Ultrabright Green-Emitting Nanoemulsions Based on Natural Lipids-BODIPY Conjugates

Abstract

Nanoemulsions (NEs) are water-dispersed oil droplets that constitute stealth biocompatible nanomaterials. NEs can reach an impressive degree of fluorescent brightness owing to their oily core that can encapsulate a large number of fluorophores on the condition the latter are sufficiently hydrophobic and oil-soluble. BODIPYs are among the brightest green emitting fluorophores and as neutral molecules possess high lipophilicity. Herein, we synthesized three different natural lipid-BODIPY conjugates by esterification of an acidic BODIPY by natural lipids, namely: α-tocopherol (vitamin E), cholesterol, and stearyl alcohol. The new BODIPY conjugates were characterized in solvents and oils before being encapsulated in NEs at various concentrations. The physical (size, stability over time, leakage) and photophysical properties (absorption and emission wavelength, brightness, photostability) are reported and showed that the nature of the lipid anchor and the nature of the oil used for emulsification greatly influence the properties of the bright NEs.

Keywords: BODIPY; brightness; fluorescence; fluorescence correlation spectroscopy; nanoemulsions.

Figure 1

Green emitting BODIPYs used for dye-loaded nanomaterials.

Scheme 1

Synthesis of three natural lipids-BODIPY conjugates from: α-tocopherol: BDP-Toco; cholesterol: BDP-Chol and Stearyl alcohol: BDP-C₁₈.

Figure 2

Normalized absorption (top) and emission (bottom) spectra of BDP-Toco (A,D), BDP-Chol (B,E) and BDP-C₁₈ (C,F) in various solvents and oils at 1 μM. Excitation wavelength was 470 nm.

Figure 3

Composition and formulation of bright fluorescent NEs based on BDP-lipid conjugates.

Figure 4

Properties of the BDP-loaded NEs in MCT (left) and in VEA (right). (A,B) Average size measured by DLS, solid bars are freshly prepared NEs and dashed bars are NEs after two weeks (indicated with the notation “−2w” in the insets). PDI averaged between 0.13 for MCT and 0.05 for VEA based NEs. Quantum yields of BDP-loaded NEs upon wt % increase (C,D) and their corresponding relative brightness (E,F), expressed as φ ×[BDP] (unit: Molar), taking into account the molecular weight of the conjugates and regardless of the NEs’ size.

Figure 5

(A) Photostability of 1 wt % dye-loaded NEs under continuous illumination. (B) Photostability of BDP-Toco loaded NEs with increase in loading wt % under continuous illumination. λ_Ex was 470 nm for A and 500 nm for B. The fluorescence was monitored at 600 nm.