Article pubs.acs.org/Langmuir
Activation of Oxygen-Mediating Pathway Using Copper Ions: FineTuning of Growth Kinetics in Gold Nanorod Overgrowth Wenqi Liu,†,§ Hui Zhang,†,§ Tao Wen,†,§ Jiao Yan,†,§ Shuai Hou,†,§ Xiaowei Shi,†,‡ Zhijian Hu,† Yinglu Ji,*,† and Xiaochun Wu*,† †
CAS Key Laboratory of Standardization and Measurement for Nanotechnology, National Center for Nanoscience and Technology, Beijing 100190, P. R. China § University of the Chinese Academy of Sciences, Beijing 100190, P. R. China ‡ Central South University, Hunan 410083, P. R. China S Supporting Information *
ABSTRACT: Growth kinetics plays an important role in the shape control of nanocrystals (NCs). Herein, we presented a unique way to fine-tune the growth kinetics via oxidative etching activated by copper ions. For the overgrowth of gold nanorods (Au NRs), competitive adsorption of dissolved oxygen on rod surface was found to slow down the overgrowth rate. Copper ions were able to remove the adsorbed oxygen species from the Au surface via oxidative etching, thus exposing more reaction sites for Au deposition. In this way, copper ions facilitated the overgrowth process. Furthermore, Cu2+ rather than Cu+ acted as the catalyst for the oxidative etching. Comparative study with Ag+ indicated that Cu2+ cannot regulate NC shapes via an underpotential deposition mechanism. In contrast, Ag+ led to the formation of Au tetrahexahedra (THH) and a slight decrease of the growth rate at similar growth conditions. Combining the distinct roles of the two ions enabled elongated THH to be produced. Copper ions activating the O2 pathway suggested that dissolved oxygen has a strong affinity for the Au surface. Moreover, the results of NC-sensitized singlet oxygen (1O2) indicated that the absorbed oxygen species on the surface of Au NCs bounded with low-index facets mainly existed in the form of molecular O2.
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INTRODCTION Noble metal nanoparticles (NPs) have attracted extensive research interest in various fields such as plasmonics, catalysis, biochemical assays, and disease theranostics.1 NP shape plays a unique role in tailoring their properties, for instance, localized surface plasmon resonance (LSPR) enhanced properties and exposure facet-dependent catalytic behaviors.2 Manipulating the growth kinetics is crucial to obtain desired particle shapes. For example, slow growth prefers thermodynamically more stable morphologies with low-index facets, such as {111}-bounded octahedra (OCT). By accelerating the growth process, kinetically controlled products such as {221}-faceted trisoctahedra could be acquired.3,4 The Xia group pioneered the kinetic control by introducing oxidative etching in the polyol synthesis of noble metal nanostructures.5 For instance, taking advantage of the subtle difference in reactivity between twinned and single-crystal nanocrystals (NCs), they selectively removed twinned seeds from the growth system using oxidative etching by the Cl−/O2 pair and obtained final products dominated by single-crystal NCs. 6 Using cubic Au NCs, Lu et al. demonstrated the formation of tetrahexahedral Au−Pd coreshell NCs, where the oxidative etching of the Pd surface by the Cl−/O2 pair was suggested to favor the emergence of {730} faces.7 Quite recently, the Xia group further demonstrated this © 2014 American Chemical Society
strategy in shape transformation of Pd NCs. Pd atoms at the corners of cubic Pd NCs were preferentially removed via oxidative etching. The resultant Pd ions inclined to deposit on the {100} facets, leading to the formation of OCT. Careful tuning of the ratio of etching and growth enables to achieve nano-OCT with desirable sizes.8 In the seed-mediated growth method, the Mirkin group obtained unprecedented control in NC uniformity and purity by employing iterative reductive growth and oxidative dissolution strategy.9 These efforts have pointed out that utilization of oxidative etching may provide a unique platform in controlling the purity, size, structure, and shape of NCs.10 Recently, we have unraveled a novel role of Cu2+ in the seedmediated growth of gold nanorods (Au NRs):11 they can catalyze the oxidative etching of small Au seeds by dissolved oxygen in acidic growth solution (pH ∼2). The etched Au seeds become more active, thus leading to an obvious increase in the growth rate of the Au NRs. Under optimal growth rate, great improvement in the size uniformity of the Au NRs was achieved. Furthermore, using the Cu2+-assisted oxidative Received: July 3, 2014 Revised: September 22, 2014 Published: September 22, 2014 12376
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Figure 1. Cu2+-mediated formation of Au OCT seeded by Au NRs. Extinction spectra evolution in the absence (A) and presence (B) of copper ions and the effect of PDCA on growth process (C); time traces of Ext400 nm value under different concentrations of copper ions (D). The insets are SEM images of the final products. Growth conditions: [CTAB] = 0.032 M, [HAuCl4] = 0.58 mM, [AA/HAuCl4] = 1.1, [Au NRs] = 0.06 nM, [Cu2+] = 0, 0.6, 1.2, and 1.8 mM, respectively. The growth temperature (T) is kept at 50 °C.
provides a convenient way to monitor the growth or etching process. At a reaction temperature of 50 °C, the growth rate, denoted as the increase in extinction values at 400 nm (Ext400 nm) per min, was 1.1 × 10−4 min−1. There was a quite long induction period of ∼150 min. It took 3 days for Au+ ions to be fully reduced. The final Ext400nm on the third day was similar to that obtained at AA/HAuCl4 ratio of 10 after 1h (Figure S1). When Cu2+ was added in the growth solution, the growth rate was increased obviously as demonstrated in Figure 1B. At 0.6 mM Cu2+, the growth rate was increased to 2.4 × 10−4 min−1. The induction period was shortened to ∼25 min. In our experiments, the variations in growth rate did not change the NC morphology. In the final products of the Au OCT, copper was undetectable based on the element analysis from the energy-dispersive X-ray spectroscopy (EDX) and inductively coupled plasma-mass spectrometry (ICP-MS) (see the Supporting Information Tables S1 and S2 and Figure S2). Surface sensitive X-ray photoelectron spectroscopy (XPS) measurements (Figure 5) also found no copper. During the growth process, addition of a strong ligand for Cu2+, 2,6pyridinedicarboxylicacid (PDCA), immediately slowed the growth rate (Figure 1C,D), verifying the role of copper ions in accelerating growth. Figure 1D shows Ext400nm value vs growth time under different concentrations of copper ions. As expected, the growth rate accelerated with increasing copper ions. We noticed that in the presence of Cu2+, when the growth finished, a slow and gradual decrease in Ext400nm was observed, which is due to the Cu2+-catalyzed etching as we will discuss later. Very interestingly, several recipes have been used to form Au OCT from Au NRs, but the intermediates are different. Herein, without adding shape regulators, thermodynamically driven growth was elegantly demonstrated. The initial Au NRs have four {110} and four {100} side facets, while the tips are enclosed by four {111} and four {110} planes.21 First, the most
etching strategy, we realized reshaping of the Au NRs and etching of the Au, Ag, and Pd NCs under much milder conditions.12 It therefore inspired us to explore whether such oxidative etching can be utilized to manipulate the overgrowth of large size Au NCs. In the case of small spherical Au seeds (∼3 nm diameter), we found that copper ions act only in low pH conditions by adding extra acids, where the oxidative capability of dissolved O2 is greatly enhanced. Thus, it will be helpful if such a strategy could be extended to work in more general growth conditions. Apart from assisting oxidative etching, Cu2+ was also introduced by several groups to the synthesis of metal NCs with different shapes.13−15 Compared with Cu2+, Ag+ is a better-known shape regulation ion.16,17 In the case of Ag+, facet-selective passivation leads to shape control. Although the exact shape regulation mechanism of Ag+ is still elusive, a popular one is the underpotential deposition (UPD) mechanism. Therefore, it is interesting to see whether copper ions can act similarly to Ag+ as a shape regulator.
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RESULTS AND DISCUSSION Formation of Au OCT in the Presence of Cu2+: Increasing Growth Rate in Weak Acidic Growth Condition. We found that when the ratio of ascorbic acid (AA) and HAuCl4 was around 1.1 the overgrowth rate of Au NRs was very slow even at elevated temperatures. The slow growth favored the formation of a thermodynamically stable morphology, Au OCT (Figure 1A inset). The evolution of extinction spectra demonstrates the typical features of the rodsto-OCT transition. For Au NCs, the region from 350 to 450 nm in the extinction spectra reflects the interband transitions, which are independent of particle shape, so the extinction value at this range can be used to evaluate the total amount of Au atoms. At a given Au NR concentration, the extinction value can reflect the volume change of the Au NRs.18−20 It therefore 12377
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I{220}. In the final products Au OCT, both I{111}/I{220} and I{111}/I{200} are enhanced. The catalytic role of copper ions was verified by the decreased activation energy (Ea) of the Au OCT formation. Growth rates were obtained at different growth temperatures (55 to ∼75 °C) using the change of Ext400nm with time. Using the Arrhenius plot, the Ea in the absence of copper ions is 121 kJ/mol (Figure 3A). In the case of adding Cu2+, Ea reduces to 93 kJ/mol. Introduction of copper ions to the growth process greatly reduced the Ea. We suggest that the dissolved oxygen competes with Au ions for the Au surface. Once they occupy the reactive sites, Au deposition will be impeded as we observed a long induction time and slowed growth rate in Figure 1A. In order to understand the role of the dissolved oxygen, we further investigated Au overgrowth under an O2-deficent environment. By purging the reaction solution with nitrogen gas (99.99%) for 30 min, the amount of dissolved oxygen decreased from the original 9.48 to 1.75 mg/L (20 °C). In the absence of Cu2+, removal of O2 resulted in a slight acceleration of the growth, indicating reduced oxygen adsorption (Figure 3B red square). In the presence of Cu2+, a significant increase in growth rate was observed by removing O2 (Figure 3B purple triangle). So, because of the reduced dissolved oxygen amount in the growth solution, less O2 molecules are available to compete with Au+ for the Au surface, thus the Au deposition is greatly accelerated. On the contrary, in the case of dissolved oxygen-induced etching, O2 removal will lead to less etching. Without adding AA and HAuCl4, the Au NRs got slightly etched by dissolved oxygen (Figure S4B). Introduction of Cu2+ aggravated oxidative etching as expected (Figure S4D). It explains the slow decrease of Ext400nm after growth in the presence of Cu2+ (Figure 1D). Decreasing etchant O2 content obviously alleviates etching (Figure S4E). High resolution XPS spectrum of O 1s exhibits an asymmetric peak (Figure 3C), with a tail toward the lowenergy region. The asymmetric shape was fitted into two peaks. The peak at 532.2 eV corresponds to the adsorbed water, while the weaker (530.7 eV) peak should be attributed to the adsorbed oxygen species on the Au NR surface.24 In all, the Au NR overgrowth picture can be described as follows. Dissolved oxygen has a high affinity for rod surface; once they occupy the reactive sites of the rod surface, the deposition of Au is hindered. Upon adding copper ions, they can catalyze the removal of the adsorbed oxygen species from the rod surface, thus exposing more reactive sites for Au deposition. In this way, copper ions facilitate the overgrowth. This process is schematically shown in Figure 3D. The solubility of dissolved oxygen in water at 60 °C is ∼5 mg/L (∼0.16 mM), 3.6 times lower than Au+; it can still predominate Au+ in the competitive adsorption to Au surface. It indicates a high affinity of dissolved oxygen for the Au surface. In the polyol synthesis of Ag nanowires, the Xia group also observed that copper ions could scavenge adsorbed oxygen species and thus accelerate the growth of the nanowire.25 They suggested that Cu+ lost electron to adsorbed oxygen species. In order to clarify which valence state of Cu took effect, we studied the role of cuprous ions in the growth and etching of Au NRs (Figure S5). Different from polyol synthesis, in our case, Cu+ does not accelerate etching (Figure S5B). In the overgrowth of Au NRs, Cu+ was rapidly oxidized to Cu2+ by Au+, which accelerated Au deposition (Figure S5D). Therefore, Cu2+ was responsible for the oxidative etching. Our previous work reported Cu2+mediated growth in low pH values (∼2) and high CTAB concentration (0.1 M). The former enhances the oxidation
unstable {110} facets at the tip and side of the rod grow more quickly, become smaller, and finally disappear, leaving a rod bounded with four {100} side facets and four {111} facets at each end-caps. It leads to the arrowhead NRs observed after growing 1 and 4h (Figure 2A b and c). Such intermediates are
Figure 2. Thermodynamically driven growth of Au OCT. (A) SEM images of Au NR seeds (a), intermediate NCs (b and c), and final products (d); (B) Schematic illustration of the shape transformation from rod to octahedron; (C) XRD patterns of Au NRs (a), intermediates (c), and final products (d).
different from those obtained in the presence of PVP22 or Ag+,23 where {110} facets were preserved. Then, four {100} side surfaces continue growing and leave the eight {111} facets as the final exposed facets (Figure 2A d). The shape transition is shown schematically in Figure 2B. The X-ray powder diffraction (XRD) pattern substantiates the shape evolution (Figure 2C). The intermediates and final products show obviously preferred orientation by comparing the intensity ratios of {111}, {220}, and {200} crystalline planes (see Table S3). In comparison with Au NRs seeds, the enlargement of {100} facets in arrowhead NRs leads to the increase of I{200}/ 12378
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Figure 3. (A) Arrhenius plot of the growth rate versus reciprocal of temperature for the overgrowth of the Au NRs in the absence (dark square) and presence (red circle) of 0.6 mM Cu2+; (B) Time traces of Ext400nm during the overgrowth of Au NRs under different reaction conditions ([Cu2+] = 0.6 mM); (C) High resolution O 1s XPS spectrum analysis showing the presence of oxygen species on the surface of Au NRs (the spectrum is fitted with two Lorentzian peaks); (D) schematic diagram of accelerated Au deposition via Cu2+-activated O2 pathway.
Figure 4. Distinct roles of Ag+ and Cu2+ in mediating the growth of the Au NRs. Extinction spectra (A and B) of the NCs obtained under different amounts of Ag+ ([Cu2+] = 0 mM) (A) and those under different amounts of Cu2+ with [Ag+] = 0.6 mM (B); (C) SEM images of NCs (the labels correspond to those in spectra A and B); (D) Ext400nm during the overgrowth of Au NRs under different growth conditions. Growth conditions: [CTAB] = 0.032 M, [HAuCl4] = 0.58 mM, [AA/HAuCl4] = 1.1, [Au NRs] = 0.06 nM, and T = 60 °C.
capability of dissolved oxygen. The latter provides Br‑ to form stable AuBr2− complexes. In all, they form a more oxidative growth environment. Herein, we further extend this strategy to less oxidative growth environments (pH 4 and low CTAB). According to different growth conditions, the concentration of Cu2+ should also change correspondingly. Herein, the
employed Cu2+ amount is larger than that in the previous study (< 0.1 mM). Our results indicate that the Cu2+-activated O2 pathway could be a universal way in manipulating NC growth. Formation of Elongated Au Tetrahexahedra (THH) in the Coexistence of Cu2+ and Ag+. In the formation of Au 12379
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Figure 5. XPS survey scan of Au OCT and Au ETHH prepared with the help of Cu2+ (A) and high resolution XPS spectra for Cu 2p (B), Ag 3d (C), and Br 3d (D), respectively. C 1s binding energy of 284.6 eV is used to calibrate the binding energy of other elements.
the Br/N atom ratio in Au ETHH is ∼1.3. The excess of Br should bind with silver. Considering their different roles, the combination of the two ions can give a synergistic effect and offer more possibilities in fine-tuning the morphology of NPs. As shown in Figure 6, in
OCT, we did not find the UPD effect of Cu2+. In order to better compare the role of the two metal ions, we introduced Ag+ to the recipe of Au OCT formation. Different from Cu2+, addition of Ag+ slightly decreased the growth rate. The growth temperature was therefore increased to 60 °C to accelerate the growth process. In the absence of Cu2+, Ag+ led to the formation of THH, verifying its shape-regulating role. It forms sharp contrast to Cu2+, which plays no role in shape transformation. The product yields were above 90%. Increasing the amount of silver ions (Figure 4A) can slightly elongate the THH (Figure 4A and Ca and b). Proper growth rate is important for the formation of elongated tetrahexahedra (ETHH). No ETHH was produced by increasing the AA/ HAuCl4 ratio above 1.6 (Figure S6). At fixed concentration of silver ions, copper ions can lead to more effective elongation of THH (Figure 4B and Cc and d). Growth kinetics curves clearly demonstrate the difference of the two ions: Ag+ slightly reduces the growth rate, whereas Cu2+ can obviously accelerate the growth (Figure 4D). XPS measurements (Figure 5) also distinguish their difference. In both Au OCT and ETHH, XPS measurements indicate the existence of Au, Ag, C, N, Br, and O. Elements of C, N, and Br mainly come from the surfactant CTAB. In both samples, the content of Cu element on the particle surface can be ignored (Cu 2p binding energy around ∼933 eV). The small amount of surface Ag content in OCT is from the Au NRs. In contrast, surface Ag content in ETHH is high. The Ag/N ratio is 0.11 in Au OCT. It increases to 0.38 in ETHH (Table S4). The results indicate that Ag+ and Cu2+ indeed act differently in the overgrowth of Au NRs. In Au OCT, the Br/N atom ratio is ∼0.9, close to the stoichiometric ratio of Br/N ratio in CTAB. The Ag/Br ratio is quite low, suggesting that the Br signal is mainly from the CTAB bilayer on the Au surface. In contrast,
Figure 6. Tailoring NC morphologies in the coexistence of Ag+ and Cu2+. SEM images of Au NCs (A) and corresponding extinction spectra (B) obtained at different concentration of Ag+. Growth conditions: [CTAB] = 0.032 M, [HAuCl4] = 0.58 mM, [AA/HAuCl4] = 1.1, [Au NRs] = 0.06 nM, [Cu2+] = 1.8 mM, [Ag+] = 0, 0.05, 0.1, 0.2, 0.6 mM, T = 60 °C.
the presence of Cu2+, increasing Ag+ can elongate the THH more effectively. THH/ETHH structures with high-index facets are desired for high reactive activity. They have been obtained using small sphere Au seeds mediated growth by several groups.26−28 Herein, we accidently got them directly from the overgrowth of the Au NRs. The method we present here 12380
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Figure 7. TEM images (Aa, b, and c) and extinction spectra (Ad) of Au dog-bones (a), Au NRs (b), and Au@Pd nanobars (c); heating and cooling curves of the NCs (B); (C) NCs-assisted degradation of ABDA in dark at 60 °C, inset: spectral evolution of ABDA in the presence of the Au@Pd nanobars; (D) ABDA degradation by NCs photosensitization upon 808 nm laser irradiation at room temperature, inset: spectral evolution of ABDA in the presence of the Au dog-bones. [NCs] = 33 pM.
and dog-bone exhibited a fast ∼10% decrease in A380nm within the initial 10 min. Prolonged heating gave no further change in A380nm. Likely, Au ETHH bounded by high-index facets also produced 1O2 upon heating. In contrast, the Au OCT bounded by {111} facets did not assist 1O2 production (Figure S7C). We suggest that there exist some high energy surface reactive sites21 on Au NR and dog-bone, and some of them are stabilized by adsorbed molecular O2. It reminds us very long induction period for the formation of the Au OCT in the absence of Cu2+ (Figure 1A). With the help of heating, some adsorbed O2 molecules turned to 1O2. Upon laser irradiation (Figure 7D), all NCs exhibited assisting ABDA degradation with different degrees. The degradation magnitude by light activation was larger than thermal activation. The Au NR had the largest absorbance and therefore had the maximum T increase within 30 min (16 °C). The maximum temperature was slightly higher than 40 °C but lower than 60 °C of water-bath heating (Figure 7B). Therefore, the degradation of ABDA is mainly due to surface plasmon resonance (SPR) excitation, rather than thermal activation. The Au dog-bone exhibited the highest efficiency of 1O2 albeit slightly lower SPR excitation efficiency in comparison with the Au NR. We hypothesize that there are more high energy surface reactive sites on the dog-bone, which are beneficial for dynamic O2 adsorption. It consists of the observation of Cu2+-assisted etching of Au dog-bones by dissolved oxygen.12 They changed to cylindrical rods after etching. From the viewpoint of SPR excitation, Au is advantageous over Pd. It therefore induces more efficient 1O2
provides a flexible way to prepare Au THH/ETHH structures with tailorable aspect ratios and sizes. Probing the Existing State of Adsorbed Oxygen Species on the Surface of Au NCs. Recent experimental observations point out that some noble metal NPs (Ag, Pt, Pd, and Au), including Au NRs, are a new kind of singlet oxygen (1O2) photosensitizers.29−34 For 1O2 production, dissolved oxygen must adsorb on the metal surface in the form of molecular oxygen. As shown above, via the Cu2+-activated oxygen pathway, we found that dissolved oxygen has a strong tendency to adsorb on the Au surface. Therefore, it is interesting to see whether we can distinguish the adsorbed oxygen species with the help of 1O2 detection reaction. 1 O2 can oxidize 9,10-anthracenediyl-bis (methylene) dimalonic acid (ABDA) irreversibly to an endoperoxide which causes a decrease in the ABDA absorption band. As LSPR excitation of the NCs will cause a temperature rise, we first investigated the production of 1O2 upon heating using water bath. In addition, we employed Au@Pd nanobars as a control because cubic Pd NCs were found to enable the spin-flip of adsorbed O2 and thus produce 1O2 efficiently in the dark.31 At 30 °C, addition of NCs caused negligible change in ABDA absorbance (A380nm) within 1 h. The reaction temperature was therefore increased to 60 °C. As shown in Figure 7C, ABDA itself is stable at 60 °C. The Au@Pd led to a gradual and continuous decrease with heating time. Without laser assistance, Pd NCs themselves acted as an efficient sensitizer for 1O2, in agreement with the reported result.31 The Au NR 12381
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mL. Then, 0.6 mL of freshly prepared 0.01 M NaBH4 solution was quickly added under vigorous stirring. The seed solution was then kept at room temperature and should be used within 2−5 h. The typical growth solution of Au NRs consisted of 100 mL of 0.1 M CTAB, 2 mL of 25 mM HAuCl4,1 mL of 1 M H2SO4, 1 mL of 10 mM AgNO3, and 800 μL of 0.1 M AA. After the color of the growth solution changed from orange to colorless, 240 μL of the seed solution was added. The mixture was then left undisturbed at room temperature for about 12 h. After preparation, the Au NRs were separated from the growth solution by centrifugation (12000 rpm for 5 min) twice. The precipitation was collected and redispersed in 100 mL of deionized water. Cu Ions Mediated Overgrowth of Au NRs: Formation of Au OCT and ETHH. In the case of Au OCT, 1 mL of purified Au NRs suspension was mixed with 2 mL of CTAB (0.1 M) and diluted with deionized water to a total volume of 6 mL. Then, 150 μL of 24 mM HAuCl4 and 39.6 μL of 0.1 M AA were added to the solution. After that, a certain amount (36, 72, and 108 μL) of 100 mM Cu2+ aqueous solution was added. The mixture was then kept in a 50 or 60 °C water bath to initiate the overgrowth. For the synthesis of Au ETHH, different volumes (3, 6, 12, 24, 36, and 54 μL) of 100 mM Ag+ aqueous solution were added. After reaction, the final products were purified by centrifugation (10000 rpm for 5 min) twice and then used for structure characterization and composition analysis. Detection of Singlet Oxygen. 1O2 production was monitored by chemical oxidation of ABDA in aqueous solutions. The ABDA solution without NCs was used as a control. The sample solution was prepared by combining 100 μL of 0.5 nM NCs with 1.5 mL of ABDA (100 μM). Each NC sample was divided into two parts. One part was put in a 60 °C water bath to study the thermal activation of 1O2; the other part was irradiated with cw laser to investigate SPR-induced 1O2. The ABDA absorbance at 380 nm was recorded at 1 min interval for 1 h for water bath samples and 5 min interval for photoirradiation samples for 30 min. The laser power was 1.145 W measured by a laser power meter. Measurement of the Heating and Cooling Curves. The sample solution was prepared by combining 100 μL of 0.5 nM NCs with 1.5 mL of deionized water. They were irradiated with a 808 nm laser at room temperature for 30 min and cooled by switching off the laser. The solution temperature was monitored by a thermocouple connected to a digital thermometer.
production via SPR-induced sensitization. On the other hand, due to low efficiency of SPR excitation, Au OCT and ETHH give less 1O2 (Figure S7). The Au ETHH shows a higher yield than the OCT, possibly related to their high-index facets. Considering that some high energy surface reactive sites on Au NRs and dog-bones could produce 1O2 in dark, we concluded that the adsorbed oxygen species on the surface of Au NCs bounded by low-index facets were mainly in the form of molecular O2. Upon SPR excitation, these Au NCs are potent 1 O2 photosensitizers. As for the Au NCs bounded by high-index facets, more studies are needed considering the possibility of atom oxygen adsorption due to the high reaction activity of high-index facets.
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CONCLUSIONS In conclusion, we extended the Cu2+-activated O2 pathway to the overgrowth of Au NRs. Our results indicate it is a universal way to manipulate growth kinetics as dissolved oxygen naturally exists in aqueous solution and many other solvents. Comparative study with Ag+ exhibits distinct roles of Cu2+ and Ag+. At often employed growth conditions, Ag+ indeed acts as shape regulator and leads to the formation of THH/ETHH structures with high-index facets. In contrast, Cu2+ accelerates the growth rate via catalyzing the removal of adsorbed oxygen species. Combination of the two ions provides a unique way to tailor the aspect ratio of ETHH. Copper ions activated O2 pathway suggests that dissolved oxygen has a strong affinity for the Au surface. With the help of a 1O2 probe, the adsorbed oxygen species on the surface of Au NCs bounded with lowindex facets are found to be mainly molecular O2, pointing out the great potential of these Au NCs as SPR-induced 1O2 photosensitizers.
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EXPERIMENTAL SECTION
Materials. Sodium borohydride (NaBH4), cetyltrimethylammonium bromide (CTAB), chlorauric acid (HAuCl4·3H2O), silver nitrate (AgNO3), L-ascorbic acid (AA), 2,6-pyridinedicarboxylicacid (PDCA), 9,10-anthracenediyl-bis (methylene) dimalonic acid (ABDA), and palladium chloride (PdCl2) were purchased from Alfa Aesar and used as received. Sulfuric acid (H2SO4) and copper(II) chloride dehydrate (CuCl2·2H2O) were at least analytical reagent grade and purchased from Beijing Chemical Reagent Co. (Beijing, China). Milli-Q water (18 MΩ cm) was used for all solution preparations. Instruments. UV−vis−NIR absorption spectra were recorded on a Varian Cary 50. Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) images were obtained with a TecnaiG2 20 S-TWIN operating at an acceleration voltage of 200 kV and a Hitachi S-4800 operating at an acceleration voltage of 10 kV, respectively. Elemental analysis was performed with energy-dispersive X-ray spectroscopy (EDX) from SEM and inductively coupled plasmamass spectrometry (ICP-MS, PerkinElmer Elan DRC II). X-ray diffraction (XRD) patterns were acquired using a Rigaku MiniFlex Xray diffractometer equipped with a Cu Kα radiation source operated at 50 kV/300 mA. Scanning transmission electron microscopy (STEM) and energy-dispersive X-ray analysis spectroscopy (EDX) element mappings were conducted under a high-angle annular dark field (HAADF) mode from Tecnai G2F20 U-TWIN. A cw diode laser at 808 nm (GCSLS-05-7W00) was from Daheng Optics. XPS spectra were collected using an ESCALAB250Xi system. The dissolved oxygen meter (SG9-ELK from Mettler Toledo) was used to measure the dissolved oxygen contents before and after degassing with high purity N2. Synthesis of Au NRs. The Au NRs were synthesized by the seedmediated growth method. CTAB-capped Au seeds were prepared by the following procedure: 7.5 mL of 0.1 M CTAB solution was mixed with 100 μL of 25 mM HAuCl4 and deionized water was added to 9.4
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ASSOCIATED CONTENT
S Supporting Information *
Extinction spectra details of Au NRs overgrowth and etching under different conditions, additional SEM and TEM images, EDX, XRD, XPS, and ICP-MS data, NCs-assisted degradation of ABDA. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The work was supported by the National Key Basic Research Program of China (2012CB934001 and 2011CB932802) and the National Natural Science Foundation of China (Grant Nos. 91127013 and 21173056).
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REFERENCES
(1) Jain, P. K.; Huang, X. H.; El-sayed, I. H.; El-sayed, M. A. Noble Metals on the Nanoscale: Optical and Photothermal Properties and Some Applications in Imaging, Sensing, Biology, and Medicine. Acc. Chem. Res. 2008, 41, 1578−1586.
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dx.doi.org/10.1021/la502623t | Langmuir 2014, 30, 12376−12383
