Research Article pubs.acs.org/journal/ascecg
Diet Culture Inspired Facile Nanoengineering Fengshun Li,† Miaoxing Liu,‡ Xiangwei Song,‡ Chaowen Xue,† Fanrong Ai,† Chen Li,§ Hongbo Xin,† and Xiaolei Wang*,†,‡ †
Institute of Translational Medicine, NanChang University, Hong Gu Tan New District, 1299 XueFu Road, NanChang 330088, China ‡ College of Chemistry, NanChang University, Hong Gu Tan New District, 1299 XueFu Road, NanChang 330088, China § Department of Orthopedic Surgery, The Second Affiliated Hospital of Nanchang University, 1 minde Road, NanChang 330006, China S Supporting Information *
ABSTRACT: Inspired from some time-honored recipes, we modified the current classical nanoengineering processes to explore some new features. The synthesis of silver nanoparticles (Ag NPs) was selected here as the model system. Inspired from the distilling technology of white spirits (Er Guo Tou), the size and properties of the obtained Ag NPs could be adjusted effectively without any additional agent. Moreover, according to the characteristic of a flowers jelly, polyethylene glycol encapsulated Ag was synthesized, which exhibited longer storage, lower skin toxicity, and unique touch triggered releasing features. Its relative applications on virtual reality (VR) glasses were also demonstrated with the aid of a 3D printing constructed paintbrush. The proposed strategy has a certain universality, which can also be applied to other kinds of nanomaterials. This study not only explored a novel nanoengineering conception but also solved some common problems in the current nanomaterials, such as poor dispersion, easy agglomeration, and high toxicity. More importantly, the whole process could be accomplished in an environmentally friendly and energy-saving manner, which thus paved the way for a green avenue to explore functional nanomaterials. KEYWORDS: Nanoengineering, Antibacterial, Silver Nanoparticles, Low toxic, 3D printing
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INTRODUCTION
the release spontaneously consistent with the actual frequency of usage?
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Diet is the paramount material base for humans to survive. Only when men began to use fire on food, they entered into the age of civilization. Nowadays, with the development of food culture, a simple ingredient, such as chicken, could be cooked by a variety of methods. It is the result that human beings have been experimenting with and developing for thousands of years. Compared with this time-honored catering art, nanotechnology, which only had a short development for several decades, is still in its infancy. There is still much room for its improvement. In this article, we derived inspiration from two typical traditional Chinese diet techniques, and for the first time, we explored these unique cooking ideas in the optimization of nanomaterials’ preparation. The preparation of silver nanoparticles (Ag NPs) was selected as the model system. Ag NPs are a relatively well investigated nanomaterial with broad-spectrum antibacterial activities.1−5 However, despite numerous efforts having been devoted to the improvement of this material, there were still several important problems that needed to be addressed. For instance, how do you prevent the aggregation of the Ag NPs solution during long time storage? How do you adjust the diameter of Ag NPs without extra toxic chemical reagents?6,7 Furthermore, in practical usage, how do you reduce its toxicity to human skin during direct contact? More ambitiously, is it possible to realize the intelligent release of this material, making © 2017 American Chemical Society
EXPERIMENTAL SECTION
Preparation of the Second Dispersed (TSD) Ag NPs. First, 3 g of PEG 4000 and 1 g of soluble starch was dissolved in 50 mL of deionized water in turn and heated with stirring for a few minutes. To form a homogeneous mixture solution (A), it was saved in a 4 °C refrigerator for at least 3 h. Second, 3 g of PEG 4000, 2.5 g of AgNO3, and 1 g of soluble starch were dissolved in turn in 100 mL of deionized water. It must be heated with stirring for 5 min after each reagent was dissolved in the solution (B). Then the reaction was placed in an ultrasonic power (560 W, temperature 70 °C) for 5 min. Third, 50 mL of the solution was take from B to A. Lastly, they were ultrasounded together for a few minutes. Preparation of the Painting. 5 g of PEG 1000 and 7 mL of PEG 600 were mixed and heated with stirring until melted. Then, 1 mL of TSD was added into the mixture and heated with stirring until the color turned black. Antibacterial Activity. First, 5 mL of Luria−Bertani (LB) broth, 100 μL of the as-prepared bacterial suspension, and 1 mL of sample were added into a 10 mL tube by turn. As the control, PBS was used instead of samples. Then, the tubes were placed in an orbital shaker at 37 °C for 6 h. Second, the mixture was diluted for different degrees, and 50 μL of the Received: May 15, 2017 Revised: June 28, 2017 Published: July 24, 2017 7979
DOI: 10.1021/acssuschemeng.7b01522 ACS Sustainable Chem. Eng. 2017, 5, 7979−7984
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Figure 1. (a) Schematic of the synthetic route of the second dispersed Ag NPs. The inset was the source of the inspiration. Typical TEM image and corresponding size distribution histogram (inset) of (b) TFD and (c) TSD. (d) XRD patterns of the TFD and TSD. The asterisk (*) represents the weak diffraction peaks of TFD. (e) One typical group of the antibacterial results of TFD and TSD against S. aureus.
Figure 2. (a) Photograph of a flowers jelly; carrageenan made triangular jelly (b) was melted and formed a circular disk (d); after the temperature changed repeatedly, the color of pure Ag NPs in aqueous solution changed (e and f); corresponding UV−vis absorption spectra changes (g); the color of PEG-Ag changed (h and i) after repeatedly changing the temperature; corresponding UV−vis absorption spectra changes (j). uniform solution was coated on the solid LB agar. Plate counting method was used for both TSD and antibacterial stability of PEG-Ag.8 Release Profiles. A total of 0.009 g of methylene blue, as the alternative of Ag, was dissolved in 100 mL of deionized water, and the mixture was stirred until completely dissolved. Then 5 g of PEG 1000 and 7 mL of PEG 600 were heated with stirring. Next, we put 1 mL of methylene blue solution into the PEG mixed solution. Finally, it was saved in a 4 °C refrigerator after stirring well. We took a few grams each time.
NPs in the dispersant (Figure. 1a). By which means, the concentration of Ag NPs precursors was dramatically reduced, which effectively depressed the relative Ostwald ripening behavior in the dispersant.9,10 In the hydrothermal process, the reaction temperature, reaction time, and many other conditions can change the particles’ size. “Once dispersion” is the representation of the ordinary method, where the size of particles becomes larger and larger due to intermolecular collisions. When the “twice dispersion” step was added, the established growth pattern was disrupted. This suddenly decreased concentration made nanoparticles disperse better,11,12 so that the size and properties of the obtained Ag NPs could be adjusted simply without changing the protective agent or the reducing agent. From the pictures of transmission electron microscopy, we can see the size of the second dispersed (TSD) was about 2.74 ± 0.73 nm, accounting for 70% of the merely one dispersed sample (TFD, 3.94 ± 1.25 nm) under the same
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RESULTS AND DISCUSSION Characterization of TSD Ag NPs. Our research started with a famous Chinese liquor brand: Er Guo Tou (inset of Figure 1a). Different than the preparation of traditional liquor, Er Guo Tou emphasized its unique “second distilled process”, which resulted in a delicate and subtle flavor. Inspired by this manufacturing feature, we added a second dispersed process to the nucleated Ag 7980
DOI: 10.1021/acssuschemeng.7b01522 ACS Sustainable Chem. Eng. 2017, 5, 7979−7984
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Figure 3. Antibacterial studies of PEG-Ag against (a) E. coli and (b) S. aureus; after each alternative heating and cooling treatment; (c) release profiles of PEG in different temperatures; (d) release profiles of PEG in different media. Each experiment was repeated at least 3 times.
Aiming at the problem of agglomeration, we got inspiration from flowers jelly. In place of carrageenan, polyethylene glycol (PEG) was applied here to synthesize and preserve Ag NPs. PEG is a kind of temperature sensitive material with good biocompatibility. With the aid of PEG, Ag NPs were sealed in the solid phase at room temperature. The agglomeration21 caused by the frequent collisions between Ag NPs in liquid medium thus could be avoided. Thereby the storage time of the Ag NPs could be effectively prolonged. When the temperature rose, for example, after direct contact with people, the PEG would switch from the solid state to liquid immediately, so as to release the inner Ag NPs. The purpose of touching triggered antibacterial activity and thus could be achieved. Subsequent studies indicated that these PEG protected Ag NPs (PEG-Ag) also possessed impressive stability.22,23 After the same alternate heating and cooling treatment, there was no obvious changes in UV absorption or the average particle size. The antimicrobial activity and releasing behavior of the as prepared PEG-Ag were then studied separately. After alternative heating and cooling for seven cycles, the antimicrobial activity of PEG-Ag was stable for both Gram-negative bacteria Escherichia coli (E. coli) and Gram-positive bacteria Staphylococcus aureus (S. aureus). Especially, the antimicrobial activity against S. aureus was almost 100%. It is well-known that pure Ag NPs have good inhibition against S. aureus.24,25 Herein, PEG-Ag still maintained this good property. The releasing behavior of PEG-Ag is depicted in Figure 3c. When the temperature reaches 37 °C, the encapsulated Ag exhibited obvious burst release phenomenon in 3 min. While when the temperature was stable at 29 °C, there would be no significant release phenomenon. These results indicated that PEG-Ag was highly sensitive to temperature change. To further prove this point, we carried out a uniform and slow heating treatment to the PEG-Ag system in the new parallel experimental group (initial temperature = 29 °C, rising 2°/min). In this case, the release rate of heated PEG-Ag was obviously improved 3 min later. One thing should be noted was that artificial sweat could increase the releasing rate of PEG-Ag significantly (Figure 3d). Portable electrical devices, such as smart phones, wearable bracelets, and VR glass have intensively proliferated in the last three years. Due to the long contact with
conditions (Figure 1b,c). This result agreed with the relative XRD test. Compared to TFD results, the XRD spectrum of the TSD sample was not obvious, which could be explained by the smaller size induced lower atomic ordering.13 More interestingly, we also found that, in addition to size differences, the antimicrobial properties of TSD Ag NPs for S. aureus were also significantly better than those for TFD Ag NPs. On the other hand, according to the E. coli tests, the antimicrobial diversities between TSD Ag NPs and TFD Ag NPs were not obvious. Such antibacterial test results have been repeated over three times, but its detailed mechanism remains to be further explored. We hypothesized that this might be related to the differences in cell membranes composition between Gram-negative E. coli and Gram-positive S. aureus.14 In addition, we further prepared Ag NPs by dispersion for three times. The corresponding particle size statistics showed that the average size of “the third dispersed” Ag NPs was about 2.97 ± 1.27 nm. Comparing with TSD, the diameters of Ag NPs were not significantly changed. However, the concentration of the obtained Ag NPs was too low to carry out the next experimental steps. In this case, TSD Ag NPs were thus recommended for further study (Figure S1). Characterization of PEG-Ag. The most interesting part of this study was inspired by flowers jelly, which (Figure 2a) is an elegant and beautiful dessert. The method of making jelly is not complicated, which mainly uses the solid−liquid switch of carrageenan in different temperatures (Figure 2b,c). Foods (such as flowers) thus can be saved in a transparent gel. As fragile as flowers, some small silver nanoparticles with high activity are easy to agglomerate15 after long-term storage. Some Ag NPs were also especially sensitive to temperature changes.16,17 As a practical example, average 1.95 ± 0.56 nm sized Ag NPs were prepared through a classical green synthesis protocol by using starch and glucose (Figure 2e).18,19 These small sized particles were then treated with alternate heating (65 °C for 15 min) and cooling (4 °C for 50 min) tests for 7 independent cycles. After the whole process was completed, the color of the solution was obviously deepened (Figure S3a).20 The subsequent TEM study revealed that the average size of Ag nanoparticles was increased to about 3.13 ± 0.85 nm (Figure 2f), and the corresponding UV absorption was also significantly increased (Figure 2g). 7981
DOI: 10.1021/acssuschemeng.7b01522 ACS Sustainable Chem. Eng. 2017, 5, 7979−7984
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more directly animal skin experiments. The results showed that this dual ink could quickly show significant color changes after contact with the skin within 10 min (Figure 5b). Furthermore, after coveraging of PEG-Ag based inks over 10 h, the skin still remained smooth without any observable lesions (Figure 5c). The results of H&E staining also showed no significant difference with the original group. However, under the same conditions, pure Ag NPs based ink quickly oxidized the skin surface and formed a certain amount of dark spots. The above results indicated that PEG not only has the characteristics of temperature controlled release but also reduces the potential toxicity of high active Ag NPs effectively,28,29 which is suitable for surface modification of various kinds of portable electronic devices.30 As a practical application, we applied this dual-ink to the surface of virtual reality (VR) glass. A crosssectional micrograph was processed by pseudo color; the upper layer was the temperature sensitive ink, and the lower layer was the PEG-Ag layer (Figure 6a). Infrared imaging proved that, after 20 min wearing, the surface of VR glass quickly rised from 18 to 34 °C. At the same time, Ag NPs in PEG-Ag can be released quickly, cleaning up the harmful microorganisms directly at the contact sites (inset of Figure 6e). Meanwhile, a remarkable color change appeared as a reminder of releasing.
the skin, coupled with the infiltration of sweat and the appropriate temperature, the surfaces of these devices are prone to bacterial enrichment,26,27 which may become the main media of various of infectious diseases in the future. However, the above results showed that the as prepared PEG-Ag can be used as an intelligent coverage of these devices with contact releasing characteristics. When not wearing them, PEG can achieve the long-term preservation of Ag NPs, while in intense exercise (temperature rising and sweating), a large number of antibacterial NPs could be spontaneously released to suppress the device surface bacteria and odor. Owing to its impressive antibacterial stability, the use of PEG-Ag is repeated and sustainable. Preparation of the 3D Customized “Paintbrush”. For the convenience of use, we utilized the three-dimensional printing technology to design and produce a “paintbrush”, in which PEG-Ag acted as the ink. Figure 4a−d shows the
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CONCLUSIONS In summary, diet is one of the oldest cultures and technologies of humankind. Numerous recipes are repeated or modified by billions of people every day, most of which can be accomplished in a regular laboratory without expensive instruments or large energy consumption. Abundant feasible experiences are thus accumulated. By using a second dispersion process, the average diameter of the Ag was remarkably decreased. In order to prove the universality of this “second dispersion”, we routinely synthesized Au nanoparticles. Subsequently, the diameter of the obtained Au nanoparticles also reduced from 15.59 ± 2.02 nm (classical method with single dispersion process) to about 14.16 ± 1.84 nm. In addition, the dispersion of the nanoparticles was also improved (Figure S6). Moreover, we further extended the TSD method to the preparation of silver nanocubes and gold nanorods. As a result, the proposed TSD strategy also exhibited a significant effect on the size and morphology of the obtained nanoparticles (Figures S7 and S8). Therefore, TSD could be a general strategy to modulate the size or shape of various inorganic metal nanoparticles. On the other hand, the asprepared PEG-Ag exhibited longer storage, lower skin toxicity, and unique touch triggered releasing features. Preparation of Ag NPs is recognized as an extensively investigated area of nanomaterials. However, the present study indicated that, even some “old” areas for nanomaterial technology, are perhaps still “naive” for diet culture. More importantly, the present study also reminded us of some valuable contents of the diet culture in green manufacturing and sustainable chemistry. All of the proposed processes could be accomplished in an environmentally friendly and energy-saving manner, which thus paves the way for a green avenue to explore functional nanomaterials. With proper enlightenment, there are still many interesting recipes that can be further explored by current material researchers, such as “vodka” (deliberate distilled production), “hui guo rou” (double cooked meat by using alternative oil and water heating), and fried ice cream.
Figure 4. (a) Schematic of disassembled modules of the paintbrush; (b) schematic of a paintbrush that has been assembled, including 1, 2, double 4, and 5; (c and d) the paintbrush with different combinations; (e) the practical photograph of the four modules of the paintbrush fabricated via 3D printing; (f) photograph of paintbrush when used with dual ink equally. Inset is the top view.
schematics of the paintbrush. Through the combination among different modules, 2−3 types of ink could be painted simultaneously with defined proportions and stacking order. Figure 4e shows the optical photograph of the corresponding module combination when combining two kinds of ink equally. Apart from PEG-Ag ink, we added a temperature sensitive ink to the other module. The color-changing range agreed with the PEG-Ag release temperature, so as to indicate the releasing of PEG-Ag visually. Animal Tests of Ink. The biocompatibility of this dual ink painting was also evaluated. According to the “3Rs” principle of animal experimental management, it refers to the replacing and reducing of animals in experiments and refining procedures to make them less harmful. We should start the experiment on the cellular level. However, when the cck8 experiment was performed, the experimental reliability interfered by PEG caused flocculent substances (Figure S2). Accordingly, we had to use 7982
DOI: 10.1021/acssuschemeng.7b01522 ACS Sustainable Chem. Eng. 2017, 5, 7979−7984
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Figure 5. (a) Without any treatment, the photo of the exposed smooth skin of a mouse back and the relative H&E stained image; (b) PEG-Ag and temperature sensitive ink were put in the back of the mouse, fixed with transparent medical film; (c) photo and the relative H&E image of the mouse back after treatment with PEG-Ag containing dual ink over 10 h; (d) photo of the back of the mouse after treatment with Ag NPs for about 20 min, where many black spots appeared.
Figure 6. (a) Cross-section micrograph of the dual-ink; photograph before wearing at room temperature (b) and corresponding infrared thermal image (c); photo of it being used (d); after wearing (e); and corresponding infrared thermal image (f).
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ASSOCIATED CONTENT
Notes
S Supporting Information *
The authors declare no competing financial interest.
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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b01522. Additional experimental details as discussed in the text. (PDF)
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ACKNOWLEDGMENTS
This work was supported by the National Natural Science Foundation of China (21461015 to X.W., 81270202 and 91339113 to H.X., and 51102131 to F.A.); National Key Basic Research Program of China (2013CB531103 to H.X.); the Project of Science and Technology of Jiangxi Provincial Education Department (Nos. KJLD14010 and 20153BCB23035 to X.W. and 20142BAB216033 to F.A.); the Major Program of Natural Science Foundation of Jiangxi Province (No. 20161ACB21002 to X.W.), and Nanchang
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Xiaolei Wang: 0000-0003-3403-1174 7983
DOI: 10.1021/acssuschemeng.7b01522 ACS Sustainable Chem. Eng. 2017, 5, 7979−7984
Research Article
ACS Sustainable Chemistry & Engineering
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University Seed Grant for Biomedicine. The abstract graphic is painted by Fengshun Li.
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