Adsorption Studies of Organophosphonic Acids on Differently

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Adsorption studies of organophosphonic acids on differently activated gold surfaces Katrin Niegelhell, Simon Leimgruber, Thomas Griesser, Christian Brandl, Boril Chernev, Robert Schennach, Gregor Trimmel, and Stefan Spirk Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.5b04467 • Publication Date (Web): 26 Jan 2016 Downloaded from http://pubs.acs.org on February 2, 2016

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Adsorption studies of organophosphonic acids on

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differently activated gold surfaces

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Katrin Niegelhell†,‡, Simon Leimgruber†,#,‡, Thomas Grießer§, Christian Brandl¥, Boril

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Chernev¥, Robert Schennach┴, Gregor Trimmel†, Stefan Spirk†* †

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Graz University of Technology, Institute for Chemistry and Technology of Materials, Stremayrgasse 9, 8010 Graz, Austria

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#

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Polymer Competence Center Leoben GmbH, Roseggerstraße 12, 8700 Leoben, Austria

University of Leoben, Institute for Polymer Chemistry, Otto Glöckel-Straße 2/IV, 8700 Leoben, Austria

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¥

Graz University of Technology, Institute for Electron Microscopy and Nanoanalysis,

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Steyrergasse 17, 8010 Graz, Austria.

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┴Graz University of Technology, Institute of Solid State Physics, Petersgasse 16/II, 8010

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Graz, Austria

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Members of NAWI Graz and the European Polysaccharide Network of Excellence (EPNOE)

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‡

contributed equally

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Keywords

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Gold; Surface Plasmon Resonance; Phosphonic Acid; Phosphonate; Self Assembled

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Monolayer

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Abstract

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In this study, the formation of self assembled monolayers consisting of three

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organophosphonic acids (vinyl-, octyl- and tetradecylphosphonic acid) from isopropanol

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solutions onto differently activated gold surfaces is studied in-situ and in real time using

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multi-parameter surface plasmon resonance (MP-SPR). Data retrieved from MP-SPR

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measurements revealed similar adsorption kinetics for all investigated organophosphonic

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acids (PA). The layer thickness of the immobilized PA is in the range of 0.6-1.8 nm

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corresponding to monolayer like coverage and correlates with the length of the hydrocarbon

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chain of the PA molecules. After sintering the surfaces, the PA are irreversibly attached onto

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the surfaces as proven by X-ray photoelectron spectroscopy, attenuated total reflection

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infrared and grazing incidence infrared spectroscopy. Potential adsorption modes and

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interaction mechanisms are proposed.

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Introduction

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For more than three decades, self-assembled monolayers (SAMs) have been playing an

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important role in nanotechnologies because of their ability to form well-defined

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monomolecular organic coatings in a very elegant way.1 Depending on the molecules,

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functional groups, the alkyl chain length and the surface roughness, more or less densely

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packed and well-organized layers are formed. Thus, surface properties of a wide variety of

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materials can be tailored for applications in numerous technologies such as corrosion-resistant

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systems, sensors, organic electronic devices and adhesion promoters.2–4 Typically, molecules

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capable to form monolayers consist of a (mostly polar) head group attached to an alkyl chain,

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which often bears additional functional groups. SAMs are ordered assemblies of such

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molecules that are formed spontaneously by the specific adsorption of the head group onto a

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solid surface with the tail group pointing towards the ambient environment.

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Up to now, one of the best studied self-assembled monolayer system comprises thiols on

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gold.5 Although SAMs on the basis of thiols can be used for a wide range of applications, they

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have some major drawbacks. They lack long-term stability due to oxidation under ambient

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conditions, detach readily from the surface in aqueous environments over time, and are

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limited to certain metal surfaces that react with the thiol head group. In addition, most thiols

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are harmful for the environment and cause a large ecological footprint, while exhibiting a

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rather high potential toxicity. Some of these problems can be avoided by the use of

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(organo)silane terminated molecules, which are, in contrast to thiols, mainly deployed on

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oxidic substrates. SAMs based on (organo)silanes show higher physical and chemical

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stability, however, the assembly of silane monolayers is highly sensitive to the process

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parameters such as temperature, solvent, water content and deposition time.6 Alternatively, in

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recent years phosphonic acids have attracted considerable attention for the built-up of SAMs.

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Phosphonic acid monolayers are able to produce robust, well-anchored thin layers on a variety

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of oxide surfaces via an acid-base condensation reaction.7 These monolayers show interesting ACS Paragon Plus Environment

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properties in terms of utilization for protection or adhesion layers8,9 and for electronic

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devices.2,10,11 Phosphonic acids are established chemicals that are mainly used as complexing

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agents in laundry detergents, as stabilizers for peroxides in the textile- and paper industry and

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as corrosion inhibitors4,9,12, ion exchangers13–15 and catalysts11 in many technical applications

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due to their strong chelation properties.13 Surprisingly, there are only a few reports that

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investigate the adsorption behavior of PA at interfaces and their interaction with metal

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substrates in detail. In order to extend their applicability, fundamental studies of the

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phosphonic acid adsorption behavior on metal surfaces have to be performed. In this paper,

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different PA are deposited onto gold surfaces from solution by application of simple

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activation procedures of the surfaces and their adsorption isotherms are investigated in situ

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using MP-SPR.

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Experimental Methods

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Multi-Parameter Surface Plasmon Resonance. Two-wavelength surface plasmon

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resonance measurements were performed with an SPR Navi 200-L instrument equipped with

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two light source pairs providing 670 and 785 nm (by BioNavis Ltd., Tampere, Finland). SPR

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sensors (gold-coated sensors (d∼50 nm) with a chromium adhesion layer (∼2 nm)) were

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obtained from BioNavis Ltd. In order to remove adventitious carbon, SPR sensors were

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cleaned before use by different cleaning methods, namely treatment with (a) O2-Plasma (10

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min at 100 W (Femto, Plasma-Surface-Technologies, Diener Electronics, Germany)), (b)

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freshly generated ammonia by immersion into boiling (80°C) NH4OH(30%)/H2O2(30%)/H2O

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(in a ratio of 1:1:5 (v/v/v) for10 min), (c) 2.5 M NaOH (2 h) or (d) piranha solution (freshly

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prepared from sulfuric acid and hydrogen peroxide in a 3:1 ratio (v/v) over a period of 15

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minutes). Afterwards the SPR slides are rinsed with MilliQ water, dried in a stream of

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nitrogen and mounted in the sample holder. After equilibration in air for ca 20 min,

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isopropanol was injected and the SPR slide was allowed to equilibrate for another 20 min.

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Then the organophosphonic acids, dissolved in isopropanol were pumped over the gold ACS Paragon Plus Environment

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surfaces (c = 5 mg·ml-1, flow rate 0.1 ml·min-1) and allowed to adsorb for a period of 7 min.

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Afterwards, the slides were rinsed with isopropanol for 30 min in order to remove loosely

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bound material followed by drying in a stream of nitrogen. For the measurements, full angular

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scans were acquired and the shifts in the SPR angle were used to model the data using

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multilayer models by using the Winspall 3.02 software. The two wavelengths and two-

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medium cross point analyses were performed by using Microsoft Office Excel 2010. All two

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wavelength SPR experiments were processed using the BioNavis Dataviewer software.

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X-ray Photoelectron Spectroscopy (XPS). XPS spectra were recorded using a Thermo

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Scientific instrument equipped with a monochromatic Al-Kα X-ray source (1486.6 eV). High

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resolution scans were acquired at a pass energy of 50 eV and a step size (resolution) of 0.1

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eV. Wide scans were acquired with a pass energy of 100 eV and a step size of 1.0 eV. All

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spectra have been normalized to the Au 4f7/2 peak. Photo-electrons were collected using a

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take-off angle of 90 º relative to the sample surface. Charge compensation was performed

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with an argon flood gun. The peaks were fitted using a Gaussian/Lorenzian mixed function

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employing Shirley background correction (Software Thermo Avantage v5.906). All analyses

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were performed at room temperature.

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Atomic Force Microscopy (AFM). AFM imaging was performed in atomic force

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microscopy tapping mode with a Veeco multimode scanning probe microscope (Bruker,

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USA). The images were scanned using silicon cantilevers (NCH-VS1-W, Nanoworld,

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Switzerland) with a resonance frequency of 320 kHz and a force constant of 42 N·m-1. All

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images were processed using Gwyddion software package.

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Attenuated Total Reflection – Infrared Spectroscopy (ATR-IR). ATR-IR spectra were

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obtained using a Bruker 66v/S instrument equipped with a MCT (mercury cadmium telluride)

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detector and a Golden Gate ATR adaptor (Specac). IR spectra were measured with 10 minutes

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scan time and a resolution of 4 cm-1 in the range of 4000-500 cm-1. Data were analyzed with

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the OPUS 4.0 software. ACS Paragon Plus Environment

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Grazing Incidence Infrared Spectroscopy. The Grazing Incidence Reflection (GIR)-

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measurements were recorded on a IR-Microscope Hyperion 3000 with an FT-IR-

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Spectrometer Tensor 27, the accumulations were 200, the spectral resolution was 4 cm-1 and a

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total of three independent measurements were collected for each sample.

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Theoretical Basis

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There are only a few methods that are capable to study the build-up of SAMs at the solid

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liquid interface in real time. The major problem is to generate a response signal from a

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relatively small amount of sample in the presence of a bulk liquid. For larger molecules (e.g.

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polymers, proteins, polysaccharides), QCM-D is an excellent option and allows to determine

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the mass of adsorbed material according to the Sauerbrey equation as well as the viscoelastic

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properties of the layer simultaneously.16,17 By viscoelastic modeling, the layer thickness can

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be determined and adsorption kinetics can be evaluated. However, for smaller molecules the

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QCM-D method is less suitable due to the smaller mass of the adsorbed species making an

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unambiguous detection often very difficult. This problem is overcome by other methods such

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as surface plasmon resonance spectroscopy (SPRS) which detects changes in the refractive

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index near to the metal surface (ca. 100 nm).18 It is based on the interaction of light with metal

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surfaces creating surface plasmon waves. The origin of these waves is the oscillation of

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charge densities on the surface of metals caused by freely moving electron gas. These

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oscillations are most pronounced close to the metal's surface, in particular when these surfaces

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are in contact with an attached insulating layer.

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For the acquisition of SPR spectra, a high refractive index prism, a coherent light source

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(typically a laser), a sensor slide (e.g. glass slide with a thin gold layer) and a detection system

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(e.g. photodiodes) are required. Modern SPR devices can accumulate spectra in dependence

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of the incidence angle of the light, which allows to investigate adsorption processes in

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different environments (e.g. air, water, alcohols) in a single measurement. Two phenomena

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can be observed at different angular shifts, namely total internal reflection (TIR) and ACS Paragon Plus Environment

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resonance. TIR refers to the reflection of the incident light beam, which occurs when the

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whole energy of the surface plasmons is transferred to the energy of the reflected light wave.

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On the other hand side, resonance is observed at the so called SPR angle where all the energy

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from the incident light beam is able to interact with the surface plasmon waves. This angle

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depends on a variety of factors, namely the nature of the metal, the wavelength of the incident

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light beam, the optical properties of adsorbed layers as well as the properties of the

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surrounding medium, which is in contact with the outermost layer.

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For the studying of adsorption processes in-situ, the use of microfluidic cells allows to

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pump an adsorbate with a defined flow rate over the surfaces of interest. During the

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adsorption, the SPR angle is shifted and the SPR curves before and after adsorption can be

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evaluated to determine the layer thickness and refractive index of the attached layer by fitting

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procedures.19 However, with a single light source it is often difficult to determine the layer

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thickness precisely since it is highly dependent on the refractive index as well. If the

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refractive index of the adsorbed layer is not known, a continuum of minima for the surface

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plasmon wave vector ksp are obtained during fitting resulting in similar goodness (Eq. 1).

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k SP ∝ n * d

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In order to tackle this problem, different approaches have been described in literature,

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namely the two media and the two/three wavelength (also called two colors) approach.20 The

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two media approach acquires SPR curves in different environments e.g. using water and air. If

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the differences in n are large enough, then a unique solution can be easily derived for the final

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layer by plotting the continuum solutions of measurements in different media in a single

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graph. The intersection point of the two continuum solutions yields the refractive index at the

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used wavelength of the light as well as the unique layer thickness d of the film (d1=d2=d).

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(1)

k SP 1 = n1 * d1 and k SP 2 = n2 * d 2

(2,3)


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The second approach is to perform the measurements at different wavelengths. As for the

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two media approach, a set of continuum solutions for kSP is obtained for each wavelength.

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Since the refractive index is a function of the wavelength, a shifting of the curves by the

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chromatic dispersion dn/dλ yields unique solutions for n and d at the intersection point of the

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continuum solutions.

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k SP1 = nλ 1 * d and k SP 2 = nλ 2 * d

(4,5) where

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dn   n λ 2 =  nλ 1 + (λ2 − λ1 )  dλ  

(6) and

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dn   (λ2 − λ1 )  * d k SP1 = nλ 1 * d and k SP 2 =  nλ1 + dλ  

(7,8)

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The disadvantage of this approach is that dn/dλ is often unknown and must be determined

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either by other methods, e.g. the two media approach or by ellipsometry. In fact, the use of

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multiple wavelengths for SPR measurements would allow for the determination of Cauchy

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parameters as well instead of using linear approximations of dn/dλ. It should be noted here,

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that the evaluation of data using these approaches only makes sense if the investigated layers

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do not absorb light at the wavelengths used for the SPR evaluations i.e. k = 0 (which is true

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for most organic compounds). If k ≠ 0, then a unique solution of kSP does exist making the

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above mentioned approaches unnecessary.

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Results and Discussion

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In this paper, the two media approach using two wavelengths is employed to determine the

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adsorption behavior of three different organophosphonic acids (Figure 1) on differently

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activated gold surfaces in situ. As organophosphonic acids, vinylphosphonic (VPA),

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octylphosphonic (OPA) and tetradecylphosphonic acid (TPA) have been used, which feature

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the same chemistry but differ in the length of the alkyl chain. As a consequence, monolayers

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with different layer thicknesses should be formed on the activated gold surfaces.


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Figure 1. Structures and schematic representation of possible binding modes (on oxidic

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surfaces) of organophosphonic acids used for the adsorption onto gold surfaces and

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comparison of the theoretical length of the whole molecules. (a-c, a: vinylphosphonic acid,

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0.55 nm, b: octylphosphonic acid, 1.2 nm, c: tetradecylphosphonic acid, 2.0 nm).

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The experiments were designed in a way that the organophosphonic acids have been

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dissolved in isopropanol (c = 5 mg·ml-1) and allowed to adsorb on the activated gold surfaces

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under a steady flow of solution over the gold surfaces (flow rate: 0.1 ml·min-1). Pre-

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experiments to use lower concentrations of organophosphonic acids resulted in rather long

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adsorption times, while higher concentrations are not feasible due to the low solubility of the

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long-chain organophosphonic acids in isopropanol. In order to obtain information using the

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two media approach, the amount of deposited material in dry state before and after adsorption

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as well as before and after rinsing with isopropanol is compared. All these experiments are

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simultaneously performed at two wavelengths (670 and 785 nm) (Figure S1). The influence of

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the pretreatment of the gold surfaces was investigated by comparing four different treatments

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(piranha, 2.5 M NaOH, O2-plasma, NH4OH/H2O2/H2O) and their effect on the adsorption of

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PA has been studied.


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Comparison of adsorption of TPA on differently activated gold surfaces. As reported in

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literature, phosphonic acids do not significantly adsorb on non-activated gold surfaces.21

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Figure 2 shows the adsorption profile of TPA on such a surface in dependence of the surface

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plasmon resonance angle.

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Figure 2. Adsorption of tetradecylphosphonic acid on untreated gold surface.

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It can be clearly seen that after the rinsing step, the SPR angle nearly reaches its initial

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value which is in line with literature stating rather poor adsorption of organophosphonic acids

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on non-activated gold surfaces. In contrast, for all the surfaces that have been activated,

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higher adsorption was observed albeit the extent strongly depends on the activation procedure.

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The O2-plasma treatment for instance as well as the exposure of the gold substrates to 2.5 M

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NaOH leads to slightly increased adsorption of the PA on the surfaces compared to the non-

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activated ones, while the piranha and NH4OH/H2O2/H2O treated surfaces exhibited both a

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much higher interaction capacity (Figure 3).


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Figure 3. Change in resonance angle due to adsorption of tetradecylphosphonic acid (c = 5

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mg·ml-1 in isopropanol, flow rate = 100 µl·min-1, T = 25°C) a) O2-plasma, b)

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NH4OH/H2O2/H2O, c) 2.5 M NaOH, d) piranha treated sensor slide. All experiments have

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been performed in 3 parallels.

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Since the treatments also induce changes in the roughness of the materials, which in turn

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can affect adsorption dramatically, AFM images have been acquired as well to explore the

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surface morphology after the different activation procedures. Less surprisingly, the outcome is

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that harsh conditions result in rougher surfaces, while mild treatments, e.g. by oxygen plasma,

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yield surfaces that resemble very much the untreated gold surfaces in terms of Rq roughness

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(Figure 4). Interestingly, the morphology of the surfaces does not have a significant influence

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on the adsorption of the organophosphonic acids since the roughest surface (piranha solution,

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Rq 4.8 nm) has the same affinity to phosphonic acids like a very smooth one

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(NH4OH/H2O2/H2O, Rq: 1.3 nm). While the adsorption capacity seems to be independent of


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the roughness, the kinetics has to some extent a correlation with the roughness of the surfaces.

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The smoothest surfaces (a, b) show a very similar kinetics whereas the surfaces featuring a

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higher roughness have a significantly higher adsorption rate.

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Figure 4. AFM topography images of differently treated gold surfaces. a) O2-plasma, b)

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NH4OH/H2O2/H2O, c) 2.5 M NaOH, d) piranha-treated sensor slides e) non-treated sensor

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slide. Image size is 10 x 10 µm2. Rq = RMS roughness. Corresponding cross sections can be

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found in the SI (Fig. S6).

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Besides roughness, the activation procedures may also affect the surface chemistry of the

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substrates either by direct oxidation and the formation of gold oxides or by dissolution of

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silicates at low pH values from the used glassware, followed by precipitation/adsorption on

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the gold surfaces.22 Both scenarios would have a major impact on the adsorption behavior of

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the phosphonates and therefore XPS studies have been performed before and after deposition

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of the TPA on the activated gold surfaces. Unexpectedly, the composition of the surfaces did

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not change after either mild or harsh treatments (as shown in the supporting info, Figure S2).

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Neither traces of gold oxides nor any sign of silicates have been detected on the surfaces. In

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addition to XPS, infrared spectroscopic studies showed that there are neither any Si-O-Si

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vibration bands at 1090 cm-1 (stretching) and 812 cm-1 (bending) in the spectra of a piranha

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treated gold slide (Figure 10,b).23


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Since our aim is to form a monolayer, for further studies the piranha procedure has been

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used for the adsorption experiments of the other organophosphonic acids in order to maximize

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their adsorption onto gold surfaces.

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Adsorption of VPA, OPA and TPA on piranha-treated gold surfaces. Figure 5 shows

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the adsorption behavior of the organophosphonic acids on piranha-treated gold surfaces at two

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different wavelengths in dependence of the SPR angle. It can be clearly seen that the

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adsorption profile of all investigated PA is very similar and rather fast. Even after a few

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minutes a stable plateau is reached, followed by a rinsing step which removes loosely bound

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material. In order to obtain continuum solutions for layer thickness and refractive indexes, the

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SPR curves before adsorption and after rinsing have been analyzed by the Winspall program.

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In all cases, the refractive index is varied and the resulting layer thickness is calculated. This

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procedure is done for the curves obtained in isopropanol and after drying with air. The fitted

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curves are plotted in a single graph and the intersection point of these determinations yields

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the layer thickness and the refractive index, respectively (Eq.7,8). Since this is done at two

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different wavelengths, potential errors due to the fitting procedure are reduced significantly

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and variations in the layer thicknesses obtained from the two wavelengths in a single

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measurement are < 0.1 nm for these experiments.


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,

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Figure 5. Adsorption curves of different organophosphonic acids (c = 5 mg·ml-1 in

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isopropanol) on gold substrates studied by SPR at 670 (compact) and 785 nm (dotted),

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respectively, with a flow rate of 0.1 ml·min-1, a-c, a) vinylphosphonic acid, b)

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octylphosphonic acid, c) tetradecylphosphonic acid.

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The layer thicknesses obtained by this method are in good agreement with the calculated

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length of the organophosphonic acid molecules. However, the determined layer thicknesses of

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TPA is slightly smaller than the calculated length of the stretched PA. Probably, the PA with


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longer alkyl chains are slightly bent on the surfaces resulting in a lower thickness of the

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SAMs.

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Figure 6. Two media approach for the determination of the layer thickness and refractive

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indexes at two different wavelengths (left: 670 nm, right: 785 nm) for TPA. Data for

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vinylphosphonic acid and octylphosphonic acid data can be found in the Supporting

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Information (Figure S3). All experiments have been performed in 3 parallels.

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Besides the layer thickness, the two-wavelength approach allows for the determination of

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the refractive indices at the investigated wavelengths (Figure 6). Interestingly, the monolayers

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show an anomal dispersion effect, i.e. the refractive index is increasing with higher

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wavelengths. This very interesting phenomenon has not been reported so far from phosphonic

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acids but probably it is based on the roughness of the substrates since the other possibility for

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such a behavior is absorption of light at the investigated wavelengths which is not the case for

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any of the investigated organophosphonic acids. Generally, this effect originates from an

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entrapment of light in a porous substrate and for other materials it has been exploited to create

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waveguides (e.g. on the basis of SiO2) for fiber optics.24 An overview on the refractive

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indexes and the layer thicknesses of the organophosphonic layers is depicted in Table 1.

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Table 1. Overview on the determined unique n and d values for the different

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organophosphonic acids adsorbed on piranha-treated gold surfaces.


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VPA OPA TPA

d (λ=670 nm) [nm] 0.68±0.05 1.41±0.14 1.73±0.07

d (λ=785 nm) [nm] 0.69±0.06 1.45±0.15 1.71±0.04

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n @ 670 nm [1] 1.4087±0.0008 1.4016±0.0009 1.3960±0.0002

n @785 nm [1] 1.4102±0.0008 1.4053±0.0014 1.4012±0.0011

302 303

Interestingly, all the organophosphonic acids can be removed from the gold surfaces by a

304

simple rinsing step with water or by exposure to high vacuum conditions indicating a rather

305

weak interaction with the surface. However, a simple tempering step for one hour at 90°C

306

leads to an irreversible attachment of the PA onto the gold substrates.

307

XPS and mode of adsorption. In order to confirm the presence of the PA on the surfaces

308

after the tempering step, XPS spectra have been acquired. Figure 7 depicts a comparison of

309

the P2p peaks of the different PA. These XPS studies clearly reveal the presence of

310

phosphorus whereas the reference surface (piranha-treated gold slide, isopropanol rinsed)

311

does not give any signal (Figure S4). In addition, those surfaces that have been rinsed with

312

water without tempering do not give any signal too.

313

The binding energy of the P2p peaks is very close to the bulk material (133.58 vs 133.98 eV)

314

indicating that phosphorus has not changed its oxidation state. This is a very important

315

observation since a possible reduction of the PA would create a phosphane exhibiting a lone

316

pair capable of interacting with the gold surfaces. However, this possibility can be excluded

317

by the results derived from XPS measurements.


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Figure 7. XPS spectra showing the P2p peaks from the different organophosphonic acid

320

monolayers (a-c, a) vinylphosphonic acid, b) octylphosphonic acid, c) tetradecylphosphonic

321

acid) and a blank gold SPR slide after rinsing with isopropanol (d).

322

On the other hand, phosphonic acids readily form (mono)layers on a variety of oxide

323

substrates such as SiO2, Al2O3, or CuO for instance.9,25 On these substrates, surface hydroxyl

324

groups are present, which react with the OH groups of the phosphonic acid in order to form a

325

covalent bond. Several reports have been made that correlate XPS data to the binding mode of

326

PA onto surfaces. In principle, there are three possible binding mechanisms on oxidic

327

surfaces, namely mono-, di- and tridentate (Figure 1).7,26 These different modes can be

328

potentially distinguished in the O1s XPS spectra. For adsorption in a tridentate fashion, only

329

one O1s XPS signal is expected since all the three oxygen atoms are equal in terms of

330

electronic situation, whereas a mono-/bidentate mechanism for instance is indicated by two

331

peaks. As shown in Figure 8 for example, the XPS spectrum of the powder is compared to the


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332

annealed films and a clear difference can be observed. For the powder two peaks at 531.38

333

and 532.78 eV are observed corresponding to the P=O and the two P-OH groups whereas for

334

the monolayer only one peak at is observed. On first glance, this would point at a tridentate

335

binding mechanism but peak fitting clearly reveals that there are two signals hidden in this

336

peak, which is a strong indication for a mono- or bidentate binding mode (Figure 9).

337 338

Figure 8. a) Comparison XPS spectra of the O1s peaks from TPA bulk material and TPA

339

monolayer on gold. b) Au4f5/2 and Au 4f7/2 peaks of non-treated and piranha-treated sensor slides

340

before and after coating with TPA.

341


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342 343

Figure 9. XPS spectra of the O1s peaks from the different organophosphonic acid monolayers

344

(a-c, a) vinylphosphonic acid, b) octylphosphonic acid, c) tetradecylphosphonic acid) and a

345

blank gold SPR slide (d). Note that the weak intensity of the O1s spectra in (d) originate

346

either from water or other contaminants from the atmosphere.

347

Interestingly, the VPA/OPA SAMs exhibit a different composition in terms of the P=O/P-O

348

ratio compared to TPA. While for VPA/OPA a clear bidentate mechanism can be argued,

349

situation is different for TPA. Probably, the TPA undergoes at least partially self-

350

condensation due to steric constraints on the solid surfaces forming P-O-P units, thus reducing

351

the amount of P-O bonds in comparison to the P=O bonds. For TPA, probably a mix between

352

monodentate type interaction of TPA-oligomers and a bidentate interaction of monomeric

353

TPA with the surface takes place leading to the observed XPS results.

354

In fact, the binding mode seems to depend strongly on the nature of the base substrate as

355

well as on the adsorption conditions. Moreover, in literature the formation of a covalent bond


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356

is often realized by a sintering step at elevated temperatures, which in the case of phosphonic

357

acids would enable a cross-linking between free OH groups of the PA with OH groups present

358

on the gold surfaces.27 Since the gold slides used in this work are subjected to cleaning

359

procedures, which aim at removing adventitious carbon from the surface28, the chemistry on

360

the outermost layer of the gold surfaces may be altered and may contain traces of gold oxides.

361

There are a few reports that investigate the formation of gold oxides on gold surfaces upon

362

exposure to oxidizing agents.29,30 These reports clearly show that commonly used procedures

363

such as NH4OH/H2O2/H2O, piranha and oxygen plasma treatment may lead to the formation

364

of gold oxides on gold surfaces although the mechanism is not fully understood. The extent

365

and amount of gold oxides that are formed are strongly dependent on the cleaning procedure

366

and of course affects the performance of the sensor slides in affinity studies as shown

367

recently.31 Since in this paper piranha cleaned surfaces are used, a very thin layer of gold

368

oxide on the surface of the sensor slide could be present. The formed Au2O3 on the surfaces

369

should provide hydroxyl groups32 which are capable to serve as anchoring points for the

370

attachment of the phosphonic acids. However, the XPS spectra did not give any useful results

371

in the detection of oxygen on the piranha-treated gold surfaces. The Au4f peaks of the piranha

372

and untreated gold slides are essentially identical exhibiting binding energies of 83.98 (4f7/2)

373

and 87.68 (4f5/2) eV (Figure 8).

374

In the case of present gold oxides, peaks at 85.5 (4f7/2) and 89.4 (4f5/2) eV would arise and

375

even at low concentrations these should be detectable as shown in another publication.30 In

376

order to confirm any kind of impurities, the experiments have been done also on freshly

377

sputtered gold samples which gave the same results: any other elements (e.g. sulfur) that

378

could favor the adsorption of PA on these activated gold surfaces could not be detected.

379

IR spectroscopy and mode of binding. In addition to XPS experiments, ATR-IR

380

measurements have been performed using a highly sensitive detector. In Figure 10, ATR-IR

381

spectra of TPA adsorbed onto piranha treated gold surfaces are depicted showing different ACS Paragon Plus Environment

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382

features. IR bands in the region of 3000-2750 cm-1 (2917 cm-1 νasCH2: 2876 cm-1 νsC-H of

383

CH3: 2849 cm-1 and νsCH2 at 1463 cm-1) can be unambiguously assigned to different C-H

384

stretching vibrations of the alkyl chains of TPA which corroborate the XPS and SPR data. In

385

contrast to Luschtinetz et al. who proposed a tridentate binding mode of phosphonic acids on

386

Al2O3 surfaces due to the absence of P=O vibrational bands7, signals attributed to P=O and P-

387

O(H) vibrations are observed in the IR spectra of PA on gold surfaces which could be

388

resolved in GI-IR experiments. For the TPA derivative, an interesting feature is noticed after

389

the sintering step. New bands appear which can be attributed to condensation products of the

390

phosphonic acid (ν(P=O)-O-P at 1263 and νP-O-P at 966 cm-1). Additionally the P=O group

391

of the monomeric TPA slightly shifts to smaller wavenumbers (1165 cm-1). Therefore, as

392

already postulated by XPS, mono- (TPA oligomers) and bidentate (VPA, OPA, TPA) binding

393

mechanisms can be proposed. Characteristic P-OH vibration bands of the free phosphonic

394

acid at approximately 2700 and 2300 cm-1 (Figure 10, c) are visible in the spectra of the bulk

395

material, but absent in the spectra of all PA monolayers. Full spectra of all PA adsorbed on a

396

piranha treated gold substrate and a blank are shown in Figure S5.


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397 398

Figure 10. ATR-IR spectra of the TPA monolayers (a, b) and bulk TPA (c) on piranha

399

cleaned SPR slides as well as GI-IR spectra of TPA on piranha treated gold before and after

400

tempering (d). Please note that intensities between individual bands may vary due to the

401

different acquisition modes and effects stemming from

402

At the moment, we speculate that the adsorption is favored by a thin hydration layer present

403

on the gold surface which is the anchoring point for the organophosphonic acids by hydrogen

404

bonding. During the tempering step, covalent bonds are formed allowing for a stable

405

monolayer coating. This would also explain why the formed monolayer can be rinsed away

406

with water before tempering since hydrogen bonds between water molecules are stronger than

407

a hydrogen bond between the organophosphonic acids and water. Further, XPS on a blank,

408

piranha treated SPR slide features an O1s peak which has the same binding energy (532.8 eV)

409

as reported for water monolayers on gold (O1s: 532.9 eV).33 However, the intensity of this


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410

peak is very weak and may be attributed to other contaminants from the ambient atmosphere

411

as well.

412

Conclusion

413

In this paper, the adsorption of different PA on activated gold surfaces to form self

414

assembled monolayers has been investigated by SPR, XPS and IR spectroscopy (ATR, GI). In

415

terms of maximum adsorption on the surfaces a piranha pre-treatment was most effective. We

416

propose a mixed mono/bidentate (TPA) and bidentate (VPA, OPA) adsorption mode that is

417

firstly stabilized by hydrogen bonding and subsequently irreversibly fixed by a tempering

418

step. Although it is not completely clear how the adsorption works in very detail, the results

419

may lead to the development of new applications for organophosphonic acids on the basis of

420

noble metal surfaces rather than metal oxides. This would not only allow to use these

421

materials directly in transistors without any anchoring layer for instance (reducing the number

422

of processing steps) but also in other fields where gold surfaces play a major role such as in

423

sensing devices or electrochemistry.

424

Author Information

425

Corresponding Author

426

*E-mail [email protected], Tel +43 316 87332284

427

Acknowledgement

428

The authors would like to acknowledge financial support from NAWI GRAZ Infrastructure

429

Grant and the People Programme (Marie Curie Actions – Career Integration Grants) of the

430

European Union’s Seventh Framework Programme (FP7/2007-2013) under REA grant

431

agreement n°618158 (PhotoPattToCell). Part of this work has been performed in the project

432

V1.06 of the Polymer Competence Center Leoben funded by the Austrian Government and

433

the State Governments of Styria and Upper Austria within the COMET program.


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434

Associated Content

435

Supporting Information available: Example for SPR curves used for fitting procedures,

436

Extensive XPS data and AFM cross section. This material is available free of charge via the

437

internet at http://pubs.acs.org.

438

References

439 440

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Adsorption Studies of Organophosphonic Acids on Differently

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