Exploring the excellent photophysical and electrochemical properties of graphene quantum dots for complementary sensing of uranium

Graphene quantum dots (GQDs) possessing excellent photophysical and electrochemical properties have been exposed here, for the first time, to develop the low cost, non-toxic, metal-free, sustainable real-life adaptable sensors for strategically important and environmentally concerned metals such as uranium(U). The Forster resonance energy transfer involving f-orbitals of U as well as the nonspecific electrostatic association between GQDs and ions of U quench the photoluminescence of the GQDs resulting into photophysical detection of U in a pure solution down to 0.56 mu g L-1. The electrochemically reduced GQDs are also found to be more electro-catalytically active than gold and it could detect U down to 2 mu g L-1 in aqueous solution having high ionic strengths. This complementary dual-purpose sensor is applicable to determine U in pure solution as well as in the presence of other metal ions in the ground water. Further, we have re-evaluated the kinetic models viz. Nicholson and Klingler-Kochi equations from their respective roots and unambiguously showed neither of the equations is suitable for universal kinetic analysis in electrochemistry. We have persuasively proposed a universal electrodic equation eliminating the perplexities of the traditional kinetic equations. This article paves the unconventional way for environmentally clean synthesis of GQDs possessing excellent photophysical and electrochemical properties expected from the complementary sensor.