Evaluation of hemp stem retting water for carbon dots production and their phyto-activity

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Water-retting hemp stem yields high-quality fiber but generates wastewater causing environmental pollution. In the present study, the retting water of hemp stem was used to synthesize carbon dots (HP-CDs). The obtained HP-CDs not only exhibited similar fluorescence emission characteristics to that of widely studied citric acid-based CDs (CA-CDs) but also contained nitrogen elements, primarily presented in the form of graphitic and pyridinic structures. Furthermore, the effect of HP-CDs on hemp growth and physiological characteristics were examined at six concentration levels (0, 10, 100, 500, 1000, and 2000 mg⋅L-1) in comparison with those of CA-CDs. Results indicated that HP-CDs were non-phytotoxic across the tested concentration levels. In contrast, CA-CDs exhibited concentration-dependent phytotoxicity, with concentration higher than 500mgL−1 resulted in adverse effects, including increased activity of superoxide dismutase (SOD) and increased concentration of malondialdehyde (MDA) in leaves, necrotic symptom at the leaf edges, reduced root volume, and decreased dry weight accumulation. In a subsequent experiment, plants grown aeroponically with 100mgL−1 of HP-CDs promoted nitrogen uptake, and resulted in a higher leaf and stem dry weight than the plants at 0mgL−1 by 36.4% and 37.8%, respectively, which was comparable with the effects of CA-CDs. In conclusion, CDs derived from hemp retting water exhibited low biological toxicity and demonstrated similar plant growth-promoting effect as chemically derived CDs. Therefore, hemp stem retting water holds potentioal as a source for carbon dot production, and thereby supporting the sustainable hemp industry.