A Low-Cost Double-Bridge Microfluidic Sensor for Water Salinity Measurement
Authors: Razavi Seyedmahmoodreza , Doostmohammadi Ali , Rezai Pouya 
Published: May 2020

Introduction

Sensors are at the core of any interaction with the physical world, and their success or restriction drives or hinders the advancement in numerous areas of science. This is especially true for microfluidics that require vigorous sensing systems with low detection limits and broad dynamic ranges [1].
The quantity of salt in the water can be described as salinity. It is expressed at a specific temperature by electrical conductivity per unit distance (μS / cm) [2]. Salt concentration in urine, blood, water, and different beverages is routinely analyzed through standard analytical techniques, but these methodologies are relatively expensive and can only provide discontinuous “one-shot” measurements inside a well-equipped laboratory [3]. Water salinity is one of the most significant water quality indices. Salinity not only impacts the human body's health, but also seriously effects industrial productions and agricultural activities. Much more interest has recently been shown in the surveillance and measurement of salinity owing to the threat posed by secondary salinization to viable irrigated agricultural production induced by irrigation salts. Microfluidic and point-of-care sensors for measuring the physical and chemical characteristics of fluids are capable of addressing the above challenges [4]. However, microsensors for measuring fluids salinity are mostly difficult to fabricate and relatively expensive [5].
In this paper, we introduce a low-cost and simple microfluidic device (Figure 1) (similar to our previous work but for characterization of Thermoresistive behavior of Three-Dimensional Silver-Polydimethylsiloxane (Ag-PDMS) Microbridges in a Mini-channel [6]) for monitoring the salinity of water based on electrical conductivity measurement between two electrically-conductive metal contacts attached from their ends to the microchannel sidewalls (called microbridges).

Method

A two-part thermoplastic negative mold was 3D-printed (Objet260-Connex, Stratasys, USA) as shown in Fig.2. PDMS base and curing agent were mixed properly at 10:1 ratio, casted over the replication molds and cured on hotplate (75℃, 1.5hr). PDMS layers were peeled off the molds and 90µm-diameter Copper wires (Remington Industries, Johnsburg, USA) were tightened around wire template rods of the bottom-layer. These wires formed electrical microbridges across the width of the channel. After 30s plasma treatment at 900mTorr, the top-layer PDMS was aligned and bonded to the bottom-layer while wire contacts were sandwiched between the two half-layers. The fluid resistances were characterized in-channel with the experimental setup of Fig. 3. Current sweep measurements were conducted at room temperature to determine the electrical resistance between microbridges. To achieve this, an electrical current ranging 1nA-1μA was supplied between the metal microbridges with 1s intervals and voltage drops were recorded. Resistances were calculated by dividing the recorded voltage drops by the currents applied. Measurements were carried out for brine water with 14 different concentrations of NaCl salt (0.025-10mg/L). Flow rate was kept constant at 1ml/min during the measurements.

Results and Conclusions

The measured mean resistances in different salt concentrations are summarized in Table I. The measured resistances are plotted versus salt concentrations in Fig. 4. Using this calibration diagram, the salinity of fluids can be easily determined in applications of this device. Compared to a recently-developed photonic crystal fiber-based salinity sensor [7], our device seems less expensive, easy to fabricate. Our detection limit is 0.025 ppm which is 400 times better than the fiber-based sensor (10 ppm). Based on our data, our device is able to measure salinities between 0.025 and 10 ppm. The sensitivity of our device increases at lower concentrations of salt which makes it ideal for fluids with low salinity.
Similar calibration diagrams can be developed, and resistance can be correlated with other characteristics of fluids such as their temperature, viscosity, and bio-marker dependent electrical conductivity. Therefore, our microfluidic device can be used as a simple and inexpensive tool to measure various physical and chemical properties of fluids.

References

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Figure 1