Article Cite This: Macromolecules XXXX, XXX, XXX−XXX
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Light-Sensitive Gas Sensors Based on Thiol-Functionalized N‑Isopropylacrylamide Polymer−Gold Nanoparticle Composite Films Yuqing Cozzens,† Diane M. Steeves,‡ Jason W. Soares,‡ and James E. Whitten*,† †
Department of Chemistry, University of Massachusetts Lowell, Lowell, Massachusetts 01854, United States U.S. Army Combat Capabilities Development Command Soldier Center, Natick, Massachusetts 01760, United States
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‡
ABSTRACT: Random and block N-isopropylacrylamide (NIPAM)/cysteamine copolymers and a thiol-terminated NIPAM homopolymer have been synthesized, characterized, and covalently linked to gold nanoparticles. Electrical conductivities of their films have been measured as a function of temperature and laser irradiation wavelength in the presence of water, methanol, and hexanes vapors. For the homopolymer composite, the distance between particles is so large that the conductivity is negligible. For the block copolymer composite, changes in dielectric constant dominate film conductivity due to coating of the gold particles. The random copolymer composite film changes conductivity dramatically in the presence of water and methanol vapors upon heating because of swelling/shrinking of the NIPAM. Laser irradiation with light matching the energy of the surface plasmon resonance of the gold nanoparticles causes dramatic changes in the conductivity of the composite film in the presence of water vapor. However, minimal changes occur for the corresponding film of the block copolymer.
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Wohlten and Snow first demonstrated that films of monolayer-protected gold nanoparticles (AuNPs) change their electrical resistance upon exposure to various gases and vapors.13 Their work inspired additional chemiresistor studies that included thiol-protected AuNPs, interlinked AuNP networks,14 and poly(2-hydroxyethyl methacrylate) inkjet printed on top of citrate-protected AuNPs.15 The mechanism of the transduction is sorption of vapor molecules into the film, which causes swelling.16,17 Ordinarily, the conductivity decreases (i.e., the resistance increases) as the gold nanoparticles become further apart. However, in some cases, an increase in conductivity has been observed due to changes in the dielectric properties of the film.13,18−21 This effect tends to dominate for polar vapors such as water or alcohols or when the distance between the gold cores is large.18,19 These types of sensors are generally completely reversible, regardless of whether the change is due to swelling or changes in permittivity of the medium, with the conductivity returning to its original value upon removal of the analyte vapor.18,19 Selectivity may be achieved by using more than one film of different functionalized gold nanoparticles, since the ligands affect how well the vapor is solubilized or partitioned from the gas to the condensed phase.
INTRODUCTION A large body of literature exists related to thermal responsive polymers,1−8 with the most common being copolymers of poly(N-isopropylacrylamide), known as “PNIPAM”. At room temperature, the pendant amide groups of this polymer form hydrogen bonds with water, resulting in an extended polymer chain conformation (coil).5,6 Upon a temperature increase, water molecules have enough energy to dissociate from the amide groups, and the polymer chains collapse into globules, which typically consist of single, collapsed polymer chains. This coil-to-globule shrinking and expulsion of water by PNIPAM copolymers is reversible, with a low critical solution temperature (LCST) of ca. 32 °C.7,8 Volume changes of the hydrogel as great as 8000% have been reported.4 Aqueous solutions of the polymers typically become cloudy above the LCST. Upon cooling below this temperature, they swell again by reabsorption of water, and a clear solution is re-established. The optical properties of gold nanoparticle/PNIPAM composites have been investigated,9 and the possibility of using laser light to actuate them via heat transfer has also been studied. Examples of such research include gold nanoparticles attached to mesoporous silica loaded with drugs, with the gold/silica structure covered by PNIPAM that changes shape and releases the drug upon laser irradiation.10 Other applications include formation of a light-activated microlens from a composite of a gold nanoparticles and PNIPAM copolymer11 and fabrication of a light-activated mechanical actuator.12 © XXXX American Chemical Society
Received: December 11, 2018 Revised: March 2, 2019
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DOI: 10.1021/acs.macromol.8b02638 Macromolecules XXXX, XXX, XXX−XXX
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Synthesis of HOOC-PNIPAM-CTA, 1. Polymerization of NIPAM was conducted at 70 °C under an argon atmosphere, employing C12CTA-COOH as the RAFT CTA and AIBN as the radical initiator with [NIPAM]:[C12-CTA-COOH]:[AIBN] = 200:1:0.1 at 70 °C in 1,4-dioxane for 4 h, following known protocols.22−25 A typical RAFT polymerization procedure was as follows. NIPAM (3.834 g, 33.8 mmol), C12-CTA-COOH (0.0612 g, 0.168 mmol), AIBN (2.76 mg, 0.0168 mmol, 1.5 mL solution of 9.2 mg of AIBN in 5 mL of 1,4dioxane), and 1,4-dioxane (10.5 mL) were placed in a 20 mL vial equipped with a magnetic stirring bar. The solution was purged with argon for 30 min, and the reaction vial was sealed and placed in a preheated oil bath at 70 °C for 4 h. The polymerization was quenched by cooling in dry ice and exposing the solution to air. The polymer was precipitated in cold ether, then filtered, and dissolved in acetone. The dissolution−precipitation cycle was repeated three times. The product HOOC-PNIPAM-CTA, 1, was filtered and dried under vacuum at room temperature overnight. The conversion was 100% (determined by confirming disappearance of NIPAM monomer proton NMR peaks), and the yield was 78%, with an obtained mass of 3.038 g. It was characterized by 1H NMR in CDCl3 and GPC with DMF as eluent in 0.1 M LiCl at 50 °C (Mn = 32K, PDI = 1.11). Synthesis of HOOC-PNIPAM-b-PNASI-CTA, 2. This was synthesized by RAFT polymerization in 1,4-dioxane at 70 °C for 24 h under an argon atmosphere using HOOC-PNIPAM-CTA, 1, as the macrochain-transfer agent in a 20 mL vial equipped with a magnetic stirring bar. The ratios used were [NASI]:[Macro-CTA-COOH]:[AIBN] = 30:1:0.1. NASI (0.0635 g, 0.376 mmol), HOOC-PNIPAM-CTA, 1 (0.376 g, 0.0125 mmol), AIBN (0.205 mg, 0.00125 mmol), and 1,4dioxane (2.5 mL). The polymerization was quenched by cooling in dry ice and exposing the solution to air. The polymer was precipitated in cold ether, then filtered, and dissolved in acetone. The dissolution− precipitation cycle was repeated three times. The product was filtered and dried under vacuum at room temperature overnight. The conversion was 100%, with a yield of 66%, with an obtained mass of 0.290 g. This resulting polymer 2 was characterized by 1H NMR in CDCl3 and GPC with DMF as eluent with 0.1 M LiBr at 50 °C (Mn = 36K, PDI = 1.22). Synthesis of HOOC-PNIPAM-b-cysteamine, 3. This was synthesized by postfunctionalization of HOOC-PNIPAM-b-PNASI-CTA, 2, by dissolving 2.90 g of it (combined from several synthetic runs to prepare 2) in 10 mL of MC. Excess cysteamine hydrochloride (0.426 g, 3.76 mmol) (10 times the molar equivalents of NASI used in polymer 2) was dissolved in MC (10 mL), mixed with TEA (0.42 mL) to deprotonate the hydrochloride, and added to the reaction mixture. The combined solution was degassed by bubbling with argon gas for 30 min, and the reaction was stirred for 24 h under an argon atmosphere at room temperature. DTT (an equal amount as cysteamine hydrochloride used) was added to disrupt the disulfide bonds. The desired block copolymer, 3, was precipitated in cold diethyl ether, filtered, and vacuum-dried, and 2.32 g was obtained, with a yield of 80%. The polymer was purified by dissolving in deionized H2O and dialyzed using 6000−8000 molecular weight cutoff membrane, against deionized H2O for 1 day, against a 5 mM HCl solution for 2 days, and against deionized H2O for another 1 day. The resulting polymer was freeze-dried and stored for further use. Synthesis of HOOC-PNIPAM-SH, 4. This was synthesized by aminolysis of HOOC-PNIPAM-CTA, 1, in the presence of propylamine.22−25 The process is also depicted in Scheme 1. In a 20 mL vial, 1 (0.376 g, 0.0125 mmol) was dissolved in 10 mL of MC together with propylamine (10-fold excess of 1). The resulting solution was deoxygenated by purging with argon for 30 min, then sealed, and stirred at room temperature for 24 h. The yellow color of the solution gradually faded to colorless. DTT was added to disrupt the disulfide bonds in the resulting product. The purification procedure of polymer 4 was identical to polymer 3. The purified polymer was freeze-dried and stored for further use. The yield was 91.3%, with an obtained mass of 0.344 g. Synthesis of PNIPAM-stat-PNASI, 5. This was synthesized, as shown in Scheme 2, via free radical polymerization using AIBN as the initiator.26−30 NIPAM (3.0 g, 26.51 mmol) and NASI (0.4982 g, 2.95
In consideration of the electrical sensors that have been fabricated from monolayer-protected gold nanoparticles, the ability of PNIPAM-based polymers to swell in the presence of polar molecules, and the thermal responsiveness of PNIPAM, it seemed interesting to explore the possibility of attaching PNIPAM to gold nanoparticles and attempting to heat them with laser light. In this study, gold nanoparticles have been covalently linked to a thiol-functionalized PNIPAM homopolymer and PNIPAM-cysteamine block and random copolymers. The morphology and electrical conductivity properties of films of these composites have been studied, and the possibility of fabricating a laser-sensitive chemical vapor sensor based on films of these composites has been investigated. It is found that the properties of the composite films and their vapor sensing properties depend dramatically on the structure of the polymer (e.g., random versus block). For the random copolymer, a laser whose wavelength matches the energy of the surface plasmon resonance of the gold nanoparticles is shown to selectively actuate the chemical sensing of water vapor. However, its electrical conductivity changes negligibly under laser irradiation in the presence of pure nitrogen, methanol, or hexanes vapors.
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EXPERIMENTAL SECTION
Materials. Reagents and solvents were purchased from SigmaAldrich. 2,2′-Azobisisobutyronitrile (AIBN, 99%), 2-(dodecylthiocarbonothioylthio)-2-methylpropionic acid (C12-CTA-COOH, 98% HPLC grade), N-acryloxysuccinimide (NASI, 90%), cysteamine hydrochloride, dithiothreitol (DTT), triethylamine (TEA), and methylene chloride (MC) were used as received. N-Isopropylacrylamide (NIPAM, 99+%) was recrystallized twice using hexanes and dried under vacuum. Anhydrous 1,4-dioxane (99+%) was passed through a column of basic alumina prior to use. Citrate-protected gold nanoparticles (Au NPs, Sigma-Aldrich, stock no. 741957) with ca. 10 nm diameter, chloroform-d (Sigma-Aldrich, 99.8% D), and deuterium oxide (Sigma-Aldrich 99.9% D) were used as received. Syntheses. Figure 1 is a depiction of the three different thiolterminated PNIPAM polymers synthesized for this study. The
Figure 1. Thiol-functionalized random, block, and homo PNIPAM polymers synthesized in this study. The dark and light spheres represent NIPAM and cysteamine monomers, respectively. Not shown are the single carboxylic acid groups on the NIPAM end of the chains of the block and homopolymer. PNIPAM-b-PNASI precursor copolymer was synthesized by a twostep method (Scheme 1). The PNIPAM-CTA polymer was prepared first and used as a macro-chain-transfer agent to polymerize the NASI monomer. The final thiol-containing PNIPAM block copolymer (PNIPAM-b-cysteamine) was obtained by PNIPAM-b-PNASI postfunctionalization, by reaction between NASI and cysteamine. The details follow. B
DOI: 10.1021/acs.macromol.8b02638 Macromolecules XXXX, XXX, XXX−XXX
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Scheme 1. Reversible Addition−Fragmentation Chain Transfer (RAFT) Homopolymerization and Block Copolymerization of N-Isopropylacrylamide (NIPAM) and N-Acryloxysuccinimide (NASI) with a Trithiocarbonate as the Chain Transfer Agenta
a
The values of m and n are 280 and 22, respectively, as discussed in the text. 10 times the moles of polymer 5 used) was dissolved in 10 mL of MC. Triethylamine (TEA) (0.42 mL) was added to the cysteamine hydrochloride solution to decomplex the hydrochloride from the cysteamine, and this was mixed with the polymer 5 solution. The combined solution was degassed by bubbling with nitrogen for 30 min, and the reaction was stirred for 24 h under a nitrogen atmosphere at room temperature. DTT (equal moles of cysteamine hydrochloride used) was added to disrupt any disulfide bonds. The final product, 6, was obtained, precipitated in cold diethyl ether, filtered, and vacuum-dried. For further purification, it was dissolved in deionized H2O and dialyzed using 6000−8000 g/mol molecular weight cutoff membrane against deionized H2O for 1 day, against 5 mM HCl solution for 2 days, and against deionized H2O for another day. Polymer 6 was freeze-dried and stored for further use (weight = 1.366 g; yield = 61%; Mn = 60K; PDI = 1.82). Preparation of Gold Nanoparticle (AuNP) Composites. Au NP/PNIPAM-stat-cysteamine, AuNP/PNIPAM-b-cysteamine, and AuNP/PNIPAM-SH composites were synthesized as follows. In each case, 10 mg of the polymer, which represents an excess, was mixed with 5 mL of gold nanospheres (corresponding to ca. 3 × 1013 Au particles, according to the vendor data sheet) in 5 mL of DI water. The mixture was ultrasonicated (Bransonic model 2510R-DTH with a power of 100 W at 42 kHz) for 5 min and stirred for 24 h. After centrifugation at 12000 rpm for 35 min, the particle composites settled to the bottom of the vial. Excess ligands and side products in the supernatant solution were decanted. The precipitate composite particles were redispersed in 5 mL of DI water, sonicated, and centrifuged again for further purification. This procedure was repeated three times. The final gold nanoparticle composite was redispersed in 0.5 mL of DI water with stirring and stored for further use. The AuNP/polymer composites were found to form stable colloidal dispersions, and particles did not settle out of solution even after several weeks. To compare their temperature dependence via 1H NMR, samples of PNIPAM-b-cysteamine and AuNPs/PNIPAM-bcysteamine were both freeze-dried to remove water after purification and then dissolved in deuterium oxide.
Scheme 2. Free Radical Polymerization of the Random Copolymer of NIPAM and NASI, with AIBN as the Initiatora
a
The values of m and n are 467 and 52, respectively, as discussed in the text.
mmol), with a feed ratio of 9:1, were dissolved in 38 mL of 1,4dioxane in a 100 mL three-neck flask. AIBN (1 mol % of the total monomers, 48.37 mg, 0.295 mmol) was added after the monomer solution was degassed by bubbling with nitrogen for 30 min. The polymerization proceeded for 24 h at 65 °C and was quenched by cooling in ice and exposed to air. The resulting polymer 5 was precipitated in an excess of cold diethyl ether, filtered, and vacuumdried at room temperature overnight (3.303 g; yield = 93%). Synthesis of PNIPAM-stat-cysteamine, 6. This was synthesized by postfunctionalizing PNIPAM-stat-PNASI, 5, using known protocols.26−30 Polymer 5 (1.630 g) was dissolved in 10 mL of methylene chloride (MC). Excess cysteamine hydrochloride (0.345 g, which is C
DOI: 10.1021/acs.macromol.8b02638 Macromolecules XXXX, XXX, XXX−XXX
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Figure 2. Experimental setup for conductivity measurements of the polymer films as a function of temperature and laser irradiation upon exposure to solvent-saturated nitrogen.
Figure 3. 1H NMR spectra of HOOC-PNIPAM-CTA (1), HOOC-PNIPAM-b-PNASI-CTA (2), HOOC-PNIPAM-b-cysteamine (3), and HOOC-PNIPAM-SH (4) in CDCl3. The peaks at ca. 2.2−2.3 and 3.75 ppm in the spectra of 1 and 2 are due to trace acetone and 1,4-dioxane, respectively. 7.5 mm). An oven temperature of 50 °C was used, and the solvent was DMF with 0.1 M LiBr at a flow rate of 1.0 mL/min. The concentration of the samples was 3 mg/mL, which equilibrated overnight. The flow marker was toluene, and PMMA was used as a calibration standard. Absorbance spectra were acquired using a Lambda 1806 UV−vis spectrometer. Transmission electron microscopy (TEM) images of 2 mg/mL aqueous solutions drop-cast onto 200 mesh carbon-coated copper grids were obtained using a Philips EM 400t microscope with 100 kV acceleration.
Characterization. Proton nuclear magnetic resonance (1H NMR) spectra were recorded on a Bruker 500 MHz spectrometer using CDCl3 as the solvent to confirm the polymer structures. D2O as solvent was used to study the thermoresponsiveness of gold nanoparticles coated with PNIPAM-b-cysteamine. Size exclusion chromatography (SEC) measurements employed an Agilent 1260 equipped with a differential refractometer and a series of the following columns: 1x PLgel mixed guard (50 mm × 7.5 mm); 1x PLgel mixed-C (300 mm × 7.5 mm); 1x PLgel mixed-D (300 mm × D
DOI: 10.1021/acs.macromol.8b02638 Macromolecules XXXX, XXX, XXX−XXX
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Figure 4. 1H NMR spectra of PNIPAM-stat-PNASI (5) and PNIPAM-stat-cysteamine (6) in CDCl3 (7.26 ppm).
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Conductivity/Sensor Measurements. Films of the AuNP/ PNIPAM composites were prepared by drop-casting from aqueous solutions onto commercial interdigitated array (IDA) microelectrodes (Microsystems, Inc., part no. M1450110) to measure their electrical resistances. These consisted of 50 pairs of gold electrodes on a quartz substrate with the following dimensions: 15 μm electrode width, 15 μm spacing, 4800 μm overlap length, and 1500 Å electrode thickness. The drop-casting procedure consisted of placing one or two drops of the composite solution onto the microelectrode array, allowing it to air-dry, and repeating the process about five times. The average thicknesses of the AuNP/PNIPAM-cysteamine composite films on the IDA electrodes were determined using a Wyko NT 2000 optical profilometer as 11.2 ± 2.5, 3.3 ± 1.0, and 4.2 ± 1.5 μm for the block, random, and thiol-terminated copolymer films, respectively. Conductivity measurements were performed as a function of temperature and laser irradiation upon exposure to different solvent vapors using the custom-built setup shown in Figure 2. Nitrogen carrier gas was bubbled through the solvent of interest (hexanes, methanol, or deionized water), with the solvent at ∼23 °C. The nitrogen gas stream, saturated with the solvent vapor, then flowed into a hermetically sealed 3.5 × 3.5 × 1.5 in.2 aluminum chamber equipped with entrance and exit valves. The concentration of the solvent in the nitrogen was estimated based on its vapor pressure at 23 °C. The film could be heated during the conductivity measurements by placing the entire aluminum box on a hot plate. A type K thermocouple for measuring the temperature was held in mechanical contact with the interior of the aluminum chamber via a screw. A quartz window was placed over a hole that had been drilled into the box such that laser light could be shined through the quartz window and onto the composite-coated IDA. It was sealed with a low vapor pressure, high temperature two-part epoxy/resin. Five diode lasers with wavelengths of 405, 462, 520, 635, and 785 nm were used to irradiate the sample through the window. The currents that drive these diode lasers were set via potentiometers such that each laser delivered 50 ± 5 mW of power, as measured by an optical power meter. The conductivities of the films were measured by applying a bias of 1.0 V and measuring the current that flowed through the two legs of the IDA using a Keithley model 2400 source meter.
RESULTS AND DISCUSSION Properties of the Copolymers. 1H NMR spectroscopy was used to characterize the structures of HOOC-PNIPAMCTA, HOO-PNIPAM-b-PNASI-CTA, HOOC-PNIPAM-bcysteamine, HOOH-PNIPAM-SH, PNIPAM-stat-PNASI, and PNIPAM-stat-cysteamine. Their spectra are shown in Figures 3 and 4. The polymer backbone proton peaks fall between 1.5 and 2.4 ppm and are labeled as “a”, “b”, “c”, and “d” in Figure 3. The peaks at ca. 1.0 and 4.0 ppm belong to the methyl and methylene protons of NIPAM, respectively, and are labeled “e” and “f”. The proton of the NH group is not observed due to rapid exchange with the solvent. For NASI, the characteristic peak is at ca. 2.8 ppm and due to the −CH2−CH2− protons of the ring; it is labeled “g”. For cysteamine, characteristic peaks are at 2.70 ppm (“h”, −NHCH2CH2SH) and 3.44 ppm (“i”, −NHCH2CH2SH). The disappearance of the 3.33 ppm peak from the CTA (“j”, −SC(S)SCH2C11H23) due to thiol functionalization of HOOC-PNIPAM-CTA with propylamine to form HOOC-PNIPAM-SH indicates quantitative conversion of the terminal moieties. The molar ratios of the NIPAM and cysteamine units in PNIPAM-stat-cysteamine and PNIPAM-b-cysteamine were calculated to be 9:1 and 13:1, respectively, based on the integral ratio of resonance signals at 4.0 ppm (peak f) and 2.7−2.8 ppm (peak h). The molecular weight (MW) and polydispersity indices (PDI) were measured by SEC/GPC. When combined with the relative areas of the NMR peaks, it is possible to determine the average number of NIPAM and cysteamine units in each polymer chain. The results are summarized in Table 1 and included in the labels of Figure 1. In the case of the random copolymer, the average number of NIPAM and cysteamine units in each polymer chain is 467 and 52, respectively. For the block copolymer, these respective values are 280 and 22. Finally, for the homopolymer terminated with a thiol, there are on average 280 NIPAM units per chain. Properties of the AuNP/Polymer Composites. The thermoresponsiveness of the PNIPAM280-b-cysteamine22 block E
DOI: 10.1021/acs.macromol.8b02638 Macromolecules XXXX, XXX, XXX−XXX
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line broadening, as similarly observed by Canzi et al.31 in NMR studies of dodecanethiol attached to AuNPs. TEM illustrates that for similarly prepared samples the AuNP/PNIPAM467-stat-cysteamine52 and AuNP/PNIPAM280b-cysteamine22 composites have a tendency to form clusters, as shown in Figure 6. In contrast, the Au NP/PNIPAM280-SH composite leads to a very dispersed film. To understand the agglomeration behavior, idealized “cartoons” of the AuNP/ polymer composites are shown in Figure 7. For the random copolymer composite, the PNIPAM chains are likely looped due to the multiple cysteamine binding sites of the polymer chains (Figure 7a). For the block copolymer composite, the cysteamine units of a single polymer chain block numerous gold sites and limit the number of PNIPAM chains that can extend from it, as depicted in Figure 7c. In both cases, the number of extended PNIPAM chains, which act as spacers for adjacent gold nanoparticles, is limited and agglomeration occurs. In contrast, for the Au NP/PNIPAM280-SH composite (Figure 7b), a high surface density of thiols with attached polymer chains forms on the nanoparticles because one thiol group per PNIPAM chain is available to bind to the nanoparticles; site blocking of the gold AuNP surface does not occur. This results in gold nanoparticles surrounded by dense, extended PNIPAM chains that protect the AuNPs from agglomeration. Figure 8 shows the absorbance spectra of Au nanoparticles coated with the three different thiol-functionalized PNIPAM polymers in aqueous solutions. All have maxima at ca. 525 nm, which is the same as that of citrate-protected gold nanoparticles. This absorbance peak is due to the AuNP surface plasmon resonance and may be employed to heat the nanoparticle composite with laser radiation, as will be discussed shortly. Vapor Sensing. The conductivities of drop-cast films of the three AuNP/PNIPAM composites on IDAs have been measured using 1.0 V bias. The film of the AuNP/PNIPAMSH composite exhibits immeasurably low current flow (ca. 0.1 nA) for all vapors studied, consistent with the particles being very disperse. The current flow through the AuNP/PNIPAM-
Table 1. GPC Results of Molecular Weights and Polydispersity Indices for PNIPAM and ThiolFunctionalized PNIPAM Polymers polymer
Mn (g/mol)
Mw (g/mol)
PDI
HOOC-PNIPAM280-CTA (1) HOOC-PNIPAM280-b-NASI22-CTA (2) HOOC-PNIPAM280-b-cysteamine22 (3) HOOC-PNIPAM280-SH (4) PNIPAM467-stat-cysteamine52 (6)
32122 35838 37003 31759 59610
35814 43751 50315 35456 108589
1.11 1.22 1.35 1.11 1.82
copolymer and the gold AuNP/PNIPAM280-b-cysteamine22 composite dissolved in D2O has been studied using variable temperature 1H NMR. As shown in Figure 5a, heating from 25 to 40 °C results in gradual shifts of the characteristic PNIPAM block copolymer proton peaks to higher field, likely due to changing distances between nearby protons as the polymer changes conformation. Their loss of intensity occurs due to restricted mobility and fast relaxation times, as the polymer chains are transformed from liquid to globular state. The almost complete disappearance of the NMR peaks upon heating in the range of 35−40 °C, and their reappearance upon cooling, are consistent, within experimental error, with the known LCST of ca. 32 °C.7,8 Corresponding data for the AuNP composite sample are shown in Figure 5b. In this case, the NMR peaks show minimum intensity at ca. 65 °C, indicating that attachment of the polymer to the gold nanoparticles causes an increase in its LCST, which has also been confirmed visually. The origin of the increased LCST likely is impediment of expulsion of water from the PNIPAM chains due to the presence of the gold particles. Furthermore, the NMR peaks do not decrease in intensity as dramatically as for the unattached block copolymer, suggesting that the chains do not collapse as fully. In the case of the AuNP composite, the absence of cysteamine α and β methylene peaks (in the magnified 25 °C scans of Figure 5) is consistent with coordination of the thiol groups to the AuNPs and extensive
Figure 5. 1H NMR as a function of cooling/heating for (a) PNIPAM280-b-cysteamine22 and (b) AuNP/PNIPAM280-b-cysteamine22. The experiments were performed in D2O in the order from bottom to top. Magnified regions between 2.4 and 3.4 ppm that contain methylene peaks of the cysteamine groups, which are absent for the AuNP composite sample, are included above the (bottom) 25 °C spectra. F
DOI: 10.1021/acs.macromol.8b02638 Macromolecules XXXX, XXX, XXX−XXX
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Figure 6. TEM images of films of (a) AuNP/PNIPAM467-stat-cysteamine52 composite, (b) AuNP/PNIPAM280-SH composite, and (c) AuNP/ PNIPAM280-b-cysteamine22 composite. The inset in (c) is a magnified image of the dashed region; its dimension from the left edge to the right one is ca. 80 nm.
Figure 7. Idealized binding of the cysteamine-containing polymers to gold nanoparticles: (a) Au NP/PNIPAM-stat-cysteamine, (b) Au NP/ PNIPAM-SH, and (c) Au NP/PNIPAM-b-cysteamine.
and the results are shown in Figure 9. For water vapor, the current increases from
