Colorimetric and coulometric dual-mode sensing based on electrochromic Prussian blue device for solid-contact ion-selective electrodes

The colorimetric and coulometric dual-mode sensing strategies based on electrochromic Prussian blue (PB) device are proposed for reading out the potentiometric responses of the solid-contact ion-selective electrodes (ISEs). The mechanism is that the transformation between the bleached state and the colored state for the PB-based electrochromic device is modulated by using the solid-contact Ca 2+ -ISE with changing the ion activities under the control of a constant potential, and a transient current related to the oxidation-reduction reaction of PB is also obtained. Results show that the concentration range of 1.0 × 10 −6 -1.0 × 10 −2 Ca 2+ can be qualitatively and semi-quantitatively detected through observing the color change via the naked eye or analyzing the red-green-blue (RGB) values by a smartphone and the corresponding integrated charges of the PB-based electrochromic device under the control of the applied potential. These proposed readout strategies are not only free from the influences of colored and turbid samples, but also can keep the inherent selectivity for the solid-contact Ca 2+ -ISE, since the signal detection based on the PB-based electrochromic device and the ion recognition based on the solid-contact Ca 2+ -ISE are separately placed. This work provides a simple, fast, and visual readout strategy for solid-contact ISEs, and expands the sensing concepts for reading out the potentiometric responses.