Reversible Assembly and Disassembly of Micelles by a Polymer That

Jun 12, 2012 - Kingshuk Dutta , Suparna Das , Patit Paban Kundu. Journal of Applied Polymer Science 2016 133 (10.1002/app.v133.15), n/a-n/a ...
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Reversible Assembly and Disassembly of Micelles by a Polymer That Switches between Hydrophilic and Hydrophobic Wettings Kingshuk Dutta,†,‡ Rajashree Y. Mahale,†,‡ Arulraj Arulkashmir,† and Kothandam Krishnamoorthy*,† †

Polymers and Advanced Materials Laboratory, CSIR-National Chemical Laboratory-Pune, Pune, Maharashtra, India 411008 S Supporting Information *

ABSTRACT: Supramolecular complexes involving nanoscopic amphiphilic assemblies (AAs) and polyelectrolytes have been used to prepare a variety of materials, wherein the dynamic AAs retain the structural features, but the polyelectrolytes undergo conformational changes. Here we show that a charge bearing rigid conjugated polymer can alter the structural features and disassemble AAs. We also demonstrate reversible assembly and disassembly of AAs by controlling the number of charges on the rigid polymer. During the disassembly, the guest molecules sequestered in the AAs are released. The rate of release has been modulated by changing the morphology of the charge bearing polymer. Concomitant to the AAs disassembly, the polymer surface becomes hydrophobic due to the binding of the amphiphiles on the charges of the polymer backbone. By controlling the charges on the polymer, the surface wettability was varied gradually from hydrophilic to hydrophobic. bonds of the polymer backbone.29−31 Although the oxidized conjugated polymers (OCPs) have not been classified as polyelectrolytes, it does comprise positive charges that can electrostatically interact with species having opposite charges.32,33 Besides, upon charge generation, the benzenoid form of the conjugated polymer is converted to the quinoid form, which increases the rigidity of the polymer backbone.34−36 Thus, OCPs fulfill the requirements that we conceived to enforce changes in the structures of AAs. If the OCPs (charge bearing quinoid form) are allowed to interact with anionic AAs, the headgroup is likely to bind with the charges on OCPs and remains attached with the polymer backbone, which would result in the disassembly of AAs (Figure 1b). The OCPs facilitated disassembly of AAs should convert the hydrophilic OCPs surface to hydrophobic due to the presence of an alkyl chain on the polymer surface (Figure 1b). These two concomitant processes open up unprecedented possibilities for reversible assembly/disassembly of AAs, site specific cargo release, and switchable surface modification. Chemicals, biomolecules, temperature, light, and magnetic fields have been widely explored as stimuli for disassembly of AAs.37−39 In this contribution, we report (i) reversible assembly and disassembly of AAs, (ii) disassembly of AAs and release of encapsulated cargo, (iii) modulation of rate of disassembly and cargo release by varying the surface morphology of OCPs, (iv) reuse of OCPs surface for repetitive disassembly of AAs, and (v) controlled switching of the polymer surface from hydrophilic to hydrophobic wettabilities.

1. INTRODUCTION Polyelectrolytes have been widely used as scaffold for rendering mechanical stability to rather dynamic amphiphilic assemblies (AAs) and prepare materials with specific architecture,1−4 nanoscopic containers,5 and surfaces with low interfacial energy.6,7 The Coulombic attraction between oppositely charged polyelectrolytes and AAs with additional assistance from hydrophobic interaction and favorable entropy changes initiates a cooperative aggregation process that eventually leads to materials with structural features similar to the AAs (Figure 1a).4,8 Furthermore, it has been found that the structural features of AAs remain unaltered upon interaction with biopolymer electrolytes having a semirigid backbone such as DNA.9−12 Similar phenomena were observed while interacting AAs with conjugated polyelectrolytes.13−16 In fact, amphiphiles and AAs were used to modulate the geometric conformation and in turn the electronic properties of conjugated polyelectrolytes.17−23 It is rather surprising to note that the rigidity of the polymer backbone, which increases from vinyl polyelectrolytes through biopolymer electrolytes to conjugated polyelectrolytes, has very little effect on the structure of AAs. The common features of these polyelectrolytes are the charges, solubility in water, and coil-like structure in solution. In fact, the coil-like structure in solution permits these polyelectrolytes to adapt a new conformation and envelope the AAs (Figure 1a).8−12,14−16,24−27 Bearing these factors in mind, we hypothesized that a charge bearing, insoluble polymer that can resist conformational changes would enforce changes in the structural features AAs. Conjugated polymers (CPs) without side chain functionalities are insoluble in common solvents.28 These CPs are charge neutral in their native state, but charges can be generated at will by removing electrons from the π © 2012 American Chemical Society

Received: April 30, 2012 Revised: June 11, 2012 Published: June 12, 2012 10097

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Figure 1. a) Cartoon showing polyelectrolyte enveloping AAs. b) Cartoon showing rigid OCPs disassembling AAs. c) The size of SDS AAs at critical micelle concentration as determined by DLS. d) Unaltered size of SDS AAs upon exposure to NCPs as determined by DLS. monomer and initiator. To prepare NCPs, 200 mg of OCPs was stirred in 25 mL of hydrazine hydrate for 12 h. Hydrazine hydrate reduces the OCPs to NCPs. The NCPs were then washed thoroughly with water. Similarly, NCPs nanofibers were prepared by treating the OCPs nanofibers with hydrazine hydrate. 2.3. Methods and Measurements. The micelles were prepared by dissolving 6 mM SDS in deionized water, which was prefiltered through 30 nm polycarbonate membrane. The pyrene encapsulation was carried out by following the reported procedure, and the concentration of pyrene was maintained at 10−6 M for all the experiments.42 Pyrene exhibits multiple emission peaks, and the ratio of the peak intensity at 372 nm (I372) to that at 384 nm (I384) provides information about the environment of the probe. For this study, pyrene was entrapped in SDS AAs, and I372/I384 was determined to be 0.97, which corresponds to the presence of pyrene in a hydrophobic environment.42 To study the disassembly and release of guest molecules, 100 mg of OCPs was added to the SDS AAs solution, and the mixture was allowed to stand quiescent. For the DLS measurement, the supernatant liquid was taken out using a syringe and filtered through a 600 nm polycarbonate membrane (SPI pore). We carried out this step as a precaution to avoid any interference from the OCPs during the DLS measurements. Please note that the size of the assemblies that we are studying is 6 nm, which is 100 times smaller than the pores (600 nm) of the membrane. Therefore, we anticipated the assemblies to be unaffected during the filtration process. Indeed, we were gratified to note that the size of the assemblies is not affected by this filtration step as evidenced from the DLS data in Figure S2 (Supporting Information). For UV−vis spectra and fluorescence emission spectra, the supernatant solution was taken out without disturbing the underlying insoluble polymer (either OCPs or NCPs). The TEM images of the polyaniline were obtained by dropping the sample, which was dispersed in isopropyl alcohol on a carbon coated copper grid (400 grid). OCPs films were prepared by electropolymerization of 0.1 M aniline dissolved in 0.1 M HCl by applying a constant potential of 0.6 V vs Ag/AgCl reference electrode. The working (geometric area - 1 cm2) and counter electrodes (geometric area - 2 cm2) were Pt foils. For reversible disassembly and assembly studies, the OCPs film coated electrode was dipped in pyrene encapsulated SDS AAs. After 3 h, an aliquot was withdrawn and the emission spectrum was recorded. The

2. EXPERIMENTAL SECTION 2.1. Chemicals and Instruments. The analytical grade chemicals aniline, ammonium persulfate, sodium dodecyl sulfate (ACS reagent), cetyl trimethyl ammonium bromide, and pyrene were purchased from Sigma-Aldrich and used as received. Reagent grade HCl and isopropyl alcohol were purchased from Loba Chemie. The deionized water was collected from Millipore Q Gard water purifier and further purified by filtering through a 30 nm polycarbonate membrane purchased from SPI pore. The 30 nm pore membrane was mounted on a stainless steel filter holder, which was then fitted in plastic syringe. A CH Instruments 600D potentiostat/galvanostat was used for electrochemical measurements and electropolymerization of aniline. The working and counter electrodes were fabricated using Pt foil (99.9% purity) purchased from Arora Matthey Ltd. UV−vis spectra were recorded with a Jasco U Best V-570 UV−vis spectrophotometer. Fluorescence spectra were recorded with a Cary Eclipse Fluorescence spectrophotometer. The particle size of the assemblies was recorded with a Brookhaven 90plus Particle Size Analyzer. AFM images were recorded with MM AFM LN supplied by Veeco Multimode in taping mode. Gold coated silicon substrates were used as substrates for AFM imaging. TEM imaging was done with a Jeol 1200 EX transmission electron microscope. The carbon coated copper grids (400 grids) were obtained from Ted Pella. Water drop contact angles were measured in a Digidrop Contact Angle Meter. 2.2. Synthesis of Polymers. Oxidized conducting polymer (OCPs), polyaniline, was synthesized by chemical polymerization of aniline using ammonium persulfate (APS) as initiator.40 For this purpose 100 mM aniline was dissolved in 1 M HCl solution, and subsequently 100 mM APS was added. The reaction was allowed to proceed for 24 h, and then the reaction was stopped by filtering the polymer using 200 nm pore nylon membrane, followed by washing the initiator and unreacted monomer. Polyaniline nanofibers were synthesized by following the interfacial polymerization procedure.41 Briefly, aniline (100 mM) was dissolved in chloroform and allowed to stand quiescent in a sample vial. To that solution, 100 mM APS dissolved in 1 M HCl was added. After five minutes, the polyaniline formation at the chloroform water interface is visible. The reaction was allowed to proceed for 24 h, and then the fibers were collected by filtration in a 200 nm pore nylon membrane. The polymer nanofibers were then washed with copious amounts of water to remove unreacted 10098

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Figure 2. a) Comparison of percentage of pyrene released upon interaction with OCPs bulk powder and OCPs nanofibers. b) TEM image showing the morphology of OCPs nanofibers. c) BET curve used to calculate the surface area of nano OCPs and d) bulk OCPs. solution was poured into the solution with the OCPs coated electrode. A potential of −0.2 V was applied to eject the SDS unimers from the OCPs surface into the solution. The aliquot was withdrawn to record the emission spectrum to show the formation of SDS AAs ejected from the OCPs that eventually encapsulated the pyrene in the water.

charges of the SDS AAs, the assemblies should not disaggregate in the presence of neutral conducting polymers (NCPs). To examine this possibility, NCPs was added to the AAs solution, and the assembly size was determined by DLS. The size of the assemblies was found to be 6 nm and remained constant over a period of 48 h (Figure 1d). This confirms that the disassembly process is exclusively due to the electrostatic attractive interaction between OCPs and SDS AAs. To further validate this, cationic micelle, cetyltrimethylammonium bromide (CTAB) was allowed to interact with OCPs, and the assembly size was monitored. In this experiment, both the AAs and the OCPs have positive charges, hence, the electrostatic attraction is unlikely. Indeed, the DLS measurement indicated that the 6 nm CTAB assemblies44 did not undergo any alteration (Figure S2, Supporting Information). Similarly, the CTAB micelle size was unaffected while it was exposed to NCPs. All these results point to the fact that the disassembly process is solely due to the polyvalent attractive interaction between negatively charged AAs and positively charged rigid polymer. It would be attractive, if this very method can be used to deliver guest molecules sequestered in the assemblies. The disassembly and concomitant release of sequestered guest molecules were determined as a function of time using pyrene, which exhibits multiple emission peaks, and the ratio of the peak intensity at 372 nm (I372) to that at 384 nm (I384)

3. RESULTS AND DISCUSSION To test our hypothesis on the disassembly of AAs, we have chosen sodium dodecyl sulfate (SDS) as the amphiphile because of the presence of anionic moieties which can, therefore, interact with the positive charge bearing OCPs. The OCPs induced disassembly process was first studied using dynamic light scattering (DLS). The aggregate size of AAs prepared using 6 mM SDS solution was found to be 6 nm (Figure 1c and Figure S1, Supporting Information).43 In order to monitor the disassembly of AAs, 100 mg of dry oxidized polyaniline (OCPs) powder was added to 5 mL of SDS solution, and these quantities were maintained in all forthcoming experiments. Upon addition of OCPs, the peak corresponding to 6 nm disappeared, which indicates the disassembly of AAs. From the correlation data, it was not possible to calculate any particle size, which confirms the absence of AAs in the solution (Figure S1, Supporting Information). If the disassembly is due to the attractive interaction between the positive charges of OCPs and negative 10099

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Figure 3. a) Cartoon showing reversible assembly and disassembly of AAs as a function of applied potential. b) Fluoresence emission spectra of pyrene in an aqueous phase (I372/I384 = 1.45) upon disassembly by OCPs film. c) Fluoresence emission spectra of pyrene encapsulated in SDS AAs that are formed from the surfactants ejected from the OCPs surface (I372/I384 = 0.94).

provides information about the environment of the probe.42 For this study, pyrene was entrapped in SDS AAs, and I372/I384 was determined to be 0.97, which corresponds to the presence of pyrene in a hydrophobic environment. Upon addition of OCPs to the pyrene sequestered SDS AAs the I372/I384 started increasing from 0.97 and reached a value of 1.5 in 60 min, which indicated that the new environment faced by pyrene was hydrophilic (Figure S3, Supporting Information). This change in the environment of pyrene is due to the disassembly of SDS AAs and concomitant release of cloistered pyrene into water. The same experiment was carried out using NCPs, and no alteration in I372/I384 of pyrene was observed, which corroborates the fact that the assemblies remain intact in the presence of NCPs (Figure S3, Supporting Information). UV−vis spectroscopy was used to quantify the amount of pyrene released due to the disassembly of the SDS AAs upon interaction with OCPs. The gradual decrease in the absorption intensity at 338 nm indicates the release of pyrene from SDS AAs (Figure S4, Supporting Information).42,43 About 96% of pyrene was released upon the disassembly of AAs, and the release profile is shown in Figure 2a. On the other hand, the unaltered UV−vis spectral features of pyrene indicate that the SDS AAs did not disassemble, and the guest molecules were not released in the presence of NCPs. From these experiments, it is apparent that the AAs disassemble only when involved in electrostatic attraction with insoluble and rigid OCPs surface.

The release percentage value is also indicative of the fact that the released pyrene is essentially in the aqueous environment and not entrapped in the amphiphile modified CPs. This further proves the absence of any reformed assemblies comprising OCPs and amphiphiles. If it is a surface bound phenomenon, the release kinetics should vary with changes in the OCPs surface morphology. In order to verify this, OCPs nanofibers were synthesized by interfacial polymerization, having a diameter of about 50 nm (Figure 2b).41 Contrary to this, polyaniline synthesized by conventional solution polymerization yielded large quantities of bulk powder. The nanofibers synthesized by interfacial polymerization were then added to the SDS AAs, and the dimension of the assemblies was determined. The 6 nm SDS AAs were disassembled as can be seen from the DLS correlation data (Figure S5, Supporting Information). To study the rate of the release, these nanoscopic fibers were added to pyrene encapsulated SDS AAs solution and release of pyrene was monitored as a function of time. We were pleased to note that the disassembly and concomitant release was completed in 15 min, which is faster by about three times as compared to the bulk OCPs release (Figure 2a). Usually, change in morphology from bulk to nano would increase the surface area significantly. If that is true, nanofibers induced guest molecules release should have been much faster than that observed in this study. Surprisingly, we did not observe such rapid release. To further 10100

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Figure 4. a) Percentage of pyrene released from SDS AAs upon disassembly induced by OCPs film. b) Percentage release of pyrene as a function of the number of surface regenerations.

investigate this, surface area of bulk OCPs and nano OCPs were determined by BET surface area measurements. The surface area of nano OCPs and bulk OCPs was found to be 45 m2/g (Figure 2c) and 20 m2/g (Figure 2d), respectively, which suggests the possibility of the presence of nanofibers in bulk OCPs. Indeed, meticulous TEM imaging of bulk OCPs showed the presence of nanofibers (Figure S6, Supporting Information). It has also been reported that the polyaniline synthesized by conventional methods did contain nanofibers along with bulk powder.47 Consolidating the results of surface area measurements and guest molecule release studies, the ∼65% decrease in release time is a result of ∼50% increase in surface area of the nanofibers. A control experiment was done using NCPs nanofibers, which showed infinitesimal release, as expected. The results so far corroborate the fact that the disassembly and release of payload occurs only when electrostatic attraction between rigid OCPs and AAs is in operation. Thus, the mere presence of CPs is not adequate to trigger disassembly and release of guest molecules. This essentially means that the CPs can be placed at a location passively and activated by generating positive charges on the CPs whenever desired. To accomplish reversible assembly and disassembly of AAs, the polymer needs to be coated on substrate and charge regeneration has to be carried out. Toward this objective, polyaniline was electrodeposited on Pt foil by applying a constant potential of 0.6 V vs Ag/AgCl.48 At this potential, the polymer bears delocalized positive charges, hence it is a film version of OCPs. The polymer film was then dipped in a solution containing SDS AAs, and the size of the assemblies in an aliquot was monitored by DLS. The disassembly of AAs was confirmed by the disappearance of a peak corresponding to 6 nm, and then a reduction potential of −0.2 V was applied with respect to a quasi reversible Pt reference electrode. This potential is suffice to convert OCPs to NCPs that in turn would result in the ejection of SDS amphiphiles into the solution from the polymer surface. Now, the concentration of SDS in the solution would reach the critical micelle concentration (CMC) if all the SDS molecules that were bound to the OCPs surface were released. To check the formation of the AAs, aliquot withdrawn from the solution was subjected to DLS analysis. We were gratified to note a peak corresponding to 6 nm in the DLS histogram indicating the formation of SDS AAs. The cartoon in Figure 3a depicts the whole process of assembly and

disassembly as a function of applied potential. To demonstrate the release and encapsulation of guest molecules upon disassembly and assembly of SDS AAs, we carried out the following experiment. OCPs film coated electrode was immersed in pyrene encapsulated SDS AAs solution and left quiescent. Emission spectrum of the solution was recorded to determine the environment of pyrene. The I372/I384 was found to be 1.45, which indicates the disassembly of SDS AAs and the presence of pyrene in an aqueous environment (Figure 3b), and then the OCPs film was reduced to NCPs, which ejects the SDS surfactants into the solution. Upon reaching the CMC, the SDS AAs are likely to sequester the pyrene in the solution. If that occurs, the I372/I384 of pyrene should indicate the presence of the guest molecule in a hydrophobic environment. Indeed, we were pleased to note the I372/I384 of 0.94, which confirms the presence of pyrene in a hydrophobic environment (Figure 3c). With these experiments, we have demonstrated the reversible assembly and disassembly of AAs and corresponding encapsulation and release of guest molecules. To further extend this approach toward release of guest molecules by reusing the OCPs surface, we studied the release of pyrene from AAs. The change in I372/I384 confirms the disassembly and concomitant release of the guest molecules from the AAs upon interaction with OCPs coated substrate. UV−vis absorption spectroscopy was used to quantify the released pyrene, and it was found to be 93% (Figure 4a). The polymer coated Pt foil was subsequently reduced at −0.2 V to convert the polymer to its neutral state (NCPs), which is not capable of inducing disassembly. Then the polymer film was reoxidized at 0.6 V and utilized for further disassembly of AAs. The experiment and the analysis were repeated to elucidate the films reusability. We were able to reuse the electrode over a period of five days and over ten cycles for the purpose of disassembling and releasing the guest molecules, after which the quantity of the release decreased, possibly owing to the degradation of the polymer (Figure 4b). The CTAB AAs used for control experiments did not disassemble, which further confirms the methods specificity. AAs assembly and disassembly, release of payload, and reuse of OCPs surface for repetitive disassembly have been unambiguously proven by the aforementioned experiments. Next, we were interested in studying the change in surface properties of OCPs upon interaction with AAs. The OCPs surface is likely to be hydrophilic due to the presence of 10101

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amine) (Figure S9, Supporting Information). The branched structure was chosen to impart hydrophobicity to the hydrophilic voids between the SDS molecules. The CA increased from 100° to 155° (Figure 5f). The OCPs treated with branched amino alkyl ester alone showed a CA of 50° (Figure S10, Supporting Information), which is 105° less than that observed for OCPs treated with SDS and Behera’s amine. Furthermore, the CA was found to be 100°, if the acid analogue of Behera’s amine was used (Figure S10, Supporting Information). The carboxylic acid and amine terminals (Figure S9, Supporting Information) cannot impart hydrophobicity to the voids, hence the CA remain close to OCPs surface treated with SDS AAs. These experiments confirm our hypothesis of filling the hydrophilic voids between the alkyl chains of SDS molecules protruding from the OCPs surface as a means to increase hydrophobicity. Often, hydrophobicity enhancement has been attributed to increase in the surface roughness,51−53 hence it is necessary to test any microscopic changes on the polymer surface upon interaction with SDS AAs. Atomic force microscopy (AFM) imaging of OCPs (Figure 6a), OCPs

positive charges on the polymer (Figure S7, Supporting Information). It is necessary to recall that the OCPs surface was prepared by oxidizing the polymer at 0.6 V vs Ag/AgCl, wherein the polymer is likely to have a maximum number of positive charges. On the other hand, NCPs have no positive charges because they are prepared by applying a reduction potential of −0.2 V vs Ag/AgCl to OCPs. One of the attractive features of CPs is it provides a handle to control the number of positive charges by varying the applied potential.49,50 Thus, by controlling the positive charges on the polymer, one can control the number of anions that bind with the polymer surface. In our experiments, the number of anionic surfactants that bind with the polymer is a function of the number of positive charges generated by the applied potential. Considering this, it is reasonable to anticipate that the hydrophobicity of the surface scales as a function of the amount of surfactant binding on the polymer surface. To test this, we studied the contact angle (CA) of a drop of water on the polymer film surface, which gives a measure of surface hydrophobicity. The CA on the OCPs surface was found to be 6°, a value typical of hydrophilic surfaces (Figure 5a). We measured the CA of

Figure 6. a) AFM image showing the morphology of OCPs surface. b) AFM image of the surface of OCPs treated with SDS AAs. c) AFM image of OCPs treated with SDS AAs, which was then treated with Behera’s amine.

Figure 5. a) Digital image showing water droplet on OCPs surface. b) Image showing water droplet on NCPs surface. c) CA of water droplet on CPs oxidized at 0.2 V and treated with SDS AAs. d) CA of water droplet on CPs oxidized at 0.4 V and treated with SDS AAs. e) CA of water droplet on CPs oxidized at 0.6 V and treated with SDS AAs. f) CA of water droplet on CPs oxidized at 0.6 V, treated with SDS AAs, and subsequently treated with Behera’s amine.

treated with SDS AAs (Figure 6b), and OCPs treated with SDS AAs and Behera’s amine (Figure 6c) was carried out to discover any changes in the morphology of the surfaces. No discernible changes in the morphology were observed in AFM images confirming the change in surface hydrophobicity is due to the noncovalent attachment of SDS surfactants and Behera’s amine. So far CPs based superhydrophobic surfaces have been prepared using tedious synthesis and patterning procedures.54−59 We have demonstrated that surface wettability of CPs can be controlled at ease by varying the applied potential and dipping in SDS AAs.

NCPs, which was also found to be about 6° (Figure 5b), and then the oxidation potential of 0.2 V was applied to NCPs which generates positive charges on the NCPs surface. After that, the film was immersed in SDS AAs, and then the CA of water drop was measured. The CA for this surface was found to be 30° (Figure 5c). By repeating the same procedure, but by applying a potential of 0.4 V, a surface with CA of 60° was obtained (Figure 5d). This was further increased to 80° by applying 0.5 V to NCPs and dipping in SDS AAs. By applying 0.6 V and treating the film with SDS AAs a contact angle of 100° was achieved (Figure 5e). Upon interaction of SDS AAs with OCPs surface, the positive charges are neutralized by the negative charges of the SDS surfactants and concurrently the alkyl chains protrude from the polymer surface. This leads to low surface energy and results in an increase in hydrophobicity. It is enticing to increase the CA to ≥150° (superhydrophobic surface). OCPs surfaces prepared by applying potentials above 0.6 V and dipping in SDS AAs did not increase the CA beyond 100°. This is likely due to the formation of fully oxidized polyaniline (OCPs) at 0.6 V. We hypothesized that the CA can be increased by filling the hydrophilic voids between the SDS molecules (Figure S8, Supporting Information). This was achieved by immersing a OCPs film with CA of 100° in a solution containing di-tert-butyl 4-amino-4-(3-tert-butoxy-3oxopropyl)heptanedioate (branched amino alkyl ester, Behera’s

4. CONCLUSIONS In summary, we have utilized a charge bearing, insoluble, and rigid conjugated polymer to disassemble AAs. Furthermore, the disassembly and assembly of AAs were accomplished by switching CPs between its charged and neutral states. The rate of disassembly was controlled by modulating the morphology of the CPs. During the disassembly of AAs, the encapsulated cargo can be released, and the process can be repeated several times by regenerating the charges on the CPs. Upon disassembly, the amphiphiles which were constituents of AAs are bound to the OCPs that impart hydrophobicity to the polymer. By controlling the charge on the polymer and subsequently dipping in AAs, the surface hydrophobicity was varied between hydrophilic to hydrophobic wettings. 10102

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ASSOCIATED CONTENT

S Supporting Information *

Dynamic light scattering hitogram, absorption spectra, emission spectra, and contact angle data. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions ‡

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. These authors contributed equally. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to CSIR and Dr. Sivaram for funding (NWP 12). We thank Drs. B. L. V. Prasad, Guruswamy, S. Bhat, K. Sreekumar, and R. Banerjee for allowing us to access their equipment. R.Y.M. and A.A. thank CSIR for a fellowship.

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