Hg(II) sensing based on functionalized carbon dots obtained by direct laser ablation

The synthesis of carbon nanoparticles obtained by direct laser ablation [UV pulsed laser irradiation (248 nm, KrF)] of carbon targets immersed in water is described. Laser ablation features were optimized to produce carbon nanoparticles with dimensions up to about 100 nm. After functionalization with NH2–polyethylene-glycol (PEG200) and N-acetyl-l-cysteine (NAC) the carbon nanoparticles become fluorescent with excitation and emission wavelengths at 340 and 450 nm, respectively. The fluorescence decay time was complex and a three-component decay time model originated a good fit (χ = 1.09) with the following lifetimes: τ1 = 0.35 ns; τ2 = 1.8 ns; and τ3 = 4.39 ns. The fluorescence of the carbon dots is sensitive to pH with an apparent pKa = 4.2. The carbon dots were characterized by 1H NMR and HSQC and the results show an interaction between PEG200 and the carbon surface as well as a dependence of the chemical shift with the reaction time. The fluorescence intensity of the nanoparticles is quenched by the presence of Hg(II) and Cu(II) ions with a Stern–Volmer constant (pH = 6.8) of 1.3 × 105 and 5.6 × 104 M−1, respectively. As such the synthesis and application of a novel biocompatible nanosensor for measuring Hg(II) is presented.