S. Grover, A. Deshpande, T. Ravindran, J.N. Krishnan
BITS Pilani K K Birla Goa Campus,
Keywords: microfluidics, water analysis, heavy metal ion, spiropyran
Summary:Simple, cost- effective water analysis facilities are essential in developing countries like India . Heavy metal ion contamination is one of the major causes of chemical pollution , like many rivers in India. In general, characterization techniques such as atomic absorption spectrometry, inductively-coupled mass spectrometry involve huge machinery, inflated costs and large sample requirements for analysis of heavy metal ions. Microfluidic technologies for water analysis allow us to achieve precise environmental analysis. The advantages of microfluidic devices include the potential for on/offsite monitoring with low sample and reagent consumption and rapid detection of compounds. In this study, a microfluidic device is designed for identifying and quantifying the amount of heavy metal ions present in water (Ex. copper, the maximum permissible limit is 1.3 ppm). The design consists of four modules such as sample inlet, mixing, detection and separation module (Figure 4). Integration of these modules into a microfluidic device provides a platform for entrapment of heavy metal ions present in water sample. Serpentine channels  with 300 µm of width were chosen for mixing module whereas detection module was made wider with 3 mm of width for ease of detection. The separation module consists of ‘Y’ shape with a width of 300 µm so that separated streams can be collected at the outlet. Here, a photochromic compound called Spiropyran is used for metal ion detection. Spiropyran changes to its polar and colored form, merocyanine under UV light of wavelength 350 nm (Figure 2).The heavy metal ions can be trapped only by this merocyanine form. The UV light acts as an optical switch entrapping ions onto the surface of the device. The sample flowing out from the separation module can further be tested using an electrochemical sensor to determine the final concentration of ions in the sample. Benito-Lopez et al used channel walls modified by Spiropyran. They used Spiropyran with a carboxyl group that can attach to the amine group of the modified channel wall . Such Spiropyran with carboxyl group is not readily available and has to be synthesized. However, Rumaisa et al have found that metal ions have the capacity to break the C-O bond to form the open ring merocyanine form establishing a powerful tool for heavy metal ion detection . The microfluidic platform that is developed would allow Spiropyran to be injected with the sample reducing the time for surface modification. This model can be extended for multiplex analysis of metal ions leading to better selectivity, with the use of photo multiplier tubes. High throughput can also be achieved by removing UV light as it converts the merocyanine back to Spiropyran and releases the bonded metal ions. Figure 1 represents the experimental setup consisting of syringe pump for injection of sample and microfluidic device under an optical microscope. Simulation studies on velocity, pressure and concentration of the samples were carried out using COMSOL Multiphysics® as shown in Figure 5. This research provides a powerful tool for the selective detection of heavy metal ions both qualitatively and quantitatively.