The detection of environmental changes has received significant attention. For example, there is much interest in polymeric hydrogels that respond to environmental stimuli because of their potential applications in drug delivery [1
], chemical and biosensing [7
], adsorption [9
], shape-control [10
], and color tuning [11
] The physical and chemical properties of these responsive polymer hydrogels can be altered by light, mechanical force, pH, and temperature [14
Among the hydrogels, poly(N
-isopropylacrylamide) (PNIPAM) is well known for its excellent thermo responsiveness [15
]. PNIPAM has a lower critical solution temperature (LCST) of around 32 °C in an aqueous medium. Below LCST, the polymer is water-soluble because its polymer chains are fully extended with random coil conformation. Because of the hydrophobic interaction, PNIPAM shrinks at temperatures above the LCST. This feature renders PNIPAM-based hydrogels suitable for use in drug-delivery systems. We attempted to investigate the relationship between shrinking and swelling and fluorescence changes of fluorescent PNIPAM hydrogels in aqueous solutions. Most fluorescent PNIPAM hydrogels were prepared via copolymerization of PNIPAM with a fluorescent monomer [17
], but fluorescent monomers showed lower stability and brightness than conjugated polymer dots (Pdots) [18
]. The quantum dots (QDs) in PNIPAM hydrogel were also investigated and QD fluorescence in the hydrogel was changed by LCST [19
]. However, QDs had low biocompatibility because of the heavy metal in QDs.
Pdots were derived from conjugated polymers (CPs) and their high fluorescence was inherited from their pristine CPs. In addition, they have various versatile properties, such as uniform dispersion in water and easy surface functionalization [20
]. Thus, a number of Pdots with various chemical structures were investigated for their application in chemo- and biosensing [24
We are reporting on the thermoresponsive fluorescence tuning of Pdots in a PNIPAM-based hydrogel. Thermally responsive fluorescence tuning in the hydrogel was reported using poly(2-isopropyl-2-oxazoline) copolymerized with various fluorescent dyes to attain various emission colors [31
]. Graphene oxide was used as a fluorescence quencher to achieve fluorescence tuning according to temperature, in which a PNIPAM copolymer controlled the distance between the quencher and the polymeric dye [32
]. However, the use of Pdot-hybridized hydrogel for fluorescence tuning has rarely been reported. To investigate the thermo responsiveness, a hybrid material based on Pdots and PNIPAM was synthesized, in which Pdots were linked with PNIPAM after allyl-group functionalization in a Pdot surface. The Pdots were covalently immobilized in PNIPAM, enabling stable fluorescence modulation without a release from the PNIPAM matrix. The changes in fluorescence intensity, as well as in the hydrodynamic diameter of the hybrid material, were investigated, resulting from the LCST behavior of PNIPAM. Upon heating the hybrid material above the LCST, the fluorescence intensity decreased mainly because of shrinkage of the PNIPAM coils, in which the fluorescence was recovered after cooling to room temperature. The hydrogel-based, fluorescence-tunable material has great potential for various applications in sensing.
2.1. Materials and Instrumentation
All chemicals were purchased from Sigma-Aldrich (St. Louis, MO, USA) and solvents were purchased from Samchun Chemicals (Seoul, Korea). All reagents were used without further purification unless otherwise noted. Maleic anhydride and benzoyl peroxide were purified by recrystallization before use. The inhibitor in styrene was removed using an inhibitor remover column (Sigma-Aldrich). The 1H NMR and 13C NMR data were obtained on a Bruker Fourier-300 spectrometer (Bruker, Karlsruhe, Germany). Elemental analysis (EA) was performed with a CE Instruments EA-1112 elemental analyzer (CE Instruments, Milan, Italy). The Fourier transform infrared (FT-IR) spectra were obtained on a Bruker Tensor 27 spectrometer (Bruker, Karlsruhe, Germany). The ultraviolet-visible (UV-vis) absorption spectra were recorded on a PerkinElmer Lambda 35 spectrometer (PerkinElmer, Waltham, MA, USA). The photoluminescence spectra with variation in temperature were taken using a Varian Cary Eclipse spectrometer (Agilient, Santa Clara, CA, USA) with a PCB-1200 temperature controller. The molecular weights (MWs) of the polymers were determined by gel permeation chromatography (GPC), with tetrahydrofuran (THF) as eluent with a polystyrene standard. Zeta-potentials and size distributions were measured by dynamic light scattering (DLS, Zetasizer Nano ZS, Malvern, Worcestershire, UK). Scanning electron microscopy (SEM) images were obtained using a Hitachi S-4800 instrument (Hitachi, Tokyo, Japan). Transmission electron microscopy (TEM) images and energy-dispersive X-ray spectroscopy (EDS) data were taken using a JEM-3011, JEOL apparatus, Tokyo, Japan.
2.2. Synthesis of a Blue-Emitting CP (BCP)
M1 (0.448 g, 0.91 mmol) and M4 (0.330 g, 1 mmol) were dissolved in tetrahydrofuran (THF) containing an aqueous 2 M potassium carbonate solution (3 mL) under argon atmosphere. After addition of tetrakis (tripenylphosphine)palladium (0) (0.0526 g, 0.045 mmol), the reaction mixture was stirred at 100 °C for 30 h. After the reaction, the mixture was cooled and added to methanol (300 mL) and the precipitate was isolated by filtration. The precipitates were extracted with acetone for 24 h in a Soxhlet apparatus to remove oligomers and catalyst residues. After drying under vacuum, a gray powder was obtained (yield 0.15 g, 38%). 1H NMR (300 MHz, CDCl3): 7.71 (s, 2H), 7.10 (m, 4H), 3.99 (s, 4H), 1.75 (s, 8H), 1.41 (m, 16H), 0.88 (m, 6H) ppm. 13C NMR (CDCl3): 129.08, 77.32, 77.20, 76.73, 69.60, 40.83, 31.86, 29.39, 26.12, 22.66, 14.10 ppm. FT-IR (KBr pellet, cm−1): 2924-2852 (C-H), 1469 (C=C), 1209 (aromatic C-H) Anal Calcd. For C30H46O2: C, 82.14%; H, 10.57%. Found. C, 81.72%; H, 9.76%.
2.3. Synthesis of a Green-Emitting CP (GCP)
M1 (1.05 g, 2.1 mmol), M2 (0.07 g, 0.24 mmol), and M4 (0.86 g, 2.6 mmol) were dissolved under an argon atmosphere in a mixture of THF containing an aqueous 2 M potassium carbonate solution (5 mL). The subsequent procedure was identical to that used for BCP. After drying under vacuum, a gray powder was obtained (yield 0.74 g, 80%). 1H NMR (300 MHz, CDCl3): 7.71 (s, 3.78 H), 7.10 (m, 2.22 H), 3.99 (s, 3,56 H), 1.76 (s, 7.12 H), 1.48 (m, 7.12 H), 1.27 (m, 7.12 H), 0.87 (m, 5.34) ppm. 13C NMR (CDCl3): 150.48, 129.07, 77.86, 77.20, 76.68, 69.60, 31.82, 29.89, 26.12, 22.67, 14.10 ppm. FT-IR (KBr pellet, cm−1): 2924-2852 (C-H), 1610 (C=N), 1492 (C=C), 1207 (aromatic C-H) Anal Calcd. For C26.5H36.5N0.2O1.8: C, 82.23%; H, 10.23%; N, 0.79%. Found. C, 81.14%; H, 9.51%; N, 0.78%.
2.4. Synthesis of a Red-Emitting CP (RCP)
M1 (0.47 g, 0.95 mmol), M3 (0.19 g, 0.41 mmol), and M4 (0.45 g, 1.36 mmol) were dissolved under an argon atmosphere in a mixture of THF containing an aqueous 2 M potassium carbonate solution (3 mL). The subsequent procedure was identical to that used for BCP (yield 0.78 g, 73%). 1H NMR (300 MHz, CDCl3): 8.15 (s, 1.26 H), 7.7 (m, 2.2 H), 7.6 (m, 0.74 H), 7.4 (m, 0.74 H), 7.1 (s, 0.74 H), 7.0 (m, 0.74 H), 6.9 (m, 0.74 H), 2.03 (t, 2.52 H), 1.85 (m, 5.04 H), 1.72 (s, 5.04 H), 1.51 (s, 5.04 H), 0.87 (d, 3.78 H) ppm. 13C NMR (CDCl3): 77.44, 77.22, 76.59, 31.82, 29.71, 22.67, 14.12 ppm. FT-IR (KBr pellet, cm−1): 2922-2852 (C-H), 1737 (C-O), 1608 (C=N), 1485 (C=C), 1253 (C=S), 1205 (aromatic C-H) Anal Calcd. For C25.0H28.9N0.7O1.3S1.1: C, 75.88%; H, 7.29%; N, 2.61%; S, 8.97%. Found. C, 74.01%; H, 7.31%; N, 2.16%; S, 8.13%.
2.5. Synthesis of Poly(Styrene-Co-Maleic Anhydride) (PSMA)
Maleic anhydride (2.94 g, 0.03 mmol), styrene (3 mL, 0.0258 mmol), and dibenzoyl peroxide (BPO, 0.03 g, 0.12 mmol) were dissolved in toluene (40 mL) at room temperature under an argon atmosphere. The solution was stirred at 80 °C for 2 h. After the polymerization, the mixture was cooled to room temperature and poured in methanol (200 mL). The precipitates were isolated by filtration and washed with methanol three times. After drying under vacuum, a white powder was obtained (yield 5.2 g, 92%). 1H NMR (300 MHz, CDCl3): 7.4 (s, 0.53 H), 7.2 (m, 2.12 H), 3.3 (s, 0.94 H), 2.8 (s, 1.06 H), 2.3 (m, 0.53 H) ppm. FT-IR (KBr pellet, cm−1): 3030–2925 (C-H), 1857–1780 (C=O), 1496–1454 (C=C), 1222 (C-O-C). Anal Calcd. For C6.04H5.06O1.47: C, 71.64%; H, 5.00%. Found. C, 70.13%; H, 5.01%.
2.6. Fabrication of Polymer Nanoparticles (NPs)
Each CP (BCP, GCP, and RCP) was dissolved in THF (1 mg/mL). PMAS was dissolved separately in THF (1 mg/mL). The CP (250 μL) and PSMA solutions (150 μL) were added to THF (4.65 mL). After complete dissolution, the solution was quickly injected into water (10 mL) under sonication. After 10 min sonication, THF was removed by nitrogen blowing and the mixture was concentrated to 5 mL at 75 °C. During nitrogen blowing, carboxylic acid was generated on the NP surface. After filtration with a 0.2 μm filter, nanoparticle solution was obtained. The polymer nanoparticles (Pdots) were denoted as BPdots, GPdots, and RPdots from their pristine BCP, GCP, and RCP, respectively.
2.7. Pdots Coated with Allyl Amine (AA)
1-Ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC) and N-hydroxysuccinimide (NHS) were separately dissolved in water (2 mg/mL). AA was also dissolved in water (1 mg/mL). The EDC (200 μL), NHS (200 μL), and AA solutions (200 μL) were added to the Pdot solution (5 mL). After the addition, the solution was gently shaken for 24 h to prevent Pdot aggregation. The mixture was purified by dialysis for 24 h using a dialysis membrane with a MW cut off (MWCO) of 2000. The various AA-coated Pdots were denoted as [email protected]
, [email protected]
, and [email protected]
originating from BPdots, GPdots, and RPdots, respectively.
2.8. Pdot-Embedded Hydrogel
’-methylenebisacrylamide (BIS), and sodium dodecyl sulfate were dissolved in aqueous [email protected]
solution (5 mL) under argon. The mixture was heated to 70 °C and potassium persulfate (KPS) was added to the mixture for initiation. The mixture was stirred for 4 h and cooled to room temperature. After filtration through a 0.45 μm filter, the mixture was dialyzed for 72 h using a dialysis membrane with 2000 MWCO. The products were denoted as [email protected]
, [email protected]
, and [email protected]
, which were fabricated from [email protected]
, [email protected]
, and [email protected]