ARTICLE pubs.acs.org/Langmuir
Magnetically Switchable Bioelectrocatalytic System Based on Ferrocene Grafted Iron Oxide Nanoparticles Ru Peng, Wenjing Zhang, Qin Ran, Cong Liang, Li Jing, Siqiu Ye, and Yuezhong Xian* Department of Chemistry, East China Normal University, Shanghai 200062, China
bS Supporting Information ABSTRACT: A simple and versatile method for the introduction of redox unites onto the surface of magnetic nanoparticles has been developed based on “click” chemistry. Azide-functionalized Fe2O3 magnetic nanoparticles were synthesized and further reacted with ethynylferrocene via Cu(I)-catalyzed azide alkyne 1,3-dipolar cycloaddition (CuAAC) reaction. The functionalized magnetic nanoparticles were characterized using a powder X-ray diffractometer (XRD), transmission electron microscope (TEM), Fourier transform infrared spectroscope (FTIR), and vibrating sample magnetometer (VSM). The resulting materials have properties of both magnetism and electrochemistry, and the electrochemical properties of the nanoparticles are dependent on the features of ethynylferrocene, while the magnetic properties remain independent of ethynylferrocene. Because of the magnetism of Fe2O3 nanoparticles and the electrocatalytic activity of ferrocene unites, a recyclable, magneto-switchable bioelectrocatalytic system for glucose oxidation in the presence of glucose oxidase is developed by alternate positioning of an external magnet, and the system has a linear response for glucose biosensing over the range of 1.0-10.0 mM.
’ INTRODUCTION Magnetic nanoparticles are well-established nanomaterials with diameter below 20 nm exhibiting interesting size-dependent superparamagnetism.1 In the past decades, magnetic nanoparticles have become a hot research topic and attracted great interest because of their potentially very useful applications. Because of their low toxicity and ability to be manipulated by an external magnetic force, magnetic nanoparticles have been widely applied in magnetic resonance imaging (MRI),2 targeted drug delivery,3 magnetic separation of DNA, proteins, and cells,4 and cancer treatments by hyperthermia and immunoassays.5 Several chemical methods have been successfully established to prepare well-dispersed magnetic nanoparticles, such as coprecipitation of iron salts in alkaline solution,6 thermal decomposition of oxygen-rich ferric compound or metal carbonyls,7 reverse micelle microemulsion approach,8 ultrasound irradiation,9 hydrothermal or solvothermal method,10 and spray pyrolysis techniques.11 Among the above-mentioned methods, coprecipitation is the most common route to prepare magnetic nanoparticles on a large scale because of its ease in dealing and scaling up. However, pure magnetic nanoparticles are intrinsically unstable over time, and small particles tend to form agglomerates to reduce the energy associated with high ratio of surface area to volume. In addition, naked metallic nanoparticles are highly chemical active and easy to be oxidized in air, which results in poor magnetism and dispersibility. The functionalization of the magnetic nanoparticles is an important way to improve the stability and dispersibility. Moreover, integrating two or more components with magnetic r 2011 American Chemical Society
nanoparticles will result in multifunctional capabilities and increase the applicability of the materials. It is desirable to seek a general strategy for synthesis multifunctional magnetic nanoparticles with well stability, dispersibility and solubility, and place-exchange reaction has been proved to be an effective way to impart tunable functionality and solubility to magnetic particles. For example, nitrilotriacetic acid was coated onto magnetic nanoparticles via metal-sulfur bonds and further used as general agents to selective binding histidine-tagged peptides and proteins.12 Dopamine was served as a robust anchor on the surface of iron oxide shell, and the resulting magnetic nanoparticles demonstrated high specificity for protein separation.13 Amino-, carboxylic acid-, and poly(ethylene glycol)terminated saline ligands also could be exchanged and made the hydrophobic superparamagnetic nanoparticles water-dispersible.14 Click chemistry, which was introduced by Sharpless,15 winner of the 2001 Nobel Prize in chemistry, is an advanced and reliable synthetic strategy and has widespread applications, such as drug discovery,16 biomacromolecule modifications,17 surface functionalization,18 and so on. One of the most popular reactions within the click chemistry is the Cu(I)-catalyzed azide alkyne 1, 3-dipolar cycloaddition (CuAAC) reaction at room temperature. This reaction has attracted wide attention for its high reaction efficiency under mild conditions, and the resulting 1,4-disubstituted Received: October 8, 2010 Revised: December 13, 2010 Published: February 07, 2011 2910
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Langmuir 1,2,3-triazole ring is thermally stable and relatively inert to hydrolysis, oxidation, and reduction. Because of the excellent performance, click reaction has become an elegant protocol to prepare magnetic nanoparticle-based functional materials. The group of Reiser reported the synthesis of magnetic carboncoated cobalt nanoparticles tagged with complex molecules utilizing click reaction, and the resulting materials could be used as recyclable heterogeneous catalyst in organic synthesis.19-22 Prosperi and co-workers reported one-pot biofunctionalization of γ-Fe2O3 nanoparticles with lactose and human serum albumin via diazo transfer followed by in-situ CuAAC reaction.23 Folic acid- and cysteine-functionalized Fe3O4 nanoparticles were synthesized from iron(III) 3-allylacetylacetonate through in-situ hydrolysis and ligand modification by applying the principle of click chemistry.24,25 The group of Rotello demonstrated coassembly of alkyne- and azide-functionalized iron oxide nanoparticles at the water-oil interface and covalently linked using click chemistry under ambient conditions to create magnetic colloidosomes.26 In this work, γ-Fe2O3 magnetic nanoparticles functionalized with redox active molecule were synthesized via click chemistry. First, azide-containing organic phosphate was synthesized and acted as an anchor which could bind to the magnetic nanoparticles surface. Then, ethynylferrocene was grafted onto the surface of magnetic nanoparticles via CuAAC reaction. The resulting materials have properties of both magnetism and electrochemistry and can be used as recyclable magneto-controlled bioelectrocatalyst. The ferrocene-tagged magnetic nanoparticles can be easily confined on electrode surface by means of the external magnet, and the biocatalytic function is activated. In contrast, the electrical communication is switched off without the external magnet. Moreover, the biocatalyst can be recycled easily because of the excellent magnetism of γ-Fe2O3. The switching of the biocatalytic activity and recyclable usage of the functional nanoparticles by means of the external magnet could provide a simple, green, and convenient strategy for bioelectrocatalysis.
’ EXPERIMENTAL SECTION Chemicals. FeCl2 3 4H2O was purchased from Sinopharm Chemical
Reagent Co, Ltd. Ethynylferrocene was purchased from Sigma-Aldrich. R-Bromoisobutyryl bromide was purchased from Alfa Aesar. Magnetic nanoparticles were synthesized using previously established methods.27 Other chemicals were of analytical grade and used without further purification. Synthesis of Fe2O3 Nanoparticles. First, Fe3O4 nanoparticles were prepared by coprecipitation of FeCl2 and FeCl3 (Fe(II)/Fe(III) ratio = 0.5) in alkaline medium. Then, the obtained Fe 3O4 nanoparticles were dissolved in HNO3 to obtain Fe2O3 nanoparticles. The detailed synthetic process is listed in the Supporting Information.
Synthesis of Fe2O3 Nanoparticles Functionalized with Azide Group. The general route for modification Fe2O3 nanoparticles with azide group is demonstrated in Scheme 1. The detailed synthetic process is listed in the Supporting Information. Synthesis of Ferrocene Grafted Fe2O3 Nanoparticles. Ferrocence grafted Fe2O3 nanoparticles were synthesized via click chemistry, which is shown in Scheme 2. The detailed synthetic process is listed in the Supporting Information. Characterization. Powder X-ray diffraction (XRD) patterns of the samples were recorded on a Rigaku D/Ultima IV X-ray diffractometer (Rigaku, Japan), which was operated at 35 kV and 40 mA at a scan rate of 0.4 deg/s and 2θ ranges from 10° to 90° at room temperature using Cu KR radiation (λ = 0.1542 nm). The morphologies of the samples were observed by a JEM-2010 analytical transmission electron microscope
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Scheme 1. Synthesis of Fe2O3 Nanoparticles Terminated with Azide Group
Scheme 2. Synthesis of Ferrocene Grafted Fe2O3 Nanoparticles
(TEM, JEOL, Japan) at 200 kV, and the samples were dissolved in water and dropped onto copper grids. Fourier transform infrared (FTIR) spectroscopy was conducted on a Thermo Nicolet Nexus 870 FTIR spectrometer between 500 and 4000 cm-1. Magnetic measurements of as-synthesized nanoparticles were recorded in a Quantum Design vibrating sample magnetometer (mmvFTB). A diamagnetic plastic tube was filled with the powder sample, and then the packed sample was put into a diamagnetic plastic straw and packed into a minimal volume for magnetic measurements. Electrochemical experiments were performed on a CHI-832 electrochemical analyzer (CHI) with a three-electrode system, with a modified ITO as the working electrode, a saturated calomel electrode (SCE) as the reference electrode, and a gold electrode as the auxiliary electrode. All potentials in this paper referred to the SCE reference electrode.
’ RESULTS AND DISCUSSION XRD Characterization. According to the literature, the reddishbrown color of these powders suggests the formation of γ-Fe2O3.28 The crystal structure of synthesized nanoparticles was further investigated using XRD measurement using Cu KR radiation. Figure 1 illustrates the XRD patterns for the magnetic nanoparticles with and without ferrocene grafting. In the 2θ range of 10°-90°, five characteristic peaks for Fe2O3 (2θ = 31.60°, 35.70°, 43.24°, 56.04°, and 62.80°) were observed for the magnetic nanoparticles without ferrocene grafting (Figure 1a), and the peak positions at the corresponding 2θ value were indexed as (220), (311), (400), (511), and (440), respectively.29 The XRD patterns match well that of γ-Fe2O3 (Powder Diffraction file, JCPOS card no. 25-1402). The (311) plane of γ-Fe2O3 located at 2θ = 35.7° was used to calculate the mean crystallite size using Scherer’s equation,30 and the average particle size was about 18 nm. After click reaction, the XRD patterns for ferrocene grafted Fe2O3 nanoparticles are shown in Figure 1b. The broad band located at 20°-30° might ascribe to the presence of ferrocene. In addition, the characteristic peak of γ-Fe2O3 nanocrystals still remains, indicating the crystal structures of the magnetic nanoparitcles do not change after click reaction. TEM Characterization. As shown in Figure 2a, the as-synthesized γ-Fe2O3 nanoparticles show relatively monodisperse, cubic 2911
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shape with uniform average diameter about 12 nm, which is also agreement with the observation of XRD. The morphology of the ferrocene grafted magnetic nanoparticles (Figure 2b) does not show significant change after click reaction when compared with the pristine Fe2O3 nanoparticles. FTIR Characterization. Figure 3 shows the FTIR spectra of the magnetic nanoparticles. The peak at 2113 cm-1 is due to the antisymmetric stretch of azide group,31 indicating the successful modification of magnetic nanoparticles with -NdNdN-. The peak at 1740 cm-1 ascribes to the CdO stretch of the ester in the N3-magnetic nanoparticles.32 The peak located at 1022, 1269, and 1647 cm-1 might be ascribed to the vibration of P-O, PdO,33 and C-N stretch of N3-magnetic nanoparticles.34 The FTIR spectrum of Figure 3b shows the stretching of NdNdN- almost completely disappearing after click reaction, which suggests the success and high efficiency of the CuAAC reaction.35 Even after click reactions, the bands from 1250 to 990 cm-1 are still present, implying that the ligand attached to the particles is still intact. Magnetic Measurements. As shown in Figure 4a,b, the solubility and stability of the pristine and ferrocene grafted Fe2O3 nanoparticles are very well. The magnetic response of these nanoparticles is easily and quickly visualized by holding the sample close to a small magnet as shown in Figure 4c,d. This “action at a distance” provides tremendous advantages for many applications such as bioseparation,4 recyclable catalysis,36 and controlled delivery.5 Magnetic measurements of as-synthesized nanoparticles were recorded in a Quantum Design vibrating sample magnetometer. The magnetization curves of the as-synthesized Fe2O3 and ferrocene grafted Fe2O3 nanoparticles, measured at room temperature, are presented in Figure 5A. The data show no hysteresis and no saturation up to a magnetic field of 15 000 Oe, indicating that the as-prepared Fe2O3 nanoparticles are
superparamagnetic. It is in contrast with those works of Kim,37 Yu,38 and Wang39 in which the ferromagnetic response and hysteretic magnetization behavior at room temperature are found. However, the superparamagnetic behavior is in accordance with those of other Fe2O3 nanostructures, for example, amorphous iron oxide nanoparticles prepared by sonochemical synthesis,40 nanorods synthesized by template-free hydrothermal method,41 and nanotubes prepared from ferritin protein.42 It is well-known that the size of the product is related to its property. As the size of the hematite particle decreases to the nanometer scale, due to the nanoscale confinement, materials can exhibit unusual magnetic behaviors that are quite different from those of conventional bulk materials. According to the literature, when the size of the magnetic particles decreases, the particles change from multidomain to single domain.43 If the single domain particles become small enough, the magnetic moment in the domain fluctuates in direction because of thermal agitation, which leads to superparamagnetism.44,45 It suggested that the as-obtained nanoparticles are right at the fringe of the superparamagnetic limit.46 Furthermore, the fielddependence curve of the ferrocene grafted Fe2O3 nanoparticles behaves in a similar manner without hysteresis, and the coercive force and remnant magnetization are zero, even at high applied fields, which is also characteristically similar to the typical superparamagnetic materials.47 As is well-known, alignments of sublattices can occur while the magnetic material is below a certain critical temperature, which is called the Curie temperature (Tc). Magnetite has a Tc of 585 °C,48 where Tc is defined as the temperature above which a ferromagnetic or ferrimagnetic material becomes paramagnetic.49 As can be seen in Figure 5B, Fe2O3 nanoparticles have a Tc of 452 °C, while
Figure 1. XRD patterns of as-prepared samples γ-Fe2O3 nanoparticles (a) and ferrocene grafted Fe2O3 nanoparticles (b).
Figure 3. FTIR spectra of N3-Fe2O3 nanoparticles (a) and the ferrocene grafted Fe2O3 nanoparticles (b).
Figure 2. TEM images of Fe2O3 nanoparticles (a) and the ferrocene grafted Fe2O3 nanoparticles (b). 2912
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Figure 4. Photographs of Fe2O3 nanoparticles (a) and ferrocene grafted Fe2O3 nanoparticles (b) dispersed in water solution and magnetic response of Fe2O3 nanoparticles (c) and ferrocene grafted Fe2O3 nanoparticles (d) dispersed in water with a permanent magnet.
Figure 5. (A) Room temperature hysteresis loops of the as-synthesized Fe2O3 (a) and ferrocene grafted Fe2O3 nanoparticles (b) at 27 °C and (B) magnetic properties of γ-Fe2O3 (a) and ferrocene grafted Fe2O3 nanoparticles (b) obtained with increasing temperature in an applied field of 360 Oe.
Figure 6. (A) Cartoon represents the electrochemical setup for ferrocene grafted Fe2O3 nanoparticles with a permanent magnet. (B) cyclic voltammograms of the ITO electrode before (a) and after modification with ferrocene grafted Fe2O3 nanoparticles (b) by an external magnet in 0.1 M LiClO4 of CH3CN solution.
the Tc of ferrocene grafted Fe2O3 nanoparticles is about 502 °C. A paramagnetic material requires an externally applied magnetic field to retain any magnetization. Above Curie temperature, magnetic particles lose their long-range order of atomic magnetic moments and become paramagnetic. In the presence of magnetic fields, these particles align in the direction of the applied field. In the micrometer sample size, these particles align in single domain and become “superparamagnetic”. Electrochemical Measurements. In order to investigate the electrochemical characteristics of ferrocene grafted Fe2O3 nanoparticles, the as-synthesized nanoparticles are successfully assembled onto the surface of the indium tin oxide (ITO) electrode with the help of a permanent magnet (Figure 6A), and the electrochemical responses are shown in Figure 6B. No redox
peak is observed at the bare ITO electrode (Figure 6B, curve a); however, the ferrocene grafted Fe2O3 nanoparticles modified ITO electrode yields a pair of well-defined redox peaks which are the characteristic of the redox process of Fe(III)/Fe(II) couple resulting from ferrocene (Figure 6B, curve b). The anodic peak potential (Epa) and the cathodic peak potential (Epc) are located at 0.444 and 0.338 V, respectively, with a formal potential E0 = (Epa þ Epc)/2 at 0.391 V, which is considerably more positive than that of free ferrocene (E0 = 120 mV).50 It might ascribe to the conjugation effect of the triazole ring and the magnetic nanoparticles, which reduces the electron density of the ferrocene and makes the oxidation of the ferrocene unit more difficult than that of unsubstituted metallocenes. In addition, the peak-to-peak separation is ca. 106 mV, indicating a relatively fast and quasi-reversible electron 2913
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Scheme 3. Reversible Switch “ON” and “OFF” of Ferrocene Grafted Fe2O3 Nanoparticles Mediated Bioelectrocatalytic Oxidation of Glucose via Glucose Oxidase by the Attraction/ Retraction Magnetic Nanoparticles onto the ITO Electrode Surface
Figure 7. Cyclic voltammograms of ferrocene grafted Fe2O3 nanoparticles modified ITO in solution of LiClO4 and CH3CN at scan rates of 100 (a), 150 (b), 200 (c), 250 (d), 300 (e), 350 (f), 400 (g), 450 (h), and 500 (i) mV/s. Inset: plot of the reduction and oxidation peak currents against the square root of the scan rates.
transfer. Furthermore, the couple of redox peaks also indicate that ferrocene has been successfully grafted onto Fe2O3 nanoparticles via click reaction. Figure 7 displays an overlaps of cyclic voltammograms of ferrocene grafted Fe2O3 nanoparticles with scan rates over the range from 100 to 500 mV/s. The reduction and oxidation peak currents exhibit a linear relationship with the square root of scan rate (inset of the Figure 7, R = 0.999), indicating a diffusionally limited electrochemical process.51 Magneto-switchable electrocatalysis/bioelectrocatalysis, pioneered by Willer and co-workers, are new topics that examine the effect of an external magnetic field on electrocatalytic and bioelectrocatalytic processes of functionalized magnetic particles associated with electrodes and can be developed to design magneto-controlled molecular electronics and bioelectronics.52 We find that ferrocene grafted Fe2O3 magnetic nanoparticles also can be used as recyclable, magneto-switchable bioelectrocatalyst for glucose oxidation, which is shown in Scheme 3. A permanent magnet is used as induce magnetic field, and ferrocene grafted Fe2O3 magnetic nanoparticles are attracted to the ITO electrode while the magnet is located at the back side of the working electrode. On this condition, the characteristic redox wave of the ferrocene units is observed, and the ferrocene-mediated biocatalytic oxidation of glucose via glucose oxidase can be achieved. While the magnet is positioned at the bottom side of electrochemical cell, the magnetic nanoparticles are retracted from the ITO electrode and the bioelectrocatlytical process is terminated. That is to say, the bioelectrocatalysis process can be reversibly switched between “ON” and “OFF” states by alternating positioning of the external magnet besides and below the electrochemical cell. Figure 8 shows the magnetic control of the bioelectrocatalytic oxidation of glucose with glucose oxidase and ferrocene grafted magnetic nanoparticles. Magnetic attraction of the nanoparticles to the ITO electrode results in the oxidation of the ferrocene units and activation of glucose oxidase. The bioelectrocatalytic oxidation of glucose is observed with the obvious increasement of the anodic currents (Figure 8, curve c). Remove the magnetic nanoparticles from the ITO electrode leads to switch off the
Figure 8. Cyclic voltammograms of bare ITO electrode in 0.1 M PBS (pH = 7.0) (a) and in presence of 10.0 mM glucose and 1.0 mg/mL glucose oxidase (b) and cyclic voltammograms of ferrocene grafted Fe2O3 magnetic nanoparticles attracted onto (c) and retracted from (d) the ITO electrode surface in the presence of 10.0 mM glucose and 1.0 mg/mL glucose oxidase.
bioelectrocatalytic cycle (Figure 8, curve d). The current of “switch-on” is almost 3 times that of “switch-off”. The control experiments reveal that naked magnetic particles do not activate the enzyme for the bioelectrocatalytic oxidation of glucose, implying that the ferrocene units mediate the electrical activation of glucose oxidase. The magneto-switchable bioelectrocatalysis was further investigated by differential pulse voltammetry, which is shown in Figure 9. When the ferrocene grafted magnetic nanoparticles are attracted onto ITO electrode, an oxidation peak located 0.42 V is observed (Figure 9, curve b), implying the activation of bioelectrocatalysis. While the ferrocene grafted magnetic nanoparticles are retracted from the electrode, the disappearance of oxidation peak indicates the inhibition of bioelectrocatalysis. By cyclic positioning of the external magnet besides and below the cell, 2914
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material is available free of charge via the Internet at http:// pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail
[email protected]; Tel 86-21-62233954; Fax 86-21-62232627.
Figure 9. Differential pulse voltammograms of ITO electrode in 0.1 M PBS (pH = 7.0) in the presence of 10.0 mM glucose, 1.0 mg/mL glucose oxidase, and ferrocene grafted magnetic nanoparticles attracted onto (a) and retracted from (b) the electrode surface.
Figure 10. Cyclic voltammograms of 0 (a), 1.0 (b), 3.0 (c), 6.0 (d), and 9.0 mM (e) glucose at ITO electrode modified with ferrocene grafted Fe2O3 magnetic nanoparticles in presence of 1.0 mg/mL glucose oxidase.
the electrical response of the ferrocene sites is reversibly switched between “ON” and “OFF” condition, respectively. Figure 10 shows the cyclic voltammograms of different concentrations of glucose at ferrocene grafted Fe2O3 magnetic nanoparticles modified ITO electrode in presence of 1.0 mg/mL glucose oxidase in 0.1 M PBS (pH = 7.0) buffer solution. The oxidation peak currents increase linearly with the concentrations of glucose over the range of 1.0-10.0 mM, indicating the bioelectrocatalytic system has potential applications in blood glucose sensing.
’ CONCLUSIONS In this work, redox active ethynylferrocene is successfully grafted to the azide group functionalized Fe2O3 nanoparticles via click reaction with a high yield. The resulting ferrocene grafted Fe2O3 nanoparticles demonstrate excellent magnetic and electrochemical characteristics and can be used as magnetswitchable bioelectronics. ’ ASSOCIATED CONTENT
bS
Supporting Information. Details of the synthesis of Fe2O3 nanoparticles and ferrocene grafted Fe2O3 nanoparticles. This
’ ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (No. 20875031), Shanghai Rising-Star Program (No. 09QH1400800), New Century Excellent Talents in University (No. NCET-09-0357), and Shanghai Municipal Education Commission (No. 08ZZ25). ’ REFERENCES (1) Correa-Duarte, M. A.; Grzelczak, M.; Salgueirin o-Maceira, V.; Giersig, M.; Liz-Marzan, L. M.; Farle, M.; Sierazdki, K.; Diaz, R. J. Phys. Chem. B 2005, 109, 19060–19063. (2) Lee, E.; Kim, D. K.; Jang, N. K.; Jeong, Y. Y.; Jon, S. Y. J. Am. Chem. Soc. 2006, 128, 7383–7389. (3) Shkilnyy, A.; Munnier, E.; Herve, K.; Souce, M.; Benoit, R.; Cohen-Jonathan, S.; Limelette, P.; Saboungi, M. L.; Dubois, P.; Chourpa, I. J. Phys. Chem. C 2010, 114, 5850–5858. (4) Liu, J. C.; Tsai, P. J.; Lee, Y. C.; Chen, Y. C. Anal. Chem. 2008, 80, 5425–5432. (5) Hayashi, K.; Moriya, M.; Sakamoto, W.; Yogo, T. Chem. Mater. 2009, 21, 1318–1325. (6) Lyon, J. L.; Fleming, D. A.; Stone, M. B.; Schiffer, P.; Williams, M. E. Nano Lett. 2004, 4, 719–723. (7) Gao, J. H.; Gu, H. W.; Xu, B. Acc. Chem. Res. 2009, 42, 1097– 1107. (8) L opez Perez, J. A.; Lopez Quintela, M. A.; Mira, J.; Rivas, J.; Charles, S. W. J. Phys. Chem. B 1997, 101, 8045–8047. (9) Teo, B. M.; Chen, F.; Hatton, T. A.; Grieser, F.; Muthupandian Ashokkumar, M. Langmuir 2009, 25, 2593–2595. (10) Chaianansutcharit, S.; Okorn Mekasuwandumrong, O.; Praserthdam, P. Cryst. Growth Des. 2006, 6, 40–45. (11) Wang, J. N.; Zhang, L.; Yu, F.; Sheng, Z, M. J. Phys. Chem. B 2007, 111, 2119–2124. (12) Li, Y. C.; Lin, Y. S.; Tsai, P. J.; Chen, W. Y.; Chen, Y. C. Anal. Chem. 2007, 79, 7519–7525. (13) Xu, C. J.; Xu, K. M.; Gu, H. W. S.; Zheng, R. K.; Liu, H.; Zhang, X. X.; Guo, Z. H.; Xu, B. J. Am. Chem. Soc. 2004, 126, 9938–9939. (14) Palma, R. D.; Peeters, S.; Van Bael, M. J.; Heidi Van den, Rul; Bonroy, K.; Laureyn, W.; Mullens, J.; Borghs, G.; Guido Maes, G. Chem. Mater. 2007, 19, 1821–1831. (15) Kolb, H. C.; Finn, M. G.; Sharpless, K. B. Angew. Chem., Int. Ed. 2001, 40, 2004–2021. (16) Moses, J. E.; Moorhouse, A. D. Chem. Soc. Rev. 2007, 36, 1249– 1262. (17) Mamidyala, S. K.; Finn, M. G. Chem. Soc. Rev. 2010, 39, 1252–1261. (18) Collman, J. P.; Devaraj, N. K.; Chidsey, C. E. D. Langmuir 2004, 20, 1051–1053. (19) Sch€atz, A.; Grass, R. N.; Kainz, Q.; Stark, W. J.; Reiser, O. Chem. Mater. 2010, 22, 305–310. (20) Sch€atz, A.; Grass, R. N.; Stark, W. J.; Reiser, O. Chem.—Eur. J. 2008, 14, 8262–8266. (21) Sch€atz, A.; Hager, M.; Reiser, O. Adv. Funct. Mater. 2009, 19, 2109–2115. (22) Sch€atz, A.; Long, T. R.; Grass, R. N.; Stark, W. J.; Hanson, P. R.; Reiser, O. Adv. Funct. Mater. 201010.1002/adfm.201000959. (23) Polito, L.; Monti, D.; Caneva, E.; Delnevo, E.; Russo, G.; Prosperi, D. Chem. Commun. 2008, 5, 621–623. 2915
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