Emulsion‐Templated Poly(N‐Isopropylacrylamide) Shells Formed by Thermo‐Enhanced Interfacial Complexation

Abstract

The encapsulation of fragile biomacromolecules is crucial in many biotechnological applications but remains challenging. Interfacial complexation (IC) in water-in-oil emulsions proves to be an efficient process for the formation of protective polymer layers at the surface of capsuleprecursor water droplets. In this work, the enhancement of conventional IC by introducing thermoresponsive poly(N-isopropylacrylamide) (PNIPAM) strands in the interfacial polymer layer is described. Surfactant-polymer IC is implemented in water-in-fluorocarbon oil emulsions between a watersoluble poly(L-lysine)-g-poly(N-isopropylacrylamide) cationic copolymer (PLL-g-PNIPAM) and an oil-soluble anionic surfactant. Fluorescence imaging demonstrates that the thermal collapse transition of PNIPAM strands, triggered by gentle heating, induces an enrichment of the polymer layer initially formed by IC. Spontaneous co-precipitation of nanoparticles initially dispersed in the aqueous cores-with no specific treatment-is also achieved upon PNIPAM transition. This process is leveraged to irreversibly segregate these nanoparticles in the interfacial polymer layer, resulting in gel-like mixed shells. Thermo-enhancement of conventional IC is thus a promising approach for the straightforward formation, strengthening, and functionalization of capsule shells. As implemented in mild conditions, thermo-enhanced IC is additionally compatible with the encapsulation of proteins, opening new opportunities for delivery systems of biomacromolecules.

UCST-Type Polymer Capsules Formed by Interfacial Complexation

“…This effect may be exploited to prepare shells doped with nanoheaters such as gold particles. The capture of nanoparticles in interfacial adlayers was previously reported with PNIPAM-grafted PLL and ascribed to the general propensity of PLL derivatives to adsorb onto most (anionic) particles, combined with the thermal phase transition of the PNIPAM macrografts. As a proof of concept, 50 nm NeutrAvidin-coated fluorescent nanoparticles (fluoNPs, materials in SI) were added in the aqueous phase containing PLL- g -PAAMAN prior to emulsification at 60 °C.…”

“…We assessed, in addition, the capture of nanoparticles into the shell layer as promising features for encapsulation of nanoheaters and the future design of remotely controlled delivery systems. This approach extends a method previously implemented in our group for the design of LCST-type capsules based on poly­( N -isopropylacrylamide) (PNIPAM) . Here, the polymer made of PAAMAN chains grafted on a cationic poly­(lysine) (PLL) was dissolved in an aqueous solution at high temperature and emulsified in a fluorinated oil containing the anionic surfactant Krytox.…”

“…First, the fluorescence of 8-anilinonaphtalene-1-sulfonic acid (ANS) was used as an extrinsic polarity/hydrophobicity probe . In the absence of PAAMAN, ANS fluorescence was homogeneously distributed in the droplet (a reference imaging of interface covered by the hydrophilic PLL- g -PEG is shown in ref ), suggesting the absence of an interaction of the probe with interfacial complexes. Figure A shows confocal micrographs of a representative droplet containing PLL- g -PAAMAN (nonfluorescent) and ANS, below and above the cloud point.…”

“…This approach extends a method previously implemented in our group for the design of LCST-type capsules based on poly(N-isopropylacrylamide) (PNIPAM). 37 Here, the polymer made of PAAMAN chains grafted on a cationic poly(lysine) (PLL) was dissolved in an aqueous solution at high temperature and emulsified in a fluorinated oil containing the anionic surfactant Krytox. A mixed surfactant/polymer layer spontaneously formed at the surface of water droplets (Figure 1).…”

Quantitative Microscale Thermometry in Droplets Loaded with Gold Nanoparticles

“…Interestingly, the effective dn/ dT became closer to that of pure water and showed less sensitivity to temperature as the size of the droplet decreased, likely because smaller droplets (with higher surface/volume ratio) contained significantly lower bulk concentrations of PLL-g-PNIPAM due to the sequestration/immobilization of most of the polymer in the shell formed at the periphery of the droplets. 38 Eventually, CGM was used to estimate not only the temperature rise but also the effective dn/dT profile within the droplet cores. Using the appropriate values of heating power and dn/dT in the simulations, the experimental δl profiles could be perfectly fitted with numerical profiles as shown in Figure 6b for the droplet under study in Figure 1…”