Figure and Table Captions Scheme 1. Schematic illustration for

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New Analytical Methods

Visual Assay of Glutathione in Vegetables and Fruits Using Quantum Dot Ratiometric Hybrid Probes Aimin Chen, Xiao Peng, Zaifa Pan, Kang Shao, Jing Wang, and Maohong Fan J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jafc.8b00662 • Publication Date (Web): 04 Jun 2018 Downloaded from http://pubs.acs.org on June 4, 2018

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is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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Journal of Agricultural and Food Chemistry

Figure and Table Captions Scheme 1. Schematic illustration for preparation of the ratiometric nanoprobe and the sensing mechanism for detection of GSH. Figure 1. TEM images of (A) PL emission spectra (λex = 380 nm) of (a) CdSe@SiO2–NH2, (b) NALC-CdTe QDs, and (c) CdSe@SiO2@CdTe NPs. The inset photos express the corresponding fluorescence colors under a 365 nm UV lamp. (B) CdSe@SiO2–NH2, (C) NALC-CdTe QDs, and (D) CdSe@SiO2@CdTe NPs. Figure 2. Fluorescence decay curves (λex = 405 nm, measured at the maximum of the fluorescence) of CdSe@SiO2@CdTe NPs, Before quenching (black line), Quenched by Hg2+ (red line), Restored by GSH (blue line). Figure 3. A. Effects of sample pH (0.01 M PBS) on the PL intensity ratios (I619/I535) of Hg2+ (1.2 μM) quenched CdSe@SiO2@CdTe NPs (50 μg/mL) and after restored with GSH (10 μM). B. Hg2+ concentration dependent PL quenching (red line) of CdTe QDs in ratiometric probe (50 µg/mL) and subsequent PL restoration (black line) by GSH (3 μM). All solutions were prepared in a 0.01 M PB of pH 8.0. Figure 4. Selectivity of the Hg2+ quencher ratiometric probe to some coexisting species in the PB (pH 8.0, 0.01 M), including Na+ (1000 µM), K+ (1000 µM), Mg2+ (1000 µM), Ca2+ (1000 µM), L-Cysteine (Cys, 45 µM), proline (Pro, 100 µM), Phenylalanine (Phe, 100 µM), DL-Methionine (Met, 100 µM), natural -carotene (β-C, 100 µM), Tyrosine (Tyr, 100 µM), DL-malic acid (MA, 100 µM), vitamin E acetate (VE, 100 µM), citric acid (CA, 100 µM), glucose (Glu, 100 µM) and ascorbic acid (AA, 100 µM ). The fluorescence photos were selectivity of the Hg2+ quencher

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ratiometric probe to some coexisting species in the PBS (pH 8.0, 0.01 M). Figure 5. (A) Recovery of PL spectra at different concentrations of GSH after the Hg2+ (1.2 μM) quenching CdSe@SiO2@CdTe NPs (50 µg/mL). The concentrations of GSH from bottom to top are 0, 0.1, 0.5, 1, 2, 3, 5, 7, 10, 15, 20 and 30 µM, respectively. The fluorescence photos were taken under a UV lamp under the excitation of 365 nm. (B) Plot of the PL intensity ratio (I619/535) of the Hg2+ (1.2 μM) quenching nanohybrid as a function of the GSH concentration. Table 1. Comparison of the analytical performance of the sensing systems for the detection of GSH. Table 2. Applications of the proposed method for analysis of various food samples spiked with different amounts of GSH.


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Scheme 1. Schematic illustration for preparation of the ratiometric nanoprobe and the sensing mechanism for detection of GSH



Figure 1. TEM images of (A) PL emission spectra (λex = 380 nm) of (a) CdSe@SiO2–NH2, (b) NALC-CdTe QDs, and (c) CdSe@SiO2@CdTe NPs. The inset photos express the corresponding fluorescence colors under a 365 nm UV lamp. (B) CdSe@SiO2–NH2, (C) NALC-CdTe QDs, and (D) CdSe@SiO2@CdTe NPs.


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Figure 2. Fluorescence decay curves (λex = 405 nm, measured at the maximum of the fluorescence) of CdSe@SiO2@CdTe NPs, original probe (black line), quenched by Hg2+ (red line) and recovered by GSH (blue line).



Figure 3. A. Effects of sample pH (0.01 M PBS) on the PL intensity ratios (I619/I535) of Hg2+ (1.2 μM) quenched CdSe@SiO2@CdTe NPs (50 μg/mL) and after restored with GSH (10 μM). B. Hg2+ concentration dependent PL quenching (red line) of CdTe QDs in ratiometric probe (50 µg/mL) and subsequent PL restoration (black line) by GSH (3 μM). All solutions were prepared in a 0.01 M PBS of pH 8.0.


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Figure 4. Selectivity of the Hg2+ quencher ratiometric probe to some coexisting species in the PB (pH 8.0, 0.01 M), including Na+ (1000 µM), K+ (1000 µM), Mg2+ (1000 µM), Ca2+ (1000 µM), L-Cysteine (Cys, 45 µM), proline (Pro, 100 µM), Phenylalanine (Phe, 100 µM), DL-Methionine (Met, 100 µM), natural -carotene (β-C, 100 µM), Tyrosine (Tyr, 100 µM), DL-malic acid (MA, 100 µM), vitamin E acetate (VE, 100 µM), citric acid (CA, 100 µM), glucose (Glu, 100 µM) and ascorbic acid (AA, 100 µM ). The fluorescence photos were selectivity of the Hg2+ quencher ratiometric probe to some coexisting species in the PBS (pH 8.0, 0.01 M).



Figure 5. (A) Recovery of PL spectra at different concentrations of GSH after the Hg2+ (1.2 μM) quenching CdSe@SiO2@CdTe NPs (50 µg/mL). The concentrations of GSH from bottom to top are 0, 0.1, 0.5, 1, 2, 3, 5, 7, 10, 15, 20 and 30 µM, respectively. The fluorescence photos were taken under a UV lamp under the excitation of 365 nm. (B) Plot of the PL intensity ratio (I619/535) of the Hg2+ (1.2 μM) quenching nanohybrid as a function of the GSH concentration.


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Table 1. Comparison of the analytical performance of the sensing systems for the detection of GSH reference

probes

methods

modes

linear range

detection

(μM)

limit (nM)

ref.52

AA-Au NPs

colorimetry

turn-on

0.25-2.5

100

ref.53

Ag+–TMB

colorimetry

turn-off

0.05-8

50

ref.54

Au NPs–Hg(II)

colorimetry

turn-off

0.025–2.28

17

ref.55

MoS2/rGO

single emission

turn-off

60-700

25000

ref.56

Ag nanoclusters

single emission

turn-off

0.5–6

380

ref.57

Carbon dots–MnO2

single emission

turn-off

0–2500

300

ref.58

CDs-Br

single emission

turn-off

0–34

140

ref.59

CdTe−Hg(II)

single emission

turn-off

0.6-20

100

ref.60

Mn-doped ZnS QDs

single emission

turn-on

0.3-280

97

ref.61

C QDs

single emission

0-0.1

43

ref.62

DPP-NO2

single emission

turn-on

30-90

61.4

ref.63

Carbon dots-gold NPs

dual-mode

turn-on

0.1-6

50

2+

N- CQDs-RhB-Hg

dual-emission

turn-on

0.08–60

20

CdSe@SiO2@CdTe

dual-emission

turn-on

0-10

42

ref.64 this study



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Table 2. Applications of the proposed method for analysis of various food samples spiked with different amounts of GSH samples dark-shinned grape

pineapple

cucumber

tomato

initial amount

standard added

(mg/100 g)

GSH (µM)

1.79

1.13

1.17

6.24

recovery (%)

av recovery (%)

RSD (%)

2.0

102.3-105.5

104.0

1.10

4.0

99.2-103.5

101.5

1.47

6.0

98.3-102.5

99.1

1.42

2.0

99.5-103.1

100.7

1.59

4.0

97.4-103.6

101.3

2.53

6.0

96.6-99.2

97.6

1.13

2.0

97.8-104.1

101.5

2.43

4.0

96.4-100.9

98.7

1.58

6.0

95.6-101.2

98.1

2.11

2.0

96.2-102.6

98.7

3.42

4.0

97.6-103.1

101.1

2.30

6.0

98.9-99.5

99.1

0.27


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1

Visual Assay of Glutathione in Vegetables and Fruits Using

2

Quantum Dot Ratiometric Hybrid Probes

3

Aimin Chen,†,‡,* Xiao Peng, † Zaifa Pan, † Kang Shao, † Jing Wang, †,*

4

and Maohong Fan‡,§,*

5

6

†.

7

310014, China.

8

‡.

9

Laramie, WY 82071, USA.

College of Chemical Engineering, Zhejiang University of Technology, Hangzhou

Departments of Chemical and Petroleum Engineering, University of Wyoming,

10

§.

11

Corresponding author: Aimin Chen, Email: [email protected]

School of Energy Resources, University of Wyoming, Laramie, WY 82071, USA.

12

Jing Wang, Email: [email protected]

13

Maohong Fan, Email: [email protected]

14

15

16

17

18

19

20



21

ABSTRACT: Future food safety monitoring with simple, fast and visual methods has

22

become increasingly important. Accordingly, this work was designed to construct a

23

new-style dual-emission ratiometric fluorescent probe (CdSe@SiO2@CdTe) for

24

visual assay of glutathione (GSH) with a “turn on” strategy. After adding Hg2+, the

25

red fluorescence of the outer CdTe QDs was quenched through both electron transfer

26

and ion-binding processes. Upon the addition of glutathione, the red fluorescence

27

occurred again owing to the strong affinity between GSH and Hg2+, whereas the inner

28

green fluorescence of CdSe QDs kept unchanged, leading to a clearly recognizable

29

fluorescence color change (from green to orange-red). In the concentration range from

30

0.1 to 10 µM, the relative fluorescence intensity ratios (I619/I535) showed an excellent

31

linear correlation with the concentration of GSH, and the detection limit was as low as

32

42 nM under optimal conditions. Meanwhile, the quantum dot ratiometric hybrid

33

probes were successfully applied for direct visual sensing GSH in real vegetable and

34

fruit samples.

35

KEYWORDS: Quantum dots, ratiometric probes, mercury ions, glutathione, visual

36

detection, silica nanoparticle

37 38 39 40 41 42 43 44 45 46 47 48


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49

INTRODUCTION

50

Glutathione (GSH), a small molecule peptide composed of three amino acids, is the

51

major free thiol present in virtually all animal cells. It plays a significant role in

52

antioxidant defense and immune maintenance during life activities including

53

participation in bioreduction reaction of organisms, maintenance of enzyme activity,

54

preventing detoxification and oxidation/nitrification stress.1-4 Although human body

55

can produce GSH, it is far from meeting the needs of all kinds of activities of the body.

56

The absence of GSH can lead to pressure, aging and a series of diseases, such as

57

Alzheimer, Parkinson, Diabetes, HIV, cancer, ect.5, 6 Therefore, the intake of foods

58

containing GSH is of great help to human health. Development of convenient

59

technology for monitoring GSH of foods is of great significance for food safety and

60

disease diagnosis. 7, 8

61

Over the last few decades, various analytical methods have been applied to the

62

mensuration of GSH, including mass spectrometry,9 high-performance liquid

63

chromatography,10 electrochemistry,11 colorimetry,12 and fluorescence.13 Among of

64

them, fluorescence techniques have been

65

specificity, high sensitivity,14 and the wide range of material sources including

66

quantum dots and organic dyes, 15, 16 metal clusters,17 carbon dots (CDs),18 and so on.

67

For instance, Wang et al designed a fluorescent probe based on gold nanoparticles to

68

detect GSH with high selectivity and ultra-sensitivity.19 Li et al. prepared Mn-doped

69

CdTe quantum dots-methyl viologen nanohybrid to detect GSH，which has the

70

advantages of a short detection time and the selective detection of GSH compared

widely explored due to their simplicity,



71

with other biothiols.20 However, the main features of these fluorescent nanoprobes

72

were the single-signal mode (turn-off or turn-on)，and often affected by concentration

73

of the probe, incident light, and external environment. Recently, ratiometric

74

fluorescence analysis method has attracted great interest because of its dependability

75

and anti-interference ability by simultaneous measurement of two fluorescence signals

76

under single wavelength excitation.21-23 For instance, An’s group reported a probe

77

combined by Rhodamine 6G and MPA-CdTe for highly sensitive detection of GSH

78

via the fluorescence resonance energy transfer mechanism with a LOD of 15 nM.24

79

Qin et al. prepared two ratiometric fluorescent probes including nitrophenol groups

80

which exhibited outstanding excellent selectivity for GSH and have been successfully

81

applied to the determination of GSH in cells.25 Despite these breakthroughs, there are

82

still many deficiencies in the construction of dual signal transmitter for GSH,

83

especially in designing mensurable spectral responses in the emitting intensity and/or

84

wavelength. Therefore, it is still a challenging project to detect GSH accurately with

85

dual-emission fluorescent nanoprobe.

86

Herein, we constructed a new-style dual-emission ratiometric fluorescent probe

87

(termed as CdSe@SiO2@CdTe) for visual detection of glutathione combined with

88

“turn on” strategy. As described in Scheme 1, the green fluorescence CdSe QDs were

89

coated in silica spheres for covalent coupling of red fluorescence CdTe QDs modified

90

by N-acetyl-L-cysteine (NALC), in oreder to achieve both adjustable fluorescence

91

intensity ratios and color. This ratiometric fluorescent probe has the following

92

advantages. Firstly,

the dual-stabilizer modified CdSe QD was used as a reference


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93

instead of frequently-used CdTe QDs because

94

atoms to MPA and SHMP could lead to QDs with excellent PL properties after silica

95

coating.26,

96

fluorescence not only can be effectively quenched by Hg2+ through both electron

97

transfer and ion-binding process but also be recovered readily in presence of the GSH.

98

Meanwhile, compared with commonly used MPA stabilizer, NALC is more

99

environmentally friendly and stable which has a remarkable advantage for practical

100

applications.28, 29, 30 Based on the above superiority, the ratiometric fluorescent probe

101

has been well applied to detect GSH in vegetable and fruit samples with high

102

sensitivity and selectivity.

27

Also, the signal reporter is CdTe QDs modified by NALC, whose

103

104

the bonding of surface cadmium

MATERIALS AND METHODS

105

Chemicals and Reagents. Cadmium chloride (CdCl2·2.5H2O, 99.95%), sodium

106

hexametaphosphate ((NaPO3)6, SHMP, 99%), sodium selenite (Na2SeO3, 99%),

107

sodium tellurite (Na2TeO3, 99.9%), 3-mercaptopropionic acid (MPA, 99%), tetraethyl

108

orthosilicate (TEOS, 99%), L-glutathione reduced (GSH, 98%), N-acetyl-L-cysteine

109

(NALC, 99%), DL-malic acid (99%), L-Cysteine (99%), Z-D-Tyrosine (98%),

110

phenylalanine, DL-methionine (99%), L-Proline (99%), natural β-carotene (96%),

111

3-aminopropyltriethoxysilane (APTES, 99%), vitamin E acetate (96%) and

112

L-histidine (99%) were ordered from Aladdin Regent Co., Ltd. Sodium borohydride

113

(NaBH4, 98%) was obtained from the Shanghai Ling Feng Chemical Reagent Co., Ltd.

114

Hydrazine hydrate (N2H4·H2O, 85%), ammonium hydroxide (NH3·H2O, 25%), and

115

other frequently-used salts were acquired from Sinopharm Chemical Reagent Co., Ltd.



116

Phosphate buffered saline (PBS) were obtained by combining PB solutions with 9%

117

NaCl. Ascorbic acid (AA, 99.7%) was obtained from the Guangdong Guanghua

118

Polytron Technologies Inc. All the related chemical reagents were of analytical grade

119

and deionized water from a Millipore water purification system (≥18 MΩ.cm, Milli-Q,

120

Millipore) was used for preparing the solution.

121

Instrumentation. Transmission electron microscopy (TEM) images were

122

investigated on FEI-F20 electron microscopy (FEI Corporation, USA) instrument

123

operating at 200 kV. Fourier-transform infrared (FTIR) data were recorded with a

124

Bruker model VECTOR22 Fourier-transform spectrometer using KBr pressed disks

125

(Bruker Corporation, Germany). X-ray photoelectron spectroscopy (XPS) was carried

126

out using a photoelectron spectrometer called AXIS Ultra DLD (Kratos Corporation,

127

Japan). Absorption spectra were gauged by SHIMADZU UV-2550 UV-Vis

128

spectrometer. Fluorescence decay curves were obtained by the time related single

129

photo counting technique on the combined steady-state and lifetime spectrometer

130

(Edinburgh Analytical Instruments, FLSP920). Photoluminescence (PL) spectra were

131

collected with a Horiba JY FluoroLog-3 spectrometer with an integrating sphere

132

attachment.

133

Synthesis of green-emitting dual-stabilizers-modified CdSe QDs. The synthesis

134

of CdSe QDs was based on a reported method.31 For a typical synthesis, 0.80 mL

135

CdCl2 solution (0.20 M) was diluted using 50 mL H2O in a flask; then in the magnetic

136

stirring, 34.60 µL MPA and then 72.50 mg SHMP were added into the solution. The

137

pH of the solution was regulated to 9.0 by using 1.0 M NaOH solution, followed by

138

injection of 0.80 mL Na2SeO3 solution (0.02 M). Subsequently, the obtained solution

139

was refluxed at 100 °C for 5 min and added 3.67 mL N2H4·H2O into the mixture. The

140

green CdSe QDs solution was obtained by refluxing for about 9 h. After


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141

centrifugation and washing with ethanol for several times, the depurated products

142

were obtained. Finally, the depurated CdSe QDs were dispersed and kept in deionized

143

water at 4 °C for next use.

144

Synthesis of amino group functionalized CdSe@SiO2 composite. Amino capped

145

CdSe@SiO2 fluorescent nanospheres were prepared by using a modified stöber

146

method.32 Specifically, 10 mL water was added toCdSe QDs (from 10 mL stocked

147

CdSe solution) for increasing dispersion,

148

and stirring it for 10 min. Then, 0.5 mL NH3·H2O (25-28 wt%) and 0.5 mL TEOS

149

was injected into the above solution and kept stirring for 12 h. Finally, 100 µL APTES

150

was introduced into in the mixed system and stirring for 12 h again. After

151

centrifugation, the precipitate was collected and cleaned using ethanol and deionized

152

water in sequence until the fluorescence of supernatant liquid disappeared. The

153

obtained amino capped CdSe@SiO2 fluorescent nanospheres were dispersed in

154

deionized water for experimental use.

followed by mixing it with 40 mL ethanol

155

Synthesis of red-emitting NALC-stabilized CdTe QDs. NALC-stabilized CdTe

156

QDs were prepared according to previous reports and improvements.33 Under the

157

protection of nitrogen flow, 0.0784 g NALC was added to 50 mL CdCl2 solution

158

(9.14×10-4 g/mL) and the pH of the solution was regulated to 11.0 by using 1.0 M

159

NaOH solution. Then 0.1075 g trisodium citrate dihydrate, 8.85 mg Na2TeO3 and 0.04

160

g NaBH4 were added into the above mixture in sequence. After refluxing at 100 °C for

161

14 h, the red CdTe QDs was obtained, followed by purification with by centrifugation

162

after adding the same volume of ethanol. The depurated CdTe QDs were dispersed

163

and maintained in deionized water at 4 °C for next use.



164

Synthesis

of

CdSe@SiO2@CdTe

ratiometric

probe.

Page 18 of 38

The

red-emitting

165

NALC-stabilized CdTe QDs could covalently connect to the silica surface on the

166

basis of the reported literature.34 Briefly, 15 mg EDC was added to the 2 mL of CdTe

167

solution under magnetic stirring for 20 min. After adding 12 mg CdSe@SiO2–NH2 to

168

the above solution, the mixture was kept stirring in the dark for 12 h at room

169

temperature. By further centrifuging and washing with deionized water, the

170

dual-emission

171

CdTe@SiO2@CdSe ratiometric probes were dispersed and kept in deionized water at

172

4 °C for next use.

silica

nanocomposites

were

obtained.

The

depurated

173

Detection of GSH in buffer solution. The procedure for detecting GSH was as

174

follows. In a 3 mL calibrated test tube, 1 mL of PBS (pH 8.0, 10 mM), 0.15 mL of

175

ratiometric fluorescence probe (1.0 mg/mL) and 36 µL of Hg2+ (0.1 mM) were

176

injected in sequene. Then, different amounts of GSH (0–30 µM) were added to the

177

test tubes in turn and diluted to 3 mL by deonized water. After mixed solution was

178

kept at room temperature for 15 min, it was tested using the PL spectrum at the

179

excitation of 380 nm.

180

Detection of GSH in vegetables and fruits. The samples of vegetables and fruits

181

were prepared according to a previous report with a few modifications.35 First, all the

182

samples were cleaned with ultrapure water. Then the samples were weighed after they

183

were dried in the air, followed by rupture and gnash in a high speed blender. The

184

resulting supernatant was filtered using a 0.22 µm microporous filters and diluted to

185

the detection range with PBS (pH 8.0, 10 mM). One hundred micro filtration samples

186

with different concentrations were put in the texting solution, and their PL spectra

187

were detected under the same condition.


Page 19 of 38


188

RESULTS AND DISCUSSION

189

Characterization of CdSe@SiO2@CdTe ratiometric hybrid probes. The optical

190

properties and structure of ratiometric hybrid probes were studied detailedly. The PL

191

spectra of CdSe@SiO2–NH2 NPs, NALC-CdTe QDs, and the ratiometric hybrid

192

probes are presented in Figure 1A. As curve a shown, the CdSe@SiO2–NH2 NPs

193

indicates an obvious emission peak centered at 533 nm as well as strong green

194

fluorescence under UV light (a of inset). For CdTe QDs, a PL ultimate peak appeared

195

at 609 nm (see curve b) with bright red fluorescence under UV light (b of inset). By

196

mixing the CdSe@SiO2–NH2 NPs with the CdTe QDs in a certain proportion,

197

ratiometric fluorescence probes were obtained with two PL peaks of 535 nm and 609

198

nm, respectively, showing orange red under UV light (c of inset). TEM images were

199

also investigated to verify the proper connection between the CdTe QDs and the

200

silicon dioxide spherical particles. As illustrated in Figure 1B, the as-prepared CdSe

201

QDs@SiO2 had uniform spherical structure with a diameter of 48.4±4.3 nm. The silica

202

coating could impede the immediate contact of the internal QDs with the outer

203

solvents during the connection and the subsequent test, generating a steady and

204

credible reference signal.36 As showed in Figure 1C, the prepared NALC-CdTe QDs

205

were nearly monodisperse with homogeneous size of 4.1±0.5 nm. A great deal of dark

206

“QD islands” were dispersed evenly on the surface of each silica sphere (shown in

207

Figure 1D), suggesting that the CdTe QDs had been successfully connected to the

208

silica shells due to the covalent link. Moreover, the FT-IR spectrum was taken to

209

obtain further structural insights about CdSe@SiO2@CdTe ratiometric hybrid probes.



210

In Figure S1B, the peaks of CdSe@SiO2–NH2 NPs at 1045.1 cm−1 and 788.6 cm−1

211

were attributed to Si–O–Si stretch vibration. The new peak 1392.3 cm−1 appeared

212

after CdTe QDs was covalently connected to CdSe@SiO2–NH2 NPs. Compared with

213

Figure S1A, we can get that the peak represented the C–C stretching vibrations from

214

NALC.37 From CdSe@SiO2 to CdSe@SiO2@CdTe, the absorption peak of –NH2

215

shifted from 1637.7 cm-1 blue to 1628.8 cm-1, which might be the result of hydrogen

216

bonding between –NH2 on the surface of silicon dioxide and –COOH on the surface

217

of CdTe QDs.38-40 For further demonstrating

218

the surface of CdSe@SiO2, XPS characterization was conducted to detect surface of

219

CdSe@SiO2 before and after conjugation. In Figure S2, compared with the original

220

CdSe@SiO2, the emergence of new N1s, Te3d and Cd (3d, 4d) peaks in the

221

CdSe@SiO2@CdTe were attributed to NALC and CdTe respectively. The above

222

analysis indicated that the ratiometric hybrid probes were successfully obtained.41

the conjugation of outer layer QDs on

223

224

Sensing mechanism of CdSe@SiO2@CdTe ratiometric hybrid probe. Some

225

previous researches had reported that quenching quantum dots with Hg2+ with two

226

different mechanisms for quenching effect: Yan’s group showed that the quenching of

227

CdS QDs by Hg2+ was due to the formation of size-quantized HgS on the surface of

228

the particle, that is, the formation of ionic bonds leads to fluorescence quenching.42

229

Xia and co-workers reported the quenching mechanism of Hg2+ was complex:

230

Ion-binding and electron transfer co-existed, but electron transfer played a dominant

231

role.43 In our experiment, NALC played a crucial role, which was directly connected


Page 20 of 38

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232

to the surface of CdTe QDs by sulfydryl. NALC contained sulfydryl and imino groups

233

which could be combined with Hg2+. The steric hindrance effect of NALC might

234

hinder the bonding between Hg2+ and sulfydryl.44 So, we assumed that these two

235

mechanisms also appeared in the quenching process. After Hg2+ was added, as

236

exhibited in Figure S3, the PL intensity of outer CdTe QDs decreased accordingly. In

237

addition, it was found that the addition of Hg2+ caused the red shift of the fluorescence

238

peak (up to 13 nm), which indicated that Hg2+ combined sulfydryl to generate HgS.45

239

Furthermore, this hypothesis was also verified by UV–Vis characterization. As

240

demonstrated in Figure S4, the maximum absorption peak had a red shift after

241

addition of Hg2+. When mixing GSH to the above system, it was found that a blue

242

shift of fluorescence peak occurred. In Figure 4A, the fluorescence intensity could

243

restore to the original 92% as GSH concentration reached 30 µM. The blue shift had

244

only 3 nm, which also confirmed that the electron transfer was the main cause of the

245

quenching process. Compared with the combination of imino group and Hg2+, the

246

combination of sulfydryl and Hg2+ was much stronger. According to the competitive

247

relationship of combination, after GSH was added, Hg2+ combined with imino group

248

in NALC would be replaced with the GSH. However, Hg2+ combined with sulfydryl

249

in NALC almost could not be released. Chen’s study also well verified our conjecture

250

that electron transfer played a dominant role in the quenching reaction.46

251

In order to analyze the quenching recovery performance of outer CdTe QDs, the

252

test of fluorescence lifetime was investigated. Because the fluorescence intensity of

253

the probe did not change at the 535 nm (Figure 4A), we studied the fluorescence



254

lifetime by monitoring 609 nm. The decays of all the samples were multiexponential.

255

After fitted using the third-order equation, the data are presented in Table S1. Figure 2

256

presents the fluorescence decay curves, including original probe (black line),

257

quenched by Hg2+ (red line) and recovered by GSH (blue line). After adding Hg2+ into

258

the original probe system, the lifetime of outer CdTe QDs was substantially decreased

259

from 22.76 to 13.95 ns. The decreasing of lifetime was owing to the electron transfer

260

from QDs to Hg2+.47 In order to restore the lifetime of CdTe QDs, addition of GSH is

261

a key element to motivet the separation of Hg2+ from probe through a strong

262

interaction between Hg2+ and GSH, which resulted the restore of the lifetime from

263

13.95 to 23.84 ns.

264 265

Optimizations of experimental conditions

266

Choice of pH. The appropriate pH buffering system is vital to achieving a

267

favorable analytical property for the fluorescent detection of GSH. So, we studied the

268

quenching and recovery process in PBS buffer (10 mM) respectively in order to find

269

the most suitable pH. In Figure 3A, the pH range changed from 5.5 to 9. Each sample

270

was added with the same concentration of Hg2+ (1.2 µM), the fluorescence intensity

271

increased slightly with the increase of pH and tended to be steady at the vicinity of pH

272

8.5; after adding 10 µM GSH, the fluorescence intensity increased greatly and tended

273

to be stable at pH 8.0. It was found that the interaction between GSH and Hg2+ in

274

alkaline medium was stronger than that in acidic solution, which was consistent with

275

previous report.48 According to the above results, the optimal pH was 8.0 for detection


Page 22 of 38

Page 23 of 38


276 277

condition. Choice of Hg2+ concentration.

The effect of Hg2+ on the test sensitivity of the

278

probe under pH 8.0 was evaluated. As displayed in Figure 3B, the PL intensity of

279

outer CdTe QDs significantly decreased with the increasing concentration of Hg2+

280

(0.2−1.5 µM). In addition, when the amount of GSH was the same, the PL intensity

281

ratio (∆(I619/I535)) of the probe (red line) enhanced gradually and reached the

282

maximum value at 1.2 µM. With the further increase in Hg2+, the

283

drop. Based on the above results, 1.2 µM of Hg2+ was chosen as the optimal

284

quenching concentration for ratiometric detection.

change started to

285

286

Other influencing factors. In order to achieve a superior performance of

287

ratiometric probe, a series of tests were carried out, including photostability,

288

long-term stability, reaction time, and concentration of GSH. As shown in Figure S5A,

289

the stability of the probe was examined under UV light. After ten continuous lighting

290

with 5 min for each time, it was found that the relative fluorescence intensity ratios

291

(I609/I535) of the probe had not changed essentially. Furthermore, indicated in Figure.

292

S5B, after 90 days' static position, there were no obvious changes concerning the

293

fluorescence intensity ratio (I609/I535) of the probe, indicating the high long-term

294

stability. The gauging of reaction time is depicted in Figure S6A. With the increasing

295

of Hg2+ concentration, the fluorescence intensity of the CdTe QDs decreased rapidly.

296

After about 5 minutes, the fluorescence intensity tended to be stable. In addition, the

297

binding of GSH to Hg2+ was accompanied by the separation of NALC and Hg2+.



298

Therefore, the reaction time was longer, and the fluorescence recovery was almost

299

unchanged after 10 minutes (Figure S6B). We also investigated the effect of

300

concentration of GSH (0−60 µM) over the probe and the results are presented in

301

Figure S7. The fluorescence intensity of probe had no change basically as the

302

concentration of GSH increased, which eliminated GSH effect on the probe itself.

303

Selectivity of the ratiometric hybrid probe. For the sake of estimating the

304

selectivity of this probe to detect GSH, it is necessary to detect some biological

305

molecules in real samples which might coexist with GSH. As shown in Figure 4, Na+,

306

K+, Mg2+, Ca2+, proline, phenylalanine, Tyrosine and vitamin E acetate had little effect

307

on the reaction system. Citric acid, glucose and ascorbic acid caused fluorescence

308

quench lightly in accompaniment with the deep green color. DL-Methionine and

309

natural β-carotene led to fluorescence recover slightly. However, cysteine is an

310

exception, which could restore fluorescence to a certain extent. It was speculated that

311

the binding force between GSH and Hg2+ was far greater than cysteine binding to the

312

Hg2+ .49, 50 The design of interaction between GSH and Hg2+ enabled to recognize

313

GSH from other small sulfydryl species, which was different from reported

314

techniques.51 The desirable results indicated higher selectivity of this probe which

315

would have a potential application prospect.

316

317

Detection of GSH by CdSe@SiO2@CdTe ratiometric probe. Then, we

318

conducted some more experiments to study the change of fluorescence intensity of

319

probe with the increase of the amount of GSH (Figure 5A). With the concentration of


Page 24 of 38

Page 25 of 38


320

GSH increasing from 0 to 30 µM, the PL peak at 619 nm increased, however it barely

321

changed at 535 nm. In Figure 5B, fluorescence intensity ratio (I619/I535) expressed a

322

perfect linear relationship in the GSH concentration range (0-10 µM), the linear

323

regression equation of calibration curve was expressed as I619/I535 =0.4262+0.0833c,

324

in which c represents the concentration of GSH, the detection limit for GSH was

325

calculated as 42 nM (S/N=3). Compared with other researches in the reported

326

literatures for GSH detection, this fluorescent method shows either comparable or

327

even better response (Table 1). Moreover, under a UV light (365 nm), the variety of

328

fluorescence intensity ratio also led to obvious change of the fluorescence color from

329

green to orange-red (inset of Figure 5A). The obvious color change made it as a visual

330

assay of GSH advantageous. In order to prove the superiority of the ratiometric

331

fluorescence for visual assay, we did the single fluorescence experiment. As shown in

332

Figure S8, a single red NALC-CdTe QD probe was applied for the detection of GSH

333

compared with ratiometric probe. However, it was difficult to find out the

334

fluorescence color change of the single CdTe probe (inset of Figure S8). The above

335

results distinctly confirmed that the ratiometric fluorescence probe provided higher

336

creditability and sensitivity as a visual assay than a single fluorescence probe did.

337

338

Detection of GSH in vegetables and fruits. The feasibility was explored for

339

detecting glutathione in real samples such as fruits and vegetables by using the

340

ratiometric fluorescent probe. Before testing samples, the effect of sample matrix on

341

the detection system was analyzed. After adding a certain amount of food samples, the



342

fluorescence of the system had not changed basically (Figure S9). So it could also

343

eliminate the influence of matrix on the whole testing system. In addition, we added

344

different amounts of GSH (2.0, 4.0 and 6.0 µM) to conduct the recovery experiment

345

and detailed results were displayed in Table 2. The percent recoveries of GSH in

346

diluted samples were between 96.2% and 105.5%, satisfying with the quantitative

347

analysis in the actual samples. The relevant fluorescence pictures were taken under

348

the same UV light condition (365 nm) (Figure S10). These results proved that the

349

designed fluorescence technique could be further developed as a platform for visual

350

assay of GSH in actual samples. The present method was applied to analyze different

351

amounts of GSH in various food samples.

352

In conclusion, in view of the ratiometric QDs nanohybrid, we have designed a

353

simple, ingenious and highly selective fluorescence signal analysis method to detect

354

GSH. The fluorescence of outer CdTe QDs was quenched by inducing Hg2+ due to the

355

existence of electron transfer and ion-binding. After the addition of GSH, the outer

356

fluorescence recovered while the internal CdSe QDs fluorescence remained

357

unchanged, which leaded to fluorescence color change from green to orange-red.

358

Selective experiments showed that even with sulfur-containing amino acids, the

359

detection of glutathione could not be interfered, and this technique was successfully

360

applied to visual assay of GSH in real fruits and vegetables. We expect that this work

361

will spread out the way for the development of new visual technique for future food

362

safety analysis.

363

AUTHOR INFORMATION


Page 26 of 38

Page 27 of 38


364

Corresponding Authors

365

* Phone/Fax: 0086-571-88320284; E-mail: [email protected]

366

* Phone/Fax: 0086-571-88320238; [email protected]

367

* Phone/Fax: 001-307- 7665633; E-mail: [email protected]

368

ORCID

369

Aimin Chen: 0000-0002-9189-3726

370

Jing Wang: 0000-0003-3185-1724

371

Maohong Fan: 0000-0003-1334-7292

372

Funding

373

This work was supported by the Key projects of Zhejiang Natural Science Foundation

374

(LZ18B050002) and Public Project of Zhejiang Province (2017C33172).

375

Notes

376

The authors declare no competing financial interest.

377

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Figure and Table Captions Scheme 1. Schematic illustration for

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