A Bottom‐Up Approach to Red‐Emitting Molecular‐Based Nanoparticles with Natural Stealth Properties and their Use for Single‐Particle Tracking Deep in Brain Tissue

Abstract

Fluorescent nanoparticles dedicated to bioimaging applications should possess specific properties that have to be maintained in the aqueous, reactive, and crowded biological environment. These include chemical and photostability, small size (on the scale of subcellular structures), biocompatibility, high brightness, and good solubility. The latter is a major challenge for inorganic nanoparticles, which require surface coating to be made water soluble. Molecular‐based fluorescent organic nanoparticles (FONs) may prove a promising, spontaneously water‐soluble alternative, whose bottom‐up design allows for the fine‐tuning of individual properties. Here, the critical challenge of controlling the interaction of nanoparticles with cellular membranes is addressed. This is a report on bright, size‐tunable, red‐emitting, naturally stealthy FONs that do not require the use of antifouling agents to impede interactions with cellular membranes. As a proof of concept, single FONs diffusing up to 150 µm deep in brain tissue are imaged and tracked.

Ultrabright Red to NIR Emitting Fluorescent Organic Nanoparticles Made from Quadrupolar Dyes with Giant Two-Photon Absorption (2PA) in the NIR Region. Confinement Effect on Fluorescence and 2PA and Tuning of Surface Properties

“…This result is in accordance with their stealthy behavior previously observed toward HeLa cells. 20 Most interestingly, we observed the same lack of interactions for Q BDtF FONs. Q BDvF FONs, on the other hand, could readily be observed accumulated inside of cells.…”

“…We had previously reported that the remarkable absorption and significant fluorescence quantum yield of Q BDF let its FONs reach a giant 1P brightness of 1 × 10 7 M −1 cm −1 . 20 We now report that these FONs also display a huge 2P brightness of 6 × 10 5 GM. Interestingly, the excellent properties of Q BDtF FONs described above make them our second brightest probe, with the foremost advantage that both its absorption and emission are red-shifted as compared to Q BDF .…”

“…33 Second because it was meant to circumvent antiparallel arrangement of dipolar dyes driven by dipole−dipole interactions, which is deleterious to both fluorescence and 2PA. 34 As previously described for the Q BDF dye, 20 the quadrupolar dyes display a benzothiadiazole (BDTA) acceptor moiety at their center, propeller-shaped diphenylamine donor moieties at their ends, and two alkylated fluorene motifs as hydrophobic, conjugated connectors linking the donors with the acceptor. The aim of choosing bulky propeller-shaped donor end groups and introducing steric hindrance with the alkyl chains was to hinder close π-stacking of the chromophores (i.e., formation of H-aggregates) within nanoparticles, which would lead to vanishing fluorescence.…”

“…Taking a different approach toward addressing nanobio interactions, we recently reported on the use of a quadrupolar red fluorescent dye that yields spontaneously stealthy FONs. 20 We hypothesized that the quadrupolar structure of the dye, combined with its bulkiness and strong hydrophobic character could be in part responsible for the stealthy behavior of its FONs. Adding to this interesting property, the quadrupolar design hints at the possibility that these FONs may also be strong 2P absorbers.…”

Stealth Luminescent Organic Nanoparticles Made from Quadrupolar Dyes for Two-Photon Bioimaging: Effect of End-Groups and Core

“…However, controlling the fate of FONs in cellular environments has been seldom addressed. We have recently reported on two dye designs, bis-dipolar [ 28 ] and quadrupolar [ 29 ], that yield spontaneously stealth FONs. We then modulated the π-conjugation of the quadrupolar template of the composing chromophores to yield FONs of red-shifted emission.…”