Open Access
Add to BibSonomy
Abstract
In this paper, thermally-stable, biocompatible and flexible luminescent films with self-healing capability were prepared by combining graphene quantum dots (GQDs) and silk fibroin (SF). Two methods were carried out to combine GQDs and SF: one is to feed silkworms with a GQDs diet and collect their cocoons; the other is to mix GQDs with SF solution directly. Then feeding GQDs/SF composite film and GQDs/SF mixing films of different mass ratios were prepared by natural air drying. The morphology, structure and performance of the SF films were characterized in detail. Results showed that the mixing composite films are flexible, luminescent and thermally stable. The more GQDs in the film, the stronger the luminescence is, in our case. The mixing films exhibit different light color under different excitations, the same with GQDs. However, fed GQDs in the silkworm seem to be playing a quenching effect on SF. It also showed that no new chemical bonds were formed between GQDs and SF, and the two were physically mixed, either by feeding or direct solution mixing. In addition, this fluorescent film also has the particular ability of self-healing by a drop of water. This new biocompatible film may have broad applications in fields of flexible display, biosensing, drug delivery and tissue engineering.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Corrections
1 May 2020: A typographical correction was made to the author affiliations.
1. Introduction
Flexible luminescent films have promising applications in solar cells, luminescent screens of field emission microscope, display devices, LEDs, implantable devices and so on [1–7]. For example, W. Chen et al. used NaYF4: Yb/Er//PMMA composite films as back reflecting up-conversion films in organic solar cells, and enhanced the photocurrent and photoelectric conversion efficiency [1]. S. Yu et al. introduced a novel double remote micro-patterned phosphor film into a remote phosphor down-light lamp. The angular color uniformity and luminous efficiency was greatly improved [2]. Y. Tan et al. demonstrated an ultra-stable structure of CsPbBr3 nanostructures embedded in CsPb2Br5 matrix with reversible photoluminescence (PL) and enhanced PL intensity, which was used for flexible instantaneous display [3]. In addition, fluorescent film sensors offer more advantages than fluorescence probes used only in solution in terms of practicality. Film sensors can be used repeatedly, easily made into devices, and exert no contamination to the analyte solution.
Common substrates for flexible luminescent films are mostly synthetic polymers, including polymethyl methacrylate (PMMA), polydimethylsiloxane (PDMS), polycarbonate (PC), polystyrene (PS), and polyethylene terephthalate (PET), etc. Those polymers are neither renewable nor degradable, and their biocompatibility is also yet to improve. With the fast development of biocompatible and implantable optoelectronic devices, a biocompatible, degradable and sustainable substrate material is in demand.
Compared to polymers, silk fibroin (SF) is superior as related to biocompatibility, biodegradability and implantability. It is a natural macromolecular fibrous protein derived from silkworm cocoons. It could be conveniently processed into thin films, which possess good mechanical and optical properties, and act as substrates for various applications [8–12]. For example, X. Qin et al. developed a SF film covered with silver nanowire network, whose conductivity can be as low as 6.9Ω/sq, and transmittance is 60-80% in the visible light range. The films also showed potentials as their resistance is almost linearly temperature-dependent [12]. In addition, various methods have been proposed to produce functional silk, specifically those with color and luminescence, through post-processing steps as well as biological approaches [13–17]. S. Putthanarat et al. prepared thin films of silk doped with green fluorescent protein (GFP). The GFP molecules maintain their nonlinear optical properties and the films exhibit nonlinear attenuation of femtosecond near-IR pulses and appear to be moderately resistant to optical damage [15]. N. Lin et al. fabricated functionalized silk films with tunable luminescence properties. By balancing the strength of the relative blue and yellow light emissions, white-light-emitting silk hybrid films could be achieved [16]. N. Qi et al. spin-coated β-NaYF4:Yb,Er nanostructures on SF substrate and obtained a fluorescent SF film [17].
At present, organic dyes, phosphors and quantum dots are commonly used as luminescent materials. Graphene quantum dots (GQDs) possess very good chemical stability, biocompatibility, PL and low toxicity, and have huge application potentials in optoelectronics, biomedications, environment and food safeties [18–23]. As GQDs also have an excitation-wavelength-dependent emission, it can display different colors in bioimaging under different excitations [19–20].
In this paper, GQDs are combined with SF to explore its special properties as a flexible, biocompatible and fluorescent film. Two different ways were investigated to incorporate GQDs into SF films: one is to feed silkworm with GQDs and get their cocoons. The other is to mix GQDs with SF solution directly. The film properties were thoroughly investigated. Compared with other flexible luminescent films, our composite film is biocompatible, biodegradable and implantable. In addition, it has the specific capability of self-healing by only a drop of water, which cannot be possible for other common films.
2. Experimental
2.1 Materials and methods
Our GQDs were provided by Professor Dengyu Pan of Shanghai University. TEM images indicated that the thickness of the GQDs ranges from 0.5 to 4 nm with an average thickness of 1.64 ± 0.68 nm, illustrating that the GQDs mostly consist of 1-4 graphene layers. The lateral sizes of GQDs are in the range of 1.8-3.6 nm (2.5 ± 0.5 nm on average). TEM image also identifies the high-quality crystalline structure evidenced by the typical 6-fold symmetric diffraction patterns. The GQDs show superior optical properties including strong excitonic absorption bands extended to ∼530 nm, bright PL at 510 nm with a quantum yield of up to 42%, and a wide PL excitation spectrum. They also exhibit an excitation-dependent emission behavior, which has been extensively observed in GQDs and carbon dots. The detailed GQDs characterization was previously reported [24–25]. Deionized (DI) water was added to GQDs sample, followed by ultrasonication. The obtained 0.5 mg/mL solutions were uniformly sprayed on fresh mulberry leaves, which were then used as diets for silkworms in fourth instars directly. Their cocoons were collected thereafter. The other silkworm cocoons were provided by Professor Hongsheng Song of Shanghai University.
The whole GQDs/SF film fabrication process is shown in Figs. 1(a) and 1(b). Briefly, the preparation of SF solution requires degumming, dissolving, dialysis, etc [8,12,26]. The dialyzed solution was centrifuged at 9000 r/min for 20 min at 4℃ and the upper transparent solution was obtained, and its concentration is set to 40 mg/mL. The solution was placed in a refrigerator at 4℃ for storage.
Fig. 1. Preparation of GQD/SF composite films and PL spectrum of the solutions. (a) Schematic showing the natural process to incorporate GQDs into silk by feeding silkworms with mulberry leaves spray-coated with GQDs solutions. (b) Preparation process of the directly mixed SF/GQDs films. (c) PL spectrum of GQDs/SF solutions with different mixing ratios. (d) PL spectrum of GQDs in water.
GQDs Feeding/SF films were fabricated from SF solution of cocoons produced by silkworms fed with GQDs. In comparison, pure SF solution and 60 mg/ml GQDs solution were mixed directly for SF/GQDs solution. It has been found during experiment that fluorescence concentration quenching occurs when the GQDs concentration is too high, so the mass ratio of GQDs to SF is limited to 0∼1.6:100. As shown in Fig. 1(c), in the range of 300-450 nm, PL of SF solution dominates; as the mass ratio of GQDs to SF increases from 0.5:100 to 1:100, the PL intensity in 450∼600 nm increases; while as it increases from 2:100 to 8:100, the intensity in 450∼600 nm decreases. Especially when the concentration is 8:100, the intensity almost quenches thoroughly. As GQDs concentration increases, they tend to agglomerate and particle size and mass will increase. According to the quantum well theory, its energy level difference will decrease and so does the emission light energy. Correspondingly the peak wavelength of emission will be red-shifted, as shown in Fig. 1(c). PL of pure GQDs water solution is shown in Fig. 1(d).
Specially, 0, 10, 20, 40, 80, 160, 320 µL of 60 mg/mL GQDs solutions were added into 30 mL SF solutions(40 mg/mL) separately, and GQDs/SF ratios of 0:100, 0.05:100, 0.1:100, 0.2:100, 0.4:10, 0.8:100, 1.6:100 would be obtained. The mixed solution was ultrasonicated for 20 min. 6 mL of each GQDs/SF solutions were added in petri dishes (the diameter is 5 cm), and then placed in a fume hood for natural air drying (3∼4 days). Different GQDs/SF composite films were obtained (named as GQDs/SF0:100, GQDs/SF0.05:100, GQDs/SF0.1:100, GQDs/SF0.2:100, GQDs/SF0.4:100, GQDs/SF0.8:100, GQDs/SF1.6:100). 3 sets of parallel experiments were performed for each sample.
2.2 Characterizations
The film morphology, composition and characteristics were tested with different equipment and instruments. Its morphology was characterized by a scanning electron microscope (SEM SU3500, Hitachi, Japan). Fluorescence spectra was recorded at room temperature by a fluorescence spectrophotometer (CKX53, Olympus, Japan), UV-vis spectra was recorded by a UV-vis spectrophotometer (100UV-VIS, Varian Australia Pty Ltd), FT-IR spectra was recorded by a FTIR spectrometer (Nicolet, Thermo Fisher), its phase structure was recorded by X-ray diffraction (2θ=5°∼45°, D/MAX-2550, Rigaku Corporation, Japan), and its mechanical/ viscoelastic properties at 30∼190℃ was characterized by dynamic thermomechanical analysis (DMA-Q800, TA Instruments, USA).
3. Results and discussion
3.1 Film morphology and characterization
As the mass ratio of GQDs/SF increases, color of corresponding films will deepen from transparent to deep yellow, as shown in Fig. 2. Color of the GQDsFeeding/SF film was not significantly different from that of GQDs/SF0:100 film by visual observation.
Fig. 2. SF films. a: GQDsFeeding/SF film; b∼h: GQDs/SF0:100, GQDs/SF0.05:100, GQDs/SF0.1:100, GQDs/SF0.2:100, GQDs/SF0.4:100, GQDs/SF0.8:100, GQDs/SF1.6:100 films.
The thickness of the SF film was determined to be in the range of 50-90 µm by SEM. Both feeding and mixing will change film morphology, as seen from Fig. 3 that the pure SF film has a smooth cross section and a dense structure, and the direct GQDs/SF mixing films have relatively rough cross sections, probably a result of nonuniform distribution of GQDs in the film, as shown in Fig. 4. The GQDsFeeding/SF film cross section demonstrates a clear uniform mesh pattern, and is more lustrous, which is obviously different from the pure SF film, which may be a proof of the fed GQDs.
Fig. 3. SEM scans of cross sections of the composite films. a: GQDsFeeding/SF film; b∼h: GQDs/SF0:100, GQDs/SF0.05:100, GQDs/SF0.1:100, GQDs/SF0.2:100, GQDs/SF0.4:100, GQDs/SF0.8:100, GQDs/SF1.6:100 films. Inset in a is the magnified indication of the cross section.
Fig. 4. Cross section of GQDs/SF0.8:100 film (left) and GQDs/SF1.6:100 film (right) taken at 1 K magnification.
3.2 Fluorescence and transmittance performance
As shown in Fig. 5(A), fluorescence of the GQDsFeeding/SF film is the weakest, GQDs/SF0:100 film exhibits weak fluorescence, and the more GQDs added, the stronger the film fluorescence in our case. Fluorescence of GQDs/SF0:100 film and GQDsFeeding/SF film comes from the endogenous fluorescence group in the SF, while the fluorescence in the composite films is the sum of the endogenous fluorescence of SF and that of GQDs. Apparently, GQDs plays a leading role in the SF fluorescence effect, and direct mixing SF with GQDs gives better fluorescence than GQDs feeding. More work is needed to investigate why the GQDsFeeding/SF film displays weaker fluorescence than pure SF film. However, this might be beneficial, as in bioimaging, autofluorescence of biotic tissues is always an interference factor.
Fig. 5. Fluorescence and transmittance of the films. (A) Different SF films with 365 nm ultraviolet radiation. a∼h: GQDsFeeding/SF, GQDs/SF0:100, GQDs/SF0.05:100, GQDs/SF0.1:100, GQDs/SF0.2:100, GQDs/SF 0.4:10, GQDs/SF0.8:100, GQDs/SF1.6:100 films. (B) Films under ultraviolet, blue and green radiations. a∼d: GQDs/SF0:100, GQDs/SF0.2:100, GQDs/SF1.6:100, GQDsFeeding/ SF films. (C) Transmittance of different GQDs/SF solutions in 200-400 nm. (D) Transmittance of different SF films at 275 nm.
In addition, as shown in Fig. 5(B), the films exhibit different color under different excitations characterized by a fluorescent microscope, the same with GQDs previously reported [27]. Basically, when the excitation lights are UV (<400nm), blue (450-500 nm) and green (500-570 nm), the emission lights are blue (450-500 nm), green (500-570 nm) and red (630-760 nm), respectively. It indicates that the GQDs kept its fluorescence properties and enhanced the composite films’ fluorescence.
The absorption peak of the mixed solution was determined to be 275 nm by absorption spectrum analysis, as shown in Fig. 5(C). The transmittance of different SF films at 275 nm is shown in Fig. 5(D), with all of them above 47%. As the GQDs/SF ratio increases, the film transmittance decreases slightly, but the Least-Significant Difference between different SF films is less than 0.05, indicating that the amount of GQDs added to SF does not exert significant influence on film transmittance at 275 nm. By reducing the film thickness, the light transmittance can be improved.
3.3 Compositional analysis
Figures 6(a)–6(b) are infrared spectra of GQDs/SF films, similar to previous reported results [28–29]. It can be seen that all GQDs/SF composite films exhibit an infrared spectrum similar to that of pure SF film. The characteristic peak positions of the acylamide I band, the acylamide II band and the acylamide III band are the same, respectively, at 1637 cm-1 (acylamide I, corresponding to α-helix), 1513 cm-1 (acylamide II, close to the empirical value of 1515 cm-1 corresponding to β-sheet) and 1234 cm-1 (acylamide III, corresponding to random coil).
Fig. 6. (a) FTIR spectra of GQDs/SF films. (b) FTIR spectra of pure SF film, GQDs/SF1.6:100 film and GQDsFeeding/SF film.
As GQDs increased, no new characteristic peaks appeared, indicating that SF and GQDs are physically mixed, and there is no chemical bond between them. The interaction between them may be van der Waals force. The derivative spectrum analysis shows that the content of GQDs in the composite has no significant effect on the secondary structure of SF.
3.4 XRD analysis
Figure 7(a) shows the XRD pattern of different SF films within a scanning range of 5°∼45°, similar to previous publications [30]. It can be seen from the figure that the XRD patterns of all SF films are narrow peaks, the diffraction peaks appear mainly at 22.2°, and there is also a weak diffraction peak at 12.2°, indicating that the structure form is dominated by Silk-I (corresponding to α-helix and random curl). It was also observed that there was a weak diffraction peak at 16.7° and 20.7°, indicating that Silk-II (corresponding to β-sheet) structures were also present. The results showed that SF in different films were mixed structures, mostly Silk-I. The addition of GQDs had no obvious effect on the structure of SF, indicating that α-helix and random coil were dominant in the secondary structures of all SF films, consistent with the conclusion of infrared spectroscopy.
Fig. 7. (a) X-ray diffraction pattern of different SF films. (b) Storage modulus vs. temperature of different films: pure SF film, GQDs/SF1.6:100 film and GQDsFeeding/SF film
3.5 Thermodynamic analysis
The storage modulus of pure SF film, GQDs/SF1.6:100 film, and GQDsFeeding/SF film vs. temperature are shown in Fig. 7(b). It can be seen from the figure that, in the range of 30∼180°C, the storage modulus of pure SF film increases sharply with temperature increases, then remains stable and then drops sharply around 180℃. The storage modulus of GQDs/SF1.6:100 film and GQDsFeeding/SF film showed similar trend: first slowly increases in the range of 30-170℃, rises sharply after 170℃, and then drops rapidly. It is indicated that the thermal stability of GQDs/SF1.6:100 and GQDsFeeding/SF films is significantly better than pure SF film in a certain temperature range.
It can also be seen from the figure that, there is no big difference between the storage modulus of the three films in the range of 40-60℃. It also indicates that the secondary structure contents of the films are not much different, which is consistent with the XRD and FTIR analysis. In the range of 30-160℃, the storage modulus of the three films increases with temperature, and the stiffness also increases, indicating that temperature increase can make α-helix and the random SF transform to β-sheet. Meanwhile, the storage modulus of GQDs/SF1.6:100 film and GQDsFeeding/SF film are mostly much lower than that of pure SF film, indicating that introducing GQDs into the film help inhibiting the transform of α-helix and the random SF to β-sheet. The storage modulus vs. temperature curves of GQDs/SF1.6:100 and GQDsFeeding/SF films are quite similar, which may be used as one important proof that GQDs have entered silk by directing feeding the silkworm.
3.6 Self-healing capability
Different with polymer films, SF films has the ability of self-healing [31–33]. Three kinds of GQDs/SF films were tested (GQDsFeeding/SF, GQDs/SF0.05:100 and GQDs/SF1.6:100 films, respectively). First, samples with sizes of $5\textrm{mm} \times 15\textrm{mm}$. were cut from whole films and into two halves. Each piece was placed on glass substrate, which was covered with a layer of plastic wrap in advance to prevent sticking of the SF film. The two film halves were moved adjacent to each other and a drop of water was dropped at the seams. The film pieces will become sticky and wrinkled due to water absorption, and when water completely evaporated, the two halves combined and formed a complete film, as shown in Fig. 8.
Fig. 8. The self-healing process. (a) GQDs/SF0.05:100 film, (b) GQDsFeeding/SF film and (c) GQDs/SF1.6:100 films.
As SF came into contact with water, the SF chain swelled up and the film softened, increasing viscoelasticity of the film. This creates a good condition for physical fusing of separated halves. Reversible hydrogen bond reconstruction also helps the self-healing process, which formed between the polar groups of SF side chains and SF coated GQDs, and the other polar groups of SF side chains. Owing to dynamic nature of the intrinsic hydrogen bonds, small cracks within the film can be healed readily when broken bonds form again at the fracture. Apparently, GQDs feeding and mixing did not hinder the self-healing capability of SF. As seen in Fig. 8 that flat surfaces of SF films were wrinkled due to humidity, the healing performance will be improved by optimizing applied water amount in our future work as too much water damages film quality.
As for the film biodegradability, silk studies in vitro have demonstrated that proteases such as chymotrypsin will cleave the less-crystalline regions of the protein to peptides which are then capable of being phagocytosed for further metabolism by the cell. Furthermore, protease cocktails and chymotrypsin (known to be produced by macrophages) are capable of enzymatically degrading silk [34]. The specific characteristics of this film suggest its application potentials in biosensing, drug delivery, tissue engineering, solar cell and displays.
4. Conclusions
In this paper, two approaches were implemented to prepare flexible and luminescent GQDs/SF composite films: feeding and direct mixing. Characteristics and properties of the films were tested and compared. The results showed that no new chemical bonds were formed between GQDs and SF, and the two were physically mixed. Though direct feeding, the GQDs does not exert enhancing effect on film fluorescence, however, the fluorescence effect of SF film could be greatly enhanced by mixing SF with GQDs. In a specific range, the fluorescence effect will strengthen with increasing GQDs mixing. Adding GQDs could also improve film thermal stability. Particularly, small cracks and fractures in the film would be able to self-heal by a drop of water. For possible applications in biosensing, drug delivery and tissue engineering, more work is still needed, such as improving the film quality, uniformity and repeatability.
Funding
National Natural Science Foundation of China (51205245, 61573236); Specialized Research Fund for the Doctoral Program of Higher Education of China; Scientific Research Foundation for Returned Scholars of Ministry of Education.
Disclosures
The authors declare no conflicts of interest.
References
1. W. Chen, Y. Hou, A. Osvet, F. Guo, P. Kubis, M. Batentschuk, B. Winter, E. Spiecker, K. Forberich, and C. J. Brabec, “Sub-Bandgap Photon Harvesting for Organic Solar Cells Via Integrating up-Conversion Nanophosphors,” Org. Electron. 19, 113–119 (2015). [CrossRef]
2. S. D. Yu, Z. T. Li, G. W. Liang, Y. Tang, B. H. Yu, and K. H. Chen, “Angular Color Uniformity Enhancement of White Light-Emitting Diodes by Remote Micro-Patterned Phosphor Film,” Photonics Res. 4(4), 140–145 (2016). [CrossRef]
3. Y. S. Tan, R. Y. Li, H. Xu, Y. S. Qin, T. Song, and B. Q. Sun, “Ultrastable and Reversible Fluorescent Perovskite Films Used for Flexible Instantaneous Display,” Adv. Funct. Mater. 29(23), 1900730 (2019). [CrossRef]
4. B. Chen, H. P. Xie, S. Wang, Z. Y. Guo, Y. F. Hu, and H. Z. Xie, “Uv Light-Tunable Fluorescent Inks and Polymer Hydrogel Films Based on Carbon Nanodots and Lanthanide for Enhancing Anti-Counterfeiting,” Luminescence 34(4), 437–443 (2019). [CrossRef]
5. K. G. Mane, P. B. Nagore, and S. R. Pujari, “Synthesis, Photophysical, Electrochemical and Thermal Investigation of Anthracene Doped 2-Naphthol Luminophors and Their Thin Films for Optoelectronic Devices,” J. Fluoresc. 28(5), 1023–1028 (2018). [CrossRef]
6. D. M. Chao, Y. M. Yang, X. T. Jia, and E. B. Berda, “Rationally-Designed Multi Responsive Fluorescent Switching Polymer Films,” Dyes Pigm. 167, 77–82 (2019). [CrossRef]
7. W. Q. Xie, K. X. Yu, and Y. X. Gong, “Preparation of Fluorescent and Antibacterial Nanocomposite Films Based on Cellulose Nanocrystals/Zns Quantum Dots/Polyvinyl Alcohol,” Cellulose 26(4), 2363–2373 (2019). [CrossRef]
8. H. Y. Yu, F. Wang, Q. C. Liu, Q. Y. Ma, and Z. G. Gu, “Structure and Kinetics of Thermal Decomposition Mechanism of Novel Silk Fibroin Films,” Acta. Phys-Chim. Sin. 33(2), 344–355 (2017). [CrossRef]
9. K. Min, M. Umar, H. Seo, J. H. Yim, D. G. Kam, H. Jeon, S. Lee, and S. Kim, “Biocompatible, Optically Transparent, Patterned, and Flexible Electrodes and Radio-Frequency Antennas Prepared from Silk Protein and Silver Nanowire Networks,” RSC Adv. 7(1), 574–580 (2017). [CrossRef]
10. C. N. Lemos, C. Cubayachi, K. Dias, J. N. Mendonça, N. P. Lopes, N. A. J. C. Furtado, and R. F. V. Lopez, “Iontophoresis-Stimulated Silk Fibroin Films as a Peptide Delivery System for Wound Healing,” Eur. J. Pharm. Biopharm. 128, 147–155 (2018). [CrossRef]
11. D. K. Kim, B. R. Sim, J. I. Kim, and G. Khang, “Functionalized Silk Fibroin Film Scaffold Using Β-Carotene for Cornea Endothelial Cell Regeneration,” Colloids Surf., B 164, 340–346 (2018). [CrossRef]
12. X. Z. Qin, Y. Peng, P. P. Li, K. Cheng, Z. Z. Wei, P. Liu, N. Cao, J. Y. Huang, J. J. Rao, J. B. Chen, T. Wang, X. M. Li, and M. Liu, “Silk Fibroin and Ultra-Long Silver Nanowire Based Transparent, Flexible and Conductive Composite Film and Its Temperature-Dependent Resistance,” Int. J. Optomechatronics 13(1), 41–50 (2019). [CrossRef]
13. N. C. Tansil, L. D. Koh, and M. Y. Han, “Functional Silk: Colored and Luminescent,” Adv. Mater. 24(11), 1388–1397 (2012). [CrossRef]
14. M. Q. Chu and G. J. Liu, “Fluorescent Silkworm Silk Prepared Via Incorporation of Green, Yellow, Red, and near-Infrared Fluorescent Quantum Dots,” IEEE Trans. Nanotechnol. 7(3), 308–315 (2008). [CrossRef]
15. S. Putthanarat, R. K. Eby, R. R. Naik, S. B. Juhl, M. A. Walker, E. Peterman, S. Ristich, J. Magoshi, T. Tanaka, M. O. Stone, B. L. Farmer, C. Brewer, and D. Ott, “Nonlinear Optical Transmission of Silk/Green Fluorescent Protein (Gfp) Films,” Polymer 45(25), 8451–8457 (2004). [CrossRef]
16. N. B. Lin, F. Hu, Y. L. Sun, C. X. Wu, H. Y. Xu, and X. Y. Liu, “Construction of White-Light-Emitting Silk Protein Hybrid Films by Molecular Recognized Assembly among Hierarchical Structures,” Adv. Funct. Mater. 24(33), 5284–5290 (2014). [CrossRef]
17. N. Qi and B. Zhao, “Preparation and characterization of β-NaYF4:Yb, Er composite fluorescence silk fibroin film,” Textile Auxiliaries. 34(8), 15–19 (2017). [CrossRef]
18. X. X Zhao, F. Zhao, and Y. Luo, “Graphene quantum dots in biomedical applications:challenges and perspectives,” International Journal of Laboratory Medicine 1(4), 192–199 (2017). [CrossRef]
19. Z. J. Fan, Y. Y. Nie, Y. Wei, J. Y. Zhao, X. Z. Liao, and J. X. Zhang, “Facile and Large-Scale Synthesis of Graphene Quantum Dots for Selective Targeting and Imaging of Cell Nucleus and Mitochondria,” Mater. Sci. Eng., C 103, 109824 (2019). [CrossRef]
20. A. Tok, T. D. Nguyen, O. Geuli, L. P. Yeo, D. Mandler, and S. Magdassi, “Additive-Free Electrophoretic Deposition of Graphene Quantum Dots Thin Films,” Chemistry – A European Journal (2019).
21. H. L. Tran and R. A. Doong, “Sustainable Fabrication of Green Luminescent Sulfur-Doped Graphene Quantum Dots for Rapid Visual Detection of Hemoglobin,” Anal. Methods 11(35), 4421–4430 (2019). [CrossRef]
22. Y. H. Wang, W. Y. Weng, H. Xu, Y. Luo, D. Guo, D. Li, and D. W. Li, “Negatively Charged Molybdate Mediated Nitrogen-Doped Graphene Quantum Dots as a Fluorescence Turn on Probe for Phosphate Ion in Aqueous Media and Living Cells,” Anal. Chim. Acta 1080, 196–205 (2019). [CrossRef]
23. X. F. Wang, G. G. Wang, J. B. Li, Z. Liu, W. F. Zhao, and J. C. Han, “Towards High-Powered Remote Wled Based on Flexible White-Luminescent Polymer Composite Films Containing S, N Co-Doped Graphene Quantum Dots,” Chem. Eng. J. 336, 406–415 (2018). [CrossRef]
24. L. Wang, B. Wu, W. T. Li, Z. Li, J. Zhan, B. J. Geng, S. L. Wang, D. Y. Pan, and M. H. Wu, “Industrial Production of Ultra-Stable Sulfonated Graphene Quantum Dots for Golgi Apparatus Imaging,” J. Mater. Chem. B 5(27), 5355–5361 (2017). [CrossRef]
25. P. Feng, B. J. Geng, Z. Cheng, X. Y. Liao, D. Y. Pan, and J. Y. Huang, “Graphene Quantum Dots-Induced Physiological and Biochemical Responses in Mung Bean and Tomato Seedlings,” Rev. Bras. Bot. 42(1), 29–41 (2019). [CrossRef]
26. N. Qi, B. Zhao, S. D. Wang, S. S. Al-Deyabc, and K. Q. Zhang, “Highly Flexible and Conductive Composite Films of Silk Fibroin and Silver Nanowires for Optoelectronic Devices,” RSC Adv. 5(63), 50878–50882 (2015). [CrossRef]
27. C. R. Yen, X. L. Hu, P. Guan, T. T. Hou, P. Chen, D. W. Wan, X. L. Zhang, J. Wang, and C. L. Wang, “Highly Biocompatible Graphene Quantum Dots: Green Synthesis, Toxicity Comparison and Fluorescence Imaging,” J. Mater. Sci. 55, 1198–1215 (2019). [CrossRef]
28. Q. Wang, C. Y. Wang, M. C. Zhang, M. Q. Jian, and Y. Y. Zhang, “Feeding Single-Walled Carbon Nanotubes or Graphene to Silkworms for Reinforced Silk Fibers,” Nano Lett. 16(10), 6695–6700 (2016). [CrossRef]
29. S. J. Ling, Z. M. Qi, D. P. Knight, Z. Z. Shao, and X. Chen, “Synchrotron Ftir Microspectroscopy of Single Natural Silk Fibers,” Biomacromolecules 12(9), 3344–3349 (2011). [CrossRef]
30. C. X. Xie, W. J. Li, Q. Q. Liang, S. T. Yu, and L. Li, “Fabrication of Robust Silk Fibroin Film by Controlling the Content of Beta-Sheet Via the Synergism of Uv-Light and Ionic Liquids,” Appl. Surf. Sci. 492, 55–65 (2019). [CrossRef]
31. Q. Wang, S. J. Ling, X. P. Liang, H. M. Wang, H. J. Lu, and Y. Y. Zhang, “Self-Healable Multifunctional Electronic Tattoos Based on Silk and Graphene,” Adv. Funct. Mater. 29(16), 1808695 (2019). [CrossRef]
32. S. J. Ling, Q. Zhang, D. L. Kaplan, F. Omenetto, M. J. Buehler, and Z. Qin, “Printing of Stretchable Silk Membranes for Strain Measurements,” Lab Chip 16(13), 2459–2466 (2016). [CrossRef]
33. D. D. Chen, D. R. Wang, Y. Yang, Q. Y. Huang, S. J. Zhu, and Z. J. Zheng, “Self-Healing Materials for Next-Generation Energy Harvesting and Storage Devices,” Adv. Energy Mater. 7(23), 1700890 (2017). [CrossRef]
34. G. H. Altman, F. Diaz, C. Jakuba, T. Calabro, R. L. Horan, J. Chen, H. Lu, J. Richmond, and D. L. Kaplan, “Silk-based biomaterials,” Biomaterials 24(3), 401–416 (2003). [CrossRef]
