Complex Photobleaching Behavior of Semiconducting Polymer Dots

Abstract

Semiconducting polymer dots (Pdots) are a type of conjugated polymer (CP) nanoparticle with highly advantageous fluorescence properties and a growing breadth of applications in bioanalysis and imaging. Exceptional brightness is a widely demonstrated advantage of Pdots. Resistance to photobleaching (PB) is an often-touted advantage but is far less studied. Here, we present a detailed study on the PB of Pdots. The effects of the particle size, pH, oxygen level, and anti-fade agents on PB were examined through ensemble and single-particle fluorescence measurements for multiple Pdot compositions. Physical changes during PB were assessed by nanoparticle tracking analysis, gel electrophoresis, Förster resonance energy transfer, and other fluorescence quenching experiments. Spectral shifts, increases in the fluorescence lifetime, reductions in quantum yield, and changes in electrophoretic mobility were some of the properties observed to vary upon PB. Loss of fluorescence under continuous illumination was from formal bleaching and also from the introduction of quenching mechanisms. Oxygen-dependent and oxygen-independent mechanisms of PB were suggested by the data, with the balance of these mechanisms dependent on both the CP and the illumination intensity. The oxygen-independent mechanism appeared to be intrinsic to the volume of the CP and may be limited by exciton–exciton annihilation at high illumination intensity. The oxygen-dependent mechanism appeared to bleach the Pdots from the surface of the particle inward, forming a bleached shell, and introduced new functional groups. Overall, the PB of Pdots was a complex process with photophysical and chemical alterations, and homogeneous and heterogeneous effects, dependent on particle properties and experimental conditions. Changes induced by PB have implications for many biological applications of Pdots, and the results of this study will help guide the development of Pdot materials with greater resistance toward bleaching.

Self-Reporting Conjugated Polymer Nanoparticles for Superoxide Generation and Detection

“…PFBT-derived CPNs exhibit bright fluorescence 2 and are more photostable than some other CPNs. 3 , 24 These properties make PFBT an excellent fluorescence resonance energy transfer (FRET) donor to doped dyes, as we 39 − 42 and others 18 , 43 , 44 have demonstrated. CPNs prepared from PFBT or closely related polymers have been also doped with photosensitizers to generate singlet oxygen for photodynamic therapy via FRET.…”

“…Alternatively, CPNs in the T 1 state can donate an electron to oxygen to form the superoxide radical anion (O 2 •– ) and a CPN-based hole polaron as products. Known as Type I photosensitization, this pathway can be a desirable addition to PDT applications in hypoxic environments because superoxide generation consumes less oxygen than singlet oxygen generation. , However, the same reaction is undesirable in many fluorescence-based applications because photogenerated hole polarons act as fluorescence quenchers, reducing CPN fluorescence intensity. − Hole polarons are also implicated in the photodegradation mechanisms of some conjugated polymers …”

“…21−23 Hole polarons are also implicated in the photodegradation mechanisms of some conjugated polymers. 24 We developed an interest in superoxide generation by CPNs after observing that some organic dyes photobleached to a greater extent when doped onto CPNs than when studied in homogeneous solution. Superoxide is a well-known contributor to the photocatalytic degradation of organic dyes, 25 and ROS generation has been identified as a treatment method for textile dye effluent.…”

“… 21 − 23 Hole polarons are also implicated in the photodegradation mechanisms of some conjugated polymers. 24 …”