Simultaneous ionic cobalt sensing and toxic Congo red dye removal: a circular economic approach involving...

A highly fluorescent quinone-capped silver hydrosol (AgOSA) was obtained using salicylaldehyde and an ionic silver solution. Such metal-enhanced fluorescence was efficiently quenched with Congo red dye (CR), producing CRAgOSA, due to the strong silver-sulfur interaction, replacing the capping of quinone (oxidized salicylaldehyde). The introduction of cobalt ions restored the fluorescence by engaging CR (CoCRAgOSA). Cobalt-induced fluorescence enhancement was 8.3 times higher than that of AgOSA due to the freeing of CR and the release of self-quenching of excess quinone molecules in CoCRAgOSA. The mammoth and selective fluorescence enhancement with ionic cobalt assisted in designing a turn-on ionic cobalt sensor with a limit of detection (LOD) of 9.4 × 10^-11 M and a linear detection range (5 × 10^-5 to 10^-9 M). Moreover, toxic CR dye was eliminated by quinone-capped silver nanoparticles and Co²⁺ due to chemisorption. Not only the fluorimetric sensing of ionic cobalt but also the colorimetric sensing of Hg²⁺ was designed due to the simultaneous aggregation of AgNPs and complexation with CR induced by Hg²⁺ (LOD 1.36 × 10^-5 M and linear detection range from 1.00 × 10^-4 to 5 × 10^-7 M). We applied our sensing method to estimate ionic cobalt and mercury in natural samples. The experiment was a unique case of circular economy, where a toxic dye was used for making a nanosensor.