Carbon-Based Solid-State Calcium Ion-Selective Microelectrode and Scanning Electrochemical Microscopy: A Quantitative Study of pH-Dependent Release of Calcium Ions from Bioactive Glass

Solid-state ion-selective electrodes are used as scanning electrochemical microscope (SECM) probes because of their inherent fast response time and ease of miniaturization. In this study, we report the development of a solid-state, low-poly(vinyl chloride), carbon-based calcium ion-selective microelectrode (Ca²⁺-ISME), 25 μm in diameter, capable of performing an amperometric approach curve and serving as a potentiometric sensor. The Ca²⁺-ISME has a broad linear response range of 5 μM to 200 mM with a near Nernstian slope of 28 mV/log[a_Ca²⁺]. The calculated detection limit for Ca²⁺-ISME is 1 μM. The selectivity coefficients of this Ca²⁺-ISME are log K_Ca²⁺,_A = -5.88, -5.54, and -6.31 for Mg²⁺, Na⁺, and K⁺, respectively. We used this new type of Ca²⁺-ISME as an SECM probe to quantitatively map the chemical microenvironment produced by a model substrate, bioactive glass (BAG). In acidic conditions (pH 4.5), BAG was found to increase the calcium ion concentration from 0.7 mM ([Ca²⁺] in artificial saliva) to 1.4 mM at 20 μm above the surface. In addition, a solid-state dual SECM pH probe was used to correlate the release of calcium ions with the change in local pH. Three-dimensional pH and calcium ion distribution mapping were also obtained by using these solid-state probes. The quantitative mapping of pH and Ca²⁺ above the BAG elucidates the effectiveness of BAG in neutralizing and releasing calcium ions in acidic conditions. (Graph Presented).