The main theme of the research being undertaken in my group is an exploration of the forces controlling the interaction of small molecules with surfaces. Our focus is on solution phase species at the solid/liquid interface. Our strategy is one in which we develop model systems where a smaller chemical species acts as an analogue of the more complex molecules in solution. We focus in particular on problems in which H-bonding interactions, fluorous interactions and the origin of charge at surfaces play an important role. Why is developing a fundamental understanding of such interactions important? Interfacial chemistry underlies many of the processes that take place when purifying mixtures of compounds, the partitioning of trace nutrients in the environment or when trying to detect biomolecules when testing for diseases such as cancer. In what follows, we discuss how we measure forces at surfaces using chemical force spectrometry; some of the techniques we have available for characterising surfaces, and some of the research projects ongoing within the group.

Chemical Force Spectrometry

To directly measure interaction forces between molecules, we use a technique called chemical force spectrometry: in this novel form of scanning probe experiment, atomic force microscope (AFM) tips are modified with appropriate functional groups and the adhesive and repulsive interactions between these modified tips and an appropriate substrate are measured. The animation shows that as the AFM tip approaches the sample, adhesive forces bend (displace) the tip towards the surface, until we reach the point at which the tip “snaps” into contact with the sample. The tip is then pulled away from the sample and eventually “snaps off” the sample. The system can be treated as a spring, following Hooke’s law
F = k · x(1)
in that the force, F, required to extend the tip is proportional to the displacement, x, of the tip as it bends towards the sample. Knowing the force constant, k, of the cantilever, allows us to determine the pull-off force – this is on the order of pN to nN. This method not only allows surface imaging with chemical specificity it also, by attaching molecules of interest to the AFM tip, provides us with a means of simulating the interactions of solution phase species with a solid surface.

An example of this technique in action is shown in Figure 1, which shows a chemical force titration: that is, the measurement of the adhesion force between tip and sample as a function of solution pH. Curves for two tip-sample combinations are shown. The first, in green, is for a monoprotic phosphate compound which forms a self-assembled monolayer on a Au surface. The phosphate end group has a solution pKa of about 4.7. We can see in the force titration curve that a maximum adhesion force is observed at a similar pH. Above this pH tip and sample are deprotonated and negatively charge, hence the drop in adhesive force, as the two electrostatically repel one another. Below this pH, the forces also drop off. At pH = pKa, tip and sample are each 50% ionized. This affords the opportunity to form a maximum number of ionic H-bonds: bonding between an (RO)2-POOH and (RO)2-POO- species. These ionic H-bonds turn out to be quite strong, and hence a maximum in the force titration curve indicates the surface pKa of the species under examination. In the case of the diprotic phosphate, in blue, we see two maxima in the force titration profile as first one, and then the other OH site is ionised. This indicates surface pKa values of 4.0 and 7.5 for the diprotic species. Thus, chemical force spectrometry, and chemical force titrations can give us information on both the types of ionisable groups present at the surface, and information on the acid-base equilibria taking place there.

Figure 1 Chemical force titration experiment using self-assembled monolayers of a monoprotic and diprotic phosphate species.

X-ray photoelectron spectroscopy

The characterisation of appropriate model systems for self-assembly onto the AFM tip is a significant challenge. One means of doing this is using X-ray photoelectron spectroscopy. Our laboratory is equipped with a Microlab Surface Analysis system for carrying out photoelectron spectroscopy measurements. Photoelectron spectroscopy is in essence an application of Einstein’s photoelectric effect: probing the band structure of a solid material by irradiating the sample with a beam of photons. These photons eject electrons (the photoelectron) from the solid, and the kinetic energy (Ek) of the outgoing photoelectrons is directly related to the energy of the irradiating photon source (hν) and the binding energy (EB) of the electron in the solid [Equation 2]. Our XPS system uses a switchable Mg or Al Kα line at 1253.6 eV or 1486.6 eV, respectively. At these energies, it is primarily core electron energy levels that are probed. Valence level spectra can be collected, but at relatively low resolution.
Ek = EB - hν(1)
As seen in Figure 2, the ejected photoelectrons are collected and analyzed as a function of their kinetic energy using a photoelectron spectrometer. This consists of a series of electrostatic lenses and steering voltages to selectively separate electrons of varying kinetic energies and bring them to an electron multiplier where they are detected. All this takes place under ultra high vacuum (UHV) conditions of <10-9 mbar pressure. Any non-volatile solid sample can be analysed, although non-conducting materials may require some special techniques to properly analyse.

Figure 2 Diagram of an X-ray photoelectron spectrometer.
A typical XP spectrum is a plot of the number of photoelectron intensity as a function of binding energy. Figure 3 shows the photoelectron spectra from a polycrystalline Pd foil which acts as the catalyst in the coupling reaction of p-nitroiodobenzene to a pinacol ester of phenyl boronic acid:

Scheme 1 Coupling of p-nitroiodobenzene and a pinacol ester of phenyl boronic acid.
XPS shows that the boron species does not appear to adsorb or react directly with the surface Pd, since any XPS signal from boron is absent, regardless of conditions. After the bare Pd foil (A) is placed under reaction conditions in the presence of both p-nitroiodobenzene and the boronic ester, significant changes (B) in the Pd spectrum are observed with a shoulder appearing at higher binding energies, corresponding to a Pd species at a high oxidation state. Comparing these spectra to those taken for Pd foil heated in the absence of reactants (C) or in the presence of only p-nitroiodobenzene (D) allows us to reach some significant conclusions as to the reaction mechanism and the species involved. Further examination of the corresponding N 1s and I 3d XP spectra, an intermediate species R-Pd-I intermediate is identified. Two nitrogen-containing species are also identified in the N 1s spectra, an NO2 and an amine, probably arising from adsorption of the dimethylamine base used during the reaction. Finally, comparison to the spectra from a PdI2 salt (E) leads to the conclusion that PdI2 is not directly formed on the surface during this reaction. Together with related scanning electron microscopy data, the XPS data establish that the Pd metal reacts with RI to form a R-Pd-I species which then desorbs to perform the Suzuki coupling reaction with the boronic ester in solution, demonstrating the use of XPS in illustrating the mechanism of heterogeneous reactions.

Figure 3 XP spectra of Pd, N and I for a Pd foil catalyzing the Suzuki-Miyaura coupling reaction. Identification of various spectra is noted in the text.

Research Projects

Our group is pursuing several projects in which the interaction of solution phase species with surfaces plays a crucial role. The first of these is adsorption at metal oxide and mineral surfaces, including mechanisms for the formation of biofilms, through adhesion of bacteria to primarily iron oxide surfaces. The second is the separation of enantiomers through the formation of a diastereomeric complex of a chiral molecule (often a drug precursor) in solution to an immobilized chiral molecule on the surface of a chromatographic column, a so-called chiral stationary phase, in chiral chromatography. In the final project, we will look at how polymers may be surface-modified to interact (or not interact!) with biomolecules in polymer-based microscale analytical devices. Please see the pages below for more information: