Liquid Crystal-Infused Porous Polymer Surfaces: A “Slippery” Soft Material Platform for the Naked-Eye Detection and Discrimination of Amphiphilic Species

We
report the design and characterization of liquid crystal (LC)-infused
porous polymer membranes that can detect and report on the presence
of natural and synthetic amphiphiles in aqueous solution. We demonstrate
that thermotropic LCs can be infused into nanoporous polymer membranes
to yield LC-infused surfaces that exhibit slippery behaviors in contact
with a range of aqueous fluids. In contrast to conventional liquid-infused
surfaces (LIS) or slippery liquid-infused porous surfaces (SLIPS)
prepared using isotropic oils, aqueous solutions slide over the surfaces
of these LC-infused materials at speeds that depend strongly upon
the composition of the fluid, including the presence, concentration,
or structure of a dissolved surfactant. In general, the sliding times
of aqueous droplets on these LC-infused surfaces increase significantly
(e.g., from times on the order of seconds to times on the order of
minutes) with increasing amphiphile concentration, allowing sliding
times to be used to estimate the concentration of the amphiphile.
Additional experiments revealed other intrinsic and extrinsic variables
or parameters that can be used to further manipulate droplet sliding
times and discriminate among amphiphiles of similar structure. Our
results are consistent with a physical picture that involves reversible
changes in the interfacial orientation of anisotropic LCs mediated
by the interfacial adsorption of amphiphiles. These materials thus
permit facile “naked-eye” detection and discrimination
of amphiphiles in aqueous samples using equipment no more sophisticated
than a stopwatch. We demonstrate the potential utility of these LC-infused
surfaces for the unaided, naked-eye detection and monitoring of amphiphilic
biotoxins in small droplets of fluid extracted directly from cultures
of two common bacterial pathogens (Pseudomonas aeruginosa and Staphylococcus aureus). The ability
to translate molecular interactions at aqueous/LC interfaces into
large and readily observed changes in the sliding times of small aqueous
droplets on surfaces could open the door to new applications for antifouling,
liquid-infused materials in the context of environmental sensing and
other fundamental and applied areas.