Effect of Polymer Spacer Length in FRET-Based Fluorescent Donor-Acceptor Sensing System

In contrast to traditional fluorescent resonance energy transfer (FRET) acceptors, two-dimensional (2D) materials potentially create the long-range energy transfer due to the strong dipole-surface energy transfer. Stimuli-responsive polymers can be an efficient tunable spacer between the FRET donor and 2D acceptor because of their conformational change to the diverse stimuli that allows the dynamic change of distance in a wide range (approximately 1-25 nm). Thus, controlling the length of polymer spacer is the most crucial to maximize the FRET signal in response to the stimuli, because FRET is dominantly distance dependent. In this paper, we present a fluorescent, thermo-responsive block copolymer grafted 2D nanosheets to retain dynamic fluorescence quenching through the change of grafted polymer length. Distance dependent FRET efficiency is studied according to the molecular weight and areal chain density of polymers. The relationship between thermally-responsive FRET and polymer behavior is theoretically calculated and elucidated by measurements of time-resolved fluorescence. Consequently, we first suggest the FRET distance between organic dye and 2D nanosheets and find optimum condition of grafting polymers to maximize the fluorescence response as a function of temperature.