Silica Thin Film Interface: Electrochemical and

Abstract. Abstract Image. This article reports chemical stability studies of a gold film electrode coated with thin silicon oxide (SiOx) layers using ...
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Langmuir 2006, 22, 10716-10722

Stability of the Gold/Silica Thin Film Interface: Electrochemical and Surface Plasmon Resonance Studies† Sabine Szunerits,*,‡ Yannick Coffinier,§,| Se´bastien Janel,§,| and Rabah Boukherroub*,§,| Laboratoire d’Electrochimie et de Physicochimie des Mate´ riaux et des Interfaces (LEPMI), CNRS-INPG-UJF, 1130 rue de la piscine, BP 75, 38402 St. Martin d’He` res Cedex, France, Institut de Recherche Interdisciplinaire (IRI), and Institut d’Electronique, de Microe´ lectronique et de Nanotechnologie (IEMN), Cite´ Scientifique, AVenue Poincare´ - BP. 60069, 59652 VilleneuVe d’Ascq, France ReceiVed March 24, 2006. In Final Form: June 2, 2006

This article reports chemical stability studies of a gold film electrode coated with thin silicon oxide (SiOx) layers using electrochemical, surface plasmon resonance (SPR) and atomic force microscopy (AFM) techniques. Silica films with different thicknesses (d ) 6.4, 9.7, 14.5, and 18.5 nm) were deposited using a plasma-enhanced chemical vapor deposition technique (PECVD). For SiOx films with d g 18.5 nm, the electrochemical behavior is characteristic of a highly efficient barrier for a redox probe. SiOx films with thicknesses between 9.5 and 14.5 nm were found to be less efficient barriers for electron transfer. The Au/SiOx interface with 6.4 nm of SiOx, however, showed an enhanced steady-state current compared to that of the other films. The stability of this interface in solutions of different pH was investigated. Whereas a strongly basic solution led to a continuous dissolution of the SiOx interface, acidic treatment produced a more reticulated SiOx film and improved electrochemical behavior. The electrochemical results were corroborated by SPR measurements in real time and AFM studies.

1. Introduction The use of analytical sensing based on electrochemical techniques next to the principle of surface plasmon resonance has found widespread applications in different research domains. The SPR sensing technique is based upon the utilization of a noble metal (typically gold or silver) to generate a surface plasmon electromagnetic field, which is used to probe changes in the optical properties upon binding or adsorption reactions occurring in the vicinity of the surface. In electrochemical SPR (E-SPR), the gold film deposited on glass and coupled to a prism is used at the same time as the surface plasmon resonance medium and as the working electrode. To date, there have been several reports in the literature devoted to the study of electrochemical processes using SPR including studies of the electrochemical double layer,1,2 the investigation of the electrochemical doping/dedoping process,3-5 the detection of trace metals,6 studies of the diffusion and adsorption reactions,7 and electrical field enhanced studies.5,8-11 The functionalization of the metallic interface with †

Part of the Electrochemistry special issue. * To whom correspondence should be addressed. E-mail: [email protected]; [email protected]. ‡ CNRS-INPG-UJF. § Institut de Recherche Interdisciplinaire (IRI). | Institut d’Electronique, de Microe ´ lectronique et de Nanotechnologie (IEMN). (1) Chao, F.; Costa, M.; Tadjeddine, A. J. Electroanal. Chem. 1992, 329, 313. (2) Gordon, J. G.; Ernst, S. Surf. Sci. 1980, 101, 499. (3) Baba, A.; Park, M.-K.; Advincula, R. C.; Knoll, W. Langmuir 2002, 18, 4648. (4) Georgiadis, R.; Peterlinz, K. A.; Rahn, J. R.; Peterson, A. W.; Grassi, J. H. Langmuir 2000, 16, 6759. (5) Kang, X.; Cheng, G.; Dong, S. Electrochem. Commun. 2001, 3, 489. (6) Chinowsky, T. M.; Saban, S. B.; Yee, S. S. Sens. Actuators, B 1996, 35-36, 37. (7) Iwasaki, Y.; Horiuchi, T.; Morita, M.; Niwa, O. Surf. Sci. 1999, 427-428, 195. (8) Hanken, D. G.; Corn, R. M. Anal. Chem. 1997, 69, 3665. (9) Heaton, R. J.; Peterson, A. W.; Georgiadis, R. Proc. Natl. Acad. Sci. 2001, 98, 3701. (10) Iwasaki, Y.; Horiuchi, T.; Niwa, O. Anal. Chem. 2001, 73, 1595.

organic monolayers makes SPR an important instrumentation for biological and medical applications.11-19 Among the wide range of materials used for electrode modification, silica-based materials have attracted considerable attention for chemical and biological sensing because of the high reactivity of the surface silanol groups, enabling the immobilization of different molecules through silane coupling chemistry.20-23 Silica films can be deposited on gold electrodes by the thermal evaporation of silicon dioxide in the presence of oxygen at low pressure24 by using a multitarget magnetron sputtering system25 or plasma-impulsed chemical vapor deposition.26 Low-temperature sol-gel chemistry has been used since the mid 1980s for the fabrication of modified gold and silver interfaces.21-24,27-31 (11) Peterson, A. W.; Wolf, L. K.; Georgiadis, R. J. Am. Chem. Soc. 2002, 124, 14601. (12) Schlecht, U.; Nomura, Y.; Bachmann, T.; Karube, I. Bioconjugate Chem. 2002, 13, 188. (13) Thiel, A. J.; Fructos, A. G.; Jordan, C. E.; Corn, R. M.; Smith, L. M. Anal. Chem. 1997, 69, 4948. (14) Georgiadis, R.; Peterlinz, K. A.; Peterson, A. W. J. Am. Chem. Soc. 2000, 122, 3166. (15) Peterlinz, K. A.; Georgiadis, R. M.; Herne, T. M.; Tarlov, M. J. J. Am. Chem. Soc. 1997, 119, 3401. (16) Smith, L. M.; Wanat, M. J.; Cheng, Y.; Barreira, S. V. P.; Fructos, A. G.; Corn, R. M. Langmuir 2001, 17, 2502. (17) Peterson, A. W.; Heaton, R. J.; Georgiadis, R. Nucleic Acids Res. 2001, 29, 5163. (18) Nelson, B. P.; Grimsrud, T. E.; Liles, M. R.; Goodman, R. M.; Corn, R. M. Anal. Chem. 2001, 73, 1. (19) Veiseh, M.; Zareie, M. H.; Zhang, M. Langmuir 2002, 18, 6671. (20) Tsionsky, M.; Lev, O. Anal. Chem. 1995, 67, 2409. (21) Walcarius, A. Chem. Mater. 2001, 13, 3351. (22) Lev, O.; Wu, Z.; Bharathi, S.; Glezer, V.; Modestov, A.; Gun, J.; Rabinovich, L.; Sampath, S. Chem. Mater. 1997, 13, 3351. (23) Etienne, M.; Walcarius, A. Electrochem. Commun. 2005, 7, 1449. (24) Kambhampati, D. K.; Jakob, T. A. M.; Robertson, J. W.; Cai, M.; Pemberton, J. E.; Knoll, W. Langmuir 2001, 17, 1169. (25) Liao, H. B.; Wen, W.; Wong, G. K. L. J. Appl. Phys 2003, 93, 4485. (26) Pfuch, A.; Heft, A.; Weidl, R.; Lang, K. Surf. Coat. Technol., in press. (27) Opallo, M.; Kukulka, J. Electrochem. Commun. 2000, 2, 394. (28) Sayen, S.; Walcarius, A. Electrochem. Commun. 2003, 5, 341. (29) Walcarius, A.; Mandler, D.; Cox, J.; Collinson, M. M.; Lev, O. J. Mater. Sci. 2005, 15, 3663.

10.1021/la060793o CCC: $33.50 © 2006 American Chemical Society Published on Web 07/13/2006

Stability of the Gold/Silica Thin Film Interface

There are several drawbacks associated with the deposition techniques cited above. For example, thermal evaporation leads to unstable SiO2/gold interfaces upon immersion in water, which limits its use for analytical applications.24 By depositing 2 nm of titanium between gold and the silicon dioxide layer, Grane´li and Kasemo reported the formation of stable films.32,33 However, for electrochemical detection, this adhesion layer adds a further unwanted kinetic barrier. The sol-gel technique was successfully used to prepare stable hydrophobic silicate films using mercaptosilanes as molecular adhesives between metal and the oxide layer.23,31,34 The thickness of the deposited silica films is between 3 and 250 nm.24,30,31 The thickness of these layers can be varied by changing either the concentration of the tetramethoxysilane solution used and spin-coated on the hydrolyzed 3-(mercaptopropyl)trimethoxysilane on gold or the rotation speed used during spin coating.24 However, even though relatively flat surfaces are formed, a random distribution of 200-600 nm hole diameter is reported.31 Recently, we have found that thin layers of SiOx can be deposited on gold films (with no intervening adhesion layer) using plamsa-enhanced chemical vapor deposition (PECVD).35 The technique is based on the decomposition of a mixture of silane gas (SiH4) and nitrous oxide (N2O) near the substrate surface, enhanced by the use of a vapor containing electrically charged particles or plasma at 300 °C. The thickness of the silica layer was controlled by the reaction time and ranged from 7 to 40 nm while the refractive index was regulated by the stoichiometry of the film. We showed by SPR that the intensity of the reflected light detected on these silica films on gold decreased when the silica film layer increased and that the incident angle was shifted to higher angles. More interestingly, the silica films exhibited very good stability in both organic and aqueous solutions as well as in piranha solution at 80 °C. The piranha treatment is often used to generate surface silanol groups required for silane coupling chemistry. We have found that this treatment did not induce any thickness or SPR response changes and produced a considerable number of Si-OH groups that were successfully coupled to perfluorodecyltrichlorosilane.35 Furthermore, the electrochemical behavior of a thin gold film electrode coated with a 9.7-nm-thick silicon oxide layer was recently investigated, and the SiOx layer was found to be efficient for the protection of the gold electrode from anodic dissolution.36 In this article, we report on the electrochemical and SPR characteristics of gold/SiOx interfaces with different silica film thicknesses d (d ) 6.4, 9.7, 14.5, and 18.5 nm). Furthermore, the stability of a Au/SiOx composite with a 6.4-nm-thick silicon oxide layer in strongly acidic, neutral, and highly basic solutions has been studied using E-SPR and AFM. An important aspect regarding the analytical applications of silica-modified thin gold films is related to the possibility of durable functionalization by organic groups accessible to the target analytes from the solution. The 6.4-nm-thick silicon oxide layer was treated with two different organosilanes (3-aminopropyltrimethoxysilane and octadecyltrichlorosilane), and the changes in SPR and electrochemical behavior using cyclic voltammetry were studied. (30) Bharathi, S.; Nogami, M.; Ikeda, S. Langmuir 2001, 17, 1. (31) Niedziolka, J.; Palys, B.; Nowakowski, R.; Opallo, M. J. Electroanal. Chem. 2005, 578, 239. (32) Garne´li, A.; Rydstrom, J.; Kasemo, B.; Hook, F. Biosens. Bioelectron. 2004, 20, 498. (33) Glasmastar, K.; Larsson, C.; Hook, F.; Kasemo, B. J. Colloid Interface Sci. 2002, 246, 40. (34) Goss, C. A.; Charych, D. H.; Majda, M. Anal. Chem. 1991, 63, 85. (35) Szunerits, S.; Boukherroub, R. Langmuir 2006, 22, 1660. (36) Szunerits, S.; Boukherroub, R. Electrochem. Commun. 2006, 8, 439.

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2. Experimental Section 2.1. Materials. Potassium chloride (KCl), potassium hexacyanoferrocyanide (Fe(CN)64-), sodium hydroxide, sulfuric acid, hydrogen peroxide, hexane, chloroform, carbon tetrachloride, ethanol, and acetone were obtained from Aldrich and used without further purification. 3-Aminopropyltrimethoxysilane (APTES) and octadecyltrichlorosilane (OTS) were purchased from Gelest Inc. (France). 2.2. Preparation of the Gold/SiOx Composite Slides. Substrate electrodes were prepared by the vacuum deposition of 5 nm of titanium and 50 nm of gold onto cleaned glass slides (76 × 26 × 1 mm3, n ) 1.58 at λ ) 633 nm CML, France). Prior to silica film deposition, the gold samples were first degreased in 2-propanol and acetone in an ultrasound bath at room temperature, rinsed copiously with Milli-Q water, and dried under a stream of nitrogen. The gold slides were then heated in a plasma chamber at 300 °C at a pressure of 0.005 Torr for 1 h. SiOx layers were synthesized by plasmaenhanced chemical vapor deposition in a Plasmalab 800Plus (Oxford Instruments, U.K.). The growth conditions used were as follows: substrate temperature, 300 °C; gas mixture, SiH4 (3% in N2) and N2O (the gas flows were 260 and 700 sccm for SiH4 and N2O, respectively); total pressure in the reactor, 1 Torr; and power, 10 W at 13.56 MHz. Under these experimental conditions, the deposition rate was 414 Å min-1, and the silica films display a refractive index of 1.48. We have adjusted the silica films’ thicknesses by varying the deposition time. For electrochemical measurements, only half of the gold-coated interface was coated with silicon dioxide to ensure electrical contact with the gold interface. 2.3. Silanization of the Gold/SiOx Interface. Au/SiOx interfaces with a 6.4 nm silica layer were first cleaned by UV/ozone to remove any organic contaminants on the surface and then reacted with a 10-2-10-3 M solution of octadecyltrichlorosilane (OTS) in hexane/ CCl4 (v/v: 70/30) for 2 h at room temperature. The resulting surfaces were rinsed with CHCl3 and dried under a stream of nitrogen. Amine-terminated Au/SiOx surfaces were prepared by chemical treatment of the clean surface with 3% 3-aminopropyltrimethoxysilane (APETS) in methanol/water (95/5 v/v) 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.37 For E-SPR measurements, only half of the gold-coated interface was coated with silica to ensure electrical contact with the gold interface. A UV-ozone treatment of the gold part (by masking the derivatized Au/SiOx part) was used after chemical reaction with organosilanes to remove any physisorbed organic molecules, which can hinder the electrical contact. The stability of the gold samples coated with 6.4 nm silica layers was examined in 0.1 M H2SO4 (pH 1) and 0.1 M NaOH (pH 14) aqueous solutions as well as phosphate buffer solutions (pH 4 and 10, Radiometer, France). A solution of pH 13 was prepared by adding sulfuric acid to a 0.1 M NaOH solution, and the pH was determined using a pH meter (PHN330T, Tacussel). The samples were immersed in the solution for 2 h at room temperature, rinsed copiously with Milli-Q water, and then dried under a stream of nitrogen. 2.4. Instrumentation. 2.4.1. Electrochemical SPR. Electrochemical experiments were performed using an Autolab potentiostat 30 (Eco Chemie, Utrecht, The Netherlands). The electrode cell is not a conventional three-electrode cell but rather the double-channel cell of Autolab ESPRIT Instruments (Eco Chimie, Utrecht, The Netherlands), allowing simultaneous surface plasmon resonance and electrochemical measurements to be performed. The configuration of this equipment is described elsewhere.38,39 In short, polarized laser light (λ ) 670 nm) is directed to the bottom side of the sensor disk via a hemispheric lens placed on a prism (N-BAF-3 having a refractive index of n ) 1.58), and the reflected light is detected using (37) Duburcq, X.; Olivier, C.; Desmet, R.; Halasa, M.; Carion, O.; Grandidier, B.; Heim, T.; Stie´venard, D.; Auriault, C.; Melnyk, O. Bioconjugate Chem. 2004, 15, 317. (38) Wink, T.; Van Zuilen, S. J.; Bult, A.; van Bennekom, W. P. Chem. Commun. 1998, 70, 827. (39) Kooyman, R. P. H.; Lenferink, A. T. M.; Eenik, R. G.; Greve, J. Anal. Chem. 1991, 63, 83.

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Figure 1. Reflectivity versus angle of incidence curves for a 50nm-thick gold film on glass with a 5 nm titanium adhesion layer without (black) and with different thicknesses of SiOx: 6.4 nm (blue), 9.7 nm (green), 14.5 nm (grey), and 18.5 nm (red). Experimental parameters, dotted lines; simulated SPR curves, solid lines. Fitting parameters: n(prism) ) 1.58 and n(gold) ) 0.282 + i3.45 with d ) 50 nm, n(titanium) ) 2.36 + i3.112 with d ) 5 nm, and n(SiOx) ) 1.48 a photodiode. The angle of incidence is varied using a vibrating mirror with a frequency of 44 Hz. SPR curves were scanned on the forward and backward movements of the mirror, and the minima in reflectance was determined and averaged. The instrument is equipped with an electrochemical open cuvette system of 20-150 µL sample volume where an Ag/AgCl reference electrode, a platinum counter electrode, and a fixed contact point to the gold layer of the sensor chip are incorporated. The active electrode surface is 0.068 cm2. 2.4.2. AFM Measurements. The samples were imaged with a Dimension 3100 model AFM (Veeco, Santa Barbara, CA) 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 resonant frequencies of ∼250 kHz were used. All AFM images were acquired in tapping mode at a constant force of 5-50 pN. 2.4.3. Ellipsometry. Spectroscopic ellipsometry data in the visible range was obtained using a UVISEL Jobin Yvon Horiba spectroscopic ellipsometer equipped with DeltaPsi 2 data analysis software. The system acquired a spectrum ranging from 2 to 4.5 eV (corresponding to 300 to 750 nm) with 0.05 eV (or 7.5 nm) intervals. Data were taken using an angle of incidence of 70°, and the compensator was set at 45.0°. Data were fitted by regression analysis to a film-onsubstrate model as described by their thickness and their complex refractive indices. The values given are averaged over five measurements taken on different spots on the surface. 2.4.4. Contact Angle Measurements. Water contact angles were measured using deionized water. We used a remote-computercontrolled goniometer system (DIGIDROP by GBX, France) to measure the contact angles. The accuracy is (2°. All measurements were made in the ambient atmosphere at room temperature.

3. Results and Discussion 3.1. SPR Characteristics of Gold/SiOx Films. Au/SiOx composites with different silica layer thicknesses were prepared using the PECVD technique. The process is based on the chemical decomposition of a gas mixture of SiH4 and N2O in a plasma reaction at 300 °C. Under these experimental conditions, the deposited film thickness was controlled by the reaction time (deposition rate was 414 Å min-1). To form different SiOx thicknesses, deposition times varied from 9 to 27 s. The thickness of the silica layer in the resulting Au/SiOx composites was evaluated using ellipsometry, and mean values of 6.4 ( 0.5, 9.7 ( 0.5, 14.5 ( 0.5, and 18.5 ( 0.5 nm were obtained. Surface plasmon resonance was used further to investigate the optical properties of the film composites. Figure 1 shows the reflectivity

Szunerits et al.

versus angle spectra for a 50 nm thin gold film (black line) deposited on a glass slide with a 5 nm titanium adhesion layer. The angle where the surface plasmon minimum occurs shifts from Θ ) 66.03° (0 nm) to Θ ) 67.07° (6.4 nm), Θ ) 67.66° (9.7 nm), Θ ) 68.39° (14.5 nm), and Θ ) 69.01° (18.5 nm) by using water as the dielectric medium. Besides a shift in the resonance angle, the intensity of the resonance minimum is increasing with increasing SiOx film thickness: 0.052 (0 nm), 0.055 (6.4 nm), 0.055 (9.7 nm), 0.058 (14.5 nm), and 0.058 (18.5 nm). Gold-silicon oxide interfaces with a SiOx thickness greater than 60 nm did not show any SPR minimum on the glass prism used (n ) 1.58) using the Autolab-ESPRIT SPR instrument because the detectable angles were outside the detection limit.35 As discussed later, for electrochemical sensing purposes silicon dioxide films of d g 18.5 are of no particular interest because the surface shows a high blocking effect for electron transfer. 3.2. Measurement of the Kinetic Hindrance Using Cyclic Voltammetry. Cyclic voltammetry (CV) of an electroactive redox couple is a valuable tool for testing the kinetic barrier properties of surface-modified electrodes. The electron transfer between the solution species (in our case Fe(CN)64-/3-) and the gold electrode may occur by tunneling either through the SiOx barrier or through the nanoscopic defects on the barrier layer. Parts A and B of Figure 2 compare the voltammetric responses obtained prior to the SiOx film deposition and after the PECVD deposition of SiOx layers with different thicknesses. As expected, Fe(CN)64is oxidized on the bare thin-film electrode in a one-electron process. The higher peak separation of ∆Ep ) 130 mV compared to that of a massive gold electrode (∆Ep ) 60 mV) is due to the higher resistivity of the thin gold film. The four interfaces investigated can be divided into three categories, a phenomenon that is typical for chemically modified electrical interfaces.23,31,40,41 For SiOx films with thicknesses greater than 18.5 nm (d g18.5 nm) (Figure 2C), the behavior is characteristic of highly efficient barriers for the redox probe. The voltammetric signal and the current detected are largely suppressed (iss ) 0.6 µA). This blocking effect was maintained by leaving the interface for more then 4 h in the aqueous solution containing the redox mediator, in agreement with the existence of a highly dense oxide film, preventing the degradation of the hydrophilic SiOx thin layer with time. For SiOx films with thicknesses between 9.5 and 18.5 nm (9.5 nm < d < 18.5 nm), the anodic current is considerably decreased (iss ) 3-1.8 µA). The presence of the SiOx layer limits the diffusion of Fe(CN)64- species to the modified gold electrode interface but does not completely hinder the electron transfer. In both cases, sigmoidal waves instead of peak-shaped signals as in the case of pure gold surface are recorded. This suggests the predominance of radial diffusion instead of the classical linear mode of mass transfer at millimeter-sized electrodes.42,43 The interface with 6.4 nm of SiOx, however, shows an enhanced steady-state current (iss ) 45 µA) compared to that of the thicker films, but the voltammetric signal is less intense (by approximately 2.5 times) than those recorded on bare electrodes. This behavior is characteristic of barriers with low packing density and coverage and a high concentration of defect sites. The shape of the i-E signal remains sigmoidal, suggesting that the interface can be considered to be an assembly of ultramicroelectrodes, which are the defect sites. This effect was (40) Cannes, C.; Kanoufi, F.; Bard, A. J. Langmuir 2002, 18, 8134. (41) Mokrani, C.; Fatisson, J.; Gue´rente, L.; Labbe´, P. Langmuir 2005, 21, 4400. (42) Amatore, C.; Saveant, J. M.; Tessier, D. J. Electroanal. Chem. 1983, 147, 39. (43) Amatore, C. Electrochemistry at Ultramicroelectodes. In Physical Electrochemistry: Principles, Methods, and Applications; Rubinstein, I., Ed.; New York, 1995; Chapter 4

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Figure 2. Cyclic voltammetry curves for (A) a 50-nm-thick polycrystalline gold film on glass with a 5 nm titanium adhesion layer (dotted line) and the same electrode covered with 6.4 nm SiOx (dashed line). (B) Gold film with 9.7 nm SiOx (solid line), 14.5 nm SiOx (point-dashed line), 18.5 nm SiOx (point-point-dash-dash line). (C) Change in current as a function of SiOx thickness at E ) 0.5 V/Ag/AgCl; solution, 10 mM Fe(CN)64- in KCl (0.1 M/water); scan rate, 0.05 V s-1; A ) 0.068 cm2.

maintained as in the case of the 18.5-nm-thick SiOx layer, and no degradation of the layer was observed with time. 3.3. Stability of Gold/SiOx Interfaces at Acidic and Basic Conditions. For potential electrochemical applications and SPR sensing, the modified electrodes have to withstand severe chemical treatments in some cases. One example is the generation of surface silanol groups required for silane coupling chemistry using piranha treatment. We have already demonstrated that gold/silica interfaces exhibited very good stability in both organic and aqueous solutions as well as in piranha solution.35 Here, the influence of different pH solutions on the stability and morphology of the silica film was investigated. Indeed, the reaction of bases with silica has been investigated in the past and is well documented. 44,45 SPR, electrochemistry, and AFM techniques were used to determine the stability of bare gold covered with a 6.4 nm silica layer in different pH solutions. Figure 3 shows the change in the minimum resonance angle over 2 h of immersion of the Au/SiOx sample with a 6.4-nmthick silica layer in aqueous solutions of different pH. As can be seen, the resonance minimum angle stays constant in the pH range between 1 and 10 over a 2 h period, which implies that the optical thickness of the layer is constant and the film is stable. When the sample was immersed in solutions of pH 13 and 14, a net decrease in the resonance angle was observed. This variation can be attributed to the change in the SiOx film thickness. While at pH 13, the decrease in the resonance angle takes place only for the first 40 min and stays constant after that, at pH 14 this stabilization is observed only after 100 minutes. These results indicate that the SiOx film is not peeling off of the surface but is rather slowly and continuously dissolving with time. (44) Despas, C.; Walcarius, A.; Bessie`re, J. Langmuir 1999, 15, 3186. (45) Tripp, C. P.; Kazmaier, P.; Hair, M. L. Langmuir 1996, 12, 6407.

Figure 3. Comparison of the minimum resonance angle of the 6.4 nm SiOx film deposited on gold for solutions of pH (A) 1, (B) 4, (C) 10, (D) 13, and (E) 14. The gold/SiOx samples were immersed in the solution for 2 h at room temperature.

The gold/SiOx interfaces before (dotted line in Figure 4) and after immersion (dashed line in Figure 4) in different aqueous solutions of different pH were also investigated by cyclic voltammetry using Fe(CN)64-/3- as the redox couple. The electrochemical response was compared to that of an unmodified thin gold film (solid line in Figure 4). The electrochemical behavior of the gold/SiOx (6.4 nm) is not altered under the influence of solutions between pH 4 and 10 (Figure 4A). The presence of the silica film limits the diffusion of Fe(CN)64- to the modified gold electrode as discussed before. However, acidic (Figure 4B) and basic treatments (Figure 4C) result in different cyclic voltammograms. In the case of pH 14, the mass transfer recorded on the surface before and after immersion in 0.1 M NaOH for 2 h changed from a sigmoidal to a peak-shaped wave with an increase in the anodic current and the observation of a cathodic wave. By comparing the i-E curve with that of a pure gold thin electrode, the characteristics of an unmodified electrode

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Figure 4. Study on the pH influence on the stability of a 6.4-nm-thick SiOx layer (dotted lines) on a 50 nm thin gold film on glass (full lines) with a 5 nm titanium adhesion layer using cyclic voltammetry. (A) No acid/base treatment (dotted line), after 2 h at pH 4 (dashed line) and pH 10 (solid line in inset); (B) after 2 h at pH 1 (0.1 M H2SO4) (dashed line); (C) after 2 h at pH 14 (0.1 M NaOH) (dashed line).

Figure 5. AFM images of a gold/SiOx layer before (A) and after immersion in aqueous solutions at pH 1, 2 h (B) and pH 14, 2 h (C)

are approached. The defects sites are so dense that the interface can no longer be considered to be an assembly of separated microelectrodes but rather a microelectrode array where the diffusion layers developed at each microelectrode are overlapping. Macroelectrode behavior is the result, where the surface is given by the size of the defects, which are the microelectrodes, and the inter-electrode distance between these sites.46 Acidic conditions change the voltammogram in a very different manner (Figure 4B). The i-E curve is peak-shaped, suggesting a change from radial diffusion to linear diffusion, whereas the recorded oxidative peak current is unchanged. This behavior, together with the SPR results, suggests clearly that the morphology of the SiOx interface has changed. It seems that the acid treatment has catalyzed the Si-O-Si formation and thus induced film reticulation (Scheme 1). This reticulation closes the nanometric defect sites in the film, and a homogeneous, completely “crack-free” interface is obtained. This assumption was made on the basis of the silica film composition obtained by the PECVD technique. We do (46) Szunerits, S.; Tam, J.; Thouin, L.; Amatore, C.; Walt, D. R. Anal. Chem. 2003, 75, 4382.

Scheme 1. Reticulation of SiOx under Acidic Conditions

believe that the surface of the film is rich in Si-OH and probably Si-H bonds, which are easily transformed to Si-O-Si bonds under acidic catalysis. The electrochemical and SPR results were corroborated by AFM measurements. Figure 5A displays the AFM image of an as-deposited gold film covered with 6.4 nm SiOx. The polycrystalline nature of the underlying gold film is preserved after SiOx deposition, with a film roughness of 1.44 nm. Immersion of the Au/SiOx interface in an acidic medium (pH 1) for 2 h at room temperature did not alter the morphology or the roughness of the silica film significantly (rms ) 1.62 nm after acid treatment) (Figure 5B). However, the AFM image of the Au/SiOx substrate subjected to basic treatment (2 h immersion in 0.1 M NaOH, pH 14) shows a pitted surface with dissolution areas of between 500

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Figure 6. Reflectivity versus angle of incidence curves for a thin gold film (50 nm) deposited on glass with a 5 nm titanium adhesion layer (A) and a 50 nm gold film covered with 6.4 nm SiOx and modified with 3-aminopropyltrimethoxysilane (B, black line) or octadecyltrichlorosilane (C, red line), Experimental parameters, dotted lines; simulated SPR curves, solid lines. Table 1. Contact Angles surface

contact angle/deg

unmodified gold thin film gold/SiOx (6.5 nm) gold/SiOx/APTES gold/SiOx/OTS

78