FRET-Integrated Polymer Brushes: What is the Most Sensitive Architecture for Probing Chain Conformation?

Polymer brush surfaces offer exciting possibilities as surface-based sensing devices, where changes in brush conformation can be used as a basis to sense stimuli. Recently, Förster resonance energy transfer (FRET) chemistry has been integrated into functional polymer brushes, which allowed for conformational changes to be spatially probed due to nanoscale proximity changes between paired fluorophores within the brushes. However, the FRET fluorophores can be arranged in a variety of different architectures within brushes (e.g., on chain ends, dispersed within, etc.), which leads to the question: what is the most sensitive arrangement of FRET fluorophores within a brush for probing conformation? Herein, we address this question from multiple directions. We devise a mathematical model of a brush which considers FRET architectures as continuous bodies of defined fluorophore density, and derive the FRET efficiency as a function of distances between bodies. For experimental based parameters (brush height, grafting density, etc.), we find that diblock random copolymer architectures are the most sensitive, followed closely by donor fluorophores dispersed within the brush, with acceptors on the chain ends. We complement this by coarse grained molecular dynamics simulations of model brush systems, and calculate the FRET efficiency as a function of brush height. The results are consistent with our model, where we find the same two brush architectures to be the most sensitive. Lastly, we compare these architectures for sensitivity from recent experimental measurements, which demonstrated reasonable agreement with both our model and simulation. Our results will inform the field on the best ways of integrating FRET chemistry into functional polymer brushes for greatest conformational sensitivity.