Ciphers and invisible ink – many of us experimented with these when we were children. A team of Chinese scientists has now developed a clever,tall-tech version of "invisible ink". As reported in the journal Angewandte Chemie, the ink is based on carbon nitride quantum dots. Information written with this ink is not visible under ambient or UV light; however, and it can be seen with a fluorescence microplate reader. The writing can be further encrypted or decrypted by quenching or recovering the fluorescence with different reagents.
Source: phys.org