A novel security ink that emits intense red colour when exposed to 254 nm wavelength UV and emits green colour soon after the UV source is turned off has been synthesised by a team of researchers from the Delhi-based National Physical Laboratory (CSIR-NPL). The emission of red is due to fluorescence while green is due to phosphorescence phenomenon. Both red and green can be clearly seen with the naked eye under ambient conditions.
The red colour is emitted at 611 nm wavelength while the green is emitted at 532 nm. The ink has the potential to be used as a security feature on currency notes and passports.
Two pigments
“To the best of our knowledge, this is the first report of an ink that contains two pigments that emit different colours at very different wavelengths when exposed to UV light of a particular wavelength,” says Dr. Bipin Kumar Gupta from NPL who led the team of researchers. The results were published in the Journal of Materials Chemistry C.
Unlike in other materials, the ink shows phosphorescence as the emission of the red pigment is not quenched by the green pigment while the UV lamp is on as the two have very different emission wavelengths — 611 nm for red and 532 nm for green. Also, when exposed to 254 nm UV light, the excitation spectrum of one does not cover the other.
Hydrothermal processing
The team first synthesised the pigments that emit red and green colours. For synthesising the red pigment, sodium yttrium fluorite doped with europium through hydrothermal method. For the green pigment, the researchers mixed strontium aluminium oxide and doped it with europium and dysprosium.
“We have to use two dopants for the green pigment as continuous generation of photons is needed for phosphorescence. In this case, the europium provides the electrons while dysprosium provides the holes. The electrons and holes recombine to create photons,” explains Dr. Gupta.
The red and green pigments synthesised separately are mixed in 3:1 weight ratio and heated to 400 degree C for three hours. “Annealing [heating] at 400 degree C ensures that the rods [of sodium yttrium fluorite red pigment] adhere to the spheres [of the strontium aluminium oxide green pigment],” says Dr. Gupta. “If the two pigments are mixed without annealing then the two pigments would separate out during ink formation and the desired property of the ink to produce dual emission with single excitation will not be possible.”
The ink is prepared by dispersing the two pigments that have been mixed at a high temperature in a commercially available polyvinyl chloride (PVC) medium and vigorously stirred for an hour.
“The advantage of having the rods sticking to the spheres is that the rods don’t cover the spheres completely and so both the pigments are exposed to UV irradiation,” says Amit Kumar Gangwar from CSIR-NPL and first author of the paper. “In the core-shell structure that we tried, the shell tends to block UV excitation and so the emission from the core is reduced.”
Lasting phosphorescence
While the green phosphorescence is seen even if the ink is briefly exposed to UV radiation, exposing the ink to UV for 15 minutes ensures that the phosphorescence lasts for about four hours.
The researchers found the images printed on ordinary paper using the ink exhibits excellent physical durability and chemical stability. There was no noticeable change in emission from the images even at the end of six months when exposed to high (42 degree C) and low (10 degree C) temperatures and high humidity. The emission showed no changes when the images were exposed to various bleaching solutions. “We carried out accelerated testing and found that the images to be stable for more than 20 years,” says Dr. Gupta.
