Unlocking New Applications for Thermally Activated Delayed Fluorescence Using Polymer Nanoparticles

ConspectusThermally activated delayed fluorescence (TADF) materials are widely used in organic light-emitting diodes, but their long emission lifetimes also make them ideal for use in bioimaging probes, fluorescent sensors, and phototheranostics. Unfortunately, their development toward these applications has been restricted by the poor compatibility of most TADF materials with aqueous conditions. This problem can be addressed by encapsulating TADF dyes into nanoparticles that form stable aqueous suspensions, while preserving─or even enhancing─the photophysical properties of the TADF emitters they contain. Free of heavy metals, these nanoparticles can exhibit reduced cytotoxicity, improved oxygen tolerance, and higher brightness compared to unencapsulated TADF emitters.This Account discusses recent advances in the development and application of TADF-active polymer nanoparticles for use as biocompatible imaging probes and stimuli-responsive materials. We demonstrate that the encapsulation of TADF emitters into semiconducting polymer dots (Pdots), aggregated organic dots (a-Odots) and glassy organic dots (g-Odots) results in nanoparticles with tunable sizes, surface chemistries, and optoelectronic properties. These nanomaterials are particularly well-suited for bioimaging applications, with customizable bioconjugate chemistry and emission profiles ranging from deep-blue to near-infrared. As TADF materials generate long-lived emission, time-resolved fluorescence imaging can be used to obtain images with high signal-to-noise ratios against background biological autofluorescence. Materials with multiphoton absorbance properties capable of near-infrared excitation are also presented. We further discuss strategies for chemically modifying the nanoparticles' coronas using biomolecules and cell-penetrating motifs to target biological structures both internal and external to the cell. With these strategies, high-contrast imaging of human breast, liver, and kidney cancer cells has been achieved. The ability to target TADF-active nanoparticles to specific tissues, such as cancer cells, has also spurred advances in phototheranostics, which harness the ability for TADF materials to produce cytotoxic singlet oxygen upon irradiation with light.Lastly, as TADF materials can exhibit changes in emission with response to oxygen, temperature, and solvent conditions, we discuss how polymer integration can be used to generate highly effective probes for environmental stimuli. These sensors have potential applications as both in vitro nanosensors and in vivo molecular reporters for disease detection. As an alternative type of sensor, TADF materials have also been employed as reporters in electrochemiluminescence assays for biomarker detection. To explore stimulus–polymer interactions on a fundamental level, the incorporation of TADF materials into bottlebrush polymers is also discussed. Overall, this Account demonstrates that polymer nanoparticles containing TADF emitters have great potential to spur new technologies in fluorescence imaging, sensing, and bioanalysis.