Skin Collagen Fiber-Biotemplated Synthesis of Size-Tunable Silver

After the adsorption of Ag+, the peak from the HO–C group in the O1s XPS ..... of Size-Tunable Silver Nanoparticle-Embedded Hierarchical Intertextur...
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Skin Collagen Fiber-Biotemplated Synthesis of Size-Tunable Silver Nanoparticle-Embedded Hierarchical Intertextures with Lightweight and Highly Efficient Microwave Absorption Properties Junling Guo,†,‡ Xiaoling Wang,† Xuepin Liao,*,† Wenhua Zhanga,‡ and Bi Shi*,†,‡ †

Department of Biomass Chemistry and Engineering, Sichuan University, Chengdu 610065, China National Engineering Laboratory for Clean Technology of Leather Manufacture, Sichuan University, Chengdu 610065, China

‡

S Supporting Information *

ABSTRACT: The bioinspired approach to the construction of well-ordered microstructures is a crucial intersection of the branches of materials science and biotechnology. In this study, size-tunable silver nanoparticles (Ag NPs) have been successfully prepared on a skin collagen fiber (SCF) biotemplate, which shows a hierarchical interwoven structure in nature having novel, special, and highly compact interlaced, biosupported Ag NPs chains. Conductivity measurements indicate that these novel Ag NPs/SCF@BT composites are semiconductive and have a typical percolation threshold of 0.85% (w/w) silver fraction. The enhancement of dielectric loss properties of Ag NPs/SCF@BT can be expressed by the Debye dipolar polarization model with three kinds of coexistent dielectric polarizations, in which the unique multiple reflection and scattering absorption characteristics are due to the special natural mesostructure of the SCF biosupport. Subsequently, it was found that the reflection loss (RL) values of the Ag NPs/[email protected] composite can be achieved in the whole X-band (exceeding −10 dB), the C-band, and some part of the S-band (exceeding −5 dB) with thicknesses from 2.0 to 5.0 mm. An important feature of the present work is that the specific gravity of our Ag NPs/SCF@BT composite is only 1/10 that of inorganic absorbing materials because of the special construction of this novel composite from biological tissue. Based on the promising properties of these biohybirds, the present work will hopefully lead to the development of new, lightweight, low-cost, flexible, and highly efficient microwave absorption materials based on biologic SCF-derived composites.

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INTRODUCTION With the explosive development of information technology using electromagnetic waves in the gigahertz (GHz) range, serious electromagnetic interference (EMI) problems have emerged. Therefore, considerable attention has been devoted to effective microwave-absorbing materials with lightweight and highly efficient absorption properties, which are crucially important for both civil and military purposes.1−4 Conventional microwave absorbents, such as magnetic or metal particles, usually have high specific gravity and complex synthesis formulations, which have somewhat limited their practical applications.5 Some new microwave absorption materials, such as carbon nanotube (CNT)-based composite and conducting polymers, also display good microwave absorption properties, but the fabrications of these materials always involve complex processes.6,7 Thus, it is desirable to develop a novel microwave absorption material with the following characteristics: lightweight, low-cost, convenient to synthesize, and highly efficient absorption properties. Recently, considerable research attention has been focused on supported nanoparticle (NP) composite materials that exhibit efficient microwave absorption properties due to their © 2012 American Chemical Society

strong electric dipolar polarizations and/or magnetic coercive force.8,9 It is generally believed that the microwave absorption properties of these nanoparticle composites often strongly depend on the morphologies of their supports and the sizes of the nanoparticles. Particularly, the nanoparticle support with hierarchical structure can lead to the enhancement of microwave absorption ability from the formation of unique multiple reflections within the mesostructure of absorbers.12 Therefore, a variety of synthetic methods, such as vapor growth and hydrothermal synthesis, have been developed so far for the syntheses of microwave absorbers with hierarchical structure.10−12 However, almost of all these approaches suffer from complicated preparation processes and high costs, problems which impede large-scale production and subsequent applications. There is strong motivation for the development of easy and low-cost parallel manipulation techniques. Biomolecules can self-assemble into complex, well-defined, and extended hierarchical superstructures due to their natural Received: January 3, 2012 Revised: March 10, 2012 Published: March 12, 2012 8188

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Scheme 1. Skin Collagen Fiber-Biotemplated Synthesis of AgNP-Embedded Hierarchical Intertexture Composite

molecular recognition capability.13 Recently, some biological systems, such as oyster shells, diatoms, DNA chains, viruses, and peptide chains, have been explored as building blocks in the bottom-up fabrications of new organic−inorganic composites with advanced structures and functionalities.14−18 It is well documented that collagen has a high level of molecular organization.19 The most abundant type of collagen, type I collagen fiber, is composed of three helical polypeptides. Five of these molecules align longitudinally with an overlap of approximately one-quarter of the molecular length to form a microfibril. The staggered triple helix arrangement is then assembled into collagen fibrils. These basic building blocks are combined, oriented, and laid up to form a higher ordered structure in collagen fiber with complicated geometrical morphologies to suit the requirements of tissue. In addition, it has been proved that at least 12% (wt %) water is contained in the skin collagen fiber, which is hydrogen bonded with collagen molecules to form water-mediated hydrogen bonded bridges. The specific water can efficiently dissipate the thermal energy orginating from the microwave. Therefore, the temperature of skin collagen fiber-based absorbers will not change significantly when exposed to microwave radiation.20,21 Inspired by this, we intend to use skin collagen fiber (SCF) as a biotemplate for the synthesis of silver nanoparticle (Ag NP)embedded hierarchical intertextures with lightweight and highly efficient microwave absorption properties. However, although SCF itself contains some stabilizing groups for NPs, our previous investigation indicated that gold nanoparticles (Au NPs) directly stabilized by collagen still lacked sufficient stability.22 Therefore, to further increase the stability and to determine the proper immobilizing amount of Ag NPs, research should be directed toward the chemical modification of SCF. In leather manufacturing, plant tannins, natural polyphenols widely distributed in plants, have been traditionally used as tanning agents for more than 100 years because of their high reactivity with SCF. Furthermore, according to the chemical structures of tannins, they contain abundant orthophenolic hydroxyls, thus exhibiting specific affinity to many metal ions. In addition, the molecular backbones of tannins consist of rigid aromatic rings, which can provide effective steric effects to prevent the aggregation of NPs and thus act as an effective stabilizer. Indeed, plant tannins have been traditionally used as an effective tanning agent in leather manufacture to increase the thermal stabilization of collagen fiber. Accordingly, plant tannins should be very useful for the synergistic construction of a stable linkage between SCF and NPs.

In the present work, we present a new strategy to use skin collagen fiber as a biotemplate for the synthesis of size-tunable Ag NP-embedded hierarchical intertextures with lightweight and highly efficient microwave absorption properties, in which bayberry tannin (BT, a typical condensed tannin) was first grafted on skin collagen fiber to serve as the disperser and stabilizer for Ag NPs. Subsequently, the microstructure, electrical conductivity, electromagnetic parameters, and microwave absorption properties of the as-prepared biohybirds were well investigated. To the best of our knowledge, this is a break from tradition in that biological tissue can be used as a biotemplate for the synthesis of hierarchical intertextures with microwave absorption properties. These biohybrids may be suitable candidates for microwave absorption materials because they are lightweight, inexpensive, and flexible and have highly efficient microwave absorption properties.

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EXPERIMENTAL SECTION

Materials Synthesis. A schematic drawing of the preparation of Ag NPs/SCF@BT composites is shown in Scheme 1. Detailed experimental procedures are described as follows. First, skin collagen fiber (SCF) was prepared from cattle hide according to the procedures in our previous work.23 In brief, the cattle hide was dehaired, fleshed, defated, limed, and delimed at room temperature to remove proteoglycan and other noncollagenous substances. The obtained SCF matrix was dried in vacuum and pulverized into powder. Second, the bayberry tannin (BT)-grafted SCF (SCF@BT) was prepared via Mannich reaction. A certain amount of BT was dissolved in 50.0 mL of deionized water and then mixed with 1.0 g of SCF, which was prepared in the previous step. The mixture was stirred at 298 K for 2 h. Then, 0.5 mL of glutaraldehyde solution (2.0 wt %, used as a cross-linking agent between SCF and BT) was added to the mixture and stirred at 318 K at pH 6.5 for 6 h. Subsequently, the BT-grafted SCF (SCF@BTx) was collected after filtering and fully washing with deionized water. The concentration of BT in the reaction solution before and after grafting reaction was analyzed by ultraviolet−visible spectra (UV−vis, Shimadzu UV-3600, Japan). The grafting degree of BT in SCF@BTx was defined as x = amount of BT grafted on CF (g)/amount of SCF (g). Herein, the grafting degree of BT in the prepared SCF@BT was 0.05, 0.18, and 0.35, respectively. Third, the resultant SCF@BT was suspended in 100 mL of AgNO3 aqueous solution, where the concentration of Ag+ was 0.2 g·L−1, and the Ag+ absorbed on the SCF@BTx was reduced 8189

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by dropwise adding 0.2 mol·L−1 NaBH4 solution. Finally, Ag NPs/SCF@BTx was collected, fully washed with deionized water, and dried in vacuum at 303 K for 24 h. On the basis of the measurements of inductively coupled plasma atomic emission spectroscopy (ICP-AES, Perkin-Elmer Optima 2100 DV), the Ag content of Ag NPs/SCF@BTx was determined to be 1.58%, 1.83%, and 1.88% (w/w), respectively. Characterization Techniques. The structural morphology of Ag NPs/SCF@BTx was observed by field emission scanning electron microscopy (FESEM, Hitachi 4700, Japan). Transmission electron microscopy (TEM) of Ag NPs/SCF@BT composites was carried out by using a Tecnai G2 F20 (TEM, FEI, The Netherlands) operating at 200 kV. The X-ray diffraction patterns of Ag NPs/SCF@BT were recorded by using a Cu Kα wide-angle X-ray diffraction pattern diffractometer (XRD, Philips X′Pert Pro-MPD, The Netherlands). Xray photoelectron spectra (XPS, Shimadzu ESCA-850, Japan) of Ag NPs/SCF@BT were recorded by employing Mg Kα Xradiation (ℏv = 1253.6 eV) and a pass energy of 31.5 eV. Peaks from all the high-resolution core spectra were fitted with XPSPEAK 4.1 software, using mixed Gaussian−Lorentzian functions. Ultraviolet−visible diffuse reflectance spectra (UV− vis DR) were recorded by means of a UV−vis spectrophotometer (UV−vis, UV-3600, Shimadzu, Japan) equipped with an integrating sphere and using BaSO4 as a reference. Fourier transform infrared spectroscopy (FT-IR, Perkin-Elmer, MA) analyses were carried out by using an attenuated total reflectance spectrophotometer. Direct-current (DC) conductivity of these composites was measured by a two-probe technique using a Keithley 6105 resistivity test device. For the measurements of microwave absorption properties, the experiments were carried out based on the transmission line technique, where AgNPs-BTx@CF composites were pressed into a toroidal shaped mold (φout: 7.00 mm; φin: 3.00 mm) for the measurements of complex permittivity (εr = ε′ − jε″) and complex permeability (μr = μ′ − jμ″) by using a vector network analyzer (VNA, Agilent E8363B, Agilent, Santa Clara, CA) in the frequency range of 0.5−18.0 GHz. Actually, the complex permeability and complex permittivity were deduced from the scattering parameters, reflection coefficient (S11), and transmission coefficient (S 21 ), respectively, using a typical Nicholson-Ross algorithm.24 The reflection loss of microwave (RL) has been calculated by using Matlab software (a registered trademark of The MathWork, Inc.) based on the measurements of complex permittivity and complex permeability using transmission line theory.

Figure 1. UV−vis DR spectra (a) and FT-IR images (b) of SCF, [email protected] and Ag NPs/[email protected].

color change of SCF from white to brown. Additionally, the grafting of BT on SCF was also confirmed by attenuated total reflectance fast Fourier transformation infrared (ATR FT-IR) spectrophotometry. As shown in Figure 1b, the adsorption band of [email protected] around 3390 cm−1 appears to be broadened compared with that of SCF, which is mainly due to the strong hydrogen bond interaction between the phenolic hydroxyl groups of BT and the amino/amide groups of SCF. Meanwhile, the appearance of a new adsorption peak at 1120 cm−1 is ascribed to the C−O−H stretching vibration of the phenolic hydroxyls of BT.22 Subsequently, the obtained SCF@ BTx biomatrix was mixed with AgNO3 aqueous solution at pH 6.0. After the adsorption of Ag+, the peak from the HO−C group in the O1s XPS spectrum of SCF@BTx shifts from 532.9 eV to a higher binding energy of 534.5 eV, as shown in Figure 2, which suggested an electron donating−accepting interaction

Figure 2. O1s core level XPS spectra for [email protected] (a) and Ag+/ [email protected] (b).

between Ag+ and the orthophenolic hydroxyls of BT.22 Finally, the Ag+ that interacted with SCF@BT was completely reduced to Ag0 by sodium borohydride (NaBH4), which was confirmed by the energy position of the Ag 3d XPS spectrum in Figure S1 in Supporting Information. In addition, the formation of Ag NPs also can be proved by the characteristic surface plasmon resonance (SPR) peak of Ag NPs centered at 350 nm. Compared with the FT-IR spectrum of SCF and [email protected], the stretching vibration peak of the phenolic hydroxyls of Ag NPs/[email protected] at 3390 cm−1 appears to be relatively more narrow than that of BT@CF, which should be attributed to the interactions of phenolic hydroxyls with AgNPs. Additionally, the in-plane deformation vibration of the phenolic hydroxyls (1120 cm−1) is weaker than that of BT@CF, which also indicated that BT interacted with AgNPs through its adjacent phenolic hydroxyls. The typical XRD patterns of SCF (a), [email protected] (b), and Ag NPs/[email protected] composites (c) are illustrated in Figure 3. All these samples exhibit a broad signal at about 2θ = 23°, which is attributed to the amorphous polymer phase of SCF. As expected, there is no crystal diffraction peak in SCF and [email protected], while a number of

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RESULTS AND DISCUSSION Skin collagen fiber (SCF) was prepared from cattle skin according to the procedures in our previous work (see also Experimental Section).23 A certain amount of bayberry tannin (BTx) was grafted on the SCF with different grafting degrees of BT (x = 0.05, 0.18, and 0.35 in wt/wt, respectively). The grafting of BT on the SCF matrix can be visually witnessed from the color of the SCF that gradually changed from white to brown, which was determined by a UV−vis diffused reflection (UV−vis DR) spectrophotometer. Figure 1a presents the UV− vis DR spectra of SCF, [email protected], and Ag NPs/[email protected]. The SCF only has two relatively weak absorption peaks, located at 210 and 280 nm, mainly ascribed to the polypeptide chains and the benzene rings in the side chains, respectively.22 In comparison, [email protected] exhibits relatively enhanced absorbance in the range of 200−800 nm, which is consistent with the 8190

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absorption characteristic which can result in improved values of microwave absorption.12 The transmission electron microscopy (TEM) images of Ag NPs/SCF@BTx with different grafting degrees of BT are presented in Figure 5. The distinctive D-period formed by the

Figure 3. X-ray diffraction (XRD) patterns of CF (a), [email protected] (b), and Ag NPs/[email protected] (c).

prominent Bragg reflections appear in the XRD pattern of Ag NPs/[email protected] that could be indexed as the face-centered cubic (fcc) structure of silver with the corresponding diffraction peaks of (111), (200), and (220) planes (JCPDS-4784).22 Moreover, the crystallite size of AgNP was calculated to be 6.5 ± 1.5 nm based on the width of the XRD peak at 2θ = 37.7° by using the Scherrer equation. As shown in Figure 4a, the field emission electron microscope (FESEM) image shows the special hierarchical Figure 5. Transmission electron microscopy (TEM) images and particle size distributions of Ag NPs/SCF@BT with different grafting degrees of BTx: x = (a, b) 0.05, (c) 0.18, and (d) 0.35.

“quarter stagger” arrangement of collagen molecules is clearly observed (Figure 5a), demonstrating that the assembly of Ag NPs on the SCF did not destroy the bioinherent hierarchical structure of SCF. As shown in Figure 5b, the statistical analysis of Ag NPs/SCF@BTx with the lowest BT grafting degree of 0.05 indicated that the average particle diameters of the Ag NPs were relatively large and that the size distribution was wide (14.2 ± 8.9 nm). Furthermore, it was also observed that some aggregations of Ag NPs were dispersed on the surface of SCF, which suggests that BT with a relatively low grafting degree is not efficient enough for the stabilization and dispersion of Ag NPs. Markedly, when the grafting degree of BT increased to 0.18, the average particle diameter of Ag NPs was drastically decreased, as shown in Figure 5c. On the basis of the statistical analysis of Ag NPs/[email protected], the particle size of Ag NPs decreased to 6.2 ± 1.5 nm and the size distribution was mainly in the range of 1.0 to 8.0 nm. When the grafting degree of BT reached 0.35, smaller sizes of Ag NPs with a narrower size distribution (4.5 ± 0.8 nm) were formed, as shown in Figure 5d, which shows a consistent trend that the particle size of Ag NPs on SCF decreases as the grafting degree of BT increases. In addition, the purity of Ag particles was demonstrated by energy dispersive X-ray analysis (EDX) (Figure S2 in Supporting Information), and it was found that only the element Ag was indicated in these particle regions (element Cu is ascribed to the TEM supporting grid). From above, the observation by TEM suggested that the diameter and size distribution of Ag NPs can be facilely controlled by varying the grafting degree of BT. Figure 6 shows the high resolution transmission electron microscopy (HRTEM) images of an individual Ag NP (in Ag NPs/[email protected]) embedded in the SCF biomatrix. The inset in Figure 6a displays the corresponding fast Fourier transform (FFT) pattern of this individual Ag NP, where the clear six-fold rotational symmetry spots give rise to a diffraction pattern corresponding to the single crystal with face-center-cubic (fcc)

Figure 4. . Field emission electron microscopy (FE-SEM) image of Ag NPs/[email protected] at different magnifications (a−c). Elemental maps of C (d), O (e), and Ag (f) in Ag NPs/[email protected], respectively.

interwoven structures of the Ag NPs/[email protected] composite, which consist of interlaced biofibers of SCF biomatrix. Moreover, in the high magnification image of Ag NPs/SCF@ BT0.35 composites (Figure 4b), the geometrical morphology of Ag NPs/SCF@BT composites is still in a well-defined fibrous state at approximately 1.0−4.0 μm in diameter, which is assigned on the basis of the diameter of the bundle of natural SCF.19 Importantly, the energy dispersive X-ray (EDS) mapping analyses of the Ag NPs/[email protected] composite in Figure 4c−f reveal that the Ag mapping image (f) has a shape similar to that of C (d) and O (e) elements, which directly confirms that the Ag NPs were highly dispersed onto the surface of the SCF biotemplate and maintained the same geometrical morphology of SCF biofibers, indicating a novel, special, and highly compact interlaced Ag NPs chains. Compared with the common microwave-absorbing counterparts of bulk structure, the hierarchical fibrous structure of collagen fiber will lead to a unique multiple scattering 8191

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where σ is the conductivity of the composite, p is the silver mass fraction, pc is the percolation threshold, and β is the critical exponent. As shown in the inset of Figure 6, the log (σ) versus log ((p − pc)/pc) plot indicated that the conductivity of Ag NPs/SCF@BT agrees very well with the percolation behavior predicted by the power law in eq 1. The straight line in this figure with pc = 0.85% and β = 0.06 gives a good fit to the data with a correlation coefficient of 0.96. The percolation threshold is found to be quite low, which is only 0.85 wt % silver, indicating a very efficient dispersion of Ag NPs onto the SCF. The analysis of the conductivity suggested that the embedding of Ag NPs onto the SCF was found to be in good agreement with percolation net system, where the Ag NPs were randomly distributed on the SCF, thus forming a threedimensional circuit intertube tunneling pathway network, which is in favor of the electron transport crossing the energy gap of the SCF biomatrix.26,27 On the other hand, according to the law of transmit line theory, the microwave absorption property of materials mainly depends on suitable conductivity, which is neither conductor due to the mismatch of impedance nor insulator due to the disability of energy dissipation. On the basis of the analysis of conductivity measurements, the conductivity of Ag NPs/SCF@BT composites is typically in a semiconductive state. Therefore, the electric property of these composites is suited for the design of microwave absorption materials. As important parameters for microwave electromagnetic properties, the real and imaginary parts of complex permittivity (εr) and permeability (μr) were measured in the frequency range of 2.0−18.0 GHz. Concerning intrinsic electric conductivity, biological systems often do not exhibit the desired physical properties. Therefore, as shown in Figure 8a and Figure 8b, the values of real parts (ε′) and the imaginary part (ε″) of complex permittivity of SCF are almost constant over the 2−18 GHz range with a slight fluctuation (ε′ ≈ 4.0 and ε″ ≈ 0.2). Similarly, the values of ε′ and ε″ for [email protected] are still about 4.0 and 0.5, respectively, which suggest that the grafting of BT on SCF cannot efficiently improve the dielectric storage and dielectric loss ability of SCF. However, the values of ε′ and ε″ for Ag NPs/[email protected] show a dramatic increase when the grafting degree of BT increased from 0.05 to 0.35, suggesting an enhancement of dielectric loss ability compared with ε″ ≈ 0.2 for the original SCF biologic system. The dielectric loss tangent (tan δ) is presented as a function of frequency, which is shown in Figure S3. The loss tangent, also called the attenuation constant, is the ratio of the imaginary part (ε″) to the real part (ε′) of relative complex permittivity, indicating the loss of dielectric. The values of loss tangent of SCF, [email protected], and Ag NPs/SCF@BTx are all on the order of magnitude of 10−1, suggesting that they are all dielectric materials. Compared with the significant changes in complex permittivity, the complex permeability (μ′ and μ″) of Ag NPs/ SCF@BTx is almost the same as that of pure SCF and SCF@ BT (Figure S3 in Supporting Information), indicating that there is no (or minor) magnetic loss contribution for Ag NPs/ SCF@BT to microwave absorption. It is usually believed that the dielectric loss in the range of 2.0−18.0 GHz mainly results from the electric dipolar polarizations of absorbers. In the polarization processes, a mass of electromagnetic wave energy is irreversibly transformed

Figure 6. (a) High resolution transmission electron microscopy (HRTEM) images of an individual Ag NP embedded in the SCF. The inset in part a shows the corresponding fast Fourier transform (FFT) patterns of the image. (b) High magnification TEM images of selected area in part a.

structure. With closer observation of these crystal lattices in Figure 6b, it can be seen that the lattice fringes could be indexed to the {200} and {111} reflections along the [011] orientation, in which the visible lattice fringes with a diameter of 2.4 Å could be assigned to the {111} planes, and the other interlaced fringes with a diameter of 2.0 Å can be assigned to the {200} planes, respectively.25 The HRTEM observation reveals the structural features of these Ag NPs on the SCF, and it also demonstrates that the Ag NPs are well-crystallized after reduction by NaBH4. Although the pure SCF biomatrix has a naturally occurring hierarchical biosupramolecular structure which can lead to the enhancement of microwave absorption ability due to the formation of unique multiple reflections within the mesostructure of absorbers, biological systems, such as SCF, often do not exhibit the desired dielectric properties concerning their intrinsic electric conductivity. Therefore, the embedding of Ag NPs onto the SCF biomatrix can certainly increase the electric conductivity, σ, of Ag NPs/[email protected]. Figure 7 displays the

Figure 7. Direct-current conductivity (σ) vs mass fraction (p) of Ag NPs/[email protected] measured at room temperature. Inset: log−log plot for (σ) vs ((p − pc)/pc) for the same composites. The straight line in the inset is a least-squares fit to the data using eq 1, returning the best fit values pc = 0.85 wt % and β = 0.06.

direct-current (DC) conductivity, σDC, of Ag NPs/[email protected] as a function of mass fraction (p) of silver measured at room temperature. The conductivity of Ag NPs/[email protected] displays a dramatic increase of fourth-order magnitude when the silver mass fraction approaches 0.9 wt %. The inset in Figure 7 shows that the electrical conductivity obeys the power law equation:

σ ∝ (p − pc )β

(1) 8192

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Figure 9. . The plots of ε′ versus B0 (B0 = ε″/f) of (a) SCF and (b) Ag NPs/[email protected].

relaxation time (τ = 1/2πk) of SCF associated to the Debye electric dipolar polarization is 0.079 × 10−9 s and 0.192 × 10−9 s. Furthermore, combined with eqs 3 and 4, the Debye polarization frequencies of SCF were calculated to be 2.01 and 0.83 GHz, respectively. The analysis of the Debye dipolar polarization model for SCF suggested that the collagen molecule contains some natural electric dipoles and molecular bounded charges. This result is in good agreement with the previous observation by other researchers.29,30 Moreover, when the collagen fiber is exposed to microwave irradiation, the natural electric dipoles and molecular bounded charges in the collagen molecule can be partly polarized under the electromagnetic field, which enhances microwave absorption over a wide frequency band. On the other hand, it also can be seen that Ag NPs/[email protected] presents good linear variations. This results suggested that the Ag NPs/[email protected] composite was in good agreement with the Debye electric dipolar polarization model. Moreover, the curve of ε′ versus B0 for Ag NPs/SCF@ BT0.4 can be fitted as a sum of three beelines with different slopes, and the corresponding relaxation time (τ) and Debye polarization frequencies can also be calculated as 0.035 × 10−9 s (4.52 GHz), 0.018 × 10−9 s (9.05 GHz), and 0.063 × 10−9 s (2.54 GHz), respectively. Some points in these plots, marked by a circle, cannot be fitted by the Debye dipolar polarization model equation, which suggested that some other dielectric relaxation mechanisms may also operate in SCF and Ag NPs/ [email protected], such as an electron migration mechanism and charge carrier hopping process. The multidielectric polarization phenomena of Ag NPs/ SCF@BT probably result from the following reasons: (1) it is reasonable that the Ag NPs embedded in the SCF have a large amount of crystal surface defect, where most of the electrons are localized in a small domain and do not act as the conduction electrons.31 The localized electrons are certainly accumulated at the defective sites, leading to an asymmetry of charge distribution.32 This high density surface charge accumulated on the Ag NP defect would act as a dipole that will be tuned with incident microwaves and contribute to a defective site polarization. (2) As heterogeneous structures, Ag NPs and SCF have different resistivities, and the interfacial polarization phenomena are widely adopted to explain the high permittivity in heterogeneous structures. Considering the high density of defects on Ag NPs, the asymmetry distribution of electrons can be considered to be a boundary layer capacitor, in which the free electrons of Ag NPs are localized and accumulate a mass of space charge at the metal−biomatrix

Figure 8. Frequency dependence of (a) real (ε′) and (b) imaginary (ε″) parts of relative complex permittivity for (A) SCF, (B) SCF@ BT0.35, and Ag NPs/SCF@BTx with various grafting degrees of BT, (C) x = 0.05, (D) x = 0.18, and (E) x = 0.35, respectively.

to Joule thermal energy. This process can be expressed by the Debye dipolar polarization model equation:28 ε − ε∞ εr = ε∞ + s = ε′(f ) + i ε″(f ) 1 + j 2πf τ (2) where τ is the relaxation time, f is the frequency of electromagnetic wave, and εs and ε∞ are the stationary optical dielectric constant, respectively. From eq 2, following equations can be deduced. εs − ε∞ ε′(f ) = ε∞ + 1 + (2πf )2 τ2 ε″(f ) =

2πf τ(εs − ε∞) 1 + (2πf )2 τ2

the and the

(3)

(4)

With suitable rearrangement of eqs 3 and 4, it can be found that ε′ is a function of ε″ and f, which can be expressed by the equation:

ε′ =

ε″ + ε∞ 2πf τ

(5)

Therefore, it can be inferred that if the dielectric loss is only a consequence of Debye electric dipolar polarization, the plot of ε′ versus B0 (B0 = ε″/f) would show a linear relation, and the relaxation time (τ) and the Debye polarization frequency can be calculated based on the linear fitting process. Figure 9 shows the plots of ε′ vs B0 (B0 = ε″/f) of SCF and Ag NPs/SCF@ BT0.4. It can be observed that some parts of SCF data present approximate linear variations which can be fitted as a sum of two beelines with different slopes (k = 1/2πτ), obtaining k1 = 2.01 and k2 = 0.83. Based on the linear fitting results, the 8193

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interfaces, thus producing the interfacial polarization.33 On the basis of these two polarization mechanisms, it is logical to explain the effects of the grafting degree of BT on the value of ε′ and ε″ for Ag NPs/SCF@BT. When the grafting degree of BT increased, the particle sizes of Ag NPs become smaller, associated with the increase of crystal surface defect and interfacial area, thus leading to the stronger defective site polarization and interfacial polarization. (3) Being among the most advanced materials made of biomolecular building blocks, collagen fiber has a high level of molecular organization and special hierarchical fibrous nanostructure. When the incident microwave enters Ag NPs/SCF@BT, energy dissipation can take place within the hierarchical intertextured structure of SCF and exhibits a unique multiple reflection and scattering absorption characteristic. To reveal the microwave absorption properties of the assynthesized Ag NPs/SCF@BT, the reflection loss (RL) values of Ag NPs/SCF@BT were calculated based on the relative complex permittivity (εr) and complex permeability (μr) according to the transmit line theory.34 Z in = Z 0 μr/εr tanh[j(2π/c) μrεr fd]

(6)

R(dB) = 20log|(Z in − Z 0)/(Z in + Z 0)|

(7)

where Zin is the input impedance of the absorber, Z0 is the impedance of air, f is the frequency of the microwave, d is the thickness of the microwave absorption material, c is the velocity of light, and μr and εr are the complex permeability and permittivity of the microwave absorption material, respectively. According to the analysis of relative complex permittivity (ε′ and ε″), it can be concluded that the grafting degree of BT on SCF is one of the crucial parameters that affects the electromagnetic properties of Ag NPs/SCF@BTx, which certainly leads to the difference in microwave absorption intensity and peak position. Therefore, we studied the RL values of Ag NPs/SCF@BTx with various grafting degrees of BT at the same thickness of 2.0 mm. Figure 10a shows that the absorption peak of Ag NPs/[email protected] (A) is relatively narrow, and the maximum intensity of microwave absorption is −15.0 dB. As for Ag NPs/[email protected] (B), the absorption peak becomes wider than that of Ag NPs/[email protected]. A substantially enhanced absorbing behavior occurs for Ag NPs/ [email protected] (C). Note that RL values exceeding −10 dB are achieved in the whole X-band (8.0−12.0 GHz) from 7.8 to 12.0 GHz. The excellent wide-band absorption property of Ag NPs/ [email protected] has a broader frequency range than those previously reported.8,9 In general, a lower RL mainly depends on the stronger dielectric loss. However, the efficient frequency bandwidth of RL, especially when RL values exceed −10 dB, depends on the impedance matching condition. Therefore, the highly efficient microwave absorption properties of the Ag NPs/SCF@BT composite are the result of suitable balance between the dielectric loss property and the impedance matching condition. In our case, the excellent microwave absorption properties of Ag NPs/SCF@BT composites also can be ascribed to the multiple scattering and reflecting property of hierarchical structure in collagen fiber. In addition, the microwave absorption intensity and peak position also strongly depend on the thickness of absorber. Figure 10b shows the RL values of Ag NPs/[email protected] with different thicknesses from 2.0 to 5.0 mm. The peak position of Ag NPs/[email protected] shifts to the lower frequency side with an increase in thickness. For common microwave absorption

Figure 10. (a) Reflection loss (RL) values of Ag NPs/SCF@BT with various grafting degrees of BT in a thickness of 2.0 mm: (A) 0.05, (B) 0.18, and (C) 0.35, respectively. (b) Reflection loss (RL) values of Ag NPs/[email protected] with different thicknesses from 2.0 to 5.0 mm.

materials, the absorption ability in the S-band (2.0−4.0 GHz) and C-band (4.0−8.0 GHz) is usually very weak, which is not suitable for wide-band microwave absorption applications. However, note that the absorption intensity of as-prepared Ag NPs/[email protected] exhibited excellent performance even in the low frequency ranges of the S-band and C-band. When the thickness of Ag NPs/[email protected] was 3.0 mm, an RL value exceeding −5 dB can be achieved in the range of 5.0−11.0 GHz, which almost covers the whole C-band and X-band. When the thickness increased to 4.0 mm, an RL value exceeding −5 dB covers the range of 3.5−7.0 GHz, which contains most of the C-band. When the thickness increased to 5.0 mm, it covers the range of 2.8−5.0 GHz, which contains most of the S-band and some of the C-band. Herein, an important feature of the present work is that the density of asprepared Ag NPs/SCF@BT is only about 1/10 that of inorganic absorbing materials, which is due to the special construction of this novel composite using biological tissue. Therefore, Ag NPs/SCF@BT shows promise for use as lightweight, low-cost, and highly efficient microwave absorption material.

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CONCLUSION

In summary, novel and interesting Ag NPs/SCF@BT composites were synthesized based on the biotemplate of skin collagen fiber (SCF). In these Ag NPs/SCF@BT composites, size-tunable silver nanoparticles (Ag NPs) were embedded in the hierarchical intertexture of SCF. TEM analysis indicated that the diameter and size distribution of Ag NPs can be facilely controlled by varying the grafting degree of bayberry tannin (BT) on SCF. Further electric conductivity measure8194

dx.doi.org/10.1021/jp300048e | J. Phys. Chem. C 2012, 116, 8188−8195

The Journal of Physical Chemistry C

Article

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ments showed that these Ag NPs/SCF@BT composites are electrically semiconductive (10−6 to 10−5 S/cm) and have a typical percolation threshold of 0.85% silver fraction. According to the analysis of relative complex permittivity (ε′ and ε″), the enhancement of dielectric loss properties of Ag NPs/SCF@BT can be ascribed to the multiple Debye dipolar polarizations which include defective site polarization, interfacial polarization, and the unique multiple reflection and scattering absorption characteristic of SCF. Subsequently, the RL values of the Ag NPs/[email protected] composite can be achieved in the whole Xband (exceeding −10 dB), the C-band (exceeding −5 dB), and some part of the S-band (exceeding −5 dB) with different thicknesses from 2.0 to 5.0 mm. We are currently investigating the electromagnetic properties of other SCF-based nanoparticle composite materials such as ZnO/SCF, MnO2/SCF, and bimetallic core−shell nanoparticles/SCF. The present work will lead to the development of new, lightweight, low-cost, flexible, and highly efficient microwave-absorbing materials based on biologic SCF-derived composites.

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ASSOCIATED CONTENT

S Supporting Information *

XPS, EDS, and complex permeability spectra. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +86-28-85400382. Fax: +86-28-85400356. E-mail: [email protected] (X.L.); [email protected] (B.S.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support provided by the National Natural Science Foundation of China (21176161, 2097611) and A Foundation for the Author of National Excellent Doctor Dissertation of China (FANEDD200762). We especially thank Prof. L. Chen in the Engineering Research Center of Stealth Materials and Technology, State Key Laboratory of Electronic Thin Films and Integrated Devices (China), for the microwave adsorption measurements.

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dx.doi.org/10.1021/jp300048e | J. Phys. Chem. C 2012, 116, 8188−8195