Lab-on-a-chip device for soil nutrient measurements

 Georgios Kokkinis1,2, Guenther Kriechhammer1, Daniel Scheidl1, Martin Smolka3
             1
              Pessl Instruments, Werksweg 107, A-8160 Weiz, Austria, e-mail:
                                georgios.kokkinis@metos.at
                 2
                   TU Wien, Gusshausstrasse 27-29, 1040 Vienna, Austria
        3
          Joanneum Research GmbH, Franz-Pichlerstraße 30, A-8160 Weiz, Austria



       Abstract. Capillary electrophoresis is known as a fast and easy method for ion
       analysis. Implemented as lab-on-a-chip costs can be reduced drastically. This
       allows for commercial applications in soil nitrate. A known issue, however, is
       the injection variability caused by chip-to-chip differences as well as by
       samples varying in viscosity and overall ion strength and 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 as a separation additive
       in our background electrolyte.




       Keywords: Lab-on-chip, capillary electrophoresis, microfluidics, precision
       agriculture




1 Introduction

Fertilizers containing nitrate, phosphate and potassium are indispensable for modern
agriculture and necessary to nourish the current world population. The calculated
fertilizer demand for the year 2015 alone is around 186.6 million tons. This
calculation combines the single amounts of N, P2O5 and K2O and the annual demand
is expected to grow beyond 199 million tons by 2019 (F.A.O. 2015).
    Although there is an undeniable need for fertilizers in industrial agriculture,
production as well as utilization have an enormous ecological and social impact.
Furthermore fertilizers are often overused, i.e. applied in quantities that don’t
increase yield any further a while increasing negative impacts. According to recent
estimates about 60% of applied fertilizers are in fact overuse and therefore
dispensable. Farmers have a genuine interest to be as resource efficient as possible.
    In this paper we present a novel lab-on-a-chip device, which allows for fast and
easy measurement of soil nutrients and does not require any laboratory skills. The
only remaining requirement is taking samples of the field. The sample is then
inserted into the device and measured in an automated manner.




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2 Material and Methods

We developed an easy-to-produce foil chip. The two PET foils have each a 125µm
thickness. Channels were carved out by laser milling resulting in a 50µm depth
resulting in 10% surface variance (5µm surface errors). The open side channel would
theoretically allow for hydrostatic sample leakage but to the highly viscous nature of
our buffer this did not occur (Bidulock, 2015)
  The channels were carved out of the foil by laser milling. Two PET foils
comprising 125µm each were heat bonded under vacuum. Electrodes for contactless
conductivity detection (Zemann, 1998) were fabricated, after bonding, by inkjet
printing of conductive ink. A fabricated chip is seen in Figure 1. The chip was
introduced in an interchangeable manner in the device containing the fluidic and
electronic peripherals. More details about the basic configuration of the proposed
platform has been reported by Smolka et al (2017).




Fig. 1. The new developed chip: a) the separation channel, b) the detection electrodes, c) the
buffer inlet and outlet and e) the sample channel.



3 Results


A series of experiments were carried out in the device in order to evaluate its
performance. The measured probes consisted of laboratory prepared samples of three
analytes i.e. Cl, Br and NO3 and calcium lactate (present in soil samples due to its use
during extraction). The analyte concentrations were cross-checked using an ion
chromatographer (Sigma-Aldrich, USA). A single probe measurement is seen in the




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electropherogram of Figure 2. Figure 3 reports the measured versus the nominal
value of the probes for a range of concentrations, which furthermore exhibits the
second order polynomial response of the sensor.




Fig. 2. Measurement of a single probe. The peaks correspond to Cl-, Br- and NO3-
respectively.




Fig. 3. Measured values versus nominal values of the probes for a range of concentrations.



4 Conclusions

In this paper we have presented the development of a lab-on-a-chip platform for soil
nutrient sensing. The device performed with relative errors of the order of 5% in a
range of concentrations ranging from 28 – 2220 µM. The response was fitted using a
second order polynomial equation.




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References

1. F.A.O. 2015. “World Fertilizer Trends and Outlook to 2018.” Food and
   Agriculture Organization of United Nations, 66p.
2. C. E. Bidulock, A. van den Berg and J. C. T. Eijkel, Electrophoresis, 2015, 36,
   875–883.
3. M. Smolka, D. Puchberger-Enengl, M. Bipoun, A. Klasa, M. Kiczkajlo, W.
   Śmiechowski, P. Sowiński, C. Krutzler, F. Keplinger and M. J. Vellekoop,
   Precis. Agric., 2017, 18, 152–168.
4. J. Zemann, E. Schnell, D. Volgger and G. K. Bonn, Anal. Chem., 1998, 70, 563–
   567.




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