Fluorescent poly(styrene-block-vinylpyrrolidone) nanobeads for optical sensing of pH

Nanosensors for pH are prepared by staining poly(styrene-block-vinylpyrrolidone) nanobeads with lipophilic pH indicators based on fluoresceins and 1-hydroxypyrene-3,6,8-trisulfonate. Fine tuning of the pH-sensing properties is achieved by varying substituents on the pH indicator. Fluoresceins based on octadecylester- and 5(6)-N-alkyl-carboxamido derivatives show significantly different sensing properties due to lactonization occurring in the latter. The nanobeads show pKa values varying from 5.6 to 7.7, possess low cross-sensitivity to the ionic strength, and are potentially suitable for application in biology, biotechnology, marine science, and other fields.