Association with Hg²⁺ enhances the hydrophobicity and triggers the cloud point extraction of ∼4 nm-diameter gold nanoparticle probes functionalized with mercaptopropionic acid and homocystine, which results in the color change of the TX-114-rich phase from colorless to red, and therefore provides a novel approach for visual and colorimetric detection of Hg²⁺ with ultrahigh sensitivity and selectivity.