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Polyethyleneimine-capped CdS Quantum Dots for Sensitive and Selective Detection of Nitrite in Vegetables and Water Hui-Hui Ren, You Fan, Bin Wang, and Li-Ping Yu J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jafc.8b01951 • Publication Date (Web): 17 Jul 2018 Downloaded from http://pubs.acs.org on July 18, 2018

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Journal of Agricultural and Food Chemistry

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Polyethyleneimine-capped CdS Quantum Dots for Sensitive and

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Selective Detection of Nitrite in Vegetables and Water

3

Hui-Hui Rena, You Fana, Bin Wanga, Li-Ping Yua,b*

4

a

5

b

6

Tianjin University, Tianjin 300350, China

7

* Corresponding author. E-mail: [email protected], Fax: 86-22-27403475

Department of Chemistry, School of Science, Tianjin University, Tianjin 300350, China National Demonstration Center for Experimental Chemistry & Chemical Engineering Education，

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1

ACS Paragon Plus Environment


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ABSTRACT

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In the present work, polyethyleneimine-capped CdS quantum dots (PEI-CdS QDs) with

11

bright green fluorescence were synthesized and applied for sensitively and selectively

12

detecting the nitrite in vegetable and water samples. Highly fluorescent and

13

environment-friendly PEI-CdS QDs (quantum yield about 8%) with diameters of ca. 5

14

nm were easily synthesized by using hyperbranched PEI as functional polymer.

15

Formation of the PEI-CdS QDs was verified by transmission electron microscopy and

16

UV–vis spectroscopy. The fluorescence intensity of the as-synthesized PEI-CdS QDs was

17

enhanced pronouncedly by the increasing amount of PEI and was stable when the pH

18

ranged from 5.0 to 9.0. Our results demonstrated that the fluorescence of the PEI-CdS

19

QDs was effectively quenched by the nitrite in a rather wide linear range of

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1.0×10-7−1.0×10-4 M while efficiently avoiding the interferences from nitrate ions and

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other commonly coexisting anions of nitrite in the vegetable samples. The detection limit

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of the present method was lower than the maximum limit of the nitrite in drinking water

23

(6.5×10-5 M) ruled by the World Health Organization, which is significant to the

24

application of the method.

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2


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INTRODUCTION

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Nitrite has always been used as additive agent in foods and its misusage has caused

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an excessive amount of nitrite in vegetables, fruits, and natural water. However, the rapid

29

increasing of nitrite in foods and waters is extremely harmful to public health.1, 2 Toxic

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nitrite can be converted from relatively non-toxic nitrate according to report.3 Nitrite can

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interfere with the intracorporal oxygen transport system, leading to methemoglobinemia,

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which is very dangerous for babies and pregnant women. Furthermore, carcinogenic

33

N-nitrosamines can be formed when nitrite react with amides and secondary amines in

34

the stomach.4 Because of these toxic effects, nitrite restrictions through thresholds were

35

set by some institutions.5 For example, 6.5×10-5 M is set to be the maximum limit of

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nitrite in drinking water, which is recommended by the World Health Organization. To

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date,

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chemiluminescent,6, 7 spectrofluorimetric,8, 9 electrochemical,10, 11 chromatographic and

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capillary electrophoresis

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sensitivity, selectivity, simplicity, and feasibility for the detection of trace amounts of

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nitrite, for instance, some reported works couldn’t distinguish nitrite from the common

42

coexisting nitrate ions.14, 15 Spectrofluorimetric methods have attracted interest owing to

43

their high selectivity and sensitivity, as well as simple operability and low-cost

44

advantages.4 In spectrofluorimetric methods, nitrite usually reacted with a reagent or

45

acted as a catalyst for various types of the chemical reactions and their detection

46

performance was highly dependent on the organic fluorophores.16,

various

methods

have

12, 13

been

reported

to

determine

nitrite

including

methods. However, some methods still have limitations on

3


17

In addition, the


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inherent compositional toxicity of the organic fluorophores limited their applications.

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Therefore, exploration of more novel effective and environment-friendly fluorescent

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materials is very important for the determination of nitrite.

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As a new kind of fluorescent material, quantum dots with various advantages over

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organic dyes such as good resistant to chemical degradation, outstanding photochemical

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stability and excellent fluorescence properties have come to the fore and made inspiring

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achievements in the analytical field.18 Hence, spectrofluorimetric methods based on

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quantum dots have great potentialities to solve the problems caused by organic

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dyes-based methods. The use of quantum dots has been widespread in biological area but

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few research works report the application of quantum dots in food science.

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reported a selective, simple, and rapid method to determine organophosphorus pesticides

58

in vegetable samples based on the highly selective and sensitive fluorescence

59

enhancement of water-soluble CdTe/CdS core-shell quantum dots. 20 Sozer et al. prepared

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a kind of water soluble CdSe/ZnS core/shell quantum dots containing carboxyl

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terminated groups, which was used for imaging of gluten network in flat bread and zein

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in corn extrudates.

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probes to detect biomolecules,22 drug molecules,23 and metal cations, such as mercury

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ion,24 copper ion,25 cadmium ion,26 and lead ion.27

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fluorescence probe, which was simply established via a electrostatic self-assembly

66

method by using folic acid and polyethyleneimine-coated CdS/ZnS QDs. And their

21

19

Chen et al.

What’s more, CdS quantum dots were often used as fluorescent

Zhang et al. developed a turn-on

4


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CdS/ZnS QDs were demonstrated to be selective and sensitive for targeted imaging of the

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folate receptor over-expressed cancer cells in a turn-on mode.18

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Herein, we will report an efficient detection method of nitrite in vegetables and

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water using polyethyleneimine-capped CdS quantum dots (PEI-CdS QDs). In recent

71

years, polyethyleneimine (PEI) was often used to modify nanoparticles for sensing or

72

detection.28-30 In this work, high fluorescence PEI-CdS QDs with diameters of ca. 5 nm

73

were firstly synthesized by an environment-friendly and low-cost one-pot aqueous-phase

74

synthesis method. The preparation conditions including nucleation time, reaction

75

temperature, Cd/S ratio, PEI molecular weight and PEI concentration were optimized in

76

detail to obtain highly fluorescent PEI-CdS QDs. We found the fluorescence of PEI-CdS

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QDs exhibited great ability for sensitive and rapid detection of nitrite through selectively

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fluorescence quenching by nitrite in a wide linear concentration range. To the best of our

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knowledge, this report represents the first detection of nitrite in vegetables and water by

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utilizing CdS quantum dots. The mechanism of fluorescence quenching was preliminarily

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explored and the present strategy is expected to expand the application of quantum dots.

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EXPERIMENTAL SECTION

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Materials

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Polyethyleneimine (branched) with a molecular weight (Mw) of 25000 gmol−1 was

85

purchased from Aldrich. Polyethyleneimine (branched) with the Mw of 600, 1800, 10000,

86

70000 gmol−1 were purchased from Alfa Aesar. Sodium nitrite (A. R.) was purchased 5



87

from Tianjin Kewei Company. CdCl2·2.5H2O (G. R.) and Na2S·9H2O (G. R.) were

88

purchased from Tianjin Yuanli Chemical Company. To prevent oxidation and hydrolysis,

89

CdCl2 and Na2S aqueous solutions were prepared just before use. Other anionic and metal

90

salts were of analytical reagent.

91

Synthesis of PEI-CdS QDs

92

The PEI-CdS QDs were synthesized by an environmental friendly one-pot

93

aqueous-phase synthesis method according to a modified procedure.31 In a typical

94

synthesis, 0.648 mL CdCl2 aqueous solution (1.0 × 10-1 M) was firstly added drop by

95

drop into 54 mL PEI polymer aqueous solution in a flask. Then, the mixed solution was

96

stirred at a certain temperature in a water bath with magnetic stirring. One hour later,

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6.480 mL Na2S aqueous solution (1.0 × 10-2 M) was added into the flask for the growth

98

of nanocrystals. After certain growth time, extra CdCl2 solution was added and stirred for

99

another 1 h to get a final PEI-CdS QDs solutions. The as-synthesized pale-yellow

100

PEI-CdS QDs solutions were stored in the refrigerator or lyophilized before use. When

101

use, 0.2 M borax buffer solution (BBS, pH 7.4) was used to dilute the as-synthesized

102

PEI-CdS QDs solution to certain times.

103

Characterizations

104

The morphology of PEI-CdS QDs was characterized by a transmission electron

105

microscopy (TF20, FEI). Fluorescence spectra of the PEI-CdS QDs without/with nitrite

106

(NO﹣2 ) were recorded on a photoluminescence spectrometer (Cary Eclipse, Varian) and 6


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the excitation and emission slit width were 5nm and 10 nm, respectively. UV-vis spectra

108

of the PEI-CdS QDs were recorded on a UV-vis spectrophotometer (T6, China Purkinje

109

General). An infrared spectrometer (Avatar FT-IR360, Nicolet) was utilized to

110

characterize the infrared spectra of the PEI-CdS QDs and PEI. Zeta potential of the

111

PEI-CdS QDs was recorded on a Maerwen Mano ZS Zeta potentiometric measuring

112

instrument.

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NO﹣2 detection based upon the quenching of PEI-CdS QDs

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All the NO﹣2 determination were implemented in the 0.2 M BBS at room temperature.

115

The obtained PEI-CdS QDs solution was diluted to 20 times by 0.2 M BBS. Then, 10 µL

116

aqueous solution of various NO﹣2 was added to 3 mL of the above solution to prepare NO﹣2

117

standard solutions from 0 M to 1.0×10-4 M. All standard solutions were shaken uniformly

118

by hands before fluorescence measurements. The relationship of F0/F of PEI-CdS QDs to

119

NO ﹣2 concentrations within the range of 1.0×10-7−1.0×10-4 M was investigated. The

120

selective experiments were carried out by recording fluorescence spectra of the PEI-CdS

121

QDs containing other anions under the same conditions.

122

Samples preparation

123

A 0.2 µm membrane was used to filter tap water samples collected at local lab, then

124

the NO﹣2 in samples was determined using the same procedure described as the NO﹣2

125

detection based on the quenching of PEI-CdS QDs.

7



126

For vegetables samples, 100 mL of deionized water and 10.0 g of the vegetables

127

were put in a juicer to obtain a broken vegetable tissue homogenate. The homogenate was

128

heated under stirring for 20 min. After cooled to room temperature or after a night, the

129

homogenate was centrifuged and filtered using filter paper and 0.2 µm filter membrane in

130

succession before detection. NO﹣2 in samples was detected under the same conditions as

131

standard solutions. And the found NO﹣2 in the recovery tests is the value after a correction

132

by the blank sample.

133

RESULTS AND DISCUSSION

134

Optimization of the synthesis of PEI-CdS QDs

135

Scheme 1 illustrates the synthesis of PEI-CdS QDs and detection process of NO﹣2 in

136

the present work. Here, a one-pot aqueous-phase synthesis method was used to synthesize

137

PEI-CdS QDs. Firstly, the CdCl2 solution was added into the PEI solution. CdS

138

nanocrystals started growing when S2− was added into the PEI and CdCl2 mixture

139

solution. After the growth of CdS nanocrystals, extra Cd2+ was added to cap the formed

140

quantum dots. To establish the optimum synthesis conditions, various conditions were

141

optimized using a series of univariate approaches, which included nucleation time,

142

reaction temperature, Cd/S ratio, PEI Mw and PEI concentration.

8


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143

Scheme 1. Schematic illustration of the synthesis of PEI-CdS QDs and detection process

144

of NO﹣2 . NH2

N

N

NH2

H 2N

N

H2N

NH NH 2

N

H2N

H N NH2 2 HN N

N

PEI

NO2-

NH2

CdS H N

N N

N

H 2N NH2

NH2

NH2

N

NH 2

N H 2N

NH H2N

2

N

H 2N

NH 2 H N 2

N

H2N

NH2

N

NH2

em

N N

Cd2+ S2Cd2+

N

N

N

N

NH 2

N

H2N

NH2

N

NH2

N

ex

N H2N

NH2

H2N

H 2N

H2N

N N H

N

N

H2N

NH2 NH2

N NH2

CdS/PEI H 2N

H2N

NH2 NH2

N

ex

N

H2N

N

N

N

N

H2N H 2N

HN NH2 2 HN N

N

NH H2N NH 2

CdS

N

H 2N

N

N

N

H2N NH 2

NH2

NH2

N

NH 2 N

H2N

NH2 H N 2

N

H 2N

NH2

N

NH2

em

N

N N H H2N

N

N

NH2 NH2

N NH2

CdS/PEI

145

The nucleation time of CdS nanocrystals started when S2− was added into the PEI

146

and CdCl2 mixture solution. The fluorescence intensity showed a slight decrease when

147

the nucleation time increased from 0.5 to 2 h (Figure 1a). The fluorescence was strong

148

when the CdS nanocrystals have grown for 0.5 h, which demonstrated that the growth of

149

CdS nanocrystals was fast. As a result, 0.5 h was selected as the nucleation time.

150

Figure 1b shows the fluorescence spectra of PEI-CdS QDs obtained under different

151

temperatures. The PEI-CdS QDs obtained at 30-60 °C all could emit fluorescence. But a

152

gradual red-shift of peak position and a slight change of the fluorescence intensity were

153

observed when the preparation temperatures changed. Considering the fluorescence

154

intensity and feasibility, 40 °C was chosen as the reaction temperature in the following

155

experiments. 9



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The total Cd/S ratio was 1:1 before extra Cd2+ was added. Figure 1c shows an

157

additional Cd2+ could evidently increase the fluorescence intensity of PEI-CdS QDs. The

158

fluorescence intensity increased approximately 175% when the Cd/S ratio increased from

159

1:1 to 3:1. Further increase of Cd/S ratio only caused a slight change of the fluorescence

160

intensity. The increase of fluorescence intensity after the supplemental addition of Cd2+

161

can be tentatively explained by the formation of non-radiative combination centers and

162

the Schottky barrier around quantum dots.31 In view of the fluorescence intensity and

163

reagent consumption, the optimal Cd/S ratio was selected as 3:1.

300

(b)

Fluorescence intensity

(a) 250

600

o

30 C o 40 C 50oC 60oC

250

550

(c) 500 Fluorescence intensity

300 Fluorescence intensity

(c)

(b) 350

(a)

200 150 100 50

200

450 400 350 300 250 200

0

150 20

40

60

80

Growth time/min

100

120

400

450

500

550

600

650

wavelength(nm)

700

1

2

3 4 Cd/S ratio

5

164

Figure 1. Effects of (a) nucleation time, (b) reaction temperature, and (c) the Cd/S ratio

165

on the fluorescence intensity of PEI-CdS QDs.

166

Given that the PEI-CdS QDs synthesized by using different Mws of PEI may exhibit

167

different sizes and ligand loading amounts,32 the effect of Mws of PEI on the

168

fluorescence intensity of PEI-CdS QDs was investigated. The results in Figure 2a show

169

that the greatest fluorescence intensity was achieved when the Mw of PEI was 25000

170

g/mol. Figure 2b shows the colors of PEI-CdS QDs obtained using different PEI Mws. It

171

can be seen that PEI-CdS QDs solution with smaller PEI Mw showed a darker yellow 10


6

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172

color, which illustrated the probably differences of particle sizes. We calculated the

173

average diameters of PEI-CdS QDs according to their ultraviolet absorption spectra

174

(Figure S2 of the Supporting Information).33 It was demonstrated that the sizes of

175

PEI-600 and PEI-1800 capped CdS QDs were calculated as 3.0 and 2.9 nm, respectively.

176

However, the particle sizes of PEI-10000, PEI-25000, PEI-70000 capped CdS QDs were

177

smaller (shown in Table 1). Possibly, the PEI with a higher Mw showed greater

178

dispersibility to limit the growth of QDs, which led to a higher PEI loading amount and a

179

smaller particle size.32 PEI-25000 thus was chosen for the synthesis of PEI-CdS QDs. (a)

540

(b)

500 520

400

λem/nm


600

300 200

PEI: 0.5×10-5 M→6.0×10-5 M

500

100 0 0

2

4 6 c(PEI)×10-5/M

8

10

480 12

180

Figure 2. Effects of Mws of PEI on the (a) fluorescence intensity and (b) colors of

181

PEI-CdS QDs.

182

Table 1. Calculated sizes of PEI-CdS QDs synthesized using PEI of different Mws. Mw of PEI (g/mol)

600

1800

10000

25000

70000

389

384

355

360

363

3.0

2.9

2.2

2.3

2.4

Maximum Absorbance Peak (nm) Size (nm)

11



(b)

(a) Fluorescence intensity

1000 Mw(PEI)=600 Mw(PEI)=1800 Mw(PEI)=10000 Mw(PEI)=25000 Mw(PEI)=70000

800 600 400 200 0

183

400

450

500 550 600 wavelength(nm)

650

700

184

Figure 3. Effects of PEI concentrations on (a) fluorescence intensity and emission

185

wavelength of PEI-CdS QDs and (b) color changes of PEI-CdS QDs.

186

Amine units of PEI were presumed to coordinate with Cd2+ in the synthesis process

187

of PEI-CdS QDs. Since all the PEI used here have the same number of amine units, the

188

ratio of PEI to CdS was considered to be represented by the concentration of PEI.31 The

189

effect of the PEI concentration on the fluorescence intensity of PEI-CdS QDs thus was

190

investigated here. It can be seen from Figure 3a that the fluorescence intensity of

191

PEI-CdS QDs increased enormously as the concentration of PEI changed gradually from

192

0.5×10-5 M to 6.0×10-5 M, whereas it obviously decreased when the concentration

193

reached 11.0×10-5 M. When using PEI with the same Mw, increasing the ratio of PEI led

194

to higher PEI loading amount and stronger fluorescence intensity of PEI-CdS QDs, which

195

was agreed with the reported results in literature.31 Concerning the emission wavelength,

196

increasing the concentration of PEI led to an obvious blue-shift of the maximum peak

197

position. In view of the need of strong fluorescence intensity for applications, 6.0×10-5 M

198

was chosen as the optimal PEI concentration for the synthesis of PEI-CdS QDs with an 12


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199

emission wavelength at 490 nm.

200

Characterizations and fluorescence stability of the PEI-CdS QDs

201

Figure 4a reveals the fluorescence excitation, emission and UV−vis absorption

202

spectra of the obtained PEI-CdS QDs. Obviously, the UV−vis absorption spectrum (left)

203

shows a peak at 360 nm, which is in line with the fluorescence excitation peak of

204

PEI-CdS QDs (middle). A fluorescence emission peak (right) at ca. 490 nm can be

205

distinctly noticed when the PEI-CdS QDs are excited at 360 nm.

206

FT-IR of PEI and CdS-PEI QDs also have been characterized. As shown in the

207

Figure S4 of the Supporting Information, peaks of the N-H stretching vibration, the N-H

208

bending vibration, the C-N stretching vibration, and the N-H out-of-plane bending

209

vibration can be obviously observed. Additionally, it can be seen that the N-H stretching

210

vibration peak of CdS-PEI QDs at 3432 cm-1 had an 80 cm-1 red shift compared with that

211

of PEI, which indicated that the association degree of amino group in CdS-PEI QDs was

212

lighter than that of pure PEI in all probability.

213

Figure S5a of the Supporting Information reveals the typical TEM image of

214

PEI-CdS QDs. The average particle size of PEI-CdS QDs is ca. 5 nm according to TEM

215

(Figure S5b of the Supporting Information). The particle sizes are larger than that

216

calculated from UV–vis spectrum (2.28 nm), which may be ascribed to the difficult

217

dispersion of quantum dots resulted from large amount of PEI.

218

The quantum yield (QY) of PEI-CdS QDs was obtained based on a comparison 13



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method.34 Quinine sulfate in 0.1 M H2SO4 (literature QY is 55%) was chosen as a control

220

to calculate the quantum yield of our PEI-CdS QDs dissolved in deionized water at

221

different concentrations. After calculation, the fluorescence quantum yield of the

222

PEI-CdS QDs we synthesized under the optimal condition is about 8% (Figure S6 of the

223

Supporting Information). (b) 0.20 Em 0.15

800 600

0.10

400 0.05 200 0.00

0


Ex

Abs

A


1000

(c) 800

600

700

500 Fluorescence intensity

(a)

600 500 400 300 200 100

300

400 500 wavelength(nm)

600

700

300 200 100 0

0 200

400

0

2

4

6 8 Time/Day

10

12

2.23 3.78 4.58 6.23 7.73 8.77 9.54 11.28 -pH

224

Figure 4. (a) Fluorescence excitation, emission, and UV−vis absorbance spectra of

225

PEI-CdS QDs. Effects of (b) time and (c) the pH on the fluorescence intensity of the as-

226

synthesized PEI-CdS QDs.

227

Fluorescence stability is one considerable performance of fluorescent materials for

228

their applications. After synthesized, the PEI-CdS QDs aqueous solution was kept at 4 °C.

229

It was found that PEI-CdS QDs had a good fluorescence stability since the fluorescence

230

intensity remained almost unchanged within 12 days (Figure 4b). The PEI-CdS QDs

231

fluorescence intensity was generally stable when pH varying from 5.0 to 9.0 but was

232

partially quenched in strong acid and strong alkali (Figure 4c). When the PEI-CdS QDs

233

solution became acidic, more N+ groups were formed from amino groups, which made it 14


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234

easy for PEI-CdS QDs to get rid of the protection of polymers and congregate with each

235

other. Once adjusting solution to strong basic, the fluorescence intensity decreased owing

236

to lacking Cd2+ because Cd2+ tends to form into Cd(OH)2 under an alkaline

237

environment.31

238

NO﹣2 detection

239

Responses of PEI-CdS QDs to a variety of anions were investigated considering the

240

positively charged PEI resulted from a great quantity amino groups in its branched

241

structure. 35, 36 In our experiments, it was found that NO﹣2 could quench the fluorescence

242

intensity of the PEI-CdS QDs dramatically. As it can be seen from Figure S7a of the

243

Supporting Information, the fluorescent intensity of PEI-CdS QDs reduced in the

244

presence of 0.10 mM NO﹣2 , demonstrating the possibility of analytical applications of

245

PEI-CdS QDs for NO﹣2 detection.

246

The effect of pH on the fluorescence quenching of PEI-CdS QDs in 0.1 mM NO﹣2

247

was investigated when pH was in the range of 2.0 to12.0. The results shown in Figure

248

S7b of the Supporting Information indicates that pH 7.4 was appropriate to obtain a

249

greatest fluorescence quenching. Thus, 0.2 M BBS (pH 7.4) was used to ensure the pH

250

for fluorescent detection of NO﹣2 using PEI-CdS QDs.

251

The analytical performance of the PEI-CdS QDs for the detection of NO﹣2 was

252

investigated. Figure 5a reveals the fluorescence spectra of PEI-CdS QDs in the presence

253

of various NO﹣2 ranging from 0 to 0.10 mM, which indicated the fluorescence intensity of 15



254

the PEI-CdS QDs was gradually decreased when the NO﹣2 concentration increased. The

255

relationship of F0/F of PEI-CdS QDs to NO﹣2 concentrationswas shown in Figure 5b. A

256

linear relationship (R2 = 0.999) was obtained when the investigated NO﹣2 concentration

257

ranged from 1.0×10-7 to 1.0×10-4 M, demonstrating the practicability of making use of

258

PEI-CdS QDs for detecting NO﹣2 by the present method. Under the optimal conditions, a

259

detection limit of 0.05 µM was achieved, illustrating promising practicability of sensitive

260

determination of NO﹣2 in real samples.

261

Figure 5c shows effects of different anions (including SO2-3 , CO2-3 , Br-, NO-3 , ClO-,

262

Ac-, F-, Cl-, SO2-4 , HCO-3 ) generally existing in samples with the same concentration of 0.1

263

mM on the fluorescence intensity of PEI-CdS QDs. As shown, a much greater quenching

264

in fluorescence intensity was observed for PEI-CdS QDs when NO﹣2 was added, while no

265

big changes were found when other investigated anions were added except for SO2-3 . The

266

interference of SO 2-3 can be avoided by using Ba2+ to chemically precipitate SO 2-3

267

because Ba2+ had no significant effect on the fluorescence of PEI-CdS QDs as shown in

268

Figure S8 of the Supporting Information.

16


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Page 17 of 30


800

0 M NaNO2

2.0

600

0.4 (F0-F)/F0

10-4 M NaNO2

400

0.5

y=10220.62x+1.02 R2=0.999

1.8 F0/F


(c) 0.6

(b) 2.2

(a)

1.6

0.3 0.2

1.4 200

0.1

1.2

0.0

0

1.0 400

450

500 550 600 wavelength(nm)

650

700

-0.1

0

2

4 6 c(NaNO2)/M/10-5

8

10

l r c F O NO 2 SO 3 CO 3 KB KNO 3 aCl NaA Na KC a 2SO 4HCO 3 N Na Na 2 Na 2 N Na

269

Figure 5. (a) Fluorescence spectra of PEI-CdS QDs in the presence of NO﹣2 of various

270

concentrations. (b) Linear relationship of F0/F of PEI-CdS QDs to NO﹣2 concentrations in

271

the range of 1.0×10-7−1.0×10-4 M. (c) Effects of different anions on the fluorescence

272

intensity of PEI-CdS QDs (all investigated anions was 0.1 mM).

273

We applied the above method to detect NO﹣2 in water and vegetables. A blank sample

274

was used to correct the possible interferences from the sample matrix and no found NO﹣2

275

was detected for the fresh sample. Compared with fresh samples, significant signal

276

changes were found in overnight samples and considered as found NO﹣2 in this work. The

277

difference response between overnight sample and fresh sample resulted from the NO﹣2

278

converted from nitrate. The recoveries of the present method were in the range of 82.6%

279

and 116.9% after a correction by the blank sample (Table 2). Compared with other works,

280

our method has the obvious advantages of low detection limit and wide linear range

281

(Table 3). Besides, the present method is simple, fast, and environment-friendly.

282

17



283

Page 18 of 30

Table 2. Analytical results of NO﹣2 in samples NO﹣2 added

NO﹣2 Found

Sample

Tap water

Cabbage1(fresh)

Recovery (%)

(M)

(M)

0

-

-

-

1.0×10-5

9.6×10-6

4.1

96.1

2.0×10-5

2.0×10-5

5.2

100.3

6.0×10-5

6.1×10-5

3.4

102.2

1.0×10-5

9.2×10-6

3.8

92.2

6.0×10-5

5.0×10-5

3.2

82.8

4.7×10-6

5.2

-

1.0×10-6

1.1×10-5

7.3

116.9

1.0×10-5

2.6×10-5

3.1

82.6

2.0×10-5

2.1×10-5

5.2

104.2

8.0×10-5

7.2×10-5

4.2

89.6

Cabbage2(overnight) 0

Lettuce(fresh)

RSD (%)

284 285

18


Page 19 of 30


286

Table 3. Comparison of analytical performances for determination of NO﹣2 using different

287

methods and reagents Reagent/detector

Method

Linear

LOD (µM) Reference

Range (µM) Sulfonazo III

Catalytic-spectrophotometric 0.16–6.1

0.12

37

method Na2CO3/H2O2/H+

Chemiluminescence method

N/A

0.1

38

IL-SWCNT

Electrochemical method

1.0–12.0

0.1

39

RP ion-pair HPLC

Chromatographic method

0–2000

0.2

40

UV

Capillary electrophoresis

35–3500

17

41

OPD

Fluorometric method

0.9–17.4

0.3

16

N-CNDs

Fluorometric method

0−1000

1.0

42

Rh 6G-SiO2

Fluorometric method

2–60

1.2

43

Neutral red

Fluorometric method

0.9–4.3

0.2

44

TAAlPc

Fluorometric method

21–840

7

45

2,3-DAN

Fluorometric method

0–30

0.4

46

PEI-CdS QDs

Fluorometric method

0.1-100

0.05

This

electrode

work 288

19



289

Synthesis strategy and fluorescence quenching mechanism

290

According to literatures, PEI shell of the quantum dots not only acted as good

291

surface passivation ligands for the synthesis of PEI-CdS QDs but also contributed to its

292

fluorescence enhancement.47 We supposed the excellent analytical performance of the

293

PEI-CdS QDs was attributed to PEI, which acted as functional polymer for both

294

stabilizing quantum dots and interacting with NO﹣2 . We performed the chromogenic

295

experiments using the Nitrite Detection Box to verify the hypothesis. As shown in Figure

296

S9 of the Supporting Information, the solution color of 1.0×10-5 M NO﹣2 in BBS was

297

purple (b), and its concentration was known in the correct range by comparing with the

298

standard color card. However, 1.0×10-5 M NO﹣2 in PEI-CdS QDs solution was colorless (a),

299

which indicated there was no free NO﹣2 in the solution after it was mixed with PEI-CdS

300

QDs. The fluorescence quenching of PEI-CdS QDs thus was considered to be resulted

301

from the interaction between the PEI-CdS QDs and NO﹣ 2.

302

In order to know the fluorescence quenching type of PEI-CdS QDs quenched by

303

nitrite, the Stern-Volmer equation (I0/I = 1+ Ksv [Q]) was utilized for further analysis.

304

Figure S10a of the Supporting Information reveals the obtained Stern-Volmer plot

305

exhibits a good linear relationship at 25 °C (Ksv = 10403 L·mol−1, R2 = 0.990).

306

Experimental results at different temperatures indicated that the Ksv decreased with the

307

increasing temperature (Ksv = 7805 L·mol-1 at 40 °C, Ksv = 4602 L·mol-1 at 60 °C).

308

Figure S10b of the Supporting Information shows that there was a change between the 20


Page 20 of 30

Page 21 of 30


309

UV absorption spectra of the PEI-CdS QDs in the absence and presence of NaNO2. The

310

above results revealed it was a static quenching process in the PEI-CdS QDs/NO﹣2

311

fluorescence system.42 The quenching of the PEI-CdS QDs may be due to the

312

non-luminescent ground state complexes formed by the fluorescent molecule PEI-CdS

313

QDs and the quenching agent NaNO2.

314

More specifically, when NO﹣2 was added into PEI-CdS QDs solution , the positive

315

charge on the surface of PEI-CdS QDs (Zeta Potential 33.7 mV) can attract the NO﹣2

316

owing to electrostatic interaction so that the NO﹣2 and the PEI-CdS QDs were in full

317

contact to react.

318

absorption of PEI-CdS QDs from 363 nm to 356 nm after the addition of NO﹣2 , which

319

demonstrated that NO﹣2 could change the electronic structure of PEI-CdS QDs (Figure

320

S10b).48 Therefore, the interaction of NO﹣2 and PEI-CdS QDs probably resulted in electron

321

transfer between PEI-CdS QDs and NO﹣2 , which led to fluorescence quenching. 49

322

ASSOCIATED CONTENT

323

Supporting Information

324

Effect of the ratio of Cd/S on the fluorescence spectra of PEI-CdS QDs (Figure S1),

325

effect of Mws of PEI on the ultraviolet absorption spectra of PEI-CdS QDs (Figure S2),

326

effect of the concentrations of PEI on the fluorescence spectra of PEI-CdS QDs (Figure

327

S3), FT infrared absorption spectrum of PEI and PEI-CdS QDs (Figure S4), typical TEM

328

image and particle size distribution of PEI-CdS QDs (Figure S5), fluorescence and

35

The UV–vis spectra showed a blue-shift of the characteristic

21



329

absorbance of PEI-CdS QDs (Figure S6), fluorescence spectra of BBS, NO﹣2 in BBS,

330

PEI-CdS QDs in BBS and PEI-CdS QDs + NO﹣2 in BBS and the effect of pH on the

331

fluorescence quenching of PEI-CdS QDs in NO﹣2 (Figure S7), effect of Ba2+ on the

332

fluorescence intensity of PEI-CdS QDs (Figure S8), the color responses to the solution of

333

NaNO2 in BBS and NaNO2 in PEI-CdS QDs using Nitrite Detection Box (Figure S9), the

334

obtained Stern−Volmer plot at different temperature and UV absorption spectrum of

335

PEI-CdS QDs in the absence and presence of NO﹣2 (Figure S10).

22


Page 22 of 30

Page 23 of 30


336

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475 29



476

TOC

NH2

N

N

N

H 2N

NH NH2

N

H2N

NH2

N

H2N

N

HN NH2 2 HN

H2N H2N

NO2-

NH H2N NH2

CdS HN

N N

2

N

H 2N NH2

NH2

N N H H2N

NH2 NH2

N

ex

N

H2N

N

N

N

H2N H 2N

NH2 H N 2 NH2

N N

NH 2

CdS

N N

N

N

H2N NH 2

NH2

NH2

N

NH 2

N H2N

NH H2N

H2N

N

H 2N

NH2

N

HN NH2 2 HN

em

N N

N N H H2N

N

N

NH2 NH2

N

N

N

CdS/PEI H2N

H2N

NH2

N

NH 2

N

PEI

NH2 H N 2

N N

H 2N

NH2

N

NH2

N N

em

N N

Cd2+ S2Cd2+

N

NH 2

N

H2N

N

N

NH2

N

ex NH2

N H2N

NH2

H2N

H 2N

H2N

NH2

CdS/PEI

30


NH2 NH2

N NH2

Page 30 of 30

Polyethyleneimine-capped CdS Quantum Dots for ... - ACS Publications

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