Quantitative characterization of fluorophores in multi-component nanoprobes by single-molecule fluorescence.

Multi-modal nanoparticles incorporating fluorophores are increasingly being used for medical applications. The number of fluorophores incorporated into the nanoparticles during synthesis is stochastic, leaving some nanoparticles devoid of fluorophores. Determining the number, the brightness and the photostability of the fluorophores incorporated, and the percentage of labeled nanoparticles (labeling efficiency) remains challenging. We have determined the aforementioned quantities for two synthesized multi-modal nanoparticles by exploiting the photobleaching of fluorophores at the single-molecule level using a total internal reflection fluorescence microscope. Labeling efficiency was determined by fitting the distribution of incorporated fluorophores with a statistical model and verified by independent experiments.