Gold Nanorods as Surface-Enhanced Raman Spectroscopy (SERS

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Gold Nanorods as Surface-Enhanced Raman Spectroscopy (SERS) Substrates for Rapid and Sensitive Analysis of Allura Red and Sunset Yellow in Beverages Yiming Ou, Xiaohui Wang, Keqiang Lai, Yiqun Huang, Barbara Rasco, and Yuxia Fan J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jafc.8b00007 • Publication Date (Web): 28 Feb 2018 Downloaded from http://pubs.acs.org on March 2, 2018

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Journal of Agricultural and Food Chemistry

Gold Nanorods as Surface-Enhanced Raman Spectroscopy (SERS) Substrates for Rapid and Sensitive Analysis of Allura Red and Sunset Yellow in Beverages Yiming Oua†, Xiaohui Wang a†, Keqiang Laia,b, Yiqun Huangc, , Barbara A Rascod, Yuxia Fana,b* a

College of Food Science and Technology, Shanghai Ocean University, No. 999 Hucheng Huan Road, LinGang New City, Shanghai, China 201306.

b

Engineering Research Center of Food Thermal Processing Technology, Shanghai Ocean University, No. 999 Hucheng Huan Road, LinGang New City, Shanghai 201306, China

c

School School of Chemistry & Biological Engineering, Changsha University of

Science & Technology, Changsha, Hunan 410076, China d

School of Food Science, Washington State University, Pullman, WA 99165, USA.

*Corresponding author: Telephone, (86-21)6190-0370; Fax, (86-21)6190-0365; E-mail, yxfan@ shou.edu.cn. †

These authors contributed equally to this work.

ACS Paragon Plus Environment


1

Abstract

2

Synthetic colorants in food can make a potential threat to the human health. In

3

this study, surface-enhanced Raman spectroscopy (SERS) coupled with gold nanorods

4

as substrates is proposed to analyze allura red and sunset yellow in beverages. The

5

gold nanorods with different aspect ratios were synthesized and its long-term stability,

6

SERS activity, and the effect of the different salts on SERS signal were investigated.

7

The results demonstrate that gold nanorods have a satisfactory stability (stored up to 28

8

days). SERS coupled with gold nanorods exhibit stronger sensitivity. MgSO4 was chosen to

9

improve the SERS signal of sunset yellow and no salts could enhance the SERS signal

10

of allura red. The lowest concentration was 0.10 mg/L for both colorant standard

11

solutions. The successful prediction results using SERS were much closer to those

12

obtained by HPLC for sample in beverages. SERS combined with gold nanorods

13

shows potential for analyzing food colorants and other food additives as a rapid,

14

convenient, and sensitive method.

15 16

Key words: surface-enhanced Raman spectroscopy; gold nanorods; beverages; colorant; rapid analysis


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Introduction

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Food colorants are essential to the food manufacturing industry, because they

19

affect the product color and thus the acceptability of the final products. They are

20

grouped into synthetic, natural, nature identical and caramel colors 1. Natural

21

colorants usually come from animal or plants tissues, they are safe to humans but are

22

more susceptible to fading and less stable to light and other chemicals. In

23

comparison, synthetic colorants are most widely used because they are brighter,

24

more stable, and more cost-efficient, however, some of them, such as azo-dyes,

25

present a potential risk to human health, especially for children in the case of

26

excessive consumption. Sunset yellow, known in the United States as FD&C Yellow

27

No. 6 (Acceptable daily intake (ADI): 2.5 mg/kg bw) and allura red, known in the

28

United States as FD&C Red No. 40 (ADI: 7 mg/kg bw), have been widely used as

29

food and beverage additives 2. They have been studied to be associated with

30

increased hyperactivity in children ages 8 – 9 and genotoxicity in murine models

31

with memory and learning deficits of the offspring 3. The permissible concentrations

32

of both colorants are from 20 – 500 mg/kg in China (GB/2760-2014) 4 or 25 – 400

33

mg/kg by Codex Alimentarius Commission (CAC) depending on the type of food 5.

34

High-performance

liquid

chromatography

(HPLC)

6-7

,

Liquid

8-9

35

chromatography-mass spectrometry (LC-MS)

, column solid-phase extraction

36

combined with UV–Vis spectrophotometry

37

used to determine the colorants in food. However, they have many disadvantages,

10

, and an electric method


11

have been


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such as labor-intensive, time-consuming expensive, poor repeatability and accuracy,

39

or complicated sample pretreatment 12-13.

40

Surface-enhanced Raman spectroscopy (SERS), which enhances the Raman

41

signals by molecules adsorbed on rough metallic nanostructured surface based on

42

electromagnetic and chemical theory

43

as 1010 to 1011 or higher 17-18. Rapid development of SERS has been achieved during

44

the last five years as evidenced by the explosive development of nanoscience and

45

nanotechnology. However, whether SERS can achieve a much broader application

46

depends greatly on the sensitivity of SERS and the reproducibility of the substrate 19.

47

SERS, a potential detection technique, has been applied as an attractive alternative

48

method to analyze biological and chemical target compounds due to its high

49

sensitivity and specificity 20-22.

14-16

. The enhancement factor can be as much

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Gold nanorods, which have anisotropic optical properties, have recently

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attracted a lot of attention due to their distinctive optical properties and long-term

52

stability at room temperature

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plasmon bands, they are very sensitive to the aspect ratio (AR) of nanorods, and

54

have been explored to increase the contribution of electromagnetic enhancement

55

mechanisms at a desired wavelength

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with ARs of 2.4, 5.8, and 16, and then used crystal violet as the probe to investigate

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their SERS activity. Au-Ag nanorods were synthesized and proved to possess SERS

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activity using sodium salicylate as an adsorbate probe

59

factors that affect the reproducibility of Au NRs and the long term and the short-term

23

. Gold nanorods show longitudinal and transverse

24

. Smitha et al.

25

26


synthesized Au nanorods

. John et al.

27

studied the

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stability of Au NRs in various conditions with UV-Vis spectroscopy. Under the

61

optimum conditions, the Au NR samples were stable for at least nine months.

62

Nguyen et al. 28 used gold nanorods and graphene as key materials to fabricate SERS

63

substrates for the detection standard solution system of pesticides. Alsammarraie et

64

al.29 and Zhang et al.

65

substrates for detection pesticides in fruit juice and milk. However, few studies have

66

been published that use Au nanorods to determine these two colorants at a series of

67

concentrations or in food samples. Reports of the Au nanorods stability characterized

68

by the SERS intensity of the probe molecule were also limited.

30

reported using standing gold nanorod arrays as SERS

69

The main purpose of this work was to develop a sensitive, rapid and convenient

70

SERS method coupled with gold nanorods for detection and determination of allura

71

red and sunset yellow in beverages. Au nanorods with different ARs as the substrates

72

were prepared, and its SERS activity, long-term stability, and the effect of the

73

different salts on SERS signal were investigated. Allura red and sunset yellow

74

standard solution and actual beverage samples can be detected directly or after

75

simply dilution, filtration. Partial least squares (PLS) models and the linear

76

relationship of SERS intensity of some characteristic peaks with the actual

77

concentrations were developed for quantitative analyses of allura red and sunset

78

yellow in standard solution and beverage samples.

79

2. Materials and methods

80

2.1 Preparation and characterization of gold nanorods



81 82

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2.1.1 Seed preparation Gold nanorods with different ARs were synthesized via a seed-mediated 24

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surfactant-directed approach described by Murphy

. In a typical procedure,

84

spherical gold seed with diameter of 3.5 – 4 nm were prepared by reducing 2.5 × 10-4

85

M chloroauric acid in 10 mL, 0.1 M CTAB (Hexadecyl trimethyl ammonium

86

Bromide, 99%, J&K) with 600 µL ice-cold 0.01 M NaBH4 (99%, Aldrich). After the

87

addition of this strong reducing agent, the color of the gold seed solution turned

88

brown, and the solution was stirred vigorously for 10 min before being transferred to

89

the incubator at 25°C for 2.5 h of aging.

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2.1.2 Growth of the gold nanorods

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For the synthesis of the nanorods, the as-prepared gold seed (12 µL) was added

92

to the growth solution containing CTAB (9.5 mL, 0.1 M), AgNO3 (20, 50 or 80 µL,

93

0.01 M), HAuCl4 (0.5 mL, 0.01 M), and ascorbic acid (55 µL, 0.1 M) under stirring.

94

The CTAB acted as a soft bilayer template for the nanorods growth with the aid of

95

AgNO3, then ascorbic acid as a mild reducing agent made it possible for the Au3+ to

96

be reduced to Au+; finally, the gold seed initiated the reaction of Au+ to Au0

97

mixture was left undisturbed overnight (14 – 16 h) at 27 °C, during which the

98

colorless solution turned blue, green, or brown depending on the amount of the

99

AgNO3 added.

100

22

. The

2.1.3 Au nanorod characterization

101

UV-Vis absorbance spectroscopy (UV3000PC, MAPADA Instruments Ltd.,

102

Shanghai, China) was applied to analyze the optical properties of the Au nanorods


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with different ARs. Transmission electron microscopy (TEM, JEM-2100F, JEOL

104

Ltd., Tokyo, Japan) was used to analyze particle size (statistic results from average

105

diameter of 100 particles) and surface morphology.

106

2.2 Preparation of the standard solution

107

Sudan IV (≥96%, Sigma-Aldrich) was dissolved in a mixture of water and

108

acetonitrile (1:1, v/v) (HPLC reagent; Sigma-Aldrich) to prepare a series of standard

109

solutions ranging from 0.08 – 5.00 mg/L. Allura red yellow (≥96%, Sigma-Aldrich)

110

or sunset yellow (≥96%, Sigma-Aldrich) were dissolved in the deionized water to

111

prepare a series of standard solutions ranging from 0.1 – 10.00 mg/L.

112

2.3 Beverage sample preparation and HPLC detection

113

Thirteen types of beverages were purchased from a local supermarket in

114

Shanghai. For allura red, five beverages (cocktail with peach flavor, energy drink,

115

pulpy strawberry juice, cranberry juice, and cranberry wine) contained the colorant.

116

The no-colored cocktail with mixed fruit flavor, red-colored vitamin water and grape

117

fruit juice were chosen as the controls. For sunset yellow, five beverages (orange

118

sprite, apple vinegar, pulp mango juice, pulp orange juice, orange vodka) contained

119

the colorant and the non-colored cocktail with mixed fruit flavor was chosen as the

120

control. Transparent samples could directly be analyzed by SERS and those pulpy

121

beverages should be passed through a 0.22 µm polytetrafluoroethylene (PTFE)

122

microporous film (SCAA-114) before the SERS measurement. Dilution is necessary

123

if the matrix interfered with the target signal to some extent. HPLC detection was

124

referenced to the regulation released by the China entry-exit inspection and



125

quarantine 31.

126

2.4 Spectral measurement

127

A Nicolet DXR microscopy Raman spectrometer (Thermo Fisher Scientific Inc.

128

Waltham, MA) with a He-Ne 633 nm laser source was used to collect SERS spectra

129

(400 – 2000 cm-1; resolution 4.7 cm-1) of Sudan IV, allura red, and sunset yellow.

130

The laser power used for collection was 5 mW. The accumulation of five scans with

131

two seconds for each scan was applied.

132

To obtain the SERS spectra, as-prepared Au nanorods were firstly centrifuged

133

for 10 min at 12000 rpm for twice to remove redundant CTAB and redispersed in

134

deionized water. Then, the nanorods were mixed with the standard solution of the

135

colorants or beverages sample in the ratio of 1:1 for Sudan IV and sunset yellow, 1:2

136

for allura red. The mixture was vortexed for six seconds to ensure of full interaction

137

of the colorants with the Au nanorods surface. Finally, 5 µL of the mixture was

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placed onto a microscope glass slide and dried at 50°C to evaporate the solvent. For

139

the study of the influence of the salts on the SERS intensity, sulfate salts Al2(SO4)3,

140

K2SO4, MgSO4 and the chlorine salts CaCl2 and MgCl2 were added immediately

141

after the colorant was added in the Au nanorods in the ratio of 10:10:1 (substrate:

142

colorant: salts). To make sure of representative measurement, 40 spectra from

143

random points of four baths (10 for each one) of substrates were recorded and

144

averaged.

145

2.5 Data analysis

146

OMINIC software (Thermo Nicolet Corp., Madison, WI, USA) was used for


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spectral acquisition. Delight 3.2.1 (Dsquared Development Inc., LaGrande, OR,

148

USA) was then used for quantitative analysis of the data. Four batches of substrate

149

were used to respectively collect ten spectra at random locations, and then the ten

150

spectra were averaged as one spectrum, that is, four averaged spectra for each

151

concentration (0.1 – 10.0 mg/L) were used to build the partial least squares (PLS)

152

model or to determine whether there were linear relationships between Raman

153

intensities of characteristic peaks and colorant concentrations of the standard

154

solutions or the beverage samples. PLS regression was employed to established

155

linear regression models between the spectra data and the actual concentration of the

156

allura red and sunset yellow in standard solutions or in the beverage samples. The

157

predictability of the model can be evaluated by the coefficient of determination (R2),

158

the root mean square errors (RMSE), and the ratio of performance to deviation (RPD)

159

of leave-one-out cross-validation.

160

3. Results and discussion

161

3.1 Characterization and stability of Au nanorods

162

Fig. 1 shows the UV-Vis spectra of the three gold nanorods, they all have two

163

plasmon bands, transverse plasmon band around 520 nm and longitudinal plasmon

164

bands red shifted from 624 nm to 715 nm with the amount of AgNO3 increasing

165

from 20 μL to 80 μL. This shape and position of the plasmon bands has a

166

relationship with the shape and size of the nanoparticles. The position of the

167

longitudinal plasmon band red-shifts, and the transverse plasmon band position stay



168

relatively constant at approximately 520 nm as the aspect ratio of the nanorods

169

increases 24. The results showed the nanorods grew longitudinally as the amount of

170

AgNO3 increased.

171

The TEM images in Fig. 2 further confirmed this result, and all the nanorods

172

were uniformly distributed except for very few nanospheres. The AR of the nanorods

173

increased from 1.8 (40.1 ± 3.8 nm by 21.8 ± 2.1 nm), 2.4 (41.3 ± 2.6 nm by 17.5 ±

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1.6 nm) to 3.5 (46.2 ± 2.9 nm by 13.1 ± 1.1 nm) with increased amounts of AgNO3.

175

This is probably due to the silver under potential deposition mechanism. The silver

176

deposition rate on crystallographic facets varied; the facets which have faster silver

177

deposition followed by strong CTAB binding and thus slower gold growth lead to

178

preferential growth of gold at the ends 32.

179

The stability of the nanorods with AR 1.8 was investigated. Fig. 1 shows that

180

when the Au nanorods remained in the growth solution for seven days, both the

181

absorption peak position and intensity changed significantly over times. The

182

longitudinal plasmon band in UV-Vis spectra would blue-shift, only transverse

183

plasmon remained and the intensity increased after seven days of storage if no

184

treatment was performed after the synthesis. The reason for this change is that the

185

nanorods continued to grow at a much lower rate because quantities of unreacted

186

Au3+ and Ag+ remained in the solution. Therefore, for long-term stability, twice

187

centrifugation was necessary to remove the subsequent nanorod growth. The

188

nanorods could be kept as long as 28 days after the twice centrifugation at

189

approximately 25°C. For the UV-Vis spectra recorded (Fig.1), there was no shift in


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the wavelength at the maximum peak position and no significant change in peak

191

intensity.

192

3.2 Activity of Au nanorods

193

Fig. 3 (a) showed the SERS activity of the Au nanorods with Sudan IV as the

194

probe molecule. The lowest concentration detected was 0.1 mg/L, which is slighter

195

better than the 0.2 mg/L with Au@Ag nanospheres as the substrate

196

characteristic bands at 456, 630, 989, 1099, 1200, 1232, 1386, 1487 and 1598 cm-1

197

could be clearly identified. The substrate itself has two major characteristic bands at

198

759 cm-1 and 1448 cm-1, which did not interfere with the signal of the Sudan IV

199

molecule. Meanwhile, the stability of the SERS activity for the Au nanorods with

200

AR 1.8 was investigated. Sudan IV was detected at 1 and 0.1 mg/L. SERS intensity

201

of peaks at 1099 cm-1 for Sudan IV and 1448 cm-1 for the substrate was recorded to

202

evaluate the sensitivity and stability change of the substrates as Fig. 3 (b) shows. Not

203

much variation was found within the peak intensity of the probe molecule or the

204

substrate. This is owing to the surfactant CTAB double layers around the gold

205

nanorods, which keep it monodispersed and steady in the solution. The concentration

206

of CTAB should not be too low in case of slight aggregation of Au NRs with less

207

repulsive force and not too high in avoid of more susceptibility to crystallization

208

when the room temperature dropped 23, 27.

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3.3 Optimal conditions for SERS detection of allura red and sunset yellow

33

. All of the

210

Fig. 4 (a) shows some representative SERS spectra of allura red at 10, 5 and 1

211

mg/L with Au nanorods in AR 1.8, 2.4 and 3.5. At the concentration of 10 mg/L,



212

there is no significant difference for the three gold nanorods, while as the

213

concentration decreased, gold nanorods of AR 2.4 show an advantage. Then, the

214

influence of sulfate salts Al2(SO4)3, K2SO4, MgSO4 and the chlorine salts CaCl2 and

215

MgCl2 on the SERS intensity are investigated with gold nanorods of AR 2.4 at the

216

allura red concentration of 5 mg/L. The result showed that when chlorine salts are

217

added, the signal of the allura red (Fig. 4 (b)) disappeared, and with the sulfate salts,

218

the colorants signal intensity remained the same or decreased compared with those to

219

which no salts were added.

220

Fig. 4 (c) shows that the intensity of 10 mg/L sunset yellow was no more than

221

1000 A.U, while the enhanced effect of Au nanorods (AR 1.8) was slight higher than

222

the other two. Then, the above salts were also added to the Au nanorods of AR 1.8 to

223

investigate whether the salts could improve or inhibit the SERS signal of sunset

224

yellow. The results indicate that all the sulfate salts could improve the SERS signal

225

in varying degrees and MgSO4 demonstrated the best results followed by Al2(SO4)3,

226

K2SO4. The SERS intensity of sunset yellow with MgSO4 added into the

227

colorant-substrate mixture was almost 10 times higher than that of without MgSO4.

228

But the signal of sunset yellow disappeared with chlorine salts added. So MgSO4

229

was chosen to improve the SERS signal of sunset yellow.

230

The CTAB bilayer on the gold nanorods could interact with the analyte,

231

allowing the analyte to enter the enhanced region of the gold nanorods surface.

232

Through the hydrophobic interaction between the analyte and CTAB, the analyte

233

could enter the hot spot region of one gold nanorod, the parallel superposition of two


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234

gold nanorods, or the end-to-end of two gold nanorods, or between the gold

235

nanorods

236

solution of metal colloid, ‘salt-induced aggregation’, is a common method to achieve

237

aggregation and provide more ‘hot spots’. In this study, the different salts could

238

improve or inhibit the SERS signal for the different colorants, which was related to

239

the probable mechanism of interaction between the substrate and the analyte.

23, 34-35

, as schematically depicted in Fig. 5. The addition of salt to a

240

Base on the mechanism of interaction between the substrate and the analyte,

241

two different results could occur because of the salt added. One is that the colorants

242

could disrupt part of the CTAB bilayer around the Au nanorods to get improved

243

accessibility to the metal surface and thus obtain a desirable SERS signal 34, such as

244

allura red molecule, which has strong affinity with gold nanorods through the azo

245

group (C–N=N–C), thus resulting in an excellent SERS response on gold nanorods 36.

246

On this condition, some types of salts added could cause gold nanorods colloidal

247

aggregation, which might break the balance of allura red and gold nanorod soltuion,

248

and also change the interaction of allura red with the metal surface. Thus, SERS

249

signal could not improve or could get worse. By contrary, sunset yellow had limited

250

ability to penetrate into or disturb the CTAB bilayer, which reduced sunset yellow

251

molecules adsorbed on the metal surface from the CTAB bilayer. The choice of salts

252

as aggregation-inducing agent

253

appropriate way for SERS hot spots generation and prevented deactivation of local

254

field enhancement. This function depends on the type of salts, amount of salts and

255

the properties of the analyte molecule. From our results, sulfate salts are better than

37-39

could assist their interaction in a more



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256

chlorine salts, and the intensity varied depending on the metal cations of the salts.

257

This indicates that it is the cations and the anions that play important roles in the

258

SERS enhancement.

259

3.4. Analysis standard solution of allura red and sunset yellow

260

The allura red molecule contains different functional groups, such as N=N,

261

naphthalene ring structure, SO3, and CH3, which have different vibration

262

characteristics. The most prominent peaks of allura red appeared at 1580 cm-1 and

263

1495 cm-1 due to C-C stretching vibration and C=C stretching in the benzene ring.

264

The other major characteristic peaks are attributed to the deformation vibration of

265

N=N (1405 cm-1), the C-O plane stretching vibration of C-O on the naphthalene ring

266

and benzene ring (1267 cm-1), the SO2 symmetrical stretching vibration (1219 cm-1),

267

the benzene ring symmetrical breathing vibrations and stretching vibration of C-H

268

(753 cm-1), and the rocking vibration of C-H (489 cm-1)

269

substrate of AR 2.4, the SERS spectra of allura red standard solution at a series of

270

concentrations are shown in Fig. 6 (a).

40

. With the optimal

271

Sunset yellow possesses characteristic spectra at 1592 cm-1 (CC stretching

272

vibration), 1497 cm-1 (C-H and N-H in plane bending), 1386 cm-1 (–OH and –SO3Na

273

in plane bending), 1224 cm-1 (CC in plane bending and C-N stretching vibration),

274

and 1166 cm-1 (N=N stretching vibration)

275

proper salts, Fig. 6 (b) shows the SERS spectra of the sunset yellow standard

276

solution at a series of concentrations.

277

41

. With the optimal substrate and the

Based upon the major characteristic bands in SERS spectra of standard


Page 15 of 39


278

solutions, allura red and sunset yellow could be visually identified at as low as 0.1

279

mg/L. In addition, the intensities of major characteristic peaks of SERS were

280

dependent upon the concentration of colorants. A linear relationship was observed

281

between amount of colorant and the spectral data within a certain range of

282

concentration. The PLS models were developed for allura red and sunset yellow

283

standard solution at the concentration ranging from 0.1 – 5.00 mg/L and 0.1 – 10.00

284

mg/L, respectively (Table 1). The linear correlation between the actual and predicted

285

concentrations of allura red (n=32) and sunset yellow (n=36) have obtained with R2

286

ranging from 0.925 and 0.982, RMSE from 0.40 and 0.48 mg/L and RPD from 3.6

287

and 7.4. The linear relationship between the analyte concentration and the intensity

288

of some primary peaks was also established (four independent spectra for each

289

concentration). Table 2 shows the results with R2 = 0.948 – 0.987, and RPD = 4.3 –

290

9.1. The best relationship was obtained based on the strongest peak for allura red of

291

1219 cm-1 and sunset yellow of 1592 cm-1, respectively. This indicated the

292

possibility for quantitative analysis of allura red and sunset yellow with SERS in

293

food samples.

294

3.5. Analysis of allura red and sunset yellow in beverages

295

Fig. 7 showed the representative SERS spectra of the colorants added into the

296

non-colored cocktail with different concentrations and their comparison with those

297

of standard solutions. For the allura red (a-d), the SERS intensity of non-colored

298

cocktail with the colorant at 10 or 5 mg/L was almost 10% lower than the

299

corresponding standard solutions. This indicated that the matrix has little influence



300

on the detection of the allura red. Thus, no dilution was made for the beverage

301

sample containing allura red. For the sunset yellow (e – i), the matrix effect was

302

obvious, and the signal for the 10 mg/L sunset yellow added into the no-colored

303

cocktail was 70% lower than that of standard solution. If the concentration increased

304

to the 20 mg/L, the SERS intensity was still lower because of the saturation effects.

305

However, if the colorant at 10 and 5 mg/L were added after the dilution of the

306

beverage by the same amount of water (equal to 20 and 10 mg/L for the original

307

beverage), the matrix influence would be decreased and a 1:1 dilution was necessary

308

for the beverage sample containing sunset yellow before the analysis.

309

Fig. 8 (a) presented the acquired representative spectra for the beverages with or

310

without allura red. All beverages that contained allura red could be detected. Also, no

311

color or red-color control (vitamin water and grape fruit juice) did not show

312

characteristic peaks of the colorant. The quantitative analysis allura red in actual

313

beverages was achieved by the PLS model and best linear regression of standard

314

solution because of minimal matrix effects. Fig. 8 (b) shows that a similar result that

315

could also be obtained for sunset yellow. The samples need to be diluted before

316

SERS analysis, so the new PLSR models and linear relationship was established for

317

determination of sunset yellow in beverages. The results exhibited good

318

predictability with R2 of 0.965 and 0.969, RMSE of 0.64 and 0.58 mg/L, and RPD

319

5.3 and 5.7 for PLSR model and linear regression, respectively.

320

The actual concentration of colorants in thirteen beverages was determined by

321

HPLC. This SERS prediction results are compared with HPLC in Table 3. Except for


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322

three beverages (cranberry juice, cranberry wine and orange juice), the other

323

predictions were much closer to those obtained by HPLC. T-test was used to evaluate

324

the difference between SERS and HPLC prediction. P-values of 0.132 or 0.178 were

325

calculated based on HPLC and each SERS prediction group, which was so much

326

higher than 0.05. The t-test results were not statistically significant. However, SERS

327

predictions for cranberry juice, cranberry wine and orange juice are significantly

328

different with HPLC results. For cranberry juice and cranberry wine, the natural

329

pigment of raw material appears strong red, and there are pulps in orange juice. The

330

matrix concentration or the natural pigment of the raw material could seriously

331

interfere with the SERS detection, which might be the primary reason for the three

332

beverages detection inaccurate. Then, different multiple diluents were also studied.

333

When the dilution ratio was respectively two times, fifteen times, and ten times for

334

cranberry juice, cranberry wine, and orange juice, the SERS predicted concentration

335

(2.88, 24.01, 19.28 mg/L) was close to that obtained by HPLC (3.55, 24.98, 21.12

336

mg/L). The results showed that the sample preparation was dependent on whether

337

the juice was pulpy and the type of the colorant. However, the SERS technique is

338

desirable for quantitative analysis of the colorants in beverages and has broad

339

potential application in the field of food safety.

340

4. Conclusions

341

In this study, gold nanorods with three different ARs were compared to achieve

342

the strongest enhancement effect for allura red and sunset yellow, and the influence



343

of the sulfate and chlorine salts were investigated. Gold nanorods show high

344

sensitivity and stability. The AR 2.4 or 1.8 of gold nanorods was selected as the

345

optimal substrate for allura red or sunset yellow. MgSO4 was chosen to improve the

346

SERS signal of sunset yellow. Quantitative analyses of allura red and sunset yellow

347

in beverages could be realized by this method with no or little sample preparation,

348

which has a higher prediction accuracy. The whole process was convenient, cost

349

efficient and quick. The methodology here could extend to similar studies on other

350

limited or prohibited food additives in food and may serve as the basis for further

351

investigation of determining ultra-traces of specific species in complex systems.


Page 18 of 39

Page 19 of 39


352

Acknowledgement

353

This research was supported by the National Natural Science Foundation of

354

China (No. 31501558), the Training Program for College Teachers in Shanghai

355

(A1-2061-17-000117), and the Agricultural Research Center, Washington State

356

University.



Page 20 of 39

357

References

358

1. Downham, A.; Collins, P., Colouring our foods in the last and next millennium. Int.

359

J. Food. Sci. Tech. 2000, 35 (1), 5-22.

360

2. Carocho, M.; Barreiro, M. F.; Morales, P.; Ferreira, I. C. F. R., Adding molecules

361

to food, pros and cons: A review on synthetic and natural food additives. Compr. Rev.

362

Food Sci. F. 2014, 13 (4), 377-399.

363

3. Wrolstad, R. E.; Culver, C. A., Alternatives to those artificial FD&C food colorants.

364

Annu. Rev. Food Sci. T. 2012, 3, 59-77.

365

4. GB 2760-2014. Food safety national standards for the usage of food additives.

366

http://www.cirs-group.com/food/downloads/china_national_standards_gb_download,

367

(Last accessed on December 21, 2017)

368

5. Codex Alimentarius Commission (CAC). General standard for food additives.

369

http://www.fao.org/fao-who-codexalimentarius/codex-texts/dbs/gsfa/en/,

370

accessed on December 21, 2017)

371

6. Chai, W.; Wang, H.; Zhang, Y.; Ding, G., Preparation of polydopamine-coated

372

magnetic nanoparticles for dispersive solid-phase extraction of water-soluble

373

synthetic colorants in beverage samples with HPLC analysis. Talanta. 2016, 149,

374

13-20.

375

7. Gosetti, F.; Chiuminatto, U.; Mazzucco, E.; Calabrese, G.; Gennaro, M. C.;

376

Marengo, E., Non-target screening of Allura Red AC photodegradation products in a

377

beverage through ultra high performance liquid chromatography coupled with hybrid

378

triple quadrupole/linear ion trap mass spectrometry. Food Chem. 2013, 136, 617-623. ACS Paragon Plus Environment

(Last

Page 21 of 39


379

8. Gosetti, F.; Chiuminatto, U.; Mazzucco, E.; Calabrese, G.; Gennaro, M. C.;

380

Marengo, E., Identification of photodegradation products of Allura Red AC (E129) in

381

a

382

quadrupole-time-of-flight mass spectrometry. Anal Chim Acta. 2012, 746, 84-89.

383

9. Gosetti, F.; Frascarolo, P.; Mazzucco, E.; Gianotti, V.; Bottaro, M.; Gennaro, M. C.,

384

Photodegradation of E110 and E122 dyes in a commercial aperitif: A high

385

performance liquid chromatography–diode array–tandem mass spectrometry study. J

386

Chrom A. 2008, 1202, 58-63.

387

10. Unsal, Y. E.; Soylak, M.; Tuzen, M., Column solid-phase extraction of sunset

388

yellow and spectrophotometric determination of its use in powdered beverage and

389

confectionery products. Int. J. Food Sci. Tech. 2012, 47 (6), 1253-1258.

390

11. Yu, L.; Shi, M.; Yue, X.; Qu, L., A novel and sensitive hexadecyltrimethyl

391

ammonium bromide functionalized graphene supported platinum nanoparticles

392

composite modified glassy carbon electrode for determination of sunset yellow in soft

393

drinks. Sensor. Actuat. B-Chem. 2015, 209, 1-8.

394

12. Pang, G. F.; Fan, C. L.; Liu, Y. M.; Cao, Y. Z.; Zhang, J. J.; Li, X. M.; Li, Z. Y.;

395

Wu, Y. P.; Guo, T. T., Determination of residues of 446 pesticides in fruits and

396

vegetables by three-cartridge solid-phase extraction-gas chromatography-mass

397

spectrometry and liquid chromatography-tandem mass spectrometry. J. Aoac. Int.

398

2006, 89 (3), 740-771.

399

13. Luo, H.; Huang, Y.; Lai, K.; Rasco, B. A.; Fan, Y., Surface-enhanced Raman

400

spectroscopy coupled with gold nanoparticles for rapid detection of phosmet and

beverage

by

ultra

high

performance


liquid

chromatography–


401

thiabendazole residues in apples. Food Control. 2016, 68, 229-235.

402

14. Cao, Y.; Zhang, J.; Yang, Y.; Huang, Z.; Long, N. V.; Fu, C., Engineering of

403

SERS substrates based on noble metal nanomaterials for chemical and biomedical

404

applications. Appl. Spectrosc. Rev. 2015, 50 (6), 499-525.

405

15. Haynes, C. L.; Mcfarland, A. D.; Duyne, R. P. V., Surface-enhanced Raman

406

spectroscopy. Anal. Bioanal. Chem. 2012, 394 (7), 1717-1718.

407

16. Cialla, D.; März, A.; Böhme, R.; Theil, F.; Weber, K.; Schmitt, M.; Popp, J.,

408

Surface-enhanced Raman spectroscopy (SERS): progress and trends. Anal. Bioanal.

409

Chem. 2012, 403 (1), 27-54.

410

17. Ru, E. C. L.; Blackie, E.; M. Meyer, A.; Etchegoin†, P. G., Surface enhanced

411

Raman scattering enhancement factors: A comprehensive study. J. Phys. Chem. C.

412

2007, 111 (37), 13794-13803.

413

18. Blackie, E. J.; Ru, E. C. L.; Etchegoin, P. G., Single-molecule surface-enhanced

414

Raman spectroscopy of nonresonant molecules. J. Am. Chem. Soc. 2009, 131 (40),

415

14466-14472.

416

19. Lin, X. M.; Cui, Y.; Xu, Y. H.; Ren, B.; Tian, Z. Q., Surface-enhanced Raman

417

spectroscopy: substrate-related issues. Anal. Bioanal. Chem. 2009, 394 (7),

418

1729-1745.

419

20. Aoki, P. H. B.; Furini, L. N.; Alessio, P.; Aliaga, A. E.; Constantino, C. J. L.,

420

Surface-enhanced Raman scattering (SERS) applied to cancer diagnosis and detection

421

of pesticides, explosives, and drugs. Rev. Anal. Chem. 2013, 32 (1), 55-76.

422

21. Bantz, K. C.; Meyer, A. F.; Wittenberg, N. J.; Im, H.; Kurtulus, O.; Lee, S. H.;


Page 22 of 39

Page 23 of 39


423

Lindquist, N. C.; Oh, S.-H.; Haynes, C. L., Recent progress in SERS biosensing. Phys.

424

Chem. Chem. Phys. 2011, 13 (24), 11551-11567.

425

22. Zheng, J.; He, L., Surface-enhanced Raman spectroscopy for the chemical

426

analysis of food. Compr. Rev. Food Sci. F. 2014, 13 (3), 317-328.

427

23. Becker, R.; Liedberg, B.; Kall, P. O., CTAB promoted synthesis of Au

428

nanorods--temperature effects and stability considerations. J. Colloid Interf. Sci. 2010,

429

343 (1), 25-30.

430

24. Murphy, C. J.; Thompson, L. B.; Chernak, D. J.; Yang, J. A.; Sivapalan, S. T.;

431

Boulos, S. P.; Huang, J.; Alkilany, A. M.; Sisco, P. N., Gold nanorod crystal growth:

432

From seed-mediated synthesis to nanoscale sculpting. Curr. Opin. Colloid In. 2011,

433

16 (2), 128-134.

434

25. Smitha, S. L.; Gopchandran, K. G.; Smijesh, N.; Philip, R., Size-dependent

435

optical properties of Au nanorods. Prog. Nat. Sci. 2013, 23 (1), 36-43.

436

26. Philip, D.; Gopchandran, K. G.; Unni, C.; Nissamudeen, K. M., Synthesis,

437

characterization and SERS activity of Au-Ag nanorods. Spectrochim. Acta. A. 2008,

438

70 (4), 780-784.

439

27. John, C. L.; Strating, S. L.; Shephard, K. A.; Zhao, J. X., Reproducibly synthesize

440

gold nanorods and maintain their stability. RSC Adv. 2013, 3 (27), 10909-10918.

441

28. Nguyen, T. H. D.; Zhang, Z.; Mustapha, A.; Li, H.; Lin, M., Use of graphene and

442

gold nanorods as substrates for the detection of pesticides by surface enhanced Raman

443

spectroscopy. J. Agric. Food Chem. 2014, 62 (43), 10445-10451.

444

29. Alsammarraie FK and Lin MS, Using standing gold nanorod arrays as



445

surface-enhanced Raman spectroscopy (SERS) substrates for detection of carbaryl

446

residues in fruit juice and milk. J. Agri. Food Chem. 2017, 65:666-674.

447

30. Zhang, Z.; Yu, Q.; Li, H.; Mustapha, A.; Lin, M., Standing gold nanorod arrays

448

as reproducible SERS substrates for measurement of pesticides in apple juice and

449

vegetables. J. Food Sci. 2015, 80 (2), 450-458.

450

31. SNT 1743-2006, Determination of allure red AC, carmosine, brillint blue FCF,

451

sunset yellow FCF in food-high performance liquid chromatogarphic method.

452

http://down.foodmate.net/standard/sort/4/11617.html, (Last accessed on December 21,

453

2017).

454

32. Orendorff, C. J.; Murphy, C. J., Quantitation of metal content in the

455

silver-assisted growth of gold nanorods. J. Phys. Chem. B. 2006, 110 (9), 3990-3994.

456

33. Ou, Y.; Pei, L.; Lai, K.; Huang, Y.; Rasco, B. A.; Wang, X.; Fan, Y., Rapid

457

analysis of multiple Sudan dyes in chili flakes using surface-enhanced Raman

458

spectroscopy coupled with Au–Ag core-shell nanospheres. Food Anal. Method. 2017,

459

10 (3), 565-574.

460

34. Guerrini, L.; Jurasekova, Z.; del Puerto, E.; Hartsuiker, L.; Domingo, C.;

461

Garcia-Ramos, J. V.; Otto, C.; Sanchez-Cortes, S., Effect of metal-liquid interface

462

composition on the adsorption of a cyanine dye onto gold nanoparticles. Langmuir.

463

2013, 29 (4), 1139-1147.

464

35. Wei, D.; Qian, W., Facile synthesis of Ag and Au nanoparticles utilizing chitosan

465

as a mediator agent. Colloid. Surface. B. 2008, 62 (1), 136-42.

466

36. Wu, DY.; Zhao, LB.; Liu, XM.; Huang, R.; Huang, YF.; Ren, B.; Tian, ZQ.,


Page 24 of 39

Page 25 of 39


467

Photon-driven charge transfer and photocatalysis of p-aminothiophenol in metal

468

nanogaps: a DFT study of SERS. Chem. Commun. 2011, 47, 2520-2522.

469

37. Bell, S. E. J.; Sirimuthu, N. M. S., Surface-enhanced Raman spectroscopy (SERS)

470

for sub-micromolar detection of DNA/RNA mononucleotides. J. Am. Chem. Soc.

471

2006, 128 (49), 15580-15581.

472

38. Nascimento, F. C.; Carneiro, C. E. A.; Santana, H. D.; Zaia, D. A. M., The effect

473

of artificial seawater on SERS spectra of amino acids-Ag colloids: An experiment of

474

prebiotic chemistry. Spectrochim. Acta. A. 2014, 118 (2), 251-259.

475

39. Otto, A.; Bruckbauer, A.; Chen, Y. X., On the chloride activation in SERS and

476

single molecule SERS. J. Mol. Struct. 2003, s661–662 (1), 501-514.

477

40. Xie, Y.; Li, Y.; Niu, L.; Wang, H.; Qian, H.; Yao, W., A novel surface-enhanced

478

Raman scattering sensor to detect prohibited colorants in food by graphene/silver

479

nanocomposite. Talanta. 2012, 100, 32-37.

480

41. Xie, Y.; Chen, T.; Cheng, Y.; Wang, H.; Qian, H.; Yao, W., SiO2@Au

481

nanoshells-based SERS method for detection of sunset yellow and chrysoidine.

482

Spectrochim. Acta. A. 2014, 132, 355-360.



483

Figure captions

484

Figure 1. UV-Vis spectra of the Gold nanorods synthesized with (a) 20, (b) 50 and (c)

485

80 μL AgNO3.

486

Figure 2. The TEM images of Au nanorods with different aspect ratio: (a) and (b) AR

487

1.8 , (c) and (d) AR 2.4 , (e) and (f) AR 3.5.

488

Figure 3. (a)Representative SERS spectra of Sudan IV with Au nanorods of AR 1.8 as

489

the substrate; (b) the intensity of 1099 cm-1peak in the SERS spectra of 1 and 0.1

490

mg/L Sudan IV standard solution and the intensity of 1444 cm-1 peak in the SERS

491

spectra for the substrate stored for different days.

492

Figure 4. Representative SERS spectra of (a) 10, 5 and 1 mg/L allura red standard

493

solution with different gold nanorods as the substrate; (b) the influence of salts on the

494

detection of 5 mg/L allura red standard solution; (c) the influence of different gold

495

nanorods and salts on the detection of 10 mg/L sunset yellow standard solution.

496

Figure 5. Probable mechanism of interaction between the substrate and the analyte.

497

Figure 6. Representative SERS spectra of (a) allura red standard solution at a series of

498

concentrations with the optimal Au nanorods; (b) sunset yellow standard solution at a

499

series of concentrations with the optimal Au nanorods and the salts of MgSO4.

500

Figure 7. Representative SERS spectra of (a) allura red standard solution (10 mg/L);

501

(b) allura red in the no-colored cocktail (10 mg/L); (c) allura red standard solution (5

502

mg/L); (d) allura red in the no-colored cocktail (5 mg/L); (e) sunset yellow standard

503

solution (10 mg/L); (f) sunset yellow in the no-colored cocktail (10 mg/L); (g) sunset

504

yellow in the 1:1 diluted no-colored cocktail (5 mg/L); (h) sunset yellow in the


Page 26 of 39

Page 27 of 39


505

no-colored cocktail (20 mg/L); (i) sunset yellow in the 1:1 diluted no-colored cocktail

506

(10 mg/L).

507

Figure 8. Representative SERS spectra of (a) allura red in actual beverage samples; (b)

508

sunset yellow in actual beverage samples.



509

Page 28 of 39

Table 1. PLS models developed by standard solution of two colorants Samples

Concentration range a

n

LV b

R2CV c

RMSE d

RPD

Allura red

0.10 – 5.00

32

4

0.925

0.45

3.6

Sunset yellow

0.10 – 10.00

36

4

0.982

0.48

7.4

510

a

The unit is mg/L ;b LV:Latent variable;c R2CV refers to correlation coefficient for cross-validation;

511

d

The unit for RMSE is mg/L


Page 29 of 39


512

Table 2. Linear correlation between the colorant concentration with some specific peaks

Allura red

Sunset yellow

Raman shifta

LEb

R2

RMSE c

RPD

1219

y=921.46x+392.25

0.955

0.35

4.7

1267

y=867.43x+405.29

0.954

0.35

4.7

1495

y=896.84x+378.36

0.948

0.38

4.3

1580

y=816.8x+0.0939

0.950

0.37

4.5

1166

y=392.53x+49.447

0.984

0.48

8.1

1386

y=829.34x+93.903

0.983

0.49

7.8

1497

y=494.76x+176.13

0.976

0.59

6.5

1592

y=913.62x+131.96

0.987

0.43

9.1

513

a

The unit is cm-1;b LE, Linear equation; x, concentration of allura red standard solution; y, the

514

Raman intensity of allura red at the specific wavenumber;c The unit for RMSE is mg/L



515

516

Page 30 of 39

Table 3. SERS and HPLC prediction of allura red or sunset yellow concentration in beverages

a

Beverage name

Colorant

HPLC (mg/L)

PLS model (mg/L)

Linear regression (mg/L)

Peach cocktail

allura red

2.07±0.03

2.98

2.23

Energy drink

allura red

1.93±0.06

1.25

1.56

Strawberry juice

allura red

2.22±0.13

1.99

1.40

Cranberry juice a

allura red

3.55± ±0.10

1.22

1.49

Cranberry wine

allura red

38.28± ±0.06

-0.58

-0.15

Vitamin water

n.d.

n.d.

n.d.

n.d.

Grape fruit juice

n.d.

n.d.

n.d.

n.d.

Cocktail (no colour)

n.d.

n.d.

n.d.

n.d.

Orange sprite

sunset yellow

24.98±0.08

18.89

18.11

Apple vinegar

sunset yellow

10.38±0.07

8.99

10.97

Mango juice

sunset yellow

11.00±0.11

10.58

12.89

Orange juice

sunset yellow

21.12± ±0.07

10.18

9.63

Orange vodka

sunset yellow

15.55±0.04

8.18

12.26

There had a quite different between SERS and HPLC prediction.


Page 31 of 39


517 518

Figure 1. UV-Vis spectra of the gold nanorods synthesized with (a) 20, (b) 50 and (c)

519

80 μL AgNO3.



520 521 522

Figure 2. TEM images of Au nanorods with different aspect ratios (ARs): (a) and (b)

523

AR 1.8, (c) and (d) AR 2.4 , (e) and (f) AR 3.5.


Page 32 of 39

Page 33 of 39


524 525

Figure 3. (a) Representative SERS spectra of Sudan IV with Au nanorods of AR 1.8

526

as the substrate; (b) the intensity of the 1099 cm-1 peak in the SERS spectra of 1 and

527

0.1 mg/L Sudan IV standard solution and the intensity of the 1444 cm-1 peak in the

528

SERS spectra for the substrate stored for different days.



529

530

531 532

Figure 4. Representative SERS spectra of (a) 10, 5 and 1 mg/L allura red standard

533

solution with different gold nanorods as the substrate; (b) the influence of salts on the

534

detection of 5 mg/L allura red standard solution; (c) the influence of different gold

535

nanorods and salts on the detection of 10 mg/L sunset yellow standard solution.


Page 34 of 39

Page 35 of 39


536 537

Figure 5. Probable mechanism of interaction between the substrate and the analyte.



538 539

Figure 6. Representative SERS spectra of (a) allura red standard solution at a series of

540

concentrations with the optimal Au nanorods; (b) sunset yellow standard solution at a

541

series of concentrations with the optimal Au nanorods and the salts of MgSO4.


Page 36 of 39

Page 37 of 39


542 543

Figure 7. Representative SERS spectra of (a) allura red standard solution (10 mg/L);

544

(b) non-colored cocktail spiked with allura red (10 mg/L); (c) allura red standard

545

solution (5 mg/L); (d) non-colored cocktail spiked with allura red (5 mg/L); (e) sunset

546

yellow standard solution (10 mg/L); (f) non-colored cocktail spiked with sunset

547

yellow (10 mg/L); (g) 1:1 diluted non-colored cocktail spiked with sunset yellow (5

548

mg/L); (h) non-colored cocktail spiked with sunset yellow (20 mg/L); (i) 1:1 diluted

549

non-colored cocktail spiked with sunset yellow (10 mg/L).



550 551

Figure 8. Representative SERS spectra of (a) allura red in actual beverage samples; (b)

552

sunset yellow in actual beverage samples.


Page 38 of 39

Page 39 of 39


553

Graphic for table of contents

554


Gold Nanorods as Surface-Enhanced Raman Spectroscopy (SERS

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