Synthetic guanine crystals, with the same magnetic controllable reflection property as a biogenic guanine crystal from fish scales, were prepared using a classical Ostwald ripening method for crude crystals, from the aqueous sodium hydroxide solution of a commercially available synthesized guanine powder. The resulting synthetic guanine crystals with an average size of several tens of micrometers were in the same crystal system as the biogenic guanine crystals under measurement by X-ray diffraction (XRD). However, XRD patterns of water-floating crystals showed that the correlation between the growing direction and reflecting surface in the synthetic crystals is different from that in the biogenic crystals. Therefore, the synthetic crystals were ground by an agate mortar for refinement of its optical and magnetic-orientation characters. As a result, we realized a fast-magnetic orientation against the vertical field, which is related to the magnetic control of light reflection, the same as the biogenic guanine crystal behavior.

Guanine, which is a simple molecule with a molecular weight of only 151, is one of the most important molecules for the manipulation of light in living system.1–8 There is a possibility that the use of guanine crystals may lead to a novel micro optical device with a higher ordered function, such as an artificial iridosome or a material exhibiting structural color. Recently, we reported that biogenic guanine crystals can be used as micro optical devices that can control the reflection characteristics by using the external magnetic field response accompanied by the anisotropy of the diamagnetic susceptibility of guanine crystals.5,9–11 For example, the reflection property of guanine crystal can be switched through applying or removing a magnetic field by using the perpendicular magnetic orientation character, as shown in Fig. 1(b).9 However, guanine crystals that can be used as magnetic switchable reflectors can only be collected from living creatures, such as fish, because the shape and size of synthetic guanine crystals prepared using an artificial method are much poorer than those of biogenic crystals. A notable reason for the difficulty in the preparation of synthetic guanine crystals by an artificial method is the low water-solubility.12 Several groups have succeeded in recrystallizing synthetic guanine under basic aqueous solution, because the solubility can be increased by using the acid-dissociation equilibrium of a guanine molecule.12–14 The resulting synthetic guanine crystals have the same crystal structure as that of biogenic guanine crystals, but these external shapes are insufficient for use in magnetic switching optical devices, as described above. It is not necessary to make identical crystals as the biogenic crystals, but an external shape that provides reflection and perpendicular magnetic rotation is required for the creation of a magnetic switchable micro optical device.

FIG. 1.

Schematic illustration of rotation manner of crystals under a magnetic field: (a) In-plane rotation and (b) perpendicular rotation. Rotation direction depends on the sample and direction of the external magnetic field. Optical microscopy images of synthetic guanine crystals (top) and images indicating the orienting direction of the long axis of the crystals (bottom): (c) Before applying a magnetic field, (d) after applying a horizontal magnetic field of 130 mT, and (e) after applying a vertical magnetic field of 150 mT by using permanent magnets.

FIG. 1.

Schematic illustration of rotation manner of crystals under a magnetic field: (a) In-plane rotation and (b) perpendicular rotation. Rotation direction depends on the sample and direction of the external magnetic field. Optical microscopy images of synthetic guanine crystals (top) and images indicating the orienting direction of the long axis of the crystals (bottom): (c) Before applying a magnetic field, (d) after applying a horizontal magnetic field of 130 mT, and (e) after applying a vertical magnetic field of 150 mT by using permanent magnets.

Close modal

In this study, we have optimized the conditions to prepare a synthetic guanine crystal with an external shape that provides reflection and perpendicular magnetic rotation similar to the magnetic orientation in a biogenic crystal.9 Synthetic guanine crystals were first prepared from a basic aqueous solution at pH 13 and were ripened by using a classical Ostwald ripening method. Next, the crystallographic information regarding the correlation between growing direction and reflecting surface of the ripened synthetic guanine crystal were estimated using X-ray diffraction (XRD) measurements of the water-floating crystals. Finally, crystal grinding was applied to the synthetic guanine crystal for refinement towards our purpose.

Crystallization from synthesized high-grade guanine (077-01692, Fujifilm Wako, Japan) was performed according to our previously reported procedure.11 The synthesized guanine (12.6 g) was dissolved in an aqueous solution of sodium hydroxide (1 L) with a pH of 13 at the boiling point, and crude crystals were obtained after cooling to ambient temperature with a cooling rate of 10 °C/day. The temperature of the dispersion containing the crude crystals was heated again to ca. 100 °C for growth of the crystals. After ripening for several weeks, the ripened guanine crystals were collected by filtration after cooling to room temperature. The resulting crystals were washed with water/methanol and dried in vacuo, and then were used as the “synthetic guanine crystals” for the following measurements. Characterization of the shape of the synthetic crystal was performed by optical microscopy. Crystallographic information of the crystals, which were dried or floating in water, was obtained by using an XRD analysis.

After checking the general character of the synthetic guanine crystals without further modification, the synthetic guanine crystals were gently ground on an agate mortar in a lateral stretch manner by using an agate pestle for half-an-hour to improve its external shape of the synthetic crystal. Drastic changes were observed from the results of XRD and magnetic orientation, as described in the next section.

The crude crystals with an average size of 10 μm were boiled at ca. 100 °C in an aqueous sodium hydroxide solution at pH 13 to increase the crystal size in a classical Ostwald ripening condition over one week. The size of the resulting guanine crystal was significantly larger than that before ripening. Although the major external formation of the crystal was rod-like, which was the same as the previously reported synthetic crystals,12–14 plate-like crystals showing interference fringes, which are a sign of high-quality crystals without cracks, were also obtained (Fig. 1(c)).

For XRD measurements, two types of samples were prepared for both the synthetic guanine crystal and the biogenic guanine crystal from the scales of a goldfish.5 Here, we defined two types of samples depending on preparation methods before the XRD measurements: The type I sample is dried crystals obtained after drying the water dispersion of crystals on the surface of a flat glass plate, and the Type II sample is floating crystals in a water droplet on the surface of the glass plate covered with a wrap film. XRD measurement of the crystals floating in water (Type II) was possible by irradiating X-rays from above the water surface. The plots of XRD patterns of the two types of samples of the synthetic and biogenic crystals are shown in Fig. 2. All clear XRD peaks could be assigned to the crystal in the anhydrous guanine-phase,3,15 which proved that the synthetic guanine crystals belonged to the same crystal structure as the biogenic crystals. This agreement was also confirmed by Fourier-transform infrared spectroscopy. As a feature of the synthetic crystals, the intensity corresponding to the (122¯) plane became large and dominated along with the (002)/(011) plane, which could not be distinguished because their peak positions overlapped, in contrast to the biogenic crystals.4 These results suggested that the synthesized crystals grew with a preferential orientation that resulted in flat (122¯) and (002)/(011) planes.

FIG. 2.

Diagrams of XRD patterns of (a) dried (Type I) and (b) water-floating (Type II) synthetic guanine crystals, and of (c) dried (Type I) and (d) water-floating (Type II) biogenic guanine crystals. The diffraction peaks marked by open triangles (∆) arise from the wrap used to mount the sample. Optical microscopy images are shown as insets in (a) and (b), respectively.

FIG. 2.

Diagrams of XRD patterns of (a) dried (Type I) and (b) water-floating (Type II) synthetic guanine crystals, and of (c) dried (Type I) and (d) water-floating (Type II) biogenic guanine crystals. The diffraction peaks marked by open triangles (∆) arise from the wrap used to mount the sample. Optical microscopy images are shown as insets in (a) and (b), respectively.

Close modal

The difference between the XRD patterns were observed depending on the presence of water (Fig. 2(a) and (b)) in the case of the synthetic crystals, whereas the two patterns were matched in the case of the biogenic crystals and only the peak from (102) was observed (Fig. 2(c) and (d)). The biogenic crystals exhibited an elongated hexagonal plate having a smooth broad surface of the (102) plane and the averaged size was 20 μm×5 μm×100 nm (thickness). Such a plate-formed guanine crystal was aligned so that the (102) plane was parallel to the water surface because the crystals could float in water because of its extremely thin structure. Therefore, even when the floating biogenic crystals were dried, the (102) plane was stacked parallel to the substrate surface, as was observed from XRD. However, the synthetic crystals mainly exhibited a rectangular shape with a larger thickness compared with the biogenic crystals. The dried synthetic crystals in the Type I samples should be stacked so that the plane orientation of the crystal faces adopt various directions after drying to exhibit many XRD peaks that originate from the resulting randomness (Fig. 2(a) inset). However, in the case of the floating synthetic crystal in the Type II sample, the crystals tended to float in water with a flat surface facing up (Fig. 2(b) inset).

The magnetic orientation experiments of the synthetic guanine crystals dispersed in water by using a permanent magnet were conducted. The optical microscopy images of the crystals under a horizontal magnetic field of 130 mT and vertical magnetic field of 150 mT are shown in Fig. 1(b) and (c), respectively. The (122¯) plane and (002)/(011) plane made angles of 55.8° and 61.7°/76.0° with the (102) plane, which meant that the (102) plane was arranged inclined to the upper surface of the crystals. When a horizontal magnetic field was applied, the synthetic crystals rotated and turned the longitudinal direction of the crystal towards 90° from the field direction, which supported the growth of the crystal planes that was discussed above. It should be noted that a quick horizontal rotation can be realized in synthetic guanine crystals. This is because that the (102) plane layered with a large thickness has a high diamagnetic anisotropy energy, and therefore a large torque is generated to make the (102) plane parallel to the magnetic field, while the biogenic guanine crystal is in a thin layer with a thickness of ca. 100 nm. However, a magnetic response is not observed in most synthetic crystals under a vertical field of 150 mT. The synthetic crystals exhibited the same magnetic orientation as the biogenic crystals reported previously; however, their frequency was not large. It was proposed that the reason for the poor magnetic response of the synthetic crystals to the vertical field compared with the biogenic crystals is caused by the difference in the correlation between the external shape and the molecular alignment in the crystal.2 Such a difference should result in a different magnetic field response from the biologic crystals.

To improve the magnetic response of the synthetic crystals through the refinement of its external shape, the synthetic guanine crystals were ground by using an agate mortar to obtain cracked crystals, because several synthetic crystals immediately after the ripening had bundled together, as shown in Fig. 1. It was surprisingly obvious that the (102) peak became dominant and no remarkable peaks of the other planes were observed in the XRD measurements (Fig. 3(c), and see also Fig. 2(a) for a comparison). Moreover, the XRD pattern of the ground crystal did not change after floating the crystal in water (Fig. 3(d)), which was the same trend as the biogenic guanine, as shown in Fig. 2(c) and (d). These results indicated that the synthetic guanine crystal could be cleaved along the (102) plane after physical grinding and its general crystal structure was maintained, even if the crystals were exposed to violent conditions such as a mechanical grinding. Through the notable toughness of the synthetic crystal, the shape of the crystal could be optimized to a flat thin plate after physical grinding, which was close to the shape of the biogenic guanine crystal.

FIG. 3.

SEM image of crystal (a) before and (b) after mechanical grinding. XRD patterns of (c) dried ground synthetic guanine crystal, which is the same sample shown in (b), and (d) ground crystal floating in water (Type II sample). The diffraction peak marked by open triangle (∆) arises from the wrap used to mount the sample.

FIG. 3.

SEM image of crystal (a) before and (b) after mechanical grinding. XRD patterns of (c) dried ground synthetic guanine crystal, which is the same sample shown in (b), and (d) ground crystal floating in water (Type II sample). The diffraction peak marked by open triangle (∆) arises from the wrap used to mount the sample.

Close modal

The magnetic orientation behavior of the ground synthetic crystals is shown in Fig. 4. The longitudinal direction of the ground guanine crystal did not clearly orient along the horizontal magnetic field (Fig. 4(b)) compared with the crystal before grinding shown in Fig. 1(d), probably because the b-axis (1st easy axis) direction of the ground guanine crystal in the (102) plane became irrelevant to the isotropic external shape of the crystal. In contrast, many ground crystals were oriented perpendicular to the field and gravity under a vertical magnetic field (Fig. 4(c)). The behavior against the vertical field, which is related to the magnetic control of light reflection, was the same as the biogenic guanine crystal behavior.

FIG. 4.

Optical microscopy images of a ground synthetic guanine crystal (a) before applying a magnetic field, (b) after applying a horizontal magnetic field of 130 mT, and (c) after applying a vertical magnetic field of 150 mT by using permanent magnets. Sectional area of crystals marked by white arrows decreased under the vertical magnetic field by perpendicular rotation as shown in Fig. 1(a).

FIG. 4.

Optical microscopy images of a ground synthetic guanine crystal (a) before applying a magnetic field, (b) after applying a horizontal magnetic field of 130 mT, and (c) after applying a vertical magnetic field of 150 mT by using permanent magnets. Sectional area of crystals marked by white arrows decreased under the vertical magnetic field by perpendicular rotation as shown in Fig. 1(a).

Close modal

Here, we prepared a synthetic guanine crystal showing a magnetic controllable reflection property similar to biogenic guanine crystals. A classical Ostwald ripening for crude crystals, composed of a synthesized guanine powder, and a mechanical grinding technique were applied to refine the shapes of the resulting synthetic guanine crystal. It is surprising that the crystals with a practical property were obtained with a rough approach, which can be termed as a top-down approach. An optimization of the procedure by modifications of the recrystallization and ripening conditions is currently in progress.

This work was supported by JST CREST (grant number: JPMJCR16N1), Japan, and JSPS KAKENHI (grant number JP16K05759), Japan.

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