Revisiting galvanic replacement between silver nanoparticles and mercury(II) ions in a cellulose membrane intended for optical assay application: Some new insights into silver-mercury interaction

Abstract

Modification in optical responses of plasmonic metal nanoparticles by galvanic replacement reactions with metal ion contaminants has underpinned the development of Localised Surface Plasmon Resonance (LSPR) based sensors during past decade. However, the studies reported so far mostly use nanoparticles in the form of colloidal solution with inadequate understanding on the sensing mechanisms. We herein revisit the galvanic reaction between silver nanoparticles and Hg2+ ions with aims to develop a cellulose based optical test strip for selective assay of Hg2+ ions in aqueous samples as well as to comprehend the mechanisms of LSPR modification of AgNPs in interaction with Hg. The contact of cellulose membrane hosted AgNPs with Hg2+ ions follows classical galvanic reaction and a linear correlation between the measured absorbance of AgNPs with Hg2+ concentration has been found, which formed the basis of the quantitative assay. The performance of the sensor test strip has been verified for three real water samples of environmental and domestic origin. The results obtained are within 1–6 % accuracy with 94–106 % recovery. The detection limit of the present test strip has been found to be 0.3 ppb, which is lower than a large number of Hg2+ sensor developed so far. Using combined small angle X-ray scattering, calculated extinction coefficient and electronic structure calculations of Ag-Hg system by density functional theory computations, it has been demonstrated that at lower mercury concentration (below 1 ppm), the AgNPs undergo coalescence to form bigger nanoparticles. However at higher concentration (1 ppm and above), the Hg atoms undergo an exceptional “coat to droplet” conversion on AgNP surface. The associated changes in LSPR of AgNPs have been explained using these models.