Electrochemical Fabrication of Surface Chemical Gradients in Thiol Self-Assembled Monolayers with Tailored Work-Functions Giulia Fioravanti,* ,† Francesca Lugli, § Denis Gentili, ‡ Vittoria Mucciante, † Francesca Leonardi, ‡ Luca Pasquali, ∥,#,∇ Andrea Liscio, ⊥ Mauro Murgia, ‡ Francesco Zerbetto, § and Massimiliano Cavallini* ,‡ † Dipartimento di Scienze Fisiche e Chimiche, Universita ̀ dell’Aquila, Via Vetoio 1, 67100 L’Aquila, Italy § Dipartimento di “Chimica G. Ciamician”, Universita ̀ di Bologna, V. F. Selmi 2, 40126 Bologna, Italy ‡ Istituto per lo Studio dei Materiali Nanostrutturati (ISMN) and ⊥ Istituto per la Sintesi Organica e la Fotoreattivita ̀ (ISOF), Consiglio Nazionale delle Ricerche (CNR), Via P. Gobetti 101, 40129 Bologna, Italy ∥ Dipartimento di Ingegneria “Enzo Ferrari”, Universita ̀ di Modena e Reggio Emilia, Via Vignolese 905, 41125 Modena, Italy # CNR - Istituto Officina dei Materiali, S.S. 14, km 163.5 in Area Science Park, I-34012 Trieste, Italy ∇ Department of Physics, University of Johannesburg, PO Box 524, Auckland Park, 2006, South Africa * S Supporting Information ABSTRACT: The studies on surface chemical gradients are constantly gaining interest both for fundamental studies and for technological implications in materials science, nano- fluidics, dewetting, and biological systems. Here we report on a new approach that is very simple and very efficient, to fabricate surface chemical gradients of alkanethiols, which combines electrochemical desorption/partial readsorption, with the withdrawal of the surface from the solution. The gradient is then stabilized by adding a complementary thiol terminated with a hydroxyl group with a chain length comparable to desorbed thiols. This procedure allows us to fabricate a chemical gradient of the wetting properties and the substrate work-function along a few centimeters with a gradient slope higher than 5°/cm. Samples were characterized by cyclic voltammetry during desorption, static contact angle, XPS analysis, and Kelvin probe. Computer simulations based on the Dissipative Particle Dynamics methods were carried out considering a water droplet on a mixed SAM surface. The results help to rationalize the composition of the chemical gradient at different position on the Au surface. ■ INTRODUCTION Surface chemical gradients are the focus of growing interest both in terms of fundamental studies and for technological implications in materials science, 1 nanofluidics, 2−6 technolog- ical application of dewetting, 7 and biological systems. 8,9 Several methods have been proposed for the fabrication of chemical gradients on a solid substrate. They include chemical vapor deposition, 10−13 solution controlled deposition, 14−17 contact printing, 18−20 photoirradiation, 21−23 photodegra- tion, 24,25 thermal treatment, 26 and others. Many of these techniques are based on the surface modification/deposition of a self-assembled monolayers (SAMs) adsorbed on a sub- strate. 27,28 Usually these methods include an initially controlled gradual immersion (or extraction) of the surface into (from) a solution containing the molecules able to form the SAM. The result is a surface that features a chemical gradient consisting of an incomplete monolayer in which the coverage is tuned by the dipping time of the surface in the solution. Incomplete SAMs are usually not very stable and tend to reorganize over time. In order to stabilize the incomplete SAM, samples are filled by a second complementary SAM obtained by immersing the surface into a solution with a second complementary molecule, which favors the formation of a stable high density SAM. Unlike many studies that report the formation of a binary monolayer coating, 29,30 derived from the coadsorption of two different species without precise control over the composition, the formation of a surface chemical gradient could be obtained with extreme accuracy. Currently, efforts are directed toward the control of the directionality and the chemical composition of the gradients obtained by using two different species, through a controlled assembly (from the liquid phase 6,10,15,17 ) or by post-treatment of the monolayer via an external stimulus (thermal, 11,26 radiative, 24,31 etc.). Varying the composition of binary hydro- philic−hydrophobic SAMs allows tailoring of the wetting properties of a surface in a continuous manner. This property Received: April 10, 2014 Revised: September 10, 2014 Published: September 15, 2014 Article pubs.acs.org/Langmuir © 2014 American Chemical Society 11591 dx.doi.org/10.1021/la5013928 | Langmuir 2014, 30, 11591−11598