Low-cost precision agriculture for sustainable farming using paper-based analytical devices

Abstract

The United Nations estimates that by 2030, agricultural production must increase by 70% to meet food demand. Precision agriculture (PA) optimizes production through efficient resource use, with soil fertility being crucial for nutrient supply. Traditional nutrient quantification methods are costly and time-consuming. This study introduces a rapid (15 min), user-friendly, paper-based platform for determining four essential macronutrients-nitrate, magnesium, calcium, and ammonium-using colorimetric methods and a smartphone for data reading and storage. The sensor effectively detects typical soil nutrient concentrations, showing strong linearity and adequate detection limits. For nitrate, the RGB method resulted in an R ² of 0.992, a detection range of 0.5 to 10.0 mmol L^-1, and an LOD of 0.299 mmol L^-1. Calcium quantification using grayscale displayed an R ² of 0.993, a detection range of 2.0 to 6.0 mmol L^-1, and an LOD of 0.595 mmol L^-1. Magnesium was best quantified using the hue color space, with an R ² of 0.999, a detection range of 1.0 to 6.0 mmol L^-1, and an LOD of 0.144 mmol L^-1. Similarly, ammonium detection using the hue color space had an R ² of 0.988, a range of 0.5 to 2.5 mmol L^-1, and an LOD of 0.170 mmol L^-1. This device enhances soil fertility assessment accessibility, supporting PA implementation and higher food production.

Fig. 1. μPAD manufacturing process. (A) μPAD fabrication: patterns were drawn in AutoCAD software. The wax was then deposited on Whatman #1 chromatographic paper by the wax printing method. When placed in an oven at 110 °C for 1 min, the printed paper allows the wax to melt and permeate the paper. (B) Reagents immobilization: firstly, the reagents for magnesium (xylidyl blue and EBT), calcium (murexide), and ammonium (salicylate and hypochlorite) detection were added to the paper. Then the sample was added to each spot and filmed 15 min later.

Fig. 2. Representation of the device assembly for the nitrate test. The sulfanilamide and NED solutions were deposited in distinct fluidic layers. Zinc was added to the reduction channel and immobilized with adhesive tape. After the reagents were dried, the device was folded. The sample was added to the sampling zone and allowed to react for 15 min for color development. The cross-sectional view shows the sample path (indicated by the gray-to-pink arrows): sample was added onto fluidic layer 1 on the left side via sampling zone 1 and percolated through layer 2, arriving at the reduction channel on layer 3 and then returned to layer 1 on the right, emerging on the detection zone 2. The black region represents the hydrophobic wax barrier, and the white part represents the hydrophilic paper forming the path through which the sample percolates. The dotted black line only indicates the different layers. Dimensions: 5 mm diameter circles and reduction channel 15 mm length × 4 mm width.

Fig. 3. Detection of calcium in the proposed μPAD. (A) Calibration curve obtained for Ca²⁺ in concentrations ranging from 2.0 to 6.0 mmol L⁻¹. (B) Structure of murexide and the murexide–Ca²⁺ complex.

Fig. 4. Detection of magnesium in the proposed μPAD. (A) Calibration curve obtained for Mg²⁺ in concentrations ranging from 1.0 to 6.0 mmol L⁻¹. (B) Structure of (i) Xylidyl blue before the interaction with the analyte; (ii) the region of Xylidyl blue responsible for binding to Mg²⁺ before and after the interaction; and (iii) EBT before and after binding to Mg²⁺.

Fig. 5. Detection of ammonium in the proposed μPAD. (A) Calibration curve obtained for ammonium in concentrations ranging from 0.5 to 2.5 mmol L⁻¹. (B) Reaction between (i) ammonium and hypochlorite, forming monochloramine; (ii) monochloroamine and salicylate, forming 5-aminosalicylate and (iii) 5-aminosalicylate being oxidized in the presence of nitroprusside catalyst forming the blue colored indosalicylate compound.

Fig. 6. Detection of nitrate in the proposed μPAD. (A) Calibration curve obtained for ammonium in concentrations ranging from 0.5 to 10.0 mmol L⁻¹. (B) Reaction between (i) nitrite and sulfanilamide, forming a diazo compound; (ii) diazo compound and NED, forming an azo dye with a red-purple color, allowing the quantification of nitrate.