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Journal of Hazardous Materials 265 (2014) 89– 95
Contents lists available at ScienceDirect
Journal of Hazardous Materials
jou rn al hom epage: www.elsev ier .com/ locate / jhazmat
etection of the mycotoxin citrinin using silver substrates and Ramanpectroscopy
heeraj K. Singha, Erdene-Ochir Ganbolda, Eun-Min Chob, Kwang-Hwi Choc,oseok Kimd, Jaebum Chooe, Sehun Kimf, Cheol Min Leeg, Sung Ik Yangb,ang-Woo Jooa,∗
Department of Chemistry, Soongsil University, Seoul 156-743, South KoreaCollege of Environment and Applied Chemistry, Kyung Hee University, Yongin 446-701, South KoreaSchool of Systems Biomedical Science, Soongsil University, Sangdo-dong, Dongjak-gu, Seoul, South KoreaDepartment of Physics, Sogang University, Seoul 121-742, South KoreaDepartment of Bionano Engineering, Hanyang University, Sa-1-dong 1271, Ansan 426-791, South KoreaMolecular-level Interface Research Center and Department of Chemistry, KAIST, Daejeon 305-701, South KoreaInstitute of Environmental and Industrial Medicine, Hanyang University, 17 Haengdang-dong, Seongdong-gu, Seoul 133-791, South Korea
i g h l i g h t s
The mycotoxin citrinin was detectedusing Ag substrates and Raman spec-troscopy.Prepared Ag substrates were charac-terized by electron microscopic tools.Density functional theory calculationpredicted the most stable geometryon Ag.
g r a p h i c a l a b s t r a c t
r t i c l e i n f o
rticle history:eceived 7 July 2013eceived in revised form 20 October 2013ccepted 19 November 2013vailable online 23 November 2013
a b s t r a c t
We detected a trace amount of the mycotoxin citrinin using surface-enhanced Raman scattering (SERS) onsilver nanoparticle (Ag NP) surfaces. The SERS substrate on hydrophobic Teflon films was also introducedto observe the citrinin peaks. A broad band at ∼1382 cm−1, which was ascribed to the symmetric car-boxylate stretching mode, was observed in addition to an antisymmetric carboxylate stretching modeat ∼1568 cm−1 in the Raman spectra. The spectral feature indicated that citrinin would adsorb on Ag
eywords:itrininycotoxin
ilver substratesaman spectroscopyensity functional theory calculations
NPs via its carboxylate form. Based on density functional theory (DFT) calculations, vibrational modeanalysis was performed to compare the Raman spectra of citrinin. DFT calculations also predicted thata bidentate bridge configuration through O15 and O16 atoms in citrinin would be the most stable onthree Ag atoms. After treating with Ag NPs, observation of citrinin peaks was attempted in fungal cells ofPenicillium citrinum. This work may provide useful insights into the direct observation of the hazardouscitrinin mycotoxin using SERS by understanding its adsorption behaviors on Ag surfaces.
∗ Corresponding author. Tel.: +82 2 820 0434.E-mail addresses: [email protected] (C.M. Lee), [email protected] (S.I. Yang),
[email protected] (S.-W. Joo).
304-3894/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.jhazmat.2013.11.041
© 2013 Elsevier B.V. All rights reserved.
1. Introduction
Research has paid much attention toward the detection of
mycotoxin because of its effects in environmental and healthissues [1,2]. Citrinin is a nephrotoxic contaminant produced byseveral fungal species, including Penicillium and Aspergillus [3,4].Toxicologically, citrinin is associated with harmful synergistic
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donation from metal to its antibonding �* orbital [26]. This may beevidenced by the observation of the strong metal-oxygen band at
0 D.K. Singh et al. / Journal of Ha
ffects, such as induction of DNA damage in cultured renal cells [5].his toxin has been found to cause the environmental and healthroblems in a variety of agricultural commodities [6].
Structural information is useful in estimating the toxicity ofycotoxins [7]. Theoretical studies have provided helpful insights
nto the stability of chemical structures and properties of myco-oxins [8–10]. Because of the unique chemistry of citrinin, it haseen the subject of several experimental structural investigations11]. The molecular structure of citrinin has not been studied exten-ively, however, despite the relatively simple conformation with itsow molecular weight. The p- and o-quinine methide tautomericquilibrium of citrinin [11] was calculated using the quantumechanical method [12].There have been several analytical methods [13] such as
C–MSMS [14], flow cytometric immunoassay [15], amperometricetection [16], and microfluidic electrochemical immunosensors17] in order to detect citrinin. Because these methods requireime-consuming and expensive sample pretreatment and deriva-ization steps by a trained expert, it is necessary to develop time-nd cost-effective method for the detection of citrinin.
Surface enhanced Raman scattering (SERS) can detect a tracemount of organic hazardous contaminants with a high sensitiv-ty [18–20]. SERS has the advantage of clarifying chemical identityf biomolecules and environmental pollutants adsorbed on metalurfaces. The analysis of spectral features has provided detailednformation at high resolution on interfacial adsorption structurend surface reactions. Recently, SERS was applied to detect myco-oxins [21] in combination with density functional theory (DFT)alculations [22].
In this study, we report the detection of the citrinin by usingERS. DFT calculation was also performed to estimate the bind-ng energies to determine the adsorption structures on metalanoparticle surfaces. The experimental results will be helpful ineveloping a new scheme for the detection of mycotoxin.
. Materials and methods
.1. Preparation of Ag NPs
Citrinin (≥98%) and spermine (≥97%) were purchased fromigma Aldrich. The other used chemicals were reagent-grade. Torepare Ag NPs, 1 mL sodium citrate (0.57 g/50 mL) is added to50 mL the boiling solution of 1.1 mM Ag nitrate stirring on a hot-late [23].
We made a 10−2 M stock solution of citrinin in ethanol, whiche then used to perform SERS measurements in the presence ofetal NPs. For this purpose, the 1 �L volume of 10−3 M citrinin
olution was mixed with 100 �L of Ag NPs (the final concentrationas 5 × 10−6 M) and allowed to react for 3 min. We then mixed 1 �L
olume of 10−2 M aggregating agents of sodium hydroxide (NaOH),odium chloride (NaCl), or spermine. After 3 min, we performedhe SERS measurements. The round septum polytetrafluoroethy-ene (Teflon) film made with a radius of 4 mm was purchased fromhimadzu to prepare the SERS platform by a drop-casting method24]. The SERS-active film was also generated by eroding the Aglate via HNO3.
.2. Physical characterization
The morphologies of Ag NPs were checked using a JEOL JEM-010 transmission electron microscope. The hydrodynamic radius
nd the surface potential were measured with an Otsuka ELSZ-2nalyzer. Raman spectra were obtained using a Renishaw Ramanonfocal system model 1000 spectrometer equipped with anntegral microscope (Leica DM LM) attached with a dark-fieldus Materials 265 (2014) 89– 95
microscopy set-up [25]. The pH value of Ag NPs was measured by aThermo Orion 3 benchtop pH meter. The 632.8 nm radiation froma 20 mW air-cooled He–Ne laser (Melles Griot Model 25 LHP 928)was used as the excitation source for the Raman experiments. Adata acquisition time of approximately 30 s was used in the Ramanmeasurements to detect citrinin. The scanning electron microscopy(SEM) images of the Ag plates were obtained using a Carl ZeissSigma microscope.
2.3. Quantum mechanical calculations
DFT calculations were performed using a Gaussian 03 package.Geometry optimization of the citrinin adsorbate on Ag atom com-plexes was carried out at the level of B3LYP. LANL2DZ basis setswere used to model the metal atoms. To predict the vibrational fre-quencies of citrinin in the gas phase, the basis set of 6-31 + G(d,p)was used. Geometry optimization for the two tautomeric structuresof citrinin was conducted starting from many possible orientationsas shown in Fig. 1. Three plausible binding sites are consideredbetween silver atoms and oxygens of the carboxyl groups in citrinin.
2.4. Cell culture of NPs in Penicillium citrinum
We purchased P. citrinum (KCCM 60384, ATCC 36382) from theKorean Culture Center of Microorganisms (Seoul, Korea). The fungiwere grown on potato dextrose agar (PDA) medium containing200 g of sliced potato, 20 g of dextrose, and 20 g of agar powder.For detection of citrinin in live cells, fungi pre-incubated for 2–3days were treated with a few drops (∼10 �L) of colloidal Ag NPs.After mixing with the mounting media, the fungi species sampledfrom the spot where Ag NPs were dropped and placed on a spec-troscopic slide glass. The sample was subsequently sandwiched bythe cover glass. The DFM-Raman experiments of the prepared fungiwere performed on the microscopy stage [25].
3. Results and discussion
3.1. Physical characterization of Ag NPs
Fig. 2(a) and (b) shows the TEM image of Ag NPs with a size dis-tribution of 10–100 nm. The high-resolution TEM image of a singleAg NP revealed a multi-crystalline structure. The quasi-elastic lightscattering (QELS) measurements of Ag NPs showed that the aver-age diameter of Ag NPs was 52.2(±0.6) nm as shown in Fig. 2(c).Almost 90% of the Ag NPs ranged from 10 to 50 nm according to thenumber distribution. The pH value of the citrate-reduced Ag NPswas measured to be 8.0 (±0.5). It has been reported that the car-boxylic acid may adsorb on the negatively-charged Ag NP surfacesvia its carboxylate form [26]. We checked the adsorption of benzoicacid on the same negatively charged Ag NPs. Since the pKa value[27] of benzoic acid is reported to be 4.17, it is also expected to haveanionic forms at the neutral pH state of Ag NPs. The strong symmet-ric carboxylic stretching band was observed at ∼1373 cm−1 in theirSERS spectra along with the Ag–O band at ∼248 cm−1. The obtainedSERS spectra indicated a binding of the carboxylic acid moleculeson the Ag NPs. Referring to these data, despite the same negativecharges, it is probable that the carboxylate may adsorb on Ag due tothe chemical binding via either its oxygen lone pairs or the electron
200–250 cm−1 in the SERS spectra [28]. On the other hand, the aro-matic ring of citrinin may interact with the Ag surfaces via its �orbital. According to the accumulated data, citrinin may adsorb onthe Ag NPs with the negative charge.
D.K. Singh et al. / Journal of Hazardous Materials 265 (2014) 89– 95 91
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Fig. 1. (a) Structure of citrinin. The labeling of the atom number is derived from
It was possible to perform the SERS as an excitation source of33 nm. The fluorescence bands of citrinin may locate at 510 nm29]. It may be advantageous to use a long wavelength to avoid thenterference and sample damage of the biological mixtures as inhe case of the recent Raman studies of mycotoxins [22,30]. The
itrate-reduced Ag NPs also played the SERS-active platform underhe irradiation at 632.8 nm in our recent report [31]. In fact theres still considerable absorption at the tail of the surface Plasmonand of Ag NPs to observe Raman signals at 632.8 nm under ourig. 2. (a) TEM image of Ag NPs. (b) High resolution TEM image of the magnified view of
he Ag plate. The scale bar is 30 nm.
ference [12]. The three plausible binding sites are marked as “T1”, “T2”, and “B”.
experimental conditions instead of 514.5 nm. Fig. 2(d) shows theSEM image of the SERS active Ag plate roughened by HNO3.
3.2. Plausible adsorption structures based on DFT calculations
Table 1 summarizes the binding energy and bond distances asillustrated in Fig. 1(b). The o-quinone methide form was chosen tostudy the energetic stability of citrinin on Ag NPs. In the previousreport, the p-quinone methide form was calculated to be slightly
a single Ag NP. (c) Size distribution of Ag NPs based on QELS data. (d) SEM image of
92 D.K. Singh et al. / Journal of Hazardo
Table 1Binding energies of the Ag atoms of citrinin and the Ag–O bond distances of thethree plausible binding site configurations of T1 , T2 , and B.
Configurations B.E. (kcal/mol) Metal-citrinin distance (Å)
T1 (O13, O16) 69.8 Ag–O13 2.39Ag–O16 2.24
T2 (O14, O15) 70.0 Ag–O14 2.40Ag–O15 2.25
B (O15, O16) 81.1 Ag–O15 2.26Ag–O16 2.26
Binding energy (BE) of a complex is defined as: �E = [Ecomplex–(the sum of Eindividual)]wo
mGst
FvO
here, Ecomplex is the ground state energy of the metal-citrinin complex and the sumf Eindividual is the sum of the ground state energies of the citrinin and metal entity.
ore stable from the B3LYP/6-31G(d,p) [12]. Our DFT calculatedibbs free energy (�G) results indicate that p-quinone methide is
lightly more stable than o-quinone methide by 0.04 kcal/mol fromhe B3LYP/6-31 + G(d,p). Moreover the calculated Raman spectrumig. 3. Optimized structures of citrinin on three Ag atoms. Tilt structures of (a) “T1”ia O13 and O16 atoms and “T2” via O14 and O15 atoms. (c) Bridge structure “B” via15 and O16 atoms.
us Materials 265 (2014) 89– 95
of the p-quinone methide form in Fig. 4(a) matches the experimen-tal Raman spectrum in Fig. 4(b) quite well.
A recent theoretical work was reported on the calculation ofcitric acid on Ag surfaces [32]. Bridged bindentate structures werepredicted to be more stable than the chelate bidentate structuresas in the case of benzoic acid on TiO2 surfaces [33]. Referring to thecarboxylate stretching mode, citrinin appeared to adsorb on Ag viaits carboxylate form. To predict the binding energy of citrinin on AgNPs, we performed the three atom cluster model.
As shown in Fig. 3, the most stable structure was predicted tobe a bidentate bridging perpendicular structure via O15 and O16atoms with the binding energy of 81.1 kcal/mol. On the other hand,the two “T1” and “T2” structures via O13 and O16 atoms and viaO14 and O15 atoms were calculated to be almost identical as 69.8and 70.0 kcal/mol, respectively. Both the o- and p-quinone methideform may coexist on the Ag surfaces. Regardless of the p- and o-tautomers, the binding energies are calculated to be the same, if thebinding sites of Ag atoms and oxygen atoms in citrinin are identical.These two tilt structures were predicted to be less favorable thanthe bidentate bridging “B” structure. One of the two Ag–O distanceswas predicted to be longer, suggesting weaker binding on Ag atoms.On the other hand, the two Ag–O distances were calculated to bethe same for the “B” structure.
In the bidentate bridging structure, the three atoms and citrininappeared to lie in the same plane. If the tilt angle was measuredbetween the quinone ring plane and the perpendicular line withrespect to the bond of the two Ag atoms, the angle between thequinone and the two Ag atoms was almost zero for the “B” struc-ture, whereas the angles were approximately 57◦ for the two tiltstructures of “T1” and “T2”.
It has to be mentioned that anionic adsorbates may be more sta-ble than the neutral state. Considering the pKa value (2.3) of citinin[34] and the neutral pH values of Ag NPs, citrinin is presumed tohave anionic states as in the case of anthraquinoline carboxylate onmetal surfaces [35]. Despite these pH conditions, our DFT calcula-tion results suggested the carboxylate binding in the neutral state,which could explain the experimental Raman data.
3.3. SERS of citrinin
Fig. 4(a) and (b) show the simulated and OR spectra of citrinin.The strong band at 1635 cm−1 may be ascribed to the ring stretchingband. The band at 1537 cm−1 may be assigned to the ring stretchingband in the in-plane O–H and C–H bending modes. Fig. 4(c) and (d)show the SERS spectra on the Ag plate and Teflon film, respectively.On the Ag plate, we could observe the weak SERS peaks at 1574 and1618 cm−1. More prominent features could be obtained using thehydrophobic Teflon film as reported [24].
At the concentration of 10−5 M in the SERS spectrum as shownin Fig. 4(e), most spectral band positions could match with theordinary Raman (OR) spectrum. The results showed that the aggre-gating agent of NaCl was more efficient in yielding strong SERSpeaks than either NaOH or spermine. Along with the Ag–O bandat 237 cm−1, the strong and broad band at 1382 cm−1 that can beascribed to the symmetric carboxylate band, indicated that citrininadsorbed on Ag NPs via its bidentate bridging form. The band at1568 cm−1, which was also assigned to the antisymmetric carbox-ylate band, supported this conclusion. This view is consistent withthe theoretical calculations predicting that the bidentate bridgingstructure is the most stable form. It has to be mentioned that thering C–H stretching mode at ∼3051 cm−1 was not observed. It hasto be mentioned that the possibility of the ring plane may have a
rather flat orientation on Ag surfaces referring from the absenceof the ring C–H band and the redshift the ring mode from 1635 to1616 cm−1 upon adsorption on Ag surfaces. Table 2 shows a sum-mary of the vibrational positions with the appropriate assignments.
D.K. Singh et al. / Journal of Hazardous Materials 265 (2014) 89– 95 93
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Fig. 4. (a) DFT calculated spectrum of p-quinine methide. (b) Ordinary Raman (OR)spectra of citrinin. SERS spectra of citrinin on (c) an Ag plate and (d) a Teflon film, andin Ag NPs at the concentrations of (e) 10−5 M and (f) 10−6 M on Ag NPs. (g) Ramanspectrum of pristine Ag NPs.
Table 2Spectral data and vibrational assignments for citrinin.
Cal. OR SERS Assignment
237 Ag–O362 388 343 (ring)422 432 437 (C–C–C)470 480 468 (Trigonal C–C–C)557 577 (ring)622 637 656 (ring breathing)662 657 (ring)685 676 680 (COOH) + (ring)765 761 762 � (C–CH3)800 800 800 � (C–CH3) + (C–O–C)869 871 861 (ring) + � (C–CH3)943 959 977 � (C–H)1043 1060 1071 � (C–CH3)1121 1154 1161 � (C–CH3) + � (C–H)1207 1204 1202 � (C–O) + (COOH)1275 1274 1281 � (C–H)1346 1344 � (C–H)1366 1373 1363 (O–H)
1382 �s (COO)1453 1450 1422 � (C–CH3)1490 1490 1490 � (C–C)1531 1537 1533 � (C = O) + �(O–H) + (C–H)
1568 �as (COO)1577 1584 (O–H) + �asym (C = C)1638 1635 1616 (O–H) + �sym (C = C)
2929 2932 �as (CH3)2980 �s (CH3)3051 � (CH)
Unit in cm−1, Scale factor: 0.974 for p-quinone methide. Assignment is based on thepresent DFT calculation at the B3LYP/6-31 + G(d,p) level. SERS spectrum on AgNPs.Abbreviations: �: stretching; ˇ: in-plane bending; �: out-of-plane bending.
Fig. 5. (a) DFM image and SERS spectrum of Penicillium citrinum. The arrow indicates the position where the Raman spectrum was obtained. (b) DFM image of P. citrinumafter treating with Ag NPs. SERS spectrum of P. citrinum. The arrows indicated the positions, where the Ag NP particulate was identified by the DFM image and the Ramanspectrum was obtained.
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As the concentration of citrinin decreased, the spectral inten-ities became quite weakened. At 10−6 M, most strong bandsecame barely discernible. By increasing the data acquisition timey focusing the spectral band, the signal to noise ratio may be
mproved. Under our experimental conditions, the detection limitas estimated to be around 10−6 M. Fig. 4(g) shows the background
pectrum of Ag NPs. We prepared the SERS substrate on a clean glassnd a Teflon septum by the dropping cast method.
We estimated the average enhancement factor (EF) calculatedccording to the following formula [36].
F = (ISERS/NSERS)(IOR/NOR)
here ISERS and IOR are the SERS intensity and ordinary Ramanntensity of the same band of citrinin, and NSERS and NOR rep-esent the corresponding number of molecules probed in theocused incident laser spot. Here, the ∼1620 cm−1 band havinghe highest peak intensity was selected for such a calculation,hich can be assigned to the aromatic stretching vibration. In
ig. 4, we obtained that the ISERS and IOR are 2200 and 10,300ounts at the Raman band of ∼1620 cm−1, respectively. In theR spectrum, the neat solid sample was used, whereas the dis-
olved concentration was 10−5 M in the SERS spectrum. Theolumes of the irradiated laser spots in the OR and SERS exper-ments were identical. Assuming the density of ∼1 g cm−3 forhe solid citrinin, the enhancement factor may be estimated as2200) × (105) × 1/(10,300) × (1/250) × (103) = 8.5 × 104. Althoughhis is not such strong enhancement as the organic dye of Rho-amine 6G [36], it is sufficient enough to directly observe a tracemount of citrinin.
.4. Fungal cell experiments
Fig. 5 shows the fungal Raman peaks of Penicillium citrinum. Thepectral band positions are consistent with the previous report [37].he bands at 1616, 1457, 1359, and 1204 cm−1 can be ascribed tohe ergosterol peak, methyl deformation band, beta glucan peaks,nd phospholipid peaks, respectively. We also checked whetherhe PDA medium should adsorb on Ag surfaces. The SERS bands at063 and 1125 cm−1, which can be ascribed to the C–O stretchingf the dextrose, were observed to indicate the adsorption of the cellulture medium on Ag. Since citrinin should displace the adsorbedomponents from the PDA medium, the SERS peaks may not bes strong as the pristine Ag NPs prepared by the citrate-reductionethod.As shown in Fig. 5(b), although we attempted to observe any
itrinin peaks after treating Ag NPs, only fungal peaks appeared.f the exhumed citrinin diffused from the fugal cell, it may haveeen caused by the interference via potato dextrose agar (PDA)ell culture medium. On the other hand, the amount of produceditrinin by P. citrinium [38] may not be sufficiently high enoughbove 1 �M to observe SERS spectra. The results indicated that thebservation of an in vivo live cell was not facilitated because ofhe interference from cell culture medium. However, the resultsid not indicate whether the SERS method would be impossibleo apply the detection of the mycotoxin citrinin in the environ-
ent. In future research, we plan to detect citrinin by improvinghe sensitivity of our Raman spectrometer and fabricating novelanostructures in yielding a more enhancement. Nevertheless, ourethod provides useful insights into the direct observation of the
azardous citrinin mycotoxin using SERS.
. Conclusions
Monitoring of hazardous citrinin mycotoxin could be achievedsing Ag NPs and SERS within a few minutes. Vibrational
[
us Materials 265 (2014) 89– 95
assignment using the DFT theory provided a good match with theobserved spectra. Citrinin was not easily detected in fungal cell cul-ture media with the application of Ag NPs. Despite the interferenceof the cell culture media and requiring higher sensitivity, a directdetection of citrinin would be possible by simply obtaining SERSspectra of citrinin either in aqueous solutions or on the Teflon films.
Acknowledgement
We acknowledge the financial support from the Korea Ministryof Environment as “Environmental Health R&D Program. The NanoMaterial Technology Development Program also supported thiswork, through the National Research Foundation of Korea, fundedby the Ministry of Education, Science, and Technology (grant num-ber 2012035286). This research was also partially supported by theAgency for Defense Development through Chemical and BiologicalDefense Research Center. This work was supported by the NationalResearch Foundation (NRF) grant funded by the Korea Govern-ment (MEST) Nos.2011-0017435 and the Korea government [MSIP][No.20090083525].
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