Synthesis of Bio‐Inspired Guanine Microplatelets: Morphological and Crystallographic Control

Abstract

b-Phase anhydrous guanine (b-AG) crystalsa re one of the most widespreado rganic crystals to construct optical structures in organisms. Currently,n os ynthetic method is available that allows for producing guanine crystals with similarc ontrol in size, morphology,a nd crystallographya si nb iological ones. Herein, af acile one-step synthesis route to fabricateb io-inspired guaninem icroplatelets with (100) exposing planesi na lmost pure b-phase is reported. Thes ynthesis is based on ap recipitation process of ag uanine sodium hydroxide solution in formamide with poly(1-vinylpyrrolidone-co-vinyl acetate) as a morphologicala dditive. Due to their uniform size (ca. 20 mm) and thickness (ca. 110nm), the crystals represent the first synthetic guanine microplatelets that exhibit strong structuralc olorationa nd pearlescent lusters. Moreover,t his synthesis route was utilized as am odel system to investigate the effects of guanine analogues, including uric acid, hypoxanthine, xanthine,a denine,a nd guanosine, during the crystallization process. Our results indicate that the introduction of guanine analogues not only can reduce the required synthesis temperature but also provide av ersatile control in crystal morphologya nd polymorph selection between the a-phase AG (a-AG)a nd bAG. Turbiditye xperiments showt hat the bAG microplatelets are formed with af ast precipitation rate in comparison to a-AG, suggesting that the formation of bAG crystals follows ak inetically driven process.

Bioinspired Crystallization of Guanine

“…Synthetic guanine microplatelets exposing the (100) face in almost pure β-AG form were fabricated for the first time by using poly(1vinylpyrrolidone-co-vinyl acetate) (P(VP-co-VA)) as a morphological modifier in formamide via a facile one-step process. 35 The obtained samples were elongated hexagonal microplatelets, similar to biological β-AG microplatelets (Figure 1). The thickness of the obtained guanine microplatelets is about 110 nm according to the AFM image (Figure 1d), slightly thicker than that of the biological β-AG microplatelets extracted from carp fish, while cryo-SEM tests for fish scales showed a much smaller thickness (∼20 nm).…”

“…It is proposed that PVP K30 may adsorb onto the (100) plane of α-AG, contributing to the formation of microplatelets. Pure phase of α-AG microplatelets can also be obtained in the mixed solvents of formamide and H 2 O (volume ratio, 5:1) in the presence of guanine analogues such as guanosine and adenine at RT . The obtained single-crystalline α-AG microplatelets have regular-hexagon shape with layered internal microstructure, similar to the hexagonal β-AG microplatelets extracted from sapphirine …”

“…Single-crystalline β-AG microplatelets can be synthesized in organic solutions via a one-step process or two-step process using amorphous guanine as precursor. Synthetic guanine microplatelets exposing the (100) face in almost pure β-AG form were fabricated for the first time by using poly­(1-vinylpyrrolidone- co -vinyl acetate) (P­(VP- co -VA)) as a morphological modifier in formamide via a facile one-step process . The obtained samples were elongated hexagonal microplatelets, similar to biological β-AG microplatelets (Figure ).…”

“…(a) Optical microscopy image, (b) SEM image, (c) PXRD pattern, (d) AFM image, (e) TEM image, and (f) SAED of single-crystalline β-AG microplatelets; polarized microscopy imaging of (g) β-AG microplatelets and (h) a prism-like guanine crystal. Reproduced with permission from ref . Copyright 2020 Wiley-VCH.…”

“…First, there is still a long way to go to control the crystallization process, the polymorph, the size, and morphology of guanine crystals. For example, the thickness for the synthetic β-AG microplatelets is about 110 nm, still much thicker than the biological β-AG microplatelets from carp fish scales, which are about 100 nm according to AFM analysis, while the crystal thicknesses of the guanine crystals extracted from Koi fish scales range from 26 to 34 nm . Although the synthesis of guanine crystals with morphology of nanoplatelets and single and twinned β-AG microplatelets have been realized, the other morphologies of guanine existing in biological systems (such as white widow spiders and panther chameleons) have not been synthesized in a controlled method. , Second, the formation mechanism of the synthetic single-crystalline and twinned β-AG microplatelets is still not known very well.…”