Size‐Controllable Synthesis and Functionalization of Ultrafine Polymeric Nanoparticles

A facile approach to ultrafine hydrophilic polymeric nanoparticles within 10 nm is developed by anhydrous or aqueous inverse microemulsion polymerization of N,N-dimethylacrylamide. Since auto- precipitation of products occurs without any demulsifiers, the recycling of surfactants is feasible. The auto-precipitation mechanism is proposed and discussed. Functionalization of the nanoparticles is achieved by employing them as nanoreactors to form hybrids with titanium tetraisopropoxide. Nanoparticles with sizes below 10 nm exhibited remarkable properties due to their precise and ultrafine dimensions.1 To date, there are numerous reports on the studies of inorganic particles within 10 nm.2 In case of polymeric nanoparticles (PNPs), only few researches involved particles with very small sizes,3 while the most reported sizes were larger than 10 nm.4 So far, facile and economical preparation of PNPs smaller than 10 nm is still challenging. Microemulsion polymerization is an effective approach to hydrophobic PNPs below 100 nm.5 Inverse microemulsion polymerization (IMP) has been used for the synthesis of hydrophilic PNPs.6 The pioneering work can be traced to Schulman who discovered7 and coined the term microemulsion.8 In IMP system, sizes of hydrophilic PNPs could be controlled by varying water content in the inverse micelles,9 where the polymerizations occurred.10 However, the minimum of adjusted range for PNP sizes was restricted by the amount of unavoidable water when using solid monomers, e.g. acrylamide and its derivatives that have to be dissolved in water.11 In this work, we developed an anhydrous IMP process to achieve the minimum PNP sizes by using a liquid hydrophilic monomer N,N-dimethylacrylamide (DMAA). The formation of PDMAA nanoparticles via anhydrous IMP is illustrated in Figure 1(A). Ultrafine PNPs of 2.6 nm were obtained in the anhydrous system. Unlike traditional IMP, the products precipitated spontaneously without addition of any demulsifier (Figure S1). The mechanism of auto-precipitation was proposed and demonstrated. In general, the large usage of surfactant and demulsifier in traditional IMP12 is an obstacle to economical industrialization applications. The auto-precipitation in our modified IMP avoided the contamination by impurities and allowed the recycling of the surfactants, which might facilitate the large-scale preparation of PNPs. To the best of our knowledge, this represents the first report of auto-precipitation of PNPs with such a small size via IMP. Moreover, the sizes of PDMAA nanoparticles could be controlled in the range from 2.6 to 10 nm by carefully adjusting the water content in the process (Figure 1(B)). Further functionalization of the PNPs was achieved by employing them as templates or nano- reactors for the complexation with titanium tetraisopropoxide (TTIP), which resulted in the organic/inorganic hybrids. Illustration of the formation of the PDMAA nanoparticles. (A) Polymerization via anhydrous inverse microemulsion. (B) The process of size-control by adjusting water content. In order to prepare ultrafine PNPs, we modified the traditional IMP by employing liquid DMAA as the hydrophilic monomer for anhydrous polymerization (the details were described in the experimental section). In the system before polymerization, the number-average diameter of the inverse micelles was 2.0 nm, determined by dynamic light scattering (DLS) (Figure S2). It should be noted that the measured diameter exactly matched with the theoretical size of the micelles calculated by Equations 1.1 and 1.2 in the Supporting Information (SI). The detected micelles were much smaller than those reported in literatures by conventional IMP.13 White flocculent precipitates appeared in the solution after the polymerization of DMAA for a certain duration (see Figure S1). The purification of the product was done by washing with hexane for three times. The formation of PDMAA was structurally confirmed by infra-red spectrum and energy dispersive spectrum analysis on a scanning electron microscope (SEM/EDS). The details were described in the SI (Figure S3 and S4). These results demonstrated that the anhydrous IMP of DMAA was successfully performed. In order to visualize their nanostructures, the white flocculent precipitates were diluted and directly dropped onto a carbon-coated copper grid for high resolution-transmission electron microscope (HRTEM) observation. Large-sized fiber-like aggregates were observed in the HRTEM image (Figure 2). We assumed that, the produced hydrophilic PNPs aggregated with each other after polymerization due to the flocculation performance of PDMAA,14 and the aggregation led to the rearrangement and departure of the surfactants surround the PNPs. When the aggregates reached a certain size, they couldn't be stabilized by surfactants anymore and then precipitated from the nonpolar phase (n-hexane). This is the reason why we observed fiber-like aggregates in the HRTEM image. To further confirm this hypothesis, extra n-hexane was added into the system. Interestingly, addition of solvent accelerated the precipitation of products (Figure S1d and S1e). It was explained that the addition of n-hexane diluted the concentration of surfactants in the solution and thus facilitated the departure of surfactants from the surface of PNPs, which finally resulted in a faster precipitation rate. To confirm the removal of surfactant sodium bis(2-ethylhexyl) sulfosuccinate (AOT) during the auto-precipitation, the chemical compositions of AOT, purified PNPs, and auto-precipitated PNPs were characterized by IR (Figure 3). It showed that the chemical compositions of the auto-precipitated product were almost same with those of the purified PNPs. All the data demonstrated the removal of most AOT during the auto-precipitation. The schematic diagram and more detailed description were presented in the SI (Figure S5). In addition, IMPs of other liquid hydrophilic monomers, such as acrylic acid, were also investigated. However, only the PDMAA products could spontaneously precipitated from the system without any demulsifier in our modified procedure. After the removal of products by centrifugation, the remaining supernatant was assumed as polymerization system containing almost all the original surfactants which could be recycled. The detailed illustrations were described in Figure 4 and the SI. Indeed, when new monomer and initiator were added into the remaining system, fresh PDMAA products with similar sizes were received without addition of any extra surfactants. Essential requirements for traditional IMP are the large usage of surfactants and demulsifiers. Therefore, our procedure offers the feasibility of recycling of the surfactants, which would attract great attention because it is a low-cost, large-scale and environmentally friendly approach. HRTEM images of the PDMAA aggregates. The white flocculent precipitates of PDMAA particles were diluted and directly dropped onto a carbon-coated copper grid for HRTEM observation after auto-precipitation. IR spectrum of (a) surfactant AOT; (b) auto-precipitated product received by centrifugalization without further purification; (c) PNPs after purification with hexane (shown in Figure S3). The sample powder (1–2 mg) was mixed with dry KBr (200 mg) and then compacted into a tablet for IR measurement. Illustration of recycled utilization of the surfactants during inverse microemulsion polymerization. To characterize the single-particle, the aqueous dispersion of the white precipitates was treated with ultrasonication first and then placed onto a copper grid for measurement by HRTEM. Monodispersed particles were observed and their diameters were read directly from HRTEM image (Figure 5a). The number-average diameter (d) calculated from 50 particles was approximately 2.6 nm. The sizes of PNPs measured by HRTEM were slightly larger but were still consistent with those of the micelles determined by DLS before polymerization. (a) HRTEM images of PDMAA nanoparticles produced via anhydrous IMP, scale bar is 10 nm. (b) Ti-complexed nanoparticles after complexation with TTIP (d = 2.6 nm, anhydrous polymerization), scale bar is 10 nm. (c) Typical HRTEM images of Ti-complexed nanoparticles with clear crystal lattice stripe inside, scale bar is 3 nm. (d) Image showed Ti-complexed particles with crystal lattice stripe in different direction, scale bar is 4 nm; inset was the Fourier diffraction pattern of it analyzed on HRTEM. Next, we investigated the effect of water content on the IMP process, especially on the PNP sizes (Figure 1B). It's well known that the sizes of PNPs in general IMP are in accordance with those of the inverse micelles, which are determined by the mole ratio of water to surfactant w (n[water]/n[surfactant]).9, 15 Therefore, high water content would enlarge the inverse micelles to form lager inner space for polymerization (Figure 1). Experiments were performed by adjusting the water content of the process with the same surfactant content. Table 1 summarizes the number-average diameters of the PNPs prepared under different water contents (w = 0.5, 1, 5, 10, 15). When directly read from HRTEM images (Figure S6), the corresponding diameters of the PNPs were 3.0, 4.5, 6.4, 8.4 and 10.2 nm, respectively. Obviously, the PNP sizes grew with the increase of water content. These results demonstrated that tuning the PNP sizes by varying water content was feasible in our modified IMP process. Therefore, the size-controllable synthesis was realized. However, the PNP sizes could not be measured when w (n[water]/n[surfactant]) reached 20 (mol/mol) because the obtained nanoparticles appeared in various shapes (Figure S6f). When w was more than 20, the system was transformed into two phases, indicating the destruction of IMP system. Nevertheless, the PNP sizes in a particular range can be controlled by the accurate adjustment of the water content in the IMP system. The above synthesized products are pure organic nanoparticles and the functionalization of them is of particular interest in exploration of their potential properties. Since the PDMAA particles are hydrophilic, we expected that the PNPs could act as in-situ templates or nano-reactors for further complexation or loading.16 To functionalize the PNPs, titanium tetraisopropoxide (TTIP) was incorporated into the hydrophilic PDMAA particles through coordination bonds (the details were described in the experimental section). The HRTEM images in Figure 5b represented the morphology of Ti-complexed particles obtained on PNPs templates with an average diameter of 2.6 nm (Figure 5a). Obviously, compared with the original template PNPs that have little electron contrast, the Ti-complexed particles possessed the same size and shape but exhibited a more legible outline. During the complexation, the hydrophilic PNPs served as nano-sized reactors for the sol-gel process of TTIP because the concentrated HCl solution and TTIP were incorporated into them (Figure S7). As a result, the hydrolysis and condensation of TTIP occurred within the PNPs, leading to the formation of Ti-complexed particles.16, 17 Consequently, the original PNPs were transformed into organic/inorganic hybrids. The hybrids exhibited better contrast than the original template PNPs in HRTEM images. In addition, EDS analyzed on HRTEM (HRTEM/EDS) was employed for elementary analysis of Ti-complexed nanoparticles (Figure 6) and fully demonstrated the successful complexation with TTIP. Energy Dispersive Spectrum analyzed on HRTEM of the Ti-complexed nanoparticles (bar of the HRTEM image is 30 nm). For further confirmation, larger PNPs (d = 6.4 nm) were used as templates as well for the complexation with TTIP. As shown in Figure S8, sizes of the complexes matched well with those of their templates. Moreover, crystal lattice stripe were observed clearly in each Ti-complexed particle (Figure 5c). Fourier diffraction pattern analyzed on HRTEM (Figure 5d) demonstrated the good orientation of the Ti-complexed particles, indicating possible crystallization. The crystallization was further supported by X-ray diffraction (XRD) and X-ray photoelectron spectrum (XPS) studies (Figure S9 and S10). Thus, the functionalization of different sized PNPs was achieved. It represents a novel technique to realize the size-controllable synthesis of ultrafine organic/inorganic hybrids. In summary, ultrafine hydrophilic PNPs was achieved by developed IMP of DMAA. The produced hydrophilic PDMAA nanoparticles aggregated with each other and then auto-precipitated from the nonpolar phase. Since demulsifier was not needed in the system, the recycling of the surfactant was feasible. Our procedure represents the first example of auto-precipitation of products via IMP. Moreover, size-controlling of the PNPs within 2–10 nm was realized by varying the water content. Further functionalization of the PNPs was achieved by employing them as templates or nano-reactors to form complexes with TTIP, thus resulting in ultrafine organic/inorganic hybrids. The size-controllable synthesis of ultrafine PNPs and their functionalization open a new avenue for the inverse microemulsion system. Future work on the biological application of the modified nanoparticles in gene delivery is in progress. General Procedure: The IMP of DMAA emulsified by sodium bis (2-ethylhexyl) sulfosuccinate (AOT) was carried out in n-hexane by using N,N'-methylenebisacrylamide (MBA) as the cross-linking agent, with redox initiated system of ammonium peroxodisulfate (APS)/N,N,N′,N′-tetramethyl-ethylenediamine (TMEDA). Inverse Microemulsion Polymerization: AOT (5.6 g, 12.6 mmol) was dissolved in n-hexane (20 mL). Then DMAA (557 mg, 5.6 mmol) and MBA (45 mg, 0.3 mmol) were added to the solution, where an anhydrous inverse microemulsion system formed. When extra water was added into the system, an aqueous inverse microemulsion was applied. After well dispersed by ultrasound, the mixture was then stirred at 25 °C for 30 min under N2 atmosphere. Subsequently, APS (100 mg/mL, 10 μL) and TMEDA (10 μL) were added to initiate the polymerization. The polymerization was carried out at 35 °C for 12 h. Finally extra solvent (n-hexane) was added to accelerate the precipitation of the product particles. The white solid product was collected by centrifugation, rinsed with hexane for several times and dried under vacuum. TTIP Complexation: 3 mg PDMAA was dissolved in 4 g THF, TTIP (35 mg, 0.12 mmol) and HCl (37%, 40 mg) were added into the solution within 3 min, and then the mixture was stirred at room temperature for 1 h. Supporting Information is available from the Wiley Online Library or from the author. This work was financially supported by the “National Science Foundation of China” (21174012, 51103008, 51221002) and the “New Century Excellent Talents Award Program from Ministry of Education of China (NCET-10-0215)” and the Doctoral Program of Higher Education Research Fund (20100010120006, 20120010110008) and Chinese Universities Scientific Fund (ZZ1208). As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re-organized for online delivery, but are not copy-edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.