Electrostatic loading and photoredox-based release of molecular cargo from oligoviologen-crosslinked microparticles

Abstract

Although on-demand cargo release has been demonstrated in a wide range of microparticle platforms, many existing methods lack specific loading interactions and/or undergo permanent damage to the microparticle to release the cargo. Here, we report a novel method for electrostatically loading negatively charged molecular cargo in oligoviologen-crosslinked microparticles, wherein the cargo can be released upon activation by visible light. A water-in-oil (W/O) emulsion polymerization method was used to fabricate narrowly dispersed microparticles crosslinked by a dicationic viologen-based dimer and a poly(ethylene glycol) diacrylate. A zinc-tetraphenyl porphyrin photocatalyst was also polymerized into the microparticle and used to photochemically reduce the viologen subunits to their monoradical cations through a visible-light-mediated photoredox mechanism with triethanolamine (TEOA) as a sacrificial reductant. The microparticles were characterized by microscopy methods revealing uniform, spherical microparticles 481 ± 20.9 nm in diameter. Negatively charged molecular cargo (methyl orange, MO) was electrostatically loaded into the microparticles through counteranion metathesis. Upon irradiation with blue (450 nm) light, the photo-reduced viologen crosslinker subunits lose positive charges, resulting in release of the anionic MO cargo. Controlled release of the dye, as tracked by absorption spectroscopy, was observed over time, yielding release of up to 40% of the cargo in 48 h and 60% in 120 h in single dynamic dialysis experiment. However, full release of cargo was achieved upon transferring the microparticles to a fresh TEOA solution after the initial 120 h period.

Figure 1 –

Photoinduced electron transfer (PET) within oligoviologen-crosslinked microparticles containing zinc porphyrin photocatalysts results in a reduction and reoxidation mechanism that facilitates release of electrostatically-bound dye molecules in response to blue light (450 nm).

Figure 2 –

a) Microparticle ingredients in their respective oil or water phase, b) Microparticle emulsion and polymerization scheme initiated by ammonium persulfate (APS), c) (Left) Scanning Electron Microscopy (SEM) image of as-synthesized microparticles (2,000x magnification), (Right) SEM image (10,000x magnification) of red box region shown in the 2000x image.

Figure 3 –

a) Loading of MO dye onto 2V-St•4Cl crosslinker. b) ¹H NMR of as-synthesized dimer crosslinker, methyl orange dye, and loaded small molecule after metathesis. ¹H NMR, (CD₃)₂SO,500MHz. *Trace NH₄PF₆ from synthesis of 2V-St•4Cl washed away during loading of MO.

Figure 4 –

a) Illustration showing counteranion metathesis as a method for loading methyl orange (MO) into MP-1. b) Optical microscopy images of microparticles before (left) and after (right) loading of MO. c) Scanning electron microscopy (SEM) images of microparticles before (left) and after (right) loading of MO. 10,000x magnification (top) with a zoomed image of the red box at 65,000x magnification (bottom).

Figure 5 –

Photoinitiated release of methyl orange over time. a) MO release scheme and pictured setup of dialysis tubing containing loaded microparticles before, during, and after irradiation with 450 nm light. b) Percent release of MO from microparticle samples and controls over 120 h.