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Tricolor emissive CDs with multiple-cores@shell structure were synthesized by one-pot hydrothermal route. The surface Aphen moieties endow the as-synthesized AC-CDs with promising pH-sensitive function to prepare fluorescent pH test paper towards qualitative and quantitative detection of pH in the solution. CDs can be used as a cell imaging marker and efficient cellular labelling to detect intracellular pH due to its unique tricolor emission property. • Tricolor emissive CDs with multiple-cores@shell structure were synthesized by one-pot hydrothermal route. • The as-obtained CDs were used to prepare fluorescent pH test paper with promising pH-sensitive feature. • CDs can be used as a cell imaging marker and efficient cellular labelling to detect intracellular pH. • Unique tricolor property of CDs can avoid the fluorescent dyes and protein fluorescence interference for cell imaging. Recently, fluorescent pH test paper has been extensively explored for visual assays due to its easy preparation, low-cost, and portable feature. In addition, intracellular pH also plays important roles in different pathological and physiological processes for understanding cellular functions and providing critical assistance for early disease diagnosis. Herein, the tricolor emission multiple-cores@shell carbon dots (CDs) were synthesized via one-pot hydrothermal method using 5-amino-1,10-phenanthroline (Aphen) and citric acid (CA) as the carbon sources. The fluorescence emission of CDs showed an interesting pH dependence due to the special structure and properties, which can make the CDs to be used as the pH test paper to qualitatively and quantitatively detect pH value in a wide pH range. At the same time, because of good cell compatibility and low toxicity, CDs can be used as a new type of cell imaging marker to detect intracellular pH. The unique tricolor emission property of CDs can well avoid the fluorescent dyes and protein fluorescence interference. [ABSTRACT FROM AUTHOR]