Manganese Detection in Water Samples via Photometrix Colorimetric Analysis on Smartphones, by using Silver Nanoparticles Prepared from Okara Extract

Abstract Soybean residue (okara) extract combined with trisodium citrate was applied as reducing and stabilizing agents for the green synthesis of silver nanoparticles (AgNPs). Okara contained large quantities of biomolecules affecting the property of the synthesized AgNPs as a detection sensor. This work presented the application of the AgNPs as a colorimetric probe for Mn 2+ detection. A free mobile application called PhotoMetrix that can be downloaded onto smartphones was developed for a color analysis. The developed approach provided a linearity range for Mn 2+ detection of 0.025–5.0 ppm with the limit of detection and limit of quantification of 0.01 and 0.03 ppm, respectively, under optimal conditions. The reaction time was 9 minutes with no need for pH adjustment. The developed method was applied for Mn 2+ determination in filtered water samples, and the results were in good agreement with the standard method, atomic absorption spectroscopy. The binding sites of interaction between molecular functional groups surrounding the surface of AgNPs and Mn 2+ ions were characterized to elucidate the detection selectivity, and it was found that carbonyl and hydroxyl groups were key to the sensor activity.