Carbon nanotubes modified with fluorescein derivatives for pH nanosensing

Multi-walled carbon nanotubes (MWCNTs) functionalized with carboxylic acid (MW-COOH) were investigated as macromolecular carriers for pH nanosensors. The activation of carboxylic groups with thionyl chloride (SOCl2) followed by reaction with a family of fluorescein ethylene glycol derivatives led to the covalent anchoring of the dyes to the CNT sidewall. This type of functionalization was found suitable for preserving the fluorescence properties of the dye, while providing at the same time higher water solubility to the modified macromolecular systems as compared with pristine carbon nanotubes. The use of polyether spacers between the dye and the nanotube surface was used to reduce fluorescence quenching effects. Fluorescence measurements conducted on fluorescein functionalized CNTs dispersed in water showed pH dependence in the 4.5–8 pH range with good sensitivities. Dye-excitation was obtained by means of a laser diode with a peak emission at 488 nm. An optical fiber orthogonally placed in relation to the excitation beam and connected to a spectrum analyzer, made it possible to record the fluorescence spectra of the nanostructured material.