Design and synthesis of carbon dots to make an invisible anti-counterfeiting marker

Fluorescent inks are among the prominent and important compounds, appreciated to which the authenticity and validity of a document or package can be checked in a fraction of a second. They are widely used by government institutions to protect financial, economic and defense systems in law enforcement operations. The main use of invisible markers in law enforcement applications is to covertly mark items subject to a criminal offense, allowing the marked item to be easily identified.

The current research is practical in terms of its purpose and experimental-laboratory in terms of its method. In this article, the synthesis and evaluation of the efficiency of the invisible marker based on photoluminescent nanoparticles are analyzed. Trisodium citrate with different amounts of urea was subjected to ultrasonication and then stirred at 50 degrees under nitrogen gas conditions. The final solution is placed inside a microwave with a power of 900 watts and a temperature of 250 degrees Celsius for 5 minutes. Then it is kept under vacuum conditions for 24 hours. Next, solid and insoluble particles are separated. Finally, the strained solution is dried in the oven at 60 degrees. The performance of optimized photoluminescent nanoparticles was investigated and evaluated through fluorescence spectrometer, ultraviolet absorption spectroscopy, zeta potential, scanning electron microscope and infrared spectroscopy.

The quantum yield obtained for the synthesized carbon dots was 0.44 and the quantum efficiency is very suitable for fluorescent compounds. The solution of carbon dots has two absorption peaks with maximum absorption at 326 and 404 nm. Carbon dots have the maximum emission at the excitation wavelength of 365 nm, and their emission is in the range of 400 to 600 nm with the maximum intensity at the wavelength of 436 nm. Also, the synthesized carbon dots, at the excitation wavelengths in the visible region, have an emission of about 520 nanometers. The FTIR results show that there are amine and carboxyl groups on the surface of the synthesized carbon dots, which shows that the surface of the carbon dots is well functionalized. The zeta potential for the synthesized carbon dots shows the value of -10.8, which makes the carbon dots stable in the solution phase and do not stick to each other. According to the electron microscope images, the synthesized carbon dots have a smooth and spherical surface. Also, the size of the particles has been calculated to be around 20.80 nm.

The synthesis method in the microwave environment was chosen due to the higher production efficiency and more uniform size with the property of exposure of nanoparticles in the solid phase. Laboratory results showed that the emission wavelength of nanoparticles is in the region of 520 nm and their applicability as invisible tracking in different environments is possible. The prepared carbon dots are hydrophilic, stable and harmless with high fluorescence performance to make invisible markers. The luminescence and emission efficiency of the synthesized compound, in solution and solid state, was investigated by applying ultraviolet radiation and was successfully confirmed. Therefore, this type of carbon dot nanoparticles can be an excellent option for the development of invisible security markers for law enforcement and security that can reduce fraud. Stability and tunable optical properties can develop promising security elements for object authentication.