Fine Tuning of the Photoacoustic Generation Efficiency by Aggregation-Caused Quenching and Excitation Energy Transfer in Bodipy-Labeled Polylactide Nanoparticles

The relationship between the fluorescence decrease by aggregation-caused quenching (ACQ) and the photoacoustic generation efficiency in polylactide-Bodipy nanoparticles (PLA-Bodipy NPs) is demonstrated. PLA NPs with different PLA-Bodipy loadings (from 2.5 to 50% by weight) were studied using a calibrated photoacoustic spectrophotometer. We demonstrate the presence of two photoacoustic emission regimes thanks to the determination of the global photoacoustic efficiency (GPAE), which represents the averaged photoacoustic generation efficiency over the whole studied band (680–870 nm). In the monomer regime, below 10% by weight of PLA-Bodipy, the fluorescence emission from the Bodipy monomer limits the GPAE. Above 10% of PLA-Bodipy, in the ACQ regime, nonradiative deactivations from the aggregates are predominant and GPAE reaches a high value of 93%. We also introduce the photoacoustic brightness BPA, as the product of the GPAE by the molar extinction coefficient of NPs. When Bodipy is aggregated, high nanoparticle extinction coefficients (2.4 × 108 L·mol–1·cm–1 for NP-50%) and high GPAE values are reached, leading to ultrabright NPs (22 × 107 L·mol–1·cm–1). Finally, we show that high laser fluences (1–3.5 mJ·cm–2) can significantly reduce the photoacoustic signal by ground state depopulation at the band maximum excitation (−20% for NP-2.5%). This nonlinear effect can be highly reduced with NPs in the ACQ regime. Above 10% PLA-Bodipy, NPs exhibit intense photoacoustic brightness and low signal loss by nonlinear effects. A mathematical fit of the absorption and photoacoustic excitation spectra allowed us to introduce the band photoacoustic efficiency (BPAE) to spectrally differentiate the averaged photoacoustic efficiency at the band maximum BPAEred and in the vibrational shoulder BPAEblue. The BPAEred highly depends on the laser fluence due to high-ground state depopulation, whereas the BPAEblue does not change with the fluence because of a lower laser fluence saturation.