Synthesis of Gold Nanoplates with Bioreducing Agent Using Syringe

Nov 8, 2012 - ... and the pH based on a syringe-pumps apparatus. The dimensions of the obtained Au nanoplates were measured using TEM and AFM...
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Synthesis of Gold Nanoplates with Bioreducing Agent Using Syringe Pumps: A Kinetic Control Guowu Zhan, Lanting Ke, Qingbiao Li, Jiale Huang, Dan Hua, Abdul-Rauf Ibrahim, and Daohua Sun* Department of Chemical and Biochemical Engineering, College of Chemistry and Chemical Engineering, Fujian Provincial Key Laboratory of Chemical Biology, Xiamen University, Xiamen 361005, China ABSTRACT: Anisotropic Au nanoplates are particularly important owing to their unusual properties. Herein, we describe a plant-mediated bioreduction method to increase the yield of Au nanoplates and shorten the reaction time through a kinetically manipulated procedure. More specifically, the reduction rate was controlled by modulating experimental factors such as the addition mode and rate of the feed solutions, the temperature, and the pH based on a syringe-pumps apparatus. The dimensions of the obtained Au nanoplates were measured using TEM and AFM. The single-crystalline structure was demonstrated by HRTEM, SAED, and XRD. The results of XPS, FTIR, and TG analyses indicated strong affinity of the biomolecules binding to the Au nanoplate facets. In particular, the nanoplate films exhibited strong surface plasmon absorbance in the near-infrared range of 700−3000 nm, vital for optical applications. Furthermore, we propose a mechanism for this formation following the timeresolved studies.

1. INTRODUCTION It is well-known that the shape of Au nanostructures can dramatically affect their physical and chemical properties.1 Among various morphologies, two-dimensional Au nanoplates are of particular interest because of their remarkable physicochemical properties, such as unique optical features, high conductivity, thermal stability, and special catalytic capabilities, compared with their spherical counterparts.2,3 As a result, the construction of Au nanoplates has been the focus of intensive research for a long time, and Au nanoplates have been explored in a wide variety of applications ranging from biosensors to optical imaging, surface-enhanced Raman scattering, cancer hyperthermia, and architecture coating.4−6 In the past decade, a library of strategies for fabricating Au nanoplates has been developed, including photochemical routes; thermal organic synthesis; and seed-mediated, microwave-assisted, and ultrasound-assisted aqueous chemical reduction methods.7,8 However, most of these synthetic procedures are environmentally unsound and somewhat tedious, involving the reduction of Au precursors in the presence of harsh chemical reductants with additional surfactants or capping agents, through growth on additional seeds, or through sphere−plate shape transformation. Another important issue in the fabrication of Au nanoplates is the growth mechanism. As such, considerable research efforts have been directed toward elucidating the complex issue surrounding their platelike nucleation and growth.9 Much encouraging progress has been achieved over the past century, which was summarized in a comprehensive review by Millstone et al.,8 wherein mechanisms for the formation of the Au nanoplates were classified into crystallographic and redox chemistry methods. However, to date, the exact driving forces behind the growth of Au nanoplates are not fully understood, as the clear elucidation of their formation mechanism is still elusive. Admittedly, there are traditional experimental difficulties in their syntheses, and seemingly similar preparatory procedures sometimes yield very different morphologies or even result in contradictory conclusions.10 In this regard, understanding the close relation© 2012 American Chemical Society

ship between the kinetics of the reduction of the Au precursor and the Au particle growth is vital to realizing and harnessing the morphology of Au nanostructures.11 Recently, the concept of pure green chemistry has stimulated the mushrooming development for biological syntheses of Au nanoparticles.12−14 Biological systems, such as lemongrass,15 Aloe vera,16 seaweed,17,18 alfalfa,19 Escherichia coli,20 and Dolichomitriopsis diversiformis21 have been explored as ecofriendly alternatives to biologically toxic reducing and stabilizing agents for the synthesis of Au nanoplates. For instance, Shankar and co-workers performed many helpful studies in this area, developing a biological method to produce Au nanoplates with a high yield of 45% using lemongrass leaves extract.15 In another case, Montes et al. successfully prepared anisotropic triangular Au nanoplates with a size range of 500−4000 nm and thicknesses of 15−30 nm through the reduction of HAuCl4 solution by alfalfa water extracts.19 Most recently, the use of some isolated pure biobased molecules (e.g., tannic acid,22 coenzyme,23 serum albumin protein24) has been reported for this kind of synthesis. However, in addition to the minimal understanding of their formation mechanisms, all of these biological procedures require relatively longer times to complete the reactions. Therefore, this article describes a reliable and purely green synthesis procedure for Au nanoplates using Cacumen Platycladi (CP) extract as the reductant. In previous studies, directly mixing CP extract with aqueous HAuCl4 routinely produced pure spherical Au nanoparticles, whereas nanoplates were sparsely obtained.25,26 Interestingly, herein, the same starting materials (i.e., CP extract and HAuCl4) were employed for the fabrication of nanoplates in high yield based on a syringe-pump apparatus. In addition, detailed investigations on the experimental factors Received: Revised: Accepted: Published: 15753

May 26, 2012 October 15, 2012 November 8, 2012 November 8, 2012 dx.doi.org/10.1021/ie302483d | Ind. Eng. Chem. Res. 2012, 51, 15753−15762

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Figure 1. (a−c) UV−vis−NIR spectra of Au sols prepared by addition modes (a) A, (b) B, and (c) C. (d−f) Representative TEM images of the Au nanoparticles prepared by (d) mode A at an addition rate of 300 mL/h, (e) mode B at an addition rate of 450 mL/h, and (f) mode C at an addition rate of 90 mL/h. The reaction temperature and pH for all samples were 90 °C and 3.41, respectively.

maintained with stirring for an additional 30 min after the completion of feeding. 2.3. Characterization. TEM, HRTEM, and SAED characterizations were performed on a Phillips Analytical FEI Tecnai 30 electron microscope (300 kV). UV−vis−NIR spectra were collected on a Varian Cary 5000 spectrometer. XRD analysis was conducted on a Phillips Panalytical X’pert Pro diffractometer equipped with Cu Kα radiation (40 kV, 30 mA). AFM observations were performed using an Agilent MI 5500 microscope in contact mode. XPS data were collected on a PHI-Quantum 2000 spectrometer employing a monochromatized microfocused Al X-ray source. FTIR spectra were recorded on a Nicolet Avatar 660 spectrometer. TG studies were carried out on a Netzsch TG209F1 thermobalance under flowing air atmosphere.

(addition mode/rate, temperature, and pH) were carried out. The obtained Au nanoplates were characterized by a variety of methods, including transmission electron microscopy (TEM), high-resolution TEM (HRTEM), selected-area electron diffraction (SAED), atomic force microscopy (AFM), X-ray diffraction (XRD), UV−vis−NIR (NIR, near-infrared) spectroscopy, X-ray photoelectron spectroscopy (XPS), Fourier-transform infrared (FTIR) spectroscopy, and thermogravimetric (TG) analysis. Additionally, a possible formation mechanism of the Au nanoplates following a time-resolved analysis is proposed.

2. EXPERIMENTAL SECTION 2.1. Materials. Chloroauric acid (HAuCl4, AR) and hydrochloric acid (HCl, AR) were purchased from Sinopharm Chemical Reagent Co. Ltd. (Beijing, China) and used as received. Sundried CP was purchased from Xiamen Jiuding Drugstore (Xiamen, China). 2.2. Synthesis of Au Nanoplates. To obtain the CP extract, 1 g of screened powder CP was dispersed in 300 mL of deionized water with a shaker (30 °C, 150 rpm) for 4 h. The mixture was then filtered, and the filtrate (i.e., CP extract) was used for the experiments. In a typical synthesis of Au nanoplates, an empty two-neck flask (100 mL) was preheated in an oil bath (equipped with magnetic stirring) at 60 °C for 5 min. Feed solutions of Au precursor (HAuCl4, 1 mM, 30 mL) and CP extract (30 mL) were simultaneously injected into the flask through two separate syringe pumps at the same addition rate (60 mL/h). Other addition rates can be employed while keeping the total amount of feed added constant (60 mL). The reaction mixture was

3. RESULTS AND DISCUSSION 3.1. Influence of Experimental Parameters. It was found that the growth of Au nanoplates and spherical Au particles was highly sensitive to the synthesis conditions and that the reduction rates could be kinetically adjusted by experimental parameters.27 Therefore, as discussed in the following subsections, the influences of experimental parameters (addition mode/rate, reaction temperature, pH) were systematically investigated, which will provide valuable insight into the morphology evolution resulting from the kinetically adjusted factors. 3.1.1. Influence of Addition Mode/Rate. In the literature, the synthesis and morphology control of Au nanoplates are largely limited to parameters such as metal precursor-to-reductant ratio, 15754

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Figure 2. (a−c) UV−vis−NIR spectra of Au sols synthesized at different temperatures and feeding conditions (mixing mode C, pH 3.41): (a) low addition rate of 18 mL/h, (b) moderate addition rate of 60 mL/h, and (c) high addition rate of 120 mL/h. (d−f) Corresponding TEM images of Au nanoparticles synthesized under three optimal sets of conditions: (d) condition 1, (e) condition 2, and (f) condition 3. (g,h) Statistical analyses of (g) the edge length of nanoplates and (h) the particle size of spherical nanoparticles prepared under conditions 1−3.

As illustrated in Figure 1a, UV−vis−NIR spectra of the Au sols synthesized by mode A show a single surface plasmon resonance (SPR) band centered at ca. 530 nm regardless of the addition rate of Au precursor. Indeed, the corresponding TEM image (Figure 1d) suggests that the reaction produced spherical nanoparticles with a mean size of 16.5 nm without nanoplates. Note also that mode A (rapid addition of metal precursor to reductant solution) is the synthesis style used in most literature reports.28 Surprisingly, in the case of mode B (Figure 1b), the synthesized Au sols showed distinct second SPR bands centered in the NIR region under some conditions of CP addition rates, attributable to the formation of Au nanoplates.29 The representative TEM

temperature, and types of reductant or capping agent. Less attention is paid to the mixing mode of Au precursors and reductants. Herein, the influence of addition mode/rate of the feed solutions on the Au morphology was investigated, and three different mixing modes were applied: designated mode A, involving the addition of Au precursor to CP extract; mode B, involving an inverted addition of the CP extract to the Au precursor; and mode C, involving the simultaneous injection of the two feed solutions (CP extract and Au precursor) with equal addition rates. Note that identical amounts of feed were used in each addition mode to understand the net effect. 15755

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Figure 3. (a) UV−vis−NIR absorption spectra of Au sols synthesized at varying Au precursor pH values. (b) TEM image of Au nanoplates prepared at a Au precursor pH of 2.81. (c) Plot of Au nanoplate yield versus pH (3.41−2.58). (d,e) Changes in (d) edge length and (e) particle size of Au nanoparticles as the pH was changed from 3.41 to 2.81. The reaction temperature was 60 °C, and the addition mode and rate were mode C and 60 mL/h, respectively.

image in Figure 1e confirms this fact, and the yield of Au nanoplates was 13%, defined by statistical analysis of the relative population of platelike particles (triangular/hexagonal). Subsequently, addition mode C was tested (Figure 1c), and the resulting Au sols displayed two clearly differentiated SPR bands in the visible and NIR regions that are associated with spherical nanoparticles and anisotropic nanoplates, respectively. The corresponding TEM image (Figure 1f) confirms the formation of Au nanoplates with a higher yield of 24%. Moreover, the statistical analysis indicates that the average edge length of the Au nanoplates increased from 82.7 ± 30.6 to 127.8 ± 47.1 nm, and the spherical nanoparticle size increased from 48.7 ± 8.7 to 55.8 ± 10.3 nm when the mixing conditions were adjusted from mode B to mode C. Note that the as-described method is timesaving. The total reaction time is typically within 1 h (90 °C) or 3 h (30 °C), rather shorter than in previous reports (6−48 h).4,5,15,16,18 To our knowledge, this is the first example of biosynthesis of Au nanoplates using syringe pumps, and the results suggest that it is possible to modulate the morphology of Au nanoparticles by simply altering the addition mode (or rate) of the feed solutions (Au precursor and/or CP extract). 3.1.2. Influence of Reaction Temperature. Aside from the addition mode/rate, we investigated the effects of temperature on the Au morphology based on addition mode C (simultaneous feeding of the two reactants at equal addition rates). As shown in

Figure 2a, when a low addition rate (18 mL/h) was used, the SPR spectral properties of the Au sols varied considerably at different temperatures. Considering that the intensity of the SPR band in the NIR region (NIR-SPR) is closely associated with the quantity of Au nanoplates,8 this indicates that the optimal reaction temperature for the formation of Au nanoplates was 30 °C. Interestingly, different addition rates allowed for different optimal reaction temperatures. For a moderate addition rate (60 mL/h, Figure 2b), the intensity of the NIR-SPR band initially increased as the temperature rose from 30 to 60 °C, but quite unexpectedly decreased as the temperature continued to increse to 90 °C. Therefore, 60 °C was the best temperature for the formation of anisotropic nanoplates as inferred from the maximum intensity of the NIR-SPR band. In comparison, when the high addition rate (120 mL/h, Figure 2c) was used, we found that the intensity of the NIR-SPR band increased straight as elevating the reaction temperature. Obviously, 90 °C was the best reaction temperature on this occasion. For simplicity, the three optimal combinations of temperatures and addition rates are referred to as condition 1 (30 °C reaction temperature with the low addition rate of 18 mL/h), condition 2 (60 °C with the moderate rate of 60 mL/h), and condition 3 (90 °C with the high rate of 120 mL/h). Corresponding TEM images for Au nanoparticles synthesized under conditions 1−3 are presented in Figure 2d−f, with the matching edge lengths and particle sizes summarized in Figure 2g,h, respectively. As shown, the yields of 15756

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Figure 4. (a) Temporal evolution of UV−vis−NIR spectra of Au sol. The reaction temperature was 60 °C, the pH of the Au precursor was 3.41, and the addition mode and rate were mode C and 60 mL/h, respectively. (b) Average nanosphere particle size and nanoplate edge length as functions of reaction time.

enabled the formation of Au nanoplates. Hence, it is feasible to increase the production of Au nanoplates by decreasing the pH of the Au precursor to a certain extent. All of the experiments performed showed that the formation of the Au nanoplates is kinetically controlled and highly sensitive to the investigated parameters. It is most likely, however, that a relatively low reduction rate is favorable for the growth of Au nanoplates. 3.2. Formation Mechanisms of the Au Nanoplates. To obatin insight into the formation mechanism of the Au nanoplates, the time evolution of the Au sols was studied by UV−vis−NIR spectroscopy and TEM. Judging from the UV− vis−NIR spectra (Figure 4a), it is clear that, with successive feeding, there was a gradual red shift of the nanoplate resonances (in NIR region) accompanied by an increase in intensity. The initial size of the Au nanoparticles became progressively larger, and the size evolution is shown in Figure 4b. Hence, the formation of the Au nanoplates should have passed through the initial formation of small nanoplates and grown into greater edge lengths. Evidence for this phenomenon is presented by the TEM images (Figure 5a−f), wherein gradual shape evolutions from small spherical nanoparticles to large spherical nanoparticles and from short-side nanoplates to long-edge Au nanoplates are clearly visible. Interestingly, based on careful HRTEM analyses (Figure 5g), we were able to observe that there were some small atom clusters (∼3.2 nm) during the feeding addition stage (prior to 30 min). We hypothesized that the atom clusters, as in the case of our experiments, could easily be adsorbed on the lateral faces of the nanoplates following the most energetically favorable direction and undergoing somewhat of a “fusing” process, thus leading to plate growth. This hypothesis was supported by the observation of some intermediate nanoplates with stepped edges. As illustrated in Figure 5h,i, the arrows show the fusing of several atom clusters on the active growth sites of the Au nanoplates. The atom clusters can be successively produced and persist during the whole feeding stage because of the stepwise feature of mode C. TEM observations demonstrated that the quantity of intermediate particles decreased after the completion of feeding (30 min) and finally disappeared in the final stage (60 min), yet with all smooth plate edges. Similarly, Shankar et al. reported a detailed study on the sintering of small “liquidlike” spherical nanoparticles (i.e., atom clusters) to biogenic nanoplates.15 Xie et al. found some rudimentary nanoplates with corrugated edges,

the Au nanoplates from the three conditions are 24%, 22%, and 24%, respectively with corresponding average edge lengths of 187.1 ± 76.4, 102.5 ± 38.9, and 105.6 ± 40.3 nm and average spherical particle sizes of 68.1 ± 13.6, 48.4 ± 9.5, and 53.1 ± 7.0 nm. Taking these results together, we conclude that the growth kinetics of the Au nanoplates is highly sensitively to both the reaction temperature and the addition rate: High temperature leads to rapid reduction of Au3+, thus decreasing the reactant concentration, whereas fast feeding addition supplements the consumption of the reactant, counterbalancing the concentration. The optimal conditions (1−3) reveal that, for higher yields of Au nanoplates, high temperatures must be implemented with high addition rates, whereas low temperatures must be applied with low addition rates. In addition, it seems that suitable temperature and concentration of the Au precursor (and/or CP extract) are essential to the formation of the anisotropic nanoplates. This guideline will become more apparent after we explain the plausible formation mechanism of the nanoplates later in this article (vide infra, section 3.2). 3.1.3. Influence of pH of Au Precursor. It is known that pH is another crucial factor influencing the reduction rate, because the reduction potentials of Au species change with varying pH.30,31 As shown in Figure 3a, a subtle pH variation from pH 3.41 (original HAuCl4 solution) to 2.58 had a pronounced effect on the SPR bands of the resulting Au sols (in terms of both position and intensity). The absorption in the NIR region due to Au nanoplates significantly increased in intensity with decreasing pH and shifted in the red direction, accompanied by a decrease in the SPR bands in the visible region (spherical nanoparticles). Indeed, TEM analysis (Figure 3b) revealed that the yield of the Au nanoplates could be modulated to as high as 39% at pH 2.81. However, decreasing the pH further from 2.81 to 2.58 decreased the yield instead of leading to a continuous increase (Figure 3c). Moreover, the statistical analysis (Figure 3d,e) confirmed the increase of both the average spherical particle size (from 48.4 ± 9.5 to 55.2 ± 11.7 nm) and the average edge length of the nanoplates (from 102.5 ± 38.9 to 162.1 ± 87.2 nm), when the pH was adjusted from 3.41 to 2.81. The color change of the reaction mixture for pH 2.81 occurred over a longer period of time compared to that at the original pH (3.41), which reflected a slowing of the crystal nucleation and growth.32 Therefore, a lowpH environment allowed for a decrease of the reduction rate and 15757

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Figure 5. TEM images of Au nanoparticles synthesized at (a) 5, (b) 10, (c) 20, (d) 30, (e) 40, and (f) 60 min. The insets are the corresponding highermagnification images. (g) HRTEM image of the atom clusters with corresponding particle sizes (inset histogram). (h,i) TEM images of two typical intermediate nanoplates, with arrows showing the fusing of several atom clusters on the active growth sites of the Au nanoplates.

indicating the attachment of nanoclusters on the biosynthesized Au nanoplates.24 As documented in the literature, the seed structure governs the shape of the final nanostructures, and twinned crystal seeds are conceived to propagate nanoplates.33 On the basis of this hypothesis, the proposed mechanism for the formation of Au nanoplates prepared by the bioreduction procedure is depicted in Scheme 1, which includes three basic stages: (i) reduction, (ii) nucleation, and (iii) growth. (i) In the reduction stage, Au precursors are reduced with CP extract to generate Au atoms. The results reported herein suggest that the experimental parameters (e.g., addition mode/rate, temperature, pH) are critical to the reduction

rate. Admittedly, the reduction rate (r1) dictates the period of the nucleation stage and the amount of Au3+ ions in growth stage, thus ultimately affecting the morphology of the Au nanostructure. (ii) In the nucleation stage, two competing reactions for the formation of either twinned or untwinned seeds occur. The twinned crystal seeds are formed by coalescence between two parallel {111}-type planes33 and thus need a longer induction time to ensure effective collision, compared with untwinned seeds. Following this step, the rate of spherical nucleation (r2) is hypothesized to be higher than the rate of the plate nucleation (r3), supporting the fact that fast reduction of the Au precursor shortens the 15758

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Scheme 1. Schematic Diagram of the Formation of Au Nanoplates by the Mechanism of Kinetically Controlling Bioreduction Described Herein

Figure 6. (Left) Typical AFM image of two Au nanoplates and (right) height profile along the dashed line.

Figure 7. (a) Typical HRTEM image of a single Au nanoplate; the inset is the corresponding SAED pattern. (b) XRD patterns of the obtained Au nanoparticles and reference [International Centre for Diffraction Data (ICCD), PDF2 01-089-3697]. (c) FTIR spectrum of the purified Au nanoparticles.

time of the nucleation stage and favors the rapid formation of the spherical (untwinned) seeds, consequently

suppressing the formation of twinned seeds (i.e., nanoplates). This is the reason that use of addition mode A 15759

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Figure 8. XPS (a) Au 4f, (b) C 1s, and (c) O 1s core-level spectra recorded from purified Au nanoplates. (d) Simultaneous TG and differential TG (DTG) curves of the corresponding nanoplates.

consume more Au3+, to coordinate the growth requirement of Au nanoplates, high addition rates are required to compensate the consumption. Likewise, the obtained result that the more acidic environment (pH 2.81) favored the formation of Au nanoplates whereas only spherical nanoparticles were produced in alkaline solutions17 supports this proposed mechanism. Abundant Au3+ remaining at lower pH is beneficial to increase the rate of nanoplate growth (r5). According to this mechanism, we conclude that any factors that affect the reduction rate, in principle, would affect the final morphology of the nanostructure. In particular, the stepwise addition of the Au precursor along with a reductant (such as CP extract) most favors the growth of Au nanoplates. 3.3. Characterization of the Au Nanoplates. 3.3.1. AFM. Figure 6 shows contact-mode AFM image of monodisperse Au nanoplates. As shown, the two Au nanoplates have triangular geometries with an average edge length of 258.2 nm. Height profile analysis indicates that the average thickness of the Au nanoplates in this case was 36.1 nm, and their top faces were comparatively flat. 3.3.2. HRTEM and SAED. Figure 7a shows an HRTEM image of an individual Au nanoplate. The basal spacing of the lattice is 2.34 Å, in agreement with the (111) lattice fringes of the Au crystal, demonstrating the single-crystal structure in the entire surface of the plates without any defects. The single-crystalline structure was further confirmed by SAED (inset in Figure 7a). The hexagonal nature of the diffraction spots supports the fact that Au nanoplates are highly [111]-oriented, with the planar face perpendicular to the electron beam.27 3.3.3. XRD. A typical XRD pattern for the as-prepared Au nanoplates is shown in Figure 7b, and the peaks are assigned to the diffraction of (111), (200), (220), (311), and (222)

produced only spherical nanoparticles, as the excess reductant (CP extract) resulted in the reduction reaction proceeding so quickly as to suppress the formation of twinned seeds. Similarly, decreasing the pH led to relatively lower reduction rate, thus prolonging the nucleation time and inducing more twinned seeds. Consequently, a higher yield of nanoplates would be achieved, consistent with the reported investigations. (iii) In the growth stage, competition between the growth of twinned and untwinned seeds occurs. In general, Au3+ can take part in the further growth of both twinned and untwinned seeds by heterogeneous reaction on the preferred surface, leading to crystal growth. However, Au3+ is more favorable for the lateral growth of twinned seeds (i.e., nanoplates) than untwinned seeds (i.e., spherical ones), resulting in preferential deposition on Au nanoplates at a higher rate.34 Hence, we conceive that the specific rate of nanoplate growth (r5) is much higher than the rate of nanosphere growth (r4). The concentration of Au3+ is affected by both the reduction rate and feeding injection conditions. Consequently, syringepump-based mode C is an excellent choice for preparing Au nanoplates in higher yield because of its ability to add Au ions in stepwise mode, quite different from the traditional addition mode (direct mixing without control of the addition rate). Aside from Au3+, atom clusters are another driving force promoting the growth of Au nanoplates (r6), as mentioned above. Nanoplates in solution continue growing until both the Au3+ and atom clusters are exhausted. Following this mechanism, it is reasonable to explain the reported results that high reaction temperatures must be implemented with high addition rates (Figure 2a−c): Because high temperatures 15760

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reflections of face-centered-cubic (fcc) metallic Au structure (ICCD, PDF2 01-089-3697). The intensity ratio of (111) to (200) peaks is much higher than the standard value (4.8 versus 2.2), implying that the Au particles were dominated by {111} facets. 3.3.4. FTIR Spectroscopy. To verify the presence of some organic groups on the surface of the Au nanoplates, FTIR analysis was conducted. The nanoparticles were purified by six cycles of centrifugation and rewashing treatments to separate the Au from any unbound biomolecules. As illustrated in Figure 7c, three well-differentiated bands at 1096, 1390, and 1610 cm−1 were observed and assigned to the vibrations of C−O, carboxyl groups, and aromatic C−C, respectively.15,25,35 This finding emphasizes that the biomolecules were able to adhere efficiently at the Au nanoplate facet, and it is reasonable to assume the existence of strong interactions between them. 3.3.5. XPS and TG. The surface composition and chemical state of the purified Au nanoplates was investigated by XPS.4,18,36 Panels a−c of Figure 8 show the Au 4f, C 1s, and O 1s core levels, respectively. The Au 4f7/2 spectrum could be decomposed into two components centered at binding energies (BEs) of 84.0 and 84.6 eV, which could be assigned to Au0 (∼89%) and Au+ (∼11%), respectively. In contrast, the C 1s and O 1s spectra are more complex, and could both be decomposed into four chemically distinct peaks [i.e., C 1s peaks at BEs of 284.2, 285 (adventitious carbon atoms), 286.2, and 288 eV and O 1s peaks at BEs of 531.4, 532.1, 532.7, and 533.5 eV). These BE peaks correspond to three carbon species and four oxygen species, or rather carbon−oxygen bonds (e.g., carbonyl, carboxyl groups, etc.). Apparently, these results further verify the presence of biomolecules on the Au surface, consistent with the FTIR analysis. In addition, concentration analysis indicates the Au, C, and O had surface atomic percentages of 8.5, 63.9, and 27.4 at. %, respectively. Mindful that XPS results reveal only surface information, the actual content of biomolecules was about 3.8 wt % in the bulk structure, as revealed by TG measurements (Figure 8d). Moreover, the decomposition temperature of the binding biomolecules was found to be 380 °C from differential TG (DTG) analysis. 3.3.6. NIR Region Absorption. It is difficult to reliably measure both the position and the intensity of the SPR bands of Au sols beyond 1400 nm because of the absorbance of water.7 Hence, the Au nanoplates were deposited on a glass substrate by solution casting and drying. As shown in Figure 9, Au films on the glass all exhibited golden colorations. The results clearly demonstrate that the film of spherical nanoparticles showed a single absorption peak centered at 590 nm (curve a). By contrast, Au films containing nanoplates were found to exhibit a noticeable absorbance in the NIR region of 700−3000 nm. As expected, the Au film from the higher yield of nanoplates showed an obvious enhancement in NIR absorption over the entire wavelength range scanned, based on a comparison of curves b (22%) and c (39%) (the same coating layer). Because of this strong absorption in the NIR region, Au nanoplate films could be promising candidates for functional NIR absorbers, architecturecoating materials, and clinical applications.

Figure 9. UV−vis−NIR spectra recorded from different Au films: glasses coated with (a) pure spherical Au nanoparticles, (b) 22% Au nanoplates, and (c) 39% Au nanoplates. Inset: Photographs of the corresponding Au films and the uncoated glass substrate (3 cm × 1 cm).

feed solutions, temperature, and pH using a syringe-pumps apparatus. The yield of Au nanoplates can be as high as 39% for the simultaneous injection of CP extract and Au precursor at 60 mL/h, for a pH of 2.81 and a temperature of 60 °C. The assynthesized nanoplates exhibited strong surface plasmon absorbance in the near-infrared region of 700−3000 nm. A possible mechanism for the formation of Au nanoplates was proposed that includes three basic stages: reduction, nucleation, and growth. The biogenic Au nanoplates are promising for applications because of their unique optical features, thermal stability, and biocompatibility.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: (+86) 592-2189595. Fax: (+86)592-2184822. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by NSFC Projects 21206140, 21106117, 21036004, and 20976146 and NSF-Fujian Project 2010J05032.

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REFERENCES

(1) Burda, C.; Chen, X. B.; Narayanan, R.; El-Sayed, M. A. Chemistry and properties of nanocrystals of different shapes. Chem. Rev. 2005, 105, 1025−1102. (2) Kan, C. X.; Wang, G. H.; Zhu, X. G.; Li, C. C.; Cao, B. Q. Structure and thermal stability of gold nanoplates. Appl. Phys. Lett. 2006, 88, 071904. (3) Sun, Y.; Lei, C. Synthesis of out-of-substrate Au−Ag nanoplates with enhanced stability for catalysis. Angew. Chem., Int. Ed. 2009, 48, 6824−6827. (4) Shankar, S. S.; Rai, A.; Ahmad, A.; Sastry, M. Controlling the optical properties of lemongrass extract synthesized gold nanotriangles and potential application in infrared-absorbing optical coatings. Chem. Mater. 2005, 17, 566−572. (5) Singh, A.; Chaudhari, M.; Sastry, M. Construction of conductive multilayer films of biogenic triangular gold nanoparticles and their application in chemical vapour sensing. Nanotechnology 2006, 17, 2399− 2405. (6) Jiang, Y. Q.; Horimoto, N. N.; Imura, K.; Okamoto, H.; Matsui, K.; Shigemoto, R. Bioimaging with Two-Photon-Induced Luminescence

4. CONCLUSIONS In summary, we have reported a novel strategy for the synthesis of Au nanoplates from the kinetically controlled bioreduction of HAuCl4 by Cacumen Platycladi extract. The distinct feature is that the yield of Au nanoplates can be tuned by modulating experimental parameters such as the addition mode/rate of 15761

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dx.doi.org/10.1021/ie302483d | Ind. Eng. Chem. Res. 2012, 51, 15753−15762