Measuring the fluorescent quantum efficiency of indocyanine green encapsulated in nanocomposite particulates

We present results of a fluorescent quantum efficiency (Φ_F) study on the encapsulation of the near-infrared dye indocyanine green (ICG) in bioresorbable calcium phosphate nanoparticles (CPNPs). The Φ_F (described as the ratio of photons emitted to photons absorbed) provides a quantitative means of describing the fluorescence of an arbitrary molecule. However, standard quantum efficiency measurement techniques provide only the Φ_F of the smallest fluorescing unit-in the case of a nanoparticle suspension, the nanoparticle itself. This presents a problem in accurately describing the Φ_F of fluorophores embedded in an inorganic nanoparticle. Combining the incidence of scattering with an evaluation of the differences in local electric field and photochemical environment, we have developed a method to determine the Φ_F of the constituent fluorescent molecules embedded in such a nanoparticle, which provides a more meaningful comparison with the unencapsulated fluorophore. While applicable to generic systems, we present results obtained by our method for the ICG-CPNP in a phosphate buffered 0.15 M saline solution (PBS, pH 7.4)-specifically, Φ_{F,free dye} = 0.027 ± 0.001, Φ_F,particle = 0.053 ± 0.003, and for the individual encapsulated molecules, Φ_{F, molecule} = 0.066 ± 0.004. The method developed also provides insight into the influences of encapsulation and key parameters to engineer resonant enhancement effects from the emission of the encapsulated fluorophores corresponding to an eigenmode of the embedding particle for tailored optical properties. (Some figures in this article are in colour only in the electronic version)