Controlled synthesis of twinning beta-form anhydrous guanine nanoplatelets in aqueous solution

Biogenic twinning beta-phase anhydrous guanine nanoplatelets are utilized as optical materials in organisms. However, it is still a puzzle how to produce crystalline guanine platelets in aqueous solutions since guanine has a low solubility in water. Herein, the controlled synthesis of twinning guanine nanoplatelets was realized for the first time in an aqueous solution in the presence of poly(1-vinylpyrrolidone-co-vinyl-acetate) via ammonia volatilization. The synthesized rectangular twinning guanine nanoplatelets with an exposed (100) face were beta-form anhydrous guanine (beta-AG), and the twinning angle of the two c-axes was about 83 degrees, similar to the biogenic twinning beta-AG nanoplatelets. The length and thickness of the twinning beta-AG nanoplatelets were about 2 mu m and 42 +/- 3 nm, respectively. There were about five to seven layers observed for the synthesized twinning beta-AG nanoplatelets, with each layer being as thin as 6 +/- 2 nm. It was proposed that amorphous guanine particles are formed as intermediate precursors, which dissolve and recrystallize to single-crystalline beta-AG nanoplatelets, and then multilayered twinning beta-AG nanoplatelets are formed via pi-pi interaction with the assistance of the guanine-quartet assembly. The obtained twinning beta-AG exhibited a high reflection intensity in the visible and near infrared region and a brilliant pearlescent luster, indicating that they could potentially be applied as optical materials. This work provides a new design strategy to synthesize particular organic molecular crystals with low solubility in water and a novel formation mechanism for twinning organic crystals.