- Email: [email protected]
Facile synthesis of gold nanorod-decorated silk fibroin spheres with enhanced NIR-sensitive photo-thermal activity
Optik - International Journal for Light and Electron Optics 188 (2019) 193–199
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.com/locate/ijleo
Original research article
Facile synthesis of gold nanorod-decorated silk fibroin spheres with enhanced NIR-sensitive photo-thermal activity
T
⁎
Li Guo, Song Chen
College of Materials Science and Engineering, Taiyuan University of Technology, Taiyuan, 030024, China
A R T IC LE I N F O
ABS TRA CT
Keywords: Gold nanorods Silk fibroin Composite spheres Photo-thermal activity
Novel gold nanorod-decorated silk fibroin (SF@GNR) spheres were synthesized through in situ decoration of gold nanorod (GNRs) on silk fibroin (SF) spheres. SF spheres were synthesized by freezing and deforesting the SF/ethanol solution and then incubated in GNR suspension at the concentration of 0–0.4 mg/mL to produce SF@GNR spheres. Microstructure and photo-thermal activity of the resultant SF@GNR spheres were evaluated. SEM and TEM observations revealed that the SF@GNR spheres were spherical in shape with the diameter of 1068 ± 98 nm. Their surface was decorated with lots of GNRs. XRD patterns showed that SF@GNR spheres had the typical peaks at 38°, 44° and 64° for GNR components and 21° and 24° for SF components. The formation of SF@GNR spheres was mainly attributed to the electrostatic interaction between negatively charged SF spheres and positively charged GNRs. Under the irradiation of 808 nmnear-infrared (NIR) light, the temperature of the suspension of SF@GNR spheres was increased from 25 °C to 50 °C, indicating that the SF@GNR spheres were NIR-sensitive and showed the photo-thermal activity. Moreover, it was found that the photo-thermal activity of SF@GNR spheres were controllable by tuning length/width aspect ratio of GNRs and concentration of GNRs in the staring system.
1. Introduction Photo-thermal therapy has received an increasing interest in the treatment of cancers [1,2]. Under the irradiation of the nearinfrared (NIR) lights, the photo-thermal materials convert the light energy to heat energy for killing the cancer cells. NIR light could penetrate the human tissues without serious effect [1,2]. Therefore, NIR-sensitive photo-thermal materials have received considerable attention in the photo-thermal therapy. Gold nanorods (GNRs) are one type of the most popular photo-thermal materials and show strong NIR-sensitive photo-thermal activity because of their unique isotropic crystalline structure and strong surface plasmon resonance in the region of the NIR light [3]. GNRs are normally synthesized via the seed-growth route with the assistance of cetyltrimethylammonium bromide (CTAB) as stabilizer and show the controlled size and well-defined rod-like morphology [4]. However, it has been extensively demonstrated that GNRs present strong cytotoxicity due to the presence of CTAB on their surface and a direct application of GNRs is thus not recommended for photo-thermal therapy [5]. Therefore, GNRs have been extensively utilized as functional components and doped in or immobilized on other types of materials such as hydroxyapatite [4], silica spheres [5], sodium alginate [6], and polyelectrolyte [7] to produce novel composites with excellent photo-thermal activity for photothermal therapy. Silk fibroin (SF) is the one of the structural components in the natural worm and shows excellent biocompatibility and ⁎
Corresponding author. E-mail address: [email protected] (S. Chen).
https://doi.org/10.1016/j.ijleo.2019.05.058 Received 13 March 2019; Accepted 20 May 2019 0030-4026/ © 2019 Elsevier GmbH. All rights reserved.
Optik - International Journal for Light and Electron Optics 188 (2019) 193–199
L. Guo and S. Chen
Fig. 1. Schematic illustration of preparation of SF, SF spheres, and SF@GNR spheres.
biodegradability [8]. Compared with the other well-known biopolymers such as chitosan [9], collagen [10], and gelatin [11], SFbased materials have unique aqueous stability due to the presence of β-sheet crystalline structure [8]. Thus, SF-based materials have found a variety of applications such as drug delivery system [9] and tissue regeneration [10]. Various forms of SF-based materials have been synthesized, including SF spheres [11], SF nanofibers [12], SF membranes [13], and SF scaffolds [14]. Compared with other forms of SF materials, SF spheres could be easily synthesized through simply freeze-drying and deforesting the ethanol/SF solution and showed the well-defined spherical morphology and tailored size [15]. Recent researches indicate that SF spheres could be utilized as supportive platform for immobilization of various therapeutic molecules such as anti-cancer drugs [16], genes [17], and antibiotics [18]. Inspired by those studies, the SF spheres would be also highly expected as supportive platform for immobilization of GNRs. However, to the best of our knowledge, the immobilization of GNRs on SF spheres has not been explored. In this study, the novel SF@GNR spheres were synthesized through immobilization of GNRs on SF spheres. SF spheres were prepared from ethanol and SF via a freeze-drying and deforesting route, while GNRs were synthesized via the seed-growth route with the assistance of CTAB. Microstructure of the obtained SF@GNR spheres was examined due to transmission electron microscopy and X-ray diffractometry. Photo-thermal activity of SF@GNRs was evaluated by monitoring the temperature change in the suspension of SF@GNR spheres under the irradiation of 808-nm NIR light.
2. Materials and methods 2.1. Preparation of SF solution SF was extracted from natural silk fibers (Fig. 1). Briefly, appropriate amount of raw Bombyx mori cocoons were treated in 2 L of boiled Na2CO3 solution (0.5%, w/w) at 100 oC for 30 min, taken out and washed with water to obtain the degummed silk fibers. Subsequently, the degummed silk fibers were dissolved in 136 mL of LiBr solution (9.3 mol/L) at 40 °C for 2 h and then filtrated. SF solution was then dialyzed against water over a period of 3 days at room temperature and freezing-dried. The obtained SF sponge was dissovled in water to obtain SF solution (3%, w/v).
2.2. Synthesis of GNRs GNRs were synthesized via a conventional seed-mediated growth route as described in our previous study [4]. Briefly, 5.0 mL of CTAB solution (0.2 mol/L) was mixed with 5.0 mL of HAuCl4 (0.5 mmol/L) in a conical flask under stirring. Then, 0.60 mL of fresh ice-cold NaBH4 solution (0.01 mol/L) was injected to the mixed solution with stirring. After 2 min, the solution was kept for 2 h at room temperature. Next, 2.5 mL of AgNO3 (2 mmol/L or 4 mmol/L), 50 mL of HAuCl4 (1 mmol/L), 0.8 mL of HCl (1 mol/L) solution, 0.7 mL of ascorbic acid (0.0788 mol/L) and 0.12 mL of gold seed solution were added successively to 50 mL of CTAB (0.2 mol/L) and the mixture was kept stirring for 2 min. The mixture was incubated in a thermostat water bath overnight. The resultant GNRs were collected by centrifugation and then re-suspended in water. According to the concentration of AgNO3, GNRs were coded as GNR02 for 2 mmol/L of AgNO3 and GNR04 for 4 mmol/L of AgNO3.
2.3. Synthesis of SF spheres SF spheres were synthesized by freezing and deforesting the ethanol/SF solution [15]. 0.25 mL of ethanol was added to 5 mL of SF solution under gentle stirring (100 rpm) at room temperature and the mixed solution was incubated at -20 °C for 18 h. Then, the frozen SF turned into a milky suspension after it was defrosted at room temperature. SF spheres were collected by centrifugation at 12,000 rpm for 30 min, washed with water, and finally lyophilized at −10 °C. 194
Optik - International Journal for Light and Electron Optics 188 (2019) 193–199
L. Guo and S. Chen
2.4. Immobilization of GNRs on SF spheres SF@GNR spheres were synthesized through in situ decoration of GNRs on SF spheres (Fig. 1). SF spheres were incubated with the suspension of GNRs (0.1 mg/mL, 0.2 mg/mL, 0.4 mg/mL) at room temperature and the mixture was kept slight shaking in a shaking bath for 10 h at room temperature to produce SF@GNR spheres. SF@GNRs composite spheres were collected by centrifugation and washed with water, and finally lyophilized at −10 °C. 2.5. Characterization Ultraviolet-Visible (UV–vis) absorption spectra were collected in the wavelength range of 400–1000 nm on a UV-8000S (Shanghai Metash, China) spectrophotometer. Transmission electron microscopy (TEM) measurements were performed on a JEM-1200EX transmission electron microscopy (JEOL, Japan) operating at 80 kV. Size and morphology of the as-synthesized spheres were observed by a JSM-7100 F field-emission scanning electron microscope (FESEM, JEOL, Japan). X-ray diffraction (XRD) patterns of the as-synthesized spheres were identified with a Bruker D8 discover X-ray diffractometer. Fourier transform infrared spectrophotometer (FT-IR) spectra were measured on a Bruker TENSOR instrument using the KBr pellet technique. Zeta-potential analysis was taken using a Malvern Zetasizer Nano ZS90 instrument. 2.6. Photo-thermal activity To measure the photo-thermal performance of SF@GNRs spheres, 10 mg of SF@GNR spheres were suspended in 0.5 mL of PBS solution placed in the quartz cuvette and irradiated with 808 nm fiber coupled diode laser, while the SF spheres dispersed in PBS solution were used as the control. The laser power was fixed at 2 W, while the beam diameter was 5 mm. Temperature was measured with a thermo coupler over a period of 12 min. 3. Results and discussion 3.1. Characterizations of GNRs GNRs were synthesized via a conventional seed-mediated growth approach as previously described [4]. As can be seen in Fig. 2, both types GNR02 (Fig. 2a) and GNR04 (Fig. 2b) exhibited rod-shaped morphology despite the concentration of AgNO3 in the growth solution. The length/width aspect ratio for GNR02 was 2.5, while that for GNR04 was 4.1. Fig. 2c shows UV spectra of the suspension of GNR02 and GNR04. Both GNR02 and GNR04 showed two peaks. One was located in the region of 500˜550 nm due to the transverse localized surface plasmon resonance (TLSR), while the other was located in the region of 600˜900 nm due to the longitudinal localized surface plasmon resonance (LSPR). The presence of both peaks further confirmed the successful synthesis of GNRs. It could be seen that by increasing the concentration of AgNO3 from 2 mmol/L to 4 mmol/L, the LSPR band red shifted dramatically from 652 nm for GNR02 to 804 nm for GNR04. Also, the solution color turned from blue for GNR02 to red for GNR04 (Inset).
Fig. 2. TEM images of (a) GNR02, (b) GNR04, and (c) UV spectra of GNR02 and GNR04. Inset images: suspensions of GNR02 and GNR04. 195
Optik - International Journal for Light and Electron Optics 188 (2019) 193–199
L. Guo and S. Chen
Fig. 3. SEM images of (a) SF spheres and their photograph (Inset) and (b) SF@GNR spheres and their photograph (Inset) and TEM images of (c) SF spheres and (d, e) SF@GNR spheres. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
3.2. Characterizations and formation mechanism of SF@GNR spheres SF spheres have been successfully prepared by self-assembling of SF in the ethanol [15]. As shown in Fig. 3(a), it is found that the obtained SF spheres exhibited spherical morphology and had the diameter of 1108 ± 281 nm. After incubation with the suspension of GNRs, the as-synthesized SF@GNR spheres also maintained the spherical morphology of SF spheres and had the similar diameter of 1068 ± 98 nm (Fig. 3b). However, a significant difference in surface morphology was clearly observed. Numerous GNRs were well deposited on the surface of SF spheres and no obvious agglomeration of GNRs was observed, indicating that SF spheres served as supportive platform for immobilization of GNRs. A significant difference in color was clearly observed between SF spheres (white, Fig. 3a, Inset) and SF@GNR spheres (Red, Fig. 3b, Inset), further confirming the successful immobilization of GNRs on the SF spheres. To further reveal the structural evolution, TEM images of both SF spheres and SF@GNR spheres were also shown in Fig. 3. SF spheres were spherical in shape with the diameter of 1184 ± 249 nm (Fig. 3c). After decoration of GNRs, SF@GNR spheres were also spherical in shape and had the similar diameter (Fig. 3d). However, a significant difference in surface morphology was observed. The surface of SF spheres was comparatively smooth, while that of SF@GNR spheres was coated with numerous GNRs. A high magnification image of SF@GNR spheres clearly showed that there was no serious agglomeration of GNRs on SF@GNR spheres (Fig. 3e). This result was consistent with the results from SEM images (Fig. 3a and b). Fig. 4 shows XRD patterns of SF spheres and SF@GNR spheres. SF spheres exhibited the characteristic peaks at 21° and 24°, both 196
Optik - International Journal for Light and Electron Optics 188 (2019) 193–199
L. Guo and S. Chen
Fig. 4. XRD patterns of SF and SF@GNR spheres.
of which were normally assigned to ß-sheet crystalline structure. After decoration of GNRs, SF@GNRs also presented the similar two peaks for ß-sheet crystalline structure. However, three new peaks at 38°, 44° and 64° was observed for SF@GNR spheres and assigned to (111), (200) and (220) diffractive plane of GNRs, respectively [4], further indicating that GNRs were decorated on SF spheres. It should be noted that decoration of GNRs has no significant effect on the crystalline structure of SF spheres. To further reveal the formation mechanism, FT-IR spectra of both SF spheres and SF@GNR spheres were shown in Fig. 5. SF spheres showed the characteristic peaks at 1632 cm−1 (eC]O) and 1525 cm−1 (eNeH), both of which were assigned to the β-sheet structure. SF@GNR spheres have the similar spectrum. No significant peak shift was observed, implying that the interaction between SF spheres and GNRs had no chemical reaction. This would strongly suggest that the GNRs were physically fixed to the SF sphere surface. Actually, both had different zeta potentials: i.e., +32 mV for GNRs and −13.5 mV for SF spheres. Thus, such a large difference in the potential should conclude that SF@GNR spheres were formed primarily due to the electrostatic interaction, the route of which was schematically illustrated in Fig. 6. 3.3. Photo-thermal activity Fig. 7 shows the temperature curves for both SF spheres and SF@GNR spheres exposed to the irradiation of NIR light. When the irradiation time was increased from 0 to 12 min, no significant different increase in temperature was found for SF spheres, indicating that SF spheres were not NIR-sensitive and had no photo-thermal activity. However, irrespective of the type of SF@GNR spheres, the
Fig. 5. FT-IR spectra of SF spheres and SF@GNR spheres. 197
Optik - International Journal for Light and Electron Optics 188 (2019) 193–199
L. Guo and S. Chen
Fig. 6. A possible formation mechanism for SF@GNR spheres and their NIR-sensitive photo-thermal activity.
Fig. 7. Temperature curves of the suspension of SF spheres and SF@GNR spheres after exposed to irradiation of 808-nm NIR light.
temperature was increased, indicating that SF@GNR spheres were NIR-sensitive and had photo-thermal activity (Fig. 6). Because of insensitivity of SF to NIR, the photo-thermal activity of SF@GNR spheres should be attributed to the GNRs. Moreover, Fig. 7 confirmed that SF@GNR04 induced higher maximum temperature (47 °C) than the SF@GNR02 (45 °C) at the same concentration of GNRs of 0.2 mg/mL. This concludes that the larger photo-thermal activity for SF@GNR04 than that for SF@GNR02. It would be natural that the larger content (0.4 mg/mL) of the SF@GNR04 derived the higher maximum temperature (50 °C). Therefore, the photo-thermal activity of SF@GNR spheres was dependent on the length/width ratio of GNRs and concentration of GNRs in the starting reaction system. It has been extensively accepted that the temperature more than 42 °C is useful to kill the cancer cells [1–3]. From the temperature curve, the present SF@GNR spheres would be applicable to be photo-thermal materials. 4. Conclusions In conclusion, NIR-responsive SF@GNRs spheres were prepared successfully by a simple but efficient route based on the electrostatic interactions between SF spheres and GNRs. Under an 808 nm laser irradiation, the SF@GNRs spheres were NIR-sensitive and their photo-thermal activity could be tailored through adjustment of length/width aspect ratio of GNRs and the concentration of GNRs in the starting reaction solution. Acknowledgements We gratefully acknowledge the financial support from the Shanxi Scholarship Council of China (Grant No. 2016-024) and Natural Science Foundation of Shanxi Province (Grant No. 201801D121087). References [1] Y. Chen, L. Wang, J. Shi, Two-dimensional non-carbonaceous materials-enabled efficient photothermal cancer therapy, Nano Today 11 (2016) 292–308.
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[2] Y.N. Liu, K.L. Ai, J.H. Liu, M. Deng, Y.Y. He, L.H. Lu, Dopamine-melanin colloidal nanospheres: an efficient near-infrared photothermal therapeutic agent for in vivo cancer therapy, Adv. Mater. 25 (2013) 1353–1359. [3] X.H. Huang, I.H. El-Sayed, W. Qian, M.A. El-Sayed, Cancer cell imaging and photothermal therapy in the near-infrared region by using gold nanorods, J. Am. Chem. Soc. 128 (2006) 2115–2120. [4] S. Chen, X.X. Du, T.L. Wang, L. Jia, D. Huang, W. Chen, Synthesis of near-infrared responsive gold nanorod-doped gelatin/hydroxyapatite composite microspheres with controlled photo-thermal property, Ceram. Int. 44 (2018) 900–904. [5] S. Shen, H. Tang, X. Zhang, J. Ren, W. Yang, Targeting mesoporous silica-encapsulated gold nanorods for chemo-photothermal therapy with near-infrared radiation, Biomaterials 34 (2013) 3150–3158. [6] K. Mitamura, T. Imae, N. Saito, O. Takai, Fabrication and structure of alginate gel incorporating gold nanorods, J. Phys. Chem. C 112 (2008) 416–422. [7] H. Chen, Y. Di, D. Chen, K. Madrid, M. Zhang, C. Tian, L. Tang, Y. Gu, Combined chemo- and photo-thermal therapy delivered by multifunctional theranostic gold nanorod-loaded microcapsules, Nanoscale 7 (2015) 8884–8897. [8] Y. Qi, H. Wang, K. Wei, Y. Yang, R.Y. Zheng, I.S. Kim, K.Q. Zhang, A review of structure construction of silk fibroin biomaterials from single structures to multilevel structures, Int. J. Mol. Sci. 18 (2017) 237. [9] H. Knidri, R. Belaabed, A. Addaou, A. Laajeb, A. Lahsini, Extraction, chemical modification and characterization of chitin and chitosan, Int. J. Biol. Macromol. 120 (2018) 1181–1189. [10] A. Ferreira, P. Gentile, V. Chiono, G. Ciardelli, Collagen for bone tissue regeneration, Acta Biomater. 8 (2012) 3191–3200. [11] E. Wenk, A.J. Wandrey, H.P. Merkle, L. Meinel, Silk fibroin spheres as a platform for controlled drug delivery, J. Control. Release 132 (2008) 26–34. [12] X. Zhang, M.R. Reagan, D.L. Kaplan, Electrospun silk biomaterial scaffolds for regenerative medicine, Adv. Drug Deliv. Rev. 61 (2009) 988–1006. [13] Y. Cai, J. Guo, C. Chen, C. Yao, S.M. Chuang, J. Yao, I.S. Lee, X. Kong, Silk fibroin membrane used for guided bone tissue regeneration, Mater. Sci. Eng. C Mater. Biol. Appl. 70 (2017) 148–154. [14] R. Nazarov, H.J. Jin, D.L. Kaplan, Porous 3-D scaffolds from regenerated silk fibroin, Biomacromolecules 5 (2004) 718–726. [15] Z. Cao, X. Chen, J. Yao, L. Huang, Z. Shao, The preparation of regenerated silk fibroin microspheres, Soft Matter 3 (2007) 910–915. [16] A. Khalid, A.N. Mitropoulo, B. Marelli, S. Tomljenovic-Hanic, F.G. Omenetto, Doxorubicin loaded nanodiamond-silk spheres for fluorescence tracking and controlled drug release, Biomed. Opt. Express 7 (2015) 132–147. [17] O. Madkhali, G. Mekhail, S.D. Wettig, Modified gelatin nanoparticles for gene delivery, Int. J. Pharm. 554 (2019) 224–234. [18] E.M. Pritchard, T. Valentin, B. Panilaitis, F. Omenetto, D.L. Kaplan, Antibiotic-releasing silk biomaterials for infection prevention and treatment, Adv. Funct. Mater. 23 (2013) 854–861.
199
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.com/locate/ijleo
Original research article
Facile synthesis of gold nanorod-decorated silk fibroin spheres with enhanced NIR-sensitive photo-thermal activity
T
⁎
Li Guo, Song Chen
College of Materials Science and Engineering, Taiyuan University of Technology, Taiyuan, 030024, China
A R T IC LE I N F O
ABS TRA CT
Keywords: Gold nanorods Silk fibroin Composite spheres Photo-thermal activity
Novel gold nanorod-decorated silk fibroin (SF@GNR) spheres were synthesized through in situ decoration of gold nanorod (GNRs) on silk fibroin (SF) spheres. SF spheres were synthesized by freezing and deforesting the SF/ethanol solution and then incubated in GNR suspension at the concentration of 0–0.4 mg/mL to produce SF@GNR spheres. Microstructure and photo-thermal activity of the resultant SF@GNR spheres were evaluated. SEM and TEM observations revealed that the SF@GNR spheres were spherical in shape with the diameter of 1068 ± 98 nm. Their surface was decorated with lots of GNRs. XRD patterns showed that SF@GNR spheres had the typical peaks at 38°, 44° and 64° for GNR components and 21° and 24° for SF components. The formation of SF@GNR spheres was mainly attributed to the electrostatic interaction between negatively charged SF spheres and positively charged GNRs. Under the irradiation of 808 nmnear-infrared (NIR) light, the temperature of the suspension of SF@GNR spheres was increased from 25 °C to 50 °C, indicating that the SF@GNR spheres were NIR-sensitive and showed the photo-thermal activity. Moreover, it was found that the photo-thermal activity of SF@GNR spheres were controllable by tuning length/width aspect ratio of GNRs and concentration of GNRs in the staring system.
1. Introduction Photo-thermal therapy has received an increasing interest in the treatment of cancers [1,2]. Under the irradiation of the nearinfrared (NIR) lights, the photo-thermal materials convert the light energy to heat energy for killing the cancer cells. NIR light could penetrate the human tissues without serious effect [1,2]. Therefore, NIR-sensitive photo-thermal materials have received considerable attention in the photo-thermal therapy. Gold nanorods (GNRs) are one type of the most popular photo-thermal materials and show strong NIR-sensitive photo-thermal activity because of their unique isotropic crystalline structure and strong surface plasmon resonance in the region of the NIR light [3]. GNRs are normally synthesized via the seed-growth route with the assistance of cetyltrimethylammonium bromide (CTAB) as stabilizer and show the controlled size and well-defined rod-like morphology [4]. However, it has been extensively demonstrated that GNRs present strong cytotoxicity due to the presence of CTAB on their surface and a direct application of GNRs is thus not recommended for photo-thermal therapy [5]. Therefore, GNRs have been extensively utilized as functional components and doped in or immobilized on other types of materials such as hydroxyapatite [4], silica spheres [5], sodium alginate [6], and polyelectrolyte [7] to produce novel composites with excellent photo-thermal activity for photothermal therapy. Silk fibroin (SF) is the one of the structural components in the natural worm and shows excellent biocompatibility and ⁎
Corresponding author. E-mail address: [email protected] (S. Chen).
https://doi.org/10.1016/j.ijleo.2019.05.058 Received 13 March 2019; Accepted 20 May 2019 0030-4026/ © 2019 Elsevier GmbH. All rights reserved.
Optik - International Journal for Light and Electron Optics 188 (2019) 193–199
L. Guo and S. Chen
Fig. 1. Schematic illustration of preparation of SF, SF spheres, and SF@GNR spheres.
biodegradability [8]. Compared with the other well-known biopolymers such as chitosan [9], collagen [10], and gelatin [11], SFbased materials have unique aqueous stability due to the presence of β-sheet crystalline structure [8]. Thus, SF-based materials have found a variety of applications such as drug delivery system [9] and tissue regeneration [10]. Various forms of SF-based materials have been synthesized, including SF spheres [11], SF nanofibers [12], SF membranes [13], and SF scaffolds [14]. Compared with other forms of SF materials, SF spheres could be easily synthesized through simply freeze-drying and deforesting the ethanol/SF solution and showed the well-defined spherical morphology and tailored size [15]. Recent researches indicate that SF spheres could be utilized as supportive platform for immobilization of various therapeutic molecules such as anti-cancer drugs [16], genes [17], and antibiotics [18]. Inspired by those studies, the SF spheres would be also highly expected as supportive platform for immobilization of GNRs. However, to the best of our knowledge, the immobilization of GNRs on SF spheres has not been explored. In this study, the novel SF@GNR spheres were synthesized through immobilization of GNRs on SF spheres. SF spheres were prepared from ethanol and SF via a freeze-drying and deforesting route, while GNRs were synthesized via the seed-growth route with the assistance of CTAB. Microstructure of the obtained SF@GNR spheres was examined due to transmission electron microscopy and X-ray diffractometry. Photo-thermal activity of SF@GNRs was evaluated by monitoring the temperature change in the suspension of SF@GNR spheres under the irradiation of 808-nm NIR light.
2. Materials and methods 2.1. Preparation of SF solution SF was extracted from natural silk fibers (Fig. 1). Briefly, appropriate amount of raw Bombyx mori cocoons were treated in 2 L of boiled Na2CO3 solution (0.5%, w/w) at 100 oC for 30 min, taken out and washed with water to obtain the degummed silk fibers. Subsequently, the degummed silk fibers were dissolved in 136 mL of LiBr solution (9.3 mol/L) at 40 °C for 2 h and then filtrated. SF solution was then dialyzed against water over a period of 3 days at room temperature and freezing-dried. The obtained SF sponge was dissovled in water to obtain SF solution (3%, w/v).
2.2. Synthesis of GNRs GNRs were synthesized via a conventional seed-mediated growth route as described in our previous study [4]. Briefly, 5.0 mL of CTAB solution (0.2 mol/L) was mixed with 5.0 mL of HAuCl4 (0.5 mmol/L) in a conical flask under stirring. Then, 0.60 mL of fresh ice-cold NaBH4 solution (0.01 mol/L) was injected to the mixed solution with stirring. After 2 min, the solution was kept for 2 h at room temperature. Next, 2.5 mL of AgNO3 (2 mmol/L or 4 mmol/L), 50 mL of HAuCl4 (1 mmol/L), 0.8 mL of HCl (1 mol/L) solution, 0.7 mL of ascorbic acid (0.0788 mol/L) and 0.12 mL of gold seed solution were added successively to 50 mL of CTAB (0.2 mol/L) and the mixture was kept stirring for 2 min. The mixture was incubated in a thermostat water bath overnight. The resultant GNRs were collected by centrifugation and then re-suspended in water. According to the concentration of AgNO3, GNRs were coded as GNR02 for 2 mmol/L of AgNO3 and GNR04 for 4 mmol/L of AgNO3.
2.3. Synthesis of SF spheres SF spheres were synthesized by freezing and deforesting the ethanol/SF solution [15]. 0.25 mL of ethanol was added to 5 mL of SF solution under gentle stirring (100 rpm) at room temperature and the mixed solution was incubated at -20 °C for 18 h. Then, the frozen SF turned into a milky suspension after it was defrosted at room temperature. SF spheres were collected by centrifugation at 12,000 rpm for 30 min, washed with water, and finally lyophilized at −10 °C. 194
Optik - International Journal for Light and Electron Optics 188 (2019) 193–199
L. Guo and S. Chen
2.4. Immobilization of GNRs on SF spheres SF@GNR spheres were synthesized through in situ decoration of GNRs on SF spheres (Fig. 1). SF spheres were incubated with the suspension of GNRs (0.1 mg/mL, 0.2 mg/mL, 0.4 mg/mL) at room temperature and the mixture was kept slight shaking in a shaking bath for 10 h at room temperature to produce SF@GNR spheres. SF@GNRs composite spheres were collected by centrifugation and washed with water, and finally lyophilized at −10 °C. 2.5. Characterization Ultraviolet-Visible (UV–vis) absorption spectra were collected in the wavelength range of 400–1000 nm on a UV-8000S (Shanghai Metash, China) spectrophotometer. Transmission electron microscopy (TEM) measurements were performed on a JEM-1200EX transmission electron microscopy (JEOL, Japan) operating at 80 kV. Size and morphology of the as-synthesized spheres were observed by a JSM-7100 F field-emission scanning electron microscope (FESEM, JEOL, Japan). X-ray diffraction (XRD) patterns of the as-synthesized spheres were identified with a Bruker D8 discover X-ray diffractometer. Fourier transform infrared spectrophotometer (FT-IR) spectra were measured on a Bruker TENSOR instrument using the KBr pellet technique. Zeta-potential analysis was taken using a Malvern Zetasizer Nano ZS90 instrument. 2.6. Photo-thermal activity To measure the photo-thermal performance of SF@GNRs spheres, 10 mg of SF@GNR spheres were suspended in 0.5 mL of PBS solution placed in the quartz cuvette and irradiated with 808 nm fiber coupled diode laser, while the SF spheres dispersed in PBS solution were used as the control. The laser power was fixed at 2 W, while the beam diameter was 5 mm. Temperature was measured with a thermo coupler over a period of 12 min. 3. Results and discussion 3.1. Characterizations of GNRs GNRs were synthesized via a conventional seed-mediated growth approach as previously described [4]. As can be seen in Fig. 2, both types GNR02 (Fig. 2a) and GNR04 (Fig. 2b) exhibited rod-shaped morphology despite the concentration of AgNO3 in the growth solution. The length/width aspect ratio for GNR02 was 2.5, while that for GNR04 was 4.1. Fig. 2c shows UV spectra of the suspension of GNR02 and GNR04. Both GNR02 and GNR04 showed two peaks. One was located in the region of 500˜550 nm due to the transverse localized surface plasmon resonance (TLSR), while the other was located in the region of 600˜900 nm due to the longitudinal localized surface plasmon resonance (LSPR). The presence of both peaks further confirmed the successful synthesis of GNRs. It could be seen that by increasing the concentration of AgNO3 from 2 mmol/L to 4 mmol/L, the LSPR band red shifted dramatically from 652 nm for GNR02 to 804 nm for GNR04. Also, the solution color turned from blue for GNR02 to red for GNR04 (Inset).
Fig. 2. TEM images of (a) GNR02, (b) GNR04, and (c) UV spectra of GNR02 and GNR04. Inset images: suspensions of GNR02 and GNR04. 195
Optik - International Journal for Light and Electron Optics 188 (2019) 193–199
L. Guo and S. Chen
Fig. 3. SEM images of (a) SF spheres and their photograph (Inset) and (b) SF@GNR spheres and their photograph (Inset) and TEM images of (c) SF spheres and (d, e) SF@GNR spheres. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
3.2. Characterizations and formation mechanism of SF@GNR spheres SF spheres have been successfully prepared by self-assembling of SF in the ethanol [15]. As shown in Fig. 3(a), it is found that the obtained SF spheres exhibited spherical morphology and had the diameter of 1108 ± 281 nm. After incubation with the suspension of GNRs, the as-synthesized SF@GNR spheres also maintained the spherical morphology of SF spheres and had the similar diameter of 1068 ± 98 nm (Fig. 3b). However, a significant difference in surface morphology was clearly observed. Numerous GNRs were well deposited on the surface of SF spheres and no obvious agglomeration of GNRs was observed, indicating that SF spheres served as supportive platform for immobilization of GNRs. A significant difference in color was clearly observed between SF spheres (white, Fig. 3a, Inset) and SF@GNR spheres (Red, Fig. 3b, Inset), further confirming the successful immobilization of GNRs on the SF spheres. To further reveal the structural evolution, TEM images of both SF spheres and SF@GNR spheres were also shown in Fig. 3. SF spheres were spherical in shape with the diameter of 1184 ± 249 nm (Fig. 3c). After decoration of GNRs, SF@GNR spheres were also spherical in shape and had the similar diameter (Fig. 3d). However, a significant difference in surface morphology was observed. The surface of SF spheres was comparatively smooth, while that of SF@GNR spheres was coated with numerous GNRs. A high magnification image of SF@GNR spheres clearly showed that there was no serious agglomeration of GNRs on SF@GNR spheres (Fig. 3e). This result was consistent with the results from SEM images (Fig. 3a and b). Fig. 4 shows XRD patterns of SF spheres and SF@GNR spheres. SF spheres exhibited the characteristic peaks at 21° and 24°, both 196
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Fig. 4. XRD patterns of SF and SF@GNR spheres.
of which were normally assigned to ß-sheet crystalline structure. After decoration of GNRs, SF@GNRs also presented the similar two peaks for ß-sheet crystalline structure. However, three new peaks at 38°, 44° and 64° was observed for SF@GNR spheres and assigned to (111), (200) and (220) diffractive plane of GNRs, respectively [4], further indicating that GNRs were decorated on SF spheres. It should be noted that decoration of GNRs has no significant effect on the crystalline structure of SF spheres. To further reveal the formation mechanism, FT-IR spectra of both SF spheres and SF@GNR spheres were shown in Fig. 5. SF spheres showed the characteristic peaks at 1632 cm−1 (eC]O) and 1525 cm−1 (eNeH), both of which were assigned to the β-sheet structure. SF@GNR spheres have the similar spectrum. No significant peak shift was observed, implying that the interaction between SF spheres and GNRs had no chemical reaction. This would strongly suggest that the GNRs were physically fixed to the SF sphere surface. Actually, both had different zeta potentials: i.e., +32 mV for GNRs and −13.5 mV for SF spheres. Thus, such a large difference in the potential should conclude that SF@GNR spheres were formed primarily due to the electrostatic interaction, the route of which was schematically illustrated in Fig. 6. 3.3. Photo-thermal activity Fig. 7 shows the temperature curves for both SF spheres and SF@GNR spheres exposed to the irradiation of NIR light. When the irradiation time was increased from 0 to 12 min, no significant different increase in temperature was found for SF spheres, indicating that SF spheres were not NIR-sensitive and had no photo-thermal activity. However, irrespective of the type of SF@GNR spheres, the
Fig. 5. FT-IR spectra of SF spheres and SF@GNR spheres. 197
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Fig. 6. A possible formation mechanism for SF@GNR spheres and their NIR-sensitive photo-thermal activity.
Fig. 7. Temperature curves of the suspension of SF spheres and SF@GNR spheres after exposed to irradiation of 808-nm NIR light.
temperature was increased, indicating that SF@GNR spheres were NIR-sensitive and had photo-thermal activity (Fig. 6). Because of insensitivity of SF to NIR, the photo-thermal activity of SF@GNR spheres should be attributed to the GNRs. Moreover, Fig. 7 confirmed that SF@GNR04 induced higher maximum temperature (47 °C) than the SF@GNR02 (45 °C) at the same concentration of GNRs of 0.2 mg/mL. This concludes that the larger photo-thermal activity for SF@GNR04 than that for SF@GNR02. It would be natural that the larger content (0.4 mg/mL) of the SF@GNR04 derived the higher maximum temperature (50 °C). Therefore, the photo-thermal activity of SF@GNR spheres was dependent on the length/width ratio of GNRs and concentration of GNRs in the starting reaction system. It has been extensively accepted that the temperature more than 42 °C is useful to kill the cancer cells [1–3]. From the temperature curve, the present SF@GNR spheres would be applicable to be photo-thermal materials. 4. Conclusions In conclusion, NIR-responsive SF@GNRs spheres were prepared successfully by a simple but efficient route based on the electrostatic interactions between SF spheres and GNRs. Under an 808 nm laser irradiation, the SF@GNRs spheres were NIR-sensitive and their photo-thermal activity could be tailored through adjustment of length/width aspect ratio of GNRs and the concentration of GNRs in the starting reaction solution. Acknowledgements We gratefully acknowledge the financial support from the Shanxi Scholarship Council of China (Grant No. 2016-024) and Natural Science Foundation of Shanxi Province (Grant No. 201801D121087). References [1] Y. Chen, L. Wang, J. Shi, Two-dimensional non-carbonaceous materials-enabled efficient photothermal cancer therapy, Nano Today 11 (2016) 292–308.
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