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

•A cellulose nanocrystal-based upconversion fluorescent nanoprobe was constructed.•Cellulose nanocrystal could improve stability and water dispersibility of nanoprobe.•The probe exhibited excellent selectivity and sensitivity for intracellular pH.•The biocompatibility probe enabled pH monitoring and imaging in live cells.