Efficient and Generic Preparation of Diverse Polyelectrolyte Nanogels by Electrostatic Assembly Directed Polymerization

Abstract

Open AccessCCS ChemistryRESEARCH ARTICLE1 Dec 2020Efficient and Generic Preparation of Diverse Polyelectrolyte Nanogels by Electrostatic Assembly Directed Polymerization Peng Ding, Jianan Huang, Cheng Wei, Wei Liu, Wenjuan Zhou, Jiahua Wang, Mingwei Wang, Xuhong Guo, Martien A. Cohen Stuart and Junyou Wang Peng Ding State Key Laboratory of Chemical Engineering, and Shanghai Key Laboratory of Multiphase Materials Chemical Engineering, East China University of Science and Technology, Shanghai 200237. Google Scholar More articles by this author , Jianan Huang State Key Laboratory of Chemical Engineering, and Shanghai Key Laboratory of Multiphase Materials Chemical Engineering, East China University of Science and Technology, Shanghai 200237. Google Scholar More articles by this author , Cheng Wei State Key Laboratory of Chemical Engineering, and Shanghai Key Laboratory of Multiphase Materials Chemical Engineering, East China University of Science and Technology, Shanghai 200237. Google Scholar More articles by this author , Wei Liu State Key Laboratory of Chemical Engineering, and Shanghai Key Laboratory of Multiphase Materials Chemical Engineering, East China University of Science and Technology, Shanghai 200237. Google Scholar More articles by this author , Wenjuan Zhou State Key Laboratory of Chemical Engineering, and Shanghai Key Laboratory of Multiphase Materials Chemical Engineering, East China University of Science and Technology, Shanghai 200237. Google Scholar More articles by this author , Jiahua Wang State Key Laboratory of Chemical Engineering, and Shanghai Key Laboratory of Multiphase Materials Chemical Engineering, East China University of Science and Technology, Shanghai 200237. Google Scholar More articles by this author , Mingwei Wang State Key Laboratory of Chemical Engineering, and Shanghai Key Laboratory of Multiphase Materials Chemical Engineering, East China University of Science and Technology, Shanghai 200237. Google Scholar More articles by this author , Xuhong Guo State Key Laboratory of Chemical Engineering, and Shanghai Key Laboratory of Multiphase Materials Chemical Engineering, East China University of Science and Technology, Shanghai 200237. Google Scholar More articles by this author , Martien A. Cohen Stuart State Key Laboratory of Chemical Engineering, and Shanghai Key Laboratory of Multiphase Materials Chemical Engineering, East China University of Science and Technology, Shanghai 200237. Google Scholar More articles by this author and Junyou Wang *Corresponding author: E-mail Address: [email protected] State Key Laboratory of Chemical Engineering, and Shanghai Key Laboratory of Multiphase Materials Chemical Engineering, East China University of Science and Technology, Shanghai 200237. Google Scholar More articles by this author https://doi.org/10.31635/ccschem.020.202000354 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesTrack Citations ShareFacebookTwitterLinked InEmail Nanogels hold promise as soft and functional carriers and nanoreactors, but whether they will ever reach the stage of large-scale application depends crucially on efficient production methods, and on what interesting properties and functionalities they can have. In particular, nanogels consisting of highly charged polyelectrolyte have not yet been very much explored. Here, the authors present a novel and generic strategy for controlled and efficient synthesis of a large diversity of polyelectrolyte nanogels. The method is based on polymerizing an ionic monomer in the presence of an oppositely charged polyion-neutral diblock copolymer as template, while adding a cross-linker. The growing polymer chains assemble with the template, forming polyion complex micelles, which dissociate upon increasing salt concentration. Subsequent separation yields nanogels with well-controlled size and properties, and free template polymers that can be used again. Our design can be applied generally to a wide range of both cationic and anionic monomers, as well as various cross-linkers. Scaled-up production presents no problems as increasing monomer concentration (hundreds of mM) and reaction volume (up to 1 L) hardly compromise product quality. Moreover, the obtained nanogels with their well-controlled size, morphology, chemistry, and cross-linking degree perform well as soft nanocarriers and catalytic nanoreactors. Download figure Download PowerPoint Introduction Nanogels are hydrogel particles in the nanosize range (10–100 nm). They consist of cross-linked hydrophilic polymer swollen with significant amounts of water,1 and they can take up, transport, and release water-borne active ingredients. Due to their unique structure and properties, such as small and well-controlled size, permeability, high loading capacity, biocompatibility, and multiple responses to external stimuli, they offer application potential in the context of drug delivery, diagnostics, sensors, catalysts, and so forth.2–6 Because a reliable and economical fabrication procedure is crucial for successful application, several synthetic strategies have been developed. These strategies can be classified into two categories: (1) post-cross-linking of polymeric precursors and (2) direct polymerization of monomers.7–9 For the first approach, macromolecular precursors with reactive groups should be available, which must be introduced into a confining medium (droplet, micelle, or polyion complex [PIC] micelle), post-cross-linked, and finally purified and/or transferred to a good solvent (water, as a rule) to obtain the functional nanogel.10–13 This approach is reliable but has as drawbacks (1) the need for delicate molecular design (taking solubility issues and/or selective cross-linking into account) and (2) often labor-intensive multistep polymer synthesis. For the second approach, one often follows typical polymer colloid fabrication methods. One can carry out polymerization in inverse, mini- and microemulsions14–17; in principle, this method allows the use of a wide range of monomers, and therefore has been applied widely for preparing various nanogels. Although this approach bypasses the synthesis of a precursor, the problem of solvent change always remains, which implies undesired use of organic solvents and surfactants.18 Alternatively, one can make use of (surfactant-free) dispersion polymerization in water, but this method is restricted to systems with a high content of “Lower Critical Solution Temperature” monomers, because these generate insoluble polymer at the (high) temperature of polymerization, while becoming water soluble at the (low) temperature of application.19–21 There are only a few monomers that satisfy this requirement, among which poly(N-isopropylacrylamide) has been widely used. We conclude from this survey that the available methods either have many disadvantages, or are geared toward one or few polyelectrolytes, and therefore do not provide a simple and generic way to prepare a diversity of highly charged polyelectrolyte nanogels. Yet, we expect that such nanogels have promising properties because charge interactions allow a high degree of control and manipulation, both internally (uptake and release) and externally (barrier crossing and surface interactions). They would seem ideal for dealing with charged cargo, which covers specific small ions, a host of biomolecules (e.g., enzymes and RNA/DNA), numerous drug molecules, fluorescent dyes, and a diversity of nanoparticles, and also for designing targeted nanogel/substrate interactions of various kinds (adsorption, wetting, and lubrication) are targeted. Therefore, designing a new strategy toward controlled and efficient synthesis of polyelectrolyte nanogels, preferably for a variety of anionic as well as cationic monomers, is highly relevant. When aiming at polyelectrolyte nanogels with proper control of size and morphology the first point of attention is to find an appropriate confining structure.22 A promising starting point is the recently developed polymerization induced self-assembly (PISA) route for preparing polymeric nanoparticles.23–26 In particular, the variation where polymerization takes place in the presence of charged-neutral diblock copolymer, denoted as polymerization induced electrostatic self-assembly (PIESA), provides a well-controlled environment in which polymerization of charged monomers occurs. The copolymer’s charged block acquires a double role, not only allowing for self-assembly to occur, but also acting as a template.27,28 Moreover, as this kind of self-assembly is driven by electrostatic interaction, PIESA should work for any oppositely charged polymer/monomer pair. To proceed to nanogel synthesis, two more steps must be made. First, a cross-linker must be included during polymerization. Not only does this transform the newly formed polymer into a chemical network, but it also facilitates separation. Second, a salt-induced dissociation of the complex into pure nanogel product and template, followed by physical separation, must be put in place. This constitutes our electrostatic assembly directed polymerization (EADP) method. Experimental Methods General procedure for preparation of nanogel: typically, poly(acrylic acid-b-ethylene oxide) (PAA-b-PEO; 8.2 mg), cationic dimethyl aminoethyl methacrylate (DMAEMA) monomer (6.3 mg, 0.04 mmol), methacryloyl-bis-acryl amide (MBA; 0.62 mg, 0.004 mmol), and 2-hydroxy-2-methyl-1-propanone (0.131 mg, 8 × 10–4 mmol) were dissolved in water (2 mL) in a 10 mL Schlenk tube. The solution was adjusted to pH 6.5 using 3.0 M HCl. The tube was sealed and deoxygenated by bubbling with nitrogen for 30 min. Then the tube was exposed to UV light for 3 h. The reaction was stopped by exposure to air. Nanogels with different size were synthesized according to the aforementioned recipe and reaction conditions, but using different salt concentrations (0, 20, 40, 60, 80, 100, and 120 mM). Nanogels with different cross-linkers were synthesized according to the aforementioned recipe and reaction conditions, but using different cross-linker (N,N'-bis(acryloyl) cystamine [BAC], ethylene glycol dimethacrylate [EGDMA], and N,N'-(1,2-dihydroxyethylene)bisacrylamide [DHEA]). Other experimental details and characterization methods are available in Supporting Information. Results and Discussion Scheme 1a shows the concept: a polyion-neutral diblock copolymer is used as template for the polymerization of an oppositely charged monomer, together with a cross-linker. The nascent charged polymer aggregates electrostatically with the template into, for example, PIC micelles.29–32 Increasing the salt concentration screens the electrostatic interaction, causing the diblock copolymer template to be released from the polymeric network.33 Separation follows either by simple centrifugation/filtration or ultrafiltration with a suitable semipermeable membrane, after which the formed nanogels can be collected and the template copolymer recycled. Our strategy relies on classical and robust aqueous free-radical polymerization (no need for living polymerization methods, i.e., chain transfer agents), and allows to synthesize nanogels from many different ionic monomers with both positive and negative charges, so that “libraries” can be constructed. The nanogel size can be tuned by varying the salt concentration during the polymerization, and its functionality can also be regulated simply by selecting appropriate ionic monomers and cross-linkers, demonstrating that the approach is both facile and generic. Scheme 1 | (a) Schematic representation of the nanogel preparation and template recycling. (b) Examples of chemical structures of template, monomer, and cross-linker used in this study. (c) Selected applications of polyelectrolyte nanogels discussed in this study. Download figure Download PowerPoint Our first example concerns PAA-b-PEO as the anionic template, DMAEMA as the cationic monomer, and MBA as the cross-linker, respectively (Scheme 1b). We follow the rate of DMAEMA conversion using 1H NMR, which allows us to measure the concentration of remaining monomer. As shown in Figure 1a, the DMAEMA conversion in the presence of 20 mM NaCl, both with and without template, initially increases rapidly with reaction time, to slow down somewhat after 1 h. Interestingly, at a given time the conversion with template is always higher than that without template, indicating a clear rate-enhancing effect of the template. The conversions level off after 3 h, such that the templated reaction shows nearly complete conversion (91%), whereas in the template free case only 54% conversion is reached ( Supporting Information Figure S1). Figure 1b presents sizes of the micelles obtained by polymerization of DMAEMA in the presence of PAA-b-PEO, with and without the cross-linker MBA (throughout the paper given as mole fraction with respect to monomer, here 10%). Both have very similar hydrodynamic radii of ∼36 nm, and a narrow size distribution; even the light scattering intensities are rather close. Apparently, cross-linking hardly affects properties like size and mass (Figure 1c). Figure 1 | (a) Kinetic plots of DMAEMA conversion with and without PAA-b-PEO template. (b) Size distribution of micelles prepared with and without 10% MBA cross-linkers. (c) Light scattering intensity of synthesized micelles vs. salt concentration. (d) Autocorrelation decay functions of micelles prepared with 0% and 10% cross-linker, respectively, at 1.5 mM NaCl. Nanogel synthesis was carried out at pH 6.5, charge mixing ratio 1∶1, cross-linker 10%, and 20 mM NaCl. Download figure Download PowerPoint As may be expected, including a cross-linker influences the salt response of the micelles. For micelles without cross-linker, the light scattering intensity decreases with increasing NaCl concentration; the particles fall apart completely around 700 mM, as indicated by the very low intensity at the plateau (Figure 1c), and the disappearance of the correlation decay curve at high salt concentration (Figure 1d). For micelles with 10% cross-linker, the scattering intensity also decreases upon increasing salt concentration, but in contrast to the non-cross-linked case it reaches a finite plateau of about 20% of the original value. As the free template molecules contribute very little to the scattering, this remaining scattering must come from the cross-linked PDMAEMA nanogels that have released their PAA-b-PEO template molecules (Figures 1c and 1d). These results confirm that the MBA was successful as cross-linker, making PDMAEMA into a nanogel network. Moreover, adding salt is indeed seen to dissociate the assemblies and release the PAA-b-PEO template from the PDMAEMA nanogels. We separated template and nanogel by centrifugation/filtration with a semipermeable membrane (Molecular Weight Cut Off of 100 kDa), and we investigated the structure and relevant properties of the purified nanogels. The amount of cross-linker was fixed at 10% of the monomer concentration. The nanogels that we obtain after removing the PAA-b-PEO template are slightly smaller (Rh ∼ 30 nm) than their complex precursors, but they keep their narrow size distribution and spherical morphology. This can be inferred from dynamic light scattering data, in particular from CONTIN analysis ( Supporting Information Figure S2a), and from the absence of angular dependence of the diffusion coefficient ( Supporting Information Figure S2b). The 1H NMR spectrum of the purified PDMAEMA nanogels displays typical chemical shifts solely from DMAEMA, but no peaks from the PAA-b-PEO template, confirming the complete release of the PAA-b-PEO from the PDMAEMA nanogels ( Supporting Information Figure S2c). The FT-IR spectrum of the PDMAEMA nanogel shows no obvious peak at 2876 cm−1, which also suggests the complete removal of the PAA-b-PEO template. Moreover, it exhibits distinctive peaks at 2825 and 2769 cm−1 (C−H stretching of methyl group), 1725 cm−1 (C=O stretching), and 1146 cm−1 (C−N stretching), confirming the presence of the PDMAEMA in the nanogel ( Supporting Information Figure S2d).34,35 The quantitative analysis of 1H NMR for both separated nanogel and supernatant indicates that around 80% of the converted DMAEMA monomers is incorporated in the nanogels, whereas ∼20% monomers are converted into free polymer and removed during separation ( Supporting Information Figure S2e). It is well known that the strength of the charge interaction and, therefore, the properties of polyelectrolyte complexes are sensitive to ionic strength.36–38 In Figure 2a, we show the effect of added salt on nanogel formation. The average size increases from 26.4, without added salt, to 81.4 nm at 120 mM NaCl, but no broadening of the size distribution occurs over this range (Figure 2b). The angular-dependent light scattering results indicate that nanogels prepared at different salt concentrations are all spherical ( Supporting Information Figure S3). The Transmission Electron Microscope (TEM) images (Figures 2c–2e) also confirm roughly spherical nanogel particles and increased size with increasing salt concentration. Apparently, when salt weakens the electrostatic interactions between the charged building blocks, this leads to cross-linked PIC micelles with a bigger core and higher aggregation number ( Supporting Information Figure S4), that is, bigger nanogels as a result. Figure 2 | (a) Hydrodynamic radius and (b) size and size distribution of PDMAEMA nanogels prepared at different salt concentrations. (c–e) TEM images of phosphotungstic acid-stained PDMAEMA nanogels prepared at different salt concentrations [(c) 0 mM, (d) 60 mM, and (e) 120 mM]. Nanogel synthesis was carried out at pH 6.5, charge mixing ratio 1∶1 and cross-linker 10%. Download figure Download PowerPoint The cross-linker amount and type are also key factors in tuning the properties of the nanogels. We varied the fraction of the MBA cross-linker from 7.5% to 20%. As shown in Figure 3a, the amount of MBA had little effect on the size or size distribution of the nanogels, but it obviously influenced the swelling degree of the nanogel. This makes sense, as a higher proportion of cross-linkers would increase the average number of cross-links in a particle, consequently reducing the ability of the material to swell upon changing pH (Figure 3b). We also conducted experiments examining the type of the cross-linker. In addition to MBA, we selected another three cross-linkers with different chemical structures and properties, namely BAC, EGDMA, and DHEA ( Supporting Information Table S1). As shown in Figure 3c, the nanogels produced using these different cross-linkers showed no obvious differences in size or size distribution, indicating that the influence of the cross-linker on the polymerization of our polyelectrolyte nanogels is negligibly small. On the other hand, different cross-linkers can introduce new properties into the nanogels. For example, the BAC cross-linker (with disulfide bonds) makes the nanogels redox-responsive so that they disintegrate entirely upon adding glutathione (Figure 3d); this would not be possible using a cross-linker lacking S–S bonds. Figure 3 | (a) Size and size distribution of PDMAEMA nanogel prepared at different proportion of MBA cross-linker. (b) Hydrodynamic radius of PDEAEMA nanogels prepared at different proportion of the MBA cross-linker as a function pH. (c) Size and size distribution of PDMAEMA nanogel prepared with different cross-linkers. (d) Intensity change of PDMAEMA nanogels prepared with 10% MBA and 10% BAC, respectively, in the presence of 10 mM glutathione. Nanogel synthesis was carried out at pH 6.5, charge mixing ratio 1∶1, and 20 mM NaCl. Download figure Download PowerPoint As our design relies on the electrostatic interaction between the template and the monomers, the chemical nature of the ionic monomer should not qualitatively change the polymerization and assembly process. Indeed, we have tried 10 different cationic monomers with different chemical structures ( Supporting Information Figure S5), and they all form similar well-defined nanogels with hydrodynamic radii in the range of 30–40 nm, and narrow size distributions (Figure 4a). We also inverted the charges of the template and the monomer; that is, we used a cationic PDMAEMA-b-PEO template, and acrylic acid or acrylamido propylsulphonate (AMPS) as weak or strong anionic monomer, respectively. As shown in Figure 4b and Supporting Information Figures S6a–S6d, the hydrodynamic radii of the PAA and PAMPS nanogels are 18.2 and 21.7 nm, respectively, and again both have a narrow size distribution. These findings confirm that our EADP route with added cross-linker is quite generally applicable to a wide range of both cationic and anionic monomers; these findings highlight the robustness and wide scope of our synthesis strategy. The EADP method allows making nanogels at significant concentrations. Figure 4c shows that it is possible to prepare nanogels at up to 300 mM monomer concentration (total solids concentration 10 wt %), while the particle size and size distribution remain the same. There is no obvious limit (except availability of template) to the reaction volume; we went up to 1 L, and still obtained identical nanogels with similar size and size distribution. Obviously, working with larger volumes forced us to adopt established separation techniques, like ultrafiltration,39,40 and appropriate membranes developed for such volumes. The 41.8 cm2 setup provided good separation at a speed of a few liters per hour under a pressure of about 1 bar. All the filtrates and retentates were analyzed by light scattering and 1H NMR; the data confirmed required purity for the nanogel ( Supporting Information Figures S7a–S7d). Therefore, separation poses no problems for efficient production of high-purity polyelectrolyte nanogels. The filtrate contains the template and a little free polycation which can be removed in an extra step (see Supporting Information section “Recycle of the template” and Supporting Information Figure S8). By way of test, the same PAA-b-PEO template has been recycled in three consecutive batch reactions, and always produced similar PDMAEMA nanogels with well-controlled sizes and size distributions (Figure 4d). Figure 4 | (a) Size and size distribution of nanogels prepared from different cationic monomers (see Supporting Information for monomer structures). (b) Size and size distribution of anionic PAA and PAMPS nanogels prepared with PDMAEMA-b-PEO as the template. (c) Size and size distribution of PDMAEMA nanogels prepared at different monomer concentrations and large volume (1 L). (d) Size and size distribution of PDMAEMA nanogels prepared from recycled PAA-b-PEO template. Nanogel synthesis was carried out at pH 6.5, charge mixing ratio 1∶1, cross-linker 10%, and 20 mM NaCl. Download figure Download PowerPoint Our discussion thus far has considered the robustness and of our synthesis strategy. In the of this we on properties and applications of the nanogels (Scheme 1c). In this we that the chemical structure of the may functional properties of polyelectrolyte complexes and In this the between weak and strong ionic groups is relevant. By way of example, we PDMAEMA with they are that PDMAEMA the cationic whereas has a strong cationic The differences show up in (a) swelling (Figure which is pH for PDMAEMA but not so for (b) (Figure which is again pH for PDMAEMA but not so for For the and anionic nanogels, similar were ( Supporting Information Figures and in chemical structure also the of This is by differences in intensity of the an fluorescent up in different nanogels, namely PDMAEMA and The first three of these have between the and the charged and but the one a These differences with weak from the first three strength but very strength from the one (Figure Figure | (a) Hydrodynamic (b) of PDMAEMA and nanogels as a function of pH. (c) of an by nanogels from monomers with various chemical (d) in of product concentration as a function of time, of free and in PDMAEMA nanogel. (e) of over time in the presence of [email protected] nanogel. Nanogel synthesis was carried out at pH 6.5, charge mixing ratio 1∶1, cross-linker 10%, and 20 mM NaCl. Download figure Download PowerPoint We finally present two of the use of polyelectrolyte nanogels as soft catalytic nanoreactors. is a with an point of which can be up well in cationic nanogels, in particular the We have at pH and mM and that under these the loading is or molecules per nanogel particle can be in Supporting Information Figure We investigated the of as a typical for the catalytic of the The is that displays a more than higher as with the free (Figure the of from the PDMAEMA nanogel environment to of have been in PIC micelles which hold similar complexes in the For in the such as or polymer at the of the Our example concerns an namely nanoparticles, in cationic PDMAEMA nanogel. The groups of PDMAEMA may such as or multiple To we prepared in where the was first introduced into the PDMAEMA nanogel followed by a have shown that this method leads to Indeed, TEM images show that the of nm) is in with the nanogels ( Supporting Information Figure the between the nanogels contains hardly any free nanoparticles, indicating that the must be with the nanogels. The particles perform well as for the of to by (Figure In we present a to prepare polyelectrolyte nanogels by which aqueous free-radical polymerization of ionic monomers with cross-linker, in the presence of an oppositely charged polyion-neutral diblock copolymer as template. This leads to well PIC micelles which dissociate upon increasing salt concentration, a of the template diblock copolymer and the newly formed nanogel an appropriate separation we obtain pure nanogels with well-controlled sizes and