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Natural Quercetin AIEgen Composite Film with Antibacterial and Antioxidant Properties for in situ Sensing of Al3+ Residues in Food, Detecting Food Spoilage and Extending Food Storage Times Ting He, Hui Wang, Zhijun Chen, Shou-Xin Liu, Jian Li, and Shujun Li ACS Appl. Bio Mater., Just Accepted Manuscript • DOI: 10.1021/acsabm.8b00128 • Publication Date (Web): 03 Aug 2018 Downloaded from http://pubs.acs.org on August 13, 2018

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ACS Applied Bio Materials

Natural Quercetin AIEgen Composite Film with Antibacterial and Antioxidant Properties for in situ Sensing of Al3+ Residues in Food, Detecting Food Spoilage and Extending Food Storage Times Ting He,a, ‡ Hui Wang, a,b,‡ Zhijun Chen a,*, Shouxin Liu a, Jian Li a and Shujun Li a,* a. Key Laboratory of Bio-based Material Science and Technology of Ministry of Education., Northeast Forestry University, Hexing Road 26, Harbin 150040, P.R. China. b. Key Laboratory of Wood Science and Technology, Zhejiang Agriculture and Forestry University, Wusu Road 666, Hang zhou 311300, P.R. China. ‡

T. He and H. Wang contributed equally to this work.

KEYWORDS Quercetin, AIEgen composite film, food storage, Al3+ residues in food, food spoilage ABSTRACT

ACS Paragon Plus Environment

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Functional materials that can both accurately and conveniently sense food quality and improve food storage times would provide important benefits. To achieve a functional material which can both sense food quality fluorescently and extend food storage times, a multifunctional film incorporating the natural AIEgen, quercetin has prepared via physically mixing quercetin and polyvinyl alcohol in this paper. As-prepared film was firstly investigated on the physical properties. The film had nice mechanical performance and water/CO2 permeability. Optical properties of the film were subsequently determined by UV-Vis and fluorescent spectra. Asprepared film was transparent and showed good AIE enhancement when exposed to foods containing Al3+ residues and to seafood containing biogenic amines, produced during spoilage. The film also showed good antibacterial and antioxidant activity, which enabled it to be successfully used as a coating to extend food storage times. The film has the potential to be used as a new smart food packaging material, which could both indicate food quality and extend shelf life. 1. INTRODUCTION Issues surrounding food safety and food storage have become major global concerns, especially in developing countries. 1 As well as the possibility of toxic food additives, many foods have an inconveniently short shelf life. 2 For example, seafood is very easily spoiled on storage. 3 Many traditional Chinese snacks, including deep-fried dough sticks and steamed buns, are prepared using aluminum potassium sulfate dodecahydrate, which means that these foods always contain residual Al3+. 4 Many different materials for improving food safety, including food preservatives, 5-6

antimicrobial packaging 7-8 and stimuli-responsive materials,

9-13

have been developed, either

to extend food storage times or for in situ detection of food quality. Some materials can be used both for sensing food quality via a color change and for extending shelf life.

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Although color


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changes allow easy visualization of food spoilage, a system that showed luminescence changes upon exposure to food spoilage would represent a significant improvement since luminescent materials can sensitively differentiate guest species.

15-18

Recently, Alam et al. reported that

aggregation-induced emission luminogens (AIEgens) could be used as fluorescence sensors for detecting biogenic amines formed during the spoilage of seafood.

19

AIEgens have weak

fluorescence when well-dispersed but emit strong fluorescence when aggregated. 20-23 Compared with traditional luminogens, which suffer from aggregation-induced quenching problems, AIEgens show high sensitivity to guest species in the solid state.

19, 24-26

Although the use of

AIE-based fluorescence technology for sensing food quality represents important progress, some challenges remain. Crucially, AIEgens for sensing food quality should be green and easily prepared. Synthetic AIEgens typically required toxic starting materials and complicated syntheses, giving rise to the threat of toxic residues in the food. The complicated syntheses of synthetic AIEgens will also hinder their future large scale application. Another shortcoming of existing AIEgen-based materials is that they are unable to extend food storage times, although this is equally as important as sensing food quality. Our group recently reported that the natural product quercetin shows AIE fluorescence.

27

Quercetin is a phenolic compound, which has

antibacterial and antioxidant properties, 28-29 indicating potential application in food preservation. Since quercetin has good biocompatibility and is easily prepared from biomass materials, quercetin AIEgens should be ideal materials for improving food storage and sensing food quality. Despite this, no attention has been paid to this area. Prompted by the attractive properties of this luminogen, we have now prepared a quercetin AIEgen composite film (QACF) and aim to provide a new multifunctional material for both efficiently sensing food quality and extending food storage times (Scheme 1).


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Scheme 1. Preparation of QACF and schematic illustration of applications in food storage, in situ sensing of Al3+ in food and detection of biogenic amines. 2. EXPERIMENTAL PROCEDURES 2.1. Materials. Quercetin was extracted from Sophora japonica as described previously. 27 PVA (average degree of polymerization = 1750 ± 50) was purchased from Sigma-Aldrich, Shanghai, China. All solvents were purchased from Sigma-Aldrich, Shanghai, China or from Tokyo Chemical Industry, Shanghai, China. Escherichia coli and bacillus subtilis were obtained from the Chinese Center of Industrial Culture Collection (CICC), China. 2.2. Characterization. UV-Vis absorption spectra were recorded over the range 200-800 nm using a TU-1901 ultraviolet-visible double-beam spectrophotometer (Persee General Instrument Co., Ltd., Beijing, China). Photoluminescence (PL) measurements were performed using a Fluomax 4 spectrofluorometer (Horiba, New Jersey, USA). Tensile strength (TS) and elongation at break (EB%) were measured using a LDX-200 (Meike, Beijing, China). CO2 permeability was recorded using a VAC-VBS (Labthink, Jinan, China). 2.3. Preparation of QACF. Quercetin (7 mg，w = 0.23%) was dissolved in THF (2 mL), and distilled water (8 mL) was added to the solution. PVA (3 g) was dissolved in distilled water (80


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mL) under magnetic stirring for 2 h at 90 oC and the solution was then cooled to room temperature. The quercetin solution was mixed with the PVA solution and the mixture was stirred for 30 min, poured onto a glass plate and allowed to dry naturally. The dried film was placed in a drier at 20 oC and 36% humidity for 24 h before testing. 2.4. Migration test. Migration of QACF in ethanol: QACF was placed in a 95% ethanol solution.

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The released quercetin from QACF was determined through monitoring absorbance

coefficiency at 374 nm at 2 h, 4 h, 6 h, 12 h, 24 h, 48 h and 72 h (25 oC, pH = 6.6). Migration of QACF on the food: the tested food was covered by QACF. Then, the food was washed by ethanol and the concentration of quercetin in ethanol was determined by UV-Vis spectra (absorbance coefficiency at 374 nm). 2.5. Transmission of QACF. The light barrier properties value of QACF (1 cm × 4 cm) was measured over the range 800-200 nm, with three parallel determinations for each sample. 2.6. Mechanical Testing. The films were cut into 1 cm × 10 cm pieces and tested at an average speed of 50 mm/min, with five parallel measurements for each sample. 2.7. CO2 permeability. The CO2 permeability was determined using a CO2 permeability tester (Model: VAC-VBS, Labthink, Jinan, China). The samples to be tested were cut into round pieces by a metal sampler. Before on-machine testing, it was necessary to put the samples in the saturated magnesium chloride solution for 24 h. 2.8. Measurement of water vapor permeability. The water vapor permeability of the films were determined according to the standard method of ASTM with slight modification. flask was used in the test.

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Glass

Permeability area of the glass flask was 28.26 cm2. For the

experiment, 30 g CaCl2 was placed in the glass flask and dried at 200 oC. After that, the flask was


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covered with test film with the assistance of 85% paraffin and 15% beewax. Then the flask was put in incubator with the condition of RH 65%, 26 oC. The water vapor permeability was calculated according to following equation (Eq.1):

WVP =

WVTR × D WVTR × D = ∆P S × (RH1 − RH2)

Eq. 1

Where the WVTR is the water vapor transmission rate (g/m2·s), D is the thickness (mm), ∆P is the water vapor pressure difference on both sides of the test film, S is the saturated vapor pressure of the water at tested condition (Pa), RH1 is the relative humidity outside the test film, RH2 is the relative humidity inside the test film. 2.9. Sensing of QACF to Al3+ and putrescine. 0.025 mL Al3+ solution (0.6 mg/mL, 1.2 mg/mL, 2.4 mg/mL, 3.6 mg/mL, 4.8 mg/mL) was dropped on the QACF (1 cm×1 cm) for sensing. The putrescine solution (5 ppm，100 ppm，700 ppm) was dropped on the QACF (2 cm×2 cm) for sensing. 2.10. Evaluation of Antioxidant Activity. Quercetin (80 mg) was dissolved in methanol (100 mL) and the solution was then further diluted with methanol to provide solutions with concentrations of 0.6 mg/mL, 0.4 mg/mL, 0.2 mg/mL, 0.1 mg/mL, 0.05 mg/mL and 0.025 mg/mL. The standard solution of quercetin (50 µL) was added to DPPH (4 mL, 0.04 mg/mL) at room temperature and the mixture was stirred for 30 min. The absorbance of the mixture was then measured at 517 nm. Pieces of QACF (1 cm × 8 cm) were placed in a DPPH (4 mL, 0.04 mg/mL). The absorbance of the solution was monitored at 517 nm every 30 min. Radical scavenging activity (RSA) was calculated using the equation (Eq. 2) : Inhibition (%) = (A0 - Ac)/A0 × 100%

Eq. 2


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Where A0 and Ac refer to the absorbance of DPPH at 517 nm in the absence and presence of quercetin solution or QACF.

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2.11. Determination of Antibacterial Activity. The antibacterial activity of QACF was determined using growth inhibition curves for E. coli and B. subtilis. The bacteria were cultivated in Luria-Bertani broth (LB), containing NaCl (5 g/L), tryptone (10 g/L) and beef extract (3 g/L) at 37 oC with shaking for 12 h. The cutures were diluted with fresh LB at an initial OD600 of 0.18. QACF (18 cm × 4 cm) and PVA films (18 cm × 4 cm) were placed in the bacterial solutions and OD600 values were measured every 2 h. 2.12. Salmon experiments. Fresh salmon was purchased from a local supermarket. Fresh salmon was put in the Petri dishes covered by a cling film with QACF (2 cm×2 cm) at room temperature and 5 oC, respectively. Photos of the sample under UV irradiation (365 nm) were taken at 0 h, 4 h and 8 h, respectively. 2.13. Fruit experiments. Banana and apples were treated by brushing pure PVA solution or QACF solution (0.23% quercetin according to the total mass of PVA), respectively. All the treated banana and apple groups were stored at 25 oC. 3. RESULTS AND DISCUSSION QACF was prepared by physically mixing quercetin and polyvinyl alcohol (PVA), and its basic physical performance as a film was investigated. Migration experiments showed that only 1.15% of the quercetin had been released after 48 h in 95% EtOH and that no more quercetin was released when the test time was extended to 72 h. The migration experiment was also conducted in the food. For the deep-fried dough sticks, steamed buns and fruit, ~ 0.14 %, 0.12 % and 0.16% quercetin were released during 2 h. For the salmon, ~ 0.34% quercetin was released during 8 h.


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Quercetin was thus stably fixed in the PVA matrix, likely because of hydrogen bonds between the hydroxyl groups of PVA and the phenolic groups of quercetin. FT-IR spectra also showed OH telescopic vibration peak at ~3400 cm-1 increased after doping of quercetin (Figure S1), which might be caused by strengthened hydrogen bond formed between quercetin and PVA molecular chains. The water vapor permeability and gas permeability of QACF were increased compared with pure PVA film (Table 1). Quercetin forms nanostructures in aqueous solution, which might disturb the integrity of the PVA film, leading to increases in water vapor permeability and gas permeability. The mechanical performance of QACF was also investigated. The tensile strength (TS) and elongation at break (EB%) of the QACF film were 35.28 MPa and 207.8%, respectively. Generally, QACF was stable, transparent and permeable, and had good mechanical performance. Table 1. Physical properties of QACF and PVA films Film

Tensile

Elongati

CO2 permeability

water vapor

strength

on at

coefficient

permeability

(MPa)

break

(cm3/m2·24 h·0.1MPa)

(g·mm/s·m2·Pa)

(%) PVA

39.31

154.38

1.223×10-6

3.1359×10-11

QACF

35.28

207.8

6.166×10-6

4.7305×10-11

3.1. Optical properties. As-prepared QACF was transparent (Figure 1a, inset). Compared with pure PVA film, transmission of QACF was weak in the UV region (200–400 nm) because of the intrinsic absorbance of quercetin (Figure 1a). The fluorescence properties of QACF were subsequently investigated (Figure 1b). As a starting point, the AIE of quercetin was studied in


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aqueous PVA solution. Quercetin was dissolved in THF, which, unlike water, is a good solvent for quercetin. On addition of aqueous PVA solution to the quercetin solution, fluorescence enhancement was observed (Figure 1b). This showed that quercetin produced AIE in a PVAcontaining system and established that QACF would also produce AIE. The fluorescence spectrum of QACF was then measured. QACF showed fluorescence emission at ~ 530 nm, which was similar to the AIE of quercetin in PVA solution (Figure 1c). This similarity demonstrated that the fluorescence of QACF originated from the AIE of quercetin. QACF showed a greatly intensified fluorescence, compared to its aggregates in liquid solution (Figure S2), which might due to its stronger aggregation in the solid film. Another point also confirmed fluorescence of QACF came from AIE. Quercetin in a well dissolved condition has two emission peaks including enol (at 450 nm) and keto emission (at 530 nm) (Figure 1b).27 The intensity of its enol and keto was almost 1:3 (Figure 1b). While its enol emission decreased and keto emission increased in the aggregation condition and the ratio of enol and keto was ~ 1:100 (Figure 1b). Fluorescence of QACF showed a very obvious stronger keto emission (Figure 1c). All these results demonstrated QACF had AIE active fluorescence.


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Figure 1. a) Transmission of QACF from 200 nm to 800 nm. Inset: image of QACF on top of a flower, scale bar = 1 cm. b) Fluorescence emission of quercetin (20 µg/mL) in mixtures of THF and PVA solution (excitation wavelength = 360 nm). c) Fluorescence emission of QACF (excitation wavelength = 360 nm). 3.2. Fluorescence sensing of Al3+ and putrescine. Having demonstrated that QACF produces AIE, preliminary experiments were carried out to investigate the sensitivity of the emission to Al3+ and putrescine. Quercetin showed marked AIE enhancement upon addition of Al3+ (Figure 2a), which was similar to its behavior under molecular dispersion conditions in solvent.

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Quercetin also showed a nice selectivity to Al3+ and it did not show an obvious fluorescent enhancement to other cations. (Figure S3). The relationship fitted the linear equation: y = 182 x − 79 (R = 0.99), where y is the fluorescence at 506 nm measured at a given Al3+ concentration (0.8-5.6 ppm) and x represents the concentration of Al3+ added (Figure S4). The detection limit (3 s/K, s = standard deviation of the blank signal, K = 182) was ~ 0.2 ppm. Quercetin showed an AIE enhancement upon addition of putrescine, a model compound for biogenetic amine (Figure 2b).19 The relationship fitted the linear equation: y = 104 x + 143, where y is the fluorescence at 540 nm measured at a given putrescine concentration (0.6 - 3.0 ppm) and x represents the concentration of putrescine added (Figure S5). The detection limit (3 s/K, s = standard deviation of the blank signal, K = 104) was ~ 0.35 ppm. The AIE enhancement may be attributable to coordination-induced restriction of rotation of aromatic rings. The whole picture was proposed like this. AIE of quercetin was due to demonstrated crystallization-promoted keto emission.

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Addition of Al3+ or putrescine formed coordination bond or hydrogen bond with phenolic hydroxyl moeities, which might “glue” adjacent aromatic rings and further locked the motions of aromatic rings in the aggregated state and induced an AIE enhancement. Having demonstrated AIE enhancement for quercetin upon addition of Al3+/putrescine, the fluorescence behavior of QACF was investigated in the presence of Al3+ and putrescine. QACF showed AIE enhancement upon addition of Al3+ (Figure S6 and S7), with as little as 0.6 mg/mL of Al3+ triggering AIE enhancement visualized by naked eye (Figure 2c). The detection limit (3s/k, k = 23565) of QACF to Al3+ was 0.002 mg/mL (2 ppm). QACF also showed AIE enhancement to putrescine (Figure S8 and S9). The detection limit (3s/k, k = 629) of QACF to putrescine was 0.06 ppm. These results demonstrate that QACF could be used for potentially detecting Al3+ and biogenic amines in food.


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Figure 2. a) Fluorescence spectra of quercetin (10 µg/mL) upon addition of different concentrations of Al3+ in THF/H2O (20/80) (excitation wavelength = 360 nm). b) Fluorescence spectra of quercetin (10 µg/mL) upon addition of different concentrations of putrescine in THF/H2O (20/80) (excitation wavelength = 360 nm). c) Images of QACF upon addition of different concentrations of Al3+ (0.6 - 4.8 mg/mL) and different concentrations of putrescine (5 ppm - 700 ppm).


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3.3. Antioxidant and antibacterial properties. Having demonstrated the sensing applications of QACF, we next investigated its antibacterial activity. Gram-negative Escherichia coli and Gram-positive Bacillus subtilis were chosen as model microorganisms for the experiment. QACF showed a nice antibacterial capacity to E. coli and B. subtilis (Figure 3a) and inhibition zone diameter was 17.5 mm and 22.5 mm, respectively. As a control experiment, pure PVA film was also tested for the antibacterial property and no obvious inhabitation zone was observed for this control group (Figure 3a). To further study the antibacterial capacity of QACF, QACF with different doping amount of quercetin was incubated with bacterial cultures and optical density at 600 nm (OD600) was measured to determine inhibition of bacterial growth (Figure 3b and 3c). The antibacterial activity of QACF against E. coli and B. subtilis is shown in Figures 3a and 3b. In the absence of QACF, both E. coli and B. subtilis grew rapidly and OD600 values reached ~ 2 after incubation for 10 h (Figure 3b and 3c). The growth curves of E. coli and B. subtilis were slowed in the presence of QACF (Figure 3b and 3c), demonstrating that QACF had antibacterial activity. High doping amount of quercetin facilitated the performance of antibacterial of QACF (Figure 3a and 3b). 0.23% quercetin composite QACF showed a most obvious antibacterial effect to E. coli and B. subtilis, compared to the ones with 0.17% and 0.10% quercetin. This antibacterial mechanism of QACF might be due to the reduction of quercetin. Bacterial cell biofilm was reduced by quercetin and , then, their structures altered, which finally caused the inhibition to E. coli and B. subtilis .35 The antioxidant capacity of QACF was determined by measuring its ability to scavenge 2,2-diphenyl-1-picrylhydrazyl (DPPH) radicals. The radicalscavenging activity of quercetin was > 90% at a concentration of 0.8 mg/mL (Figure S10), indicating effective antioxidant ability. Although QACF was able to efficiently scavenge DPPH radicals, the speed of scavenging was slow (Figure S11), with only 33.71% of the radicals


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inhibited after 8 h (Figure S11). These results confirm that QACF would provide prolonged inhibition of radicals, which is necessary for food coating materials.

Figure 3. a) Image of E. coli in the presence of PVA film and 0.23% quercetin composite QACF (upper), inhibition zone diameter = 17.5 mm, scale bar = 20 mm, image of B. subtilis in the presence of PVA film and 0.23% quercetin composite QACF (down), inhibition zone diameter = 22.5 mm, scale bar = 20 mm. b) Optical density at 600 nm (OD600) of E. coli in the presence of PVA film and QACF as a function of time. c) OD600 of B. subtilis in the presence of PVA film and QACF as a function of time.


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3.4. Applications. Encouraged by its Al3+/putrescine-triggered AIE enhancement, and its antibacterial and antioxidant properties, we next tested whether QACF could be used for fluorescence sensing of Al3+ in food products, for detecting biogenic amines formed during seafood spoilage, and for extending food storage times. Its ability to sense Al3+ in deep-fried dough sticks and steamed buns was investigated first. When freshly purchased deep-fried dough sticks and steamed buns were covered with QACF, AIE enhancement was observed immediately (Figure 4). The AIE enhancement following contact with deep-fried dough sticks was especially intense (Figure 4, top panel), indicating that QACF provides a rapid method for in situ sensing of Al3+ in food. QACF was also able to detect biogenic amines. The AIE of QACF was noticeably enhanced when the film was placed inside a tightly closed package of salmon at room temperature (25 oC ) for up to 8 h (Figure 4, center panel), with a smaller effect observed at 5 oC (Figure 4, bottom panel). QACF can thus serve as a sensitive sensor for detecting spoilage of seafood.

Figure 4. Images of QACF (left), QACF exposed to steamed buns (center) and QACF exposed to deep-fried dough sticks (right). Images of QACF incubated with salmon for 0 h, 4 h and 8 h at 25 oC (top) and 5 oC (bottom).


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Bananas and apples were chosen as model fruits to investigate whether QACF can extend food storage times. After five days, bananas coated with QACF retained their original yellow color and did not develop black spots (Figure 5a), whereas untreated bananas, or bananas coated with only PVA, became black and decayed. Similarly, apple slices coated with QACF did not noticeably deteriorate after 2 h, whereas untreated apples, or apples treated with only PVA, turned brown (Figure 5b). These experiments with bananas and apples clearly demonstrate that coating with QACF has the potential to improve the shelf life of fruit.

Figure 5. a) Images of bananas over 5 days without coating (top), coated with PVA (center) and coated with QACF (bottom). b) Images of apples at 0 h and 2 h without coating (left), coated with PVA (center) and coated with QACF (right). 4. CONCLUSION We have prepared a composite film (QACF), based on the natural AIEgen quercetin, which has both antibacterial and antioxidant properties. QACF showed a sensitive AIE enhancement that could be used in situ to detect both Al3+ residues in food and the presence of biogenic amines formed during the spoilage of seafood. Because of its antibacterial and antioxidant properties, QACF coating was also able to extend the shelf life of bananas and apples. Since QACF can be


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easily and cheaply prepared, it could be produced on a large scale and used to extend food storage times and sense food quality. QACF thus has huge potential to be developed as a smart and green packaging material, which could both indicate food quality and extend shelf life by preventing spoilage. This work was expected to provide a new, smart, convenient and green material that could be used for both improving the shelf life of fruit and for sensing food quality. ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: . Details of the figures related to: FT-IR of QACF and blank PVA; Fluorescence emission of QACF and quercetin (20 µg/mL) in mixtures of THF and PVA solution; Fluorescence intensity of quercetin (10 µg/mL) with the addition of various metal ions (100 µmol/L) at 491 nm in THF/H2O (20/80); Linear fitting of fluorescence intensity of quercetin at 540 nm and concentration of putrescine in THF/H2O; Fluorescence spectra of QACF upon addition of different concentrations of Al3+; Linear fitting of fluorescence intensity of QACF at 542 nm upon addition of different concentrations of Al3+; Fluorescence spectra of QACF upon addition of different concentrations of putrescine; Linear fitting of fluorescence intensity of QACF at 544 nm upon addition of different concentrations putrescine; Scavenging of DPPH radicals as a function of quercetin concentration in methanol; Scavenging of DPPH radicals as a function of time in the presence of QACF in methanol.

AUTHOR INFORMATION


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Corresponding Author *E-mail address: [email protected] (Z.C.) *E-mail address: [email protected] (S.L.) Author Contributions Z. Chen and S. Li conceived the idea and designed the experiments. T. He. prepared and characterized the materials and took the photos in TOC and scheme. H. Wang performed the bacterial experiments and took the photos in the Figures in the manuscript. S. Liu supported the fluorescence measurement. J. Li supported anti-oxidation experiments. T. He and H. Wang contributed equally to this work. Funding Sources This work was supported by the National Key Research and Development Program of China (2016YFD0600806), the Central Universities (2572017EB07), the Heilongjiang Science Fund for Distinguished Young Scholars (JC2017003). Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the National Key Research and Development Program of China (2016YFD0600806), the Central Universities (2572017EB07), the Heilongjiang Science Fund for Distinguished Young Scholars (JC2017003). We are grateful for the funding. ABBREVIATIONS


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