Fig. 2: Morphological characterization of the blank microneedle patch, microneedle patch modified with the Au layer, microneedle patch modified with the PEDOT layer, and microneedle patch modified with ion-selective layers.

a From left to right: blank steel microneedles, blank microneedles modified with Au layer, microneedles modified with the PEDOT ion conversion layer, and SEM images at different magnifications. b–d Optical and SEM characterizations of microneedle ion electrodes: b Ca2+-sensing microneedle electrode, c K+-sensing microneedle electrode, and d Na+-sensing microneedle electrode