Green Synthesis of Fluorescent Carbon Dots for Selective Detection of

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Green Synthesis of Fluorescent Carbon Dots for Selective Detection of Tartrazine in Food Samples Hua Xu, Xiupei Yang, Gu Li, Chuan Zhao, and Xiangjun Liao J. Agric. Food Chem., Just Accepted Manuscript • Publication Date (Web): 08 Jul 2015 Downloaded from http://pubs.acs.org on July 9, 2015

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Journal of Agricultural and Food Chemistry

1

Green Synthesis of Fluorescent Carbon Dots for Selective

2

Detection of Tartrazine in Food Samples

3

Hua Xu,† Xiupei Yang,* ,† Gu Li,† Chuan Zhao,† and Xiangjun Liao*,‡

4 5

†

College of Chemistry and Chemical Engineering, China West Normal University, Nanchong 637000, P.R. China

6 7

‡

Exposure and Biomonitoring Division, Health Canada, 50 Colombine Driveway, Ottawa, K1A 0K9 Canada

8

ABSTRACT: A simple, economical, and green method for the preparation of water-soluble,

9

high-fluorescent carbon quantum dots (C-dots) has been developed via hydrothermal process

10

using aloe as a carbon source. The synthesized C-dots were characterized by atomic force

11

microscope

12

spectrophotometer, UV-vis absorption spectra as well as fourier transform infrared

13

spectroscopy (FTIR). The results reveal that the as-prepared C-dots were spherical shape with

14

an average diameter of 5 nm and emit bright yellow photoluminescence (PL) with a quantum

15

yield of approximately 10.37%. The surface of the C-dots was rich in hydroxyl groups and

16

presented various merits including high fluorescent quantum yield, excellent photostability,

17

low toxicity and satisfactory solubility. Additionally, we found that one of the widely-used

18

synthetic food colorants, tartrazine, could result in a strong fluorescence quenching of the

19

C-dots through a static quenching process. The decrease of fluorescence intensity made it

20

possible to determine tartrazine in the linear range extending from 0.25 to 32.50 µM, This

21

observation was further successfully applied for the determination of tartrazine in food

22

samples collected from local markets, suggesting its great potential towards food routine

23

analysis. Results from our study may shed light on the production of fluorescent and

24

biocompatible nanocarbons due to our simple and environmental benign strategy to

25

synthesize C-dots in which aloe was used as a carbon source.

26

KEYWORDS: Carbon quantum dots, tartrazine, aloe, fluorescence quench

(AFM),

transmission

electron

microscopy

27

ACS Paragon Plus Environment

(TEM),

fluorescence


28

■ INTRODUCTION

29

Tartrazine is one of the widely used synthetic food colorants that can be found in certain food

30

products such as candies, beverages, bakery products and dairy products.1,2 However, some

31

reports have revealed that tartrazine may cause adverse health effects like changes in hepatic

32

and renal parameters, reproductive toxicity, as well as neurobehavioural poisonousness when

33

it is excessively consumed.3,4 Therefore, the food industry must strictly control and regulate

34

the content of tartrazine in foods, which necessitates an interest in the development of an

35

efficient measurement technique to determine tartrazine in foods in terms of rapidness,

36

simplicity and sensitivity.

37

Until now, various instrumental techniques which analyzed tartrazine in foodstuff products

38

have been increasingly employed, which include thin-layer chromatography (TLC) method,5

39

electrochemical sensor,6 spectrophotometry,7 and high performance liquid chromatography

40

(HPLC).8 Nevertheless, these methods may not be suitable for routine monitoring due to they

41

require sophisticated equipments and time consuming sample preparation. As a result, the way

42

to develop a simple, economical, fast and reliable assay of tartrazine has been a challenge for

43

analytical researchers.

44

Recently, carbon quantum dots (C-dots), which are a new class of fluorescent nanomaterials

45

with a size less than 10 nm，have received much attention owning to their good water

46

solubility, excellent photostability, low toxicity and favorable biocompatibility.9,10 The

47

application of C-dots have been explored in fluorescent biosensing and in vivo bioimaging,

48

food detection together with food-packing domain.11-13 C-dots also served as reasonable

49

candidates for future nanodevices, cellular imaging, and biomedicine.14,15 Over the past years,

50

several methods have been developed for the synthesis of C-dots, including arc discharge,16

51

laser ablation,17,18 electrochemical oxidation19 and microwave irradiation.20 However,


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hydrothermal carbonization has provided great advancement over existing physical methods

53

which is due to its simplicity and production of C-dots with good quantum yield. Recently,

54

hydrothermal carbonization of chitosan, orange peels, coffee grounds, and grass have been

55

successfully applied to synthesize fluorescent C-dots which could be probes for recognizing

56

various chemical species and cells in vitro and in vivo.21-24 All of these proved that

57

hydrothermal is an eco-friendly, facile and classical route for the synthesis of C-dots in

58

aqueous media. From the point of material preparation, there is an urgent need to locate new

59

carbon source for simple, economical, and green synthesis of C-dots.

60

In this work, a facile and green method for the preparation of fluorescence C-dots by

61

hydrothermal treatment of aloe and the application has been proposed. Based on the

62

fluorescence quenching, the prepared C-dots can serve as an effective sensor for sensitive and

63

selective determination of tartrazine. The use of the synthesized C-dots for detection has been

64

validated by measuring the concentration of tartrazine in food samples collected from the

65

local supermarket.

66 67

■ EXPERIMENTAL PROCEDURE

68

Materials. Aloe was obtained from the potted plants in our laboratory and washed with

69

water for further use. Dichloromethane (CH2Cl2, 99.5%) was purchased from Aladdin

70

Industrial Corporation (Shanghai, China). Tartrazine (C16H9N4Na3O9S2, 87%), sunset yellow

71

(C16H10N2Na2O7S2,

72

amaranth (C20H11N2Na3O10S3, 85%) was received from Aladdin Chemistry Co. Ltd.

73

(Shanghai, China). Sodium dihydrogen phosphate (NaH2PO4·H2O) and disodium hydrogen

74

phosphate dodecahydrate (Na2HPO4·12H2O) was obtained from Tianjin Fuchen Chemical

75

Reagents Co., Ltd. (Tianjin, China). All chemicals were of analytical reagent grade and used

85%),

erioglaucine

disodium

salt

(C37H34Na2N2O9S3,85%)


and


76

without further purification. The ultrapure water used throughout the experiments was

77

purified through an UPH-II-20T up water purification system (Chengdu Ultrapure Technology

78

Co. Ltd , Chengdu, China).

79

Apparatus and Characterization. The AFM analysis was carried out on a

80

Multimode/Nanoscope (Veeco Corporation, USA) on a tapping mode with a RTESP-Veeco

81

cantilever on a platinum coated mica substrate. All absorption spectra were recorded on a

82

Shimadzu UV-2550 UV-vis absorption spectrophotometer (Kyoto, Japan). Fluorescence

83

measurements were conducted with a Cary Eclipse fluorescence spectrophotometer (Varian,

84

Palo Alto, CA, USA). The infrared spectra were obtained on a Nicolet 6700 Fourier transform

85

infrared (FTIR) spectrometer (Thermo Electron Corporation, USA) with passed KBr pellet at

86

room temperature.

87

Synthesis of fluorescent C-dots. The C-dots were prepared by hydrothermal treatment of

88

fresh aloe in water. In a typical procedure, 5 g of aloe was added into 25 mL of water and then

89

the mixture was transferred into a 25-mL Teflon-lined autoclave and was heated at 180 ℃for

90

a period of 11 h. After heating, the autoclaves were allowed to naturally cool down at fume

91

hood on a heat-resistant plate and the resulted yellow solution was filtrated with 0.22 µm

92

membrane followed by washing with dichloromethane to remove the unreacted organic

93

moieties. Finally, the upper light yellow aqueous solution containing C-dots was collected and

94

stored at 4 ℃ for further characterization and use.

95

Quantum yield measurements. The quantum yield of the as-synthesized C-dots was

96

measured on the basis of a procedure described previously.25 Rhodamine 6G aqueous solution

97

was used as a reference standard, which the quantum yield was 0.95 at 488 nm reported by the

98

literature. Absolute values of the quantum yield were calculated according to the following

99

equation:


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100


Φ x = Φ std

I x Astd η x2 2 Ax I std η std

101

where Φ is the quantum yield of the as-prepared C-dots , A is the absorbance and I is the

102

corrected emission intensity at the excitation wavelength, η is the refractive index of the

103

solvent. The subscript “std” and “x” refer to reference standard with known quantum yield

104

and for the C-dots solution, respectively. For the sake of reducing effects of re-absorption

105

within the sample on the observed emission spectrum, the absorbance values (A) of all

106

solutions in the 10 mm cuvette were always controlled under 0.1.

107

Sample pretreatment. Candy, steamed buns made of corns and honey were selected as

108

test samples because tartrazine may be as a colorant added into them. All samples were

109

obtained from the local supermarket in Nanchong, China. The candy or honey sample (10.0 g)

110

was crushed and subsequently dissolved in hot water (~60 ℃). The resulted solution was

111

transferred and diluted to 50 mL volumetric flask. The diluted solution was filtered through a

112

0.45 µm filter membrane for subsequent use. The 10.0 g steamed buns and certain amount of

113

water were added into a 100 mL beaker and then the mixture was blended by electric mixer

114

and extracted with ultrasonic for 15 min, respectively. After extraction, the mixture was

115

centrifugated at 12000 rpm for 10 min followed by transferring the supernatant and diluting to

116

50 mL. The diluted solution was also filtered through a 0.45 µm filter membrane for

117

subsequent use. The above sample pretreatment method is reference to literature with some

118

minor modifications.26

119

Detection of tartrazine in food samples. The tartrazine detection procedure was carried

120

out in phosphate buffer (PB) (30 mM, pH 6.0) at 5 ℃. In a typical run, 450 µL of C-dots

121

solution was added into 500 µL of PB, followed by adding 1000 µL of sample solution and

122

mixing thoroughly. The resulted mixture was reconstituted to 4 mL with water. After reaction

123

time of 5 min at 5 ℃, the spectra were recorded under excitation at 441 nm with the slit



124 125

widths setting at 10/10 nm. All the recoveries were calculated based on the equation below: Recovery = (Cmeasured - Cinitial) / Cadded

126 127

■ RESULTS AND DISCUSSION

128

Optimization of the synthesis conditions. In order to make the excellent performance of

129

the synthesized C-dots, we have optimized the time and temperature of the synthesis simply

130

which the results shown in Figure S1 and Figure S2. From Figure S1 we can see clearly that

131

the fluorescence intensity gradually increased with the reaction time up to 11 h while

132

decreased when the time exceeded 11h. So 11 h was choosed as the optimal reaction time.

133

Simultaneously, as displayed in Figure S2 the fluorescence intensity increased with the

134

reaction temperature rise. We finally choosed 180 °C as the optimal temperature owning to

135

when the temperature exceed 180 °C, the fluorescence intensity increase not that obviously.

136

Characterization. Figure. 1 shows the typical AFM image of the as-synthesized C-dots

137

solution. It reveals that the C-dots are well dispersed in solution with spherical shape and have

138

an average size of 5 nm approximately. Similarly, Figure S3 shows the typical TEM image of

139

the C-dots. It can be seen that the C-dots are in the monodispersion state and the size of the

140

them is consistent with the results of AFM.

141

The absorption (black line) and emission spectra (green line) of the as-synthesized C-dots

142

were shown in Figure 2. A peak at 278 nm exhibited in the UV-vis absorption spectrum,

143

which was attributed to n- π* transition of C=O and π- π* transition of C=C.27 The

144

photoluminescent (PL) spectrum shows an optimal emission peak at about 503 nm when

145

excited at 441 nm. The inset photograph in Figure. 2 indicates the C-dots aqueous solution

146

under visible(a) and under UV illumination at 365 nm (b). The bright yellow PL of the

147

C-dots under UV light is strong enough to be seen with the naked eye while when added some

148

tartrazine the fluorescence quenched obviously (c). The full width at half maximum (FWHM)


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149

is 100 nm, suggesting that the relatively small size distribution of C-dots, which was

150

consistent with AFM and TEM data and approximately equal to that of most reported

151

C-dots.28,29 The strong fluorescence can be caused by the surface energy traps in the C-dots

152

that become emissive upon stabilization.17

153

To further investigate the optical properties, PL emission spectrum of the C-dots was

154

recorded at progressively increased excitation wavelengths (Figure. 3). It can be observed that

155

a red-shift was attributed in the emission spectra of C-dots from 443nm to 525 nm with

156

increasing excitation wavelengths, accompanied with the decrease of the fluorescence

157

intensity, revealing that the fluorescence of C-dots is strongly dependent on the excitation

158

wavelength. This finding is in substantial agree with that of Vaibhavkumar.30,31 To investigate

159

the components, surface groups and structure of the as-synthesized C-dots, EDS and FT-IR

160

have been carried out. Figure S4 shows the as-prepared C-dots are mainly composed of C, H,

161

O and N. As shown in Figure 4, a characteristic absorption bands of the −OH stretching

162

vibration mode at about 3400 and 1073 cm−1 could be observed. The band at 2923 cm−1

163

corresponds to the C−H stretching mode.32 In addition, the peaks appear at 1590 and 1400

164

cm−1 may be caused by the asymmetric and symmetric stretching vibration of COO−,

165

respectively. These findings provide evidence that both the hydroxyl and carboxylic groups

166

were originated from carbohydrates in the aloe.

167

It is well known that the photostability of C-dots plays a key role in sensitive fluorescence

168

detection. In this connection, we studied the emission behavior of the C-dots under

169

continuous UV light illumination at 365 nm for 120 min. It was also noticed that as shown in

170

Figure S5 the photobleaching of C-dots is not observed and the fluorescence intensity of

171

C-dots is remained constant even after 120 min of continuous UV light illumination,

172

indicating the good photostability of C-dots. Using rhodamine 6G as a reference, a PL

173

quantum yield (QY) of 10.37% was measured. Table S1 shows the comparison of the optical



174

properties and applications of the C-dots derived from aloe with the reported methods. It can

175

be seen that the present method are green, simple and have a relatively high quantum yield. At

176

the same time, it is worth mentioning that the as-prepared C-dots emitting strong yellow

177

fluorescence while the most reported carbon dots blue and it can be a sensitive fluorescent

178

probe for colorants detention.

179

Design Principle of the Sensor. Under the same experimental conditions, the fluorescence

180

spectra of the C-dots alone and the system of C-dots with tartrazine were recorded,

181

respectively. As shown in Figure S6, the C-dots presented strong fluorescence at 503 nm

182

when excited at 441 nm. Upon the addition of tartrazine, the fluorescence intensity of the

183

prepared C-dots decreased significantly. Basing on the fluorescence quenching, we speculated

184

that a facile fluorescence sensor for the determination of tartrazine could be constructed. The

185

synthetic strategy for C-dots and the principle of tartrazine sensing are schematically

186

presented in Scheme 1.

187

Mechanism of Fluorescence Quenching. Broadly speaking, various kinds of molecular

188

interactions with the quencher molecule can reduce the fluorescence quantum yield, such as

189

electron or energy transfer, collisional quenching, excited-state reaction and ground-state

190

complex formation The quenching mechanisms are usually divided into dynamic quenching

191

which resulting from collision and static quenching, resulting from the formation of a

192

ground-state complex between the fluorescence material and quencher. On the other hand,

193

they could be distinguished by some additional formations such as the relationship between

194

the quenching and viscosity, temperature and life time measurements.33 In general, the

195

dynamic fluorescence quenching constants will increase with the raise of the system

196

temperature due to the energy transfer efficiency and the effective collision times between

197

molecules will also increase. On the contrary, the values of the static fluorescence quenching

198

constants will decrease with the raise of temperature. Let us suppose that the mechanism is


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199 200


the dynamic quenching, it can be described by the following Stern-Volmer equation:34 F0 / F = 1 + K SV [Q ] = 1 + K qτ 0 [Q ]

201

where F0 and F are the C-dots fluorescence intensities at 503 nm in the absence and presence

202

of tartrazine, respectively. KSV and Kq are the Stern-Volmer quenching constant and the

203

bimolecular quenching constant, respectively. [Q] is the concentration of tartrazine and τ0 is

204

the average lifetime of the C-dots without any other fluorescence quencher, with a general

205

value of 10-8 s. Figure 5 shows the fluorescence intensities of the C-dots analyzed by plotting

206

F0/F versus [Q] at 278, 288, 298 and 308 K. Table 1 summarizes the calculated KSV and Kq

207

values for each temperature. As shown, the KSV is inversely correlated with temperature and

208

the value of Kq is far larger than 2.0 ×1010 L·mol-1·s-1, which is the maximum scatter collision

209

quenching constant. These findings indicate that the quenching process may be caused by

210

static quenching. Additionally, the UV-vis spectra of the prepared C-dots alone and the system

211

of the C-dots with tartrazine are illustrated in Figure S7. As can be seen from this figure, with

212

the addition of tartrazine, the absorbance intensity of the C-dots at 280 nm increases, and with

213

a blue shift. This observation indicate that the formation of ground-state complexes is

214

generated due to the interaction between tartrazine and C-dots.

215

Optimization of Experimental Conditions. With the purpose of investigating the

216

sensitivity, precision and selectivity of the analytical method, the parameters including the

217

media pH, dosage of C-dots, reaction temperature and incubation time were systematically

218

optimized for the system.

219

The Effect of the solution pH on the fluorescence quenching of C-dots in the presence of

220

tartrazine is shown in Figure 6(a). An increase in pH from 4.0 to 6.0 results in the increased

221

fluorescence quenching efficiency (represented as F0/F, where F0 and F are the fluorescence

222

intensities of the C-dots at 503 nm before and after the addition of tartrazine, respectively.)

223

whereas a further increase in pH from 6 to 7.5 leads to a gradual decrease. Such observation



224

suggests that the fluorescence intensity of the C-dots strongly depends on the pH value of the

225

system. Our results are consistent with those of C-dots functioned with hydroxyl and

226

carboxylic/carbonyl moieties.10,19,32 Consequently, we selected 6.0 as the optimal pH for our

227

study.

228

The effect of the dosage of C-dots on the fluorescence quenching efficiency is presented in

229

Figure 6(b). The fluorescence quenching efficiency gradually increased with the dosage of

230

C-dots up to 450 µL. When the dosage of C-dots exceeded 450 µL, the fluorescence

231

quenching efficiency decreased. Therefore, 450 µL was used as the optimal dosage for further

232

performance

233

Figure 6(c) shows the fluorescence curves of the system at different temperatures. As the

234

temperature increased from 5 ℃ to 35 ℃ , the fluorescence quenching efficiency decreased

235

gradually. Among the temperatures studied, the maximum fluorescence intensity efficiency

236

was achived at 5 ℃. Hence, 5 ℃ is selected as the optimum reaction temperature.

237

The effects from incubation time on the fluorescence intensity of the system is shown in

238

Figure 6(d). No significant changes in F0/F was observed after incubation time of 1 min. In

239

order to ensure the consistency of the whole experiment, it is important to record the stable

240

fluorescence signal. Thus, 5 min is conservatively chosen as the optimum incubation time.

241

Analytical Performance for tartrazine Sensing. Sensitivity. The dependence of F0/F on

242

the different concentrations of tartrazine under the identical conditions is shown in Figure 7.

243

As displayed, the fluorescence quenching efficiency of C-dots gradually decreases with an

244

increase in the concentration of tartrazine. As shown in inset of Figure 7, the decrease in

245

fluorescence quenching efficiency was exhibited a linear response to the tartrazine

246

concentration in the range of 0.25-32.50 µM, which was consistent with the photograph of the

247

solutions under UV light. The calibration curve can be depicted as F0/F= 0.9604+ 0.0577X

248

( X is the concentration of tartrazine, µM) with a correlation coefficient of 0.9986. The


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249

relative standard deviation (RSD) was 0.25% through five parallel determinations (n=5) at a

250

fixed tartrazine concentration of 10.00 µM, indicating the excellent reliability of this sensor.

251

The detection limit is estimated to be 73 nM at a signal-to-noise ratio of 3.

252

In table 2, we compared the experimental results with those reported methods for tartrazine

253

detection. As shown in Table 2, our developed assay exhibits a wider linear range and lower

254

RSD compared to some methods. Our method can be an alternative to others for the

255

determination of tartrazine in samples, notwithstanding the limit of detection (LOD) from us

256

is not the smallest in Table 2, It is worth mentioning almost all of the reported sensors need

257

special equipment, a sophisticated technique, or complicated operations. By contrast, the

258

sensor we developed here has its own features including low cost in instrument, simplicity in

259

operation and fast response, which makes it more applicable for routine analysis of the

260

tartrazine in foods.

261

Selectivity. To evaluate the selectivity of this sensing system, we examined the

262

fluorescence response of the system to tartrazine at a concentration of 5.0 µM with the

263

presence of co-existing foreign substances such as K+, Ca2+, Zn2+, Fe3+, HCO-3 , NO-2 , glutamic

264

acid, glutathione (GSH), citric acid, phenylalanine, starch, tartaric acid, vitamin C, glucose,

265

and lactose, sunset yellow, erioglaucine disodium salt, amaranth under the same conditions.

266

As shown in Figure 8, for the present study, various different substances were added in the

267

test solution at the amount of 100-times tartrazine initially and the ratio would be gradually

268

reduced when the interference presented. It was found that some of the materials, such as Fe3+,

269

sunset yellow, erioglaucine disodium salt and amaranth, could be only allowed at relatively

270

lower levels. Nevertheless, the concentration of these substances were much lower than the

271

allowed levels in food samples. Meanwhile, most of the common excipients in foods could be

272

tolerated at high concentrations up to 100-times. That is to say the established strategy own a

273

highly selectivity toward tartrazine detection.



274

Application in Food Samples. The developed approach was employed to detect the trace

275

level of tartrazine in some food samples. The results for the pre-treated food samples spiked

276

with known amounts of standard tartrazine are shown in Table 3. The recoveries for the

277

intra-day and inter-day ranged from 88.6% to 103.4% and 87.3% to 106.6%, respectively. All

278

of these results indicate that the accuracy and reliability of the proposed method can be

279

applied to the determination of tartrazine in food samples.

280

In summary, the C-dots based on aloe was synthesized via a simple and green method.

281

Without further chemical modification, the synthesized C-dots have been applied to the

282

sensitive and selective detection of tartrazine in some food samples. This new C-dots

283

described here may extend their great potential for cell imaging and drug delivery applications

284

due to the simplicity of their synthesis procedure and the use of affordable and environmental

285

friendly aloe as carbon sources.

286 287

■ ASSOCIATED CONTENT

288

Supporting Information description.

289

Experimental procedures for EDS, supplementary figures of EDS, fluorescence spectra and

290

absorbance spectra of the C-dots and the system of C-dots-tartrazine. This material is

291

available free of charge via the Internet at http://pubs.acs.org.

292 293

■ AUTHOR INFORMATION

294

Corresponding Author

295

*(X.Yang) Tel./Fax: +86-817-2568081. E-mail: [email protected] and (X. Liao) Tel./Fax:

296

613-415-2098. E-mail [email protected]

297

Funding

298

No.


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299

Notes

300

The authors declare no competing financial interest.

301

Abbreviations Used

302

C-dots, carbon quantum dots; TEM, transmission electron microscopy; AFM, atomic force

303

microscope; FTIR, fourier transform infrared spectroscopy; EDS, energy dispersive

304

spectrometry;

305

chromatograph; FWHM, full width at half maximum; PB, phosphate buffer; PL,

306

photoluminescent; QY, quantum yield; LOD, limit of detection; RSD, relative standard

307

deviation; GSH, glutathione.

TLC,

thin-layer

chromatography;

HPLC,

high

performance

liquid

308 309

■ Acknowledgments

310

The authors thank the Natural Science Foundation of China (21277109) and the Program for

311

Young Scientific and Technological Innovative Research Team in Sichuan Province

312

(2014TD0020) for research grants. The authors thank Prof. Martin M.F. Choi of the

313

Department of Chemistry, Hong Kong Baptist University, for valuable suggestions and

314

fluorescence spectra study.

315 316

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hybrid and its application for simultaneous determination of sunset yellow and tartrazine.

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(7) Sahraei, R.; Farmany, A.; Mortazavi, S. S. A nanosilver-based spectrophotometry

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(36) Zhang, Y. Y.; Hu, L. T.; Liu, X.; Liu, B.F.; Wu, K. B. Highly-sensitive and rapid

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(38) Dominguze, F. B.; Diego, F. G.; Mendez, J. H. Determination of sunset yellow and tartrazine by differential pulse polarography. Talanta. 1990, 37, 655-658. (39) Zhang, W. K.; Liu, T.; Zheng,X. J.; Huang, W. S.; Chidan Wan, C. D.

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Surface-enhanced oxidation and detection of Sunset Yellow and Tartrazine using

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multi-walled carbon nanotubes film-modified electrode. Colloids Surf. B. Biointerfaces. 2009,

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solid phase spectrophotometry using partial least squares multivariate

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coupled with diode-array detector. Anal. Chim. Acta. 2007, 583, 103-110.

445


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Page 19 of 32


446

Figure Captions

447

Figure 1. AFM images of the C-dots.

448

Figure 2. Uv-vis absorption (black line) and fluorescence emission (green line) spectra of the

449

C-dots. The inset shows the photographic images of C-dots under visible light (a), C-dots

450

under ultraviolet light (b), and C-dots with tartrazine (22.5 µΜ) under ultroviolet light (c).

451

Figure 3. Fluorescence emission spectra of C-dots obtained at different excitation

452

wavelengths progressively increasing from 370 nm to 480 nm with a 10 nm increment.

453

Figure 4. FT-IR spectrum of C-dots.

454

Figure 5. Stern-Volmer plots for the system of C-dots-tartrazine under the temperature of 278

455

K，288 K，298 K, 308 K, respectively. Where F0 and F are the fluorescence intensity of C-dots

456

in the absence and presence of tartrazine. Conditions: C-dots, 450 µL and PB, 30 mΜ,

457

pH=6.0.

458

Figure 6. Effect of (a) pH of buffer solution, (b) dosage of C-dots, (c) reaction temperature

459

and (d) reaction time on fluorescence quenching efficiency of the C-dots-tartrazine system.

460

Where F0 and F are the fluorescence intensity of C-dots in the absence and presence of

461

tartrazine. Conditions: PB, 30 mM; tartrazine, 10 µΜ

462

Figure 7. Fluorescence emission spectra of C-dots in the presence of different concentrations

463

of tartrazine in 30 mΜ PB (pH=6.0). From a to l: 0.00, 0.25, 0.75, 2.50, 3.75, 5.00, 7.50,

464

12.50, 17.50, 22.50, 27.50, 32.50 µΜ. C-dots, 450 µL. (Inset) The insets shows the

465

photographic images of the corresponding solutions under UV light and the relationship curve

466

between F0/F and concentration of tartrazine.

467

Figure 8. Effects of potentially interfering substances. Conditions: C-dots, 450 µL; PB, 30

468

mM, pH=6.0; tartrazine, 5.0 µΜ. (0) noninterrerence, (1) glucose, 500 µΜ, (2) lactose, 500

469

µΜ, (3) starch, 500 µΜ, (4) citric acid, 500 µΜ, (5) tartaric acid, 500 µΜ, (6) ascorbic acid,

470

500 µΜ, (7) glutamic acid, 250 µΜ, (8) phenylalmine, 250 µΜ, (9) NO 2 , 500 µΜ, (10)

471

HCO 3 ,500 µΜ, (11) Ca2+, 500 µΜ, (12) Zn2+,500 µΜ, (13) K+, 500 µΜ, (14) Fe3+, 25 µΜ, (15)

472

sunset yellow, 5.0 µΜ, (16) erioglaucine disodium salt, 25 µΜ, (17) amaranth, 5.0 µΜ.

-

-

473



Page 20 of 32

474

Table 1. Stern-Volmer Quenching Constants for the Interaction of C-dots and Tartrazine

475

at Different Temperatures

pH

T (K)

KSV (L· mol-1)

Kq (L· mol-1 · s-1)

R

SD

6.0

278

5.663×104

5.663×1012

0.9984

0.0278

6.0

288

5.213×104

5.213×1012

0.9960

0.0402

6.0

298

5.178×104

5.178×1012

0.9967

0.0361

6.0

308

4.734×104

4.734×1012

0.9939

0.0452

476


Page 21 of 32


477

Table 2. Comparison of the proposed method with other methods for determination of

478

tartrazine Method

R2

Linear range (µM)

LOD (nM)

RSD%

Ref.

Graphene and mesoporous TiO2 Electrochemical sensor

0.02-1.18

0.994

8

2.70

26

Spectrophotometry method

0.00131-0.67

0.992

0.56

0.98

23

Electrochemical detection

0.11-56

-

56

3.12

17

Electrochemical detection

0.05-20

-

14.3

-

18

Electrochemical sensor

0.00936-0.37

0.994

2.8

4.3

22

High-performance liquid chromatography

0.0934-9.34

0.999

18.5

4.3

35

Alumina microfibers-based electrochemical sensor

0.005-0.14

0.998

2.0

4.7

36

Gold nanoparticles carbon paste electrode

0.05-1.6

0.997

2

1.1

37

Differential pulse polarography

0.19-19

0.999

30

-

38

Multi-walled carbon nanotubes film-modified electrode

0.37-74.8

0.990

187

5.2

39

Solid phase spectrophotometry

0.094-1.22

0.998

-

4.00

40

Capillary zone electrophoresis

5.6-178

0.995

2430

-

41

Thin-layer chromatography

74.9-356

0.992

-

0.03

5

Reversed-phase high-performance liquid chromatography

0.011-39.3

0.999

3.5

-

42

Fluorescence analysis

0.25-32.5

0.998

73

0.25

479 480


This work


481

Page 22 of 32

Table 3. Recovery test and precision of the analysis of tartrazine in food samples Intra-day Food samples

Steamed buns

Honey

Candy

Detected (µΜ)

NDb

NDb

4.80

Spiked (uM)

Inter-day

Founda (uM)

Recovery (%)

RSD (%)

Founda (uM)

Recovery (%)

RSD (%)

1.00

1.00±0.07

99.9

2.8

1.06±0.09

106.6

4.2

5.00

4.96±0.07

99.2

0.6

4.99±0.07

99.8

0.6

7.00

7.00±0.05

100.0

0.3

7.02±0.13

100.3

0.7

1.00

1.04±0.11

103.4

3.9

1.05±0.15

105.4

5.8

5.00

4.96±0.03

99.1

0.2

5.00±0.07

99.9

0.6

7.00

7.04±0.10

100.6

0.7

7.00±0.18

100.0

1.0

3.00

7.48±0.15

88.6

0.8

7.44±0.08

87.3

0.4

5.00

9.44±0.06

92.3

0.2

9.51±0.12

93.6

0.5

7.00

11.12±0.12

90.0

0.5

11.16±0.10

90.4

0.4

a

Value = mean ±S.D (n=5).

b

Not detectable.

482 483


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484


Figure 1

485 486



487

Figure 2

488 489


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490


Figure 3

491



492

Figure 4

493 494


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Page 27 of 32

495


Figure 5

496 497



498

Figure 6

499


Page 28 of 32

Page 29 of 32

500


Figure 7

501 502



503

Figure 8

504 505


Page 30 of 32

Page 31 of 32


506

507 508

Scheme 1. Scheme of the synthetic strategy for C-dots and the principle of tartrazine

509

sensing.



1

Graphic for manuscript

2


Page 32 of 32

Green Synthesis of Fluorescent Carbon Dots for Selective Detection of

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