An anti-counterfeiting strategy based on thermochromic pigment activated by highly Yb3+ doped photothermal particles

The counterfeiting of products is one of the main crimes combated by private companies and governments worldwide. Therefore, numerous anti-counterfeiting mechanisms must be designed, and the photothermal properties of rare-earth doped materials provide the opportunity to formulate increasingly complex strategies. This work presents a series of highly Yb3+ doped Sr/Zr hybrid oxide particles with distinctive photo-responsive features that allow the design of an innovative anti-counterfeiting system in combination with a thermochromic pigment implemented by screen printing technique. Upon exposure of the samples to 975 nm laser diode (LD) irradiation, a fast increase of the temperature was observed, reaching temperatures from 82 to 215 °C according to the excitation power, and thus revealing the encrypted code during some seconds in a reversible way. This unique photothermal response was attributed mainly to the high amount of doping, and the inherent low thermal conductivity of the developed matrices. The reliability of the proposed system was systematically tested under different excitation wavelengths and powers, and upon exposition to an extrinsic source of heat. Such evaluation demonstrated the high-security level of the system, whose decoding can be performed only with specific light in a narrow region of the near-infrared (NIR) spectrum, additionally avoiding the use of more sophisticated equipment. The great advantages, both in manufacturing and decoding, allow our system to be implemented in anti-counterfeiting printing methods. In addition, we propose two models, the first to describe the origin of the high-temperature increase in our samples and the second to describe the dynamic temperature behavior.