Water-soluble acidochromic dyes lipophilized with quaternary ammonium cations as tunable hydrogen chromoionophores for polymeric optical sensors

Colorimetric ion-selective bulk optodes generate an analytical signal using chromoionophores, which are mainly derivatives of acid-base indicators lipophilized by introducing long-chain substituents. However, this method of lipophilization can be cumbersome due to the complex structure of the parent compounds and intricacy of the product purification. Here, we explore a promising approach to increase the lipophilicity of water-soluble acidochromes by ion-pairing them with lipophilic counterions. Lipophilic ion pairs of several sulfonaphthalein dyes with quaternary ammonium cations are synthesized and used as chromoionophores in coextraction-based PVC–DOS optodes. It is shown that the nature of the acidochrome and counterion determine the lipophilicity and acidity of the resulting ion pair. The partition coefficients of the obtained dyes are in the range from 10⁴ to 10⁶. The pK_a values for bromothymol and bromophenol blue ion pairs in the polymeric phase are estimated to be 16.0 ± 0.1 and 10.7 ± 0.6, respectively. The predicted and experimental pH-response of the optodes containing synthesized ion pairs is largely independent of the background electrolyte activity in contrast to conventional optodes.