Tailoring Fluorescence Brightness and Switching of Nanoparticles through Dye Organization in the Polymer Matrix

Fluorescent nanoparticles (NPs) help
to increase spatial and temporal resolution in bioimaging. Advanced
microscopy techniques require very bright NPs that exhibit either
stable emission for single-particle tracking or complete on/off switching
(blinking) for super-resolution imaging. Here, ultrabright dye-loaded
polymer NPs with controlled switching properties are developed. To
this aim, the salt of a dye (rhodamine B octadecyl ester) with a hydrophobic
counterion (fluorinated tetraphenylborate) is encapsulated at very
high concentrations up to 30 wt % in NPs made of poly­(lactic-co-glycolic acid) (PLGA), poly­(methyl methacrylate) (PMMA),
and polycaprolactone (PCL) through nanoprecipitation. The obtained
35 nm NPs are nearly 100 times brighter than quantum dots. The nature
of the polymer is found to define the collective behavior of the encapsulated
dyes so that NPs containing thousands of dyes exhibit either whole
particle blinking, for PLGA, or stable emission, for PMMA and PCL.
Fluorescence anisotropy measurements together with small-angle X-ray
scattering experiments suggest that in less hydrophobic PLGA, dyes
tend to cluster, whereas in more hydrophobic PMMA and PCL, dyes are
dispersed within the matrix, thus altering the switching behavior
of NPs. Experiments using a perylene diimide derivative show a similar
effect of the polymer nature. The resulting fluorescent NPs are suitable
for a wide range of imaging applications from tracking to super-resolution
imaging. The findings on the organization of the load innside NPs
will have impact on the development of materials for applications
ranging from photovoltaics to drug delivery.