Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 144 (2015) 125–130

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Rapid surface enhanced Raman scattering detection method for chloramphenicol residues Wei Ji a,⇑, Weirong Yao b a b

School of Food Science and Technology, Guangdong Ocean University, Zhanjiang 524088, Guangdong, PR China State Key Laboratory of Food Science and Technology, School of Food Science and Technology, Jiangnan University, Wuxi 214122, Jiangsu, PR China

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 The detection limit of the method

reached 0.1 lg/mL.  The whole detection time was about

10 min, which could meet market demands.  The method was of high sensitivity, accuracy and was suitable for large amounts of food sample test.

a r t i c l e

i n f o

Article history: Received 21 October 2014 Received in revised form 28 January 2015 Accepted 5 February 2015 Available online 16 February 2015 Keywords: Chloramphenicol (CAP) Gold colloidal nanoparticles Surface enhanced Raman scattering (SERS) Quantitative analysis Density functional theory (DFT)

a b s t r a c t Chloramphenicol (CAP) is a widely used amide alcohol antibiotics, which has been banned from using in food producing animals in many countries. In this study, surface enhanced Raman scattering (SERS) coupled with gold colloidal nanoparticles was used for the rapid analysis of CAP. Density functional theory (DFT) calculations were conducted with Gaussian 03 at the B3LYP level using the 3-21G(d) and 6-31G(d) basis sets to analyze the assignment of vibrations. Affirmatively, the theoretical Raman spectrum of CAP was in complete agreement with the experimental spectrum. They both exhibited three strong peaks characteristic of CAP at 1104 cm 1, 1344 cm 1, 1596 cm 1, which were used for rapid qualitative analysis of CAP residues in food samples. The use of SERS as a method for the measurements of CAP was explored by comparing use of different solvents, gold colloidal nanoparticles concentration and absorption time. The method of the detection limit was determined as 0.1 lg/mL using optimum conditions. The Raman peak at 1344 cm 1 was used as the index for quantitative analysis of CAP in food samples, with a linear correlation of R2 = 0.9802. Quantitative analysis of CAP residues in foods revealed that the SERS technique with gold colloidal nanoparticles was sensitive and of a good stability and linear correlation, and suited for rapid analysis of CAP residue in a variety of food samples. Ó 2015 Elsevier B.V. All rights reserved.

Introduction Chloramphenicol (CAP) is an amide alcohols antibiotic (Seen in Fig. 1), which was widely used in livestock and aquaculture and ⇑ Corresponding author at: School of Food Science and Technology, Guangdong Ocean University, East Huguangyan 524088, Zhanjiang City, Guangdong, PR China. Tel.: +86 13536395190. E-mail address: [email protected] (W. Ji). http://dx.doi.org/10.1016/j.saa.2015.02.029 1386-1425/Ó 2015 Elsevier B.V. All rights reserved.

can efficiently inhibit bacterial growth. However, it has been proven to cause aplastic anemia, lack of granular leukocytes and adverse reactions due to tissue residues in food animals [1,2]. In many countries, such as China, the USA and many European countries, CAP was banned from animal feed and treatment, with maximum residue limits (MRI) of 0 mg/kg [3,4]. As a relatively cheap antibiotic, many businesses still use CAP illegally. However, with current methods, CAP residue is very difficult to detect, especially rapid detection is not concerned. In order


W. Ji, W. Yao / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 144 (2015) 125–130

boiling, For reduction, a 1% solution of C6H5O7Na32H2O (2 mL) was added by vigorously stirring (500 rpm) until the solution color was a stable wine red. The resultant solution was further refluxed for 15 min, which generated a wine red suspension. The cooled gold colloidal nanoparticles suspension (particle diameter 50 nm) was gradually cooled to room temperature while stirring in preparation for SERS detection. SERS measurements

Fig. 1. Structure of CAP.

to quickly detect CAP residues in animal derived foods and to ensure consumer safety, it is necessary to develop a rapid CAP detection method. Currently, the common analytical methods for CAP are mainly chromatography-based, including high performance liquid chromatographic (HPLC) [5,6], liquid chromatography mass spectrometry (LC–MS) [7] or ELISA [8]. However, these methods are time-consuming and labor intensive, requiring complex sample treatment procedures and well-trained personnel in order to be performed accurately and consistently. To rapidly identify the CAP residue, it is critically important to develop simpler, quicker, and more sensitive analytical methods. Since surface enhanced Raman scattering (SERS) phenomenon was discovered in the 1970s [9,10], SERS has been used to enhance the scattering effect markedly, overcoming the low sensitivity of the classic Raman technique [11]. Characteristic information about various chemical and biochemical components in a complex system can be obtained from the ‘‘fingerprint-like’’ Raman spectra, and little or no sample preparation is required. SERS has emerged as a powerful technique for the detection of various analytes [12]. Recently, SERS has also succeeded in evaluating food safety and quality, including rapid detection of antibiotics such as malachite green [13], nitrofuran antibiotic [14], tetracycline [15] and other prohibited additions [16]. In this study, the objective was to study CAP vibration spectra and to identify assignments of its vibration peaks by density functional theory (DFT) calculations, and to establish the SERS method coupled with gold colloidal nanoparticles as the substrate for rapid analysis of CAP residues in food samples. Materials and methods Instruments and chemicals A portable Raman spectrometer RamTracerÒ-200-HS (OptoTrace Technologies Co., Ltd., Silicon Valley, CA, USA) was used for Raman spectra analysis and a centrifuge Force-1624 (Yibei Innovative Science and Technology Co. Ltd. of Beijing, China) and tissue homogenizer 04254-00 (CoLe-Parmer Corporation, USA) were used for sample preparation. The reagents that were used included chloramphenicol (99.0%, Sigma–Aldrich Chemicals, St. Louis, MO, USA), KAuCl42H2O (99%, Alfa, USA) and C6H5O7Na32H2O (99%, Sinopharm Chemical Reagent Co., Ltd., China). Other commercially available chemicals and reagents were of analytical grade. Pork, chicken, beef, and lamb meat were purchased from a local supermarket in Wuxi city in China and kept at the refrigerator of 4 °C prior to use. SERS active substrates The SERS active gold colloidal nanoparticles were prepared by a citrate reduction method [17,18]. KAuCl4 (20 mg) was dissolved in 50 mL of ultrapure water and heated in 120 °C oil bath until

All analytes were deposited onto clean glass slides. SERS measurements were carried out in a dark environment with a scanning power of 300 mWat, 785 nm duration of 10 s, a scanning range of 300–2000 cm 1 and resolution of less than 2 cm 1. Sample preparation 5.0 g (accurate to 0.1 g) of CAP was dissolved in 10 mL ethyl acetate. The homogenate was mixed for 2 min, and then centrifuged for 10 min at 10,000 r/min. The supernatant was rotary evaporated in water bath at 50 °C until nearly dry. The residue was dissolved in 5 mL ultrapure water and 5 mL n-hexane was added and the mixture was vortex oscillated for 1 min. With the resultant biphasic solution, the n-hexane phase was firstly removed with a pipet, with any residual n-hexane removed by nitrogen. The residue constant volume was 10 mL with ultrapure water. The final solution was filtered using an 0.45 lm microporous membrane prior to analysis [19,20]. 0.01 g standard of CAP was dissolved in 10 mL of triple distilled water as stock solution. The stock solution was ultrasonically vibrated for 10 min and then gradually diluted in series with ultra pure water to final concentrations of 100, 50, 10, 1 and 0.1 lg/mL. Recovery rate of CAP Different amounts of CAP were added to 5 g samples of chick meat and treated according to sample preparation methods (see Section ‘‘Sample preparation’’) and SERS measurements (see Section ‘‘SERS measurements’’). Density functional theory (DFT) The geometry optimization and normal mode calculations for CAP were performed with the Gaussian 03 program. Becke’s threeparameter hybrid exchange function (B3) [21] and the correlation function of Lee, Yang and Parr (LYP) [22] were employed in the DFT calculations. Two different basis sets were used, 3-21G(d,p) and 6-31G(d,p). The harmonic vibration wavenumbers for the Raman spectrum were analytically calculated for the fully optimized molecular geometry. The results were viewed using Gauss View 5.0 software (Gaussian, Inc., PA, USA). Statistical analysis The data are presented as mean ± standard deviations. The mean value and its standard deviation have been calculated using one-way analysis of variance [23]. All statistical Raman spectra shown were measured under optimum conditions in this study. Results and discussion Comparison between theoretical and experimental Raman spectra of CAP CAP molecular is collectively composed of a nitro phenyl, a propylene glycol and a dichloro ethyl amide group. The differing

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Fig. 3. Normal Raman spectra of water (a), CAP solution in case of triple distilled water as solvent (100 lg/mL) (b) and CAP solid (c). Fig. 2. Comparison of the theoretical (a) and the experimental (b) Raman spectra of CAP (The upper left corner is the TEM of gold colloidal nanoparticles and the balland-stick model of CAP).

structures between the groups produce different Raman peaks on the Raman spectra. The DFT method is useful for calculating the optimal molecular structure and vibrational spectra of compounds from Raman spectra. The inset of Fig. 2 showed that the theoretical Raman spectra of chloramphenicol were in a complete agreement with its experimental counterpart. The vibrational fundamentals were analyzed by comparing the measured modes with those from the literature [24], in combination with the results of DFT calculations. The assignments and frequencies of selected peaks from the theoretical and experimental Raman spectra are given in Table 1. Effects of solvents on the enhancement function The normal Raman spectra Before the SERS method was used to detect CAP in solution, the need for this enhancement and its potential were first examined. As shown in Fig. 3, the normal Raman spectrum of the CAP solution was almost the same as the spectra of water, with no clear peaks. This suggested that an additional step was needed in order to enhance the Raman signals in the CAP solution and to improve the detection limit. Due to its excellent SERS enhancement

capabilities, gold colloidal nanoparticles were used as a SERS substrate in our study. Effects of different solvents on SERS of CAP Optimizing the solvent is one of the most important factors affecting Raman signal strength in a SERS system [25]. The polarity of the solvent and the complexity of the molecular structure greatly influence the SERS signal. Four kinds of solvents, methanol, ethanol, N,N-dimethyl formamide (DMF) and ultrapure water, were used to analyze SERS measurements of CAP (Fig. 4). The characteristic CAP peaks at 1344 cm 1, 1102 cm 1 had clearly been enhanced when methanol was used as the solvent, although signal interference between methanol and the main CAP peaks was large, especially at 1596 cm 1 (Fig. 4A). Fig. 4B showed that spectral peaks of CAP in ethanol had much overlap with peaks derived from ethanol. As shown in Fig. 4C, SERS signals of CAP at 620 cm 1, 930 cm 1, 1104 cm 1, 1134 cm 1 and 1486 cm 1 are clearly enhanced by DMF, although there cannot was strong interference with the DMF signals. Fig. 4D displayed that ultrapure water not only obviously enhances the peak intensity of CAP at 860 cm 1, 1102 cm 1, 1344 cm 1, 1558 cm 1 and 1596 cm 1, but it contributes very little signal interference. Ultrapure water was considered the best of the four solvents for SERS analysis of CAP.

Table 1 Comparison of the experimental and theoretical Raman spectra of CAP and peak assignments. No.


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

360 400 620 754 840 860 930 970 1010 1102 1240 1288 1344 1464 1486 1516 1558 1596 1678

m = stretch; d = bending; Ph = phenyl.


Theoretical/cm 349 403 577 711 832 868 916 936 1014 1106 1194 1308 1344 1446 1480 1508 1556 1660 1658


Peak assignment

m(17C16O) m(CC) m(23NH + CO) d(28C–Cl2); m(CH + 27C28C) m(N–O) m(Ph–H) m(Ph–CH3 + Ph–H) m(Ph–CH3 + Ph–H) d(Ph–H); m(Ph–H) d(14C15O); m(Ph–H) m(O–H) m(15O17H + 18C23N + 20CH2) d(11NO2 + Ph–H); m(16C17H + 14CH + 18CH + 20CH2) d(C18C20); m(C20H2 + C25O26) m(14CH + 11NO2); d(Ph–CH3) m(Ph–CH3) m(20CH2) d(25C28O + 25C23N) d(23N24H + 27C + 28C); m(23N24H + 27C + 28C)


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Fig. 4. The influence of different solvents (A: methanol; B: ethanol; C: DMF; D: ultrapure water.) on SERS signals of CAP (a: normal Raman spectrum of CAP solid; b: SERS of the solvent; c: SERS of 50 lg/mL CAP solution).

Fig. 5. SERS spectra of the ratio of the gold colloidal nanoparticles and CAP solid.

Volume ratio of analytical system The SERS effect occurs mainly due to the charge transfer and electromagnetic effect of gold colloidal nanoparticles in the analyte.

Fig. 6. The effect of absorption time after the mixture of CAP and gold colloid nanoparticle on their SERS spectra.

When focused on the metal surface, the laser light can be absorbed by the molecules on the metal surface, and charge is transferred between metal and molecule, changing the polarization of the

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A series of CAP and gold colloidal nanoparticles solutions mixed for different lengths of time were analyzed by SERS. The results are shown in Fig. 6, where the extension of waiting time clearly influences the Raman effect. When the mixing time was at an extended 6 min, the enhancement effect was optimal. When the mixing time was extended for a further minute (7 min), the effect was weakened. Quantitative analysis of CAP by SERS

Fig. 7. Normal Raman spectra of CAP solid (a) and SERS spectra of CAP solution in case of triple distilled water as solvent 0.1 lg/mL (b), 1 lg/mL (c), 10 lg/mL (d), 50 lg/mL (e), 100 lg/mL (f).

Limitation of detection Fig. 7 depicts the effects of varying CAP concentration (100 lg/ mL, 50 lg/mL, 10 lg/mL, 1 lg/mL, 0.1 lg/mL) of CAP on the SERS spectra, where the ratio of CAP to gold colloidal nanoparticles and the mixing time were kept optimal at 6 min and 10:14 respectively. It was clear that the CAP still exhibited enhanced characteristic peaks at 1102 cm 1, 1344 cm 1 and 1596 cm 1 even at a concentration of 0.1 lg/mL. Thus the SERS detection limit of CAP in ultrapure water can reach 0.1 lg/ml, with characteristic CAP peaks at 1102 cm 1, 1344 cm 1 and 1596 cm 1 being used for qualitative analysis. Furthermore, it could also be seen that the peak intensity of CAP SERS spectra at 1344 cm 1 shows great linear correlation with CAP concentration (Fig. 8), making it the ideal candidate as the index peak for quantitative analysis of CAP.

system. Thus, SERS signals are increased dramatically [26]. In this study, CAP and gold colloidal nanoparticles are used as a mixed system for SERS detection. Thus, the concentration of fold colloidal nanoparticles relative to CAP used in the SERS system is one of the factors those influence the intensity of recorded SERS peaks in the CAP spectra. The effect of the ratio of the gold colloidal nanoparticles and CAP on SERS spectra is shown in Fig. 5. When the ratio of CAP and gold colloidal nanoparticle was tested in series from 10:8 to 10:16, a well-defined shape of the SERS spectra with maximum enhancement effect was achieved at a ratio of 10:14, with both lower and higher concentrations impairing the quality.

The regression equation Fig. 8 shows the correlation between CAP concentration and its SERS peak intensity at 1344 cm 1. A linear regression was established using the data collected measures SERS peak intensity at 1344 cm 1 from CAP at 0.1 lg/mL, 1 lg/mL, 10 lg/mL, 50 lg/mL, and 100 lg/mL, exhibiting a good linear correlation (R2 = 0.9802). These results revealed the potential of using the SERS peak at 1344 cm 1 for the accurate and consistent quantitative analysis of CAP residues in foods.

Absorption time

The precision of the method

It has been found that the mixing time between material substrate and test solution influences the SERS enhancement effect. When the substrate material was mixed with gold nanoparticles solution under test conditions, the solution had a certain degree of aggregation, which leaded to an enhancement effect at active hot spots [11]. So the activated time of hot spots was another key factor of SERS detection.

The analysis results of CAP residues of pork, chicken, beef and lamb meat samples are shown in Fig. 9 and summarized in Table 2. The mean recovery rates following 6 separate analysis were calculated as shown, CAP residues were undetectable at the concentration used 0.1 lg/mL in Pork, beef and lamb meat samples, while the chicken meat SERS spectrum had clear characteristic peaks at 1102 cm 1, 1344 cm 1 and 1596 cm 1, and the CAP residue was detected.

Fig. 8. Linear correlation between the signal intensity of SERS spectra at 1344 cm



Fig. 9. SERS spectra of CAP solution and CAP residues in 4 samples. (1: pork; 2: lamb meat; 3: beef; 4: chicken meat).


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Table 2 Test on the recovery rate of the method. Sample

CAP content detected (lg/kgFW)

Additive amount of CAP (lg)

Measured amount (lg/kg FW) 1






 ± SD X

The average recovery (%)




0.1 0.2 1

15.34 31.56 178.67

15.89 32.67 180.24

16.56 33.78 179.22

16.88 32.99 178.45

16.45 33.78 179.45

16.23 33.89 183.56

16.23 ± 0.55 33.11 ± 0.91 179.93 ± 1.89

81.13 82.78 89.97

3.36 2.74 1.05



0.1 0.2 1

15.56 31.33 177.53

16.78 32.41 178.98

15.67 31.78 174.65

15.88 32.78 177.84

16.32 33.89 174.36

16.67 33.78 178.87

16.04 ± 0.50 32.66 ± 1.04 177.04 ± 2.04

80.73 81.65 88.52

3.15 3.18 1.15



0.1 0.2 1

15.78 32.56 184.44

16.23 31.45 185.56

16.53 31.78 187.23

16.78 32.89 184.78

15.89 32.87 188.67

15.45 31.22 185.35

16.11 ± 0.50 32.13 ± 0.74 186.00 ± 1.62

80.55 80.32 93.00

3.08 2.30 0.87


0.1 0.2 1

117.72 145.56 292.06

115.08 146.98 289.17

115.62 138.34 288.99

118.89 145.07 288.82

116.42 147.89 286.94

115.56 148.98 285.35

116.55 ± 1.47 147.14 ± 1.56 288.56±2.27

83.18 91.89 90.14

1.26 1.06 0.79


Note: FW means fresh weight.

Recovery rate of CAP As can be seen in Table 2, all four samples’ recovery rates are over 80% and the variation coefficient (CV%) was between 0.79% and 3.36%. CAP residues in the chicken meat sample were detected at 120.12 lg/kg with a coefficient of variation was between 0.79% and 1.26%. This highlights the repeatability and reliability of the method for the use of quantitative analysis of large numbers of different food samples. Conclusions Gold colloidal nanoparticles used as a kind of SERS substrate can significantly increase Raman spectra signals of CAP, its characteristic peaks shape and SERS sensitivity can be greatly improved by the optimization of solvents, the volume ration of analytical ratio of substrate: analyte system and absorption time. The detection limit could reach at the levels of 0.1 lg/mL in using optimal conditions for enhancing the CAP spectra. Quantitative analysis of CAP residues in foods revealed that SERS technique with gold colloidal nanoparticles was sensitive and of a good stability and linear correlation. It was well suitable for rapid analysis of CAP residue of a great deal of food samples. Acknowledgments This article is supported by the National Natural Science Foundation of People Republic of China (No. 21203076), National Key Technology R&D Program in the 12th Five year Plan of China (No. 2012BAD36B02), Key Technology R&D Program in Jiangsu province (No. BE2012631), Open Project of State Key Laboratory of Supramolecular Structure and Materials (sklssm 201328), the Fundamental Research Funds for the Central Universities (JUSRP51309A) and the Priority Academic Program Development of Jiangsu Higher Education Institution (PAPD). References [1] P. Rajchgot, C.G. Prober, S. Soldin, et al., Chloramphenicol in the newborn infant, Prog. Clin. Biol. Res. 135 (1983) 421–426. [2] N.J. Shepard, N.G. Samaras, Chloramphenicol-induced aplastic anemia, Oral Surgery Oral Med. Oral Pathol. 29 (5) (1970) 689–782. [3] A. Pengov, V.C. Flajs, T. Zadnik, et al., Distribution of chloramphenicol residues in lactating cows following an external application, Anal. Chim. Acta 529 (2005) 347–351. [4] Commission Decision of 13 March 2003 amending Decision 2002/657/EC as regard the setting of minimum required performance limits (MRPLs) for certain residues in food of animal origin, EC, 2003–181.

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Rapid surface enhanced Raman scattering detection method for chloramphenicol residues.

Chloramphenicol (CAP) is a widely used amide alcohol antibiotics, which has been banned from using in food producing animals in many countries. In thi...
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