Controlling Photoswitching via pcFRET in Conjugated Polymer Nanoparticles

Abstract

Nanoparticles prepared from conjugated polymers (CPNs or Pdots) are powerful fluorophores for sensing and imaging applications, including those that require fluorescence photoswitching. We and others have developed CPNs doped with photochromic dyes that act as acceptors in only one of their two forms via fluorescence resonance energy transfer (pcFRET). We recently developed visible-light-responsive CPNs doped with a reverse photochromic spirooxazine dye that has a thermally stable merocyanine form. Off-to-on fluorescence switching occurs when the merocyanine is switched to its nonquenching spirooxazine form. The on-state fluorescence intensity is more than 100 times greater when photoswitching occurs via FRET from the CPNs than when the dyes are excited directly. Temporal control of fluorescence intensity is achieved using a single-color input of variable intensity for fluorescence excitation (read-only mode) and photoswitching (read–write mode). Here, we present photokinetic measurements and simulations that establish the origin of the observed fluorescence behavior. We find that highly efficient CPN-to-merocyanine energy transfer sensitizes the photochromic reaction, which has a quantum yield of 0.03 in the absence of FRET. Simulated fluorescence trajectories reveal that the low photochromic quantum yield is the necessary condition for single-color intensity control in this system. From the simulations, we determine that the product of the photochromic quantum yield and the energy transfer efficiency (Φ × E) must be no more than 0.05 to produce analogous fluorescence behavior. These findings suggest that the CPNs could be used as an amplifier to drive other low-quantum-yield photoreactions to completion with intensity control.

Maximizing Amplified Energy Transfer: Tuning Particle Size and Dye Loading in Conjugated Polymer Nanoparticles

“…Figure D shows that a sample irradiated at 590 nm (direct pathway) takes far longer (10×) to complete the cycloreversion reaction than the same sample irradiated at 455 nm (FRET pathway) at the same photon flux. The dramatic increase in reaction rate observed in the FRET pathway is consistent with our previous observations in other dye dopants sensitized by CPNs . We previously demonstrated that the exceptional light-harvesting ability of the CPNs and highly efficient energy transfer to dye dopants are responsible for the accelerated reaction kinetics .…”

“…Applications of dye-doped CPNs have largely been focused on the fluorescence output: dyes are used to shift the CPNs’ emission wavelength to lower energy, act as a receptor for fluorescence sensing applications, or modulate fluorescence intensity. , In our recent study of fluorescence intensity modulation in dye-doped CPNs, we determined that FRET sensitization of a low-quantum-yield photochromic dye could drive the dye’s slow, inefficient reaction nearly all the way to completion in just seconds . Our findings are emblematic of an emerging body of literature that focuses on what sensitization by CPNs can do for dyes rather than what dyes can do for CPNs in the form of emission tuning.…”

“…These samples, denoted as CPN 15 , CPN 20 , and CPN 26 , are in the range over which FRET efficiency has previously been shown to increase with increasing particle size when acceptor dyes are doped throughout the particles. , The size distributions are broad (15 ± 11, 20 ± 13, and 26 ± 15 nm), but the data below show that the three populations demonstrate significant differences in energy transfer behavior. The number of polymer chains per nanoparticle can be estimated from the particle size and the polymer MW as described previously . CPN 15 , CPN 20 , and CPN 26 are composed of 8, 19, and 41 polymer chains, respectively.…”

“…5,20 In our recent study of fluorescence intensity modulation in dye-doped CPNs, we determined that FRET sensitization of a low-quantum-yield photochromic dye could drive the dye's slow, inefficient reaction nearly all the way to completion in just seconds. 35 Our findings are emblematic of an emerging body of literature that focuses on what sensitization by CPNs can do for dyes rather than what dyes can do for CPNs in the form of emission tuning. For example, several groups have demonstrated that CPNsensitized dyes can produce singlet oxygen for photodynamic therapy applications.…”

“…The dramatic increase in reaction rate observed in the FRET pathway is consistent with our previous observations in other dye dopants sensitized by CPNs. 35 We previously demonstrated that the exceptional light-harvesting ability of the CPNs and highly efficient energy transfer to dye dopants are responsible for the accelerated reaction kinetics. 35 However, the CPN size and dye loading that maximize this effect remain unknown.…”