Rapid Microfluidic Electrochemical Sensor for the Detection of Heavy Metal Ions in Water Sample
Authors: Gupta Krishna , Agarwal Arya , Nambi Krishnan Jegatha 
Published: December 2020

Heavy metals are naturally found in the earth’s crust. A relatively dense metal or metalloid which is known for its potential toxicity in an environmental context is termed as Toxic Heavy Metal. They include cadmium, mercury, lead, arsenic, copper, silver etc. to name a few. Human activities like mining, production of industrial wastes, vehicle emissions, manufacturing paints, disposal of microplastics in water bodies are few examples of how these metals are exposed to open vicinities that lead to their direct interaction with living beings. Plants uptake heavy-metal polluted water is then consumed by animals. Ingestion of these plants and animal further becomes the largest source of heavy metal concentration in humans. They are hard to metabolize that causes them to bioaccumulate in the system of the living being. These metals bind to vital cellular components, such as structural proteins, enzymes, and nucleic acids, and interfere with their proper functioning. The symptoms and effects vary as per the concentration of the metal or metal compound. In most cases, carcinogenic effects have been observed in central and peripheral nervous system, and circulatory system of the organism. Therefore, there is a need to develop a fast, convenient, accurate, and cost-effective microfluidic sensor platform for heavy metal ions in water bodies of both urban and rural scenarios.

In this project, a microfluidic device design (as shown in the Figure 1) is developed adapting a logical sequence of microchannels and microstructures representing the mixing, detection, and separation zones with consideration of desired operation. This is followed by incorporation of an optical and electrochemical sensing system for increased precision of heavy metal ion detection. The electrochemical system includes three electrodes such as working electrode, counter electrode and reference electrode. The potential at working electrode start with -0.4 V and switching potential at 1.2 V while the counter electrode at 0 V and standard reference electrode at 0.23 V. The three-electrode cell configuration consists of a nanoporous gold film (NPGF) as working electrode, Pt wire as the counter electrode and Ag/AgCl as the reference electrode. The electrode behavior is investigated using cyclic voltammetric studies in the presence of the electrolyte 10mM K3Fe(CN)6 in 0.1M KCl. The inlet velocity achieved through hydrodynamic focusing facilitates the rapid detection of heavy metal ions in the electroanalysis unit.

In this study, a simulation model has been created using COMSOL Multiphysics® Modelling Software using both laminar flow physics for Fluid Transport and Electroanalysis modules to obtain Cyclic Voltammetry results for heavy metal ion detection within the microfluidic platform. The transport properties of water are used for simulation studies. Diffusion coefficient for the two species K3Fe(CN)6 and KCl (K3Fe(CN)6)= 7.6x10-10 m2/s ; (KCl) = 2.47x10-9 m2/s. Starting concentrations of the two species, C1 (K3Fe(CN)6) = 10 mM = 10 mol/m3and C2 (KCl) = 0.1M = 100 mol/m3. The scan rate applied is 0.1V/s. Figure 2 represents the results for the simulation model. These simulation results are in good agreement with the experimental results of the same. Further investigation on the microfluidic sensor for selective detection of heavy metal ions in water samples is in progress.

Figure 1