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Article
Highly sensitive and selective determination of tertiary butylhydroquinone in edible oils by competitive reaction induced “on-off-on” fluorescent switch Xiaoyue Yue, Wenxin Zhu, Shuyue Ma, Shaoxuan Yu, Yuhuan Zhang, Jing Wang, Yanru Wang, Daohong Zhang, and Jianlong Wang J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jafc.5b05340 • Publication Date (Web): 08 Jan 2016 Downloaded from http://pubs.acs.org on January 11, 2016
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Journal of Agricultural and Food Chemistry
Highly sensitive and selective determination of tertiary butylhydroquinone in edible oils by competitive reaction induced “on-off-on” fluorescent switch Xiaoyue Yue, Wenxin Zhu, Shuyue Ma, Shaoxuan Yu, Yuhuan Zhang, Jing Wang, Yanru Wang, Daohong Zhang, Jianlong Wang*
College of Food Science and Engineering, Northwest A&F University, Yangling 712100, Shaanxi, China
Corresponding Author *Tel.: +86 29-8709-2275; Fax: +86 29-8709-2275. E-mail address:
[email protected] (JL, Wang)
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ABSTRACT As one of most common synthetic phenolic antioxidants, tertiary
2
butylhydroquinone (TBHQ) has received increasing attention due to the potential risk
3
of TBHQ for liver damage and carcinogenesis. Herein, a simple and rapid fluorescent
4
switchable methodology was developed for high selective and sensitive determination
5
of TBHQ by utilizing the competitive interaction between the photo-induced electron
6
transfer (PET) effect of carbon dots (CDs) / Fe(Ⅲ) ions and the complexation
7
reaction of TBHQ / Fe(Ⅲ) ions. This novel fluorescent switchable sensing platform
8
allows determining TBHQ in a wider range from 0.5 to 80 µg mL-1 with a low
9
detection limit of 0.01 µg mL-1. Furthermore, high specificity and good accuracy with
10
recoveries ranging from 94.29 to 105.82% in spiked edible oil samples are obtained
11
with the present method, confirming its applicability for the trace detection of TBHQ
12
in complex food matrix. Thus, the present method provides a novel and effective
13
fluorescent approach for rapid and specific screening of TBHQ in common products,
14
which is beneficial for monitoring and reducing the risk of TBHQ overuse during
15
food storage.
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Keywords: TBHQ; Fluorescence detection; Carbon dots; PET effect; Complexation
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INTRODUCTION
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Tertiary butylhydroquinone (TBHQ) is one of most common synthetic phenolic
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antioxidants, which can prevent edible oil and lipid food putrefaction and
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deterioration.1 Due to their excellent antioxidant property, chemical stability, low cost
24
and availability, TBHQ is very popular with food manufacturers. Although TBHQ has
25
better antioxidant property and has been permitted to be used in foods up to a
26
maximum limit of 200 mg kg-1 in some countries such as China, the United States,
27
Australia, Brazil, New Zealand and Philippines, it is banned in the European Union
28
and Japan due to their potential risks such as liver damage and carcinogenesis.
29
to ensure human health and food safety in the food industry, various analytical
30
methods have been proposed to assay TBHQ in food samples, such as
31
high-performance liquid chromatography (HPLC),5 gas chromatography–mass
32
spectrometry (GC-MS),6 and capillary electrophoresis.7 These traditional analytical
33
methods are highly accurate and widely used for TBHQ determination, however,
34
they suffer from more or less inherent drawbacks such as laborious procedures,
35
time-consuming processing and expensive instruments, which limit their practical
36
applications. Therefore, it is indispensable to develop a simple, rapid and inexpensive
37
method for the detection of TBHQ.
2-4
So,
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Possessing the advantages of simplicity, lower-cost, high sensitivity and less
39
cell-damaging, fluorescence analytical techniques have been widely used in detection 3
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of various substances.8-11 Fluorescence carbon dots (CDs), as newly emerging
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carbon-based nanomaterials are superior in many respects including good aqueous
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solubility, stable photo-luminescence (PL), low toxicity, resistance to photo bleaching,
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high fluorescent activity and excellent biocompatibility,
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widely used in fluorescent analysis. Recently, several CDs based switchable
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fluorescence sensors have been reported for detection of L-cysteine,
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receptor,15 glutathione,
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fluorescence switchable sensors can be constructed using various
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chemical and biological functions, which mainly focus on biomolecule in
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biological system. For food systems, fluorescent sensors have been also gradually
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developed for rapid detection of food additives and harmful substances. For example,
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Chen’s team constructed a fluorescent sensor for rapidly sensing of foodborne
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pathogens based on upconversion nanoparticles,
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carbon dots-based fluorescent sensor for selective detection of tartrazine in food
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samples.10 Wang’s team have successfully developed optosensing platform for
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detecting tocopherol in rice20 and Wu established fluorescent sensor for the
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simultaneous determination of Mycotoxins21 .However, to the best of our knowledge,
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TBHQ fluorescence sensors applied in food system has never been reported.
16
ascorbic acid,
17
biothiol
19
18
12-14
which make carbon dots
12
folate
and other molecules. These
and Liao’s team
physical,
developed a
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According to previous reports, the fluorescence of carbon dots can be quenched
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by Fe(Ⅲ) irons via PET effect17, 22. Meanwhile, it is well known that complexation
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reaction will probably happen when phenolic hydroxyl exists with Fe(Ⅲ) irons. 4
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Considering that there are two phenolic hydroxyl groups in TBHQ molecule, we
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conclude that it is possible to construct an “on-off-on” fluorescent sensor based on a
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competitive reaction occurring between PET effect and complexation reaction. What's
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more, to the best of our knowledge, there is hardly any available report on the
65
quantification of TBHQ using CDs-based fluorescent sensor.
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Inspired by the above investigations, this work aimed to construct a novel, rapid
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and inexpensive switchable fluorescent sensing platform for the rapid detection of
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TBHQ in edible oil samples based on the competitive interaction between the PET
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effect and the complexation reaction. Combining the advantages of fluorescent
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nanomaterial with competitive reaction, this simple, rapid and low-cost analytical
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method offers a very effective fluorescent probe for sensitive and selective detection
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of TBHQ with a wide linear range, low detection limit and satisfactory recoveries.
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Besides, we further demonstrate the feasibility through employing the developed
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assay strategy for the detection of TBHQ spiked in edible oil samples and verify
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herein the reliability of the designed sensor by comparing the testing results of
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fluorescent sensor with that of HPLC (high performance liquid chromatograph).
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MATERIALS AND METHODS
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Reagents. TBHQ, BHA (butylated hydroxyanisole) and BHT (butylated
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hydroxytoluene) were purchased from Sigma-Aldrich (Sigma-Aldrich, Shanghai,
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China). Citric acids (CA), ethylenediamine (EDA) and metal salts (AgNO3, CaCl2,
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MnCl2·4H2O,
CdCl2,
BaCl2,
Cu(NO3)2·5H2O,
CoCl2·6H2O,
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Hg(ClO4)·3H2O, CrCl3·3H2O and AlCl3) were obtained from Sinopharm Chemical
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Reagent Co. Ltd
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isopropanol used in HPLC were of chromatographic grade and other chemicals were
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of analytical reagent grade. Besides, the water used in the process of TBHQ detection
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by HPLC was Wahaha purified water purchased from local market. Doubly distilled
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water was used throughout other experiments.
(Shanghai, China). Acetic acid, methanol, acetonitrile and
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Apparatus. Fluorescence spectra were performed on a PE LS-55 spectrometer
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with a slit width of 5 nm for both excitation and emission to obtain suitable
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fluorescent
intensity.
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JEM-3010
transmission electron microscope (TEM). UV / Vis absorption
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measurements were performed on a shimadzu UV-2550 spectrophotometer (Japan).
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Fourier transform infrared (FT-IR) spectroscopy was obtained from a Vetex70 FT-IR
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spectroscope (Bruker Corp., Germany). X-ray photo-electron spectroscopy (XPS)
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analysis was obtained on a Thermo ESCALAB 250 spectrometer. HPLC
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measurements were performed
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liquid chromatograph with an UV/ Vis detector.
Morphology
on
characterizations
were
obtained
with
a
Shimadzu LC-10AVPPLUS high performance
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Preparation of Fluorescence CDs. CDs were synthesized by microwave
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assisted pyrolysis method according to previous report with some modifications. 23
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Briefly, 0.2 g citric acid was dissolved in 5 mL water and 1 mL ethylenediamine was
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added by the dropwise under vigorous magnetic stirring. After that, the mixture was
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pyrolyzed in a microwave oven over medium heat for 2 min and high heat for another 6
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2 min to form a brown product. Finally, anhydrous ethanol was added and the sample
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was sonicated until muddy dispersion was formed. Attributed to the poorer solubility
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of CDs in ethanol than that of CA and EDA, the purified CDs can be separated after
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several cycles of centrifugation (10000rpm, 5min) and wash with anhydrous ethanol.
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Finally, the purified CDs was dried at 60 °C, dissolved with a small amount of water
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sample and then freezing dried overnight at -50 °C to obtain dried powder.
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Real samples preparation and HPLC detection. Samples preparation and
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HPLC detection were based on the standard method GB/T 21512-2008 of China.
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Briefly, edible oil sample (2 g) was weighed accurately and added to a 40 mL
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centrifuge tube and 6 mL 95% ethanol was added to the centrifuge tube. Then, the
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mixture was vibrated for about 1 min and layered under 90 °C water bath. The
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supernatant was transferred into another centrifuge tube and
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re-extracted twice. The supernatant was concentrated by vacuum rotatory evaporator
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and then diluted with acetonitrile and iso-propanol to 10 mL. Finally, the above
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solution was filtered before for HPLC determination. Chromatographic conditions:
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flow rate: 0.8 ml/min; Injection Volume: 20 µL; column temperature: room
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temperature 30 °C; ultraviolet detector wavelength: 280 nm, organic phase:
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methanol-acetonitrile (1:1); aqueous phase: 1.8% acetic acid solution.
the residues was
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Fluorescence Assay of TBHQ. For fluorescence assay, the standard or real
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(extracts of edible oils) TBHQ solutions was prepared with anhydrous ethanol. 1 mL
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of water and 1mL anhydrous ethanol was added with 1 mL of the CD dispersion (0.5 7
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µg mL-1) at room temperature in 4mL fluorescent cuvette as blank control group. For
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TBHQ determination, 1 mL of ferric chloride hexahydrate (1mM) was added into 1
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mL of the CD dispersion ( 0.5 µg mL-1) at room temperature and mixed with 1 mL
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standard TBHQ solutions with different concentrations or sample extract. After
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incubation for 10 min, the fluorescence intensity of the solution was measured with an
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excitation at 360 nm. The fluorescence of CDs could be quenched in the presence of
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Fe(Ⅲ) due to the photo-induced electron transfer effect occurring between Fe(Ⅲ)
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ions and CDs. Subsequently, the fluorescence of the CDs–Fe(Ⅲ) ions system was
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recovered gradually with the addition of TBHQ due to their strong complexation
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reaction with Fe(Ⅲ) ions and the quantitive detection of TBHQ was achieved
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according to the degree of fluorescence restoration of CDs. All experiments were
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conducted at room temperature and repeated at least three times.
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RESULTS AND DISCUSSION
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The detection principle of the switchable fluorescent sensor. The principle of
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this fluorescence sensor is shown in Scheme 1. Emission of a photon, fluorescence,
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follows HOMO (Highest Occupied Molecular Orbital) to LUMO (Lowest
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Unoccupied Molecular Orbital) excitation of an electron in a molecule. If this
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emission is efficient, the molecule may be termed a fluorophore.24 Various other
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interactions may also affect the emission process, which are significant in regard to
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analytical applications of fluorescence. When CDs serving as a fluorophore in this
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work was excited, the electrons on the surface of CDs leap into the excited state from 8
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the ground state. Due to the instability of excited electronic state, these electrons will
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return to the ground state, accompanied by efficient energy transfer, giving rise to the
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generation of fluorescence. After the introduction of Fe(Ⅲ)
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excited state enters the unfilled orbit of Fe(Ⅲ) ions. Such PET provides a mechanism
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for nonradiative deactivation of the excited state, leading to a decrease in emission
150
intensity or “quenching” of the fluorescence. 25, 26 Subsequently, with the introduction
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of TBHQ, TBHQ will compete with CDs for Fe(Ⅲ) ions, which will lead to that the
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fluorescence of the CDs–Fe(Ⅲ )
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introduction of TBHQ due to their strong complexation reaction with Fe(Ⅲ) ions.
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According to the above principles, we infer that it is probable to design an
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“on-off-on” fluorescent sensing platform based on a competitive reaction occurring
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between PET effect and complexation reaction.
ions, an electron in the
ions system is recovered gradually with the
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<Scheme 1> >
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Characterization of Morphology and optical properties of CDs. In general,
159
the optical properties are largely dependent on the size, shape and surface state of
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nanomaterials.27 TEM was applied to characterize the morphology of the as-prepared
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CDs by one-step microwave-assisted pyrolysis. As shown in Figure. 1A, CDs
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distributed evenly and well separated from each other. The corresponding nanoparticle
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size distribution histogram obtained by counting 100 particles CDs reveals that CDs
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have a narrow size ranging from 2 to 5 nm, as displayed in Figure 1B and Figure 1C.
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According to previous reports, these CDs of this size probably have excellent optical 9
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characteristics and excellent biocompatibility and will be greatly applicable in
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fluorescence analysis.28-30
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Furthermore, we investigated the remarkable optical properties of the as-obtained
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CDs by the UV-vis absorption and the steady-state fluorescent spectra. From Figure
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1D, it is seen that the maximum fluorescent excitation wavelength and emission
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wavelength of the CDs were 360 and 455 nm, respectively. Additionally, the UV/Vis
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spectrum shows that the CDs exhibit a significant UV/Vis absorption band centered at
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approximately 360 nm (Figure 1D), which is corresponding to the optimum
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fluorescence excitation peak and should be attributed to the n−π* transition of C=O or
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C–OH on the surface of the CDs. Another UV/Vis absorption band is centered at
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approximately 230 nm, which is a typical characteristic of CDs, arising from the π–π*
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transition of aromatic C=C bonds,27, 31 further indicating the successful synthesis of
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CDs. FT-IR spectrum was acquired to determine the surface state of CDs. As shown
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in Figure 1E, the peaks at 3421 cm-1 and 2925 cm-1 are assigned to stretching
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vibrations of the C−OH and C−H, respectively. The peaks at 1654 and 1560 cm-1 can
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be attributed to the stretching vibration of C=O and N−H, respectively. Bands at 1294
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cm-1 are attributed to the stretching vibration of C−O. The above results indicate that
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the as-prepared CDs are surrounded by –COOH and –OH groups. These hydrophilic
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surface groups enable the as-prepared CDs to exhibit good water-solubility and can
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improve the stability and hydrophilicity of the CDs in an aqueous system.13
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<Figure 1> > 10
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XPS was used to identify the surface chemical composition and elemental
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distribution of CDs. As shown in Figure 2, XPS spectra revealed that these
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nanoparticles are comprised of three main elements carbon, oxygen and nitrogen
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and a limited mount of Si element which may come from the silicon substrate
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during XPS test. The high-resolution spectra of C 1s (Figure 2B) displayed three
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intensive peaks at around 284.5 eV, 285.6 eV, 288.0 eV, which are attributed to
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C−C, C−N, and C=O, respectively.
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O 1s spectrum (Figure 2C) should be assigned to C–O and C–OH/C–O–C groups,
195
respectively. In the high-resolution N1s spectrum (Figure 2D), two peaks appeared
196
at 399.4 and 400.3 eV can be ascribed to C–N–C and N–(C)3 groups,
197
respectively.13 The above observations imply that citric acid was oxidized and
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carbonized during the process of microwave pyrolysis, resulting in the successful
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synthesis of CDs with lots of hydrophilic groups including hydroxyl, carbonyl and
200
carboxyl group. Importantly, the presence of those hydrophilic groups is not only
201
in favour of achieving higher water solubility but also benefit to facilitate electron
202
transfer between CDs and Fe(Ⅲ) ions.
30
Two peaks at 531.0 eV and 532.2 eV in the
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<Figure 2> >
204
Optimization of analytical conditions and mechanism analysis. It has been
205
reported that the absorption of the excitation or emission light by absorbers may
206
reduce the fluorescence intensity of the fluorophor.33 Aiming to further explore the
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optical properties of the as-prepared CDs and optimize analytical condition, we 11
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investigated the excitation-dependent photoluminescence behavior. As shown in
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Figure 3A, the highest fluorescent intensity appeared at 360 nm, which was selected
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as the excitation wavelength for the following experiments. On the other hand, when
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CDs were excited at different excitation wavelength the photoluminescence peak of
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the resultant CDs remained almost unchanged, which is of benefit to reducing the
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effect of auto fluorescence during applications.34 Such excitation-dependent
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photoluminescence behavior may be related to the different surface states of the
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CDs.23,31, 34
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To understand the mechanism of the interaction between the CDs / Fe(Ⅲ)
217
complex and TBHQ, the time-dependent fluorescence responses of the CDs
218
dispersion upon addition of 1mM Fe(Ⅲ) ions and time-dependent fluorescence
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recovery of CDs in the presence of Fe(Ⅲ) ions upon addition of TBHQ were also
220
measured. As displayed in Figure 3B, When only Fe(Ⅲ) ions were added to the
221
reaction system(1mL CDs and 1mL anhydrous ethanol), the fluorescence intensity
222
instantly decreased at the beginning of incubation, which should be ascribed to the
223
rapid PET effect happened between CDs and Fe(Ⅲ) ions. In details, when a lone
224
electron pair is located in an orbital of the fluorophore itself or an adjacent molecule
225
and the energy of this orbital lies between those of the HOMO and LUMO, efficient
226
electron transfer of one electron of the pair to the hole in the HOMO created by light
227
absorption may occur, followed by transfer of the initially excited electron to the lone
228
pair orbital. Such PET provides a mechanism for nonradiative deactivation of the 12
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excited state, leading to a decrease in emission intensity.
And then the fluorescence
230
intensity gradually leveled off because the process of electron transfer has been
231
accomplished. In contrast, when 1 mL TBHQ (100 µg mL-1) was added to the CDs /
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Fe(III) reaction system, the fluorescence responses of the CDs increased dramatically
233
at the initial stage because TBHQ can react with Fe(Ⅲ)
234
from the surface of the CDs, making the fluorescence of CDs restore quickly.
235
More specifically, fluorescence quenching as a result of PET may be recovered if it is
236
possible to involve the lone pair in a bonding interaction. In this work, the recovery of
237
fluorescence should be ascribed the coordination reaction can happen between TBHQ
238
and Fe(III) ions. Thus, on one hand, protonation or binding of a metal ion effectively
239
places the electron pair in an orbital of lower energy and inhibits the electron-transfer
240
process and on the other hand, TBHQ compete with CDs for Fe(III) ions, which make
241
a rapid recovery of the fluorescence. Afterwards, the fluorescence intensity
242
gradually leveled off, indicating that the competitive interaction between CDs / Fe(III)
243
system and TBHQ / Fe(III) system reached equilibrium. This experimental
244
phenomenon well vindicated the feasibility of the principle that we used to design the
245
switchable “on-off-on” sensing platform for TBHQ detection.
ions and take them away
246
The pH value of the solution is another key factor affecting the sensing system,
247
because the initial fluorescence intensity (in the absence of Fe (Ⅲ)) and the quenched
248
fluorescence intensity (in the presence of Fe (Ⅲ)) of the CDs are both
249
pH-dependent.33 Therefore, to achieve sensitive detection, the effect of pH values on 13
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the fluorescence intensity of CDs was also investigated. As shown in Figure 3C, the
251
fluorescence intensity increases with the increase of pH, which could be ascribed to
252
the protonation of the carboxyl groups on the surface of the CDs, thus weakening the
253
electrostatic repulsion between CDs and rendering them unstable. For another aspect
254
of stability, a basic environment will result in the formation of insoluble ferric
255
hydroxide.25 Hence, we selected a weak acidic condition (pH 5.5, 10 mM) for
256
subsequent detection.
257
To further confirm the feasibility of the strategy we proposed, a control
258
experiment was carried out. As exhibited in Figure 3D, CDs displayed high
259
fluorescence intensity at 455 nm and the fluorescence intensity decreased significantly
260
after the addition of Fe(III) to the CDs. As anticipated, upon the addition of TBHQ to
261
the CDs/Fe(III) system, the fluorescence intensity at 455 nm increased remarkably.
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During the whole detection procedures, carbon dots acts as PET donor, and Fe(Ⅲ)
263
ions serve both as a PET acceptor and a bridge between PET effect and complexation
264
reaction, thus constructing a switchable “on-off-on” sensor. Additionally, there is no
265
change in fluorescence intensity when the TBHQ solution was added to the CDs
266
dispersion alone. It's worth mentioning that no detectable fluorescence was observed
267
in the TBHQ and Fe(III) solution alone at 455 nm. These results verify that Fe(III)
268
can effectively quench the fluorescence of CDs and then the preferential complexation
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of TBHQ with Fe(III) ions induces the recovery of fluorescence, which is a good
270
match with the principle of this fluorescence sensor, confirming the feasibility of the 14
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strategy we proposed.
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<Figure 3> >
273
Fluorescence detection of TBHQ. It is well known that linear range, detection
274
limit and sensitivity are three important factors to evaluate the analytical performance
275
of sensors. The calibration curve was constructed under the optimal conditions and as
276
seen in Figure 4A, the fluorescent intensity enhanced with the increasing of the
277
concentrations of TBHQ in the range of 0.5 to 80 µg mL-1. When the concentration of
278
TBHQ beyond 80 µg mL-1, the fluorescent intensity keeps unchanged, which may be
279
due to the fact that the binding between CDs and Fe(III) and the complexing between
280
Fe(III) and TBHQ achieve a balance, leading to maximum recovery of the
281
fluorescence produced by CDs. As shown in Figure 4B, F-F0 was in a linear
282
relationship along with concentrations of TBHQ and the linear regression equation is
283
y (F-F0) = 3.118 x + 3.298 with a correlation coefficient of 0.994, where F0 is the
284
fluorescence intensity of CDs in the presence of 1mM ferric ions and F is the
285
recovered fluorescence intensity after the addition of TBHQ. The limit of detection
286
(LOD) was estimated to be
287
3S0/S criterion (based on three times signal-to-noise ratio), where S is the slope of the
288
calibration curve and S0 represents the standard deviation of a blank (n=6). As
289
exhibited in Table 1, the LOD of the proposed method is much lower than that of
290
previously report obtained by conventional instrumental methods with a much wider
0.01 µg mL-1, as calculated according to the
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linear range. Moreover, the detection limit of the developed fluorescent sensor is quite
292
lower than the maxium level for TBHQ in edible oils permitted by most of countries,
293
implying the developed method has a great potential for further applications.
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<Figure 4> >
295
<Table 1> >
296
Interference effect. Selectivity is another key parameter to evaluate the
297
analytical performance of fluorescent sensor. Fluorescence response of CDs was
298
inspected in the presence of other similar oil-solubility antioxidants including BHA
299
and BHT. As shown in Figure 5, the introduction of BHA and BHT to CDs/Fe (III)
300
system presents negligibly fluorescent recovery of CDs. Besides, we also examined
301
the fluorescence intensity changes in the presence of representative metal ions under
302
the same conditions, including Ag+, Ba2+, Ca2+, Cr2+, Co2+, Pb2+, Cu2+ and Hg2+. From
303
Figure 5A and Figure 5B, although Cu2+ displays weak fluorescence quenching and
304
Hg2+ induces a substantial decrease in the fluorescence intensity of CDs, the
305
fluorescence intensity of CDs can not be restored after the addition of TBHQ, which
306
should be on account of that there is no chemical reaction between TBHQ and both
307
two metal ions. Thus, these potential oil-soluble antioxidants and metal ion
308
interferences showed negligible effects on the signal for the detection of TBHQ,
309
demonstrating the high specificity of the proposed method for TBHQ determination.
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<Figure 5> > 16
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Practical applications in edible oils of the fluorescence sensor. It is a very key
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problem whether the constructed sensing platform could be used in real samples and
313
that
314
can be applied in practical application. To confirm the feasibility of this “on-off-on”
315
fluorescent sensor, the edible samples spiked with standard solutions containing
316
different concentrations of TBHQ at five levels (0, 50, 100, 150 and 200 µg g -1) were
317
detected. As summarized in Table 2, for the three samples, the relative standard
318
deviation (RSD, %) of the repeated measurements ranged from 1.29 to 4.13%,
319
suggesting that this developed method has favorable precision. And the recovery
320
values were also good, ranging from 94.29 % to 105.82 %, providing evidence that
321
the results obtained by the fluorescent sensor were relative accurate and reproducible.
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These results illustrate that the designed method has a great potential for the
323
quantitive detection of TBHQ in complex food matrix. Besides, to verify the
324
reliability of the results detected by fluorescence sensor, traditional HPLC was used to
325
validate the results obtained by the fluorescent sensing platform. From Figure 6, we
326
can observe that the retention time of TBHQ was about 11.8 min and the intrinsic
327
concentration of edible oil sample detected by HPLC is 112.5 µg g-1, which is well
328
consistent with the result gained by the developed fluorescence sensor, indicating the
329
proposed sensor has a good reliability.
will
determine
whether
this
kind
330
<Figure 6> >
331
<Table 2> > 17
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In summary, we have successfully designed a facile and effective “on-off-on”
333
fluorescence switchable strategy for the rapid detection of TBHQ. This method
334
combines the advantages of CDs fluorescence with the competitive interaction
335
between the PET effect of carbon dots (CDs) / Fe(Ⅲ) ions and the complexation
336
reaction of TBHQ / Fe(Ⅲ) ions, enabling an ultrasensitive and highly selective
337
determination of TBHQ. The developed approach provides a wide linear range from
338
0.5 µg mL-1 to 80 µg mL-1 and a low detection limit (0.01 µg mL-1), which is much
339
better than that of previously report obtained by conventional instrumental methods
340
and much lower than the maxium level for TBHQ in edible oils permitted by most of
341
countries. Furthermore, favorable accuracy, satisfactory stability and reproducibility
342
are achieved for the detection of TBHQ in edible oils samples, indicating a great
343
potential for monitoring and reducing the risk of TBHQ overuse during food storage.
344
Meantime, the present design combines the advantages of fluorescent nanomaterial
345
with competitive reaction, which may be as reference for the fluorescent sensor
346
design of other food additive.
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ABBREVIATIONS USED
348
TBHQ, tertiary butylhydroquinone; BHA, butylated hydroxyanisole; BHT, butylated
349
hydroxytoluene; PET, photo-induced electron transfer; XPS, X-ray photoelectron
350
spectroscopy;
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Ultraviolet–visible spectroscopy; HPLC, high performance liquid chromatography;
352
HPLC–DAD, high performance liquid chromatography with diode array detector;
FT-IR,
Fourier
transform
infrared
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spectroscopy;
UV-Vis,
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HPLC–FLD, high performance liquid chromatography with fluorescence detector;
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AuNPs / GCE, gold nanoparticles modified bare glasy carbon electrode; MWCNT /
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GCE, Multi walled carbon nanotubes modified bare glasy carbon electrode; HMDE,
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hanging mercury-drop electrode; MWCNT / SPE, Multi walled carbon nanotubes
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modified screen printed electrode; RSD, relative standard deviation; SD, standard
358
deviation
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AUTHOR INFORMATION
360
Corresponding Author
361
*Tel.: +86 29-8709-2275; Fax: +86 29-8709-2275.
362
E-mail address:
[email protected] (JL, Wang)
363
Funding Sources
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This research was financed by Grants from the New Century Excellent Talents in
365
University (NCET-13-0483), Open Fund of State Key Laboratory of Electroanalytical
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Chemistry (SKLEAC201301), the Shaanxi Provincial Research Fund (2014KJXX-42,
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2014K02-13-03, 2014K13-10), the Yangling district research fund (2014NY-35) and
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Fundamental Research Funds for the Northwest A&F University of China
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(2014YB093, 2452015257).
370
Note
371
The authors declare no competing financial interest.
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TABLES
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Table 1 Comparison of the analytical performance of several commonly selected
504
methods for the determination of TBHQ Linear range
LOD
(µg mL-1)
(ng mL-1)
HPLC–DAD
7.99~22.98
180.0
35
HPLC–FLD
0.40~12.80
20.70
36
AuNPs / GCE
0.20~2.80
79.00
37
MWCNT / GCE
0.66~16.6
5.31
38
HMDE
0.17~1.68
5.69
39
MWCNT / SPE
0.166~1.66
81.00
40
Fluorescence detection
0.5~80
10.00
This work
Methods
Reference
HPLC
electroanalytical method
505
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Table 2 Comparison of performance: analysis of TBHQ in commercial edible oil samples using calibrations modeled with the HPLC and Fluorescence analysis Added spiked
Detected concentration by
Detected by fluorescence
RSD by fluorescence
Recovery by
concentration (µg
HPLC (mean±SD, µg g-1)
detection (mean±SD, µg
detection (%)
fluorescence detection
g-1)
g-1)
(%)
0
118.35±5.90
119.80±3.22
2.69
101.23
50
170.15±1.97
179.90±7.40
4.13
105.82
100
215.95±4.93
222.28±6.30
2.83
102.93
150
270.50±3.68
278.02±3.59
1.29
102.78
200
309.60±2.17
291.92±7.73
2.40
94.29
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Scheme and Figure Captions: Scheme 1. The sensing principle of the detection process for TBHQ Figure 1. A: Low-resolution TEM of CDs; B and C: high-resolution TEM of the as-synthesized carbon quantum dots (CQDs). D: UV−vis absorption (black line) , PL excitation (blue line) and emission (red line) spectra of the aqueous dispersion of the nanoparticles ; E: FTIR spectra of CDs. The inset of (B) shows the particle size distribution histogram. Figure 2. A: XPS of CD; B–D: High-resolution XPS spectra of (B) C 1s, (C) O 1s and (D)N Figure 3. A: Emission spectra of fluorescence CDs at different excitation; B: Time-dependent fluorescence responses of the CD dispersion upon addition of 1mM Fe(III) ions (red line) and the time-dependent fluorescence recovery of CDs in the presence of Fe(III)
ions (1mM) upon addition of 100 µg mL-1 TBHQ (black line). C:
Emission spectra of fluorescence CDs at different pH; inset: the relationship between pH and fluorescence intensity. D:Fluorescence emission spectra of CDs under different conditions. Figure 4. A: The fluorescence emission spectra of CDs in the presence of Fe (III) ions (1mM) upon addition of TBHQ with different concentrations. B: Plot of the fluorescence recovery factor (F –F0) versus concentrations of TBHQ (Inset is a linear region). Figure 5. A: Fluorescence response of CDs in the presence of different interfering 29
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substances (the concentrations of metal ions and antioxidants are all 100 µg mL-1); B:fluorescence recovery factor (F –F0) versus different interfering substances Figure 6. (A)Representative chromatograms of standard sample containing 100 µg mL-1 TBHQ; (B) HPLC of the real edible oil samples. Experimental details are described in the text.
Scheme 1.
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Figure 1.
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Figure 2.
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Figure 3.
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Figure 4.
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Figure 5.
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Figure 6.
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Table of Contents Graphic ( TOC) In this work, we developed a new analytical method for sensitive and selective detection of TBHQ based on a facile and effective “on-off-on” fluorescence switchable strategy.
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