Reversible Fluorescence Switching of Spiropyran-Conjugated Biodegradable Nanoparticles for Super-Resolution Fluorescence Imaging

We report on the reversible fluorescence switching of biodegradable nanoparticles of spiropyran-terminated poly(epsilon-caprolactone) (SP-PCL) for super-resolution fluorescence imaging. SP-PCL was synthesized via ring-opening polymerization using hydroxyl-containing SP derivative as the initiator. SP-PCL solution in THF or dioxane exhibits fast photochromism from colorless to blue upon UV irradiation due to the transformation of SPs in SP-PCL into merocyanines (MCs). Although both SP-PCL solution and MC-PCL solution do not fluoresce, SP-PCL nanoparticle dispersion fabricated via nanoprecipitation in aqueous media, in which SP molecules were embedded into the hydrophobic PCL matrix, displays considerable green emission at 530 nm at an excitation wavelength of 420 nm. Upon <420 nm irradiation, the resulting MC-PCL nanoparticles show strong red emission at 650 nm when excited at 420 nm. SP-PCL nanoparticles display green red dual-color intrinsic fluorescence switching upon alternated UV/vis illumination. Green emission from SP in SP-PCL nanoparticles is observed before UV irradiation while red emission from MCs in MC-PCL nanoparticles after UV irradiation. For both SP-PCL and MC-PCL nanoparticles, the critical excitation wavelength is determined at 420 nm, at which the photoinduced interconversion of MC- and SP-forms are found to be at equilibrium. Positive and inverse photoisomerizations monitored using time-dependent fluorescence spectra show that blue light excitation above 420 nm yields green emission of SPs in SP-PCL nanoparticles while light irradiation below 420 nm imparts photoisomerization (SP to MC) and thus red emission of MCs in MC-PCL nanoparticles. Green and red fluorescence can be optically switched and imaged under fluorescent microscopy. Biodegradable SP-PCL nanoparticles are demonstrated to be promising photoswitchable fluorophores for localization-based super-resolution microscopy, evidencing by resolving nanostructures with sub-50 nm resolution in poly(vinyl alcohol) (PVA) film and live cells.