Mercuric ion in aqueous media is extremely toxic to the environment and organisms. Surface plasmon resonance-based optical sensors have been reported that can detect low concentrations as low as fg analytes per cm(2) of the sensor surface. Moreover, the ellipsometry, which detects the changes in polarization states of reflected light from the sensor surface, can be combined with SPR technique to figure out the changes in dielectric media beneath the sensor surface. In this paper, this technique called as surface plasmon resonance-enhanced total internal reflection was used to make trace monitoring of mercuric ion in aqueous media. A protein and an aptamer-based biosensor were developed for this purpose. Bovine serum albumin and mercury-specific aptamer were immobilized on sensor chip via self-assembly routes. The limit of detection was 40.7 nM Hg2+ for protein sensor and 26 pM Hg2+ for anti-Hg aptamer-based sensor. The selectivity of the protein sensor was lower than anti-Hg aptamer sensor when Pb2+ ion was used as the competent ion.