Characterization of dynamics of encapsulated small molecule Nile red in biodegradable polymer using 2D IR methods

Biodegradable polymers are seen as a promising option for drug delivery. Here, we applied 2D IR methods with scattering removal to characterize the vibrational dynamics of encapsulated Nile red in amphiphilic block copolymer polyethylene glycol-block-polycaprolatone. Nile red was chosen because it is a commonly used hydrophobic dye molecule. Comparing the absorptive spectra, the splitting of diagonal peaks in nonrephasing spectra confirms the encapsulation of Nile red in the presence of polymer droplets. Pump-probe lifetime data exhibit multiple exponential decays and oscillations, which were fitted to quantify the encapsulation. Waiting time dependent 2D IR spectra were acquired and simulated to characterize the dynamics of the encapsulated Nile red. From these results, we can determine that a portion of the Nile red population, in the presence of polymer, exhibits distinctly different vibrational dynamics, consistent with encapsulation within coacervate microdroplets.