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(a) The procedures for the preparation and testing of the probe 1 paper strips. (b) Schematic illustration of cell phone-based signal treatment for SA quantification. (c) Photographs of the test paper (probe 1, 1 mM) exposure to various concentrations of SA (0 mM; 1 mM; 2 mM; 4 mM; 6 mM; 8 mM; 10 mM) by using natural light with a white background and UV light (365 nm) with a black background. (d) The plot of R/B values against SA concentrations in the range of 0–10 mM.
