Density Functional Theory Investigation of Graphene Functionalization with Activated Carbenes and Its Application in the Sensing of Heavy Metallic Cations

Anthropogenic activities presently generate undesirable industrial and house waste byproducts such as heavy metallic cations (HMCs), which are then often released into the environment, despite being harmful to human beings. Developing new materials that can detect and capture HMCs is therefore highly desirable for wastewater remediation, especially if using cheap starting raw materials such as carbon. To shed theoretical light in this direction, we use density functional theory simulations to study the functionalization of pristine graphene with prototypical carbenes, RC(O)CH, with R = -OCH3 (2a), -OH (2b), -ONa (2c), and -Ph (2d), and explore their use in sensing and capturing toxic HMCs (here, we focus on the most common and harmful ones: Cd2+, Hg2+, and Pb2+). We first demonstrate that, starting from activated diazomethanes, RC(O)CHN2 (1a-d), and graphene as precursors, it is possible to yield substituted cyclopropanes tethered to graphene, here modeled as a 6 × 6 supercell (g6×6), via a [2 + 1]─cycloaddition reaction. Projected density of states and band structure calculations show that the cycloaddition reaction induces a band gap opening of the graphene, which can be used to tune it for electronic sensing devices. These nanomaterials (g6×6/3a-d) favorably interact with the metallic cations through coordination bonds with interaction energies varying from -1.18 to -2.75 eV. Differences in the electronic charge density following HMC adsorption reveal regions of electron depletion or gain induced by these interactions. Based on energetic and electronic structure analysis, we suggest that g6×6/3b-d are good candidates to detect HMCs. All the four nanomaterials show a higher affinity toward Pb2+, as rationalized by a synergic interaction with the graphene substrate, suggesting that they can be used for Pb2+ sensing and removal. Notably, we demonstrate that once and only once the proper computational approach is employed, the accuracy of our predictions of a larger interaction strength of Pb2+ with respect to Cd2+ is validated via the agreement with available experimental data on g6×6/3b. Finally, we predict that by varying the pH, the g6×6/3b,c pair can be employed to differentially sense Cd2+ and Hg2+ cations.