Fluorescent pH nanosensors: Design strategies and applications

Accurate measuring of pH is of great significance for various research areas ranging from environmental to chemical and biological sciences. In the past decades, there has been growing interest in the use of nanoparticles (NPs) for pH measurement, especially for intracellular pH sensing and imaging. In this regard, a number of different NP-based fluorescent pH sensors have been developed, which can be classified into three major categories including (I) fluorescent NPs with direct or indirect responses to pH, (II) nonfluorescent NPs that are used only as scaffold and carriers for pH-sensitive fluorescent dyes, and (III) nonfluorescent NPs whose pH-responsive structural change is converted into the fluorescence signal of their conjugated dyes. This review is a complete coverage of all NPs used so far for fluorescence pH sensing. The authors of this review invite readers to find all design strategies for employing semiconductor quantum dot, nanoclusters, carbon-based dots, polymer dots, upconversion NPs, fluorescent metal-organic frameworks, metallic NPs, silica NPs, polymer NPs, micellar NPs, nanogels, and protein NPs for this purpose. (C) 2019 Elsevier B.V. All rights reserved.