Disclosed is a method for detecting toxic metal ions in a sample. The method includes: a) preparing a solution of organic acid-bound gold nanoparticles; b) adding a sample containing toxic metal ions to the solution prepared in a) to allow the gold nanoparticles to aggregate; c) dropping the reaction solution obtained in b) onto a silicon substrate and drying the reaction solution such that the gold nanoparticle aggregates are immobilized on the silicon substrate; and d) analyzing the characteristics of the gold nanoparticles immobilized on the silicon substrate. The method enables the detection of even a trace amount of toxic metal ions in a sample with high sensitivity. Therefore, the method can be applied to the management of water quality in food service providers and hospitals, the measurement of contaminants in water supply and drainage systems, and the management of industrial wastewater. Furthermore, the method is expected to be widely applicable to water purifiers and the food and beverage industry in the future.