Abstract
In this work, we investigated the sensing performance of epitaxial graphene on Si-face 4H-SiC (EG/SiC) for liquid-phase detection of heavy metals (e.g., Pb and Cd), showing fast and stable response and low detection limit. The sensing platform proposed includes 3D-printed microfluidic devices, which incorporate all features required to connect and execute lab-on-chip (LOC) functions. The obtained results indicate that EG exhibits excellent sensing activity towards Pb and Cd ions. Several concentrations of Pb2+ solutions, ranging from 125 nM to 500 µM, were analyzed showing Langmuir correlation between signal and Pb2+ concentrations, good stability, and reproducibility over time. Upon the simultaneous presence of both metals, sensor response is dominated by Pb2+ rather than Cd2+ ions. To explain the sensing mechanisms and difference in adsorption behavior of Pb2+ and Cd2+ ions on EG in water-based solutions, we performed van-der-Waals (vdW)-corrected density functional theory (DFT) calculations and non-covalent interaction (NCI) analysis, extended charge decomposition analysis (ECDA), and topological analysis. We demonstrated that Pb2+ and Cd2+ ions act as electron-acceptors, enhancing hole conductivity of EG, due to charge transfer from graphene to metal ions, and Pb2+ ions have preferential ability to binding with graphene over cadmium. Electrochemical measurements confirmed the conductometric results, which additionally indicate that EG is more sensitive to lead than to cadmium.
Highlights
Nowadays, among water pollutants, heavy metals (HMs) are considered as the most serious source to pollute the biosphere, posing a significant threat to human health, because they are non-biodegradable and accumulate in soft tissues [1]
In the experiments described in this work, the graphene response to metal-containing liquid phase has been measured for three different cases: (i) adsorption of individual cadmium ions, (ii) adsorption of individual lead ions, and (iii) simultaneous adsorption of both metals
It was revealed that the adsorption order of heavy metal ions on graphene is changed from Cd2+ > Pb2+ for gas-phase to Pb2+ < Cd2+ for water, respectively
Summary
Among water pollutants, heavy metals (HMs) are considered as the most serious source to pollute the biosphere, posing a significant threat to human health, because they are non-biodegradable and accumulate in soft tissues [1]. Nanomaterials-based sensors are promising in the detection of heavy metals due to their large surface area, high catalytic efficiency, high surface reactivity, and strong adsorption capacity For all of these reasons, graphene is one of the best transducer materials because it exhibits extreme sensitivity thanks to its unique properties, such as every atom being available for interaction with adsorbing molecules [32], the high carrier mobility [33], and the high electronic conductivity even when very few charge carriers are present [34]. The use of nanomaterials in the design of chemical sensors has improved their limit of detection (LoD), reproducibility, and due to the unique properties of nanoscale materials, have opened avenues for miniaturization, which has led to the emergence of lab-on-chip (LOC) technology [35]
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