Letter pubs.acs.org/Langmuir
Bioinspired Synthesis of a Hollow Metallic Microspiral Based on a Spirulina Bioscaffold Xiaoliang Zhang, Mei Yu, Jianhua Liu,* and Songmei Li School of Materials Science and Engineering, Beihang University, Beijing 100191, China S Supporting Information *
ABSTRACT: Bioinspired synthesis approaches aim to take advantage of the morphology and structural features of biological materials for the development of functional micro/ nanodevices. In this Letter, we report that a unicellular algae known as a Spirulina was applied as a bioscaffold for the synthesis of hollow metallic Cu microspirals with length of 200−300 μm. The electroless deposition method was employed to cover the spirulina forming the spiral. The nanomechanical properties of the spiral were investigated by using the nanoindentation technique. The results showed the hardness and elastic modulus of the spiral were 0.63−0.68 GPa and 12.35−12.63 GPa, respectively. Other metallic or alloy spirals could also be synthesized by using the spirulina as a bioscaffold with low cost and high reproducibility, and the obtained spirals could be promising materials as functional micro/nanodevices for microelectromechanical systems.
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synthesize hollow micro/nanopartices.18−20 Very recently, plasmid DNA separated from Bacillus hosts was also applied as an effective bioscaffold for the synthesis of metallic nanoparticles and nanorings.21 The spirulina, in the shape of perfect spiral coil, is a microscopic unicellular algae, and it exhibits prolific reproductive capacity and easy preparation and handling.22 In this letter we report the synthesis and nanomechanical characterization of metallic Cu microspirals using spirulina as a bioscaffold.
INTRODUCTION Biological materials serving as scaffolds have been introduced in materials design and preparation.1−4 Biological materials are usually environmentally friendly, abundant in nature, and exhibit multiple size scales that vary from nano and micro to macro.5 They display an astonishing variety of complex structures that are difficult to obtain even with the most technologically advanced synthetic methodologies.6 The most significant characteristic of biological materials is high morphology reproducibility because of the self-assembling and self-reproducing properties of biological systems. Building on these characteristics, nanosized biomolecules and microsized microorganisms have successfully served as scaffolds for the synthesis of novel micro/nanomaterials with different shapes and scales.7−12 The metallic Cu microspiral is a promising material that could be used as a functional microinductor and microspring for the micro/nanoelectronics and interlocking reinforcement for composite materials because the metallic Cu exhibits low resistivity, high electron migration resistance, and increased stability at elevated temperatures.13,14 The mechanical properties, however, are also significant for further developments and applications of spiral micro/nanostructures. Recently, the nanoindentation technique has been established as an important tool to study the mechanical properties of both block solids and micro/nanostructures.15,16 In the nanoindentation technique, a loading−unloading curve is obtained and the hardness and elastic modulus are obtained by the Oliver and Pharr method.17 To date, the preparation and mechanical characterization of these complex structures remains a tremendous challenge. In previous work, we have reported that bacteria can be used as a biotemplate to © 2012 American Chemical Society
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RESULTS AND DISCUSSION The electroless deposition method was applied to synthesize the Cu microspiral based on the spirulina bioscaffold. The synthesis process of the spiral is illustrated in Figure 1. The spirulina (Figure S1) used in this work was a kind of green plant, and it was cultured in a liquid nutrient medium under abundant light to promote photosynthesis. Before the electroless deposition, the spirulina was treated with a colloidal Pd/Sn catalyst leading to noncovalent catalyst adsorption on the surface of spirulina (Figure 1c). The “one-step”20 method was used to coat the spirulina and to make it suitable for the subsequent Cu deposition (See Supporting Information). The catalyst-treated spirulina was then immersed in the alkaline electroless deposition solution to deposit a confluent metallic Cu film covering the spirulina (Figure 1d). As described in the Supporting Information, CuSO4·5H2O was used as the source of Cu2+ ions and formaldehyde was used as a reducing agent to Received: November 7, 2011 Revised: January 15, 2012 Published: January 17, 2012 3690
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Figure 1. Schematic for the synthesis of the metallic Cu microspiral: (a) the green circle corresponds to a cross section of a spirulina, (b) and (c) the introduction of catalyst Pd onto the surface of spirulina, and (d) electroless deposition of Cu onto the surface of the spirulina.
reduce the Cu2+ ions to metallic Cu deposited on the surface of the spirulina. The Cu-coated spirulina was collected on a membrane filter and the morphology and dimensions of the collected spirulina were studied by scanning electron microscopy (SEM), as shown in Figure 2. The SEM images indicated that the spirulina exhibited a helical shape averaging ca. 10 μm in diameter and 200−300 μm in length. Observation by SEM images of the metallic spirulina confirmed that the metallic Cu was successfully deposited on the surface of the spirulina forming a Cu microspiral, as shown in Figure 3. The interior of the spiral is hollow and has an inner diameter of ∼10 μm, as shown in the inset of Figure 3b. The thickness of the coating on the surface of the spirulina was estimated to be approximately 2 μm (Figure S2). The main component of the spirulina is moisture. In the drying process, the moisture of the spirulina was vaporized and the volume of the spirulina was shrunk forming the inner hollow structure within the spiral. The surface of the spiral was typically a rough structure (Figure 3b), which was traced to three sources. First, during the catalysis process, pure Pd clusters were formed and adsorbed on the surface of the spirulina and subsequently served as nucleation sites for the deposition of the metallic Cu coating. The metallic Cu developed only at these nucleation sites and, ultimately, formed a continuous structure on the surface of the spirulina. Second, the rough structure is a major characteristic of the electroless deposition. The electroless deposition process is an autocatalytic reaction that, once there is an initial nucleation of Cu, proceeds indefinitely without requiring a contiguous ground plane of metal.23 Similar rough structures were also observed by Shukla et al.24 and Price et al.14 during electroless deposition of Cu on the surface of other
substrates. Lastly, during the electroless deposition process, the Cu2+ ions were reduced by formaldehyde to be metallic Cu and were deposited onto the surface of catalyst-coated spirulina. The hydrogen gas generated in the electroless deposition process was adsorbed on the surface of spirulina. With the effect of the adsorbed hydrogen gas, the spirulina floated in the electroless deposition solution leading to uneven deposition of metallic Cu on the spirulina surface. Energy-dispersive X-ray (EDX) spectroscopy confirmed that the spiral was composed mainly of Cu element (Figure S3). The nanoindentation hardness and elastic modulus of the spiral were determined from the loading−unloading data in the nanoindentation technique. Nanoindentation hardness, H, is the mean pressure that a material will support under loading.13 Thus, the hardness is determined from the maximum loading divided by the projected contact area of the indentation from the loading−unloading data as P H = max A
(1)
where A is the projected contact area and Pmax is the maximum indentation loading. According to the Oliver and Pharr method, the elastic modulus of a sample that exhibits plastic deformation during loading is determined from the initial unloading. Thus, the elastic modulus is calculated by fitting the unloading curve to a power-law relation. The unloading stiffness is obtained from the slope of the initial portion of the unloading curve, S = dP/dh. Based on the relationships developed by Sneddon,25 the indentation of the elastic half spaced by any punch was described as a solid of revolution of a smooth function, a geometry independent relation involving contact stiffness and 3691
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Figure 3. SEM images of the metallic Cu microspiral at low (a) and high (b) magnification, respectively. The inset in (b) shows the hollow structure of the spiral.
affects the nanoindentation measurements. However, if the indentation depth does not exceed 30% of the product’s diameter, the effects on the measurements of the hardness and elastic modulus can be ignored.26 A series of nanoindentation measurements were performed on a single spiral. Figure 4 shows the typical loading−unloading curves for the ethoxyline resin substrate (Figure 4a) and a single spiral (Figure 4b). It is noted that the ethoxyline resin substrate and the spiral produced different indentation loading−unloading curves. The inset of Figure 4b shows a SEM image taken after the nanoindentation measurement of the spiral. The resulting triangular indent was clearly observed. The loading−unloading data of the spiral was used to calculate the hardness and elastic modulus of the spiral, which were 0.63−0.68 GPa and 12.35− 12.63 GPa, respectively. The results were reproducible over multiple tests. Comparing with bulk Cu with hardness value of 3.02 GPa27 and elastic modulus value of 108−135 GPa,28 the hardness and elastic modulus of the spiral were relatively low. The hardness and elastic modulus of the electroless deposition Cu films on the other solid substrate are 1.5 and 120 GPa,29 which are also lower than the bulk Cu. The reduction of the hardness and elastic modulus of the spiral in this study were traced to two sources. On one hand, the reductions were attributed to the high surface-to-volume ratio of the spiral. Unlike the atoms locked in the lattice of bulk Cu, there were some defects or pores in the Cu coating of the spiral. The surface atoms were less constrained, making the spiral easier to deform and, consequently, leading to relatively lower hardness and elastic modulus. On the other hand, the spiral had a hollow structure and the nanoindenter tip penetrated the Cu coating and into the interior of the spiral. The top wall of the hollow spiral worked as a membrane that was bent under the indentation loading.
Figure 2. SEM images of the collected spirulina at low (a) and high (b) magnification, respectively.
contact area. Therefore, the elastic modulus is derived as follows: S = 2β
A Er π
where β is a constant that depends on the geometry of indenter (β = 1.034 for a Berkovich indenter),7 and Er is reduced elastic modulus which accounts for the fact that elastic deformation occurred in both the sample and indenter. Er is given by 1 − vi2 1 1 − v2 = + Er E Ei
(2)
the the the the
(3)
where E and v are the elastic modulus and Poisson’s ratio for the sample, and Ei and vi are the same quantities for the indenter. For diamond, Ei and vi are 1141 GPa and 0.07, respectively. The spiral was embedded in the ethoxyline resin substrate to fix them and prevent them from moving or rolling during nanoindentation measurements (Figure S4). The embedded samples were ground and polished. A Berkovich diamond nanoindenter tip was used to perform the nanoindentation measurements of the ethoxyline resin substrate and a single spiral. The nanoindenter tip was shifted to the center of a single spiral to perform an indentation and the nanoindenter recorded the loading and unloading of the nanoindenter tip. The peak depth of the nanoindentation measurements was 1000 nm or about 10% of the spiral diameter. The product dimension 3692
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Letter
AUTHOR INFORMATION
Corresponding Author
*Tel.: 86-010-82317103, Fax: 86-010-82317103, E-mail: liujh@ buaa.edu.cn. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (No. 51001007), Fundamental Research Funds for the Central Universities, and Innovation Foundation of BUAA for PhD Graduates.
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Figure 4. Loading−unloading curves for (a) the ethoxyline resin substrate and (b) the metallic Cu spiral. The inset in image (b) is an SEM image of a Berkovich indentation on the spiral.
In conclusion, a hollow metallic Cu microspiral with length of 200−300 μm was successfully synthesized using a spirulina microorganism as a bioscaffold. The obtained spiral maintained the original shape of the spirulina. The interior of the spiral has a hollow structure with an inner diameter of ∼10 μm and the thickness of the coating on the spirulina surface was estimated to be approximately 2 μm. The hardness and elastic modulus of the spiral were studied by using a nanoindenter, which indicated that the hardness and elastic modulus of the obtained spiral were 0.63−0.68 GPa and 12.35−12.63 GPa, respectively. Although the metallic Cu spiral was synthesized based on the spirulina bioscaffold with low cost and high reproducibility in this work, this simple approach can also be used to synthesize other metallic spirals such as Co, Ni, Ag, or alloys to vary the magnetic, electrical, chemical, and physical properties. The obtained spiral could be a promising material for functional micro/nanodevices for microelectromechanical systems (MEMS) applications and as an interlocking reinforcement for composite materials.
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ASSOCIATED CONTENT
S Supporting Information *
Detailed experimental approaches, the amounts of reagents used in this work, and additional figures. This material is available free of charge via the Internet at http://pubs.acs.org. 3693
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(29) Chang, S. Y.; Lee, Y. S.; Chang, T. K. Mater. Sci. Eng., A 2006, 423, 52−56.
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