Ion-selective electrodes (ISEs) are widely used for many important applications. Traditional ISEs use inner filling solutions, which must be optimized to obtain low detection limits. This optimization is often sample specific, which is not ideal for most applications. Here, the inner filling solution is replaced by three-dimensionally ordered macroporous (3DOM) carbon as an intermediate layer between an ionophore-doped membrane and a metal contact in a solid contact ISE. After optimizing the membrane composition and electrode conditioning technique, a subnanomolar detection limit for Ag+ was achieved. These electrodes also show excellent long-term stability, as well as resistance to interference from oxygen and light.