhance efficiency of single AFB1molecule absorbed on Ag nanoparticles.

II.COMPUTATIOANAL METHODS

The simple microscopic active adsorption site modes are adopted to simulate the AFB1molecule adsorbed on the Ag nanoparticle.Two adsorption sites of AFB1are considered in this work.The structures of AFB1and AFB1-Ag complex are optimized using 6-311G(d,p) basis set for C,H,O atoms and Lanl2dz[18]ECP basis set for Ag atom with 1s-4p core kept frozen at the B3LYP level[19-21].The B3LYP method is a hybrid HF/DFT method using a combination of Becke’s threeparameter exchange functional(B3)with the Lee-Yang-Parr(LYP)generalized gradient correlation functional. The Berny gradient method[22]is employed in complete geometry optimization for AFB1and AFB1-Ag complexes.The SERS spectra and pre-resonance Raman spectra of AFB1molecule are calculated at the same functional and basis set.The absorption spectra and the excited state electronic structures are calculated to explore the resonance incident light wavelength by using the time-dependent density functional theory(TD-DFT)[23]at the same level.To better match experimental vibrational frequencies,the vibrational scaling factor[24]0.98 is adopted.All of the computations are performed by the Gaussian 09 program[25],in which the f i ne grid(75302)is the default for evaluating integrals numerically.

The Raman activity or Raman scattering factor Sp(in a.u.)is determined by:

where αpand γpare isotropic and anisotropic polarizabilities.Spis directly obtained by Gaussian 09 program.

III.RESULTS AND DISCUSSION

A.Geometrical structure

Due to the strong electronic negativity,atom O is much easier to interact with metal Ag than atoms C and H.Therefore two dif f erent adsorption sites of AFB1molecule,denoted as a site and b site shown in Fig.1,are selected to explore the adsorbing properties of the AFB1molecule adsorbed on the Ag nanoparticle.More signif i cant structural perturbations are found when AFB1molecule adsorbed on Ag nanoparticle through a site than b site.The O-Ag bond length(R1,R2)are 0.278 and 0.304 nm for AFB1-Agacomplex,respectively,shorter than that for complex AFB1-Agb(0.315 and 0.357 nm)in Table I.The bond lengths near the Ag surface have changed obviously,and the other bonds far away from the Ag surface have not changed signif i cantly.Compared to AFB1molecule,the bond length C12-O13 in AFB1-Agacomplex is increased obviously by 0.005?A(from 1.219?A to 1.224?A),and C23-O22 bond length is lengthened,while the bonds C8-C12,C8-C23,and C23-O21 become shorter.For the AFB1-Agbcomplex,the bonds C1-O14,C15-O19, and C18-O19 are all lengthened.The other bond lengths,bond angles,and dihedral angles agree approximately with the ones in the AFB1molecule.The optimized parameters of AFB1molecule,including bond lengths,bond angles,and dihedral angles for the ground state are in good agreement with N-V’s calculated results[26]and experimental results[27].

FIG.1 Scheme of AFB1,AFB1-Aga(adsorption in a site) complex and AFB1-Agb(adsorption in b site)complex.

TABLE I Binding properties between AFB1and Ag atom.

The adsorbed energy is def i ned as:

When the AFB1molecule is adsorbed on Ag nanoparticle through the a adsorption site,the adsorbed energy is-16.22 kJ/mol,while for AFB1-Agbcomplex,the adsorbed energy is much smaller than the AFB1-Agbcomplex(-6.37 kJ/mol,Table I).Therefore,the a adsorption site is a more appropriate site for the AFB1molecule adsorbed on the Ag nanoparticles.Only the AFB1-Agacomplex is discussed in the following section.

Because of the coupling interaction,the charges are redistributed between the AFB1molecule and Ag atom. 0.105 e for the AFB1-Agacomplex and 0.052 e for the AFB1-Agbcomplex are transported from the AFB1molecule to Ag,which result in the static polarizability along x-axis being more increased,listed in Table II. Compared to the single AFB1molecule,the static polarizabilities of two complexes are increased obviously. The average static polarizabilities are increased from 246.11 to 323.68.The largest change is found for the AFB1-Ag complex in the xx components of the static polarizability,corresponding to C=O stretching vibrational mode.The Raman intensity is proportional to the square of the molecular induced dipole moment, while the molecular induced dipole moment is P=α·E, where α is the molecular polarizability and E is the external electric f i eld.Thus when the molecular polariz-ability is increasing,the Raman intensity is also greatly enhanced.The static polarizabilities are one of decisive factors of the ground state chemical enhancement which is not associated with any excitation of the moleculemetal system.The calculated static polariability in Table II predicts that the static chemical enhancement will be found in the surface-enhanced Raman spectra due to the static polarizability changes.

TABLE II Calculated static polarizability in a.u.hαi=(αxx+αyy+αzz)/3.

TABLE III Comparison of vibration modes between AFB1and AFB1-Ag complex.f is frequency and I is intensity.

B.The Raman spectra of AFB1-Ag complex

The surface enhanced Raman e ff ect is observed in the Raman spectra of AFB1-Ag complex compared to the normal Raman spectrum(NRS)of single AFB1molecule,shown in Fig.2.The Raman peaks frequencies,the Raman intensities and the corresponding vibration assignments are listed in Table III.The profi le of SERS for AFB1-Ag complex is consistent with the normal Raman spectrum,however,obvious enhanced Raman intensities are found at some vibrational modes.The vibration modes assignments in this work are consistent with the experimental results, which are obtained at the 785 nm excitation wavelength[17].The Raman enhanced factor(IEF)is given by IEF=ISERS/INRSat the corresponding vibrational modes.

FIG.2 The Raman spectra of AFB1molecule and AFB1-Ag complex.

According to the surface plasmon resonance theory [28],the vibrations along the direction perpendicular to the absorption surface are expected to occur more enhancement than the vibrations in the parallel direction.For the single AFB1molecule and the AFB1-Ag complex,the peaks with the maxima enhanced intensity are found at 1596 and 1588 cm-1,whose Raman intensities are up to 609,belonging to the C2=C4 and C=Ostretching vibrations of cyclopentene ring with O moving toward silver surface.The C=O(cyclopentene ring and pyrane ring)stretching vibrations of AFB1-Ag complex are found at 1613 and 1740 cm-1,whose Raman intensities are 103 and 101,respectively.While the two vibrational modes are found at 1693 and 1756 cm-1in experiment[17].The C17=C18 stretching vibration in the complex is found at 1636 cm-1,with the blue shift of 16 cm-1compared to the experiment.The C-C-C stretching vibration is found at 1235 cm-1,with vibrational direction along with x-axis,resulting the Raman enhancement factors up to 120.The other vibrational modes agreed approximately with the ones in the AFB1molecule.

For AFB1-Ag complex,the static polarizabilities incensement along x-axis can result in the NRS spectrum intensity enhancement due to the static chemical enhancement.Compared to the NRS of the single AFB1molecules,the enhanced SERS ef f ects mainly result from the chemical environment modif i cation when the AFB1molecule is adsorbed on Ag-nanoparticle.The charge redistribution and the structural perturbation lead to greater increases of the static polarizabilities in the complex.Therefore,the enhancement mechanism of SERS for AFB1molecule can be ascribed to the ground state chemical enhancement.

C.Pre-resonance Raman spectra of AFB1-Ag complex

When the incident light wavelength is close to the molecule electronic excitation energy,the incident electronic f i eld can excite the electron transition to induce resonance,which leads to the Raman scattering signal intensity enhanced by a factor up to 104-106.This process is referred as the resonance Raman scattering, in which the enhancement is proportional to the oscillator strength of the electron transition.For the AFB1-Ag complex,the metal-molecule charge transfer(CT)is found due to the interaction between the molecule and the metal when the molecule is adsorbed on the metal nanoparticle surface.

According to the absorption spectra of the AFB1-Ag complex,266 and 482 nm incident wavelengths,in the proximity of the two absorption maxima of AFB1-Ag complex,are chosen to explore the pre-resonance spectrum of AFB1-Ag complex.Meanwhile 785 and 1064 nm incident wavelength,which are away from resonance absorption of the AFB1-Ag complex but correspond to the S5and S3electron excitation states,are selected to compare the pre-resonance Raman spectra. The total enhancement factors are up to 102-103in pre-resonance Raman spectra of the AFB1-Ag complex at four chosen incident wavelengths.The pre-resonance Raman spectra and molecular orbital corresponding to charge transfer between AFB1molecule and Ag atom are shown in Fig.4.Due to the charge transfer resonances between the molecule and the atom Ag,the most enhancement factor of C=O stretching vibration is up to 102at 1740 cm-1at the 266 nm incident light,the stretching vibration of C-C and C=O at 1588 cm-1, the enhancement factor is only 63.But for 482 nm incident wavelength,the most enhancement factor 104is found at 1531 cm-1,corresponding to C-C stretching and stretching vibration of C-C-C with the wiggle of C-H.The C-C and C=O stretching vibration at 1588 cm-1along x-axis,the enhancement factor is up to 102.The stretching vibration of the bonds near the Ag atom(C-C,C-O-C)and C=O stretching are also obviously enhanced up to 10.

Compared to incident wavelength 266 and 482 nm, the maximum enhancement factor is up to 103at 785 nm incident wavelength,which is C-C stretching vibration with the wiggle of C-H at 1531 cm-1along x-axis,because of electric transfer between HOMO and LUMO of AFB1-Ag complex.The C=C and C=O stretching vibration is enhanced up to 102at 1588 cm-1,the C=O stretching vibration modes are found at 1740 cm-1,their enhancement factor are only up to 10.For 1064 nm excitation energy,the most enhancement factor is 102,corresponding to C-C stretching vibration and ring(s)skeleton vibration at 1531 cm-1.The enhancement Raman intensities at 892,1031,1461,1588,and 1593 cm-1are also up to 102,corresponding to the pyrane ring breath vibration and C-C,C-C-C,C-O-C stretching vibration mode with ring deformation,ring(s)skeleton vibration,and the C=O stretch peaks,respectively.Therefore,when the incident light is away from resonance absorption of excitation energy,the more vibratioanal modes enhanced simultaneously.

Compared to the SERS intensities of isolated AFB1, the enhancement due to the charge transfer resonance contributes an additional 2-4 orders of magnitude to the chemical enhancement at four chosen incident wavelengths.It is proven that the CT mechanism signif icantly contributes to the enhancement of pre-resonance Raman intensity,which is also used to explain the dependence of the certain bands in SERS experiments on the electrode potential[29,30].

IV.CONCLUSION

The SERS and pre-resonance spectra of the AFB1molecule are studied by DFT method.Raman scattering intensity not only depends strongly on the local chemical environment of adsorption site but also depends on the incident excitation wavelength.The geometry structure and the adsorption energy show that a site of AFB1molecule is a more favorable adsorption site than b site.When AFB1molecule is adsorbed on silver nanoparticle by a site,the enhancement factor of AFB1-Ag complex is up to 103compared to normal Raman spectrum of the isolated AFB1molecule,which results from a great change of the perpendicular polar-izabilities due to the chemical environment modif i cation in AFB1-Ag complex.

FIG.3 The pre-resonance Raman spectra of AFB1-Ag complex at four dif f erent incident light wavelengths of 266,482,785, and 1064 nm.The molecular orbitals corresponding to charge transfer between AFB1molecule and Ag atom are also shown.

For the pre-resonance Raman spectra at the incident wavelength 266,482,785,and 1064 nm,the enhancement factors at some specif i c vibration modes are up to 103,which are mainly attributed to the charge-transfer excitation resonance enhancement but not the plasmon resonance of the nanoparticle because Ag atom or cluster is too small to have a real plasmon resonance.The SERS enhancement mechanism of AFB1-Ag complex can be ascribed to the chemical enhancement in which the static chemical enhancement of ground state and the charge transfer resonance enhancement of excitation state work is together due to the chemical interaction between the AFB1molecule and the Ag nanoparticle.The stronger SERS enhancements ef f ects should be predicted for the real nanoparticles because of the combination of the chemical enhancement and the electromagnetic enhancement.

V.ACKNOWLEDGMENTS

This work was supported by the National Natural Science Foundation of China(No.11174237),the National Basic Research Program of China(No.2013CB328904), and the Application Basic program of Sichuan Province (No.2013JY0035).

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?Author to whom correspondence should be addressed.E-mail：hongyanw@home.swjtu.edu.cn,Tel.：+86-28-87600963, FAX：+86-28-87601357