Studies on the effect of solvents on self-assembled monolayers formed from organophosphonic acids on indium tin oxide.
ABSTRACT The preparation of self-assembled monolayers (SAMs) of organophosphonic acids on indium tin oxide (ITO) surfaces from different solvents (triethylamine, ethyl ether, tetrahydofuran (THF), pyridine, acetone, methanol, acetonitrile, dimethyl sulfoxide (DMSO), or water) has been performed with some significant differences observed. Cyclic voltammetry (CV), X-ray photoelectron spectroscopy (XPS), scanning tunneling microscopy (STM), and contact angle measurement demonstrated that the quality of SAMs depends critically on the choice of solvents. Higher density, more stable monolayers were formed from solvents with low dielectric constants and weak interactions with the ITO. It was concluded low dielectric solvents that were inert to the ITO gave monolayers that were more stable with a higher density of surface bound molecules because higher dielectric constant solvents and solvents that coordinate with the surface disrupted SAM formation.
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ABSTRACT: The modification of surfaces by the deposition of a robust overlayer provides an excellent handle with which to tune the properties of a bulk substrate to those of interest. Such control over the surface properties becomes increasingly important with the continuing efforts at down-sizing the active components in optoelectronic devices, and the corresponding increase in the surface area/volume ratio. Relevant properties to tune include the degree to which a surface is wetted by water or oil. Analogously, for biosensing applications there is an increasing interest in so-called "romantic surfaces": surfaces that repel all biological entities, apart from one, to which it binds strongly. Such systems require both long lasting and highly specific tuning of the surface properties. This Review presents one approach to obtain robust surface modifications of the surface of oxides, namely the covalent attachment of monolayers.Angewandte Chemie International Edition in English 05/2014; 53(25):6322–6356. · 13.45 Impact Factor
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ABSTRACT: It is essential to obtain stable self-assembled monolayers (SAMs) for their applications to electrochemical (bio)sensors. Two commonly used methods for preparing phosphonate SAMs were compared in detail for the first time. SAMs of phosphonates with different alkyl chains were formed on indium–tin oxide (ITO) electrodes using the dipping method and the T-BAG method (tethering by aggregation and growth). In addition, two different post-assembly washing methods were assessed. The stability of the SAMs measured by their charge-transfer blocking abilities were investigated using cyclic voltammetry and electrochemical impedance spectroscopy. The SAMs were tested to assess their stability against ultrasonic washing and their long-term stability in phosphate-buffered saline. Only the phosphonate with the longest alkyl chain (octadecylphosphonic acid, ODPA) was stable to the ultrasonic washing, with the charge-transfer blocking ability of SAMs prepared from decyl- and hexadecylphosphonic acid (DPA and HDPA) being significantly reduced after the process owing to damage of the monolayer. Moreover, the ODPA SAMs gave similar X-ray photoelectron spectroscopic data, irrespective of the method of preparation and washing process used, providing further evidence of the stability of this monolayer. An increase in the length of the alkyl chain of the phosphonate (i.e., the length of the dielectric SAMs) decreased the double-layer capacitance and increased the charge-transfer resistance (blocking ability) against a redox reaction of Fe(CN)63-/Fe(CN)64-. After 7 days of immersion in phosphate-buffered saline, the ODPA SAMs prepared by the dipping method maintained their blocking ability to a greater extent than those prepared using the T-BAG method.Journal of Electroanalytical Chemistry. 01/2014; 712:8–13.
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ABSTRACT: The deposition of a prototypical phosphonic acid from an ethanol solution onto indium-tin oxide is studied using XPS to assess the chemisorption kinetics and the purity of the film deposited, and UPS is employed to determine the electronic structure changes induced by the modifier. Room temperature (r.t.) vs. 75 °C depositions are compared, as well as the effect of using air plasma (AP) pretreatment vs. utilizing only detergent-solvent cleaning (DSC) prior to immersion. It is concluded that the order of adsorption kinetics and film purity follows the trend AP 75 °C > DSC 75 °C > AP r.t. > DSC r.t., while the work function obtained for each immersion time depends on whether the substrate is plasma pretreated or not and on the molecular dipole, which saturates at high coverages.Physical Chemistry Chemical Physics 01/2014; · 3.83 Impact Factor