Fig. 4: Analysis of the crown ether functional layer selectivity via XPS.

Normalized narrow scan XPS spectra at the photonic chip surface by subtracting the spectra prior to ion interaction from that of after ion interaction (normalization). The ions tested are a, Na⁺, b, K⁺, c, Mg²⁺, d, Li⁺, e, Zn²⁺, f, Ca²⁺, g, Fe²⁺, h, Cu²⁺, i, Al³⁺, j, Sn²⁺, k, Cd²⁺, and l, Pb²⁺ at 100 ppb, in DI water. The pH of all the analyte is maintained at 6.8 (see Methods).