Ultrabright and Bioorthogonal Labeling of Cellular Targets Using Semiconducting Polymer Dots and Click Chemistry

Abstract

semiconducting polymer; nanoparticle; click chemistry; bioorthogonal labeling; fluorescenceimagingClick chemistry describes a powerful set of chemical reactions that are rapid, selective, andproduce high yields.[1] The most recognized of these reactions is the copper (I)-catalyzedazide-alkyne cycloaddition, which has been applied to diverse areas, ranging from materialsscience to chemical biology.[2–8] For biological applications, both azido and alkyne groupsare considered to be bioorthogonal chemical reporters because they do not interact with anynative biological functional groups. As a result, these bioorthogonal reporters can beincorporated into a target biomolecule using the cell’s biosynthetic machinery to providechemical handles that can be subsequently tagged with exogenous probes. Thebioorthogonal reporters are complementary to genetically encoded tags, such as greenfluorescent protein (GFP),[9] and provide a powerful approach to tag biomolecules withoutthe need of direct genetic encoding. Bioorthogonal labeling via click chemistry is highlysensitive with low background despite the complex cellular environment. In practice,however, the sensitivity is constrained by the abundance of the target molecules, the labelingefficiency of the chemical reporters, and the performance of the exogenous probes.[7] Inalmost all cases, bright and photostable probes are highly desirable, particularly for long-term tracking and sensitive detection of low-abundance biomolecules.Fluorescent nanoparticles such as quantum dots (Qdots) exhibit improved brightness andphotostability over traditional fluorescent dyes.[10–12] In the context of click chemistry,however, the copper catalyst irreversibly quenches Qdot fluorescence and prevents theirusage in the various applications based on copper-catalyzed click chemistry.[13] Because ofcopper’s cytotoxicity, copper-free bioorthogonal approaches, such as the Staudinger ligationand the strain-promoted azide-alkyne cycloaddition, have been developed for live cell and