Electrochemical synthesis of nitrogen-doped carbon dot inks and evaluation of their use for anticounterfeiting applications

This work investigates the optical properties of nitrogen-doped carbon dots (N-CDs) obtained by a bottom-up electrochemical approach and their potential for anticounterfeiting applications. The N-CDs were readily synthesized by application of voltage to an aqueous solution of urea and sodium citrate for 2 h, followed by a filtering and drying step. The N-CDs exhibited a strong and tunable blue to green wavelength excitation-dependent fluorescence emission and a quantum yield of 20%. The N-CDs also displayed solvatochromism, concentration-dependent fluorescence emission properties and high stability across a wide range of NaCl concentrations (0-5 M), temperatures (4-40 °C), and pHs (2-9). Fluorescent inks were obtained by dispersing concentrated N-CDs in various matrices, and their suitability for anticounterfeiting applications was tested by performing optical and accelerated aging analysis of fluorescent marks obtained by pen writing, brushing, and inkjet printing of the inks on paper. In all cases the written features were invisible under daylight and displayed bright fluorescence emission, which remained stable upon storage in the dark for several months and showed good resistance to accelerated aging processes. This novel in depth analysis performed on electrochemically produced CDs shows a route for the fast and low-cost production of fluorescent markers for applications in information security and flexible optics.