The structure-activity relationship of hydrophilic carbon dots regulated by the nature of precursor ionic liquids

The preparation of hydrophilic carbon dots (HCDs) with imidazolium dicyanamide ionic liquids (ILs) as precursor revealed a unique structure-activity relationship for the IL-HCDs. Their hydrophilicity, fluorescence nature and cytotoxicity are closely correlated to the alkyl side chain length of the imidazolium cationic moiety. (1) The hydrophilicity of the precursor ILs decreases with the alkyl chain length of their imidazolium cations (from ethyl, butyl, hexyl, octyl to decyl). On the contrary, that of the IL-HCDs increases with the alkyl chain length due to the emergence of COC, NH₂ moiety. (2) The passivation effect of alkyl chain plays a dominative role in the enhancement of quantum yield (QY, from 4.6% to 48.0%) of IL-HCDs. The doping of nitrogen-containing moieties contributes marginally. (3) The increase of alkyl chain length leads to the weakening of IL-HCDs/bovine serum albumin (BSA) affinity with a decrease on the quenching constants from 12.59 × 10⁴ to 1.779 × 10⁴ L mol^-1. (4) The cytotoxicity of IL-HCDs increases with the length of alkyl chain in the imidazolium cation, though the hydrophilicity of IL-HCDs is increased. In addition, the cytotoxicity of IL-HCDs/BSA is lower than that of IL-HCDs. The protective effect of BSA in the IL-HCDs/BSA 'protein corona' could be utilized to improve the biocompatibility of IL-HCDs.