Development of New Localized Surface Plasmon Resonance

Feb 22, 2010 - This article reports on the fabrication and characterization of plasmonic interfaces composed of a sandwiched structure comprising a ti...
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Development of New Localized Surface Plasmon Resonance Interfaces Based on Gold Nanostructures Sandwiched between Tin-Doped Indium Oxide Films Joanna Niedziozka-J€onsson,*,† Fatiha Barka,†,‡ Xavier Castel,§ Marcin Pisarek, Nacer Bezzi,‡ Rabah Boukherroub,† and Sabine Szunerits*,† †

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Institut de Recherche Interdisciplinaire (IRI, USR-3078) Parc de la Haute Borne, 50 avenue de Halley, BP 70478, 59658 Villeneuve d’Ascq, France, and Institut d’Electronique, de Micro electronique et de Nanotechnologie (IEMN, CNRS-8520), Cit e Scientifique, Avenue Poincar e, 59652 Villeneuve d’Ascq, France, ‡ Laboratoire de Technologie des Mat eriaux et de G enie des Proc ed es (LTMGP), Universit e Abderrahmane Mira de Bejaia, Targa Ouzemour, 06000 B ejaia, Alg erie, §Institut d’Electronique et de T el ecommunications de Rennes (IETR -UMR CNRS 6164), 18 rue Henri Wallon, PB 406, 22004 Saint-Brieuc, Cedex 1, France, and Institute of Physical Chemistry, Polish Academy of Sciences (Physical Chemistry of Materials Center), Kasprzaka 44/52, 01-224 Warsaw, Poland Received September 4, 2009. Revised Manuscript Received January 19, 2010 This article reports on the fabrication and characterization of plasmonic interfaces composed of a sandwiched structure comprising a tin-doped indium oxide (ITO) substrate, gold nanostructures (Au NSs), and a thin ITO film overcoating. The change in the optical characteristics of the ITO/Au NSs/ITO interfaces as a function of the ITO overlayer thickness (dITO = 0-200 nm) was followed by recording UV-vis transmission spectra. The influence of the thickness of the ITO overcoating on the position and shape of the plasmonic signal is discussed. The possibility to functionalize the ITO/Au NSs/ITO interfaces chemically is demonstrated by covalently linking ethynyl ferrocene to azide-terminated ITO/Au NSs/ITO interfaces. The resulting interfaces were characterized using X-ray photoelectron spectroscopy (XPS), electrochemical (cyclic voltammetry and differential pulse voltammetry) techniques, and UV-vis transmission spectroscopy.

1. Introduction The recent decade has witnessed an enormous research effort directed toward the understanding and utilization of the unique and tunable optical properties of metallic nanostructures. The development of plasmonic-based interfaces has been driven mainly by the possibility of numerous exciting applications including sensors1,2 and was enabled by the rapid progress in nanotechnology and nanoscale science allowing for successful synthesis and characterization. The influence of metal particle shape, size, composition, and interparticle distance on the plasmonic characteristics has been largely discussed in the literature.3 Furthermore, it is well established that the position of the localized surface plasmon resonance (LSPR) band is sensitive to changes in the environment around the metallic nanostructures. Changes caused by interfacial reactions in the local dielectric constant of the medium surrounding the nanostructures have made LSPR useful for the detection of molecular interactions.4-6 The same concept, a change in the local refractive index, applies to LSPR interfaces coated with thin transparent or semitrans*To whom correspondence should be sent. (J.N.-J.) E-mail: joanna. [email protected]. Phone: þ33 3 2 62 53 17 22. (S.S.) E-mail: [email protected]. Phone: þ33 3 2 62 53 17 25. Fax: þ33 3 2 62 53 17 01.

(1) Stuart, D. A.; Haes, A. J.; Yonzon, C. R.; Hicks, E. M.; Van Duyne, R. P. IEE Proc.: Nanobiotechnol. 2005, 152, 13. (2) Aslan, K.; Lakowicz, J. R.; Geddes, C. D. Curr. Opin. Chem. Biol. 2005, 9, 538. (3) El-Sayed, M. A.; Eustis, S. Chem. Soc. Rev. 2006, 35, 209–217. (4) Anker, J. N.; Paige Hall, W.; Lyandres, O.; Shah, N. C.; Zhao, J.; Van Duyne, R. P. Nat. Mater. 2008, 7, 442. (5) Larsson, E. M.; Alegret, J.; Kall, M.; Sutherland, D. S. Nano Lett. 2007, 7, 1256. (6) Kalyushny, G.; Vaskevich, A.; Schneeweiss, M. A.; Rubinstein, I. Chem.; Eur. J. 2002, 8, 3850.

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parent films. In the case of oxide-based overcoatings, the thin coating films will allow the stabilization of the metal nanostructures and thus obtaining a reliable sensing platform having stable and reproducible optical properties. Indeed, the poor adhesion of noble metal nanoparticles to most inorganic oxide substrates induces morphological changes and aggregation upon exposure to solvents and analytes. This leads to a change in the optical properties of the structure, causing uncertainty in any detection scheme based on the refractive index sensitivity. This poor stability has been known for silver and gold island films.7,8 The other important feature of oxide-based LSPR sensing platforms is the possibility to use well-established oxide-related surface chemistry for further coupling organic or biological molecules.9,10 Such nanocomposite thin films formed by noble metal nanoparticles embedded in or coated with a dielectric matrix can exhibit LSPR responses due to collective excitations of conducting electrons in the metal nanoparticles when photons are coupled to the metal particle/dielectric interface.11 Such hybrid LSPR platform should thus result in a wide range of potential applications in different areas. Surprisingly, the formation and application of such hybrid LSPR structures is still in its infancy. To date, there are only a few examples of oxides deposited on LSPR platforms. Thin films of aluminum oxide (Al2O3) were deposited on gold nanostructures using a one-step radio-frequency (7) Malinsky, M. D.; Kelly, K. L.; Schatz, G. C.; Van Duyne, R. P. J. Am. Chem. Soc. 2001, 123, 1471–1482. (8) Luo, Y.; Ruff, J.; Ray, R.; Gu, Y. L.; Ploehn, H. J.; Scrivens, W. A. Chem. Mater. 2005, 17, 5014. (9) Szunerits, S.; Das, M. R.; Boukherroub, R. J. Phys. Chem. C 2008, 12, 8239. (10) Ruach-Nir, I.; Bendikov, T. A.; Doron-Mor, I.; Barkay, Z.; Vaskevich, A.; Rubinstein, I. J. Am. Chem. Soc. 2007, 129, 84. (11) Prasad, P. N., Ed. Introduction to Biophotonics; Wiley-Interscience: New York, 2004.

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magnetron cosputtering technique, and the LSPR platform was used for Pd2þ sensing.12 Van Duyne and co-workers used atomic layer deposition (ALD) to deposit 1-600 monolayers of Al2O3 on silver nanotriangles.13 Rubinstein’s group suggested the deposition of ultrathin silica films (1.5 nm) by a sol-gel procedure on gold nanoislands.10 It involves a two-step procedure based on the self-assembly of a monolayer of 3-mercaptopropyl trimethoxysilane on the Au NSs, followed by deposition of the silica layer from a sodium silicate solution for 1 to 2 h at 90 °C. Our group has recently shown that the deposition of SiOx overcoatings on gold nanostructures allows DNA hybridization detection to be performed with a detection limit of ∼50 nM. The plasmonic responses of such hybrid LSPR structures depend, in addition to the particle shape, size, composition, and density, on the thickness of the dielectric coating.15,16 We have shown that even when glass/gold nanostructures (Au NSs) were coated with silicon dioxide (SiOx) thin films (310 nm thick) plasmonic signals can still be detected.9 These findings were recently verified through theoretical calculations using the Lorentz-Drude model.17 The interest in such multilayer interfaces is due to the possibility of sensitive long-range sensing with important implications in biological studies as well as the possibility to link ligands to the oxide surface in a stable and reproducible manner. In this article, the effect of tin-doped indium oxide (ITO) coatings on the plasmonic properties of ITO interfaces modified with gold nanostructures (Au NSs) is investigated. The interest in developing ITO overlayers is multiple. The presence of a conducting ITO overlayer creates a LSPR-active interface, which can serve simultaneously as a working electrode in an electrochemical setup. In contrast to LSPR interfaces10 coated with nanoporous silica and thin SiOx films, electrochemistry on ITObased LSPR interfaces should not be influenced by the presence of the overcoating.10 This opens the possibility to study electrochemical and optical phenomena in parallel as shown for classical SPR interfaces.18,19 In parallel with the work on the formation of the gold/ITO propagating SPR interface, ITO overlayers were deposited by thermal evaporation in this article, with the main difference being that the planar gold interface was replaced by gold nanostructures. Like silicon dioxide, the surface of ITO contains hydroxyl groups that can be used to link functional groups to the interface. Here the covalent linking of ethynyl ferrocene to azide-terminated glass/Au NSs/ITO hybrid LSPR platforms using click chemistry will be presented.

2. Experimental Section 2.1. Materials. ITO (sheet resistivity 15-25 Ω square-1), potassium hexacyanoferrate(II) [K4Fe(CN)6], potassium chloride (KCl), N,N-decyclohexylcarbodiimide (DCC), 4-dimethylaminopyridine (DMAP), ethynyl ferrocene, 3-sodium ascorbate, copper (12) Gao, S.; Koshizaki, N.; Koyama, E.; Tokushisa, H.; Sasaki, T.; Kim, J.-K.; Cho, Y.; Kim, D.-S.; Shimizu, Y. Anal. Chem. 2009, 81, 7703–7712. (13) Whitney, A. V.; Elam, J. W.; Zou, S.; Zinovev, A. V.; Stair, P. C.; Schatz, G. C.; Van Duyne, R. P. J. Phys. Chem. B 2005, 109, 20522. (14) Haes, A. J.; Zou, S.; Schatz, G. C.; Van Duyne, R. P. J. Phys. Chem. B 2004, 108, 6961. (15) Haes, A. J.; Hall, W. P.; Chang, L.; Klein, W. L.; Van Duyne, R. P. Nano Lett. 2004, 4, 1029. (16) Rindzevicius, T.; Alaverdyan, Y.; K€all, M.; Murray, W. A.; Barnes, W. L. J. Phys. Chem. C 2007, 111, 11806. (17) Galopin, E.; Noual, A.; Niedziolka-J€onsson, J.; J€onsson-Niedziolka, M.; Akjouj, A.; Pennec, Y.; Djafari-Rouhani, B.; Boukherroub, R.; Szunerits, S. J. Phys. Chem. C 2009, 113, 15921. (18) Szunerits, S.; Castel, X.; Boukherroub, R. J. Phys. Chem. C 2008, 112, 10883. (19) Castel, X.; Boukherroub, R.; Szunerits, S. J. Phys. Chem. C 2008, 112, 15813.

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Article sulfate (CuSO4 3 5H2O), dichloromethane (DCM), and tetrahydrofuran (THF) were purchased from Sigma-Aldrich. 4-Azidobenzoic acid was obtained from TCI Europe. 3-Aminopropyltrimethoxysilane (APTMS) was obtained from Gelest.

2.2. Preparation of Gold Nanostructures on ITO Substrates. ITO/Au NSs. ITO slides (76  26  1 mm3) were cleaned with acetone and isopropanol and then rinsed with Milli-Q water and dried under a stream of nitrogen. A 2 nm thin gold film was deposited by thermal evaporation (0.1 A˚ s-1) at a base pressure of 10-7-10-8 Torr using a MECA 2000-1 system (Plassys) with about 10% accuracy. Postdeposition annealing of the gold film was performed under a stream of nitrogen at 500 °C for 1 min using a rapid thermal annealer (Jipelec Jet First 100). The reproducibility of the Au evaporation was evaluated by measuring the LSPR signals of a batch of eight samples. The standard deviation in the wavelength (λmax) and maximum absorption (Imax) are typically 2 nm and 0.02 abs units, respectively. 2.3. Deposition of the ITO Overlayer. ITO overcoatings were deposited onto the ITO/Au NSs interface using rf sputtering (Plassys MP 450S) at 8  10-8 mbar (turbomolecular rotary pump system).14,16 The deposition chamber contains an In2O3SnO2 (In2O3 90% w/w, SnO2 10% w/w 99.999% purity) ceramic sputtering target (75 mm in diameter). The deposition temperature is measured with a thermocouple set behind the sample holder. ITO deposition is carried out at a rf power of 13.56 MHz under an oxygen/argon atmosphere using the following parameters: rf power = 38 W, total pressure = 0.012 mbar, O2/ Ar ratio = 0.051, deposition rate = 0.6 nm min-1, and substrate temperature = 25 °C.

2.4. Surface Functionalization. 2.4.1. Silanization with APTMS. A low-pressure mercury arc lamp (UVO cleaner, no.

42-220, Jelight, P = 1.6 mW cm2, distance from sample = 3 mm) was used to form surface hydroxyl groups on the ITO/Au NSs/ITO hybrid interfaces. Amine-terminated ITO/Au NSs/ITO hybrid interfaces were prepared by chemical treatment of the clean surface with 3% 3-aminopropyltrimethoxysilane (APTMS) in methanol/water (v/v 95/5) for 30 min under sonication. The interfaces were then washed with methanol, water (two times), and methanol and annealed for 20 min at 110 °C.20 2.4.2. Azide Termination. 4-Azidobenzoic acid (2 mmol), N,N-dicyclohexylcarbodiimide (2.2 mmol), and 4-dimethylaminopryridine (DMAP, 0.66 mmol) were dissolved in dry dichloromethane (DCM) (20 mL). The amine-terminated ITO/Au NSs/ ITO hybrid interfaces were immersed in the solution and left at room temperature for 24 h under a nitrogen atmosphere. The samples were then washed with dichloromethane (CH2Cl2) (5 min, two times), ethanol (5 min, two times), and finally with water and dried under a stream of nitrogen.

2.4.3. “Clicking” Ferrocene to Azide-Terminated Surface. The azide-terminated ITO/Au NSs/ITO surface was immersed in 15 mL of an ethanol/water (1/2) solution of ethynyl ferrocene (2 mM), CuSO4 3 5H2O (100 μM), and sodium ascorbate (150 μM) and kept for 24 h at room temperature. The resulting surface was washed with ethanol and water and dried under a stream of nitrogen.

2.5. Instrumentation. 2.5.1. Electrochemical Measurements. Cyclic voltammetry (CV) was performed with an Autolab potentiostat 30 (Eco-Chemie, Utrecht, The Netherlands). The ITO working electrode was sealed against the bottom of a singlecompartment electrochemical cell and a copper plate. Electrical contact between the ITO and the plate was made with copper selfadhesive tape. A platinum mesh and a silver wire were used as a counter electrode and a reference electrode, respectively. The scan rate was 50 mV s-1, and the active surface area was 0.04 cm2. Differential pulse voltammograms were obtained using the following parameters: step potential = 5 mV, modulation amplitude =25 mV, modulation time 0.05 s, and interval time 0.5s (20) Szunerits, S.; Coffinier, Y.; Janel, S.; Boukherroub, R. Langmuir 2006, 22, 10716.

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Figure 1. SEM images of (A) ITO/AuNSs, (B) ITO/AuNSs/10 nm ITO, and (C) ITO/AuNSs/200 nm ITO.

3. Results and Discussion 3.1. Formation of ITO/Au NSs/ITO Hybrid Interfaces. Figure 1A shows an SEM image of an ITO substrate after thermal evaporation of a 2 nm gold film and rapid thermal postannealing. The resulting gold nanostructures display average particle diameters of 12 ( 5 and height of 9 ( 3 nm determined by AFM measurements. They are uniformly distributed over the ITO surface. Soft rf sputtering was used for the deposition of the ITO overcoatings.21 In the case of a 10-nm-thick ITO overcoating, the structures of the Au NSs are still visible in the

SEM image (Figure 1B). Increasing the thickness of the ITO overcoating layer led to a significant increase in the surface roughness and the formation of ITO crystallites (Figure 1C). The surface roughness, calculated as an arithmetic average roughness (rms), increased from 2.32 to 4.26 nm for 10 and 200 nm ITO overcoating thicknesses, respectively. Similar results were obtained on silver interfaces coated with ITO protective films.18 3.2. Plasmonic Response of ITO/Au NSs/ITO Hybrid Interfaces. Absorption UV-vis spectra of ITO/Au NSs interfaces with different ITO thicknesses when immersed in a cuvette with water are presented in Figure 2A. In the absence of ITO overcoating, the ITO/Au NSs interface displays a LSPR signal with a maximum wavelength position of λLSP = 588 nm. The presence of ITO overlayers changes the overall absorption spectra of the hybrid interfaces. Up to d(ITO) < 30 nm, a red shift in λLSP is observed (Figure 2B). Thereafter, λLSP is blue shifted for ITO thicknesses of up to d(ITO) ≈ 90 nm. A further increase in the ITO coating thickness reveals a second red shift up to d(ITO) ≈ 180 nm, where a second blue shift becomes visible. The resonances (corresponding to the maximum of the amplitude of the oscillations) and the antiresonances (corresponding to the minimum of the amplitude of the oscillations) are similar to the classical Fabry-Perot cavity, which allows the determination of the periodicity of the oscillation dp using the following relation: dp = λmax/2n, where n is the refractive index of the dielectric layer. This results theoretically in an oscillation periodicity of dp(ITO) ≈ 588/2  2.0 ≈ 147 nm and compares well with experimentally found dp(ITO) ≈ 110 nm. The oscillation behavior could originate from electronic interactions among the gold nanostructures, the supporting interface, and the dielectric overcoating. Recent studies demonstrated that interparticle plasmon coupling can give rise to pronounced shifts in the LSPR wavelength with respect to that observed for an isolated particle.22,23 These investigations showed that the coupling strength decays with the inverse cube of the separation distance between the plasmonic centers. Also, the LSPR wavelength shift decay length is ∼0.2 times the particle diameter. In our

(21) Legeay, G.; Castel, X.; Benzerga, R.; Pinel, J. Phys. Status Solidi C 2008, 5, 3248.

(22) Jain, P. K.; Huang, W.; El-Sayed, M. A. Nano Lett. 2007, 7, 2080. (23) Gunnarsson, L.; Rindzevicius, T.; Prikulis, J.; Kasemo, B.; Kall, M.; Zou, S.; Schatz, G. C. J. Phys. Chem. B 2005, 109, 1079.

2.5.2. UV-Vis Spectrometer. Absorption spectra were recorded using a Perkin-Elmer Lambda UV/vis 950 spectrophotometer in polystyrene cuvettes with an optical path length of 10 mm. The wavelength range was 400-800 nm. 2.5.3. Scanning Electron Microscopy (SEM). SEM images were obtained using a Zeiss ULTRA 55 electron microscope equipped with a thermal field emission emitter and three different detectors (EsB detector with a filter grid, a high-efficiency In-lens SE detector, and an Everhart-Thornley secondary electron detector). 2.5.4. Atomic Force Microscopy (AFM). The samples were imaged using a Dimension 3100 model AFM (Veeco) equipped with a Nanoscope IV controller (Digital Instruments) under ambient conditions. Single-beam silicon cantilevers (AFM-TM Arrow, Nanoworld) with spring constants of ∼42 N m-1 and resonance frequencies of ∼250 kHz were used. All AFM images were acquired in tapping mode. 2.5.5. X-ray Photoelectron Spectroscopy (XPS). The chemical composition of the samples’ surface was characterized by an X-ray photoelectron spectroscope (Microlab 350) using Al Ka nonmonochromated radiation (1486.6 eV; 300 W) as the excitation source. The pressure during analysis was 1.0  10-9 mbar. The binding energy of the target elements (C1s, In3d, Sn3d, N1s and O1s) was determined at a pass energy of 40 eV, with a resolution of 0.83 eV, using the binding energy of carbon (C1s, 285 eV) as the reference. A linear or Shirley background subtraction was made to obtain the XPS signal intensity. The peaks were fitted using an asymmetric Gaussian/Lorentzian mixed function.

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Figure 2. (A) UV-vis transmission spectra of the ITO/AuNSs/ ITO interface in water with different ITO overlayer thicknesses (0, 10, 30, 60, 90, 120, 150, 180, and 200 nm). (B) Variation of λLSPR with increasing ITO overlayer thickness in air (red dots) and water (b).

case, the particle length is about ∼12 nm, giving a decay length of ∼3 nm. The deposited ITO films have thicknesses that are thus too large to induce significant electronic coupling between the gold nanostructures and the ITO surface. Optical interference thus plays the main role and is the reason for the observed oscillations. Indeed, McCreery et al. observed similar phenomenon on Ag nanoparticles deposited on SiOx layers with different thicknesses on a conducting Si interface.24 They showed that the transmission of a 9 nm silver film on glass is about 30% at λmax, indicating that a significant portion of the incident light is transmitted to the underlying material. As a consequence, a large portion of the light incident on the metal islands will be reflected, producing spatially distributed standing electric field patterns. The characteristics of this standing wave are determined by the optical properties and the thickness of the materials used. The peak fwhm is ∼80 ( 2 nm for an uncoated ITO/Au NSs interface. For ITO/Au NSs/ITO interfaces with dITO = 10-30 and 90-120, (24) Shoute, L. C. T.; Bergen, A. J.; Mamoud, A. M.; Harris, K. D.; McCreery, R. L. Appl. Spectrosc. 2009, 63, 133.

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the fwhm is about ∼90 ( 4 nm. The smallest peak fwhm is found for the ITO/Au NSs/ITO interface with dITO =150, and the largest fwhm of ∼134 ( 4 nm is found for dITO = 60 and dITO >200 nm. The reason for this broadening is still not clear but could result from additional absorption of the slightly colored ITO overlayer. In contrast to glass/Au NSs/SiOx LSPR platforms9 (Supporting Information), additional optical features next to the main plasmonic peak λLSPR are observed on ITO/Au NSs/ITO interfaces (Figure 2A). A rather broad second absorption band between 400 and 550 nm is present in all cases. For a 180 nm ITO film, even a third absorption peak can be clearly identified. The origin of these bands is not plasmonic-based but is simply a result of classical Fabry-Perot cavities due to uncompensated thin film interferences. Indeed, the positions of these bands do not change when immersed into different solvents, different from the main plasmonic peak λLSPR, and the detected maximum wavelength depends on the incident angle of light. Figure 2B shows the change in the maximum wavelength when the LSPR hybrid interfaces were immersed in water (n = 1.333) instead of air (n = 1.000). The different interfaces seem to respond differently when in contact with a medium of higher refractive index. A shift of 26 nm was observed on the ITO/Au NSs interface whereas in the case of ITO/Au NSs/ITO coated with 30- and 180-nm-thick ITO layers optical shifts of 15 and 20 nm were recorded, respectively. The other interfaces showed shifts in λLSPR that were smaller than 5 nm. The ITO/Au NSs/ITO (30 or 180 nm) interfaces seem to be thus most suitable as chemical sensing platforms. 3.3. Electrochemical Characterization of ITO/Au NSs/ ITO Hybrid Interfaces. Cyclic voltammetry using Fe(CN)64as a redox couple was performed to analyze the conductive properties of ITO/Au NSs/ITO interfaces with increasing ITO thickness. Well-developed anodic and cathodic peaks were obtained for a bare ITO electrode (Figure 3A) and ITO/Au NSs/ ITO hybride interfaces with d(ITO) > 60 nm (Figure 3B). A peakto-peak separation of ΔEp = 180 ( 15 mV was determined for bare ITO with an apparent electron-transfer rate constant of kapp = 0.003 cm s-1. However, ΔEp = 200 ( 15 mV and kapp = 0.0025 ( 0.0005 cm s-1 were estimated for ITO/Au NSs/ITO (d(ITO) = 60-200 nm). This is rather typical of rough electrode material because of the presence of uncompensated resistance within the ITO overlayer.25 The ITO substrate modified with only gold nanostructures shows smaller currents as compared to bare ITO, and ΔEp spreads to 270 mV (Figure 3A). This might be due to a lack of electrical contact between the metal nanostructures and the ITO substrate as found for metallic interparticle distances larger than 5 nm, resulting in increased capacitive current.26 Increasing the ITO overlayer thickness leads to an increase in the detected current and electron-transfer rate. The i-E curves suggest that a minimal thickness of d(ITO) ≈ 60 nm is required to provide good electrical contact. With classical SPR interfaces (Ti/ Ag/ITO), a minimal thickness of d(ITO) ≈ 10 nm supplied welldeveloped anodic and cathodic peaks.18 This lower value is coherent for interfaces with a smoother surface. The UV-vis signal of this LSPR interface coated with 60 nm ITO is rather broad. The best compromise in terms of good electrochemical characteristics and acceptable optical properties would thus be an ITO/Au NSs/ ITO (150 nm) interface. This interface also showed improved sensibility to very large bulk refractive index changes as observed (25) McKenzie, K. J.; Niedziolka, J.; Paddon, C. A.; Marken, F.; Rozniecka, E.; Opallo, M. Analyst 2004, 129, 1181. (26) Collier, C. P.; Saykally, R. J.; Shiang, J. J.; Henrichs, S. E.; Heath, J. R. Science 1997, 277, 1978.

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Figure 3. Cyclic voltammograms of (A) bare ITO (black) and an ITO electrode modified with Au NSs (blue). (B) ITO/Au NSs/ITO as a function of the ITO overlayer thickness: 10 nm (black), 30 nm (gray), 60 nm (blue), and 200 nm (red) in an aqueous solution of 10 mM Fe(CN)64-/0.1 M KCl, v = 50 mV s-1, A = 0.04 cm2; experimental data is represented by the solid lines, and fitted curves were produced from DigiSim (dotted lines).

Figure 4. Schematic illustration of ITO/Au NSs/ ITO (30 nm) surface functionalization using click chemistry.

with air and water (Figure 2B). This seems to be related to favorable conditions for the generation of LSPR far-field phenomena that can be fed back to the plasmonic nanostructures.5 3.4. Linking of Ethynyl Ferrocene. Figure 4 illustrates the surface-functionalization strategy employed. To form a sufficient number of surface hydroxyl groups, the ITO/Au NSs/ ITO (30 nm) LSPR platform was treated for 10 min with UV/ozone.27 After silanization with APTMS, chemical coupling of 4-azidobenzoic acid resulted in the incorporation of an azide termination on ITO/Au NSs/ITO. The click derivatization of the azide-terminated ITO/Au NSs/ITO surface with ethynyl ferrocene was performed in an ethanol/water solution in the presence of copper sulfate and ascorbic acid.28,29 Water contact angle measurements were used to examine the macroscopic evolution in the wetting properties of the ITO/Au NSs/ITO surface upon clicking ferrocene moieties. The initial ITO/Au NSs/ITO surface exhibits a water contact angle of θ = 30°. Photochemical oxidation (UV/ ozone for 10 min) of the ITO/Au NSs/ITO substrate yields a (27) Manesse, M.; Stambouli, V.; Boukherroub, R.; Szunerits, S. Analyst 2008, 133, 1097. (28) Das, M. R.; Wang, M.; Szunerits, S.; Gengembre, L.; Boukherroub, R. Chem. Commun. 2009, 2753. (29) Rostovtsev, V. V.; Green, L. G.; Fokin, V. V.; Sharpless, K. B. Angew. Chem., Int. Ed. 2002, 41, 2596.

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surface termination with a hydrophilic character. The contact angle decreased significantly to θ = 10°. Silanization with APTMS resulted in a water contact angle of θ = 40 ( 2°. Incorporation of the azide function led to an increase in the water contact angle to θ = 53 ( 2°. After clicking ferrocene groups to the azide terminal groups, the contact angle dropped again to θ = 14 ( 2°. X-ray photoelectron spectroscopy is a valuable tool for evaluating the changes in the surface chemical composition and bonding occurring during surface derivatization. Figure 5 displays the XPS survey spectra of ITO/Au NSs/ITO surfaces before and after clicking ferrocene. It shows peaks due to indium at about 17 eV (In 4d), 150 eV (In 4s), 444 eV (In 3d), 665 eV (In 3p), and 703 eV (In 3s) and small contribution due to doping with Sn at 493 eV (Sn 3d) as well as a peak at 532 eV due to O 1s. An additional band at 285 eV due to C 1s from surface contamination is observed. After silanization of the terminal hydroxyl groups with APTMS, additional peaks at ∼400 eV and ∼100 eV due to N 1s (-NH2) are observed, in agreement with the chemical composition of the molecule (Figure 5b). From the high-resolution XPS of the N 1s, the peak can be deconvoluted into two bands at 399.2 eV due to free amines (-NH2) and a smaller contribution at 400.6 due to protonated amine groups (-NH3þ) (Figure 6a).30 (30) Zhang, F.; Srinivasan, M. P. Langmuir 2004, 20, 2309.

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Figure 7. Differential pulse voltammetry of ITO/Au NSs/30 nm ITO in 10 mM ethynyl ferrocene, in 0.1 M TEAPF6 in acetonitrile (black), and in 0.1 M TEAPF6 in acetonitrile after clicking ethylene ferrocene (blue). Figure 5. (a) XPS survey spectra of ITO/Au NSs/ITO interfaces (b) after silanization with APTMS, (c) after incorporating 4-azidobenzoic acid, and (d) after clicking ethynyl ferrocene onto the azide-terminated surface.

Figure 6. High-resolution XPS spectra of the N 1s band (a) after silanization with APTMS, (b) after incorporating 4-azidobenzoic acid, and (c) after clicking ethynyl ferrocene on the azide-terminated surface.

The reaction of the terminal amine groups with azidobenzoic acid increased the overall nitrogen, carbon and oxygen contents (Figure 5c). The broad N 1s signal in the high-resolution scan was fitted and deconvoluted into three peaks: 400.4 eV (NdNdN), 401.2 eV (-HN-CdO), and 402.6 eV (NdNdN) with a ratio of 2.7:1.3:1, which is close to the expected ratio of 2:1:1 (Figure 6b). Upon click reaction of ethynyl ferrocene to the azideterminated interface, high-resolution N 1s shows a broad signal centered at 400 eV, which was fitted and deconvoluted into three peak at 398.8 eV (N-NdN), 400.1 eV (NdN), and 401.2 eV (-HN-CdO) in a ratio of 1.1:2:1, consistent with the formation of surface-confined triazole groups (Figure 6c). The clicking of the ferrocene units did not reveal the presence of Fe in the XPS spectrum (Figure 5d). The Fe 2p3/2 and Fe 2p1/2 bands occur at Langmuir 2010, 26(6), 4266–4273

711.7 and 725.6 eV, respectively. This overlaps with the bands of indium (Figure 5d). The conducting nature of the ITO overlayer together with the optical properties of ferrocene, which absorbs at ∼450 nm, allows the detection of the ferrocene units using differential pulse voltammetry (DVP) as well as UV/vis absorption spectroscopy. 3.4.1. Differential Pulse Voltammetry (DPV). DPV is a derivative of linear sweep voltammetry, where a series of regular voltage pulses are superimposed on the potential linear sweep. The faradaic current is sampled just before the potential is changed, decreasing the effect of the charging current considerably. This very sensitive electrochemical technique was employed to characterize the LSPR interface before and after the click reaction. A single peak with a redox potential of E0 ≈ 0.7 V versus Ag/AgCl is observed on the ITO/Au NSs/ITO interface when immersed in a solution of ethynyl ferrocene in acetonitrile (Figure 7). This potential is higher than that observed for the oneelectron oxidation of unsubstituted ferrocene (E0 ≈ 0.5 V vs Ag/ AgCl) and is mainly linked to the electron-rich triple bond. DPV on the ITO/Au NSs/ ITO (30 nm) interface with clicked ferrocene shows a redox peak at Ep1 ≈ 0.54 V versus Ag/AgCl. This is slightly higher than for bound ferrocene moieties reported on gold (E 0 = 0.33-0.46 V vs Ag/AgCl).31 The thickness of the ITO overlayer had no significant influence on the position of E0 and is thus only interface-dependent. An analysis of the change in peak currents as a function of scan rate allows the assessment that the ferrocene moieties are surface bound rather than absorbed. The linear dependence of the anodic peak current with the scan rate v rather than with v1/2 suggests a surface redox process. The ferrocene surface coverage Γ was estimated from the current intensity of the DVP peak (Figure 7) using eq 1 ipa ¼

ð1 - RÞnna F 2 Aν Γ 2:718RT

ð1Þ

where ipa is the anodic peak current, R is the charge-transfer coefficient (assumed to be 0.5), n is the total number of electrons, na is the number of electrons involved in the rate-determining step, F is the Faraday constant, and v is the scan rate. Assuming n = na = 1 electron, a surface coverage of Γ = (5.16 ( 0.9)  1014 molecules cm-2 is obtained. This compares well to the surface (31) Devaraj, N. K.; Miller, G. P.; Ebina, W.; Kakaradov, B.; Collman, J. P.; Kool, E. T.; Chidsey, C. E. D. J. Am. Chem. Soc. 2005, 127, 8600.

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Figure 9. UV-vis transmission spectra of the ITO/Au NSs/ ITO interface (30 nm) before (black) and after clicking ethynyl ferrocene (blue) in acetonitrile.

sample is diminished by the amount of absorbing material in its path (e.g., the concentration of the analyte cferrocene, the optical path length d, and the probability that the photon of that particular wavelength will be absorbed by the material, i.e., the extinction coefficient ε). A ¼ εdcferrocene

Figure 8. (A) Absorption spectra of ethynyl ferrocene solutions in acetonitrile at different concentrations (0 (;), 6 (gray line), 12 (pink line), 25 (blue line), 50 (green line), and 100 (red line ) μM) (B) and in the presence of the ITO/Au NSs/ITO (30 nm) interface. (C) Calibration curves without (b), in the presence of ITO (blue dots), and in the presence of the ITO/AuNSs/ITO (30 nm) interface (pink dots).

coverage reported on BDD using the same grafting step of Γ = (3.46 ( 0.5)  1014 molecules cm-2 28 and is higher than the reported Γ = 0.78  1014 molecules per cm-2 on gold using a click chemistry approach.32 3.4.2. UV-Vis Absorption Spectroscopy. Acetonitrile solution of ferrocene absorbs at λ = 450 nm (Figure 8A). According to the Beer-Lambert law (eq 2), the amount of light emerging from a (32) Collman, J. P.; Devaraj, N. K.; Chidsey, C. E. D. Langmuir 2004, 20, 1051.

4272 DOI: 10.1021/la903330d

ð2Þ

Linear behavior between the absorption intensity at λmax and the ferrocene concentration in solution is observed with a limit of detection (LOD) of 4 ( 0.1 μM (Figure 8C). The same correlation is observed when the absorption of a ferrocene solution was determined in the presence of an ITO/Au NSs/ITO interface. Although the plasmonic band at λLSPR ≈ 650 nm was not affected by the presence of different concentrations of ferrocene (Figure 8B), the absorption band at ∼450 nm due to the absorption of ferrocene molecules increased linearly with increasing ferrocene concentration (Figure 8C). The baseline of the absorption spectrum of the ITO/Au NSs/ITO interface immersed in acetonitrile solution was used (Figure 8B, 0 μM concentration of ferrocene). To rule out any adsorption effects of ferrocene on the ITO interface, which will influence the calibration curve of ITO/ Au NSs/ITO, we looked at the correlation between the increase in the absorption band at ∼450 nm due to the increase in ferrocene concentration. Figure 8C shows that a linear correlation is observed with a sensitivity comparable to the case where no ITO interface was present. This is an indication that no real adsorption phenomena are taking place. In addition, taking the interface out of the most concentrated ferrocene solution, washing it with water, and recording the UV/vis spectrum results in the overlayer of the initially recorded UV/vis spectrum. These results show that no absorption effects are observed. The UV/vis spectra of ITO/Au NPS/ ITO (30 nm) interfaces before and after clicking ferrocene units are finally shown in Figure 9. Subtracting the absorption signal of the ITO/Au NSs/ ITO interface from the modified one leads to an increase in the absorption band by 0.0009 unit. Using the Beer-Lambert law with ε = 96.5 cm-1 M-1 and d = 1.83 cm and an area of illumination of 1.25  10-4 cm-1, a surface concentration of Γ = (3.84 ( 2.45)  1014 molecules cm-2 is obtained. This is in accordance with the values obtained by DPV.

4. Conclusions The preparation of hybrid plasmonic interfaces consisting of ITO/Au NSs/ITO has been demonstrated. The obtained Langmuir 2010, 26(6), 4266–4273

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interfaces showed optical signals for thin as well as for thick ITO overcoatings. This is thus another example of LSPR interfaces offering the possibility of short- and long-range sensing. The different interfaces show sensitivity when in contact with media of different refractive indexes. ITO/Au NSs/ITO interfaces with overlayer thicknesses of 30 and 180 nm are more sensitive to refractive index changes than are other interfaces. These interfaces also exhibited good electrical conductivity for interfaces with ITO overlayers with a minimum thickness of 60 nm. The best compromise in terms of good electrochemical characteristics and acceptable optical properties would be an ITO/Au NSs/ITO (120 nm) interface, allowing such an interface to be used in the future in electrochemical-based LSPR experiments. Here the focus was additionally on the possibility of using the presence of the ITO hydroxyl groups for further chemical modification. An easy way to incorporate azide functions on the ITO/Au NSs/ITO interface was developed. This interface

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allows the linking of functional groups bearing triple bonds using the Cu(I)-catalyzed click chemistry approach. The covalent linking of ferrocene is used as a proof of concept. UV/vis transmission spectroscopy was used with differential pulse voltammetry to calculate the final surface coverage of ferrocene units. Acknowledgment. The Agence Nationale de la Recherche (ANR JCJC 2006), the Centre National de la Recherche Scientifique (CNRS), and the Nord-Pas-de Calais region are gratefully acknowledged for financial support. We thank Dr. Martin J€onsson-Niedziozka for recording the SEM images. Supporting Information Available: Comparison of the variation in λLSPR determined in water on ITO/AuNSs/ ITO (black) and glass/Au NSs/SiOx (red) interfaces with the increase in the dielectric overlayer. This material is available free of charge via the Internet at http://pubs.acs.org.

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