A Facile and Template-Free Method to Prepare Mesoporous Gold

Jun 21, 2008 - ... Shenyang 110004, China, Research Center for Compact Chemical Processes, ... National Institute of Advanced Industrial Science and T...
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A Facile and Template-Free Method to Prepare Mesoporous Gold Sponge and Its Pore Size Control Gaowu W. Qin,*,† Juncheng Liu,§,⊥ Tatineni Balaji,§,¶ Xiaoning Xu,† Hideyuki Matsunaga,§ Yukiya Hakuta,§ Liang Zuo,† and Poovathinthodiyil Raveendran*,§,‡ Key Laboratory for Anisotropy and Texture of Materials (Ministry of Education), Northeastern UniVersity, Shenyang 110004, China, Research Center for Compact Chemical Processes, National Institute of AdVanced Industrial Science and Technology (AIST), Sendai 983-8551, Japan, and Department of Chemistry, UniVersity of Calicut, Kerala 673-635, India ReceiVed: January 31, 2008; ReVised Manuscript ReceiVed: April 26, 2008

A facile, one-step, template-free method is reported to tune β-D-glucose-stabilized Au nanoparticles (NPs) into 3-D network nanowires (NWs) and thus mesoporous gold sponge on a large scale by precise control of the solution pH in aqueous medium at room temperature. The Au NPs appear to undergo sequentially linear aggregation and welding initially, and then, they randomly cross link into self-supporting, three-dimensional networks with time. The mesoporous gold sponge thus formed, on a millimeter to centimeter scale, exhibits high specific surface area (11.9 m2/g) and small pore sizes of 5-30 nm, as well as high thermal stability up to ∼120 °C, above which it undergoes sintering and reorganization into macroporous gold sponge with pore sizes of ∼1-4 µm depending on annealing temperatures but still with mesoporous walls/filaments. 1. Introduction Assembly of nanoparticles of noble metals such as gold into three-dimensionally networked porous architectures has been a challenge in nanomaterial science in the recent years.1 Such nanomaterials have potential applications in advanced catalysis, biofiltration, sensing, electrochemistry, fuel cells, and so on.2–6 Current methods for the generation of meso- or macroporous gold include dealloying7,8 of the Au-Ag alloy and templating using colloidal crystals, polymers and surfactants,9–12 and biomaterials such as dextran.13 Generally, these methods involve cumbersome procedures for the removal of the sacrificial metal or organic templates using concentrated acids or calcination, thus raising environmental concerns; sometimes, in these processes, it is hard to control their final compositions, which is significant in their applications such as sensing or catalysis. It is well-known among chemists that D-glucose (both R- and β- forms) can reduce several metal ions to their zero oxidation states (Tollens’s test, for example). Recent studies14 have demonstrated that this nontoxic and mild reducing agent can be used in the synthesis of metal nanoparticles (NPs) in aqueous medium. It was also shown that glucose by itself can stabilize metal nanoparticle systems.15 We recently reported a one-step route to prepare nano-Au colloid in an alkaline aqueous solution of glucose with monodisperse stability over several months.15 In this work, we reported that, on the basis of the pH of the original glucose solution, one can synthesize stable NPs or induce their linear welding into the nanowires (NWs) or three* To whom correspondence should be addressed. Phone: +86-2483683772. Fax: +86-24-83686455. E-mail: [email protected]. (G.W.Q.); Phone: +91-944-7790167. Fax: +91-494-2400269. E-mail: [email protected]. (P.R.). † Northeastern University. § National Institute of Advanced Industrial Science and Technology. ‡ University of Calicut. ⊥ Current Address: Department of Chemistry, Auburn University, Auburn, Alabama 36849. ¶ Current Address: Department of Chemistry, NCAT State University, Greensboro, North Carolina 27410.

dimensionally extended, self-supporting, mesoporous networks of NWs (mesoporous Au sponge) with high surface area and thermal stability. 2. Experimental Section Synthesis of Au Nanoparticles, Nanowires, and Mesoporous Sponges. All of the chemicals were used as received (Sinopharm Chemical Reagent Co. Ltd.) and without any further purification. Au nanoparticles were synthesized by reducing HAuCl4 (99.9% in purity) using β-D-glucose (99% in purity) as the reducing agent under alkaline conditions, and the Au NWs as well as mesoporous Au sponges were obtained by controlling the solution pH. In a typical processing, 108 mg of β-D-glucose was added into 20 mL of deionized water to get 0.03 M glucose aqueous solution at room temperature, and then, the pH value of the solution (called the initial pH value in this paper, except when otherwise noted) was adjusted by adding a 0.1 and/or 1 M NaOH aqueous solution to the desired value, kept for 20 min with magnetic stirring. Finally, 80 µL of 0.05 M HAuCl4 aqueous solution was dropped into the basic glucose solution and continuously stirred for ∼5 min. Au NPs, NWs, and sponges are straightforwardly dependent on the initial pH, and stable Au nanoparticles become a colloid at the pH of 10.5-11.4, nanowires at the pH of 11.5-11.8, and mesoporous Au sponges at the pH of 11.9-12.4. For the Au nanowires and sponges, their Au recovery ratios are higher than 98%. Characterization. Transmission electron microscopy (TEM) studies of the samples were carried out by using TECNAI G2 type field emission TEM (FE-TEM) on standard Cu grids covered by an amorphous carbon thin film containing the samples. Scanning electron microscopy was performed on JSM6500F to observe the Au mesoporous sponges at the operation voltage of 20 kV, together with energy dispersion of X-ray spectra for analysis of the composition of the Au sponges. X-ray diffraction studies were carried out by using Rigaku 2000 at 40kVand40mAwithCuKRirradiation.Brunauer-Emmett-Teller (BET) surface areas were calculated from the N2 adsorption-

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desorption measurements using a BELSORP 28SA automatic gas adsorption apparatus after preheating the sample for three hours at each temperature studied. The UV-vis-NIR experiments were performed at 350-1300 nm on Shimadzu UV-3150 spectrophotometer. 3. Results In a typical experiment, we prepared several samples of 0.03 M D-glucose solutions (20 mL) in deionized water and added different volumes of a 0.1 and/or 1.0 M NaOH stock solution to get the desired initial pH, in the range of 10-12.5, to which then we added about 80 µL of 0.05 M HAuCl4 solution. The system was continuously stirred for about 5 min, resulting in the formation of the Au NPs (indicated by the purple color). It is observed that at first, the pH lowers drastically, as anticipated (neutralization), but even after prolonged duration, one observes a slow but continuous decrease in the pH (Supporting Information, Figure S1). Thus, in the present work, we report the initial pH of the glucose solution (except when otherwise noted) prior to the addition of the HAuCl4 solution. We observed that glucose solutions with different pH values give rise to a significant difference in the stability of the colloidal NP dispersions and the particle assembly behaviors. For example, solutions with an initial pH in the range of 10.5-11.4 result in the formation of stable, nonaggregated Au NP dispersions (average particle size ) 8.0 nm; σ ) 1.8 nm), corresponding to the final pH of the nano-Au colloids, 3.5-10.5. These dispersions were found to be stable over 6 months and did not show any signs of aggregation, as shown in Figure 1a. On the other hand, at very high initial pH (>12.5), the nanoparticles form a normal aggregate within 30 min, and each large aggregated sphere (up to tens to hundreds of nanometers in diameter) with many Au NPs attached each other closely, together with some coarsened Au nanowires, as shown in Figure 1c. However, in the intermediate pH range, that is, in a very narrow initial pH window of 11.5-12.4, we observed that the nanoparticles weld into one-dimensional and three-dimensional networks of significance, and Figure 1b shows a case when pH ) 11.9. Finally, it results in mesoporous Au sponge with a homogeneous pore size varying from several to tens of nanometers, as discussed later in detail. The extent of networking varies depending on the pH. For the initial pH range of 11.5-11.8, we observed their selforganization into stable NW-like sequential assembles, and above that (11.9-12.4), they first linearly weld into small NWs and then slowly network and finally precipitate out as a whole thin film of continuous, three-dimensional networks of nanogold or nanoporous sponge over a period of 12 h. Visual observation of the kinetics of the self-welding from Au NPs to NWs and then to nanoporous sponge at pH 12.0 is presented in Figure 2 a-f. The formation of Au 3-D NWs cross linked in the strong basic glucose solution is also evidenced by in situ UV-vis-NIR absorption spectra (Figure 2g), which show a decrease in the intensity of the surface plasmon absorption peak of the individual Au NPs at about 535 nm, accompanied by an increased absorption in the near-IR region with time. The surface plasmon absorption peak at 535 nm corresponds to light resonance along the transverse direction of the Au NWs, while the enhanced flat absorption in the NIR region should arise from the collective surface plasmon resonance along the longitudinal direction of each NW in three dimensions. This kind of UV-vis-NIR characteristic on Au nanowires has been well documented so far.16

Figure 1. TEM observation of the pH-dependent stability of Au colloids in 20 h; (a) pH ) 11.0, showing a stable and monodisperse Au colloid, (b) pH ) 11.9, showing 3-D Au nanowires networks, and (c) pH ) 12.8, showing very large welded and aggregated Au large particles (inserted picture), together with coarsened Au nanowires.

Transmission electron microscope (TEM) images of the dispersions at different maturing times (Figure 2h-k) also support this sequentially linear welding of the Au NPs. It appears that the gray-black dispersions primarily contain NWs of Au. TEM images of the NWs, the thin film, and the sponge reveal a closely welded assembly with significant interparticle contact regions. Higher-resolution TEM images (Figure 3) show welldefined grain boundaries between the individual, crystalline NPs, oriented along random crystallographic axes. TEM images of the film (and sponge) show similar welding between the NWs. The high-resolution SEM images of the mesoporous gold at different magnifications are shown Figure 4a and b. Here, it should be mentioned that the mesoporous Au sponge prepared in this study has a significant difference in microstructure when compared with those of the dealloyed ones or dextran-templating calcinated ones. The nanofilaments in the Au-Ag dealloyed sponge are usually of single crystalline, inherent from the parent large grains.7 In contrast, the nanofilaments in this study are linearly welded by many small Au nanoparticles, resulting in an abundance of grain boundaries, which are expected to provide

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Figure 2. Transformation from Au NPs to mesoporous Au networks via nanowires; (a-f) optical images of the Au NP dispersions (starting pH 12.0) at different times after formation, showing the transformation from NPs to NWs to mesoporous thin film and to one mesoporous sponge (upon shaking); (g) UV-vis-NIR absorption spectra at different times corresponding to the transformation from NPs to NWs; TEM images of (h) Au NPs (12 min), (i) Au NWs (3 h), and (j) mesoporous Au thin films (12 h); (k) TEM image (magnification of (j)) showing the self-supporting nature of the mesoporous network in (e) and (f).

Figure 3. High-resolution TEM images showing the 3-D network; (a) close connectivity between the individual particles and (b) higher magnification of (a), showing particle crystallinity and the well-defined grain boundaries.

a larger number of high chemical activity sites for either catalysis or sensors. Also, the dealloyed mesoporous Au sponge has some residual Ag trapped inside even after strong acid etching; in most cases, the Ag content inside is hard to control, varying with acid and etching conditions.7,8 However, the Au mesoporous sponge developed in this study is of high-purity gold with only traces of organics (mainly from the glucose), as shown by

the SEM-EDX study (Figure 4c). Further, the XRD study further verifies that the Au mesoporous sponge is composed of NPs of face-centered cubic (FCC) structure, with a mean grain size of about 9.0 ( 1.3 nm calculated by using the Scherrer formula, consistent with the TEM observations above. The porous sponge prepared by dextran templating and calcination13 can be of arbitary macroscopic size and shape, but

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Figure 4. SEM observation of Au mesoporous sponge; (a) low magnification and (b) higher magnification, (c) EDX analysis showing high-purity Au, and (d) XRD pattern showing a FCC structure.

both the filaments and pores are in the micron size range, with the maxium Brunauer-Emmett-Teller (BET) surface area of about 0.4 m2/g. In comparison, the mesoporous sponge developed in this study has a BET surface area of about 11.9 m2/g, about 30 times that developed by the former method. High surface area is another important consideration for the utilization of these materials in most applications such as sensors and catalysts. An optical reflectance study also (Supporting Information, Figure S2) shows significantly low reflectance of this mesoporous Au film, suggesting important applications such as optical limiting or low-reflectance functional films. It is observed that for the as-synthesized mesoporous Au sponge, in general, the pores are multimodal (typical pore sizes being in the 5-30 nm range), as one would anticipate from their formation by the linear welding of NWs and the selfsupporting networks thus formed. It is also important to note that the surface morphology of the mesoporous Au thin film or sponge closely resembles that of the mesoporous Au produced by dealloying Au-Ag alloys.7,8 Temperature dependence studies (Figure 56) reveal a significant reduction in the BET surface area above 120 °C. This is attributed to the sintering of the NWs and thus the reconstruction of the mesoporous gold. High-resolution SEM observation shows the microstructure change of the initial mesoporous Au sponge after annealing at 160 (Figure 6a,b) and 380 °C (Figure

Figure 5. Temperature dependence of the BET surface area of the mesoporous Au sponge.

6c,d), respectively, for 3 h. Even after the heat treatment at 160 °C, the porous microstructure is retained, with a relatively homogenuous pore size of 1-3 µm. However, closer examination reveals that the pore walls (or filaments) are mesoporous with the pore size of tens to hundreds of nanometers. It is different from that by the dextran-templating method,13 where the filaments are usually solid with a diameter of 1-5 µm depending on the calcination temperatures. Even when annealed at 380 °C, the Au sponge remains a little larger with a pore size of 2-4 µm, but the ligaments are still mesoporous, as shown

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Figure 6. Microstructure of mesoporous Au sponge annealed at high temperatures for 3 h, (a) 160 and (c) 380 °C, and (b) and (d) are higher magnification of (a) and (c), respectively.

in Figure 6d. As a result, it has a specific surface area as high as 1.8 m2/g. The pore size in the Au sponge, varying from mesoporous to macroporous, could thus be easily tailored by the appropriate annealing processes, but the initial 3-D mesoporous microstructure plays an essential role in the following annealing processes to control the final pore size and size distribution. It is important to note that a metallic luster was observed after heat treatment at 380 °C. 4. Discussion The sequentially linear welding of the glucose-protected NPs into NWs and 3-D networks is of fundamental importance. Particularly, it is important to address what governs the linear welding of these NPs, compared to their normal aggregation into bulk gold (as usually observed in the absence of any protecting agent or in the very high pH environment in this study). In the previous study15 on the self-assembly of glucoseprotected Pt NPs, we did not observe significant welding areas between the individual NPs and attributed it to a hydrogenbond-driven self-assembly. In comparison, herein, we observed significant welding between the individual Au NPs with welldefined grain boundaries. We also noticed that several factors such as pH and the glucose concentration can alter the extent of the linear welding behaviors. Generally, we observed that lower pH and larger concentrations of the glucose solution increase the stability of the Au NPs. On the other hand, low concentrations of glucose at low pH result in the normal aggregation into very large Au particles rather than NWs. The simplest plausible explanation for such sequential linear welding would be the partial protection (or partial “stripping”) of the individual NP surfaces. Such unprotected, naked regions of the

Au NPs are extremely reactive due to the high surface energy and tend to weld with similar, partially “stripped” NPs (such as undergoing Brownian motion in the aqueous dispersion), thereby reducing their surface energy. The effect of high pH may contribute to the linear welding of Au NPs in two ways: First, at higher pH, the equilibrium between the cyclic and the noncyclic structure of the glucose shifts more toward the noncyclic form, and this may reduce the effective surface coverage by the individual glucose units. Second, the binding constant for the sugar-metal NP system may also vary with the pH. The stability of the Au nanoparticles may be also related to the formation of the gluconic acid; the -COOH would have a stronger interaction with the Au surface than the -OH arising from the different polarity. This could be evidenced by the long stability (of several months) when the Au colloid is made at room temperature at weakly alkaline or acidic conditions. Actually, the gluconic acid containing the -COOH function group can also be derived from the slow oxidization of the glucose itself, in the presence of O2 and under the alkaline condition by the following reaction, and Au NPs would speed the reaction in this case due to its significant catalysis, even at room temperature.17

CH2OH-(CHOH)4-CHO + ½O2 f CH2OH-(CHOH)4COOH (1) However, in the case of Au colloids prepared under strongly alkaline conditions, the alkaline ion -OH- inevitably reacts with the functional group -COOH to form -COO-. At the same time, an equilibrium between -OH and -O- on the nanogold surface, as the pH is varied, has been confirmed recently by Sylvestre.18

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Figure 7. The volume diffusivity of Au nanoparticles at different temperatures (300-500 K) (a) and the surface diffusion distance assuming the duration time 1000 s after the collision by using surface diffusivity (T ) 300, 400 K) provided that the surface diffusivity is 6 orders of magnitude of the volume diffusivity.

When the solution pH is below the pK value of the hydroxylated surface of Au nanoparticles, that is, in acid or weak basic environment, -OH protection is dominant for the sugar-protected Au colloids. Furthermore, the Au-O- group is dominant at a high pH.18 Considering our case of the Au nanoparticle colloid in the acid or weak alkaline solution, the Au nanoparticle surface is completely covered by -OH and -COOH, and thus, they should be of stable colloids. However, for the case of Au nanowire formation in the strong alkaline solution, the Au nanoparticle surface would be covered by -COOH, -OH, -COO-, and -O-. Once the newly formed -COO- ions, by the reaction of -COOH and -OH-, meet the neighboring -COO- or -O-, the strong electrostatic repulsing force thus generated makes both of them leave from the Au nanoparticle surface, therefore leaving behind some naked surface area in an instantaneous time. As a result, these naked surface areas would provide some possible sites for connecting with the similar area of the neighboring collided Au nanoparticles. The contacting between the two naked areas may be through either Brownian motion of the Au nanoparticles and/ or the hydrogen bond of the -OH functional group of the glucose and gluconic-acid-capped two neighboring Au nanoparticles. Then, a fundamental question arises on how the collided Au nanoparticles with the similar naked (or bare) areas weld together in the aqueous solution. One possible cause might be thought to be due to concurrent deposition of the reduced Au atoms by the left AuCl4- ions. Actually, the reduction kinetics of the AuCl4- ions are strongly dependent on the pH value for a given glucose concentration, but the incubation time in this study for the Au nanoparticle formation varies from ∼10 s in weak basic solution to ∼1 s or less in strong basic solution. For example, in the case of pH ) 12.0, once the Au nanoparticles form within about 1 s, the plasma absorption peak position initially at 535 nm shifts a little to the blue side, not to the red side, during the several-hour process of their connection into nanowires, as shown in Figure 2g. It thus implies that there are few AuCl4- ions left for the connection into NWs. As a matter of fact, it is hard for the the AuCl4- ions to be stable in this strong reduction environment for several hours. Considering a similar phenomenon, observed by Kiely et al.,19 that the alkanethiol-stabilized bimodal Au nanoparticles (4.5 and 7.8 nm) undergo connection into a semicontinuous network on a TEM grid after exposure to air for several months at room temperature, it is thus reasonable for us to postulate a general physical cause which may play a major role in the welding process of Au nanoparticles, that is, a diffusion-driven welding process. The driving force for the welding is the surface freeenergy decrease of the naked area of Au particle surfaces. As

evidenced by HR-TEM in Figure 3, the connected area between the two Au nanoparticles becomes one large-angle grain boundary. Provided that the general grain boundary energy is ∼1 J/m2 and the naked metal surface energy is ∼2-3 J/m2, then one grain boundary generated would cause a surface freeenergy decrease of ∼3-5 J/m2 once the two Au nanoparticles weld. The self-diffusivity of bulk gold DV is (3.1 × 10-6)exp(-164800/ RT) m2/sec,20 where R is the gas constant and T is the temperature. When considering the nanoparticle size effect on diffusion, Jiang et al.21 concluded that the volume diffusion activation energy Q decreases with nanoparticle size, and thus, the diffusivity generally enhances for the nanomaterials. They added one pre-exponential factor β to modify the volume diffusion activation energy of the nanoparticles, and deduced21

(

β ) exp

-2Svib(∞) 3R(d/6h - 1)

)

(2)

where Svib(∞) is the bulk melting entropy, 9.3826 J/molK for gold,22 d is the nanoparticle diameter, and h is the Au atom diameter, 0.2884 nm. The volume diffusivity dependence on Au nanoparticle size has thus been calculated, as shown in Figure 7a, together with the effect of temperature. The volume diffusivity of the Au nanoparticles drastically increases depending on their sizes, especially for the particle size less than 10 nm. Once the two Au nanoparticles are collided and connected by metal bonding without departing, the welding process will be controlled by the surface diffusion of Au atoms. Considering that the surface or interface diffusivity DS is 105-107 times that of the volume diffusvity for the bulk alloy systems,23 the main principle is assumed to be also held for nanomaterials, and DS ) 106DV is supposed in this study. We found the linear welding of the Au nanoparticles happens within several minutes to several hours, depending on the basicity of the solution, and thus assumed the diffusion time to be τ ) 1000 s after the collision of the two Au particles. Then, the diffusion distance of the gold atoms is about 0.1-14.6 nm for 5.0-8.0 nm Au particles during this period at room temperature, which is comparable to a grain boundary width of ∼0.5-1 nm. Consequently, it suggests the two Au nanoparticles could be welded by rapid surface diffusion, and the smaller sized Au particles are mainly responsible for this welding process because of their higher diffusivity. Indeed, detailed observation on the welding parts of the Au nanoparticles, as shown in Figure 3b, shows the main characterisitics of the concave surface, as usually

10358 J. Phys. Chem. C, Vol. 112, No. 28, 2008 happened in diffusion-controlled sintering of powders in conventional diffusion-assisted powder metallurgy. The absorption peaks in the UV-vis spectrograms at 535 nm have a little blue shift, suggesting the particle size decreasing to some extent. Both of these observations support our postulation above on diffusion-driven welding process. Another contribution should also be mentioned for this welding process, that is, visible light irradiation may enhance this welding process. Gold and silver nanoparticles experience morphology changes under laser irradiation with wavelengths of 300-800 nm, which has been well observed and documented in detail.24 The shape change or fusion of nanoparticle colloids, even fragmentation, happens depending on the feature of capping agents, solution thermal conductivity, absorption characteristic of nanoparticles and also the intensity of the laser. It arises from the electron-photon coupling and photon-photon relaxation in Au nanoparticles, resulting in their temperature rising. Radt et al.25 recently exploited this feature for drug deliver systems toward tumor therapy by incorporating Au nanoparticles in a polymer capsule shell as a heater and thus breaking the capsule to release medicine targeting toward tumors with irradiation of near-infrared light. This is direct evidence of the temperature rising of Au nanoparticles with laser irradiation. As shown in Figure 3g, the absorption of the Au colloids in this study in the visible light band increases with linear welding processing, and this linear welding further improves the absorption of the irradiated light and finally contributes to a higher temperature rise locally. As a result, it remarkably increases the diffusivity of gold atoms, as calculated in Figure 7, and therefore speeds the welding of the connected Au nanoparticles. Even so, the solid conclusion on the mechanism of how pH and glucose concentration separately tune Au NPs into NWs remains a question rather than an answer, and it should be further studied in the future. 5. Conclusions In summary, we have demonstrated a general, simple, template-free, and environmentally friendly route for the synthesis of high-surface-area, mesoporous Au sponges by the pH-assisted linear welding of glucose-protected (plausibly, partially unprotected) Au NPs into NWs and further cross linking in three dimensions in basic glucose aqueous solution at room temperature. This suggests that simple biomolecules such as D-glucose present tremendous opportunities in the organization of nanoparticles into three-dimensional networks such as mesoporous Au and other noble metal/alloy sponges. Acknowledgment. This work was supported by Ministry of Education (China) under the project of No. 108039, No.

Qin et al. IRT0713 and No. B07015. P.R. thanks JSPS for financial assistance. The authors also fondly acknowledge the great support from Late Prof. Yutaka Ikushima during the course of this work. Supporting Information Available: Plot of pH variation with time and an optical reflectance spectrum. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Li, M.; Schnablegger, H.; Mann, S. Nature 1999, 402, 393. (b) Whitesides, G. M.; Grzybowski, B. Science 2002, 295, 2418. (c) Chandrasekharan, N.; Kamat, P. V. Nano Lett. 2001, 1, 67. (2) Noort, D.; van Mandenius, C. F. Biosens. Bioelectron. 2000, 15, 203. (3) Basheer, C.; Swaminathan, S.; Lee, H. K.; Valiyaveettil, S. Chem. Commun. 2005, 409. (4) Peng, X.; Koczkur, K.; Nigro, S.; Chen, A. Chem. Commun. 2004, 2872. (5) Maaroof, A. I.; Cortie, M. B.; Smith, G. B. J. Opt. A: Pure Appl. Opt. 2005, 7, 303. (6) He, X.; Antonelli, D. Angew. Chem., Int. Ed. 2002, 41, 214. (7) Ding, Y.; Kim, Y. J.; Erlebacher, J. AdV. Mater. 2004, 16, 1897. (8) Erlebacher, J.; Aziz, M. J.; Karma, A.; Dimitrov, N.; Sieradzki, K. Nature 2001, 410, 450. (9) Zhang, H.; Hussain, I.; Brust, M.; Cooper, A. I. AdV. Mater. 2004, 16, 27. (10) Bartlett, P. N.; Baumberg, J. J.; Birkin, P. R.; Ghanem, M. A.; Netti, M. C. Chem. Mater. 2002, 14, 2199. (11) Velev, O. D.; Tessier, P. M.; Lenhoff, A. M.; Kaler, E. W. Nature 1999, 401, 548. (12) Jiang, P.; Cizeron, J.; Bertone, J. F.; Colvin, V. L. J. Am. Chem. Soc. 1999, 121, 7957. (13) Walsh, D.; Arcelli, L.; Ikoma, T.; Tanaka, J.; Mann, S. Nat. Mater. 2003, 2, 386. (14) Raveendran, P.; Fu, J.; Wallen, S. L. J. Am. Chem. Soc. 2003, 125, 13940. (15) (a) Liu, J. C.; Raveendran, P.; Qin, G. W.; Ikushima, Y. Chem. Commun. 2005, 2972. (b) Liu, J. C.; Raveendran, P.; Qin, G. W.; Ikushima, Y. Chem.sEur. J. 2006, 12, 2132. (16) Zhong, Z.; Patskovskyy, S.; Bouvrette, P.; Luong, J. H. T.; Gedanken, A. J. Phys. Chem. B. 2005, 108, 4046. (17) Thompson, D. T. Nanotoday 2007, 2, 40. (18) Sylvestre, J. P.; Kabashin, A. V.; Sacher, E.; Meunier, M.; Luong, J. H. T. J. Am. Chem. Soc. 2004, 126, 7176. (19) Kiely, C. J.; Fink, J.; Brust, M.; Bethell, D.; Schiffrin, D. J. Nature 1998, 396, 444. (20) Okkerse, B. Phys. ReV. 1956, 103, 1246. (21) Jiang, Q.; Zhang, S. H.; Li, J. C. Solid State Commun. 2004, 130, 581. (22) Handbook of Metals, 4th revised version; Japan Institute of Metals, Eds. Maruzen Press: Tokyo,2004. (23) Kaur I.; Mishin Y.; Gust W. Fundamentals of Grain and Interphase Boundary Diffusion, 3rd revised version; John Wiley & Sons Ltd.: New York, 1995. (24) Kamat, P. V. J. Phys. Chem. B 2002, 106, 7729. (25) Radt, B.; Smith, T. A.; Caruso, F. AdV. Mater. 2004, 16, 2184.

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