Construction of Polyelectrolyte-Responsive Microgels, and

Jul 23, 2014 - This simple yet feasible design augurs well for the promising applications in controlled release fields. ζ-Potential values of P2VP mi...
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Construction of Polyelectrolyte-Responsive Microgels, and Polyelectrolyte Concentration and Chain Length-Dependent Adsorption Kinetics Jun Yin,*,† Shengyu Shi,† Jinming Hu,‡ and Shiyong Liu*,‡ †

Key Laboratory of Advanced Functional Materials and Devices, Anhui Province, Department of Polymer Material and Engineering, School of Chemistry and Chemical Engineering, Hefei University of Technology, Hefei 230009, China ‡ CAS Key Laboratory of Soft Matter Chemistry, Department of Polymer Science and Engineering, Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei, Anhui 230026, China S Supporting Information *

ABSTRACT: We report on the construction of a polyelectrolyte-responsive system evolved from sterically stabilized protonated poly(2-vinylpyridine) (P2VPH+) microgels. Negatively charged sodium dodecylbenzenesulfonate (SDBS) surfactants could be readily internalized into the cationic microgels by means of electrostatic interactions, resulting in microgel collapse and concomitant formation of surfactant micellar domains (P2VPH+/SDBS)-contained electrostatic complexes. These internal hydrophobic domains conferred the opportunity of fluorescent dyes to be loaded. The obtained fluorescent microgel complexes could be further disintegrated in the presence of anionic polyelectrolyte, poly(sodium 4-styrenesulfonate) (PNaStS). The stronger electrostatic attraction between multivalent P2VPH+ microgels and PNaStS polyelectrolyte than single-charged surfactant led to triggered release of the encapsulated pyrene dyes from the hydrophobic interiors into microgel dispersion. The process was confirmed by laser light scattering (LLS) and fluorescence measurements. Furthermore, the entire dynamic process of PNaStS adsorption into P2VPH+ microgel interior was further studied by stopped-flow equipment as a function of polyelectrolyte concentration and degree of polymerization. The whole adsorption process could be well fitted with a doubleexponential function, suggesting a fast (τ1) and a slow (τ2) relaxation time, respectively. The fast process (τ1) was correlated well with the approaching of PNaStS with P2VPH+ microgel to form a nonequilibrium complex within the microgel shell, while the slow process (τ2) was consistent with the formation of equilibrium complexes in the microgel deeper inside. This simple yet feasible design augurs well for the promising applications in controlled release fields.

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INTRODUCTION The interactions between polyelectrolyte or polypeptides and ionic surfactants of opposite charge have been studied extensively long ago, which are more interesting than pure surfactants from both physicochemical and application points of view.1−10 Strong electrostatic attraction and hydrophobic interaction between oppositely charged polyelectrolytes and surfactants occur at surfactant concentrations several orders of magnitude lower than the critical micelle concentration (CMC) values.11−13 However, most of previously reported work focused on the phase behavior and the binding of surfactant to the polyion chains, demonstrating that the introduction of surfactant into a polyelectrolyte solution often resulted in a remarkable change of morphological characteristics, even phase separation at low concentrations.7,14−16 © 2014 American Chemical Society

Polyelectrolyte composition, charge density, and size exert intercorrelated effects on such binding behavior as well as the subsequent phase transition behavior. For example, Choi and Kim used fluorescence measurements to indicate that the critical aggregation concentration (CAC) of CnTAB was lower when a higher molar mass of poly(acrylic acid) (PAA) was utilized.17 Moreover, in recently reported literature, Thayumanavan et al.2 found a protein-responsive system that protein binding to the poly(potassium acrylate) could result in the release of pyrene molecule from the hydrophobic interiors of the poly(potassium acrylate)−CTAB complex due to the Received: May 16, 2014 Revised: July 22, 2014 Published: July 23, 2014 9551

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2.0; the monomethoxy-capped poly(ethylene glycol) methacrylate (PEGMA) stabilizer in this design plays the role of stabilizing the microgels against nonspecific aggregation.41,42 Electrostatic attraction between P2VPH+ microgels and SDBS could lead to reduced hydrodynamic diameter and formation of the surfactant-contained (P2VPH+/SDBS) microgel complex. These electrostatic complexes could be further triggered by disintegration upon PNaStS addition due to the competitive binding to P2VPH+ microgels, resulting in spontaneous release of the encapsulated guest molecules from the hydrophobic interiors. The guest molecules-releasing process was monitored by fluorescence probed by pyrene molecules. Subsequently, the entire dynamic process of PNaStS adsorption into P2VPH+ microgel interior was further studied by stopped-flow equipment as a function of polyelectrolyte concentration and degree of polymerization, which has been confirmed to be competent and efficient for the dynamic process, especially for the initial rapid kinetic process.41,46

stronger electrostatic attraction between polyelectrolyte and multicharged protein than single-charged surfactant. Gels have been widely studied both with regard to their physical properties and also in exploiting their massive potential applications.18,19 Microgels can also trap appropriate molecules in their swollen state by means of some attraction between the adsorbate and the cross-linked polymer network.20−25 Fan et al.26 reported on the absorption of anionic sulfonate groupscontained organic salts into cationic poly(vinylpyridine) (PVP) microgels and observed decreased microgel size and even largescale aggregates when the organic salts were beyond a limiting concentration. The effect of hydrocarbon chain length on the interaction between modified organic salts and poly(Nisopropylacrylamide-co-acrylic acid) microgels then was investigated as a follow-up work by the same research group.27 Two principal factors have been presented concerning what kind of solutes can be trapped inside the microgels:13 (i) a sufficient attractive interaction between the solute and microgel network; and (ii) solute-induced microgel deswelling occurs, especially for molecules that are large, for example, polymers or even nanoparticles. As we know, the adsorption of both ionic and nonionic surfactant molecules into microgels had been widely reported.28−30 The electrostatic attraction between charged groups and hydrophobic interactions between the hydrocarbon tails of the surfactant molecules and the hydrophobic moieties in microgel particles contributed significantly to the adsorption mechanism. The uptake and release of molecules into and from microgels has tremendous application in drug delivery and separations technology.31−34 In recent years, stimuli-responsive microgels have been widely studied with regard to their fundamental properties and potential applications in drug delivery and as sensors.35−37 pH-responsive microgel dispersions have been the subject of significant interest for the development of advanced carrier material. For example, pHresponsive chitosan-based microgels containing methotrexate had been designed by Zhang et al.38 in that exposure to low pH environments could lead the chitosan to swell and release the encapsulated drug subsequently. Similar research was conducted for a series of drugs such as doxorubicin,39 dibucaine,32 and benzylamine.40 Bradley et al.12 also reported the uptake and release of anionic surfactant, sodium dodecylbenzenesulfonate (SDBS), into and from poly(vinylpyridine)−poly(2(dimethylamino)ethyl methacrylate) (PVP−PDMAEMA) cationic core−shell microgels, and the absorbed amount of SDBS within the microgels could be tuned by pH change. Although numerous studies on the interaction of polyelectrolyte and ionized microgels with surfactants have been extensively investigated up to now, the interaction of polyelectrolyte with oppositely charged microgels was rare; especially the corresponding kinetic process has not yet been well understood. Moreover, aside from directly utilizing the responsive microgels for the uptake and release of adsorbates, there has been less work reported on the polyelectrolyte triggered release of encapsulated components from specific microgels or other aggregates.2 Inspired by the above aspects concerning electrostatic attraction between polyelectrolyte or ionized microgels and surfactants, we herein constructed a polyelectrolyte-responsive system consisting of negatively charged poly(sodium 4-styrenesulfonate) (PNaStS) and sodium dodecylbenzenesulfonate (SDBS) surfactant, and sterically stabilized protonated poly(2-vinylpyridine) (P2VPH+) microgels with a diameter of ∼1270 nm at pH

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EXPERIMENTAL SECTION

Materials. Sodium 4-styrenesulfonate (NaStS), 4-(bromomethyl)benzoic acid (BMB), sodium dodecylbenzenesulfonate (SDBS), copper(I) chloride, pyrene, and 2,2′-bipyridine (bpy) were purchased from Sigma-Aldrich and used as received. Methanol was dried and further distilled just before use. Water was deionized with a Milli-Q SP reagent water system (Millipore) to a specific resistivity of 18.0 MΩ cm. All of the other chemical reagents were obtained from Sinopharm and used as received unless otherwise denoted. Sample Preparation. The preparation of such sterically stabilized pH-responsive poly(2-vinylpyridine) (P2VP) microgels with diameter of ∼1270 nm at pH 2.0 has been previously described in detail, and the physical parameters are also summarized in previously reported literatures.41 After extensive dialysis at pH 2.0 (MW cutoff ∼14 000 Da) for 3 days, P2VP microgels were further diluted to desired concentrations for subsequent studies. Poly(sodium 4-styrenesulfonate) (PNaStS) was synthesized using atom transfer radical polymerization (ATRP) in protic media according to a previous literature procedure.43 Briefly, into a glass ampule equipped with a magnetic stirring bar, NaStS monomer (0.87 g, 3.78 mmol; target DP = 40), water (4.5 mL), methanol (1.5 mL), and BMB initiator (20.62 mg, 0.095 mmol) were charged. NaOH (1 M) was used to adjust the pH to 10 to dissolve the initiator in its sodium salt form (NaBMB), and the aqueous mixture was purged with nitrogen gas for 30 min. CuCl (9.5 mg, 0.095 mmol) and bpy ligand (29.67 mg, 0.19 mmol) were then introduced under the protection of nitrogen gas flow to generate the reaction at 25 °C. After being stirred for 24 h, the dark brown solution was quenched by aerated methanol and water. The solution was then passed through a silica gel column, yielding a colorless solution. After being concentrated by a rotary evaporator, the solution was precipitated into an excess of THF to remove any unreacted initiator, monomer, and ligand. Following the same procedure, a series of PNaStS with varying DP was also prepared. Aqueous gel permeation chromatography (GPC) analysis was used to confirm the molecular weights (M n ) and molecular weight distributions (Mw/Mn) of the homopolymers and summarized in Table 1. Characterization. Gel Permeation Chromatography (GPC). Molecular weights, Mn, and molecular weight distributions, Mw/Mn, were determined by aqueous gel permeation chromatography (GPC) equipped with an Agilent 1260 pump and an Agilent G1362A differential refractive index detector. The eluent was ultrapure water with 50 mM NaCl and 10 mM acetic acid at a flow rate of 1.0 mL/min. A series of low polydispersity PEG standards were employed for calibration. Laser Light Scattering (LLS). A commercial spectrometer (ALV/ DLS/SLS-5022F) equipped with a multitaudigital time correlator (ALV5000) and a cylindrical 22 mW UNIPHASE He−Ne laser (λ0 = 9552

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equipped with three 10 mL step-motor-driven syringes (S1, S2, and S3), which can be operated independently to carry out single- or double-mixing. The stopped-flow device is attached to a MOS-250 spectrometer; kinetic data were fitted using the Biokine program provided by Bio-Logic. Both the excitation and the emission wavelengths were adjusted to 335 nm with 10 nm slits. Using FC08 or FC-15 flow cells, typical dead times are 1.1 and 2.6 ms, respectively. The solution temperature was maintained at 25 °C by circulating water around the syringe chamber and the observation head. Prior to loading into the motor-driven syringes, all aqueous solutions were clarified by passing through 2.0 μm Millipore Nylon filters.

Table 1. Summary of Aqueous Gel Permeation Chromatography (GPC) Data Obtained for NaStS Homopolymers Prepared Using the NaBMB Initiator sample IDa

target Dp

Mn (g/mol)

Mw/Mn

exp. Dp

S1 S2 S3

10 20 40

3100 5800 9500

1.36 1.31 1.24

15 28 46

a

The relative molar ratios of NaBMB:CuCl:bpy were 1:1:2 for all entries.43 All reactions were carried out in a mixture of water and methanol (v/v = 3:1) at 25 °C for 24 h.

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632 nm) as the light source was employed for dynamic LLS measurements. Scattered light was collected at a fixed angle of 90° for duration of 5 min. Distribution averages and particle size distributions were computed using cumulants analysis and CONTIN routines. The final microgel concentration was fixed at 0.01 g/L. All data were averaged over three consecutive measurements. Fluorescence Measurements. Fluorescence spectra were recorded using a RF-5301/PC (Shimadzu) spectrofluorometer. The temperature of the water-jacketed cell holder was controlled by a programmable circulation bath. The slit widths were set at 5.0 nm for both the excitation and the emission. The critical micelle concentration (CMC) was determined by a fluorescence technique.44,45 A calculated volume of the pyrene solution in acetone was added to a series of volumetric flasks, acetone was removed by a mild blow of nitrogen gas, fixed volume P2VP microgel dispersions with various amount of surfactant were then added to the volumetric flasks, and the pyrene concentration was fixed at 5 × 10−7 mol/L. All of the samples were sonicated for 15 min and then allowed to stand for 1 h before fluorescence measurements. The %dye release was calculated from a decrease in emission intensity (374 nm) of the pyrene by using the following equation: %dye release = (1 − I/I0) × 100, where I0 is the initial emission intensity, and I is the emission intensity after adding PNaStS. All of the measurements were done in triplicate, and the average results are reported. Stopped-Flow Measurements. Stopped-flow studies were carried out using a Bio-Logic SFM300/S stopped-flow instrument. It is

RESULTS AND DISCUSSION

Hydrophobic moieties in microgels do not swell readily in water, and a significant degree of charge buildup inside such microgels may be required before they can begin to swell. For weak acids and bases, one-half of the potentially ionizable groups are ionized when the pH equals the pKa.46 The pKa of the 1.0% cross-linked sterically stabilized P2VP microgels was determined to be ∼4.1, lower than that of ∼4.9 estimated for linear-P2VP homopolymers, which is due to the higher degree of protonation required to swell microgels.41 Judiciously adjusting the pH value of P2VP microgel dispersion to 2.0, the pyridine groups-contained microgels were fully protonated with positive charges. The degree of ionization of P2VP microgels was investigated by zeta potential as a function of pH in water and 0.01 M NaCl solution, respectively (see Supporting Information Figure S1). As expected, protonation of the P2VP microgels led to strongly cationic character at low pH values with zeta potentials ranging from +28 to +0.8 mV and +24 to +0.9 mV in the pH range from 2.0 to 6.0 in water and 0.01 M NaCl solution, respectively. Only weakly anionic character was observed at higher pH values. At pH 2.0, the P2VP microgels are completely protonated and fully swollen with a zeta potential value of +28 mV; if we define the degree of

Figure 1. Dependence of intensity-average hydrodynamic diameter (⟨Dh⟩) and scattered light intensities (SLI) obtained for the P2VPH+ microgels as a function of the (a) concentration (DP = 28) and (b) DP of PNaStS (25 μM) in water at pH 2.0 and 25 °C. Dependence of ⟨Dh⟩ and SLI obtained for the (P2VPH+/SDBS) microgels as a function of the (c) concentration (DP = 28) and (d) DP of PNaStS (20 μM) in the presence of 200 μM SDBS in water at pH 2.0 and 25 °C. 9553

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Scheme 1. Schematic Illustration of the Sequential Adsorption of Anionic Surfactant (SDBS) and Solubilization of Chromophore (Pyrene) into Sterically Stabilized Cationic Poly(2-vinylpyridine) (P2VPH+) Microgels at Low pH As Well As Subsequent Anionic Polyelectrolyte (PNaStS) Regulated Release Process

of SDBS in the concentration range of 0−200 μM, lower than the CMC value of SDBS in pH 2.0 (see Supporting Information Figure S2b). The results here also confirmed that the P2VPH+ microgels were well-dispersed in aqueous dispersion without obvious aggregation. However, further addition of SDBS caused reswelling of microgels gradually. That this should be attributed to the hydrophobic association between the alkyl chains of surfactant tails is of crucial dominance at a high SDBS concentration instead of the initial electrostatic attraction between the charged headgroup of SDBS and P2VPH+ within the microgel particles, producing net bulk negative charges within the microgel particles and therefore reswelling microgels.12 It should be noted that the ⟨Dh⟩ of microgels reaches a minimum value (780 nm) at the SDBS concentration of 200 μM, which is significantly lower than the CMC of SBDS in water; therefore, SDBS molecules can be released once the electrostatic interaction between the P2VPH+ microgels and SDBS is interrupted. In a recent paper, the absorption of organic salts containing varying sulfonate groups into cationic PVP microgel particles at pH 2 was studied.26 For multicharged polyelectrolyte (PNaStS), dynamic light scattering (DLS) in Figure 1a revealed the minimum ⟨Dh⟩ (910 nm) and maximum SLI values of P2VPH+ microgels when the concentration of added PNaStS (DP = 28) reached 15 μM, and no obvious change at a higher polyelectrolyte concentration range of 15−25 μM. The difference between the size change of P2VPH+ microgels upon addition of SDBS (see Supporting Information Figure S2) and PNaStS here should be ascribed to the lack of hydrophobic tail chains in PNaStS as explained above. In principle, the abundant charged groups within polyelectrolyte guarantee the higher adsorption capacity of the P2VPH+ microgels toward the PNaStS than SDBS, resulting in the

ionization at pH 2.0 to be 100%, one-half degree of ionization (+14.6 mV) could be obtained at pH 4.0, which was well in accordance with the pKa value (4.1) determined by acid titration previously.20 The electrostatic repulsion between the polymer chains and osmotic pressure mainly governs the extent of microgels swelling. As detailed previously, it is easier for the hydrophobic micelle-like aggregates of the anionic surfactant molecules to form around a positive charge group than through purely hydrophobic association with the hydrophobic moieties.2,12,13 In the current situation, the critical micelle concentration (CMC) and critical aggregation concentration (CAC) of SDBS in DI water, pH 2.0 solution, and P2VPH+ microgel dispersion at pH 2.0 and 25 oC, respectively, were determined by the inflection point observed in the plot of I1/I3 (the ratio of the intensities of the first and third vibronic peaks in the fluorescence spectrum) from pyrene emission spectra (see Supporting Information Figure S2). The results showed that the CAC (1.20 × 10−5 M) of P2VPH+−SDBS complex is approximately 2 orders of magnitude lower than that of SDBS itself in pure water (1.21 × 10−3 M) or in pH 2.0 solution (4.5 × 10−4 M),47 in accord with the results reported by Thayumanavan et al. and suggested the strong interaction between P2VPH+ and SDBS.2 At an intermediate concentration between these two values, it was thus expected that the SDBS molecules could be released if the interaction between the polymer and the surfactant was actuated. Next, the absorption of the anionic surfactant (SDBS) into sterically stabilized P2VPH+ microgels at pH 2.0 and 25 oC was studied. Similar to recent literature,12,13 decreased hydrodynamic diameter, ⟨Dh⟩, and correspondingly increased scattered light intensities (SLI) for protonated P2VPH + microgels at pH 2 and 25 °C were observed with the addition 9554

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Figure 2. (a) Emission spectra (λex = 339 nm) obtained for (P2VPH+/SDBS/Py) microgel dispersions upon addition of varying concentration of PNaStS (DP = 28) in water at pH 2.0 and 25 °C. (b) Pyrene release percent from (P2VPH+/SDBS/Py) microgels as a function of PNaStS concentration (DP = 28). (c) Emission spectra obtained for (P2VPH+/SDBS/Py) microgel dispersions upon addition of PNaStS (25 μM) with different DP in water at pH 2.0 and 25 °C. (d) Pyrene release percent of (P2VPH+/SDBS/Py) microgels as a function of the DP of PNaStS (25 μM).

discussion above, that shorter polyelectrolyte chains suffer less hindrance during the adsorption process and resulted in larger size of microgels, shown in Figure 1d. That PNaStS (DP = 28) triggered release of guest molecules from microgel hydrophobic interiors could also be verified by fluorescence measurements probed by solubilized pyreneloaded (P2VPH+/SDBS) microgels, (P2VPH+/SDBS/Py) microgels. Figure 2a showed the emission spectra obtained for aqueous dispersion of (P2VPH+/SDBS/Py) microgels as a function of the PNaStS concentration change at pH 2.0 and 25 °C. The fluorescence intensity at 374 nm decreased from 0.59 to 0.14 with the increased concentration of PNaStS, which was likely due to the fact that the pyrene fluorophores were excreted from the hydrophobic interiors of (P2VPH+/SDBS/ Py) microgels as a result of disappeared hydrophobic domains within the microgels formed by SDBS. Note that the pyrene dye is very sparingly soluble in water and always precipitated, and the relative emission intensity is much lower in water as compared to in the hydrophobic interiors within microgels,2 although all of the pyrene molecules remain in the system. As mentioned above, the pyrene molecules can be released when the hydrophobic microaggregates are broken by the stronger electrostatic attraction between P2VPH+ and multicharged PNaStS than single-charged SDBS. Note that PNaStS is not capable of quenching pyrene emission. The corresponding dye release percent is shown in Figure 2b; 74.82% of the solubilized pyrene molecules could be extruded out of the microgels when the concentration of PNaStS reached 25 μM, as evidenced by the weakened fluorescence emission as compared to that of the initial fluorescence emission intensity without PNaStS. This result accorded well with the literature reported that proteininduced disassembly of the pyrene-loaded poly(potassium acrylate)−CTAB complex, which resulted in the decreased emission intensity of pyrene and 78% dye release, was observed.2 Consistent with our expectation inspired by the

more screening of opposite charges and smaller size. However, there is a significant exclusion volume contribution to this interaction other than the electrostatic attraction, which substantially hinders the diffusion and deep penetration of polyelectrolyte into the microgel inner cores, although the long PNaStS chain possesses greater interaction with P2VPH+ microgel particles than SDBS. Moreover, the effect of polyelectrolyte volume on the ⟨D h⟩ values of P2VPH + microgels was investigated. From Figure 1b, smaller ⟨Dh⟩ could be obtained upon addition of PNaStS with short chain length (DP = 15) as compared to that with long chain lengths (DP = 28 and 46) at a fixed concentration (25 μM), in line with the analysis above that PNaStS with the shortest chain length as well as the smallest volume can enhance the uptake of polyelectrolytes into microgel interiors with least resistance. Inspired by the work concerning the protein-responsive system,2 we studied the effect of polyelectrolyte (PNaStS, DP = 28) concentration on the ⟨Dh⟩ and SLI values of (P2VPH+/ SDBS) microgels preloaded with 200 μM of SDBS in water at pH 2.0 and 25 °C. Figure 1c showed the observed reswelling of (P2VPH+/SDBS) microgels and decreased SLI upon gradual addition of PNaStS (0−10 μM). Absorption of PNaStS into microgels led to the disruption of original attraction between P2VPH+ and SDBS, and rebuilt a stronger electrostatic attraction between P2VPH + and PNaStS, termed as (P2VPH+/PNaStS) microgels (shown in Scheme 1). The replaced SDBS molecules (noted: the total amount of SDBS used is below the CMC values at both pH 2.0 and 7.0 in the current situation) would spontaneously diffuse out of microgels and into the dispersion medium, staying as single molecules.2 However, not all of the negative charges along the polyelectrolyte chain could be neutralized by P2VPH+, generating a localized net bulk negative charge within the microgel particles and reswelling of microgels. Again, exclusion volume played a significant role here in accordance with the 9555

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Figure 3. Variation in scattered light intensity for (a) long time scales and (b) short time scales for P2VPH+ microgel dispersions after mixing with varying concentrations of PNaStS (DP = 28) at pH 2.0 and 25 °C upon a stopped-flow technique. The final PNaStS concentration dependence of characteristic relaxation times (c) τ1 and (d) τ2 obtained from double-exponential fitting of dynamic traces obtained after mixing P2VPH+ microgel dispersions with different concentrations of PNaStS. The final microgel concentration was fixed at 0.01 g/mL.

process according to previous knowledge, especially for the initial rapid kinetic process.48,49 The principle of relaxation experiment using a basic stoppedflow is simple and has been described in previous reports in detail.50 Figures 3a,b showed the time-dependent scattered light intensities for P2VPH+ microgel dispersions after mixing with varying concentrations of PNaStS (DP = 28) upon a stoppedflow technique at pH 2.0 and 25 °C in long and short time ranges, respectively. Without addition of PNaStS, the positive dynamic trace of scattered light intensity is maintained as a straight line, and no relaxation process was discernible (see reference in Figure 3a). Once the concentration of PNaStS added was above 2.5 μM, relaxation processes with quite large positive amplitudes could be typically achieved, revealing an abrupt increase of scattered light intensities within the first 10 s and then reaching a plateau value gradually. The strong electrostatic attraction between P2VPH+ and PNaStS dramatically weakens the repulsive forces between neighboring chains and leads to collapse of the microgel network. Here, charge neutralization of the microgel exterior occurs first, which leads to the formation of a collapsed, relatively dense outer shell and an abrupt increase in scattered light intensity, and is then followed by a slow neutralization process of the swollen inner cores. The change in scattered light intensity at 90° includes two synergistic contributions: (i) the shrinking microgels cause an increase in scattered intensity due to their higher refractive index difference relative to water; and (ii) absorption of PNaStS increases the overall mass of P2VPH+ particles, leading to an additional increase in scattered intensity. Note that the shrunk microgel size and enhanced scattered light intensity upon the addition of PNaStS are in well accordance with the static data obtained by LLS (see Figure 1a). The time dependence of the dynamic traces can be converted to a normalized function as our previous report.48 Single- and double-exponential functions were used for fitting the typical

results in Figure 1b and d, the PNaStS with different chain lengths did respond differently at identical concentrations (25 μM, see Figure 2c). The trend in response to variations in PNaStS is that shorter PNaStS chains resulted in much more dyes releasing from (P2VPH+/SDBS/Py) microgels. Specifically, only about 37% dye release was observed with PNaStS (DP = 46), as compared to 78.9% with the DP of 15 (see Figure 2d). However, LLS and fluorescent results of surfactant molecules or polyelectrolyte chains that induced microgel size and fluorescence intensity change presented above mainly can only impart the insights of final equilibrium states, whereas the whole dynamic process during polyelectrolyte adsorption is unclear. Some literature reported the real-time observations of the dynamic process for the permeation of polyelectrolyte through multilamellar charged membrane, such as DNA to cationic lipid, which is crucially important and extensively investigated in the areas ranging from biomimetic in bioengineering to clinical gene therapy. Adsorption of either SDBS or PNaStS into P2VPH+ microgels should occur from microgel exterior first and then be followed by a slow neutralization process of the swollen inner cores, which leads to the formation of a collapsed and relatively dense outer shell, resulting in a radial gradient distribution of surfactants or polyelectrolyte within microgels. Note that not all of the surfactants could be released from the microgels and went into dispersion medium,2 as evidenced by the residual pyrene fluorescence emission (Figure 2a,c). Exclusion volume played a significant role here in that SDBS suffered less hindrance than PNaStS chains during the adsorption process and could diffuse deeper into the microgels. This prompted us to consider the dynamic process in detail. To address this issue, the entire dynamic process was further monitored by stopped-flow kinetic studies, which has been confirmed to be competent and efficient for the dynamic 9556

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Figure 4. Comparison of time-dependent scattered light intensity curves for (a) long time scales and (b) short time scales obtained for P2VPH+ microgel dispersions after mixing with PNaStS (DP = 15, 28, and 46) at pH 2.0 and 25 °C upon a stopped-flow technique. The DP of PNaStS dependence of characteristic relaxation times (c) τ1 and (d) τ2 obtained from double-exponential fitting of dynamic traces of P2VPH+ microgel dispersions after mixing with different PNaStS.

kinetics curves obtained by mixing P2VPH+ microgel dispersion with 20 and 2.5 μM of PNaStS solution, respectively. It was found that the single-exponential function cannot fit well, especially for the first 10 s. Empirically, such a dynamic trace could be well fitted by a double-exponential function (see Supporting Information Figure S4), exhibiting two characteristic relaxation times, τ1 < τ2. As stated above, the adsorption process involves two stages: the first fast stage is the approaching of polyions, which leads to nonequilibrium complex formation within the microgel shell within the first 10 s; the second slower stage is correlated to formation of the equilibrium complexes through polyion interchange reactions in the inner cores of microgels. The corresponding doubleexponential fitting results are shown in Figure 3c,d. The characteristic relaxation times τ1 decreased from 3.6 to 0.7 s and τ2 decreased from 107 to 56 s when the concentration of PNaStS (DP = 28) increased from 2.5 to 20 μM, respectively. The adsorption process here can be explained as spontaneous and entropy-driven, in accordance with the idea reported by Schlenoff et al. that stemmed from mixing polyelectrolyte complexes and multilayers.51 Next, the influence of chain lengths of PNaStS (DP = 15, 28, and 46) on the adsorption rate into the microgel interior was examined. It is well-established that a larger exclusion volume has a greater effect on the adsorption extent prominently as well as the final microgel size (see Figure 1). Figure 4a,b showed both long and short time scales scattered light intensity curves obtained from mixing P2VPH+ microgel dispersions with PNaStS of various DP (15, 28, and 46) at pH 2.0 and 25 °C upon a stopped-flow technique. As expected, in comparison with the equilibrium times of P2VPH+ and PNaStS with DP of 15 and 46 (samples 1 and 3 in Table 1), the equilibrium time of the former was much faster, as evidenced by the detectable scattered light intensity loss in the initial time scale (see Figure 4b). Moreover, the final scattered light intensity of the former

was also higher than that of the latter at the same time range, which could be ascribed to the less resistance for PNaStS (DP = 15) diffusing into the microgel interior. The higher scattered light intensity corresponded to the smaller microgel size due to the higher refractive index difference relative to water. This result agreed well with the SLI changes observed by LLS and also gathered to further authenticate the above data. The corresponding DP dependence of τ1 and τ2 obtained from double-exponential fitting of recorded dynamic traces is shown in Figure 4c,d. Both τ1 and τ2 possessed positive amplitudes. Upon increasing the PNaStS chain lengths from 15 to 28 and 46, the obtained τ1 and τ2 generally increased from 3.0 to 3.7 s and 67 to 107 s, respectively, with increasing polyelectrolyte chain lengths, indicating that the exclusion volume of polymer chains dominates the adsorption process. It is worth noting that the negligible change of τ1 during the first fast process is presumably due to the less hindrance of PNaStS chains during penetrating into the microgel corona and independence of DP in the current case. During the second slow process, increased density and osmotic pressure in the microgel interior restricted penetration of the PNaStS chains, especially for the longest chain length (DP = 46), leading to larger size of microgels, which is in accordance with the concept proposed by Dormidontova et al.52

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CONCLUSION A polyelectrolyte-responsive system was constructed, consisting of negatively charged PNaStS and SDBS, and sterically stabilized protonated P2VPH+ microgels at acidic condition. Electrostatic attraction between P2VPH+ microgels and SDBS could lead to reduced hydrodynamic diameter and form surfactant contained (P2VPH+/SDBS) microgel complex. LLS and fluorescence measurements revealed that PNaStS binding to P2VPH+ microgels resulted in disruption of the complex and release of the encapsulated pyrene molecules from the 9557

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hydrophobic interiors due to the stronger electrostatic attraction between polyelectrolyte and multicharged P2VPH+ microgels than single-charged surfactant. Stopped-flow afforded the entire dynamic process of PNaStS adsorption into the P2VPH+ microgel interior when the polyelectrolyte concentration and degree of polymerization varied; two characteristic relaxation times were achieved by a double-exponential function. This simple yet feasible design augurs well for the promising applications in controlled release fields.

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ASSOCIATED CONTENT

S Supporting Information *

ζ-Potential values of P2VP microgels in water and 0.01 M NaCl solution as a function of pH, plot of I1/I3 from pyrene emission spectra in various solutions, average hydrodynamic diameter (⟨Dh⟩) and scattered light intensities (SLI) of P2VPH+ microgels, and double-exponential fitting of typical dynamic traces. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the National Natural Scientific Foundation of China (NNSFC) Project (21274137, 51273190, 91027026, 51033005, and 51303044), Fundamental Research Funds for the Central Universities (2013HGCH0013), Anhui Provincial Natural Science Foundation (1408085QE80), and Specialized Research Fund for the Doctoral Program of Higher Education (SRFDP, 20123402130010, and 20130111120013) is gratefully acknowledged.

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REFERENCES

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