A First Step towards Determining the Ionic Content in Water with an Integrated Optofluidic Chip Based on Near-Infrared Absorption Spectroscopy

Gerwin W Steen,Adam D Wexler,Elmar C Fuchs,Herman L Offerhaus

doi:10.3390/opt1020014

Abstract

In this work, we present a feasibility study of integrated optofluidic chips to measure the ionic content in water using differential absorption spectroscopy. The second overtone of the OH-stretch vibration of water is used as indicator for both the type and concentration of the dissolved ions. The optofluidic chips are based on silicon nitride (TripleX) containing Mach–Zehnder interferometers (MZI) with two 5 cm sensing paths for the sample and reference arms, respectively. Simulations show that, theoretically, the determination of both the type and concentration of a mixture of four electrolytes is possible with the techniques presented. However, the performance of the chips deviated from the expected results due to the insufficient reproducibility and precision in the fabrication process. Therefore, at this early stage, the chips presented here could only determine the ion concentration, but not differentiate between the different ion types. Still, this work represents the first steps towards the realization of an online and real-time sensor of ionic content in water.

Highlights

Mach–Zehnder Interferometer (MZI) and interferometers showed rather large variations in the fringe spacing following procedure was used: first, 200 baselines were recorded after both fluidic channels were for the same optical design between the different wafers
We have shown that with interferometric techniques, the qualitative and quantitative measurements of ions in aqueous solutions with an optofluidic chip are possible
The initial experimental results revealed that the fabricated chips operated in the desired single mode

Summary

Introduction

The Relevance of Detecting Ionic Content in Water. A large variety of dissolved ions (ionic content) are present in drinking water. Some of the ions are essential and beneficial (e.g., Cu2+ , Cr3+ , and Zn2+ ) though toxic in excess, while others are toxic in trace amounts (e.g., Pb2+ , Hg2+ , Cd2+ , Tl3+ ) and have no known biochemical value [1]. The adverse effect of consuming unhealthy water is cumulative. Between 35 and 77 million people in Bangladesh were poisoned, with 407 deaths, occurring over a period of nine years due to the contamination of groundwater with arsenic compounds [2]

Methods

Results

Conclusion