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A novel fluorescent sensors for sensitive detection of nitrite ions
Journal Pre-proof A novel fluorescent sensors for sensitive detection of nitrite ions
Min Yang, Yujia Yan, Huanxian Shi, Enzhou Liu, Xiaoyun Hu, Xu Zhang, Jun Fan PII:
S0254-0584(19)30938-1
DOI:
https://doi.org/10.1016/j.matchemphys.2019.122121
Article Number:
122121
Reference:
MAC 122121
To appear in:
Materials Chemistry and Physics
Received Date:
19 June 2019
Accepted Date:
31 August 2019
Please cite this article as: Min Yang, Yujia Yan, Huanxian Shi, Enzhou Liu, Xiaoyun Hu, Xu Zhang, Jun Fan, A novel fluorescent sensors for sensitive detection of nitrite ions, Materials Chemistry and Physics (2019), https://doi.org/10.1016/j.matchemphys.2019.122121
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Journal Pre-proof
A novel fluorescent sensors for sensitive detection of nitrite ions Min Yanga1, Yujia Yana1, Huanxian Shia, Enzhou Liua, Xiaoyun Hub, Xu Zhangd, Jun Fanc,*
aSchool
of Chemical Engineering, Northwest University, Xi’an 710069, P. R. China.
bSchool
of Physics, Northwest University, Xi’an 710069, P. R. China.
cCollege dState
of Food Science and Engineering, Northwest University, Xi’an 710069, P. R. China.
Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, Wuhan Center for
Magnetic Resonance, Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences, Wuhan 430071, P. R. China.
*Corresponding author: Prof. Jun Fan E-mail: [email protected] 1: Min Yang and Yujia Yan co-first authors; they contributed equally to the work
Journal Pre-proof Abstract In this work, the fluorescent sensors based ternary ZnCdS quantum dots (QDs) were constructed for nitrite (NO2-) detection, and ternary ZnCdS QDs were synthesized by a rather facile, low-cost and effective aqueous synthesis method using air-stable compounds. Experimental results indicated that the ZnCdS QDs fluorescence (PL) emission intensity was decreased gradually with the introduction of NO2-. Importantly, the F/F0 (F and F0 were the PL intensity in the presence and absence of NO2- at 505 nm, respectively) was linearly proportional to the NO2- to 700 µM, the equation of linear regression was y=-0.123x+1.1525, which showed a better performance to the detection of NO2-. Moreover, there was no interference to other interfering substances including metal ions and anion ions. Thus, we applied the ZnCdS QDs fluorescence sensors to detect the contents of NO2- in ham sausage and the results showed that the contents of NO2- in ham sausage was 18.45 mg/kg. Therefore, this test method had the advantages of simplicity, rapidity and high sensitivity.
Keywords: ZnCdS QDs, fluorescence sensors, nitrite, ham sausage
Journal Pre-proof 1. Introduction It is common knowledge that nitrite (NO2-) as a kind of food additive and preservative agent, has been widely used in the food and beverages manufacturing processes [1-3]. Iron in human hemoglobin is ferrous iron[4], which binds with oxygen and delivers oxygen to every part of the body when the blood circulates. However, a chemical reaction will happen when excessive NO2- is taken into the blood, which leading to the transformation of hemoglobin to methemoglobin [5]. At this moment, the phenomenon of hypoxia will occur due to methemoglobin cannot carry oxygen in the human body [6,7]. Currently, the Legislation of the European Union suggests the maximum permissive level of NO2- for meat products to be 50 mg/kg [8]. In order to strictly monitor the content of NO2-, several strategies have been designed and utilized to detect NO2- in water or biological samples [9], such as chromatography method [10], chemiluminescence [11-14], capillary electrophoresis [15], electrochemistry [16-18] and spectrophotometry [19]. Unfortunately, most of these methods are usually demanded complicated treatment, toxic solvents and expensive equipment. Compared to other detection methods, fluorescent sensors are preferred because they have higher signal-to-noise ratio and high sensitivity [20-23]. Hence, many researchers have attempted to exploit novel fluorescent sensors for the detection of NO2-. At present, the fluorescent sensors based semiconductor quantum dots (QDs) have attracted extensively interest for the unique optical and electrical properties compared with other semiconductor materials, such as broad absorption spectra and narrow emission spectra, excellent photostability, size-tunable or composition-dependent optical performances, high photoluminescence quantum yield, and large molar extinction coefficients [24-29]. The sensitivity detection of biomolecules and inorganic ions based on binary QDs fluorescence sensors have been reported [30-34]. For instance, the quickly detection of bovine serum albumin based a new kind of QDs fluorescence sensors has been designed by Yu et al [35]. In addition, the optical properties of ternary alloyed ZnCdS QDs can be regulated by changing Zn/Cd molar ratio, which leads to the ZnCdS QDs semiconductor materials possess unique performances differ from those of binary CdS and ZnS NCs and from those of their bulk counterpart [36]. These advantages enable ternary alloyed ZnCdS QDs to become an ideal fluorescent sensor for chemical and biological assay. In this study, we achieved the detection of NO2- using water-soluble ternary alloyed ZnCdS QDs fluorescent sensors for the first time. The as-prepared ZnCdS QDs with high-quality, excellent optical performance and the synthetic method was simple, low-cost and effective. The PL intensity
Journal Pre-proof of ZnCdS QDs was gradually decreasing with the increasing of NO2- concentrations (scheme 1), and the F/F0 (at 505 nm) was linearly proportional to the NO2- concentrations in the range from 180 µM to 700 µM. Moreover, the selectivity of the QDs for the detection of NO2- has been investigated, and the results showed that the fluorescence sensors had a good specificity. Finally, we applied this fluorescent sensors to detect the content of NO2- in ham sausage, these results revealed that quantitative fluorescence detection of NO2- in the actual sample was accomplished by the water-soluble ternary alloyed ZnCdS QDs.
2. Experimental section 2.1. Materials Zinc acetate dihydrate (Zn(CH3COO)2·2H2O, 99%), zinc chloride (ZnCl2), aluminium nitrate nonahydrate (Al(NO3)3·9H2O, 99%), ferrous sulfate heptahydrate (FeSO4·7H2O, 99%), magnesium carbonate (MgCO3, 99%) and sodium chloride (NaCl) were bought from Tianjin Hengxing Chemical reagents Reagent Co., Ltd. Calcium chloride (CaCl2) was purchased from Hedong District, Tianjin Hongyan Reagent Factory. Lead acetate trihydrate (Pb(CH3COOH)3·3H2O, 99.5%), Sodium sulfide (Na2S, 99%), L-ascorbic acid (AA) and sodium nitrite (NaNO2) were provided from Tianjin Tianli Chemical Ltd. Potassium chloride (KCl) was purchased from Tianjin chemical reagent NO 3 plant. 3-mercaptopropionic acid (MPA, 98%) was bought from Aladdin Reagent company. Cadmium chloride dihydrate (CdCl2·2H2O, 99%) was provided from China Pine Chemical Reagent Factory (Zhengzhou). Sodium hydroxide (NaOH, 98%) was purchased from Tianjin Ruijin special Chemical Co., Ltd. Ethanol absolute (analytical regent) was provided from Tianjin Fuyu Fine Chemical Co., Ltd. Zinc sulfate (ZnSO4·7H2O) was purchased from Local state-owned Xi'an paint factory. Glycine (Gly), Folic acid (FA), Glucose, and L-cysteine (Lys) were purchased from Xiya Reagents. Serine (Seri) was obtained from Shanghai blue season Technology Development Co., Ltd. Bovine Serum Albumin (BSA), L-Asparic acid (APs), L-Phenylalanine (Phe) and L-Alanine (Ala) were purchased from Xi'an Ke Hao Biological Engineering Co., Ltd. Citric acid (CA) was purchase from Tianjin KGM Chemical Reagent Co., Ltd. Cobalt nitrate hexahydrate (Co(NO3)2·6H2O, AR), (Ni(NO3)2·6H2O, 98%) and Sodium sulfite (Na2SO3) were obtained form shanghai Aladdin biochemical technology co., Ltd. The ham sausage was obtained from the supermarket. 2.2. Synthesis of ZnCdS QDs
Journal Pre-proof The water-soluble ternary alloyed ZnCdS QDs were synthesized in aqueous solution using MPA as stabilizer according to our previous study [37]. Typically, the cationic precursor solution was obtained by mixing CdCl2, Zn(CH3COO)2, and MPA in 100 mL of ultrapure water and the molar ratio of (Zn2++Cd2+)/MPA was 1:2. Then, NaOH solution (1 mol/L) was added dropwise to adjust the pH of the system to alkalinity before the mixture was stirred at room temperature for 20 min. Continue stirring for 30 min, the Na2S solution (0.1 mol/L) was added drop by drop to the mixture under adequately mixing. The ternary alloyed ZnCdS QDs was obtained by reacting at 100 ℃ for 90 min. Furthermore, the ZnCdS precipitates could be obtained by adding ethanol to the mixture and centrifugation, then washing repeatedly with water to remove unreacted materials (including the excess capping molecules and ions). Finally, the ZnCdS QDs were dissolved in the ultrapure water for further use. 2.3. Detection of nitrite using ZnCdS QDs NaNO2 aqueous solution with various concentrations was freshly prepared before detect. 0.4 mL ZnCdS QDs (Zn/Cd molar ratio was 0.8:0.2 and the concentration of QDs was 0.017 mol/L calculated using the total amount of cations) and NaNO2 solution with different concentrations were transferred into 10 mL colorimetric tube, then diluted with deionized water to graduation lines and the mixture was thoroughly mixed. After 5 min reaction at room temperature, the PL spectra were recorded at excitation wavelength of 365 nm. Moreover, the following interfering substances were used to research the selectivity of the ZnCdS QDs to NO2- detection: Lys, AA, CA, Pb2+, Ni2+, S2-,SO32-, Na+, K+, Ca2+, Mg2+, NO3-, SO42- and so on. The solution was mixed with 0.4 mL 0.017 mol/L ZnCdS QDs in a 10-fold concentration. All mixture was reacted 5 min at room temperature, and the PL spectra were recorded at excitation wavelength of 365 nm. 2.4. Preparation of real samples Firstly, 5 g of shredded ham sausages was put into a 50 mL beaker, and 12.5 mL of saturated borax solution was added and stirred vigorously. Then the mixture was washed into a 250 mL volumetric flask with 150 mL of deionized water above 70 ℃ and heated it out in a boiling water bath for 15 min. Finally, 2.5 mL of ZnSO4 aqueous solution (1 mol/L) was dripped to precipitate the protein in the ham sausage. After cooling to room temperature, the distilled water was added to the mark line and placed 10 min, and the solution to be tested will be obtained after filtration.
Journal Pre-proof 2.5. Characterization The UV-vis absorption spectra were acquired with an ultraviolet and visible (UV-vis) ratio recording spectrophotometer (Shimadzu UV-3600) in the wavelength range of 200-900 nm. The energy dispersive X-ray spectroscopy (EDS) was taken with the JEOL JSM-6390A system. The high resolution transmission electron microscopy (HRTEM) of the ternary ZnCdS QDs were conducted using a Hitachi H-800 electron microscope at an acceleration voltage of 200 kV with a CCD camera. Fourier transform Infrared (FTIR) absorption spectra were performed using Perkin Elmer Frontier FTIR in the mid IR range with ATR accessory. X-ray diffraction (XRD) were investigated using a Rigaku D/MAX 3C powder diffract meter (Cu K as radiation source, λ=0.15406 nm). PL spectra of the QDs were performed from 200 to 900 nm using an F-7000 fluorescence spectrometer Record (Hitachi, Japan) equipped with a 150 W Xenon light source, a 1.0 cm quartz battery, a thermostat bath and the slit width was 5 nm.
3. Results and discussions 3.1. Characterization of the ternary ZnCdS QDs The size and morphology of ternary ZnCdS QDs were characterized by the high resolution transmission electron microscopy. The HRTEM images of ZnCdS QDs showed that the as-prepared ZnCdS QDs were monodisperse, nearly spherical nanoparticles and the average particle size of ZnCdS QDs have been concluded as 4.45 nm from particle size distribution plot based on the TEM images. In the inset of Fig. 1 (a), it could be found that the obvious lattice fringes with the interplanar spacing of 0.32 nm, which corresponded to the (1 1 1) lattice planes of the cubic zinc blende. In addition, the XRD patterns (Fig. 1 (b)) of ternary ZnCdS QDs displayed three obvious diffraction peaks, which could be indexed to the (1 1 1), (2 2 0), and (3 1 1) planes of cubic blend structure and matched with the standard JCPDS data (JCPDS No. 05-0566 and No. 10-0454 for ZnS and CdS, respectively) and the result was consistent with other study [38]. What's more, it was
clearly that
the XRD diffraction peaks of ZnCdS QDs were between pure CdS and ZnS. Additionally, no CdS/ZnS nanomaterials was formed due to the fluorescence spectrum were not red-shift which usually occurred accompany with the formation of core-shell materials Infrared absorption spectrum was also used for analyzing surface functional groups of the ternary ZnCdS QDs. According to the reports in the literature [39], there are three distinct characteristic absorption peaks for pure MPA at 3304 cm-1, 1580 cm-1 and 1407 cm-1, which assigned to the
Journal Pre-proof stretching vibrations of -OH and the asymmetrical and symmetrical stretching vibration of -COOH, respectively. In the FTIR spectra (Fig. 2(a)) of the ternary ZnCdS QDs, the absorption band at about 3425 cm-1 represented the -OH stretching mode, the peak at 1562 cm-1 and 1400 cm-1 were attributed to the anti-symmetry and symmetric stretching of the -COOH, respectively. Furthermore, the disappearance of the obvious absorption band of -SH group at about 2662 cm-1, which suggested that a strong covalent bond could be formed between the -SH and metal ions and the successful modification of MPA on the surface of the QDs. Elemental analysis by EDS demonstrated that Cd, Zn, S, O and C were present in the ternary ZnCdS QDs sample (Fig. 2(b)), which also verified that MPA was capped on the surface of the ZnCdS QDs. 3.2. Optical properties of the ternary ZnCdS QDs The UV-vis absorption and PL spectra for ternary ZnCdS QDs samples with different Zn/Cd molar ratios has been studied in our previous report [38]. As shown in Fig. 3, the absorption edges of ZnCdS QDs had obvious blue-shift compared with pure CdS which owing to the increase of the QDs band gap energy when the Zn2+ ions were incorporated in CdS crystals, and between the binary CdS and ZnS QDs, which agreed with other studies [40-42]. In addition, the emission peak of the as-prepared ZnCdS QDs (Zn/Cd molar ratio was 0.8:0.2) was 505 nm, and no emission peaks of CdS or ZnS were found in the ZnCdS QDs fluorescence emission spectra, which indicated that there was no CdS or ZnS nanomaterials were formed in our synthesis system, and also no CdS/ZnS nanomaterials because the formation of core-shell materials tend to be accompanied by the red shift of the fluorescence spectrum. 3.3. Nitrite detection using ZnCdS QDs as fluorescent sensors We selected the ZnCdS QDs of the Zn/Cd molar ratio was 0.8:0.2 as fluorescent sensors for the detection of NO2-, on account of the response of ZnCdS QDs with other Zn/Cd molar ratio to NO2were not ideal, which could be due to the fact that the optical properties of the ternary alloyed QDs was affected by its elemental composition [38]. As shown in Fig. 4, the PL intensity of the ZnCdS QDs was decreased gradually with the increase of NO2- concentrations from 180 µM to 700 µM. We speculated that the probable reason was that the nonradiative energy or electron transfer may be strengthened when the NO2- were introduced into the ZnCdS QDs solution. On the other hand, some anions and metal ions can easily lead to the aggregation of nanoparticles, which also weaken the PL intensity [2]. In order to certify this detection mechanism, the UV-vis absorption spectra of ZnCdS
Journal Pre-proof QDs in the absence and presence of NO2- and TEM image of ZnCd0S QDs after addition of NO2have been characterized. As shown in Fig. 5, the absorption intensity of ZnCdS QDs decreased significantly after NO2- was added to ZnCdS QDs solution, indicating the interaction between NO2and ZnCdS QDs. In addition, the morphology of the ZnCdS QDs after the addition of NO2- was characterized. As shown in Fig. 4(b), the agglomeration of ZnCdS QDs was obtained. Therefore, this may be due to the fact that NO2- causes ZnCdS QDs agglomeration when NO2- interacts with ZnCdS QDs. Additionally, the equation of linear regression was y=-0.123x+1.1525, where y was the F/F0 and x was the NO2- concentrations, with a correlation coefficient of 0.9903. The detection limit was calculated as 219.3 μM (S/N=3). To further investigate the specificity of the ZnCdS QDs fluorescent sensors for the detection of NO2-, the effect of potential interfering substances on the system were investigated. It could be clearly seen from Fig. 6 that only the NO2- could lead to the PL intensity quench. Most importantly, fluorescence signal of the system was not obvious when adding representative metal ions and anion ions including Ca2+, Na+, Zn2+, NO3-, SO42-, Cl- and so on. The results illustrated that the ternary ZnCdS QDs were highly selective sensors for NO2-. Furthermore, the response time of ternary ZnCdS QDs to NO2- was studied and the results showed that the required response effect of the experiment can be achieved when the response time was 5 minutes (Fig. 7), which demonstrated the rapid detection of NO2- could be completed by ternary ZnCdS QDs. Therefore, compared with other sensors for NO2- [43-44], the ZnCdS QD-based sensor has demonstrated obvious advantages because it can be easily synthesized and also showed good performance for sensing NO2- (Table 1). 3.4. Detection of NO2- in real samples To determine the feasibility of analysis in actual samples, the prepared ZnCdS QDs was used to detect the concentration of NO2- in ham sausage with the standard addition methods. The results were listed in Table 2, which showed the recovery was larger than 100% for ham sausage and all relative standard deviations (RSD) for four repeated detection were less than 5%, and the results showed that the content of NO2- in sausage was 18.45 mg/kg, which indicated that the detection of the content of NO2- in sausage could be achieved by the ZnCdS QDs fluorescent sensors. In addition, in order to study the stability of ZnCdS QDs sensor for detection of NO2- in real samples, the F/F0 value of the ZnCdS QDs sensor solution containing a certain amount of the analyte and standard
Journal Pre-proof solution after being placed for a certain period of time is characterized (Fig. 8), the results showed that it has good stability.
4. Conclusions In a word, we reported water-solution ternary alloyed ZnCdS QDs were synthesized by an aqueous method and the fluorescence sensors were constructed for the detection of NO2-. The PL intensity of ZnCdS QDs in response to NO2- was investigated, the results illustrated that the response PL intensity of ZnCdS QDs was gradually decreased with the increase of NO2- concentrations from 180 µM to 700 µM. The regression equation was y=-0.123x+1.1525 with a linear coefficient R2 of 0.9903 and the detection limit (S/N=3) was estimated to be 219.3 μM. Furthermore, the investigation of interference showed that the fluorescent sensors of ternary alloyed ZnCdS QDs has highly selective for NO2-, and the prepared ZnCdS QDs was used to detect the NO2- in ham sausage to determine the feasibility of analysis in actual samples, the results indicated the high accuracy of the ZnCdS QDs fluorescent sensors for NO2- detection in actual samples. Acknowledgements We are very grateful for the support of the National Natural Science Foundation of China (21476183 and 21676213), the Project funded by China Postdoctoral Science Foundation (2016M600809), and the Open Research Fund of State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, Chinese Academy of Sciences (T151702).
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Journal Pre-proof Captions: Sch. 1. The schematic diagram of the preparation process of ZnCdS QDs and its determination of nitrite. Fig. 1. The HRTEM images (a) and the XRD patterns (b) of ZnCdS QDs. Inset (a): the lattice fringes with the interplanar spacing of 0.32 nm and the particle size distribution plot. Fig. 2. The FT-IR spectra (a) and EDS analysis (b) of ZnCdS QDs. Fig. 3. The UV-vis absorption (solid) and PL spectra spectra (dash) of ZnCdS QDs. Fig. 4. The PL spectra of ZnCdS (Zn/Cd molar ratio was 0.8:0.2) with increasing concentrations of NO2- (a). Plots of the relationship between fluorescence intensity F/F0 and NO2- concentration (b). Inset (b): expanded linear region (180 µM-700 µM) of the calibration curve. Plots of the relationship between fluorescence intensity F/F0 of ZnCdS QDs (Zn/Cd molar ratio was 0.6:0.4) (c) and ZnCdS QDs (Zn/Cd molar ratio was 0.5:0.5) (d) and NO2- concentration (180 µM-700 µM). Fig. 5. UV-vis absorption spectra of Zn0.8Cd0.2S QDs in the absence and presence of NO2- (a), TEM image of Zn0.8Cd0.2S QDs after addition of NO2- (b) Fig. 6. The fluorescence responses of ZnCdS QDs to NO2- and other interfering substances. Fig. 7. The effect of response time of ternary ZnCdS QDs to NO2- on detection results. Fig. 8. The stability of ZnCdS QDs sensor for detection of NO2- in real samples. Table 1. Comparison of the performance of different methods for the detection of NO2Table 2. Determination results of NO2- in actual samples.
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Journal Pre-proof Table 1 Linear range
Detection limit
(μmol/L)
(μmol/L)
Cu nanoclusters
0.125~125
0.0036
[38]
Carbon dots
0.1~10
0.05
[39]
Carbon nanofibers
5~300
3
[40]
This work
180~700
0.780
Probes
Reference
0.7
F/F0
0.6 0.5 0.4 0.3
0
15
30
45
60
75
90
Time (min) Fig. 8.
Table 2 Sample
Ham sausage
Added
Total after addition
Recovery
RSD
(mg/kg)
(mg/kg)
(%)
(%)
0
17.871
-
4.09
3.45
21.581
107.54
3.37
6.90
25.637
112.55
2.81
13.80
32.846
108.5
4.45
Journal Pre-proof Highlights
The water-soluble ZnCdS QDs were synthesized by aqueous synthesis method.
The fluorescent sensors based ZnCdS QDs were constructed for NO2- detection.
The content of NO2- in ham sausage was detected using ZnCdS QDs.
Min Yang, Yujia Yan, Huanxian Shi, Enzhou Liu, Xiaoyun Hu, Xu Zhang, Jun Fan PII:
S0254-0584(19)30938-1
DOI:
https://doi.org/10.1016/j.matchemphys.2019.122121
Article Number:
122121
Reference:
MAC 122121
To appear in:
Materials Chemistry and Physics
Received Date:
19 June 2019
Accepted Date:
31 August 2019
Please cite this article as: Min Yang, Yujia Yan, Huanxian Shi, Enzhou Liu, Xiaoyun Hu, Xu Zhang, Jun Fan, A novel fluorescent sensors for sensitive detection of nitrite ions, Materials Chemistry and Physics (2019), https://doi.org/10.1016/j.matchemphys.2019.122121
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Journal Pre-proof
A novel fluorescent sensors for sensitive detection of nitrite ions Min Yanga1, Yujia Yana1, Huanxian Shia, Enzhou Liua, Xiaoyun Hub, Xu Zhangd, Jun Fanc,*
aSchool
of Chemical Engineering, Northwest University, Xi’an 710069, P. R. China.
bSchool
of Physics, Northwest University, Xi’an 710069, P. R. China.
cCollege dState
of Food Science and Engineering, Northwest University, Xi’an 710069, P. R. China.
Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, Wuhan Center for
Magnetic Resonance, Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences, Wuhan 430071, P. R. China.
*Corresponding author: Prof. Jun Fan E-mail: [email protected] 1: Min Yang and Yujia Yan co-first authors; they contributed equally to the work
Journal Pre-proof Abstract In this work, the fluorescent sensors based ternary ZnCdS quantum dots (QDs) were constructed for nitrite (NO2-) detection, and ternary ZnCdS QDs were synthesized by a rather facile, low-cost and effective aqueous synthesis method using air-stable compounds. Experimental results indicated that the ZnCdS QDs fluorescence (PL) emission intensity was decreased gradually with the introduction of NO2-. Importantly, the F/F0 (F and F0 were the PL intensity in the presence and absence of NO2- at 505 nm, respectively) was linearly proportional to the NO2- to 700 µM, the equation of linear regression was y=-0.123x+1.1525, which showed a better performance to the detection of NO2-. Moreover, there was no interference to other interfering substances including metal ions and anion ions. Thus, we applied the ZnCdS QDs fluorescence sensors to detect the contents of NO2- in ham sausage and the results showed that the contents of NO2- in ham sausage was 18.45 mg/kg. Therefore, this test method had the advantages of simplicity, rapidity and high sensitivity.
Keywords: ZnCdS QDs, fluorescence sensors, nitrite, ham sausage
Journal Pre-proof 1. Introduction It is common knowledge that nitrite (NO2-) as a kind of food additive and preservative agent, has been widely used in the food and beverages manufacturing processes [1-3]. Iron in human hemoglobin is ferrous iron[4], which binds with oxygen and delivers oxygen to every part of the body when the blood circulates. However, a chemical reaction will happen when excessive NO2- is taken into the blood, which leading to the transformation of hemoglobin to methemoglobin [5]. At this moment, the phenomenon of hypoxia will occur due to methemoglobin cannot carry oxygen in the human body [6,7]. Currently, the Legislation of the European Union suggests the maximum permissive level of NO2- for meat products to be 50 mg/kg [8]. In order to strictly monitor the content of NO2-, several strategies have been designed and utilized to detect NO2- in water or biological samples [9], such as chromatography method [10], chemiluminescence [11-14], capillary electrophoresis [15], electrochemistry [16-18] and spectrophotometry [19]. Unfortunately, most of these methods are usually demanded complicated treatment, toxic solvents and expensive equipment. Compared to other detection methods, fluorescent sensors are preferred because they have higher signal-to-noise ratio and high sensitivity [20-23]. Hence, many researchers have attempted to exploit novel fluorescent sensors for the detection of NO2-. At present, the fluorescent sensors based semiconductor quantum dots (QDs) have attracted extensively interest for the unique optical and electrical properties compared with other semiconductor materials, such as broad absorption spectra and narrow emission spectra, excellent photostability, size-tunable or composition-dependent optical performances, high photoluminescence quantum yield, and large molar extinction coefficients [24-29]. The sensitivity detection of biomolecules and inorganic ions based on binary QDs fluorescence sensors have been reported [30-34]. For instance, the quickly detection of bovine serum albumin based a new kind of QDs fluorescence sensors has been designed by Yu et al [35]. In addition, the optical properties of ternary alloyed ZnCdS QDs can be regulated by changing Zn/Cd molar ratio, which leads to the ZnCdS QDs semiconductor materials possess unique performances differ from those of binary CdS and ZnS NCs and from those of their bulk counterpart [36]. These advantages enable ternary alloyed ZnCdS QDs to become an ideal fluorescent sensor for chemical and biological assay. In this study, we achieved the detection of NO2- using water-soluble ternary alloyed ZnCdS QDs fluorescent sensors for the first time. The as-prepared ZnCdS QDs with high-quality, excellent optical performance and the synthetic method was simple, low-cost and effective. The PL intensity
Journal Pre-proof of ZnCdS QDs was gradually decreasing with the increasing of NO2- concentrations (scheme 1), and the F/F0 (at 505 nm) was linearly proportional to the NO2- concentrations in the range from 180 µM to 700 µM. Moreover, the selectivity of the QDs for the detection of NO2- has been investigated, and the results showed that the fluorescence sensors had a good specificity. Finally, we applied this fluorescent sensors to detect the content of NO2- in ham sausage, these results revealed that quantitative fluorescence detection of NO2- in the actual sample was accomplished by the water-soluble ternary alloyed ZnCdS QDs.
2. Experimental section 2.1. Materials Zinc acetate dihydrate (Zn(CH3COO)2·2H2O, 99%), zinc chloride (ZnCl2), aluminium nitrate nonahydrate (Al(NO3)3·9H2O, 99%), ferrous sulfate heptahydrate (FeSO4·7H2O, 99%), magnesium carbonate (MgCO3, 99%) and sodium chloride (NaCl) were bought from Tianjin Hengxing Chemical reagents Reagent Co., Ltd. Calcium chloride (CaCl2) was purchased from Hedong District, Tianjin Hongyan Reagent Factory. Lead acetate trihydrate (Pb(CH3COOH)3·3H2O, 99.5%), Sodium sulfide (Na2S, 99%), L-ascorbic acid (AA) and sodium nitrite (NaNO2) were provided from Tianjin Tianli Chemical Ltd. Potassium chloride (KCl) was purchased from Tianjin chemical reagent NO 3 plant. 3-mercaptopropionic acid (MPA, 98%) was bought from Aladdin Reagent company. Cadmium chloride dihydrate (CdCl2·2H2O, 99%) was provided from China Pine Chemical Reagent Factory (Zhengzhou). Sodium hydroxide (NaOH, 98%) was purchased from Tianjin Ruijin special Chemical Co., Ltd. Ethanol absolute (analytical regent) was provided from Tianjin Fuyu Fine Chemical Co., Ltd. Zinc sulfate (ZnSO4·7H2O) was purchased from Local state-owned Xi'an paint factory. Glycine (Gly), Folic acid (FA), Glucose, and L-cysteine (Lys) were purchased from Xiya Reagents. Serine (Seri) was obtained from Shanghai blue season Technology Development Co., Ltd. Bovine Serum Albumin (BSA), L-Asparic acid (APs), L-Phenylalanine (Phe) and L-Alanine (Ala) were purchased from Xi'an Ke Hao Biological Engineering Co., Ltd. Citric acid (CA) was purchase from Tianjin KGM Chemical Reagent Co., Ltd. Cobalt nitrate hexahydrate (Co(NO3)2·6H2O, AR), (Ni(NO3)2·6H2O, 98%) and Sodium sulfite (Na2SO3) were obtained form shanghai Aladdin biochemical technology co., Ltd. The ham sausage was obtained from the supermarket. 2.2. Synthesis of ZnCdS QDs
Journal Pre-proof The water-soluble ternary alloyed ZnCdS QDs were synthesized in aqueous solution using MPA as stabilizer according to our previous study [37]. Typically, the cationic precursor solution was obtained by mixing CdCl2, Zn(CH3COO)2, and MPA in 100 mL of ultrapure water and the molar ratio of (Zn2++Cd2+)/MPA was 1:2. Then, NaOH solution (1 mol/L) was added dropwise to adjust the pH of the system to alkalinity before the mixture was stirred at room temperature for 20 min. Continue stirring for 30 min, the Na2S solution (0.1 mol/L) was added drop by drop to the mixture under adequately mixing. The ternary alloyed ZnCdS QDs was obtained by reacting at 100 ℃ for 90 min. Furthermore, the ZnCdS precipitates could be obtained by adding ethanol to the mixture and centrifugation, then washing repeatedly with water to remove unreacted materials (including the excess capping molecules and ions). Finally, the ZnCdS QDs were dissolved in the ultrapure water for further use. 2.3. Detection of nitrite using ZnCdS QDs NaNO2 aqueous solution with various concentrations was freshly prepared before detect. 0.4 mL ZnCdS QDs (Zn/Cd molar ratio was 0.8:0.2 and the concentration of QDs was 0.017 mol/L calculated using the total amount of cations) and NaNO2 solution with different concentrations were transferred into 10 mL colorimetric tube, then diluted with deionized water to graduation lines and the mixture was thoroughly mixed. After 5 min reaction at room temperature, the PL spectra were recorded at excitation wavelength of 365 nm. Moreover, the following interfering substances were used to research the selectivity of the ZnCdS QDs to NO2- detection: Lys, AA, CA, Pb2+, Ni2+, S2-,SO32-, Na+, K+, Ca2+, Mg2+, NO3-, SO42- and so on. The solution was mixed with 0.4 mL 0.017 mol/L ZnCdS QDs in a 10-fold concentration. All mixture was reacted 5 min at room temperature, and the PL spectra were recorded at excitation wavelength of 365 nm. 2.4. Preparation of real samples Firstly, 5 g of shredded ham sausages was put into a 50 mL beaker, and 12.5 mL of saturated borax solution was added and stirred vigorously. Then the mixture was washed into a 250 mL volumetric flask with 150 mL of deionized water above 70 ℃ and heated it out in a boiling water bath for 15 min. Finally, 2.5 mL of ZnSO4 aqueous solution (1 mol/L) was dripped to precipitate the protein in the ham sausage. After cooling to room temperature, the distilled water was added to the mark line and placed 10 min, and the solution to be tested will be obtained after filtration.
Journal Pre-proof 2.5. Characterization The UV-vis absorption spectra were acquired with an ultraviolet and visible (UV-vis) ratio recording spectrophotometer (Shimadzu UV-3600) in the wavelength range of 200-900 nm. The energy dispersive X-ray spectroscopy (EDS) was taken with the JEOL JSM-6390A system. The high resolution transmission electron microscopy (HRTEM) of the ternary ZnCdS QDs were conducted using a Hitachi H-800 electron microscope at an acceleration voltage of 200 kV with a CCD camera. Fourier transform Infrared (FTIR) absorption spectra were performed using Perkin Elmer Frontier FTIR in the mid IR range with ATR accessory. X-ray diffraction (XRD) were investigated using a Rigaku D/MAX 3C powder diffract meter (Cu K as radiation source, λ=0.15406 nm). PL spectra of the QDs were performed from 200 to 900 nm using an F-7000 fluorescence spectrometer Record (Hitachi, Japan) equipped with a 150 W Xenon light source, a 1.0 cm quartz battery, a thermostat bath and the slit width was 5 nm.
3. Results and discussions 3.1. Characterization of the ternary ZnCdS QDs The size and morphology of ternary ZnCdS QDs were characterized by the high resolution transmission electron microscopy. The HRTEM images of ZnCdS QDs showed that the as-prepared ZnCdS QDs were monodisperse, nearly spherical nanoparticles and the average particle size of ZnCdS QDs have been concluded as 4.45 nm from particle size distribution plot based on the TEM images. In the inset of Fig. 1 (a), it could be found that the obvious lattice fringes with the interplanar spacing of 0.32 nm, which corresponded to the (1 1 1) lattice planes of the cubic zinc blende. In addition, the XRD patterns (Fig. 1 (b)) of ternary ZnCdS QDs displayed three obvious diffraction peaks, which could be indexed to the (1 1 1), (2 2 0), and (3 1 1) planes of cubic blend structure and matched with the standard JCPDS data (JCPDS No. 05-0566 and No. 10-0454 for ZnS and CdS, respectively) and the result was consistent with other study [38]. What's more, it was
clearly that
the XRD diffraction peaks of ZnCdS QDs were between pure CdS and ZnS. Additionally, no CdS/ZnS nanomaterials was formed due to the fluorescence spectrum were not red-shift which usually occurred accompany with the formation of core-shell materials Infrared absorption spectrum was also used for analyzing surface functional groups of the ternary ZnCdS QDs. According to the reports in the literature [39], there are three distinct characteristic absorption peaks for pure MPA at 3304 cm-1, 1580 cm-1 and 1407 cm-1, which assigned to the
Journal Pre-proof stretching vibrations of -OH and the asymmetrical and symmetrical stretching vibration of -COOH, respectively. In the FTIR spectra (Fig. 2(a)) of the ternary ZnCdS QDs, the absorption band at about 3425 cm-1 represented the -OH stretching mode, the peak at 1562 cm-1 and 1400 cm-1 were attributed to the anti-symmetry and symmetric stretching of the -COOH, respectively. Furthermore, the disappearance of the obvious absorption band of -SH group at about 2662 cm-1, which suggested that a strong covalent bond could be formed between the -SH and metal ions and the successful modification of MPA on the surface of the QDs. Elemental analysis by EDS demonstrated that Cd, Zn, S, O and C were present in the ternary ZnCdS QDs sample (Fig. 2(b)), which also verified that MPA was capped on the surface of the ZnCdS QDs. 3.2. Optical properties of the ternary ZnCdS QDs The UV-vis absorption and PL spectra for ternary ZnCdS QDs samples with different Zn/Cd molar ratios has been studied in our previous report [38]. As shown in Fig. 3, the absorption edges of ZnCdS QDs had obvious blue-shift compared with pure CdS which owing to the increase of the QDs band gap energy when the Zn2+ ions were incorporated in CdS crystals, and between the binary CdS and ZnS QDs, which agreed with other studies [40-42]. In addition, the emission peak of the as-prepared ZnCdS QDs (Zn/Cd molar ratio was 0.8:0.2) was 505 nm, and no emission peaks of CdS or ZnS were found in the ZnCdS QDs fluorescence emission spectra, which indicated that there was no CdS or ZnS nanomaterials were formed in our synthesis system, and also no CdS/ZnS nanomaterials because the formation of core-shell materials tend to be accompanied by the red shift of the fluorescence spectrum. 3.3. Nitrite detection using ZnCdS QDs as fluorescent sensors We selected the ZnCdS QDs of the Zn/Cd molar ratio was 0.8:0.2 as fluorescent sensors for the detection of NO2-, on account of the response of ZnCdS QDs with other Zn/Cd molar ratio to NO2were not ideal, which could be due to the fact that the optical properties of the ternary alloyed QDs was affected by its elemental composition [38]. As shown in Fig. 4, the PL intensity of the ZnCdS QDs was decreased gradually with the increase of NO2- concentrations from 180 µM to 700 µM. We speculated that the probable reason was that the nonradiative energy or electron transfer may be strengthened when the NO2- were introduced into the ZnCdS QDs solution. On the other hand, some anions and metal ions can easily lead to the aggregation of nanoparticles, which also weaken the PL intensity [2]. In order to certify this detection mechanism, the UV-vis absorption spectra of ZnCdS
Journal Pre-proof QDs in the absence and presence of NO2- and TEM image of ZnCd0S QDs after addition of NO2have been characterized. As shown in Fig. 5, the absorption intensity of ZnCdS QDs decreased significantly after NO2- was added to ZnCdS QDs solution, indicating the interaction between NO2and ZnCdS QDs. In addition, the morphology of the ZnCdS QDs after the addition of NO2- was characterized. As shown in Fig. 4(b), the agglomeration of ZnCdS QDs was obtained. Therefore, this may be due to the fact that NO2- causes ZnCdS QDs agglomeration when NO2- interacts with ZnCdS QDs. Additionally, the equation of linear regression was y=-0.123x+1.1525, where y was the F/F0 and x was the NO2- concentrations, with a correlation coefficient of 0.9903. The detection limit was calculated as 219.3 μM (S/N=3). To further investigate the specificity of the ZnCdS QDs fluorescent sensors for the detection of NO2-, the effect of potential interfering substances on the system were investigated. It could be clearly seen from Fig. 6 that only the NO2- could lead to the PL intensity quench. Most importantly, fluorescence signal of the system was not obvious when adding representative metal ions and anion ions including Ca2+, Na+, Zn2+, NO3-, SO42-, Cl- and so on. The results illustrated that the ternary ZnCdS QDs were highly selective sensors for NO2-. Furthermore, the response time of ternary ZnCdS QDs to NO2- was studied and the results showed that the required response effect of the experiment can be achieved when the response time was 5 minutes (Fig. 7), which demonstrated the rapid detection of NO2- could be completed by ternary ZnCdS QDs. Therefore, compared with other sensors for NO2- [43-44], the ZnCdS QD-based sensor has demonstrated obvious advantages because it can be easily synthesized and also showed good performance for sensing NO2- (Table 1). 3.4. Detection of NO2- in real samples To determine the feasibility of analysis in actual samples, the prepared ZnCdS QDs was used to detect the concentration of NO2- in ham sausage with the standard addition methods. The results were listed in Table 2, which showed the recovery was larger than 100% for ham sausage and all relative standard deviations (RSD) for four repeated detection were less than 5%, and the results showed that the content of NO2- in sausage was 18.45 mg/kg, which indicated that the detection of the content of NO2- in sausage could be achieved by the ZnCdS QDs fluorescent sensors. In addition, in order to study the stability of ZnCdS QDs sensor for detection of NO2- in real samples, the F/F0 value of the ZnCdS QDs sensor solution containing a certain amount of the analyte and standard
Journal Pre-proof solution after being placed for a certain period of time is characterized (Fig. 8), the results showed that it has good stability.
4. Conclusions In a word, we reported water-solution ternary alloyed ZnCdS QDs were synthesized by an aqueous method and the fluorescence sensors were constructed for the detection of NO2-. The PL intensity of ZnCdS QDs in response to NO2- was investigated, the results illustrated that the response PL intensity of ZnCdS QDs was gradually decreased with the increase of NO2- concentrations from 180 µM to 700 µM. The regression equation was y=-0.123x+1.1525 with a linear coefficient R2 of 0.9903 and the detection limit (S/N=3) was estimated to be 219.3 μM. Furthermore, the investigation of interference showed that the fluorescent sensors of ternary alloyed ZnCdS QDs has highly selective for NO2-, and the prepared ZnCdS QDs was used to detect the NO2- in ham sausage to determine the feasibility of analysis in actual samples, the results indicated the high accuracy of the ZnCdS QDs fluorescent sensors for NO2- detection in actual samples. Acknowledgements We are very grateful for the support of the National Natural Science Foundation of China (21476183 and 21676213), the Project funded by China Postdoctoral Science Foundation (2016M600809), and the Open Research Fund of State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, Chinese Academy of Sciences (T151702).
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Journal Pre-proof Captions: Sch. 1. The schematic diagram of the preparation process of ZnCdS QDs and its determination of nitrite. Fig. 1. The HRTEM images (a) and the XRD patterns (b) of ZnCdS QDs. Inset (a): the lattice fringes with the interplanar spacing of 0.32 nm and the particle size distribution plot. Fig. 2. The FT-IR spectra (a) and EDS analysis (b) of ZnCdS QDs. Fig. 3. The UV-vis absorption (solid) and PL spectra spectra (dash) of ZnCdS QDs. Fig. 4. The PL spectra of ZnCdS (Zn/Cd molar ratio was 0.8:0.2) with increasing concentrations of NO2- (a). Plots of the relationship between fluorescence intensity F/F0 and NO2- concentration (b). Inset (b): expanded linear region (180 µM-700 µM) of the calibration curve. Plots of the relationship between fluorescence intensity F/F0 of ZnCdS QDs (Zn/Cd molar ratio was 0.6:0.4) (c) and ZnCdS QDs (Zn/Cd molar ratio was 0.5:0.5) (d) and NO2- concentration (180 µM-700 µM). Fig. 5. UV-vis absorption spectra of Zn0.8Cd0.2S QDs in the absence and presence of NO2- (a), TEM image of Zn0.8Cd0.2S QDs after addition of NO2- (b) Fig. 6. The fluorescence responses of ZnCdS QDs to NO2- and other interfering substances. Fig. 7. The effect of response time of ternary ZnCdS QDs to NO2- on detection results. Fig. 8. The stability of ZnCdS QDs sensor for detection of NO2- in real samples. Table 1. Comparison of the performance of different methods for the detection of NO2Table 2. Determination results of NO2- in actual samples.
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Sch. 1
Fig. 1.
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Fig. 2.
Fig. 3.
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Fig. 4.
Fig. 5.
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Fig. 6.
Fig. 7.
Journal Pre-proof Table 1 Linear range
Detection limit
(μmol/L)
(μmol/L)
Cu nanoclusters
0.125~125
0.0036
[38]
Carbon dots
0.1~10
0.05
[39]
Carbon nanofibers
5~300
3
[40]
This work
180~700
0.780
Probes
Reference
0.7
F/F0
0.6 0.5 0.4 0.3
0
15
30
45
60
75
90
Time (min) Fig. 8.
Table 2 Sample
Ham sausage
Added
Total after addition
Recovery
RSD
(mg/kg)
(mg/kg)
(%)
(%)
0
17.871
-
4.09
3.45
21.581
107.54
3.37
6.90
25.637
112.55
2.81
13.80
32.846
108.5
4.45
Journal Pre-proof Highlights
The water-soluble ZnCdS QDs were synthesized by aqueous synthesis method.
The fluorescent sensors based ZnCdS QDs were constructed for NO2- detection.
The content of NO2- in ham sausage was detected using ZnCdS QDs.