Fig. 5: Selectivity, validation, and reproducibility of the sensor.

a, c Selectivity studies. Sensor responses in the presence of interfering ions (Na⁺ and K⁺) along with and without 0.5, 1.0-, and 2.0-mM concentrations of both interfering ions. The corresponding sensor responses against interfering ions are shown in (a). The RSD is found as 6.53% indicating the sensors’ high selectivity. b Sensor test results with Fatal Bovine Serum (FBS) in the presence and absence of Ca²⁺. For this, FBS was diluted to different concentrations such as 1: 1 (FBS: water), 1:10, 1: 100, and 1:200. Further, these diluted concentrations of FBS were spiked with 2 mm of Ca²⁺ concentration. The corresponding calculated values of OCP vs FBS samples. c Sensor results with whole milk samples. The whole milk obtained from Walmart Inc. was diluted to four different concentrations such as 100%, 80%, 60%, and 30%. With dilution, the sensor response was found to be decreased as the concentration of Ca²⁺ decreased. d Reproducibility test of the Ca-sensor. Four identical Ca-sensors were chosen to test the reproducibility of the sensor and the OCP of each sensor was recorded in the presence of 0.2 mm of Ca²⁺. The sensor’s reproducibility was calculated by evaluating with RSD of 6.68%. Error bars are the standard deviation (SD) of three repeated measurements (n  =  3 biologically independent experiments). Error bars, mean ± SD. Source data are provided as a Source Data file.