Size-tunable hydrophilic cerium oxide nanoparticles as a ‘turn-on’ fluorescence sensor for the rapid detection of ultralow concentrations of vitamin C

Abstract

The novel perspective of cerium dioxide as a fluorescence sensor has been demonstrated in the present study. The green coloured emission associated with the nano-dimensions of ceria has been adopted as an analytical tool to sense vitamin C, which is a biologically important molecule, in dilute concentrations. Ultrafine ceria nanoparticles of average size 2.2 nm have been fabricated by a surfactant assisted thermal decomposition strategy. The particular fashion of attachment of the oleic acid surfactant with ceria resulted in the surface hydrophobicity of the nanoparticles which in turn prevents their interaction with a hydrophilic molecule like vitamin C in the reaction media. In order to tackle the incompatibility of the nanoparticles with water, a hydrophilic surfactant coating has been grafted over their surface via bilayer surface functionalisation. The success of the accomplished strategy has been confirmed by thermogravimetric analyses, zeta potential and contact angle measurements. The redox properties of ceria and its optical properties served as a probe to quantify vitamin C in the concentration range 10⁻⁷ to 10⁻⁴ M with a very low limit of detection (LoD) of 500 nM. The designed sensor exhibits a rapid ‘turn on’ fluorescence response within 30 seconds and the reversibility of its fluorescence even after 5 cycles of vitamin C addition corroborates its reusability. The high selectivity of the sensor to detect vitamin C again highlights its suitability as an analytical tool. The realistic application of the sensor has also been displayed by the quantification of vitamin C in pharmaceutical formulations within acceptable error limits.