Copper is an indispensable trace element for human health. Too much or too little intake of copper ion (Cu2+) can lead to its own adverse health conditions. Therefore, detection of Cu2+ is always of vital importance. In this work, a simple sensor was developed for rapid detection of trace Cu2+ in water, in which L-cysteine (Cys) as a molecular probe was self-assembled on a gold interdigital electrode to form a monolayer for specific capture of Cu2+. The interfacial capacitance of interdigital electrode was detected to indicate the target adsorption level under an AC signal working as the excitation to induce directed movement and enrichment of Cu2+ to the electrode surface. This sensor reached a limit of detection of 4.14 fM and a satisfactory selectivity against eight other ions (Zn2+, Hg2+, Pb2+, Cd2+, Mg2+, Fe2+, As3+, and As5+). Testing of spiked tap water was also performed, demonstrating the sensor's usability. This sensor as well as the detection method shows a great application potential in fields such as environmental monitoring and medical diagnosis.