Electrochemical Detection of Cd(II) in Environmental Samples Using Nano-Ities
Authors: N Ahmed Muzammil , Zhou Wemdy , Strini Miriam , Pathirathna Pavithra 
Published: November 2022

Heavy metals such as cadmium, lead, and arsenic have caused global health concerns that continue to rise yearly, which lead to several harmful diseases that primarily target organs such as kidneys and liver among other vital organs. Because of these well-known harmful effects, regulatory authorities have prescribed “permissible” levels of these metals in food, drinking water, and other possible sources to limit human exposure. However, increasing industrialization and inefficient recycling systems for industrially produced metal wastes have led to an increase in heavy metal exposure to humans. These toxins also bioaccumulate across the food chain, leaving us vulnerable to their harmful effects. Hence it is of great interest to develop systems that can detect metals efficiently to help set up effective metal mitigation protocols.

Most conventional metal detecting tools, although highly accurate, require extensive sample pre-treatment steps which alter the speciation of the metal; a critical parameter for determining its toxicity. As(III) is known to be more toxic than As(V) and as such, they have separate medical remedies targeted toward the particular metal species. However, a conventional technique such as ICPMS is unable to differentiate between them. Furthermore, these conventional techniques require equipment that is bulky, expensive, and not user-friendly and limit real-time 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. Low cost and user-friendliness will ensure that the sensor is within the economic and technological reach of most of the population and the portability of the sensor will enable testing in areas that are hard to access via a stationary lab. Such attributes coupled with accurate information on metal speciation will make an ideal metal sensor that will significantly aid the fight against heavy metal exposure.

This study uses ion transfer between two immiscible electrolyte solutions (ITIES) to develop a Cd(II) sensor. Electrochemistry at ITIES is less complicated than other electrochemical techniques as it is based on the transfer of ions and does not include redox reactions; making it more attractive. Our electrode is a borosilicate glass electrode that is pulled using a carbon dioxide laser puller with an inner radius of ~300 nm. The nano-scale interface of our sensor follows a hemispherical diffusion regime which allows us to have a high mass transfer rate, which is essential for fast kinetic measurements. The nano-interface can also withstand various complex matrices consistently making it ideal for field applications. An ionophore- 1-10 phenanthroline was used to facilitate the Cd(II) transfer across the nano-interface. The sensor was calibrated n various matrices such as potassium chloride and artificial seawater to show its capability to withstand complex matrices without fouling. Stability and selectivity tests were done to showcase the sensor’s performance. It can also successfully detect Cd(II) when it is present in a complex form with strong ligands such as EDTA and NTA, etc. Our sensor’s analytical performance passed the ultimate test when we were able to accurately detect the dissolved Cd(II) ion concentrations in a water sample collected from the Indian River Lagoon in Melbourne, FL. The results from this test were in close agreement with the results reported by another research group that used ICPMS to quantify the amount of Cd(II) dissolved in the same environmental sample. However, ITIES does not require any sample pretreatment steps which is required for ICPMS.

Thus, this study shows great promise for the development of an ideal electrochemical metal sensor for environmental samples. To the best of our knowledge, this is the first time a nanometer-scale glass electrode with ITIES to detect Cd(II) ions in complex matrices is being reported. Future studies will focus on the detection of metals in urine and blood samples and develop this into a portable point of care device.