Innovative colorimetric biosensing platform has been realized by synthesis of silver nanoparticles in silk-fibroin matrix derived from domesticated silkworm species . As-synthesized nano-biocomposite dispersion was characterized by UV-Vis spectroscopy and transmission electron microscopy. In a pilot attempt to develop hassle free on-site screening protocol, such green hybrid systems were explored for colorimetric detection of broad range of metal ion targets, viz. toxic heavy metals such as mercury and chromium (which adversely affect hydrosphere, lithosphere, anthroposphere and biosphere) as well as relatively less-toxic metals like copper and iron in a solution. Quite interestingly, our simple biointerfacial-sensing approach reveals highly selective colorimetric sensor response down to ppb level for mercury ions analyte.