13 October 2011
Scientists from China have created nanoparticles with dual mode colour for anti-counterfeiting ink, making it harder to imitate than current inks.
Lehui Lu and colleagues from the Chinese Academy of Sciences, Changchun, have designed dual mode fluorescent lanthanide doped nanocrystals to make the ink. The nanocrystals display upconversion, in which particles absorb light of one wavelength and emit light of a shorter wavelength, and downconversion, in which a high energy photon is split into two lower energy photons. These are triggered by near infrared and ultraviolet light, respectively, to produce different colours. The crystals would make the ink difficult to replicate if used on important documents as an anti-counterfeiting measure.
Traditional anti-counterfeiting materials only emit one colour so are more easily replicated. Including more colours involves mixing different nanocrystals, which could affect ink quality. Now, ‘colour tuning can be achieved from a single nanocrystal, avoiding a decrease in the ink’s quality’, says Lu.
The team bound the nanocrystals to oleic acid, which stabilises them in organic solvents so that they can be applied to paper. They tested the crystals by stamping the ink onto a transparent film. In daylight, the effect was invisible. However, when they shone infrared light on the film the stamped section was clearly seen, with a bright green upconversion emission. When they changed to ultraviolet light, the colour changed to blue because of the downconversion.
Exposing film stamped with the ink to near infrared light caused a green emission, while under ultraviolet light, a colour change from green to blue was seen
The nanocrystals could also be developed for use in biological imaging, as Yadong Li, an expert in lanthanide-doped nanocrystals from Tsinghua University, P. R. China, points out. ‘The near infrared emission is suitable for in vivo imaging, owing to the weak autofluorescence background and deeper penetration,’ he says.
The next step for Lu is to increase the nanocrystals’ quantum yield by increasing the number of molecules participating in the process. ‘Compared to traditional organic dyes,’ explains Lu, ‘the quantum yield of oleic acid-stabilised lanthanide doped fluoride nanocrystals is relatively low. Improving the quantum yield is a big challenge.’
Designing lanthanide-doped nanocrystals with both up- and down-conversion luminescence for anti-counterfeiting
Yanlan Liu, Kelong Ai and Lehui Lu, Nanoscale, 2011