The present article reports the application of hexamethylsilazane (HMDS) modified filter paper for ultrasensitive detection of Hg 2+ , Co 2+ and Zn 2+ . By chemical vapor deposition of HMDS, a highly hydrophilic filter paper was fabricated to a low wetting (hydrophobic) substrate. The water contact angle (θ) of modified paper was ~128 , whereas scanning electron and atomic force microscopy confirmed the surface modification. Using chromogenic reagents, a one-step assay for aforementioned ions was demonstrated onto pristine as well as hydrophobic paper. The assay was completed in less than 10 min and the end-result was in form of a color change that could be easily read by the naked eye. The limit of detection on modified paper was 0.5 ppb, which was 5-order of magnitude superior to that observed on pristine paper. The proposed method was successfully applied for semi-quantitative determination of Hg 2+ ions in real wastewater samples.