Article pubs.acs.org/Langmuir
Self-Assembled Monolayers on Gold of β‑Cyclodextrin Adsorbates with Different Anchoring Groups Alejandro Méndez-Ardoy, Tom Steentjes, Tibor Kudernac, and Jurriaan Huskens* Molecular Nanofabrication Group, MESA+ Institute for Nanotechnology, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands S Supporting Information *
ABSTRACT: We designed multivalent β-cyclodextrin-based adsorbates bearing different anchoring groups aiming to yield stable monolayers with improved packing and close contact of the cavity to the gold surface. Toward this end the primary rim of the β-cyclodextrin was decorated with several functional groups, namely iodide, nitrile, amine, isothiocyanate, methyl sulfide, and isocyanide. Monolayers formed by these adsorbates were characterized by contact angle measurements, surface plasmon resonance spectroscopy, polarization modulation infrared reflection adsorption spectroscopy, X-ray photoelectron spectroscopy, and electrochemistry. The nature of the anchoring group influenced the adsorption kinetics, thickness, layer stability, number of anchoring groups bounded to the surface, and packing in the resulting monolayers. Therefore, chemical manipulation of multivalent adsorbates can be used to modify the properties of their monolayers.
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INTRODUCTION
Improving and modulating the adsorption capabilities of multivalent molecules without hampering the electrical properties remains a challenge. In this sense, tuning of the affinity of the headgroup for the substrate might be used for improving the lateral mobility, although comparative information about the effect of their chemical nature in the self-assembly properties is in general scarce.15−17 Furthermore, this concept may take advantage from the fact that variations of the nature of the headgroup can change the electron transport characteristics of the metal−organic interface.18,19 Here we report the synthesis of a series of β-CD adsorbates bearing different anchoring groups and their self-assembly on gold. The anchoring groups were selected based on their different affinities for gold and their facile implementation onto the CD scaffold to ensure the homogeneity of the final derivatives and eventually the electrode−headgroup contact. Concretely, β-CD adsorbates fully functionalized at their primary rim with methyl sulfide,20 isocyanide,21 isothiocyanate,15,22 nitrile,23 amine,24 and iodide25,26 functionalities were chosen. These adsorbates show lower affinities for gold in comparison to thiols and provide a convenient set of derivatives well suited for comparative studies. The kinetics of the selfassembly process and the structure and electrochemical properties of the monolayer were studied for the presented set of molecules.
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Cyclodextrins (CDs) are cyclic oligosaccharides constituted of 6, 7, or 8 (α-, β-, and γ-CDs, respectively) D-glucopyranose units linked by α-(1 → 4) bonds featuring a basket-like structure. These have been used extensively as a platform for the design of adsorbates that assemble to form monolayers with recognition capabilities. In this sense, they have found a wide range of versatile applications including the oriented immobilization of proteins,2 supramolecular thin film deposition,3 or as a valuable model for studying multivalent host−guest interactions.4 Nonetheless, the immobilization of the CDs, mostly based on the incorporation of a varying number of thiol or thioether moieties, is far from straightforward, and the anchoring groups and linker have a great impact on the coverage, orientation, and packing and by extrapolation on the host−guest chemistry due to heterogeneous orientation of the hosts.5−13 While inclusion of multiple, preferably symmetrically displayed head groups increases the possibility of achieving orientation of the cyclodextrin core, the strong multivalent interaction with the substrate hampers lateral mobility and self-healing.14 The use of multivalently exposed strong gold-binding groups such as thiols result in a reduced lateral mobility and thus poor packing of the resulting monolayers.13 Substitution of thiols with long alkyl thioethers with a lower affinity to gold improved the lateral mobility of the molecules.10 The increased mobility of the molecules and the interacting additional alkyl chains led to an increase of surface coverage and an improved order. However, the incorporation of the alkyl chains creates an insulating layer that hampers their use in applications where electron transfer from or to the electrode is required. © 2014 American Chemical Society
Received: January 14, 2014 Revised: February 26, 2014 Published: March 4, 2014 3467
dx.doi.org/10.1021/la500172a | Langmuir 2014, 30, 3467−3476
Langmuir
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Article
RESULTS Figure 1 shows the molecular structures of the set of β-CD adsorbates bearing different anchoring groups on the primary
The preparation of the per-isocyanide-β-CD 6 was carried out according to the strategy shown in Scheme 1b, consisting of N-formylation followed by dehydration. We protected permanently the secondary rim as methyl ethers in order to avoid dehydration of the hydroxyl groups and to facilitate the purification procedure. Derivative 9 was treated with a preformed mixture of formic acid and N,N′-dicyclohexylcarbodiimide (DCC) in dichloromethane (DCM) to give a mixture of rotamers 10. After dehydration with PCl3 in the presence of an excess of diisopropylamine (DIPA) under careful control of the pH, adsorbate 6 was isolated in a 63% yield. The presence of the isocyano groups was confirmed by a peak at 159 ppm in the 13C NMR spectrum as well as an absorption band at 2148 cm−1 in the infrared attenuated total reflectance (IR-ATR) spectrum corresponding to stretching of the isocyanide moiety (Figures S4 and S5). Self-assembled monolayers of adsorbates 2, 4, and 5 were prepared by immersion of freshly cleaned gold substrates in 0.1 mM adsorbate solutions in THF−H2O 4:1 for 12−24 h at room temperature, except for adsorbates 1, 3, and 6, which were dissolved in THF−MeOH 4:1, water (pH = 7), and dry degassed DCM, respectively, for solubility reasons. The resulting monolayers were characterized by contact angle goniometry, surface plasmon resonance (SPR) spectroscopy, polarization-modulation infrared reflection adsorption spectroscopy (PM-IRRAS), X-ray photoelectron spectroscopy (XPS), and electrochemistry. Table 1 summarizes the most relevant SAM properties.
Figure 1. Structure of the β-CD-based adsorbates used in this study.
rim of the CD core. Heptaiodo β-CD adsorbate (1)27 provides the synthetic intermediate for the other derivatives. Adsorbates 2,28 3,27 and 429 were prepared according to published routes with some minor modifications. The synthesis of the per-methyl sulfide β-CD 5 was reported previously.9 However, the final desired product was obtained only in an inseparable mixture with incompletely functionalized derivatives. In other to circumvent the separation problem, an alternative strategy based on the temporary protection of the secondary rim with acetyl groups was developed (Scheme 1a). Scheme 1. Synthesis of Adsorbates 5 and 6
Table 1. Characterization of the Self-Assembled Monolayers of the β-CD Adsorbates on Gold adsorbate 1 2 3 4 5 6
a
(I) (CN) (NH2) (NCS) (SMe) (NC)
θa/θra (H2O, deg)
Cmlb (F/m2)
RCTc (Ω)
thicknessd (nm)
45/
