Fluorescent vesicular nanointerface: construction and ratiometric sensing behavior toward CD44 protein

Abstract

A fluorescent vesicular nanointerface for CD44 protein sensing has been achieved by using amphiphilic Eu3+ complexes and derivatives of a pair of fluorophores, fluorescein and coumarin. Both Eu3+ complex and coumarin were designed to have hydrophobic tails, which facilitated their co-assembly to form vesicles (EuL3+/CA). The negatively charged fluorescein-labeled hyaluronic acid (HA-FA) could bond onto EuL3+/CA surface via coulombic interaction. The vesicles usually emit in the blue region, but they exhibited green fluorescence after HA-FA was bonded to the vesicle surface. This is due to the fluorescence resonance energy transfer (FRET) process between coumarin and fluorescein. As the CD44 protein was introduced to the system, intensity of the green emission of the assembly decreased, while the blue emission enhanced. This phenomenon is accounted for the specific binding between CD44 and hyaluronic acid thereby prohibiting the FRET process between HA-FA and EuL3+/CA. The vesicular nanointerface was found to be sensitive and selective to CD44 sensing (DL=0.179 μg/mL) while the amino acids and proteins tested did not affect the fluorescent properties of the assembly. What is more, the assembly could not only be used to detect CD44 in buffer solution, but also be applied to detect CD44 in cancer cells.