Herein, a fully lipophilic ionic liquid (IL) comprising a lipophilic fluorescein anion and a trihexyltetradecylphosphonium cation was synthesized and used as the plasticizer for a plasticized poly(vinyl chloride) (PVC) membrane optode. Systematic investigation of the alkyl chain length of the fluorescein anion proved the significance of lipophilicity for obtaining the reversible absorbance measurements. A PVC membrane fabricated with the synthesized lipophilic IL was observed to comprise an unusually high dye concentration (915 mmol kg(-1)) and exhibited good sensitivity as well as response time in its sensor performance. The sensitivity of the presented PVC membrane was 26-fold higher than that of a conventional optode membrane with the same membrane thickness and the same lipophilic dye of typical dye content (1 wt%). The response time was observed to be >120-fold faster by using a significantly thinner PVC membrane (approx. 140 nm). Heparin is known to be a polyanionic anticoagulant, and the presented PVC membrane exhibited an extremely fast response (20-150 seconds) to the heparin in diluted serum within the required concentration region. Thus, the lipophilic IL-based dye could significantly improve the sensor performance in conventional optodes, especially for an analyte showing slow diffusion, such as macromolecular heparin.