We report the design and synthesis of newly functionalized graphene hybrid material that can be used for selective membrane-free potentiometric detection of alkali metal ions, represented by potassium ions. Reduced graphene oxide (RGO) functionalized covalently by 18-crown[6] ether with a dense surface coverage is achieved by the intro-duction of a flexible linking molecule. The resulting hybrid composite is highly stable and is capable of detecting potas-sium ions down to micro-molar ranges with a selectivity over other cations (including Ca2+, Li+, Na+, NH4+) at concentrations up to 25 mM. This material can be combined further with disposable chips, demonstrating its promise as an effective ion-selective sensing component for practical applications.