Regular Article
Synthesis of polymer nanoparticles via electrohydrodynamic emulsification-mediated self-assembly

https://doi.org/10.1016/j.jcis.2020.10.108Get rights and content

Abstract

Hypothesis

Electrospray can rapidly produce fine, organic solvent-in-water emulsions in the absence of surfactant via electrohydrodynamic emulsification (EE), a reverse configuration of traditional electrospray. This paper investigates whether EE can produce high-quality nanocomposites comprised of block co-polymers and organic nanoparticles (NPs) via the interfacial instability (IS) self-assembly method. Surfactant-free approaches may increase encapsulation efficiency and product uniformity, process speed, and ease of downstream product purification.

Experiments

All particles were produced using EE-mediated self-assembly (SA) (EE-SA). Particles were produced using poly(lactic-co-glycolic acid) (PLGA) polymers as proof of concept. Then, block copolymer (BCP) micelles were synthesized from polystyrene-block-poly(ethylene oxide) (PS-b-PEO) (PS 9.5 kDa:PEO 18.0 kDa) in the presence and absence of superparamagnetic iron oxide nanoparticles (SPIONs) or quantum dots (QDs). Encapsulant concentration was varied, and the effect of encapsulant NP ligands on final particle size was investigated.

Findings

EE-SA generated both pure polymer NPs and nanocomposites containing SPIONs and QDs. PLGA particles spanned from sub- to super-micron sizes. PS-b-PEO NPs and nanocomposites were highly monodisperse, and more highly loaded than those made via a conventional, surfactant-rich IS process. Free ligands decreased the size of pure BCP particles. Increasing encapsulant levels led to a morphological transition from spherical to worm-like to densely loaded structures.

Introduction

Nanoparticles (NPs), including quantum dots (QDs) and superparamagnetic iron oxide nanoparticles (SPIONs), display unique properties resulting from their small size. In the past few decades, substantial research has demonstrated the usefulness of NPs in biological and biomedical applications [1]. In particular, NPs have been used in fluorescence imaging, multiplexed imaging, magnetic resonance imaging, magnetic hyperthermia, photodynamic therapy, and magnetically-induced cell manipulation and separation [2], [3], [4], [5], [6], [7], [8], [9]. Many research groups [9], [10], [11], [12], including our own [13], have demonstrated synthesis routes that produce high quality NPs, but many of these methods are performed in organic solvent. For biological applications, these NPs must be transferred from the organic to the aqueous phase.

One of the most widely used approaches for aqueous phase transfer is the formation of nanocomposites via encapsulation or coating procedures [14], [15], [16], [17], [18]. Such nanocomposites can incorporate multiple NPs in a larger nanostructure, while maintaining the desired properties of the original NPs. For example, some of the earliest QD imaging work described QDs encapsulated in silica, which provides good water solubility when the surface is functionalized with either thiol or amine groups [19]. More recently, we [16], [17], [18] and others [14], [20], [21] have used lipid-polymers or amphiphilic block copolymers (BCPs) to generate micelles encapsulating multiple NPs, including both QDs and SPIONs, to achieve dual fluorescent-magnetic functionality.

BCP nanocomposites can be synthesized by a number of different approaches. Two important methods are the water addition and interfacial instability (IS) processes [22]. In water addition, the polymer and cargo (if any) are dissolved in a water-miscible organic solvent such as tetrahydrofuran (THF), and the addition of an anti-solvent (i.e., water) induces polymer nanocomposite formation [23]. In contrast, the interfacial instability method (IS) begins with emulsification of a volatile, water-immiscible organic solvent, containing the BCP and cargo, into an aqueous phase containing surfactant. The presence of surfactant molecules at the oil-water droplet interface reduces the interfacial tension between phases. As the volatile organic solvent evaporates, the droplets shrink, and the surfactant concentration increases, leading to interfacial instability and ejection of smaller droplets containing BCPs (and cargo). Eventually, the organic solvent evaporates sufficiently for BCPs to obtain their critical micelle concentration, inducing BCP self-assembly [22]. These approaches are, however, still largely practiced at the bench scale. The challenge remains to develop scalable synthesis platforms that produce sufficient high-quality nanocomposites to meet the demands of biological applications.

One technique for high yield production of nanocomposites is the flash nanoprecipitation (FNP) process developed by Prud’homme and co-workers, a scalable version of the water addition process [24]. In FNP, a jet mixer (i.e., confined impingement jet mixer, multi-inlet vortex mixer) promotes nanoprecipitation of organic actives and BCPs that are dissolved in a water-miscible solvent by rapid mixing with water [24]. FNP is a kinetically-driven process in which nanostructure size and uniformity depends on the degree of mixing. In particular, to ensure homogeneous nucleation kinetics, the mixing time (τmix) should be shorter than both the encapsulant nucleation and growth time (τng) and the BCP aggregation time (τagg). When τngτagg, encapsulation is highly effective (nearly 99.9%) [25]. Under the right conditions, FNP can produce high quality, monodisperse products at high throughput [25].

Alternatively, we demonstrated an electrospray-assisted interfacial instability process (Aero-IS) as a scalable approach for producing polymer nanocomposites [26], [27]. Aero-IS uses a coaxial electrospray configuration with an inner flow of water-immiscible, organic solvent containing the nanocomposite constituents and an outer flow of surfactant-rich aqueous solution. Compound spray droplets ejected by the electrospray are captured in an aqueous phase, forming a fine emulsion from which micellar nanocomposites are generated via the IS process. Despite our success in translating bench scale IS processes to a semi-continuous platform, Aero-IS has a number of disadvantages. In particular, as in most IS processes, large amounts of surfactant are used to stabilize the emulsion; the ratio of surfactant to BCP can be on the order of 100:1. Thus, surfactant can compete with BCPs for hydrophobic cargo encapsulation. In addition, this approach presents significant downstream purification challenges [28]. Finally, the Aero-IS process involves NP aerosolization, albeit within micron-sized droplets, and as such, raises environmental, health, and safety concerns (EHS) [29].

Most electrospray applications, including Aero-IS, spray conductive liquids into non-conductive media, such as air or insulating oils. This electrospray configuration, namely conductive-in-nonconductive electrospray, has been used to encapsulate inorganic nanoparticles [27], therapeutics [30], as well as living cells [31], [32]. Although more challenging, the opposite configuration, i.e., spraying a non-conductive phase into a more conductive continuous medium, has also been demonstrated [33], [34], [35], [36], [37]. Specifically, Sato et al. [33], [34], [35] and Tsouris et al. [36], [37] describe electrohydrodynamic (EHD) atomization of non-conductive fluids (i.e., air, carbon tetrachloride) into more conductive fluids (i.e., alcohol solution, distilled water) [33], [35], [38]. Based on their approach, we recently developed a technique, EHD mixing-mediated nanoprecipitation, (EM-NP) [39] to rapidly disperse water-miscible solvents into water. In particular, spraying of tetrahydrofuran (THF) solutions containing BCP and NPs into water induced hydrophobic component precipitation and BCP/NP nanocomposite formation [39]. EM-NP is clearly related to FNP processes [24], since both synthesis routes are based on the batch-scale water addition method [40]. The primary distinction between FNP and EM-NP is that, in EM-NP, mixing is driven by EHD flow rather than high velocity liquid stream interactions in a confined geometry [41]. Both EM-NP and FNP require use of water-miscible organic phases, and the ability to encapsulate hydrophobic cargo depends on the characteristic times associated with mixing, BCP aggregation, and cargo nucleation and growth. Thus, solubility limitations and time scales may restrict the nanocomposites that can be produced using this route.

To address these limitations, here, we extended the EM-NP platform to develop a novel electrospray configuration, EHD emulsification-mediated self-assembly (EE-SA) for high throughput manufacturing of BCP nanocomposites using immiscible solvents. EHD-emulsification (EE) generates a fine emulsion of water-immiscible organic solution (i.e., CHCl3) containing BCPs and hydrophobic encapsulants in water. BCP self-assembly in the emulsified solution then proceeds via the IS process to yield micellar nanocomposites. Importantly, in contrast to traditional IS processes and Aero-IS, aqueous phase surfactants are not required to decrease stream surface tension. In fact, EE emulsification preferably occurs in surfactant-free water to avoid introduction of conductive contaminants (i.e., residual salts generally present in commercial surfactants) that rapidly lead to short circuits that prevent spray formation. This EE-SA platform was used to synthesize polylactic-co-glycolic acid (PLGA) solid nanoparticles (SNPs), a common product of standard electrospray systems [42], as well as BCP micelles and micellar nanocomposites encapsulating either QDs or SPIONs. For each of these materials, we characterized the resultant particle size distribution parameters, morphology, degree of NP encapsulation, and demonstrated the subtle effects that stabilizing ligands associated with inorganic NPs can have on BCP nanocomposites. These results establish EE-SA as a scalable, surfactant-free, oil-in-water emulsification platform for micelle and nanocomposite formation.

Section snippets

Materials

Poly(DL-lactide-co-glycolide) with a molecular weight of 50-70 kDa was purchased from Lactel Absorbable Polymers. Poly(styrene-b-ethylene oxide) block co-polymers (PS 9.5 kDa:PEO 18.0 kDa) with different (–OH, –COOH, and –NH2) terminal groups were purchased from Polymer Source Inc. (Montreal, Canada). Hydrophobic SPIONs in chloroform (nominal size: 5 nm) and powdered, hydrophobic CdSe/ZnS QDs (nominal size: 5 nm; λem = 550 nm or 600 nm) were purchased from Ocean Nanotech (San Diego, CA).

Electrohydrodynamic emulsification (EE) design principles and feasibility

In standard implementations of electrospray, a conductive liquid is sprayed into a non-conductive fluid medium, typically gas or an insulating oil, by applying a high voltage between the liquid being sprayed and a counter electrode [46], [47], [48]. Within a certain range of flow rates and voltages, the electric stresses at the interface between the two fluids balance interfacial tension, deforming the shape of the fluid interface at the electrospray needle exit into a Taylor cone [49]. Fluid

Conclusion

Here, a new process, electrohydrodynamic-emulsification mediated self-assembly (EE-SA), is presented for polymer NP and composite synthesis, including synthesis of PLGA SNPs, micelles, and micelles encapsulating QDs or SPIONs (i.e., MultiDots or SuperMags). These materials have potential for broad application in energy, biomedical, and consumer product fields. In this process, electrospray is employed in a reverse configuration (N-in-C) in which the organic phase is sprayed into a continuous,

CRediT authorship contribution statement

Kil Ho Lee: Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Validation, Visualization, Writing - review & editing. Megan Ireland: Investigation, Validation. Brandon L. Miller: Conceptualization. Barbara E. Wyslouzil: Conceptualization, Formal analysis, Funding acquisition, Project administration, Resources, Supervision, Visualization, Writing - review & editing. Jessica O. Winter: Conceptualization, Funding acquisition, Project administration, Resources,

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

Funding: This research was funded by the National Science Foundation under grant CMMI-1344567. Parts of this work were carried out in the Characterization Facility, University of Minnesota, a member of the NSF-funded Materials Research Facilities Network (www.mrfn.org) via the MRSEC program. This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under

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