Development of Biosensor-Based Device for Urea Concentration Measurement in Industrial Fertilizer Samples
Authors: Dagys Marius , Gureviciene Vidute , Razumiene Julija 
Published: February 2020

In this presentation, we will demonstrate creation process (R&D and engineering) of biosensor and device for direct urea concentration measurement in industrial fertilizer samples. The analyzer is fully automatic, fast, not requiring professional assistance, compact and mobile. The breakthrough technology enabling the construction of such device is amperometric urea biosensor electrode, re-purposed from the biosensor suited for monitoring hemodialysis patients developed earlier by our team. There is no direct competitor for the device – fertilizer production and supply companies have to measure urea in their products by manual, labor-intensive techniques such as Kjeldahl method, where single measurement takes at least an hour, thus creating a beneficial market window for rapid and accurate industrial urea analyzer.
Direct urea concentration measurement in water media has been a scientific challenge for decades. It is possible to measure urea concentration by spectrophotometry, but only in transparent, homogeneous and single compound solutions. A target media, concentrated ammonium nitrate – urea solution (UAN), is widely used fertilizer standard in agriculture, and in this case rapid determination of urea in product is complicated. A good technique for such urea measurement would be biosensors; however, urea breakdown does not involve electron exchange with external electrode, so for a long time the potentiometric electrodes were employed, where urea breakdown causes polarization changes of enzyme-covered surface. Such sensors exhibit fast electrochemical response, but have drawbacks - influence of media pH and high ionic strength to measurement errors, low sensitivity in low analyte concentrations (usually not less than 0.01 mM). Another option, ion-sensitive field-effect transistor electrodes adequately measure the current proportional to urea concentration, but usually lack reliability and are prone to permeability problems.
Our suggested technology enabling the construction of such device - amperometric urea biosensor, constructed as a membrane to be fitted on working electrode surface. Its benefits - stability, insensitivity to pH or high ionic strength, low production cost, substrate selectivity, can be used constantly in enzyme-degrading fertilizer media for at least two weeks. It works at 0.2 V vs NHE potential, thus does not suffer of potentially significant electrochemical noise. It can measure urea concentration by declared accuracy of 5 %, when the automatic calibration is performed every 3 hours. The biosensor element – a membrane, fitted on the end of graphite electrode – works for 3 months, resulting in single measurement price under one cent. The reiterative measurement is implemented by magnetically stirred flow-through electrochemical cell with opening on top for sample entry. A double-flow peristaltic pump is designed to wash the cell and pre-fill the buffer solution for introduction of 50 µl of 1:1000 (v/v) pre-diluted sample.
Currently, our team has developed the technology up to a final product trial batch of biosensor and analyzer device. In the presentation we will demonstrate the biosensor design and electrochemical properties, as well as design of device appliances developed such as electrochemical cell and double-flow peristaltic pump.

Figure 1