Abstract
The key to ultrabright fluorescent nanomaterials is the control of dye emission in the aggregated state. Here, lipophilic rhodamine B derivatives are assembled into nanoparticles (NPs) using tetraphenylborate counterions with varied fluorination levels that should tune the short-range dye ordering. Counterion fluorination is found to drastically enhance the emission characteristics of these NPs. Highly fluorinated counterions produce 10-20 nm NPs containing >300 rhodamine dyes with a fluorescence quantum yield of 40-60% and a remarkably narrow emission band (34 nm), whereas, for other counterions, aggregation caused quenching with a weak broad-band emission is observed. NPs with the most fluorinated counterion (48 fluorines) are ∼40-fold brighter than quantum dots (QD585 at 532 nm excitation) in single-molecule microscopy, showing improved photostability and suppressed blinking. Due to exciton diffusion, revealed by fluorescence anisotropy, these NPs are efficient FRET donors to single cyanine-5 acceptors with a light-harvesting antenna effect reaching 200. Finally, NPs with the most fluorinated counterion are rather stable after entry into living cells, in contrast to their less fluorinated analogue. Thus, the present work shows the crucial role of counterion fluorination in achieving high fluorescence brightness and photostability, narrow-band emission, efficient energy transfer and high intracellular stability of nanomaterials for light harvesting and bioimaging applications.
Highlights
Designing brightly fluorescent nanomaterials is the key step for obtaining fluorescent nanoparticles (NPs) for imaging applications.[1,2] Quantum dots are considered as the brightest materials among inorganic NPs, since their cores of ∼5 nm diameter can exhibit extinction coefficients up to 106 M−1 cm−1 together with high quantum yields (40–100%).[3,4] for imaging applications, QDs require a robust water-compatible shell, which makes them much larger with a hydrodynamic radius usually around 15–25 nm
We hypothesized that the level of fluorination and the size of the counterion could change the molecular arrangement of the dyes and vary the inter-fluorophore distance
According to fluorescence correlation spectroscopy these NPs emit like 300–500 rhodamines together, so that their brightness should be around 2.4 × 107 M−1 cm−1
Summary
Designing brightly fluorescent nanomaterials is the key step for obtaining fluorescent nanoparticles (NPs) for imaging applications.[1,2] Quantum dots are considered as the brightest materials among inorganic NPs, since their cores of ∼5 nm diameter can exhibit extinction coefficients up to 106 M−1 cm−1 together with high quantum yields (40–100%).[3,4] for imaging applications, QDs require a robust water-compatible shell, which makes them much larger with a hydrodynamic radius usually around 15–25 nm. 105–3 × 105 M−1 cm−1 for molecules with a size of only ∼1 nm and that they emit with high quantum yields (20–100%).[5] the organization of organic dyes in the form of watercompatible NPs should lead to nanomaterials with comparable or better brightness than QDs of the same size These organic fluorescent nanomaterials are potentially biodegradable in contrast to QDs, making them attractive for biomedical imaging applications.[1,2] Several organic systems have already been developed: conjugated polymer NPs,[6,7] dye-doped polymer NPs8–14 and dye-based NPs.[10,11,15,16,17] Conjugated polymer NPs are probably the brightest organic nanomaterials,[6,18,19] but, they lack biodegradability, due to their polymer backbone made from carbon–carbon bonds. The major problem of dye-based particles is aggregation-caused quenching (ACQ), originated from aggregation of usually flat fluorophores into non-fluorescent pistacked structures (H-aggregates).[20,21] In recent years, several solutions to the ACQ problem were proposed, where fluorophores exhibit unique arrangements in the solid phase, so that self-quenching is prevented.[11,17,22] The first one exploits
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