Rational design of electrochemical sensors based on quinone derivatives adsorbed on graphene for the detection of [Cd(CN)₄]²⁻

Although cyanide is essential in mining operations, its high toxicity to both human health and the environment makes it an extraction agent that requires continuous <i>in situ</i> monitoring. This can be achieved through electrochemical sensors, which enable optimal detection of cyanide and related species without the need for time-consuming sample preparation steps. Graphene-based electrochemical sensors can be enhanced through non-covalent functionalization, involving the adsorption of a modifier onto the substrate surface <i>via</i> π-π interactions. In this study, we explored the effect of incorporating quinone derivatives onto a graphene substrate using a density functional theory (DFT) approach, coupled with a methodology based on the variation of the electronic density gradient (<i>igmh</i>). This approach aims to identify novel materials for the electrochemical detection of the tetracyanocadmate ion, [Cd(CN)₄]^2-, from WAD-CN (weak acid dissociable cyanide). First, we quantify the noncovalent contacts between the quinone and the graphene support through a fragment-based calculation. Subsequently, we focus on the coordination bond strength involving Cd²⁺ and the quinones attached to graphene. Then, we evaluate the effect of incorporating electron-donating substituents, which would directly lead to stronger coordination bonds with the metal center. The results reveal that an optimal balance between the modifier's anchoring on the substrate and its coordination strength toward Cd²⁺ can be achieved by functionalizing the graphene surface with 3-hydroxy-<i>o</i>-benzoquinones substituted at the 4-position with electron-donating groups. This suggests that experimental efforts conducted in this direction could lead to the development of electrochemical sensors with lower detection limits.