This is an open access article published under an ACS AuthorChoice License, which permits copying and redistribution of the article or any adaptations for non-commercial purposes.
Research Article Cite This: ACS Cent. Sci. XXXX, XXX, XXX−XXX
http://pubs.acs.org/journal/acscii
Direct Observation of the Interplay of Catechol Binding and Polymer Hydrophobicity in a Mussel-Inspired Elastomeric Adhesive Sukhmanjot Kaur,# Amal Narayanan,# Siddhesh Dalvi, Qianhui Liu, Abraham Joy,* and Ali Dhinojwala* Department of Polymer Science, The University of Akron, Akron, Ohio 44325, United States
ACS Cent. Sci. Downloaded from pubs.acs.org by 5.188.216.176 on 10/09/18. For personal use only.
S Supporting Information *
ABSTRACT: Marine organisms such as mussels have mastered the challenges in underwater adhesion by incorporating posttranslationally modified amino acids like L-3,4-dihydroxyphenylalanine (DOPA) in adhesive proteins. Here we designed a catechol containing elastomer adhesive to identify the role of catechol in interfacial adhesion in both dry and wet conditions. To decouple the adhesive contribution of catechol to the overall adhesion, the elastomer was designed to be cross-linked through [2 + 2] photo-cycloaddition of coumarin. The elastomer with catechol moieties displayed a higher adhesion strength than the catechol-protected elastomer. The contact interface was probed using interface-sensitive sum frequency generation spectroscopy to explore the question of whether catechol can displace water and bond with hydrophilic surfaces. The spectroscopy measurements reveal that the maximum binding energy of the catechol and protected-catechol elastomers to sapphire substrate is 7.0 ± 0.1 kJ/(mole of surface O−H), which is equivalent to 0.10 J/ m2. The higher dry and wet adhesion observed in the macroscopic adhesion measurements for the catechol containing elastomer originates from multiple hydrogen bonds of the catechol dihydroxy groups to the surface. In addition, our results show that catechol by itself does not remove the confined interstitial water. In these elastomers, it is the hydrophobic groups that help in partially removing interstitial water. The observation of the synergy between catechol binding and hydrophobicity in enabling the mussel-inspired soft adhesive elastomer to stick underwater provides a framework for designing materials for applications in tissue adhesion and moist-skin wearable electronics.
■
INTRODUCTION Mussels, caddisflies, sandcastle worms, barnacles, and many other species use biological adhesives to form interfacial bonds to various substrates in the presence of water,1−4 and among these examples, the mussel holdfast is one of the most wellstudied biological adhesive system.5−7 Mussels secrete a series of adhesive proteins which solidify to form the byssus threadthe holdfast of mussels.8 The byssus thread comprises of more than 15 different mussel foot proteins (Mfp),9−12 and among them, a set of low molar mass (95°, Table S1) and are in contact with a hydrophilic surface, and hence the contact area is expected to be patchy. Perhaps a patchy contact could also form as the draining of interstitial water requires more time. If this was the case, increasing the contact time should increase the dry molecular contact and thus lead to higher adhesion.66 Figure S4B shows the work of adhesion at contact equilibration times of 0.5, 3, and 15 min. The underwater adhesion values remain lower than those measured in dry contact, indicating that the kinetics of drainage is not playing an important role in underwater adhesion. The deprotected elastomer showed higher underwater adhesion as compared to the protected elastomer. Since the percentage reduction in the adhesion between dry and wet contacts for both protected (∼72%) and deprotected (∼79%) elastomers are similar, we infer that the fraction area of patchy water contact is similar for both elastomers. From the analysis of adhesion measurements and SFG spectra, we propose an overall model for the underwater G
DOI: 10.1021/acscentsci.8b00526 ACS Cent. Sci. XXXX, XXX, XXX−XXX
Research Article
ACS Central Science adhesion for these elastomers (Figure 5). Regardless of the presence of catechol, both elastomers form patchy contact with hydrophilic substrates underwater. The wet patches which contain water (blue) between the polymer and substrate constrain the adhesion. The dry patches (yellow) are true contacts between the polymer and substrate which provide underwater adhesion.66,74 We anticipate that the patches are smaller than the resolution of the optical probes (less than microns). Both the protected and deprotected elastomers succeed in contacting the substrate underwater. However, the interfacial strength of the dry patch of deprotected elastomer is higher due to the localized multimodal interactions of O−H groups in catechol with the sapphire substrate, resulting in higher underwater interfacial adhesion than the protected elastomer. We have reported recently that a hydrophilic adhesive with catechol does not adhere underwater compared to a relatively hydrophobic adhesive without catechol groups.49 Therefore, for effective underwater adhesion, initially the polymer should make interfacial contact with the surface and remove bound water, which in this case is achieved by a conformable hydrophobic adhesive. Also, polar functional groups with strong interfacial interactions are essential to display significant adhesion underwater. If we use hydrophobic PDMS in contact with hydrophilic substrates, we observe patchy contact. However, the wet adhesion is very weak for PDMS (Figure S4C) due to the absence of strong interfacial polar interactions present in mussel-inspired catechol polymers. Additionally, the hydrophobic functional groups in Mfp19 and mussel-inspired polymers77 have also been shown to improve adhesion by shielding catechol groups from oxidation reactions.
achieving similar patchy contact underwater. The formation of patchy contact between a hydrophobic and hydrophilic interface underwater is consistent with the previous studies.66,78 The higher adhesion strength of the deprotected elastomer is due to multimodal bonding of catechol with the hydroxylated surface. Hence, the interplay of hydrophobicity to remove water from the interface and the role of catechol in forming multiple hydrogen bonds are proposed from the combination of SFG spectroscopy and adhesion measurements in this study. Although the synergistic effect of lysine to displace cations and water from the surface for enhanced interfacial binding of catechol is also essential to understand the adhesion of Mfp, our current focus was to study the importance of hydrophobicity in improving underwater adhesion of mussel-inspired polymers.79,80 In the future, the critical understanding of the interplay of hydrophobicity and mussel-inspired dihydroxy chemistry in synthetic adhesives can be put to use in designing adhesives for applications in tissue adhesion where the presence of moisture or physiological fluids cannot be avoided.
CONCLUSION We have used polymers with well-defined chemistry, JKR geometry for adhesion measurements, and interface-sensitive SFG spectroscopy to understand the role of catechol in underwater adhesion of mussel-inspired polymers. Having both the elastomers of similar bulk mechanical properties, but a difference in only the surface-active catechol groups being protected and deprotected, helped to identify the importance of the dihydroxyl groups and the hydrophobic groups in adhesion. The dry adhesion of deprotected elastomer is higher than the protected elastomer. Direct probing of the contact interface using SFG spectroscopy reveals that the strength of the acid− base interactions of polar groups with surface O−H groups on the sapphire substrate is very similar for both protected and deprotected elastomers. The stronger adhesion strength obtained from macroscopic measurements can be explained by the dihydroxy chemistry of catechol despite similar interaction strength as observed in SFG spectra. The energy required to break multiple bonds concurrently is higher than the sequential breaking of monodentate bonds of similar strength, thus increasing the adhesion strength of deprotected elastomer compared to protected elastomer. In wet environment, the deprotected elastomer has a higher adhesion compared to protected elastomer. The SFG experiments reveal that the presence of “liquid-like” water in the contact interface for both the polymers. The SFG and adhesion data combined show the interplay of hydrophobicity and catechol binding to enhance adhesion. Both elastomers have a patchy contact underwater. The protected and deprotected elastomers have a similar water contact angle (similar hydrophobicity) and similar decrease in wet adhesion compared to dry adhesion indicating the assistance of hydrophobicity in
*E-mail:
[email protected] (A. J.). *E-mail:
[email protected] (A. D.).
■
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscentsci.8b00526.
■
■
The data used for supporting the observations in the article (Scheme S1, Table S1 and Figures S1−S9) (PDF)
AUTHOR INFORMATION
Corresponding Authors
ORCID
Abraham Joy: 0000-0001-7781-3817 Ali Dhinojwala: 0000-0002-3935-7467 Author Contributions #
S.K. and A.N. equally contributed in the work. S.K. and A.N. synthesized the polymers and performed the measurements. S.D. provided guidance during the adhesion measurement, and Q.L. performed rheology measurements. S.K., A.N., A.J., and A.D. designed the experiments, analyzed the data, and wrote the manuscript. Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS We acknowledge funding from the National Science Foundation (NSF) (DMR Awards 1508440 and DMR 1610483), the University of Akron start-up funds, and the Ohio Soybean Council Foundation Graduate Scholarship 2016-17 (S.K.). We thank Mr. Steven Mankoci for the assistance with fluorescence microscopy. Authors also acknowledge Dr. Nishad Dhopatkar, Mr. Michael Wilson, Dr. Saranshu Singla and Mr. Tanmay Jain for valuable discussions.
■
REFERENCES
(1) Kamino, K. Underwater adhesive of marine organisms as the vital link between biological science and material science. Mar. Biotechnol. 2008, 10 (2), 111−121. (2) Stewart, R. J.; Ransom, T. C.; Hlady, V. Natural underwater adhesives. J. Polym. Sci., Part B: Polym. Phys. 2011, 49 (11), 757−771.
H
DOI: 10.1021/acscentsci.8b00526 ACS Cent. Sci. XXXX, XXX, XXX−XXX
Research Article
ACS Central Science
(25) Anderson, T. H.; Yu, J.; Estrada, A.; Hammer, M. U.; Waite, J. H.; Israelachvili, J. N. The contribution of dopa to substrate-peptide adhesion and internal cohesion of mussel-inspired synthetic peptide films. Adv. Funct. Mater. 2010, 20 (23), 4196−4205. (26) Danner, E. W.; Kan, Y. J.; Hammer, M. U.; Israelachvili, J. N.; Waite, J. H. Adhesion of mussel foot protein mefp-5 to mica: an underwater superglue. Biochemistry 2012, 51 (33), 6511−6518. (27) Yu, J.; Wei, W.; Menyo, M. S.; Masic, A.; Waite, J. H.; Israelachvili, J. N. Adhesion of mussel foot protein-3 to TiO2 surfaces: the effect of pH. Biomacromolecules 2013, 14 (4), 1072−1077. (28) Utzig, T.; Stock, P.; Valtiner, M. Resolving Non-Specific and Specific Adhesive interactions of catechols at solid/liquid interfaces at the molecular scale. Angew. Chem., Int. Ed. 2016, 55 (33), 9524−9528. (29) Das, P.; Reches, M. Revealing the role of catechol moieties in the interactions between peptides and inorganic surfaces. Nanoscale 2016, 8 (33), 15309−15316. (30) Li, Y.; Liu, H.; Wang, T.; Qin, M.; Cao, Y.; Wang, W. Singlemolecule force spectroscopy reveals multiple binding modes between dopa and different rutile surfaces. ChemPhysChem 2017, 18 (11), 1466−1469. (31) Li, Y.; Qin, M.; Cao, Y.; Wang, W.; Li, Y. Single molecule evidence for the adaptive binding of DOPA to different wet surfaces. Langmuir 2014, 30 (15), 4358−4366. (32) Wang, J. J.; Tahir, M. N.; Kappl, M.; Tremel, W.; Metz, N.; Barz, M.; Theato, P.; Butt, H. J. Influence of binding-site density in wet bioadhesion. Adv. Mater. 2008, 20 (20), 3872−3876. (33) Sedo, J.; Saiz-Poseu, J.; Busque, F.; Ruiz-Molina, D. Catecholbased biomimetic functional materials. Adv. Mater. 2013, 25 (5), 653− 701. (34) Brubaker, C. E.; Messersmith, P. B. Enzymatically degradable mussel-inspired adhesive hydrogel. Biomacromolecules 2011, 12 (12), 4326−4334. (35) Meredith, H. J.; Jenkins, C. L.; Wilker, J. J. Enhancing the adhesion of a biomimetic polymer yields performance rivaling commercial glues. Adv. Funct. Mater. 2014, 24 (21), 3259−3267. (36) Shao, H.; Weerasekare, G. M.; Stewart, R. J. Controlled curing of adhesive complex coacervates with reversible periodate carbohydrate complexes. J. Biomed. Mater. Res., Part A 2011, 97A (1), 46−51. (37) Sparks, B. J.; Hoff, E. F. T.; Hayes, L. P.; Patton, D. L. Musselinspired thiol-ene polymer networks: influencing network properties and adhesion with catechol functionality. Chem. Mater. 2012, 24 (18), 3633−3642. (38) Stepuk, A.; Halter, J. G.; Schaetz, A.; Grass, R. N.; Stark, W. J. Mussel-inspired load bearing metal-polymer glues. Chem. Commun. 2012, 48 (50), 6238−6240. (39) Westwood, G.; Horton, T. N.; Wilker, J. J. Simplified polymer mimics of cross-linking adhesive proteins. Macromolecules 2007, 40 (11), 3960−3964. (40) Lee, H.; Lee, B. P.; Messersmith, P. B. A reversible wet/dry adhesive inspired by mussels and geckos. Nature 2007, 448 (7151), 338−341. (41) North, M. A.; Del Grosso, C. A.; Wilker, J. J. High strength underwater bonding with polymer mimics of mussel adhesive proteins. ACS Appl. Mater. Interfaces 2017, 9 (8), 7866−7872. (42) White, J. D.; Wilker, J. J. Underwater bonding with charged polymer mimics of marine mussel adhesive proteins. Macromolecules 2011, 44 (13), 5085−5088. (43) Yu, M. E.; Hwang, J. Y.; Deming, T. J. Role of L-3,4dihydroxyphenylalanine in mussel adhesive proteins. J. Am. Chem. Soc. 1999, 121 (24), 5825−5826. (44) Mu, Y. B.; Wu, X.; Pei, D. F.; Wu, Z. L.; Zhang, C.; Zhou, D. S.; Wan, X. B. Contribution of the polarity of mussel-inspired adhesives in the realization of strong underwater bonding. ACS Biomater. Sci. Eng. 2017, 3 (12), 3133−3140. (45) Ahn, B. K. Perspectives on mussel-inspired wet adhesion. J. Am. Chem. Soc. 2017, 139 (30), 10166−10171. (46) Xu, H.; Nishida, J.; Ma, W.; Wu, H.; Kobayashi, M.; Otsuka, H.; Takahara, A. Competition between oxidation and coordination in
(3) Taylor, S. W.; Kammerer, B.; Bayer, E. New perspectives in the chemistry and biochemistry of the tunichromes and related compounds. Chem. Rev. 1997, 97 (1), 333−346. (4) Waite, J. H. Mussel adhesion - essential footwork. J. Exp. Biol. 2017, 220 (4), 517−530. (5) Sagert, J.; Sun, C.; waite, J. H. Chemical subtleties of mussel and polychaete holdfasts. In Biological Adhesives; Smith, A. M.; Callow, J. A., Eds.; Springer: Berlin, 2006; pp 125−143. (6) Silverman, H. G.; Roberto, F. F. Understanding marine mussel adhesion. Mar. Biotechnol. 2007, 9 (6), 661−681. (7) Waite, J. H.; Tanzer, M. L. Polyphenolic substance of mytilusedulis - novel adhesive containing l-dopa and hydroxyproline. Science 1981, 212 (4498), 1038−1040. (8) Petrone, L.; Kumar, A.; Sutanto, C. N.; Patil, N. J.; Kannan, S.; Palaniappan, A.; Amini, S.; Zappone, B.; Verma, C.; Miserez, A. Mussel adhesion is dictated by time-regulated secretion and molecular conformation of mussel adhesive proteins. Nat. Commun. 2015, 6, 8737. (9) Qin, X. X.; Waite, J. H. Exotic collagen gradients in the byssus of the mussel Mytilus-edulis. J. Exp. Biol. 1995, 198 (3), 633−644. (10) Taylor, S. W.; Chase, D. B.; Emptage, M. H.; Nelson, M. J.; Waite, J. H. Ferric ion complexes of a DOPA-containing adhesive protein from Mytilus edulis. Inorg. Chem. 1996, 35 (26), 7572−7577. (11) Zhao, H.; Waite, J. H. Coating proteins: Structure and crosslinking in fp-1 from the green shell mussel Perna canaliculus. Biochemistry 2005, 44 (48), 15915−15923. (12) Zhao, H.; Waite, J. H. Linking adhesive and structural proteins in the attachment plaque of Mytilus californianus. J. Biol. Chem. 2006, 281 (36), 26150−26158. (13) Zhao, H.; Robertson, N. B.; Jewhurst, S. A.; Waite, J. H. Probing the adhesive footprints of Mytilus californianus byssus. J. Biol. Chem. 2006, 281 (16), 11090−11096. (14) Waite, J. H.; Hansen, D. C.; Little, K. T. The glue protein of ribbed mussels (Geukensia demissa): a natural adhesive with some features of collagen. J. Comp. Physiol., B 1989, 159 (5), 517−525. (15) Wei, W.; Tan, Y.; Martinez Rodriguez, N. R.; Yu, J.; Israelachvili, J. N.; Waite, J. H. A mussel-derived one component adhesive coacervate. Acta Biomater. 2014, 10 (4), 1663−1670. (16) Waite, J. H. Adhesion a la Moule. Integr. Comp. Biol. 2002, 42 (6), 1172−1180. (17) Levine, Z. A.; Rapp, M. V.; Wei, W.; Mullen, R. G.; Wu, C.; Zerze, G. H.; Mittal, J.; Waite, J. H.; Israelachvili, J. N.; Shea, J. E. Surface force measurements and simulations of mussel-derived peptide adhesives on wet organic surfaces. Proc. Natl. Acad. Sci. U. S. A. 2016, 113 (16), 4332−4337. (18) Lin, Q.; Gourdon, D.; Sun, C. J.; Holten-Andersen, N.; Anderson, T. H.; Waite, J. H.; Israelachvili, J. N. Adhesion mechanisms of the mussel foot proteins mfp-1 and mfp-3. Proc. Natl. Acad. Sci. U. S. A. 2007, 104 (10), 3782−3786. (19) Wei, W.; Yu, J.; Broomell, C.; Israelachvili, J. N.; Waite, J. H. Hydrophobic enhancement of dopa-mediated adhesion in a mussel foot protein. J. Am. Chem. Soc. 2013, 135 (1), 377−383. (20) Yu, J.; Kan, Y. J.; Rapp, M.; Danner, E.; Wei, W.; Das, S.; Miller, D. R.; Chen, Y. F.; Waite, J. H.; Israelachvili, J. N. Adaptive hydrophobic and hydrophilic interactions of mussel foot proteins with organic thin films. Proc. Natl. Acad. Sci. U. S. A. 2013, 110 (39), 15680−15685. (21) Yu, J.; Wei, W.; Danner, E.; Ashley, R. K.; Israelachvili, J. N.; Waite, J. H. Mussel protein adhesion depends on interprotein thiolmediated redox modulation. Nat. Chem. Biol. 2011, 7 (9), 588−590. (22) Yu, J.; Wei, W.; Danner, E.; Israelachvili, J. N.; Waite, J. H. Effects of interfacial redox in mussel adhesive protein films on mica. Adv. Mater. 2011, 23 (20), 2362−2366. (23) Lee, H.; Scherer, N. F.; Messersmith, P. B. Single-molecule mechanics of mussel adhesion. Proc. Natl. Acad. Sci. U. S. A. 2006, 103 (35), 12999−13003. (24) Wei, W.; Petrone, L.; Tan, Y.; Cai, H.; Israelachvili, J. N.; Miserez, A.; Waite, J. H. An underwater surface-drying peptide inspired by a mussel adhesive protein. Adv. Funct. Mater. 2016, 26 (20), 3496−3507. I
DOI: 10.1021/acscentsci.8b00526 ACS Cent. Sci. XXXX, XXX, XXX−XXX
Research Article
ACS Central Science
(68) Zhang, L.; Tian, C.; Waychunas, G. A.; Shen, Y. R. Structures and charging of alpha-alumina (0001)/water interfaces studied by sumfrequency vibrational spectroscopy. J. Am. Chem. Soc. 2008, 130 (24), 7686−7694. (69) Zhou, J.; Anim-Danso, E.; Zhang, Y.; Zhou, Y.; Dhinojwala, A. Interfacial water at polyurethane-sapphire interface. Langmuir 2015, 31 (45), 12401−12407. (70) Even, M. A.; Wang, J.; Chen, Z. Structural information of mussel adhesive protein Mefp-3 acquired at various polymer/Mefp-3 solution interfaces. Langmuir 2008, 24 (11), 5795−5801. (71) Leng, C.; Liu, Y. W.; Jenkins, C.; Meredith, H.; Wilker, J. J.; Chen, Z. Interfacial structure of a dopa-inspired adhesive polymer studied by sum frequency generation vibrational spectroscopy. Langmuir 2013, 29 (22), 6659−6664. (72) Yeh, I. C.; Lenhart, J. L.; Rinderspacher, B. C. Molecular dynamics simulations of adsorption of catechol and related phenolic compounds to alumina surfaces. J. Phys. Chem. C 2015, 119 (14), 7721−7731. (73) Dhopatkar, N.; Defante, A. P.; Dhinojwala, A. Ice-like water supports hydration forces and eases sliding friction. Sci. Adv. 2016, 2 (8), No. e1600763. (74) Chaudhury, M. K.; Whitesides, G. M. Direct measurement of interfacial interactions between semispherical lenses and flat sheets of poly(dimethylsiloxane) and their chemical derivatives. Langmuir 1991, 7 (5), 1013−1025. (75) Kirpat, I.; Goksel, Y.; Karakus, E.; Emrullahoglu, M.; Akdogan, Y. Determination of force-free wet adhesion of mussel-inspired polymers to spin labeled surface. Mater. Lett. 2017, 205, 48−51. (76) Defante, A. P.; Burai, T. N.; Becker, M. L.; Dhinojwala, A. Consequences of water between two hydrophobic surfaces on adhesion and wetting. Langmuir 2015, 31 (8), 2398−2406. (77) Seo, S.; Das, S.; Zalicki, P. J.; Mirshafian, R.; Eisenbach, C. D.; Israelachvili, J. N.; Waite, J. H.; Ahn, B. K. Microphase behavior and enhanced wet-cohesion of synthetic copolyampholytes inspired by a mussel foot protein. J. Am. Chem. Soc. 2015, 137 (29), 9214−9217. (78) Defante, A. P.; Nyarko, A.; Kaur, S.; Burai, T. N.; Dhinojwala, A. Interstitial water enhances sliding friction. Langmuir 2018, 34 (13), 4084−4094. (79) Maier, G. P.; Rapp, M. V.; Waite, J. H.; Israelachvili, J. N.; Butler, A. Adaptive synergy between catechol and lysine promotes wet adhesion by surface salt displacement. Science 2015, 349 (6248), 628− 632. (80) Rapp, M. V.; Maier, G. P.; Dobbs, H. A.; Higdon, N. J.; Waite, J. H.; Butler, A.; Israelachvili, J. N. Defining the catechol-cation synergy for enhanced wet adhesion to mineral surfaces. J. Am. Chem. Soc. 2016, 138 (29), 9013−9016.
cross-linking of polystyrene copolymer containing catechol groups. ACS Macro Lett. 2012, 1 (4), 457−460. (47) Yang, J.; Cohen Stuart, M. A.; Kamperman, M. Jack of all trades: versatile catechol crosslinking mechanisms. Chem. Soc. Rev. 2014, 43 (24), 8271−8298. (48) Clancy, S. K.; Sodano, A.; Cunningham, D. J.; Huang, S. S.; Zalicki, P. J.; Shin, S.; Ahn, B. K. Marine bioinspired underwater contact adhesion. Biomacromolecules 2016, 17 (5), 1869−1874. (49) Xu, Y.; Liu, Q.; Narayanan, A.; Jain, D.; Dhinojwala, A.; Joy, A. Mussel-inspired polyesters with aliphatic pendant groups demonstrate the importance of hydrophobicity in underwater adhesion. Adv. Mater. Interfaces 2017, 4 (22), 1700506. (50) Gnanaguru, K.; Ramasubbu, N.; Venkatesan, K.; Ramamurthy, V. A study on the photochemical dimerization of coumarins in the solidstate. J. Org. Chem. 1985, 50 (13), 2337−2346. (51) Maddipatla, M. V. S. N.; Wehrung, D.; Tang, C.; Fan, W. Z.; Oyewumi, M. O.; Miyoshi, T.; Joy, A. Photoresponsive coumarin polyesters that exhibit cross-linking and chain scission properties. Macromolecules 2013, 46 (13), 5133−5140. (52) Govindarajan, S. R.; Xu, Y.; Swanson, J. P.; Jain, T.; Lu, Y. F.; Choi, J. W.; Joy, A. A Solvent and initiator free, low-modulus, degradable polyester platform with modular functionality for ambienttemperature 3D printing. Macromolecules 2016, 49 (7), 2429−2437. (53) Johnson, K. L.; Kendall, K.; Roberts, A. D. Surface energy and contact of elastic solids. Proc. R. Soc. London, Ser. A 1971, 324 (1558), 301−313. (54) Gent, A. N. Adhesion and strength of viscoelastic solids. Is there a relationship between adhesion and bulk properties? Langmuir 1996, 12 (19), 4492−4496. (55) Chen, Z.; Shen, Y. R.; Somorjai, G. A. Studies of polymer surfaces by sum frequency generation vibrational spectroscopy. Annu. Rev. Phys. Chem. 2002, 53, 437−465. (56) Guyotsionnest, P.; Hunt, J. H.; Shen, Y. R. Sum-frequency vibrational spectroscopy of a langmuir film - study of molecularorientation of a two-dimensional system. Phys. Rev. Lett. 1987, 59 (14), 1597−1600. (57) Kurian, A.; Prasad, S.; Dhinojwala, A. Direct measurement of acid-base interaction energy at solid interfaces. Langmuir 2010, 26 (23), 17804−17807. (58) Prasad, S.; Zhu, H.; Kurian, A.; Badge, I.; Dhinojwala, A. Interfacial segregation in polymer blends driven by acid-base interactions. Langmuir 2013, 29 (51), 15727−15731. (59) Gokhale, S.; Xu, Y.; Joy, A. A library of multifunctional polyesters with ″peptide-like″ pendant functional groups. Biomacromolecules 2013, 14 (8), 2489−2493. (60) Liu, Z.; Hu, B. H.; Messersmith, P. B. Acetonide Protection of Dopamine for the Synthesis of Highly Pure N-docosahexaenoyldopamine. Tetrahedron Lett. 2010, 51 (18), 2403−2405. (61) Li, G. L.; Zhang, H.; Sader, F.; Vadhavkar, N.; Njus, D. Oxidation of 4-methylcatechol: Implications for the oxidation of catecholamines. Biochemistry 2007, 46 (23), 6978−6983. (62) Burzio, L. A.; Waite, J. H. Cross-linking in adhesive quinoproteins: studies with model decapeptides. Biochemistry 2000, 39 (36), 11147−11153. (63) Hsu, P. Y.; Dhinojwala, A. Contact of oil with solid surfaces in aqueous media probed using sum frequency generation spectroscopy. Langmuir 2012, 28 (5), 2567−2573. (64) Ahn, B. K.; Lee, D. W.; Israelachvili, J. N.; Waite, J. H. Surfaceinitiated self-healing of polymers in aqueous media. Nat. Mater. 2014, 13 (9), 867−872. (65) Gautam, K. S.; Kumar, S.; Wermeille, D.; Robinson, D.; Dhinojwala, A. Observation of novel liquid-crystalline phase above the bulk-melting temperature. Phys. Rev. Lett. 2003, 90 (21), 215501. (66) Nanjundiah, K.; Hsu, P. Y.; Dhinojwala, A. Understanding rubber friction in the presence of water using sum-frequency generation spectroscopy. J. Chem. Phys. 2009, 130 (2), No. 024702. (67) Yurdumakan, B.; Harp, G. P.; Tsige, M.; Dhinojwala, A. Template-induced enhanced ordering under confinement. Langmuir 2005, 21 (23), 10316−10319. J
DOI: 10.1021/acscentsci.8b00526 ACS Cent. Sci. XXXX, XXX, XXX−XXX
