Article pubs.acs.org/Macromolecules
Poly(N‑isopropylacrylamide)/Gold Hybrid Hydrogels Prepared by Catechol Redox Chemistry. Characterization and Smart Tunable Catalytic Activity Gema Marcelo,*,† Mar López-González,‡ Francisco Mendicuti,† M. Pilar Tarazona,† and Mercedes Valiente† †
Universidad de Alcalá, 28871 Alcalá de Henares, Madrid, Spain Instituto de Ciencia y Tecnología de Polímeros (ICTP-CSIC), C/Juan de la Cierva 3, 28006 Madrid, Spain
‡
ABSTRACT: PNIPAM hydrogels functionalized with gold nanoparticles were prepared by making use of catechol redox chemistry. For this purpose catechol groups were introduced in the PNIPAM network during the cross-linking polymerization process (PNIPAM− catecholx hydrogel). These groups act as reducing agents of HAuCl4, which enables the functionalization of the PNIPAM with gold nanoparticles (PNIPAM−catecholx@Au hydrogel). The rheological study shows that catechol groups reinforce the hydrogel structure. A stronger effect was observed after the functionalization of the hydrogel with gold nanoparticles. This influence could be easily observed since the variation of G′ with the MBA mole fraction was fitted to a powerlaw expression G′ ∼ xMBAa, with a = 1.6 for PNIPAMx hydrogels, a = 2.3 for PNIMPAM−catecholx hydrogels and a = 3.2 for PNIMPAM−catecholx@Au hydrogels. The capability of the PNIPAM−catecholx@Au hydrogel to act as a tunable catalyst was demonstrated with a model reduction reaction. The half-lifetime at 25 °C was 10.5 min; however, at 38 °C the half-lifetime was 133 min.
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INTRODUCTION Poly(N-isopropylacrylamide) (PNIPAM) is the most studied and employed thermosensitive polymer. It undergoes a sharp coil-to-globule transition and a subsequent phase separation above its lower critical solution temperature (LCST, ca. 32 °C).1−23 This feature makes PNIPAM a very attractive candidate for the preparation of hydrogel materials with a wide range of applications such as catalysis,4 sensor devices5 and drug delivery systems,6−8 membranes,9 and microvalve devices10 among others. However, its poor mechanical properties in the swollen state1,11−15 and weak shrinking behavior16 limit the application domain. One important strategy to reinforce the mechanical properties of a PNIPAM hydrogel is to introduce inorganic materials in its network.17−20 Organic−inorganic hybrid materials combine the mechanical strength characteristic of the inorganic part and the required properties of the thermoresponsive PNIPAM network. Apart from the desired effect on the reinforcement of the hydrogel network, the introduction of inorganic material provides new functionalities to the PNIPAM hydrogel. For instance, the preparation of hydrogels that combine an inorganic material that transforms an external stimulus in heat with a PNIPAM thermoresponsive network widens the application range of PNIPAM hydrogels, because the PNIPAM phase transition could be triggered by external stimuli.21−23 Recent literature reveals that there are a growing number of papers describing the synthesis of new hybrid © XXXX American Chemical Society
inorganic/thermoresponsive hydrogel materials with promising applications in different material areas, but relatively few manuscripts investigate the mechanical behavior of these new materials. Gold nanoparticles (Au NPs) display a strong absorption band in the UV−visible−NIR light region; the physical origin of this phenomenon is the collective oscillation of the conduction-band electrons induced by the interacting electromagnetic field, which is known as localized surface plasmon resonance (LSPR).24 This feature makes gold nanoparticles very interesting for applications in different fields such as catalysis,25 electronics,26 photonics,27 biomedicine (photo thermal therapy)28 and sensing.29 Combining the sensitivity of a stimuli-responsive polymer with inorganic metal nanoparticles, especially gold nanoparticles, could result in the formation of composite hydrogels with attractive synergistic properties, in which the gold nanoparticles (Au NPs) are stabilized by the hydrogel networks. Moreover, changes in the PNIPAM network below and above LCST can modify the activity properties of gold nanoparticles.30 This issue has recently generated great interest especially in the catalysis area, in which the ability to shrink/swell can modify the performance of the catalyst supported in the hydrogel structure.31 Received: June 12, 2014 Revised: August 1, 2014
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dx.doi.org/10.1021/ma501214k | Macromolecules XXXX, XXX, XXX−XXX
Macromolecules
Article
Table 1. MBA Content and Swelling Degree in PNIPAMx and PNIPAM−Catecholx Hydrogels with Characterization Results: Storage Moduli and Onset DSC Temperature of All Hydrogels hydrogel PNIPAM1 PNIPAM2 PNIPAM3 PNIPAM−catechol1 PNIPAM−catechol2 PNIPAM−catechol3 PNIPAM−catechol4 PNIPAM−catechol5 PNIPAM−catechol1@Au PNIPAM−catechol2@Au PNIPAM−catechol3@Au PNIPAM−catechol4@Au PNIPAM−catechol5@Au
MBA mole fraction 6.56 2.10 4.03 2.63 4.50 9.41 3.04 4.75
× × × × × × × ×
swelling degree (%)
G′ (kPa)
onset (DSC) (°C)
76.6 29.0 21.0 48.4 − 38.0 11.3 9.5
4.6 187 1569 5 60 350 2965 5155 80 173 710 4338 6726
33 33 33 25 25 25 25 25 25 25 25 25 25
−4
10 10−3 10−3 10−4 10−4 10−4 10−3 10−3
nitrophenol to 4-aminophenol at both above and below PNIPAM LCST.
Up to now, hybrid PNIPAM−gold gels have generally been prepared in two stages with several protocols such as (i) the separate preparation of Au NPs and hydrogels and then combining the two physically or chemically. The incorporation of Au NPs into the PNIPAM network by a thermal “breathingin” process based on the shrinking of the hydrogels at a temperature above LCST to be later swollen below LCST in an aqueous solution containing Au NPs.32 The cross-linking copolymerization of NIPAM with monomers containing either thiol or dithio groups have been reported to build hydrogels to be subsequently used as templates for the growth of gold nanoparticles in the presence of an external reducing agent.33,34 (ii) Mixing preformed Au NPs with a hydrogel precursor followed by gelation.35−37 (iii) A new methodology has recently been reported allowing the in situ synthesis of gold nanoparticles within the PNIPAM structure at the same time as the network is formed by the γ radiation of NIPAM and gold salt.38 In the last years, catechol-containing molecules are being studied for their exceptional binding affinities to various substrates39 and their unique reductive properties40,41 which permit the formation of metallic nanoparticles in the presence of catechol derivative species. The key feature of the catechol groups’ redox activity is their rapid oxidative self-conversion into their quinone forms by releasing protons and electrons under mild reductive conditions. For instance, the redox potential of catechol to 1,2-benzoquinone (−0.795 V)42 is substantially more negative than the redox potential of citrate (−0.5 V) widely used in gold nanoparticle formation.43 This difference in the reduction potential resulted in the fast formation of Au NPs when catechol groups were used. We selected catechol redox chemistry as a new strategy to prepare PNIPAM hydrogels containing gold nanoparticles in their structure in the absence of any other external reducing agent. In the present work, the cross-linking polymerization of NIPAM with a catechol methacrylamide monomer is carried out in order to functionalize the PNIPAM network with catechol groups. These new hydrogel materials are characterized in terms of rheologial and swelling behavior. The effect of the gold nanoparticles in the network structure and their localization are detailed by a complete characterization: SEM, XRD, DSC, and rheology study. We combine PNIPAM with gold nanoparticles in the same hydrogel structure to obtain a smart catalyst system. The capability of PNIPAM hydrogels containing gold nanoparticles to act as a smart catalyst system is studied with a model reduction reaction, the reduction of 4-
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MATERIALS AND METHODS
Materials. Dopamine hydrochloride, borax (Na2B4O7·10H2O), Nisopropylacrylamide (NIPAM), magnesium sulfate (MgSO4), 2,2′azobis(isobutyronitrile) (AIBN), N,N′-methylenebis(acrylamide) (MBA), methacryloyl chloride, sodium borohydride (NaBH4), chlorhydric acid (HCl), and sodium hydroxide (NaOH) were all purchased from Aldrich and used as received. All organic solvents dimethylformamide (DMF), ethyl acetate (EtAc), were purchased from Scharlau. Hydrogen tetrachloroaurate (III) (HAuCl4) was obtained from Alfa Aesar. Measurements and Equipment. Cryo scanning electron microscopy (SEM) measurements were performed using a SEM Zeiss 960 microscope equipped with a Cryotrans CT-1500 from Oxford. UV−vis spectra were recorded in a PerkinElmer Lambda 16 spectrophotometer. DSC experiments of the swollen hydrogels were carried out in a PerkinElmer DSC 6 model. The temperature range was 5 to 55 °C, and the heating rate was 10 °C/min. The rheological measurements were performed at 20 °C, in a Carri-Med CSL2100 controlled stress rheometer with cone−plate configurations (4 cm 1° and 2 cm 2°, depending on the viscosity of the samples). Temperature was controlled at ±0.1 °C by a Peltier system. Oscillation experiments were performed: structural parameters such as the storage modulus, G′, and the loss modulus, G″, were recorded as a function of frequency ( f). All of these measurements were carried out under the regimen of linear viscoelasticity; i.e. the material parameters were independent of the applied stress. When the linear viscoelastic region was established, measurements were performed as a function of frequency at constant stress. Thermogravimetric analyses (TGA) were carried out on a TGA Q500−0885 instrument from TA Instrumental Analysis. Dynamic experiments were performed at a heating rate of 10 °C/min from room temperature up to 800 °C under a nitrogen atmosphere (50 cm3/min). X-ray diffraction (XRD) patterns were recorded in the reflection mode by using a Bruker D8 Advance diffractometer provided with a PSD Vantec detector (from Bruker, Madison, WI. Cu Kα radiation (λ = 0.1542 nm) was used, operating at 40 kV and 40 mA. The parallel beam optics was adjusted by a parabolic Göbel mirror with a horizontal grazing incidence Soller slit of 0.12° and a LiF monochromator. The equipment was calibrated with different standards. A step scanning mode was employed for the detector. The diffraction scans were collected within the range of 2θ = 4−80°, with a 2θ step of 0.024° and 0.5 s per step. Synthesis of the Catechol Methacrylamide Derivative. The catechol methacrylamide monomer (catechol-methacrylamide) was prepared and characterized according to a previously reported strategy.44,45 B
dx.doi.org/10.1021/ma501214k | Macromolecules XXXX, XXX, XXX−XXX
Macromolecules
Article
Scheme 1. Illustration Showing the Hydrogel Formation and Functionalization with Au NPs
Synthesis of Hydrogels: PNIPAM−Catecholx and PNIPAMx Hydrogels. Hydrogels were synthesized from solutions containing NIPAM (16.5 mmol), catechol−methacrylamide (0.65 mmol), and MBA in a total volume of 2 mL (1.6 mL of DMF and 0.4 mL of water) and the MBA content was varied. The composition of the polymerization mixtures is shown in Table 1. The monomer feed content was constant in all polymerizations, mole content = 96% NIPAM and 4% catechol−methacrylamide; AIBN was used as the radical initiator. Upon AIBN addition (2 mg), the mixtures were degassed by bubbling argon for 30 min. The polymerization was initialized by warming up, reaction was maintained at 64 °C overnight. Also, hydrogels containing only NIPAM (16.5 mmol) were prepared with the same procedure described above. See Table 1. All hydrogels were swollen in deionized water at 25 °C for 2 weeks in order to eliminate the monomer excess and reach the swelling equilibrium. The swelling degree is defined as swelling degree (%) = (Ws/Wd) × 100, where Ws is the weight of water in the swollen gel and Wd is the dry weight of the gel. Synthesis of Hydrogels Containing Gold Nanoparticles: PNIPAM−Catecholx@Au Hydrogels. The incorporation of gold into the hydrogel structure was performed by putting a slice of the swollen PNIPAM−catecholx hydrogel (1.8 g) in contact with 1.5 mL of HAuCl4 water solution (14 mM). The solution was absorbed almost instantaneously (
