Mercury and its compounds are widely distributed in the environment and have a significant negative impact on human health. In this paper, we report the development of a rapid and facile method for the detection of mercury ions (Hg2+) using heteroepitaxially synthesized unmodified silver nanoparticle-based smart probes using UV–Vis spectrophotometer and also through the naked eye by means of a paper-based sensor strip. The silver nanoparticles were prepared by heteroepitaxial growth method using gold seed nanoparticle of ~2.4 nm size as the template. The silver is grown on the seed particles by reducing the silver–ammonia complex using glucose, which resulted in Glu–AgNPs having an average size of 14.65 ± 3.53 nm. The sensing of mercury ions was carried out in aqueous solution and the reaction response was monitored by UV–Vis spectrophotometer. The interaction of Hg2+ with Glu–AgNPs resulted in a significant drop in the absorbance at 402 nm along with a prominent color change (from bright yellow to colorless) and wavelength shift (blue shift). The limit of detection (LOD) of this assay was found to be 100 nM (i.e., 20 ppb) with a good linearity in the concentration range of 100–10 mM. To further ease the detection process and make it field deployable, we attempted to develop a paper-based sensor strip by immobilizing Glu–AgNPs on a paper strip. Upon interaction with mercury solution of varying concentrations, the decoloration of the spots could be observed easily through naked eyes, with the limit of detection under sub-optical conditions being 1 µM. © 2017, The Author(s).