We used K + PLNs to quantify K + in FBS. After mixing the FBS with K + PLNs (1-to-1 v/v) and adjusting for IFE, the K + level was found to be 1.9 ± 0.8 mM. These results show that ion-selective PLNs are a promising tool for the evaluation of real clinical samples -even those with background autofluorescence.