Mechanofluorescent Polymer Brush Surfaces that Spatially Resolve Surface Solvation

Abstract

Polymer brushes, consisting of densely end-tethered polymers to a surface, can exhibit rapid and sharp conformational transitions due to specific stimuli, which offer intriguing possibilities for surface-based sensing of the stimuli. The key toward unlocking these possibilities is the development of methods to readily transduce signals from polymer conformational changes. Herein, we report on single-fluorophore integrated ultrathin (<40 nm) polymer brush surfaces that exhibit changing fluorescence properties based on polymer conformation. The basis of our methods is the change in occupied volume as the polymer brush undergoes a collapse transition, which enhances the effective concentration and aggregation of the integrated fluorophores, leading to a self-quenching of the fluorophores’ fluorescence and thereby reduced fluorescence lifetimes. By using fluorescence lifetime imaging microscopy, we reveal spatial details on polymer brush conformational transitions across complex interfaces, including at the air–water–solid interface and at the interface of immiscible liquids that solvate the surface. Furthermore, our method identifies the swelling of polymer brushes from outside of a direct droplet (i.e., the polymer phase with vapor above), which is controlled by humidity. These solvation-sensitive surfaces offer a strong potential for surface-based sensing of stimuli-induced phase transitions of polymer brushes with spatially resolved output in high resolution.

Dipolar Dispersion Forces in Water–Methanol Mixtures: Enhancement of Water Interactions upon Dilution Drives Self-Association

“…At face value, mixtures of water and short-chain alcohols are seemingly simple; small, polar, hydrogen bonding molecules that are macroscopically soluble. Yet the complexities in their structures and thermodynamics leaves a missing link toward better understanding broader solvation processes of polar solutes in water , and leveraging their use in self-assembly and functional devices. − In this sense, the use of such small molecule systems (lower alcohols with water) provide an ideal prototype for studies of more complex mixtures . For this prototype, more definitive links need to be made between specific in-liquid structures and the free energy of the fluid.…”

Luminescent Surface‐Tethered Polymer Brush Materials

“…Upon grafting to a substrate and introduction of a “poor solvent”, the polymer brushes would collapse resulting in an increased local concentration of the rhodamine fluorophore, causing concentration‐based quenching. This enabled monitoring of the local pressure by fluorescence lifetime imaging microscopy, as the fluorescence lifetime decreases as a result of the self‐quenching [96] …”

“…This enabled monitoring of the local pressure by fluorescence lifetime imaging microscopy, as the fluorescence lifetime decreases as a result of the self-quenching. [96]…”

Spatially Resolving Polymer Brush Conformation: Opportunities Ahead

“…[6,17] Other polymers, such as PNIPAM, are inert toward conjugation on the polymer backbone. Instead, more complex methods of end-group substitution, [18,19] surface tethering, [20] or copolymerization of fluorescent monomers [21][22][23] need to be pursued. An exotic range of fluorescent monomers have been reported, including organic light-emitting diodes.…”

“…An exotic range of fluorescent monomers have been reported, including organic light-emitting diodes. [24,25] Without condoning the difficulties in integrating fluorophores, the resulting polymer brushes can readily reveal information on polymer conformation based on altered optical properties if the spatial distribution of fluorophores (i.e., fluorophore-fluorophore proximity) [22] and integrated concentrations [23] are carefully chosen.…”