Fabrication of highly luminescent red-emissive carbon dots and their enhancement of cotton growth promoted by La³⁺

Red-emissive carbon dots (RCDs) were fabricated using a simple solvothermal method with Nile blue A sulphate and malic acid. The obtained RCDs exhibited good water solubility, excellent stability and a high luminescent quantum yield. The effect of RCDs on the growth of cotton sprouts was further explored, and it was found that the RCDs could effectively promote growth by enhancing root vitality. Furthermore, it was found that adding lanthanum to the RCD solutions could achieve better promotion, with the optimum concentration measured to be 0.02 mg mL^-1 of RCDs for mixed solutions, which was much lower than that of the RCD solution alone (0.03 mg mL^-1). Further analysis results showed that adding lanthanum could enhance the absorption of RCDs by the cotton sprouts, which might be induced by the enhancement of endocytosis in root cells triggered by lanthanum. This work not only provides novel long-wavelength emissive carbon dots with excellent biological activity but also holds positive significance for plant physiology and agricultural production.