Parameters Affecting Synthesis of Carbon Dots for Quantitation of Copper Ions
Carbon dots (C dots) have been prepared from histidine hydrochloride in the absence or presence of halides (Cl, Br, and I) at various potentials up to 10 V. The time-evolution UV-vis absorption and photoluminescence (PL) spectra provide more detailed information about the formation of C dots. Upon increasing reaction time from 1 to 120 min, more and more C dots are formed, leading to increased PL intensity. The halides play two important roles in determining the formation of C dots; controlling the reaction rate and surface states. When compared to chloride and bromide, iodide has a greater effect on varying surface states and inducing PL quenching through intersystem crossing. The PL intensities of the four types of C dots all decrease upon increasing Cu2+, Hg2+, and Ag+ concentrations although Cu2+ has greater quenching efficiency. In the presence of 0.8 mM I-, the C dots prepared in the presence of I- over the other three types of C dots are slightly better for quantitation of Cu2+. Fourier transform infrared spectroscopy, cyclic voltammetry, electrochemical impedance spectroscopy, and X-ray photoelectron spectroscopy results of the C dots reveal the interactions of Cu2+ with the surface ligands (imidazole and histidine). The C dot probe is selective toward Cu2+ over the tested metal ions such as Hg2+ and Ag+, with a limit of detection of 0.22 µM for Cu2+ at a signal-to-noise ratio of 3. Practicality of this probe has been validated by the analyses of tap, lake, and sea water samples, with negligible matrix effects.