<p>A novel approach for developing an optical sensor membrane tailored for the detection of thorium ions (Th⁴⁺) has been explored. The novelty of designed optode is created by embedding Eriochrome Cyanine R (ECR), tri-<i>n</i>-octyl phosphine oxide (TOPO), and sodium tetraphenylborate into a plasticized poly(vinyl chloride) matrix with <i>o</i>-nitrophenyloctyl ether as the plasticizer. ECR plays a dual function as both an ionophore and a chromoionophore, while TOPO facilitates the complexation of Th⁴⁺ ions with ECR. The membrane composition has been precisely tailored to achieve maximum efficiency. The proposed membrane demonstrates a linear dynamic range for Th⁴⁺ ions between 6.0 × 10⁻⁸ and 4.5 × 10⁻⁵ M, with detection and quantification limits of 1.75 × 10⁻⁸ M and 5.9 × 10⁻⁸ M, correspondingly. No significant interference from various anions and cations was detected during the assessment of Th⁴⁺. The optode represented excellent stability and a rapid response time of approximately 5.0&#xa0;min, with no evidence of ECR leaching. The current optode exhibits high selectivity for Th⁴⁺ ions and can be regenerated by treatment with 0.1 M HCl. It has been successfully utilized to determine Th⁴⁺ levels in both biological and environmental samples.</p> Graphical Abstract <p></p>