Letter pubs.acs.org/macroletters
Magnetite-Loaded Thermosensitive Nanogels for Bioinspired Lubrication and Multimodal Friction Control Guoqiang Liu,†,‡ Meirong Cai,† Xiaolong Wang,*,† Feng Zhou,*,† and Weimin Liu† †
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, P. R. China ‡ University of Chinese Academy of Sciences, Beijing 100049, P. R. China S Supporting Information *
ABSTRACT: The ability to control friction is quite attractive for many applications. Other than mechanical/physical methods to control friction, this letter shows how materials chemistry can regulate friction effectively. Magnetite-loaded thermosensitive poly(N-isopropylacrylamide) nanogels (Fe3O4@PNIPAM) were synthesized as nanoparticulate soft matter to reduce friction when it is used as an additive in aqueous lubricant. Interestingly, friction can be multiply regulated by temperature, magnetism, and near-infrared light through manipulating the colloidal properties of multifunctional composite nanogels in bulk solution and at the frictional interface.
A
control by external triggers,13 which is an issue of bioinspiration from nature yet superior. In this paper, we have synthesized a thermosensitive nanogel incorporated with magnetic nanoparticles. It possesses integrated responsiveness to temperature, magnetism, and near-infrared light and behaves differently in bulk solution and at that frictional interface so that it realizes effective, multiple friction regulation. Magnetic nanoparticles have gained increasing attention due to their potential applications in magnetic resonance imaging,14 targeted drug delivery,15 separation,16 photothermal therapy,17 and application in tribology.18 However, magnetically regulated friction with magnetic polymeric nanocomposites has not yet been reported. Here, magnetite-loaded thermosensitive poly(N-isopropylacrylamide) nanogels (Fe3O4@PNIPAM, abbreviated as MTNGs) were synthesized by the hydrophilic treatment of oleic acid capped Fe3O4 nanoparticles with NaIO419 and the subsequent emulsifier-free emulsion polymerization, as shown in Scheme 1a. The transmission electron microscopic (TEM) characterization shows that the monodispersed hydrophilic Fe3O4 nanoparticles have a mean size of 10 ± 2 nm in diameter (Figure S1a). After polymerization, there were some black nanoparticles in the interior of PNIPAM nanogels (Figure S1b), indicating successful encapsulation of Fe3O4 nanoparticles inside the nanogels. The composite nanogels have an average size of about 70 nm. The hydrodynamic diameter of Fe3O4 nanoparticles measured by dynamic light scattering (DLS) was 12 nm (Figure S1c), consistent with that obtained from TEM, whereas the hydrodynamic diameter of Fe3O4@
t biological interfaces, such as the eye−eyelid, organs, and intima of vessel−blood, articular cartilage usually exhibits extremely low friction (0.001−0.03),1 which is due to the soft and wet nature of tissues and interstitial fluid, where solvated water is strongly bound to polar functional groups and acts as a lubricant.2 From the viewpoint of material science, cartilage tissue is a kind of polymeric hydrogel in which a large amount of water is interposed,3 and synovial fluid is a viscous liquid that acts as a lubricant and shock absorber for the cartilage surfaces of joints.4 They both synergistically contribute to biological lubrication.5 In recent years, considerable effort has been expended to develop synthetic aqueous lubricants for biological tribo-systems, mainly hydrated soft matter as lubricating materials.4,6 Spencer et al. have systematically investigated the aqueous lubrication of poly(L-lysine)-graf t-poly(ethylene glycol)7 and pointed out the synthetic polymer lubricants inspired by biological systems may play a key role in water-based lubrication.8 Lee et al. reported a series of amphiphilic copolymers as aqueous lubricants.9 Grinstaff et al. synthesized a large-molecular-weight polyanion for highly efficacious synthetic biolubricants.4a Banquy et al. utilized a bioinspired bottle-brush polymer to achieve low friction.6d From an even broader perspective, biological friction is in substantial measure related to the hydration lubrication of interfacial cells; i.e., the frictional contact force occurs in the cellular mechanosensing ranges.10 However, biomimicking lubricating materials at the cell scale are scarcely reported. Nanogel,11 a soft nanoparticulate material, possessing remarkable colloidal stability, rheological viscoelasticity, and biocompatibility, is a promising candidate for aqueous lubrication.12 In addition to achieving ultralow friction, it is also required to know its tolerance to the surrounding environment and ideally to arrive at friction © 2016 American Chemical Society
Received: November 27, 2015 Accepted: January 5, 2016 Published: January 7, 2016 144
DOI: 10.1021/acsmacrolett.5b00860 ACS Macro Lett. 2016, 5, 144−148
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ACS Macro Letters
of MTNGs in aqueous media can be assigned to boundary lubrication (Figure S6d). The exact mechanism of nanoparticles as a lubricant for enhancing lubricating performance is still not very clear.18e,21 In our study, the good lubricating effect of the magnetite-loaded nanogels can be explained from three perspectives. After the addition of MTNGs, the hydrated layer can form surrounding the hydrophilic PNIPAM chains at room temperature (∼23 °C). Because it is difficult to extrude the hydrated layer from polar groups and due to the reverse osmotic pressure when pressing swollen polymer chains, such a hydrated sheath can support a large amount of pressure. Therefore, the hydrated layer still has a fluid manner under shear to bring about low friction. Second, the MTNGs increase the viscosity of water significantly so that the suspension can form stable elastohydrodynamic fluid film between friction pairs.22 The increase of viscosity and the formation of a boundary-absorbed layer both contributed to the good lubricating effect of MTNGs. From the perspective of the frictional mode, the spherical nanogels can serve as rolling bearings, and the friction between contacts will change from sliding friction to rolling friction; thus, the COF reduced prominently. In addition to a good lubricating effect, our particular interest was multiply mediated aqueous lubrication with intelligent nanocomposites, as illustrated in Scheme 1c. If the above proposed lubricating mechanisms are accurate, dehydration of nanogels induced by raising temperature will lead to high friction. The temperature dependence of the hydrodynamic diameter of MTNGs was investigated by the DLS technique. As shown in Figure 1a, the hydrodynamic diameter showed a marked
Scheme 1. (a) Synthesis of MTNGs, (b) H-Bonding Mechanism for the Hydration/Dehydration of PNIPAM and NIR Laser-Triggered Dehydration of the Hybrid Nanogels Using the Photothermal Effect, and (c) Regulation of Friction with Temperature, Magnetic Field, and NIR Laser Using Nanogels
PNIPAM nanogels (113 nm, Figure S1d) was much larger than that obtained from TEM, which can be attributed to the hydration of PNIPAM. The composition of MTNGs was further confirmed by fourier-transform IR (FTIR) and X-ray photoelectron spectrometry (XPS). The characteristic peak of Fe−O appeared at 578 cm−1. The broad peak at 3600−3200 cm−1 was associated with the stretching vibration of N−H groups, and the peak at 1646 cm−1 corresponded to −CO of the amide group (Figure S2). In addition, there is an obvious change in the relative element fraction of Fe 2p, O 1s, and C 1s from XPS of Fe3O4@PNIPAM (Figure S3). The FTIR and XPS evidently reveal the combination of PNIPAM nanogels and Fe3O4 nanoparticles. The crystalline structure of MTNGs by X-ray diffraction (XRD, Figure S4) exhibited a polycrystalline feature, the same as Fe3O4 nanoparticles. The quantitative composition of oxidized oleic acid, PNIPAM, and Fe3O4, measured by thermogravimetric analysis (TGA, Figure S5), is about 13.47%, 58.69%, and 27.84%, respectively, indicating a high encapsulation efficiency of Fe3O4 by nanogels. The tribological property of MTNGs in aqueous media was first investigated as a function of concentration, normal load, and sliding speed (Figure S6). Instead of using traditional hard contacts (e.g., steel−steel), the friction pairs are an upper soft PDMS (Sylgard 184, Dow Corning) hemisphere sliding against a lower silicon wafer. The PDMS hemisphere was utilized to mimic the elasticity of joint cartilage.9,20 It is clear that the coefficient of friction (COF) of MTNG suspension (0.5 g/mL) showed a marked decrease in comparison with that of pure water (from 0.087 to less than 0.05). Meanwhile, with increasing concentration or sliding speed, the COF decreased gradually to 0.02 (Figure S6a and b). In contrast, the COF showed a gradual increase with increasing the normal load (Figure S6c). From the Stribeck curve, the tribological behavior
Figure 1. (a) Hydrodynamic diameter of MTNGs as a function of temperature. The inset is a representative image of MTNGs. (b) The variation of COF as a function of temperature. The friction tests were performed at the sliding speed of 200 mm/min under the normal load of 5 N. The concentration of MTNGs suspension is 0.5 g/mL.
decrease (from ∼115 to ∼72 nm) as the temperature increased from 20 to 45 °C, dropping especially sharply around 32 °C, which was consistent with the volume phase transition temperature of PNIPAM. This collapse/swelling behavior was attributed to the hydration/dehydration transition caused by the formation and dissociation of a hydrogen bond, as illustrated in Scheme 1b. Figure 1b displayed the variation of COF as a function of temperature. When lubricated by the MTNG suspension, the COF increased gradually with the increased temperature (from ∼0.026 to ∼0.17). Especially, there was a remarkable increase around the LCST. The increased temperature destroyed the hydrated layer surrounding the MTNGs. The MTNGs changed from hydrophilic to hydrophobic. Therefore, the hydration lubrication of MTNGs was weakened, and the higher temperature led to higher COF. In other words, the COF can be regulated by the changed temperature. 145
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ACS Macro Letters In addition to heat, the magnetic field may also result in an increase of COF. The magnetic properties of MTNGs and the electromagnetic chuck were first evaluated. Figure 2a displays
heat generated around nanoparticles can be quickly transferred to PNIPAM and induces dehydration collapse of PNIPAM chains, providing a feasible and easy way to change friction. To observe the photothermal effect of the as-prepared magnetic nanogels, the nanogel suspensions with a series of concentrations were irradiated with an 808 nm laser at a power density of 2 W/cm2 in comparison with deionized water. As shown in Figure 3a, under irradiation for 10 min, the temperature of the
Figure 2. (a) Magnetic hysteresis curves of Fe3O4 nanoparticles and MTNGs. (b) The variation of COF as a function of voltage. The friction tests were performed at the sliding speed of 200 mm/min under the normal load of 5 N. The concentration of MTNG suspension was 0.5 g/mL.
the hysteresis loop of Fe3O4 and MTNGs. It is seen that the MTNGs have no obvious remanence or coercivity, suggesting a superparamagnetic property. The saturation magnetization value (Ms) of pure Fe3O4 nanoparticles reached 60.8 emu/g. After being encapsulated in PNIPAM nanogels, MTNGs reduced Ms to 17.1 emu/g. As shown in the inset of Figure 2a, the MTNG nanogels can be separated from the MTNG colloidal solution by applying a regular magnetic field. However, the separation process was somewhat slow, and the complete separation took about 3 min because of good colloidal stability. An electromagnet chuck was employed to provide a stable magnetic field during the friction test. The intensity of the magnetic field near the surface can be regulated through the direct current and could be measured by a teslameter. As shown in the inset (Figure 2b), the intensity of the magnetic field shows a linear relationship with voltage. The glass cuvette containing MTNG suspension was fixed above the electromagnet chuck. Figure 2b shows the variation of COF as a function of voltage. When lubricated with the MTNG suspension, the COF increased gradually with increasing voltage (from 0.025 at 0 V to 0.15 at 24 V) and returned back when reducing the applied voltages. When the magnetic field was applied to the friction system, the stability of the colloidal dispersion system was disrupted, as shown in the inset of Figure 2a, resulting in nanogel accumulation. Generally, the homogeneous colloidal aqueous dispersion brought about low friction, while the inhomogeneous colloidal dispersion resulted in high friction. More importantly, the higher magnetic strength led to higher frictional drag for the magnetite-loaded nanogels, which can be attributed to the “viscous effect”. That is, when the strength of the magnetic field is enhanced, the movement of magnetic nanogels is inhibited, leading to the increased friction force between nanogels and thereby the increased COF. In short, the aggregation and viscous effect cooperatively increased the COF. On the contrary, with the magnetic strength decreased, the aggregation and viscous effect were both weakened and the friction restored to a low level. Fe3O4 nanoparticles, as a plasmonic photosensitizer, are highly efficient photothermal conversion and thermal conductivity materials. When dispersed in polymer matrices, Fe3O4 nanoparticles can absorb and convert NIR light into thermal energy and act as nanometer-sized heaters to raise the temperature of composites.23 The interesting scenario of MTNGs under NIR-light irradiation is that the photothermal
Figure 3. (a) Temperature elevation of different concentrations of MTNGs as a function of irradiation time. (b) The variation of COF as a function of irradiation time. (c) The variation of COF as a function of simultaneous irradiation time and voltage. The friction tests were performed at the sliding speed of 200 mm/min under the normal load of 5 N. The concentration of MTNG suspension was 0.5 wt %.
dispersion exhibited a remarkable increase, while no obvious temperature increase was observed for pure deionized water. The temperature of the suspension was raised more profoundly at the higher MTNG concentration, which implies the excellent photothermal effect of the magnetic nanogels. In the tribological measurement, the glass cuvette was irradiated by an NIR laser from one side. As shown in Figure 3b, after irradiation with an NIR laser, the COF had a marked increase with the irradiation time; i.e., longer irradiation time led to the higher COF. When the irradiation was stopped and the MTNG lubricant was cooled to ambient temperature, the COF restored to the low level. The change of COF from a low level to a high level was directly attributed to the hydration/dehydration of MTNGs, as a result of elevated temperature due to photothermogenesis of Fe3O4 cores under NIR. The magnetic nanogel suspensions were heated from inside colloidal particles, which was different from the conventional heating mode from solution to colloidal particles. The heat induced by the photothermal mode will directly act on the colloidal particles and efficiently induces volume phase transition of thermosensitive nanogels. Therefore, the photothermal effect induced by NIR light is not only a light-induced heating mode instead of the conventional mode but also an efficient way to regulate the interfacial friction. In addition, the NIR laser and magnetic field may jointly impact friction. As shown in Figure 3c, under the same magnetic field, the COF increased gradually with the irradiation time and leveled off at a high COF value. After the same irradiation time, the COF exhibited a gradual increase with the increasing strength of the magnetic field (applying voltages). 146
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Goldberg, R.; Kampf, N.; Klein, J. Faraday Discuss. 2012, 156, 217− 233. (3) (a) Yasuda, K.; Ping Gong, J.; Katsuyama, Y.; Nakayama, A.; Tanabe, Y.; Kondo, E.; Ueno, M.; Osada, Y. Biomaterials 2005, 26, 4468−4475. (b) Kagata, G.; Gong, J. P.; Osada, Y. J. Phys. Chem. B 2002, 106, 4596−4601. (4) (a) Wathier, M.; Lakin, B. A.; Bansal, P. N.; Stoddart, S. S.; Snyder, B. D.; Grinstaff, M. W. J. Am. Chem. Soc. 2013, 135, 4930− 4933. (b) Sivan, S.; Schroeder, A.; Verberne, G.; Merkher, Y.; Diminsky, D.; Priev, A.; Maroudas, A.; Halperin, G.; Nitzan, D.; Etsion, I.; Barenholz, Y. Langmuir 2010, 26, 1107−1116. (5) Gong, J. P.; Kurokawa, T.; Narita, T.; Kagata, G.; Osada, Y.; Nishimura, G.; Kinjo, M. J. Am. Chem. Soc. 2001, 123, 5582−5583. (6) (a) Coles, J. M.; Chang, D. P.; Zauscher, S. Curr. Opin. Colloid Interface Sci. 2010, 15, 406−416. (b) Liu, G.; Cai, M.; Zhou, F.; Liu, W. J. Phys. Chem. B 2014, 118, 4920−4931. (c) Pettersson, T.; Naderi, A.; Makuska, R.; Claesson, P. M. Langmuir 2008, 24, 3336−3347. (d) Banquy, X.; Burdynska, J.; Lee, D. W.; Matyjaszewski, K.; Israelachvili, J. J. Am. Chem. Soc. 2014, 136, 6199−6202. (e) Dėdinaitė, A. Soft Matter 2012, 8, 273−284. (7) (a) Muller, M. T.; Yan, X.; Lee, S.; Perry, S. S.; Spencer, N. D. Macromolecules 2005, 38, 5706−5713. (b) Muller, M. T.; Yan, X.; Lee, S.; Perry, S. S.; Spencer, N. D. Macromolecules 2005, 38, 3861−3866. (8) Lee, S.; Spencer, N. D. Science 2008, 319, 575−576. (9) (a) Ron, T.; Javakhishvili, I.; Jankova, K.; Hvilsted, S.; Lee, S. Langmuir 2013, 29, 7782−7792. (b) Javakhishvili, I.; Røn, T.; Jankova, K.; Hvilsted, S.; Lee, S. Macromolecules 2014, 47, 2019−2029. (10) (a) Angelini, T. E.; Dunn, A. C.; Urueña, J. M.; Dickrell, D. J.; Burris, D. L.; Sawyer, W. G. Faraday Discuss. 2012, 156, 31−39. (b) Jin, Z.; Dowson, D. Friction 2013, 1, 100−113. (c) Schweitzer, Y.; Kozlov, M. M. Soft Matter 2013, 9, 5186−5195. (11) (a) Ramos, J.; Forcada, J.; Hidalgo-Alvarez, R. Chem. Rev. 2014, 114, 367−428. (b) Seiffert, S. Angew. Chem., Int. Ed. 2013, 52, 11462− 11468. (c) Lu, Y.; Ballauff, M. Prog. Polym. Sci. 2011, 36, 767−792. (d) Sorrell, C. D.; Carter, M. C. D.; Serpe, M. J. Adv. Funct. Mater. 2011, 21, 425−433. (e) Schmidt, S.; Zeiser, M.; Hellweg, T.; Duschl, C.; Fery, A.; Möhwald, H. Adv. Funct. Mater. 2010, 20, 3235−3243. (12) (a) Yue, Y. F.; Haque, M. A.; Kurokawa, T.; Nakajima, T.; Gong, J. P. Adv. Mater. 2013, 25, 3106−3110. (b) Yue, Y.; Kurokawa, T.; Haque, M. A.; Nakajima, T.; Nonoyama, T.; Li, X.; Kajiwara, I.; Gong, J. P. Nat. Commun. 2014, 5, 4659. (c) Liu, G.; Wang, X.; Zhou, F.; Liu, W. ACS Appl. Mater. Interfaces 2013, 5, 10842−10852. (13) (a) Sweeney, J.; Hausen, F.; Hayes, R.; Webber, G. B.; Endres, F.; Rutland, M. W.; Bennewitz, R.; Atkin, R. Phys. Rev. Lett. 2012, 109, 155502. (b) Li, H.; Wood, R. J.; Endres, F.; Atkin, R. J. Phys.: Condens. Matter 2014, 26, 284115. (c) Wu, Y.; Pei, X.; Wang, X.; Liang, Y.; Liu, W.; Zhou, F. NPG Asia Mater. 2014, 6, e136. (d) Wu, Y.; Wei, Q.; Cai, M.; Zhou, F. Adv. Mater. Interfaces 2015, 2, 1400392. (14) (a) Li, X.; Li, H.; Liu, G.; Deng, Z.; Wu, S.; Li, P.; Xu, Z.; Xu, H.; Chu, P. K. Biomaterials 2012, 33, 3013−3024. (b) Zhu, H.; Shang, Y.; Wang, W.; Zhou, Y.; Li, P.; Yan, K.; Wu, S.; Yeung, K. W.; Xu, Z.; Xu, H.; Chu, P. K. Small 2013, 9, 2991−3000. (15) (a) Sun, C.; Lee, J. S.; Zhang, M. Adv. Drug Delivery Rev. 2008, 60, 1252−1265. (b) Kim, J.; Lee, J. E.; Lee, S. H.; Yu, J. H.; Lee, J. H.; Park, T. G.; Hyeon, T. Adv. Mater. 2008, 20, 478−483. (16) (a) Ma, W.; Zhang, Y.; Li, L.; Zhang, Y.; Yu, M.; Guo, J.; Lu, H.; Wang, C. Adv. Funct. Mater. 2013, 23, 107−115. (b) Zhao, L.; Qin, H.; Hu, Z.; Zhang, Y.; Wu, R. a.; Zou, H. Chem. Sci. 2012, 3, 2828−2838. (c) Zhang, Y.; Ma, W.; Li, D.; Yu, M.; Guo, J.; Wang, C. Small 2014, 10, 1379−1386. (17) (a) Zhou, Z.; Sun, Y.; Shen, J.; Wei, J.; Yu, C.; Kong, B.; Liu, W.; Yang, H.; Yang, S.; Wang, W. Biomaterials 2014, 35, 7470−7478. (b) Shen, S.; Kong, F.; Guo, X.; Wu, L.; Shen, H.; Xie, M.; Wang, X.; Jin, Y.; Ge, Y. Nanoscale 2013, 5, 8056−8066. (18) (a) Uhlmann, E.; Spur, G.; Bayat, N.; Patzwald, R. J. Magn. Magn. Mater. 2002, 252, 336−340. (b) Zhao, L.; Qin, H.; Hu, Z.; Zhang, Y.; Wu, R. a.; Zou, H. Chem. Sci. 2012, 3, 2828−2838. (c) Chen, W.; Huang, W.; Wang, X. Tribol. Int. 2014, 72, 172−178. (d) Bombard, A. J. F.; de Vicente, J. Wear 2012, 296, 484−490.
Importantly, the COF mediated by simultaneous NIR laser and magnetism was always higher than that under either NIR laser or magnetism. In this case, not only the aggregation and separation of MTNGs were enhanced by the magnetic field but also the hydration layer around MTNGs was destroyed by the heat generated by NIR irradiation. These two factors led to a higher COF. In other words, the joint stimuli induced a more significant regulation for COF than did a single stimulus. In conclusion, a facile yet efficient route has been developed to fabricate magnetite-loaded thermosensitive nanogels that can act as an efficient aqueous lubricant for compliant contacts in biotribological systems, and ultralow COF was achieved. Importantly, the magic nanogels offer different possibilities to tune the lubricating properties by heat, magnetic field, and NIR irradiation. The hydration or dehydration phase transfer of nanogels induced by traditional heating leads to thermally adjusted lubrication. The photothermogenesis effect of Fe3O4 nanoparticles under NIR-light irradiation allows heating nanogels from the interior, and thus friction is reversibly switched more efficiently. Aggregation/redispersing of nanogels in the presence/absence of an external magnetic field induces magnetic-controlled friction. The multimodal friction tunability conferred by functional nanogels broadens the perspective of aqueous lubrication. Moreover, in the biomedical field, using the functional nanogels offers a possibility for simultaneous articular lubrication, magnetic resonance imaging, and photothermal therapy. This multiply regulated tribological property in aqueous lubrication may lead to new strategies to develop intelligent lubrication materials for biomedical and engineering applications.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.5b00860. Synthesis of Fe3O4 nanoparticles and MTNGs; TEM, DLS, FTIR, XPS and XRD spectra and TGA curves of Fe3O4 nanoparticles and MTNGs; the variation of COF of MTNG aqueous suspension as a function of concentration, normal load, and sliding speed; typical stribeck curve used for defining the lubrication system (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was financially supported by NSFC (21434009, 51305428, and 51335010), 973 projects (2013CB632300), and CAS (KJZD-EW-M01 and Western Light Program).
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REFERENCES
(1) (a) Gong, J. P. Soft Matter 2006, 2, 544−552. (b) Neu, C. P.; Komvopoulos, K.; Reddi, A. H. Tissue Eng., Part B 2008, 14, 235−247. (2) (a) Raviv, U.; Giasson, S.; Kampf, N.; Gohy, J.-F.; Jerome, R.; Klein, J. Nature 2003, 425, 163−165. (b) Klein, J. Friction 2013, 1, 1− 23. (c) Gaisinskaya, A.; Ma, L.; Silbert, G.; Sorkin, R.; Tairy, O.; 147
DOI: 10.1021/acsmacrolett.5b00860 ACS Macro Lett. 2016, 5, 144−148
Letter
ACS Macro Letters (e) Song, X.; Qiu, Z.; Yang, X.; Gong, H.; Zheng, S.; Cao, B.; Wang, H.; Möhwald, H.; Shchukin, D. Chem. Mater. 2014, 26, 5113−5119. (19) (a) Chen, H.; Qi, B.; Moore, T.; Colvin, D. C.; Crawford, T.; Gore, J. C.; Alexis, F.; Mefford, O. T.; Anker, J. N. Small 2014, 10, 160−168. (b) Si, J. C.; Xing, Y.; Peng, M.-L.; Zhang, C.; Buske, N.; Chen, C.; Cui, Y. L. CrystEngComm 2014, 16, 512−516. (20) (a) Wei, Q.; Cai, M.; Zhou, F.; Liu, W. Macromolecules 2013, 46, 9368−9379. (b) Liu, G.; Liu, Z.; Li, N.; Wang, X.; Zhou, F.; Liu, W. ACS Appl. Mater. Interfaces 2014, 6, 20452−20463. (21) (a) Shi, G.; Zhang, M. Q.; Rong, M. Z.; Wetzel, B.; Friedrich, K. Wear 2004, 256, 1072−1081. (b) Hernandez Battez, A.; Fernandez Rico, J. E.; Navas Arias, A.; Viesca Rodriguez, J. L.; Chou Rodriguez, R.; Diaz Fernandez, J. M. Wear 2006, 261, 256−263. (22) (a) Smith, A. M.; Parkes, M. A.; Perkin, S. J. Phys. Chem. Lett. 2014, 5, 4032−4037. (b) Smith, A. M.; Lovelock, K. R. J.; Gosvami, N. N.; Licence, P.; Dolan, A.; Welton, T.; Perkin, S. J. Phys. Chem. Lett. 2013, 4, 378−382. (23) (a) Zhu, C. H.; Lu, Y.; Chen, J. F.; Yu, S. H. Small 2014, 10, 2796−2800. (b) Zhao, Y.; Song, W.; Wang, D.; Ran, H.; Wang, R.; Yao, Y.; Wang, Z.; Zheng, Y.; Li, P. ACS Appl. Mater. Interfaces 2015, 7, 14231−14242. (c) Hou, C.; Zhang, Q.; Wang, H.; Li, Y. J. Mater. Chem. 2011, 21, 10512−10517. (d) Chu, M.; Shao, Y.; Peng, J.; Dai, X.; Li, H.; Wu, Q.; Shi, D. Biomaterials 2013, 34, 4078−4088. (e) Lin, L. S.; Cong, Z. X.; Cao, J. B.; Ke, K. M.; Peng, Q. L.; Gao, J.; Yang, H. H.; Liu, G.; Chen, X. ACS Nano 2014, 8, 3876−3883. (f) Lai, B. H.; Chen, D. H. Acta Biomater. 2013, 9, 7573−7579.
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