A smartphone-integrated imaging device for measuring nitrate and phosphate in soil and water samples

This research presents a smartphone-integrated imaging device to detect nitrate (NO₃^–) and phosphate (PO₄^3-) in soil and water samples for agricultural input management and environmental pollution monitoring. This low-cost and field-deployable device uses a smartphone with a digital camera to take a color photograph of the extracted solution under test. The sensing principle is based on the standard colorimetric phenoldisulfonic acid and ascorbic acid methods for NO₃^– and PO₄^3- in soil and water samples, respectively. A 3D printed device that houses the sensor's optical source and other optical components is coupled to the phone's built-in rear camera and turned into a compact handheld system. An android application, 'SMART NP,' is also developed to quantify the concentrations of NO₃^– and PO₄^3- in soil and water samples using the Value (V) component of the Hue Saturation and Value (HSV) color space model. The results obtained with the proposed device were comparable to laboratory-grade spectrophotometer data. The limit of detection of the device for NO₃^– in soil, NO₃^– in water, PO₄^3- in soil, and PO₄^3- in water were calculated as 0.1 mg L^-1, 0.07 mg L^-1, 0.001 mg L^-1, and 0.02 mg L^-1, respectively, with good accuracy (≤1% bias) and precision (∼2% residual standard deviation). The device exhibited a sensitivity of 0.17 mg L^-1, 0.12 mg L^-1, 0.026 mg L^-1, and 0.49 mg L^-1 for NO₃^– in soil, NO₃^– in water, PO₄^3- in soil, and PO₄^3- in water, respectively. Overall, this sensing device has the potential to assist farmers or scientists in easily measuring NO₃^– and PO₄^3- concentrations in soil and water media, without the need for laboratory equipment.