Abstract A molecular valve, consisting of poly(acrylic acid) gel-coated Au mesh, was developed based on volume change of the gel in response to cation concentration. The valve closed when concentration of cations such as H + , Na + , K + , Ca 2+ , Cu 2+ , or Al 3+ was low, whereas opened upon increase in its concentration. The valve re-closed when water was flowed. The concentration where the valve opens was found to increase in the order of Al 3+ , Ca 2+ , and Na + (2 × 10 −4 , 5 × 10 −4 , and 6 × 10 −3 M, respectively). The response to Cu 2+ ion showed similar behaviour, but the opening concentration was ca. 2 × 10 −4 M, which is lower than that of Ca 2+ ion. The valve appeared to close over the pH range from 3 to 12, whereas to open below and above it. The fastest response time to open the valve (less than 1 min) was obtained for a solution of pH 1–2. The valve showed repeatability at least 25 cycles upon successive loading of a solution of pH 2 and water. Effects of anions and pressure were also studied.