Article pubs.acs.org/cm
Hybrid Poly(urethane−urea)/Silica Nanocapsules with pH-Sensitive Gateways Matthew A. Hood, Umaporn Paiphansiri, David Schaeffel, Kaloian Koynov, Michael Kappl, Katharina Landfester, and Rafael Muñoz-Espí* Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany S Supporting Information *
ABSTRACT: We have produced hybrid poly(urethane−urea)/silica nanocapsules with controlled molecular-scale regimes of silica that break upon introduction into basic media. The miniemulsion technique used is simple and scalable but yields complex molecular-scale morphologies that create molecular gates for the release of hydrophilic components. The hybrid nanocapsules displayed no microphase separation, indicating the formation of microscopically mixed regions of silica and poly(urethane−urea). Using atomic force microscopic techniques, we characterize the mechanical properties of individual capsules and identify the tailorability of the capsule modulus by changing the ratio of isocyanate to silica in the precursor mixture. The compositions of the hybrids were confirmed by infrared spectroscopy and thermogravimetric analysis. The change in size of a nanocapsule with pH and time was monitored by fluorescence correlation spectroscopy to evaluate their potential as nanocontainers and show a pH-responsive release.
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requires longer times than silica at pH > 13.15 Polyurethane and silica nanocomposites have shown drastically enhanced mechanical properties in films, and it is expected that this may be transferred to nanocapsule morphologies.16 The formation of silica capsules by emulsion processes has been largely investigated for direct (oil-in-water) systems. The high concentration of precursor in the droplets of the dispersed phase quickly hydrolyzes when in contact with the aqueous continuous phase, forming well-ordered capsule morphologies.17−19 However, many functional molecules are watersoluble, so that inverse systems must be used. In an inverse miniemulsion, the aqueous dispersed phase is added to a continuous oil phase. In such systems, the formation of welldefined capsules becomes more difficult than in direct emulsions because the alkoxide precursor is added from the outside (i.e., to the continuous oil phase) and high concentrations are not localized at the interface.20 The addition of a charged cationic surfactant, such as cetyltrimethylammonium bromide (CTAB), increases the interaction by increasing the time that TEOS is at the interface and drives the formation of nanosized droplets.21−23 Unfortunately, upon addition of surfactant, it has been observed that silica networks tend to be porous.8,24 To prevent pores from releasing chemical agents, composite materials have been made by formation of a porous silica network coated after with a polymer matrix, to fill the pores and prevent the release until a stimulus that degrades or swells the polymer is applied.25 Recently, Cao et al. 26 had shown the fabrication of
INTRODUCTION Controlled release of functional molecules into caustic environments is difficult, requiring complex synthetic pathways to place responsive regimes into a capsule shell.1 Release is achieved when the shell breaks/opens due to a stimulus in the environment that acts as a catalyst for change. Controlled release is a desirable trait in biological applications, such as sitespecific release of drugs (e.g., chemotherapeutics delivered to a tumor cell).1−4 Industrial applications also seek to gain benefit from responsive nanocontainers. Unfortunately, unlike biological systems, many industrial applications require capsules to be robust, containing the contradictory properties of high hardness and toughness (as the capsules must be able to protect the additives from large external forces), extreme pH values, and varying thermal conditions for a given amount of time before release is desired. To prevent the final product from having properties differing from that of the pure material, a capsule should also be nanosized and preferably degrade to molecular components before use. Hybrid nanocapsules and nanocomposites have been a trending topic in recent years.5 Many commercially scalable nanocontainers take advantage of silica (SiO2) as the main component of the shell. Silica is a cheap and hard material used as a filler in many composite systems. The facile formation of silica materials from the tetrafunctional alkoxide precursor tetraethyl orthosilicate (TEOS) has been well-studied in the design of capsule and particle systems.6−9 Poly(urethane−urea) (PUU) is often selected to add toughness to hybrid systems.10−12 PUU is a polymer with highly tailorable properties, good chemical resistance, and high mechanical toughness.13,14 It should be noted that the carbamate and urea bonds are also susceptible to hydrolysis, but the degradation © XXXX American Chemical Society
Received: February 22, 2015 Revised: May 21, 2015
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DOI: 10.1021/acs.chemmater.5b00690 Chem. Mater. XXXX, XXX, XXX−XXX
Article
Chemistry of Materials
nanocapsules in cyclohexane (solid content of 0.1 wt %). Additional cyclohexane was added until the χ2 goodness-of-fit measurements were below a value of 1, indicating an accurate representation of the particle size and a distribution mostly free from aggregates. Morphology of the nanocapsules was analyzed by scanning electron microscopy (SEM) on a Zeiss 1530 Gemini LEO microscope with an accelerating voltage of 1 kV. The morphology, size, and shell thickness of the capsules was studied by transmission electron microscopy (TEM) on a JEOL 1400 microscope with an accelerating voltage of 120 kV. The relative amount of silica to PUU was measured by Fourier transmission infrared spectroscopy (FTIR). Samples were measured for 32 scans with a resolution of 2 cm−1 on a PerkinElmer Spectrum BX spectrometer. Thermal gravimetric analysis (TGA) was carried out on a Mettler Toledo ThermoSTAR TGA/SDTA85 thermobalance under air with a flow rate of 20 mL/min. Samples were heated from room temperature to 850 °C with a heating rate of 10 °C/min. Mechanical and Phase Behavior of Hybrid PUU/Silica Nanocapsules. The extent of microphase separation in PUU/silica hybrid nanocapsules was evaluated by phase-contrast atomic force microscopy (AFM), while the mechanical stiffness of individual nanocapsules was evaluated by AFM. Samples were prepared by dilution of nanocapsule/cyclohexane suspensions to 0.1 wt % solid content and then drop-cast onto silicon wafers. Phase contrast images were collected on a Veeco Multimode Tuna AFM in tapping mode using Olympus OMCL-AC160TS-W2 cantilevers with a nomial spring constant of 42 N/m and a resonance frequency of 300 kHz. Mechanical characterization by AFM was done with a JPK Nanowizard 3 (JPK Instruments, Berlin, Germany), using Olympus OMCL-AC 240TS cantilevers with a nominal spring constant of 2 N/m and a resonance frequency of 70 kHz. True spring constants of cantilevers were determined by the thermal noise method and were found to be 1.8 ± 0.2 N/m.27 To probe the relative mechanical stiffness of the nanocapsules, force versus distance curves were taken on top of single capsules as well as on the silicon substrate, which served as a reference material. The AFM tip was positioned on top of the nanocapsules by first imaging the samples in the intermittent contact mode. The tip was then retracted a few micrometers from the surface and the AFM was switched to force spectroscopy mode, to take force versus piezo position curves on top of nanocapsules. For each type of capsule, at least five different capsules were probed, and on each capsule, at least 25 force versus distance curves were recorded. Afterward, imaging in intermittent contact mode was repeated to exclude errors in tip positioning due to drift. Reference force curves on the hard silicon substrate were used to determine the deflection sensitivity of the AFM cantilevers and allow conversion of force versus piezo position curves taken on the nanocapsules into force versus distance curves by use of self-written LabView software.28 To compare the relative mechanical stiffness, the slopes of the force versus deformation curves recorded on the different nanocapsules were normalized to those taken on the silicon substrate. Nanocapsule pH-Responsive Degradation Behavior. The pHresponsive degradation of the nanocapsules was analyzed by fluorescent correlation spectroscopy (FCS) to measure the hydrodynamic radii, RH, of nanocapsules under various basic pH conditions and times. Basic KOH solutions with concentrations of 0.1 and 0.25 M were prepared, relating to pH values of ∼13 and 13.5 based on theoretical calculations. Nanocapsules, which are hydrophobic as prepared, had to be redispersed in water for degradation experiments. Redispersion was achieved with the aid of the redispersing agent Lutensol AT50 [a nonionic hexadecyl-modified poly(ethylene oxide) surfactant with 50 ethylene oxide units], which adsorbs to the nanocapsules with the hydrophobic block and imbues a degree of hydrophilicity to the nanocapsules. An equal volume of cyclohexane was added to 2 mL of water containing 1 wt % Lutensol AT50. The suspensions were then lyophilized. Dried nanocapsules containing Lutensol AT50 (8 mg) were added to 300 μL of ultrapure water and 0.1 M KOH solution, or 0.25 M
submicrometer-sized silica capsules produced by inverse miniemulsion requiring an aqueous phase containing either ammonia (pH 12.5) or HCl (pH 0.2) to catalyze the silica formation. In the absence of any catalyst, capsule formation was not achieved.7 In the present work, we have used the miniemulsion process to create poly(urethane−urea)/silica hybrid nanocapsules using a simple one-pot method with simultaneous interfacial polymerization and sol−gel precipitation. This simultaneous in situ formation of both polymer and inorganic component is novel and particularly simple, especially when compared to the combination of a polymer matrix to preformed silica particles. The hybrid nanocapsules act as containers for release of deliverable molecules. Individual capsules were shown to have hybrid mechanical and chemical-resistive properties by using atomic force microscopic techniques and fluorescence correlation spectroscopy to demonstrate the stiffness of individual capsules and their propensity to break at different times in a caustic medium. Nanocapsules were produced by the inverse (water-in-oil) miniemulsion technique. By using this facile and direct mixing of highly available reactive components, toluene diisocyanate (TDI) and TEOS, we propose a scalable method to produce capsules comprising an interpenetrating network of PUU and silica. We show that, by simply changing the ratio of TDI to TEOS, the properties of degradability in a caustic environment and of mechanical robustness of nanosized capsules can be easily tailored.
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EXPERIMENTAL SECTION
Materials. Tetraethyl orthosilicate (TEOS, 98%, Sigma−Aldrich), toluene diisocyanate (TDI, 95%, Sigma−Aldrich), glycerol (87 wt % in water, Merck), linear polyethylenimine (PEI, Sigma−Aldrich, Mw = 346 g/mol), cetyltrimethylammonium bromide (CTAB, ≥98%, Carl Roth), polyglycerol polyrincoleate (PGPR, Danisco), cyclohexane (99%, VWR), Lutensol AT50 (BASF), potassium hydroxide (≥85%, Riedel deHaën), and sulforhodamine 101 (SR101, 95%, Sigma− Aldrich) were used as received. Ultrapure Milli-Q water with 18.5 MΩ· cm resistivity was used in all experiments. Preparation of Hybrid PUU/Silica Nanocapsules. A water-inoil miniemulsion was prepared by addition of 650 μL of an aqueous dispersed phase to 6.25 g of a continuous phase containing 55 mg (0.88 wt %) of PGPR in cyclohexane. Stock solutions of the aqueous phase were composed of 1.1 g of PEI, 1.1 g of 87% glycerol aqueous solution, 535 mg of CTAB (5.35 wt % of aqueous phase), and approximately 10 mg of SR101 dye; ultrapure water was added so that the total weight of aqueous phase was 10 g. Each mixture was preemulsified by stirring for 30 min and emulsified by ultrasonication (1/4-in. sonication tip, 70% intensity, 3 min with pulses of 30 s and pauses of 20 s) under cooling in an ice−water bath to prevent side reactions. Teflon-coated magnetic stir bars were added to the sonicated emulsions and placed on a magnetic stirrer at 300 rpm. Pure PUU, silica, or hybrid PUU/silica capsules were prepared by addition of the reactants (TDI, TEOS, or TDI and TEOS, respectively) mixed with solutions of 0.88 wt % PGPR in cyclohexane, so that the total volume of reactants and cyclohexane was maintained at 2 mL. The reactant solutions were added directly and all at once to the stirring miniemulsion suspensions and allowed to react overnight. Nanocapsule formation was halted by centrifugation of the original suspension for 30 min at 4000 rpm and resuspended in 10 mL of fresh cyclohexane. Washed nanocapsules were dried by lyophilization for characterization. Characterization Methods. Average diameter and distributions of the nanocapsules were measured by dynamic light scattering (DLS) at 25 °C, using a PSS Nicomp Particle Sizer 380 in 90° scattering mode. Dispersions for scattering were prepared from dilutions of the B
DOI: 10.1021/acs.chemmater.5b00690 Chem. Mater. XXXX, XXX, XXX−XXX
Article
Chemistry of Materials Scheme 1. Formation of Nanocapsules by Inverse Miniemulsiona
a
Reaction of TEOS and TDI at the water/oil interface after emulsification leads to the nanocapsule morphology. Capsules lyophilized with Lutensol AT50 allow for redispersion in water. Addition of KOH initiates capsule breakage, which is controlled by the size of silica gates, produced by tailoring the chemical composition of the nanocapsule shell.
are possible; first NCO forms amines and CO2 with water prior to urea formation.14 SEM and TEM micrographs confirmed the capsule morphology and that their size was in the submicrometer range (Figure 1). The diameter of the nanocapsules was influenced by the polymerization kinetics of the reactants in the water-in-oil system. Silica nanocapsules had a larger size than pure PUU and hybrid PUU/silica nanocapsules, as listed in Table 1. Hybrid nanocapsules generally possessed diameters between those of pure silica and pure PUU. In addition, PUU and hybrid capsules had a shell thickness greater than that of the pure silica capsules. Pure PUU nanocapsules tended to be smaller in the SEM and TEM images than from DLS and could reflect some aggregation behavior in the cyclohexane medium. These differences can also be caused by deviations in the DLS data for larger-sized particles and not perfect sphererical geometries. Interestingly, we did not observe any clear microphase separation from TEM images, that is, shell contrast remains the same over the entire nanocapsule. Phase-contrast AFM micrographs for nanocapsules of pure silica and sample H1 appeared to also possess this homogeneous behavior, indicating that no microphase separation existed for the nanocapsules (Figure 2). The silica is most likely incorporated into the PUU network during the production of silicic acid, which occurs during hydrolysis of TEOS (Scheme 2). Oligomeric and polymeric chains of silica are formed and then incorporated into the PUU, leading to the formation of molecular-sized regimes/gates of silica that enhance the hybrid network but are too small to be observed by AFM or TEM. The presence of PUU and silica in the nanocapsules was characterized by FTIR (Figure 3). The FTIR spectrum of pure PUU shows an absorbance band at 1542 cm−1, corresponding to N−H wagging. Bands around 1630 and 1650 cm−1 may be associated with amide I stretching from urea bonds. Similarly, a small shoulder at 1730 cm−1 is assigned to urethane ν(CO). Bands at 2940 and 2879 cm−1 are characteristic of ν(CH2) asymmetric and symmetric methylene stretching. A broad signal at 3340 cm−1 illustrates the stretching vibration of -OH groups. In pure silica nanocapsules, a number of peaks related to ν(Si−O−Si) are observed at 460, 800, and 1070 cm−1. An additional band at 943 cm−1 is from ν(Si−OH).13,22 The pure silica nanocapsule spectrum also possesses methylene stretching around 2900 cm−1 due to the organic modifiers, including PEI and glycerol. As the TDI content was increased with respect to constant TEOS content, the intensity of the PUU bands increased and the intensity of characteristic silica bands decreases, indicating a change in composition of the hybrid nanocapsules. Thermal degradation behavior of the hybrid nanocapsules and their relative silica content was determined by TGA, shown
KOH solution, and allowed to mix for 24 h. The solutions were diluted further for FCS measurements. FCS experiments were performed on a commercial setup (Zeiss, Germany) consisting of an inverted microscope, model Axiovert 200, coupled with the FCS module ConfoCor 2. A C-Apochromat 40 × /1.2 W water immersion objective was used in all measurements. The fluorophores were excited with a HeNe laser (λ = 543 nm) and the emission was collected after filtering with a LP560 long-pass filter. For detection, an avalanche photodiode, enabling single photon counting, was used. As sample cell, eight-well, polystyrene-chambered cover glass (Laboratory-Tek, Nalge Nunc International) was applied. For each solution, 20 measurement cycles with a total duration of 200 s were carried out. Time-dependent fluctuations of the fluorescence intensity δI(t) were detected and evaluated by autocorrelation analysis, yielding the diffusion coefficient and hydrodynamic radius of the fluorescent species.29 Calibration of the FCS observation volume was done with rhodamine 6G, a dye with a known diffusion coefficient. A proposed scheme for nanocapsule formation and degradation is outlined in Scheme 1.
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RESULTS AND DISCUSSION We have produced nanocapsules under mild basic conditions by using oligomeric PEI, which, like CTAB, contributed to the trapping of negatively charged silicic acid, Si(OH)4, at the water/oil interface in order to produce silica capsules.21 The pH of the aqueous phase was 11.7−11.9. Glycerol was added to increase the hydrophilic retention of the particles, preventing osmotic pressure differences from causing the aqueous phase to take part in Ostwald ripening prior to nanocapsule formation. Samples with different ratios of TEOS to TDI were prepared and labeled H1, H2, H3, and H4, as reported in Table 1, corresponding to higher silica content with increasing value after H. PEI and glycerol both react with TDI; secondary and primary amines from the PEI are expected to react first to form urea bonds and the hydroxyl groups to form urethane units. An excess of TDI was required, as side reactions of TDI with water Table 1. Nanocapsule Size, Percent Residue, and Precursor Composition for Pure Silica, pure PUU, and Hybrid Samples (H1−H4).a sample silica H4 H3 H2 H1 PUU
DHb (nm)
weight850 °Cc (wt %)
TEOSd (μL)
TDId (μL)
± ± ± ± ± ±
34.9 18.3 14.5 7.20 8.30 7.10
650 500 500 500 500 0
0 30 60 90 125 125
400 310 320 270 290 300
170 110 100 110 120 110
a
Intensity-weighted Gaussian distributions of hydrodynamic diameter are reported. bDetermined by DLS. cResidual mass present in TGA at 850 °C. dOut of a total of 2 mL with PGPR/cyclohexane solution. C
DOI: 10.1021/acs.chemmater.5b00690 Chem. Mater. XXXX, XXX, XXX−XXX
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Chemistry of Materials
Figure 1. SEM images of (a) pure silica, (b) H4, and (c) pure PUU nanocapsules, and TEM images of (d) pure silica, (e) H4, and (f) pure PUU nanocapsules.
of nanocapsules, as the onset of decomposition of PUU is above 180 °C.13 Typical force−distance curves for different nanocapsules and for the reference silicon substrate are shown in Figure 5. When the tip is more than 8 nm away from the sample surface, no significant deflection of the cantilever is observed. This part of the curve defines the zero force baseline. Upon closer approach, attractive van der Waals forces lead to a jump in contact of the AFM tip onto the sample surface. With further approach, the AFM tip is pushed upward by the sample and the force between the AFM tip and sample increases. This leads to a deformation of the sample due to the applied force, corresponding to negative values of the distance. The zero distance was taken as the intersection between the zero force line with the contact part of the force−distance curve. For the silicon substrate and pure silica nanocapsules, there is hardly any deformation detectable, as expected for such stiff samples. If a tip radius of 10 nm is assumed, one would expect a deformation of less than 0.1 nm for the silicon wafer at an applied force of 10 nN. The sample H1 is slightly more deformable than the PUU nanocapsules, whereas the H4 hybrid nanocapsules (higher silica amount of the hybrid systems) have a mechanical stiffness that is just in between urethane and silica. The average relative stiffness values for all force versus distance curves are summarized in Table 2. To study the encapsulation and release properties of the nanocapsules, a small fluorescent molecule, sulforhodamine 101, was encapsulated during the inverse emulsion process. The use of fluorescent species as a model for drug molecules is common, as it enables the use of sensitive and selective fluorescent techniques to study the encapsulation and release properties of a nanocarrier. In our case, monitoring of dye release from the nanocapsules by conventional means (e.g., centrifugation or dialysis of suspensions to remove the capsules, followed by fluorescent spectroscopy to estimate the amount of released dye) was not possible, since nearly all the dye molecules were trapped in the shell of the nanocapsules during synthesis.30 Polar dyes with amino, sulfonate, and carboxylate groups also possibly react
Figure 2. AFM phase-contrast images of (a) pure silica and (b) H1 nanocapsules. No microphase separation is observed.
in Figure 4 (first derivatives of the thermogram are shown in Figure S1, Supporting Information). TGA data confirm that the final silica content at 850 °C is not directly linked with the ratio of TDI to TEOS (Table 1). The degradation observed in pure silica capsules corresponds to the organic modifiers used to regulate capsule formation. The addition of TDI at values greater than 90 μL led to a final silica weight loss of ∼7.5 wt %, a value close to that from pure PUU nanocapsules because the TDI reaction at the water/oil interface was faster than TEOS. Multiple organic thermal degradations were observed below 350 °C in Figure 4. The first derivative aided in defining one shift of low-degrading species at ∼180 °C to higher temperatures with the addition of TDI, indicating that glycerol, PEI, and possibly other molecules in the aqueous phase are being polymerized and taking part in the chemical composition D
DOI: 10.1021/acs.chemmater.5b00690 Chem. Mater. XXXX, XXX, XXX−XXX
Article
Chemistry of Materials Scheme 2. Proposed Formation of Hybrid PUU/Silica Nanocapsulesa
a
Small regions of oligosilica react with TDI to form small interconnected domains not observed by phase separation. These molecular gates of silica were broken open by hydrolysis. Increasing PUU content reduced the nanocapsule breakage by making the gates smaller.
Figure 4. TGA thermogram of pure silica, pure PUU, and hybrid PUU/silica nanocapsules H1−H4 with decreasing ratio of PUU to silica.
Figure 3. Transmission FTIR of pure silica, pure PUU, and hybrid PUU/silica nanocapsules H1−H4 with decreasing ratio of PUU to silica.
sulfonated rhodamine with no ester due to its possible hydrolytic cleavage at basic pH. The sensitive (recognition of single fluorescent molecules) technique fluorescence correlation spectroscopy (FCS) was applied to monitor in situ the degradation of nanocapsules with sulforhodamine trapped within the shell at pH 13 and 13.5.29
with TDI or TEOS. For this reason we expected that a large proportion of the dye will migrate to the interface of the waterin-oil emulsion. Further entrapment at the interface occurred due to dye complexation with cationic PEI, CTAB, and possible hydrogen bonding with glycerol. As isocyanates show high reactivity with primary and secondary amines, we chose a E
DOI: 10.1021/acs.chemmater.5b00690 Chem. Mater. XXXX, XXX, XXX−XXX
Article
Chemistry of Materials
Figure 5. Approaching force vs distance curves recorded on different samples. Upon close approach (
