In this phase, we assemble the device using readily available materials such as an N95 mask, salt, a coffee filter, interdigitated electrodes, and an LCR meter to validate the concept.

Prototyping: We will extract the electret from an N95 Mask, which is a cheap and accessible filter (Figure 3). However, the electret sheet extracted from an N95 mask is generally not porous enough to allow water to pass through. So we will process the water using salt (NaCl) to density-separate the microplastics from other sediment in the water. We will then use a coffee filter to extract the microplastics and transfer them to the electret surface. We will place the electret on a sensitive interdigitated electrode and use an LCR meter to measure the capacitance across the electrodes.

Testing:

Step 1: Density Separation 

Combine table salt (NaCl) with water and stir until the mixture is uniform (Figure 1). Allow it to sit so that dirt can settle at the bottom of the container, while microplastics will float to the top. Carefully pour off the top layer, which contains only the microplastics, into a coffee filter to separate them from the water (Figure 2). Use distilled water to rinse the coffee filter, ensuring that any remaining salt and impurities are washed away. This step is essential because the salt's conductivity can lead to inaccurate readings. 

Step 2: Transferring microplastics to the electret

Place the dried coffee filter in a clean glass beaker. Rinse it with a small volume of distilled water (approximately 1–3 mL), focusing on the area where the particles are trapped to ensure they are washed into the beaker. Using a pipette, add small droplets of this suspension directly onto the electret window, extracted from the N95 mask, above the electrodes, and allow it to dry (Figure 3). Ensure the microplastics are completely dried out, as even small traces of water can affect the accuracy of the readings.

Step 3: Measure change in capacitance.

Measure the initial capacitance of the interdigitated electrodes using an LCR meter. This measured value will serve as the baseline for any changes in capacitance that occur after the introduction of the microplastics. Next, transfer the extracted microplastics from the coffee filter to the electret filter. Finally, measure the capacitance of our built device’s electrode and compare it to the initial capacitance measured at the beginning of the process.

Step 4: Calibration Process

Perform the above process for known microplastic concentrations in water: 0.1 mg, 0.2 mg, and 0.25 mg. Compute the change in capacitance for each concentration. Plot the measured capacitance on the y-axis against the microplastic concentrations on the x-axis. Apply a polynomial curve fit to the plotted points using established algorithms. Now, given the change in capacitance, the microplastic quantity in the test water sample can be estimated using the polynomial.

To evaluate the viability of our proposed method, we have established the following criteria:

1. Accuracy: A maximum error margin of 10% in estimating the amount of microplastics.

2. Low Cost: The total material cost of the system should be under $200.

3. Rapid Detection: The detection process should take no longer than 30 minutes.

4. Sensitivity: The system must be capable of measuring microplastic particles within the range of 100 to 500 µm. 

5. Portability: The device must be designed to fit within a 20 cm x 20 cm square.

6. Minimal Equipment: Should operate independently, without additional methods.

7. Environmental Compatibility: The system must function effectively with freshwater, seawater (after desalination), and wastewater samples.