The solid-state ion-selective electrode (ISE) has been widely used in various applications due to its inherent properties of fast response time and ease of miniaturization. We have developed a new type of solid-state carbon-based ion-selective membrane to detect and quantify monovalent and divalent ions such as H⁺, Ca²⁺. Multiwalled Carbon nanotubes (MCNTs) have been used as carbon source in these carbon-based potentiometric sensors. The sensors were also miniaturized in the size of 25 to 40-micron diameter (μ-ISE). The Ca²⁺-μ-ISE showed fast response time (~1 sec or less), low limit of detection (~1 μM) and broad linear range (5 μM to 200 mM). In addition, the Ca²⁺-μ-ISE was proven to be insensitive towards redox molecules while demonstrating excellent selectivity towards major interfering ions (such as Na⁺, K⁺, and Mg²⁺, with logK_Ca²⁺_,A = -5.5 to -6.0). The sensors were later used as a probe in Scanning Electrochemical Microscopy (SECM) to characterize Ca²⁺-releasing bioactive glass as well as monitoring real-time bacterial metabolic exchange in quorum sensing pathways. The recent development of this carbon-based miniaturized potentiometric sensors (Ca²⁺ and H⁺) and their application in live bacterial or biological systems would be presented at the meeting.