Atomic Force Microscopy Study of new Sensing Platforms: Cucurbit[n]uril (n = 6, 7) on Gold Elías Blanco,* Carmen Quintana, Lucas Hernµndez, † Pedro Hernµndez Departamento de Química Analítica y Anµlisis Instrumental, Facultad de Ciencias, Universidad Autónoma de Madrid, 28049 Madrid, Spain † In memoriam *e-mail: elias.blanco@uam.es Received: July 19, 2012 Accepted: October 17, 2012 Published online: December 5, 2012 Abstract This work presents the comparative study of the spontaneous adsorption of cucurbit[6]uril and cucurbit[7]uril on gold by means of atomic force microscopy. A faster cucurbit[6]uril adsorption was not achieved by the use of a cu- curbit[6]uril solution with sodium salts to increment its solubility than by the treatment with a saturated one in water. However, sodium ions promoted the cucurbit[6]uril polymerization on the previously adsorbed layer. The electrode reaction of hydroquinone became irreversible when a cucurbit[6]uril modified gold electrode was used. Cucurbit[7]uril adsorption was faster than for the smaller homologue and multilayers were generated too with high adsorption times. Keywords: Adsorption, Cucurbituril, Gold, Monolayers DOI: 10.1002/elan.201200379 Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/elan.201200379. 1 Introduction Self-assembled monolayers (SAMs) are defined as orient- ed monolayer films formed by spontaneous adsorption of molecules on surfaces. They have multiple applications as in electronics [1] or analytical chemistry where, for exam- ple, SAMs of molecules with good inclusion properties can be used in the construction of sensor devices in which the analytes are specifically recognized, as in the case of the use of a modified b-cyclodextrin [2]. Cucurbit[n]urils (n = 5–8, 10) are a group of cyclic, pumpkin-shaped mole- cules composed of n glycoluril units bridged by methylene groups, that show high affinity towards inorganic and or- ganic cations. The structure of CB[n] is depicted in Figure 1. They present a common height portal-to-portal of 0.91 nm and a cavity diameter of 0.44, 0.58, 0.73 and 0.88 nm for n = 5, 6, 7 and 8, respectively. The two sym- metric entrances are lined by carbonyl oxygen atoms and are slightly smaller than the cavity (0.2 nm). Their general inclusion properties are high affinity and highly selective, constrictive binding interactions with cationic species, es- pecially ammonium ions, driven by a combination of ion- dipole interactions, hydrogen bonds, and hydrophobic effect [3,4]. These properties were first elucidated for CB[6] in strong acidic solutions [5] and latter in aqueous solutions of different salt concentration [6]. These solu- tions permit the study of its host-guest chemistry due to their poor solubility in pure water (around 3  10 À5 M for CB[6], less than 110 À5 M for CB[8], a moderate solubil- ity for CB[5] and CB[7], 2–3  10 À2 M), CB[6] solubility increased up to 0.066 M in 0.2 M sodium sulfate for CB[6] [7]. The larger homologues CB[7] and CB[8] in- clude bigger guests as ferrocene [8,9] and tryptophan, re- spectively, fact that we applied to develop an electro- chemical sensor for tryptophan [10]. Three general strategies have been reported to anchor CB[n] to surfaces, two of them use previously modified surfaces. One is based on the inclusion complex forma- tion between preadsorbed molecules and CB[n] and ac- complished the reversible threading and dethreading of CB[6] and the attachment of several CB[8] molecules to the surface [11,12]. The second one uses functionalized SPECIAL ISSUE Fig. 1. Schematic representation of CB[n] molecules. Electroanalysis 2013, 25, No. 1, 263 – 268  2013 Wiley-VCH Verlag GmbH &Co. KGaA, Weinheim 263 Full Paper