Effect of spontaneous degradation of ion-selective optodes receptor materials on sensors performance. Where is the catch?

Interest in application of biodegradable polymers as matrix for ion-selective sensors, including optodes is likely to increase. However, finding an alternative for plasticized poly(vinyl chloride) or polyacrylates, requires among others consideration if spontaneous changes in these materials affect the analytical performance of sensors. In this work we study ion-selective optodes prepared using biodegradable matrix: p-polycaprolactone and acetyl tributyl citrate using classical constituents: optically silent ionophore and chromoionophore as optical transducer. The effect of spontaneous changes of sensor constituents on its performance was studied. It was confirmed that hydrolysis of receptor is occurring leading to change of the matrix composition. Surprisingly it was found that the key factor affecting performance of sensor is spontaneous transformation of the dye embedded within it, typically considered as stable within the sensor matrix. N-octadecanoyl-Nile blue, i.e. chromoionophore I, undergoes acidic hydrolysis leading to formation of Nile blue and Nile red, and ultimately resulting in decreased sensor performance. The hydrolysis of chromoionophore in plasticized polycaprolactone matrix is significantly affected by incorporation of carbon dioxide to the solution containing nanoprobes. As model sensors hydrogen or potassium ions selective optodes were tested.