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Time–temperature chromatic sensor based on polydiacetylene (PDA) vesicle and amphiphilic copolymer
Sensors and Actuators B 150 (2010) 406–411
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Time–temperature chromatic sensor based on polydiacetylene (PDA) vesicle and amphiphilic copolymer MaLing Gou, Gang Guo, Juan Zhang, Ke Men, Jia Song, Feng Luo, Xia Zhao, ZhiYong Qian ∗ , YuQuan Wei State Key Laboratory of Biotherapy and Cancer Center, West China Hospital, West China Medical School, Sichuan University, Chengdu 610041, Sichuan, PR China
a r t i c l e
i n f o
Article history: Received 20 April 2010 Received in revised form 16 June 2010 Accepted 20 June 2010 Available online 7 July 2010 Keywords: Functional materials Sensor Polydiacetylene Thermochromism Amphiphilic polymers
a b s t r a c t Polydiacetylene (PDA) is an excellent color-changing indicator. In this paper, we demonstrate a new and interesting kind of time–temperature chromatic sensor based on PDA vesicle and amphiphilic polymer. In presence of amphiphilic polymers, PDA vesicles could gradually transit from blue to red, which was irreversible and regularly depended on the temperature, time, and properties of amphiphilic polymer (including the concentration, molecular weight, molecular structure, hydrophobicity, and thermosensitive property). Higher concentration of amphiphilic polymers resulted in lower color-transition temperature and faster color-transition. Also, higher temperature led to faster color-transition. The colorchanging of PDA/amphiphilic polymer matrix may have resulted from the higher temperature and gradual insertion of amphiphilic polymer into PDA, synergistically perturbing the conformation of PDA. The prepared PDA/F127, PDA/F68, PDA/L35 and PDA/Tween-20 aqueous matrix showed potential application in time–temperature chromatic sensors at around 10–50 ◦ C. This work provides a new method to design time–temperature chromatic sensors. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Irreversible color-changing indicators have potential application in monitoring the temperature history of perishables. Polydiacetylene (PDA) is -conjugated polymer that has alternating double- and triple-bond groups in the main polymer chain. Disruption of the extensively delocalized enyne back bones of molecularly ordered PDA side-chains induces a blue-to-red color change [1]. PDA has been extensively investigated as a sensor matrix, owing to its brilliant blue-to-red color-transition behavior that takes place in response to environmental changes including heat (thermochromism), organic solvents (solvatochromism), mechanical stress (mechanochromism), and ligand–receptor interactions (affinochromism) [2–9]. PDAs are already used to fabricate sensors for the detection of various substances, such as virus, lipophilic enzymes, peptides, ions, antibody, protein, oligonucleotide, etc. [10–15]. As well as the very interesting optical properties, PDAs are attractive sensory materials as they (1) can be prepared from self-assembled crystalline or semi-crystalline states of diacetylene monomers, (2) are produced by UV or irradiation of self-assembled diacetylenes without the need for chemical initiators or catalysts, and (3) are readily prepared
∗ Corresponding author. Tel.: +86 28 85164063; fax: +86 28 85164060. E-mail address: [email protected] (Z. Qian). 0925-4005/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2010.06.041
in aqueous solution in the form of nanostructured liposomes, vesicles and wires, and can thus PDAs be readily generated in matrix formats for diagnostics purpose. Among the various chromatic transitions of PDAs, the most extensively explored properties are thermally induced color-transitions [16]. Due to the interesting thermochromism, PDAs have been widely used as temperature sensors [17–19]. To our knowledge, lots of agents are perishable and should be kept at low temperature. It may be necessary to know the temperature history of a perishable product when we receive it, particularly whether the product has encountered a time–temperature procedure possibly affecting the quality of the product in question. Though PDAs attract great attention as temperature sensors, little exists in the literature regarding the time–temperature chromatic sensors based on PDAs [20]. Previous reports indicated that amphiphilic molecules could induce the color-transition of PDA [21]. In this paper, based on the understanding of interactions between PDA and amphiphilic polymers, we demonstrate a new and interesting kind of time–temperature chromatic sensor based on PDA vesicle and amphiphilic polymer, which could be readily generated, and could work between 10 and 50 ◦ C. The demonstrated chromatic sensors have great potential application in determining the time–temperature history of perishable bio-products, such as vaccines, bio-drugs, bio-kits, etc., which is particularly important after transportation and storage.
M. Gou et al. / Sensors and Actuators B 150 (2010) 406–411
2. Experimental 2.1. Materials
407
camera, and data about the color of the PDA/amphiphilic polymer aqueous matrix was processed according to Fig. S2 (see support information).
Poly(ethylene glycol) (PEG4000, Mn = 4000) was purchased from Fluka (USA). Also, 10,12-pentacosadiynoic acid (PCDA) was purchased from Fluka (USA), and further purified by dissolving in dichloromethane and then filtered to remove polymerized monomers before use. Pluronic F127, F68 and L35 were purchased from Sigma (USA). Tween-20 and dichloromethane were purchased from KeLong Chemicals (Chengdu, China). See Fig. S1 (see supporting information) for the molecular structure of F127, F68, L35 and Tween-20.
2.4. Quantifying the color response (CR)
2.2. Preparation and characterization of PDA vesicles
where PB = Ablue /(Ablue + Ared ). A is the absorbance at either the blue component (640 nm) or the red component (550 nm) of visible spectrum. PB0 is the control, blue-ratio of pure PDA solution at 0 ◦ C, while PB1 is the value of sample exposed to different temperature for different time.
In preparation, 40 mg of PCDA was dissolved in CH2 Cl2 followed by solvent evaporation. After adding 20 mL of distilled water, the suspension was probe ultra-sonicated for 10 min at approximately 75 ◦ C. After the ultra-sonication, the solution was cooled and then stored overnight at 4 ◦ C to induce crystallization of lipid membranes. Polymerization was carried out under irradiation at 254 nm wavelength for 5 min in ice bath. At last, the obtained blue PDA vesicles were stored at 4 ◦ C for further use. The particle size of prepared PDA vesicles was 128 ± 5 nm, which was determined by the diffraction particle size detector (Nano-ZS, Malvern Instrument, UK). The result was the mean of three test runs. 2.3. Preparation of PDA/amphiphilic polymer aqueous matrix Afterward, 0.1 mL of PDA vesicle solution was mixed into 0.9 mL of amphiphilic polymer aqueous solution at 0 ◦ C, followed by placing the vessel in a water bath at different temperatures. The color of the PDA/amphiphilic polymer aqueous matrix was taken by digital
Finally, 0.15 mL of sample was added into the well of 96-well plate, and the absorption value at 640 and 550 nm was immediately calculated using a spectrophotometer (M5, Molecular Corporation). To quantify the extent of the blue-to-red color-transitions, the CR (%) was calculated using the following equation: CR (%) =
PB − PB 0 1 PB0
× 100
3. Results and discussion In this paper, we demonstrate a novel time–temperature chromatic sensor, which is schematically presented in Scheme 1. After the blue PDA vesicle was prepared by UV irradiation of self-assembled colorless PCDA vesicle, hydrophobic segments of amphiphilic polymers were gradually inserted into PDA vesicles (due to hydrophobic interaction). With this, the temperature synergistically perturbed the conformation of PDA, changing the color of PDA, a process which was dependent on the temperature, time and properties of amphiphilic polymers. Pluronic polymers are triblock copolymers in the form of poly(ethylene glycol)–poly(propylene glycol)–poly(ethylene glycol) (PEG–PPG–PEG), which have surface activity and can insert
Scheme 1. Schematic representation of the time–temperature chromatic sensor based on PDA vesicle and amphiphilic polymer.
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M. Gou et al. / Sensors and Actuators B 150 (2010) 406–411
Fig. 2. Quantitative determination of the colorimetric response (CR) of PDA/F127 aqueous matrix.
Fig. 1. The color of PDA/F127 aqueous matrix after exposure to different temperature for different time.
into lipid monolayer [22]. Pluronic F127 has the molecular structure of PEG99 –PPG67 –PEG99 and molecular weight of 12,700. After the PDA/F127 was exposed to a different temperature for different time, its color was recorded, with results presented in Fig. 1. The naked eye visible color-transition of PDA/F127, from blue to red, took placed at 20–50 ◦ C, which was dependant on the temperature, time and concentration of F127. It could be observed that higher concentration of F127 resulted in lower color-transition temperature and faster color-changing. Meanwhile, higher temperature resulted in faster color-changing. Additionally, we quantified the colorimetric response (CR) of PDA/F127 (see Fig. 2). When the concentration of F127 was kept constant (20%, w/w), the CR increased with time and temperature as shown in Fig. 2a. When the reaction time was kept constant (2 h), the CR increased with increase in temperature and concentration of F127 (Fig. 2b). When the temper-
ature was kept constant (30 ◦ C), the CR increased with increase in time and concentration of F127 as presented in Fig. 2c. These results point to the potential application of PDA/F127 as an interesting time–temperature chromatic sensor. Usually, PDA vesicles were blue below 50 ◦ C, and could transit from blue to red at or above ca. 60 ◦ C in seconds. Here, F127 amphiphilic polymer contributed to decrease the color-transition temperature of PDA. The temperature and interaction between PDA and F127 likely play the key roles in the gradual color change of PDA/F127 aqueous matrix. Previously, numerous investigations have been carried out to gain a better understanding of the origin of PDA thermochromism. The results of recent studies strongly suggest that the release of mechanical strain, developed on the side chains during polymerization, induced a partial distortion of the conjugated p-orbital arrays. This distortion led to a shortening of the effective p-conjugation length, and was the main factor responsible for the thermally promoted color-transitions [16,23]. This study also addressed the effect of hydrophilic poly(ethylene glycol) (PEG) on the color of PDA, in order to understand the interaction between PDA and F127. In the presence of hydrophilic PEG4000 (20%, w/w) blue PDA could not transit to red, even after 5 days exposure to 30 ◦ C temperature. But in the presence of amphiphilic F127 (PEG–PPG–PEG) at the concentration of 5% (w/w), the color-transition of PDA was observed at 30 ◦ C. So, we suspect that hydrophobic interactions between hydrophobic PPG segments of F127 and hydrophobic core of PDA vesicles might be the main interaction between PDA and F127. To our knowledge,
M. Gou et al. / Sensors and Actuators B 150 (2010) 406–411
409
Fig. 3. The color of PDA/F68 (a) and PDA/L35 (b) after exposure to different temperature for different time.
PCDA has a hydrophilic head (carboxy group), and PDA vesicles have a hydrophilic shell. Hydrophobic interactions between PDA and F127 could not occur before F127 molecules diffused through the hydrophilic shell into the hydrophobic core of PDA vesicles. Previously, Su et al. believed that, in presence of CTAB surfactants, color-transition of PDA from blue to red was mainly due to the insertion of alkyl chain of CTAB into the hydrophobic domain perturbed the conformation of the conjugated polymer backbone [21]. In our results, the temperature induced partial distortion of the conjugated p-orbital arrays and the insertion of hydrophobic PPG segments of F127 into PDA, synergistically perturbing the conformation of PDA leading to color-changing of PDA; this is likely the key reason of color-transition of PDA/F127 aqueous matrix. Otherwise, the insertion of F127 into PDA vesicles might mainly be a diffusion process, and the speed of diffusion might be dependent on the temperature, concentration of F127, and so on. Based on the above hypothesis, the timing of F127 insertion into PDA is likely correlated to the color-transition of PDA/F127 aqueous matrix. In order to better understand this phenomenon, the thermochromism of PDA/F68 (PEG76 –PPG29 –PEG76 , Mn = 8400) and PDA/L35 (PEG11 –PPG16 –PEG11 , Mn = 1900) was also studied (Fig. 3). Effects of F68 and L35 on the color of PDA vesicles indicated that both PDA/F68 and PDA/L35 showed visible temperatureand polymer concentration-dependent color-changing, and higher
polymer concentration resulted in lower color-transition temperature of PDA. From PDA/F127 to PDA/F68 to PDA/L35, the correlation between time and color change was gradually weakened. This may be because the insertion of PEG–PPG–PEG in to PDA was related to the molecular weight, and higher molecular PEG–PPG–PEG led to slower insertion and slower color-transition of PDA. Amphiphilic polymers with more hydrophobicity should result in stronger hydrophobic interaction between PDA and amphiphilic polymers. At 30 ◦ C, the capacity of PEG–PPG–PEG polymers to induce color-transition of PDA vesicle was: F127 > F68 > L35, although the hydrophobicity was: L35 > F127 > F68. So, not only hydrophobicity but also molecular weight of PEG–PPG–PEG polymer would affect its capacity to induce color-changing of PDA. Otherwise, many properties of PEG–PPG–PEG aqueous solution were temperaturedependent, including micellization, surface activity, insertion into lipid monolayer, etc. [24,22]. Regarding PEG–PPG–PEG copolymers, higher temperature always resulted in lower CMC, stronger hydrophobicity, and stronger insertion into lipid monolayer. Among F127, F68 and L35, the most sensitive to temperature is F127. The CMC of F127 is 3.174 mM at 20 ◦ C, while the CMC is 0.006 mM at 40 ◦ C [22], which may mean that at a concentration as low as 0.1% of F127 the PDA would transit to red at 50 ◦ C. Later on, we also studied the correlation between the PDA/Tween-20 aqueous matrix color change and both the temper-
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ature and concentration of Tween-20 (Fig. 4). Between 10 and 30 ◦ C, a clear correlation between time and PDA/Tween-20 color change could be observed, although the molecular weight of Tween-20 was 1226 that was lower than 1900 of L35. This indicates that the molecular structure of amphiphilic polymer has an important effect on the thermochromism of PDA/amphiphilic polymer aqueous matrix. In summary, the above results suggest that the visible colorchanging of PDA/amphiphilic aqueous matrix was irreversible and regularly dependent on the temperature, time, and properties of amphiphilic polymers (including the concentration, molecular structure, molecular weight, hydrophobicity, and thermo-sensitive property). This suggests that the PDA/amphiphilic polymer aqueous matrix could be applied in a time–temperature chromatic sensor at a temperature around 10–50 ◦ C. Through adjusting the properties of the amphiphilic polymer, PDA/amphiphilic polymer matrix would manifest different colors according to the temperature and time, allowing the precise determination of time–temperature history through the combination of fast color-changing and slow color-changing sensors based on PDA/amphiphilic polymer. 4. Conclusion Amphiphilic polymers could decrease the color-transition temperature of PDA vesicles in water, and the irreversible color-changing of PDA/amphiphilic polymer matrix was regularly dependent on the temperature, time, and properties of amphiphilic polymer (including the concentration, molecular structure, molecular weight, hydrophobicity and temperaturesensitive property), which make the PDA/amphiphilic polymer aqueous matrix novel time–temperature chromatic sensors. The demonstrated PDA/F127, PDA/F68, PDA/L35 and PDA/Tween-20 matrix showed potential application in time–temperature chromatic sensors at around 10–50 ◦ C. This study provides a novel method to designing time–temperature chromatic sensor based on PDA vesicles and amphiphilic polymers. Acknowledgements This work was financially supported by New Century Excellent Talents in University (NCET-08-0371), Specialized Research Fund for the Doctoral Program of Higher Education (SRFDP 200806100065), National 863 project (2007AA021902), National Natural Science Foundation (NSFC20704027), and Chinese Key Basic Research Program (2010CB529906). We thank Angela Merriam (Sichuan University, China) for correcting this paper in English. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.snb.2010.06.041. References
Fig. 4. The color of PDA/Tween-20 aqueous matrix after exposure to different temperature for different period.
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Biographies MaLing Gou received his PhD in July 2010, from Sichuan University, China. His research interest is in the area of nanobiotechnology, sensing materials research, and nanomedicine. Now he has published more than 20 research articles in the international journals. Gang Guo received his PhD in July 2007, from Sichuan University, China. His research interest is in the area of functional polymer. Juan Zhang is a postgraduate student in Sichuan University. Her research interest is in the area of nanobiomaterials. Ke Men is a postgraduate student in Sichuan University. His research interest is in the area of nanobiotechnology. Jia Song is a postgraduate student in Sichuan University. Her research interest is in the area of nanobiomaterials. Feng Luo is a professor in West China Hospital, Sichuan University. His research interest is cancer therapy and cancer diagnosis. Xia Zhao is a professor in West China Second Hospital, Sichuan University. Her research interest is cancer therapy and cancer diagnosis. ZhiYong Qian received his PhD in July 2003, from Chengdu Institute of Organic Chemistry, Chinese Academy of Sciences, PR China. Now, Prof. ZhiYong Qian is head of the nanobiomaterials group at State Key Laboratory of Biotherapy, West China Hospital, Sichuan University. He is the associate editor of two international journals, including Journal of Biomedical Nanotechnology and Advanced Science Letters. Prof. Qian is in charge of some projects, including National High-Tech Project of China (863-Project), National Natural Science Foundation project, and Specialized Research Fund for the Doctoral Program of Higher Education. Prof. Qian is focused on the research of nanobiotechnology, functional polymers, and advanced drug delivery systems. YuQuan Wei is the academician of Chinese Academy of Sciences, the Cheung Kong Scholar, and The National Science Fund for Distinguished Young Scholar. And Prof. Wei is the vice-president of Sichuan University, head of State Key Lab of Biotherapy. Prof. Wei is the associate editor of “Human Gene Therapy”, the premier journal in the field of gene therapy. He is the principal scientist of two 973 projects. And he is the principal of some 863 projects, and National Natural Science Foundation project. Prof. Wei has published more than 100 articles in the international journals, is a named inventor of over 30 patents. His research interest is cancer therapy and cancer diagnosis.
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Time–temperature chromatic sensor based on polydiacetylene (PDA) vesicle and amphiphilic copolymer MaLing Gou, Gang Guo, Juan Zhang, Ke Men, Jia Song, Feng Luo, Xia Zhao, ZhiYong Qian ∗ , YuQuan Wei State Key Laboratory of Biotherapy and Cancer Center, West China Hospital, West China Medical School, Sichuan University, Chengdu 610041, Sichuan, PR China
a r t i c l e
i n f o
Article history: Received 20 April 2010 Received in revised form 16 June 2010 Accepted 20 June 2010 Available online 7 July 2010 Keywords: Functional materials Sensor Polydiacetylene Thermochromism Amphiphilic polymers
a b s t r a c t Polydiacetylene (PDA) is an excellent color-changing indicator. In this paper, we demonstrate a new and interesting kind of time–temperature chromatic sensor based on PDA vesicle and amphiphilic polymer. In presence of amphiphilic polymers, PDA vesicles could gradually transit from blue to red, which was irreversible and regularly depended on the temperature, time, and properties of amphiphilic polymer (including the concentration, molecular weight, molecular structure, hydrophobicity, and thermosensitive property). Higher concentration of amphiphilic polymers resulted in lower color-transition temperature and faster color-transition. Also, higher temperature led to faster color-transition. The colorchanging of PDA/amphiphilic polymer matrix may have resulted from the higher temperature and gradual insertion of amphiphilic polymer into PDA, synergistically perturbing the conformation of PDA. The prepared PDA/F127, PDA/F68, PDA/L35 and PDA/Tween-20 aqueous matrix showed potential application in time–temperature chromatic sensors at around 10–50 ◦ C. This work provides a new method to design time–temperature chromatic sensors. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Irreversible color-changing indicators have potential application in monitoring the temperature history of perishables. Polydiacetylene (PDA) is -conjugated polymer that has alternating double- and triple-bond groups in the main polymer chain. Disruption of the extensively delocalized enyne back bones of molecularly ordered PDA side-chains induces a blue-to-red color change [1]. PDA has been extensively investigated as a sensor matrix, owing to its brilliant blue-to-red color-transition behavior that takes place in response to environmental changes including heat (thermochromism), organic solvents (solvatochromism), mechanical stress (mechanochromism), and ligand–receptor interactions (affinochromism) [2–9]. PDAs are already used to fabricate sensors for the detection of various substances, such as virus, lipophilic enzymes, peptides, ions, antibody, protein, oligonucleotide, etc. [10–15]. As well as the very interesting optical properties, PDAs are attractive sensory materials as they (1) can be prepared from self-assembled crystalline or semi-crystalline states of diacetylene monomers, (2) are produced by UV or irradiation of self-assembled diacetylenes without the need for chemical initiators or catalysts, and (3) are readily prepared
∗ Corresponding author. Tel.: +86 28 85164063; fax: +86 28 85164060. E-mail address: [email protected] (Z. Qian). 0925-4005/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2010.06.041
in aqueous solution in the form of nanostructured liposomes, vesicles and wires, and can thus PDAs be readily generated in matrix formats for diagnostics purpose. Among the various chromatic transitions of PDAs, the most extensively explored properties are thermally induced color-transitions [16]. Due to the interesting thermochromism, PDAs have been widely used as temperature sensors [17–19]. To our knowledge, lots of agents are perishable and should be kept at low temperature. It may be necessary to know the temperature history of a perishable product when we receive it, particularly whether the product has encountered a time–temperature procedure possibly affecting the quality of the product in question. Though PDAs attract great attention as temperature sensors, little exists in the literature regarding the time–temperature chromatic sensors based on PDAs [20]. Previous reports indicated that amphiphilic molecules could induce the color-transition of PDA [21]. In this paper, based on the understanding of interactions between PDA and amphiphilic polymers, we demonstrate a new and interesting kind of time–temperature chromatic sensor based on PDA vesicle and amphiphilic polymer, which could be readily generated, and could work between 10 and 50 ◦ C. The demonstrated chromatic sensors have great potential application in determining the time–temperature history of perishable bio-products, such as vaccines, bio-drugs, bio-kits, etc., which is particularly important after transportation and storage.
M. Gou et al. / Sensors and Actuators B 150 (2010) 406–411
2. Experimental 2.1. Materials
407
camera, and data about the color of the PDA/amphiphilic polymer aqueous matrix was processed according to Fig. S2 (see support information).
Poly(ethylene glycol) (PEG4000, Mn = 4000) was purchased from Fluka (USA). Also, 10,12-pentacosadiynoic acid (PCDA) was purchased from Fluka (USA), and further purified by dissolving in dichloromethane and then filtered to remove polymerized monomers before use. Pluronic F127, F68 and L35 were purchased from Sigma (USA). Tween-20 and dichloromethane were purchased from KeLong Chemicals (Chengdu, China). See Fig. S1 (see supporting information) for the molecular structure of F127, F68, L35 and Tween-20.
2.4. Quantifying the color response (CR)
2.2. Preparation and characterization of PDA vesicles
where PB = Ablue /(Ablue + Ared ). A is the absorbance at either the blue component (640 nm) or the red component (550 nm) of visible spectrum. PB0 is the control, blue-ratio of pure PDA solution at 0 ◦ C, while PB1 is the value of sample exposed to different temperature for different time.
In preparation, 40 mg of PCDA was dissolved in CH2 Cl2 followed by solvent evaporation. After adding 20 mL of distilled water, the suspension was probe ultra-sonicated for 10 min at approximately 75 ◦ C. After the ultra-sonication, the solution was cooled and then stored overnight at 4 ◦ C to induce crystallization of lipid membranes. Polymerization was carried out under irradiation at 254 nm wavelength for 5 min in ice bath. At last, the obtained blue PDA vesicles were stored at 4 ◦ C for further use. The particle size of prepared PDA vesicles was 128 ± 5 nm, which was determined by the diffraction particle size detector (Nano-ZS, Malvern Instrument, UK). The result was the mean of three test runs. 2.3. Preparation of PDA/amphiphilic polymer aqueous matrix Afterward, 0.1 mL of PDA vesicle solution was mixed into 0.9 mL of amphiphilic polymer aqueous solution at 0 ◦ C, followed by placing the vessel in a water bath at different temperatures. The color of the PDA/amphiphilic polymer aqueous matrix was taken by digital
Finally, 0.15 mL of sample was added into the well of 96-well plate, and the absorption value at 640 and 550 nm was immediately calculated using a spectrophotometer (M5, Molecular Corporation). To quantify the extent of the blue-to-red color-transitions, the CR (%) was calculated using the following equation: CR (%) =
PB − PB 0 1 PB0
× 100
3. Results and discussion In this paper, we demonstrate a novel time–temperature chromatic sensor, which is schematically presented in Scheme 1. After the blue PDA vesicle was prepared by UV irradiation of self-assembled colorless PCDA vesicle, hydrophobic segments of amphiphilic polymers were gradually inserted into PDA vesicles (due to hydrophobic interaction). With this, the temperature synergistically perturbed the conformation of PDA, changing the color of PDA, a process which was dependent on the temperature, time and properties of amphiphilic polymers. Pluronic polymers are triblock copolymers in the form of poly(ethylene glycol)–poly(propylene glycol)–poly(ethylene glycol) (PEG–PPG–PEG), which have surface activity and can insert
Scheme 1. Schematic representation of the time–temperature chromatic sensor based on PDA vesicle and amphiphilic polymer.
408
M. Gou et al. / Sensors and Actuators B 150 (2010) 406–411
Fig. 2. Quantitative determination of the colorimetric response (CR) of PDA/F127 aqueous matrix.
Fig. 1. The color of PDA/F127 aqueous matrix after exposure to different temperature for different time.
into lipid monolayer [22]. Pluronic F127 has the molecular structure of PEG99 –PPG67 –PEG99 and molecular weight of 12,700. After the PDA/F127 was exposed to a different temperature for different time, its color was recorded, with results presented in Fig. 1. The naked eye visible color-transition of PDA/F127, from blue to red, took placed at 20–50 ◦ C, which was dependant on the temperature, time and concentration of F127. It could be observed that higher concentration of F127 resulted in lower color-transition temperature and faster color-changing. Meanwhile, higher temperature resulted in faster color-changing. Additionally, we quantified the colorimetric response (CR) of PDA/F127 (see Fig. 2). When the concentration of F127 was kept constant (20%, w/w), the CR increased with time and temperature as shown in Fig. 2a. When the reaction time was kept constant (2 h), the CR increased with increase in temperature and concentration of F127 (Fig. 2b). When the temper-
ature was kept constant (30 ◦ C), the CR increased with increase in time and concentration of F127 as presented in Fig. 2c. These results point to the potential application of PDA/F127 as an interesting time–temperature chromatic sensor. Usually, PDA vesicles were blue below 50 ◦ C, and could transit from blue to red at or above ca. 60 ◦ C in seconds. Here, F127 amphiphilic polymer contributed to decrease the color-transition temperature of PDA. The temperature and interaction between PDA and F127 likely play the key roles in the gradual color change of PDA/F127 aqueous matrix. Previously, numerous investigations have been carried out to gain a better understanding of the origin of PDA thermochromism. The results of recent studies strongly suggest that the release of mechanical strain, developed on the side chains during polymerization, induced a partial distortion of the conjugated p-orbital arrays. This distortion led to a shortening of the effective p-conjugation length, and was the main factor responsible for the thermally promoted color-transitions [16,23]. This study also addressed the effect of hydrophilic poly(ethylene glycol) (PEG) on the color of PDA, in order to understand the interaction between PDA and F127. In the presence of hydrophilic PEG4000 (20%, w/w) blue PDA could not transit to red, even after 5 days exposure to 30 ◦ C temperature. But in the presence of amphiphilic F127 (PEG–PPG–PEG) at the concentration of 5% (w/w), the color-transition of PDA was observed at 30 ◦ C. So, we suspect that hydrophobic interactions between hydrophobic PPG segments of F127 and hydrophobic core of PDA vesicles might be the main interaction between PDA and F127. To our knowledge,
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Fig. 3. The color of PDA/F68 (a) and PDA/L35 (b) after exposure to different temperature for different time.
PCDA has a hydrophilic head (carboxy group), and PDA vesicles have a hydrophilic shell. Hydrophobic interactions between PDA and F127 could not occur before F127 molecules diffused through the hydrophilic shell into the hydrophobic core of PDA vesicles. Previously, Su et al. believed that, in presence of CTAB surfactants, color-transition of PDA from blue to red was mainly due to the insertion of alkyl chain of CTAB into the hydrophobic domain perturbed the conformation of the conjugated polymer backbone [21]. In our results, the temperature induced partial distortion of the conjugated p-orbital arrays and the insertion of hydrophobic PPG segments of F127 into PDA, synergistically perturbing the conformation of PDA leading to color-changing of PDA; this is likely the key reason of color-transition of PDA/F127 aqueous matrix. Otherwise, the insertion of F127 into PDA vesicles might mainly be a diffusion process, and the speed of diffusion might be dependent on the temperature, concentration of F127, and so on. Based on the above hypothesis, the timing of F127 insertion into PDA is likely correlated to the color-transition of PDA/F127 aqueous matrix. In order to better understand this phenomenon, the thermochromism of PDA/F68 (PEG76 –PPG29 –PEG76 , Mn = 8400) and PDA/L35 (PEG11 –PPG16 –PEG11 , Mn = 1900) was also studied (Fig. 3). Effects of F68 and L35 on the color of PDA vesicles indicated that both PDA/F68 and PDA/L35 showed visible temperatureand polymer concentration-dependent color-changing, and higher
polymer concentration resulted in lower color-transition temperature of PDA. From PDA/F127 to PDA/F68 to PDA/L35, the correlation between time and color change was gradually weakened. This may be because the insertion of PEG–PPG–PEG in to PDA was related to the molecular weight, and higher molecular PEG–PPG–PEG led to slower insertion and slower color-transition of PDA. Amphiphilic polymers with more hydrophobicity should result in stronger hydrophobic interaction between PDA and amphiphilic polymers. At 30 ◦ C, the capacity of PEG–PPG–PEG polymers to induce color-transition of PDA vesicle was: F127 > F68 > L35, although the hydrophobicity was: L35 > F127 > F68. So, not only hydrophobicity but also molecular weight of PEG–PPG–PEG polymer would affect its capacity to induce color-changing of PDA. Otherwise, many properties of PEG–PPG–PEG aqueous solution were temperaturedependent, including micellization, surface activity, insertion into lipid monolayer, etc. [24,22]. Regarding PEG–PPG–PEG copolymers, higher temperature always resulted in lower CMC, stronger hydrophobicity, and stronger insertion into lipid monolayer. Among F127, F68 and L35, the most sensitive to temperature is F127. The CMC of F127 is 3.174 mM at 20 ◦ C, while the CMC is 0.006 mM at 40 ◦ C [22], which may mean that at a concentration as low as 0.1% of F127 the PDA would transit to red at 50 ◦ C. Later on, we also studied the correlation between the PDA/Tween-20 aqueous matrix color change and both the temper-
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ature and concentration of Tween-20 (Fig. 4). Between 10 and 30 ◦ C, a clear correlation between time and PDA/Tween-20 color change could be observed, although the molecular weight of Tween-20 was 1226 that was lower than 1900 of L35. This indicates that the molecular structure of amphiphilic polymer has an important effect on the thermochromism of PDA/amphiphilic polymer aqueous matrix. In summary, the above results suggest that the visible colorchanging of PDA/amphiphilic aqueous matrix was irreversible and regularly dependent on the temperature, time, and properties of amphiphilic polymers (including the concentration, molecular structure, molecular weight, hydrophobicity, and thermo-sensitive property). This suggests that the PDA/amphiphilic polymer aqueous matrix could be applied in a time–temperature chromatic sensor at a temperature around 10–50 ◦ C. Through adjusting the properties of the amphiphilic polymer, PDA/amphiphilic polymer matrix would manifest different colors according to the temperature and time, allowing the precise determination of time–temperature history through the combination of fast color-changing and slow color-changing sensors based on PDA/amphiphilic polymer. 4. Conclusion Amphiphilic polymers could decrease the color-transition temperature of PDA vesicles in water, and the irreversible color-changing of PDA/amphiphilic polymer matrix was regularly dependent on the temperature, time, and properties of amphiphilic polymer (including the concentration, molecular structure, molecular weight, hydrophobicity and temperaturesensitive property), which make the PDA/amphiphilic polymer aqueous matrix novel time–temperature chromatic sensors. The demonstrated PDA/F127, PDA/F68, PDA/L35 and PDA/Tween-20 matrix showed potential application in time–temperature chromatic sensors at around 10–50 ◦ C. This study provides a novel method to designing time–temperature chromatic sensor based on PDA vesicles and amphiphilic polymers. Acknowledgements This work was financially supported by New Century Excellent Talents in University (NCET-08-0371), Specialized Research Fund for the Doctoral Program of Higher Education (SRFDP 200806100065), National 863 project (2007AA021902), National Natural Science Foundation (NSFC20704027), and Chinese Key Basic Research Program (2010CB529906). We thank Angela Merriam (Sichuan University, China) for correcting this paper in English. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.snb.2010.06.041. References
Fig. 4. The color of PDA/Tween-20 aqueous matrix after exposure to different temperature for different period.
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Biographies MaLing Gou received his PhD in July 2010, from Sichuan University, China. His research interest is in the area of nanobiotechnology, sensing materials research, and nanomedicine. Now he has published more than 20 research articles in the international journals. Gang Guo received his PhD in July 2007, from Sichuan University, China. His research interest is in the area of functional polymer. Juan Zhang is a postgraduate student in Sichuan University. Her research interest is in the area of nanobiomaterials. Ke Men is a postgraduate student in Sichuan University. His research interest is in the area of nanobiotechnology. Jia Song is a postgraduate student in Sichuan University. Her research interest is in the area of nanobiomaterials. Feng Luo is a professor in West China Hospital, Sichuan University. His research interest is cancer therapy and cancer diagnosis. Xia Zhao is a professor in West China Second Hospital, Sichuan University. Her research interest is cancer therapy and cancer diagnosis. ZhiYong Qian received his PhD in July 2003, from Chengdu Institute of Organic Chemistry, Chinese Academy of Sciences, PR China. Now, Prof. ZhiYong Qian is head of the nanobiomaterials group at State Key Laboratory of Biotherapy, West China Hospital, Sichuan University. He is the associate editor of two international journals, including Journal of Biomedical Nanotechnology and Advanced Science Letters. Prof. Qian is in charge of some projects, including National High-Tech Project of China (863-Project), National Natural Science Foundation project, and Specialized Research Fund for the Doctoral Program of Higher Education. Prof. Qian is focused on the research of nanobiotechnology, functional polymers, and advanced drug delivery systems. YuQuan Wei is the academician of Chinese Academy of Sciences, the Cheung Kong Scholar, and The National Science Fund for Distinguished Young Scholar. And Prof. Wei is the vice-president of Sichuan University, head of State Key Lab of Biotherapy. Prof. Wei is the associate editor of “Human Gene Therapy”, the premier journal in the field of gene therapy. He is the principal scientist of two 973 projects. And he is the principal of some 863 projects, and National Natural Science Foundation project. Prof. Wei has published more than 100 articles in the international journals, is a named inventor of over 30 patents. His research interest is cancer therapy and cancer diagnosis.