Controlled Orientation of Asymmetric Copper Deposits on Carbon

Sep 19, 2012 - bipolar electrochemistry that allows generating asymmetric particles with a highly controlled spatial orientation of a metal deposit on...
0 downloads 0 Views 1MB Size
Download PDF
Article pubs.acs.org/JPCC

Controlled Orientation of Asymmetric Copper Deposits on Carbon Microobjects by Bipolar Electrochemistry Zahra Fattah,†,‡ Patrick Garrigue,†,‡ Véronique Lapeyre,†,‡ Alexander Kuhn,†,‡ and Laurent Bouffier*,†,‡ †

Univ. Bordeaux, ISM, UMR 5255, F-33400 Talence, France CNRS, ISM, UMR 5255, F-33400 Talence, France

‡

ABSTRACT: Asymmetric particles of various chemical compositions have attracted great attention because of their application potential in different areas, ranging from photosplitting of water to autonomous swimmers. In this context, the spatial arrangement of the different components can be of major importance. We report here a bulk procedure based on bipolar electrochemistry that allows generating asymmetric particles with a highly controlled spatial orientation of a metal deposit on a carbon substrate. Two fundamentally different topologies of the metal deposit can be obtained as a function of controllable experimental parameters like the orientation and amplitude of the electric field as well as the viscosity of the medium.

■

INTRODUCTION Over the past decade, nanotechnology has witnessed a revolution with respect to the synthesis of asymmetric nanoand microobjects, such as multicompartment, patchy, and Janus particles, through improving synthetic methods to access a variety of different material combinations with novel morphologies and properties. The interest in such asymmetrically functionalized particles arises from the high application potential of these particles in many areas including display technologies,1 self-healing materials, drug delivery,2 sensing, and electronic devices.3 Different strategies have been reported to prepare these complex objects such as microfluidic techniques,4 protection/ deprotection mechanisms,5 Langmuir−Blodgett deposition,6 metal striping,7 particle lithography,8 and microcontact printing.9 However, the majority of these approaches are based on using interfaces or surfaces to break the symmetry,10 which limits the production quantity because the modification occurs in a 2-D reaction space. Therefore, many efforts are devoted to scale up the production yield by using bulk procedures.11−14 Among the many different approaches, one promising method, initially described by Fleischmann et al.,15 is based on the concept of bipolar electrochemistry (BPE).16−18 It can, in principle, be used to generate asymmetric particles in a 3-D electrochemical cell without needing interfaces or surfaces to break the symmetry.19−21 This original approach found interesting applications that have been reviewed recently22,23 not only in the fields of analytical chemistry and materials science but also for other purposes, such as electronic24 and microfluidic devices,25 patterning,26 propulsion,27 and selfregeneration28 of objects and as driving force in electrochemiluminescent reactions.23 © 2012 American Chemical Society

In the present work we take advantage of BPE to functionalize in an asymmetric way carbon substrates with copper electrodeposits in a bulk solution, with a precise control of the deposition site and the direction of the metal growth. We have validated the concept at two different scales by using millimeters-long carbon fibers and micrometric carbon tubes. On the microscale, two different topologies of copper deposits have been investigated, exhibiting a metal growth either parallel or diagonal with respect to the symmetry axis of the carbon tubes. Finally, a sequential combination of both orientations was successfully investigated. Such controlled topologies might be of importance for the design of electronic devices, where a variety of specific electric connections are required, especially on the microscale, when conventional electrodeposition approaches are experimentally challenging because of the necessity of a direct electric contact between the conductive objects, which need to be modified, and the external power supply. Unlike traditional electrochemistry, BPE occurs when a conductive object is immersed in an electrolytic solution under the influence of an external electric field. As soon as a sufficient electric field is applied across the solution, reduction and oxidation reactions will be induced at both extremities of the object, even though there is no direct contact between the object and the power supply.15 The applied external electric field leads to an apparent polarization potential difference between the opposite sides of the object with respect to the solution given by the following equation: Received: June 29, 2012 Revised: September 17, 2012 Published: September 19, 2012 22021

dx.doi.org/10.1021/jp3064118 | J. Phys. Chem. C 2012, 116, 22021−22027

The Journal of Physical Chemistry C

Article

Figure 1. Schematic representation of the bipolar electrochemical cells designed for asymmetric modification of (a) carbon fibers and (b) carbon microtubes (CMTs) with copper metal.

ultrapure water (18.2 MΩ•cm). The carbon fibers were introduced into a glass capillary, and a sufficient electric field was applied to polarize the carbon fibers and trigger electrodeposition of copper at one end of the carbon fibers. Asymmetric Modification of Carbon Microtubes. The CMTs were produced according to a procedure described in a previous report.29 Copper acetate monohydrate (98%), dimethyl sulfoxide (DMSO, ACS spectrophotometer grade ≥99.9%), and absolute ethanol (≥99.8%) were purchased from Sigma-Aldrich. All experiments were performed in a home-built glass cell30 composed of a centered inner compartment and two outer compartments (Figure 1b) separated by two sintered porous glass membranes (3 mm thickness, porosity 2 and d = 1.4 cm between both separators). A cathode and an anode (gold plates) were positioned in the outer compartments of the cell with an effective distance of 4.3 cm between both electrodes. The inner part of the cell was surrounded at the bottom by an outer cooling compartment containing liquid to control the temperature. The suspension of CMTs is prepared by adding 0.1 mg of CMTs to 1 mL of 10 mM Cu(OAc)2 in DMSO solution. The suspension formation was accelerated by sonicating the mixture for a short time (40 s) to avoid excessive breaking of the tubes. This suspension was directly used to fill the inner part of the cell and employed for the bipolar electrodeposition process. Pure DMSO was put in the two outer electrode compartments. This means that in the beginning the two outer compartments have a quite high resistivity, which will change gradually as a function of time by migration and diffusion of ions from the inner compartment through the membranes. A first potential step was applied to align the CMTs in the electric field, and after 30 s, cold ethanol was poured in the cooling compartment to increase the viscosity of the DMSO, which led to the freezing of the CMT orientation in the cell. A second potential step was then applied to perform the electromodification of CMTs with copper metal. Characterization of the Copper Modified Carbon Substrates. The modified carbon fibers were collected from the glass capillary after BPE on a conductive substrate or TEM grid, and the sample was washed several times with ultrapure water to remove salt residues. After drying, the sample was investigated by scanning electron microscope (SEM) (Hitachi, TM-1000) and high-resolution SEM (JEOL, 840A). For CMTs, the suspension containing the modified CMTs was collected between two glass slides at the end of the bipolar experiments and observed under a transmission optical

(1)

ΔV = E × l

with E representing the applied electric field and l representing the length of the object. ΔV has to be in a first-order approximation at least equal to the difference between the formal potentials of the two involved redox reactions. In the case of a bipolar electrode, immersed in an aqueous cupric solution, the following half reactions need to be considered: Copper reduction at one side: Cu 2 + + 2e− ↔ Cu°(s) E° = + 0.34 V/NHE

(2)

Oxidation of water at the opposite side: H 2O(l) ↔ 2H+(aq) + + 2e−

1 O2 (g) E° = +1.23 V/NHE 2 (3)

In this case, the polarization has to generate a potential difference of approximately ΔVmin = E°(O2 /H 2O) − E°(Cu II/Cu 0) = 0.89 V

(4)

to trigger both reactions simultaneously at the two extremities of the conducting substrate. We will show here how to modify asymmetrically carbon fibers and carbon microtubes (CMTs) with a copper deposit at one extremity and controlling at the same time the orientation of this metal deposit with respect to the substrate axis. This can be achieved through tuning the direction of the applied potential or by adjusting the viscosity of the surrounding medium. For these proof-of-principle experiments, we explored essentially two different topologies of the modified objects that will be referred to as centered and noncentered topology as a function of the orientation of the metal deposit with respect to the main axis of the carbon substrate.

■

EXPERIMENTAL SECTION Electrodeposition of Copper on Carbon Fibers. The carbon fibers (10 μm diameter, grade: P100) were obtained from Goodfellow; copper(II) sulfate pentahydrate (99.995% purity) was purchased from Sigma-Aldrich. The experiments were performed in a setup analogue to what has been previously reported.19 All compartments of the electrochemical cell (Figure 1a) were filled with a solution of 10 mM CuSO4 in 22022

dx.doi.org/10.1021/jp3064118 | J. Phys. Chem. C 2012, 116, 22021−22027

The Journal of Physical Chemistry C

Article

microscope (Zeiss, Imager M1). Further characterization of the samples was performed by SEM.

■

RESULTS AND DISCUSSION Modification of Carbon Fibers. Precise site-selective modification of carbon objects has been investigated by controlling the alignment of these objects in the electric field during the bipolar electrodeposition. In a proof-of-principle experiment we first test the concept on a millimetric scale using carbon fibers. The minimum polarization along the conducting carbon fiber that is necessary to achieve the electrodeposition is ∼0.9 V, and, according to eq 1, an electric field of 360 V/m needs to be applied to trigger the deposition of copper onto one extremity of a 2.50 mm long carbon fiber. This simple calculation corresponds to a theoretical situation, but a large deviation of the potential threshold value can be expected, as the experimental conditions are far from standard conditions regarding concentrations, temperature, partial gas pressures, and pH. Therefore, we applied a higher electric field (700 V/ m) to be sure to modify asymmetrically this carbon fiber with a copper cluster. To produce a centered topology, the alignment of the carbon fiber should be perfectly parallel with respect to the direction of the electric field during the bipolar electrodeposition process because the nucleation will start at the point of maximum polarization. After filling the glass capillary and the two connected compartments with CuSO4 solution, the carbon fiber was inserted manually into the capillary and oriented mechanically until being perpendicular to both electrodes (therefore parallel with respect to the field lines). The electric field was applied for an appropriate time (40 min) to produce a copper deposit of a sufficient size for ease of characterization, but it is noteworthy that the deposition time is adjustable. One can note that the applied potential is almost twice as high as the thermodynamic threshold value required for the redox reaction. This value is, however, still moderate enough to limit the formation of oxygen bubbles at the anodic side of the fiber. In fact, the slow production of these bubbles allows their continuous dissolution in the electrolyte, thus preventing the evolution of too big bubbles, which might disturb the experiment and ultimately stop the current flow because the capillary diameter is relatively narrow (1 mm). Investigation of this carbon fiber at the end of the experiment under SEM shows clear evidence of asymmetric deposition of copper at one end of the fiber with a perfectly centered localization of this microcluster with respect to the fiber axis (Figure 2a). The chemical composition of the Cu deposit was further analyzed by energy-dispersive X-ray spectroscopy (Figure 2b). A strong signal around 1 keV, besides two other signals at 8 and 9 keV, was observed, which confirms that the deposit is copper. The weak signal assigned to carbon comes from the underlying carbon fiber and the supporting grid, whereas the gold signal most likely originates from the gold sputtering of the sample prior to SEM imaging. We followed the same procedure to prepare carbon fibers with a noncentered topology of the metal deposit. For that, a 2.45 mm long carbon fiber (almost the same length as the first fiber modified with a centered topology) was inserted into the capillary and mechanically oriented with a significant angle between the fiber axis and the direction of the applied electric field. From the previous experiment, one can predict that a value of 720 V/m should be enough to modify this carbon fiber. However, the effective length, which is responsible for the polarization of the fiber, is significantly shorter than the real

Figure 2. (a) Scanning electron microscope (SEM) image of a carbon fiber (2.5 mm long) modified with a centered copper cluster. (b) Energy-dispersive X-ray (EDX) spectra recorded for the deposit on the end of the carbon fiber.

length (2.45 mm) because it depends on the orientation angle of the fiber in the field. Figure 3a schematically illustrates two carbon fibers with the same length but different orientations. The first one (orange) is aligned parallel to the electric field, so the effective length (leff) of this fiber is the same as the real length (l). The second fiber (golden) has an angle α with respect to the electric field vector; therefore, the effective length equals the real length multiplied by cosine α. As a consequence, in the second case, a higher electric field needs to be applied to ensure a comparable driving force for the electrodeposition. Therefore, an electric field of 800 V/m was applied for 40 min to modify this fiber with a noncentered deposit (Figure 3b, inset). Because the resulting topology depends, in principle, on the angle of orientation of the carbon fiber in the field, we also used a shorter fiber (1.86 mm) that allowed a higher α in the narrow capillary (1 mm diameter). With this second carbon fiber, we could generate the maximum polarization at the very edge of the extremity of the fiber using an electric field of 920 V/m. As a result, a well-pronounced noncentered localization of the microcluster has been observed by SEM (Figure 3b). From these modified macroscopic objects, we conclude that BPE is well-adapted to asymmetrically modify the carbon fibers with copper metal and control the topology of the deposit by the alignment of the fibers in the electric field. This is a first key step towards generating a highly controlled orientation of the deposit with respect to the fiber axis. After these preliminary 22023

dx.doi.org/10.1021/jp3064118 | J. Phys. Chem. C 2012, 116, 22021−22027

The Journal of Physical Chemistry C

Article

namic threshold value required for the modification. At the end of the first potential step and before the second potential step, the viscosity of the medium surrounding the tubes should be increased to immobilize the tubes at a certain angle with respect to the orientation of the initially applied electric field. The sample of CMTs used in this work contains a population of tubes with a length ranging from 10 to 30 μm. To estimate a suitable value for the initial potential step, we monitored the alignment of the tubes under a transmission optical microscope by gradually increasing the applied potential between the two electrodes. An electric field of 12 kV/m was found to be large enough to orientate the tubes along the direction of the electric field. Figure 4a shows three carbon tubes with a length of 15 to

Figure 3. (a) Schematic illustration of the effect of orientation angle on the effective length of carbon fibers in the electric field. (b) SEM of a modified carbon fiber (1.86 mm long) with a noncentered topology of copper deposit. Inset: magnified SME image of noncentered copper modified carbon fiber (2.45 mm long); scale bar: 100 μm.

experiments, we explored the possibility of extending this concept to objects with dimensions on the microscale. Polarization and Alignment of Carbon Microtubes in the Electric Field. CMTs have been selected as suitable substrates because they are large enough to be visualized by standard optical microscopy for tracking the alignment of the CMTs in the electric field and characterization of the metal electrodeposits. It is indeed more complicated to modify microscale objects in comparison with millimetric ones with such a specific topology because it is not possible to align them at a certain angle with respect to the electric field by a simple mechanical manipulation as in the case of the macroscopic carbon fibers. Figure 1b shows the bipolar electrochemical cell that has been designed to align electrically the CMTs in the field. The cathode is facing the anode in the outer compartments that have been filled with DMSO. The inner compartment was filled with the CMT suspension. Two separator membranes were included in the setup to minimize convection that originates from the Joule effect and the bubbles released at the electrodes. Cold ethanol was added to the lower outside reservoir to freeze the DMSO at a certain moment of the experiment. The mechanism of the CMT alignment during the experiment depends on two parameters: the applied potential and the viscosity of the surrounding medium. Two potential steps were applied instead of a single linear potential ramp. The first potential step should be high enough to orientate the carbon tubes; however, it has to be lower than the threshold value necessary for the bipolar electrodeposition. The second potential step should be higher than the thermody-

Figure 4. Optical micrographs showing the CMTs alignment: (a) before and (b) during application of the first potential step. Inset: magnified SEM image of unmodified carbon microtube.

20 μm that adopt a random orientation in a DMSO solution before applying the field. As soon as an appropriate electric field is applied (Figure 4b), the tubes get polarized and orientate in a perfect parallel alignment with respect to the applied field. Although a field of 12 kV/m seems to be high, such a value is much lower than the theoretical threshold value necessary for bipolar electrodeposition of copper (45 kV/m for a CMT of 20 μm according to eq 1). The second tunable parameter governing the orientation of the tubes is the viscosity, which in turn depends on many other factors such as the nature of the solvent, temperature, nature, and concentration of the electrolyte. The main reason for choosing DMSO was its relatively high viscosity (1.996 cP at 20 °C) compared with water (0.894 cP at 25 °C), the high freezing point (18 °C), and the good miscibility with water, which is essential during the collection of the modified objects at the end of the experiment. In this way, it has been possible to maintain the orientation of the tubes once they have been aligned by the first electric field. Controlled Modification of CMTs by Electric-Field Orientation. The previous step to align the CMTs is essential for modifying the microtubes with a coaxial topology of the copper deposit. When changing slightly the design of the electrochemical cell it is possible to change the orientation of the electric field between the first and the second potential step. A new set of electrodes consisting of two independent and 22024

dx.doi.org/10.1021/jp3064118 | J. Phys. Chem. C 2012, 116, 22021−22027

The Journal of Physical Chemistry C

Article

the field of the second potential step is aligned parallel to the cell axis by connecting the four electrodes. As a consequence, in this case, the CMTs axis and the electric field will not be parallel during the electrodeposition step but tilted by a certain angle. This is the key element for generating now a maximum polarization at the edge of the carbon tube tip. The nucleation of the copper deposit at this point of maximum polarization results in a noncentered metal cluster. For both types of experiments described above, the solution has been collected after the second potential step; the tubes were washed with water, and their morphology has been investigated by optical and SEM. Figure 6 shows SEM images

addressable electrodes, separated by a piece of insulating material (such as glass), has been integrated in the cell setup. This leads to a final set of four electrodes instead of two (Figure 5). The inner part of the cell was filled with a suspension of

Figure 6. Selection of SEM pictures showing toposelective bipolar electromodification of carbon microtubes with a centered copper deposit.

Figure 5. Schematic illustration (top view) of the electrode connections used in the bipolar cell to control CMTs alignment in the electric field and the resulting deposits.

of four representative CMTs with different length collected from the first experiment, all modified at one end with a centered deposit. Practically ∼40% of the CMTs were modified with copper (global yield), and ∼75% of these tubes exhibited a centered deposit. The nonquantitative conversion yield has several origins. The first reason is the possible detachment of the copper deposit from the tube during the washing procedure. Second, the conductivity of the tubes, which varies depending on their morphology and the defects, might be in some cases too low for bipolar electrodeposition. In the extreme case of an insulating tube, no deposition can occur, and even for conducting tubes their intrinsic conductivity has to be larger than that of the surrounding medium to induce efficient deposition. The third reason is linked to the calculation of the potential drop as a function of CMTs size. The shortest tubes might experience a potential difference between the two ends that is below the threshold value. Looking at the SEM pictures of the resulting CMTs from the second experiment (Figure 7), it is obvious that the siteselective noncentered topology is highly controllable. The total percentage of the modified CMTs was estimated to be ∼35% by optical microscopy. 70% of them are modified with a noncentered topology. The presence of the CMTs modified with a centered deposit mode is most likely due to the local melting of DMSO around these microtubes, which cannot be directly observed by eye. In that case, the tubes can partially or totally realign under the effect of the field applied during the second potential step. The total product yield of this experiment (35%) is lower than the previous one (40%) for modifying the tubes with a centered metal topology. This difference is quite expected according to what we discussed

CMTs and Cu(OAc)2 in DMSO, whereas pure DMSO was used for the electrode compartments. To synthesize coppermodified tubes with a centered topology of the deposit, all four electrodes were connected, and the first potential step was applied with a slightly higher amplitude (14 kV/m for 30 s) than what we previously estimated (Figure 5a). The latter value was selected to be sure that the shortest tubes also get wellaligned. Then, the viscosity of the solvent (DMSO) was increased in the middle compartment containing the tubes. Half a minute was long enough to allow the orientation of all tubes in the field. During this time, a solution of cold ethanol (about −10 °C) was poured in the compartment located under the center part. This led to an increase in viscosity and eventually to freezing of the CMT suspension. In this way, it was possible to keep the alignment of the carbon tubes parallel to the field before the electrodeposition step. The second potential step was also applied using simultaneously the four electrodes with an electric field of 90 kV/m for 8 min. With this alignment mode of the tubes, a symmetric maximum polarization around the extremity of the tube was generated, triggering the nucleation of the deposit at this point and finally leading to objects modified with a centered metal cluster. During the experiment, the DMSO was maintained at low temperature. After the electrodeposition, the solution was allowed to warm up before collecting and characterizing the CMTs. When the same procedure is employed, but connecting only two of the four electrodes during the first potential step, a diagonal electric field with respect to the cell axis is generated (Figure 5b). After aligning and freezing of the tube orientation, 22025

dx.doi.org/10.1021/jp3064118 | J. Phys. Chem. C 2012, 116, 22021−22027

The Journal of Physical Chemistry C

Article

Figure 7. Selection of copper-modified CMTs (SEM images) with a noncentered topology of the deposit.

concerning the orientation angle (Figure 3a) between the CMTs and the applied field, which changes the effective length of the tubes. Because we applied the same potential difference in both experiments, the number of CMTs that are polarized with a sufficient potential for electrodeposition is higher in the case of the tubes aligned parallel to the external electric field than the others, which were in a diagonal alignment with respect to the field. Toward More Complex Structures of the Copper Deposit. In a last set of experiments it was possible to modify CMTs with a more complex and tunable copper structure by combining sequentially both topologies (centered and noncentered electrodeposition). An experiment analogue to the one used to prepare the noncentered deposit has been carried out using the same conditions in terms of electric field (the electrodeposition lasts for 6.5 min.), but the DMSO was allowed to melt during the second potential step. Therefore, the alignment of the tubes is diagonal with respect to the electric field during the first part of the electrodeposition process (as long as DMSO is frozen), but as soon as the solution starts to melt, a reorientation of the carbon tubes occurs. Investigation of the collected tubes at the end of the experiment revealed a copper deposit with a typical L shape, resulting from this change of orientation. Figure 8 shows SEM images of representative CMTs exhibiting such an L-shape structure. This last experiment illustrates that the concept of modulating the alignment of the objects in the electric field is a very efficient strategy to generate controlled topologies of copper clusters at one end of the CMTs when using BPE to trigger the deposition.

Figure 8. SEM images for CMTs modified with a controlled sequence of a first noncentered and a second centered copper deposit.

(1) a fully centered topology, (2) a fully noncentered topology, and (3) a combination of centered and noncentered modification. This versatility of modification in such a real bulk phase procedure might be interesting for industrial applications, such as connecting electronic devices.31 Because no interface is required, this means that a significantly higher product yield can be achieved in comparison with 2-D approaches, as many objects can be modified simultaneously. Moreover, the principle of aligning the objects in the electric field to control the mode of modification might be extended to a variety of other substrates and materials, leading to a large library of microscale hybrid materials.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: Laurent.Bouffi[email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

■

CONCLUSIONS We have demonstrated in this work for the first time that BPE is not only a powerful tool to control the side of metal modification of microobjects (Janus objects) but also that it is possible to fine-tune the local orientation of the deposit by carefully choosing the experimental conditions. We modified carbon fibers and CMTs with copper at one end. The BPE principle was adapted to control the toposelectivity of the metal deposit by applying two successive potential steps and adjusting the viscosity of the reaction medium. By manipulating the orientation of the applied electric field, we could align the CMTs in a suitable orientation prior to the electromodification. Three different topologies of copper clusters with respect to the main axis of the modified object were produced respectively:

Funding Sources

Financial support from the European ECW fellowship program is acknowledged for Z.F. The work has also been partially supported by the ANR program Emergence (Project PROJANUS) under contract ANR-2011-EMMA-007-01 as well as by the maturation fund of the University of Bordeaux (Aquitaine Valo). Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We would like to thank Chompunuch Warakulwit and Jumras Limtrakul for providing the carbon microtubes. 22026

dx.doi.org/10.1021/jp3064118 | J. Phys. Chem. C 2012, 116, 22021−22027

The Journal of Physical Chemistry C

■

Article

REFERENCES

(1) Xu, C.; Xie, J.; Ho, D.; Wang, C.; Kohler, N.; Walsh, E. G.; Morgan, J. R.; Chin, Y. E.; Sun, S. Angew. Chem., Int. Ed. 2008, 47, 173−176. (2) Yoshida, M.; Lahann, J. ACS Nano 2008, 2, 1101−1107. (3) Walther, A.; Müller, A. H. E. Soft Matter 2008, 4, 663−668. (4) Nie, Z.; Li, W.; Seo, M.; Xu, S.; Kumacheva, E. J. Am. Chem. Soc. 2006, 128, 9408−9412. (5) Perro, A.; Reculusa, S.; Pereira, F.; Delville, M.-H.; Mingotaud, C.; Duguet, E.; Bourgeat- Lamid, E.; Ravaine, S. Chem. Commun. 2005, 5542−5543. (6) Petit, L.; Manaud, J.-P.; Mingotaud, C.; Ravaine, S.; Duguet, E. Mater. Lett. 2001, 51, 478−484. (7) Kline, T. R.; Paxton, W. F.; Mallouk, T. E.; Sen, A. Angew. Chem., Int. Ed. 2005, 44, 744−746. (8) Snyder, C. E.; Yake, A. M.; Feick, J. D.; Velegol, D. Langmuir 2005, 21, 4813−4815. (9) Cayre, O.; Paunov, V. N.; Velev, O. D. Chem. Commun. 2003, 18, 2296−2297. (10) Gu, H.; Yang, Z.; Gao, J.; Chang, C. K.; Xu, B. J. Am. Chem. Soc. 2005, 127, 34−35. (11) Jiang, S.; Schultz, M. J.; Chen, Q.; Moore, J. S.; Granick, S. Langmuir 2008, 24, 10073−10077. (12) Mokari, T.; Sztrum, C. G.; Salant, A.; Rabani, E.; Banin, U. Nat. Mater. 2005, 4, 855−863. (13) Loget, G.; Kuhn, A. J. Mater. Chem. 2012, 22, 15457−15474. (14) Perro, A.; Meunier, F.; Schmitt, V.; Ravaine, S. Colloids Surf., A 2009, 332, 57−62. (15) Fleischmann, M.; Ghoroghchian, J.; Rolison, D.; Pons, S. J. Phys. Chem. 1986, 90, 6392−6400. (16) Ndungu, P. G. Ph.D. Thesis, Drexel University, Philadelphia, PA, 2004. (17) Duval, J.; Kleijn, J. M.; van Leeuwen, H. P. J. Electroanal. Chem. 2001, 505, 1−11. (18) Ulrich, C.; Andersson, O.; Nyholm, L.; Björefors, F. Angew. Chem., Int. Ed. 2008, 47, 3034−3036. (19) Warakulwit, C.; Nguyen, T.; Majimel, J.; Delville, M.-H.; Lapeyre, V.; Garrigue, P.; Ravaine, V.; Limtrakul, J.; Kuhn, A. Nano Lett. 2008, 8, 500−504. (20) Loget, G.; Lapeyre, V.; Garrigue, P.; Warakulwit, C.; Limtrakul, J.; Delville, M.-H.; Kuhn, A. Chem. Mater. 2011, 23, 2595−2599. (21) Fattah, Z.; Loget, G.; Lapeyre, V.; Garrigue, P.; Warakulwit, C.; Limtrakul, J.; Bouffier, L.; Kuhn, A. Electrochim. Acta 2011, 56, 10562− 10566. (22) Loget, G.; Kuhn, A. Anal. Bioanal. Chem. 2011, 400, 1691− 1704. (23) Mavre, F.; Anand, R. K.; Laws, D. R.; Chow, K.-F.; Chang, B.-Y.; Crooks, J. A.; Crooks, R. M. Anal. Chem. 2010, 82, 8766−8774. (24) Bradley, J.-C.; Chen, H.-M.; Crawford, J.; Eckert, J.; Ernazarova, K.; Kurzeja, T.; Lin, M.; McGee, M.; Nadler, W.; Stephens, S. G. Nature 1997, 389, 268−271. (25) Sheridan, E.; Hlushkou, D.; Anand, R. K.; Laws, D. R.; Tallarek, U.; Crooks, R. M. Anal. Chem. 2011, 83, 6746−6753. (26) Ulrich, C.; Andersson, O.; Nyholm, L.; Björefors, F. Anal. Chem. 2009, 81, 453−459. (27) Wang, Y.; Hernandez, R. M.; Bartlett, D. J.; Bingham, J. M.; Kline, T. R.; Sen, A.; Mallouk, T. E. Langmuir 2006, 22, 10451−10456. (28) Loget, G.; Kuhn, A. J. Am. Chem. Soc. 2010, 132, 15918−15919. (29) Warakulwit, C. Ph.D. Thesis, Kasetsart University, Bangkok, Thailand and University Bordeaux 1, France, 2007. (30) Loget, G.; Roche, J.; Kuhn, A. Adv. Mater. 2012, 24, 5111−5116. (31) Bradley, J.-C.; Crawford, J.; McGee, M.; Stephens, S. G. J. Electrochem. Soc. 1998, 145, L45−L47.

22027

dx.doi.org/10.1021/jp3064118 | J. Phys. Chem. C 2012, 116, 22021−22027