Controlled Synthesis of Elongated Hexagonal Uric Acid Dihydrate Crystals in Aqueous Solution with Superior Optical Properties

Uric acid is reported as a light reflecting material in the organisms, but the controlled synthesis of its crystalline forms with controlled morphology, exposed crystal planes, and enhanced optical performance remains a challenge, which limits its practical application as functional optical materials. In this work, the controlled synthesis of single-crystalline uric acid dihydrate (UAD) elongated hexagonal microplates mainly exposing the (001) plane is achieved for the first time in an aqueous solution in the presence of poly(1-vinylpyrrolidone- co -vinyl acetate) (P(VP- co -VA)) via an acid–base neutralization method. The length and width of elongated hexagonal uric acid microplates are 10–20 μm and 5–10 μm, respectively. The average thickness of them is about 300 nm. We find that two copolymers, poly(1-vinylpyrrolidone- co -2-dimethylaminoethyl methacrylate) (P(VP- co -DM)) and P(VP- co -VA), can induce the formation of elongated hexagonal uric acid crystals and stabilize the (210) and (21̅0) planes of UAD, while rectangular UAD microplates are obtained in the absence of polymer. The synthetic UAD microplates exhibit excellent optical properties, including high diffuse reflectivity in the visible and near-infrared regions and superior depolarization features, which are attributed to their high refractive index of the (001) plane and the microplates primarily exposing the (001) plane. The small thickness of the elongated hexagonal UAD microplates of about 300 nm and the high ratio of length and thickness of about 50 also contribute to the high reflectivity intensity and the strong depolarization feature of the UAD crystals. The synthetic UAD microplates with superior optical properties can be applied as functional optical materials. As far as we know, this is the first time to report the optical properties such as diffuse reflectivity and depolarization feature of synthetic uric acid crystals. This work broadens the range of functional optical crystals of synthetic purine organic molecular crystals. This work not only provides a new strategy to synthesize organic molecular crystals with a designed polymorph, morphology with particular exposing planes, and excellent optical properties but also promotes the development and application of biological-derived optical materials in related fields.