Towards the Commercialization of a Lab-on-a-Chip Device for Soil Nutrient Measurement

Abstract

In this paper, we present a soil nutrient sensor based on the capillary electrophoresis chip technology. As a product intended for commercial use in soil nutrient analysis, we focused on the analysis of -NO3 and -SO4. The sensing core of the device is the microfluidic chip. The design of chip, adapted to the needs of a portable handheld device, hinders the flow of sample in the detection area - due to non-planarity of instalment or pressure differences - via a narrow injection channel. A known issue, is the injection discrepancies caused by chip-to-chip variances and overall ion strength, thus turning quantitative analysis into a challenge. We overcame this by adopting bromide as an internal standard. In order to discriminate bromide from ubiquitous chloride in soil samples we used polyvinylpyrrolidone (PVP) as a separation additive in our background electrolyte. An in-house algorithm was developed for the identification of the measurement peaks, consisting of a baseline smoothing and subtraction along with an optimized quantification of the area under the peaks and thus the ion concentration. For the detection of the ion concentration on-chip electrodes were utilized for a capacitively coupled conductivity measurement. Tests were performed with soil sample extractions from different regions and the results were cross-referenced with an ion chromatographer. The sensor’s response had to be corrected for different ions and it exhibited a second order polynomial response with an average absolute error of 5%.