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Dynamic Imine Chemistry at Complex Double Emulsion Interfaces Dynamic Imine Chemistry at Complex Double Emulsion Interfaces The MIT Faculty has made this article openly available. Please share how this access benefits you. Your story matters. Citation Zentner, Cassandra A. et al. "Dynamic Imine Chemistry at Complex Double Emulsion Interfaces." Journal of the American Chemical Society 141, 45 (November 2019): 18048–18055 © 2019 American Chemical Society As Published http://dx.doi.org/10.1021/jacs.9b06852 Publisher American Chemical Society (ACS) Version Author's final manuscript Citable link https://hdl.handle.net/1721.1/128146 Terms of Use Article is made available in accordance with the publisher's policy and may be subject to US copyright law. Please refer to the publisher's site for terms of use. Page 1 of 10 Journal of the American Chemical Society 1 2 3 4 5 6 7 Dynamic Imine Chemistry at Complex Double Emulsion Interfaces. 8 † ‡ ‡ † 9 Cassandra A. Zentner, Francesca Anson, S. Thayumanavan,* and Timothy M. Swager* 10 †Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States 11 12 ‡Department of Chemistry, University of Massachusetts – Amherst, Amherst, Massachusetts 01003, United states 13 14 ABSTRACT: Interfacial chemistry provides an opportunity to control dynamic materials. By harnessing the dynamic 15 covalent nature of imine bonds, emulsions are generated in situ, predictably manipulated, and ultimately destroyed along 16 liquid-liquid and emulsion-solid interfaces through simply perturbing imine equilibria. We report the rapid production of 17 surfactants and double emulsions through spontaneous in situ imine formation at the liquid-liquid interface of oil/water. 18 Complex double emulsions with imine surfactants are stable to neutral and basic conditions and display dynamic behavior 19 with acid catalyzed hydrolysis and imine exchange. We demonstrate the potential of in situ imine surfactant formation to 20 generate complex surfactants with biomolecules (i.e., antibodies) for biosensing applications. Furthermore, imine 21 formation at the emulsion-solid interface offers a triggered payload release mechanism. Our results illustrate how simple, 22 dynamic interfacial imine formation can translate changes in bonding to macroscopic outputs. 23 24 25 26 Introduction specifically, provides a versatile interface with biological materials through reactions with the primary amines of 27 Double emulsions are of significant importance in food lysine residues. Imine surfactants can be generated in situ 28 applications,1 drug delivery,2-6 and cell-encapsulation.7 The by interfacial reaction between an amine and an aldehyde, 29 fabrication of complex emulsions (e.g., oil-in-oil-in-water, and we demonstrate herein that single or double 30 water-in-oil-in-water) has primarily focused on emulsions can be synthesized in one-step, bypassing post- 31 microfluidic techniques and phase separation emulsification functionalization (Figure 1a). 32 emulsification, with particular interest in developing 33 responsive double emulsion systems.8-12 For example, Reversibility of imine bond formation has proved useful 34 responsive double emulsions can provide a detectable in creating materials with dynamic behaviors including in 35 optical readout when responding to external stimuli, 2D assemblies,19-20 biomimicry,21 systems chemistry,22 23 24-26 27-28 36 making them an effective sensing platform technology.13-15 drug/receptor design, micelles, and vesicles. There 37 However, responsive emulsions that rely on pre-made have been limited examples of imine surfactant stabilized 29-32 38 surfactants limit the ability to use biomolecules as emulsions. Additionally, imine bond equilibria are pH 39 surfactants at the interface for biosensing applications. sensitive, allowing for manipulation of the bond and 40 Pre-synthesized amino-acid based surfactants are a step therefore the structures of double emulsions (Figure 1b). Although there are reported examples of in situ molecular 41 towards functional bio-compatible surfactants; however, surfactant generation for single emulsions, including 42 they lack the specificity necessary for sensing 16 deprotonation of carboxylic acids and 43 applications. Although post-emulsification metallosurfactants,33-39 and Pickering single and double 44 functionalization of responsive biological components has been reported,17-18 these processes introduce complications emulsions,30-31 we are unaware of any prior reports of 45 and additional preparation steps. An additional issue with double emulsion fabricated via in situ molecular surfactant 46 pre-made molecular surfactants is that they need to have synthesis. 47 appreciable solubility in one of the emulsion phases. This In this study, we demonstrate the fabrication and 48 solubility requirement complicates the development of manipulation of complex double emulsions as a 49 more complex surfactants. Generating surfactants from consequence of in situ formation of imines at emulsion 50 precursors via in situ interfacial synthesis during emulsion interfaces. Imine formation greatly simplifies the 51 formation prevents additional manufacturing steps and fabrication of complex double emulsions, reducing a multi- 52 provides access to surfactants that are ordinarily not used step process to a single-step 5 sec method. The dynamic 53 as a result of solubility restrictions or synthetic complexity. nature of this reaction allows the manipulation of 54 Dynamic covalent bonds provide an avenue to impart emulsions interfaces through pH changes and associated 55 materials with responsive behavior. Imine formation, imine exchange. In addition, dynamic imine formation at 56 1 57 58 59 60 ACS Paragon Plus Environment Journal of the American Chemical Society Page 2 of 10 the emulsion-solid interface produce reactive wetting and the changes in the droplet morphology, thereby creating a 1 controlled payload release (Figure 1c). dynamic system. 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Figure 2. Dynamic double emulsions. a) Morphological 16 changes of dynamic double emulsions of HC (red) and FC 17 (white) with adjustments in γ balance. Top and side view 18 images of the droplets at each morphology are shown. Typical 19 FC solvents used are denser than the HC solvent, thus phases 20 align with gravity; b) Example of commercially available HC 21 and FC-surfactants. 22 23 Figure 1: Interfacial imine formation scheme. a) Non- 24 amphiphilic amine/protein and aldehyde reagents in separate In situ surfactant and phase separation immiscible phases to form an imine surfactant through 25 emulsification interfacial formation during emulsification; b) By lowering the We report a new emulsification method that uses a 26 pH of the system or through imine exchange, the amphiphilic water-soluble amine and oil-soluble aldehyde that undergo 27 properties of the imine surfactant are cleaved causing changes 28 in morphology or emulsion stability; c) Amine functionalized a rapid in situ interfacial reaction to produce imine 29 surface and aldehyde reagent in an emulsion to form an imine surfactants (Figure 3). To achieve exclusive interfacial 30 functionalized surface and triggered payload release. imine formation, we chose the amine to be soluble only in 31 the water phase, and the aldehyde to be soluble exclusively 32 Results and Discussion in only the HC or the FC oil phases. Additionally, the amine 33 Phase separation and dynamic double emulsions and aldehyde reagents were chosen to be non-amphiphilic to prevent inherent surfactant character from generating 34 A facile fabrication of double emulsions is enabled by emulsions without imine formation. As a result, stable 35 thermally induced phase separation.10 In this method, the double emulsions require imine formation via the in situ 36 dispersed phase, comprising two immiscible oils (e.g., method. 37 hydrocarbon (HC) and fluorocarbon (FC) oils), is heated 38 above their upper critical temperature. The homogenous 39 (single phase) mixture of HC and FC oils is then emulsified 40 in an aqueous continuous phase containing surfactants. 41 Upon cooling, the two oils separate to yield double 42 emulsions with uniform compositions and a structure 43 determined by the surfactants in the system. In the present 44 studies, HC-surfactants and FC-surfactants in the 45 continuous phase stabilize the HC/water (HC/W) and 46 FC/water (FC/W) interfaces, respectively. Tuning the Figure 3. In situ formation of imine surfactants and double 47 balance of the interfacial tension (γ) at the water interfaces emulsions. Aldehyde, selectively soluble in HC or FC, is 48 of HC and FC (γHC/W and γFC/W, respectively) using HC and dissolved in the dispersed phases. HC and FC are heated above FC-surfactants generate morphologies that range from miscibility and are added to amine-enriched continuous 49 encapsulated to Janus droplets (Figure 2). The perfect phase. Interfacial imine formation promotes surfactant 50 Janus morphology (equal hemispheres) is achieved when formation and emulsification simultaneously. 51 γ = γ When γ >> γ , a higher surface area at 52 HC/W FC/W. HC/W FC/W FC/W interface over HC/W is preferred leading to HC-in- To validate our interfacial reactions, we investigated a 53 FC-in-water (HC/FC/W) encapsulated droplets, and the 54 proof-of-concept scheme with mono-functional amine inverse is necessary to attain FC-in-HC-in-water polymer monomethoxypolyethylene glycol amine (amine 55 morphology (FC/HC/W). Adjusting the mass ratio and/or 56 5, M.W. 1000) that reacts with FC-soluble
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