A microcantilever was modified with a self-assembled monolayer (SAM) of L-cysteine for the sensitively and selectively response to Cu(II) ions in aqueous solution. The microcantilever undergoes bending due to sorption of Cu(II) ions. The interaction of Cu(II) ions with the L-cysteine on the cantilever is diffusion controlled and does not follow a simple Langmuir adsorption model. A concentration of 10(-10) M Cu(II) was detected in a fluid cell using this technology. Other cations, such as Ni2+, Zn2+, Pb2+, Cd2+, Ca2+, K+, and Na+, did not respond with a significant deflection, indicating that this L-cysteine-modified cantilever responded selectively and sensitively to Cu(II).