Surface area effects on the response mechanism of ion optodes: a preliminary study

The relationship between the surface-to-volume ratio and the response mechanism of polymeric ion probes (ion optodes) is not well understood. In this work, the surface-to-volume ratio of ion optodes was systematically increased in an attempt to characterize this relationship. Several different batches of ion-selective optodes were fabricated via the solvent displacement method using sodium ionophore X, BME-44, and ETH 1001 for sodium-, potassium-, and calcium-selective optodes, respectively. Dilution of the membrane cocktail with varying amounts of an organic solvent provided a convenient tool to control the resulting particle size distribution. Specifically, ion optodes of five different size distributions were fabricated. An apparent shift of the response function on the pH scale was observed for optodes with identical composition that differed in terms of size. There was a strong correlation between the calculated specific surface area and the apparent ion-exchange constant for all three types of ion optodes. However, there was an indication that selectivity does not substantially correlate with the optode size. We hypothesize that the observed effect is caused by surface phenomena which contribute to the overall optode response. The results reported here may raise a word of caution about the application of established response models, which were developed for macroscopic ion optodes, toward probes at micrometer and submicrometer scales.