Polymeric microparticles or microcapsules have been widely used as an effective matrix to fabricate various advanced functional materials or devices for energetic, environmental, catalytic, and biomedical applications (Chen et al., 2019; Schofield et al., 2020; Özkayalar et al., 2020; Jia et al., 2022; Li et al., 2022). Basically, the fine morphology and surface reactivity of these microparticles or microcapsules are of great importance for their practical applications (Ahangaran et al., 2019). For instance, due to their higher specific surface area, the mesoporous polymeric microparticles exhibited better adsorption and catalytic performance than their counterparts of macroporous and solid microparticles (Peng et al., 2018; Zheng et al., 2019). In addition, the immobilization of bio-reactive components (antibody, enzyme, and nucleotides) on polymeric microparticles is an indispensable step for their application in various biomedical fields (Wang et al., 2014; Daly et al., 2020; Jo and Lee, 2020; Tobias et al., 2021). Although the polymeric microparticles prepared by the classical emulsion or suspension polymerization normally exhibit uniform size distribution and tunable morphology, it is still quite challenging to prepare polymer-inorganic hybrid microparticles via the conventional radical chain polymerization as the involved reactive species are prone to be quenched by metal ions or salts from inorganic precursors.

The self-assembly of amphiphilic block copolymers (BCP) is regarded as a preferred methodology to prepare polymeric-inorganic hybrid microparticles; the fine morphology and surface reactivity of the final product can be readily modulated via tuning of interface reaction between BCP matrix and inorganic precursors or nanoparticles during the self-assembly process (Yang et al., 2016; Yan et al., 2019; Jia et al., 2020). Among various self-assembly protocols, the recently developed emulsion confinement self-assembly is emerging as a powerful methodology to fabricate functional microparticles, because the introduced emulsion droplet interface would serve as an additional parameter to tune the self-assembly kinetics (Li et al., 2015; Lee et al., 2019; Shin et al., 2020). Therefore, ranges of polymeric micro/nano-structures with special morphology that are hardly obtained via conventional solvent exchange–induced self-assembly have been prepared via the emulsion confinement self-assembly. However, the wider application of emulsion confinement self-assembly, especially in large-scale production, is still limited by the tedious chemical synthesis of involved block copolymers. In addition, although there are plenty of published works dealing with the morphology modulation of polymeric microparticles by tuning block copolymer backbones structures, surfactants compositions, and concentrations (Deng et al., 2016; Shin et al., 2018; Ku et al., 2019; Hu et al., 2021; Xu et al., 2021), the research work involving the post-modification and further application of these polymeric microparticles is still quite limited.

Herein, we discovered that the polymeric microparticles with tunable surface morphology could be obtained from the emulsion confinement self-assembly of a homopolymer, which was super-engineering polyarylene ether containing pendent nitrile and carboxyl (PAE-NC). It should be observed that the synthesis of PAE-NC can be easily scaled up via industrial polycondensation polymerization. Furthermore, we found that the polymeric microparticles obtained from emulsion self-assembly of PAE-NC exhibited exterior carboxyl groups, which can be employed as reactive sites for immobilization of molecular recognition agents to prepare functional beads. More interestingly, we also found that the hydrophobic fluorescent quantum dots can be firmly embedded in the PAE-NC microcapsules, leading to the water-dispersible fluorescent beads. Considering the scalable synthesis of PAE-NC matrix, post-functionalization capacity, and stable encapsulation of susceptive functional inorganic nanoparticles, the current work would open a new way for the fabrication of advanced polymeric-inorganic hybrid microparticles.

Super-engineering thermoplastics are a kind of high-performance polymers that exhibit good thermal stability and mechanical properties due to their aromatic backbone structure as well as strong intermolecular entanglements (Park et al., 2019; Wang and Zhang, 2019; Lin et al., 2022). However, the post-modification of these high-performance polymers is quite a challenge but is indispensable for many applied scenarios (Guan et al., 2019; Boydston et al., 2020; Lu et al., 2021). For instance, the interfacial compatibility between polymer matrix and inorganic nanoparticles plays a decisive role in determining the optical, magnetic, and electric functionalities of these polymeric nanocomposites (Jia et al., 2016; Liu et al., 2022; Zhang et al., 2022), which normally requires the appropriate interfacial modification on either polymer matrix or inorganic nanoparticles. In this sense, we have designed a reactive super-engineering polymer that contained pendent carboxyl and cyano groups, which exhibited versatile reactivity toward many chemical groups of inorganic nanoparticles. Specifically, the PAE-NC was synthesized using DFBN and PPL as monomers via the nucleophilic substitution polycondensation that can be easily scaled up according to Figure 1A. In addition, we attempted to transfer the pendent nitrile of PAE-NC into reactive groups, since the alkaline hydrolysis of nitrile leads to the generation of various reactive groups (Eloy and Lenaers, 1962; Yanaranop et al., 2016; He et al., 2021a). According to the 1H NMR spectra in Figure 1B, the PAE-NC was successfully synthesized and its chemical structure was steadily modulated along with hydrolysis treatment. Moreover, the characteristic carbonyl peak of as-synthesized PAE-NC was shifted from 1,714 cm⁻¹ to around 1,696–1,699 cm⁻¹ after alkaline hydrolysis according to FTIR spectra shown in Figure 1C, which could be due to the alternation of intermolecular hydrogen bonding by hydrolysis treatment. Moreover, it should be noted that the vibration peak of the nitrile group at 2,230 cm⁻¹ still presented even after hydrolysis for 8 h, implying that it was quite challenging to realize the complete transformation of nitriles by the heterogeneous alkaline treatment, and similar results were also reported by another group (Sun et al., 2022). Anyway, the hydrolysis leads to partial nitriles of PAE-NC transformation into carboxyl groups, which would provide additional reactive sites for post-functionalization or further surface modification.

Next, the thermal properties of as-synthesized PAE-NC and hydrolyzed PAE-NC-Hx were characterized. According to Figure 2A, the glass transition temperature of PAE-NC was gradually decreased along with the increase in hydrolysis time, which was supposed to be attributed to the attenuated hydrogen bonding between nitrile and carboxyl during prolonged hydrolysis (Le Questel et al., 2000). Moreover, the TGA results in Figure 2B also exhibited decreased thermal stability with increasing hydrolysis treatment, as the decomposition temperature at 5% was decreased from 440.2°C for as-synthesized PAE-NC to 387°C after hydrolysis for 8 h. Despite slightly decreased glass transition temperature and decomposition temperature, the hydrolyzed PAE derivatives still exhibited good stability up to 350°C, which was comparable to many commercially available super-engineering polymers including polyether ether ketone (PEEK), polyethersulfone (PES), and polyphenylene sulfide (PPS).

Furthermore, the surface morphology of obtained polymeric microparticles was characterized by SEM. According to Figure 3A, the polymeric microparticles with invaginate pit morphology were obtained from the PAE-NC, and the polymeric microparticles of larger average size with a smoother surface (Figure 3B) or smaller pinhole morphology (Figure 3C) were detected from the sample obtained using PAE-NC-H1 and PAE-NC-H4, respectively. The average size information of prepared superparticles was calculated and listed in Table 1. Interestingly, the irregular membrane structures without any microparticles were observed from the PAE-NC-H8 sample (see Figure 3D), which should be attributed to the declined hydrophobic interaction derived from the enhanced water solubility of this sample after a long time of hydrolysis.

Since the polymer beads obtained from PAE-NC-H1 exhibited a more uniform size and smoother surface, they were selected as the optimized sample for constructing post-functionalized beads. As a proof-of-concept application, the PAE-NC-H1 polymer was first converted to pristine polymer beads via emulsion confinement self-assembly, then the surface carboxyl groups of self-assembled polymer beads were activated with an EDC agent (see details in experimental section 2.4), followed by covalent immobilization of protein A via classical amidation reaction to obtain bio-functionalized beads (abbreviated as PA beads). Due to the specific antibody affinity properties of protein A, the PA beads could serve as bio-separation scaffolds to enable antibody enrichment from complex bio-fluids. In addition, we also compared the antibody enrichment performance of as-prepared PA beads and commercially available agarose beads (AG in short) from cell lysis solution. It was clear that when compared with the commercial AG beads, our PA beads exhibited improved antibody enrichment and lower non-specific binding effect according to the Western blot assay results in Figure 4A and Coomassie blue staining results in Figure 4B, respectively.

In addition to emulsion confinement self-assembly, we also prepared the self-assembled PAE-NC structures of different morphology using classical solvent exchange protocol. More specifically, the solvent exchange–induced self-assembly of PAE-NC was conducted by rapid injection of polymer solution in good solvent to its non-solvents system. It should be observed that much smaller-sized polymeric nanoparticles were obtained by introducing PEN solution in DMF into mixed solvents of toluene and ethanol (volume ratio of 1:2) according to Figure 5A, and the average diameter was determined to be 81 ± 1.2 nm according to Figure 5B. In addition, we also found that the metal ions crosslinking with carboxyl groups of PAE-NC had an obvious influence on the surface morphology of solvent exchange–induced self-assembled structures. Larger-sized necklace-like aggregates were observed from the Zn²⁺ cross-linked sample (see Figure 5C), while much smaller nanoparticle networks were generated after adding Cu²⁺ to the self-assembly of PAE-NC according to Figure 5D. The totally different sizes of resultant particles between solvent exchange–induced self-assembly and emulsion confinement self-assembly should be attributed to the different driven forces and mechanisms.

Considering that both the as-synthesized PAE-NC and its hydrolyzed derivatives can be self-assembled into microparticles, the process could also be employed for encapsulation of vulnerable compounds or inorganic nanoparticles to preserve their functionalities. With this regard, the commercially available highly fluorescent quantum dots (QDs) of ZnCdSe@ZnS stabilized by the hydrophobic surface ligand of oleic acid were used as an example to evaluate the encapsulation capacity of PAE-NC matrix, as the typical red emission of hydrophobic QD could be easily quenched by water without appropriate surface modification (Pellegrino et al., 2004; He et al., 2021b). Figure 6 demonstrated the fluorescent emission spectra recorded from the aqueous solution of QD@PAE-NC composites. It was clear that the red emission peak at 627 nm of QD can be well-preserved after being encapsulated in PAE-NC microparticles. Moreover, the PAE-NC-H1 and PAE-NC-H4 also exhibited good encapsulation of QD, while the composites involving PAE-NC-H8 showed a limited encapsulation effect in terms of preserving QD fluorescence. Moreover, the variation of fluorescence emission from these QD@PAE samples was also confirmed by the inset photo.

The surface morphology characterization also proved that the composite beads using pristine PAE-NC (Figure 7A), PAE-NC-H1 (Figure 7B), and PAE-NC-H4 (Figure 7C) showed similar morphology, and their average diameter data were summarized in Table 1 as well. Lastly, the microparticles mixed with continuous membranes were observed from sample PAE-NC-H8 (Figure 7D). The different encapsulation capacity and surface morphology would be due to the attenuation of hydrophobic interaction between PAE-NC and surface ligand of QD along with increased hydrolysis treatment. It should be observed that the hydrophobic nanoparticles encapsulation capacity of PAE-NC was also applicable for other fluorescent quantum dots or even magnetic nanoparticles.

In this work, we have synthesized a super-engineering thermoplastic of PAE-NC containing pendent reactive groups. Furthermore, we demonstrated that both as-synthesized PAE-NC and its hydrolysis derivatives can be self-assembled into polymeric microparticles. The size and fine morphology of these polymeric microparticles can be easily modulated by using different self-assembly protocols or crosslinking metal ions. In addition, the prepared polymeric microparticles can be rendered with additional bio-separation functionality after post-biomodification with the biorecognition agent of protein A. Moreover, the emulsion confinement self-assembly of PAE-NC and its hydrolyzed derivatives can be employed to encapsulate hydrophobic QD and preserve its red fluorescent in an aqueous solution. It should be observed that although the encapsulation capacity of PAE-NC was evaluated using fluorescent QD for the moment, the protocol revealed in the current work is applicable to other hydrophobic functional nanoparticles. Considering the high thermal stability, abundant surface reactive groups, and encapsulation properties of PAE-NC microparticles, the current work would open a new way to fabricate microparticles or microcapsules to preserve the functionalization of various sensitive or hazardous components including explosive, radioactive agents.