Bio-Inspired Synthesis of Square-Shaped Anhydrous Hypoxanthine Nanoplatelet Crystals with Superior Optical Properties

Hypoxanthine, a purine derivative, is one of the doping components of biogenic guanine crystals and can form cocrystals with guanine in organisms. As far as we know, there is no research work related to the controlled formation of anhydrous hypoxanthine (AH) crystals, neither the relationship of the morphology nor the optical properties of hypoxanthine crystals. Herein, the controlled synthesis of square AH nanoplatelets is realized for the first time in aqueous solution in the presence of poly(N-vinylcaprolactam). The lattice parameters of synthetic hypoxanthine crystals are simulated based on experimental powder X-ray diffraction and selected area electron diffraction data, which are proposed to be AH with monoclinic symmetry. The length and thickness of AH nanoplatelets are about 1–3 μm and 65 ± 5 nm, respectively. Poly(N-vinylcaprolactam) and Soluplus, both containing caprolactam groups, can preferentially adsorb on and stabilize the (100) plane of AH crystals. The AH nanoplatelets are composed of hypoxanthine molecules with inner-layer hydrogen bonding networks and interlayer π–π stacking. Square-shaped single-crystalline AH nanoplatelets with monoclinic symmetry are proposed to be formed via the transformation of intermediate amorphous hypoxanthine nanoparticles. The AH nanoplatelets exhibit excellent optical properties including high reflectivity, depolarization, and brilliant pearlescent luster.