A Sensitive âTurn-Onâ Fluorescent Sensor for Melamine Based on

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A Sensitive “Turn-On” Fluorescent Sensor for Melamine Based on FRET Effect between Polydopamine-Glutathione Nanoparticles and Ag Nanoparticles Li Tang, Shi Mo, Shi Gang Liu, Yu Ling, Xiao Fang Zhang, Nian Bing Li, and Hong Qun Luo J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jafc.7b05245 • Publication Date (Web): 08 Feb 2018 Downloaded from http://pubs.acs.org on February 9, 2018

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Journal of Agricultural and Food Chemistry

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A Sensitive “Turn-On” Fluorescent Sensor for Melamine

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Based

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Polydopamine-Glutathione

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Nanoparticles

5

Li Tang, Shi Mo, Shi Gang Liu, Yu Ling, Xiao Fang Zhang, Nian Bing Li*, Hong

6

Qun Luo*

7

Key Laboratory of Eco-Environments in Three Gorges Reservoir Region (Ministry of

8

Education), School of Chemistry and Chemical Engineering, Southwest University,

9

Chongqing 400715, P.R. China.

on

FRET

Effect Nanoparticles

between and

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*Corresponding authors: Fax: (+86)23-6825-3237; Tel: (+86)23-6825-3237

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E-mail addresses: [email protected], [email protected]

Ag

1

ACS Paragon Plus Environment


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Abstract

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In this work, Ag nanoparticles (AgNPs) were synthesized quickly by one step

14

method utilizing polydopamine-glutathione nanoparticles (PDA-GNPs) as a reducing

15

agent. The PDA-GNPs and the generated AgNPs acted as the energy donor and

16

acceptor, respectively. Accordingly, the fluorescence of PDA-GNPs was quenched

17

based on fluorescence resonance energy transfer (FRET). In the presence of melamine,

18

the preferential combination of Ag(I) and melamine to form Ag(I) - melamine

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complex prevents Ag(I) from forming AgNPs, together with fluorescence

20

enhancement compared to the absence of melamine. Under the optimal conditions

21

including the concentration of AgNO3, reaction time, reaction temperature, and pH,

22

the fluorescence enhancement efficiency has a linear response to the concentration of

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melamine from 0.1 to 40 µM with a detection limit of 23 nM for melamine. The

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proposed method is simple, time-saving, and low-cost, which was further applied to

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detect melamine in real milk products with satisfactory results.

26 27

Keywords:

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fluorescence resonance energy transfer; fluorescent sensor; melamine detection

polydopamine-glutathione

nanoparticles;

Ag

nanoparticles;

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2


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INTRODUCTION

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Melamine (2,4,6-triamino-1,3,5-triazine) with a high nitrogen level is widely

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employed during the synthesis of paint, plastic engineering, adhesive, and other

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industrial applications.1-3 However, it is illegal to adulterate melamine with milk

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powder, feed stuffs for the purpose of artificial enhancement of the protein content by

35

virtue of its high nitrogen content.4,5 According to the related literature, the

36

hydrolyzate of melamine is cyanuric acid, which has a good combination with

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melamine to produce fewer soluble complex and then precipitate in the renal tubules,

38

leading to subsequent tissue damage.6 Long-term consumption of food with the

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content of melamine can even lead to death.7 Thus, it is important to detect this

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hazardous compound in food and feed industry. Up to now, capillary electrophoresis

41

(CE),8

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chromatography-mass

43

chromatography (HPLC),13 enzyme-linked immunosorbent assay (ELISA),14,15

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colorimetry,16-20 Fourier transform infrared spectroscopy (FT-IR),21,22 and ultraviolet

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(UV) spectroscopy23 have been applied to the detection of melamine. They are

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time-consuming and require expensive instruments as well as sophisticated sample

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pretreatment, although most of them have high accuracy and sensitivity. Thus, it is

48

necessary to build a simple, reliable method for melamine detection.

liquid

chromatography-mass spectrometry

spectrometry

(GC-MS),11,12

(LC-MS),9,10

high-performance

gas liquid

49

In recent years, depending on real-time detection, high sensitivity, and

50

operational simplicity, fluorescence analysis has gained more and more attention. It is

51

known that the fluorescence detection of melamine was almost based on silver 3



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nanoparticles or gold nanoparticles combined with mercury ion or quantum dots. 24-28

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However, the expensive price of Au nanoparticles, the toxicity of mercury ion or

54

complicated modification processes of nanoparticles impede the application of these

55

fluorescent probes. Therefore, the low-cost and environmentally friendly materials

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may be a preferred candidate.

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In this work, the reduction of PDA-GNPs was utilized to prepare the

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AgNPs/PDA-GNPs complexes in one step. Since the emission spectrum of

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PDA-GNPs overlapped with the absorption spectrum of AgNPs, the fluorescence of

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PDA-GNPs is easy to be efficiently quenched by the AgNPs via the fluorescence

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resonance energy transfer (FRET). With the existence of melamine, the combination

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of Ag(I) with melamine declined the reduction of Ag(I), together with the elimination

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of energy transfer of Ag nanoparticles. Based on this principle, a novel fluorescence

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detection method for sensing of melamine was established, which does not need any

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expensive materials, toxic reagents. The method has the advantages of simple

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operation, high sensitivity, and selectivity. The sensing mechanism is illustrated in

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Scheme 1. The high sensitive fluorescent sensor exhibited high selectivity to

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melamine over common potential interfering substances. At last, the established

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sensor was applied for the determination of melamine in real milk products with

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satisfactory results.

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MATERIALS AND METHODS

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Chemicals. 3-Hydroxytyramine hydrochloride (dopamine) was purchased from 4


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Adamas Reagent Co., Ltd. (Shanghai, China). Glutathione (GSH, reduced),

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tris(hydroxymethyl) aminomethane (Tris), silver nitrate (AgNO3), and melamine were

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received from Aladdin Reagent Co., Ltd. (Shanghai, China). The raw milk was

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obtained from local supermarkets. Hydrogen peroxide (H2O2, 30 wt %) and Zn(NO3)2

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were ordered from Chengdu Kong Chemical Reagents Factory, China. KCl, NaCl,

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FeCl3·6H2O, MgCl2·6H2O, Na2HPO4·12H2O, NaH2PO4·2H2O, glycine (Gly), serine

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(Ser), cysteine (Cys), fructose (Fru), lactose (Lac), glucose (Glu), phenylalnine (Phe),

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lysine (Lys), and leucine (Leu) were purchased from Sigma-Aldrich (USA). Tris-HCl

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(50 mM, pH 8.5) and NaH2PO4-Na2HPO4 (50 mM, pH 6.5) buffers were prepared.

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Other reagents were analytical grade and no purification treatment was required.

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Ultrapure water with a resistivity of 18.2 MΩ cm was utilized in all the procedures.

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Detection of Melamine. Freshly prepared AgNO3 solution (40 µL, 40 mM) was

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mixed with 20 µL of different concentrations of melamine solutions. Then, 100 µL of

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PDA-GNPs was added to the above solution and the mixture was diluted to 500 µL

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using a NaH2PO4-Na2HPO4 buffer (50 mM，pH 6.5), and the mixture was incubated at

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50 ◦C for 20 min. At last, under the excitation of 380 nm, the fluorescence emission

89

spectra were measured on a Hitachi F-2700 spectrofluorometer (Tokyo, Japan). Each

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sample was measured by three trials and the results show an average value. In

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addition, 250 mg mL-1 PDA-GNPs was employed under optimum conditions.

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Sample Preparation. Liquid milk and yogurt were treated according to the report:29

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First, 1.0 mL of liquid milk or yogurt was added to 3.5 mL of water and 0.5 mL of

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acetonitrile. 0.5 mL of CCl3COOH was introduced into the solution, respectively. 5



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Then the mixture was ultrasonically extracted for 15 min and centrifuged at 2500 rpm

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for 10 min. The gained supernatant was used for the following detection. In recovery

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experiments, before the pretreatment, different concentrations of melamine were

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spiked in the raw liquid milk, which was treated with the same procedures

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aforementioned.

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Infant formulas were purchased from the local supermarket. The sample was

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treated as described by Liao.30 Firstly, 100 mg of infant formulas sample was

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dissolved in 10 mL of methanol. Then 1 mL of 1% trichloroacetic acid was injected

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into the above solution and the mixture was stirred for 5 min. After that, the acquired

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solution was filtered. The residue was rinsed with methanol and then filtered again.

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The resulting solution was used for the measurement of melamine.

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Preparation of PDA-GNPs and AgNPs/PDA-GNPs Composite. The synthesis

107

of PDA-GNPs is described as follows: Typically, 4 mg of dopamine was firstly

108

dissolved in 2 mL of Tris-HCl buffer solution (pH 8.5) with magnetic stirring for 2

109

min, and then 4.8 mg of GSH was added. The mixture was continued to react for 2 h

110

with the color changing from brown to pale yellow. All the above experiments were

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carried out at room temperature. Subsequently, 200 µL of H2O2 (30 wt %) was

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injected into the above solution dropwise and the mixture was incubated via

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hydrothermal treatment at 50 °C for 7 h. For purification, the as-prepared PDA-GNPs

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solution was dialyzed against ultrapure water through a dialysis membrane (molecular

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weight cutoff 1000 Da) for 24 h. The purified product was used as a precursor to

116

subsequent experiments. The AgNPs/PDA-GNPs composite was prepared by a 6


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chemical reduction of AgNO3 with PDA-GNPs as the reducing agent. Firstly, 100 µL

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of PDA-GNPs solution was added to 360 µL of NaH2PO4-Na2HPO4 (50 mM, pH 6.5)

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buffer under vigorous stirring. Then the freshly prepared AgNO3 solution (40 µL, 40

120

mM) was introduced, and the mixture reacted at 50 ◦C for 20 min. After cooled to

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room temperature, a purplish red AgNPs/PDA-GNPs composite solution was

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obtained.

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RESULTS AND DISCUSSION

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Characterization of AgNPs/PDA-GNPs Composite. The morphology structure

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and optical properties of AgNPs/PDA-GNPs were characterized successively. The

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transmission electron microscopy (TEM) image of PDA-GNPs with a size about 3.0 ±

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0.3 nm is presented in Figure 1A. A slightly increased size of Ag/PDA-GNPs of about

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5.0 ± 0.2 nm can be seen in Figure 1B, which reveals that the generated Ag

129

nanoparticles may be attached to the surface of PDA-GNPs. The inset in Figure 1B is

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the high resolution transmission electron microscopy (HRTEM) image of

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AgNPs/PDA-GNPs, and the appearance of lattice further demonstrates that silver ions

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have been successfully reduced to silver particles. The UV-vis absorption spectra of

133

various components are shown in Figure 2A. Compared to the absorption spectra of

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melamine (curve a), AgNO3/melamine (curve b), and PDA-GNPs (curve c), after the

135

addition of AgNO3 in PDA-GNPs solution, a new peak appears at about 520 nm

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(curve d), which corresponds to the characteristic absorption of the AgNPs induced by

137

surface plasmon resonance (SPR) absorption.31 However, upon the coexistence of

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melamine, AgNO3, and PDA-GNPs, no absorption peaks can be observed at 520 nm

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(curve e), revealing that AgNPs were not formed. The above phenomena may be

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attributed to the fact that nitrogen atoms of melamine molecule are electron-donating

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and metallic silver is electron-accepting. The interaction between nitrogen atoms and

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metallic silver can result in the production of Ag(I) - melamine complexes. According

143

to surface-enhanced Raman scattering (SERS), the formation of Ag(I) - melamine is

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more reasonable than the formation of Ag(0) - melamine.32,33 Therefore, melamine

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and silver nitrate are predominantly presented as Ag(I) - melamine complexes. In

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addition, the nitrogen atoms in melamine are composed of two parts, one is

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heterocyclic nitrogen and the other is amino nitrogen. A previous work reported that

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Ag(I) tended to interact with the heterocyclic nitrogen of melamine, according to the

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comparison with the energy of conformer.31 Figure 2B is the excitation and emission

150

spectra of PDA-GNPs. Obviously, the emission spectrum of PDA-GNPs overlaps

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partially with the absorption spectrum of AgNPs. Besides, the X-ray photoelectron

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spectroscopy full scan spectrum (XPS) was performed to analyze the elements of

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AgNPs/PDA-GNPs composite. The five peaks in Figure 3A are associated with

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sulphur, carbon, silver, nitrogen, and oxygen, respectively. Figure 3B presents the

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high-resolution XPS spectrum of Ag(3d) core level in Ag/PDA-GNPs. The Ag(3d5/2)

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and Ag(3d3/2) peaks can be found at binding energies of 367.4 and 373.4 eV.

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Moreover, the slitting of the 3d doublet of Ag is 6.0 eV, indicating the formation of

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metallic silver.34-36

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Optimization of Sensing Conditions. As revealed in Figure S1, with increasing 8


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concentration of AgNO3 (5.0 - 50.0 mM), the fluorescence intensity of PDA-GNPs

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(2.5 mg mL-1) decreases gradually, and the fluorescence intensity is almost quenched

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at a concentration of AgNO3 (40 mM), which can be used in the following

163

experiments. To get high sensitivity for sensing melamine, the effects of reaction

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temperature (25 ◦C, 50 ◦C, and 80 ◦C) and reaction time (0 - 120 min) on the

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fluorescence quenching of AgNPs to PDA-GNPs were investigated. As shown in

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Figure S2, the fluorescence of the PDA-GNPs system is gradually quenched with

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increasing reaction time at different temperatures in the presence of AgNO3 (40 mM).

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After the hydrothermal reaction at 50 °C for 20 min the fluorescence intensity is

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almost quenched and remains stable within 2 h. Therefore, we selected 50 ◦C as the

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reaction temperature and 20 min as the reaction time in the following experiments.

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Besides, the influence of pH on the fluorescence intensity of Ag(I)/PDA-GNPs and

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Ag(I)/melamine/PDA-GNPs systems was also studied from pH 4.5 to 8.5. As

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displayed in Figure S3, the fluorescence maximal enhancement is achieved in pH 6.5

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solution. Naturally, a 20 mM NaH2PO4-Na2HPO4 buffer solution with pH 6.5 is

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applied in the next experiments.

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Sensing of Melamine. Under the optimal conditions mentioned above, the

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fluorescence spectra of PDA-GNPs system were measured and are shown in Figure

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4A. In the Ag(I)/melamine/PDA-GNPs system, with increasing concentration of

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melamine, the fluorescence intensity of PDA-GNPs gradually enhances. The

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relationship of the fluorescence enhancement efficiency (F - F0) / F0 and the

181

concentration of melamine is displayed in Figure 4B (where F and F0 are the 9



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fluorescence intensity of the system in the presence and absence of melamine,

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respectively). Obviously, the inset in Figure 4B shows a good calibration curve in the

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range of 0.1 - 40 µM between the concentration of melamine and the fluorescence

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enhancement efficiency of PDA-GNPs system. The calibration curve can be

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expressed as (F - F0) / F0 = 0.0287C + 0.0826 (C: µM). The detection limit (3σ/s) for

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melamine is 23 nM, These analytical parameters are comparable and even better than

188

those reported in the literature (Table S1). The stability of the method for sensing

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melamine has been investigated. Figure S4 reveals that the fluorescence enhancement

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efficiency of the AgNPs/PDA-GNPs system for the detection of melamine can remain

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stable within 40 min at room temperature. After 40 min, the fluorescence

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enhancement efficiency decreased slightly.

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Selectivity of the Method. The selectivity of constructed sensor for the sensing of

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melamine was investigated. Some possible interference involving metal ions: Fe3+, K+,

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Na+, Mg2+, Zn2+; amino acids: Gly, Ser, Cys, Lys, Leu, Phe; some sugars: Fru, Lac,

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Glu in raw milk were studied, and their impacts are displayed in Figure 5. The figure

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reveals that only melamine can turn on the fluorescence of the PDA-GNPs system

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distinctly. However, other ions and compounds do not have a significant effect on the

199

fluorescence of the PDA-GNPs system. The above results suggest that our sensor has

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an acceptable selectivity.

201

Analysis of Melamine in Raw Milk Samples. To evaluate the feasibility of the

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fluorescent sensor in real samples, the contents of melamine in liquid milk, yoghourt

203

and infant formula were tested, respectively. As melamine was not found in the above 10


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milk samples, the recovery experiments were used to evaluate the accuracy of our

205

established probe. The recovery experiments were carried out by spiking different

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concentrations of melamine in each sample, and then the fluorescence signal was

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measured. The recoveries in Table 1 are ranging from 96.2% to 104.2%,

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demonstrating that the fluorescent sensor has a good applicability.

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Fluorescence Quenching Effect of AgNPs on PDA-GNPs. The possible

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mechanism of Ag(I) reduction was studied. Some reports have claimed that the

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catechol groups of polydopamine can chelate with Ag(I) and then reduce Ag(I) to

212

Ag(0), while the catechol groups were simultaneously oxidized to quinone structure.

213

37-39

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or an acidic condition shifts to the catechol functional group, which ensures that Ag(I)

215

can be continuously reduced to AgNPs without external addition of any exogenous

216

reducing agents. Immediately, Ag binds to the nitrogen atom and the oxygen atom in

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the PDA-GNPs and forms seed precursor, which can form silver nanoparticles by the

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growth of atom by atom with the reduction of Ag(I).40 Furthermore, Jiang’s group

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utilized the reducibility of PDA to reduce noble metal ions including Ag(I).41 In order

220

to investigate whether AgNO3 can be reduced to AgNPs by GSH part in PDA-GNPs

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or not, we have measured the UV-vis absorption spectrum of GSH/AgNO3 mixture in

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NaH2PO4-Na2HPO4 buffer (50 mM, pH 6.5) after incubation for 20 min at 50 ◦C.

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Curve f in Figure 2A reveals that there is no new peak appearing in the wavelength

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range of 400-800 nm, indicating that AgNO3 cannot be reduced to Ag nanoparticles

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by GSH in the method. Thus, the reducing agents are derived from the catechol

The equilibrium between the catechol group and the quinone structure in a neutral

11



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groups in PDA-GNPs in this work. The inset in Figure S1 shows the change in color

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when different concentrations of AgNO3 are added to PDA-GNPs to form AgNPs/

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PDA-GNPs composite. It is evident that the color of the products gradually changes

229

from light yellow to pink with increasing concentration of AgNO3.

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The uniform and well-dispersed morphology of Ag/PDA-GNPs (Figure 1B) excludes

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the possibility of fluorescence aggregation-induced quenching (AIQ). In addition, as

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seen in Figure S1, the fluorescence of PDA-GNPs is quenched by AgNPs efficiently

233

in a concentration-dependent way. Moreover, the characteristic absorption band of

234

AgNPs overlaps with the fluorescence emission of PDA-GNPs (Figure 2B) from 400

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to 600 nm and the AgNPs can adhere to the surface of PDA-GNPs because of strong

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chemical bonds (Ag-N and Ag-O), which shorten the distance between AgNPs and

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PDA-GNPs. Hence, a FRET between AgNPs and PDA-GNPs occurs. Furthermore,

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similar quenching mechanism between metal nanoparticles and fluorescent materials

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has been reported.42-45

240

In conclusion, based on FRET effect between AgNPs and PDA-GNPs, a sensitive

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“turn-on” fluorescent sensor for melamine was established. Since no modification or

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labeling step of AgNPs is involved, the above method makes the fluorescent assays

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economical, rapid, and simple. The whole procedures including pre-treatment of

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sample can be completed within 1 h, which can meet the requirements of rapid

245

sensing melamine. Therefore, it is a hopeful candidate for rapid screening melamine

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in raw milk.

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AUTHOR INFORMATION

248

Corresponding Authors

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* [email protected] (N. B. Li)

250

* [email protected] (H. Q. Luo)

251

Notes

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The authors declare no competing financial interest.

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ABBREVIATIONS USED

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AgNPs, Ag nanoparticles; PDA-GNPs, polydopamine-glutathione nanoparticles;

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AgNPs/PDA-GNPs, Ag nanoparticles/polydopamine-glutathione nanoparticles; FRET,

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fluorescence resonance energy transfer; TEM, transmission electron microscopy

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image; HRTEM, high resolution transmission electron microscopy image

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FUNDING SOURCES

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This work was supported by the National Natural Science Foundation of China (No.

260

21675131, 21273174) and the Municipal Science Foundation of Chongqing City

261

(CSTC-2015jcyjB50001).

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SUPPORTING INFORMATION DESCRIPTION

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Figures S1-S4 and Table S1. This material is available free of charge via the Internet

264

at http://pubs.acs.org.

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396

FIGURE CAPTIONS:

397

Scheme 1. Schematic illustration of the sensing mechanism of melamine in the

398

absence (A) and presence of melamine (B).

399

Figure 1. TEM images of PDA-GNPs (A) and Ag/PDA-GNPs composite (B). The

400

inset is the partially magnified image of Ag/PDA-GNPs.

401

Figure 2. UV-vis absorption spectra of melamine, AgNO3/melamine, PDA-GNPs,

402

PDA-GNPs/AgNO3, PDA-GNPs/AgNO3/melamine, and GSH/AgNO3 (melamine: 20

403

µM; AgNO3: 40 mM; PDA-GNPs: 250 mg mL-1) (A). The excitation and emission

404

spectra of PDA-GNPs (Ex = 380 nm; Em = 450 nm) and the absorption spectrum of

405

AgNPs (B).

406

Figure 3. Survey scan XPS spectra of Ag/PDA-GNPs composite (A) and Ag 3d peak

407

(B).

408

Figure 4. Fluorescence spectra of the Ag/PDA-GNPs system in pH 6.5

409

NaH2PO4-Na2HPO4 buffer in the presence of 40 mM AgNO3 with increasing

410

concentration of melamine (0 - 160 µM) (Ex = 380 nm) (A) and the linear calibration

411

plot of the fluorescence enhancement efficiency((F - F0) / F0) and the concentration of

412

melamine (0 - 40 µM) (B).

413

Figure 5. Effects of some foreign substances (40 µM) on fluorescence enhancement

414

efficiency ((F - F0) / F0) of the reaction system.

415

20


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416

Table 1

417

The application of proposed method for the detection of melamine and spiked

418

recoveries in real samples.

Sample

Added (µM)

Found (µM)

Liquid milk

0

0

5

Yoghourt

Infant formula

Recovery (%)

RSD (n = 3, %)

5.07

101.4

2.46

15

15.09

100.6

4.03

25

25.02

100.1

2.69

0

0

5

5.21

104.2

3.47

15

15.32

102.1

3.98

25

25.18

100.7

4.37

0

0

5

4.81

96.2

3.32

15

15.38

102.5

4.83

25

24.86

99.4

2.98

419 420

21



421

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Scheme 1

422

22


Page 23 of 28


423

Figure 1

424 425

23



426

Page 24 of 28

Figure 2

427

24


Page 25 of 28


428

Figure 3

429 430

25



431

Page 26 of 28

Figure 4

432 433

26


Page 27 of 28


434

Figure 5

435

436

437

27



438

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GRAPHIC FOR TABLE OF CONTENTS

439

28


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