Surface plasmon resonance sensor based on gold-coated tilted fiber Bragg gratings (SPR-TFBGs) are perfectly suited for fine refractometry. Thanks to the functionalization of the gold layer, they can be used for label-free biosensing. They have been largely used for the specific detection of proteins and cells. In this work, we experimentally demonstrate that they are enough sensitive to detect a very small entity like an environmental pollutant. In this context, we report here a bio-functionalization of the SPR-TFBG with thrombin aptamers for lead ion detection. We used aqueous solutions of lead ions with increasing concentrations from 0.001 ppb to 10 ppb. Based on the affinity bending of Pb2+ ions to the thrombin aptamer, we experimentally demonstrated low detection level of lead ion concentration (0.001 ppb) while the saturation limit is meanly fixed by the physical dimension of the sensor and the binding efficiency.