Determining bioavailable trace concentrations of mercury (Hg) in water is still a challenging analytical task. In this study, we report a methodology for determining labile Hg in natural waters using newly developed sorbents. Silicon dioxide at a nanoparticle range (Si-np) and cellulose powder at a microparticle range (Cel-p), both modified with the ionic liquid trioctylmethylammonium thiosalicylate (TOMATS), have been tested as sorbents (sorb-TOMATS) for Hg(II) uptake from solution. These novel sorb-TOMATS materials were characterized, and parameters affecting the uptake were examined. A similar Hg(II) uptake efficiency (97%) and binding capacity (9 mg Hg/g) was obtained for both sorb-TOMATS, while only a 25% of Hg(II) was taken up using non-impregnated materials. Moreover, these sorb-TOMATS were effectively embedded in agarose gel and were tested as a novel binding phase for the Diffusive Gradients in Thin Films (DGT) technique. Research revealed Si(np)-TOMATS sorbent as a suitable binding phase in the DGT technique for Hg(II) measurements, since it also allowed the efficient elution of the bound Hg(II). This new binding phase showed strong linear correlation between the accumulated Hg(II) mass and deployment time, which is in agreement with the DGT principle. In summary, this novel sorbent has a great potential to improve Hg monitoring in natural waters when integrated it in the DGT design.