Novel, Ultra-Small, Robust Electrochemical Sensor for in-Situ Detection of Cd(II) in the Environment

Heavy metal contamination is a rising global concern and causes a potent threat to humans and other biotas. Toxic metal species bioaccumulate through the food chain; thus, we are highly vulnerable to exposure to devastating health hazards. Most of the conventional metal-detecting analytical tools require lengthy sample pre-treatment methods, accordingly, alter metal speciation, one of the critical parameters for determining the toxicity of the metal species. Furthermore, these traditional techniques utilize less user-friendly bulky and expensive instruments, limiting real-time metal monitoring. Consequently, the development of a low-cost, portable, and robust sensor capable of providing accurate information on metal speciation will significantly aid in establishing metal mitigation systems efficiently.

In this study, we take advantage of ion transfer between two immiscible electrolyte solutions (ITIES) to develop a Cd(II) sensor. The fundamental mechanism of electrochemistry at ITIES, is that ion transfer is ruled by the ion’s Gibbs energies of transfer for a specific aqueous-organic solvent system. Charged ions are transferred across the interface by imposing a potential difference (using a potentiostat) greater than the Gibbs energy for transfer between two phases and the resultant current can be measured as a function of applied potential. Electrochemistry at ITIES is less complicated since it doesn't involve electron transfer; hence more attractive over redox-based chemistry .A suitable ionophore is added to the organic phase to lower the energy requirement that needs to be applied via the external circuit when the Gibbs energy for ion transfer is too high (compared to that of background electrolyte solution). Moreover, this approach provides a greater selectivity for a particular target ion in the presence of a mixture of other ions.

Our electrode is a borosilicate glass electrode with an inner radius of ~300 nm. The nano-scale interface of our sensor follows hemispherical diffusion regime which allows us to have high mass transfer rate essential for fast kinetic measurements. An ionophore- 1-10 phenanthroline was used to facilitate the Cd(II) transfer across the nano-interface. We performed ITIES based cyclic voltammetry experiments with our nanosensor in various matrices, including simple electrolytes like KCl and complicated buffer solutions such as artificial seawater and artificial cerebellum fluid. We found out that our electrode shows excellent stability and capable of withstanding the complex matrices without fouling, an attractive feature of an exemplary sensor. We performed complexation studies with several ligands to show our sensor's capability to identify not just the unbound Cd(II) and the complexed, bound Cd(II). We tested our sensor with Cd(II) dissolved in a water sample collected from Indian River Lagoon, Melbourne, FL; thus, we showcase our sensor's power as an environmental monitoring tool. To the best of our knowledge, this is the first time reporting a glass electrode with a sub-nano-meter scale for Cd(II) detection in a natural environmental sample using ITIES. Our ultra-small electrode will enable us to study the kinetics of ion transfer across ITIES; thus, allowing us to modify the sensor to enhance the sensitivity and selectivity.