Determination of chondroitin sulfate in synovial fluid and drug by ratiometric fluorescence strategy based on carbon dots quenched FAM-labeled ssDNA.

•The monitor of chondroitin sulfate (CS) in synovial fluid and drug is important and demanded.•A ratiometric fluorescence assay based on positively charged nitrogen doped carbon dots and ssDNA was developed for CS.•The ratios of the fluorescence intensities for CS detection can be obtained through synchronous fluorescence spectrometry.•The assay showed a good linear relationship, low detection limit and excellent selectivity for CS.•This method successfully tested CS in synovial fluid and drug.