Fig. 3: Response performance and detection selectivity characteristics of the ion-sensing microneedle electrodes to different ions.

a Fluorescence images of the ion-sensing microneedle electrode stained with rhodamine B. b Fluorescence image showing rhodamine B deposition into porcine skin by ion-sensing microneedle electrode penetration. c Schematic showing the experimental setup of the ion-sensing microneedle electrode for continuous monitoring in vitro. d Diagram showing the linear responses of ion-sensing microneedle electrodes to different concentrations of Ca²⁺, K⁺, and Na⁺. e, f Parallel experiments on the linear responses of the ion-sensing microneedle electrodes to detect different concentrations of Ca²⁺, K⁺, and Na⁺. g Statistical analysis of the relative stabilities of the linear responses using the ion-sensing microneedle electrodes to detect different concentrations of Ca²⁺, K⁺, and Na⁺ over 5 days. h Ion-sensing microneedle electrode sensitivities to detect Ca²⁺, K⁺, and Na⁺. i–k: i Specific responses of the ion-sensing microneedle electrodes to Ca²⁺, K⁺, and Na⁺ with relative changes in the response voltage (absolute values). j Heatmap showing the specific responses of the ion-sensing microneedle electrodes to Ca²⁺, K⁺, and Na⁺ with relative changes in the response voltage (relative value). k Comparison of the interference resistance levels of the ion-sensing microneedle electrodes to Ca²⁺, K⁺, and Na⁺