Article pubs.acs.org/JPCC
Stimuli-Responsive Poly‑N‑isopropylacrylamide: Phenylene Vinylene Oligomer Conjugate Young Il Park,† Bingqi Zhang,†,∥ Cheng-Yu Kuo,† Jennifer S. Martinez,‡ Jongwook Park,§ Surya Mallapragada,∥ and Hsing-Lin Wang*,† †
Chemistry Division and ‡Materials Physics & Applications Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States § Department of Chemistry, Display Research Center, The Catholic University of Korea Bucheon, Kyunggi, Korea 420-743 ∥ Department of Chemical and Biological Engineering, 2114 Sweeney Hall, Iowa State University, Ames, Iowa 50014, United States S Supporting Information *
ABSTRACT: Phenylene vinylene trimer (OPV) and PNIPAM conjugate with stimuli-responsive optical properties has been synthesized through the formation of amide linkage between PNIPAM and carboxylic-acidterminated OPV. This material exhibits thermoresponsive optical properties as temperature exceeds the lower critical solution temperature (LCST), which is 32 °C for PNIPAM and the conjugate. This PNIPAMtrimer conjugate is fully characterized by using NMR, FT-IR, temperature-dependent UV−vis, and fluorescence spectroscopy. We have found that the polymer conjugate solution turns opaque as temperature exceeds lower critical solution temperature and a five-fold increase in fluorescence intensity as temperature increases from 20 to 70 °C. Such distinct increase in fluorescence intensity is likely due to the rigidchromism, that is, the change in optical properties due to confinement of the chromophores resulting from restriction of polymer conformational structures. The PNIPAM-trimer conjugate also shows a decrease in decay lifetime with increasing temperature, whereas OPV trimer alone shows no change in decay lifetime as a function of temperature. These unique optical properties are not observed in the trimer and PNIPAM mixture, suggesting that the stimuli-responsive optical properties can occur only in PNIPAM−trimer conjugate linked through covalent bond.
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which is ∼32 °C.2 Because the phase transition occurs at a temperature close to body temperature, PNIPAM has been extensively explored for its possible applications in tissue engineering3 and controlled gene and drug delivery.4 Besides these applications that directly take advantage of the sharp conformational transition of PNIPAM in response to temperature, the smart behavior of PNIPAM has also been utilized to trigger fluorescence change and convey particular optical properties by incorporating chromophores or fluorophores to the PNIPAM system. For example, fluorophore and PNIPAM copolymers were synthesized as a nanothermometer to sense temperature change fluorescently5,6 even at the intracellular level;7 by inclusion of other stimuli responsive moieties, this kind of device could be further developed as a probe to monitor specific chemicals, such as metal ions8 and glucose.9 Depending on the properties of fluorophores (e.g., aggregation-induced emission or quenching upon aggregation) and the manners of incorporation (e.g., copolymerization, conjugation, or physically blending), PNIPAM-induced fluorescence change has been
INTRODUCTION The synthesis, processing, and applications of conjugated polymers and oligomers have attracted tremendous interests as they show commercial applications toward optical, electrical, and electronic devices. The advancement in the synthesis of conjugated polymer bearing side-chain structures has long been used to allow tunability in optical and electronic properties that are not accessible through conventional processing routes. Recent efforts in using conjugated oligomers as part of the polymer side chain or main chain to modulate the polymer optical properties via controlling the chromophore interactions has shown promise in achieving emerging properties. John Tovar et al. have shown that the self-assembly of oligomer and peptide conjugates leads to the formation of nanoscaled structures and specific optical properties.1 Using biomolecules as templates to form hierarchical structures with conjugated oligomers to modulate the electronic and optical properties remains an area of strong interest because they offer tunability and allow complex functionality such as energy and charge transfer through control of the self-assembly. Poly-N-isopropylacrylamide (PNIPAM) is a unique biopolymer because it undergoes a corresponding coil-to-globule transition at its lower critical solution temperature (LCST), © 2013 American Chemical Society
Received: December 10, 2012 Revised: March 15, 2013 Published: March 18, 2013 7757
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Figure 1. 1H NMR data of PNIPAM compounds in DMSO-d6: (a) PNIPAM with terminated amine group, (a-1) PNIPAM with terminated amine group added trifluoroacetic acid, (b) PNIPAM-trimer conjugate, and (b-1) PNIPAM-trimer conjugate added trifluoroacetic acid.
Scheme 1. Synthetic Scheme and Chemical Structure of PNIPAM-Trimer Conjugate
thermoresponsiveness of the PNIPAM-induced change in
shown to vary in different ways and on different scales based on local polarity, hydrophobicity, and rigidity around the chromophore probe.8−10 In this manuscript, we performed synthesis and characterization of a novel thermoresponsive fluorescent polymer by coupling polyphenylene vinylene (OPV) oligomer and PNIPAM through covalent bond. PNIPAM provides a platform allowing stimuli to trigger coil−globule phase transition, which in turn significantly alters their optical properties. The
luminescence properties is carefully studied by using temperature-dependent absorbance, emission, and dynamic light scattering (DLS) spectroscopies. Because of the broad applications of oligomers and their derivatives in electronic and optical fields, this novel polymer holds great promise as a candidate for future intelligent optical devices. 7758
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Article
EXPERIMENTAL SECTION
General Experiment. All reagents and solvents were purchased from Aldrich or Fisher and were used without further purification. PNIPAM with terminated amine groups (Mn 2500) was purchased from Aldrich. 1H NMR spectrum was recorded on Bruker Avance 400 spectrometers. The IR spectrum was recorded on a NICOLET-750 FT-IR spectrometer with KBr. The optical absorption, fluorescence spectrum, and decay lifetime were obtained with Varian Cary 500, QuantaMaster 40, and TimeMaster LED TM-200 on Photon Technology International (PTI) respectively. DLS was recorded on Malvern Zetasizer. Synthesis of PNIPAM-Trimer Conjugate. COOH-trimer (50 mg, 0.13 mmol), PNIPAM terminated with amine groups (15 mg), and toluene (0.1 mL) were added; then, the tube was sealed and heated at reflux for 24 h. The reaction mixture was allowed to cool to room temperature and the solvent was removed in a vacuum dry oven. The reaction mixture was analyzed by 1H NMR, and the mixture was used without purification. PNIPAM-trimer conjugate was filtered by a 0.4 μm Nylon Syringe filter before use. 1H NMR (400 MHz, DMSO): δ 1.02 (s, 78H), 1.46−1.54 (m, 26H), 1.92−1.98 (m, 13H), 2.66 (m, 2H), 2.95 (m, 2H), 3.83−3.87 (m, 6H), 3.88 (s, 6H), 7.32−7.19 (m, 16H), 7.44−7.38 (m, 4H), 7.57−7.53 (d, 2H), 7.65−7.63 (d, 2H) (Figure 1). FT-IR (KBr, cm−1): 3305, 3074, 2972, 2934, 2875, 1651, 1545, 1459, 1387, 1367, 1242, 1173, 1131, 1046, 988, 938, 884, 839, 670 (Figure 3 and Figure S4 in the Supporting Information).
Figure 2. UV−vis spectra of COOH-trimer (black), PNIPAM with terminated amine groups (red), their mixture (green), PNIPAMtrimer conjugate (blue), and amide-trimer (Magenta).
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RESULTS AND DISCUSSION Synthesis and Characterization. PNIPAM has two amine-terminated end groups that allow us to easily attach carboxylic-acid-terminated OPV-trimer to the end of the polymer, as shown in Scheme 1. PNIPAM-trimer conjugate was synthesized via the formation of amide following the procedure previously reported in the literature.11 The structure of the final product has been fully characterized by 1H NMR, which confirmed that trimers attached to both ends of PNIPAM. As shown in Figure 1, the 1H NMR spectrum of the PNIPAM-trimer conjugate, both PNIPAM and trimer have had the corresponding proton peaks well-assigned to their chemical structure. In addition, the 1H NMR of PNIPAM terminated with amine group results shows the NH3 peak upon protonation in DMSO when trifluoroacetic acid was added. (See Figure 2a,a-1.) However the protonation of this PNIPAMtrimer conjugate reveals no change in 1H NMR, suggesting that all NH2 has reacted, indicative of the covalent bond between OPV trimer and the PNIPAM polymer. (See Figure 2b,b-1.) FT-IR absorption spectra of PNIPAM-trimer conjugate, COOH_trimer, and PNIPAM terminated with amines are shown in Figure 3, Figure S5 in the Supporting Information, and Table 1. The main peak assignments of PNIPAM-trimer conjugate are as follows: 3305 (amide, N−H stretching), 2972 (−CH3, stretching), and 1651 cm−1 (amide, CO stretching) that was shifted from 1682 cm−1 (carboxylic acid, CO stretching) in COOH_trimer and evidenced that trimer was conjugated with PNIPAM.12 The FT-IR spectra of PNIPAMOPV conjugate for the most part resemble the FT-IR of the PNIPAM except that all peaks shift 1 to 2 nm between the two, suggesting a change in microenvironment, which causes subtle change in the vibrational energy of these functional groups.
Figure 3. FT-IR of (a) PNIPAM-trimer conjugate, (b) COOH-trimer, and (c) PNIPAM with terminated amine groups.
In addition to the NMR and FT-IR spectra, we compared the UV−vis and PL spectra of PNIPAM-trimer conjugate and PNIPAM/COOH_trimer mixture to better understand how covalent bonding impacted the trimer’s optical properties. The PNIPAM/COOH_trimer mixture lacking the covalent bond exhibited optical properties differing from those of the PNIPAM-trimer conjugate. Figure 2 shows the absorbance spectra of COOH_trimer, PNIPAM/COOH_trimer mixture, PNIPAM-trimer conjugate, and amide-trimer (see Scheme S1 in the Supporting Information); PNIPAM-trimer conjugate and amide-timer exhibit similar shape and λmax in the absorption spectra. The PNIPAM-trimer conjugate has a red-shifted UV− vis as compared with the COOH_trimer and PNIPAM/ COOH_trimer mixture, which further confirms the formation of covalent bond between PNIPAM and trimer. Stimuli-Responsive Optical Properties. This PNIPAM and trimer conjugate is visually light green in water with a bluish green fluorescence color. Upon heating the aqueous solution from 30 to 50 °C, we found that the solution changes 7759
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Table 1. FT-IR of PNIPAM-Trimer Conjugate, COOH-Trimer, and PNIPAM with Terminated Amine Groupsa CO (carboxylic acid) PNIPAM-trimer conjugate COOH-trimer PNIPAM with terminated amine a
CO (amide)
C−O, O−H (carboxylic acid)
1651, 1545 1682, 1599
1291, 1411 1645, 1543
New peak: 1242 cm−1. Peak assignment: N−C (amine): 1200 cm−1, CC: 1275−1020 cm−1, C−O (carboxylic acid): 1210−1320 cm−1.
Figure 4. Temperature-dependent behavior of PNIPAM-trimer conjugate in aqueous solution at 30−50 °C (a) UV−vis and (b) photoluminescence spectra of PNIPAM-trimer conjugate in aqueous solution. The optical images of (c) PNIPAM-trimer conjugate aqueous solution under phase transition and the (d) fluorescence colors of PNIPAM-trimer conjugate solution under UV radiation.
from clear to turbid (Figure 4c) and shows a four-fold increase in the photoluminescence (PL) at elevated temperature (>50 °C, Figure 4d). This indicates that the PNIPAM-trimer conjugate exhibits similar trends in its thermoresponsive properties, as the conjugate also undergoes a phase transition at temperature greater than 33 °C, which could be evidenced by the temperature-dependent UV and PL spectra in Figure 4a,b. The LCST of PNIPAM is well-documented at temperature around 32 °C. Thus, coupling with water-soluble trimers on both ends barely altered LCST. The enhancing effect of fluorescence intensity with increasing temperature showed a steady and sensitive response in the region below 50 °C, yet it could persist to over 70 °C, where fluorescence increased more conservatively upon further heating. (Figure S1-a in the Supporting Information). As compared with the fluorescence emitted at 33 °C, a nearly five-fold enhancement at 70 °C was shown, whereas, the trimer alone did not emit higher intensity fluorescence with increasing temperature. Instead the fluorescence was maintained at the same level and started to drop at 60 °C due to poor water solubility (Figure S1-c in the Supporting Information). The above results suggest that the enhanced fluorescence emission of PNIPAM-trimer conjugate is resulting from conformational change induced by PNIPAMtrimer conjugate.
Increasing the temperature beyond the LCST leads to destruction of hydrogen bonds between amide groups of PNIPAM and water molecules. The increase in quantum yield of PNIPAM−trimer conjugate is shown in Figure 4d, in which we found that PL intensity increases from 4 × 105 to 1.6 × 106. The change in quantum yield with increasing temperature is due to chain collapse, which brings trimer molecules close to each other in a water-free, rigid microenvironment. Therefore, we did not observe the quenching of the fluorescence because we expect that chromophores coming together results in quenching of fluorescence by way of aggregate formation. On the contrary, we find a huge increase (∼400%) in emission fluorescence intensity and a slight (∼10 nm) blue shift in fluorescence emission spectra (Figure 4b). The reason should lie in the formation of the hydrophobic domain near the PNIPAM units with a decreased microenvironmental polarity.6,10d Phase transition restricts the dynamics of the OPV trimer attached to the end of PNIPAM. When conformational change occurred at high temperature, trimer fluorophores were restricted but rather were forced together to form trimer aggregates. That is why we observed an enhancement rather than quenching in emission. A schematic illustration is shown in Scheme 2 to represent how the fluorescence emission was enhanced via conformational change of PNIPAM-trimer 7760
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structures, which agrees with our previous presumption that conjugated trimers were restricted in a relatively loose and rigid microenvironment but not forming trimer aggregates. The fluorescence decay lifetime of PNIPAM-trimer conjugate is measured to allow greater understanding of the correlation between fluorescent emission and aggregation as a function of temperature. (See Figure 6a.) The PL decay time of the conjugate is shortened at higher temperature from 3.0 (20 °C) to 1.5 ns (70 °C). A slight increase in the decay rate at room temperature was, however, insufficient to allow an accurate estimation of the activation energy for this material. The shorter lifetime may also be due to the reduction of delocalization of the exciton because of higher rigidity of trimer in polymer chain.13 The decay lifetime results are consistent with the coil−globule transition at elevated temperatures. The PNIPAM/COOH-trimer, PNIPAM/amide-trimer mixture, pure COOH_trimer, and amide-trimer, however, showed minimum dependence on temperature (Figure 6b,c, Figure S3 in the Supporting Information). Decay lifetime of all trimers and PNIPAM-trimer conjugates shows a single exponential decay curve with estimated lifetime ranges from 0.9 to 3.0 ns. Moreover, as can be seen in the Figure, only the NIPAM-trimer conjugate has a decay lifetime that is highly temperaturedependent. The PNIPAM-trimer conjugate with increased quantum yield suggests an increase in the rate of radiative decay versus nonradiative decay with increasing temperature. Using temperature triggered conformational changes of PNIPAM, and hence the optical properties exemplify the use of this kind of materials toward sensing and optical applications.
Scheme 2. Schematic Illustration of Fluorescence Intensity of PNIPAM-Trimer Conjugate in Response to Temperature
conjugate upon heating. At low temperature, PNIPAM-trimer molecules are under a random coil conformation where trimers could move freely with little restriction; however, as temperature increased, PNIPAM chains collapsed and underwent a coil−globule transition, leading trimers to be restricted in a more rigid and hydrophobic microenvironment, and consequently brought about an enhancement in fluorescence emission. The enhancing effect increases with further heating in the temperature range from 30 to 60 °C. In the case of PNIPAM/COOH-trimer and PNIPAM/amide-trimer mixture, we also found an enhancement of fluorescence intensity at high temperature. Nevertheless, the enhancement was only around 150−200% and saturated at 50−60 °C. (See Figure S1-b,c in the Supporting Information.) This result implied that free trimers in solution could be restricted less effectively in the favorable microenvironment upon phase transition. This trimer-molecules-decorated PNIPAM allows accurate determination of phase-transition temperature using fluorescence spectroscopy. Our results suggest a dynamic aggregate structure at elevated temperatures. Increasing temperature could enhance the interchain interaction, as indicated by the DLS data, which show a dramatic increase in particle size from 200 nm at 30 °C to ∼300 nm as temperature increases to 50 °C (Figure 5). This should be attributed to the hydrophobic interaction among PNIPAM chains. When the PNIPAM-trimer molecules are in a random coil configuration at 20 and 30 °C, the size distributions are broader compared with when the polymer is in a condensed globule at temperature above its LCST. The particle size decreases to 265 and 256 nm as temperature reaches 60 and 70 °C, respectively. The slight shrinking of particle size with further increase in temperature can be rationalized as a result of the contraction and rearrangement of the fine structures in polymers. As compared with the size of pure PNIPAM with increasing temperature (Figure S2 in the Supporting Information), inclusion of trimer on both ends of PNIPAM seems to make it harder to form tight
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CONCLUSIONS
To summarize, we synthesized a novel temperature responsive fluorescent PNIPAM-trimer conjugate that showed significantly enhanced emission fluorescence upon heating above the LCST due to microenvironmental polarity change. Other optical properties such as decay lifetime also followed a similar trend, indicating that a temperature-triggered coil−globule phase transition could bring about new features to the polymer conjugate relative to the free fluorophore, OPV trimer in this case. Moreover, because OPV trimer could be easily modified and functionalized with biologically active moieties, the nanoscaled polymer and trimer conjugate will expand the use of OPV type of fluorophores from industry to biotechnology as a smart sensor.
Figure 5. DLS data of PNIPAM-trimer conjugate (a) volume size distributions and (b) summarized average size and distribution as a function of temperature: (i) 20, (ii) 30, (iii) 40, (iv) 50, (v) 60, and (vi) 70 °C. 7761
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Figure 6. Decay lifetime of (a) PNIPAM-trimer conjugate, (b) PNIPAM/COOH-trimer mixture, and (c) PNIPAM/Amide-trimer mixture as a function of temperature.
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ASSOCIATED CONTENT
S Supporting Information *
Molecular structure of amide-trimer conjugate; temperaturedependent PL spectra of trimers, and NIPAM-trimer conjugate from 10 to 70 °C; DLS data of PNIPAM as a function of temperature; decay lifetime curves of COOH-trimer and amide-trimer as a function of temperature; FT-IR of PNIPAM-trimer conjugate, COOH-trimer, and PNIPAM with terminated amines. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We acknowledge support of the Basic Energy Science (BES), Materials Sciences and Engineering Division, Biomolecular Materials program, U.S. Department of Energy and Los Alamos National Laboratory (LANL) Directed Research and Development Funds. Los Alamos National Laboratory is operated by Los Alamos National Security, LLC, for the National Nuclear Security Administration of the U.S. Department of Energy under contract DE-AC52-06NA25396. We acknowledge support of the Center for Integrated Nanotechnologies (CINT), a DOE Nanoscience User Facility, and the Center for Nonlinear Studies (CNLS).
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