Herein, we describe a simple laboratory experiment to address nanomaterial synthesis, plasmon resonance, and its application to detect Cu[superscript 2+] in ultralow concentrations. The proposed experiment is very visual and appealing for chemistry students, especially the undergraduate-level chemistry major. In this experiment, the aggregation of silver nanoparticles (Ag NPs) promoted by interactions between the NPs and cations in solution induces a color change in the Ag NP suspension. This visual color change can be explained by concepts of plasmon resonance and nanomaterial properties which can be addressed in this laboratory course. The experiment comprises three main parts: (1) synthesis of Ag NPs using a well-established method reported in the literature; (2) functionalization of the Ag NP surface by l-cysteine; and (3) application of the Ag NPs to detect Cu[superscript 2+] ions in ultralow concentrations. This experiment can be performed with a relatively simple laboratory infrastructure and with instrumentation that is generally widely available.