The possibility of obtaining sodium-sensitive electrolyte/oxide/semiconductor structures by low-energy implantation of Al+ and Na+ ions directly into a silica insulator is shown. The depth profiles of the ions are experimentally determined after chemical etching, using spectroscopic ellipsometry and x-ray photoelectron spectroscopy. It is shown that the narrow depth profile (40 nm) is not perturbed by thermal annealing. The optimum ion fluence needed to obtain a quasi-Nernstian response to sodium is 2 x 10(16) ions cm(-2), which corresponds to 15% sodium aluminosilicate (NaAlSi3O8) at the surface of the implanted silica insulator. Low selectivity coefficients (0.02 and 0.15) are obtained for the interfering ions K+ and H+. The sensitivity of the implanted electrolyte/oxide/semiconductor structures is still stable after testing for an entire year.