Silver nanoparticles for the colorimetric determination of electrolytes by UV–vis spectrophotometry and digital image analysis

The use of silver nanoparticles (AgNPs) for the colorimetric determination of sodium, potassium, calcium, and magnesium electrolytes was investigated as an alternative to the existing AuNP-based methods. Different synthetic routes were explored, and the characterization and purification of the obtained solutions was performed, reducing particle size from 27 ± 15 nm to 10.3 ± 1.4 nm. A procedure employing UV–Vis spectroscopy and Principal Component Analysis (PCA) is proposed to evaluate the success of the purification process. The colorimetric response was achieved with the amino acid Lysine, which causes the aggregation of NPs and leads to a visible color change from yellow to dark orange and red. UV–Vis spectroscopy and digital image analysis (DIA) were used as detection methods, obtaining good results in all cases (R² 0.987–0.998 and 0.959–0.997, respectively). By UV–Vis, the first method for Na⁺ and Ca²⁺ using AgNPs was developed, and by DIA, the first method for all four electrolytes. Finally, an extensive literature search led to the identification of several critical aspects regarding the synthesis and use of AgNPs, and some general considerations were raised.