DOI: 10.1002/chem.201200172 Robust Assembly of Dendrimers as an Active Redox-Sensing Monolayer: An Example of Oxo-Anion Sensing Yann R. Leroux, [a] Fei Hui, [a] Jaime Ruiz, [b] Didier Astruc, [b] and Philippe Hapiot* [a] Redox dendrimers are versatile supramolecular building blocks that are well-adapted to the design of functional in- terfaces. [1, 2] Their topology could be tailored before immobi- lization resulting in new properties that do not exist in the isolated moieties. [2, 3] Numerous possible applications could evolve in particular in the areas of molecular electronics, catalysis, or chemical/biological specific sensing. [4, 6] Most of the procedures that have been reported for the preparation of dendrimer films rely on self-assembly techniques (physi- sorbtion, self-assembled monolayers (SAMS) on gold, elec- trostatic interactions) leading to well-organized structures. These procedures are often marred by stability problems. [2] Covalent bonding could circumvent this difficulty, but it is difficult to conduct such an approach for the design of well- organized surfaces. Nevertheless, covalent bonding is suita- ble for the immobilization of supramolecular objects on carbon materials, as recently illustrated in reported exam- ples of applications of dendrimer surfaces in catalysis. [2c] In this context, there is great interest in the design of robust in- terfaces involving a controlled arrangement of the supra- molecular entities while retaining the inherent properties of the dendrimers. [1] We have recently proposed a global strat- egy to prepare robust active monolayers by the electrore- duction of aryldiazonium salts [7] containing a silyl protecting group, 4-[(triisopropylsilyl)ethynyl]benzenediazonium tetra- fluoroborate (TIPS-Eth-ArN 2 + ). [8] After chemical deprotec- tion, a dense and active phenylacetylene monolayer is at- tached onto the carbon surface that allows specific immobi- lization of functional groups through the well-known “click chemistry” (copper(I)-catalyzed Huisgen 1,3-dipolar alkyne azide cycloaddition). [9] Interestingly, this monolayer presents a low activation barrier to charge transfer. [8] Click chemistry was also used with success in the preparation of giant den- drimers. [1d, 10] In this paper, we combine the two strategies to covalently attach dendrimer backbones onto carbon sub- strates and introduce functionality on top of it, taking ad- vantage of the dendritic arrangement. As an example, we have considered a second-generation dendrimer G2 contain- ing 81 N 3 groups (see Scheme 1 and the Supporting Informa- tion for structures) and ferrocenyl moieties as the redox functionality. Such dendrimers display attractive possibilities for encapsulation and transport catalysis and could specifi- cally sense various anions or cations. Moreover, the ferro- cenyl group is a convenient redox label for electrochemical analyses. Functionalizations were performed on two types of carbon substrates that display similar reactivities : glassy carbon (GC) and pyrolyzed photoresist film (PPF). The first one was used as an electrode substrate in cyclic voltammetry analyses, whereas the second one was more adapted to local probe microscopy experiments because of its low roughness. The first modification step is the preparation of the anchor- ing monolayer by electroreduction of TIPS-Eth-ArN 2 + , which is followed by deprotection of the immobilized layer with nBu 4 NF. [8a] The evolution of the surface after each modification step could be followed by the cyclic voltamme- try of ferrocyanide oxidation on the modified surfaces (see Figure 1). [8] After electroreduction of TIPS-Eth-ArN 2 + , the surface is totally inhibited (Figure 1, curve 2), then becomes partially blocked after deprotection showing the functionali- zation of the carbon substrate by the phenylacetylene mono- layer (Figure 1, curve 3). The second step is the addition of the azido-terminated G2 dendrimer by click chemistry. Comparison between curves 3 and 4 shows an increase of the inhibition, confirming the attachment of the N 3 -den- drimers. A similarly modified PPF surface was examined by AFM that evidences the formation of a regular and flat film (see the Supporting Information). The thickness of this layer was estimated by AFM scratching [11] as 2.0 Æ 0.5 nm. Such a low value means that dendrimers are deformed from their globular conformation to a flat disk during immobilization and implies that only a monolayer of dendrimers is present on the surface. Several studies have indeed reported such deformations of dendrimers when they are put in contact with a surface. [12] In a last step, the redox entities are intro- duced onto the immobilized dendrimers by coupling the free N 3 termini with ethynylferrocene. Examination of the modified GC surface by cyclic voltam- metry in a blank ethanol solution shows a well-defined single reversible system (see Figure 2) and a linear variation [a] Dr. Y.R. Leroux, Dr. F. Hui, Prof. P. Hapiot Sciences Chimiques de Rennes UniversitØ de Rennes 1, CNRS UMR 6226 (Equipe MACSE), Campus de Beaulieu 35042 Rennes Cedex (France) E-mail : philippe.hapiot@univ-rennes1.fr [b] Dr. J. Ruiz, Prof. D. Astruc ISM, Univ. Bordeaux, CNRS, UMR 5255 33405 Talence Cedex (France) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201200172. Chem. Eur. J. 2012, 00,0–0  2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim These are not the final page numbers! ÞÞ &1& COMMUNICATION