Controllable Synthesis of Carbon Dots with Excitation-Wavelength-Dependent or Independent Photoluminescence for the Selective and Sensitive Detection of Co²⁺ Ions

Fluorescent carbon dots (CDs) with excitation-wavelength-dependent and excitation-wavelength-independent emission have been prepared with varying amounts citric acid (CA), ethylenediamin end-capped polyethylenimine (PEI-EC) and sodium borohydride (NaBH₄), their structure, optical and chemical sensing properties have been also systematically investigated. The varying of NaBH₄ and PEI-EC renders the structure and surface state change as well as photoluminescence (PL) properties. The results indicated that NaBH₄ made the CDs sensitive to Co²⁺ ions, whilst the PEI-EC served to increase their sensitivity. The synergistic roles that both NaBH₄ and PEI-EC played on the selective and sensitive detection of Co²⁺ were achieved via a simple adjustment of the mass ratios during synthesis. Furthermore, the PL intensity was gradually recovered until completely restored with the addition of chelating agent ethylenediaminetetraacetic acid (EDTA) to the Co²⁺ quenched CDs solutions. This work provided quantitatively preparation and controllable synthesis of excitation-wavelength-dependent or independent CDs for the development of PL based sensors.