Spirulina-Templated Metal Microcoils with Controlled Helical Structures for THz Electromagnetic Responses

Microstructures in nature are ultrafine and ordered in biological roles, which have attracted material scientists. Spirulina forms three-dimensional helical microstructure, one of remarkable features in nature beyond our current processing technology such as lithography in terms of mass-productivity and structural multiplicity. Spirulina varies its diameter, helical pitch, and/or length against growing environment. This unique helix is suggestive of a tiny electromagnetic coil, if composed of electro-conductive metal, which brought us main concept of this work. Here, we describe the biotemplating process onto Spirulina surface to fabricate metal microcoils. Structural parameters of the microcoil can be controlled by the cultivation conditions of Spirulina template and also purely one-handed microcoil can be fabricated. A microcoil dispersion sheet exhibited optically active response attributed to structural resonance in terahertz-wave region.

Microstructures in nature are ultrafine and ordered in biological roles, which have attracted material scientists. Spirulina forms three-dimensional helical microstructure, one of remarkable features in nature beyond our current processing technology such as lithography in terms of mass-productivity and structural multiplicity. Spirulina varies its diameter, helical pitch, and/or length against growing environment. This unique helix is suggestive of a tiny electromagnetic coil, if composed of electro-conductive metal, which brought us main concept of this work. Here, we describe the biotemplating process onto Spirulina surface to fabricate metal microcoils. Structural parameters of the microcoil can be controlled by the cultivation conditions of Spirulina template and also purely one-handed microcoil can be fabricated. A microcoil dispersion sheet exhibited optically active response attributed to structural resonance in terahertz-wave region. P eople have been fascinated by highly complex microstructures produced in nature 1 . Natural materials are sophisticatedly organized in response to specific functions, whose systematical organizing processes are superior to any engineering routes for smart materials in the light of multiplicity, optimal integration, and low energy consumption. In naturally occurring microstructures, important feature is their high dimensionality, which stimulates material scientists to design functional materials. Biomimetics or bioinspiration [2][3][4] , learning microstructures produced in nature, have drawn attention as one of breakthroughs on material science and processing. But profound understanding of formation mechanism established under natural selection process must be required to develop the biomimetic process. In fact, we have suggested collaborating with nature, i.e., biotemplating process, which directly borrows the natural microstructures for new material fabrications (Fig. 1). A concept of this process can lead to mass productivity with less energy. Main issue is how to control the natural microstructures. If we can find effective factors controlling the structures from various environmental conditions, the biotemplating process would have unlimited potential and will be global strategy for material development. We have focused three-dimensional helical feature in nature as biotemplate candidate 5 . Helical microalgae, Spirulina (Arthrospira platensis) 6,7 naturally shaping left-handed (LH) open helical structure and already commercialized as nutritional supplements or food materials 8 , was employed in this study. Its helical structure is very sensitive against the environmental conditions, which should be strong advantage to exploit a variety of diameter, pitch, and handedness. Development of biotemplating process using Spirulina can achieve mass production of microcoil (mcoil), which is currently manufactured by precision machining or lithography one by one. Since Chen et al. first coated the Spirulina surface with magnetic ferrite 9 , several biotemplating processes using the Spirulina have been reported in the field of material science. However, no function specific to its helical microstructure has yet been discovered.

Results
Structure-controllability of Spirulina biotemplate. The LH helix of Spirulina is a common structure found in nature and easily obtained as the stock strain from culture collections. Under the conventional cultivation condition 10 , average feature size of the LH Spirulina ( Fig. 2a) were 6 mm in wire diameter (d), 43 mm in coil diameter (D), 174 mm in free length of coil (L free ), 74 mm in coil pitch (L free /N), 2.4 in turn number (N), and 29 u in pitch angle (a) (for graphical illustration of the symbols, see Fig. S1). The number of Spirulina in 1 mL increased from 10 2 to 10 5 within one week (Methods and Fig.  S2). During the cultivation, the d, D, and L free /N remained with narrow size distributions (around 5-15% of relative standard deviation, RSD), while the L free had a higher RSD around 20% because Spirulina grows along the long axis direction (Fig. S3). It has been known that the Spirulina forms various helical features and even linear shape under different environments. In this study, the L free /N was systematically controlled by varying cultivation temperature and light intensity (Methods and SI-II) 11 . The series of LH Spirulina with five different helical features in Fig. 2 are numbered as LH template-1 to -5. Linear Spirulina often found in laboratory cultivation of regular helical strains was obtained in pure culture (Fig. 2f). Such flexible morphologies against the environment can achieve distinction as microstructured-materials separately from commonly-used genetic control.
Spirulina-based biotemplating process. We have designed the biotemplating process using electroless plating technique, which can generate smooth metal layer selectively on surface of targeting object to be plated. In this study, the process includes (i) fixation of Spirulina, (ii) Pd catalyzation (Pd nanoparticle adsorption as plating catalyst nuclei 12,13 ), and (iii) copper electroless plating (Methods, see also Supplementary SI-III). The process (i) employed general method of tissue fixation by glutaraldehyde, which cross-links amine groups of amino-acid side chains in proteins 14 . The process (ii) includes two steps with Pd ion adsorption and reduction of the Pd ion to form the metallic Pd, which can work as catalyst oxidizing reducing reagent for the metal deposition from the plating bath. For the electroless plating, bath load, i.e., the total surface area of Spirulina to amount of plating bath, was adjusted to be 200 cm 2 /L. In the laboratory scale with 1 L of plating bath, one batch from 20 mL of the Spirulina cultivation medium with 10 5 mL 21 in Spirulina concentration gave approximately 2 million mcoils (90 mg, 80% yield). The resulting mcoils were quantitatively and qualitatively characterized by ICP, XPS, XRD, and EDX-SEM at different stage of biotemplating process (for the detailed information, see supplementary SI-III). It was found that the Pd catalyst was adsorbed to the surface as well as inside the tissue of Spirulina with almost 2 vol% in volume fraction to the Spirulina. The mcoil contained the metallic Pd of .80% and a little amount of PdO x in the nanoparticles.
The copper layer deposited on the Spirulina was consisting of the metal (.90% in content) with the oxides at the surface. The mcoils looked reddish brown with metallic luster and had tubular structure with around 550 nm in thickness, whose interior space still included the fixed Spirulina template (Fig. S15). The thickness can be controlled by changing the plating time and also the bath load, here, we demonstrated with 200 cm 2 /L in the bath load (20-mL loading amount) to obtain the thickness enough to exhibit bulk-like electric property. The efficiency of Cu deposition and relationship between the bath load and the thickness were discussed in supplementary SI-IV. The optical micrographs of mcoils (LH mcoil-1 to -5) fabricated from corresponding LH template-1 to -5 are shown in Figure 2g-2k along with straight copper wire (Fig. 2l) from the linear strain. Emphasis should be placed on that the microstructures after the biotemplating processes were faithfully transferred from those of the Spirulina templates, for example, in case of LH mcoil-1, d 5   (a)-(e), LH template-1 to -5. (f) Linear strain was prepared by the pure cultivation of laboratory-derived single trichome. The biotemplating process was successfully carried out to generate the copper mcoils whose structures are followed by those of LH templates: (coil number, L free /N); (g) LH mcoil-1, The straight copper wire was also properly templated from the linear strain.

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Right-handed (RH) strains of Spirulina are rare but they are found both in nature and in culture. Reversal of helical handedness from left to right and vice versa has been observed and ascribed to genetic drift or environmental factors like temperature uplift or mechanical stress 15 . One such RH strain obtained from Earthrise Nutritionals, California, USA, was grown in pure culture from a single trichome. The RH Spirulina was more tightly coiled smaller than around 20 mm in L free /N. The RH templates and the corresponding mcoils were prepared in the same manner as the LH series ( Fig. 3 and  Table 1. Can the mcoil behave as chiral electromagnetic material? We prepared dispersion silicon sheets of LH mcoil-1 for transmission and reflection spectroscopies in the region of millimeter wave with free space method ( Fig. 4a and 4b, SI-VI). The logarithmic transmittance decreased as the concentration of mcoil was increased. Reflection components were almost constant against the  The pitch angle equals to tan 21 (L free /N)/(pD). 2 The frequency region means that the mcoil emits elliptical polarization with the opposite handedness within the range and can be predicted with L wire /N , l 0 , 2L wire /N, as defined in helical antenna array.
The wave propagates in paraffin matrix (n 5 1.5), so that the frequency is given by Fd~3 00|n l0 . 3 The frequency region was defined as the difference between two peaks of ellipticity angles. concentration change in the region of V-band and W-band (50 to 110 GHz), although multi-reflections based on the sheet thickness were observed in every sample. The sheet showed remarkable transmission loss possibly based on absorption, for example, less than 10% transmittance at 60 GHz in case of 3 wt%. A monotonous decrease in transmittance at higher frequency region led us to realize resonance specific to the mcoil structure in the region of THz wave. THz time-domain spectroscopy (THz-TDS) with nonpolarization mode was measured with the LH mcoil-1 dispersed into paraffin ( Fig. 4c and 4d). No spectral feature with negligible reflection loss was observed in the mcoil-free paraffin sheet. Considerable transmission loss was observed over the entire region from 0.2 to 3.0 THz for the paraffin sheets containing only small amount (,2 wt%) of the LH mcoil. The mcoil concentration showed linear relationship to optical density converted from the transmittance, which is consistent with Lambert-Beer law (Fig.  S19). The concentration dependence proved no anomalous radiation from imperfect dispersibility of mcoils. Furthermore, reflectance of the sheet was around 3%, so that the transmission loss was attributed to absorption of THz wave (for the spectra of all the LH mcoils with non-polarization mode, see Fig. S20). In order to evaluate optical activity of mcoil, THz-TDS combined with polarimetric analysis (THz-TDS-PA, SI-VII) 16,17 was conducted with the same sample, LH mcoil-1. Significant difference was found in transmittances against RH and LH circular polarizations (Fig 4e). The spectrum of ellipticity angle gave negative and positive bands in around 0.5 and 1.5 THz, respectively (Fig. 4f). The spectroscopic evidences supported that the mcoil-dispersion sheet exhibits optically active THz response. Here, we considered function of the structure handedness of LH mcoil for the optical activity found in the isotropic dispersion, unlike metal helix array 18,19 . Chigrin et al. 20 reported that achiral microparts can even effectively exhibit elliptical dichroism only based on their twisted configuration.
The LH and RH mcoils are optically active isomers. The variety of mcoils fabricated through systematic control of helical structures of Spirulina biotemplates enables us to experimentally evaluate structure-specific chiral electromagnetic responses. As references, it was confirmed that the dispersion sheets containing the straight copper wire and freeze-dried LH template-2 showed optical inactive. All of LH mcoils gave the similar spectral features of their ellipticities as the LH mcoil-1, i.e., negative sign at 0.5 THz and positive one above 1.0 THz (Fig. 5a). At the same peak frequencies, the RH mcoils exhibited opposite signs of ellipticities (Fig. 5b). The LH mcoil-4 and RH mcoil-1 were selected here as enantiomeric pair with similar L free /N values. The ellipticities of the enantiomeric pair obviously showed mirror-image spectra with opposite signs; 115u for LH mcoil-4, -10u for RH mcoil-1 at 2.0 THz (Fig. 5c). Separately, another sheet containing a racemic mixture of LH and RH mcoils with 1 wt% each traced the average of individual ellipticity angle spectrum, meaning no peaks with flat spectral feature (Fig.  S21). The frequency showing sign inversion in ellipticity angle spectrum resulted in peak of rotation angle (Fig. S22). Therefore, it came to light that the LH mcoil emits the RH elliptical polarization and vice versa above the sign inversion frequency. That is to say, the LH mcoil shows dextrorotation and the RH one does levorotation.

Discussion
There are three points of view to discuss the THz response observed in this study; the functional frequency region for mcoil response, the frequency at 0 u of ellipticity angle (sign inversion frequency), the degree of ellipticity angle. First, the peak of ellipticity angle at higher frequency showed a tendency to shift inversely proportionally to length of wire to make one pitch, L wire /N. The relationship between the L wire /N and the functional frequency has been well explained by helical antenna theory 21 and helix array system 22 , whose resonant wavelength is defined as L wire /N , l , 2 L wire /N. Briefly, it can be said that the mcoil sheet operates in a half-wave resonance mode (l 5 2 L wire /N) ( Table 1 and SI-VIII). In the lower frequency region, the correlation specific to the structure was not observed. The possible reason is that there are many other resonance modes such as dipole mode, internal resonance based on L free /N, and inter-coil coupling. Second, the sign inversion frequency shifted toward lower frequency as the sample was varied from loose to tight mcoils, which appeared closely related to the a. The sign inversion shift observed here was  found as having dominant self-resonant frequency, f 0 , expressed with inductance, L, and capacitance, C, i.e., f 0~1 2p ffiffiffiffiffiffi LC p . Since the tight mcoil has the shorter L free /N as well as the larger N and/or D, the LC value should be larger, consequently giving the lower f 0 . Finally, as a crucial sense, the intensities of ellipticities decreased as the L free /N became smaller, in common with both of LH and RH mcoils (Fig. 5d). It has been reported that the helix array theoretically shows the same phenomenon 22 . In an extreme case, a ring or wire shape having L free / N 5 0 or a 5 90 u, respectively, loses chirality. We are also considering a similarity between the THz chiral behavior of mcoil and exciton chirality of optically active molecule. Similar to Davydov splitting in molecular crystal, an interactive pair of dipole moments induced by electric or magnetic field change along the helix causes the resonance energy split into two. Due to their twisted configuration, both resonances are allowed and act asymmetrically electromagnetic induction, resulting in the ellipticities with opposite signs. Further discussion on the optical activity with helical antenna response is under investigation. The alignment control in the sheet may allow us to fully understand the THz responses in our system.
The biotemplating process was successfully demonstrated to develop a new class of 3D-structured material. The Spirulina-based mcoil exhibited the optical activity, of which the conditions such as the sense of wave rotation and the operation frequency were controlled by the structural parameters of mcoils. A wide distribution on the structural parameters, always attendant on the biotemplating process, would be rather effective for broadband operation. The present fabrication process can be applied for various distinctive biological tissues and microorganism with mass-productivity and a wide variety of material form, which will promise new strategy for material design.

Methods
Spirulina cultivation and shape control. The LH Spirulina (NIES-39 and -46, stock strains in National Institute for Environmental Studies) was cultivated with conventional aqueous media 10 in an open-air water tank at room temperature under fluorescent light (2,500 lx). For the shape control of Spirulina template, the cultivation temperature and light intensity were raised up to 35 uC and 7,500 lx, respectively. The LH template-1 was obtained from the standard condition and the LH template-2 to -5 were collected every fourth day from the mass cultivation medium with the controlled condition under way into the tightening of helix, decreasing of L free /N. The linear strain was prepared by pure cultivation of the single trichome occurred by longer cultivation at the constant condition for more than 2 months.
Biotemplate process. The Spirulina was collected by a nylon mesh filter with 355 mesh and resuspended into a 100 mL of phosphate buffer solution (pH 5 7.0) including 4% glutaraldehyde as tissue fixation solution. The optical density of the Spirulina suspension was adjusted to be around 2.0 at 550 nm with an optical pass length of 1 cm, which closely equals to the order of 10 5 in the number of Spirulina per 1 mL. The suspensions were left in ambient atmosphere overnight to complete fixation of Spirulina tissues. For further storage, they are kept in cool dark place so that the Spirulina can preserve their shapes and be used as the biotemplates for several years. Copper electroless plating onto the surface of Spirulina was conducted by the use of commercial products from Okuno Chemical Industries Co., Ltd.: for delipidation treatment, OPC-370 Condiclean MA; for Pd catalyzation (adsorption of Pd chloride based alkaline ionic catalyst), OPC-50 Inducer; for activation of the catalyst (reduction of Pd ion by dimethylamine borane), OPC-150 Cryster MU; for copper electroless plating, OIC Copper. All kinds of plating baths were initially prepared for 1 L. The preparation of plating baths and each processing time were listed in Table S1. A 20 mL of fixed Spirulina suspension was filtered with the nylon mesh to collect the number of Spirulina with 2 3 10 6 , corresponding to 12 mg in weight of dried Spirulina and 200 cm 2 /L in bath load. It is noted that the surface area of one Spirulina is typically around 1 3 10 4 cm 2 . The Spirulina collected on the filter was immediately added to the baths before started to dry and stirred by mechanical stirrer with 300 rpm. The filtration and rinsing with water were successively carried out to switch the bath. After the final step with OIC Copper, the resulting copper mcoils were washed well on the filter and dispersed into 100 mL of distilled water overnight. The mcoils were collected by vacuum filtration using membrane filter (0.8mm pore size, ATTP type, Isopore TM Track-Etched Membrane Filters) and dried in the atmosphere. The amount of mcoil generally averaged 90 mg, which gave 80% yield in the case of mcoil having 550 nm in the thickness of copper layer.