Article pubs.acs.org/Langmuir
Photochemical Grafting and Patterning of Organic Monolayers on Indium Tin Oxide Substrates Yan Li† and Han Zuilhof*,†,‡ †
Laboratory of Organic Chemistry, Wageningen University, Dreijenplein 8, 6703 HB Wageningen, The Netherlands Department of Chemical and Materials Engineering, King Abdulaziz University, Jeddah, Saudi Arabia
‡
S Supporting Information *
ABSTRACT: Covalently attached organic layers on indium tin oxide (ITO) surfaces were prepared by the photochemical grafting with 1-alkenes. The surface modification was monitored with static water contact angle, X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM) measurements. Hydrophobic methyl-terminated ITO surfaces can be obtained via the grafting of tetradec-1-ene, whereas the attachment of ω-functionalized 1-alkenes leads to functionalized ITO surfaces. The use of a CCGe(CH3)3 terminus allows for facile tagging of the surface with an azido group via a one-pot deprotection/click reaction, resulting in bio/electronically active interfaces. The combination of nonaggressive chemicals (alkenes), mild reaction conditions (room temperature), and a light-induced grafting that facilitates the direct patterning of organic layers makes this simple approach highly promising for the development of ITO-based (bio)electronic devices.
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INTRODUCTION Indium tin oxide (ITO) is a mixture of In2O3 and SnO2 and was initially developed as a conductive transparent electrode for materials science. Due to its favorable combination of conductive and optical properties, ITO substrates are becoming increasingly important in many areas of research. Examples include the development of organic light-emitting diode (OLED),1,2 solar cells,3,4 biosensors,5−7 and memory devices.8 The wide application of ITO has driven the interest to combine its remarkable electronic and optical properties with the well-defined functionalities of organic layers at the interface. One important application of this strategy is to decrease the charge injection barrier and to increase the interface compatibility between the ITO electrode and the deposited organic semiconductors, which results in marked improvement for the performance of molecular solar cell9 and OLED devices.10 In the past few years, many approaches to form organic layers on ITO substrates have been envisaged.2,8,11−17 For instance, the chemisorption of alkanethiols and carboxylic acids has been investigated, but the interactions between the resulting organic layers and the ITO substrate may be too weak for many practical applications.17,18 Although improvements in the layer stability have been achieved by using phosphonic acid derivatives,13,19−21 extensive heating is required to form nonlabile substrate−surface bonds, while it can also be © 2012 American Chemical Society
synthetically challenging to incorporate the phosphate group to many functional molecules.22 Silane chemistry is another approach that is widely used for the modification of ITO surface. However, the attachment of silane derivatives is sensitive to moisture both prior to and during anhydrous adsorption to the surface. Moreover, the formation of Si−O bonds is not only restricted to the surface hydroxyl groups but also between adjacent silanols,23 yielding multilayer formation and cluster deposition, which is particularly problematic for the coating on miniature devices.24,25 To achieve optimal functionality and stability of the interface, new methods to assemble and pattern organic layers on ITO are thus needed to advance these fields such that various functional molecules and/or nanomaterials can be readily immobilized on the surfaces. Inspired by the successful photografting of 1-alkenes on wetchemically etched silicon carbide,26 which is an −OH terminated surface,27 we applied this reaction to the UVinduced functionalization of glass surfaces,28 leading to the local functionalization of enclosed glass surfaces (inside wall of microchannels) with oligo-DNA 29 and even complete enzymatic cascades.30 This approach has also been widely Received: December 17, 2011 Revised: February 10, 2012 Published: February 10, 2012 5350
dx.doi.org/10.1021/la204980f | Langmuir 2012, 28, 5350−5359
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Article
high-resolution spectra were obtained under UHV conditions using monochromatic Al Kα X-ray radiation at 12 kV and 20 mA, using an analyzer pass energy of 50 eV for wide scan and 10 eV for narrow scan. The emitted electrons were collected at 30° from the surface normal. High-resolution spectra were corrected with a linear background before fitting. Atomic area ratios were determined after a baseline correction and normalizing the peak area ratios by the corresponding atomic sensitivity factors (1.00 for C1s, 1.80 for N1s, 24.2 for Ge2p3/2, 2.93 for O1s, 14.8 for Sn3d5/2, and 13.4 for In3d5/2). AFM Characterization. AFM images (512 × 512 pixels) were obtained with an MFP3D AFM (Asylum Research,Santa Barbara, CA). The imaging was performed in tapping mode in air using Olympus silicon cantilevers with a stiffness of 1.54 N/m. Images were flattened with a first-order flattening procedure using the MFP3D software. Static Water Contact Angle Measurements. The wettability of the modified surfaces was determined by automated static water contact angle measurements with the use of Krüss DSA 100 goniometer (volume of the drop of demineralized water is 3.0 μL). The reported value is the average of at least 3 droplets with the error of less than ±2° (and typically ≤1° for any value >90°). Infrared Reflection Absorption Spectroscopy (IRRAS). IRRAS were obtained with a Bruker Tensor 27 FT-IR spectrometer, using a commercial variable-angle reflection unit (Auto Seagull, Harrick Scientific). A Harrick grid polarizer was installed in front of the detector and was used for measuring spectra with p-polarized radiation with respect to the plane of incidence at the sample surface. Single channel transmittance spectra were collected at an angle of 64° using 2048 scans in each measurement. The raw data were subtracted by the data recorded on a freshly cleaned reference ITO surface, after which a baseline correction was applied to give the reported spectra. Click Reaction at the Interface Presenting TMG-Alkyne. A stock solution of ligand N,N,N′,N′-tetramethylethane-1,2-diamine (0.8 mM, 0.5 mL) and azide derivatives (20 mM, 0.5 mL) in i-propanol were combined in a reaction vial containing the TMG-modified ITO surface (1.25 × 1.25 cm2). Subsequently, sodium ascorbate (10 mM, 0.5 mL) and CuSO4 (0.8 mM, 0.5 mL) in H2O were added to the reaction mixture as the catalyst, resulting in 2 mL solution with ipropanol/H2O = 1:1. The reaction vial was filled with N2, sealed with a cap, and placed on an orbital shaker with slight shaking (70 r/min). For the attachment of ferrocene-azide (Fc-N3), the reaction was stopped after 6 h. For the immobilization of biotin-azide (biotin-N3) with an oligo(ethylene glycol) chain, the reaction was run for 12 h to ensure the complete of click reaction. The ITO wafer was rinsed with CH2Cl2 and EtOH, ultrasonic cleaned in EDTA solution (0.05%) for 5 min, washed again with Milli-Q water, and dried with a stream of N2. Electrochemical Characterization. Electrochemical measurements were performed on AUTOLAB PGSTAT10 in a custom-built electrochemical cell equipped with modified ITO substrate as the working electrode (area exposed to the solution = 1.1 cm2), a platinum mesh as the counter electrode, and silver/silver chloride junction as the reference electrode. The total surface coverage of active ferrocene centers (ΓFc) can be derived from the integration of the area under the voltammetric peaks at low scan rates giving the charge passed (Q), according to eq 1.37 Here, n is the number of electrons exchanged, F the Faraday constant, and A the electro-active surface area of the ITO electrode used in the study.
applied for the photochemical attachment of a wide range of alkene-derived monolayers onto several metal oxide semiconductors, including TiO2, SnO2, and ZnO by especially the Hamers group.31−34 The light-triggered nature of the grafting reaction enables constructively patterned metal oxide surfaces by using a simple masking technique. More importantly, the resulting organic thin film coatings on these substrates also exhibit remarkable stability in both organic and aqueous solutions.26,31 In the current study, we apply this strategy to introduce robust, covalently bound organic monolayers on ITO substrates, which are characterized by water contact angle measurements, XPS, AFM, and scanning electron microscopy (SEM). Bio/electrochemically active interfaces, which are particularly attractive for the development of ITO-based sensor devices, can be fabricated using a photochemically attached “clickable” organic layer as the platform. We demonstrate the potential of this approach by preparing ferrocene-functionalized ITO and locally biotinylated ITO surfaces that can thus be coated locally by specific proteins.
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EXPERIMENTAL SECTION
Chemicals. All chemicals, unless otherwise noted, were of analytical grade and used as received. Milli-Q water with a resistivity above 18 MΩ·cm was used to prepare aqueous solutions for stability testing and surface cleaning. Tetradecanoic acid (98%), tetradec-1-ene (>97%), deca-1,9-diyne (97%), D-biotin (>98%), and Triton X-100 were purchased from Alfa Aesar. Cy3-stravavidin was purchased from Invitrogen company, USA. N-Hydroxysuccinimide, N,N′-dicyclohexylcarbodiimide, sodium azide, 4,7,10-trioxa-1,13-tridecanediamine (97%), and n-tetrabutylammonium hexafluorophosphate (NBu4PF6) were purchased from Sigma-Aldrich. Tetradec-1-ene and deca-1,9diyne were purified by column chromatography before surface modification. Dodec-11-en-1-yn-1-yltrimethylgermane (TMG-ene)35 and azidomethylferrocene (Fc-N3)36 were synthesized by the previously reported protocols. Figure S1 in the Supporting Information demonstrates the synthetic routine of N-(11-azido-3,6,9trioxatridecanyl)biotinamide (biotin-N3). ITO Cleaning and Activation. The ITO substrates (25 × 25 mm2, Sigma Aldrich, 8−12 Ω/sq) were first cleaned in ultrasonic baths using a solvent series with increasing polarity: CH2Cl2, acetone, and methanol for 5 min each. Then, the substrates were treated in an oxidizing bath of NH4OH/H2O2/H2O (1:1:6) at 80 °C for 30 min, washed thoroughly with Milli-Q water, and dried under a stream of N2. Photochemical Attachment of Alkene-Derived Organic Layers. Prior to the photografting, the alkene was degassed by three consecutive freeze−pump−thaw cycles. A cleaned ITO substrate and the degassed 1-alkene were then transferred to an Ar-glovebox. Afterward, a fused quartz slide was used as a cover, onto which a mercury capillary lamp (6.0 mW/cm2, Jelight, Irvine, CA, USA) was placed above. The setup was packed in aluminum foil, and the ITO substrate was illuminated through the quartz for a specified time. After illumination, the substrate was removed from the glovebox, rinsed with hexane and acetone, sonicated in ethanol for 5 min, and finally dried with nitrogen. As for the photolithography, a gold contact mask as shown in Figure 6a or a 400 mesh copper grid (G400-Cu, Electron Microscopy Science Company) was placed between the ITO substrate and quartz slide. Carboxylate Chemisorption. Similar to the treatment before photochemical grafting, the ITO surface were treated with NH4OH/ H2O2/H2O (1:1:6) at 80 °C for 20 min, in order to maximize the number of −OH groups on the surface. Then, the films were immersed in a 2 mM solution of tetradecanoic acid in anhydrous CH3CN for 16 h. The resulting films were then rinsed thoroughly with freshly distilled acetonitrile and dried by a stream of nitrogen. XPS Measurements. The XPS analysis of surfaces was performed using a JPS-9200 Photoelectron Spectrometer (JEOL, Japan). The
ΓFc = Q /nFA
(1)
Binding of Cy3-Streptavidin on the Biotinylated Surface. The biotinylated surface was incubated in a solution of Cy3streptavidin (0.2 mg/mL) in PBS buffer for 30 min at room temperature. The wafer was subsequently rinsed 3 times in a 0.5% solution of Triton X-100 in PBS buffer under constant shaking on a shaking bed (80 r/min) to remove the nonspecifically bound streptavidin from the surface, followed by an additional rinse in PBS buffer (pH = 7.1) under the same shaking condition. The total rinse time was about 15 min. Samples were then rinsed briefly in DI water and dried under a stream of nitrogen. Fluorescent images were 5351
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measured with a confocal laser scanning microscope (Zeiss LSM 510 Meta).
upon exposure of the surface to air. After the UV irradiation for 16 h, both the In3d and the Sn3d intensities decrease substantially, indicating the formation of an overlayer, while the In/Sn ratio remains essentially unchanged (In:Sn ≈ 9:1). Accompanied by the reduction in the substrate signal, the C/In ratio increases from 0.6 to 3.2, which is consistent with the presence of a C14-ene-derived organic layer onto the ITO substrate. Under otherwise identical conditions, a control experiment was carried out in ambient light (>380 nm), which results in a negligible increase of the C/In ratio within experimental error, suggesting the critical role of 254 nm UV irradiation in the formation of a covalently bound organic layer. The narrow scan of C1s signal was used to probe the chemical states of carbon on the surface. Figure 2 demonstrated the C1s
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RESULTS AND DISCUSSION Photochemical Grafting of Alkyl Monolayer. The grafting of tetradec-1-ene (henceforth referred to as C14ene) was performed on ammonia/H2O2-cleaned ITO substrates, and the degree of modification was evaluated by the surface wettability. As shown in Figure 1, the static water
Figure 1. Contact angle of C14-ene-derived organic layer on ITO substrate as function of UV irradiation time.
contact angle on flat ITO substrates immediately after cleaning by ammonia/H2O2 solution was
