Design of a cellulose nanocrystal-based upconversion ratiometric fluorescent nanoprobe for pH monitoring and imaging

The monitoring of intracellular pH value is beneficial for the early diagnosis and treatment of some diseases. Here, a cellulose nanocrystal (CNC)-based upconversion ratiometric fluorescent nanoprobe for pH monitoring and imaging was constructed by chemicaliy bonding the upconversion nanoparticles donor and fluorescein isothiocyanate and rhodamine B acceptors using CNC as a bridge. The CNC bridge could improve stability and solve the problems of aggregation-caused quenching of dye on the donor surface, long donor-to-acceptor distance, and poor water dispersibility of the probe. The probe with a double internal reference of donor and rhodamine B acceptor exhibited excellent selectivity, sensitivity and visualization for intracellular pH of 5.0–7.4 with 980 nm excitation. Interestingly, the chameleon-inspired biocompatible probe based on the luminescent resonance energy transfer platform enhanced the depth of penetration and demonstrated excellent pH monitoring and imaging in living cells and in vivo. Therefore, it has promising application for the early diagnosis and treatment of some diseases.