Preparation and Characterization of Low Dispersity Anionic

Article pubs.acs.org/Langmuir

Preparation and Characterization of Low Dispersity Anionic Multiresponsive Core−Shell Polymer Nanoparticles J. P. Pinheiro,*,†,‡ Leila Moura,§ Remco Fokkink,‡ and J. P. S. Farinha*,§ †

CMQE/IBB, Departamento de Química e Farmacia/Faculdade de Ciências e Tecnologia, Universidade do Algarve, Campus de Gambelas, 8005-139 Faro, Portugal ‡ Laboratory of Physical Chemistry and Colloid Science, Wageningen University, Wageningen, The Netherlands § Centro de Química-Física Molecular and INInstitute of Nanoscience and Nanotechnology, Instituto Superior Técnico, 1049-001 Lisboa, Portugal S Supporting Information *

ABSTRACT: We prepared anionic multistimuli responsive core− shell polymer nanoparticles with very low size dispersity. By using either acrylic acid (AA) or methacrylic acid (MA) as a comonomer in the poly(N-isopropyl acrylamide) (PNIPAM) shell, we are able to change the distribution of negative charges in the nanoparticle shell. The particle size, volume phase transition temperature, and aggregation state can be modulated using temperature, pH, or ionic strength, providing a very versatile platform for applications in sensors, medical diagnostics, environmental remediation, etc. The nanoparticles have a glassy poly(methyl methacrylate) (PMMA) core of ca. 40 nm radius and a cross-linked PNIPAM anionic shell with either AA or MA comonomers. The particles, p(N-AA) and p(MA-N), respectively, have the same total charge but different charge distributions. While the p(MA-N) particles have the negative charges preferentially distributed toward the inner shell, in the case of the p(N-AA) particles the charge extends more to the particle outer shell. The volume phase transition temperature (TVPT) of the particles is affected by the charge distribution and can be fine-tuned by controlling the electrostatic repulsion on the particle shell (using pH and ionic strength). By suppressing the particle charge we can also induce temperaturedriven particle aggregation.

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INTRODUCTION Responsive nanoparticles (RNP) change their properties, such as dimension, structure, interactions, or aggregation state, in response to external stimuli (temperature, pH, pressure, ionic strength, etc.).1−10 The simplicity and versatility of preparation of polymer RNPs of different sizes, shapes, and surface properties have led to many new materials with promising applications in a range of areas from sensors, to removal of toxic substances, medical diagnostics, intelligent catalysts, microreactors, multiresponsive coatings, etc.11−13 For temperature-responsive polymers in water the balance between segment−segment interactions and segment−solvent interactions can be shifted by temperature changes, inducing a volume phase transition of the material. One such material is poly(N-isopropylacrylamide) (PNIPAM), which was first used to produce cross-linked thermosensitive microgel particles.7,8,14−17 The polymer−water interactions in the case of PNIPAM decrease upon increasing the temperature above the lower critical solution temperature (LCST), which is 32 °C for PNIPAM alone, and ca. 31−35 °C in nanoparticles.18−22 At this temperature, the PNIPAM chains undergo a reversible volume phase transition from a solvated coil to a collapsed globule. Below the LCST the chain is hydrophilic and therefore highly swollen with water, adopting a coil conformation that results from the balance between the hydrophobic interactions © 2012 American Chemical Society

between isopropyl groups and the hydrogen bonding between water and the polymer amide groups.23 Above the LCST, the PNIPAM chains collapse to a globular conformation because they are partially dehydrated due to the attractive interactions between the hydrophobic isopropyl groups.24 This triggers an entropically favorable hydrophobic aggregation of the polymer segments that causes the polymer to deswell.23−28 When PNIPAM-based materials are functionalized with pHresponsive groups, such as weak polyelectrolytes with acid or basic functional groups (carboxylic, phosphoric, or amino functional groups), their volume phase transition can be tuned over an even wider range of environmental conditions to generate fast and targeted swelling responses to multiple external stimuli (i.e., temperature, pH, ionic strength). In polymer RNPs with carboxylic acid groups, the change in the pH of the media changes the ionization degree of the polyelectrolyte moiety so that at higher pH, the osmotic pressure due to the increased ionization will result in swelling of the particles, while at lower pH the particles have less charge and therefore are less expanded.29 Received: November 18, 2011 Revised: February 23, 2012 Published: February 23, 2012 5802

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MBA and 9.5 mol % AA, relative to NIPAM) for p(MA-N) and p(NAA) nanoparticles, respectively, was added in three shots, 10 min apart, to the reaction medium at 70 °C. The mixture was allowed to react for another 2 h to obtain a final conversion of 100%. The solid content values of the final particle dispersions determined by gravimetry were 8.5% for p(MA-N) and 8.8% for p(N-AA). Polymerization conversions were determined by gravimetric analysis, from the ratio of the experimentally determined solid content and the solid content for a complete polymerization reaction calculated from mass balance. Absence of the characteristically invasive NIPAM smell in the final sample provided immediate (though qualitative) confirmation of 100% NIPAM conversion. Sample Preparation. The particle dispersions were diluted to half its original solid content with Milli-Q water and cleaned by dialysis using a Spectra/Por membrane with MWCO of 6−8000 kDa, changing the water twice a day during 2 weeks in order to remove all traces of surfactant. The ionic strength was adjusted using a NaNO3 solution prepared from solid NaNO3 (Merck, suprapur). The pH was adjusted using 1 mM buffer (MES for pH between 5.0 and 6.5 and MOPS for pH > 6.5). Stock solutions of MES [2-(N-morpholino)ethanesulfonic acid] and MOPS [3-(N-morpholino)propanesulfonic acid] buffers were prepared from the solids (Fluka, Microselect, >99.5%). Solutions prepared from HNO3 (Merck, suprapur) and NaOH (0.1 M standard, Merck) were used to adjust the pH. The pH measurements were performed using a Denver Instruments semimicro combined pH electrode (300736-1) connected to a HANNA pH 211 module. Particle Size and Size Distribution. The hydrodynamic radius of the nanoparticles (RH) was determined by dynamic light scattering (DLS) measurements using an ALV apparatus with a DPSS laser (Cobolt Samba 300 mW at 532 nm). Diluted particle dispersions (5.0 × 10−3 wt % of nanoparticles) were measured at 90° and the intensity fluctuations were analyzed automatically and in a single run by means of an ALV-7002 digital correlator. The temperature was controlled (±0.1 °C) using an Haake Phoenix-II heater/circulater with a C30P cooling bath, with Haake Sil 180 mineral oil. The temperature was read directly from the decalin bath using a Platinum Pt100 temperature sensor. Each hydrodynamic radius determination was calculated from at least 30 autocorrelation curves. Further measurements of the particle dimensions reported as Supporting Information were performed by scanning electron microscopy (SEM) (JEOL 7001F) and nanoparticle tracking analysis (NTA) (Nanosight, model LM10-HSBF).

Most responsive particles described in the literature have been prepared as microgels using simple procedures. Although their properties can be improved using controlled synthesis,30 microgels have shown some experimental limitations on their applicability, particularly due to low affinity for the absorption of some species and the difficulty in separation. On the other hand, by supporting the stimuli-responsive gels in a denser nanoparticle, some of these limitations are not so severe, and also, one can take advantage of the higher specific surface area of the nanoparticles. Stimuli-responsive core−shell nanoparticles, with a thermo-responsive PNIPAM shell, have been prepared with cores of polystyrene (PS),31 poly(methyl methacrylate) (PMMA),23,32,33 silica,34,35 etc. In this work we report the synthesis of monodisperse core− shell nanoparticles with a glassy core of PMMA and an anionic multiresponsive shell of PNIPAM with either acrylic acid (AA) or methacrylic acid (MA) comonomers, cross-linked with methylene bisacrylamide (MBA). The particle diameters are in the range of 90−260 nm, being smaller than most other responsive polymer particles described in the literature (for example 300−600 nm diameters in ref 31), thus decreasing the problem of sedimentation with time. The shell negative charge is given by the initiator and the AA or MA comonomers, so that the charge distribution should reflect not only the polymer density profile in the particle shell, but also the comonomer distribution in the chains (due to the difference in reactivity ratios of the comonomers NIPAM/AA and NIPAM/MA). We want to characterize the effect of the differences of charge distribution in the shell on the response of the RNP to external stimuli such as temperature, pH, and ionic force, since the localization of functional groups near the surface of microgel particles has been known to play a key role in regulating the phase transition behavior on these materials.36 We are also interested in studying the interaction of the shell with metal cations. This has many interesting applications, including the metal detoxification of environmental and industrial waters, taking advantage of the high surface area of the particles and the multiresponsive properties to control metal uptake and release, and using the core shell RNPs to model the behavior of environmental colloids, which often possess a hard core of metal oxides and an adsorbed soft shell of carboxylate-rich natural organic matter.

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RESULTS AND DISCUSSION Particle Synthesis. We prepared two sets of negatively charged multiresponsive polymer core−shell nanoparticles, both with a glassy PMMA core to support the cross-linked microgel shell. The shell was either of PNIPAM and MA [for p(MA-N) nanoparticles] or PNIPAM and AA [for p(N-AA) nanoparticles]. The negatively charged core−shell nanoparticles were produced by a two-stage emulsion copolymerization process. The PMMA particles obtained by batch emulsion polymerization were used as seed particles to copolymerize two different types of termoresponsive shell. The batch was allowed to run until the MMA conversion was ca. 80%. Then, a mixture of NIPAM/MBA/MA or NIPAM/MBA/AA, for p(MA-N) and p(N-AA) nanoparticles, respectively (Figure 1), was added over a 10 min interval, in order to control the formation of the shell around the PMMA core. The addition of NIPAM over a time interval starting before the complete conversion of MMA favors the formation of monodisperse particles with smooth shells covering the entire PMMA core and avoids the nucleation of PNIPAM aggregates in the water phase.39 We think the initiation of PNIPAM does not take place in the aqueous phase because, although the initiator is water-soluble, NIPAM is only added to the reaction mixture when the conversion of PMMA

EXPERIMENTAL SECTION

Thermoresponsive Core−Shell Nanoparticles. Two sets of negatively charged polymer core−shell nanoparticles, p(MA-N) and p(N-AA), were prepared by a two-stage emulsion shot copolymerization technique in water, using the monomers methylmethacrylate (MMA, from Aldrich, 99%, distilled under vacuum), N-isopropylacrylamide (NIPAM, from Acros, 99%, recrystallized in hexane), and acrylic or methacrylic acid (AA or MA, respectively, both from Aldrich, 99%, distilled under vacuum). The initiator was potassium persulfate (KPS, from Aldrich, 99% ACS reagent, used as received), and the surfactant was sodium dodecyl sulfate (SDS, from Sigma, 99% GC grade, used as received). N,N-Methylene bisacrylamide (MBA, from Kodak, electrophoretic grade, used as received) was used to cross-link the shell polymer. In the first step, the PMMA core was prepared from a mixture of SDS (0.1 g/100 g of mixture, ca. 3.8 mM, a concentration below its critical micelle concentration, cmc ∼ 8 mM),37,38 KPS (0.0501 g), and MMA (5 mL, 4.7 g) in Milli-Q water (50 mL). The mixture was kept at 70 °C, under nitrogen, until ca. 80% conversion was reached. In the second stage, a mixture of either 0.92 g/0.10 g/0.065 g NIPAM/MBA/MA (8.0 mol % MBA and 9.3 mol % MA, relative to NIPAM) or 0.90 g/0.10 g/0.055 g NIPAM/MBA/AA (8.2 mol % 5803

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dry particles (Figure SI-1, Supporting Information). On the other hand, the core−shell particles change their diameter in response to changes in temperature, pH, or ionic force, while a narrow size distribution is maintained. The breadth of the size distribution can be approximated by the normalized variance (where Γ and μ2 are the first and second moments of the second order cumulant),48 which for our samples is below 0.07 over the full temperature range. This is equivalent to a dispersion below Mw/Mn ≈ 1.2, considering that μ2/Γ2 ≈ 0.33 (Mw/Mn − 1) for hard spheres.49,50 The low particle size dispersion is also apparent in SEM images (Figure SI-1, Supporting Information) and from nanoparticle tracking analysis measurements (Figure SI-2, Supporting Information). The NTA technique follows the Brownian diffusion of each individual particle by detecting its position through the light that is scattered in a reflective microscope slide (still image shown in Figure SI-2, Supporting Information). From these data the diffusion coefficient and hydrodynamic radius of each particle is calculated. Measurements at pH = 5.5, 23 °C, and 3 mM ionic strength yielded the hydrodynamic diameter distributions shown in Figure SI-2 (Supporting Information). The calculated number-averaged diameters, (2.2 ± 0.5) × 102 nm for p(N-AA) and (2.1 ± 0.5) × 102 nm for p(MA-N), are consistent with the DLS results shown in Figures 3 and 8 below. By assuming an homogeneous composition of the samples and comparing the number-average and weight-average diameters, we can estimate dispersity index values (Mw/Mn) of 1.17 and 1.15 for p(N-AA) and p(MA-N), which compare to the value estimated from DLS, Mw/Mn ≈ 1.2. Effect of Temperature. Temperature-induced volume phase transitions have been most widely studied for microgels of PNIPAM and copolymers of PNIPAM in water. At temperatures below the LCST of PNIPAM in water (ca. 32 °C),1,18,19,21,40,41,51 the polymer chains are highly solvated owing to hydrogen bonding between the water molecules and the amide residues of the polymer. Above the LCST, the hydrogen bonds are broken and PNIPAM undergoes a coil-toglobule transition.52 In microgels and polymer nanoparticles, this coil-to-globule transition originates at the volume phase transition temperature (TVPT), above which the volume of the gel is dramatically reduced.23,53 In our case, at pH = 3.8 and low ionic strength (I = 3 mM), the hydrodynamic radius of the core−shell p(MA-N) and p(NAA) particles shows a large and broad volume phase transition, centered at temperatures TVPT ≈ 30 °C and TVPT ≈ 35 °C, respectively, which are close to the LCST of PNIPAM chains in water (Figure 2). At this pH, most of the negative charges of the particle shell coming from the AA and MA comonomers are protonated (but not the small amount coming from the initiator, KPS).32,54 At about 15 °C (far below the TVPT), the particles have the highest hydrodynamic radius, RH ≈ 80 nm for p(MA-N) and RH ≈ 90 nm for p(N-AA), because the PNIPAM chains of the particle shell adopt extended conformations due to hydrogen bonding with the water molecules. Since the hydrodynamic radius of the core is Rc = 42 nm, the length of the particle shell can be estimated as Lshell = RH − Rc = 38 nm for p(MA-N) and as Lshell = 48 nm for p(N-AA). At 45 °C (ca. 10−15 °C above the TVPT) the hydrodynamic radii of the particles are reduced to ca. RH ≈ 45 nm as a result of the collapse of the PNIPAM chains onto the PMMA core (the size recovered by SEM imaging, Figure SI-1, Supporting Information). The particle shell is thus reduced to a thickness of only Lshell = 3 nm,

Figure 1. Cartoon showing the charge distribution in p(N-AA) and p(MA-N) nanoparticles.

is already ca. 80%, so the NIPAM (and AA or MA, according to the corresponding reaction rates) reacts preferably at the particle surface where PMMA oligoradicals are still active. We note that no fresh initiator is added during the polymerization of the particle shell. In fact, the particles obtained are very monodisperse, as observed in the DLS (results below), SEM, and NTA measurements (Supporting Information), and also visually confirmed by their opalescence in water, characteristic of the formation of colloidal crystals. The difference in the reactivity of the monomers NIPAM, AA,40 and MA41 effectively controls the distribution of functional (carboxylic) groups within the three-dimensional shell matrix. First, AA is only slightly more hydrophilic than MA, so we would not expect a large effect of the difference in solubility of the monomers in water. Also, both AA and MA react via standard free radical propagation. On the other hand, while AA propagates slightly slower than acrylamide-based monomers (such as NIPAM) under the acidic reaction conditions,42 MA propagates faster than NIPAM under the same conditions.43 The difference in functional group distribution in the nanoparticle shell should reflect the difference in reactivity of these monomers, although attenuated by the fact that the shell monomers are added to the reactor over a time interval. Thus, the comonomer with the highest reactivity toward NIPAM, MA, is the least surface-localized. On the other hand, the comonomer with the slowest reactivity toward NIPAM, AA, is the most surface-localized.44−46 Therefore, while p(N-AA) is expected to have a NIPAM-rich inner shell and carboxylic groups uniformly distributed across the shell (Figure 1A), p(MA-N) should present a carboxylic-rich inner shell and a NIPAM-rich outer shell (Figure 1B). This was confirmed by Hoare and Pelton,47 who compared the electrophoretic mobility and pKa of monoacid-functionalized NIPAM based microgels to find that NIPAM-MAA gels have the lowest electrophoretic mobility and highest pKa, thus containing the most core-localized radial functional group distribution. On the other hand, NIPAM-AA exhibits higher electrophoretic mobility and lower pKa, corresponding to a more homogeneous functional group distribution. (Note that a homogeneous distribution of AA groups means that ca. 50% of all the AA groups are located in a spherical outer shell with a thickness equal to 20% of the total particle radius.) Particle Dimension. The hydrodynamic radius of the glassy PMMA core, Rc = 42 ± 1 nm, was measured by DLS of PMMA nanoparticles synthesized under similar conditions as those used to prepare the core of the core−shell particles and does not change in the interval of temperature, pH, and ionic strength used. This size was also found in SEM images of the 5804

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Figure 3. Hydrodynamic radii of the p(N-AA) nanoparticles at ionic strength 3 mM and different temperatures, for pH = 3.8 (◆), 5.0 (■), 5.5 (Δ), 6.0 (●), 6.5 (◇), 7.0 (×), and 7.5 (□).

Figure 2. Hydrodynamic radii of the p(N-AA) (◆) and p(MA-N) (◇) nanoparticles determined by DLS of diluted nanoparticle dispersions (5.0 × 10−3 wt%) at pH = 3.8 and low ionic strength (I = 3 mM), for different temperatures. Error bars are calculated from at least 30 autocorrelation curves.

corresponding to ca. 2.5% and 3.9% of the low temperature volume of the p(MA-N) and p(N-AA) particles, respectively. The particle swelling ratio, Vparticle(15 °C)/Vparticle(45 °C), calculated from the particle volume at 15 and 45 °C, is 5.6 for p(MA-N) and 8.0 for p(N-AA), corresponding to shell swelling ratios, Vshell(15 °C)/Vshell(45 °C) (discounting the PMMA core) of 26 and 39, in agreement with values obtained for hybrid silica core/cross-linked PNIPAM shell particles with similar structure.34,35 Effect of pH. The copolymerization of NIPAM with ionic monomers such as acrylic acid,40 methacrylic acid,41 vinyl pyridine,55−57 and vinyl imidazole57 has led to the preparation of multiresponsive (mostly microgel) materials and also allows some degree of tunability of the volume phase transition temperature. 53 The functionalization of PNIPAM with carboxylic acid groups, generally by incorporation of acrylic acid (AA) or methacrylic acid (MA), is of particular interest, since the resulting polyelectrolyte materials undergo volume phase transitions in response to changes in temperature, pH, and ionic strength. It is well-known that the dependence of swelling behavior on pH and ionic strength in polyelectrolyte PNIPAM microgels is due to the electrostatic interactions between the ionic groups. For example, PNIPAM microgels containing AA (monomer pKa = 4.25) or MA (monomer pKa = 4.66) expand above ca. pH = 4.5 owing to the deprotonation of the carboxylic acid groups in AA and MA, resulting in an increased electrostatic repulsion.40,41 The effect of pH on the volume phase transition temperature can be best observed at low ionic strength, for which the charge shielding is smaller. At I = 3 mM, p(N-AA) has a very clear temperature-driven volume transition at lower pH values (pH = 3.8−5.0), but for higher pH values, the ionization of AA groups increases repulsion and expands the particle shell, limiting its collapse (Figure 3). Also, in the interval pH = 3.8−5.0 the volume transition temperature increases around 10 °C. This effect can be used to control the transition temperature of the p(N-AA) particles over a 8 °C interval (from ca. 34 to 42 °C) by changing the pH from 3.8 to 5.0. The decrease in the influence of temperature on particle dimensions as the pH is increased can be better understood by plotting the particle hydrodynamic radius as a function of pH (Figure 4). While at pH = 3.8 the particle radius changes ca. 50 nm, from 90 to 42 nm at pH = 7.5 the particle radius decreases

Figure 4. Variation of the hydrodynamic radius of the p(N-AA) nanoparticles with temperature [from 15 °C (◆) to 45 °C (■) in 3 °C steps] at ionic strength 3 mM and different pH values.

only ca. 10 nm when the temperature is increased from 15 to 45 °C. At higher temperatures, where the particle shell is collapsed, we can observe a pH-driven volume transition centered at ca. pH = 5.5 (blue squares in Figure 4), due to the increase in electrostatic repulsion caused by the deprotonation of the acrylic acid. For the p(N-AA) particles at 45 °C, the hydrodynamic radius increases from ca. RH ≈ 45 nm at pH = 3.8 to ca. RH ≈ 120 nm at pH = 7.5. Effect of Ionic Strength. The increase in ionic strength of the medium screens the repulsive interactions that expand the particle shell and thus results in a decrease in particle radius (Figure 5). This shielding effect does not appreciably change the curve shape, but rather uniformly decreases the particle dimensions over the full temperature range, without affecting the volume phase transition temperature. At high ionic strength the effect of pH is highly screened (Figure 6), although charge repulsion is not completely suppressed at higher temperatures. For pH < 5 we detect extensive particle aggregation because the colloidal stability of the particles is compromised. By comparing the data in Figures 3 and 6 it is possible to observe that the difference in particle size between I = 3 and 100 mM is not constant for all the pH range (Figure 7). For higher pH values, the difference in particle size is constant, while at pH = 5.0 we observe that the size collapse is larger at 3 mM than at 100 mM. 5805

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pH is represented in Figures 3 and 8, for p(N-AA) and p(MAN), respectively. At the lower pH values tested, the volume

Figure 5. Hydrodynamic radii of the p(N-AA) nanoparticles at pH = 5.5 and ionic strengths I = 3 mM (Δ), 15 mM (◆), and 100 mM (□). The shielding of the charges results in a decrease in repulsion between the charged chains and thus a less expanded shell.

Figure 8. Hydrodynamic radii of the p(MA-N) nanoparticles at ionic strength 3 mM and different temperatures, for pH = 3.7 (◆), 5.0 (■), 5.5 (Δ), 6.0 (●), 6.5 (◇), 7.0 (×), and 7.5 (□).

phase transition temperature of p(N-AA) particles (ca. 34 °C) is slightly higher than that of p(MA-N) (ca. 30 °C). This is an indication that the localization of the charge in the outer layers of the particle shell in p(N-AA) is more effective in expanding the shell than the charge in the inner layers of the shell in p(MA-N). Since the charge density is not expected to be very different in the two cases (the polymer density profile of the shell should closely follow the one previously calculated for similar particles with a cross-linked PNIPAM shell containing positive charges),23 this effect is attributed to the higher mobility of the chain ends in the outer shell, as opposed to the more hindered chains close to the particle core. The difference for the other pH values is more clearly seen in Figure 9: while at pH = 5 the TVPT of p(MA-N) is still a few

Figure 6. Hydrodynamic radii of the p(N-AA) nanoparticles at high ionic strength (I = 100 mM) for pH = 5.0 (■), 5.5 (Δ), 6.0 (●), 6.5 (◇), 7.0 (×), and 7.5 (□). At high ionic strength the chain charges are screened and thus the effect of pH on particle size is dampened. Below pH = 5, colloid stability is compromised and the particles aggregate.

Figure 9. Comparison of the hydrodynamic radii of the p(N-AA) nanoparticles (full symbols) and the p(MA-N) nanoparticles (open symbols) at ionic strength 3 mM. While at pH = 5.0 (circles) the TVPT of p(MA-N) is lower than for p(N-AA), at pH = 7.5 (squares) there is no difference over the full temperature range because both particles are fully ionized.

Figure 7. Difference in the hydrodynamic radii of the p(N-AA) nanoparticles at I = 3 and 100 mM, for pH values 5.0 (■), 5.5 (Δ), 6.0 (●), and 7.5 (□).

Effect of Charge Distribution. Both p(N-AA) and p(MAN) have the same amount of charges, but their distribution within the shell is not the same due to the different reactivity of the AA and MA monomers. While p(N-AA) has a NIPAM-rich inner shell and a carboxylic-rich outer shell morphology, p(MAN) has a carboxylic-rich inner shell and a NIPAM-rich outer shell (Figure 1). The change in hydrodynamic radius of the nanoparticles with the temperature at I = 3 mM and different

degrees lower than for p(N-AA), at pH = 7.5 no difference is noticed between the two particle samples over the full temperature range tested. In the latter case, both particles are fully ionized and the difference in size is minimal. Similarly to what was observed for p(N-AA) (Figure 5), for p(MA-N) the increase in ionic strength screens the repulsive interactions resulting in a decrease in particle radius (Figure 5806

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lost and the particles aggregate. Rapid coagulation (visible in the 20 min time interval of the data in Figure 11) can be advantageously used for separation applications.

10). This shielding effect does not appreciably change the curve shape, but rather uniformly decreases the particle dimensions

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CONCLUSIONS In this work we describe the synthesis of two types of core− shell multiresponsive polymer nanoparticles. Both have a glassy core of PMMA and a cationic cross-linked multiresponsive shell of PNIPAM with either acrylic acid, p(N-AA), or methacrylic acid, p(MA-N). One remarkable feature of these particles is that, while their total negative charge is the same, the distribution of the charges in the shell is different because of the difference in the comonomers reactivity toward NIPAM. The least reactive commoner, AA, originates p(N-AA) nanoparticles with a NIPAM-rich inner shell and a carboxylicrich outer shell, while the more reactive MAA yields p(MA-N) nanoparticles with a carboxylic-rich inner shell and a NIPAMrich outer shell (Figure 1). At low pH values the p(MA-N) and p(N-AA) particles show the expected large and broad volume phase transition close to the lower critical solution temperature (LCST) of PNIPAM chains in water, with the shell polymer segments evolving from an extended coil conformation to a collapsed state at higher temperature. By increasing the pH, deprotonation of the carboxylic acid groups increases the total charge of the shell. The increase in electrostatic repulsion expands the shell and increases the volume phase transition temperature (TVPT) of the particles. By controlling the temperature, pH, and ionic strength of the dispersion, a fine control of the nanoparticle size can be achieved. For I = 3 mM and pH = 7.5 at ca. 15 °C, the particles have the highest hydrodynamic radius, ca. 120 nm for p(MAN) and 130 nm for p(N-AA). Decreasing the pH to 3.8 lowers the repulsion in the shell and the particle radius decreases to 90 nm for p(MA-N) and 80 nm for p(N-AA). Increasing the temperature (above ca. 40 °C) has no effect for the high pH values since the electrostatic repulsion prevents the volume phase transition, but at pH values below pH = 5.5, the collapsed stage is achieved and we obtain a radius of 45 nm for both p(MA-N) and p(N-AA), corresponding to a particle shell thickness of only Lshell = 3 nm (ca. 3% of the low temperature volume of the particle shell). The TVPT of the particles can also be fine-tuned by controlling the electrostatic repulsion on the particle shell: for example, the TVPT of the p(N-AA) particles can be changed from ca. 34 to 42 °C by changing the pH from 3.8 to 5.0. Moreover, the TVPT is also affected by the charge distribution within the shell. For instance, at pH = 3.8 and I = 3 mM, the volume phase transitions of p(MA-N) and p(N-AA) are centered at TVPT ≈ 30 and 35 °C, respectively (Figure 2). By increasing the ionic strength of the medium the effect of the shell charge on the particle size is attenuated. This screening effect of the repulsive interactions results in a decrease in particle radius, without affecting TVPT. Finally, we note that by suppressing the particle charge (e.g., at pH = 3.5 and ionic strength I = 100 mM), we observed timedependent particle aggregation above TVPT due to the loss of steric stabilization by the particle shell. The ability to fine-tune the particle radius, here between 45 and 130 nm, by playing with the shell charge distribution, the temperature, pH, and ionic strength, opens a wide range of possibilities for application of these particles in areas such as

Figure 10. Hydrodynamic radii of the p(MA-N) nanoparticles at pH = 5 and ionic strengths I = 3 mM (□), 15 mM (▲), and 100 mM (○).

over the full temperature range, without affecting the volume phase transition temperature. Compared to p(N-AA), the p(MA-N) nanoparticles not only are less expanded at all temperatures but also are more affected by the increase in ionic strength of the medium. In fact, while in p(N-AA) the increase in ionic strength from 3 to 100 mM caused a steady decrease in particle dimensions (Figure 5), for p(MA-N) the particles reach their lowest size with only 15 mM ionic strength. This difference in behavior is attributed to the higher charge density in p(MA-N), compared to p(N-AA). This originates from a volume consideration since for p(MA-N) the charges are primarily in the inner shell near the particle core and for p(NAA) the charges are primarily in the outer shell, effectively occupying a larger volume. Nanoparticle Destabilization. At pH = 3.5 (not shown in the previous figures) and high ionic strength (100 mM) we observed time-dependent particle aggregation above the volume phase transition temperature (Figure 11). Under these conditions, the carboxylic groups of AA are protonated and the small charge contribution from the sulfonic groups of the initiator is screened by the high ionic strength. At low temperature, the shell is expanded, and the particles are sterically stabilized by the shell chains. However, above the volume phase transition temperature, the steric stabilization is

Figure 11. Hydrodynamic radii of the p(N-AA) nanoparticles at pH = 3.5 and ionic strength I = 100 mM, showing time-dependent particle aggregation above the volume phase transition temperature. Data are collected over a 20 min interval after stabilization at each temperature. 5807

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sensors, medical diagnostics, separation of biomolecules, environmental remediation, etc.

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ASSOCIATED CONTENT

S Supporting Information *

SEM image showing dry p(MA-N) particles and diameter distribution of the particles in water at pH = 5.5, 23 °C, and 3 mM ionic strength, obtained by nanoparticle tracking analysis. 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] (J.P.P.), [email protected] (J.P.F.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by Fundaçaõ para a Ciência e a Tecnologia (FCT, Portugal) and POCI 2010 (FEDER) through project PTDC/CTM-NAN/115110/2009. J.P.P. acknowledges funding by IBB/CBME (LA, FEDER/POCI 2010/PEst-OE/EQB/LA0023/2011) and a sabbatical grant SFRH/BSAB/855/2008 (FCT, Portugal).

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REFERENCES

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