Article pubs.acs.org/Langmuir
Thermochromic Ion-Exchange Micelles Containing H+ Chromoionophores Xinfeng Du, Changyou Zhu, and Xiaojiang Xie* Department of Chemistry, Southern University of Science and Technology, Shenzhen 518055, P. R. China S Supporting Information *
ABSTRACT: Thermochromic composites constitute a classical subfamily of stimuli responsive materials. We report here the thermochromic effect in Pluronic F-127 (F127) micelles containing hydrophobic ion-exchanger and H+ chromoionophores. The highly versatile and reversible thermochromism is attributed to the temperature-induced hydration−dehydration of the peripheral layer of the micelles, which in turn controls the ion-exchange process between the core and the periphery of the micelles. The color typically changes abruptly within 3− 5 °C, and the color transition temperature can be tuned within 5−25 °C upon varying the F127 concentrations. This work lays the foundation of a new variety of thermochromic materials involving ion-exchange.
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INTRODUCTION Thermochromism is commonly known as the phenomenon of a color change due to a change in temperature. Beside the antiquated observation of the red to green color transition in ruby upon heating,1 other state-of-art thermochromic materials such as organic dyes,2−4 inorganic metal complexes,5,6 and cholesteric liquid crystals7,8 have also been discovered. Thermochromism has found a wide range of applications in research and everyday life ranging from intracellular thermometers to baby’s nursing bottles.9−11 In the past decades, thermochromic composites, as a highly versatile platform, have made tremendous progress.12−15 These types of materials function on various bases. Photonic crystals and nanoparticle-incorporated systems rely on the thermoreversible change in particle size,15 shape,16,17 refractive index,18,19 and interparticle distance.20 Other thermochromic mechanisms involve the formation of polymer−dye aggregates,21 dye−dye aggregates,22,23 charge transfer complex,24 and local proton equilibrium.25,26 It is worth continuing efforts to discover uncharted materials with distinct thermochromic mechanisms. Micelles based on amphipathic block copolymers such as poloxamers have previously been functionalized as ion sensors.27−30 Poloxamer micelles contain a hydrophobic core and a hydrophilic periphery. Upon increasing the polymer concentration, the micelles can self-arrange into cubic crystalline order which is temperature dependent as well.31,32 According to several previous observations, desolvation of the ethylene oxide groups in the poloxamer with increasing temperature can cause the effective volume fraction of the micelles to decrease.33,34 In this work, we present for the first time ion-exchange block copolymer micelles based on Pluronic F-127 (F127) © XXXX American Chemical Society
poloxamers as a versatile thermochromic platform. The micelles contain ion exchanger and hydrophobic pH indicator (chromoionophore) and exhibit a tunable, reversible, and sensitive color change upon varying the temperature.
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EXPERIMENTAL SECTION
Materials. Pluronic F-127 (F127), sodium tetrakis[3,5-bis(trifluoromethyl)phenyl] borate (NaTFPB), chromoionophore I, chromoionophore IV, tetradodecylammonium chloride (TDDA), sodium phosphate monobasic monohydrate, citric acid, boric acid, Trizma base, and tetrahydrofuran (THF) were obtained from SigmaAldrich. The chromophore 2-((1E,3E)-4-(4-(dimethylamino)phenyl)buta-1,3-dien-1-yl)-3-octadecylbenzo[d]thiazol-3-ium iodide (PSD) and oxazinoindoline dye (ox blue) were custom synthesized according to previous reports.29,35 All salts used were analytical grade or better. All aqueous solutions were prepared by dissolving the appropriate salts in water purified by Milli-Pore Integral 5. Generally, the ion-exchange micelle samples were prepared by adding 200 μL of THF cocktail (vide infra) into 5 mL corresponding F127 micelles. The cocktail for Figure 1 was prepared by dissolving 0.5 mg (0.28 mM) of chromoionophore I and 2.2 mg (0.83 mM) of NaTFPB in 3 mL of THF. The cocktail for Figure 2 was prepared by dissolving 0.5 mg (0.32 mM) of chromoionophore IV and 2.0 mg (0.92 mM) of TDDA in 3 mL of THF. The cocktail for Figure 3 was prepared by dissolving 0.5 mg (0.35 mM) of ox blue and 2.0 mg (0.75 mM) of NaTFPB in 3 mL of THF. The F127 micelles were prepared in the deionized water or in buffer solutions (pH 7.4 10 mM Tris-HCl buffer, pH 8.0, 2.5 mM universal buffer containing NaH2PO4, citric acid, and boric acid). The 20% (w/w) F127 micelles were prepared by mixing 10 g of F127 with 40 g of H2O or corresponding buffer in an ice-cooled glass vessel. The mixture was stirred all night at low Received: April 10, 2017 Revised: May 24, 2017 Published: May 24, 2017 A
DOI: 10.1021/acs.langmuir.7b01221 Langmuir XXXX, XXX, XXX−XXX
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temperature until all the solids were dissolved. The F127 micelles of lower concentrations were diluted from the 20% (w/w) solution. Instrumentations and Measurements. Absorption spectra were measured using an Evolution 220 UV−vis spectrometer from Thermo Fisher Scientific. The temperature inside the samples were carefully controlled with PCCU1 Peltier control and cooling unit from Thermo Fisher Scientific. The temperature values inside the sample solutions were measured additionally with a mercury thermometer to confirm the digital temperature readings. The samples were allowed 10 min after reaching the desired temperature before recording the spectra. Dynamic light scattering (DLS) was performed with 5% (w/w) concentrated F127 samples using a Brookhaven Instruments BI200SM at 10 and 25 °C. Zeta potential was measured with 5% (w/w) concentrated F127 samples on a Brookhaven BI-200SM instrument at 25 and 5 °C.
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RESULTS AND DISCUSSION As shown in Scheme 1, the ion-exchange micelles are composed of F127 incorporating hydrophobic H+ chromoionophore (C) and ion exchanger (R−). F127 is a triblock copolymer with two poly(ethylene oxide) arms on the edges, one poly(propylene oxide) chain in the middle, and an average molecular weight of 12 400 Da. Above the critical micelle concentration, micelles with a hydrophobic core and a relatively hydrophilic periphery are formed. C and R− are well-known hydrophobic molecules in ion-selective membranes,36−38 and therefore, can be readily doped into the core of the micelles. Scheme 2 shows acid−base reactions of the H+ chromoionophores and the colors of the corresponding forms. After incorporation of C and R− into the core of the micelles, an ion-exchange would occur between the core and the periphery. The ion exchange equilibrium as shown in eq 1 thus resulted in the formation of CH+, which showed a strong blue color at lower temperature. Here, the subscript aq, pre, and cor designate the aqueous solution, the peripheral layer, and the core of the micelles, respectively. However, for this ionexchange to take place, water molecules have to present in the periphery. The partition of water between the aqueous bulk solution and the periphery of the micelles (eq 2) is known to depend on temperature. As temperature went higher, water molecules gradually left the micelles and the ion exchange equilibrium was shifted to the right, causing the deprotonation of CH+ and a recovery of the magenta color, for this particular chromoionophore.
Figure 1. (a) Absorption spectra of the 15% (w/w) F127 ion-exchange micelles containing the chromoionophore as indicated (11 μM) and cation-exchanger Na+R− (33 μM) at various temperature in pH 7.4 Tris-HCl buffer. (b) Plot of the normalized absorbance at 665 nm as a function of temperature for micelles prepared with different concentrations of F127 as indicated. (c) Relationship between the color transition temperature and the F127 concentration. (d) Appearance of the 15% (w/w) F127 ion exchange micelles under ambient light at temperature values as indicated.
Figure 2. (a) Absorption spectra of 15% (w/w) F127 micelles containing the chromoionophore as indicated (13 μM) and the anionexchanger tetradodecylammonium chloride (37 μM) at various temperatures in pH 8.0 buffer solutions. (b) Plot of the normalized absorbance at 532 nm as a function of temperature. (c) Appearance of the micelles under ambient light at various temperature values as indicated.
R ‐cor + CH+cor + Na +pre ⇔ R ‐cor + H+pre + Ccor + Na +cor (1) + Hpre + OH‐pre ⇔ H 2Opre ⇔ H 2Oaq
(2)
Figure 1a shows the absorption spectra at various temperature values from micelles made of 15% (w/w) F127 in pH 7.4 buffer solutions. We noticed that the ion-exchanger R− was essential for the thermochromic effect. It was no longer observable once R− was removed (see Figure S1 in the Supporting Information). The isosbestic point in Figure 1a further confirms the acid−base reaction mechanism. Additionally, the zeta potential of the micelles remained at zero (±3 mV), indicating no dramatic change in adsorption and the spatial distribution of the charged species within. The color transition was typically achieved within 3 to 5 °C, which rendered the micelles very sensitive to detect small temperature variations. The temperature responsive windows were successfully adjusted by varying the concentrations of F127. As shown in Figure 1b, decreasing the F127 concentration generally
Figure 3. (a) Absorption spectra of the 15% (w/w) F127 ion-exchange micelles containing the chromoionophore as indicated (14 μM) and cation-exchanger Na+R− (30 μM) at various temperature in pH 7.4 Tris-HCl buffer. (b) Plot of the normalized absorbance at 653 nm as a function of temperature. Inset: Appearance under ambient light 5 and 15 °C.
B
DOI: 10.1021/acs.langmuir.7b01221 Langmuir XXXX, XXX, XXX−XXX
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Scheme 1. Schematic Illustration of the Thermochromic Mechanism of the Ion-Exchange Micelles Containing a Representative Neutral H+ Chromoionophore (C) and Cation-Exchanger (Na+ R−)
Scheme 2. Acid−Base Reactions of the H+ Chromoionophores Used in This Work and the Colors of Their Acidic and Basic forms
micelles is in agreement with previous report by Basak et al. on drug encapsulated micelles.33 Previously, a localized proton-transfer induced pH change was reported to account for thermochromic effect in a thermoreversible hydrogel.25 To evaluate whether our observation here was also due to localized pH change, an H+ chromoionophore with a phenol group (as shown in Figure 2) was incorporated into the micelles, together with an anionexchanger (tetradodecylammonium chloride). As expected, a color transition occurred as well upon changing the temperature. However, a rose color corresponding to the deprotonated form was observed at lower temperature while a yellowish color corresponding to the protonated form appeared at higher temperature. This opposite protonation/deprotonation direction (as compared to the compound used in Figure 1) clearly indicated that this thermochromism was not due to localized pH change. The deprotonated phenolate form at lower temperature was also a consequence of ion-exchange. Therefore, it is again the temperature induced hydration-dehydration that controls the thermochromism. In the case of this commonly called charged chromoionophore and anion exchanger, the underlying ion-exchange equilibrium can be expressed in eq 3, where R+cor represents the tetradodecylammonium cation.
increased the color transition temperature (inflection point) and also caused a decrease of sensitivity at very low polymer concentration (10% (w/w)), a thermoreversible hydrogel was formed upon increasing the temperature. The thermochromism was observed for both the jellifiable and nonjellifiable micelle suspensions. In fact, the formation of Pluronic hydrogels has been ascribed to the dehydration and shear thinning according to several previous reports.41,42 The redistribution of the water molecules is able to cause the microenvironmental polarity change in the micelles. Nile red was used recently to evaluate the local polarity change within the micelles.43 Here, we used a polarity sensitive dye which is more lipophilic than Nile red to confirm the subtle C
DOI: 10.1021/acs.langmuir.7b01221 Langmuir XXXX, XXX, XXX−XXX
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microenvironmental change.29 The steady-state absorption spectra as shown in Figure S5 indeed indicated a less polar micelle core at higher temperature due to loss of water. Moreover, the microscopic hydration-related ion-exchange process was also magnified at macroscopic scale on a nylon filter paper. Chromoionophore and cation exchanger used in Figure 1 was dissolved in THF and drop casted on a filter paper. As shown in Figure S6, after exposing the stained area to water, a blue color appeared as a result of the ion-exchange in eq 1. After water was vacuum evaporated, the color went back to magenta. This ion-exchange based thermochromic platform could be highly versatile considering the large number of available hydrophobic chromoionophores. The use of the charged chromoionophore in Figure 2 served as an example. Further, as shown in Figure 3, we investigated micelles incorporating oxazinoindoline-based H+ chromoionophore. This molecule exhibited an ON−OFF mode H+-activated visible absorption.35 The protonated form of this compound (shown in Scheme 2) shows a blue color in contrast to the colorless deprotonated form. As expected, at higher temperature, the solution became almost colorless, while at lower temperature, a deep blue color and 9-fold increase in the maximum absorbance was observed. The thermochromic effect described in this work is fully reversible. As shown in Figure 4, the absorption spectra of the
Article
ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.7b01221. Absorption spectra of the 15% (w/w) F127 micelles containing chromoionophore; Plot of the normalized absorbance at 665 nm as a function of temperature for F127; normalized intensity autocorrelation function C(τ) and normalized intensity based size distribution; plot showing light absorbance at 665 nm; absorption spectra of dye PSD doped F127 micelles; pictures showing the color changes of chromoionophore (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Xiaojiang Xie: 0000-0003-2629-8362 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank the Shenzhen government, the Southern University of Science and Technology and the Thousand Young Talents Program for financial support.
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REFERENCES
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Figure 4. (a) Absorption spectra of the thermochromic micelles (15% (w/w)) containing H+ chromoionophore I and cation-exchanger Na+R− recorded at 5 and 15 °C alternatively in pH 7.4 Tris-HCl buffer. (b) Absorbance at 665 nm normalized to the first measurement during the multiple temperature cycles.
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CONCLUSIONS In summary, a thermochromic platform based on ion-exchange F127 micelles was demonstrated. The thermochromism features a mechanism relevant with the temperature dependent dehydration and rehydration of the micelle periphery. The latter was coupled to the ion-exchange equilibria at the interface between the core and the periphery of the micelles. The color transitions were fully reversible and tunable. It enables the applications of a number of H+ chromoionophores for thermochromic purposes, and can potentially be extended to other chromoionophores with different selectivity. The micelles could potentially serve as temperature controlled ion sensors and optical thermometer for in vivo measurements D
DOI: 10.1021/acs.langmuir.7b01221 Langmuir XXXX, XXX, XXX−XXX
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