An elegant and handy selective sensor for ppt level determination of mercury ions

Abstract

We project a reduced graphene oxide-fluorescein sensor moiety that offers a quick, selective and sensitive optical response to the presence of a noxious heavy metal ion, mercury (Hg²⁺) in aqueous solution. A turn off and turn on optical response is envisaged by the unit on competitive adsorption of fluorescein and mercury ions on reduced graphene oxide sheets. The crux behind the sensing of Hg²⁺ ion by the reduced graphene oxide-fluorescein unit lies behind the decrease in absorbance/fluorescence intensity of the dye on complexing with reduced graphene oxide and the corresponding restoration of the optical response upon highly selective competitive adsorption of Hg²⁺ ions by reduced graphene oxide sheets. Quantification of these optical responses was attained by colorimetric and fluorimetric measurements, which yielded ppb level and ppt level mercury ion assay. The LOD values were 240 ppb and 780 ppt respectively for these analytical methods. The moiety is highly selective and specific for mercury ions over alkali, alkaline earth metals and its congeners. The significant advance in this investigation is the fabrication of a reduced graphene oxide-fluorescein unit as a qualitative read out tool for naked eye detection of Hg²⁺ ions in ppm levels noting the colour and for the visual detection of ppt levels of Hg²⁺ through fluorescence in UV light. A highly efficient GFU immobilized filter paper strip was developed which could sense Hg²⁺ under UV light in a concentration as low as ppb level. Moreover, investigations on tap water samples spiked with Hg²⁺ ions underlines that the reduced graphene oxide-fluorescein moiety can swiftly monitor Hg²⁺ levels in complex solutions and demonstrates its potential utility in practical applications.