Acid-mediated synthesis coupled with liquid–liquid extraction separation for obtaining red and orange double-color carbon dots: Application for pH, water sensing and cell-imaging

•Acid-mediated synthesis with liquid–liquid extraction assisted separation strategy was built.•The developed strategy was successfully used to obtain double-color long wavelengths CDs.•R-CDs with acid-sensitivity can be applied as sensing platform for pH and cell-imaging.•O-CDs with dual emissions can be used to water content detection in various organic solvents.