Photoluminescent security ink for continuous ink-jet printing

A photoluminescent security ink for continuous ink-jet printing, wherein said ink has a viscosity of about 1.5 mPas to about 6 mPas at 25° C., and comprises:

• a) from about 1 wt-% to about 6 wt-% of uncapped NaX1-y-zF4YbyZz nanoparticles wherein
• X is selected from the group consisting of Y3+ and Gd3+;
• Z is selected from the group consisting of Er3+, Tm3+ and Ho3+;
• 0<y+z≤0.4;
• and 0≤z≤0.1,
• and wherein the uncapped NaX1-y-zF4YbyZz nanoparticles have a mean diameter of between about 30 nm to about 70 nm as measured by scanning electron microscopy;
• b) from about 1.5 wt-% to about 10 wt-% of a dispersing agent selected from the group consisting of a polyvinyl acetal resin, a vinyl chloride copolymer containing from about 60 wt-% to about 95 wt-% of vinyl chloride, a polyurethane resin, a nitrocellulose resin, an ester cellulose resin, an alkyl (meth)acrylate polymer, and mixtures thereof;
• c) from about 80 wt-% to about 90 wt-% of an organic solvent selected from the group consisting of a ketone, an alcohol, an ester, and mixtures thereof;
• and
• d) from about 0.1 wt-% to about 1 wt-% of a conductive salt,
• wherein the weight percents are based on the total weight of the photoluminescent security ink.