Colorimetric detection of manganese(II) ions using alginate-stabilized silver nanoparticles

Marine algal polysaccharides have been recognized as the most effective and promising substrates for reduction and stabilization of metal nanoparticles. Ecofriendly synthesis of highly stable silver nanoparticles using alginate, an algal polysaccharide, is described herein. These alginate-stabilized silver nanoparticles (Alg-AgNPs) act as a label-free colorimetric sensor for quantification of manganese(II) metal ions (Mn 2+ ) with excellent selectivity and sensitivity and detection in aqueous solution in the range of 1–10 µM. On addition of Mn 2+ ions, the binding forces between Alg-AgNPs and Mn 2+ ions bring the silver nanoparticles closer, decreasing the interparticle distance and causing slight agglomeration, with a color change from pale yellow to brownish yellow. Transmission electron microscopy images of Alg-AgNPs in presence of Mn 2+ ions showed aggregation of AgNPs. Photon correlation spectroscopy showed decreases in the surface charge of Alg-AgNPs in presence of Mn 2+ ions, thus predicting cross-linking aggregation. The ratio of absorbance at 500 nm to 400 nm ( A 500 / A 400 ) versus the Mn 2+ ion concentration was linear with calibration curve of A 500 / A 400 = 0.1611 + 0.0201 × C Mn . Such selective and sensitive detection of Mn 2+ ions over other analytes appears suitable for application in point-of-use analysis of aqueous environmental samples.