Herein, a label-free colorimetric nanosensor for Hg(II) is developed utilizing the hindering effect of Hg(II) on the kinetic aspect of gold nanoparticle (AuNPs) growth on the surface of gold nanostars (AuNSs). H-AuNS probes are synthesized and modified by 2-[4-(2-hydroxyethel) piperazine-1-yl] ethanesulfonic acid (HEPES). After the formulation of the reagents and testing conditions are optimized, HEPES-capped AuNSs (H-AuNSs) demonstrates good selectivity and sensitivity towards Hg(II) determination. A H-AuNS probe, in the presence of HCl/Au(III)/H₂O₂, is capable of detecting a Hg(II) concentration range of 1.0 nM⁻100 µM, with a detection limit of 0.7 nM, at a signal-to-noise ratio of 3.0, and a visual detection limit of 10 nM with naked eyes. For practicality, the H-AuNS probe is evaluated by measuring Hg(II) in the environmental water matrices (lake water and seawater) by a standard addition and recovery study. The detection limits for environmental samples are found to be higher than the lab samples, but they are still within the maximum allowable Hg concentration in drinking water (10 nM) set by the US Environmental Protection Agency (EPA). To create a unique nanosensor, the competitive interaction between Hg(II) and Pt(IV) toward the H-AuNSs probe is developed into a logic gate, improving the specificity in the detection of Hg(II) ions in water samples.