Presence and formation of fluorescence carbon dots in a grilled hamburger

Yao Li abc, Jingran Bi abc, Shan Liu abc, Haitao Wang abc, Chenxu Yu abd, Dongmei Li abc, Bei-Wei Zhu *abc and Mingqian Tan *abc
aSchool of Food Science and Technology, Dalian Polytechnic University, Qinggongyuan 1, Ganjingzi District, Dalian 116034, China. E-mail: zhubeiwei@163.com; mqtan@dlpu.edu.cn
bNational Engineering Research Center of Seafood, Dalian 116034, China
cEngineering Research Center of Seafood of Ministry of Education of China, Dalian 116034, China
dDepartment of Agricultural and Biosystems Engineering, Iowa State University, Ames, IA 50010, USA

Received 5th May 2017 , Accepted 11th June 2017

First published on 12th June 2017


The presence of nanomaterials during food processing has attracted significant concern due to the physicochemical properties of nanomaterials. In this study, the presence and formation of nitrogen-containing fluorescence carbon dots (C-dots) in a grilled hamburger at different temperatures (220, 260, and 300 °C) were investigated during the pyrolysis process. The size and morphology of the C-dots were found to be highly dependent on the heating temperatures, which again affected the functional groups on their surface. The C-dots are strongly fluorescent with multicolor emission accompanied by a gradual decrease in fluorescence intensity. The fluorescence quantum yield of the C-dots produced at 260 °C was measured to be 23.25%. The potential cytotoxicity and biodistribution of the C-dots within live organisms were examined with the mouse osteoblasts cell line and mung bean sprout, respectively. The cell viability after 24 h incubation remained 79% for the C-dots obtained at 300 °C at a concentration of 3.2 mg mL−1, and no obvious phytotoxicity in the growing mung bean sprout was observed. The results showed an increased cytotoxicity of the C-dots formed at higher temperatures.


Introduction

Grill is a popular cooking method because of its simplicity. It generates unique aroma, flavor, and savory taste in foods that are often favored by consumers. However, in recent years, the potential health risks associated with grilled foods have caused increasing public concern.1,2 Most of the current studies are aimed at the examination of a class of acrylamide,3,4 benzopyrene,5 and heterocyclic amines,6,7 which are mainly formed from amino acids, sugars, myofibers, or aldehydes as precursors during intensive heating of foods. Human consumption of these toxicants has been shown to be associated with elevated risk of several types of cancers such as pancreatic cancer,8 prostate cancer,9 and breast cancer.10 Besides these organic chemicals, another group of agents, namely, carbon nanostructures that are also produced in grilled foods, may also be associated with potential health risks. The understanding of the potential toxicity/risks of these materials is at its infancy, and the carbon nanoparticles/nanostructures generated in grilled foods are emerging as a target for much needed research.11–13

Among carbon nanostructures, fluorescent carbon dots (C-dots) have been quite extensively investigated because of their unique properties such as high photostability, excitation-tunable fluorescence, high chemical stability, and easy functionalization.14–16 C-Dots have been utilized as imaging agents for biomedical applications,17,18 fluorescent probes for the selective detection of inorganic molecules,19,20 and novel materials for optoelectronic devices.21 They can be synthesized through both top-down and bottom-up routes.22,23 In a top-down method, C-dots are formed or broken off from large carbon structures such as laser-ablated eggs.24,25 In a bottom-up method, also known as hydrothermal synthesis, C-dots are prepared from molecular precursors such as citric acid and ethylene diamine as the carbon sources.26 Food may produce C-dot-like carbonaceous nanostructure during thermal processing. Reports have shown that C-dots are present in beverage, bread, and jaggery;27–29 however, to date, no study on the presence of C-dots in grilled meats, a very popular food category that deserves more attention, has been reported.30 Hence, in this study, we aimed at filling this knowledge gap and studying the presence and formation of C-dots in grilled foods.

Hamburger is one of the most popular meat products worldwide. It is widely served in fast food restaurants such as McDonald's and Burger King. Herein, the presence, formation, and potential toxicity of C-dots in grilled hamburgers were investigated at different temperatures. The C-dots generated at different temperatures were extracted and purified from the grilled hamburger samples, and their physicochemical properties including particle sizes, fluorescent behaviors, lifetime, elemental composition, and surface functional groups were thoroughly characterized. The impact (i.e., cytotoxicity, transport, and biodistribution) of C-dots on live organisms was studied at relatively short exposure times30 using an animal-related model (mouse osteoblasts cell line) and a fast-growing plant model (mung bean sprout).

Experimental

Materials

Beef and mung bean were purchased from local grocery market. Dialysis bags with a cut-off molecular weight at 1.0 and 3.5 kDa were purchased from Aldrich Chemical Co. (Shanghai, China). 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) was purchased from Aladdin Chemistry (Shanghai, China). Mouse osteoblast cell line was purchased from the Cell Bank of Type Culture Collection of the Chinese Academy of Sciences (Shanghai, China). Unless otherwise stated, ethyl acetate and other chemicals were purchased from commercial vendors and used without further purification.

Preparation and extraction of the C-dots

Beef was washed several times with distilled water and then cut into pieces and ground using a domestic fruit-juicer (Joyoung, JYL-C012, Shanghai, China) to make hamburger patties. The patties were then roasted on an electric heating oven (Rational, SCC-WE-101, Bavaria, Germany) at 220, 260, and 300 °C for 30 min. After being cooled down to room temperature, the hamburgers were dissolved in ethanol, and the precipitates were removed. After solvent removal, the resultant solid was dissolved in water, washed several times with ethyl acetate, and was subjected to dialysis (molecular weight cut-off of 3.5 kDa). The outside dialysis fluid was obtained and concentrated. Finally, the C-dots were obtained and purified via dialysis (molecular weight cut-off of 1.0 kDa) again to remove other impurities.

Characterization of the C-dots

The morphology and size of the C-dots were characterized using transmission electron microscopy (TEM), which was performed using a JEM2100 UHR (JEOL, Tokyo, Japan) with an electron energy of 200 kV. Fourier transform infrared (FT-IR) spectra were obtained using a PerkinElmer Spectrum TWO FT-IR infrared spectrometer (PerkinElmer, Norwalk, USA) in the wavelength range from 4000 to 1000 cm−1. Ultraviolet visible (UV-vis) absorption spectra were obtained using a UV–vis spectrophotometer (Lambda 35, PerkinElmer, Cambridge, USA). X-ray diffraction (XRD) was conducted using a diffractometer (XRD-6100, Shimadzu, Kyoto, Japan) with Cu Kα radiation (50 kV, 200 mA, λ = 0.154 nm) and a scanning step of 0.02°. X-ray photoelectron spectroscopy (XPS) was conducted using an ESCALAB 250 spectrometer (Thermo VG, Waltham, USA) with a mono X-Ray source of Al Kα excitation (1486.6 eV). Fluorescence spectra were obtained using a fluorescence spectrometer (F-2700, Hitachi, Tokyo, Japan) with a xenon lamp as the light source. The fluorescence lifetime of the C-dots was measured using a FS5 spectrofluorometer (Edinburgh Instruments Co., UK) with a 450 nm laser as the excitation source. Raman spectra were obtained using a Raman system model 1000 spectrometer (Horiba Co., Japan) with radiation at 633 nm.

Toxicity and imaging of C-dots

Mouse osteoblasts were cultured in a Defined K-SFM serum free medium containing 10% fetal bovine serum (FBS) at 37 °C, 5% CO2, and 95% humidity. The potential cytotoxicity of C-dots was evaluated via an MTT assay. Mouse osteoblasts were treated with different concentrations of 0.1 mL (0.2, 0.4, 0.8, 1.6, and 3.2 mg mL−1) of the C-dot solutions and incubated for 24 hours. The cells were then treated with 0.02 mL of freshly prepared 2.5 mg mL−1 MTT and incubated for 4 hours. The supernatant was carefully removed, and dimethylsulfoxide solution was added. The absorbance at 570 nm was then measured. Mouse osteoblasts were treated with 0.5 mg mL−1 C-dot solutions for bio-imaging after incubation for 24 h.

Mung beans were selected to have similar sizes and then sterilized by soaking in a 10% NaClO solution for 10 minutes. Subsequently, the beans were washed three times with distilled water and then arranged in beakers (50 seeds for each group). In the control experiments, pure water and 3.2 mg mL−1 C-dot solution obtained from the roast beef sample processed at 300 °C was added into the abovementioned beakers. After 5 days, the mung bean sprouts were harvested. The germination rate was calculated, and the root length and stem length were measured. The fluorescence signal distribution of the mung bean sprouts was analyzed via a laser scanning microscope.

Results and discussion

Structural characterization

The C-dots from hamburgers processed at 220, 260, and 300 °C were extracted with ethanol, purified, and characterized using high-resolution transmission electron microscopy (TEM) to reveal the formation mechanism (Fig. 1a–c). The results indicated that the C-dots exhibited good mono-dispersion. The images of the hamburger patties, as shown in Fig. 1d, f, and h, demonstrate that the charring surface area of the patties gradually expanded as the heating temperature increased. The average particle sizes of the C-dots calculated based on the TEM images were 33.6 ± 21.2, 5.1 ± 4.9, and 2.5 ± 1.6 nm, respectively (Fig. 1e, g, and i), for the patties baked at different temperatures. The particle size was large for C-dots obtained from beef patties heated at 220 °C, and much smaller particle size with a narrower distribution of C-dots obtained from the patties heated at 300 °C was observed. Furthermore, well-resolved lattice fringes with an interplanar spacing of 0.261 nm were observed for the C-dots formed at 260 °C and 300 °C, and no such obvious lattice fringes were observed for the C-dots formed at 220 °C (Fig. S1). A reasonable hypothesis is that the proteins and polysaccharides in beef patties were decomposed into large nanoparticles at low temperatures, and the large C-dots were subsequently carbonized to form small C-dots with lattice fringes at high temperatures.31 The C-dots produced at 220 °C are considered as polymer dots (PDs), which are amorphous, whereas the C-dots formed at 260 and 300 °C are considered as carbon quantum dots (CQDs) possessing obvious lattice structures.32,33 Moreover, the threshold temperature is probably between 220 °C and 260 °C. Additionally, a strong blue emission was observed from the C-dots under illustration of UV light (365 nm), indicating that the C-dots formed in beef patties were good fluorophores (Fig. 1j–l).
image file: c7fo00675f-f1.tif
Fig. 1 The TEM images of the C-dots obtained at (a) 220 °C, (b) 260 °C, and (c) 300 °C. Insets show the HRTEM images of C-dots extracted from the hamburger samples heated at 220, 260, and 300 °C. Images of the hamburger samples heated at (d) 220 °C, (f ) 260 °C, and (h) 300 °C. Particle size distribution of the C-dots obtained at (e) 220 °C, (g) 260 °C, and (i) 300 °C, respectively. Photographs of the aqueous solution of C-dots extracted from roasted beef at (j) 220 °C, (k) 260 °C, and (l) 300 °C, respectively, under UV light.

The surface chemical characteristics of the C-dots formed at different temperatures were determined via FTIR spectroscopy. As observed in Fig. 2a, the strong characteristic absorption peaks at 3200–3600 cm−1 could be assigned to the O–H (stretching) and N–H (stretching) vibrations, and the characteristic absorption peaks at 2963–2857 cm−1 are assigned to –CH3 (stretching) and –CH2 (stretching). The typical absorption peaks of –CONH are at 1655 and 1545 cm−1. The peak at 1655 cm−1 is the amide I band, and the peaks at 1545 and 1480 cm−1 are the amide II and III bands produced by the amide N–H bending and C–N stretching vibrations. All these signals suggested the presence of functional groups on the C-dot surfaces. The decline in amide II band intensity at higher heating temperatures might be attributed to the partial break-down of the CO–NH bond. C-dots formed at different temperatures were also characterized by UV–vis absorption spectroscopy (Fig. 2b). The C-dots exhibited a series of broad absorption bands in the region of 200–250 nm, mainly attributed to the π–π* electronic transitions of conjugated organic molecules on the C-dot surfaces. The absorption bands in the region of 250–350 nm are due to the n–π* electronic transitions of the p–π conjugate. As the temperature increased, the absorption at 250–350 nm significantly decreased because of the C–N bond breakage. These results were consistent with the FTIR findings.


image file: c7fo00675f-f2.tif
Fig. 2 (a) FT-IR and (b) UV-visible spectra of the C-dots processed at different temperatures.

The XRD results of the C-dots (Fig. S2) demonstrated a broad peak centered at approximately 22.6° with an interlayer spacing of 0.395 nm, which is broader than that of graphite (0.34 nm).34–36 As Raman spectroscopic signal of the C-dots is very weak, silver colloid was used as the active substrate for surface-enhanced Raman spectroscopy. The Raman spectra (Fig. S3) suggested that the C-dots consist of sp2 (1580 cm−1) and sp3 (1360 cm−1) hybridization. The oxygen groups on the C-dot surface might have enhanced the interlayer distance. Moreover, XPS analysis showed that the C-dots contained elemental carbon, nitrogen, and oxygen on their surfaces. As shown in Fig. 3a, C-dots obtained from hamburgers heated at 220 °C exhibited three peaks at 282, 397, and 529 eV, which were attributed to C1s, N1s, and O1s, respectively. Elemental analysis further established the composition of the C-dots to be C 68.68%, N 17.38%, and O 13.38%, confirmed their carbon-rich nature. Furthermore, the high-resolution C1s spectrum (Fig. 3b) demonstrated the presence of C–C bond with a binding energy at 281.9 eV, C–O or C–N at 282.9 eV, and O–C[double bond, length as m-dash]O at 285.1 eV. High resolution N1s spectrum (Fig. 3c) showed major peaks at 396.6 eV and 397.5 eV, suggesting C–N, N–H or C[double bond, length as m-dash]N pyridine-like and amide nitrogen atoms, respectively. High resolution spectrum of O1s (Fig. 3d) showed major peaks at 528.7 and 529.9 eV for C[double bond, length as m-dash]O, and C–OH or C–O–C groups, respectively.37–40 Similar results were obtained for the C-dots obtained from hamburgers heated at 260 °C and 300 °C, respectively (Fig. S4 and S5). Note that the nitrogen content of the C-dots decreased with the increase in the processing temperature (Table S1), suggesting that the C-dots partially broke-down at high temperatures, consistent with the results obtained from the FTIR and UV–vis absorption spectroscopy.


image file: c7fo00675f-f3.tif
Fig. 3 (a) XPS spectrum of the C-dots processed at 220 °C, high-resolution (b) C1s, (c) N1s, and (d) O1s spectra.

Optical characterization

The presence of fluorescent benzopyrene in C-dots was checked by thin layer chromatography (TLC), as shown in Fig. S6. The Rf value of C-dots is significantly different from that of the reference benzopyrene. No fluorescent signal of the hydrophobic poly cyclic aromatic hydrocarbon (PAH) was observed from the C-dots, as observed in the TLC image. The fluorescence only originated from the hydrophilic components.

The strong fluorescent C-dots under UV light (365 nm) showed blue, green, and red emission under the excitation of 405, 488, and 543 nm laser, respectively (Fig. S7). C-dots obtained from the hamburgers heated at 220, 260, and 300 °C, as shown in Fig. 4a, b, and c, respectively, demonstrated an excitation-dependent emission with a maximum emission wavelength at 380 nm under the excitation of 320 nm. The emission spectra shifted to a longer wavelength accompanied by a gradual fluorescence intensity decrease. The maximum emission peak shifted from 385 nm to 525 nm with a gradually decreased intensity as the excitation wavelength was changed from 300 nm to 450 nm. This behavior is believed to be caused by the surface state affecting the band gap of the C-dots.18,20,23 The oxygen- and nitrogen-containing surface groups of the C-dots might be responsible for these emissions via trapping of the excitons under excitation and the radiative recombination of the surface-trapped excitons.41


image file: c7fo00675f-f4.tif
Fig. 4 UV-visible absorption and emission spectra of the C-dots processed at (a) 220 °C, (b) 260 °C and (c) 300 °C at the excitation wavelengths progressively increasing from 300 to 450 nm.

Fluorescence quantum yields for C-dots produced at 220, 260 and 300 °C were 23.25%, 19.43%, and 15.03%, respectively, higher than those of other C-dots reported from natural carbon sources.42,43 High quantum yield of C-dots may originate from the high nitrogen residues that lead to the trapping of more excitons under excitation. Interestingly, the quantum yield of the C-dots exhibited temperature-dependency, which may be caused by the reduction of the surface-trapped excitons of the C-dots as the barbecue temperature increased, thus affecting the fluorescence quantum yields.44

The fluorescence lifetimes measured through a time-correlated single-photons counting (TCSPC) technique were 8.32, 6.49, and 7.26 ns, respectively, for the C-dots produced at 220, 260, and 300 °C (Fig. S8). The fluorescence decay data fitted well to an exponential decay model. No significant differences were observed for the fluorescence lifetimes of the C-dots obtained at different temperatures.

The existence of various surface groups on C-dots was further examined by UV-irradiation to gain insights into the structure of the C-dots (Fig. 5a–c). The fluorescence intensity of the C-dots obtained from hamburgers heated at 220, 260, and 300 °C decreased 53%, 46%, and 28%, respectively, after irradiation of 365 nm UV light for 30 min. This discrepancy in photobleaching activity suggests that the C-dots formed at lower temperatures mainly emitted surface-state fluorescence,45 whereas the C-dots formed at higher temperature mainly emitted center-state fluorescence, with fewer fluorescent molecules on the surface.46,47 As illustrated in Fig. 5d, it is reasoned that hamburgers heated at 220 °C tend to yield large-sized C-dots with more surface chemical groups that are easily etched by UV light, whereas hamburgers heated at higher heating temperatures tend to generate smaller C-dots with less surface functional groups and more stable fluorescence yield.


image file: c7fo00675f-f5.tif
Fig. 5 Emission spectra and image (inset) of the C-dots processed at (a) 220 °C, (b) 260 °C, and (c) 300 °C before and after UV-irradiation. (d) Schematic of the formation of C-dots processed at different temperature.

Toxicity and biodistribution in organisms

Potential cytotoxicity of the C-dots obtained from hamburgers was evaluated against mouse osteoblasts via an MTT test. Fig. 6a–c show the cell viability after the cells are incubated with 0.2–3.2 mg mL−1 of C-dots for 24 h. The relative cell viability remained 90% for the C-dots obtained at 220 °C with a concentration of 3.2 mg mL−1, whereas those of the C-dots obtained at 260 °C and 300 °C decreased by 15% and 21%, respectively. The C-dots formed at low-temperatures did not elicit obvious cytotoxicity at a concentration as large as 3.2 mg mL−1. Note that the results showed an increased cytotoxicity of the C-dots formed at higher temperatures. This might be due to the different carbonization degree of the C-dots at different temperatures as abovementioned. The carbon nucleus is a minor component of the C-dots formed during low-temperature processing, which is probably the main cause of cytotoxicity. Therefore, the C-dots formed during low-temperature processing did not exert potential cytotoxicity. As the number of the surface molecules of C-dots decreased during high-temperature processing, carbon nucleus constituted most of the total weight, and the C-dots consequently induced potential cytotoxic effects. This assumption will be examined in detail in our future studies.
image file: c7fo00675f-f6.tif
Fig. 6 Cytotoxicity of the C-dots processed at (a) 220 °C, (b) 260 °C, and (c) 300 °C, against mouse osteoblasts cell line at increasing concentrations from 0 to 3.2 mg mL−1. (d) Length of the root and stem of mung bean sprouts grown in a C-dot aqueous solution for 5 days. Control group was grown in water for 5 days.

Moreover, the biodistribution of the C-dots in organisms was also examined using a mouse osteoblasts cell line and mung bean sprout. Blue, green, and red fluorescence images were obtained via excitation of the C-dot-absorbed cells with 405, 488, and 543 nm lasers, respectively (Fig. 7). In vitro cell-uptake study showed that the C-dots were able to enter the cells and distributed within the cytoplasm, without showing up in cell nuclei. No significant morphological damage to the cells was observed after incubation for 24 h for all types of C-dots formed at different temperatures.


image file: c7fo00675f-f7.tif
Fig. 7 Confocal microscopy images of the C-dots in the mouse osteoblasts cells excited by 405 nm, 488 nm, and 543 nm laser: (a) to (d) treated cells with C-dots extracted from the 220 °C samples; (e) to (h) treated cells with C-dots extracted from the 260 °C samples; (i) to (l) treated cells with C-dots extracted from the 300 °C samples. Scale is 20 micron.

Furthermore, mung bean sprout was employed as a model plant to investigate the distribution of C-dots in vivo after culturing for 5 days. In the test group, the concentration of the C-dots obtained at 300 °C used was 3.2 mg mL−1, which was found to have potential cytotoxicity. The lengths of the roots and stems of 50 individual mung bean sprouts grown with/without the influence of C-dots were compared. No significant differences were observed in the lengths of the roots and stems of the mung bean sprouts between the control and test groups (Fig. 6d), indicating that the C-dots did not elicit potential phytotoxic effects on the growth of mung bean. During sprout growth, the absorption and distribution of C-dots were studied by tracking their fluorescence in the plant. As shown in Fig. 8, the size of the mung bean seeds was similar (Fig. 8a and f), and no significant differences between the mung bean sprouts grown in a C-dot solution (i.e., test) and water (i.e., control) were observed after 2 days growth (Fig. 8b and g) as well as 5 days (Fig. 8d and i), respectively. Under the excitation of 365 nm light, fluorescence was clearly seen in the roots, stem, and cotyledon (Fig. 8h and j) of the test group, whereas no visible fluorescence was observed in the control group (Fig. 8c and e). This indicates that the C-dots obtained from hamburgers could be absorbed by the mung bean sprout. In addition, laser scanning microscopy images of the mung bean sprout stem (Fig. 8l and m) show blue fluorescence in axially elongated vascular tissues (excitation of 405 nm), revealing that the C-dots distribute within the vascular system (Fig. 8k) of the mung bean sprout stem, as reported in a previous work.30 Clearly, C-dots obtained from hamburgers can be absorbed by live organisms, and further research is needed to understand their physiological effects.


image file: c7fo00675f-f8.tif
Fig. 8 Images of the mung bean seed germination process: (a) to (e) show the mung bean seed treated with deionized water, (f) to (j) represent mung bean seed treated with the solution of C-dots (3.2 mg mL−1) processed at 300 °C. (c), (e), (h), and (j) fluorescence images of mung bean sprouts under UV (365 nm). (k) Cartoon images of mung bean sprout stem vascular system. (l) Laser scanning microscopy images of transverse sections of mung bean sprout stem excited by 405 nm. Scale is 200 micron. (m) Laser scanning microscopy images of longitudinal sections of mung bean sprout stem excited by 405 nm. Scale is 100 micron.

Conclusions

This study demonstrated the presence, formation, and physicochemical properties of the C-dots in hamburgers. Different heating temperatures led to different particle sizes, surface groups, and fluorescence property in the C-dots produced. The C-dots produced at high temperatures elicited much more significant cytotoxic effects than those produced at low temperatures, but neither exhibited phototoxic effects on model plants. The C-dots showed good absorption and distributive characteristics in the cytoplasm of mouse osteoblast cells, as well as in model plants (i.e., mung bean sprouts), indicating their potential as biological fluorescent labels.

Acknowledgements

This work was supported by the National Key Research and Development Project (2017YFD0400100, 2016YFD0400404) and the National Nature Science Foundation of China (31601389).

Notes and references

  1. J. G. Lee, S. Y. Kim, J. S. Moon, S. H. Kim, D. H. Kang and H. J. Yoon, Food Chem., 2016, 199, 632–638 CrossRef CAS PubMed.
  2. J. W. Broady, 2nd, D. Han, J. Yuan, C. Liao, C. L. Bratcher, M. R. Lilies, E. H. Schwartz and L. Wang, J. Food Sci., 2016, 81, M1766–M1772 CrossRef PubMed.
  3. S. Yang, C. Cao, S. Chen, L. Hu, W. Bao, H. Shi, X. Zhao and C. Sun, J. Agric. Food Chem., 2016, 64, 9237–9245 CrossRef CAS PubMed.
  4. O. E. Adedipe, S. D. Johanningsmeier, V. D. Truong and G. C. Yencho, J. Agric. Food Chem., 2016, 64, 1850–1860 CrossRef CAS PubMed.
  5. S. Chen, T. H. Kao, C. J. Chen, C. W. Huang and B. H. Chen, J. Agric. Food Chem., 2013, 61, 7645–7653 CrossRef CAS PubMed.
  6. Q. Zhu, S. Zhang, M. Wang, J. Chen and Z.-P. Zheng, Food Funct., 2016, 7, 1057–1066 CAS.
  7. M. Zeng, Y. Li, Z. He, F. Qin and J. Chen, Meat Sci., 2016, 116, 50–57 CrossRef CAS PubMed.
  8. C. Pelucchi, V. Rosato, P. M. Bracci, D. Li, R. E. Neale, E. Lucenteforte, D. Serraino, K. E. Anderson, E. Fontham and E.A. Holly, et al. , Ann.Oncol., 2017, 28, 408–414 CAS.
  9. J. G. Hogervorst, L. J. Schouten, E. J. Konings, R. A. Goldbohm and P. A. Van Den Brandt, Am. J. Clin. Nutr., 2008, 87, 1428–1438 CAS.
  10. G. C. Kabat, A. J. Cross, Y. Park, A. Schatzkin, A. R. Hollenbeck, T. E. Rohan and R. Sinha, Int. J. Cancer, 2009, 124, 2430–2435 CrossRef CAS PubMed.
  11. Y. Chen, M. Xu, J. Zhang, J. Ma, M. Gao, Z. Zhang, Y. Xu and S. Liu, Adv. Mater., 2016, 29, 1604580–1604588 CrossRef PubMed.
  12. A. A. Alshatwi, V. S. Periasamy, P. Subash-Babu, M. A. Alsaif, A. A. Alwarthan and K. A. Lei, Environ. Toxicol. Pharmacol., 2013, 36, 215–222 CrossRef CAS PubMed.
  13. J. Li, M. Tian, L. Cui, J. Dwyer, N. J. Fullwood, H. Shen and F. L. Martin, Sci. Rep., 2016, 6, 20207 CrossRef CAS PubMed.
  14. W. Liu, C. Li, Y. Ren, X. Sun, W. Pan, Y. Li, J. Wang and W. Wang, J. Mater. Chem. B, 2016, 4, 5772 RSC.
  15. D. W. Zheng, B. Li, C. X. Li, J. X. Fan, Q. Lei, C. Li, Z. Xu and X. Z. Zhang, ACS Nano, 2016, 10, 8715–8722 CrossRef CAS PubMed.
  16. Z. Wang, B. Fu, S. Zou, B. Duan, C. Chang, B. Yang, X. Zhou and L. Zhang, Nano Res., 2016, 9, 214–223 CrossRef CAS.
  17. J. Wang and J. Qiu, J. Mater. Sci., 2016, 51, 4728–4738 CrossRef CAS.
  18. S. A. Hill, D. Benito-Alifonso, D. J. Morgan, S. A. Davis, M. Berry and M. C. Galan, Nanoscale, 2016, 8, 18630–18634 RSC.
  19. Y. Yan, H. Yu, K. Zhang, M. Sun, Y. Zhang, X. Wang and S. Wang, Nano Res., 2016, 9, 2088–2096 CrossRef CAS.
  20. S. Liu, J. Tian, L. Wang, Y. Zhang, X. Qin, Y. Luo, A. M. Asiri, A. O. Al-Youbi and X. Sun, Adv. Mater., 2012, 24, 2037–2041 CrossRef CAS PubMed.
  21. Q. Li, M. Zhou, Q. Yang, Q. Wu, J. Shi, A. Gong and M. Yang, Chem. Mater., 2016, 28, 8221–8227 CrossRef CAS.
  22. S. Zhu, L. Wang, B. Li, Y. Song, X. Zhao, G. Zhang, S. Zhang, S. Lu, J. Zhang, H. Wang, H. Sun and B. Yang, Carbon, 2014, 77, 462–472 CrossRef CAS.
  23. J. Tan, J. Zhang, W. Li, L. Zhang and D. Yue, J. Mater. Chem. C, 2016, 4, 10146–10153 RSC.
  24. J. Wang, C.-F. Wang and S. Chen, Angew. Chem., Int. Ed., 2012, 124, 9431–9435 CrossRef.
  25. Z. Zhang, W. Sun and P. Wu, ACS Sustainable Chem. Eng., 2015, 3, 1412–1418 CrossRef CAS.
  26. S. Zhu, Q. Meng and L. Wang, Angew. Chem., Int. Ed., 2013, 125, 4045–4049 CrossRef.
  27. H. Liao, C. Jiang, W. Liu, J. M. Vera, O. D. Seni, K. Demera, C. Yu and M. Tan, J. Agric. Food Chem., 2015, 63, 8527–8533 CrossRef CAS PubMed.
  28. M. P. Sk, A. Jaiswal, A. Paul, S. S. Ghosh and A. Chattopadhyay, Sci. Rep., 2012, 2, 383 Search PubMed.
  29. J. M. Aguilera, J. Agric. Food Chem., 2014, 62, 9953–9956 CrossRef CAS PubMed.
  30. W. Li, Y. Zheng, H. Zhang, Z. Liu, W. Su, S. Chen, Y. Liu, J. Zhuang and B. Lei, ACS Appl. Mater. Interfaces, 2016, 8, 19939–19945 CAS.
  31. Z. Song, T. Lin, L. Lin, S. Lin, F. Fu, X. Wang and L. Guo, Angew. Chem., Int. Ed., 2016, 55, 2773–2777 CrossRef CAS PubMed.
  32. B. Sun, B. Zhao, D. D. Wang, Y. B. Wang, Q. Tang, S. J. Zhu, B. Yang and H. C. Sun, Nanoscale, 2016, 8, 9837–9841 RSC.
  33. S. J. Zhu, Y. B. Song, X. H. Zhao, J. R. Shao, J. H. Zhang and B. Yang, Nano Res., 2015, 8, 355–381 CrossRef CAS.
  34. L. Shi, J. H. Yang, H. B. Zeng, Y. M. Chen, S. C. Yang, C. Wu, H. Zeng, O. Yoshihito and Q. Zhang, Nanoscale, 2016, 8, 14374–14378 RSC.
  35. M. Tan, X. Li, H. Wu, B. Wang and J. Wu, Colloids Surf., B, 2015, 136, 141–149 CrossRef CAS PubMed.
  36. M. Zheng, S. Ruan, S. Liu, T. Sun, D. Qu, H. Zhao, Z. Xie, H. Gao, X. Jing and Z. Sun, ACS Nano, 2015, 9, 11455–11461 CrossRef CAS PubMed.
  37. H. Gao, A. V. Sapelkin, M. M. Titirici and G. B. Sukhorukov, ACS Nano, 2016, 10, 9608–9615 CrossRef CAS PubMed.
  38. M.-L. Kung, P.-Y. Lin, C.-W. Hsieh and S. Hsieh, Nano Res., 2014, 7, 1164–1176 CrossRef CAS.
  39. L. Pan, S. Sun, L. Zhang, K. Jiang and H. Lin, Nanoscale, 2016, 8, 17350–17356 RSC.
  40. Y. Song, S. Zhu and B. Yang, RSC Adv., 2014, 4, 27184–27200 RSC.
  41. A.-M. Alam, B.-Y. Park, Z. K. Ghouri, M. Park and H.-Y. Kim, Green Chem., 2015, 17, 3791–3797 RSC.
  42. S. Sahu, B. Behera, T. K. Maiti and S. Mohapatra, Chem. Commun., 2012, 48, 8835–8837 RSC.
  43. N. Wang, Y. Wang, T. Guo, T. Yang, M. Chen and J. Wang, Biosens. Bioelectron., 2016, 85, 68–75 CrossRef CAS PubMed.
  44. Y. Hu, J. Yang, J. Tian and J.-S. Yu, J. Mater. Chem. B, 2015, 3, 5608–5614 RSC.
  45. S. Zhu, S. Tang, J. Zhang and B. Yang, Chem. Commun., 2012, 48, 4527–4539 RSC.
  46. S. Zhu, Y. Song, X. Zhao, J. Shao, J. Zhang and B. Yang, Nano Res., 2015, 8, 355–381 CrossRef CAS.
  47. Y. Song, S. Zhu, S. Xiang, X. Zhao, J. Zhang, H. Zhang, Y. Fu and B. Yang, Nanoscale, 2014, 6, 4676–4682 RSC.

Footnote

Electronic supplementary information (ESI) available. See DOI: 10.1039/c7fo00675f

This journal is © The Royal Society of Chemistry 2017