A novel fluorescent sensor for monitoring the change of pH was developed, based on a structure containing two hydroxyl groups that are sequentially deprotonated as the pH is increased: the first deprotonation increases the fluorescence intensity and the second decreases it. Compared with most reported pH sensors that can detect only whether or not the pH is below or above a specific value, our sensor could detect a specific range of pH (centered on pH 8-9) that should be useful for the study of biological microenvironment. (C) 2014 Elsevier Ltd. All rights reserved.