Neutral red dye adsorption on carbon nano-onions: viability assay interference and adduct characterisation

Carbon nano-onions (CNO) are versatile carbon nanomaterials with many potential biomedical applications. In this work, the interaction of submerged arc discharge in water (SADW) produced CNOs with the neutral red (NR) dye was studied. This dye is used in the in vitro toxicity NR assay, one of the most commonly used dye-based procedures to determine cell viability. Firstly the NR assay was carried out in murine fibroblast cell cultures exposed to CNOs. It was demonstrated that this assay produced invalid results due to the strong adsorption of NR on the CNOs. Fourier transform infrared spectroscopy (FTIR) and x-ray photoelectron spectroscopy (XPS) studies confirmed the effective adsorption of the NR on CNOs and π-π stacking as the main interaction between them. The adsorption of NR on the CNOs was evaluated by studying the decrease of the dye solution absorbance. The influence of different experimental conditions such as pH and CNOs dosage was evaluated: absorbance was found to diminish with the CNO dosage. For the maximum dosage used of 240 μg ml⁻¹, the highest absorbance drops of −85% at pH 7 and −78% at pH 4 were registered. The adsorption process was found to be described best by a pseudo-first order (PFO) kinetics model (R ² = 0.99), with a kinetic adsorption constant of k ₁ = 0.02 min⁻¹ and achieving an estimated sorption capacity of 3866 mg of dye per gram of CNOs. This is one of the highest values ever reported for dyes’ adsorption on carbon materials. Lastly, density functional theory (DFT) calculations were carried out to gain further insights into the interaction. These studies suggest a CNO highest occupied molecular orbital (HOMO)/NR lowest unoccupied molecular orbital (LUMO) electron density transfer as the main orbital interaction.