Article pubs.acs.org/Macromolecules
Facile Fabrication of Color Tunable Film and Fiber Nanocomposites via Thiol Click Chemistry Darryl A. Boyd,*,† Jawad Naciri,† Jake Fontana,† Dennis B. Pacardo,‡ Adam R. Shields,† Jasenka Verbarg,† Christopher M. Spillmann,† and Frances S. Ligler‡ †
Center for Bio/Molecular Science & Engineering, Naval Research Laboratory, 4555 Overlook Ave SW, Washington, D.C. 20375, United States ‡ Department of Biomedical Engineering, University of North Carolina at Chapel Hill and North Carolina State University, EB3, Mail Stop 7115, Raleigh, North Carolina 27695-7115, United States S Supporting Information *
ABSTRACT: A simple method for the fabrication of nanocomposite materials using thiol click chemistry is reported. The thiol click nanocomposite materials produced each displayed distinctive colors which were found to be dependent on both the ligand used to functionalize the nanoparticles and the concentration of nanoparticles in the materials. Functionalized metallic nanospheres were combined with thiol click solutions forming viscous prepolymer solutions which were then polymerized upon UV light exposure. Films were fabricated in a custom-built film mold, and microfibers were fabricated using hydrodynamic focusing in a microfluidic channel. For this study, three unique thiolated ligandsincluding a newly synthesized ligandwere used to functionalize the nanospheres, thus assisting in the facile incorporation and stability of the nanospheres within the polymers. In comparison to a previously reported method in which thiol−ene nanocomposite films were fabricated, the method reported herein reduces the fabrication time from weeks to minutes. Furthermore, the method in this report is expanded to also include fabrication of thiol−yne nanocomposites. Young’s moduli and glass transition temperatures were determined for the materials, while UV−vis spectroscopy, transmission electron microscopy, and optical analyses were also performed in order to characterize the nanocomposites.
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INTRODUCTION In recent years thiol click chemistrysometimes called “Hoyle click chemistry” in honor of the substantial contributions Professor Charles Hoyle made in establishing this research areahas earned a prominent place in materials science because of the adaptability of the thiol click reaction platform and the ease with which these reactions can be performed.1−4 Because of the well-known strength of the gold−thiol bond (∼45 kcal/mol),5 there are many reports of thiolated molecules being used to form or modify self-assembled monolayers (SAMs) deposited on various materials including gold nanospheres (AuNS).5−8 Some of these reports specifically utilize thiol click chemistry as a means of probing and immobilizing species on surfaces, including gold.9−13 Despite the use of thiol click chemistry for modification of gold surfaces, reports describing the inclusion of metallic nanoparticles (NPs) into thiol click polymer materials are not common. In one report, AuNS were incorporated into thiol− ene films using a process that required up to 3 weeks of stirring to incorporate the AuNS into the prepolymer.14 A gold nanopowder was precomplexed with a trithiol during the stirring process, followed by the addition of a trialkene to the gold−thiol dispersion. This prepolymer material was photopolymerized on glass slides upon UV light exposure. The report © 2014 American Chemical Society
concluded that the nanospheres were well dispersed within the polymer matrix.14 Recently, we reported the fabrication of thiol−yne and thiol−ene microfibers using hydrodynamic focusing in microfluidic channels.15−17 Those reports highlighted our ability to fabricate thiol click fibers with nonround cross-sectional shapes on the micrometer scale as well as the ability to perform postpolymerization modifications on the surface of the fibers by altering the prepolymer stoichiometry.15−17 The advantages of the fiber fabrication method employed included the structural uniformity of the fibers, the easily controlled production conditions, and the ability to perform surface modifications on the fibers. The possibility that inclusions, like metallic nanoparticles, could be covalently coupled into these fibers, thus altering the material properties, was intriguing. Utilizing a process for the facile phase transfer and surface functionalization of metallic NPs,18 in conjunction with the microfluidic production of thiol click microfibers,15−17 we now report a rapid, facile method for inclusion of AuNS into thiol click fibers. We also use the same prepolymer preparation to Received: August 4, 2013 Revised: November 21, 2013 Published: January 7, 2014 695
dx.doi.org/10.1021/ma401636e | Macromolecules 2014, 47, 695−704
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Article
(77%) of 2. 1H NMR (CDCl3): δ 1.28−1.40 (m, 4H), 1.8−1.93 (m, 4H), 3.42 (t, 2H), 4.03 (t, 2H), 6.93 (2H, ArH), 8.05 (2H, ArH). Synthesis of 4-(Hex-5-yn-1-yloxy)phenyl 4-((6-Bromohexyl)oxy)benzoate (3). To a mixture of 2 (2 g, 6.64 mmol), 4-(hex-5-yn1-yloxy)phenol (1.26 g, 6.66 mmol), and DMAP (68 mg, 0.54 mmol) in 80 mL of dichloromethane was added 1-[3-(dimethylamino)propyl]-3-ethylcarbiimide methiodide (EDC·CH3I) (2.65 g, 8.94 mmol). The mixture was stirred for 24 h at room temperature. After evaporation of solvent, the residue was subjected to column chromatography on silica gel with hexane/ethyl acetate (9/1) as the eluting solvent to yield 2.9 g (93%) of 3. 1H NMR (CDCl3): δ 1.28− 1.40 (m, 4H), 1.6 (m, 2H), 1.8−1.95 (m, 7H), 2.24 (m, 2H), 3.40 (t, 2H), 4.03 (t, 2H), 4.04 (t, 2H), 6.92 (4H, ArH), 7.1 (2H, ArH), 8.15 (2H, ArH). S y n t h e s i s of 4 - ( H e x - 5 - y n - 1 - y l ox y ) p h e n y l 4 - ( ( 6 Mercaptohexyl)oxy)benzoate (4). A stirred solution of 3 (2.9 g, 6.15 mmol) in 30 mL of THF was cooled to −10 °C under nitrogen. Hexamethyldisilathiane (1.4 mL, 6.68 mmol) and tetra-n-butylammonium fluoride (TBAF) (1 M solution in THF, 6.28 mL) were added. The reaction mixture was stirred at this temperature for 10 min and poured into a saturated solution of ammonium chloride. The solution was extracted with ether, washed with brine, and dried over magnesium sulfate. After evaporation of solvent, the residue was subjected to column chromatography on silica gel with hexane/ethyl acetate (9/1) as the eluting solvent to yield 2 g (77%) of 4 as white crystals. 1H NMR (CDCl3): δ 1.25−1.32 (m, 3H), 1.35−1.40 (m, 2H), 1.5−1.63 (m, 4H), 1.75−1.95 (m, 5H), 2.24 (m, 2H), 2.5 (q, 2H), 4.03 (t, 2H), 4.04 (t, 2H), 6.92 (4H, ArH), 7.1 (2H, ArH), 8.15 (2H, ArH). Anal. Calcd for C25H30O4S: C, 70.39; H, 7.09. Found: C, 70.30; H, 7.0. Synthesis of 4-(5-Hexenyloxy)phenyl 4-((6-Mercaptohexyl)oxy)benzoate (5). Ligand 5 was synthesized as previously described.18,26 Surface Functionalization of Nanospheres. Metallic nanosphere surfaces were functionalized as previously described.18 Briefly, three different ligands were used to functionalize AuNS (Chart 2). To
fabricate thiol click nanocomposite films. In addition, the control of the chemistry at the surface of the AuNS provided an opportunity to evaluate the effect of functionalizing the AuNS with different ligands on the mechanical properties of the materials as well as assess the ability to cross-link NPs to a polymer matrix via thiol click chemistry. For this purpose, the synthesis of a new ligand was performed and is fully outlined in this report. Finally, we analyzed the optical properties of the materials via optical microscopy, transmission electron microscopy (TEM), and UV−vis spectroscopy. Compared to the previously mentioned technique,14 the method reported herein drastically decreased the amount of time needed to produce thiol click materials with well distributed AuNS from weeks to minutes. Control of the composition impacted both the strength and color of the nanocomposites. The process for fabricating nanoparticlecontaining thiol click materials may find use in the production of antibacterial coatings,19,20 development of optical materials/ devices,21,22 and applications in microcontact printing and soft lithography.23−25
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EXPERIMENTAL SECTION
Materials and Techniques. Pentaerythritol tetrakis(3-mercaptopropionate) (PETMP), 1,7-octadiyne (ODY), 1,4-butanediol divinyl ether (BDDVE) (Chart 1), 2,2-dimethoxy-2-phenylacetophenone
Chart 1. Reagents Used in Fabricating Thiol Click Nanocomposites
Chart 2. Ligands Used To Functionalize 10 nm AuNS: 4 (Alkyne−Thiol Ligand); 5 (Vinyl−Thiol Ligand); 6 (1Dodecanethiol) (DMPA), 1-dodecanethiol, tetrahydrofuran (THF), chloroform, poly(ethylene glycol) (PEG), and all the starting materials used in the ligand syntheses were purchased from Sigma-Aldrich and used as received. 10 nm gold and 20 nm silver nanospheres were purchased from SPI Supplies. 1 H NMR spectra were recorded on a Varian VXR-400 spectrometer using CDCl3 (internal reference 7.26) as the solvent. All column chromatography was performed using Mallinckrodt Chemicals silica grade 62 (60−200 mesh). Analytical thin chromatography (TLC) was performed on Merck silica gel 60 F254 plates. Synthesis of Ethyl 4-((6-Bromohexyl)oxy)benzoate (1). To ethyl 4-hydroxybenzoate (7.30 g, 44 mmol), 6-bromohexan-1-ol (8 g, 44.2 mmol), and triphenylphosphine (11.52 g, 44 mmol) was added 100 mL of THF. To this mixture was added dropwise a solution of 10 mL of THF and diethyl azodicarboxylate (DEAD) (7.42 mL, 46.9 mmol). The reaction mixture was stirred overnight at room temperature. After evaporation of solvent, the residue was subjected to column chromatography on silica gel with dichloromethane as the eluting solvent. The product was further crystallized from hexane to yield 10 g (69%) of white crystals 1. 1H NMR (CDCl3): δ 1.28−1.38 (m, 7H), 1.8−1.94 (m, 4H), 3.42 (t, 2H), 4.03 (t, 2H), 4.38 (q, 2H), 6.92 (2H, ArH), 7.99 (2H, ArH). Synthesis of 4-((6-Bromohexyl)oxy)benzoic Acid (2). To a solution of 1 (10 g, 30.4 mmol) in 150 mL of methanol and 40 mL of water was added LiOH·H2O (3.75 g, 90 mmol). The reaction mixture was heated to 50 °C for 4 h. After evaporation of the solvent, the residue was neutralized by a mixture of HCl/H2O. The suspension was filtered and washed with cold water. The crude product was crystallized from a mixture of hexane/ethanol (9/1) to yield 7 g
an empty 20 mL borosilicate glass vial, 5 mg of ligand (4, 5, or 6) was added, followed by 1 mL of THF; the contents were agitated and mixed together (vial 1). In a separate 20 mL borosilicate glass vial, 1 mL of citrate-stabilized 10 nm gold nanospheres was added (vial 2). The contents of vial 1 were added to vial 2, the vial was capped, and the combined contents were shaken vigorously. The resulting solution was allowed to sit in the vial (now uncapped and open to air), causing the now functionalized (and phase transferred) nanospheres to form a thin film layer along the wall of the vial. After about 30 min the vial was decanted, leaving behind only the nanosphere thin film. To resuspend the nanospheres, 1 mL of chloroform was added to the vial, removing the nanosphere film from the vial wall. To prepare the functionalized nanospheres for incorporation into polymer materials, the chloroform nanosphere suspension was concentrated by slight heating on a hot plate at 50 °C while being blown dry with N2(g) over the top of the open vial until only a few microliters of the suspension remained. In addition to AuNS, silver nanospheres (AgNS) were also functionalized and incorporated into thiol−yne polymers (Supporting Information). 696
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Prepolymer Solution Preparation. Thiol−yne solutions were made by combining 1 equiv (0.010 mol) of ODY with 1 equiv (0.010 mol) of PETMP. Thiol−ene solutions were made by combining 2 equiv (0.010 mol) of BDDVE with 1 equiv (0.005 mol) of PETMP. Either a thiol−yne or a thiol−ene solution was added to a vial containing concentrated, functionalized nanospheres. Vials were then sonicated until the nanospheres were dispersed in the thiol click solution (
