In this study, we present a thermoplasmonic transparent ink based on a colloidal dispersion of indium tin oxide (ITO) nanoparticles, which can offer several advantages as anti-counterfeiting technology. The custom ink could be directly printed on several substrates, and it is transparent under visible light but is able to generate heat by absorption of NIR radiation. Dynamic temperature mapping of the printed motifs was performed by using a thermal camera while irradiating the samples with an IR lamp. The printed samples presented fine features (in the order of 75 μm) and high thermal resolution (of about 250 μm). The findings are supported by thermal finite-element simulations, which also allow us to explore the effect of different substrate characteristics on the thermal readout. Finally, we built a demonstrator comprising a QR Code invisible to the naked eye, which became visible in thermal images under NIR radiation. The high transparency of the printed ink and the high speed of the thermal reading (figures appear/disappear in less than 1 s) offer an extremely promising strategy toward low-cost, scalable production of photothermally active invisible labels.
Keywords: anti-counterfeiting; heavily doped semiconductor nanoparticles; indium tin oxide; ink-jet printing; thermoplasmonics.