- Email: [email protected]
Effect of amphiphilic molecules upon chromatic transitions of polydiacetylene vesicles in aqueous solutions
Colloids and Surfaces B: Biointerfaces 39 (2004) 113–118
Effect of amphiphilic molecules upon chromatic transitions of polydiacetylene vesicles in aqueous solutions Yan-lei Su, Jin-ru Li, Long Jiang∗ Key Laboratory of Colloid and Interface Science, Center for Molecular Science, Institute of Chemistry, Chinese Academy of Science, Beijing 100080, PR China Available online 19 March 2004
Abstract Effect of amphiphilic molecules upon the chromatic transitions of polymerized 10,12-pentacosadiynoic acid (PCDA) vesicles in aqueous solutions was reported. The colorimetric response of polymerized PCDA vesicles for 1-pentanol is higher than that for ethanol due to more hydrophobic property of 1-pentanol. The colorimetric response of polymerized PCDA vesicles for sodium dodecyl sulfate (SDS) and Triton X-100 is lower than that for cetyltrimethylammonium bromide (CTAB). The strong ability of CTAB to induce chromatic transition of the vesicles is related to the positively charged headgroups of CTAB, which favors approach of CTAB to the negatively charged carboxylate groups at the vesicle surface. The insertion of alkyl chain of CTAB into the hydrophobic domain perturbs the conformation of the conjugated polymer backbone and induces color change of polydiacetylene vesicles. For a series of alkylamine hydrochloric salts, the longer the alkyl chain, the stronger the ability of alkylamine to induce chromatic transition of polydiacetylene vesicles. © 2004 Published by Elsevier B.V. Keywords: Polydiacetylene vesicles; Chromatic response; Amphiphilic molecules; Interaction
1. Introduction Diacetylenes substituted with various side chains readily undergo photopolymerization to form an ene–yne alternated polymer chain under UV irradiation in a wide range of organized structures, such as single crystals, Langmuir–Blodgett (LB) films, self-assembled monolayers, liposomes or vesicles [1–10]. Optical absorption in polydiacetylene occurs via a –* absorption within the linear -conjugated polymer backbone. The conjugated polydiacetylene backbone has two spectroscopically distinct phases, designated as the blue and the red forms, that result from their excitation absorption peaks at 650 and 540 nm, respectively. Under external perturbation, such as heat or mechanical stress, the conjugated polydiacetylene backbone can undergo a drastic reversible color transition or irreversible color change from the blue to the red form [11–14]. The mechanism of these transitions is not understood in detail. It is believed that molecular conformation changes, such as side chain packing, ordering, and orientation, impart stresses ∗ Corresponding author. Tel.: +86-10-82612084; fax: +86-10-82612084. E-mail address: [email protected] (L. Jiang).
0927-7765/$ – see front matter © 2004 Published by Elsevier B.V. doi:10.1016/j.colsurfb.2003.12.005
to the polymer backbone that alter its conformation, thus changing the electronic states and the corresponding optical absorption. This unique chromatic property has made polydiacetylenes promising candidates in the development of biosensors [15–30]. The colorimetric biosensors are selfassemblies of the diacetylene lipids mixed with natural or synthetic biological receptor molecules. Polydiacetylene vesicles or Langmuir–Blodgett films decorated with virusor toxin-specific ligands undergo dramatic color changes from blue to red in direct response to pathogen binding at the interface. These biochromic effects arise from (1) multipoint interactions of the receptor at the polydiacetylene supramolecular assembly surface, and/or (2) insertion of viral membrane or toxin hydrophobic domains into the polydiacetylene membrane [31]. Amphiphilic molecules have a tendency to incorporate into a hydrophobic environment, however, the phenomenon of amphiphilic molecule insertion enabled chromatic transitions of polydiacetylene vesicles, to the best of out knowledge, has not been reported in literature. We carried out a detailed study on the interactions of polydiacetylene vesicles with amphiphilic molecules. This work has a relevance to the actions of surfactant on phophatidylcholic (PC) vesicles. Due to the complexities of the natural
114
Y.-l. Su et al. / Colloids and Surfaces B: Biointerfaces 39 (2004) 113–118
cell membranes, PC vesicles have been used extensively as models for the study of biological membrane structure and function [32–37]. It is well known that surfactant molecules adsorb upon the vesicles, however, the mechanism and means of this adsorption remain relatively obscure. In this work, the experiments have been designed to investigate the electrostatic interaction and hydrophobic interaction between the polydiacetylene vesicles and surfactants. An overall aim of this work is to evaluate the effects of various physical parameters, such as the headgroup properties of surfactants, the pH of environment, and the alkyl chain length of the amphiphilic molecules upon the surfactant–vesicle interaction. It might be significant in understanding the interactions of biological membranes with surfactants; and that in the application of biochromism in disease diagnose, amphiphilic molecules should be eliminated lest wrong decision.
2. Experimental section
tained at 6.88 with phosphate buffer. All measurements have been carried out at 20 ◦ C except the variable temperature measurement. 2.4. Colorimetric response A quantitative value for the extent of blue to red color transition is given by the colorimetric response (CR), which is defined: PB0 − PBf CR = × 100% PB0 where PB = Ablue /(Ablue + Ared ), A is the absorbance at either the blue component (≈650 nm) in the UV-Vis spectrum or the red component (≈540 nm), PB0 is the initial percent blue of the vesicle solution before addition of amphiphilic molecules, and PBf is the final percent blue obtained for the vesicle solution after addition of amphiphilic molecules. Thus, for a completely converted vesicle solution, PBf = 0 and the CR = 100%.
2.1. Materials 3. Results 10,12-pentacosadiynoic acid (PCDA) was purchased from Lancaster Co. Ethanol, 1-pentanol, sodium dodecyl sulfate (SDS), cetyltrimethylammonium bromide (CTAB), p-(1,1,3,3-tetramethylbutyl)phenoxypolyoxyethyleneglycol (Triton X-100 containing an average of 10 ethylene oxide units per molecule), and cetylamine were purchased from Beijing Chemical Reagent Corp. (China). Dodecylamine and tetracylamine were purchased from Sigma. All reagents were used as received. Aqueous solutions for SDS, CTAB, and Triton X100 (20 mmol/l) were prepared by dissolving appropriate amounts of surfactants in distilled water. Dodecylamine, tetracylamine, and cetylamine were dissolved in 0.05 mol/l HCl solution.
The UV-Vis absorption spectrum of polymerized PCDA vesicles in aqueous solution is shown in Fig. 1, which is similar to the reported results [15–17,22–30]. The polymerized PCDA vesicles show a maximum absorption at 650 nm (excitonic band) with a shoulder peak at 596 nm (vibronic absorption), which is called the blue form. Upon increasing the pH to 11.30, the intensity of the blue form absorption band decreased. In the meantime, the maximum absorbance band appears at 540 nm and the vibronic shoulder shifts to 500 nm, which is called the red form. The heat effect upon the chromatic properties of the vesicles is analogous to that of elevating pH. Fig. 2 shows the colorimetric response 0.8
2.2. Vesicle preparation
0.7 0.6
Absorbance
A 20 ml volume of a 1 mmol/l PCDA solution in chloroform was rotoevaporated to dryness, and 20 ml of distilled water was added. The suspension was heated to 60 ◦ C and sonicated. The solution was filtered immediately and then stored at 4 ◦ C overnight to induce crystallization of the lipid bilayer membranes prior to photopolymerization. Then the solution was irradiated with UV light (254 nm) to yield a blue colored vesicle solution.
(3)
(2) (1)
0.5 0.4 0.3 0.2 0.1
2.3. UV-Vis spectroscopy UV-Vis spectra of vesicle in water were taken in quartz cuvettes with a 10 mm optical path length on a two-beam JASCO UV-Vis spectrometer. Aliphatic alcohol or aqueous surfactant solution was injected into aqueous vesicle solution with a syringe. The pH of vesicle solutions was main-
0.0 400
500
600
700
800
Wavelength (nm) Fig. 1. UV-Vis absorption spectra of polymerized PCDA vesicles in aqueous solutions at 20 ◦ C, (1) the pH of solution is 6.88, (2) the pH of solution is 11.30, and (3) the added CTAB concentration is 1.48 mmol/l.
Y.-l. Su et al. / Colloids and Surfaces B: Biointerfaces 39 (2004) 113–118
response of polymerized PCDA vesicles for 1-pentanol is higher than that for ethanol. For example, the CR value is 17.33 when 0.1 ml 1-pentanol was added into 5 ml polymerized vesicle solution. However, the CR value is only 0.17 when 1 ml ethanol was added into 5 ml polymerized vesicle solution. Due to its more hydrophobic property, 1-pentanol can soluble in the micellar cores [38]. The hydrophobic domain of polydiacetylene vesicle is fit to “soluble” hydrophobic molecules that introduce colorimetric response. Ethanol is a hydrophilic molecule, which dissolves perfectly in water rather than in the hydrophobic region, so that it causes a lower CR value.
pH of aqueous solution 2
4
6
8
10
12
70 60 50
CR (%)
115
40 30 20 10
3.2. Surfactants upon chromatic transition of the vesicles
0 20
30
40
50
60
70
80
o
Temperature ( C) Fig. 2. Colorimetric response of polymerized PCDA vesicles in aqueous solutions as a function of temperature and pH.
of polymerized PCDA vesicles in aqueous solutions as a function of pH and temperature. The CR value begins to increase when the pH is above 8.5 or temperature is above 35 ◦ C. A higher CR value indicates a stronger reddish appearance of the solution, compared to the blue form. 3.1. Aliphatic alcohol upon chromatic transition of the vesicles Colorimetric response of 5 ml aqueous solution of polymerized PCDA vesicles as a function of the volume of added ethanol and 1-pentanol is shown in Fig. 3. The colorimetric Volume (mL) 0.00
0.02
0.04
0.06
0.08
Motivation from that hydrophobic 1-pentanol can induce colorimetric change of polymerized PCDA vesicles; we examine surfactants upon chromatic transition of the vesicles in aqueous solutions. The usual anionic surfactant (SDS), cationic surfactant (CTAB), and nonionic surfactant (Triton X-100) in chemical laboratory were selected, an interesting phenomenon of the effect of surfactants upon the chromatic transitions was observed. The spectrum of polymerized PCDA vesicles in aqueous solution at pH 6.88 after addition of CTAB is presented in Fig. 1; the maximum absorption band is at 510 nm, the final color of the vesicle solution is a distinct orange. Fig. 4 shows the colorimetric response of polymerized PCDA vesicles in aqueous solutions, which is plotted as a function of the concentration of added CTAB. The CR value increases dramatically with an increase of CTAB concentration; saturation of the CR value is reached when CTAB concentration is above approximate 1.0 mmol/l. The CR value is 93.18 when the concentration of CTAB is 1.48 mmol/l. It can be seen in Fig. 4 that the colorimetric response of polymerized PCDA vesicles for SDS and Triton X-100 is ap-
0.10
50 100
40
80
CR (%)
CR (%)
30
20 1-pentanol
CTAB Triton X-100 SDS
60 40
10 20
ethanol 0
0
0.0
0.2
0.4
0.6
0.8
1.0
Volume (mL) Fig. 3. Colorimetric response of 5 ml aqueous solutions of polymerized PCDA vesicles as a function of the volume of added ethanol and 1-pentanol.
0.0
0.5
1.0
1.5
2.0
2.5
Concentration (mmol/L) Fig. 4. Colorimetric response of polymerized PCDA vesicles in aqueous solutions as a function of the concentration of sodium dodecyl sulfate (SDS), Triton X-100, and cetyltrimethylammonium bromide (CTAB).
Y.-l. Su et al. / Colloids and Surfaces B: Biointerfaces 39 (2004) 113–118
parently lower than that for CTAB at the same concentration. When surfactant concentration is 1.48 mmol/l, the CR values for SDS is 4.39, for Triton X-100 is 21.57. At pH 6.88 of the aqueous solution, the carboxylate function groups at the surface of vesicles are ionized. The strong ability of CTAB to induce chromatic transition of the vesicles is related to the positively charged headgroups of CTAB, which favors approach of CTAB molecules to the negatively charged carboxylate groups at the vesicle surface. The repulsive Coulombic interaction between the negatively charged headgroups of anionic surfactant SDS and the polymerized PCDA vesicles hinders the approach of SDS molecules to the vesicles, so that it causes a low chromatic response. The electrostatic interaction gives slightly effect upon nonionic surfactant Triton X-100, while the chromatic response for Triton X-100 is moderate.
Since the colorimetric response of polymerized PCDA vesicles for CTAB is related to the electrostatic attraction, the change of the charge density on the surface of vesicles should alter the colorimetric response. The charge density on the surface of vesicles varies according to the pH of the environment. Colorimetric responses of polymerized PCDA vesicles in aqueous solutions for different pH as a function of CTAB concentration are given in Fig. 5. A decrease of pH of the vesicle solution induces a lower CR value, which confirms the chromatic transitions indeed dependent on the charge density at the vesicle surface. When CTAB concentration is 1.48 mmol/l, the CR value for pH 4.50 is 30.71, and for pH 2.80 is 12.61. The sensitivity of the vesicles to red form conversion is clearly enhanced at pH 6.88, compared to pH 4.50 and 2.80. Those results suggest a cooperative effect for pH-induced chromatic transition. That is, the repulsive Coulombic interaction between the carboxylate function groups at the surface
100 80
CR (%)
40 dodecylamine tetradecylamine cetylamine
30
20
10
0 0.0
0.5
1.0
1.5
2.0
Concentration (mmol/L) Fig. 6. Colorimetric response of polymerized PCDA vesicles in aqueous solutions as a function of the concentration of alkylamine hydrochloric salts.
3.3. pH dependence of the colorimetric response
pH = 6.88 pH = 4.50 pH = 2.80
60
50
CR (%)
116
40 20
of the vesicles is enlarged at pH 6.88, so that the barrier to further conversion is reduced. On the other hand, higher charge density on the surface of the vesicles would enhance electrostatic attractions between the CTAB and the vesicles; CTAB molecules would really approach the vesicles. 3.4. Alkylamine upon chromatic transition of the vesicles While above work demonstrates the importance of electrostatic interactions to drive incorporation of foreign molecules into membranes, small amino acid molecules did not cause color change of the polydiacetylene vesicles [22]; it is guessed that hydrophobic interactions also play a key role in the chromatic response where the amphophilic molecules insets into the hydrophobic domain of the vesicles. Colorimetric response of polymerized PCDA vesicles in aqueous solutions at pH 4.50 as a function of the concentration of alkylamine hydrochloric salts is shown in Fig. 6. For a series of alkylamine hydrochloric salts at the same concentration, the longer the hydrophobic chain, the stronger the ability of alkylamine to induce chromatic transition of polydiacetylene vesicles. The current work shows the important of hydrophobically driven alkyl chains of amphiphilic molecules insertion into the hydrophobic domain of polymerized PCDA vesicles and the CR value is correlated with the length of the alkyl chain. 4. Discussion
0 0.0
0.5
1.0
1.5
2.0
2.5
Concentration (mmol/L) Fig. 5. Colorimetric response of polymerized PCDA vesicles in aqueous solutions at different pH as a function of the concentration of CTAB.
Although numerous and wide-ranging works have been published on the study of polydiacetylenes, there is still a lack of an appropriate and clear model to explain the chromatic behavior of these polymers. Huo has proposed that a “self-folding” process to explain the chromatic changes of polydiacetylenes at the air–water interface and in the
Y.-l. Su et al. / Colloids and Surfaces B: Biointerfaces 39 (2004) 113–118
117
Fig. 7. Schematic diagram of the interaction of CTAB and the polymerized PCDA vesicle. The head group of CTAB is positive charged and PCDA is negative charged.
soild-state [7]. With increased irradiation time, the length of the polymer chain is increased; the original linear polyeneye backbone starts to “self-fold” to a zigzag structure due to the free rotation of single bonds within the polymer chain. The efficient -electron delocalization along the polyenyne backbone is interrupted by this process, leading to a chromatic change from the blue to red form of polydiacetylenes. Based on the experiment results, we proposed a model for the interaction of CTAB with the polymerized PCDA vesicle, which is illustrated in Fig. 7. Since the vesicle contains mostly negatively charged groups, it can be expected to stimulate the binding of CTAB onto it. CTAB molecule has a long alkyl chain; hydrophobic interaction would drive the alkyl chain to incorporate with the hydrophobic domain of the vesicle. The adsorbed CTAB molecule would reverse and make its alkyl chain insert into the vesicle. The insertion would result in a zigzag polymer backbone; which would effectively reduce the conjugated polymer length. It is suggested that the insertion of CTAB molecules into polymerized PCDA vesicle form a strong complex (supramolecular assembly). CTAB has a specific tendency to adsorb onto the vesicles due to the presence of negative charge on the vesicle surface. The insertion perturbs the conformation of the polymer backbone of the bilayers, which induces the chromatic response of the polymerized PCDA vesicle. In the absence of favorable interaction of electrostatic attraction, neither SDS nor Triton X-100 is an efficient binder of vesicles to perturb the conformation of the polymerized PCDA backbone. Surfactants have a tendency to form a micelle. The critical micelle concentration (CMC) is the characteristic concentration of surfactants in aqueous solution above that the appearance and development of micelles brings about sudden variation in the relation between the concentration and certain physico-chemical properties of the solution. Above the CMC, the concentration of singly dispersed surfactant molecules is virtually constant. The CMC of Triton X-100 is 0.26 mmol/l, the CMC of CTAB is 0.92 mmol/l, and the CMC of SDS is 8.3 mmol/l. The concentration of surfactants in the present work is above the CMC of Triton X-100, below the CMC of SDS, and spanning the CMC of CTAB. It can be seen in Figs. 3 and 4 that there are not durative increase of the CR values when the concentration is above the CMC of CTAB. The manner of the vesicles adsorption for CTAB is monomer as that illustrateed in Fig. 7. It is observed that the CR values do not increase when the concentration above its CMC.
In Fig. 4 for the interaction of Triton X-100 with the polydiacetylene vesicles, the concentration is remarkedly higher than the CMC, and the CR value increases with an increase of Triton X-100 concentration (which included the concentration of Triton X-100 monomer and micelle, the concentration of monomer is invariable above the CMC, whereas the concentration of micelle increase with an increase of surfactant concentration). The most plausible explanation is that Triton X-100 micelles directly attack the polymerized PCDA vesicles, since the CR values increase with an increase of Triton X-100 micelle concentration. At last, we emphasized that in Fig. 1, the maximum absorption band of red form of polymetized PCDA vesicles at pH 11.3 is at 540 nm, whereas the maximum absorption band is at 510 nm by the addition of CTAB. The energy of an electronic excitation on a linear conjugated polydiacetylene depends upon the number of carbon atoms coupled into the -bonding sequence. The higher energy means a shorter chain of the conjugated polydiacetylene backbone. The perturbation induced by insertion of CTAB into the polymerized PCDA vesicles is stronger than that by increasing temperature and elevating pH of aqueous solution.
5. Conclusions The interactions of amphiphilic molecules with vesicles in aqueous solutions were studied by UV-Vis spectroscopy. CTAB has a specific tendency to adsorb onto the polydiacetylene vesicles owing to the favorably electrostatic attraction. The hydrophobic interaction results in the structural reorientation of CTAB molecule, and the alkyl chain of CTAB insetion into the hydrophobic domain of the vesicle. The insertion perturbs the conformation of the polymer backbone of the bilayers, and introduces the chromatic response of the polymerized PCDA vesicle. Alternation of the properties of surfactants and the decrease of the charge density on the vesicle surface effectively lessen the interaction of amphiphilic molecules with the polydiacetylene vesicles and induce lower chromatic responses.
Acknowledgements We thank the National Natural Science Foundation of China (grant number 90206035) for their financial support.
118
Y.-l. Su et al. / Colloids and Surfaces B: Biointerfaces 39 (2004) 113–118
References [1] J. Song, Q. Cheng, S. Kopta, R.C. Stevens, J. Am. Chem. Soc. 123 (2001) 3205. [2] J. Song, Q. Cheng, R.C. Stevens, Chem. Phys. Lipids 114 (2002) 203. [3] Q. Cheng, M. Yamamoto, R.C. Stevens, Langmuir 16 (2002) 5333. [4] Q. Cheng, R.C. Stevens, Langmuir 14 (1998) 1974. [5] Q. Cheng, R.C. Stevens, Chem. Phys. Lipids 87 (1997) 41. [6] Q. Huo, S. Russev, T. Hasegawa, J. Nishijo, J. Umemura, G. Puccetti, K.C. Russell, R.M. Leblanc, J. Am. Chem. Soc. 122 (2000) 7890. [7] Q. Huo, K.C. Russell, R.M. Leblanc, Langmuir 15 (1999) 3972. [8] U. Jonas, K. Shah, S. Norvez, D. H Charych, J. Am. Chem. Soc. 121 (1999) 4580. [9] A. Lio, A. Reichert, D.J. Ahn, J.O. Nagy, M. Salmeron, D.H. Charych, Langmuir 13 (1997) 6524. [10] D.W. Britt, U.G. Hofmann, D. Möbius, S.W. Hell, Langmuir 17 (2001) 3757. [11] A.A. Deckert, J.C. Horne, B. Valentine, L. Kiernan, L. Fallon, Langmuir 11 (1995) 643. [12] N. Mono, H. Tamura, K. Ogawa, Langmuir 8 (1992) 594. [13] N. Mono, H. Tamura, K. Ogawa, Langmuir 7 (1991) 2336. [14] R.A. Nallicheri, M.F. Rubner, Macromolecules 24 (1991) 517. [15] D.H. Charych, J.O. Nagy, W. Spevak, M.D. Bednarski, Science 261 (1993) 585. [16] A. Reichert, J.O. Nagy, W. Spevak, D. Charych, J. Am. Chem. Soc. 117 (1995) 829. [17] J.J. Pan, D. Charych, Langmuir 13 (1997) 1365. [18] T. Peng, Q. Cheng, R.C. Stevens, Anal. Chem. 72 (2000) 1611. [19] Q. Cheng, T. Peng, R.C. Stevens, J. Am. Chem. Soc. 121 (1999) 6767.
[20] Y. Li, B. Ma, Y. Fan, X. Kong, J. Li, Anal. Chem. 74 (2002) 6349. [21] J. Song, Q. Cheng, S. Zhu, R.C. Stevens, Biomed. Microdevices 4 (2002) 213. [22] Z. Ma, J. Li, M. Liu, J. Cao, Z. Zou, J. Tu, L. Jiang, J. Am. Chem. Soc. 120 (1998) 12678. [23] Z. Ma, J. Li, L. Jiang, J. Cao, P. Boullanger, Langmuir 16 (2000) 7801. [24] B. Ma, Y. Fan, L. Zhong, X. Kong, Y. Li, J. Li, Colloid Surf. B 27 (2002) 209. [25] R. Jelinek, S. Kolusheva, Biotech. Adv. 19 (2001) 109. [26] S. Kolusheva, R. Kafri, M. Katz, R. Jelinek, J. Am. Chem. Soc. 123 (2001) 417. [27] S. Kolusheva, T. Shahal, R. Jelinek, J. Am. Chem. Soc. 122 (2000) 776. [28] S. Kolusheva, T. Shahal, R. Jelinek, Biochem. 39 (2000) 15851. [29] S.Y. Okada, R. Jelinek, D. Charych, Angew. Chem. Int. Ed. 38 (1999) 655. [30] R. Jelinek, S. Okada, S. Norvez, D. Charych, Chem. Bio. 5 (1998) 619. [31] S. Okada, S. Peng, W. Spevak, D. Charych, Acc. Chem. Res. 31 (1998) 229. [32] N. Deo, P. Somasundaran, Langmuir 19 (2003) 2007. [33] C. Rodrigues, P. Gameiro, S. Reis, J.L.F. Lima, B. de Casro, Langmuir 18 (2002) 10231. [34] M.M. Santore, D.F. Discher, Y. Won, F.S. Bates, D.A. Hammer, Langmuir 18 (2002) 7299. [35] O. López, M. Cócera, C. López-Iglesias, P. Walter, L. Coderch, J.L. Parra, A. de la Maza, Langmuir 18 (2002) 7002. [36] M. Cócera, O. López, J. Estelrich, J.L. Parra, A. de la Maza, Langmuir 16 (2000) 4068. [37] I. Reviakine, A. Brisson, Langmuir 16 (2000) 1806. [38] Y.L. Su, X.F. Wei, H.Z. Liu, Langmuir 19 (2003) 2995.
Effect of amphiphilic molecules upon chromatic transitions of polydiacetylene vesicles in aqueous solutions Yan-lei Su, Jin-ru Li, Long Jiang∗ Key Laboratory of Colloid and Interface Science, Center for Molecular Science, Institute of Chemistry, Chinese Academy of Science, Beijing 100080, PR China Available online 19 March 2004
Abstract Effect of amphiphilic molecules upon the chromatic transitions of polymerized 10,12-pentacosadiynoic acid (PCDA) vesicles in aqueous solutions was reported. The colorimetric response of polymerized PCDA vesicles for 1-pentanol is higher than that for ethanol due to more hydrophobic property of 1-pentanol. The colorimetric response of polymerized PCDA vesicles for sodium dodecyl sulfate (SDS) and Triton X-100 is lower than that for cetyltrimethylammonium bromide (CTAB). The strong ability of CTAB to induce chromatic transition of the vesicles is related to the positively charged headgroups of CTAB, which favors approach of CTAB to the negatively charged carboxylate groups at the vesicle surface. The insertion of alkyl chain of CTAB into the hydrophobic domain perturbs the conformation of the conjugated polymer backbone and induces color change of polydiacetylene vesicles. For a series of alkylamine hydrochloric salts, the longer the alkyl chain, the stronger the ability of alkylamine to induce chromatic transition of polydiacetylene vesicles. © 2004 Published by Elsevier B.V. Keywords: Polydiacetylene vesicles; Chromatic response; Amphiphilic molecules; Interaction
1. Introduction Diacetylenes substituted with various side chains readily undergo photopolymerization to form an ene–yne alternated polymer chain under UV irradiation in a wide range of organized structures, such as single crystals, Langmuir–Blodgett (LB) films, self-assembled monolayers, liposomes or vesicles [1–10]. Optical absorption in polydiacetylene occurs via a –* absorption within the linear -conjugated polymer backbone. The conjugated polydiacetylene backbone has two spectroscopically distinct phases, designated as the blue and the red forms, that result from their excitation absorption peaks at 650 and 540 nm, respectively. Under external perturbation, such as heat or mechanical stress, the conjugated polydiacetylene backbone can undergo a drastic reversible color transition or irreversible color change from the blue to the red form [11–14]. The mechanism of these transitions is not understood in detail. It is believed that molecular conformation changes, such as side chain packing, ordering, and orientation, impart stresses ∗ Corresponding author. Tel.: +86-10-82612084; fax: +86-10-82612084. E-mail address: [email protected] (L. Jiang).
0927-7765/$ – see front matter © 2004 Published by Elsevier B.V. doi:10.1016/j.colsurfb.2003.12.005
to the polymer backbone that alter its conformation, thus changing the electronic states and the corresponding optical absorption. This unique chromatic property has made polydiacetylenes promising candidates in the development of biosensors [15–30]. The colorimetric biosensors are selfassemblies of the diacetylene lipids mixed with natural or synthetic biological receptor molecules. Polydiacetylene vesicles or Langmuir–Blodgett films decorated with virusor toxin-specific ligands undergo dramatic color changes from blue to red in direct response to pathogen binding at the interface. These biochromic effects arise from (1) multipoint interactions of the receptor at the polydiacetylene supramolecular assembly surface, and/or (2) insertion of viral membrane or toxin hydrophobic domains into the polydiacetylene membrane [31]. Amphiphilic molecules have a tendency to incorporate into a hydrophobic environment, however, the phenomenon of amphiphilic molecule insertion enabled chromatic transitions of polydiacetylene vesicles, to the best of out knowledge, has not been reported in literature. We carried out a detailed study on the interactions of polydiacetylene vesicles with amphiphilic molecules. This work has a relevance to the actions of surfactant on phophatidylcholic (PC) vesicles. Due to the complexities of the natural
114
Y.-l. Su et al. / Colloids and Surfaces B: Biointerfaces 39 (2004) 113–118
cell membranes, PC vesicles have been used extensively as models for the study of biological membrane structure and function [32–37]. It is well known that surfactant molecules adsorb upon the vesicles, however, the mechanism and means of this adsorption remain relatively obscure. In this work, the experiments have been designed to investigate the electrostatic interaction and hydrophobic interaction between the polydiacetylene vesicles and surfactants. An overall aim of this work is to evaluate the effects of various physical parameters, such as the headgroup properties of surfactants, the pH of environment, and the alkyl chain length of the amphiphilic molecules upon the surfactant–vesicle interaction. It might be significant in understanding the interactions of biological membranes with surfactants; and that in the application of biochromism in disease diagnose, amphiphilic molecules should be eliminated lest wrong decision.
2. Experimental section
tained at 6.88 with phosphate buffer. All measurements have been carried out at 20 ◦ C except the variable temperature measurement. 2.4. Colorimetric response A quantitative value for the extent of blue to red color transition is given by the colorimetric response (CR), which is defined: PB0 − PBf CR = × 100% PB0 where PB = Ablue /(Ablue + Ared ), A is the absorbance at either the blue component (≈650 nm) in the UV-Vis spectrum or the red component (≈540 nm), PB0 is the initial percent blue of the vesicle solution before addition of amphiphilic molecules, and PBf is the final percent blue obtained for the vesicle solution after addition of amphiphilic molecules. Thus, for a completely converted vesicle solution, PBf = 0 and the CR = 100%.
2.1. Materials 3. Results 10,12-pentacosadiynoic acid (PCDA) was purchased from Lancaster Co. Ethanol, 1-pentanol, sodium dodecyl sulfate (SDS), cetyltrimethylammonium bromide (CTAB), p-(1,1,3,3-tetramethylbutyl)phenoxypolyoxyethyleneglycol (Triton X-100 containing an average of 10 ethylene oxide units per molecule), and cetylamine were purchased from Beijing Chemical Reagent Corp. (China). Dodecylamine and tetracylamine were purchased from Sigma. All reagents were used as received. Aqueous solutions for SDS, CTAB, and Triton X100 (20 mmol/l) were prepared by dissolving appropriate amounts of surfactants in distilled water. Dodecylamine, tetracylamine, and cetylamine were dissolved in 0.05 mol/l HCl solution.
The UV-Vis absorption spectrum of polymerized PCDA vesicles in aqueous solution is shown in Fig. 1, which is similar to the reported results [15–17,22–30]. The polymerized PCDA vesicles show a maximum absorption at 650 nm (excitonic band) with a shoulder peak at 596 nm (vibronic absorption), which is called the blue form. Upon increasing the pH to 11.30, the intensity of the blue form absorption band decreased. In the meantime, the maximum absorbance band appears at 540 nm and the vibronic shoulder shifts to 500 nm, which is called the red form. The heat effect upon the chromatic properties of the vesicles is analogous to that of elevating pH. Fig. 2 shows the colorimetric response 0.8
2.2. Vesicle preparation
0.7 0.6
Absorbance
A 20 ml volume of a 1 mmol/l PCDA solution in chloroform was rotoevaporated to dryness, and 20 ml of distilled water was added. The suspension was heated to 60 ◦ C and sonicated. The solution was filtered immediately and then stored at 4 ◦ C overnight to induce crystallization of the lipid bilayer membranes prior to photopolymerization. Then the solution was irradiated with UV light (254 nm) to yield a blue colored vesicle solution.
(3)
(2) (1)
0.5 0.4 0.3 0.2 0.1
2.3. UV-Vis spectroscopy UV-Vis spectra of vesicle in water were taken in quartz cuvettes with a 10 mm optical path length on a two-beam JASCO UV-Vis spectrometer. Aliphatic alcohol or aqueous surfactant solution was injected into aqueous vesicle solution with a syringe. The pH of vesicle solutions was main-
0.0 400
500
600
700
800
Wavelength (nm) Fig. 1. UV-Vis absorption spectra of polymerized PCDA vesicles in aqueous solutions at 20 ◦ C, (1) the pH of solution is 6.88, (2) the pH of solution is 11.30, and (3) the added CTAB concentration is 1.48 mmol/l.
Y.-l. Su et al. / Colloids and Surfaces B: Biointerfaces 39 (2004) 113–118
response of polymerized PCDA vesicles for 1-pentanol is higher than that for ethanol. For example, the CR value is 17.33 when 0.1 ml 1-pentanol was added into 5 ml polymerized vesicle solution. However, the CR value is only 0.17 when 1 ml ethanol was added into 5 ml polymerized vesicle solution. Due to its more hydrophobic property, 1-pentanol can soluble in the micellar cores [38]. The hydrophobic domain of polydiacetylene vesicle is fit to “soluble” hydrophobic molecules that introduce colorimetric response. Ethanol is a hydrophilic molecule, which dissolves perfectly in water rather than in the hydrophobic region, so that it causes a lower CR value.
pH of aqueous solution 2
4
6
8
10
12
70 60 50
CR (%)
115
40 30 20 10
3.2. Surfactants upon chromatic transition of the vesicles
0 20
30
40
50
60
70
80
o
Temperature ( C) Fig. 2. Colorimetric response of polymerized PCDA vesicles in aqueous solutions as a function of temperature and pH.
of polymerized PCDA vesicles in aqueous solutions as a function of pH and temperature. The CR value begins to increase when the pH is above 8.5 or temperature is above 35 ◦ C. A higher CR value indicates a stronger reddish appearance of the solution, compared to the blue form. 3.1. Aliphatic alcohol upon chromatic transition of the vesicles Colorimetric response of 5 ml aqueous solution of polymerized PCDA vesicles as a function of the volume of added ethanol and 1-pentanol is shown in Fig. 3. The colorimetric Volume (mL) 0.00
0.02
0.04
0.06
0.08
Motivation from that hydrophobic 1-pentanol can induce colorimetric change of polymerized PCDA vesicles; we examine surfactants upon chromatic transition of the vesicles in aqueous solutions. The usual anionic surfactant (SDS), cationic surfactant (CTAB), and nonionic surfactant (Triton X-100) in chemical laboratory were selected, an interesting phenomenon of the effect of surfactants upon the chromatic transitions was observed. The spectrum of polymerized PCDA vesicles in aqueous solution at pH 6.88 after addition of CTAB is presented in Fig. 1; the maximum absorption band is at 510 nm, the final color of the vesicle solution is a distinct orange. Fig. 4 shows the colorimetric response of polymerized PCDA vesicles in aqueous solutions, which is plotted as a function of the concentration of added CTAB. The CR value increases dramatically with an increase of CTAB concentration; saturation of the CR value is reached when CTAB concentration is above approximate 1.0 mmol/l. The CR value is 93.18 when the concentration of CTAB is 1.48 mmol/l. It can be seen in Fig. 4 that the colorimetric response of polymerized PCDA vesicles for SDS and Triton X-100 is ap-
0.10
50 100
40
80
CR (%)
CR (%)
30
20 1-pentanol
CTAB Triton X-100 SDS
60 40
10 20
ethanol 0
0
0.0
0.2
0.4
0.6
0.8
1.0
Volume (mL) Fig. 3. Colorimetric response of 5 ml aqueous solutions of polymerized PCDA vesicles as a function of the volume of added ethanol and 1-pentanol.
0.0
0.5
1.0
1.5
2.0
2.5
Concentration (mmol/L) Fig. 4. Colorimetric response of polymerized PCDA vesicles in aqueous solutions as a function of the concentration of sodium dodecyl sulfate (SDS), Triton X-100, and cetyltrimethylammonium bromide (CTAB).
Y.-l. Su et al. / Colloids and Surfaces B: Biointerfaces 39 (2004) 113–118
parently lower than that for CTAB at the same concentration. When surfactant concentration is 1.48 mmol/l, the CR values for SDS is 4.39, for Triton X-100 is 21.57. At pH 6.88 of the aqueous solution, the carboxylate function groups at the surface of vesicles are ionized. The strong ability of CTAB to induce chromatic transition of the vesicles is related to the positively charged headgroups of CTAB, which favors approach of CTAB molecules to the negatively charged carboxylate groups at the vesicle surface. The repulsive Coulombic interaction between the negatively charged headgroups of anionic surfactant SDS and the polymerized PCDA vesicles hinders the approach of SDS molecules to the vesicles, so that it causes a low chromatic response. The electrostatic interaction gives slightly effect upon nonionic surfactant Triton X-100, while the chromatic response for Triton X-100 is moderate.
Since the colorimetric response of polymerized PCDA vesicles for CTAB is related to the electrostatic attraction, the change of the charge density on the surface of vesicles should alter the colorimetric response. The charge density on the surface of vesicles varies according to the pH of the environment. Colorimetric responses of polymerized PCDA vesicles in aqueous solutions for different pH as a function of CTAB concentration are given in Fig. 5. A decrease of pH of the vesicle solution induces a lower CR value, which confirms the chromatic transitions indeed dependent on the charge density at the vesicle surface. When CTAB concentration is 1.48 mmol/l, the CR value for pH 4.50 is 30.71, and for pH 2.80 is 12.61. The sensitivity of the vesicles to red form conversion is clearly enhanced at pH 6.88, compared to pH 4.50 and 2.80. Those results suggest a cooperative effect for pH-induced chromatic transition. That is, the repulsive Coulombic interaction between the carboxylate function groups at the surface
100 80
CR (%)
40 dodecylamine tetradecylamine cetylamine
30
20
10
0 0.0
0.5
1.0
1.5
2.0
Concentration (mmol/L) Fig. 6. Colorimetric response of polymerized PCDA vesicles in aqueous solutions as a function of the concentration of alkylamine hydrochloric salts.
3.3. pH dependence of the colorimetric response
pH = 6.88 pH = 4.50 pH = 2.80
60
50
CR (%)
116
40 20
of the vesicles is enlarged at pH 6.88, so that the barrier to further conversion is reduced. On the other hand, higher charge density on the surface of the vesicles would enhance electrostatic attractions between the CTAB and the vesicles; CTAB molecules would really approach the vesicles. 3.4. Alkylamine upon chromatic transition of the vesicles While above work demonstrates the importance of electrostatic interactions to drive incorporation of foreign molecules into membranes, small amino acid molecules did not cause color change of the polydiacetylene vesicles [22]; it is guessed that hydrophobic interactions also play a key role in the chromatic response where the amphophilic molecules insets into the hydrophobic domain of the vesicles. Colorimetric response of polymerized PCDA vesicles in aqueous solutions at pH 4.50 as a function of the concentration of alkylamine hydrochloric salts is shown in Fig. 6. For a series of alkylamine hydrochloric salts at the same concentration, the longer the hydrophobic chain, the stronger the ability of alkylamine to induce chromatic transition of polydiacetylene vesicles. The current work shows the important of hydrophobically driven alkyl chains of amphiphilic molecules insertion into the hydrophobic domain of polymerized PCDA vesicles and the CR value is correlated with the length of the alkyl chain. 4. Discussion
0 0.0
0.5
1.0
1.5
2.0
2.5
Concentration (mmol/L) Fig. 5. Colorimetric response of polymerized PCDA vesicles in aqueous solutions at different pH as a function of the concentration of CTAB.
Although numerous and wide-ranging works have been published on the study of polydiacetylenes, there is still a lack of an appropriate and clear model to explain the chromatic behavior of these polymers. Huo has proposed that a “self-folding” process to explain the chromatic changes of polydiacetylenes at the air–water interface and in the
Y.-l. Su et al. / Colloids and Surfaces B: Biointerfaces 39 (2004) 113–118
117
Fig. 7. Schematic diagram of the interaction of CTAB and the polymerized PCDA vesicle. The head group of CTAB is positive charged and PCDA is negative charged.
soild-state [7]. With increased irradiation time, the length of the polymer chain is increased; the original linear polyeneye backbone starts to “self-fold” to a zigzag structure due to the free rotation of single bonds within the polymer chain. The efficient -electron delocalization along the polyenyne backbone is interrupted by this process, leading to a chromatic change from the blue to red form of polydiacetylenes. Based on the experiment results, we proposed a model for the interaction of CTAB with the polymerized PCDA vesicle, which is illustrated in Fig. 7. Since the vesicle contains mostly negatively charged groups, it can be expected to stimulate the binding of CTAB onto it. CTAB molecule has a long alkyl chain; hydrophobic interaction would drive the alkyl chain to incorporate with the hydrophobic domain of the vesicle. The adsorbed CTAB molecule would reverse and make its alkyl chain insert into the vesicle. The insertion would result in a zigzag polymer backbone; which would effectively reduce the conjugated polymer length. It is suggested that the insertion of CTAB molecules into polymerized PCDA vesicle form a strong complex (supramolecular assembly). CTAB has a specific tendency to adsorb onto the vesicles due to the presence of negative charge on the vesicle surface. The insertion perturbs the conformation of the polymer backbone of the bilayers, which induces the chromatic response of the polymerized PCDA vesicle. In the absence of favorable interaction of electrostatic attraction, neither SDS nor Triton X-100 is an efficient binder of vesicles to perturb the conformation of the polymerized PCDA backbone. Surfactants have a tendency to form a micelle. The critical micelle concentration (CMC) is the characteristic concentration of surfactants in aqueous solution above that the appearance and development of micelles brings about sudden variation in the relation between the concentration and certain physico-chemical properties of the solution. Above the CMC, the concentration of singly dispersed surfactant molecules is virtually constant. The CMC of Triton X-100 is 0.26 mmol/l, the CMC of CTAB is 0.92 mmol/l, and the CMC of SDS is 8.3 mmol/l. The concentration of surfactants in the present work is above the CMC of Triton X-100, below the CMC of SDS, and spanning the CMC of CTAB. It can be seen in Figs. 3 and 4 that there are not durative increase of the CR values when the concentration is above the CMC of CTAB. The manner of the vesicles adsorption for CTAB is monomer as that illustrateed in Fig. 7. It is observed that the CR values do not increase when the concentration above its CMC.
In Fig. 4 for the interaction of Triton X-100 with the polydiacetylene vesicles, the concentration is remarkedly higher than the CMC, and the CR value increases with an increase of Triton X-100 concentration (which included the concentration of Triton X-100 monomer and micelle, the concentration of monomer is invariable above the CMC, whereas the concentration of micelle increase with an increase of surfactant concentration). The most plausible explanation is that Triton X-100 micelles directly attack the polymerized PCDA vesicles, since the CR values increase with an increase of Triton X-100 micelle concentration. At last, we emphasized that in Fig. 1, the maximum absorption band of red form of polymetized PCDA vesicles at pH 11.3 is at 540 nm, whereas the maximum absorption band is at 510 nm by the addition of CTAB. The energy of an electronic excitation on a linear conjugated polydiacetylene depends upon the number of carbon atoms coupled into the -bonding sequence. The higher energy means a shorter chain of the conjugated polydiacetylene backbone. The perturbation induced by insertion of CTAB into the polymerized PCDA vesicles is stronger than that by increasing temperature and elevating pH of aqueous solution.
5. Conclusions The interactions of amphiphilic molecules with vesicles in aqueous solutions were studied by UV-Vis spectroscopy. CTAB has a specific tendency to adsorb onto the polydiacetylene vesicles owing to the favorably electrostatic attraction. The hydrophobic interaction results in the structural reorientation of CTAB molecule, and the alkyl chain of CTAB insetion into the hydrophobic domain of the vesicle. The insertion perturbs the conformation of the polymer backbone of the bilayers, and introduces the chromatic response of the polymerized PCDA vesicle. Alternation of the properties of surfactants and the decrease of the charge density on the vesicle surface effectively lessen the interaction of amphiphilic molecules with the polydiacetylene vesicles and induce lower chromatic responses.
Acknowledgements We thank the National Natural Science Foundation of China (grant number 90206035) for their financial support.
118
Y.-l. Su et al. / Colloids and Surfaces B: Biointerfaces 39 (2004) 113–118
References [1] J. Song, Q. Cheng, S. Kopta, R.C. Stevens, J. Am. Chem. Soc. 123 (2001) 3205. [2] J. Song, Q. Cheng, R.C. Stevens, Chem. Phys. Lipids 114 (2002) 203. [3] Q. Cheng, M. Yamamoto, R.C. Stevens, Langmuir 16 (2002) 5333. [4] Q. Cheng, R.C. Stevens, Langmuir 14 (1998) 1974. [5] Q. Cheng, R.C. Stevens, Chem. Phys. Lipids 87 (1997) 41. [6] Q. Huo, S. Russev, T. Hasegawa, J. Nishijo, J. Umemura, G. Puccetti, K.C. Russell, R.M. Leblanc, J. Am. Chem. Soc. 122 (2000) 7890. [7] Q. Huo, K.C. Russell, R.M. Leblanc, Langmuir 15 (1999) 3972. [8] U. Jonas, K. Shah, S. Norvez, D. H Charych, J. Am. Chem. Soc. 121 (1999) 4580. [9] A. Lio, A. Reichert, D.J. Ahn, J.O. Nagy, M. Salmeron, D.H. Charych, Langmuir 13 (1997) 6524. [10] D.W. Britt, U.G. Hofmann, D. Möbius, S.W. Hell, Langmuir 17 (2001) 3757. [11] A.A. Deckert, J.C. Horne, B. Valentine, L. Kiernan, L. Fallon, Langmuir 11 (1995) 643. [12] N. Mono, H. Tamura, K. Ogawa, Langmuir 8 (1992) 594. [13] N. Mono, H. Tamura, K. Ogawa, Langmuir 7 (1991) 2336. [14] R.A. Nallicheri, M.F. Rubner, Macromolecules 24 (1991) 517. [15] D.H. Charych, J.O. Nagy, W. Spevak, M.D. Bednarski, Science 261 (1993) 585. [16] A. Reichert, J.O. Nagy, W. Spevak, D. Charych, J. Am. Chem. Soc. 117 (1995) 829. [17] J.J. Pan, D. Charych, Langmuir 13 (1997) 1365. [18] T. Peng, Q. Cheng, R.C. Stevens, Anal. Chem. 72 (2000) 1611. [19] Q. Cheng, T. Peng, R.C. Stevens, J. Am. Chem. Soc. 121 (1999) 6767.
[20] Y. Li, B. Ma, Y. Fan, X. Kong, J. Li, Anal. Chem. 74 (2002) 6349. [21] J. Song, Q. Cheng, S. Zhu, R.C. Stevens, Biomed. Microdevices 4 (2002) 213. [22] Z. Ma, J. Li, M. Liu, J. Cao, Z. Zou, J. Tu, L. Jiang, J. Am. Chem. Soc. 120 (1998) 12678. [23] Z. Ma, J. Li, L. Jiang, J. Cao, P. Boullanger, Langmuir 16 (2000) 7801. [24] B. Ma, Y. Fan, L. Zhong, X. Kong, Y. Li, J. Li, Colloid Surf. B 27 (2002) 209. [25] R. Jelinek, S. Kolusheva, Biotech. Adv. 19 (2001) 109. [26] S. Kolusheva, R. Kafri, M. Katz, R. Jelinek, J. Am. Chem. Soc. 123 (2001) 417. [27] S. Kolusheva, T. Shahal, R. Jelinek, J. Am. Chem. Soc. 122 (2000) 776. [28] S. Kolusheva, T. Shahal, R. Jelinek, Biochem. 39 (2000) 15851. [29] S.Y. Okada, R. Jelinek, D. Charych, Angew. Chem. Int. Ed. 38 (1999) 655. [30] R. Jelinek, S. Okada, S. Norvez, D. Charych, Chem. Bio. 5 (1998) 619. [31] S. Okada, S. Peng, W. Spevak, D. Charych, Acc. Chem. Res. 31 (1998) 229. [32] N. Deo, P. Somasundaran, Langmuir 19 (2003) 2007. [33] C. Rodrigues, P. Gameiro, S. Reis, J.L.F. Lima, B. de Casro, Langmuir 18 (2002) 10231. [34] M.M. Santore, D.F. Discher, Y. Won, F.S. Bates, D.A. Hammer, Langmuir 18 (2002) 7299. [35] O. López, M. Cócera, C. López-Iglesias, P. Walter, L. Coderch, J.L. Parra, A. de la Maza, Langmuir 18 (2002) 7002. [36] M. Cócera, O. López, J. Estelrich, J.L. Parra, A. de la Maza, Langmuir 16 (2000) 4068. [37] I. Reviakine, A. Brisson, Langmuir 16 (2000) 1806. [38] Y.L. Su, X.F. Wei, H.Z. Liu, Langmuir 19 (2003) 2995.