Photophysical phenomena associated with carbon nanoparticles in combination with biocompatibility and readily functionalizable properties have attracted significant interest for sensing and imaging applications. A potassium ion optode based on the fluorescence quenching of carbon quantum dots (CQDs) was constructed. The CQDs were synthesized using a microwave method, citric acid and 2,2'-(ethylene-dioxy)bis(ethylamine). A quantum yield of 7.1% was calculated for the synthesized CQDs. A linear dynamic range of about one-order of magnitude with a correlation coefficient of 0.99 was obtained. The optode was applied on real samples and a 0.60-1.60% error range was obtained relative to the ion-selective electrode.