Integrated Plastic Microfluidic Device for Heavy Metal Ion Detection

Abstract

The presence of heavy metal ions in soil, air and water constitutes an important global environmental threat, as these ions accumulate throughout the food chain, contributing to the rise of chronic diseases, including, amongst others, cancer and kidney failure. To date, many efforts have been made for their detection, but there is still a need for the development of sensitive, low-cost, and portable devices able to conduct on-site detection of heavy metal ions. In this work, we combine microfluidic technology and electrochemical sensing in a plastic chip for the selective detection of heavy metal ions utilizing DNAzymes immobilized in between platinum nanoparticles (PtNPs), demonstrating a reliable portable solution for water pollution monitoring. For the realization of the microfluidic-based heavy metal ion detection device, a fast and easy-to-implement fabrication method based on the photolithography of dry photosensitive layers is proposed. As a proof of concept, we demonstrate the detection of Pb²⁺ ions using the prototype microfluidic device.

Keywords: DNAzyme; Kapton; Lab on a Chip; biosensor; heavy metal ion detection; microfabrication; microfluidics; nanoparticles.

Figure 1

The microfluidic device design in KiCad software version 4.0.0-rc1. The mixing unit is a square wave channel, which is located before the detection unit. The detection unit is an array of electrodes with an inter-finger distance (electrode gap) of 10 μm, one next to the other at a distance of 1 mm. The electrical connections for the operation of the microfluidic device are also shown in red.

Figure 2

(a) Schematic representation of the device fabrication process, including the PtNPs/DNAzymes biosensor array fabrication procedure. The PtNP deposition procedure and the immobilization of DNAzymes in between the PtNPs are depicted. (b) Actual image of the final device. As an inset, we provide an optical microscopy image of the IDEs inside the detection unit of the device (10× magnification).

Figure 3

Schematic representation of the overall experimental setup used for the detection of heavy metal ions, which can be easily integrated into a complete portable water analysis system. A peristaltic pump is used for control of flow, whereas a switch relay matrix enables the simultaneous measurement of multiple sensors.

Figure 4

(a) Evaluation of the passive mixing unit using the transparent PMMA device. (b) The absorbance spectrum of the starting blue and yellow solutions and the final (green) solution after mixing. The absorbance spectrums are in agreement with typical absorbance spectrums reported in the literature for the food coloring solutions like the ones used here, verifying the successful mixing of the two solutions.

Figure 5

Real-time measurement of Pb²⁺ using the proposed DNAzyme biosensor array (Sensor 1 response: black curve, Sensor 2 response: red curve, and Sensor 3 response: blue curve). In the beginning, while the buffer solution is flowing over the detection unit, the resistance reaches a steady state, and the baseline curve is obtained. Then, the sample is injected (t = 0 min) and reaches the detection unit in 2 min, causing a resistance increase due to the cleavage of the DNAzyme double strand caused by the presence of Pb²⁺ (reaction takes place within 1 min). This increase is recorded by all the three sensors forming the detection unit.