A method of sensing Hg and related Hg sensing system for fluid samples includes the steps of providing a sensing solution including a plurality of anisotropic Au nanoparticles, and contacting a water sample or an air sample suspected of containing Hg or a vapor stream derived from the water sample with the plurality of anisotropic Au nanoparticles. A gold amalgam compound is generated when Hg is present in the sample. The presence, and optionally the concentration, of Hg in the sample are then determined using an optical method based on a change in at least one of absorption, reflectance and scattering of the solution. In a related inventive embodiment a filter for water treatment and remediation including the removal of Hg includes a first flow through grid having a Hg reducing material thereon on an inlet side of the filter and at least one flow through second grid including a surface having amalgamating material downstream from the first grid.