Lanthanide Functionalized\nCarbon Quantum Dots for\nWhite Light Emission, pH Sensing, and Co (II) Detection

Carbon quantum dots (CQDs) with fluorescence emission\nhave been\nwidely studied for versatile applications, but facile tunability of\nthe spectral properties of CQDs by doping remains to be further explored.\nHerein, employing lanthanide ion Eu<sup>3+</sup> as a dopant and activator,\na simple and efficient synthesis route for pure CQDs and Eu-CQDs was\ndemonstrated using N, N-dimethylformamide, oleic acid, and oleylamine\nas precursors for carbon sources. In comparison, with the popular\ncitric acid precursor, the as-prepared CQDs and Eu-CQDs exhibited\nan obviously smaller particle size (1.72 ± 0.29 nm) and a more\nuniform distribution. Through systematic optimization of Eu<sup>3+</sup> doping for the Eu-CQD system, colorful light emissions in the visible\nrange were first realized under excitation of ultraviolet (UV) ∼360\nnm for promising application of UV-triggered light-emitting diodes.\nMost interestingly, this Eu-CQD solution with UV-excited colorful\nlight emissions was utilized as a visual pH sensor, experimentally\nfeaturing good repeatabilities and stabilities (pH values switching\nbetween 7 and 14). Besides, the Eu-CQD solution exhibited highly sensitive\ndetection characteristics for Co (II) ions. Due to the spectral overlapping,\nthe quenching efficiency is as high as 96.3%, and the detection limit\nis obtained as 0.139 μM, which is much lower than the maximum\nconcentration of Co (II) in the drinking water permitted by WHO. All\nof these relevant results significantly demonstrate the great potential\nof lanthanide doping in the multifunctionalization of CQDs for certain\nemerging applications like lighting, sensing, and detection.