Dual-Responsive Ionically Assembled Fluorescent Nanoparticles from

Feb 22, 2012 - *E-mail: [email protected] (Q.L.); [email protected] (X.L.). Cite this:J. Phys. ... Harnessing Poly(ionic liquid)s for Sensing Applic...
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Dual-Responsive Ionically Assembled Fluorescent Nanoparticles from Copoly(Ionic Liquid) for Temperature Sensor Kun Cui, Dandan Zhu, Wei Cui, Xuemin Lu,* and Qinghua Lu* School of Chemistry and Chemical Engineering, State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, Dongchuan Rd. No. 800, Shanghai, China, 200240 S Supporting Information *

ABSTRACT: A simple copolymer, copoly(ionic liquid), consisting of N-isopropylacrylamide (NIPAM) and 1-benzyl-4-vinylpyridine bromide (4-VPBn+Br−) units as thermoresponsive and ionic liquid parts, respectively, has been synthesized. Fluorescent nanoparticles (FNPs) were then formed under the driving forces of electrostatic interaction between 2-(4-amino-2-hydroxyphenyl) benzothiazole derivative (AHBTA) and copoly(ionic liquid) due to hydrophilic/hydrophobic balance. The fluorescent intensity of the FNPs enhanced by a factor of about 50 times when the pH was increased from 7 to 10, and was effectively doubled within 1 °C around LCST in pH 9 buffer solution, thus showing a dramatic pH and thermal dual-dependent property. The FNPs exhibited reversible fluorescence enhancement/quenching over more than five cycles, regardless of the heating/cooling process. Furthermore, the FNPS proved to be much more stable to UV light irradiation than pure fluorescence molecule AHBTA.

1. INTRODUCTION Recently, fluorescent thermometers have attracted much attention because they allow simple monitoring of solution temperature on the basis of fluorescent intensity.1−13 These fluorescent thermometers have some advantages over traditional thermometers in application where electromagnetic noise is strong, sparks could be hazardous, or the environment is corrosive.14 Practical application of fluorescent thermometers requires high sensitivity, reversibility, and reusability. The polymer-based fluorescent thermometers designed in previous works1−8 have properties capable of meeting these requirmens. In particular, the fluorescence-enhancement-type polymer sensors are good candidates because of their high signal-tonoise ratio. Uchiyama et al.1−8 have designed a fluorescent thermometer, poly(NIPAM-co-BF), consisting of N-iospropylacylamide (NIPAM) and benzofurazan (BF) units as the thermo-responsive and fluorescent parts, respectively. Shiraishi et al. have developed a series of NIPAM copolymers with different fluorescent moieties, such as boradiazaindacene (BODIPY),15 anthracene (AN),16 hemicyanine (HC),17 and rhodamine (RD). 18,19 At temperatures above 32 °C, fluorescence enhancement is seen because the less-polar domain formed inside the globule-state polymer enhances the fluorescence emission. Among conventional fluorophores, however, even those with high fluorescence efficiency suffer from some inherent limitations. Photobleaching, limited brightness, and short lifetimes are serious drawbacks in many applications.20 In the past decade, fluorescent nanoparticles have emerged as a new class of fluorophores with the potential to overcome these limitations.21 Encapsulation of organic dyes into silica nanoparticles,22,23 polymer nanoparticles,24 or micelles25 from block © 2012 American Chemical Society

copolymers improved the dispersion and emission of fluorescence dyes, decreased emission quenching, and increased the photostability of a fluorophore due to the exclusion of oxygen and solvent.26 In general, complicated synthesizing procedures, as well as the leakage of fluorescent molecules from these host systems owing to a weak retaining force between fluorescent molecules and the nanoparticles or micelles, mean that there are still many challenges. Herein, we presented a novel and facile strategy to prepare fluorescent nanoparticles (FNPs, in brief) based on the complex of copoly(ionic liquid) (CoPIL, in brief) with organic molecules, specifically 2-(4-amino-2-hydroxyphenyl) benzothiazole derivative (AHBTA, in brief). The FNPs were formed between CoPIL and AHBTA due to the hydrophilicity/ hydrophobicity balance. The fluorescent intensity of FNPs is increasing with increasing temperature due to the formation of aggregates between them. The FNPs show highly stability to UV light irradiation at both low and high temperatures, and the process of heat-induced fluorescence enhancement is reversible.

2. EXPERIMENTAL SECTION 2.1. Synthesis of AHBTA and CoPIL. 4-((4-(Benzothiazol-2-yl)-3-hydroxyphenyl)amino)-4-oxobutanoic acid (AHBTA) was synthesized according to our previous work.27 Poly(N-isopropylacryamide-co-1-benzyl-4-vinylpyridine bromide) (CoPIL) was prepared by radical polymerization (see Scheme 1 in the Supporting Information). 4-Vinylpyridine and 1-benzyl bromide were stirred together for 24 h, and the Received: December 9, 2011 Revised: February 9, 2012 Published: February 22, 2012 6077

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Figure 1. Schematic representation of the formation of FNPs between CoPIL and AHBTA.

measurements were recorded on a PTI QuantaMaster spectrofluorometer equipped with a temperature controller. Differential scanning calorimetry (DSC) was performed on a TA DSC Q2000 instrument using a heating rate of 10 °C/min in the temperature range of −60 to 150 °C. The heating cycles were performed twice, but only the result of the second cycle was reported, since the former was influenced by the mechanical and thermal history of the samples. Thermogravimetric analysis was performed using a TA TGA Q5000 instrument. Samples were heated at the speed of 20 °C/min from room temperature to 700 °C in a flowing nitrogen atmosphere. FNPs dispersed in aqueous solution were examined using a JEOL JEM-2100 transmission electron microscope (TEM) operating at 200 kV. Samples for TEM were prepared by casting a drop of solution on a carbon-coated copper grid, followed by drying in air. Dynamic light scattering (DLS) measurements were carried out on a Zetasizer Nano ZS90 from Malvern Instruments equipped with a temperature controller cell with a laser at 633 nm. The low critical solution temperatures (LCST) of the FNPs and copolymer solution were measured at 550 nm with a Cintra 10e spectrophotometer (GBC Company, Australia) equipped with a circulating water bath. The temperature at 95% light transmittance of the FNPs and copolymer solution was defined as the LCST.

precipitation was collected. The crude product was dissolved in CHCl3 again and precipitated in ethyl acetic. The polymerization was then conducted in ethanol, with initiation by AIBN at 78 °C for 24 h. The polymer was precipitated in ethyl ester. 1 H NMR (400 MHz, DMSO-d6, δ): 9.22 (H near N on the pyridine ring), 8.16 (H near the polymer backbone on the pyridine ring), 7.85−6.90 (ArH of benzyl), 5.80 (CH2 of benzyl), 3.82 (CH of isopropyl on NIPAM), 2.13−1.15 (H of polymer backbone), 1.03 (CH3 of isopropyl on NIPAM). The incorporation of 4VPBn+Br− was determined by 1H NMR. 2.2. Prepared of Fluorescent Nanoparticles. The AHBTA was neutralized with 1 mol/L NaOH aqueous solution and diluted to 2 μmol/mL stock aqueous solution. CoPIL dissolved in water or pH buffer solution was mixed with AHBTA aqueous solution according to a 1:1 charge molar ratio, such that the final concentration of CoPIL was 1 mg/mL after being diluted with water or pH buffer solution. The FNP aqueous solution was stirred for 2 h at room temperature. All FNP samples for 1H NMR, FTIR, DSC, and TGA were obtained by freeze-drying. 2.3. Instruments and Characterization. Nuclear magnetic resonance (1H NMR) studies were carried out with a Varian Mercury Plus 400 MHz spectrometer in DMSO-d6 solvent at room temperature, with chemical shift referenced versus internal standard (TMS) shifts at 0 ppm. Fourier transform IR (FTIR) spectra were recorded on a Perkin-Elmer Spectrum100 FTIR spectrometer on pressed thin transparent disks of the sample mixed with KBr. The fluorescent spectra

3. RESULTS AND DISCUSSION 3.1. Synthesis and Characterization of FNPs. To prepare thermo-responsive FNPs, the random copolymer 6078

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CoPIL was first synthesized by radical copolymerization of ionic liquid (4VPBn+Br−) and N-isopropylacryamide (NIPAM) which play the roles of ionic self-assembly component and thermo-response agent, respectively. The molar ratio of NIPAM to 4VPBn+Br− in CoPIL is 40 according to the peak area integration ratio of 3.8−5.8 ppm in 1H NMR. Then, the 2(4-amino-2-hydroxyphenyl) benzothiazole derivative (AHBTA) was chosen as the fluorescent moiety because it is sensitive to the polarity and hydrophobicity of its microenvironment. As shown in Figure 1, when NaOH neutralized AHBTA aqueous solution was added into CoPIL aqueous solution, there was electrostatic interaction between CoPIL and AHBTA, and further investigation of DLS measurement proved that FNPs were formed under the force of hydrophilic/ hydrophobic balance in the complex aqueous solution. The 1H NMR spectra of FNPs, CoPIL, and AHBTA are shown in Figure 2. It can be seen that all signals of the CoPIL and

Figure 3. Plots of the maximum of the fluorescent intensity (λem = 450 nm) vs different pHs at 40 °C (1 mg/mL).

change. Under these conditions, FNPs cannot be formed, since the complex of CoPIL and AHBTA formed through electrostatic interaction may be destroyed. This might be verified by detecting the pKa of the carboxyl of AHBTA molecules. However, it is difficult to directively measure the pKa value of AHBTA because it can only be dissolved in basic solution. On the other hand, the succinic acid can be considered to be an analogue to AHBTA in some respects due to the similarity in the structure of the carboxyl group as AHBTA, so succinic acid was chosen to estimate the approximate pKa value of AHBTA. Comparing with succinic acid (pKa 4.2),29 the electronwithdrawing groups or the extension of the ring system of the AHBTA molecule can be expected to lead to a decreased electron density in the vicinity of the proton and consequently result in a lower pKa ( 10, the fluorescent intensity remains at the maximum value and does not change obviously. These results showed that FNPs exhibited different fluorescent intensities in different buffer solutions, suggesting that the FNPs might potentially be used as a pH indicator. Low Critical Solution Temperature (LCST) of FNPs. It is necessary to first determine the LCST of FNPs before starting to study the thermo-responsive capability of FNPs. When the pH value exceeded 4, thermo-responsive FNPs could be formed, and the LCSTs of CoPIL and FNPs are 35.9 and 35.6 °C, respectively, in pH 9 buffer solution (Figure 4). The LCST of FNPs is very close to that of original CoPIL, proving that the FNPs obtained from the self-assembly of CoPIL retain the thermal dependence property of original CoPIL. Moreover, the size of FNPs determined by DLS is also found to be thermo-responsive. The pure CoPIL cannot form

Figure 2. 1H NMR spectra of FNPs, CoPIL, and AHBTA.

AHBTA are still detectable in the 1H NMR spectra of FNPs, although that of the proton of the benzyl group (Ph−CH2) of CoPIL shifts from 5.80 to 5.33 ppm because of the electrostatic interaction between CoPIL and AHBTA. Moreover, thermal analysis results of FNP samples also prove the formation of FNPs. The glass transition temperatures (Tg) of FNPs (122.9 °C) is lower than that of pure CoPIL (137.7 °C), since the incorporation of AHBTA destroyed the regularity of the polymer chain. 3.2. pH-Dependent Fluorescent Emission of FNPs. Since the fluorescent unit AHBTA we used here is sensitive to the polarity surrounding the microenvironment, we first investigated the effect of pH value on the fluorescence characteristic of FNPs in buffer solutions with various pH values. In this work, the AHBTA was neutralized with NaOH aqueous solution first in order to prepare the FNPs. The fluorescence emission peak at 450 nm is corresponding to the Ar−O− groups of deprotonated AHBTA molecules.28 Under pH < 4, it is found that the complex of CoPIL and AHBTA formed through electrostatic interaction precipitated in the buffer solution. It is because neither component of the complex is stable at low pH, and hence the FNPs cannot be formed at all under pH < 4. Thus, we studied the steady-state fluorescence emission spectra of FNPs in buffer solutions under a 450 nm emission wavelength with pH values ranging from 4 to 13. Figure 3 shows the maximum fluorescent intensity of FNPs in various pH value solutions at 40 °C. When the pH value was 4−6, the fluorescent intensity was low and without remarkable 6079

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the FNPs become drastically larger and even precipitate gradually from solution (as shown in Figure 5c,d). The reason for the sudden change in size of FNPs near the LCST is that the hydrogen bonds between the hydrophilic groups on the surface of FNPs and water molecule are broken up when the temperature is close to LCST; therefore, the aggregates are assembled due to the increasing hydrophobic property of FNPs. Consequently, when the temperature is above the LCST, the aggregates gradually turn to visible precipitation. To collect more information on the chain conformation about FNPs and to gain more insight into its thermal transitions, its 1H NMR spectra were measured in D2O at various temperatures (Figure 6). At temperatures near the LCST, not just the isopropyl pendants in NIPAM parts but whole polymer chains can quickly dehydrate. The dehydration results in the formation of compact aggregates, as evidenced by the decrease of peak intensities of a and c−f of AHBTA molecules in the NMR. Further heating the complex to above 36 °C may cause little change in the compactness of the complex aggregates because the phase transition has already completed at the LCST, despite the fact that the aggregates continue to grow in size. Indeed, the aggregates become so large that they eventually precipitate at high temperatures, whereupon an NMR spectrum can no longer be acquired. 3.3. Temperature-Dependent Fluorescence Emission of FNPs. Due to the fact that the FNPs are pH-dependent, we chose to study the effect of temperature on the fluorescence properties of FNPs in pH 9 buffer sulution, for which the FNPs show the maximun fluorescence emission intensity. Figure 7

Figure 4. Temperature dependence of optical transmittance recorded at a wavelength of 550 nm for 1 mg/mL aqueous solutions of FNPs and CoPIL in pH 9 buffer solution.

particles in solution due to its hydrophilic nature (Figure 5a). After the addition of AHBTA, however, nanoparticles were formed at appropriate pH value under the driving force of opposite charged ionic interaction and the balance between the hydrophobic and hydrophilic interactions (as shown in Figure 5b). The average size of FNPs below the LCST is 172.9 nm and the PDI is about 0.12, so the size of FNPs is uniform. The FNPs show only a modest change in size at temperature below the LCST. However, when the temperature reaches the LCST,

Figure 5. TEM images of (a) CoPIL, (b) FNPs at 16 °C, and (c) FNPs at 40 °C. (d) The size of FNPs in pH 9 buffer solution ([CoPIL] = 1 mg/ mL) at different temperatures. 6080

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at high temperature, which is different from previous literature reported.13 It must also be noted that an ionic complex between the AHBTA and CoPIL is necessary for the structure design of the heat-induced fluorescence enhancement. The fluorescent intensity of pure AHBTA in water is monotonously decreased with increasing temperature, which is presumably because there are more and more collisions occuring between the molecules of AHBTA and water molecules, leading to bleaching of the fluorescent intensity. The results show that the FNPs have excellent heat-induced fluorescence enhancement property and could be potentially used as a fluorescent thermometer. 3.4. Reversibility and UV-Stability FNP Sensor. The fluorescence enhancement/quenching of FNPs occurs reversibly, regardless of the heating/cooling process. Figure 8 shows

Figure 6. Temperature-dependent 1H NMR spectra of FNPs in D2O.

Figure 8. Change in the fluorescent intensity of FNPs in pH 9 buffer solution (1 mg/mL, λex = 390 nm), where the temperature was changed repeatedly between 25 and 40 °C.

the change of fluorescent intensity when temperature changes repeatedly between 25 and 40 °C. The results clearly show the fluorescent intensity changed reversibly over at least five cycles. This indicates that FNPs undergo smooth structure change in response to the temperature change, thereby allowing a reversible fluorescence response. Generally, organic fluorescent molecules are rapidly photobleaching, which restricts their application. Figure 9 shows the fluorescent intensity of FNPs and AHBTA in pH 9 buffer solution irradiated by a UV lamp (∼365 nm) for different times. For pure AHBTA molecules, photobleaching occurred very rapidly, especially at high temperature. However, once FNPs were formed, although slight photobleaching occurred initially, the fluorescent intensity of FNPs was much more stable to UV light irradiation than pure AHBTA.

Figure 7. Temperature-dependent fluorescent spectra of FNPs in pH 9 buffer solution (1 mg/mL, λex = 390 nm, λem = 450 nm, I0 = 1 represents the fluorescent intensity of FNPs at 16 °C). Inset: The fluorescent intensity of pure AHBTA in H2O (pH 9) at different temperatures.

represents the temperature-dependent fluorescence spectra of FNPs in pH 9 buffer solution. It is evident that a sudden change in fluorescence intensity appeared at the LCST. These changes resulted from the constitution transition of FNPs. The FNP shells formed from hydrophilic NIPAM and the −NH−CO− groups are prone to form hydrogen bonds with water molecules at a low temperature, while the fluorescent molecules AHBTA are wrapped in the FNP cores with a hydrophobic microenvironment. When the temperature is increased, the hydrogen bonds between the FNP shells and water are destroyed, leading to the rise of hydrophobicity of the microenvironment in which AHBTA resides, which in turn leads to a gradual enhancement of the fluorescent intensity in this phase. When the temperature rises up to the LCST, the hydrogen bonds are completely destroyed and the polymer chains start to shuck off the water molecules around the isopropyl groups,32 leading to the aggregation of FNPs and thus the sudden enhancement of fluorescent intensity at the LCST. The fluorescent intensity of FNPs slowly decreases with the temperature keeping on increase due to the thermal quenching of fluorescent molecules

4. CONCLUSION In this contribution, we have presented a new strategy to prepare thermo-responsive fluorescent nanoparticles. The approach is based on the ionic complex of copoly(ionic liquid) and charged fluorescent molecules. The fluorescent nanoparticles were formed due to the balance of hydrophobic and hydrophilic interaction. The fluorescent intensity of FNPs is low at pH 4.0−7.0 but is enhanced about 50-fold when the pH is increased from 7 to 10. The LCST of FNPs was slightly different from that of CoPIL. The fluorescent intensity was effectively doubled within 1 °C around the LCST and showed a 6081

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(7) Uchiyama, S.; Matsumura, Y.; de Silva, A. P.; Iwai, K. Anal. Chem. 2003, 75, 5926−5935. (8) Uchiyama, S.; Matsumura, Y. Chem. Commun. 2009, 2646−2648. (9) Hu, J.; Liu, S. Macromolecules 2010, 43, 8315−8330. (10) Liu, T.; Hu, J.; Yin, J.; Zhang, Y.; Li, C.; Liu, S. Chem. Mater. 2009, 21, 3439−3446. (11) Yin, J.; Guan, X.; Wang, D.; Liu, S. Langmuir 2009, 25, 11367− 11374. (12) Hu, J.; Li, C.; Liu, S. Langmuir 2010, 26, 724−729. (13) Tang, L.; Jin, J. K.; Qin, A.; Yuan, W. Z.; Mao, Y.; Mei, J.; Sun, J. Z.; Tang, B. Z. Chem. Commun. 2009, 4974−4976. (14) Chapman, C. F.; Liu, Y.; Sonek, G. J.; Tromberg, B. J. Photochem. Photobiol. 1995, 65, 416−425. (15) Wang, D.; Miyamoto, R.; Shiraishi, Y.; Hirai, T. Langmuir 2009, 25, 13176−13182. (16) Shiraishi, Y.; Miyamoto, R.; Hirai, T. Tetrahedron Lett. 2007, 48, 6600−6664. (17) Shiraishi, Y.; Miyamoto, R.; Hirai, T. Langmuir 2008, 24, 4273− 4279. (18) Shiraishi, Y.; Miyamoto, R.; Zhang, X.; Hirai, T. Org. Lett. 2007, 9, 3921−3924. (19) Shiraishi, Y.; Miyamoto, R.; Hirai, T. J. Photochem. Photobiol., A 2008, 200, 432−437. (20) Somers, R. C.; Bawendi, M. G.; Nocera, D. G. Chem. Soc. Rev. 2007, 36, 579−591. (21) Goesmann, H.; Feldmann, C. Angew. Chem., Int. Ed. 2010, 49, 1362−1395. (22) Zhao, X.; Bagwe, R. P.; Tan, W. Adv. Mater. 2004, 16, 173−176. (23) Cho, C. J.; Lee, S. S.; Jung, J. H. Analyst 2010, 135, 1551−1555. (24) Tian, Z.; Shaller, A. D.; Li, A. D. Q. Chem. Commun. 2009, 180− 182. (25) Ma, B.; Wu, S.; Zeng, Y.; Luo, Y.; Zhao, J.; Tong, Z. Nanotechnology 2010, 21, 195501. (26) Burns, A.; Ow, H.; Wiesner, U. Chem. Soc. Rev. 2006, 35, 1028− 1042. (27) Cui, K.; Lu, X.; Cui, W.; Wu, J.; Chen, X.; Lu, Q. Chem. Commun. 2011, 47, 920−922. (28) Wang, R. J.; Liu, D.; Xu, K.; Li, J. Y. J. Photochem. Photobiol., A 2009, 205, 61−69. (29) Parthasarathi, R.; Padmanabhan, J.; Elango, M.; Chitra, K.; Subramanian, V.; Chattaraj, P. K. J. Phys. Chem. A 2006, 110, 6540− 6544. (30) Tao, L.; Han, J.; Tao, F. M. J. Phys. Chem. A 2008, 112, 775− 782. (31) Henary, M. E.; Fahrni, C. J. J. Phys. Chem. A 2002, 106, 5210− 5220. (32) Dimitrov, I.; Trzebicka, B.; Müller, A. H. E.; Dworal, A.; Tsvetanov, C. B. Prog. Polym. Sci. 2007, 32, 1275−1343.

Figure 9. Plots of the maximum of the fluorescent intensity (λem = 450 nm) vs UV irradiation time (∼365 nm) (1 mg/mL, λex = 390 nm, pH 9).

good thermal dependent property. Furthermore, the FNPs are much more stable to UV light irradiation than pure AHBTA. This offers a new strategy for design and preparation of multiresponsive fluorescent nanoparticles, which are not easily produced by traditional chemical approaches. In addition, copoly(ionic liquid) offers a new perspective for the construction of functional nanoparticles that may find application in the fields of functional chemosensors, thermometers, and so on.

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ASSOCIATED CONTENT

* Supporting Information S

The synthetic route of CoPIL, the steady-state fluorescence excitation/emission spectra of the FNPs. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (Q.L.); [email protected] (X.L.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge financial support from the National Science Fund for Distinguished Young Scholars (50925310), the National Science Foundation of China (20874059, 50902094), 973 Project (2009CB93043 and 2011CB935700), and the Shanghai Leading Academic Discipline Project (No. B202).

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REFERENCES

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