ARTICLE pubs.acs.org/Langmuir
Tandem “Click” Reactions at Acetylene-Terminated Si(100) Monolayers Simone Ciampi,† Michael James,†,‡ Pauline Michaels,† and J. Justin Gooding*,† † ‡
School of Chemistry, The University of New South Wales, Sydney NSW 2052, Australia Bragg Institute, Australian Nuclear Science and Technology Organisation (ANSTO), Locked Bag 2001, Kirrawee DC NSW 2232, Australia
bS Supporting Information ABSTRACT: We demonstrate a simple method for coupling alkynes to alkynes. The method involves tandem azidealkyne cycloaddition reactions (“click” chemistry) for the immobilization of 1-alkyne species onto an alkyne modified surface in a one-pot procedure. In the case presented, these reactions take place on a nonoxidized Si(100) surface although the approach is general for linking alkynes to alkynes. The applicability of the method in the preparation of electrically well-behaved functionalized surfaces is demonstrated by coupling an alkyne-tagged ferrocene species onto alkyne-terminated Si(100) surfaces. The utility of the approach in biotechnology is shown by constructing a DNA sensing interface by derivatization of the acetylenyl surface with commercially available alkyne-tagged oligonucleotides. Cyclic voltametry, electrochemical impedance spectroscopy, X-ray photoelectron spectroscopy, and X-ray reflectometry are used to characterize the coupling reactions and performance of the final modified surfaces. These data show that this synthetic protocol gives chemically well-defined, electronically well-behaved, and robust (bio)functionalized monolayers on silicon semiconducting surfaces.
1. INTRODUCTION Many of the emerging applications for organicsilicon interfaces, such as molecular electronics,1 memory devices,2 photovoltaic,3 electrochemical systems,2,4 and sensors,57 are reliant on direct electronic communication with the silicon substrate. The use of siliconcarbon-linked monolayers dramatically increases the robustness of the device,810 and prevents the oxidation of silicon which tends to degrade the electronic/ electrochemical performance.1,11 Furthermore, functional organic monolayers are essential if the modification of the silicon surface is intended for sensing or biomolecular applications.1217 Hence, there is a need for a modular surface chemistry that can stabilize silicon, especially the industrially most relevant Si(100) crystal face,18 against oxidation in air and in aqueous environments. This same surface chemistry must also allow for a highly efficient and reliable bottom-up assembly approach.19,20 We have recently demonstrated a strategy to stabilize silicon that seems to satisfy all these criteria.21 The strategy involves the modification of hydrogen terminated silicon using a symmetrical R,ω-diyne (1,8-nonadiyne 1, Scheme 1). The hydrosilylation reaction of the diyne 1 with the SiH surface, which gives an exceedingly stable alkenyl linkage (SiCdC),22,23 results in a well-defined, highly stable monolayer on the silicon surface.21 The distal end of this monolayer is an ethynyl function that is then amenable to further modification with azide species via the copper(I)-catalyzed alkyneazide cycloaddition (CuAAC) reations;24,25 r 2011 American Chemical Society
the archetypal “click” reaction.26 The particularly surprising result with this surface chemistry is that the monolayer provides unprecedented protection of the underlying silicon surface against oxidation, a totally unexpected result for a device prepared on the Si(100) face. The excellent protection of the substrate is attributed to both the ethynyl functionalities at the monolayer distal end having an affinity for each other due to ππ bonding,27 and to the SiCdC linkage.22,23 The distal alkyne therefore is vital to achieve such performance. This excellent protection is demonstrated by further functionalizing the diyne-modified silicon with an azide-tagged ferrocene. Performing electrochemistry of these surfaces in aqueous solution allowed for prolonged use without any evidence of degradation.4,27 CuAAC reactions are a straightforward surface chemistry tool for making covalent connections between azide- or alkyne-tagged building blocks.24,25,2830 CuAAC reactions benefit from high selectivity, modularity, and the absence of both activation and protection/deprotection steps, and they are tolerant to a wide range of solvents and functional groups.31,32 As neither azides nor 1-alkynes are generally present in natural systems, the intrinsic orthogonality of the CuAAC reaction makes it a bioconjugation strategy of utmost importance.33,34 There are however situations where chemical procedures for alkyne-tagging are significantly Received: April 14, 2011 Published: May 10, 2011 6940
dx.doi.org/10.1021/la2013733 | Langmuir 2011, 27, 6940–6949
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ARTICLE
Scheme 1. Tandem “Click” Reactions on Acetylenyl Si(100) Electrodesa
a
Formation of surface-bound 1,3-bis(1,2,3-triazole) for the one-pot coupling of acetylene-terminated alkenyl monolayers with 1-alkyne species.
more straightforward than those for the corresponding azide functionality. A notable example is found in the nucleic acid field.3538 DNA can be modified postsynthetically using CuAAC reactions;37 however, while alkyne-bearing phosphoramidite building blocks are compatible with commercial automated solid-phase oligonucleotide synthetic protocols,37,38 the azidetagged counterparts are reduced by the phosphorus(III) atom of the phosphoramidite in a Staudinger-type reaction.39 Alkynemodified oligonucleotides can therefore be ordered at many of the custom oligo-synthesizing facilites, while azide-tagged oligonucleotides are only accessible via postsynthetic labeling40 of commercial oligos. Naively, the simpler solution to the coupling of alkyne-tagged molecules onto the surface is to modify the silicon substrate with an azide terminated monolayer. This is not generally an option because the terminal azide decomposes to a highly reactive nitrene intermediate under the conditions of monolayer formation.16,41 Moreover, this synthetic route is not a viable option in our particular case for electrochemical applications, where the good substrate protection we get from 1,8-nonadiyne 1 is likely to arise from interactions between ethynyl groups at the distal end of the monolayer.27 To date, studies into the immobilization of alkyne-tagged molecules onto nonoxidized silicon surfaces have been limited to a much debated three-step protocol involving (i) hydrosilylation of ω-halo-1-alkenes (e.g., 11-bromo-1-undecene, BrUDE), followed by (ii) nucleophilic halide/azide exchange to yield an azide-terminated surface, and (iii) “click” immobilization of the 1-alkyne species.4143 Prato and co-workers have recently demonstrated the scope and limitations of this three-step procedure in the context of chemical derivatization of silicon electrodes.43 Following the photochemical anchoring of BrUDE onto HSi(100) and
treatment with sodium azide, “click” reactions with azidedecorated surfaces were used to immobilize alkyne-tagged ferrocene derivatives and prepare electrochemically well-behaved Si(100) electrodes.43 In common with cases reported by Bedzyk and co-workers44 and by Chazalviel and co-workers,45 however, XPS data revealed not-negligible amounts of side products in the anchoring of alkyl halides.46 Long reaction times (72 h) were also required for the halide/azide exchange step.47 The purpose of this paper is to examine an alternative two-step strategy for the “clicking” of alkyne-tagged molecules onto alkyne terminated monolayers prepared on nonoxidized Si(100) surfaces (Scheme 1). The protocol involves (i) the hydrosilylation of 1,8-nonadiyne 1 onto HSi(100) to give SAM-1 and (ii) simultaneous (one-pot) immobilization of a R,ω-diazide spacer molecule (1,3-diazidopropane 4) and the 1-alkyne tagged molecule of interest onto SAM-1. The utility of the method is demonstrated by coupling an alkyne-tagged ferrocene (ethynylferrocene 2) and a commercially available alkyne-tagged oligonucleotide (alkyne-ODN 3) onto SAM-1. The attachment of the redox center (i.e., 2) is because it provides a convenient method of monitoring the progress of this surface CuAAC reaction and a rapid screening tool for optimized reaction conditions. This is because the ferrocene electrochemistry is sensitive to whether the coupled ferrocene molecules are all in the same environment, or in a range of different environments, and hence provides an indication of the uniformity of the coupling reaction. Further, such surfaces are of interest in molecular electronics and molecular memory applications.2 The attachment of an alkyne-tagged 20-mer oligonucleotide (i.e., 3)48 is to demonstrate the utility of the new coupling method for bioelectronic applications and in particular due to the growing interest in using silicon devices in biotechnology. 6941
dx.doi.org/10.1021/la2013733 |Langmuir 2011, 27, 6940–6949
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2. EXPERIMENTAL SECTION 2.1. Materials. Unless otherwise noted, commercially available reagents and solvents were were of high purity (>99.9%) and used as purchased without further purification. Hydrogen peroxide (30 wt % solid in water, Sigma-Aldrich), hydrofluoric acid (Riedel-de Ha€en, 48 wt % solid in water), and sulfuric acid (Fluka) used for wafer cleaning and etching procedures were of semiconductor grade or the highest available commercial grade. 1,8-Nonadiyne (1, Alfa Aesar, 97%) was redistilled from sodium borohydride (Sigma-Aldrich, 99þ %) under reduced pressure (79 C, 89 Torr) and collected over activated molecular sieves (Fluka, 3 Å pore diameter, 1020 mesh beads, dehydrated with indicator) and then stored under a dry argon atmosphere prior to use. Tris(benzyltriazolylmethyl)amine (TBTA, Aldrich) and N,N,N0 ,N0 -tetramethylethylenediamine (TMEDA, Aldrich) were of a 97% nominal purity. Ethynylferrocene (2, Aldrich, 97%) was purified using column chromatography (DavisilLC 60 Å, 4060 μm) with elution by hexane. Alkyne-tagged probe oligonucleotide 50 -(GGG-GCA-GTG-CCT-CACAAC-CTX)-30 (alkyne-ODN, 3), with X being 5-alkynyl-20 -deoxyuridine37 (Scheme 1), was obtained from BaseClick GmbH (Tutzing, Germany). Ultrapure sterile water was added to the lyophilized alkyneODN strand and stored at 20 C when not in use. The complementary strands oligonuclotides were purchased from GeneWorks Pty Ltd. (Adelaide, Australia). The target sequences were 50 -(AG-GTT-GTGAGG-CAC-TGC-CCC)-30 (complementary strand) and 50 -(GG-ATGGAC-GAA-GCG-CTC-AGG)-30 (noncomplementary strand). 1,3-Diazidopropane 4 was prepared from 1,3-dibromopropane (Alfa Aesar, 97%) according to literature procedures with minor modifications. In brief, to a stirred solution of 1,3-dibromopropane (5.0 g, 24.8 mmol) in N,N-dimethylformamide (80 mL), sodium azide (5.0 g, 76.9 mmol) was added in one portion while stirring under argon. The reaction mixture was heated to 65 C and the reaction continued for 48 h under argon. The reaction mixture was allowed to cool to room temperature and then diluted with diethyl ether (200 mL). The obtained suspension was washed with water (20 mL) and brine (2 50 mL) and then dried over NaSO4. Filtration and evaporation gave a pale yellow oil that was purified using column chromatography (DavisilLC 60 Å, 4060 μm) with elution by hexane to give diazide 4 as colorless oil (2.1 g, 67.3%). 1H NMR (300 MHz, CDCl3) δ: 1.811.88 (m, 2H), 3.42 (t, 4H, J = 6.5 Hz). 13C NMR (75.5 MHz, CDCl3) δ: 49.32, 28.21. IR (NaCl, cm1): 2941, 2875, 2099, 1737, 1455, 1373, 1298, 1247, 1046. Milli-Q water (>18 MΩ cm) was used to prepare electrolyte solutions and for surface cleaning and surface modification. Dichloromethane, ethanol, 2-propanol, 2-methyl-2-propanol for surface cleaning and surface modification were redistilled prior to use. Plasticware, Milli-Q water, dimethyl sulfoxide, and solutions used for the handling of oligonuclotides were either received presterilized and certified DNase- and RNasefree, or sterilized by autoclave prior to use. Prime grade single-side polished silicon wafers, 100-oriented (Æ100æ ( 0.05), p-type (boron), 525 ( 25 μm thick,
