We describe cobalt
cross-linked redox-responsive 4-arm histidine-modified
PEG (4A-PEG-His) hydrogels, which can be switched from self-healing
viscoelastic liquids to form stable elastic solids through a simple
oxidation step from Co2+ to Co3+. The dramatic
change in gel properties is quantified in rheological measurements
and is associated with the altered ligand exchange rate of the cross-linking
cobalt ions. While Co2+ forms kinetically labile coordination
bonds with low thermodynamic stability, Co3+ forms kinetically
inert and highly stable coordination bonds. Unlike the Co2+ cross-linked hydrogels, the Co3+ cross-linked hydrogels
do not dissolve in buffer and swell overtime, where they remain intact
longer with increasing gel connectivity, increasing polymer concentration
and decreasing temperature. Remarkably, these gels can even resist
the strong chelator EDTA and withstand both low and high pH due to
the low ligand exchange rates in the primary coordination sphere.
Overall, the Co2+/3+ redox pair provides an attractive
platform to produce redox-responsive materials with big deviations
in mechanical and chemical properties.
Compartmentalization
and selective transport of molecular species are key aspects
of chemical transformations inside the cell. In an artificial setting,
the immobilization of a wide range of enzymes onto surfaces is commonly
used for controlling their functionality but such approaches can restrict
their efficacy and expose them to degrading environmental conditions,
thus reducing their activity. Here, we employ an approach based on
droplet microfluidics to generate enzyme-containing microparticles
that feature an inorganic silica shell that forms a semipermeable
barrier. We show that this porous shell permits selective diffusion
of the substrate and product while protecting the enzymes from degradation
by proteinases and maintaining their functionality over multiple reaction
cycles. We illustrate the power of this approach by synthesizing microparticles
that can be employed to detect glucose levels through simultaneous
encapsulation of two distinct enzymes that form a controlled reaction
cascade. These results demonstrate a robust, accessible, and modular
approach for the formation of microparticles containing active but
protected enzymes for molecular sensing applications and potential
novel diagnostic platforms.
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