Facile, green and energy-efficient preparation of fluorescent carbon dots from processed traditional Chinese medicine and their applications for on-site semi-quantitative visual detection of Cr(VI)

[Display omitted] •A novel and facile method for preparing green CDs from processed TCM was developed.•The prepared CP-CDs had excellent fluorescence property and high photostability.•CP-CDs had excellent fluorescence response to Cr(VI) with a detection limit of 15 nM.•CP-CDs based fluorescent hydrogel sensor was used to on-site visually detect Cr(VI). Green synthesis of fluorescent carbon dots (CDs) from biological sources have attracted much attention because of their low cost, excellent bio-compatibility and renewability. However, previously reported synthesis strategies of biomass-derived CDs were commonly involved in the sophisticated equipment, high energy consumption and complicated synthetic conditions, restricting their practical application in various fields. In this work, a green, facile and low energy consumption method was demonstrated to prepare highly fluorescent CDs through room temperature one-step solvent extraction from rice fried Codonopsis pilosula (CP) which could be easily obtained commercially. The obtained CP-CDs exhibited outstanding fluorescence property (quantum yield up to 12.8 %) and high photostability. Besides, CP-CDs showed high selectivity and sensitivity of fluorescence response to Cr(VI) over a broad range of 0.03−50 μM and a satisfactory detection limit of 15 nM. The developed sensing system was applied to quantitative determination of Cr(VI) in environment water samples and industrial sewage successfully with good recovery and high precision. More interestingly, CP-CDs based fluorescent hydrogel sensor was further fabricated for on-site semi-quantitative visual Cr(VI) detection in industrial sewage, demonstrating their tremendous potential for the fast and low-cost monitoring of Cr(VI) in industrial discharges.